MODELING AND SIMULATION OF COAL GASIFICATION IN A CIRCULATING FLUIDIZED BED REACTOR

Abstract

The present investigation pertains to modeling and simulation of coal gasification in a circulating fluidized bed reactor. It basically deals with the development of mathematical model based on the principles of equilibrium and computational fluid dynamics (CFD). The equilibrium model is on Gibbs free energy minimization approach and 373 gas and 119 solid species are considered. The CFD model includes the hydrodynamics, heat and mass transfer as well as eight heterogeneous and eight homogeneous reactions. The hydrodynamics is described by Euler granular multiphase model, which includes kinetic theory of granular flow (KTGF) for constitutive relations of the dispersed solid phase. The effect of operating variables, namely, temperature, pressure, equivalence ratio (ER) and steam/coal (S/C) ratio on the performance of the gasifier in terms of hydrodynamic characteristics as well as distribution of temperature and syngas species are studied through both equilibrium and CFD models. The optimum values of the operating variables, viz., temperature, pressure, ER and S/C ratio for poly-generation i.e. electricity production and Fischer-Tropsch (FT) synthesis are suggested for an Indian coal by the equilibrium model. The preliminary study by a non-stoichiometric equilibrium model based on Gibbs free energy minimization method is performed to analyze the effect of process parameters, namely, temperature, pressure, ER and S/C ratio on gasification of high-ash Indian coal in terms of equilibrium composition of gas and solid species as well as performance parameters such as cold gas efficiency (CGE), carbon conversion (X), heat demand (HD), syngas yield (SGY) and H2/CO ratio. Further, optimum operating conditions for electricity generation and FT synthesis is found. The equilibrium model incorporates most of the ash elements to understand the ash chemistry during gasification and its effect on the performance of the gasifier. Therefore, the model considers 16 elements distributed among 373 gas and 119 solid species. The model is solved using MATLAB 2015. Temperature is the most important parameters but for temperatures greater than 1100 K, the effect of temperature is found to be small. While increase in pressure brings operational benefits, effect of pressure on the gasification performance is very small. In general, with increase in ER, oxidative species start to dominate and consequently, cold gas efficiency decreases while carbon conversion increases. Cold gas efficiency increases with iii increase in S/C ratio. Optimization results show that at T = 900 K, P = 2 atm, ER = 0.2 and S/C = 1.0, CGE = 81.43 %, HD = - 4.21 MJ/kg coal, SGY = 3.79 Litre/kg coal and H2/CO = 2.09 are achieved which fulfill the conditions for poly-generation i.e. electricity generation and FT synthesis. A 3D full-loop hydrodynamic CFD simulation is performed to study the hydrodynamics characteristics and to find out the most suitable drag model. Six drag models (EMMS, Gibilaro, Gidaspow, Schiller-Naumann, Shyamlal-O’Brien and Wen-Yu) were employed and simulated results of time-averaged solid volume fraction, solid axial velocity and static pressure is compared with experimental data, both qualitatively and quantitatively. It was concluded that EMMS model is the most suitable model since it captures the heterogeneity of the CFB and predicts results more accurately than other models. Thereafter, the CFD model, including eight heterogeneous and eight homogeneous reactions, is applied to a bubbling fluidized bed gasifier (BFBG) operating with high ash Indian coal. The simulation results were observed to agree well with experimental data with absolute normalized error within 25.9%, except for concentration of CH4 due to its low experimental values. The deviation in bed temperature was less than 5.7%. The same CFD model is then applied to a CFBG and a good agreement with experimental data was observed with absolute normalized error less than 20.9%, except for concentration of CH4. The maximum deviation in temperature was found to be 8.8%. The characteristic features of the CFB such as core-annular flow, uniform temperature etc. are predicted well by the simulation. Therefore, the model deemed to be suitable for predictions of both BFBG and CFBG. Further, a comparison between BFBG and CFBG based on the developed and validated model is presented. Keeping process parameters i.e. feed temperature and operating pressure as well as Air/Coal (A/C) and Steam/Coal (S/C) ratios constant, the velocity of the feed (Air-Steam) is increased so as to get into fast fluidization from bubbling regime. It was observed that the syngas exiting from the CFBG is richer in CO and H2 than that from BFBG. The concentration of CO2, CH4 and tar decrease as a result of the regime change. In addition, the temperature and pressure in the case of CFBG was found to be significantly higher than that in the BFBG. The syngas composition at the outlet of CFBG is observed to be closer to the equilibrium composition. A root mean square (RMS) error between the syngas composition predicted by CFD and equilibrium models was found approximately 10% and 5% for the BFBG and CFBG, respectively. iv Finally, effect of operating variables, namely, temperature, pressure, Air/Coal (A/C) ratio and Steam/Coal (S/C) ratio on the performance of the CFBG in terms of its hydrodynamic characteristics as well as distribution of temperature and concentration of syngas species is studied using the 3D full-loop CFD simulation. Effect of temperature on the hydrodynamics is found to be small. The concentration of CO and H2 increase whereas that of CO2 and H2O decrease with increase in temperature. While effect of pressure on outlet species mole fraction is negligible, gas and solid axial velocity decrease with increase in pressure. With increasing A/C ratio or decreasing S/C ratio, combustion products (CO2 and H2O) increase and gasification products (CO and H2) decrease due to increase in O2 concentration. In addition, temperature increases with increase in A/C ratio or decrease in S/C ratio. The concentration of CH4 decreases in all the cases as it is being consumed in gasification as well as combustion reactions. Thus, it was concluded that the CFBG is the most suitable for gasification of low rank, high ash fuels such as Indian coals and further studies should be undertaken in this area.