CFD modeling of oxy-coal combustion in circulating fluidized bed

CFD modeling of oxy-coal combustion in circulating fluidized bed

International Journal of Greenhouse Gas Control 5 (2011) 1489–1497 Contents lists available at ScienceDirect International Journal of Greenhouse Gas...

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International Journal of Greenhouse Gas Control 5 (2011) 1489–1497

Contents lists available at ScienceDirect

International Journal of Greenhouse Gas Control journal homepage:

CFD modeling of oxy-coal combustion in circulating fluidized bed Wu Zhou, Changsui Zhao ∗ , Lunbo Duan, Daoyin Liu, Xiaoping Chen School of Energy and Environment, Southeast University, Nanjing 210096, China

a r t i c l e

i n f o

Article history: Received 30 March 2011 Received in revised form 12 July 2011 Accepted 7 August 2011 Available online 1 September 2011 Keywords: CFD modeling CFB Oxy-coal combustion Desulfurization

a b s t r a c t Based on the Computational Fluid Dynamics (CFD) method, the previously established and validated 2dimensional model was employed to predict the oxy-coal combustion processes in a 50 kW circulating fluidized bed (CFB) at Southeast University, China. The simulated processes of coal conversion in our CFB combustion system included particle drying, dry coal devolatilization, volatile combustion, char combustion, char gasification and SO2 emission. The effects of combustion atmospheres, including air (21% O2 /79% N2 ) and oxygen/recycled flue gas (O2 /RFG) atmosphere with different O2 concentration (from 21% to 40%), were investigated on distributions of solid volume fraction, temperature and gas concentration in the riser. During combustion of Xuzhou bituminous coal in O2 /RFG modes with different O2 concentrations referred to above, about 70% volume fraction of CO2 were achieved in the wet flue gas with more than 25% volume fraction of H2 O. The rates of char gasification by CO2 and H2 O in 30% O2 /70% RFG atmosphere were obviously higher than that in air. However, compared to air combustion, CO concentration in the dense zone in 30% O2 /70% RFG atmosphere was lower because high concentration of H2 O accelerated CO combustion. Both indirect and direct desulfurization mechanisms were considered for sulfur retention by limestone and the corresponding overall reaction rates were calculated statistically. With increase in O2 inlet concentration gaseous pollutant SO2 was enriched. © 2011 Elsevier Ltd. All rights reserved.

1. Introduction Oxy-fuel circulating fluidized bed combustion technology is one of the most promising technologies for carbon capture and storage (CCS) (Wall, 2007) and it is undergoing rapid development. Numerical modeling is helpful to better understanding of the combustion processes and significant for CFB combustor scale-up. A mathematical three-dimensional CFB model, initially developed in 1989 with semi-empirical models for the concentration distributions of gas and solids components and temperatures, was modified to include the specific features under oxygen-enriched atmosphere (Krzywanski et al., 2010a,b). CFD modeling of gas–solid two-phase flow in CFB has already reached a high level (Wang et al., 2010; Lu et al., 2009) and oxygen-fired pulverized coal combustion is actively being investigated through CFD approaches (Murphy and Shaddix, 2006; Tan et al., 2006), while numerical modeling of reactive multiphase flows in oxy-fired CFB is still in an early stage (Hartge et al., 2009). Even in most of the experimental and simulation researches, O2 /CO2 atmospheres were considered for the oxygen-enriched combustion rather than O2 /RFG atmospheres.

∗ Corresponding author. Tel.: +86 25 83793453; fax: +86 25 83793453. E-mail address: [email protected] (C. Zhao). 1750-5836/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijggc.2011.08.006

In our previous study (Zhou et al., 2011), an Euler–Euler model based on the Kinetic Theory of Granular Flow (KTGF) was established to simulate the hydrodynamics of gas–solid flow in a CFB riser, coupled with heat transfer and chemical reaction sub-models. The model was validated by experimental data in air combustion in the 50 kW CFB at Southeast University, China. Detail descriptions of the experimental system can be found in the reference material (Duan et al., 2011). Using the 2-dimensional CFD model, this paper predicted the coal combustion characteristics in oxygen-fired CFB with wet flue gas recycled. The effects of combustion atmospheres, including air (21% O2 /79% N2 ) and oxygen/recycled flue gas (O2 /RFG) atmosphere with different O2 concentration (from 21% to 40%), were investigated on distributions of solid volume fraction, temperature and gas concentration in the furnace. 2. Model description Configuration and detailed dimensions of the CFB riser are shown in Fig. 1, and mesh refinement near the inlets is displayed in Fig. 2. According to the variation of cross-sectional dimensions of the riser, it is divided into three different sections, the lower zone, the middle zone and the upper zone, with heights of 0.8 m, 0.2 m and 3.2 m, respectively. The ultimate and proximate analyses of the coal tested are listed in Table 1.


W. Zhou et al. / International Journal of Greenhouse Gas Control 5 (2011) 1489–1497


Nomenclature C dp D g H h Ji K k km m p Ri R r Re S Sc Sh T t wc

v Yi

mole concentration (kmol m−3 ) particle diameter (m) diffusion coefficient (m2 s−1 ) gravity (m s−2 ) riser height (m) enthalpy (kJ kg−1 ) diffusion flux of species i (kg m−3 s−1 ) momentum exchange coefficient reaction rate constant (see Table 2) mixture thermal conductivity (w m−1 s−1 ) mass transfer rate (kg m−3 s−1 ) gas pressure (Pa) net production rate of species i (kg m−3 s−1 ) universal gas constant (J kmol−1 K−1 ) reaction rates (kmol m−3 s−1 ) Reynolds number heat source (J m−3 s−1 ) Schmidt number Sherwood number temperature (K) time (s) carbon molecular weight (kg kmol−1 ) velocity (m s−1 ) mass fraction of species i

Transition Zone

Secondary Air

Coal Inlet Dilute Zone

Dense Zone Recylce Inlet

Primary Air

Fig. 1. Configuration and detailed dimensions of the CFB riser.


Sec Air


Height / m

Subscripts m mixture g gas phase solid phase s c carbon Greek letters  density (kg/m3 ) thermal conductivity (w m−1 s−1 )  ε volume fraction  stress tensor (Pa)  viscosity (kg/m s)



0.75 Coal Inlet

0.7 2.1. Main assumptions


The Euler–Euler two phase flow model was used to simulate the oxy-coal combustion in the circulating fluidized bed. In order to decrease the impact of the strong nonlinear characteristic of the model and ensure the good convergence and acceptable computational time, the gas–solid hydrodynamic and coal combustion models are simplified as follows.





r/R Fig. 2. Mesh refinement near the inlets.

(3) Particles are assumed isothermal, inelastic, smooth and monodispersed spheres. (4) Small interaction forces such as lift force, thermophoretic force, Brownian force and virtual mass force are neglected. (5) Energy transfer due to pressure stress work and viscous dissipation are not considered. Since the Lewis number is nearly unity, the effect of enthalpy transport due to species diffusion is excluded for good computational convergence.

(1) The simulation case is assumed as two-dimensional with the furnace depth of 0.1 m. The widths of dilute and dense zones in the 2-D simulation case are determined based on the corresponding cross-section area in the 3-D riser. (2) O2 /N2 or O2 /RFG mixture flows enter the CFB riser via the bottom, secondary oxidant inlet, coal inlet or solid circulating inlet at uniform velocity. Gas density follows the incompressible ideal gas law. Table 1 Ultimate and proximate analyses of Xuzhou bituminous coal. Sample

Ultimate analysis/wt%

Xuzhou coal

Cad 58.97

Had 3.65

Oad 7.30

Nad 0.67

Sad 1.76


Proximate analysis/wt%

/MJ kg−1 23.54

FCad 47.33

Vad 25.02

Aad 25.55

Mad 2.10

W. Zhou et al. / International Journal of Greenhouse Gas Control 5 (2011) 1489–1497

2.2. Governing equations The conservation equations of mass and momentum are applied to each phase (gas and solid). The k–ε dispersed turbulence model is used. The gas turbulence is obtained using the standard k–ε model supplemented with extra terms that include the interphase turbulent momentum transfer and Tchen-theory correlations (Tchen, 1959) are used for the solid phase. The species conservation equation is used for each of species in gas and solid phases. The above equations for gas phase are shown as Eqs. (1)–(3). Considering the excellent performance of interphase heat transfer in the circulating fluidized bed and large amount of inert materials in the solid phase, the energy conservation equation as Eq. (4) is applied for the mixture enthalpy hm of two phases (Zhou et al., 2011; Wischnewski et al., 2010). As part of the comprehensive model, the complicated processes of chemical reactions are considered by setting the source terms of mass, momentum, energy and/or species transport equations when the reactants are consumed and the products are created. ∂ g) = m ˙ sg (εg g ) + ∇ · (εg g  ∂t


∂  g ) + ∇ · (εg g  g g) (εg g  ∂t s s −  g) + m ˙ sg  = −εg ∇ p + ∇ · g + εg g g + Ksg (


limited in literatures. Even for some processes, such as char combustion, it is nearly impossible to get the real experimental kinetic data of C–O2 reaction in O2 /CO2 or O2 /RFG atmospheres excluding the impacts of gasification by high concentration CO2 . To deal with this problem in our CFD simulation, we did our best to find proper and commonly used chemical kinetic data in air firing mode, and the main chemical reactions related with CO2 and H2 O are added, such as char gasification. That is to say, the effects of atmospheres on coal devolatilization and char combustion mainly reflect in char gasification. And for the sulfur retention, which is very interesting in CFB coal combustion system, direct desulfurization mechanism is considered as well as indirect desulfurization. Anyway, these data, especially the gas combustion kinetic data, should be further investigated and the results should be compared with real experiments. In most coal combustion modeling researches, moisture evaporation process was omitted or assumed to happen promptly once the coal was fed into the furnace, and volatiles were assumed to be released uniformly in the dense zone and combust immediately. It is assumed in this paper that the reaction rate constant of moisture evaporation is ten times as high as that of devolatilization (Zhou et al., 2011). Dry coal is consumed according to (R1) and reaction rate (Fu et al., 1989) is illustrated in Table 2. 1 kg drycoal → Yc kg char + Yv kg volatile + (1 − Yc − Yv ) kg ash


∂  g Yi ) = −∇ · Ji + Ri (εg g Yi ) + ∇ · (εg g  ∂t


∂  m hm ) = ∇ · (km ∇ Tm ) + Sm (m hm ) + ∇ · (m  ∂t


 h, k and T are the volume fraction, the density, the where ε, , , instantaneous velocity, the enthalpy, the thermal conductivity and the temperature, respectively. The subscript g, s and m stand for gas, ˙ sg characterizes the mass transfer solid and mixture, respectively. m from solid phase to gas phase. p is pressure shared by all phases, q is the stress–strain tensor. Ksg = Kgs is the interphase momentum exchange coefficient defined by EMMS-Matrix corrected Wen and Yu model (Lu et al., 2009; Wang and Li, 2007). Mixture den m and mixture thermal conductivity km sity m , mixture velocity    m =  , and are calculated according to m = ε   p εp p , m  p p p p

km = p εp kp , respectively. Sm characterizes the heat sources from chemical reactions. Yi , Ji and Ri are the mass fraction, mass diffusivity and mass source from reactions of species i. 2.3. Chemical reactions The solid phase consists of 7 species (dry coal, water, char, calcium carbonate CaCO3 , calcium oxide CaO, calcium sulfate CaSO4 and ash) and gas phase consists of 9 species (methane CH4 , oxygen O2 , carbon monoxide CO, carbon dioxide CO2 , water vapor H2 O, hydrogen H2 , tar, sulfur dioxide SO2 and nitrogen N2 ). In the present model, reactions related with nitrogen are not taken into account and nitrogen is considered passing directly to ash, but it is expected to be considered in the future work. Physical parameters of the mixtures obey the volume/mass-weighted-mixing law. The simulated processes of coal conversion in our CFB combustion system include particle drying, raw coal devolatilization, volatile combustion, char combustion, char gasification and gaseous pollutant (SO2 ) formation/reduction. Coal combustion process in CFB system is very complex and it’s a very big issue to execute deep analysis on all of the chemical reactions. As for chemical reactions in oxy-fired atmosphere, the kinetic data are more

(R1) The volatile matter consists of CH4 , CO, H2 O, CO2 , H2 and tar, whose fraction compositions are determined from the Loison & Chauvin model (Gungor and Eskin, 2008). The chemical formula of the tar is deduced based on the proximate and ultimate analyses of the coal tested. As homogeneous reactions ((R2)–(R5)), combustion of volatiles takes place once the volatiles release from the raw coal particles according to the reaction rates shown in Table 2 (DesrochesDucarne et al., 1998; Howard et al., 1973; Nemtsov and Zabaniotou, 2008; Heikkinen et al., 2008). The char is consumed according to heterogeneous combustion (R6) and gasification ((R7), (R8)) reactions. According to the reference Field (1970), the char combustion rate is described by both the chemical kinetic reaction rate and the diffusion rate of oxygen to the particle surface and internal pores, which is widely used in fluidized bed combustion (Gungor and Eskin, 2008; Adanez et al., 2001; Rajan and Wen, 1980). This is a shrinking coal model with mixed control for particles which lose their ashes. kc (m s−1 ) is the overall rate constant, k6 (kg m−2 s−1 kPa−1 ) is the apparent kinetic constant for surface reaction and kd (kg m−2 s−1 kPa−1 ) is the gas film diffusion rate constant (Rajan and Wen, 1980). Small changes are made to the formula in reference Gungor and Eskin (2008) to explain the reaction rate (R6) more exactly in CFD modeling. The formula 6(1 − ε)s Ychar /dp c means total surface area of char particles per unit volume. Explanations about other variables in Table 2 can be found in the nomenclature. Kinetic data of gasification rates are found in literature from Matsui et al. (Matsui et al., 1985, 1987a,b; Petersen and Werther, 2005). The sulfur in the coal is considered to be partitioned between the volatiles and char during coal devolatilization in accordance with the assumption that its concentration in the volatiles is identical to that in the dry, ash-free parent coal (Adanez et al., 2001; Uzun and Özdogan, 1998). With the combustion of tar and char, it is assumed that sulfur converts to SO2 . Both indirect ((R9), (R10)) and direct (R11) desulfurization mechanisms are considered for SO2 reduction. Reaction rate information can be found in the references Rajan and Wen (1980), Stanmore and Gilot (2005), Borgwardt and Bruce (1986), Borgwardt (1970), Snow et al. (1988) and expressions are listed in Table 2. The specific surface area SCaCO3 of CaCO3 in the


W. Zhou et al. / International Journal of Greenhouse Gas Control 5 (2011) 1489–1497

Table 2 Rates for the chemical reactions. Reaction rate r/kmol m−3 s−1

Reaction rate constant kc k1 = 4.136 × 104 (s−1 ) exp(− 0.73 × 108 /RT) (Fu et al., 1989)


r1 = k1 Crawc


0.7 r2 = k2 CO0.8 CCH


r3 = k3 CCO CH0.5O CO0.5

k3 = 1.3 × 1011 (m3 kmol−1 s−1 ) exp(− 1.255 × 108 /RT) (Howard et al., 1973)


r4 =

k4 = 1.03 × 1014 (m4.5 kmol−1.5 s−1 K1.5 )T−1.5 exp(− 0.284 × 108 /RT)a (Nemtsov and Zabaniotou, 2008)


k2 = 5.0122 × 1011 (m1.5 kmol−0.5 s−1 ) exp(− 2.0085 × 108 /RT)a (Desroches-Ducarne et al., 1998)




k4 CH1.5 CO2 2

k5 = 3.80 × 107 (m3 kmol−1 s−1 ) exp(− 0.555 × 108 /RT) (Heikkinen et al., 2008)


r5 = k5 Ctar CO2


r6 =

6εs s Ychar dp c

kc =

RT/wc b (1/kd )+(1/k6 )

k6 = 8910 (kg m−2 s−1 kPa−1 ) exp(− 1.4974 × 108 /RT) (Gungor and Eskin, 2008; Adanez et al., 2001; Rajan and Wen, 1980)

· kc CO2

kd = ShDg wc /dp RTg b , Sh = 2ε + 0.69(Re/ε) Re = udp g /g , Sc = g /g Dg , Dg = Dg0 · [T/T0 ]



Sc 1/3

[p0 /p]

Dg0 = 3.13 × 10−4 (m2 s−1 ), T0 = 1500 (K), p0 = 101325 (Pa) (R7)

r7 =

k7 CCO 2 C +K 7 C k CO2 CO2 k CO CO


k7 = 4.89 × 1010 (m3 kmol and Werther, 2005)

1+K 7


Kk7 CO = 66.0 (m3 kmol 2


r8 =

k8 CH O 2 C +K 8 C +K 8 C k H2 O H2 O k H2 H2 k CO CO

1+K 8


)Kk7 CO = 1.20 × 102 (m3 kmol −1

k8 = 2.39 × 105 (m3 kmol Matsui et al., 1985) Kk8 H



= 31.6 (m3 kmol


s c s−1 ) exp(−2.68 × 108 /RT ) MW (1 − X) (Matsui et al., 1987a,b; Petersen c


s c s−1 ) exp(−1.29 × 108 /RT ) MW (1 − X) (Petersen and Werther, 2005; c


) exp(−3.01 × 107 /RT ) Kk8 H = 5.36 (m3 kmol 2


Kk8 CO = 8.25 × 10−2 (m3 kmol [-6pt] (R9)

r9 = εs s Yc SCaCO3 k9

pe −pCO

) exp(−2.55 × 107 /RT ), X = 0.35

) exp(−5.98 × 107 /RT )

) exp(−9.61 × 107 /RT ), X = 0.5

k9 = 6.078 × 104 (kmol m−2 s−1 ) exp(− 2.05 × 108 /RT) (Stanmore and Gilot, 2005)



SCaCO3 = 1.26 (m2 g−1 ), pe = 4.192 × 1012 (Pa) exp(− 1.702 × 108 /RT) (R10)

k10 = 490 (m s−1 ) exp(− 0.175 × 108 /RT) (Rajan and Wen, 1980)

r10 = εs s YCaO sg k10 CSO2 a

sg =

−38.4Ts + 5.6 × 104 , cm2 g−1 Ts ≥ 1253 K 35.9Ts − 3.67 × 104 , cm2 g−1 Ts < 1253 K

a = exp (R11) a b c

−571 (C −1

r11 = εs s YCaCO3 sCaCO3 k11 CSO2




k11 = 0.72 (m s

(Borgwardt and Bruce, 1986; Borgwardt, 1970)

) exp(− 0.64 × 10 /RT) (Snow et al., 1988) 8


Unit of molar concentration C in this paper is kmol m and this leads to different coefficients with the references. Unit of universal gas component R in these two formulas is kJ kmol−1 K−1 , while else are J kmol−1 K−1 . Unit of all the activity energy is J kmol−1 .

tested coal was set as 1.26 m2 /g according to our previous research (Chen et al., 2007). CH4 + 3/2O2 → CO + 2H2 O


CO + 1/2O2 → CO2


H2 + 1/2O2 → H2 O


CH2.652 O0.015 S0.015 + 1.597O2 →CO2 + 1.326H2 O + 0.015SO2 (R5) C + 1/ O2 → (2 − 2/ )CO + (2/ − 1)CO2


C + CO2 → 2CO


C + H2 O → CO + H2


CaCO3 → CaO + CO2


CaO + SO2 + 1/2O2 → CaSO4


CaCO3 + SO2 + 1/2O2 → CaSo4 + CO2


2.4. Numerical considerations Parts of the initial and boundary conditions used in the simulation are listed in Table 3. The bed is initially filled with ash particles with static height of 0.4 m, where the volume fraction of the solids is 0.55. The maximum particle packing is limited to 0.63. The no-slip wall condition is used for the gas phase and partial-slip boundary condition for solid phase. The specific heat capacity of each gas

species is calculated as a piecewise-linear function of temperature and viscosity as a power law. It is assumed in the modeling that no chemical reactions occur in the return leg. So the mass fractions of the solid species remain the same from the furnace outlet to the solid recycle inlet. To maintain constant bed inventory, mass flow rate of the recycled solids is adapted at real time during the simulation based on the principle that the mass flow rates entering the furnace and leaving it are equal. Based on the heat and mass balance of experimental operation condition for our test facility, about 50% of the thermal input is absorbed by flue gas and bed materials. Rest is released along the furnace and the return leg. Since the lower and middle zones of our facility are nearly adiabatic, it is assumed that heat losses through walls of the upper zone and the return leg are 20% and 30%, respectively. As the thermal boundary conditions, heat extraction from the upper zone is modeled with an averaged heat flux through the walls and heat extraction from the return leg is modeled by setting the temperature value of the recycled solid. The thermal boundary condition is expected to be improved with more flexibility in further researches. For O2 /RFG cases, fuel feeding rate and O2 supply rates are maintained the same as those in air combustion. The volume fraction of O2 in O2 /RFG oxidant is set to change from 21% to 40%. And fractions of other oxidant species are set by calculating the flue gas compositions in real time with user-defined function codes compiled to the FLUENT software.

W. Zhou et al. / International Journal of Greenhouse Gas Control 5 (2011) 1489–1497


Table 3 Primary parameters used in simulations. Parameters




Real density of char particles Real density of ash particles Specific heat capacity of ash Particle diameter P–P restitution coefficient Specularity coefficient Inlet coal temperature Initial temperature O2 /N2 volume fraction

1280 kg/m3 2400 kg/m3 856 J/kg K 0.35 mm 0.9 0.001 298 K 1123 K 21%/79%

Coal feed rate Excess O2 coefficient Primary oxidant ratio Primary oxidant temperature Secondary oxidant temperature Heat loss through upper zone wall Wall of the lower and middle zones

8 kg/h 1.2 0.7 400 K 298 K 20% Adiabatic 21%/79% 30%/70% 40%/60%


Outlet O2 concentration



6 4


2 0 0

indicates that the time averaged variables computed from 30 s to 50 s are representative in describing the primary characteristics for the simulated cases.

30 Outlet mass flux

Outlet O2 concentraion / %

Outlet mass flux / kgm s

-2 -1


O2 /RFG volume fraction

3.1. Hydrodynamics profiles

0 10





Time / s Fig. 3. Time series of monitored variables through the outlet during simulation of air combustion.

A 5 mm × 5 mm grid is applied in the dense and transition zones and an 8 mm × 15 mm in the dilute zone. Mesh refinements near the inlets are applied with the total mesh number of nearly 8500. The time step is set as 1 × 10−4 s. For the first 10 s, gas-ash fluidization is simulated at temperature of 1123 K without coal feeding, and then coal is continuously fed into the furnace. Flue gas recycling is executed from the fifteenth second. The simulation is conducted for 50 s and time-averaged distributions of flow and combustion characteristic variables are computed for the period from 30 s to 50 s. 3. Results and discussion Fig. 3 shows the time series of mixture mass flux through the furnace outlet and O2 concentration in the flue gas from the outlet during simulation of air combustion. They are monitored for the determination of steady-state coal combustion processes. The same monitored variables are applied when dealing with other cases. It


The simulated axial distributions of both time and cross-section averaged solid volume fraction εs with different drag models and under different atmospheres during bituminous combustion are displayed in Fig. 4. Fig. 4(a) shows the hydrodynamic comparison between EMMS/Matrix corrected Wen and Yu drag model and Gidaspow model. The previous one shows better agreement with experimental data. The smaller solid volume fraction values than experiments obtained at the riser bottom are owing to the assumption of one averaged solid diameter in the modeling rather than wide particle size distribution. Solid particles with different diameters may have very different hydrodynamic characteristics. However, one more solid phase means one more set of conservation equations and large computing time. Researchers are also trying hard to deal with the momentum transfer between different solid phases. Anyway, this problem is a top priority in our further study once we have enough computer capability. With EMMS/Matrix correction, for 21% O2 /79% N2 , 21% O2 /79% RFG, 30% O2 /70% RFG and 40% O2 /60% RFG cases, the outlet mass fluxes are calculated statistically as 8.3 kg/m2 s, 9.3 kg/m2 s, 1.3 kg/m2 s and 0.6 kg/m2 s, respectively. Based on an approximate averaged lower zone combustion temperature in each real case, the superficial fluidization velocity Us is calculated as 4.013 m/s, 3.836 m/s, 2.884 m/s and 2.237 m/s, respectively. Besides the numerical error, the inconsistent between Us and outlet mass flux for 21% O2 /79% N2 and 21% O2 /79% RFG cases may due to the same EMMS/Matrix correction, which is expected to be different for different cases (different velocities and gas densities). Their impacts on drag model corrections and relationships with the outlet mass fluxes should be paid more attention to in further investigation.

b 4


O2/RFG EMMS/Matrix


Height / m


Height / m


Experamental data




21% O2 Us=3.836m/s 30% O2 Us=2.884m/s


40% O2 Us=2.237m/s

Gidaspow 1

0 0.0









0 0.0








Fig. 4. Comparison of simulated axial distribution of both time and cross-section averaged solid volume fraction εs with (a) different drag models in air and (b) different O2 concentrations in O2 /RFG mixture.


W. Zhou et al. / International Journal of Greenhouse Gas Control 5 (2011) 1489–1497


1250 1200


Mass fraction

Temperature / K

1150 1100 21% O2/79% N2


21% O2/79% N2 Expt.

O2 30%O2/70%RFG Coal: 8kg/h CO2 Excess O2 coefficient: 1.2 H2O Primary oxidant ratio: 0.7



21% O2/79% RFG


0.2 30% O2/70% RFG


40% O2/60% RFG





0.0 3






Height / m

Height / m Fig. 5. Simulated axial distributions of cross-section averaged mixture temperature at the moment of 50 s in different atmospheres.


Fig. 6. Simulated axial distributions of both time and cross-section averaged mass fractions of main gas components in 30% O2 /70% RFG atmosphere.

3.2. Temperature profiles The simulated axial distributions of cross-section averaged mixture temperature at the moment of 50 s in different atmospheres are displayed in Fig. 5. Experimental data in air atmosphere are used to validate the model and a good agreement is achieved. For the same O2 concentration, coal combustion under 21% O2 /79% RFG atmosphere yields the similar temperature trend to that in 21% O2 /79% N2 with about 50 K deficit. The reason is that water vapor and CO2 have larger molar heat capacity than that of N2 . When firing in 30% O2 /70% RFG mixture, the bed temperature is

Volume fraction in FG


For the same mass flow rate of O2 supply, less flue gas is recycled to meet higher O2 volume fraction in O2 /RFG cases. As shown in Fig. 4(b), the decreased volume of total gas flow decreases the solid concentration in the upper dilute zone. The differences of hydrodynamic characteristics also have significant effect on temperature distributions.



Simulation O2





H2O 0.2

Coal: 8kg/h Excess O2 coefficient: 1.2

0.0 79%N2




Atmosphere (with O2 for balance) Fig. 7. Comparison of main components in the flue gas among different atmospheres during bituminous combustion.

Fig. 8. Contours of char gasification rate by CO2 (R7) and H2 O (R8) at the 50 s (kmol/m3 ).

W. Zhou et al. / International Journal of Greenhouse Gas Control 5 (2011) 1489–1497


Fig. 9. Contours of CO molar fraction and CO combustion rate (kmol/m3 ) at the 50 s.

slightly higher than that in air because of higher O2 concentration. But temperature along the dilute zone decreases more obviously. That can be explained by the given certain amount of heat loss through the furnace wall in the upper dilute zone. Larger temperature drop is yielded in lower volume gas flow because of lower particle concentration in the upper zone. A dramatic temperature rise followed with drop at the elevation of about 2 m in 40% O2 /60% RFG is observed, caused by over-fire volatile combustion and constant heat flux boundary condition, respectively. It indicates that the size distribution of bed materials plays an important role in controlling the furnace temperature. To avoid the phenomenon in 40% O2 /60% RFG mixture, reasonable finer bed materials with wide-size-distribution are required. 3.3. Composition profiles Fig. 6 exhibits the simulated axial distributions of both time and cross-section averaged mass fractions of main gas components when firing in 30% O2 /70% RFG mixture. A large amount of O2 is consumed in the dense zone and an increase in O2 concentration is observed near the secondary oxidant injection location, while CO2 concentration shows the opposite trend. The distributions in other

Coal: 8 kg/h Primary oxidant raio: 0.7 21% O2 / 79% N2



SO 2 in flue gas/ mg Nm


SO2 in flue gas/ mg Nm




cases behave in a similar way. The main components in the flue gas include CO2 and H2 O, with CO2 volume fraction of about 70% on wet basis and more than 25% H2 O, as shown in Fig. 7. About 95% volume fraction of CO2 can be achieved when calculating on dry basis. As the inlet O2 concentration increases from 21% to 40%, outlet O2 concentration increases accordingly in principle, meanwhile more O2 is consumed because of improved combustion efficiency, thus a minimum outlet O2 concentration appears in the atmosphere near the 30% O2 /70% RFG. Fig. 8 shows the contours of char gasification rate by CO2 and H2 O at the 50 s in 21% O2 /79% N2 and 30% O2 /70% RFG atmospheres. It is very obvious that these two gasification reactions perform more dramatically in O2 /RFG atmosphere, especially in the dense zone. That is attributed to high solid volume fraction and high concentrations of CO2 and H2 O. Char–H2 O gasification rate in air case displays low conversion rate in the lower zone because H2 O is not formed in that region. H2 O in air combustion is formed from water evaporation, dry coal devolatilization and gas combustion, which all happen near or above the coal feeding point. Feeding coal from the left side of the furnace leads to asymmetries, which is more apparent in

Coal: 8 kg/h


Primary oxidant raio: 0.7 Ca/S: 2.5 2000


1000 0 0

79%N 2 No self-des


Ca/S: 2.5

Fig. 10. Comparison of SO2 emission considering different sulfur retention cases.




Atmosphere (with O2 for balance) Fig. 11. Comparison of SO2 emission under different atmospheres.


W. Zhou et al. / International Journal of Greenhouse Gas Control 5 (2011) 1489–1497

Fig. 12. Contours of transient molar concentrations of solid component and desulfuration rate in 30% O2 /70% RFG.

this figure. There is no such apparent phenomenon in O2 /RFG cases because H2 O are fed into the furnace uniformly from the inlets with RFG. The results also show that the gasification rate by H2 O is a little larger than that by CO2 , which is consistent with the results from other literature (Roberts and Harris, 2007). Fig. 9 illustrates the predicted contours of CO molar fraction and CO combustion rate at the 50 s in 21% O2 /79% N2 and 30% O2 /70% RFG cases. It is observed that the CO molar fraction in the dense zone in O2 /RFG atmosphere is lower than that of air because high concentration of H2 O accelerates the CO combustion, according to the reaction rate (R3) in Table 2. And the CO mass fraction for O2 /RFG atmosphere increases along the furnace height in the dense zone because of the CO formation from gasification reactions.

3.4. Pollutant emissions For the sulfur retention model, self-desulfurization mechanism of coal ash was considered by specifying the mass fraction of CaCO3 in the coal ash, which was obtained by XRF analysis. Limestone was also added to further reduce SO2 emission. Fig. 10 illustrates the SO2 emissions under air combustion with/without considering selfdesulfurization ability of ash in coal and with adding limestone. The simulated desulfurization efficiency under air achieves 87%, which is in a good agreement with experimental value, 81%. Fig. 11 illustrates the SO2 emissions under different atmospheres considering coal ash self-desulfurization with adding limestone. SO2 is enriched as O2 inlet concentration increases under O2 /RFG atmosphere. That is owing to the recycled SO2 in the RFG and the reduced total flue gas. Fig. 12 shows contours of transient molar concentration of solid components (CaO and CaCO3 ) and reaction rates (indirect desulfurization (R10) and direct desulfurization (R11)) in 30% O2 /70% RFG combustion. Concentrations of CaO and CaCO3 in the solid are relatively uniform in the furnace and both of the desulfurization rates depend more on the solid volume fractions. The average molar concentrations of CaO and CaCO3 in the riser are 0.017 and 0.011 kmol/m3 , respectively. The average reaction rates of (R10) and (R11) are 7.9E−6 kmol/m3 s and 5.3E−7 kmol/m3 s, respectively. That reveals 0.6 h and 6 h of full sulfation for the corresponding desulfurizer, which are consistent with the results reported by Borgwardt and Bruce (1986).

4. Conclusions A comprehensive coal combustion model based on Euler–Euler model was applied to simulate the oxygen-coal combustion process and sulfur emission in a circulating fluidized bed. Simulation results in 21% O2 /79% N2 atmosphere were satisfactorily validated by the experimental data. The flow behavior is obviously affected by the flue gas recycle ratio. With the same coal feeding rate and excess oxidant ratio, higher O2 inlet concentration with smaller recycle ratio leads to lower volume gas flow and more dilute particle concentration in the upper zone. A certain high O2 inlet concentration will cause a large drop in solids circulation rate and a dramatic increase in dilute zone temperature. To avoid this phenomenon, reasonable finer bed materials with wide-size-distribution are required. Temperature prediction results in this study also indicate that the one conservation equation of mixture enthalpy is reliable for predicting the heat transfer characteristics in the CFB combustor. When firing the Xuzhou bituminous coal in O2 /RFG mode with O2 concentration from 21% to 40%, 70% volume fraction of CO2 can be achieved with more than 25% H2 O volume fraction in the flue gas. With increase in O2 inlet concentration, gaseous pollutant SO2 is enriched. Indirect and direct desulfurization models show consistent results with literature in sulfur retention. This study helped understanding oxy-coal combustion mechanism and established a good foundation for further researches in oxy-coal combustion in CFB. Acknowledgements Financial supports of this work by the National Key Program of Basic Research of China (2006CB705806 and 2011CB707301) and State Key Laboratory of Coal Combustion (FSKLCC1006) are gratefully acknowledged. References Adanez, J., Gayán, P., Grasa, G., de Diego, L.F., Armesto, L., Cabanillas, A., 2001. Circulating fluidized bed combustion in the turbulent regime: modelling of carbon combustion efficiency and sulphur retention. Fuel 80, 1405–1414. Borgwardt, R.H., 1970. Kinetics of reaction of SO2 with calcined limestone. Environ. Sci. Technol. 4, 5.

W. Zhou et al. / International Journal of Greenhouse Gas Control 5 (2011) 1489–1497 Borgwardt, R.H., Bruce, K.R., 1986. Effect of specific surface area on the reactivity of CaO with SO2 . AlChE J. 32, 239–246. Chen, C., Zhao, C., Liang, C., Pang, K., 2007. Calcination and sintering characteristics of limestone under O2 /CO2 combustion atmosphere. Fuel Process. Technol. 88, 171–178. Desroches-Ducarne, E., Dolignier, J.C., Marty, E., Martin, G., Delfosse, L., 1998. Modelling of gaseous pollutants emissions in circulating fluidized bed combustion of municipal refuse. Fuel 77, 1399–1410. Duan, L., Zhao, C., Zhou, W., Qu, C., Chen, X., 2011. O2 /CO2 coal combustion characteristics in a 50 kWth circulating fluidized bed. In: Int. J. Greenh. Gas Con. Field, M.A., 1970. Measurements of effect of rank on combustion rates of pulverized coal. Combust. Flame 14, 237–248. Fu, W., Zhang, Y., Han, H., Wang, D., 1989. A general model of pulverized coal devolatilization. Fuel 68, 505–510. Gungor, A., Eskin, N., 2008. Two-dimensional coal combustion modeling of CFB. Int. J. Therm. Sci. 47, 157–174. Hartge, E.-U., Ratschow, L., Wischnewski, R., Werther, J., 2009. CFD-simulation of a circulating fluidized bed riser. Particuology 7, 283–296. Heikkinen, J.M., Venneker, B.C.H., di Nola, G., de Jong, W., Spliethoff, H., 2008. CFD simulation and experimental validation of co-combustion of chicken litter and MBM with pulverized coal in a flow reactor. Fuel Process. Technol. 89, 874–889. Howard, J.B., Williams, G.C., Fine, D.H., 1973. Kinetics of carbon monoxide oxidation in postflame gases. In: Symposium (International) on Combustion, vol. 14, pp. 975–986. Krzywanski, J., Czakiert, T., Muskala, W., Sekret, R., Nowak, W., 2010a. Modeling of solid fuels combustion in oxygen-enriched atmosphere in circulating fluidized bed boiler. Part 1. The mathematical model of fuel combustion in oxygenenriched CFB environment. Fuel Process. Technol. 91, 290–295. Krzywanski, J., Czakiert, T., Muskala, W., Sekret, R., Nowak, W., 2010b. Modeling of solid fuel combustion in oxygen-enriched atmosphere in circulating fluidized bed boiler. Part 2. Numerical simulations of heat transfer and gaseous pollutant emissions associated with coal combustion in O2 /CO2 and O2 /N2 atmospheres enriched with oxygen under circulating fluidized bed conditions. Fuel Process. Technol. 91, 364–368. Lu, B., Wang, W., Li, J., 2009. Searching for a mesh-independent sub-grid model for CFD simulation of gas–solid riser flows. Chem. Eng. Sci. 64, 3437–3447. Matsui, I., Kunii, D., Furusawa, T., 1985. Study of fluidized bed steam gasification of char by thermo gravimetrically obtained kinetics. J. Chem. Eng. Jpn. 18, 105–113. Matsui, I., Kunii, D., Furusawa, T., 1987a. Study of char gasification by carbon dioxide. 1. Kinetic study by thermogravimetric analysis. Ind. Eng. Chem. Res. 26, 91–95.


Matsui, I., Kojima, T., Kunii, D., Furusawa, T., 1987b. Study of char gasification by carbon dioxide. 2. Continuous gasification in fluidized bed. Ind. Eng. Chem. Res. 26, 95–100. Murphy, J.J., Shaddix, C.R., 2006. Combustion kinetics of coal chars in oxygenenriched environments. Combust. Flame 144, 710–729. Nemtsov, D.A., Zabaniotou, A., 2008. Mathematical modelling and simulation approaches of agricultural residues air gasification in a bubbling fluidized bed reactor. Chem. Eng. J. 143, 10–31. Petersen, I., Werther, J., 2005. Experimental investigation and modeling of gasification of sewage sludge in the circulating fluidized bed. Chem. Eng. Process. 44, 717–736. Rajan, R.R., Wen, C.Y., 1980. A comprehensive model for fluidized bed coal combustors. AlChE J. 26, 642–655. Roberts, D.G., Harris, D.J., 2007. Char gasification in mixtures of CO2 and H2 O: competition and inhibition. Fuel 86, 2672–2678. Snow, M.J.H., Longwell, J.P., Sarofim, A.F., 1988. Direct sulfation of calciumcarbonate. Ind. Eng. Chem. Res. 27, 268–273. Stanmore, B.R., Gilot, P., 2005. Review—calcination and carbonation of limestone during thermal cycling for CO2 sequestration. Fuel Process. Technol. 86, 1707–1743. Tan, Y., Croiset, E., Douglas, M.A., Thambimuthu, K.V., 2006. Combustion characteristics of coal in a mixture of oxygen and recycled flue gas. Fuel 85, 507–512. Tchen, C.M., 1959. Diffusion of particles in turbulent flow. In: Landsberg, H.E., Mieghem, J.V. (Eds.), Advances in Geophysics. Elsevier, pp. 165–174. Uzun, D., Özdogan, S., 1998. Correlations for the sulfur contents of Turkish coals exposed to ashing and devolatilization conditions at 750 ◦ C and 950 ◦ C. Fuel 77, 1599–1604. Wall, T.F., 2007. Combustion processes for carbon capture. Proc. Combust. Inst. 31, 31–47. Wang, W., Li, J., 2007. Simulation of gas–solid two-phase flow by a multi-scale CFD approach—extension of the EMMS model to the sub-grid level. Chem. Eng. Sci. 62, 208–231. Wang, W., Lu, B., Zhang, N., Shi, Z., Li, J., 2010. A review of multiscale CFD for gas–solid CFB modeling. Int. J. Multiphase Flow 36, 109–118. Wischnewski, R., Ratschow, L., Hartge, E.-U., Werther, J., 2010. Reactive gas–solids flows in large volumes—3D modeling of industrial circulating fluidized bed combustors. Particuology 8, 67–77. Zhou, W., Zhao, C.S., Duan, L.B., Qu, C.R., Chen, X.P., 2011. Two-dimensional computational fluid dynamics simulation of coal combustion in a circulating fluidized bed combustor. Chem. Eng. J. 166, 306–314.