Study of the performance, simplification and characteristics of SNCR de-NOx in large-scale cyclone separator

Accepted Manuscript Study of the performance, simplification and characteristics of SNCR de-NOx in large-scale cyclone separator Zhizhong Kang, Qixin Yuan, Lizheng Zhao, Yukui Dai, Baomin Sun, Tao Wang PII: DOI: Reference:

S1359-4311(16)32463-2 http://dx.doi.org/10.1016/j.applthermaleng.2017.04.122 ATE 10271

To appear in:

Applied Thermal Engineering

Received Date: Accepted Date:

16 October 2016 24 April 2017

Please cite this article as: Z. Kang, Q. Yuan, L. Zhao, Y. Dai, B. Sun, T. Wang, Study of the performance, simplification and characteristics of SNCR de-NOx in large-scale cyclone separator, Applied Thermal Engineering (2017), doi: http://dx.doi.org/10.1016/j.applthermaleng.2017.04.122

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Study of the performance, simplification and characteristics of SNCR de-NOx in large-scale cyclone separator Zhizhong Kang , Qixin Yuan, Lizheng Zhao, Yukui Dai, Baomin Sun, Tao Wang (Key Laboratory of Condition Monitoring and Control for Power Plant Equipment, North China Electric Power University, Beijing 102206, China) Corresponding author (Qixin Yuan). Tel: 0086-15811418492; E-mail addresses:[email protected]

Abstract: CHMKIN software was used to optimize the mechanism of SNCR reaction. Then the optimized 18-element mechanism and CFD software were combined to simulate the process of SNCR de-NOx and meanwhile the gas–solid characteristics in a cyclone separator of a supercritical CFB boiler were studied. The gas–solid characteristics and SNCR characteristics of the large-scale cyclone separator were obtained. The total pressure difference of the cyclone separator increases with the inlet velocity, and the rate of increase increases gradually. When the inlet velocity is 20 m/s, the total pressure difference is 1987.6 Pa. The classification efficiency of the cyclone separator increases with the inlet velocity, but the rate of increase decreases gradually. When the inlet velocity is 20 m/s, the classification efficiency is 66.74%. The window of the reaction temperature for the SNCR process is in the range of 1123–1323 K. The de-NOx efficiency first increases and then decreases with the increase of temperature, reaching a peak at 1223 K. At low temperature, improving the NSR has little effect on the de-NOx efficiency. When the reaction temperature is in the temperature window, improving the NSR can effectively enhance the de-NOx efficiency. But after increasing the NSR to a certain extent, the de-NOx efficiency increase will slow down. Setting NSR=1.5 is suitable.

Keywords: SNCR; Mechanism simplification; CHEMKIN; CFD; Separator characteristics

1.Introduction Ox nitride (NOx) is one of the main pollutants from power plants, causing photochemical smog pollution by chemical reaction with light and combining with the water in the air to produce acid rain. There is great international concern about NOx emission control. The circulating fluidized bed (CFB) boiler has the characteristics of low temperature in the furnace. So when the external conditions are the same, the amount of NOx generated in the furnace is low relative to the pulverized coal furnace [1]. The CFB boiler is now widely used in various countries. In order to further strengthen environmental protection, China put forward the goal of "super-low emission"——under the condition of 6% of the standard oxygen content, NOx emission concentration is less than 50 mg/Nm3; SO2 emission concentration is less than 35 mg/Nm3; soot emission concentration is less than 5 mg/Nm3. A power plant will be built with a 660 MW CFB boiler, burning low calorific value coal (coal gangue + middling). To achieve "super low emission" it is necessary to further strengthen the de-NOx unit. SNCR has the advantages of low investment, simple equipment and, most importantly, the temperature in the CFB boiler cyclone separator is just within the SNCR reaction temperature window, so a CFB boiler equipped with an SNCR de-NOx device at the cyclone separator can achieve more than 50% de-NOx efficiency[2]. A 660 MW supercritical CFB boiler is designed with SNCR de-NOx equipment installed at the cyclone separator to achieve the goal of super-low emission. The 660 MW supercritical CFB cyclone separator is characterized by its large scale. The preliminary design is to scale up the structure of the common cyclone separator, but the large scale of the structure will lead to a more complex internal flow field in the cyclone separator. In practical operation, SNCR de-NOx is largely dependent on the mixing of flue gas and reducing agent. The complexity of the flow field will have a major influence on the SNCR de-NOx. At the same time, the complexity of the flow field will make the boundary of the inner and outer swirl zone of the cyclone separator irregular, which will affect the gas-solid separation and energy loss of the cyclone separator. In this paper, we use CHEMKIN software to optimize the SNCR reaction mechanism. The optimized mechanism and computational fluid dynamics (CFD) numerical simulation are combined to study the SNCR de-NOx in the cyclone separator of a 660 MW supercritical CFB. Meanwhile, the CFD software is used to study the performance of the cyclone separator——the pressure difference and separation efficiency. It provides a valuable reference for the SNCR de-NOx and the practical operation of the large-scale cyclone separator in the supercritical CFB.

2.Simplification of SNCR reaction mechanism Although the mechanism of SNCR de-NOx has been very thoroughly studied, in practical engineering application, with the SNCR de-NOx reducing agent input from a limited number of nozzles into the reaction zone, the flow and mixing of flue gas in the cyclone separator are the key factors affecting the SNCR de-NOx efficiency[3]. Therefore, it is necessary to use the combination of CFD and SNCR chemical reaction process simulation to simulate the process of SNCR de-NOx in the cyclone separator of the CFB boiler. FLUENT itself contains a SNCR de-NOx reaction module. The reaction mechanism of this module involves 14 components, including 9 basic chemical reactions [4]. Table 1 is the reaction mechanism of the reaction module. Reactions 1—7 are the SNCR de-NOx mechanism. Reactions 8 and 9 are the decomposition of urea. Because the module is in a black box, the chemical reactions and their parameters cannot be modified. When the simulation of the chemical reaction process is combined with the CFD simulation of the flow process, the detailed chemical reaction mechanism will occupy a large memory, and thus seriously reduce the efficiency of the simulation and affect its accuracy. In order to simulate the process of SNCR de-NOx in the

cyclone separator under the coupled action of flow mixing and chemical reaction kinetics, according to the reaction conditions of the CFB boiler cyclone separator, the optimized SNCR reaction mechanism is essential. 2.1 Analysis of SNCR mechanism The chemical reaction mechanism is based on the micro level, from the point of view of the basic elements, to explore the dynamic behavior of the total reaction. The corresponding reaction rate constants are needed in the calculation of each basic element reaction: pre-exponential factor A, reaction order n and the activation energy E. Numerous studies[5-7] have found that the reducing agent itself, through rapid pyrolysis, produces NH3 through the reaction of NH3+OH↔NH2+H2O to reduce NO. NO selectively and rapidly reacts with NH 2 in the SNCR reaction temperature window. The generated products have two paths, as shown in reactions (1) and (2): NH2+NO↔NNH+OH （1） NH2+NO↔N2+H2O （2） Reaction (1) is the reaction when producing chain factors and reaction (2) is the reaction when not producing chain factors[8]. If it is short of chain factors, the self-sustaining reaction of NO and NH3 cannot be continued. So to a certain extent, the relative reaction rate of reactions (1) and (2) determines the extent of de-NOx. As a response to reactions (1) and (2), Miller and Bowman put forward the branch coefficient a showing a reaction that produces active routes. That is, reaction (1) accounts for the proportion of the total reaction rate. In order to maintain the NH3 and NO self-sustaining reactions, Miller[9] proposes that the branch coefficient a should be greater than 0.25. Reaction (1) generating OH can react with NH3 to regenerate NH2. The competitive reaction of NH2 includes a reaction between NH2 and free radicals, as shown in reactions (3) – (6): NH2+H↔NH+H2 （3） NH2+O↔HNO+H （4） NH2+O↔NH+OH （5） NH2+OH↔NH+H2O （6） The generated NH2 can be reduced and oxidized. The reduction reaction takes the dominant position at lower temperature, and the oxidation reaction at high temperature produces a greater impact. From experimental and theoretical analysis, the survival time of the NNH active route generated in reaction (1) is between 8–11 seconds. If the survival time of the NNH active route is too short, and the coefficient a is too large, it will cause an explosive reaction[10]. By superimposing reaction (1) and reaction (7), the following reaction can be obtained: NH2+NO↔N2+H+OH. This reaction not only reduces the NO but also produces chain factors. When the reaction temperature increases, the reaction (1) rate increases, producing more chain factors and yielding a more in-depth reaction and improving the de-NOx efficiency. Reaction (7) is as follows: NNH↔N2+H （7） NH free radicals are generated by the NH 2 extraction of hydrogen atoms, such as reaction (3), reaction (5) and reaction (6). At the same time, the reaction of NH radicals with NO generates N2O or N2, such as in reactions (8) and (9): NH+NO↔N2O+H （8） NH+NO↔N2+OH （9） The reaction of NO with an H or CO direct amines is shown in (10) and (11). The reaction between NO and H atoms at high temperature is dominant; however, due to the large amount of heat absorption, this almost does not occur under 1400 K. The reaction of NO and CO first generates N atoms. The rate at which the reaction occurs at low temperature is relatively slow. However, usually a higher reaction rate constant is used in

the study of the mechanism of the early stage[11]. NO+H↔N+OH （10） NO+CO↔N+CO2 （11） The formation of HNO is mainly through the reaction of NO with H 2 or the recombination of NO and H. The series of reactions of NH generated by HNO are as follows. Usually reactions (12) – (14) under the condition of the reaction are slow. Only reaction (13) has a strong competitive ability at high temperature [12]. HNO+H↔NH+OH （12） HNO+CO↔NH+CO2 （13） HNO+H2↔NH+H2O （14） Existing studies have found that when the temperature is below 1400 K, CO or H 2 in the SNCR reaction conditions can lead to a significant reduction in NO, but the mechanism remains to be further studied. 2.2 Simplification of SNCR mechanism in cyclone separator The SNCR de-NOx process is very complicated. The detailed mechanism of the elementary reaction model usually contains several hundred elementary reactions, leading to a large number of numerical solutions of the reaction kinetics equations. In 1996, Brouwer et al.[13] optimized the detailed mechanism of Miller and Bowman by means of sensitivity analysis and curve fitting. They developed a set of 6 substances, and a 7-step reaction mechanism in which NH3 was the reducing agent in the reaction and the effect of CO on the reaction process of SNCR was also considered. In 2002, Rota et al.[14] used the detailed mechanism of urea decomposition and the SNCR reaction processed with ammonia as the reducing agent to describe the process of NOx OUT reaction. In 2003, Xiaohai et al.[15], through an automatic simplification of the program code, optimized the detailed basis of the reaction mechanism as the optimized mechanism of 10 steps and 14 components for the coal-fired boiler. In 2006, Lu Zhimin[8] used a sensitivity analysis to analyze and optimize the 1989 Miller model and the Rota model. The optimized model, including a 14-step reaction from the Rota model could predict the change of NO and NH3 concentration with the change of temperature in high and low oxygen levels. CHEMKIN 4.1 software has a strong sensitivity analysis tool. Considering the flue gas composition, inlet velocity and residence time of the CFB boiler cyclone separator in the process of SNCR reaction, we make a sensitivity analysis of the 18 components: NH3, N2O, NO, NO2, NH2, HNO, H, O, OH, O2, HO2, H2O, NH, NNH, N, N2, CO, CO2. If the absolute value of the sensitivity coefficient of any one of these substances in a basic element reaction is larger, this shows that the effect of the base element reaction on the SNCR reaction system in the cyclone separator is greater. By ignoring element reactions with less absolute values of the sensitivity coefficient, we remove those intermediate components that have little effect on specific issues to achieve the purpose of optimizing the detailed reaction mechanism. In this paper, using the CHEMKIN software to push the flow reactor and adopting the consolidation mechanism of combining Øyvind Skreiberg et al.’s mechanism of ammonia oxidation with the SNCR de-NOx mechanism, with ammonia as the reducing agent, we calculate the chemical kinetics of NH3 reduction by NOx. After using the sensitivity analysis, the optimized mechanism is as Table 2: 2.3 Optimized mechanism verification In order to verify whether the optimized mechanism can accurately simulate the process of de-NOx in setting the CFB boiler cyclone separator SNCR de-NOx reaction conditions, in this paper, an SNCR de-NOx experiment is carried out under laboratory conditions. The experimental results and simulation results are compared to verify the optimized mechanism. The experiment was carried out on a quartz tube reaction system. The experiment equipment comprised a reaction gas cylinder, mass flow meter, electric resistance furnace, quartz reactor and gas analyzer (Testo 350).

The schematic diagram of the experimental system is shown in Figure 1. N2, O2, NO and NH3 were provided from reaction gas cylinders (O2 bottles, NO bottles and NH3 bottles were 1% of the content of the mixture with N2). The ratio of the various reactive gases was controlled by the mass flow meter. A thermal resistance furnace was used to heat the quartz reactor, increasing the temperature by 10K per minute at the preheating stage and setting the pre- heating time to 1.5 hours. With a uniform temperature increase, the reaction gas with a good proportion of the prior mixture was reacted in a high-temperature quartz reactor. The gas analyser was used to measure the volume concentration of the reaction gas before and after the reaction. The experiments were carried out under atmospheric pressure. The total mass flow rate of the reaction gas was constant, at about 1250 cm3/s. The NO flow in the experiment remained unchanged for 125 cm3/s. N2 in the reaction gas was the protective gas. Setting normalized stoichiometric ratio (NSR)=1.5, O2=6% conditions, the experimental results and simulation results are compared , as shown in Figure 2. From Fig. 2, when the reaction temperature is between 1073 K and 1323 K, the trend of the de-NOx efficiency curve with the change of temperature is the same. The de-NOx efficiency curve of the optimized mechanism and the de-NOx efficiency curve of Øyvind Skreiberg’s detailed mechanism are similar. But it is found that the experimental values are higher than the simulated values. The analysis shows that the reaction gas in the experiment before the reaction is fully mixed to improve the SNCR de-NOx efficiency. Under the conditions of setting the CFB boiler cyclone separator SNCR de-NOx reaction, the optimized mechanism can simulate the process of de-NOx..

3 Establishment of the calculation model 3.1 Physical model construction The size of each part of the cyclone separator and the specific structure of the cyclone separator are shown in Figure 3. The initial size of the structure is shown in Table 3.3.2 Grid division We adopt a method for partitioning the grid in the partition domain and divide the separator into 3 sub-regions: the inlet section, the exhaust pipe section, and the other section.Respectively, mesh the sub-regions. Finally, we choose a model divided into about 500,000 as the number of the grid which can get a good result. The specific grid of the cyclone separator is shown in Figure 4. 3.3 Selection of turbulence model Commonly used turbulence models are the k-ε models and RSM model. Figure 5 shows velocity nephogram of x=0 cross-section using RSM、RNG k-ε、Realizable k-ε model. Figure 6 shows velocity nephogram of z=12 cross-section using RSM、RNG k-ε、Realizable k-ε model. It can be seen from figure 5 that the simulation results using RSM model accord with the flow field distribution in cyclone separator. The deviation of simulation results using k-ε model with the decrease of axial position will become increasingly large. This is because turbulent pulsation is strong in the conical part of cyclone separator. At the same time, the k-ε model based on the same direction eddy viscosity hypothesis will show its own limitation and insufficiency. It can be seen from figure 6 that the simulation results using RSM model accord with the flow field distribution in cyclone separator. Velocity nephogram exhibits an axisymmetric form. The simulation results using RNG k-ε model are in disorder, so the simulation results do not conform to the internal flow field distribution. The simulation results using Realizable k-ε model are much better than the simulation results using RNG k-ε model. However, there is still a gap. Figure 7 shows the tangential velocity of x=0 cross-section using RSM、RNG k-ε、Realizable k-ε model.

When the air enters from the inlet of the separator, the air flow begins to accelerate. After the airflow enters the cyclone separator, the air flow continues to accelerate, reaching the maximum at the turn. Then the air

flows along the cylinder wall, and the tangential velocity decreases slowly. In the separation space below the exit section, the axial symmetry of the tangential velocity distribution is quite good, which shows the characteristics of strong swirling flow in the separator. Figure 8 shows The tangential velocity of the x=0 cross –section using RSM、RNG k-ε、Realizable k-ε model The axial velocity of air flow in the cyclone separator in the cylinder and the upper part of the cone is basically quasi symmetric distribution. But the deviation is relatively large at the lower part of the cone. The axial velocity distribution is not symmetrical along the geometric center of the cyclone separator. It has a certain eccentricity distance. This shows that in the cyclone separator at the end of the trachea, there is the phenomenon of airflow short circuit. It can be seen from the figures that the simulation results using RSM model accord with the flow field distribution in cyclone separator. RNG k-ε model was proposed by Yakhot et al[16] in 1986. The model was optimized based on the standard k-ε model. The turbulent viscosity was modified and meanwhile the swirling of the averaged flows and the swirling flow were considered. Realizable k-ε model was a newer turbulence model proposed by Shih et al[17] in 1995. Turbulent eddy viscosity coefficient was related to strain rate in Realizable k-ε model. The k-ε models are based on the isotropic property assumption. This is not in conformity with the strong spin property of the cyclone separator. The RSM model is no longer based on the isotropic property assumption and has significant advantages in simulating anisotropic turbulence [18]. The biggest difference between RSM model and the k-ε models is that RSM model completely abandons the isotropic eddy viscosity Boussinesq hypothesis and more rigorously considers streamline bending, vortex, rotation and tension rapid change. For complex flows, RSM model has higher accuracy in predicting the potential and in many cases can give results better than various k-ε models. But this prediction is limited to the reynolds stress related equation. When need to consider the anisotropy of reynolds stress, must use RSM model. Many scholars [19-21] use the RSM model to study the cyclone separator, and get results that are close to the actual situation. So we use the RSM turbulence model for the simulation. The governing equations of the RSM model are shown below. Transport equation: （15） Cij - convection term; Pij - Reynolds stress; Dij - diffusion term;  ij - stress and strain term;  ij - dissipation term. Turbulent diffusion model:

k

- Prandtl number of turbulent kinetic energy: 0.82; ul - turbulent viscosity.

Stress strain model:

ij ,1 - pressure strain term; ij , 2 - fast stress and strain term; ij,w Buoyancy model:

- wall radiation term.

Pri - turbulent Prandtl number: 0.85 Dissipative term model:2



- scalar dissipation

3.4 Difference schemes and algorithms The commonly used difference schemes are the first-order difference scheme, two-order difference scheme and QUICK difference scheme. The first-order difference scheme is easy to converge but the error is relatively large. The two-order difference scheme optimizes the accuracy error and the simulation error is reduced, but it still has some error. The QUICK difference scheme not only has stability but also carries on the further optimization to the precision cutting and further reduces the false diffusion term. The most accurate results can be obtained by using the QUICK difference scheme[22]. According to the characteristics of high swirl flow in the cyclone separator, the PRESTO pressure interpolation scheme should be adopted. The SIMPLEC coupled method is the optimization of the SIMPLE coupled method. The convergence of the high spin fluid flow problem is limited by the pressure-velocity coupling. We use the SIMPLEC pressure-velocity coupling method to accelerate the convergence of the solution and set the default values for the SNCR component equation, momentum and energy equations. 3.5 Boundary condition setting According to the 660 MWCFB cyclone separator SNCR design nozzle, we select 3 reducing agents nozzle. Two spray points are positioned at the air inlet and the tube body tangent, and the third spray point is located on the outer wall of the tube body at the same height. The reducing agent and flue gas can be well mixed. The concrete structure schematic diagram is shown in Figure 9. The specific locations are shown in Table 4. Based on the full analysis of the flow field in the cyclone separator, the model is set as follows: (1) Input: speed of input (inlet velocity), 20 m/s; (2) Exit: free flow (outflow), flow rate of 1; (3) Particle capture port: the wall, set to trap type when adding particles; (4) Wall: standard wall function, no slip wall; (5) The effect of solid particles on the SNCR reaction is not considered in the cyclone separator; (6) The main components of the flue gas are N 2, CO2, H2O, O2 and NO, and the components are calculated by the thermodynamic calculation of the boiler; (7) We select ammonia solution with a concentration of 15% as the reducing agent. NSR is set to 1.5. The liquid droplet evaporation process is set up by the DPM model. Evaporation generates gaseous NH3. The main components of flue gas are N2, CO2, H2O, O2 and NO, and each component is obtained by boiler thermodynamic calculation. Imported gas phase statistics and urea injection statistics are shown in Tabel.5 and Tabel.6. The mixing degree of reducing agent and NOx in flue gas is evaluated by investigating the concentration distribution of reducing agent. The concentration distribution of reducing agent is simulated. The calculated results are shown in Figure 10. Coordinate value indicates the mass fraction of ammonia reductant. As can be seen from Figure 10, the ammonia nitrogen molar ratio is between 1-2. It can not only take into account the mixing in the flue gas enrichment area, but also take into account the mixing of flue gas entering cyclone separator dead zone. Due to offset the effect of upward rotating airflow, the distribution of the ammonia reducing agent in the core cylinder is more uniform, which ensures the good mixing of the reducing agent and

the flue gas. The SNCR de-NOx reaction is slow. The various components of the equation and the momentum and energy equations should be solved first. On the basis of the flow field, temperature field and concentration of the cyclone separator, we solve the chemical reaction of SNCR denitrification[23]. The metal composition of the circulating ash particles entering the cyclone will have an effect on the SNCR reaction. SiO 2 and Al2O3 have little effect on SNCR reaction, but Fe2O3、Fe3O4 and CaO have an effect on SNCR [24]. But it is difficult to calculate the metal composition of the circulating ash (relating to combustion coal, operating parameters, boiler size, etc), and the content of metal components in circulating ash is less. Circulating ash has little effect on SNCR reaction, which can be ignored in Engineering Research. In the study of SNCR denitrification, only the gas phase steady flow field is considered for cyclone separator. Without considering the influence of solid particles on SNCR reaction, many scholars[4,25] use this method to study the characteristics of SNCR reaction and get accurate results. The same method is used to simplify the simulation of SNCR reaction. The SNCR de-NOx efficiency calculation is shown in formula (16): （16）

4 The SNCR characteristics of large-scale cyclone separator In the supercritical CFB boiler the SNCR equipment is installed to achieve the low-emission standard in the cyclone separator. Although a large number of scholars have done extensive research on the SNCR de-NOx system, this mainly focused on the SNCR of regular CFB boilers. When it comes to the supercritical CFB boiler, whose inner flow field is more complicated, affecting the characteristics of the SNCR definitely, few people have done this kind of research. With the method of the optimized mechanism and CFD simulation, we study the characteristics of the SNCR of the cyclone separator. 4.1 Effect of temperature SNCR reactions are affected most by temperature, where the temperature window is usually between 1123 K and 1323 K[26], and the main reaction follows the pathway shown below [27-28]: 4NH3+4NO+O2 → 4N2+6H2O (17) Under high temperature, the NH3 oxidation reactions are as follows: 4NH3+3O2 → 2N2+6H2O (18) 4NH3+5O2 →4NO+6H2O (19) When the temperature is low, the SNCR reaction is too slow to achieve the goal of de-NOx. On the other hand, under a relatively high reaction temperature, the NH3 will take pathways (18) and (19) above, which will decrease the reducer and produce NO, causing a drop in the SNCR de-NOx efficiency. As a result, for actual operation, choosing an appropriate reaction temperature is critical. The SNCR de-NOx efficiency affected by temperature is shown in Figure 11. As Figure 11 shows, the SNCR de-NOx efficiency first increases then decreases, and it reaches a peak at the temperature of 1223 K. Choosing NO transformation efficiency higher than 40%, the SNCR de-NOx reaction temperature window is 1123–1323 K. In the system based on NH3 for deoxidizing flue gas NOx, NH3 reacts with OH and H groups to generate NH2 under the optimized mechanism reaction 5, then NH2 group reduces the NO in the flue gas under the optimized mechanism reactions 6 and 7. Optimized mechanism reaction 5 is affected most by the temperature, and when the temperature is below 1123 K, some of the NH3 turns into NH2, and furthermore the optimized mechanism reactions 7 and 8 are inhibited, which decreases the SNCR de-NOx efficiency. However, at temperatures higher than 1223 K, the OH

group will increase greatly, and be oxidized into NH 2 group with simplified mechanism reactions 6 and 14, which also decreases the SNCR de-NOx efficiency. 4.2 Effect of molar ratio of ammonia and nitrogen With NH3 playing a role in the reducing agent to NOx without a catalyst, the quantity of NH 3 is critical to de-NOx. According to the principle of chemical reaction equilibrium, reaction (17) will move towards the positive direction as the quantity of NH3 increases, which will raise the SNCR de-NOx efficiency and make reactions (18) and (19) move towards the positive direction. As reactions (18) and (19) move towards the positive direction, the reducing agent will be consumed and the newly generated NO will reduce the transformation rate[29]. As a result, increasing the quantity of reducing agent will increase the cost and vice versa. So it is important to choose the appropriate quantity of reducing agent to balance the efficiency and cost in actual operation. The SNCR de-NOx efficiency affected by the NSR under different temperatures is shown in Figure 12. Figure 12 shows that, when the temperature is lower than the reaction temperature window, increasing the NSR has little effect on the SNCR de-NOx efficiency. With the NSR at 1073K from 1 to 2, the SNCR de-NOx efficiency increases from 16.3% to 21.1%; with the NSR at 1123K from 1 to 2, the SNCR de-NOx efficiency increases from 26.4% to 45.1%. This is because when the temperature is low, inhibiting the formation of OH and O groups, the NH2 content is less, resulting in a reduction in SNCR de-NOx efficiency. At low temperature, the temperature is the controlling factor of the SNCR reaction. So when the reaction temperature is low, and adding the reducing agent NH3, the SNCR de-NOx efficiency cannot be effectively improved. When the temperature exceeds 1123K, the temperature is no longer the controlling factor of SNCR and the increase of NSR can significantly increase the SNCR de-NOx efficiency. At different temperatures, the effects of the NSR on the SNCR de-NOx efficiency show the same change trend. But when the NSR increases to a certain extent, the rate of increase in SNCR de-NOx efficiency gradually slows down. Analysis shows that NH2 can generate four OH free radicals, so the NH3 has sufficient OH free radicals to produce NH 2. NH2 undergoes the chain reaction of NH2+NO generation of N2 and NH2+NO generation of NNH. At around NSR=1.5, the two chain reactions compete to reach equilibrium[30]. The NSR then continues to increase, only making the SNCR de-NOx efficiency increase slowly, and the increase in the amplitude tends to be gentle. After comprehensive consideration of the SNCR de-NOx efficiency and operational economic factors[31-33], the NSR is set to 1.5.

5 Performance of large-scale cyclone separator A large-scale cyclone will lead to the flow field becoming more disordered in the cyclone separator and will have a major effect on the energy loss and the separation of solid particles [34]. Research on the performance of large-scale cyclone separators can provide a meaningful reference for practical operation. 5.1 Study of gas flow field The tangential velocity, axial velocity and radial velocity of the x=0 cross-section are shown in Figure 13. The axial symmetry of tangential velocity distribution is better. Show the characteristics of strong swirl in separator. All the maximum tangential velocity point form an interface which divides the internal flow field of the separator into two parts. The velocity of the central area is large, so the centrifugal force is large. It is favorable for separation. While velocity of the external area is small, and the effect of the airflow carrying particles weakens, which is favorable for the particles to be captured near the wall. Partial area velocity is negative indicating secondary swirls existing. The axial symmetry of axial velocity distribution is also better. The boundary point is related to the shape of the cyclone. Radial velocity is much smaller than tangential velocity and axial velocity. Most are centripetal, and only the central vortex core has the radial flow. Scholars[35] have found that the radial velocity is mostly centripetal, between 0-3m/s. Lower part of the outlet section has the largest radial velocity, up to 7m/s.

Figure 14 is velocity vector diagram at z=12m section. As can be seen from Figure 146 after the airflow enters the cyclone separator, due to the restriction of the wall surface of the cyclone separator, the air flow turns downward to form the outer swirl. When the air reaches the bottom, the air has to flow upward to form an internal swirl. External airflow flows downward and internal airflow flows upward. Direction of rotation is the same. 5.2 Pressure distribution of large-scale cyclone separator Fig. 15 is the variation curve of the total pressure difference between the inlet and outlet of the cyclone separator at different inlet velocities. It can be seen from Figure 15 that with the increase of the inlet velocity, the pressure drop of the cyclone separator increases, and the rate of increase is also more and more powerful. The pressure drop from 295.84 Pa soon increases to 4231.2 Pa when the inlet velocity rises from 5m/s to 30m/s. From an energy point of view, increasing the inlet velocity of the cyclone separator will increase the loss of energy. This is because too high inlet speed will accelerate the wear of the cyclone separator. It should be based on the separation performance of the cyclone separator as far as possible to use a slightly lower inlet velocity, saving energy. In normal operation at the entrance speed of 20 m/s or so, the diagram shows that with the large-scale cyclone’s normal operation, the total pressure difference is 1987.6 Pa. 5.3 Classification efficiency of large-scale cyclone separator Gas-phase turbulent flow will affect the particle phase, similarly, and particles will also affect the gas phase. But the interaction between particles can be neglected. So the phase coupling stochastic trajectory model is used to simulate the particle motion[36]. Particles do high-speed rotating flow in the separator. The large particles are thrown out under the centrifugal force and fall into the dust exhaust port after several collision walls [37-38]. Set these particles to be captured; Gas fluid carries small particles which are discharged from the exhaust pipe. Set these particles to escape. Set the number of particles into the cyclone n, the number of particles captured ni. Classification efficiency is calculated by formula 20: （20） In this paper, set solid particles as spherical ash particles. Particle density is the average of ash density. According to the m=ρv, sphere volume v=4/3πr^3, the ratio of mass is proportional to the third power of particle size. The analysis of particle size includes the consideration of particle quality. Use DPM model to simulate the particle phase in Euler-Lagrangian coordinate system. The particle phase motion equation is as follows: （21） FD——Drag force； FP——Pressure gradient force：

FA——Additional mass force：

FB——Basset force：

FS——Saffman force：

FM——Maguns force： FC——Volume force In the gas-solid two-phase flow, the particle size is small and the concentration is very thin. So in this paper, fluid drag force is the main force, and the other compared with very small, can be neglected [39-40]. The particle velocity trajectories are shown in Figure 16. Classification efficiency is one of the important indexes to evaluate the separation performance of the cyclone separator. The efficiency of classification is the separation efficiency of a given particle size, which has nothing to do with the particle size at the inlet[41]. So it can be seen that evaluating the performance of the cyclone separator with classification efficiency is more significant. Without consideration of the operating conditions, the total efficiency of the cyclone separator is meaningless [42]. Because the cyclone can be completely separated from the large particles, the key lies in the separation of small particles so as to mainly simulate the small particles. The particle diameter obeys the Rosin–Rammler distribution, with particle sizes 0.05–0.1 mm, average particle size 0.075 mm, and 2000 particles are selected randomly from the particle mass flow to calculate the classification efficiency. Figure 17 is the classification efficiency change curve at different inlet velocities. As can be seen from Figure 17, when the inlet velocity increases, the classification efficiency of the cyclone separator will increase; when the inlet velocity decreases, the separation efficiency of the cyclone separator will decrease. At the same time, it can be seen that the influence of the change of inlet velocity on the classification efficiency curve is relatively large. When the inlet velocity increases, the centrifugal force of the particles is increased. But turbulence is enhanced, and the ash bucket back mixing becomes more serious, causing the small size particle classification efficiency not to obviously improve and become even lower [43]. To sum up, the entry speed is not a matter of the bigger the better. It should be based on the cleanliness of the outlet air flow requirements to select the appropriate inlet velocity which can achieve the purpose of separation, save energy and extend the life of the cyclone separator. When the inlet velocity of the normal operation is 20 m/s, the classification efficiency of the large-scale cyclone separator is 66.74%. 6 Conclusion Based on Oyvind Skreiberg’s SNCR de-NOx reaction mechanism and under the reaction conditions of the cyclone separator of the CFB, the sensitivity analysis method of CHEMKIN software is used to optimize the SNCR reaction mechanism. The optimized SNCR reaction mechanism and CFD software are combined to simulate the performance of SNCR de-NOx and the characteristics in a 660 MW supercritical circulating fluidized bed cyclone separator. The following conclusions are obtained: (1) CHEMKIN software is used to determine the degree of correlation between each reaction in the model and the components of interest and we obtain the optimized mechanism of the 18 basic elements and compare these with the experimental and detailed mechanism. It is proved that the optimized mechanism can simulate the SNCR de-NOx process in the cyclone separator of the supercritical CFB boiler. (2) At reaction temperatures of 1073–1323K, the SNCR de-NOx efficiency first increases and then decreases with the increase of temperature, reaching a peak at 1223K. The conversion rate of NO is more than 40% and the corresponding temperature is set to the temperature window. The SNCR de-NOx reaction temperature window is 1073–1323K. (3) When the temperature is lower than the reaction temperature window, temperature is the controlling factor

and improving the NSR cannot effectively improve the SNCR de-NOx efficiency. When the temperature is increased to the reaction temperature window, the NSR can effectively improve the SNCR de-NOx efficiency. But by enhancing the NSR to a certain extent, the SNCR de-NOx efficiency increase will gradually slow down. Considering the SNCR de-NOx efficiency and economic factors, the NSR is set to 1.5 as appropriate. (4) The total pressure difference of the cyclone separator increases with the inlet velocity and the rate of increase gradually increases. When the inlet velocity of the cyclone separator is 20 m/s, the total pressure difference is 1987.6 Pa. The classification efficiency of the cyclone separator increases with the inlet velocity and the rate of increase gradually decreases. When the inlet velocity of the cyclone separator is 20 m/s, the classification efficiency is 66.74%. Acknowledgment This study was supported by the Fundamental Research Funds for the Central Universities (JB2015RCY06)

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Fig. 1 Schematic diagram of the experimental system

90

SNCR de-NOx efficiency/%

85

SNCR mechanism(Oyvind Skreiberg) Experimental result Simplified mechanism(18)

80 75 70 65 60 55 50 45 1100

1150

1200

1250

1300

1350

Temperature(K)

Fig. 2 The change curve of the denitrification efficiency with temperature

Fig. 3 Schematic diagram of cyclone separator

Fig. 4 Schematic diagram of the grid of cyclone separator

RSM

RNG k-ε

Realizable k-ε

Figure 5 velocity nephogram of x=0 cross-section using RSM、RNG k-ε、Realizable k-ε model

RSM

RNG k-ε

Realizable k-ε

Figure 6 velocity nephogram of z=12 cross-section using RSM、RNG k-ε、Realizable k-ε model.

RSM

RNG k-ε

Realizable k-ε

Figure 7 The tangential velocity of the x=0 cross –section using RSM、RNG k-ε、Realizable k-ε model

RSM

RNG k-ε

Realizable k-ε

Figure 8 The axial velocity of the x=0 cross –section using RSM、RNG k-ε、Realizable k-ε model

Fig. 9 Schematic diagram of injection point positions

Figure 10 Mass fraction of ammonia reducing agent

90

SNCR de-NOx efficiency/%

80 70 60

NSR=1 NSR=1.2 NSR=1.4 NSR=1.6 NSR=1.8 NSR=2

50 40 30 20 10 1050

1100

1150

1200

1250

1300

1350

Temperature(K)

Fig. 11 Effect of temperature on SNCR de-NOx efficiency under different NSR

SNCR de-NOx efficiency /%

90 1073K 1123K 1173k 1223k 1273k 1323k

80 70 60 50 40 30 20 1.0

1.2

1.4

1.6

1.8

2.0

NSR

Fig. 12 Effect of NSR on the SNCR de-NOx efficiency at different temperatures

tangential velocity

axial velocity

radial velocity

Figure 13 The tangential velocity, axial velocity and radial velocity of the x=0 cross –section

Figure 14 velocity vector at the z=12m section

5000

Total pressure/Pa

4000 3000 2000 1000 0 5

10

15

20

25

30

Inlet velocity/m/s

Fig. 15 Variation curve of total pressure difference at different inlet velocities

Figure 16 The particle velocity trajectories

Classification efficiency/%

75 70 65 60 55 50 45

5

10

15

20

25

30

Inlet velocity/m/s

Fig. 17 Curve of different inlet velocity classification efficiency

Tab. 1 SNCR reaction mechanism (Fluent reaction module)

Serial number

Chemical reaction

1

NH3+NO→N2+H2O+H

2

NH3+O2→NO+H2O+H

3

HNCO+M→H+NCO+M

4

NCO+NO→NO+CO+H

5

NCO+OH→NO+CO+H

6

N2O+OH→N2+O2+H

7

N2O+M→N2+O+M

8

CO（NH2）2→NH3+HNCO

9

CO（NH2）2→2NH3+CO2

Tab. 2. Optimized mechanisms

Optimized equation

pre-exponential factor

activation energy

temperature exponent

O+OH<=>H+O2 2OH<=>H2O+O

2.0E14 4.3E03

0 -2486

-0.40 2.70

H+O2(+M)<=>HO2(+M) HO2+OH<=>H2O+O2

2.1E18 2.9E13

0 -497

-1.0 0

NH3+OH<=>NH2+H2O NH2+OH<=>NH+H2O

2.0E06 4.0E06

566 1000

2.04 2.0

NH2+NO<=>NNH+OH NH2+NO<=>N2+H2O NH+OH<=>N+H2O NH+NO<=>N2O+H NNH<=>N2+H NNH+O2<=>N2+HO2

8.9E12 -8.9E12 5.0E11 2.9E14 1.0E07 2.0E14

0 0 2000 0 0 0

-0.35 -0.35 0.5 -0.4 0 0

CO+OH<=>CO2+H NH2+O<=>HNO+H

1.5E07 6.6E14

-758 0

1.3 -0.5

NH+O2<=>HNO+O HNO+O2<=>NO+HO2

4.6E05 1.0E13

6500 25000

2.0 0

NO+HO2<=>NO2+OH NO2+O<=>NO+O2

2.1E12 3.9E12

-480 -238

0 0

Tab. 3 Size table of structure of cyclone separator Body diameter

Riser diameter

Diameter of

Depth of

Inlet section

Inlet section

dust discharge

insertion

height

width

Total height

Cone height

port D

Dx

Dd

S

a

b

H

Hc

8.5m

4.25m

1.5m

3.6m

8.5m

3.6m

21.8m

10.8m

Tab. 4 Injection point positions of reducing agent

Injection

x

y

z

-2.42

4.26

19.8

-2.42

4.26

15.3

-4.25

0

15.3

point 1 Injection point 2 Injection point 4

Tabel.5 Imported gas phase statistics gas flow rate(kg/s)

Temperature(K)

Composition(mass fraction%)

789.48

1173

N2:62.83%; CO2:22.79%; H2O:9.95%; O2:4.42; NO:0.01%.

Tabel.6 Urea injection statistics urea mass flow rate(kg/s)

dilution water mass flow rate(kg/s)

urea concentration(%)

0.024

0.136

15

Highlights 

The optimized mechanism of the SNCR reaction was presented and its correctness was verified by experiments.



The optimized mechanism and CFD software were combined to simulate the process of SNCR de-NOx in a large-scale cyclone separator.



CFD software was used to study gas–solid characteristics in a cyclone separator of a supercritical CFB boiler.