**1. Introduction**

In recent years, waste incineration technology has been widely used in the world by virtue of reduction (about 90% reduction in volume, 70% reduction in weight), harmlessness, and energy recovery [1,2]. Due to the complex compositions of waste, a variety of harmful gases and toxic substances will be formed during the incineration process. These are inorganic acid gases such as HCl, HF, NOx, SO2, and gaseous heavy metals such as mercury, highly toxic organic substances like Polycyclic Aromatic Hydrocarbons (PAHs), Polychlorinated Biphenyls (PCBs), Polychlorinated Dibenzo-Dioxins/Polychlorinated Dibenzofurans (PCDD/PCDFs), fly ash which equivalent to 2–5% of the original waste weight and required for special treatment [3–5]. Therefore, secondary pollution problem is urgent to be solved in the waste incineration process.

The traditional treatment of flue gas is mostly to remove the pollutants at low temperatures (100–350 ◦C), and the process is very complicated. The corrosion of superheater caused by acid gases and fly ash in incineration flue gas at high temperatures limits the further improvement of steam parameters and power generation efficiency [6]. In addition, the low temperature environment is easy to promote the re-synthesis of dioxins, and the presence of chlorine and heavy metals in fly ash provide favorable conditions for dioxins re-synthesis [7–10]. Therefore, it is necessary to pre-remove acidic gases and fly ash from flue gas at high temperatures.

The grate waste incinerator is the mainstream equipment for waste incineration [11,12]. At present, the grate waste incinerator relies on a three-stage furnace to reduce the flue gas temperature at the entrance of convective heating surface generally. In order to prevent corrosion at low temperatures, the flue gas temperature at the exit of the waste heat boiler is approximately designed to be at temperatures between 190–230 ◦C, which will limit the improvement of the efficiency of waste heat boiler. According to the structural characteristics of the grate waste incinerator, this research proposes a square adiabatic cyclone flue arranged at the first-stage furnace exit of the grate waste incinerator (as shown in Figure 1) to pre-remove the fly ash in the flue gas. In addition, calcium and ammonia can be injected into the flue to remove acidic gases synergistically, which alleviates corrosion of heating surface at high and low temperatures, as well as inhibits dioxin formation and improves furnace parameters. In order to facilitate the arrangement, the cyclone flue is designed as a square, and downward exhaust is adopted to conform to the flow direction of flue gas in the grate waste incinerator, the original grate waste incinerator can be reformed directly. There is no heating surface inside the flue, the inner layer is refractory, and the outer layer is insulation materials. These can prevent heat loss and ensure that the flue gas exists at a higher temperature, which can crack dioxins effectively. In order to understand the flow field characteristics and gas–solid separation performance of cyclone flue, this paper simulates the internal flow field and gas–solid separation performance of cyclone flue with the CFD method.

**Figure 1.** Arrangement diagram of cyclone flue in grate waste incinerator.

The adiabatic cyclone flue proposed in this paper is similar in structure to the cyclone separator, so the simulation method of the cyclone separator can be referred to. The traditional cyclone separator is circular and has an upper exhaust, which has been subjected to a large number of experiments and numerical studies by many researchers, and the research methods are relatively reasonable and reliable. Xiang et al. [13] used the Reynold's stress model (RSM) to simulate the internal flow field of cyclone separators with different heights. The results showed that as the height of the cyclone separator increases, the tangential velocity will decrease, then the centrifugal force on the particles is reduced, which will result in a decrease in dust removal performance. Raoufi et al. [14] studied the effect of the gas outlet tube structure on the separation efficiency of the cyclone separator based on the Euler–Lagrangian method; the results indicated that the separation efficiency of the separator decreases with the increase of the diameter of gas outlet tube. Moreover, Safikhani et al. [15] applied a simple pressure correction algorithm, using the Reynold's stress turbulence model and the random

walk model to study the internal flow field and particle motion trajectory of cyclone separators with different number of inlets. In addition, Sakura et al. [16] used the large eddy simulation (LES) model, based on Ansys CFX to study the gas–solid flow characteristics of cyclone separators, and compared the effects of different dust outlet shapes on separation efficiency and pressure drop. Zhang et al. [17] used the transient Reynold's stress model and discrete phase model to study the flow field and particle motion trajectory in the cyclone separator, and proposed a new cyclone separator, which can improve efficiency and reduce pressure drop. Furthermore, Parvaz et al. [18] studied the performance of a cyclone separator which had an inner cone located at the bottom of the cyclone, and simulated the influence of the inner cone with different heights and diameters on the performance of the separator.

Compared to conventional cyclone separator, there is less research on square and downward exhaust cyclones. Safikhani et al. [19] compared the internal flow field, separation efficiency of a square cyclone separator and a circular cyclone separator based on the CFD method, the results showed that the separation efficiency of the square cyclone separator is lower than the circular cyclone separator, but the pressure drop is rather small. Su et al. [20] used the Euler–Lagrange method to simulate the gas–solid flow characteristics of three inlet structures of square cyclones. Raoufi et al. [21] studied the internal flow field of a square cyclone separator by numerical simulation method, and analyzed the similarities and differences of the internal flow field of the upper exhaust type and the downward exhaust type cyclone separator. Moreover, Oh et al. [22] investigated the internal flow field and particle separation efficiency of the downward exhaust cyclone based on the Euler–Lagrange method. Fatahian et al. [23] studied the performance that using the laminarizer in the square and circular cyclones, the results suggested that square cyclone is more effective. Additionally, Mokni et al. [24] used the CFD method to study the effects of cylinder height on flow field, pressure drop and separation performance in a turbulent hydrocyclone.

Here we simulated by way of a realizable k-ε model and random walk model. Firstly, the geometric model and mathematical model are described, then the internal flow field characteristics of the cyclone flue and the influence of various structural parameters on the separation performance and pressure drop are analyzed, finally, the comprehensive performance of the cyclone flue is evaluated by technique for order preference by similarity to an ideal solution (TOPSIS) method.

### **2. Model Description**

### *2.1. Adiabatic Cyclone Flue Model*

As shown in Figure 1, the cyclone flue distributes along the width of the grate waste incinerator evenly, and each adiabatic cyclone flue works independently without any influence on each other, so only one of the flues needs to be studied. Its structure is shown in Figure 2. In this paper, the influences of the vortex finder diameter, vortex finder length, distance between vortex finder and gas outlet tube, gas outlet tube diameter on the performance of cyclone flue were studied. The dimensions of each cyclone flue are given in Table 1. In order to reduce the computational complexity, the numerical studies only were carried out on the parts above the cone. The study of Oh et al. [22] showed that neglecting the cone part has little effect on the results and can be neglected.

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**Figure 2.** Structure diagram of adiabatic cyclone flue: (**a**) Three-dimensional structure diagram, (**b**) Profile diagram.

**Table 1.** Dimension of the adiabatic cyclone flue.

### *2.2. Governing Equation*

The CFD method was used to simulate a three-dimensional flow field in the adiabatic cyclone flue. For steady incompressible flow, the continuity equation and momentum equation are as follows:

$$\frac{\partial \overline{u}\_i}{\partial \overline{x}\_i} = 0 \tag{1}$$

$$
\overline{u}\_{\dot{j}} \frac{\partial \overline{u}\_{\dot{i}}}{\partial \mathbf{x}\_{\dot{j}}} = -\frac{1}{\rho} \frac{\partial \overline{P}}{\partial \mathbf{x}\_{\dot{i}}} + \upsilon \frac{\partial\_2 \overline{u}\_{\dot{i}}}{\partial \mathbf{x}\_{\dot{j}} \partial \mathbf{x}\_{\dot{j}}} - \frac{\partial}{\partial \mathbf{x}\_{\dot{j}}} R\_{i\dot{j}} \tag{2}
$$

In the above equations, *ui* is the average velocity of the fluid; *xi* is the coordinate position; *P* is the average pressure; ρ is the fluid density; *v* is the kinematic viscosity of the fluid; *Rij* = *u i u j* , is the Reynold's stress tensor, which represents the influence of turbulence on the flow field, here, *u i* = *ui* − *ui*.

The turbulence models for cyclone separator simulation were the *k*-ε (standard, RNG, realizable) model and the Reynold's stress model (RSM) commonly. In this paper, the realizable *k*-ε model was adopted, the transport equations of turbulent kinetic energy (*k*) and turbulent dissipation rate (ε) were as follows:

$$\frac{\partial}{\partial t}(\rho k) + \frac{\partial}{\partial \mathbf{x}\_{\dot{j}}}(\rho k u\_{\dot{j}}) = \frac{\partial}{\partial \mathbf{x}\_{\dot{j}}}[(\mu + \frac{\mu\_{\mathbf{f}}}{\sigma\_k})\frac{\partial \mathbf{k}}{\partial \mathbf{x}\_{\dot{j}}}] + G\_k - \rho \varepsilon \tag{3}$$

$$\frac{\partial}{\partial \mathbf{t}} (\rho \boldsymbol{\varepsilon}) + \frac{\partial}{\partial \mathbf{x}\_{\circ}} (\rho \boldsymbol{\varepsilon} \boldsymbol{u}\_{\circ}) = \frac{\partial}{\partial \mathbf{x}\_{\circ}} [ (\mu + \frac{\mu\_{t}}{\sigma\_{\varepsilon}}) \frac{\partial \boldsymbol{\varepsilon}}{\partial \mathbf{x}\_{\circ}}] - \rho \mathbf{C}\_{2} \frac{\boldsymbol{\varepsilon}^{2}}{\mathbf{k} + \sqrt{v \boldsymbol{\varepsilon}}} \tag{4}$$

In the above equations, *Gk* represents the turbulent energy generated by the velocity gradient; μ is the dynamic viscosity of the fluid; *C*2 is a constant; σ*k*, and σε are turbulent Prandtl numbers in *k* equation and ε equation respectively.

There are two main methods simulating gas–solid two-phase flow. They are the Euler–Euler method and the Euler–Lagrange method. In order to track the trajectory of particles, the Euler–Lagrange method was used to simulate gas–solid two-phase flow. In this method, gases were regarded as continuous phase and particles as dispersed discrete phase. Because the particle concentration was small, it belonged to the dilute phase flow and the particle size was small too, so the influence of particle on flow field can be neglected, unidirectional coupling mode was adopted in this paper, and the interaction force between particles was ignored. In Lagrangian coordinates, the force balance equations of particles are as follows:

$$\frac{du\_p}{dt} = F\_D (u\_A - u\_p)\_x + \frac{g\_x (\rho\_p - \rho\_A)}{\rho\_p} \tag{5}$$

$$F\_D = \frac{18\mu\_A}{\rho\_\mathrm{F}d\_p^2} \frac{\mathbb{C}\_D \mathrm{Re}\_p}{24} \tag{6}$$

$$\mathbf{C}\_{D} = \mathbf{C}\_{1} + \frac{\mathbf{C}\_{2}}{\mathrm{Re}\_{p}} + \frac{\mathbf{C}\_{3}}{\mathrm{Re}\_{p}} \tag{7}$$

$$\text{Re}\_p = \frac{\rho\_A d\_p \left| u\_p - u\_A \right|}{\mu\_A} \tag{8}$$

In the above equations, *uA* is the fluid velocity; up is the particle velocity; μ*A* is the fluid dynamic viscosity; *dp* is the particle diameter; ρ*p* is the particle density; ρ*A* is the fluid density; *Rep* is the relative Reynold's number (particle Reynold's number); *CD* is the resistance coefficient; *C*1–*C*3 is a constant, depending on Reynolds number; *gx* is the gravitational acceleration.

### *2.3. Boundary Conditions and Numerical Schemes*

Semi-implicit method pressure-linked equations consistent (SIMPLEC) were used as a method for pressure-velocity coupling. The Pressure staggering option (PRESTO) was used for pressure interpolation. The quadratic upstream interpolation for convective kinetics (QUICK) was used for discrete difference format.

In this paper, we assumed that the gas flow enters the flue at a constant velocity, the gas phase is a high temperature flue gas, its temperature is 850 ◦C, its velocity is 20 m/s, and the particles and fluids have the same inlet velocity. The setting of specific boundary conditions are shown in Table 2, non-slip adiabatic boundary conditions are adopted for all walls.

### *2.4. Grid Independence Study*

In this paper, the structured hexahedral mesh of cyclone flue was generated by ICEM. In order to ensure the accuracy of calculation and shorten the calculation time, five different numbers of structured grids were generated to verify the irrelevance between the calculated results and the number of grids, which are 890,000 cells, 1,190,000 cells, 1,400,000 cells, 1,670,000 cells and 1,960,000 cells. Figure 3 shows the weighted average turbulence intensity (I) and weighted average turbulent kinetic energy (K) at z = 1.5 m cross section area under different mesh numbers, with the increase of the number of grids, the values of K and I decreased gradually, the differences between the 1,670,000 grids and the 1,960,000 grids were 0.17% and 0.06% respectively, indicating that the number of grids had little impact on the calculation results. The model with 1,670,000 grids is selected for numerical simulation.

**Figure 3.** The weighted average turbulent kinetic energy (*K*) and weighted average turbulence intensity (*I*) at z = 1.5 m cross-section under different grids.

### *2.5. Model Validation*

In order to verify the accuracy of the model, the flow characteristics were compared with the experimental data of Su [26]. Su et al. measured the axial velocity distribution of Plane 1 at an inlet velocity of 25.3 m/s using a particle dynamic analyzer (PDA). The geometric model is shown in Figure 4.

Figure 5 compares the computed axial velocity at different positions in Plane 1 with the corresponding experimental data of Su. The two positions were *x* = 30 mm and *x* = 90 mm. It is clearly seen that the model prediction was in close quantitative agreement with the experimental data. Overall, the adopted model can accurately and reliably predict the performance of the cyclone flue.

**Figure 4.** Geometric model of Su et al.

**Figure 5.** Comparison of simulated velocity with measurements.

### **3. Results and Discussion**

### *3.1. Characteristics of Flow Field*

Figure 6a shows the velocity distribution of the cyclone flue at *z* = 1.5 m cross section. It can be seen from the figure that the maximum velocity appeared at the entrance of the gas outlet tube. The velocity below the vortex finder was lower, because the recirculation zone was formed here. The obvious flow separation was found at the edge of the vortex finder near the inlet, resulting in a lower velocity. Oh et al. [22] also found this phenomenon in the simulation of a circular uniflow cyclone. The gas flow went into the flue and rotated around the vortex finder, flow separation appeared due to the centrifugal and inertial forces, and this phenomenon was weakened gradually in the downstream direction.

**Figure 6.** Flow field distribution at *z* = 1.5 m: (**a**) velocity contours, (**b**) static pressure contours.

Figure 6b shows the static pressure distribution of the cyclone flue at *z* = 1.5 m cross section. It can be seen from the figure that the static pressure distributed symmetrically around the central axis, which is similar to traditional cyclone separators [14,27]. Along the radius, the pressure decreased slowly, thus forming a low pressure zone in the center. The minimum static pressure appeared at the entrance of the gas outlet tube, reaching −1750 Pa. Combined with the velocity distribution, it can be seen that the fluid velocity reached the highest here, and the static pressure was converted into dynamic pressure.

Figure 7 shows the velocity vector diagram of cyclone flue at *y* = 9 m cross section. It can be seen from the figure that when the gas flow went into the flue from the inlet, the velocity increased firstly, and then decreased gradually in the course of the flow. There was a strong eddy at the corner facing the inlet, which has been confirmed in Su's discovery [20]. As the gas flow goes into the flue, it impinged the corner opposite the inlet directly, and the sharp changed the flow direction at the corner, causing the local vortex. Some particles were collected in the corner and fell down, while some particles were rebounded in the corner and rotated continuously. In the other corners, there was also a weaker local vortex.

**Figure 7.** Velocity vector at y = 9 m.

### *3.2. Particle Separation E*ffi*ciency and Pressure Drop*

Particle separation efficiency and pressure drop are two important indices to study the performance of the adiabatic cyclone flue. This paper used the control variable method to study the effects of structural parameters such as vortex finder length (*h*1), vortex finder diameter (*D*1), distance between vortex finder and gas outlet tube (*h*2), and gas outlet tube diameter (*D*2) on the separation performance and pressure drop of the cyclone flue, which provided guiding significance for the optimal design of the cyclone flue. According to the literature [28–30], the particle size of fly ash in the waste incineration flue gas ranged from 1 to 100 μm, which can be assumed that the particle size follows the Rosin–Rammler distribution.

### 3.2.1. The Influence of the Vortex Finder Length

Figure 8 shows the effects of the vortex finder length on the separation efficiency and pressure drop of the cyclone flue, and the *h*1/*a* varying from 1.1 to 1.5. It can be seen from the figure that the separation efficiency increased firstly and then decreased with the increase of the vortex finder length. There was an optimum value in the middle. When *h*1/*a* = 1.2, the separation efficiency reached its highest point. The pressure drop increased linearly with the increase of the vortex finder length, and the increase ratio was about 1.5%. The experiments conducted by Trieseh et al. also found that the pressure drop increased proportionally to the vortex finder length [31].

**Figure 8.** Effect of the vortex finder length (*h*1/*a*) on efficiency and pressure drop.

The function of the vortex finder was mainly embodied in two aspects. On the one hand, the gas flow was rotated under the guidance of the vortex finder. On the other hand, the annular space formed by the vortex finder and the body affected the rotation speed of gas flow. As the increase of the vortex finder length appropriated, enough time was ensured to separate particles to the wall. However, if the vortex finder length was too long, it caused back-flushing of the bottom due to the extension of the swirling gas flow, and resulted in a decrease in separation efficiency. The longer the length of vortex finder was, the larger the area of the wall contacted with the gas flow was. The rotational kinetic energy loss of the gas flow increased due to friction, resulting in an increase in the pressure drop.

### 3.2.2. The Influence of the Vortex Finder Diameter

Figure 9 shows the effects of the vortex finder diameter on the separation efficiency and pressure drop of the cyclone flue, and the *D*1/*A* varying from 0.33 to 0.47. It can be seen from the figure that the separation efficiency increased firstly and then decreased with the increase of the vortex finder diameter. While *D*1/*A*= 0.4, the separation efficiency was the highest, reaching to 77.2%. The pressure drop increased with the increase of vortex finder diameter, but while *D*1/*A* was higher than 0.4, the change of the diameter of vortex finder had little effect on the pressure drop. Compared with the vortex finder length, the vortex finder diameter had a weaker influence on the pressure drop.

**Figure 9.** Effect of the vortex finder diameter (*D*1/*A*) on efficiency and pressure drop.

With the increase of the vortex finder diameter, the annular cross-section area of the vortex finder was reduced, and the velocity of the flow increased when entering the flue, in addition, the centrifugal force strengthened, which helped to improve separation efficiency. In addition, the increase in velocity of flow led to an increase in the loss of rotational kinetic energy, resulting in an increase in pressure loss. However, the too large vortex finder diameter led to the impact of part of the gas flow on the vortex finder directly, resulting in the deterioration of the flow field and the rebound of particles, which was not conducive to the separation of particles.

### 3.2.3. The Influence of the Gas Outlet Tube Diameter

Figure 10 shows the effects of gas outlet tube diameter on separation efficiency and the pressure drop of the cyclone flue, and the *D*2/*A* varying from 0.33 to 0.5. It can be seen from the figure that the separation efficiency decreased with the increase of the gas outlet tube diameter. But in the range of 0.33–0.4 and 0.43–0.5, the variation range of separation efficiency was small, while *D*2/*A* increased from 0.4 to 0.43, the separation efficiency decreased from 77.2% to 75.8%. Compared with the vortex finder, the gas outlet tube diameter had a greater influence on the pressure drop. When *D*2/*A* increased from 0.33 to 0.5, the pressure drop decreased from 2065 Pa to 934 Pa, and the variation decreased gradually.

**Figure 10.** Effect of the gas outlet tube diameter (*D*2/*A*) on efficiency and pressure drop.

The entrance of the gas outlet tube was a high-speed and low-pressure area, as shown in Figure 6, and the gas flow converged here. If the diameter of gas outlet tube was small, the fly ash particles collided with the wall of gas outlet tube easily when the particles moved to the entrance of the gas outlet tube, and then falling into the hopper and being separated, which helped to improve the separation efficiency. With the increase of the diameter of the gas outlet tube, the phenomenon of the flow rate

rising sharply when the gas reached the gas outlet tube was alleviated, thus making the flow field more stable.

3.2.4. The Influence of the Distance between Vortex Finder and Gas Outlet Tube

Figure 11 shows the effects of the distance between vortex finder and gas outlet tube on the separation efficiency and pressure drop of the cyclone flue, and the *h*2/*A* varying from 0.83 to 1.5. It can be seen from the figure that the separation efficiency increased at first and then decreased with the increase of the distance between the vortex finder and the gas outlet tube. When *h*2/*A* = 1.33, the separation efficiency was the highest, reaching to 76.5%. The pressure drop decreased linearly with the increase of the distance between the vortex finder and the gas outlet tube, and the reduction ratio was about 5%.

**Figure 11.** Effect of the distance between vortex finder and gas outlet tube (*h*2/*A*) on efficiency and pressure drop.

Gauthier studied the effect of the length of the separation section (the sum of *h*1 and *h*2) on the performance of the uniflow cyclone. The results showed that the separation efficiency increased first and then decreased with the increase of the separation section [32].

As the increase in the distance between the vortex finder and gas outlet tube appropriated, it allowed enough time for particles to converge into the wall. However, the excessive distance led the particles that have been separated to re-enter the central gas flow and escape from the gas outlet tube. In addition, the longer the distance between the vortex finder and gas outlet tube was, and the shorter the length of the gas outlet tube was then the weaker the interference to the flow field was, which made the flow field more stable and decreased the pressure loss.

### *3.3. Comprehensive Performance Evaluation*

The highest separation efficiency and the lowest pressure drop cannot always exist at the same time, it is necessary to analyze and evaluate the comprehensive performance of the cyclone flue, and to ensure high efficiency and low energy loss. The TOPSIS evaluation method is a scientific method used in multi-objective decision analysis of limited schemes commonly, and it was developed by Yoon and Hwang [33]. This method has no special requirement on the sample data, it can make full use of the original data, and it can obtain better results [34–36]. Therefore, this method was used to analyze and evaluate the comprehensive performance of the cyclone flue in this study. The steps of the TOPSIS model were as follows:

(1) Establish the initial decision matrix *X* = {*xij*}. Two evaluation indexes of separation efficiency and pressure drop of cyclone flue were considered, and *xij* is the *j*-th evaluation index of the *i*-th evaluation object.

$$\mathbf{x}\_{ij} = \mathbf{x}\_{ij}^{\prime} / \sqrt{\sum\_{i=1}^{m} \mathbf{x}\_{ij}^{\prime 2}} \tag{9}$$

$$d\_i^+ = \sqrt{\sum\_{j=1}^n \left(s\_j^+ - z\_{ij}\right)^2} \tag{10}$$

$$d\_i^- = \sqrt{\sum\_{j=1}^n \left(s\_j^- - z\_{ij}\right)^2} \tag{11}$$

(6) Calculating the relative proximity (*Ci*) of each evaluation scheme to the *S*+. The equation is as follows:

$$\mathbb{C}\_{i} = d\_{i}^{-} / (d\_{i}^{+} + d\_{i}^{-}) \tag{12}$$

(7) Ranking according to the value of *Ci*. the larger the value of *Ci* is, the closer the scheme to *S*+ and the farther away from *S*−, the better the scheme is.

In this paper, the vortex finder length, the vortex finder diameter, the gas outlet tube diameter, and the distance between the vortex finder and the gas outlet tube were taken as the evaluation objects. The *Ci* values were calculated under different structural parameters. The calculation results are listed in Table 3.

**Variable 1 Variable 2 Variable 3 Variable 4** *h*1/*a Ci D*1/*A Ci h*2/*A Ci D*2/*A Ci* 1.1 0.91 0.33 0.65 0.83 0 0.33 0.04 1.2 0.73 0.37 0.48 1.00 0.25 0.37 0.21 1.3 0.43 0.40 0.39 1.17 0.48 0.40 0.42 1.4 0.22 0.43 0.11 1.33 0.74 0.43 0.63 1.5 0 0.47 0.01 1.5 0.97 0.47 0.82 ////// 0.50 0.96

**Table 3.** Comprehensive performance evaluation of cyclone flue.

According to the data in Table 3, the comprehensive performance of the cyclone flue was negatively correlated with the vortex finder diameter and the vortex finder length, while it was positively correlated with the distance between the vortex finder and the gas outlet tube, and gas outlet tube diameter. When designing the cyclone flue, the vortex finder diameter and the vortex finder length should be adopted to smaller values, but the distance between the vortex finder and the gas outlet tube, and the gas outlet tube diameter, should be taken to a larger value. In the scope of this study, when *h*1/*a* = 1.1,

*D*1/*A* = 0.33, *h*2/*A* = 1.5, *D*2/*A* = 0.50, the separation efficiency reached to 75% and the pressure drop was about 900 Pa, reaching a better comprehensive performance.

### **4. Conclusions**

In this paper, the internal flow field and performance of the adiabatic cyclone flue were studied by the CFD method. The effects of the vortex finder diameter, vortex finder length, distance between vortex finder and gas outlet tube, and gas outlet tube diameter on the separation efficiency and pressure drop of cyclone flue were also studied. Then the TOPSIS method was applied to evaluate the comprehensive performance of the cyclone flue. The main conclusions are shown as follows:

**Author Contributions:** Conceptualization, D.-m.C., J.-y.R. and L.Y.; methodology, D.-m.C., J.-y.R., J.-t.N. and Z.-q.Y.; software, D.-m.C. and J.-t.N.; validation, D.-m.C., J.-y.R. and J.-t.N.; formal analysis, Z.-q.Y. and G.P.; investigation, D.-m.C. and J.-t.N.; resources, J.-y.R., G.P. and L.Y.; data curation, D.-m.C. and Z.-q.Y.; writing—original draft preparation, D.-m.C.; writing—review and editing, J.-t.N. and J.Y.R; visualization, G.P. and Z.-q.Y.; supervision, L.Y.; project administration, J.-y.R.; funding acquisition, J.-y.R.

**Funding:** This research was funded by KEY INDUSTRIAL GENERIC TECHNOLOGY INNOVATION PROJECT OF CHONGQING, grant number Cstc2016zdcy-ztzx20006.

**Conflicts of Interest:** The authors declare no conflicts of interest.

### **Abbreviations**


**3. Land Management in European Countries**

Over the years, the narrowly defined discipline of *land policy* has evolved into *land management*. As recently as the 1980s and 1990s, *land policy* was defined and practiced as:

. . . the name suggests, [it] deals with the resource "land". It involves preparing the land for urban development and other uses that conform to the plan. It helps to ensure that the land is available to users who are suitable from an urban planning point of view. (Güttler 1997, pp. 78–91)

2 Axial coding: selection of categories/codes that appear useful for generating theory/answering a question; possibly forming "code families".

As of the 2000s, the discipline was referred to in an international context as land and property management and defined as follows:

Land and property management, as an action-oriented component of spatial development and land policy, encompasses all planning and development processes as well as evaluation and regulatory measures for the use of land and built structures. For this purpose, it uses the necessary legal instruments, economic procedures and engineering methods as well as forms of governance, thus supporting sustainable land use and the functioning of the real estate market at the same time. (Kötter et al. 2015, pp. 137–146)

Both definitions are good examples of the evolution of land management. The second one gives a brief overview of the understanding of land management in the context of this book chapter. It contains the elements and aims of land management.

#### *3.1. Process of Land Development in European Countries*

The European countries have experience in the field of land management, in the areas of land banking, land administration and land management systems (including land registration). Even if the systems of the individual countries differ in detail, generally valid statements can still be made (Williamson et al. 2010). The land development process covers a broad spectrum of tasks, from project development, the acquisition or subdivision of land, legal assessment and planning approval and construction work to the allocation of development incentives and costs. Based on planning permissions or land-use permits, the process of land development manages the transformation (e.g., in land use) of existing rural or urban areas and also realizes new building areas (e.g., districts) with new physical infrastructures (Williamson et al. 2010).

The land development process (Figure 1) is divided primarily into three main phases: (1) the initial stage, (2) the land development phase and (3) the phase of mobilization and use.

**Figure 1.** Land development process. Source: extended according to Hendricks et al. (2017), used with permission.

The land development process is usually associated with the change in land use, land tenure and land value. Every step in the development process causes an increase in the land value (realized at purchase prices) and is determined by the (free) market forces (supply and demand).

#### *3.2. Land Management Instruments in Europe*

Land management instruments are important for settlement structures and urban development. The systematic use of land management instruments supports the development, order and protection of land as a limited resource. Construction activities for housing, commerce or services and the provision of land for technical infrastructures and public facilities can be controlled.

Each country has its own understanding of land administration and therefore land management instruments are used in different ways. The overarching land management instruments are applied differently in the municipalities of each country, depending on their needs. In this case, land management instruments commonly used throughout the country are considered. The specific application at the municipal level needs to be elaborated in further research.

A comparison of the European country administration systems shows that there is no single European land administration policy (Williamson et al. 2010). Nevertheless, common land management instruments of the land administration systems within European countries can be identified.

Types of Steering Measures in Land Development

From a general land management perspective, land development instruments usable in urban expansion and urban renewal can be categorized as steering measures: (1) voluntary instruments, (2) financial instruments and (3) legal instruments, as presented in Figure 2. Steering refers the possibilities provided—i.e., that the municipalities have to promote social, economic and ecological aspects within their own administrative area.

Voluntary instruments include types of planning instruments, participatory instruments, negotiation and land banking as private interim purchases. Voluntary instruments are all instruments which are optional and not legally binding (Bouwma et al. 2015). Unlike the legal instruments, they rely on free will of the participated parties. The steering possibility of the public is only low—a lot of convincing and negotiating is required.

Public–private partnerships (PPPs) are organized between public and private sectors. The purpose of these is the joint work on urban and regional development functions that none of the partners involved can handle alone and which are of benefit to the partners involved (Schaeffer and Loveridge 2001). There are three kinds of partnerships:

In the negotiation, both parties act on an equal level. A voluntary transaction is a private purchase of land or real estate with both parties on an equal level (private law regulations). Additionally, the municipality has no superior position as in public instruments. Content of the contracts could be also a private land reallocation.

The private interim purchase is an approach of land banking, which is similar to the public interim purchase, with the difference that a private developer buys, develops and sells land of their own free will. Costs, risks and also the profits lie with the developer (Alterman 2012)—often the private interim purchase is combined with a negotiation (urban contract) with the municipality.

3 The most frequent form of public–private partnerships is a cooperative arrangement regulated by a contract.

**Figure 2.** Land development instruments. Source: Graphic by authors.

Financial instruments intervene in the economic balance of development. They include both financial incentives (direct steering—e.g., subsidies) and financial involvement (indirect steering—e.g., cost sharing, charges/fees, and taxes) in costs for urban development stakeholders. Subsidies include the financial support of individual households as well as persons or the financial support for the purchase price of land or the construction of new living space/affordable housing (Europe—e.g., European Social Fund: ESF). The financial support for new living spaces is mostly important for cities with a rise in population and a reduction in the vacancy rate (Silva and Acheampong 2015).

An alternative financial instrument is cost sharing. Responsibility for the provision of the necessary infrastructure (technical infrastructure such as roads and utilities, electricity, water, sewage or public facilities such as playgrounds, kindergartens and schools) lies in general in competence of the municipality, but their production costs can be transferred to a developer. The level of cost sharing is a matter for negotiation and will manifest in an urban contract (see below, legal instrument—contract). In contrast to a private negotiation, the municipality is superior in the process. If a municipality itself develops, they have to pay the costs for initial provision, especially for vehicular and pedestrian infrastructures (roads, paths, squares). The costs can be shifted by charging connection fees to be paid by the owners (Silva and Acheampong 2015). In some countries, the possibility of value capturing is possible for financing the development costs. The raise of value amount can be taken or used to pass on development costs (Hendricks et al. 2017).

Legal instruments are regulated by laws and are legally binding (Silva and Acheampong 2015). Three subgroups can be distinguished: planning instruments, land banking and development instruments.

Formal plans are of the planning instruments type. The function of urban land-use planning is to prepare and manage the structure and the use of land in the municipality (Healey and William 1993; Silva and Acheampong 2015). There can be subdivision in the zoning plan with general regulations (e.g., different land uses such as residential, commercial or industrial) that apply to the whole municipal territory and the binding land-use plan with detailed planning and design for individual parts of the municipal territory (Silva and Acheampong 2015). Regulatory content and liabilities vary in European countries.

Another type of legal instrument is land banking with the subcategories pre-emptive right, public purchase and public interim purchase. The pre-emptive right means that person A and person B conclude a sales contract, whereby persons could also be companies or municipalities. The pre-emptive right allows the municipality to take the place of the buyer. The agreements of the contract continue to apply (Wirth and Wolff 2012; Kaiser et al. 2016). An early and strategic land supply (purchase of land) offers the municipalities scope for action. They are independent and can mobilize and realize the areas according to their goals and ideas. Cities which have tight markets but bought land early on are now profiting from the results. Public interim purchase means that the municipality is first a buyer and then a supplier on the municipal land market. In an early phase, the municipality buys low-cost land, mainly arable land at the edge of the cities, and sells it at a higher price as building land for residential or commercial purposes. Sometimes, the arable land is held for years before a development (Alterman 2012).

One of the development instruments is expropriation, which implies that the removal of property rights by the state is only permissible in the public interest. The land is reused for common goods afterwards. The owner receives monetary compensation, but it can also be paid to another plot of land in an equivalent location. The instrument is strictly regulated (ECHR European Convention on Human Rights; Council for the Environment and Infrastructure 2017). The (public) reallocation means the redistribution of land intends to create land that is suitable for buildings or other uses in terms of location, shape and size. This procedure aims to reorganize or extend certain areas of both developed and undeveloped lands (Council for the Environment and Infrastructure 2017).

The last category in this topic is the urban contract. It is an agreement between the municipality and third parties that include preparation and implementation of urban development measures or other agreements. The municipality is superior and can use its planning right to negotiate the contents of the contract. However, the municipality is influenced by competition and high investment sums, so that it may not exploit this balance of power for economic reasons. An important part of the urban contract is the height of transferable costs from the municipality to a developer. Signing the contract is a precondition for the development of a plot (Hendricks et al. 2017).

#### *3.3. Good-Practice Examples from Europe*

In the following subsections, information of the instruments is provided (voluntary, financial and legal). The conclusion highlights their relevance in terms of supporting socially integrative cities. Here, the focus lies on legal instruments in combination with other sets of common instruments, since they provide the most direct opportunities for steering the implementation of urban development.

#### 3.3.1. The Netherlands

In the Netherlands, there are four practices for developing land. A distinction is made between the acquisition or nonacquisition of land by the municipality and whether negotiations take place with other stakeholders. In the following, only the Active Land Policy approach with contractual negotiations will be described (Fischer and Foißner 2002; Tennekes 2018). These negotiated land-use plans (instrument under study) are a frequently used instrument for the development of cities (Tennekes 2018). In order to counteract increased land prices, development costs and speculation of land, this planning approach was established as a new land management strategy in the Netherlands in the 1990s (Tennekes 2018).

#### (Core) Content of the Instrument

As private developers increasingly own land in potential development areas (land speculation in the 1990s; land often without the right to build), it had become impossible for the municipality to acquire the land at a reasonable price for building development (Tennekes 2018). The negotiated land-use plans between the municipality and private developers combine several individual land management instruments into one overall measure and are based on the principle of public–private partnerships. The obvious instruments are: (urban) contracts, formal plans as well as public interim purchase. Under the contract, the municipality can agree on different items with the developers, which can spatially vary.

There are basically two approaches of development:

• Building claim model: Private landowners and municipalities negotiate the sale and price of the land. The municipality develops the land, manages the land and reallocates the land according to future land-use claims. The divided land is re-purchased by the contracting parties at the previously negotiated price (Tennekes 2018).

• Joint venture model: Private landowners and the municipality establish a development company together under private law that bundles and develops the land. The company (public–private partnership) can then sell the land to others or develop the property itself (Tennekes 2018).

### Effect in Terms of Supporting Socially Inclusive Cities

The Dutch approach of interim purchase of land for development leads to urban development tailored to the goals and needs of the municipality. Based on urban calculations, the municipality can negotiate reasonable prices for the land. The money raised from the sale of the land can be used to cover the costs of public facilities and technical infrastructure and (affordable) housing. This procedure is transparent and uniform for all contractual partners, which in turn increases acceptance among developers. The revenue enables the municipalities to establish needs-based services (e.g., primary and secondary schools, social housing), which can contribute to the satisfaction of the population living there.

The municipality can thus actively guide urban development and integrate social aspects in urban renewal and urban expansion areas. In addition, the municipality can specifically prevent urban sprawl and maintain the security and order of the areas. Since the municipality takes over all development measures itself and has an overview of all development steps, this can lead to an accelerated planning process.

#### Stakeholders Involved in the Planning and Implementation Process

The main stakeholders in the Dutch urban development process are primarily the municipality and private developers or corporations (Tennekes 2018; Holtslag-Broekhof et al. 2018). The development-led approach, i.e., that land-use plans are drawn up through negotiations on a project-by-project basis, encourages early and close cooperation between the stakeholders in urban development. Other stakeholders are citizens who can be formally or voluntarily involved in different steps of the planning process. The voluntary participation of citizens is at the discretion of the municipality. The involvement of citizens has both potentials (co-decision, acceptance) and risks (longer time for voting, good management in coordinating the management). There may exist tensions between effectiveness and citizen participation and other democratic values. Especially in the joint venture model, municipalities have a double role—as government actors protecting the public good and as private actors invested in the venture. Demanding additional social investments may put the municipality at (financial) risk. It provides opportunities for socially integrative development, but also poses risk due to these roles.

#### Classification in Systematization

Negotiated land-use plans consist of several land management instruments. These are legal instruments (formal planning and development instruments as well as land banking). However, negotiated land-use plans as an overall measure take into account the project-based planning approach in the Netherlands and support socially inclusive cities as a part of the negotiated content.

#### Interim Conclusion Regarding Negotiated Land-Use Plans

This instrument is probably more suitable for urban expansion areas. However, its use in urban renewal areas is not excluded. The instrument especially attracts attention because of the close cooperation with private developers. The public–private partnerships enabled the municipality to cover the costs of and provide public services and social housing. As a result of discussions and negotiations with the landowners, the municipality is able to implement the city-wide goals.

#### 3.3.2. Germany

In the German context, the focus is also on legal instruments to support socially integrative cities: one example of steering an intended development process is the so-called building land strategy (instrument under study).

#### (Core) Content of the Instrument

Using the tool of a basic decision (of the municipality), the municipality can commit itself to manage a social, environmental and/or economic orientation.4 With the basic decision, the municipality already sets a future direction for the entire municipal territory. The basic decision is legally binding for the municipality. Building land strategies are basic decisions that combine the possible elements of an urban contract. Building land strategies create various benefits for municipalities and developers, such as the mobilization and conversion of building land potential and the acceleration of land development processes and the basic decision to ensure sustainable land use.

4 Objectives of municipal building land strategies are: Social, economic and urban planning objectives. Social Objectives: Improvement of housing supply through housing funding quota and housing construction quota. Economic objectives: Discounted sale of real estate; transfer of costs to developers. Urban planning objectives: Quality objectives of urban development such as building culture, urban development standards and environmental standards.

Effect in Terms of Supporting Socially Inclusive Cities

Building land strategies promotes qualified urban land use, as well as sustainable land use. The municipality chooses their core criteria in a municipal resolution which is binding and afterwards negotiated in each urban contract (e.g., a special portion of affordable housing, green aspects). These strategies offer the mobilization and conversion of building land and the acceleration of processes. The application of building land strategies allows the municipality to partially transfer the financial effort of mobilization to investors (Weitkamp et al. 2017). This, in turn, leads to a reduction in the burden on the municipal budget. Due to the contractual agreement (urban contract) and transparency in the development process, the investor also benefits from the timely use of construction rights. The cost burden for investors is usually capped and depends on an increase in the land value of the respective area to be developed (Suering and Weitkamp 2019). An advantage of building land strategies is the fixation of a transparent and uniform strategy instead of individual case decisions of urban development contracts. Furthermore, they offer fast and secure implementation of the projects, transparency and uniformity.

The investor gives his basic agreement, and subsequently the negotiations for the cost transfer take place. The contract is concluded between the municipality and the investor. After signing the contract, projects are interpreted by the public and the citizens participate. After the participation has taken place, a land-use plan is established and the investor can implement his construction project.

#### Stakeholders Involved in the Planning and Implementation Process

The investor and the municipality are primarily involved in the actual development process. Other stakeholders in a development process can be landowners, developers, financial institutions, planning and building authorities, building contractors, professional advisers and third parties. The actual basic decision and development project can be preceded by citizen participation (meetings) or informal plans. Through these preceding measures, the acceptance and satisfaction can be increased (Williamson et al. 2010; Jeschke and Weitkamp 2017).

#### Classification in Systematization

Thus, a building land strategy is a mix of legal and financial instruments (Suering and Weitkamp 2019).

#### Interim Conclusion Regarding the Building Land Strategies

The social integration in building land strategies can be realized by focusing on social objectives. Instead of making far-reaching economic demands, the municipality can focus on, e.g., social housing (Adolphs et al. 2019; Weitkamp et al. 2020). To this

extent, it uses its room for negotiation to achieve socially integrative goals. With the transfer of obligations to the private sector, the (economic) advantage does not exclusively remain with the investor. Structures that serve the common good are created based on the realization of social and technical infrastructures, housing promotion and climate aspects.

#### 3.3.3. France

In France, too, there are different instruments to support social aspects in land development. The application depends on the respective requirements and needs of the region or municipality. Therefore, not all instruments are applied equally everywhere. With a focus on social aspects (mainly social housing), there is the procedure of land banking (instrument under study), particularly selling land to housing companies in France (Cahier Pratique Documents 2014; Hendricks et al. 2017). The basis for this procedure is the strategic development policy "politique de la ville" and different laws on solidarity and urban renewal. A national authority for urban renewal is established to monitor and steer construction measures. This authority becomes a new central element of the "politique de la ville" (Bauhardt 2005; Glasze and Weber 2010).

#### (Core) Content of the Instrument

Until the 1970s, many social housing estates were built on the outskirts of cities. After that, the existing stock was considered sufficient and the focus was on subsidies in the form of housing subsidies rather than on promoting the construction of new housing. Over the years, many of the social housing estates have been neglected, with many requiring redevelopment or demolition (Glasze and Weber 2010; Reiter 2011). The need for social housing is very high again today. Therefore, the land banking procedure in the urban development process focuses on social housing.

Traditionally, local authorities sell their land to housing developers. This is to ensure the provision of affordable rental housing development. When developing land, the predefined themes and objectives of the region and municipality must be taken into account.

#### Effect in Terms of Supporting Socially Inclusive Cities

The right to housing is established in French law. The purpose of construction, planning, allocation and operation of social rental housing is to give people with low incomes access to affordable housing. This should lead to an improvement in housing conditions and a social mix in cities and neighbourhoods (Glasze and Weber 2010).

Apart from simply providing people with housing, there are numerous other effects. Some of them are described in more detail.

The land banking approach in France works against the rise in prices for real estate and land, which has been observed for several years. Further, it counteracts advancing urban sprawl. Urban sprawl often has a direct effect on infrastructure. By stopping urban sprawl, costs for infrastructure facilities can be saved at the same time. Thus, good land policy and land management can counteract the effects mentioned above. In addition, the municipality is to provide social (or adequate) housing in the long term. Land reserves in public hands, which are developed according to demand, can prevent price speculation by private developers.

#### Stakeholders Involved in the Planning and Implementation Process

The main stakeholders in this process are the local authorities and housing developers. The municipalities sell their land to the companies. After receiving the land, the housing companies are responsible for building social housing. Social housing in France is regulated by a strong involvement of the public sector (Cahier Pratique Documents 2014; Hendricks et al. 2017). The construction of new social housing depends, on the one hand, on the municipality itself as the planning authority, and on the other hand on the possibilities (e.g., financial possibilities) of the housing companies.

#### Classification in Systematization

The French example also involves a mix of legal and financial instruments to provide the population with social (or adequate) housing. Traditionally, municipalities buy land and resell it to social housing companies.

#### Interim Conclusion Regarding Land Banking

The construction of social housing to provide affordable housing for the population is heavily dependent on municipalities and housing companies. For many years, the need for social housing was covered, which led to the instrument being pushed into the background of planning. Nowadays, old strategies need to be focussed on and new strategies need to be developed to meet the increasing demand for affordable housing. In this way, the social integration of people in French cities can be successful.

#### *3.4. Comparative Consideration of Good Practice Examples from Europe*

In European countries, land management instruments are used at different stages in a development process—mostly in the first two stages of the development process, as in the three examples of the Netherlands, Germany and France. The aim of the instruments is to steer towards a sustainable land use; this can include a change of the type of usage. The most common type of urban development in Europe is a developer-led development (Hepperle et al. 2017; Gerber et al. 2018). For a developer model, two scenarios exist.

Scenario 1 (expansion or renewal): The land is owned by a developer. There is a gap between actual land use and intended land use. According to the law, the developer is only obliged to build on the gap in such a way that the type and degree of the development fit in with the surrounding buildings. The municipality does not influence the use of the area or the possibility of demanding the realization of affordable housing. If the gap raises, a land-use plan is needed for development. In this case, instruments such as a building land strategy (Germany) or negotiated land-use plans (the Netherlands) could be used.

Scenario 2 (classical expansion): The land is owned by a developer. It is arable land without building rights or greater innercity areas with land-use plans or with former uses such as, e.g., industrial use. The developer wants to develop the land into a new quarter. For this purpose, the owner needs a land-use plan to acquire a building permit. Scenario 2 describes the most common one in Europe. Especially in this scenario, the applicability of the described legally and financing instruments from the Netherlands, Germany and France can mean an added value for the support of socially integrative cities.

However, the city normally tries to combine the preparation of the land-use plan with negotiating (urban contract in Germany or negotiated land-use plan in the Netherlands) preconditions such as taking over all development costs (including public ones) or realizing affordable housing. To this extent, an urban contract can be deployed for different purposes and urban aims—e.g., preparation of infrastructure, permit agreements, reallocation contracts or realization of affordable housing—and the social integrity can be realized in the negotiation process. If the developer does not agree, there will be no planning process needed for permission. Thus, the municipality is able to use its planning right to force special conditions on the developer.

The same effect can be seen using the public interim purchase (land banking in France to promote social housing). There, the city is free to define condition for reselling their own land. However, unlike urban contracts, the city and not a developer bears the complete financial risk. It takes a financially strong city or strategic land banking to be able to carry out an interim purchase at a large scale. However, it offers the greatest steering opportunity for cities. Social integration can be carried out without hindrances within the financial frame of the municipal budget.

#### **4. Conclusions and Outlook**

Many European cities are confronted with the challenges of the rapid, sustainable and demand-oriented provision of space for the supply of people. Planning processes and land management instruments are necessary so that cities can be developed in an orderly and structured manner. Cities are aware of the versatile instruments for activating or restructuring land. The main challenge is the sensible application and combination of different instruments for socially integrative cities.

The land development process proceeds as described in Figure 1. Steering measures are carried out in the first and second phases—the Initial and Land Development Phases—of the development process. In the third phase, processes of urban development are finished. The mentioned land management instruments do not have the same scope and potential for steering socially integrative urban development. Even though the municipalities have knowledge of the legal framework, the challenge posed is the combination of the instruments to achieve a sustainable outcome.

The possibilities to influence land development through steering measures are immense. Every land development instrument offers certain advantages, but also has its limitations. In general, there are a lot of instruments to steer a social integrative city. It is up to the city if it wants to make a conscious choice of which instrument or instruments to apply to a situation. A social integrative development also depends on whether the municipality has to weigh up certain issues against other equally important urban planning issues. Here, a social integrative development may be pushed into the background or will be part of a compromise decision (as shown in the case of the negotiated land-use plan in the Netherlands). The instruments for it exist and can also be used very purposefully (support of legal instruments through financing strategies), as the cases of the Netherlands, Germany and France illustrate. Moreover, these can also be used in a very targeted manner (support for legal instruments through financing strategies).

Land management instruments (e.g., negotiations or interim purchase) are a good example of land development through their steering possibilities. The municipality decides on their aims, e.g., social integration, for the development of the land. Municipality and developer have to negotiate the detailed conditions while the municipality can act freely on their own land in terms of interim purchase (as shown in the cases of the negotiated land-use plan in the Netherlands and the building land strategies in Germany).

The land management strategies in the selected countries all include:

The general findings show that land management instruments are advantageous when supporting socially integrative urban expansion and renewal. To what extent this can happen depends on the individual context (case-by-case decision in the individual area). Thus, the general assessment provides the potentials of individual instruments, and needs reflect regarding broader practice experiences depending on specific projects.

The identified land management instruments support socially integrative matters. The combination of informal instruments (integrated urban development concepts, renewal concepts) and formal instruments (zoning plan or binding land-use plan, as shown in the three case studies) allow strategic and incremental development as well as legally binding steering.

Socially integrative urban development focuses on managing urban expansion, promoting urban density, regenerating existing urban areas and fostering dynamic communities. Strategies for managing sustainable land use must take into account the individual framework conditions for land ownership and land management systems. Land management must play a key role in the transition to urban sustainability through socially integrative cities, because its instruments are able to steer the defined urban aims—e.g., urban contracts, land-use plans (as shown in the Netherlands and Germany) or land banking (as shown in France). Different instruments are needed because of the different initial conditions in urban expansion or urban renewal—combinations of instruments allow a target-oriented development process. The social needs of each city are highly individual and the range of instruments is very diverse. Since not every instrument is equally suitable for all challenges in cities, a detailed choice of usable instruments is required.

If the city intends to develop in a sustainable manner, it shall integrate social aspects in renewal and expansion areas. Economically operating cities should be recommended to carry out demand analyses, move away from supply planning and specifically address their individual needs. This will lead to higher benefits, which could be used for realizing urban aims. This can be supported and implemented by different land management instruments. Therefore, land management instruments allow steering a social integrative process in different stages of the development process and different preconditions. Cities are recommended to use the portfolio of instruments on a case-by-case basis to enable sustainable development.

**Author Contributions:** J.S.: Conceptualization, Methodology, Analysis, Investigation, Validation, Writing—original draft, Visualization, Writing—review & editing. A.O.: Conceptualization, Writing—original draft, Supervision, Visualization, Writing—review & editing. A.W.: Conceptualization, Writing—original draft, Supervision, Visualization, Writing—review & editing.

**Funding:** This research was funded by the European Union's Horizon 2020 Research and Innovation Programme under Grant Agreement No. 770141.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

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