Coal combustion and gasification process. 

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... was primarily applied to burner and furnace design, utility boilers, waste inciner- ation, pulverized coal combustion, and it was expanded to simulate the coal gasifiers[4]. Over the last few decades, attempts have been made by scholars to simulate coal gasification and combustion. Several papers are devoted to the study of a detailed CFD analysis of coal gasification and combustion using general CFD codes[1-8]. Hill and Smoot developed a three dimensional coal combustion CFD model, PCGC-3[2]. This code was assumed to equilibrium gas phase chemistry. The turbulent flow field with the chemical reactions was coupled by integrating the equations over a probability density function. Fletcher et al. proposed a CFD analysis model to simulate the flow and reaction in an entrained flow biomass gasifier[5]. The model was based on the commercial code, CFX R . Biomass particulate was modeled by a Lagrangian approach as it enters the gasifier and releases its volatiles. Finally, the particulate undergoes gasification and combustion. The model provided information about the gas composition and temperature at the outlet. Models of finite rate chemistry in the gas phase and char reactions were added to the standard model. Watanabe and Otaka presented modeling of coal gasification reaction using CFX R via a user Fortran interface[6]. This model was used to predict the gasification performance of the entrained flow coal gasifier. This model was composed of pyrolysis, char gasification, and gas phase reaction models. The distribution of the gas temperature and composition was presented. Syred et al. indicated the development of fragmentation model for solid fuel combustion and gasification as subroutines for inclusion in CFD codes, FLUENT R [7]. They recognized that FLUENT R was well developed and had well proven routines for Lagrangian tracking of burning particles through complex flow fields. The difficulties of incorporating models of fragmentation in CFD codes were discussed. Kang et al. constructed a numerical model of the entrained flow coal gasifier to simulate the coal gasification process using FLUENT R [8]. The complicated processes were classified into simplified stages of slurry evaporation, coal devolatilization, and chemical reactions coupled with turbulent flow and heat transfer. Most of the simulation cases are the comprehensive models. Namely, fluid flow, heat and mass transfer, turbulence, chemical reaction, and reactor geometry are considered for simulation. As a result, thermal and hydrodynamic characteristics are obtained with discretized control volumes in the system. Since coal combustion and gasification take place simultaneously, multiphase modeling should be considered. A discrete phase is modeled by defining the initial position, velocity, size, and temperature of indi- vidual particles. These initial conditions are used to initiate a trajectory. The calculations are based on the force balance on the particle and on the convective and radiative heat transfer, and mass transfer from the particle, using the local continuous phase conditions as the particle moves through the flow. The Lagrange discrete phase model follows the Euler-Lagrange approach. The fluid phase is treated as a continuum to solve the time averaged Navier-Stokes equations, while the dispersed phase is solved by tracking a number of particles through the calculated flow field. The dispersed phase can exchange mass, momentum, and energy with the fluid phase[9]. Fig. 1 shows the transfer relation between discrete and continue phases. In the Euler-Euler approach, since the volume of a phase cannot be occupied by the other phases, the con- cept of phasic volume fraction is introduced. These volume fractions are assumed to be continuous functions of space and time. Their sum should be equal to one. Conservation equations for each phase are de- rived to obtain a set of equations, which have similar structure for all phases. These equations are closed by providing constitutive relations that are obtained from empirical information or by application of kinetic theory[9]. In conclusion, the governing equations between the solid phase and the fluid phase are coupled and the equations are solved with consideration of the fluid mechanics. As a result, the fluid fields and the particle trajectory are obtained at given initial and boundary conditions. When a coal particle is heated to a temperature higher than 700-1000K the coal devolatilizes. The coal particle is divided into a solid residue, the char, and volatiles. Then, homogeneous reactions of volatiles and heterogeneous reactions of char take place. In general purpose CFD code, modeling coal combustion is generally categorized into non-premixed combustion model. When coal is the only fuel in the system, coal can be modeled by two mixture fractions. One stream is used to represent the char and the other stream is used to represent volatiles. This process can be simply described as shown in Fig. 2. Devolatilization. A complete devolatilization model would describe the composition and physical state of the coal or char particle at all stages of devolatilization. However, it is impossible to predict it with numerical simulation because operating conditions have an effect on the quantity of volatiles. The composition and yield of the volatiles released are dependent on the heating rate and the temperature[5]. The yield of volatiles is significantly in excess of the value given by the proximate analysis. Volatile matter is composed of many complex substances. The composition of the volatiles is not arbitrary as it depends on the composition of the original coal. In FLUENT R , coal is simply classified into anthr- acite, coal-lv, coal-mv, coal-hv, lignite, and peat as the quantities of the fixed carbon and volatile matter. Table 1 shows the coal type and default properties provided by FLUENT R [9]. The devolatilization model is applied to a combusting particle when the temperature of the particle rea- ches the vaporization temperature and remains in effect while the mass of the particle exceeds the mass of the non-volatiles in the particle[9]. Composition of the volatiles is represented as C X H Y O Z , such as mixture of the fuel. The volatile stream composition is defined by selecting appropriate species and setting their mole or mass fraction. There are some models to describe the devolatilization in FLUENT R . They are the constant rate model, the single kinetic rate model, and the two competing rates model (Kobayashi model). Table 2 shows the devolatilization models and rate constants provided by FLUENT R [9]. If the kinetics is considered, a rate constant k should be used to model the phenomena. The reaction rate constants are defined by input of an Arrhenius type of equation (1), such as pre-exponential factor, A , and activation energy, E ...