reference will now be made in detail to the exemplary embodiments of the present disclosure , examples of which are illustrated in the accompanying drawings , wherein like reference numerals refer to the like elements throughout . the exemplary embodiments are described below , in order to explain the aspects of the present disclosure , by referring to the figures . as those skilled in the art would realize , the described exemplary embodiments may be modified in various ways , without departing from the spirit or scope of the present disclosure . accordingly , the drawings and description are to be regarded as illustrative in nature and not restrictive . in addition , when an element is referred to as being “ on ” another element , it can be directly on the other element or may be indirectly on the other element , with one or more intervening elements interposed therebetween . also , when an element is referred to as being “ connected to ” another element ; it can be directly connected to the other element , or may be indirectly connected to the other element , with one or more intervening elements interposed therebetween . fig1 illustrates an no x emission reduction system 300 , according to an exemplary embodiment of the present disclosure . referring to fig1 , the system 300 includes a driving unit , a fuel cell system 100 , and a reduction unit 130 . the reduction unit 130 may be a hydrogen selective catalytic reduction ( h - scr ) unit . the driving unit 102 includes a fuel tank 210 , a fuel line m 1 , an engine 220 , and an exhaust line m 2 . the driving unit may be incorporated into a vehicle . the fuel cell system 100 includes a fuel reformer 110 and a fuel cell stack 120 . the system 300 further includes : reformate lines r 1 and r 2 that respectively connect the reformer and the stack 120 to the exhaust line m 1 ; a bypass line b 1 that connects the fuel tank 210 to the reformer 110 ; and bypass lines b 2 and b 3 that independently connect the reformer 110 and the stack 120 . herein , such connections may be referred to as fluid communications . the fuel tank 210 stores a fuel to operate the engine 220 . the fuel is supplied to the engine 220 through the fuel line m 1 . the fuel is also supplied to the fuel reformer 110 , through the bypass line b 1 . the fuel is generally diesel fuel , and the engine 220 is generally a diesel engine , but the present disclosure is not limited thereto , as other fuels and engine types may also be used . the engine 220 combusts the fuel , and exhaust from the engine 220 is discharged through the exhaust line m 2 . the fuel reformer 110 reforms the fuel supplied from the fuel tank 210 , to generate a reformate gas . generally , in the fuel reformer 110 the fuel undergoes a water gas shift ( wgs ) reaction and / or a preferential carbon monoxide oxidation ( prox ) reaction , or the like . the fuel reformer 110 may also include a desulfurizer to remove a sulfur component from the fuel . the wgs reaction produces hydrogen and a carbon monoxide byproduct . the prox reaction reduces the concentration of the carbon monoxide , such that the reformate gas is composed of primarily hydrogen gas ( h 2 ). herein , the reformate gas may be referred to simply as hydrogen ( h 2 ). the reformate gas is supplied to anodes of the stack 120 , via the bypass line b 2 , where it is oxidized to generate electricity . the electricity can be supplied to various devices , such as components of a vehicle including the engine 220 . in particular , the reformate gas supplied to the stack 120 is oxidized into water , while electrons are collected by the anodes of the stack 120 . the stack 120 may produce an anode off gas ( aog ) which may include water vapor and h 2 that was not oxidized in the stack 120 . herein the aog may be referred to as hydrogen ( h 2 ). the aog may be supplied to the exhaust line m 2 , via the reformate line r 2 , where it is mixed with the exhaust . the resultant mixture ( mixed exhaust stream ) is then delivered to the reduction unit 230 , via the exhaust line m 2 . the aog may also be returned to the reformer 110 , via the bypass line b 3 , where it is used to operate a heat source ( not shown ), such as a burner , included in the reformer 110 . the heat source may be used to heat the reformer 110 to a preset operating temperature . the aog may also be mixed with natural gas , propane , etc , prior to being supplied to the heat source . the reduction unit 230 removes no x from the mixed exhaust stream . in particular , the reduction unit 230 includes a catalyst that facilitates a reduction reaction between the no x and the hydrogen of the mixed exhaust stream . in other words , the hydrogen from the reformate gas and / or the aog is used as a reducing agent , to reduce the no x through a preferential catalytic reduction reaction . the catalyst may be an ag / alumina catalyst , to expand an active temperature zone of the preferential catalytic reduction reaction . fig2 illustrates a no x emission reduction system 400 , according to another exemplary embodiment of the present disclosure . as shown in fig2 , the system 400 is similar to the system 300 , so only the differences will be described in detail . in particular , the system includes reformate lines r 3 and r 4 , in place of the reformate lines r 1 and r 2 . in addition , the system includes an anode off gas control valve 130 disposed on the bypass line b 3 , and a reformate gas control valve 140 disposed on the bypass line b 2 . the reformate line r 2 extends between the valve 140 and the exhaust line m 2 . the reformate line r 4 extends between the valve 130 and the exhaust line m 2 . the fuel is transferred from the fuel tank 210 to the reformer 110 and the engine 220 , via lines b 1 and m 1 , respectively . the fuel reformer 110 reforms the fuel into a reformate gas , as described above . the reformate gas control valve 140 is disposed on the bypass line b 2 , between the fuel reformer 110 and the stack 120 , and is also connected to the reformate line r 3 . the reformate gas control valve 140 selectively controls the amount of the reformate gas that is delivered from the fuel reformer 110 , to the stack 120 and to the exhaust line m 2 . the reformate gas control valve 140 may be a proportionate valve , so as to adjust the relative amounts of the reformate gas that is delivered to the stack 120 and the exhaust line m 2 . the reformate gas control valve 140 may include a pump ( not shown ) or a blower ( not shown ), etc ., in order to reinforce the delivering force . the reformate gas control valve 140 may include a check valve , etc ., in order to prevent backflow . the anode off gas control valve 130 is disposed on the bypass line b 3 , and receives the aog discharged from the stack 120 . the reformate line r 4 provides a fluid communication between the anode off gas control valve 130 and the exhaust line m 2 . the anode off gas control valve 130 controls the distribution amount , ratio , and period of the aog gas to the reformer 110 and the exhaust line m 2 , and may be similar to the reformate gas control valve 140 . the heat source ( not shown ) of the fuel reformer 110 is used to control the temperature thereof . in order to prevent improper ignition ( backfire ) of the aog , due to the temperature of the heat source , the aog may be supplied in pulses . the anode off gas control valve 130 may be used to create such pulses . as described above , the reduction unit 230 receives the reformate gas / aog and the exhaust , via the exhaust line m 2 . the reduction unit chemically reduces the no x included in the exhaust into n 2 , using hydrogen as a reducing agent . fig3 illustrates an no x emission reduction system 500 , according to an aspect of the present disclosure . as shown in fig3 , the reduction system 500 is similar to the reduction system 400 , so only differences therebetween will be described in detail . unlike the reduction system 400 , the reduction system 500 further includes another fuel tank 215 to supply fuel to the reformer 110 . as a result , the fuel reformer 110 may receive fuel from either of the fuel tanks 210 , 215 . the fuel included in the fuel tank 215 may be a fuel such as natural gas , propane , city gas , or the like , which may be more readily processed by the reformer 110 into a reformate gas , as compared to diesel fuel or gasoline . thus , quality and / or production efficiency of the reformer 110 may be increased . fig4 illustrates an no x emission reduction system 600 , according to an aspect of the present disclosure . the reduction system 600 is similar to the reduction system 400 , so only differences therebetween will be described in detail . as shown in fig3 , the reduction system includes a controller 300 that controls the operation of the valves 130 and 140 . in particular , the controller may vary the amount of hydrogen that is applied to the exhaust line , in accordance with the amount of no x produced by the engine 220 . when the reduction system 600 is employed in a vehicle , such as in a recreational vehicle , tractor trailer , or the like , the fuel cell stack 120 may be used to provide electrical power to the vehicle , when the engine 220 is not operated . in such a case , the controller 300 controls the reformate gas control valve 140 and the anode off gas control valve 130 , such that hydrogen is not supplied to the exhaust line m 2 . in addition , when the engine 220 and the fuel cell stack 120 are simultaneously operated , the controller may vary the amount of hydrogen that is applied to the exhaust line m 2 , in accordance with the amount of no x produced by the engine 220 and / or the amount of load applied to the fuel cell stack 120 . in other words , when there is a high demand for electricity , and a low load on the engine 220 , the amount of the reformate gas and / or aog that is supplied to the exhaust line m 2 may be reduced . in addition , when there is a low load applied to the fuel cell stack 120 and a high load applied to the engine 120 , the amount of the reformate gas and / or the aog supplied to the exhaust line m 2 may be increased . although a few exemplary embodiments of the present disclosure have been shown and described , it would be appreciated by those skilled in the art that changes may be made in these exemplary embodiments , without departing from the principles and spirit of the disclosure , the scope of which is defined in the claims and their equivalents .