Simulation on coal-fired supercritical CO2 circulating fluidized bed boiler: Coupled combustion with heat transfer

Simulation on coal-fired supercritical CO2 circulating fluidized bed boiler: Coupled combustion with heat transfer

Advanced Powder Technology xxx (xxxx) xxx Contents lists available at ScienceDirect Advanced Powder Technology journal homepage: www.elsevier.com/lo...

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Advanced Powder Technology xxx (xxxx) xxx

Contents lists available at ScienceDirect

Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt

Original Research Paper

Simulation on coal-fired supercritical CO2 circulating fluidized bed boiler: Coupled combustion with heat transfer Ying Cui a, Wenqi Zhong a,⇑, Jun Xiang b, Guoyao Liu c a Key Laboratory of Energy Conversion and Process Measurement and Control Ministry of Education, School of Energy and Environment, Southeast University, Xuanwu District, Nanjing, Jiangsu Province 210096, PR China b State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, PR China c Nanjing Sciyon Automation Group Co. Ltd., Nanjing 211102, PR China

a r t i c l e

i n f o

Article history: Received 1 April 2019 Received in revised form 1 August 2019 Accepted 7 September 2019 Available online xxxx Keywords: S-CO2 CFB Combustion characteristics Gas pollutant emissions Eulerian-Lagrangian approach

a b s t r a c t Using supercritical carbon dioxide (S-CO2) as the working fluid integrated in a circulating fluidized bed (CFB) boiler is a rising technology used to improve the power generation efficiency and reduce gas pollutant emissions in coal-fired power generation systems. This study established a comprehensive 3-D model based on an Eulerian-Lagrangian frame to simulate the combustion process. A new method was presented using constant heat flux as the boundary obtained from the coupled simulation of heat transfer and combustion. The gas phase was described with large eddy simulation (LES). The solid phase used the multi-phase particle-in-cell (MP-PIC) approach. Simulations were carried out in a 10 MW S-CO2 CFB boiler (with cross section area of 3.557  3 m2 and height of 21.01 m). Combustion characteristics obtained in boundary heat flux and excess air ratio were numerically investigated. Results showed that the temperature profile was relatively uniform in the whole boiler and the furnace temperature increased with the increase of boundary heat flux. Emissions of CO2 and SO2 declined with the increase of boundary heat flux while CO emission increased. An increased excess air ratio caused a decrease in furnace temperature and the rise of CO and SO2. The characteristics of combustion and pollutant emissions were optimal with the heat flux at around 25–37 kW/m2 and an excess air ratio at 1.18–1.25. Ó 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

1. Introduction With higher efficiency, wider availability of working fluid, and lower emissions, the supercritical CO2 (S-CO2) Brayton cycle is an efficient emerging technology. The cycle, when integrated with a coal-fired power system, can reach high thermal efficiency due to conditions of the turbine inlet and cycle architecture. In S-CO2, the operation of CO2 exceeds the critical point (T = 304.1 K, P = 7.37 MPa). The critical temperature can be easily achieved. Operating at high pressure allows more compact turbo machinery [1–4]. Le Moullec [5] studied power plants with a post-combustion CO2 capture mechanism and an S-CO2 Brayton cycle to discover the potential of the concept. The results, when compared with the supercritical power plants using conventional carbon capture technology, demonstrated that the levelized cost of electricity (LCOE) can be reduced by 15%; thus, the cost of avoiding CO2 emissions

⇑ Corresponding author at: Sipailou 2#, Nanjing 210096, Jiangsu, PR China. E-mail address: [email protected] (W. Zhong).

was reduced by 45%, and no transportation or storage were required. When circulating fluidized bed (CFB) technology based on an SCO2 power cycle is used in coal-fired power plants to generate electricity, the consumption of coal and the emission of pollution will be reduced. Shelton et al. [6] constructed S-CO2 recompression cycle with a CFB and carbon capture and storage (CCS), which could realize a nearly constant temperature heat source through heating the recycled flue gas and a fraction of CO2 from the MC exit. The efficiency increment of the S-CO2 cycle plant configured with reheat and intercooling could achieve 2% compared to steam Rankine cycle power. Pratt and Whitney Rocketdyne [7] developed Zero Emission Power and Steam (ZEPS) plant which adopted a pressurized fluidized bed combustor (PFBC) with combined cycle that modified S-CO2 cycle was used as the topping cycle and steam cycle was used as the bottoming cycle. The ZEPS plant efficiency could increase by 9.4% compared with oxy-combustion atmospheric boiler. Many research institutions all over the world focus on the layout of the S-CO2 Brayton cycle and the characteristics of this cycle with CFB coal combustion [8]. Dostal et al. [9] have

https://doi.org/10.1016/j.apt.2019.09.010 0921-8831/Ó 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

Please cite this article as: Y. Cui, W. Zhong, J. Xiang et al., Simulation on coal-fired supercritical CO2 circulating fluidized bed boiler: Coupled combustion with heat transfer, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.09.010

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Nomenclature Cd d D Dp f F g h Ki k ms Nu p q R Ri rp Re T u Yi X A

drag coefficient diameter (m) turbulent mass diffusion coefficient for gas (m2/s) interphase momentum-transfer coefficient particle distribution function interphase momentum-transfer rate gravitational acceleration (m/s2) specific enthalpy (J/kg) chemical reaction rate coefficient coefficient of heat transfer (W/m2K) mass of solid (kg) Nusselt number pressure (Pa) heat transfer flux (J/m2s) universal gas constant (J/molK) reaction rate (mol/m3s) particle radius (m) Reynolds number temperature (K) velocity vector (m/s) mass fraction view factor between the wall and cell area of heat transfer

demonstrated that the S-CO2 cycle outperformed the equivalent helium cycle when the turbine inlet temperature was the same. The system layout based on the Brayton cycle is simpler than the steam cycle. Various power plant suppliers and operators, such as Pratt Whitney & Rocketdyne (PWR, California, USA) Électricité de France, S.A. (EDF; Paris, France) are designing S-CO2 cycles for use in coal-fired power plants [10]. They found that innovative SCO2 topping cycles can generate the same net power generation as non-CO2 capture steam plants, while significantly reducing CO2 emissions. In recent years, Wengang Bai et al. [11] carried out the conceptual design study of a 300 MW boiler with a single reheated recompression S-CO2 Brayton cycle and conducted the boiler’s heating surfaces. Notably, the layouts of the S-CO2 Brayton cycles differ from those of steam cycles due to the physical properties of the two working fluids. Scant research has concentrated on the coupled heat transfer process of an S-CO2 wall, which studies the heat flux boundary of the furnace and the combustion characteristics of an S-CO2 CFB boiler [12]. In the simulation study of CFB boiler, many scholars have proposed several treatments of the combustion and heat transfer boundary conditions. Computational fluid dynamics (CFD) as a powerful approach is widely used in the simulation of the pulverized-coal particles combustion in industrial-scale boilers [13–16]. Nevertheless, this method is hard to operate a large mesh, so there were few reports on the three-dimensional (3D) simulation of fluid flow in the heater of the furnace. On the contrary, the one-dimensional (1D) fluid flow model, due to its simplicity and reasonable accuracy, was usually used to reveal the temperature distribution and heat transfer coefficient of the fluid. Chen Yang [17] established a 600 MW W-shaped flame boiler model to simulate combustion characteristics combined with 1D distribution parameter model of furnace water wall. In most 3D model, the method of simplifying to constant wall temperature is widely used in the treatment of boundary conditions. Jun et al. [18] developed a 3D simulation of solid waste and coal mixed combustion in a 75 t/h CFB boiler by assuming the boundary condition as a constant wall temperature. The study by Yang et al. [19] simulated the coupled heat transfer process between combustion and fluid

t L Q

time (s) characteristic length (m) mass flow (kg/s)

Greek letters h volume fraction U heat exchange e a small number on the order of 10-7 to remove the singularity l viscosity (kg/ms) q density (kg/m3) s stress tensor (Pa) D length scale g coefficient of correction Subscripts g gas phase p solid phase i the ith species cp close pack f1 hot fluid f2 cold fluid

heating in a 300 MW S-CO2 CFB. In their work, the heat flux of the thermal walls obtained in the combustion process was applied in the calculation of heat transfer coefficient according to the S-CO2 fluid model and tube wall model. The newly boundary temperature was presented when the difference between the result and that of the last step was within the error range. However, there are some hypothetical conditions in modeling, which are different from the real situations. It is of great concern to come up with advanced methods for thermal wall boundary treatment and find out the effects of different boundary conditions and operating conditions on the combustion process in S-CO2 CFB boiler in order to lay the foundation for optimization design and operation. Furthermore, the Multi-Phase Particle-In-Cell (MP-PIC) approach applied the Eulerian-Lagrangian method has proven as an advanced method by replacing real particles as millions of computational particles, and this process seldom loses the advantage of the Lagrangian method in particle phase treatment [20–23]. Up to now, few studies have conducted simulations with the heat flux boundary, and the method of coupling heat transfer and combustion was not advanced enough. Furthermore, few studies have been investigated to simulate the 3D combustion process in a coal-fired SCO2 CFB boiler by means of the MP-PIC approach. The main purpose of this study is to introduce a new method of using constant heat flux as the boundary. This method was developed from the coupled heat transfer in the S-CO2 wall and from the combustion process in a designed 10 MW S-CO2 CFB design based on the MP-PIC approach. Simulations were conducted with boundaries and operating conditions to elucidate the effects of different heat flux boundaries and excess air ratios on combustion characteristics and pollution emission characteristics. These simulations were created to generate technical guidance for industrial and experimental operation. 2. Establishment of numerical models The present 3D MP-PIC model can be divided into three major parts [24,25]: continuum model is used to describe gas phase motion and particle phase motion is described by Lagrangian

Please cite this article as: Y. Cui, W. Zhong, J. Xiang et al., Simulation on coal-fired supercritical CO2 circulating fluidized bed boiler: Coupled combustion with heat transfer, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.09.010

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model with a drag model describing momentum transfer; the combustion model includes coal devolatilization, char gasification, combustion of char and volatile; convection and radiation heat transfer model [26]. 2.1. Governing equations The gas phase can be modeled as a continuum with momentum balance equations derived from kinetic theory. The gas phase motion is simulated by the large eddy simulation (LES) approach in order to better reflect the characteristics of turbulence in several advantages. First, LES handles the transport process affected by the resolved, large-scale motions, and the modeling effort of the turbulence is simplified to the residual motion structures. Compared to Reynolds-averaged Navier-Stokes (RANS) modeling, LES approach is more accurate in complex turbulent gaseous flows. This mainly results from the explicit and direct computation of the large scale structures of the flow, which contain a significant part of the physics and have an important effect on the combustion [27]. Second, compared with direct numerical simulations (DNS), the speed of solution is increased applying LES and turbulence can be calculated with a larger Reynolds number. Third, fresh and burned gas zones, with different turbulence characteristics, are instantaneously identified at the resolved grid level, so a better description of turbulence-chemistry interactions can be expected [28,29]. The particle distribution function (PDF) is used to reflect the kinetics of solid phase. The contact force on the numerical particles simulated by the spatial gradient is easily calculated on the Eulerian grid. The exchange term in the equation conserves the mass, momentum and energy of the gas-solid phases. The interaction between gas and solid phases is described by Wen-Yu-Ergun drag model [30,31]. Governing equations of solid and gas phases are briefly illustrated in Table 1.

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model was close to the real situation, which was acceptable. Above 750 , char oxidized to gaseous products, CO, CO2 and SO2. During the complicated coal combustion process in S-CO2 CFB, when the coal was sent into the furnace, the moisture was released from coal at first and volatiles such as CO, CO2, H2, H2O and H2S, were released from coal pyrolysis process. After the devolatilization, the remaining char was burned with O2 and produced gaseous emissions like CO2, CO and SO2. Homogeneous reaction equations used in simulation are as follows:

CO2 + C(s) ! 2CO

ðR0Þ

CO + 0.5 O2 ! CO2

ðR1Þ

C(s) + 0.5O2 ! CO

ðR2Þ

C(s) + O2 ! CO2

ðR3Þ

C(s) + H2 O ! H2 + CO

ðR4Þ

2H2 + O2 ! 2H2 O

ðR5Þ

CO + H2 O ! H2 + CO2

ðR6Þ

In addition, the formation and absorption reactions are also taken into account:

H2 S + 1.5O2 ! SO2 + H2 O

ðR7Þ

CaO + SO2 + 0.5 O2 ! CaSO4

ðR8Þ

The reaction rates and kinetic parameters of these reactions are listed in Table 2. 2.3. Computational geometry and model setup

2.2. Chemical reaction models Coal particles consist of char, volatile and ash, of which the char comprises mainly carbon, ash, nitrogen, sulphur and hydrogen. The simplified mechanism was adopted in this study, and only the reactions of the carbon and sulphur were taken into consideration [32]. The model was verified by comparing with experiments in Section 2.3, and the results showed that the chemical reaction

The model coal-fired CFB with S-CO2 Brayton power cycle used in the simulation was self-designed with dimensions P = 10 MW, H = 21.01 m and S = 3  3.557 m2, as presented in Fig. 1 [33]. The S-CO2 cold wall of the chamber in this model was simplified to be the boundary thermal walls. The heights of the static bed in the chamber and the seal pot are 2.415 m and 0.525 m, respectively. The initial calculated particle number is approximately

Table 1 Governing equations of solid and gas phases. Primary governing equations for gas and solid phases

Equation for particle contact normal stress

hp þ hg ¼ 1

P s hp s ¼ max½ðhCP h p Þ;eð1hp Þ

Mass conservative equation for gas phase

Convective heat transfer coefficient between fluid and wall h ¼ ð1  f d Þh1 þ f d hd

@hg qg @t

b

þ rðhg qg ug Þ ¼ 0

Momentum conservation equation for gas phase

Equations for dimensionless numbers

@hg ug @t

Re1 ¼ gl g h1 L ¼ Nu1 ¼ 0:546Re0:5 1 þ 3:66 kg

þ rðhg ug ug Þ ¼  q1 rp  q1 F þ hg g þ q1 rs g

g

g

q ug L

h d dp kg

¼ Nug ¼ 0:525Re0:75 1

Solid-phase momentum conservation equation, Cd is determined by WenYu-Ergun model

Convective heat transfer coefficient between gases and particles

¼ Dp ðug  up Þ q1 rp þ g  hp1q rsp p g 8 q jug up j > Dp;w ¼ C d 38 qg rp hg2:65 ; hp < 0:75hcp > > p < h 0:75h Dp ¼ Dp;w þ p 0:1hcp cp ðDp;e  Dp;w Þ; 0:75hcp 6 hp 6 0:85hcp > > > : D ¼ 45 lg hp þ qg jug ug j ; 0:85h < h p;e cp p q rp q r2 h

f d ¼ 1  expð10hp =hCP Þ h p dp ¼ Nup ¼ 0:37Rep0:6 þ 0:1 k

dup dt

p p g

f

Rep ¼

qg u g dp lg

p

0:687 Re < 1000 : C d ¼ 24 Þ Re ð1 þ 0:15Re Re P 1000 : C d ¼ 0:55 2q jug up jrp Re ¼ g l g

Please cite this article as: Y. Cui, W. Zhong, J. Xiang et al., Simulation on coal-fired supercritical CO2 circulating fluidized bed boiler: Coupled combustion with heat transfer, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.09.010

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Table 2 Chemical reaction equations [18,23,32]. Reaction rate Ri (mol/m3s) R0 ¼ R1 ¼

K 0 C CO2  K 0 C 2CO K 1 C CO C O0:5 2

R2 ¼ K 2 C O2 R3 ¼ K 3 C O2 R4 ¼ K 4 C H2 O  K 4 C H2 C CO R5 ¼ K 5 C O2 C 1:5 H2 R6 ¼ K 6 C CO C H2 O  K 6 C H2 C CO2 R7 ¼ K 7 C H2 S C CO R8 ¼ hs qqs Y CaO gK 8 C SO2 CaO

Table 3 Fuel characteristics.

Reaction rate coefficient Ki 5

K0 = 1.272msTexp(1.88  10 /RT) K1 = 1.0  1015exp(1.33  105/RT) K2 = 4.34  107hsTexp(1.13  105/RT) K3 = 3.8  107hsTexp(0.555  105/RT) K4 = 1.272msTexp(1.88  105/RT) K5 = 5.159  1015msT1.5exp(0.285  105/RT) K6 = 3.552  1014exp(1.31  105/RT)T1 K7 = 5.2  108exp(0.193  105/RT) K8 = 1.1  106exp(0.595  105/ CaSO4 RT)g ¼ exp5:71X X ¼ C CaOCþC CaSO

Proximate analysis, wt%, dry ash free basis(d.a.f.)

Value

Moisture Ash Volatiles Fixed carbon C H S N O (dif.) High calorific value/(kJ/kg)

9.4 27.22 19 44.38 50.96 3.11 0.81 0.77 7.73 27,100

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1.25 [23,34]. The total air air flow rate was changed by keeping the flow rate of coal feed, primary air and loop-seal air unchanged but changing the secondary air flow rate, which led to the change of air-coal ratio. Table 4 lists the operating conditions and simulation settings. 2.4. Process of boundary treatment

Fig. 1. 10 MW S-CO2 CFB boiler model.

8.78  106 and the volume fraction was 0.55. There are a total of three coal inlets on the left wall, arranged at a height of 1.58 m. The CaO contained in ash composition can act as absorbent for acid gas, the mass fraction of which is 14.89% for coal. Silica sand, whose particle density is 2200 kg/m3, is used as the bed material. Bed material and coal particles are both wide sieve particles with the size of 0.099–0.5 mm. The characteristics of coal used in the simulation are analyzed in Table 3. The primary air was evenly fed into the furnace through the distributor plate, and there were twelve secondary air inlets which were arranged on each side of the wall with the height of 3.5 and 4.2 m. The ratio of the primary airflow volume to secondary airflow volume was 0.55:0.35, and the remaining 0.1 was set as sowing wind and loosens airflow. The primary and second airflow inlets adopted the velocity inlet boundary condition while the pressure outlet boundary condition for the cyclone outlet, and the reactor was filled completely with N2 at the beginning. The combustion characteristics of the chamber with different boundary conditions were simulated. That was to say, the thermal wall heat flux as follows: 26.744 kW/m2, 36.880 kW/m2, 51.210 kW/m2. The numerical simulation calculation of excess air ratio (a) was carried out under the three working conditions, with a set at 1.1, 1.18 and

The setting of the constant heat flux boundary cannot be realized directly due to the limitation of the MP-PIC approach and the CPFD Barracuda software function. The constant heat flux boundary conditions were selected according to the simulations of three groups of S-CO2 wall temperature boundaries in heat transfer. Fig. 2 shows the coupled simulation of the combustion and heat transfer process. To begin with, furnace surface temperature distribution was assumed as the boundary to simulate the combustion process in the furnace. The initial temperature of the S-CO2 wall on the heat transfer side was set as 550 , 600 and 650 , respectively. Data on S-CO2 wall temperatures were from the literature [33], and the boundary of heat flux was proposed on this basis. The combustion process in the furnace was simulated under temperature boundary conditions, which helped provide the information needed to develop uniform heat flux in the thermal walls. The change of heat flux with time was basically realized by changing the thermal wall temperature of the S-CO2 wall at different times in order to obtain a constant heat flux boundary. The newly calculated heat flux at the thermal walls was compared with the heat flux in the previous step. The iteration can be seen as converged if the difference between the two was within the error

Table 4 Input parameters for the simulations. Parameters

Unit

Value

Primary air inlet, Q1 Secondary air inlet, Q2

kg/s kg/s

Loop-seal coal sowing inlet, Q3 Loop-seal coal loosen inlet, Q4 Coal feed inlet, Qm Fuel consumption, Qp Solid flow rate, Qs Cyclone outlet, P Recovery coefficient Friction coefficient Particle normal-to-wall Retention coefficient Particle tangential-to-wall retention coefficient Time step Thermal wall heat flux, q

kg/s kg/s kg/s kg/s kg/s Pa – – –

5.3735 2.855 (a = 1.1), 3.602 (a = 1.18) and 4.255 (a = 1.25) 0.8956 0.2687 0.5374 1.3475 0.1263 101,325 0.9 0.3 0.3



0.99

s kW/ m2 – Pa %

0.01 26.744, 36.880 and 51.210

Excess air ratio, a Operating pressure Mass fraction of CaO in coal

1.1, 1.18 and 1.25 101,325 14.89

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Fig. 2. Simulation procedures of the combustion and heat transfer process.

range. Otherwise, the newly calculated heat flux would be assigned to the thermal walls, and the iteration would continue until convergence. With the condition of a constant wall temperature of 600 , the average heat flux was 36. 880 kW/m2, and the heat quantity distribution of the thermal wall is shown in Fig. 3(a). Due to the heat absorption of the S-CO2 wall, the heat flow shown in the figure is negative, the area with high heat flow rate is green, and the area with low heat flow rate is red. The distribution of heat flux is also reflected in the diagram since the heat flux was proportional to the heat quantity. The heat flux near the central line of the S-CO2 wall was relatively small, while the heat flux increased near the corner of the chamber. The heat flux at the bottom of the chamber was higher than that at the top. This was due to the correlation between the distribution of the thermal wall heat flux in the axial direction and the concentration distribution of the solid materials in the furnace. At the bottom of the dense phase, the concentration of materials was high, and the chemical reaction rate was faster, leading to large heat release and high heat flux. The heat flux distribution of the front and rear walls was close and the rule was suitable for the right and left walls. The upper left corner of the right wall was the interface between the furnace and the cyclone separator, where there was no S-CO2 heat exchange tube to transfer heat, so the heat flux was small at that stage. The distribution of heat flux in the front wall and the rear wall was similar, as was the left wall and right wall of the S-CO2 CFB; the front wall and the right wall were analyzed as the representatives, as shown in Fig. 3(b). The heat flux increased near both sides of the front wall but was small at the central line of the S-CO2 wall, and the heat flux distribution in the right wall was similar. This was due to the relatively slow change of the material concentration inside the chamber in the central line of the S-CO2 wall; but the superposition of the velocity field and the concentration field between the side wall and the front wall made the highest concentration in the corner. In the chamber, the flow boundary layer influenced the upward flow of the gas, and the velocity of the near-wall region was small. Therefore, the heat flux in the furnace center was smaller, and the heat flux in the corner was larger. At the position of 5.657 m and 6.257 m, the interface between chamber and cyclone separator led to a heat flux drop. The average heat flux of the front wall and the right wall were 37.4 kW/m2 and 36.8 kW/m2, respectively. The heat flux decreased with the height of the chamber. The change of heat flux was less obvious when getting closer to the bottom of the chamber [35–37]. The boundary condition of constant heat flux was basically realized by changing the temperature of the thermal wall correspond-

Fig. 3. Thermal wall heat transfer characteristics: (a) thermal wall heat quantity distribution and (b) thermal wall heat flux distribution.

ing to different times. The average heat flux of the thermal front wall corresponding to these different times was simulated, as shown in Fig. 4. It was found that the three groups of heat flux did not change much with time. Both of which fluctuated up and down at 26.744 kW/m2, 36.880 kW/m2 and 51.210 kW/m2, generally showing an upward trend. The constant heat flux boundary condition was reliable since the error was controlled within 5%.

2.5. Grid independent verifications Grid independent validation was required before data analysis, and four groups of grid domains were tested, which contained grid numbers of 49,910, 63,200, 71,454 and 83,042, respectively. Distribution of furnace axial voidage was analyzed, as shown in Fig. 5. Results indicated that there was no significant difference in the axial voidage distribution between the two grid domains with 71,454 and 83,042 grid cells. However, there was an obvious larger voidage at the bottom of the chamber with the grid domains of 49,910 and 63,200 grid cells. Also, the concentrations of the main gas emissions at the cyclone exit including O2 and CO2 in the grid domains with 49,910 and 63,200 grid numbers were different from those with 71,454 and 83,042 grid numbers, as shown in Fig. 6. The

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percentage of the latter two groups of O2 and CO2 molar fractions was very close, with a difference of only 0.2% and 0.1%, respectively. Considering the influence of calculation time and accuracy, grid domain for 71,454 grid numbers were selected in the analysis of the simulation results. 3. Results and discussion 3.1. Model validations

Fig. 4. Error analysis of constant heat flux boundary.

The numerical model was verified by comparing the simulation results with the experimental data of Fernandez et al [38] before the comprehensive research. Due to the fact that there were no experimental results of the S-CO2 CFB boiler in China and abroad, the conventional steam CFB boiler with equal power was selected as the experimental verification basis [39–41]. A 12 MW coal-fired CFB boiler was adopted in the experiment. Fig. 7 shows the concentrations of the four main gas species at the outlet of the cyclone with the comparisons of simulation results and experimental data, and the form of the average molar fraction in the area was used. There was a certain difference between the predicted gas concentration and the experimental result since the working fluids were different in the models of simulation and experiment. The predicted gas concentrations had a good agreement with experimental results, despite the fact that the amounts of CO and SO2 were very small. The results show that the validity of the model is verified since the total average relative error is less than 18%. 3.2. Effects of boundary heat flux Combustion characteristics affected by different boundary conditions including three groups of constant heat flux in the 10 MW S-CO2 CFB were investigated. In addition, the simulation results of the S-CO2 CFB boiler under the 36.880 kW/m2 heat flux boundary with the excess air ratio of 1.25 were used as typical results to analyze the temperature distribution and gas emission.

Fig. 5. Profile of axial voidage in chamber with different grid numbers.

Fig. 6. Profile of cyclone outlet gas composition with different grid numbers.

3.2.1. Furnace temperature distributions Because boundary conditions were set at different heat flux distributions (26.744 kW/m2, 36.880 kW/m2, 51.210 kW/m2), the temperature distribution of the furnace was obtained by numerical simulation, as shown in Fig. 8. The temperature range of the furnace under constant heat flux conditions was between 888 and 1111.5 K, which was lower than when temperatures ranged between 962.5 and 1186 K [33]. The furnace temperature

Fig. 7. Comparisons between simulations and experiments.

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Fig. 8. Temperature profiles in S-CO2 CFB boiler with different heat flux boundaries (a = 1.25): (a) gas temperature, q = 26.744 kW/m2, (b) gas temperature, q = 36.880 kW/m2, (c) gas temperature, q = 51.210 kW/m2 and (d) solid temperature, q = 51.210 kW/m2.

corresponding to the 51.210 kW/m2 heat flux was the highest; that corresponding to 26.744 kW/m2 was the lowest. The heat flux expression of the heat transfer process is shown as the following formula [42,43].

u ¼ kADT

ð1Þ

where the A-Heat transfer area is m ; DT = Tf1  Tf2. The difference in heat transfer temperatures between a hot fluid and a cold fluid can be expressed in K or ; k is the total heat transfer coefficient, W/(m2K), which can be deduced as 2

q ¼ u=A ¼ kDT

ð2Þ

where q denotes the heat flux. When the heat transfer coefficient, k, is constant, the heat transfer temperature difference, DT, is directly proportional to the heat flux q. The cold fluid temperature, that is, the S-CO2 working fluid, is considered to be the same as the boundary temperature in the simulation and tends to be constant. The hot fluid temperature, that is, the combustion temperature in the furnace, is proportional to the heat flux q. Hence, with an S-CO2 CFB boiler, the furnace temperature increases with the increasing boundary heat flux. From the temperature distribution diagram of the furnace0 s gas phase and solid phase, as shown in Fig. 8(c) and (d), it can be concluded that the furnace temperature was distributed relatively uniformly. The gas temperature in the lower region was a little higher than that at the top because the former concentrations of oxygen and fuel particles were higher. Peak temperature occurred near fuel inlets. Average temperatures in both the gas phase and solid phase in the chamber were about 879.25 K, which agreed with the design temperature (850–900 K). Due to the heat transfer between the S-CO2 wall and the gas-solid phases, the temperature was relatively low near the surface. Since these particles were mostly inert bed materials, the rise in temperature was mainly due to the combustion of fuel particles and volatile gas heat release. As a result, the temperature of the gas phase was slightly higher than that of the particle phase. S-CO2 differs from steam in its unique physical properties leading to the unique characteristics of heat transfer. The critical point of CO2 is 30.98 /7.38 MPa, which is much lower than that of water for about 373.95 °C/22.06 MPa [44]. The drastic variation of

physical properties in the critical region leads to unique heat transfer and compression characteristics of the S-CO2. The variation of thermal conductivity with temperature near the critical point at different supercritical pressure is shown in Fig. 9 [45]. At a certain pressure, the thermal conductivity increases rapidly with the working fluid temperature. When it reaches the peak, it decreases with the increase of temperature. This shows that the temperature of S-CO2 has a great influence on the heat transfer performance near the critical point, and the heat transfer coefficient reaches the maximum near the quasi-critical point. The thermal conductivity of S-CO2 is much higher than that of supercritical water, which is 0.418 W/(mK) [46], demonstrating that the temperature difference generated by S-CO2 is smaller when transferring the same heat. In other words, at the same temperature difference, S-CO2 transmits more heat and thus has a better cooling effect than steam. However, the heating surface area of the S-CO2 CFB boiler cooling tubes is much larger than that of the steam CFB boiler, which aims to reduce the pressure drop in the S-CO2 tubes. The furnace heat flux in the S-CO2 CFB boiler is lower than that of the steam CFB boiler.

Fig. 9. Thermal conductivity of CO2 versus temperature at different supercritical pressures.

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3.2.2. Gas composition Fig. 10 displays the distribution of the time-averaged molar fraction of gas components in the furnace central cut-down in the S-CO2 CFB boiler. Around one-third of the oxygen was consumed in the dense region where the combustion of char occurred, and the CO2 concentration rose. At the height of the fuel inlet, the CO2 content gradually increased due to the combustion of char and CO. Due to devolatilization, the CO concentration increased first; then it dropped in the dilute region due to the oxidation reaction, as shown in Fig. 10(c). An increase in thermal wall heat flux promoted the consumption of O2. The concentration of CO2 decreased when the heat flux increased, while the trend was the opposite for CO. On the one hand, the consumption rate of CO2 was higher than the formation rate at a higher boundary heat flux. On the other hand, CO2 reacted with O2 at a high heat flux and C reacted with water vapor, which led to the decrease of CO2 and increase of CO when the boundary heat flux increased. The predicted distributions of SO2 concentrations along the riser were shown in Fig. 10(d). In the dense bottom region, SO2 was mainly from the combustion of recirculating char and its concentration increased along the height of riser. At the levels of recirculating inlet, the SO2 concentration slightly decreased due to the air dilution. After the fuel was fed into the riser, the SO2 rapidly increased due to the volatile-S, which was released during

devolatilization, and then was oxidized to SO2 in the chamber and cyclone. The CaO contained in the ash absorbed a portion of the SO2, resulting in the slight decrease of SO2 concentration along the chamber height [47,48]. Fig. 11 shows four main gas compositions presented in contours of time-averaged molar fraction discussed above with the front of the boiler model cut-away. The concentration of O2 appeared high at the bottom of the chamber, shown in Fig. 11(a). It decreased with the height of the chamber due to the consumption by the combustion. Fig. 11(b) illustrates the concentration of CO2 showing the opposite profile. Particles accumulated at the bottom of the chamber. The incomplete combustion of carbon particles caused low CO2 concentration. The secondary inlets replenished a large amount of O2 causing the carbon to burn fully and the CO2 concentration to increase. The profile of CO can be seen in Fig. 11(c), as results of devolatilization and incomplete combustion of carbon, it increased near the primary inlet and decreased along the S-CO2 wall. The high concentration in the seal pot was due to the incomplete combustion caused by high particles density and rarefied air. Fig. 11(d) shows the distribution of SO2 concentration, which is high near the fuel inlets and secondary inlets, where large quantities of coal particles burned and the volatile of S was liberated. At the seal pot, the combustion of the returning coal particles also produced SO2, leading to the increase of SO2 concentration.

Fig. 10. Profiles of gas concentrations with the height of furnace (a = 1.25): (a) O2, (b) CO2, (c) CO and (d) SO2.

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Fig. 11. Concentration profiles of gas compositions obtained in 36.880 kW/m2 heat flux boundary with a = 1.25: (a) O2, (b) CO2, (c) CO and (d) SO2.

3.2.3. Comparisons of temperature and heat flux boundaries Combustion characteristics in the heat flux boundary were compared with those in the temperature boundary using data was from a previous study [33]. Furnace temperature distribution in constant heat flux boundaries (1010–1100 K) was lower than that in constant wall temperature boundaries (1080–1110 K). The value of the three groups of constant heat flux was the average of the distribution of boundary heat flux. This value was obtained from the numerical simulation of three groups of constant wall temperature conditions. The furnace temperature in a constant heat flux boundary was closer to the real situation according to the results of numerical simulation. In practice, if the cost of boiler materials and the reduction of high temperature corrosion were taken into consideration, the S-CO2 CFB boiler can be controlled under the conditions of constant heat flux and the heat flux. The thermal wall heat flux can also be reduced as far as possible. The variation of O2 concentration with the furnace height at the constant heat flux boundary was more uniform than that with the constant temperature boundary. The overall concentration was higher with the lowest O2 concentration, which appeared no less than 0.07. In the constant temperature boundaries, the consumption of O2 was faster because of the high furnace temperature. Half of the O2 concentration was consumed at about 1/3 of the furnace height. The trend of CO2 concentration variation with furnace height was consistent under two types of boundaries. The CO2 within the constant wall temperature boundaries was generated faster than that of the constant heat flux boundaries. Moreover, the CO2 concentration in the constant wall temperature boundaries at the same furnace height was higher than that in the constant heat flow boundaries. For example, at the height of 2.5 m, the CO2 concentrations produced within the boundary of a constant wall temperature can reach 0.08 and the maximum in constant heat flux boundaries is only 0.075. Concentrations of CO2 in temperature boundaries and constant heat flux boundaries at the outlet of the cyclone separator were 0.102 and 0.108 respectively. This was because the combustion temperature was higher in constant temperature boundaries than in constant heat flux boundaries. The higher the chemical reaction rate was, the more CO2 would be generated, which would in turn lead to less CO. The CO concentration within constant temperature boundaries was lower than

that of heat flux boundaries at the furnace’s same height. The CO concentration at the outlet was 0.0102 lower than that in the heat flux boundaries (0.011). There was not too much difference in the SO2 concentration between temperature boundaries and heat flux boundaries; the outlet concentrations were 1.55  105 and 1.35  105, respectively. In engineering applications and experiments, the requirements of safety and gas pollution control for coal-fired boilers can’t be achieved at the same time. The best scheme can only be selected according to actual situations. If the emphasis is to avoid overheating the furnace, the condition that the furnace boundary is in constant heat flux can be achieved, and the heat flux must be reduced to no more than 3.6 kW/m2. If controlling pollution emissions is the emphasis, the furnace wall can be operated under constant temperature conditions, which are best controlled at around 650 , and the choice of a segmented wall temperature boundary is much better. 3.3. Effects of excess air ratio 3.3.1. Furnace temperature distributions Fig. 12 reveals the average temperature distribution of the chamber cross section along the furnace height obtained in 36.880 kW/m2 heat flux boundary. The variation tendency of temperature distribution with furnace height was similar under three conditions of excess air ratios. There was an upward tendency in the average temperature of the chamber with the increase of furnace height. At the height of 1.576 m, the average temperature of the furnace increased due to high air temperature of fuel inlets (1123 K) and the rapid combustion of the char. With the increase of furnace height, S-CO2 wall absorbed a lot of heat and the furnace temperature rose slowly. The secondary air inlets were arranged at the height of 3.15 and 4.2 m in the furnace wall, and the average temperature fluctuated due to the large quantity of secondary air ejecting into the chamber with the lower temperature. The furnace temperature at the same height decreased with the excess air ratio increasing, which was caused by two main reasons. On the one hand, the rise of the excess air ratio accelerated flue gas flowing out of the furnace and promoted the rapid upward movement of the pulverized coal. On the other hand, when the ratio of air to coal

Please cite this article as: Y. Cui, W. Zhong, J. Xiang et al., Simulation on coal-fired supercritical CO2 circulating fluidized bed boiler: Coupled combustion with heat transfer, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.09.010

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Fig. 12. Average temperature distribution along furnace height under different excess air ratios (q = 36.880 kW/m2 boundary).

increased, dilution effect by N2 in the air occurred. The ignition rate of pulverized coal slowed down and the required ignition time was longer after the fuel entering the furnace.

3.3.2. Gas composition Fig. 13 (a) illustrates the distribution of O2 concentration in SCO2 CFB. The change of excess air ratio did not affect the variation trend of O2 concentration with furnace height. The O2 concentration fluctuated obviously at the height of 3.15 and 4.2 m where the secondary air was blew into the furnace. The combustion reaction of char and volatile took place and the O2 concentration decreased rapidly along the furnace height. As a result of the ascent of excess air ratio, O2 concentration increased. The increased excess air ratio offered the increase of air content in furnace. That is, the increase of O2 content. The combustion reaction rate accelerated leading to more consumption of O2. But the air supplement was larger than the consumption, resulting in a slight rise in O2 concentration. The distribution of CO2 and CO concentrations in S-CO2 CFB is presented in Fig. 13(b) and (c), respectively. With the excess air ratio increasing, the CO2 concentration increased while CO decreased. This was because O2 concentration required for combustion increased with the increase of excess air ratio, leading to acceleration of char combustion rate and the precipitating rate of gaseous C volatile, thus more CO2 was produced. When the furnace height was between 0 and 3 m, the excess air ratio was low and the coal quantity near the coal feeding inlets increased, so the CO concentration became higher. The CO concentration decreased as the

Fig. 13. Profiles of gas concentrations along the furnace in segmented temperature boundary: (a) O2, (b) CO2, (c) CO and (d) SO2.

Please cite this article as: Y. Cui, W. Zhong, J. Xiang et al., Simulation on coal-fired supercritical CO2 circulating fluidized bed boiler: Coupled combustion with heat transfer, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.09.010

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secondary air blew into the furnace, which was due to two functions of the secondary air in the area where the secondary air was ejected. On the one hand, CO could be diluted by secondary air. On the other hand, the oxygen content in this area was relatively large transforming large part of CO to CO2. The combined results led to a decrease in the amount of CO in this region. The SO2 concentration increased significantly in the range of 2.5–8 m, because there were a lot of secondary air injection into the front and back wall of the area. The O2 concentration of the flame was higher and the temperature was more than 1140 K. Intense combustion of coal particles produced a large amount of SO2. With the rise of excess air ratio, SO2 concentration increased slightly, only about 3  105, as shown in Fig. 13 (d). On the one hand, the concentration of SO2 increased with the increase in excess air concentration, which was induced by char combustion. On the other hand, when excess air increased, the reaction showed the trend of acid gas recapturing for the SO2 emission [23].

4. Conclusions In this study, an Eulerian-Lagrangian model based on the MP-PIC method was established to perform the 3D full-loop coal combustion process in a designed 10 MW S-CO2 CFB boiler. A heating surface based on the constant heat flux boundary obtained by coupling heat transfer and combustion was proposed. The study investigated the combustion characteristics in a 10 MW S-CO2 CFB boiler under the effects of different boundary conditions including different thermal wall heat fluxes and different operating conditions including excess air ratios. The primary conclusions are summarized as follows: (1) The boundary heat flux of a 10 MW S-CO2 CFB decreased along the furnace height. The boundary heat flux was relatively small near the central line of the S-CO2 wall, but increased near the corner of the chamber. The distributions of heat flux in the front and the rear walls were basically the same, and the heat flux distribution was the same for the left and right walls. (2) Within the scope of this study, the furnace temperature increased with the increasing boundary heat flux, and the CO2 and SO2 emissions decreased. The thermal wall can be controlled under the conditions of constant heat flux to between 25 and 37 kW/m2 in actual operation so that the boiler material cost is not too high and the CO gas emission is minimized. (3) A furnace temperature of 10 MW S-CO2 CFB in heat flux boundaries was lower than that in the temperature boundaries, which helped to avoid overheating. The concentration of CO2 was lower in heat flux boundaries while the concentrations of CO and SO2 were slightly higher than when formed under temperature boundary conditions. (4) In this study, under a constant heat flux boundary, furnace temperature decreased with the rise of excess air ratio, and the CO2 and SO2 emissions increased while CO decreased. The excess air ratio can be controlled under the operating condition to between 1.18 and 1.25 as far as possible in actual operation so that the boiler temperature is not too high and the gas emissions of CO and SO2 are minimized.

Acknowledgements Financial supports from the National Key Research and Development Program of China (no. 2017YFB0601802) and Key

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Research and Development Program of Jiangsu Province (no. BE2017159) are sincerely acknowledged.

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