declared on 4000 rpm engine speed. From the minimum engine speed to 2500 rpm engine speed, the increasing engine speed can be decrease the ISFC. After 2500 to 4000 rpm, the increasing engine speed can be increase the ISFC. In the original and modified direct injection diesel engine, the fuel is injected directly to engine cylinder, the lowest ISFC are 205.93 g/kW-h and 210.98 g/kW-h declared on 3000 rpm engine speed. Then, the highest ISFC are 245.981 g/kW-h and 254 g/kW-h declared on 4000 rpm engine speed. The ISFC profile shows that from the minimum engine speed at 1500 rpm to medium speed, the increasing engine speed is decrease the ISFC. From medium speed to maximum engine speed at 4000 rpm, the increasing engine speed is increase the ISFC. The conversion of diesel engine to NG engine can be reduce the ISFC in the low to medium engine speed, but in the medium engine speed to high speed can be increase the ISFC. The minimum ISFC of multi point injection NG engine is reduced 2.9% compared to direct injection diesel engine. The maximum ISFC of port injection NG engine has increase 31.53% compared to direct injection diesel engine. The increasing engine speed can be increase the ISFC of NG engine compared to both of diesel engines. It meant that the NG engine not applicable to run on high speed, but it is very good ISFC in medium speed. The combustion in high speed is not excellent, not completely and high in unburned fuel, so the engine is produce lower power than the medium speed because the NG engine is developed in low energy and low density. The

g/kW-h declared on 2500 rpm engine speed and the highest ISFC is 323.532 g/kW-h

effect of the lower energy, density and power is increase the ISFC to the higher.

Brake specific fuel consumption (BSFC) of original diesel engine, modified diesel engine and port injection NG engine are shown in Fig. 9b. The BSFC is the nominal total fuel were used

Fig. 9. Specific fuel consumption of diesel engine convert to MPI NG engine.

500 1000 1500 2000 2500 3000 3500 4000 4500 Engine Speed (rpm)

185

210

235

260

ISFC (g/kW-h)

285

310

335

O.D.E Sim. O.D.E Expr. 14.5CR D.E CNG.E Sim. CNG. E Expr. in the engine to product their brake power output. The simulation results are higher than the experimental results, it caused by the not excellently setting and reading data experiment and the assumption in simulation with not have losses. In the original and modified diesel engine, the lowest BSFC is 252 g/kW-h and 249.18 g/kW-h declared in 2500 and 2000 rpm engine speed. Then, the highest BSFC both of the diesel engines are 329.678 g/kW-h and 336.62 g/kW-h declared in 4000 rpm engine speed. In the original diesel engine, the BSFC profile shows that from 1500 rpm to 2500 rpm, the increasing engine speed can be decrease the BSFC. After 2500 rpm engine speed, the increasing engine speed can be increase the BSFC. In the modified diesel engine, the BSFC profile shows that from 1500 rpm to 2000 rpm, the increasing engine speed can be decrease the BSFC. But, after 2000 rpm engine speed, the increasing engine speed can be increase the BSFC. In the port injection NG engine, the lowest BSFC is 247.23 g/kW-h declared in 2000 rpm engine speed and the highest BSFC is 659 g/kW-h declared in 4000 rpm engine speed. From 1500 to 2000 rpm engine speed, the increasing engine speed of NG engine has been reduced the BSFC. After 2000 to 4000 rpm engine speed, the increasing engine speed can be increase the BSFC extremely. Its means, that the conversion of direct injection diesel engine to multi point injection NG engine can reduce the BSFC in lowest to medium engine speed and increase the BSFC in medium to highest engine speed. The minimum BSFC of modified direct injection diesel engine convert to port injection NG engine is reduced 0.78%. The maximum BSFC of modified direct injection diesel engine convert to port injection NG engine is increase 95.79%. The increasing engine speed can be increase the deviation point or deviation percentage of the BSFC of diesel engine compared to NG engine. In the low to medium engine speed, the unburned fuel in NG engine combustion is decrease so the out put can be product the higher torque and power. From the medium to the highest engine speed, the combustion of NG engine is not excellently, so the fuel consumption is increase because the unburned fuel is increase in the medium to highest speed. The effect of the increasing unburned fuel is can be decrease the engine brake torque and brake power. The effect of lower brake torque and brake power is increase the BSFC.

#### **4.7 Mean effective pressure of multi point injection Natural Gas engine**

The IMEP result is shown in Fig. 10a. The simulation and experimental investigation results are not similar average in 1.5 % for both of the engines. The simulation results are higher than the experimental results, it caused by the not excellently setting and reading data experiment and the assumption in simulation with not have losses. In the NG engine, the highest IMEP is 6.34952 bar declared on 2000 rpm engine speed. From the minimum engine speed to 2000 rpm engine speed, the increasing engine speed can be increase the IMEP. After 2000 rpm to maximum engine speed, the increasing engine speed can be decrease the IMEP. In the original and modified diesel engine, the highest IMEP is 7.5242 bar and 7.34 bar declared on 3000 rpm engine speed. The IMEP performance profile of diesel engines shows that from the minimum engine speed at 1500 to 3000 rpm engine speed, the increasing engine speed can be increase the IMEP, but from 3000 rpm to maximum engine speed at 4000 rpm, the increasing engine speed can be decrease the IMEP. Based on Fig.10a, the conversion diesel engine to NG engine can reduce the IMEP. The maximum IMEP of modified direct injection diesel engine convert to port injection NG engine has been reduced

Application of Natural Gas for Internal Combustion Engines 475

The investigation results on mean effective pressure performance such as indicated and break mean effective pressure of direct injection diesel engine converted to port injection NG engine are shown that the engine conversion development can be decrease the engine power performance. The reduction of the mean effective pressure in the NG engine is caused by lower energy, density and higher friction of compressed natural gas as an alternative fuel for engines. The increasing of engine speed over the medium speed can be decrease the engine mean effective pressure performance of NG engine. The highest of engine mean effective pressure of NG engine is declared in medium engine speed and the lowest mean effective pressure is declared in highest engine speed. The increasing engine speed will be increase the percentage deviation of mean effective pressure performance of

The original diesel engine cylinder pressure is higher than the modified diesel engine and CNG engine. It caused the compression ratio of NG engine is lower than the original diesel engine and the combustion energy output of diesel fuel has to produce the highest power than natural-gas fuel. Another that, the density of natural-gas fuel is lower than the diesel fuel. The increasing engine speed of the diesel engine has increase the maximum temperature in engine. Unfortunately, the increasing engine speed of NG engine has been decrease maximum temperature inin- engine. The decreasing engine speed of diesel engines has been decrease maximum temperature inin- engine. Decreasing engine speed of NG engine has increase maximum temperature in engine. Engine torque, power, mean effective pressure and efficiency performance of original direct injection diesel engine cylinder pressure is higher than the modified diesel engine and sequential port injection dedicated NG engine. It meant that the thermodynamics energies were resulted from the diesel fuel combustion is higher than the NG fuel. It caused by the hydrocarbon chain, density and energy of diesel fuel of the diesel engine is higher than the gas fuel of NG engine were ignited using to spark assistant. The fuel consumption of NG engine is higher than the diesel engine. Fuel consumption is increased because the unburned fuel is increased in the medium to the highest speed of NG engine. The effect of the increasing unburned fuel is can decrease the engine brake torque and brake power. The effect of lower brake torque and brake power increase the brake specific fuel

Aslam, M.U.; Masjuki, H.H.; Kalam, M.A.; Abdesselam, H.; Mahlia, T.M.I.; Amalina, M.A.

Bakar, R. A.; Sera, M.A.; Mun, W.H. (2002). Towards The Implementation of CNG Engine: A

(2006). An experimental investigation of CNG as an alternative fuel for a retrofitted

Literature Review Approach To Problems And Solutions. *Proc. of BSME-ASME International Conference on Thermal Engineering*. December 31, 2001 – January 2, 2002.

NG engine compared to diesel engine.

**5. Conclusion**

consumption..

**6. References**

gasoline vehicle. *Fuel* 85: 717–724.

Dhaka. BSME-ASME. 2002.

Fig. 10. Mean effective pressure of diesel engine convert to MPI NG engine.

13.54%. The increasing engine speed can be increase the deviation point or percentage deviation of the IMEP of NG engine compared to diesel engine. If the engine is run on 1500 to 4000 rpm, the conversion of modified diesel engine to port injection NG engine It meant that the NG engine not applicable to run on high speed and very good IMEP performance in low to medium speed. The combustion in high speed produces lower IMEP than the medium speed because the NG engine is developed in low energy. Effect of lower energy is causing the lower indicated torque. The lower indicated torque is can be reducing the lower IMEP.

Brake mean effective pressure (BMEP) performance of original diesel engine, modified diesel engine and port injection NG engine are shown in Fig. 10b. The BMEP is the external shaft work done per unit displacement. The simulation results are higher than the experimental results, it caused by the not excellently setting and reading data experiment and the assumption in simulation with not have losses. In the port injection NG engine, the highest BMEP is 5.21 bar declared on 1500 rpm engine speed. From 1500 to 1500 rpm, the increasing engine speed is increase BMEP. After 1500 rpm engine speed, the increasing engine speed can be decrease the BMEP. In the original and modified diesel engine, the highest BMEP are 6.148 bar and 6.22 bar declared at 2500 and 2000 rpm engine speed. The BMEP performance profile of original and modified diesel engine shows that the BMEP performance is increase if the engine speed is increased until 2500 and 2000 rpm engine speed. After 2500 and 2000 rpm engine speed, the increasing engine speed can be decrease the BMEP. The conversion of four stroke direct injection diesel engine to multi point injection NG engine can be reducing the engine BMEP performance. The maximum BMEP of modified direct injection diesel engine convert to port injection NG engine is reduced 16.18%. If the engines are run on 1500 to 4000 rpm, the conversion of diesel engine to port injection NG engine reduced BMEP 15.14%, 16.48%, 25.67%, 45.68%, 57% and 70.68%.

The investigation results on mean effective pressure performance such as indicated and break mean effective pressure of direct injection diesel engine converted to port injection NG engine are shown that the engine conversion development can be decrease the engine power performance. The reduction of the mean effective pressure in the NG engine is caused by lower energy, density and higher friction of compressed natural gas as an alternative fuel for engines. The increasing of engine speed over the medium speed can be decrease the engine mean effective pressure performance of NG engine. The highest of engine mean effective pressure of NG engine is declared in medium engine speed and the lowest mean effective pressure is declared in highest engine speed. The increasing engine speed will be increase the percentage deviation of mean effective pressure performance of NG engine compared to diesel engine.

### **5. Conclusion**

474 Advances in Natural Gas Technology

a) IMEP of NG engine b) BMEP of NG engine

0.5

500 1000 1500 2000 2500 3000 3500 4000 4500 Engine Speed (rpm)

O.D.E Sim. O.D.E Expr. 14.5CR Sim. CNG.E Sim. CNG.E Expr.

1.5

2.5

3.5

BMEP (Bar)

4.5

5.5

6.5

13.54%. The increasing engine speed can be increase the deviation point or percentage deviation of the IMEP of NG engine compared to diesel engine. If the engine is run on 1500 to 4000 rpm, the conversion of modified diesel engine to port injection NG engine It meant that the NG engine not applicable to run on high speed and very good IMEP performance in low to medium speed. The combustion in high speed produces lower IMEP than the medium speed because the NG engine is developed in low energy. Effect of lower energy is causing the lower indicated torque. The lower indicated torque is can be reducing the lower

Brake mean effective pressure (BMEP) performance of original diesel engine, modified diesel engine and port injection NG engine are shown in Fig. 10b. The BMEP is the external shaft work done per unit displacement. The simulation results are higher than the experimental results, it caused by the not excellently setting and reading data experiment and the assumption in simulation with not have losses. In the port injection NG engine, the highest BMEP is 5.21 bar declared on 1500 rpm engine speed. From 1500 to 1500 rpm, the increasing engine speed is increase BMEP. After 1500 rpm engine speed, the increasing engine speed can be decrease the BMEP. In the original and modified diesel engine, the highest BMEP are 6.148 bar and 6.22 bar declared at 2500 and 2000 rpm engine speed. The BMEP performance profile of original and modified diesel engine shows that the BMEP performance is increase if the engine speed is increased until 2500 and 2000 rpm engine speed. After 2500 and 2000 rpm engine speed, the increasing engine speed can be decrease the BMEP. The conversion of four stroke direct injection diesel engine to multi point injection NG engine can be reducing the engine BMEP performance. The maximum BMEP of modified direct injection diesel engine convert to port injection NG engine is reduced 16.18%. If the engines are run on 1500 to 4000 rpm, the conversion of diesel engine to port injection NG engine reduced BMEP 15.14%, 16.48%, 25.67%, 45.68%, 57% and 70.68%.

Fig. 10. Mean effective pressure of diesel engine convert to MPI NG engine.

500 1000 1500 2000 2500 3000 3500 4000 4500 Engine Speed (rpm)

IMEP.

2

3

4

O.D.E Sim. O.D.E Expr. 14.5CR Sim. CNG.E Sim. CNG. E Expr.

5

IMEP (Bar) .

6

7

8

The original diesel engine cylinder pressure is higher than the modified diesel engine and CNG engine. It caused the compression ratio of NG engine is lower than the original diesel engine and the combustion energy output of diesel fuel has to produce the highest power than natural-gas fuel. Another that, the density of natural-gas fuel is lower than the diesel fuel. The increasing engine speed of the diesel engine has increase the maximum temperature in engine. Unfortunately, the increasing engine speed of NG engine has been decrease maximum temperature inin- engine. The decreasing engine speed of diesel engines has been decrease maximum temperature inin- engine. Decreasing engine speed of NG engine has increase maximum temperature in engine. Engine torque, power, mean effective pressure and efficiency performance of original direct injection diesel engine cylinder pressure is higher than the modified diesel engine and sequential port injection dedicated NG engine. It meant that the thermodynamics energies were resulted from the diesel fuel combustion is higher than the NG fuel. It caused by the hydrocarbon chain, density and energy of diesel fuel of the diesel engine is higher than the gas fuel of NG engine were ignited using to spark assistant. The fuel consumption of NG engine is higher than the diesel engine. Fuel consumption is increased because the unburned fuel is increased in the medium to the highest speed of NG engine. The effect of the increasing unburned fuel is can decrease the engine brake torque and brake power. The effect of lower brake torque and brake power increase the brake specific fuel consumption..

#### **6. References**

Application of Natural Gas for Internal Combustion Engines 477

Kubesh, John. T.; Podnar, Daniel. J. (1998). Ultra Low Emissions and High Efficiency from

Lino, Paolo.; Maione, Bruno.; Amorese, Claudio. (2008). Modelling and predictive control of

Mbarawa, M.; Milton, B.E.; Casey, R.T. (2001). Experiments and modelling of natural gas

Oullette, P.; Mtui, P.L.; Hill, P.G. (1998). Numerical Simulation of Directly Injected Natural

Ouellette, Patric. (2000). High Pressure Direct Injection (HPDI) of Natural Gas in Diesel

Pischinger, S.; Umierski, M.; Hüchtebrock, B (2003). New CNG concepts for passenger

Poulton, M.L. (1994). *Alternative Fuels for Road Vehicles*, Computational Mechanics

Ren, W and Sayar, H. (2001). Influence of nozzle geometry on spray atomization and shape

Sera, M.A.; Bakar, R.A.; Leong, S.K. (2003) CNG engine performance improvement strategy

Shashikantha.; Parikh, P.P. (1999). Spark ignition producer gas engine and dedicated

Shiga, S.; Ozone, S.; Machacon, H. T. C.; Karasawa, T. (2002). A Study of the Combustion

Suga, T.; Muraishi, T.; Brachmann, T.; Yatabe, F. (2000). Potential of a natural gas vehicle as

Tilagone, R.; Monnier, G.; Chaouche, A.; Baguelin, Y. and Chauveron D.S. (1996).

Vermiglio. E; Jenskins, T.; Kleliszewski, M.; Lapetz, J.; Povinger, B.; Willey, R.; Herber, J.;

Wang, D. E and Watson, H. C. (2000). Direct injection compressed natural gas combustion

Wayne, W. S.; Clark, N. N.; Atkinson, C. M. (1998). A parametric study of knock control

Zastavniouk, Oleg. (1997). Study of Mixing Phenomena in a Dual Fuel Diesel Engine Air

compressed natural gas engine-Technology development and experimental

and Emission Characteristics of Compressed-Natural-Gas Direct-Injection Stratified Combustion Using a Rapid-Compression-Machine, *Combustion and*

Development of a High Efficiency, low Emission SI-CNG Bus Engine. *SAE Paper*.

Sahutske, K.; Blue, M, and Clark, R. (1997). Ford's SULEV Dedicated Natural Gas

a new injection system for compressed natural gas engines. *Control Engineering*

combustion ignited by a pilot diesel fuel spray. *Int. J. Therm. Sci.* 40: 927–

Gas and Pilot Diesel Fuel in a Two Stroke Compression Ignition Engine. State of

cars: High torque engines with superior fuel consumption. *SAE Paper*. 2003-01-

On-Highway Natural Gas Engine. *SAE Paper* 981394.

Alternative Fuel Technologies. *SAE Paper* 981400.

for port fuel injector. *SAE Paper*. 2001-01-0608.

Engines. Proceeding ANGVA 2000 Conference. Yokohama.

through advanced intake system. *SAE Paper*. 2003-01-1937.

performance optimization. *SAE Paper*.1999-01-3515.

*Practice*, 2008. 16 (10): 1216-1230.

936.

2264.

Publication, London.

*Flame* 129:1–10.

961080.

EEV. *SAE Paper*. 2000-01-1863.

Trucks. *SAE Paper*. 971662.

and visualization. *SAE Paper* 2000-01-1838.

strategies for a bi-fuel engine. *SAE Paper*. 980895.

Intake Manifold. *MSc Thesis*. University of Alberta.

Brombacher, E.J. (1997). Flow Visualisation of Natural Gas Fuel Injection, *MSc Thesis*,

Catania, A.E.; Misul, D.; Spessa, E.; Vassallo, A. (2004). Analysis of combustion parameters

Chiu, J.P. (2004). Low Emissions Class 8 Heavy-Duty, On-Highway Natural Gas and

Cho, H. M.; He, Bang-Quan (2007). Spark ignition natural gas engines - A review. *Energy*

Czerwinski J.; Comte P.; Janach.W.; Zuber P. (1999). Sequential Multipoint Trans-Valve-

Czerwinski, J.; Comte, P.; Zimmerli, Y. (2003). Investigations of the Gas Injection System on

Duan, S.Y. (1996). Using Natural Gas in Engines: Laboratory experience with the use

Durrel, Elizabeth.; Allan, Jeff.; Law, Donald.; Heath, John. (2000). Installation and

Evan, R.L.; Blaszczyk, J.; and Matys, P. (1996). An Experimental and Numerical Study of

Hollnagel, Carlos.; Borges, Luiz.H.; Muraro, Wilson. (1999). Combustion Development of

Hollnagel, C.; Neto, J. A. M.; Di Nardi, M. E.; Wunderlich, C.; Muraro, W. (2001).

Johansson, B. and Olsson, K. (1995). Combustion Chambers for Natural Gas SI Engines Part

Kato, K.; Igarashi, K.; Masuda, M.; Otsubo, K.; Yasuda, A.; Takeda, K.; Sato, T. (1999).

Kawabata, Y and Mori, D. (2004). Combustion diagnostics and improvement of a

Klimstra, J. (1989). Carburetors for Gaseous Fuels - On Air-to-Fuel Ratio, Homogeneity and

Kowalewicz, Andrzej. (1984). *Combustion System of High-Speed Piston I.C. Engines*. Warszawa:

Kubesh, J.T.; Podnar, D.J.; Gugliemo, K.H. and McCaw, D. (1995). Development of an

Electronically-Controlled Natural Gas-Fueled John Deere Power Tech 8.1L Engine.

prechamber lean-burn natural gas engine. *SAE Paper*. 2004-01-0979.

of natural gas fuel in IC engines. *IMechE Seminar Publication*, 1996: 39-

Development of a Direct Injection System for a Bi-Fuel Gasoline and Compressed Natural Gas. *Proceeding of ANGV Conference 2000*. October 17 – 19, 2000. Yokohama:

Combustion Chamber Design for Lean-Burn Natural Gas Engines. *SAE Paper*.

the Mercedes-Benz MY1999 CNG-Engine M366LAG. *SAE Technical Paper* 1999-01-

Application of the natural gas engines Mercedes-Benz in moving stage for the

Development of engine for natural gas vehicle. *SAE Paper*. 1999-01-0574.

multivalve bi-fuel CNG SI engine. *SAE Paper*. 2004-01-0983.

Injection for Natural Gas Engines. *SAE Paper*. 1999-01-0565.

Ganesan, V. (1999). *Internal Combustion Engines*. N.Delhi: Tata McGraw-Hill.

carnival 2001 in Salvador City. *SAE Paper*. 2001-01-3824.

I: Fluid Flow and Combustion. *SAE Paper*. 950469.

Flow Restriction. *SAE Paper* 892141.

*SAE Paper .*951940.

Wydawnictwa Komunikacji i Lacznosci..

Gasoline Engine. *SAE Paper*. 2004-01-2982.

a HD-CNG-Engine. *SAE Paper*. 2003-01-0625.

*Conversion and Management* 48: 608–618.

and their relation to operating variables and exhaust emissions in an upgraded

University of Toronto.

46.

ANGV.

961672.

3519.

1999.

**The Influence of Modified Atmosphere**

Coal technology, dominant in Poland, ensures efficient production of electricity and heat. However, a large exploitation of existing reserves and the growing demand for electricity causes interest in other available fuels, primarily natural gas. Environmentally friendly modern technologies are constantly looking for the possibility of using energy sources other than coal. Research is carried out on various innovative technologies such as CO2 capture and storage or unconventional combustion. Combustion in the oxygen-enriched gas mixtures at the first was conducted in the steel industry and metallurgy, which require very high temperatures to heat the metal and pig iron. Such a process can be classified as the

The use of natural gas in the metallurgical processes requires the precise technological regime, therefore, it is difficult to make changes in the process (Amann, Kanniche & Bouallou, 2009; Choi & Katsuki, 2001; Flamme, 2001). However, appropriately organized gas combustion process can lead to a reduction in CO2 emissions, which is required by applicable laws imposed by the European Union. The European Union environmental priorities promote the development of the new technologies of energy production from the conventional sources (coal and natural gas), e.g. oxy-combustion (Andersson & Johnsson, 2007; Czakiert et al., 2006; Davidson, 2007; H.K. Kim, Kim, Lee & Ahn, 2007; Kotowicz, 2007; Lampert & Ziębik, 2007; Li, Yan & Yan, 2009; Muskał et al., 2008; Seepana & Jayanti, 2009; Simpson & Simon, 2007; Szlęk et al., 2009; Tan et al., 2002). The oxy-fuel combustion process is conducted in an atmosphere enriched in oxygen which means that the reactor is supplied by the combustion gas mixture, in which the oxygen concentration is higher than the concentration of oxygen in the air. The study of the modified air combustion appears in scientific literature. There are various modified oxidizing atmospheres like O2/N2, O2/N2/CO2, O2/CO2. It should be emphasized that the majority of results of the oxy-combustion process refer to coal combustion, because of large amount of deposited coal in Poland (Buhre et al., 2005; Chen, Liu & Huang, 2007; Croiset, Thambimuthu & Palmer, 2000; Czakiert, Nowak & Bis, 2008; Kim et al., 2007; Normann et al., 2008). The research effort is focused on the conventional coal, fluidised bed combustion and co-generation solutions with the coal gasification (Lampert & Ziębik, 2007; Tan et al., 2002). Oxy-combustion is the subject of research in many international

future technology known as the clean combustion technologies.

**1. Introduction**

**on Natural Gas Combustion**

Małgorzata Wilk and Aneta Magdziarz *AGH University of Science and Technology, Krakow,*

*Poland*

Zhao, F.; Lai, M.; Harrington, D.L. (1995). The Spray Characteristics of Automotive Port Fuel Injection – A Critical Review. *SAE Paper*. 950506. **19**

## **The Influence of Modified Atmosphere on Natural Gas Combustion**

Małgorzata Wilk and Aneta Magdziarz *AGH University of Science and Technology, Krakow, Poland*

### **1. Introduction**

478 Advances in Natural Gas Technology

Zhao, F.; Lai, M.; Harrington, D.L. (1995). The Spray Characteristics of Automotive Port Fuel

Coal technology, dominant in Poland, ensures efficient production of electricity and heat. However, a large exploitation of existing reserves and the growing demand for electricity causes interest in other available fuels, primarily natural gas. Environmentally friendly modern technologies are constantly looking for the possibility of using energy sources other than coal. Research is carried out on various innovative technologies such as CO2 capture and storage or unconventional combustion. Combustion in the oxygen-enriched gas mixtures at the first was conducted in the steel industry and metallurgy, which require very high temperatures to heat the metal and pig iron. Such a process can be classified as the future technology known as the clean combustion technologies.

The use of natural gas in the metallurgical processes requires the precise technological regime, therefore, it is difficult to make changes in the process (Amann, Kanniche & Bouallou, 2009; Choi & Katsuki, 2001; Flamme, 2001). However, appropriately organized gas combustion process can lead to a reduction in CO2 emissions, which is required by applicable laws imposed by the European Union. The European Union environmental priorities promote the development of the new technologies of energy production from the conventional sources (coal and natural gas), e.g. oxy-combustion (Andersson & Johnsson, 2007; Czakiert et al., 2006; Davidson, 2007; H.K. Kim, Kim, Lee & Ahn, 2007; Kotowicz, 2007; Lampert & Ziębik, 2007; Li, Yan & Yan, 2009; Muskał et al., 2008; Seepana & Jayanti, 2009; Simpson & Simon, 2007; Szlęk et al., 2009; Tan et al., 2002). The oxy-fuel combustion process is conducted in an atmosphere enriched in oxygen which means that the reactor is supplied by the combustion gas mixture, in which the oxygen concentration is higher than the concentration of oxygen in the air. The study of the modified air combustion appears in scientific literature. There are various modified oxidizing atmospheres like O2/N2, O2/N2/CO2, O2/CO2. It should be emphasized that the majority of results of the oxy-combustion process refer to coal combustion, because of large amount of deposited coal in Poland (Buhre et al., 2005; Chen, Liu & Huang, 2007; Croiset, Thambimuthu & Palmer, 2000; Czakiert, Nowak & Bis, 2008; Kim et al., 2007; Normann et al., 2008). The research effort is focused on the conventional coal, fluidised bed combustion and co-generation solutions with the coal gasification (Lampert & Ziębik, 2007; Tan et al., 2002). Oxy-combustion is the subject of research in many international

The Influence of Modified Atmosphere on Natural Gas Combustion 481

The relatively high CO emissions in combustion processes of natural gas occur in the

Very rapid cooling of combustion products in the cold boundary layer of the

The formation of CO in the flame is one of the main paths of reaction in the mechanism of combustion of hydrocarbons. Fuel hydrocarbons during the chemical degradation can be partially converted to CO. The formation of CO is done quickly, right at the beginning of the flame. The proposed overall reaction of formation of CO is as follows (for gaseous and liquid hydrocarbons) and volatiles of solid fuels. (Wilk, 2002; Bartok & Sarofim, 1991)

> nm 2 2 n m C H O nCO H 2 2

Oxidation of CO is strongly catalysed by even small amounts of hydrogen or its compounds with oxygen. There are two paths of CO oxidation. Main path of oxidation takes place at T >

The second path CO oxidation takes place at T = 1000 - 1500 K and at p > 1 MPa is as follows

HO2 radical concentration is comparable to the concentration of OH radical reactions and

Competitive reactions to the above and CO oxidation reactions are the following

(1)

CO OH CO H 2 (2)

CO HO CO OH 2 2 (3)

H O M HO M 2 2 (4)

H O OH O <sup>2</sup> (5)

O H OH H <sup>2</sup> (6)

O H O OH OH <sup>2</sup> (7)

H H O OH H 2 2 (8)

HH H 2 (9)

H OH H O 2 (10)

**2.1 The mechanism of CO formation**

Inadequate mixing of air and fuel,

1500 K (at p ≈ 0,1 MPa) and is as follows

rapid reactions (Bartok & Sarofim, 1991)

recombination reaction

HO2 radical is produced in the recombination reaction:

combustion chamber.

Staging combustion for reduction of NOx emissions,

following cases:

research institutions (Chalmers University of Technology, Sweden, University of Leeds UK, CANMET Energy Technology Centre, Canada, Chicago USA Research Centre, Tokyo Institute of Technology, Japan, University of Newcastle Australia). In Poland, the research of oxy-combustion concerns mainly coal (Czestochowa University of Technology), but the natural gas research is also carried out in the field of high-temperature combustion gas HTAC technique (Normann, 2008; Seepana & Jayanti, 2009). Fuel combustion processes are the main source of environmental pollution. In many branches of industry, the main fuel is natural gas, because of the possibility of obtaining a high temperature process e.g. in the glass industry and in the manufacture of cement, the process of oxy-combustion can be applied, or air combustion enriched with oxygen. This of course raises the temperature of the combustion process, which is associated with increased concentrations of NOx. Therefore, oxy-combustion is used simultaneously with the process of eliminating nitrogen from the air combustion (largely responsible for the formation of NO) and replacing it into exhaust gas (RFG - Recycled Flue Gas.). Despite the very high temperatures in the chamber (e.g. 1600 °C in the melting process), the concentration of nitrogen oxides may be lower, due to the elimination of nitrogen from combustion air. The primary obstacle to the propagation of oxy-combustion has so far been the high cost of obtaining pure oxygen. Since the pure oxygen production technologies have been improved and costs have been reduced, oxy-combustion can be applied in many industrial processes. Industry may be interested in this technology, where the conditions in the very high temperature contribute to the formation of large amounts of thermal NO. An additional advantage of the oxy-combustion is high combustion efficiency, lower volume of exhaust gases, less fuel consumption and therefore lower CO2 and NOx. Combustion of natural gas in O2/CO2 atmosphere allows optimising the combustion process. The results of experimental studies indicate that the combustion of natural gas in O2/CO2 with recycling exhaust gases has positive effects on reducing CO2 emissions, a noticeable reduction or even elimination of NOx and improve the efficiency of the furnace (Lampert & Ziębik, 2007; M.Wilk, Magdziarz & Kuźnia, 2010). It was noted that the major advantage of this technology is the ability to apply it in an existing energy plants (H.K. Kim, Kim, Lee & Ahn, 2007; Simpson & Simon, 2007).

Therefore, the problem seems to be both interesting and promising. The complex nature of the combustion process of natural gas causes the obtained experimental results which are not always repeatable so this issue requires further study.

#### **2. Mechanisms of CO and NOx formation in natural gas combustion processes**

In many industries, because of the possibility of obtaining high temperatures, the primary fuel is natural gas, whose main component (ca. 98%) is methane. Combustion of natural gas is the source of the formation of many pollutants. During the combustion of natural gas in metallurgical furnaces the air pollutants are formed. These are nitrogen oxides, carbon oxides, and possibly trace amounts of hydrocarbons. The composition of natural gas varies slightly. The number and types of pollutants emitted from combustion are related to the composition of the fuel, the type of oxidation atmosphere used and the temperature prevailing in the combustion chamber.

#### **2.1 The mechanism of CO formation**

480 Advances in Natural Gas Technology

research institutions (Chalmers University of Technology, Sweden, University of Leeds UK, CANMET Energy Technology Centre, Canada, Chicago USA Research Centre, Tokyo Institute of Technology, Japan, University of Newcastle Australia). In Poland, the research of oxy-combustion concerns mainly coal (Czestochowa University of Technology), but the natural gas research is also carried out in the field of high-temperature combustion gas HTAC technique (Normann, 2008; Seepana & Jayanti, 2009). Fuel combustion processes are the main source of environmental pollution. In many branches of industry, the main fuel is natural gas, because of the possibility of obtaining a high temperature process e.g. in the glass industry and in the manufacture of cement, the process of oxy-combustion can be applied, or air combustion enriched with oxygen. This of course raises the temperature of the combustion process, which is associated with increased concentrations of NOx. Therefore, oxy-combustion is used simultaneously with the process of eliminating nitrogen from the air combustion (largely responsible for the formation of NO) and replacing it into exhaust gas (RFG - Recycled Flue Gas.). Despite the very high temperatures in the chamber (e.g. 1600 °C in the melting process), the concentration of nitrogen oxides may be lower, due to the elimination of nitrogen from combustion air. The primary obstacle to the propagation of oxy-combustion has so far been the high cost of obtaining pure oxygen. Since the pure oxygen production technologies have been improved and costs have been reduced, oxy-combustion can be applied in many industrial processes. Industry may be interested in this technology, where the conditions in the very high temperature contribute to the formation of large amounts of thermal NO. An additional advantage of the oxy-combustion is high combustion efficiency, lower volume of exhaust gases, less fuel consumption and therefore lower CO2 and NOx. Combustion of natural gas in O2/CO2 atmosphere allows optimising the combustion process. The results of experimental studies indicate that the combustion of natural gas in O2/CO2 with recycling exhaust gases has positive effects on reducing CO2 emissions, a noticeable reduction or even elimination of NOx and improve the efficiency of the furnace (Lampert & Ziębik, 2007; M.Wilk, Magdziarz & Kuźnia, 2010). It was noted that the major advantage of this technology is the ability to apply it in an existing energy plants (H.K.

Therefore, the problem seems to be both interesting and promising. The complex nature of the combustion process of natural gas causes the obtained experimental results which are

In many industries, because of the possibility of obtaining high temperatures, the primary fuel is natural gas, whose main component (ca. 98%) is methane. Combustion of natural gas is the source of the formation of many pollutants. During the combustion of natural gas in metallurgical furnaces the air pollutants are formed. These are nitrogen oxides, carbon oxides, and possibly trace amounts of hydrocarbons. The composition of natural gas varies slightly. The number and types of pollutants emitted from combustion are related to the composition of the fuel, the type of oxidation atmosphere used and the temperature

**2. Mechanisms of CO and NOx formation in natural gas combustion**

Kim, Kim, Lee & Ahn, 2007; Simpson & Simon, 2007).

not always repeatable so this issue requires further study.

prevailing in the combustion chamber.

**processes**

The relatively high CO emissions in combustion processes of natural gas occur in the following cases:

The formation of CO in the flame is one of the main paths of reaction in the mechanism of combustion of hydrocarbons. Fuel hydrocarbons during the chemical degradation can be partially converted to CO. The formation of CO is done quickly, right at the beginning of the flame. The proposed overall reaction of formation of CO is as follows (for gaseous and liquid hydrocarbons) and volatiles of solid fuels. (Wilk, 2002; Bartok & Sarofim, 1991)

$$\rm C\_nH\_m + \frac{n}{2}O\_2 \to nCO + \frac{m}{2}H\_2 \tag{1}$$

Oxidation of CO is strongly catalysed by even small amounts of hydrogen or its compounds with oxygen. There are two paths of CO oxidation. Main path of oxidation takes place at T > 1500 K (at p ≈ 0,1 MPa) and is as follows

$$\text{CO} + \text{OH} \rightarrow \text{CO}\_2 + \text{H} \tag{2}$$

The second path CO oxidation takes place at T = 1000 - 1500 K and at p > 1 MPa is as follows

$$\text{CO} + \text{HO}\_2 \rightarrow \text{CO}\_2 + \text{OH}\tag{3}$$

HO2 radical is produced in the recombination reaction:

$$\rm H + O\_2 + M \rightarrow HO\_2 + M \tag{4}$$

HO2 radical concentration is comparable to the concentration of OH radical reactions and rapid reactions (Bartok & Sarofim, 1991)

$$\text{H} + \text{O}\_2 \leftrightarrow \text{OH} + \text{O} \tag{5}$$

$$\text{O} + \text{H}\_2 \leftrightarrow \text{OH} + \text{H} \tag{6}$$

$$\text{O} + \text{H}\_2\text{O} \leftrightarrow \text{OH} + \text{OH} \tag{7}$$

$$\text{H} + \text{H}\_2\text{O} \leftrightarrow \text{OH} + \text{H}\_2\tag{8}$$

Competitive reactions to the above and CO oxidation reactions are the following recombination reaction

$$\text{H} + \text{H} \rightarrow \text{H}\_2\tag{9}$$

$$\text{H} + \text{OH} \rightarrow \text{H}\_2\text{O} \tag{10}$$

The Influence of Modified Atmosphere on Natural Gas Combustion 483

considerable speed only at temperatures higher than 1400 °C. The reaction of a hydrocarbon radical OH plays an important role in the combustion of humidified hydrocarbon fuels.

 If one molecule of one atom of NO and N is produced in this reaction, it immediately becomes the second particle produced by rapid reaction of NO (14). The formation of thermal NO takes place just behind the flame front in a zone of high temperatures (t > 1400 °C). The basic ways of reducing emissions of thermal NO in combustion processes are the reduction of the temperature, shortening the stay of the reagents in the zone of high temperatures and

Malte and Pratt proposed the mechanism of taking into account the role of N2O in NO formation by the following reaction at temperatures lower than 1800 K (Kordylewski, 2008

These reactions, together with the reactions (13) and (14) in the literature are called "the extended thermal mechanism". The formation of NO by a mechanism of N2O formation is particularly important at lower temperatures (T < 1200 °C) in the flames rather poor (λ > 1). Important role in the formation of N2O plays the kind and characteristics of the third body M. It can be assumed that H2O or its dissociation products (O, H, OH) affect the course of

(16)

dt (17)

dt (18)

dt (19)

O N M NO M 2 2 (20)

N O O NO NO <sup>2</sup> (21)

NO O N O 2 22 (22)

the equilibrium state, the preliminary assumption can be given:

dt

hence the rate of formation is:

and then equation (16) takes the form

k1, k2, and k3 are the rate constants of the reaction.

reducing the local concentration of N2 and O2.

(in Polish), Steele et al, 1995)

the reaction (20) (Wilk, 2002).

$$\text{H}\_2 + \text{OH} \rightarrow \text{H}\_2\text{O} + \text{H} \tag{11}$$

H and OH radicals can meet together on the walls where it comes to their exhaustion, and it causes stopping of the CO oxidation reaction. In practice it is the result of too rapid cooling of exhaust gases below 1000 K. This is the main reason of no oxidation of CO to CO2. The direct oxidation of CO by reaction:

$$\text{CO} + \text{O}\_2 \rightarrow \text{CO}\_2 + \text{O} \tag{12}$$

is very unlikely, because this reaction is very slow (the activation energy of this reaction is very high).

#### **2.2 The mechanisms of NO formation**

The primary adverse products of high temperature combustion are the nitrogen oxides NOx. Knowledge of the mechanism of the NOx formation can identify thermal and chemical conditions of furnaces and control of combustion processes, which affects prevention or reduction of the harmful substances emissions. The source of nitrogen oxides is nitrogen in the fuel and molecular nitrogen from the air. In combustion processes, there are two main types of nitrogen oxides: nitrogen monoxide, NO, and dioxide, NO2. The main component of NOx produced during natural gas combustion is NO, whose share in total NOx emissions is typically at least 95%, and the rest is NO2. The concentration of other oxides N2O, N2O3 and N2O5 is low. The amount of NOx in the exhaust gases depends mainly on the combustion temperature, excess air ratio and residence time in the reaction zone. There are four different mechanisms of the formation of NO:

During the combustion of natural gas, containing mostly methane, and not containing chemically bound nitrogen, the main way of NOx formation mechanism in natural gas combustion is the thermal mechanism.

#### **2.2.1 The thermal mechanism of NO formation**

The thermal mechanism is based on the oxidation reactions of nitrogen from the air supplied for combustion, the rate becomes significant above 1400 °C. These reactions were first described by Zeldovich (Bartok & Sarofim, 1991; Warnatz et al., 2006; Tomeczek &, Gradoń, 1997; Muzio & Quartucy, 1997;. Flamme, 1998):

$$\text{CO} + \text{N}\_2 \xrightarrow{\text{k}\_1} \text{NO} + \text{N} \quad \text{k}\_1 = 1,8 \cdot 10^{14} \exp(-318 \text{kJ} \cdot \text{mol}^{-1} \text{ (RT)}) \text{cm}^3 \text{ (} \text{(mol} \cdot \text{s}) \tag{13}$$

$$\text{N} + \text{O}\_2 \xrightarrow{\text{k}\_2} \text{NO} + \text{O} \quad \text{k}\_2 = 6,4 \cdot 10^9 \text{Texp}(-26 \text{kJ} \cdot \text{mol}^{-1} \text{ (RT)}) \text{cm}^3 \text{ (mol} \cdot \text{s}) \tag{14}$$

$$\text{NO} + \text{OH} \xrightarrow{\text{k}\_3} \text{NO} + \text{H } \text{k}\_3 = 3.8 \cdot 10^{13} \tag{15}$$

The term "the thermal NO" is connected with a very high activation energy due to a strong, triple-atomic bond in the molecule N2. It is a highly endothermic reaction that runs with

H and OH radicals can meet together on the walls where it comes to their exhaustion, and it causes stopping of the CO oxidation reaction. In practice it is the result of too rapid cooling of exhaust gases below 1000 K. This is the main reason of no oxidation of CO to CO2. The

is very unlikely, because this reaction is very slow (the activation energy of this reaction is

The primary adverse products of high temperature combustion are the nitrogen oxides NOx. Knowledge of the mechanism of the NOx formation can identify thermal and chemical conditions of furnaces and control of combustion processes, which affects prevention or reduction of the harmful substances emissions. The source of nitrogen oxides is nitrogen in the fuel and molecular nitrogen from the air. In combustion processes, there are two main types of nitrogen oxides: nitrogen monoxide, NO, and dioxide, NO2. The main component of NOx produced during natural gas combustion is NO, whose share in total NOx emissions is typically at least 95%, and the rest is NO2. The concentration of other oxides N2O, N2O3 and N2O5 is low. The amount of NOx in the exhaust gases depends mainly on the combustion temperature, excess air ratio and residence time in the reaction zone. There are

During the combustion of natural gas, containing mostly methane, and not containing chemically bound nitrogen, the main way of NOx formation mechanism in natural gas

The thermal mechanism is based on the oxidation reactions of nitrogen from the air supplied for combustion, the rate becomes significant above 1400 °C. These reactions were first described by Zeldovich (Bartok & Sarofim, 1991; Warnatz et al., 2006; Tomeczek &,

k1 <sup>14</sup> <sup>1</sup> <sup>3</sup> O N NO N k 1,8 10 exp( 318kJ mol /(RT)) cm /(mol s) 2 1

k2 9 13 N O NO O k 6,4 10 Texp( 26kJ mol /(RT)) cm /(mol s) 2 2

The term "the thermal NO" is connected with a very high activation energy due to a strong, triple-atomic bond in the molecule N2. It is a highly endothermic reaction that runs with

(13)

(14)

k3 <sup>13</sup> N OH NO H k 3,8 10 3 (15)

direct oxidation of CO by reaction:

**2.2 The mechanisms of NO formation**

four different mechanisms of the formation of NO:

**2.2.1 The thermal mechanism of NO formation**

Gradoń, 1997; Muzio & Quartucy, 1997;. Flamme, 1998):

 the thermal mechanism, the prompt mechanism, by means of N2O the fuel NOx.

combustion is the thermal mechanism.

very high).

H OH H O H 2 2 (11)

CO O CO O 2 2 (12)

considerable speed only at temperatures higher than 1400 °C. The reaction of a hydrocarbon radical OH plays an important role in the combustion of humidified hydrocarbon fuels.

The rate of formation of NO is expressed by the equation (Warnatz et al., 2006; Gardiner, 2000)

$$\frac{d[\text{NO}]}{dt} = \mathbf{k}\_1[\text{O}][\text{N}\_2] + \mathbf{k}\_2[\text{N}][\text{O}\_2] + \mathbf{k}\_3[\text{N}][\text{OH}] \tag{16}$$

Atomic nitrogen is formed by the reaction (13) and is consumed in the reaction (14) and (15), hence the rate of formation is:

$$\frac{d[\text{N}]}{dt} = \mathbf{k}\_1[\text{O}][\text{N}\_2] - \mathbf{k}\_2[\text{N}][\text{O}\_2] - \mathbf{k}\_3[\text{N}][\text{OH}] \tag{17}$$

Taking into account the fact that reactions (14) and (15) are so fast that their products reach the equilibrium state, the preliminary assumption can be given:

$$\frac{\text{d[N]}}{\text{dt}} = 0 \tag{18}$$

and then equation (16) takes the form

$$\frac{d[\text{NO}]}{dt} = 2k\_1[\text{O}][\text{N}\_2] \tag{19}$$

k1, k2, and k3 are the rate constants of the reaction.

The rate of formation of NO is controlled by the first, slow reaction of Zeldovich (13). If one molecule of one atom of NO and N is produced in this reaction, it immediately becomes the second particle produced by rapid reaction of NO (14). The formation of thermal NO takes place just behind the flame front in a zone of high temperatures (t > 1400 °C). The basic ways of reducing emissions of thermal NO in combustion processes are the reduction of the temperature, shortening the stay of the reagents in the zone of high temperatures and reducing the local concentration of N2 and O2.

Malte and Pratt proposed the mechanism of taking into account the role of N2O in NO formation by the following reaction at temperatures lower than 1800 K (Kordylewski, 2008 (in Polish), Steele et al, 1995)

$$\rm O + N\_2 + M \to N\_2O + M \tag{20}$$

$$\text{N}\_2\text{O} + \text{O} \rightarrow \text{NO} + \text{NO} \tag{21}$$

$$\rm N\_2O + O \rightarrow N\_2 + O\_2 \tag{22}$$

These reactions, together with the reactions (13) and (14) in the literature are called "the extended thermal mechanism". The formation of NO by a mechanism of N2O formation is particularly important at lower temperatures (T < 1200 °C) in the flames rather poor (λ > 1). Important role in the formation of N2O plays the kind and characteristics of the third body M. It can be assumed that H2O or its dissociation products (O, H, OH) affect the course of the reaction (20) (Wilk, 2002).

The Influence of Modified Atmosphere on Natural Gas Combustion 485

Therefore, under normal conditions of combustion (T = 1000 - 1700 °C, λ 1,3) the final of NO2 emission is low and does not exceed 5% of the total NOx emissions. Significant impact on the formation of NO2 next to a low temperature is connected with a high combustion pressure and the presence of hydrocarbons. The increase in the pressure favours the growth

The investigation of oxy-combustion process was conducted on a laboratory reactor containing a specially designed combustion burner, oxidizer preparation system, and temperature system, flow rate of combustion substrates control system, exhaust gas analysis

The study included the characterization of the basic parameters of combustion, and above all took into account the effect of oxygen and carbon dioxide concentrations in the oxidizer on the exhaust gas composition and temperature profile along the combustion chamber.

Fig. 1. Scheme of the experimental apparatus: 1 - combustion chamber, 2 - burner,

3 - rotameter, 4 - exhaust gas system, 5 - control valves, 6 - mixer, 7 - cylinder with oxygen,

In parallel, at higher temperatures, the NO2 decomposition reactions may occur:

of the concentration of NO2.

**3. Experimental apparatus**

system and exhaust gas system (Fig. 1).

8 - cylinder with carbon dioxide.

NO HO NO OH 2 2 (28)

NO H NO OH <sup>2</sup> (29)

NO O NO O 2 2 (30)

#### **2.2.2 The prompt mechanism of NO formation**

Fenimore conducting the experimental research on the combustion of rich mixtures (λ < 1) with various hydrocarbons (methane, ethane, propane) found that relatively high concentrations of NO occur in combustion zone just before the flame where the temperature does not exceed 750 °C**.** Fenimore was confident that there should be another mechanism for the generation of thermal NO and called him a "prompt" which is immediate**.** In the hydrocarbon flames there are not only O, H, OH radicals but also hydrocarbon radicals, where its highest concentration was found in the reaction zone of the flame. The hydrocarbon radicals are capable of activating N2 reaction with the formation of nitrogen oxides in the flame. Fenimore assumed that CHi hydrocarbon radicals react with nitrogen air according to the following reaction:

$$\text{CH}\_2 + \text{N}\_2 \rightarrow \text{HCN} + \text{NH} \tag{23}$$

$$\text{CH} + \text{N}\_2 \rightarrow \text{HCN} + \text{N} \tag{24}$$

$$\text{C} + \text{N}\_2 \rightarrow \text{CN} + \text{N} \tag{25}$$

Generally, these reactions can be written:

$$\text{CH}\_{\text{x}} + \text{N}\_{2} \rightarrow \text{HCN} \text{ and other radicals (CN,NH}\_{\prime}\text{N...)} \tag{26}$$

The forming of amino and cyano compounds, among which the most important are HCN, NH, and CN, is oxidized to NO in the flame with the participation of radicals H, O, OH (Glarborg, Alzueta & Dam-Johansen, 1998). The prompt NO is formed very quickly during combustion. Velocity of its formation is like combustion velocity. The amount of formed NO depends weakly on temperature, but strongly depends on the local concentration of N2. The NO prompt participates in further reactions running along the flame and lose their individuality.

#### **2.2.3 The fuel NO mechanism**

The amount of nitrogen in the fuel composition is very diverse. Nitrogen, in the gaseous fuel, is not chemically bonded with the combustible gas. However, it can occur as free molecular nitrogen N2, which is the source of thermal or prompt nitrogen oxides. It is assumed therefore, that during the gas combustion the fuel nitrogen oxides do not occur.

#### **2.2.4 The formation of NO2**

Miller and Bowman gave the most plausible explanation of the mechanism of formation of NO2. They assumed that as a result of diffusion of H radicals from the flame in the area of low temperature (T < 750 °C) and high concentration of O2 the reaction occurs (Miller & Bowman, 1989; Hori, 1986).

$$\rm H + O\_2 + M \rightarrow HO\_2 + M \tag{27}$$

At the same time from the flame to low temperature zone NO diffuses, which comes in a rapid reaction with a peroxide radical reaction of HO2 by:

$$\text{NO} + \text{HO}\_2 \leftrightarrow \text{NO}\_2 + \text{OH} \tag{28}$$

In parallel, at higher temperatures, the NO2 decomposition reactions may occur:

$$\text{NO}\_2 + \text{H} \rightarrow \text{NO} + \text{OH} \tag{29}$$

$$\rm NO\_2 + O \rightarrow NO + O\_2 \tag{30}$$

Therefore, under normal conditions of combustion (T = 1000 - 1700 °C, λ 1,3) the final of NO2 emission is low and does not exceed 5% of the total NOx emissions. Significant impact on the formation of NO2 next to a low temperature is connected with a high combustion pressure and the presence of hydrocarbons. The increase in the pressure favours the growth of the concentration of NO2.

#### **3. Experimental apparatus**

484 Advances in Natural Gas Technology

Fenimore conducting the experimental research on the combustion of rich mixtures (λ < 1) with various hydrocarbons (methane, ethane, propane) found that relatively high concentrations of NO occur in combustion zone just before the flame where the temperature does not exceed 750 °C**.** Fenimore was confident that there should be another mechanism for the generation of thermal NO and called him a "prompt" which is immediate**.** In the hydrocarbon flames there are not only O, H, OH radicals but also hydrocarbon radicals, where its highest concentration was found in the reaction zone of the flame. The hydrocarbon radicals are capable of activating N2 reaction with the formation of nitrogen oxides in the flame. Fenimore assumed that CHi hydrocarbon radicals react with nitrogen

CH N HCN NH 2 2 (23)

CH N HCN N <sup>2</sup> (24)

C N CN N <sup>2</sup> (25)

H O M HO M 2 2 (27)

CH N HCN and other radicals (CN,NH,N...) x 2 (26)

The forming of amino and cyano compounds, among which the most important are HCN, NH, and CN, is oxidized to NO in the flame with the participation of radicals H, O, OH (Glarborg, Alzueta & Dam-Johansen, 1998). The prompt NO is formed very quickly during combustion. Velocity of its formation is like combustion velocity. The amount of formed NO depends weakly on temperature, but strongly depends on the local concentration of N2. The NO prompt participates in further reactions running along the flame and lose their

The amount of nitrogen in the fuel composition is very diverse. Nitrogen, in the gaseous fuel, is not chemically bonded with the combustible gas. However, it can occur as free molecular nitrogen N2, which is the source of thermal or prompt nitrogen oxides. It is assumed therefore, that during the gas combustion the fuel nitrogen oxides do not occur.

Miller and Bowman gave the most plausible explanation of the mechanism of formation of NO2. They assumed that as a result of diffusion of H radicals from the flame in the area of low temperature (T < 750 °C) and high concentration of O2 the reaction occurs (Miller &

At the same time from the flame to low temperature zone NO diffuses, which comes in a

**2.2.2 The prompt mechanism of NO formation**

air according to the following reaction:

Generally, these reactions can be written:

individuality.

**2.2.3 The fuel NO mechanism**

**2.2.4 The formation of NO2**

Bowman, 1989; Hori, 1986).

rapid reaction with a peroxide radical reaction of HO2 by:

The investigation of oxy-combustion process was conducted on a laboratory reactor containing a specially designed combustion burner, oxidizer preparation system, and temperature system, flow rate of combustion substrates control system, exhaust gas analysis system and exhaust gas system (Fig. 1).

The study included the characterization of the basic parameters of combustion, and above all took into account the effect of oxygen and carbon dioxide concentrations in the oxidizer on the exhaust gas composition and temperature profile along the combustion chamber.

Fig. 1. Scheme of the experimental apparatus: 1 - combustion chamber, 2 - burner, 3 - rotameter, 4 - exhaust gas system, 5 - control valves, 6 - mixer, 7 - cylinder with oxygen, 8 - cylinder with carbon dioxide.

The Influence of Modified Atmosphere on Natural Gas Combustion 487

0,8 6,3 - 9,5 21 - 29 0 - 2 1231 - 1316 885 - 1277

and air mixture for each oxygen-enriched air, and the obtained values resulted from the mixture of air and oxygen, and third option concerned the study of CO2 addition to oxidizer whereas the air combustion was oxygen-enriched up to 27% and excess air ratio was: 1 = 1,25. The excess air ratio was calculated by taking into account the increased oxygen

The first investigations were conducted for three values of the air excess ratio: 1 = 1,15,

by choosing an appropriate air and oxygen flows ratio, the air was oxygen-enriched in the range 21 - 29%. Table 2 and 3 shows the average values of measured experimental

The effect of oxygen addition to the combustion air on CO concentration for three different excess air ratios 1 - 3 was presented in Figure 3. For all the cases an increase in CO concentration with increasing oxygen concentration in the oxidising mixture was observed. The higher the excess air ratio, the lower was the concentration of carbon monoxide. It was observed that a small addition of oxygen around 4% slightly increases the concentration of CO, and the addition of 6% - 8% strongly increases the CO concentration. There was no

**1 = 1,15**

 **, m3/h** O2 <sup>V</sup> **, m3/h O2, % CO, ppm NO, ppm**  8,67 0 21 175 125 7,014 0,38 25 192 231 6,317 0,534 27 230 347 5,715 0,663 29 301 533 **2 = 1,20**

 **, m3/h** O2 <sup>V</sup> **, m3/h O2, % CO, ppm NO, ppm**  9,04 0 21 161 114 7,32 0,40 25 170 228 6,59 0,56 27 224 352 5,96 0,69 29 293 521 **3 = 1,25**

 **, m3/h** O2 <sup>V</sup> **, m3/h O2, % CO, ppm NO, ppm**  9,421 0 21 112 78 7,620 0,418 25 118 206 6,866 0,580 27 179 305 6,212 0,720 29 215 458 Table 2. The results of experimental studies of natural gas combustion in oxygen enriched

Table 1. The parameters of the combustion process of natural gas.

2 = 1,20, 2 = 1,25. The flow of gas was constant Vg

**, m3/h O2, %** CO2 <sup>V</sup> **,** m3/h **<sup>T</sup>***flame***, C T***exhaust gas***, <sup>C</sup>**

(in measured point)

= 0,8 m3/h. For each value of ,

Vg

data.

Vair

Vair

Vair

atmosphere for Vg

= 0,8 m3/h, CO2 <sup>V</sup> = 0.

**, m3/h** Voxidizer

content in the mixture.

The specially designed kinetic burner, so called "pipe in pipe", was used. The inside diameter of burner was 34 mm and the outer diameter was 47 mm (Fig. 2). The combustion chamber was made from the heat-resistant steel with a length of 1310 mm and a diameter of 160 mm. Thermal isolation chamber was made of ceramic fibre with a thickness of 150 mm. Along the combustion chamber were holes, which allowed the measurement or analysis of the exhaust gas temperature inside the furnace. Oxidizer preparation system consisted of oxidizer pressure cylinders containing oxygen, air supply system with a fan and a mixer filled with the ceramic fittings, which enable more efficient mixing of the streams brought air and oxygen. The flow rates control system of combustion substrates included the rotameters and control valves. Mixing of fuel with an oxidizer took place in space between two pipes inside the burner, and was enforced by the system of holes in the outer pipe of the burner. The homogeneous mixture was introduced into the combustion chamber.

Fig. 2. Scheme of a specially designed burner.

To measure the flame temperature the thermocouple (PtRh-Pt) is installed in the wall of the chamber combined with a digital millimetre. The temperature profile along the length of the furnace was measured by NiCr-NiAl thermocouples at four points connected to a multichannel temperature recorder Czaki WRT-9 consisting of a microprocessor thermometer EMT 200, the switch places the PMP test. Concentrations of the combustion products (O2, CO, CO2, NO) were measured by the means of a gas analyser Testo 350 XL.

### **4. Results and discussion**

The investigation of the oxy-combustion of natural gas is takes into account three types of oxidizing mixtures with an increased oxygen contents: 25% O2, 27% O2, 29% O2. The parameters of the combustion process of the natural gas with the addition of oxygen to the combustion air are shown in Table 1. The study concerned the natural methane rich gas from the city with the following average composition: CH4 - 98%, C2-C4 - 0,9%, N2 - 1%, CO2 - 0,1%.

To study the combustion of natural gas in modified atmosphere three options were carried out: the first - for the selected excess air ratio, the second - assumed a steady stream of gas

The specially designed kinetic burner, so called "pipe in pipe", was used. The inside diameter of burner was 34 mm and the outer diameter was 47 mm (Fig. 2). The combustion chamber was made from the heat-resistant steel with a length of 1310 mm and a diameter of 160 mm. Thermal isolation chamber was made of ceramic fibre with a thickness of 150 mm. Along the combustion chamber were holes, which allowed the measurement or analysis of the exhaust gas temperature inside the furnace. Oxidizer preparation system consisted of oxidizer pressure cylinders containing oxygen, air supply system with a fan and a mixer filled with the ceramic fittings, which enable more efficient mixing of the streams brought air and oxygen. The flow rates control system of combustion substrates included the rotameters and control valves. Mixing of fuel with an oxidizer took place in space between two pipes inside the burner, and was enforced by the system of holes in the outer pipe of the

burner. The homogeneous mixture was introduced into the combustion chamber.

To measure the flame temperature the thermocouple (PtRh-Pt) is installed in the wall of the chamber combined with a digital millimetre. The temperature profile along the length of the furnace was measured by NiCr-NiAl thermocouples at four points connected to a multichannel temperature recorder Czaki WRT-9 consisting of a microprocessor thermometer EMT 200, the switch places the PMP test. Concentrations of the combustion products (O2, CO, CO2, NO) were measured by the means of a gas analyser Testo 350 XL.

The investigation of the oxy-combustion of natural gas is takes into account three types of oxidizing mixtures with an increased oxygen contents: 25% O2, 27% O2, 29% O2. The parameters of the combustion process of the natural gas with the addition of oxygen to the combustion air are shown in Table 1. The study concerned the natural methane rich gas from the city with the following average composition: CH4 - 98%, C2-C4 - 0,9%, N2 - 1%,

To study the combustion of natural gas in modified atmosphere three options were carried out: the first - for the selected excess air ratio, the second - assumed a steady stream of gas

Fig. 2. Scheme of a specially designed burner.

**4. Results and discussion**

CO2 - 0,1%.

Table 1. The parameters of the combustion process of natural gas.

and air mixture for each oxygen-enriched air, and the obtained values resulted from the mixture of air and oxygen, and third option concerned the study of CO2 addition to oxidizer whereas the air combustion was oxygen-enriched up to 27% and excess air ratio was: 1 = 1,25. The excess air ratio was calculated by taking into account the increased oxygen content in the mixture.

The first investigations were conducted for three values of the air excess ratio: 1 = 1,15, 2 = 1,20, 2 = 1,25. The flow of gas was constant Vg = 0,8 m3/h. For each value of , by choosing an appropriate air and oxygen flows ratio, the air was oxygen-enriched in the range 21 - 29%. Table 2 and 3 shows the average values of measured experimental data.

The effect of oxygen addition to the combustion air on CO concentration for three different excess air ratios 1 - 3 was presented in Figure 3. For all the cases an increase in CO concentration with increasing oxygen concentration in the oxidising mixture was observed. The higher the excess air ratio, the lower was the concentration of carbon monoxide. It was observed that a small addition of oxygen around 4% slightly increases the concentration of CO, and the addition of 6% - 8% strongly increases the CO concentration. There was no

Table 2. The results of experimental studies of natural gas combustion in oxygen enriched atmosphere for Vg = 0,8 m3/h, CO2 <sup>V</sup> = 0.

The Influence of Modified Atmosphere on Natural Gas Combustion 489

20 22 24 26 28 30  **O2, %**

Fig. 4. The effect of oxygen addition to the combustion air on NO concentration in the natural gas combustion process with the oxygen enriched air depending on excess air ratio.

Table 4. The results of natural gas combustion in oxygen-enriched atmosphere for

Figure 5 shows the effect of oxygen addition to the CO concentration in the combustion of

The increase of oxygen concentration in the oxidizer from 21 to 29% O2, an increase in CO concentrations up to 300% was observed. Addition of oxygen in the oxidizer does not improve the complete combustion of the gas. CO molecule is more stable than CO2 and its oxidation by oxygen is very slow, it is unknown whether pure carbon monoxide CO could be burnt (Kotowicz & Janusz, 2007). However, the oxidation of CO is possible by means of even a small concentration of hydrogen and its compounds, e.g. addition of a small

= 0,8 m3/h and

= 9 m3/h, T*flame* = 1140 - 1160 C.

natural gas in oxygen enriched air for constant flows of gas and air ( Vg

concentration of water vapour would cause the CO combustion.

In the second series, the investigations were carried out for a fixed flow of gas and air in natural gas combustion in oxygen-enriched atmosphere. The combustion air was enriched from 21 to 29% oxygen, thereby generating the excess air ratios from 1,18 to 1,63. The

 **O2, % CO, ppm NO, ppm T***exhaust***, C**  1,18 21 23 85 991 1,40 25 42 167 987 1,52 27 50 203 985 1,63 29 71 290 983

0

Vg

Vair

= 0,8 m3/h and Vair

= 9 m3/h).

results are presented in Table 3 and the graphs (Fig. 5 - 7).

100

200

300

 **NO, ppm**

400

500

600

Table 3. The results of experimental studies of natural gas combustion in CO2 and oxygen enriched atmosphere for Vg = 0,8 m3/h, 3 = 1,25; O2 = 27 %.

Fig. 3. The effect of oxygen addition to the combustion air on CO concentration in the natural gas combustion process with the oxygen enriched air depending on the excess air ratio.

difference in the concentration of CO depending on the excess air ratio used if air has been enriched to 29% O2. It should be noted that the increase in O2 concentration decreased the flow of oxidising mixture, so the observed increases of the concentrations of CO and NO are quite large. Figure 4 shows the effect of the oxygen addition to the combustion air for three different excess air ratios 1 - <sup>3</sup> of on the concentration of nitrogen oxide NO. Addition of oxygen to the oxidizer also causes an increase in NO concentration in the exhaust gas. NO concentration for all cases is at the same level.

**3 = 1,25; O2 = 27 %**  CO2 <sup>V</sup> **, m3/h CO, ppm NO, ppm**  0 179 305 0,5 182 144 1 210 73 1,5 242 43 2 355 24

Table 3. The results of experimental studies of natural gas combustion in CO2 and oxygen

20 22 24 26 28 30 O2, %

Fig. 3. The effect of oxygen addition to the combustion air on CO concentration in the natural gas combustion process with the oxygen enriched air depending on the excess

difference in the concentration of CO depending on the excess air ratio used if air has been enriched to 29% O2. It should be noted that the increase in O2 concentration decreased the flow of oxidising mixture, so the observed increases of the concentrations of CO and NO are quite large. Figure 4 shows the effect of the oxygen addition to the combustion air for three different excess air ratios 1 - <sup>3</sup> of on the concentration of nitrogen oxide NO. Addition of oxygen to the oxidizer also causes an increase in NO concentration in the exhaust gas. NO

= 0,8 m3/h, 3 = 1,25; O2 = 27 %.

enriched atmosphere for Vg

80

concentration for all cases is at the same level.

air ratio.

120

160

200

**CO, ppm**

240

280

320

Fig. 4. The effect of oxygen addition to the combustion air on NO concentration in the natural gas combustion process with the oxygen enriched air depending on excess air ratio.

In the second series, the investigations were carried out for a fixed flow of gas and air in natural gas combustion in oxygen-enriched atmosphere. The combustion air was enriched from 21 to 29% oxygen, thereby generating the excess air ratios from 1,18 to 1,63. The results are presented in Table 3 and the graphs (Fig. 5 - 7).

Table 4. The results of natural gas combustion in oxygen-enriched atmosphere for Vg = 0,8 m3/h and Vair = 9 m3/h, T*flame* = 1140 - 1160 C.

Figure 5 shows the effect of oxygen addition to the CO concentration in the combustion of natural gas in oxygen enriched air for constant flows of gas and air ( Vg = 0,8 m3/h and Vair = 9 m3/h).

The increase of oxygen concentration in the oxidizer from 21 to 29% O2, an increase in CO concentrations up to 300% was observed. Addition of oxygen in the oxidizer does not improve the complete combustion of the gas. CO molecule is more stable than CO2 and its oxidation by oxygen is very slow, it is unknown whether pure carbon monoxide CO could be burnt (Kotowicz & Janusz, 2007). However, the oxidation of CO is possible by means of even a small concentration of hydrogen and its compounds, e.g. addition of a small concentration of water vapour would cause the CO combustion.

The Influence of Modified Atmosphere on Natural Gas Combustion 491

 21 % O2 25 % O2 27 % O2 29 % O2

0.4 0.6 0.8 Lenght of the combustion chamber L, m

Fig. 7. The exhaust gas temperature profile along the length of the combustion chamber

The consequence of fuel combustion in oxygen-enriched atmospheres in O2 + CO2 system (replacing nitrogen by CO2), is the reduction of pollutants emissions, mainly NOx (Amann et. al, 2009; Li, Yan & Yan, 2009; Muskał et al., 2008). In the studied system (modified atmosphere: oxygen-enriched air combustion) lower concentrations of nitrogen oxide NO are not obtained, but on the contrary, more than threefold increase in NO concentration was observed (Fig. 6). Addition of oxygen increases the flame temperature, and therefore it also increases the NO concentration. Increased concentration of NO may also result from the larger concentration of oxygen in the reacting system, making easier the connection between

The temperature profile along the length of the furnace was also performed (Fig. 7). It was found that the exhaust gas temperature decreased along the furnace chamber in the measured points of the furnace, as well as an increased concentration of oxygen in the oxidizer. Oxygen enrichment of combustion air is done in order to raise the temperature of combustion in the furnace and to raise the growth rate of fuel combustion, which causes shortening of the flame. The reduction of the flame length explains the decrease of the exhaust gas temperature in the measured fixed points along the length of the furnace. The larger was the addition of oxygen, the flame was shorter and the temperature was lower in the test point. The flame temperature with the addition of oxygen increased from

In the course of the experiment it was observed that addition of oxygen resulted in visible changes in shape and colour of the flame, the bright crown of the burner nozzle and a very bright flame colour. Opportunity to observe these changes undoubtedly comes from reduction of the flame length. Adding oxygen to the combustion air also caused a change in

900

the air nitrogen and oxygen at high temperature.

1140 to 1160 °C.

during combustion of natural gas in oxygen-enriched air.

1000

1100

Temperature, oC

1200

1300

Fig. 5. The effect of oxygen addition on the CO concentration in the natural gas combustion in oxygen-enriched oxidizer for Vg = 0,8 m3/h and Vair = 9 m3/h.

Fig. 6. The effect of oxygen addition on the NO concentration in the natural gas combustion in oxygen-enriched oxidizer for Vg = 0,8 m3/h and Vair = 9 m3/h.

20 22 24 26 28 30 O2, %

20 22 24 26 28 30 O2, %

= 9 m3/h.

Fig. 6. The effect of oxygen addition on the NO concentration in the natural gas combustion

= 0,8 m3/h and Vair

= 9 m3/h.

Fig. 5. The effect of oxygen addition on the CO concentration in the natural gas combustion

= 0,8 m3/h and Vair

20

50

in oxygen-enriched oxidizer for Vg

100

150

200

NO, ppm

250

300

in oxygen-enriched oxidizer for Vg

30

40

50

CO, ppm

60

70

80

Fig. 7. The exhaust gas temperature profile along the length of the combustion chamber during combustion of natural gas in oxygen-enriched air.

The consequence of fuel combustion in oxygen-enriched atmospheres in O2 + CO2 system (replacing nitrogen by CO2), is the reduction of pollutants emissions, mainly NOx (Amann et. al, 2009; Li, Yan & Yan, 2009; Muskał et al., 2008). In the studied system (modified atmosphere: oxygen-enriched air combustion) lower concentrations of nitrogen oxide NO are not obtained, but on the contrary, more than threefold increase in NO concentration was observed (Fig. 6). Addition of oxygen increases the flame temperature, and therefore it also increases the NO concentration. Increased concentration of NO may also result from the larger concentration of oxygen in the reacting system, making easier the connection between the air nitrogen and oxygen at high temperature.

The temperature profile along the length of the furnace was also performed (Fig. 7). It was found that the exhaust gas temperature decreased along the furnace chamber in the measured points of the furnace, as well as an increased concentration of oxygen in the oxidizer. Oxygen enrichment of combustion air is done in order to raise the temperature of combustion in the furnace and to raise the growth rate of fuel combustion, which causes shortening of the flame. The reduction of the flame length explains the decrease of the exhaust gas temperature in the measured fixed points along the length of the furnace. The larger was the addition of oxygen, the flame was shorter and the temperature was lower in the test point. The flame temperature with the addition of oxygen increased from 1140 to 1160 °C.

In the course of the experiment it was observed that addition of oxygen resulted in visible changes in shape and colour of the flame, the bright crown of the burner nozzle and a very bright flame colour. Opportunity to observe these changes undoubtedly comes from reduction of the flame length. Adding oxygen to the combustion air also caused a change in

The Influence of Modified Atmosphere on Natural Gas Combustion 493

0 0.4 0.8 1.2 1.6 2 CO2, m3/h

0 0.5 1 1.5 2 CO2, m3/h

Fig. 10. The effect of carbon dioxide on the flame temperature in the combustion of natural

Fig. 9. The effect of carbon dioxide on the concentration of NO in the combustion of natural

0

1200

gas with oxygen-enriched oxidizer up to 27% O2 and 3 = 1,25.

1220

1240

1260

temperatura p³omienia, 0C

1280

1300

gas with oxygen-enriched oxidizer up to 27% O2 and 3 = 1,25.

100

200

NO, ppm

300

400

the sound of the furnace operation to the louder, more intense one, associated with changes in the fuel and oxidizer mixture flow within the combustion chamber.

The study of natural gas combustion was also conducted in another modified atmosphere: O2/CO2/N2. The process was studied under excess air ratio 3 = 1,25 and the oxidizer was oxygen-enriched up to 27%. CO2 was added to the oxidizer in the range of 0 to 2 m3/h.

The effect of carbon dioxide addition on the CO and NO concentrations of the studied process were investigated (Figure 8 and Figure 9). The CO concentration increased with CO2 addition. The addition of 2 m3/h of CO2 has generated two times larger CO concentration comparing to the process operated in conventional atmosphere O2/N2. NO concentration, in contrary, decreased with increasing addition of CO2. The maximum of CO2 addition (2 m3/h) decreased NO concentration ca. 12 times. The decrease of NO was connected with lower temperature obtained in the combustion chamber, because of large CO2 thermal capacity. That fact confirms the flame temperature measured close to the burner nozzle in the axes of the flame presented in Figure 11. The decrease of flame temperature was observed with increasing CO2 addition to the oxidizer. The thermal NO formation, main way of NO formation during natural gas combustion, takes place just behind the flame front in a zone of high temperature (t > 1400 °C). Therefore, the efficient method of the NO reduction is the lower range of temperature used, shortening the stay of the reagents in the zone of high temperatures and reducing the local concentration of N2 and O2.

The exhaust gas temperature profile was conducted along the combustion chamber including the CO2 addition (Figure 10). The exhaust gas temperature decreases with CO2 addition along the combustion chamber. The more CO2 is added the lower temperature is obtained.

Fig. 8. The effect of carbon dioxide on the concentration of CO in the combustion of natural gas with oxygen-enriched oxidizer up to 27% O2 and 3 = 1,25.

the sound of the furnace operation to the louder, more intense one, associated with changes

The study of natural gas combustion was also conducted in another modified atmosphere: O2/CO2/N2. The process was studied under excess air ratio 3 = 1,25 and the oxidizer was oxygen-enriched up to 27%. CO2 was added to the oxidizer in the range of 0 to 2 m3/h.

The effect of carbon dioxide addition on the CO and NO concentrations of the studied process were investigated (Figure 8 and Figure 9). The CO concentration increased with CO2 addition. The addition of 2 m3/h of CO2 has generated two times larger CO concentration comparing to the process operated in conventional atmosphere O2/N2. NO concentration, in contrary, decreased with increasing addition of CO2. The maximum of CO2 addition (2 m3/h) decreased NO concentration ca. 12 times. The decrease of NO was connected with lower temperature obtained in the combustion chamber, because of large CO2 thermal capacity. That fact confirms the flame temperature measured close to the burner nozzle in the axes of the flame presented in Figure 11. The decrease of flame temperature was observed with increasing CO2 addition to the oxidizer. The thermal NO formation, main way of NO formation during natural gas combustion, takes place just behind the flame front in a zone of high temperature (t > 1400 °C). Therefore, the efficient method of the NO reduction is the lower range of temperature used, shortening the stay of the reagents in the

in the fuel and oxidizer mixture flow within the combustion chamber.

zone of high temperatures and reducing the local concentration of N2 and O2.

the combustion chamber. The more CO2 is added the lower temperature is obtained.

The exhaust gas temperature profile was conducted along the combustion chamber including the CO2 addition (Figure 10). The exhaust gas temperature decreases with CO2 addition along

> 0 0.4 0.8 1.2 1.6 2 CO2, m3/h

Fig. 8. The effect of carbon dioxide on the concentration of CO in the combustion of natural

160

gas with oxygen-enriched oxidizer up to 27% O2 and 3 = 1,25.

200

240

280

CO, ppm

320

360

Fig. 9. The effect of carbon dioxide on the concentration of NO in the combustion of natural gas with oxygen-enriched oxidizer up to 27% O2 and 3 = 1,25.

Fig. 10. The effect of carbon dioxide on the flame temperature in the combustion of natural gas with oxygen-enriched oxidizer up to 27% O2 and 3 = 1,25.

The Influence of Modified Atmosphere on Natural Gas Combustion 495

Chen J.C., Liu Z.S., Huang J.S.: Emission characteristics of coal combustion in different

Choi G., Katsuki M. (2001). Advanced low NOx combustion using highly preheated air,

Croiset E., Thambimuthu K., Palmer A. (2000). Coal Combustion in O2/CO2 mixtures

Czakiert T., Bis Z., W. Muskała, Nowak W. (2006). Fuel conversion from oxy-fuel

Czakiert T., Nowak W., Bis Z (2008). Spalanie w atmosferach modyfikowanych tlenem,

Davidson J. (2007). Performance and cost of power plants with capture storage of CO2,

Flamme M. (1998): Low NOx Combustion Technologies for High Temperature Application.

Flamme M. (2001). Low NOx combustion technologies for high temperature applications,

Glarborg P., Alzueta M., Dam-Johansen K. (1998). Kinetic Modeling of Hydrocarbon/Nitric Oxide Interactions in a Flow Reactor. *Combustion and Flame*, Vol. 115, pp. 1-27, Hori M. (1986). Experimental Study of Nitrogen Dioxide Formation in Combustion Systems.

Kim H.K., Kim Y., Lee S.M., Ahn K. Y. (2007). NO reduction in 0.03-0.2 MW oxy-fuel

Kordylewski W. (2008). Combustion and fuels, *Oficyna wydawnicza Politechniki Wrocławskiej,*

Kotowicz J., Janusz K. (2007). Sposoby redukcji emisji CO2 z procesów energetycznych,

Lampert K., Ziębik A. (2007). Comparative analysis of energy requirements of CO2 removal

Li H., Yan J., Yan J., Anhenden M. (2009) Impurity impacts on the purification process in

Miller J. A., Bowman C. T. (1989). Mechanism and Modeling of Nitrogen Chemistry in Combustion. *Progress in Energy and Combustion Science*, Vol. 15, pp. 287-338, Muskał W., Krzywański J., Czakiert T., Sekret R., Nowak W. (2008). Spalanie w atmosferach

Muzio L.J., Quartucy G.C. (1997). Implementing NOx Control: Research to Application.

oxy-fuel combustion based CO2 capture and storage system, *Applied Energy*, Vol.

*Related Technologies*, (December 1-3), Nagoya, Japan, pp. 152-159,

Gardiner W.C. (2000). Gas-Phase Combustion Chemistry. *Springer-Verlag*, New York,

*Energy Conversion and Management*, Vol. 42, pp. 1919-1935,

from metallurgical fuel gases, *Energy*, Vol. 32, pp. 521-527,

modyfikowanych O2 i CO2, *Energetyka,* Vol. 10, pp. 669-671,

*Progress in Energy and Combustion Science*, Vol. 23, pp. 233-266,

*Energy Conversion and Management*, Vol. 42, pp. 639-652,

142, s. 266-271,

407,

538,

Polish),

1181-1188,

86, pp. 202-213,

*Energy*, Vol. 32, pp. 1163-1176,

*Institute*, Vol. 31, pp. 3377-3384,

*Rynek Energii*, Vol. 1 (in Polish),

Wrocław, Poland (in Polish),

O2/N2, O2/CO2 and O2/RFG atmosphere, Journal of Hazardous Materials 2007,

compared with air, *The Canadian Journal of Chemical Engineering*, Vol. 78, pp. 402-

combustion in a circulating fluidized bed, *Fuel Processing Technology,* Vol. 87, s. 531-

kierunki rozwoju dla kotłów CWF, *Energia i Ekologia*, Vol. 10, pp. 713-718 (in

*Proceedings of 2nd International Symposium on Advanced Energy Conversion System and*

*Twenty-first Symposium (International) on Combustion/The Combustion Institute*/,pp.

combustor using flue gas recirculation technology, *Proceedings of the Combustion*

Fig. 11. The effect of carbon dioxide on the exhaust gas temperature measured along the combustion chamber in the combustion of natural gas with oxygen-enriched oxidizer up to 27% O2 and 3 = 1,25.

#### **5. Conclusion**

In industrial processes, which are required to maintain very high temperatures such as in glass or in the production of cement, the oxy-combustion process can be used, or oxygenenriched combustion air. This of course raises the temperature of the combustion process, which is associated with an increase in the concentration of NOx. Therefore, the oxycombustion should be used simultaneously with the process of elimination of nitrogen from combustion air (largely responsible for the formation of NO). Otherwise, the addition of oxygen increases the NO concentration, and therefore undesirable effect is achieved. A mixture of oxygen and carbon dioxide by replacing the air can lead to lower concentrations of nitrogen oxides by eliminating nitrogen from combustion air.

#### **6. References**

0 m3/h CO2 0,5 m3/h CO2 1 m3/h CO2 1,5 m3/h CO2 2 m3/h CO2

0.4 0.6 0.8 Lenght of the combustion chamber L, m

Fig. 11. The effect of carbon dioxide on the exhaust gas temperature measured along the combustion chamber in the combustion of natural gas with oxygen-enriched oxidizer up to

In industrial processes, which are required to maintain very high temperatures such as in glass or in the production of cement, the oxy-combustion process can be used, or oxygenenriched combustion air. This of course raises the temperature of the combustion process, which is associated with an increase in the concentration of NOx. Therefore, the oxycombustion should be used simultaneously with the process of elimination of nitrogen from combustion air (largely responsible for the formation of NO). Otherwise, the addition of oxygen increases the NO concentration, and therefore undesirable effect is achieved. A mixture of oxygen and carbon dioxide by replacing the air can lead to lower concentrations

Amann J.-M., Kanniche M., Bouallou C. (2009). Natural gas combined cycle power plant

Andersson K., Johnsson F. (2007). Flame and radiation characteristics of gas-fired O2/CO2

Bartok W., Sarofim A. F. (1991). Fossil fuel combustion. A source book. *John Wiley and Sosns,*

Buhre B.J.P., Elliott L.K., Sheng C.D., Gupta R.P., Wall T.F. (2005). Oxy-fuel combustion

*INC*, New York, Chichester, Brisbane, Toronto, Singapore,

modified into an O2/CO2 cycle for CO2 capture, *Energy Conversion and Management*,

technology for coal-fired power generation, *Progress in Energy and Combustion*

900

of nitrogen oxides by eliminating nitrogen from combustion air.

combustion, *Fuel*, Vol. 86, pp. 636-668,

*Science*, Vol. 31, pp. 283-307,

27% O2 and 3 = 1,25.

**5. Conclusion**

**6. References**

Vol. 50, pp. 510-521,

1000

1100

Temperature, oC

1200

1300

**0**

**20**

*Japan*

**Modelling a SOFC Power Unit Using**

Completed models covering dynamic characteristics of those types of DGs are not openly available. The necessary task is to study these dynamic models based on the literature and any available operational data on DGs. Simulation of various types of DGs in a suitable software environment is the key step in analyzing the dynamic characteristics of DGs and designing the control strategies. In fact, computer simulation plays a vital role in the design and analysis of power system. Designing power systems without computer simulation is extremely laborious, time consuming, error-prone and expensive. Especially, in the new research field as DGs, computer simulation in an industrial environment with regard to the time in shortening the overall design process as compared to assembling and testing the components in the laboratory and deciding on the optimum values for components and

Among many types of FC, high-temperature fuel cells such as the solid oxide fuel cells (SOFC) have the potential for centralized power generation as well as combined heat and power. This chapter employs the SOFC model method. Especially, in this simulation, the rate of temperature change and load following ability will be included. In short, it consists of 3 main mathematical models, namely, the electrochemical model, the heat balance model and the

The electrochemical model is to calculate output voltage, to regulate the fuel and air streams

The heat balance model is to calculate thermal energy inside SOFC stack as well as operating temperature. The heat exchangers are included in this simulation to represent the practical application when using to increase the temperature of input air and methane. Because of the high SOFC operating temperature, if there have no preheat, manufacture still have to set up a preheat system using electricity from other source or a small part of SOFC output power to elevate input species temperature to prevent thermal shock which can damage materials. Therefore, using heat generated inside SOFC stack for preheat can increase overall efficiency. Finally, there are two important results that this chapter points out. The first is load following ability of SOFC power unit. The fuel cell control is achieved by adjusting the input volume of

and to represent the ability to follow the load chance of SOFC.

**1. Introduction**

controller parameters.

power conditioning unit model.

**1.1 Literature review and objective of this chapter**

**Natural Gas Fed Directly**

Nguyen Duc Tuyen and Goro Fujita

*Shibaura Institute of Technology*

## **Modelling a SOFC Power Unit Using Natural Gas Fed Directly**

Nguyen Duc Tuyen and Goro Fujita *Shibaura Institute of Technology Japan*

#### **1. Introduction**

496 Advances in Natural Gas Technology

Normann F., Andersson K., Leckner B., Johnsson F. (2008). High-temperature reduction of nitrogen oxides in oxy-fuel combustion, *Fuel*, Vol. 87, pp. 3579-3585, Seepana S., Jayanti S. (2009). Flame structure and NO generation in oxy-fuel combustion at high pressures, *Energy Conversion and Management*, Vol. 50, pp. 1116-1123, Simpson A., Simon A.J. (2007). Second law comparison of oxy-fuel combustion and post

Steele R.C., Malte P.C., Nicol D.G., Kramlich J.C. (1995). NOx and N2O in Lean-Premixed Jet-

Szlęk A., Wilk R.K, Werle S., Schaffel N. Clean technologies generating energy from coal and the prospect of flameless combustion, *Rynek Energii,* Vol*.* 4, pp. 39-45 (in Polish), Tan Y., Douglas M.A., Thambimuth K.V. (2002). CO2 capture using oxygen enhanced combustion strategies for natural gas power plants, *Fuel*, Vol. 81, pp. 1007-1016, Tomeczek J., Gradoń B. (1997). The Role of Nitrous Oxide in the Mechanism of Thermal

Warnatz J., Maas U., Diable R.W. (2006). Combustion. Physical and Chemical Fundamentals,

Wilk M., Magdziarz A., Kuźnia M. (2010). The influence of oxygen addition into air

Wilk R. (2002). Low-emission combustion. *Wydawnictwo Politechniki Śląskiej*, Gliwice, Poland, Zhang N., Lior N. (2008). Two novel oxy-fuel power cycles integrated with natural gas

reforming and CO2 capture, *Energy*, Vol. 33, pp. 340-351.

Stirred Flames. *Combustion and Flame*, Vol. 100, pp. 440-449,

pp. 3034-3045,

Technology, Vol. 125, pp. 159-180,

*Verlag*, Berlin Heidelberg,

Polish),

combustion carbon dioxide separation, *Energy Conversion and Management,* Vol. 48,

Nitric Oxide Formation within Flame Temperature Range. Combustion Science and

Modeling and Simulation, Experimentals, Pollutant Formation. *4th Edition Springer-*

combustion on natural gas combustion process, *Rynek Energii,* Vol*.* 5, pp. 32-36 (in

#### **1.1 Literature review and objective of this chapter**

Completed models covering dynamic characteristics of those types of DGs are not openly available. The necessary task is to study these dynamic models based on the literature and any available operational data on DGs. Simulation of various types of DGs in a suitable software environment is the key step in analyzing the dynamic characteristics of DGs and designing the control strategies. In fact, computer simulation plays a vital role in the design and analysis of power system. Designing power systems without computer simulation is extremely laborious, time consuming, error-prone and expensive. Especially, in the new research field as DGs, computer simulation in an industrial environment with regard to the time in shortening the overall design process as compared to assembling and testing the components in the laboratory and deciding on the optimum values for components and controller parameters.

Among many types of FC, high-temperature fuel cells such as the solid oxide fuel cells (SOFC) have the potential for centralized power generation as well as combined heat and power. This chapter employs the SOFC model method. Especially, in this simulation, the rate of temperature change and load following ability will be included. In short, it consists of 3 main mathematical models, namely, the electrochemical model, the heat balance model and the power conditioning unit model.

The electrochemical model is to calculate output voltage, to regulate the fuel and air streams and to represent the ability to follow the load chance of SOFC.

The heat balance model is to calculate thermal energy inside SOFC stack as well as operating temperature. The heat exchangers are included in this simulation to represent the practical application when using to increase the temperature of input air and methane. Because of the high SOFC operating temperature, if there have no preheat, manufacture still have to set up a preheat system using electricity from other source or a small part of SOFC output power to elevate input species temperature to prevent thermal shock which can damage materials. Therefore, using heat generated inside SOFC stack for preheat can increase overall efficiency.

Finally, there are two important results that this chapter points out. The first is load following ability of SOFC power unit. The fuel cell control is achieved by adjusting the input volume of

(Graham M. Goldin, 2009) approach this chapter ideal when taking HX into account but calculate heat balance in another way and for specific SOFC configuration. David investigated the transient behavior of a stand-alone SOFC caused by a load change in (Achenbach & Elmar, 1995), (Kourosh Sedghisigarchi, 2004), (J. Padulles, 2000). However, the built model is simple for evaluate the real response but these simplified models consider constant cell temperature. (M.Y. El-Sharkh, 2004) considers only the dynamic characteristic of Power Conditioning Unit system. (P.R. Pathapati, 2005) represents PEM dynamic model which does not consider concentration loss. (S.H.Chan, 2002) deals with HX model. A physically based model for

Modelling a SOFC Power Unit Using Natural Gas Fed Directly 499

A transient dynamic model of SOFC will be proposed in this chapter. Electrochemical and thermal simulations of a SOFC reported in all reference will be used to identify the key parameters of this SOFC system from a single cell to a *N*<sup>0</sup> single fuel cell connected in series. The cell's terminal voltage during a load change was discussed. Overall heat balance inside SOFC power unit effecting on operating temperature afterward on output voltage will be

Fuel cells will be important components of distribution system due to their high efficiency and low environmental pollution. Generally, efficiency of the fuel cells ranges from 40-60% can be improved to 80-90% in co-generation applications. The waste heat produced by the lower temperature cells is undesirable since it cannot be used for any application and thus limits the efficiency of the system. The higher temperature fuel cells have higher efficiency since the heat produced can be used for heating purposes. Due to an electric-chemical process of power generation, there is no noise develop usually in mechanical members of conventional generator. All of these features will without any doubt lead to their wide application in the power industry in the near future. Several types of fuel cells have been reported in the literatures: phosphor acid fuel cell (PAFC), solid oxide fuel cell (SOFC), molten carbonate fuel cell (MCFC) and proton exchange membrane fuel cell (PEMFC). The PAFC has been commercially used in hospitals, nursing homes, utility power plants, etc. The SOFC can be used in large-power applications such as central electricity generation station. SOFC has the

Each of these FC types differs in the electrolyte and fuel used, operating temperature and

The most important component of a FC is the fuel processor and the reformer since hydrogen is not readily available. Fossil fuels such as gasoline, natural gas and coal gases need to be processed and reformed to obtain enriched hydrogen. Natural gas is the most easily available fuel source. Bio-fuels can also be used as a source to obtain hydrogen. Biological methods such as photosynthesis and fermentation can be used to produce hydrogen. Though there are different methods to produce hydrogen, a proper and feasible method which can be

Storage of hydrogen is an important aspect of the FC systems because the fuel has to be readily available for continuous supply of electric power. Sometimes, electrical energy is used to

tubular SOFC was developed in (Caisheng Wang, 2007).

highest potential in large power application.

commercialized is not yet available.

pressure, construction materials, power density and efficiency.

**1.4 Fuel for fuel cells**

pointed out.

**1.3 Fuel cell**

gas and air and controlling real power output. The two control loops which are in SOFC itself and in the DC-DC inverter make the SOFC power unit strongly following flexible change of load. And the second is heat balance inside fuel cell system with HX included.

#### **1.2 Different modelling approaches**

Depending upon the application, different models are available in the open literature and there are large differences in the level of details in the models presented. This section presents a review of the work of selected authors relevant to the model developed in the present work. Research work of SOFCs modelling has been begun since 1980's and there have been a lot of models developed so far. Initial models were lumped mass models and there were a lot of uncertainties in the results due to lack of experimental data as well as mature approaches. Increasing experimental research during early 1990's focused many such issues and established many empirical relationships to accurately predict the performance of SOFC. Also, due to the increase in calculation capabilities, it was possible to create more detailed models. During the late 1990's, several projects were initiated for detailed single SOFC modelling. Today modelling research is pursued in both detailed single SOFC modelling and system level stack modelling. Nevertheless, lumped models still continue to attract the attention of researchers due to their simplicity and small calculation time. Lumped models are considered over detailed models when it comes to predict accurately the FCs overall thermodynamic and electrical performance. A large amount of experimental data and mathematical relations exists for components such as air and fuel compressors, heat exchangers, thus these components can be modeled fairly accurately upon the lumped approach. Accordingly, lumped models are also easier to adjust to experimental data. The disadvantage of lumped SOFC models is that they can only account for mean values of the parameters and more detailed investigation of the cell is needed to check for undesirable effects such as thermal cracking, coking or exceeding temperature limits locally. This problem may be partly solved by using a detailed model to test the validity of the results after using a lumped model for system calculations. Obviously, implementing a detailed SOFC model in the system model gives the most accurate results.

References (Achenbach & Elmar, 1995), (Wang & Nehrir, 2007) provide a basic approach for fuel cell modelling suitable for distributed generation, however not discussed in details about SOFC. A SOFC model has been developed by various researchers in (Li & Chyu, 2003), (Ali Volkan Akkaya & Erdem, 2009), (N. Lu, 2006), (David L. Damm, 2005), (Mitsunori Iwata, 2000), (Tomoyuki Ota, 2003), (Xiongwen Zhang, 2007), (Takanobu Shimada, 2009), (S. Campanari, 2004) and (A.C. Burt, 2004) taking its thermodynamic effect into consideration which concentrated on the effects of temperature changes on the output voltage response. Heat balance is considered in specific model configuration more than in general and the detail calculation seems to be complex. Some did not consider the dynamics of the chemical species. Reference (Tadashi Gengo, 2007) points out empirical responses of real model considering temperature inside SOFC combine with output voltage, current and power. (Takanobu Shimada, 2009), (D. Sanchez, 2008) considers internal *CH*<sup>4</sup> reformer, fuel and air input temperature are increased. But there is not any research caring about using SOFC exhaust to take full advantages of high operating temperature. (M. Uzunoglu, 2006), (Caisheng Wang, 2007), (Caisheng Wang, 2005) take the double layer charging effect into account but not SOFC. Recently research (Takanobu Shimada, 2009), (P. Piroonlerkgul, 2009), (Graham M. Goldin, 2009) approach this chapter ideal when taking HX into account but calculate heat balance in another way and for specific SOFC configuration. David investigated the transient behavior of a stand-alone SOFC caused by a load change in (Achenbach & Elmar, 1995), (Kourosh Sedghisigarchi, 2004), (J. Padulles, 2000). However, the built model is simple for evaluate the real response but these simplified models consider constant cell temperature. (M.Y. El-Sharkh, 2004) considers only the dynamic characteristic of Power Conditioning Unit system. (P.R. Pathapati, 2005) represents PEM dynamic model which does not consider concentration loss. (S.H.Chan, 2002) deals with HX model. A physically based model for tubular SOFC was developed in (Caisheng Wang, 2007).

A transient dynamic model of SOFC will be proposed in this chapter. Electrochemical and thermal simulations of a SOFC reported in all reference will be used to identify the key parameters of this SOFC system from a single cell to a *N*<sup>0</sup> single fuel cell connected in series. The cell's terminal voltage during a load change was discussed. Overall heat balance inside SOFC power unit effecting on operating temperature afterward on output voltage will be pointed out.

#### **1.3 Fuel cell**

2 Will-be-set-by-IN-TECH

gas and air and controlling real power output. The two control loops which are in SOFC itself and in the DC-DC inverter make the SOFC power unit strongly following flexible change of

Depending upon the application, different models are available in the open literature and there are large differences in the level of details in the models presented. This section presents a review of the work of selected authors relevant to the model developed in the present work. Research work of SOFCs modelling has been begun since 1980's and there have been a lot of models developed so far. Initial models were lumped mass models and there were a lot of uncertainties in the results due to lack of experimental data as well as mature approaches. Increasing experimental research during early 1990's focused many such issues and established many empirical relationships to accurately predict the performance of SOFC. Also, due to the increase in calculation capabilities, it was possible to create more detailed models. During the late 1990's, several projects were initiated for detailed single SOFC modelling. Today modelling research is pursued in both detailed single SOFC modelling and system level stack modelling. Nevertheless, lumped models still continue to attract the attention of researchers due to their simplicity and small calculation time. Lumped models are considered over detailed models when it comes to predict accurately the FCs overall thermodynamic and electrical performance. A large amount of experimental data and mathematical relations exists for components such as air and fuel compressors, heat exchangers, thus these components can be modeled fairly accurately upon the lumped approach. Accordingly, lumped models are also easier to adjust to experimental data. The disadvantage of lumped SOFC models is that they can only account for mean values of the parameters and more detailed investigation of the cell is needed to check for undesirable effects such as thermal cracking, coking or exceeding temperature limits locally. This problem may be partly solved by using a detailed model to test the validity of the results after using a lumped model for system calculations. Obviously, implementing a detailed SOFC model in

References (Achenbach & Elmar, 1995), (Wang & Nehrir, 2007) provide a basic approach for fuel cell modelling suitable for distributed generation, however not discussed in details about SOFC. A SOFC model has been developed by various researchers in (Li & Chyu, 2003), (Ali Volkan Akkaya & Erdem, 2009), (N. Lu, 2006), (David L. Damm, 2005), (Mitsunori Iwata, 2000), (Tomoyuki Ota, 2003), (Xiongwen Zhang, 2007), (Takanobu Shimada, 2009), (S. Campanari, 2004) and (A.C. Burt, 2004) taking its thermodynamic effect into consideration which concentrated on the effects of temperature changes on the output voltage response. Heat balance is considered in specific model configuration more than in general and the detail calculation seems to be complex. Some did not consider the dynamics of the chemical species. Reference (Tadashi Gengo, 2007) points out empirical responses of real model considering temperature inside SOFC combine with output voltage, current and power. (Takanobu Shimada, 2009), (D. Sanchez, 2008) considers internal *CH*<sup>4</sup> reformer, fuel and air input temperature are increased. But there is not any research caring about using SOFC exhaust to take full advantages of high operating temperature. (M. Uzunoglu, 2006), (Caisheng Wang, 2007), (Caisheng Wang, 2005) take the double layer charging effect into account but not SOFC. Recently research (Takanobu Shimada, 2009), (P. Piroonlerkgul, 2009),

load. And the second is heat balance inside fuel cell system with HX included.

**1.2 Different modelling approaches**

the system model gives the most accurate results.

Fuel cells will be important components of distribution system due to their high efficiency and low environmental pollution. Generally, efficiency of the fuel cells ranges from 40-60% can be improved to 80-90% in co-generation applications. The waste heat produced by the lower temperature cells is undesirable since it cannot be used for any application and thus limits the efficiency of the system. The higher temperature fuel cells have higher efficiency since the heat produced can be used for heating purposes. Due to an electric-chemical process of power generation, there is no noise develop usually in mechanical members of conventional generator. All of these features will without any doubt lead to their wide application in the power industry in the near future. Several types of fuel cells have been reported in the literatures: phosphor acid fuel cell (PAFC), solid oxide fuel cell (SOFC), molten carbonate fuel cell (MCFC) and proton exchange membrane fuel cell (PEMFC). The PAFC has been commercially used in hospitals, nursing homes, utility power plants, etc. The SOFC can be used in large-power applications such as central electricity generation station. SOFC has the highest potential in large power application.

#### **1.4 Fuel for fuel cells**

Each of these FC types differs in the electrolyte and fuel used, operating temperature and pressure, construction materials, power density and efficiency.

The most important component of a FC is the fuel processor and the reformer since hydrogen is not readily available. Fossil fuels such as gasoline, natural gas and coal gases need to be processed and reformed to obtain enriched hydrogen. Natural gas is the most easily available fuel source. Bio-fuels can also be used as a source to obtain hydrogen. Biological methods such as photosynthesis and fermentation can be used to produce hydrogen. Though there are different methods to produce hydrogen, a proper and feasible method which can be commercialized is not yet available.

Storage of hydrogen is an important aspect of the FC systems because the fuel has to be readily available for continuous supply of electric power. Sometimes, electrical energy is used to

5. No precious metals as catalysts (platinum: Pt, etc.)

based on specific applications and fuel availability.

**2.2 SOFC based power generation systems**

fundamental parts in a FC power unit.

Balance of Power Unit

Fig. 1. Structure of SOFC power generation unit

In addition, while maintaining high efficiency, to lower the temperature of the SOFC is driving development issues. Because 1000*oC* high operating temperature of the material will degrade cells, reduce the choice of construction materials. Operating temperature of 700 <sup>−</sup> <sup>1000</sup>*oC* from the conventional 500 <sup>−</sup> <sup>800</sup>*oC*, these issues are resolved smaller, lower cost, can improve endurance. High-temperature operation removes the need for a precious-metal catalyst, thereby reducing the cost. It also allows SOFCs to reform fuels internally, which enables the use of a variety of fuels and reduces the cost associated with adding a reformer to the system. Although a SOFC produces electricity, it only produces DC power and utilizes only processed fuel. Therefore, a SOFC based power generation system requires the integration of many other components beyond the SOFC stack itself. Moreover, to recover the high quality waste heat from the SOFC stack, an efficient integration of co-generation or bottoming system with the FC section is crucial for a SOFC based power generation plant. Since the balance of plant will directly impact the overall system efficiency and may cost more than the SOFC stack itself, it is obvious that the design of a SOFC power generation system involves more than the optimization of the SOFC unit with respect to efficiency or economics. It also involves balance of plant studies. With SOFC materials and stacks approaching a commercialization stage, there is a need to explore various process designs to obtain optimal efficiency and economics

Modelling a SOFC Power Unit Using Natural Gas Fed Directly 501

As other types of FCs, a SOFC produces only DC power and requires processed fuel. It also produces high quality heat due to its high operating temperature. Beyond the SOFC stack itself, a typical SOFC power system basically includes: a reformer to start the hydrogen production process, a fuel conditioner to clean up the pollutants that could otherwise poison the fuel cell elements, a power conditioner to convert direct current from the fuel cell to the appropriate voltage range and current type depending on the application, and a cogeneration or bottoming cycle to utilize the rejected heat to achieve high system efficiency. The system also requires the most common balance of plant equipments such as heat exchangers, air blower and fuel compressors, controls systems, and safety systems. Fig.1 illustrates

> Power Grid or Stand alone Load

Network Interface Controller

Network Parameters

AC Power

P and Q Commands

Power Conditioning Unit

SOFC stack

Power Unit Controller

6. Structure for carbon dioxide recovery

divide water into hydrogen and oxygen with the help of electrolysers during times of high supply and low demand. FCs have to be compact and portable for mobile applications; hence storage of hydrogen is essential for such applications. Hydrogen needs to be handled with great care because it is a highly volatile and flammable gas. It has a high leak rate due to which the gas tends to escape through small orifices, faster than the other gases. Hence storage of hydrogen plays a key role in the FC systems.

A fuel processor converts the primary fuel source (hydrocarbons) into the fuel gas (hydrogen) required by the FC stack. The processor uses a catalytic reaction to break the fuel into hydrogen and separate it from the carbon based gases. Each of the FC types has specific fuel requirements. Natural gas and petroleum liquids contain sulphur compounds and have to be desulphurized before they can be used as a fuel. The anode catalysts are intolerant of sulphur and it must be removed before it degrades catalyst performance. There is a risk of carbon formation in fuel cell systems which can be reduced by carrying out pre-reforming of the fuel gas before it is fed to the reformer reactor. Carbon monoxide can be used as a fuel for SOFC and MCFC because it can be internally converted to hydrogen whereas the PEMFC should be completely free from it. CO has high affinity for anode catalyst (especially platinum) and it prevents the flow of fuel in the PEMFC. Ammonia is a poison for all the FC types due to its adverse effects on the cell life except for SOFC, where it can be internally reformed.

Lower-temperature FCs require an external reformer to obtain the hydrogen rich fuel, thus increasing the cost and thereby reducing the efficiency. Higher temperature FCs do not require an external reformer; its high temperature allows direct conversion of natural gas to hydrogen. High temperature requires stringent materials which increases the cost of the fuel cells. Hence, researchers are working to combine the benefits of the PEMFC and the PAFC to obtain intermediate temperature cells, often referred to as high temperature PEM.

## **2. Solid oxide fuel cell**

The SOFC technology, dates from Walther Nernst, who around 1890 discovered that stabilized zirconia is an isolator at room temperature, but turns into an ionic conductor between 600 − 1000*oC* and an electronic and ionic conductor around 1500*oC*. The first SOFC based on zirconia was introduced by Baur and Preis in 1937. Since then, research on SOFC has been steadily increasing until today.

### **2.1 Fundamentals**

There are different types of FCs that have been mentioned above and are currently in use and development. Among them, SOFCs that works in the simplest structure have grown in recognition as a viable high temperature FC technology. SOFC can be improved to create a hydrogen fuel with heat inevitably occur in the cell, the power structure is a device that requires no external reformer, and without the need for transformer and reactor. The SOFC has a few typical advantages compared to other FCs.

4 Will-be-set-by-IN-TECH

divide water into hydrogen and oxygen with the help of electrolysers during times of high supply and low demand. FCs have to be compact and portable for mobile applications; hence storage of hydrogen is essential for such applications. Hydrogen needs to be handled with great care because it is a highly volatile and flammable gas. It has a high leak rate due to which the gas tends to escape through small orifices, faster than the other gases. Hence storage of

A fuel processor converts the primary fuel source (hydrocarbons) into the fuel gas (hydrogen) required by the FC stack. The processor uses a catalytic reaction to break the fuel into hydrogen and separate it from the carbon based gases. Each of the FC types has specific fuel requirements. Natural gas and petroleum liquids contain sulphur compounds and have to be desulphurized before they can be used as a fuel. The anode catalysts are intolerant of sulphur and it must be removed before it degrades catalyst performance. There is a risk of carbon formation in fuel cell systems which can be reduced by carrying out pre-reforming of the fuel gas before it is fed to the reformer reactor. Carbon monoxide can be used as a fuel for SOFC and MCFC because it can be internally converted to hydrogen whereas the PEMFC should be completely free from it. CO has high affinity for anode catalyst (especially platinum) and it prevents the flow of fuel in the PEMFC. Ammonia is a poison for all the FC types due to its

adverse effects on the cell life except for SOFC, where it can be internally reformed.

obtain intermediate temperature cells, often referred to as high temperature PEM.

Lower-temperature FCs require an external reformer to obtain the hydrogen rich fuel, thus increasing the cost and thereby reducing the efficiency. Higher temperature FCs do not require an external reformer; its high temperature allows direct conversion of natural gas to hydrogen. High temperature requires stringent materials which increases the cost of the fuel cells. Hence, researchers are working to combine the benefits of the PEMFC and the PAFC to

The SOFC technology, dates from Walther Nernst, who around 1890 discovered that stabilized zirconia is an isolator at room temperature, but turns into an ionic conductor between 600 − 1000*oC* and an electronic and ionic conductor around 1500*oC*. The first SOFC based on zirconia was introduced by Baur and Preis in 1937. Since then, research on SOFC has been steadily

There are different types of FCs that have been mentioned above and are currently in use and development. Among them, SOFCs that works in the simplest structure have grown in recognition as a viable high temperature FC technology. SOFC can be improved to create a hydrogen fuel with heat inevitably occur in the cell, the power structure is a device that requires no external reformer, and without the need for transformer and reactor. The SOFC

hydrogen plays a key role in the FC systems.

**2. Solid oxide fuel cell**

increasing until today.

has a few typical advantages compared to other FCs.

2. Easy to handle with simple structure which is composed of all solid

1. Higher efficiency compared to other FCs

4. Carbon monoxide (CO) can be used as a fuel

3. The reforming system is simple

**2.1 Fundamentals**

In addition, while maintaining high efficiency, to lower the temperature of the SOFC is driving development issues. Because 1000*oC* high operating temperature of the material will degrade cells, reduce the choice of construction materials. Operating temperature of 700 <sup>−</sup> <sup>1000</sup>*oC* from the conventional 500 <sup>−</sup> <sup>800</sup>*oC*, these issues are resolved smaller, lower cost, can improve endurance. High-temperature operation removes the need for a precious-metal catalyst, thereby reducing the cost. It also allows SOFCs to reform fuels internally, which enables the use of a variety of fuels and reduces the cost associated with adding a reformer to the system. Although a SOFC produces electricity, it only produces DC power and utilizes only processed fuel. Therefore, a SOFC based power generation system requires the integration of many other components beyond the SOFC stack itself. Moreover, to recover the high quality waste heat from the SOFC stack, an efficient integration of co-generation or bottoming system with the FC section is crucial for a SOFC based power generation plant. Since the balance of plant will directly impact the overall system efficiency and may cost more than the SOFC stack itself, it is obvious that the design of a SOFC power generation system involves more than the optimization of the SOFC unit with respect to efficiency or economics. It also involves balance of plant studies. With SOFC materials and stacks approaching a commercialization stage, there is a need to explore various process designs to obtain optimal efficiency and economics based on specific applications and fuel availability.

#### **2.2 SOFC based power generation systems**

As other types of FCs, a SOFC produces only DC power and requires processed fuel. It also produces high quality heat due to its high operating temperature. Beyond the SOFC stack itself, a typical SOFC power system basically includes: a reformer to start the hydrogen production process, a fuel conditioner to clean up the pollutants that could otherwise poison the fuel cell elements, a power conditioner to convert direct current from the fuel cell to the appropriate voltage range and current type depending on the application, and a cogeneration or bottoming cycle to utilize the rejected heat to achieve high system efficiency. The system also requires the most common balance of plant equipments such as heat exchangers, air blower and fuel compressors, controls systems, and safety systems. Fig.1 illustrates fundamental parts in a FC power unit.

Fig. 1. Structure of SOFC power generation unit

*O*

*e*

*e*

*e*

*e O*2

<sup>−</sup> <sup>→</sup> <sup>2</sup>*O*2<sup>−</sup> (1)

*O*<sup>2</sup> → *H*2*O* (3)

<sup>2</sup>*H*<sup>2</sup> <sup>+</sup> <sup>2</sup>*O*2<sup>−</sup> <sup>→</sup> *<sup>H</sup>*2*<sup>O</sup>* <sup>+</sup> <sup>4</sup>*e*<sup>−</sup> (2)

*CH*<sup>4</sup> + 2*O*<sup>2</sup> → 2*H*2*O* + *CO*<sup>2</sup> (4)

*e*

*e*

*H O*<sup>2</sup> *O*

Air flows along the cathode. When an oxygen molecule inside air flow contacts the cathode/electrolyte interface, it acquires 4 electrons from the cathode and splits into 2 oxygen

The oxygen ions diffuse into the electrolyte material and migrate to the other side of the cell where they encounter the anode. These oxygen ions travel through the porous electrolyte and react with H2 (fuel) to produce water and most importantly, electrons at the anode and the

The electrons transport through the anode to the external circuit and back to the cathode, thus an electro motive force (EMF) is generated between two electrodes. The two electrodes can be connected via an external circuit and an electrical current can be generated. At the same time,

In cases using methane as a fuel, the CH4 reacts with O2 via internal reformer which will be

Before we begin to look at how the electromotive force (EMF) and thus work is produced in a FC, it is necessary to understand some basic thermodynamic concepts. The Gibbs free energy is the energy required for a system at a constant temperature with a negligible volume, minus any energy transferred to the environment due to heat flux. Gibbs free energy is the energy available to do external work which involves moving electrons around an external circuit. In FCs, change in Gibbs free energy of formation (Δ*G*) is considered, as this change is responsible

*O*<sup>2</sup> + 4*e*

*H*<sup>2</sup> + 1 2

discuss later that produces H2O and CO2. The final reaction equation is:

*H*2

Fig. 2. Reaction process inside SOFC

ions. The reaction occurring at the cathode is:

following reaction may occur at the anode:

**2.7 Fuel cell voltage and nernst equation**

we get heat when the reaction occurs. The overall reactions are:

*H*2 *e e e*

*e*

Modelling a SOFC Power Unit Using Natural Gas Fed Directly 503

#### **2.3 Fuel processing**

Fuel processing converts a commercially available fuel to a fuel gas suitable for the SOFC anode reaction. Typical fuel processing steps include:

However, high operating temperature SOFCs can accommodate internal reforming by means of a CO-tolerant nickel catalyst, so they can operate on natural gas with minimum pre-processing of the fuel. This will not only reduce the capital cost of the SOFC system, but also can be beneficial to system efficiency because there is an effective transfer of heat from the exothermic cell reaction to satisfy the endothermic reforming reaction.

Hydrogen sulfide, hydrogen chloride and ammonia are impurities typically found in coal gas. Some of these substances maybe are harmful to the performance of SOFCs. Therefore, a SOFC system will require fuel cleanup equipment such as desulfurizer depending on the raw fuel components.

#### **2.4 Rejected heat utilization**

At 1000*C* operating temperature, SOFCs produce a tremendous amount of waste heat while generating electricity. In order to obtain the highest possible system efficiency, the heat must be recovered by producing hot water, steam, or additional electricity. In a large SOFC power system (>100MW), production of electricity via a steam turbine bottoming cycle is maybe advantageous.

#### **2.5 Power conditioning unit**

While used as a power generator, FCs usually are connected to the load or distribution system via Power Conditioning Unit basically including DC-DC converter and DC-AC inverter. Therefore, low cost and high efficiency inverters are required together with acting controllers for fast tracking of real and reactive power demands. The inverter serves as the interface between the SOFC and the power distribution system. It is controlled in order to provide real and reactive power set point tracking and to adjust the power factor as well as frequency. Transient response control equipment may also be included. The efficiency of the power conversion is typically on the order of 94 to 98%.

#### **2.6 Electrochemistry of SOFC**

Fig.2 shows the processes taking place in a SOFC with hydrogen.

The SOFC fundamentally consists of two porous electrodes (anode and cathode) separated by a ceramic electrolyte in the middle, and flow channels for fuel and air delivery and collection.

#### Fig. 2. Reaction process inside SOFC

6 Will-be-set-by-IN-TECH

Fuel processing converts a commercially available fuel to a fuel gas suitable for the SOFC

• Desulphurization, where a catalyst is used to remove sulphur contaminants in the fuel. Sulphur compounds are noxious, and they can also bind catalysts used in later stages of

• Reformation, where the fuel is mixed with steam and then passed over a catalyst to break

• Shift conversion, where the carbon monoxide reacts with steam over a catalyst to produce

However, high operating temperature SOFCs can accommodate internal reforming by means of a CO-tolerant nickel catalyst, so they can operate on natural gas with minimum pre-processing of the fuel. This will not only reduce the capital cost of the SOFC system, but also can be beneficial to system efficiency because there is an effective transfer of heat from

Hydrogen sulfide, hydrogen chloride and ammonia are impurities typically found in coal gas. Some of these substances maybe are harmful to the performance of SOFCs. Therefore, a SOFC system will require fuel cleanup equipment such as desulfurizer depending on the raw fuel

At 1000*C* operating temperature, SOFCs produce a tremendous amount of waste heat while generating electricity. In order to obtain the highest possible system efficiency, the heat must be recovered by producing hot water, steam, or additional electricity. In a large SOFC power system (>100MW), production of electricity via a steam turbine bottoming cycle is maybe

While used as a power generator, FCs usually are connected to the load or distribution system via Power Conditioning Unit basically including DC-DC converter and DC-AC inverter. Therefore, low cost and high efficiency inverters are required together with acting controllers for fast tracking of real and reactive power demands. The inverter serves as the interface between the SOFC and the power distribution system. It is controlled in order to provide real and reactive power set point tracking and to adjust the power factor as well as frequency. Transient response control equipment may also be included. The efficiency of the power

The SOFC fundamentally consists of two porous electrodes (anode and cathode) separated by a ceramic electrolyte in the middle, and flow channels for fuel and air delivery and collection.

it down into hydrogen, as well as carbon dioxide and carbon monoxide.

the exothermic cell reaction to satisfy the endothermic reforming reaction.

**2.3 Fuel processing**

components.

advantageous.

**2.4 Rejected heat utilization**

**2.5 Power conditioning unit**

**2.6 Electrochemistry of SOFC**

conversion is typically on the order of 94 to 98%.

Fig.2 shows the processes taking place in a SOFC with hydrogen.

anode reaction. Typical fuel processing steps include:

fuel reformation poisoning the catalyst.

more hydrogen and carbon dioxide.

Air flows along the cathode. When an oxygen molecule inside air flow contacts the cathode/electrolyte interface, it acquires 4 electrons from the cathode and splits into 2 oxygen ions. The reaction occurring at the cathode is:

$$\rm O\_2 + 4e^- \rightarrow 2O^{2-} \tag{1}$$