The kinetic parameters of heterogeneous catalytic and/or non-catalytic char-gas reactions (C-$H_2O$, C-$CO_2$ and C-$O_2$) are determined using a thermobalance. The effects of key experimental variables such as coal rank, particle size, temperature, heating rate, reactant gas concentration, catalyst type and the amount of catalyst loading on these rate processes are investigated.
The generalized equilibrium prediction model is developed to calculate the equilibrium composition of complex gasification products with the variation of temperature, pressure and feed condition. The cyclone gasifier model is developed to estimate the product gas composition and the carbon conversion with the variation of operating conditions. Basic calculated values of concentration and conversion of the char-$H_2O$-$O_2$ reaction in a cyclone gasifier are provided with the variation of operating conditions such as temperature and mole ratio of the reactant materials, with which to evaluate its potential application to industrial needs.
Reaction kinetics of char-$CO_2$ gasification at atmospheric pressure are investigated in a 5.5 cm-ID thermobalance reactor using four ranks of coals. Effects of coal ranks (lignite-semianthracite), particle size (0.18-1.00 mm), reaction temperature (700-900℃), and concentration (30-100 %) of the reactant gas ($CO_2$) on the conversion rate of char gasification are determined. The gasification reaction order and the reaction rate constant are determined from the conversion data, and the activation energy of the reaction is determined from the Arrhenius plot. Chemical reactivity of the char is correlated with the carbon content of the parent coals, particle size, partial pressure of $CO_2$ and reaction temperature. The experimental conversion data are well represented by the unreacted shrinking core model in which chemical reaction is the rate controlling step.
Three different catalysts ($K_2CO_3$, $Na_2CO_3$, $Li_2CO_3$) and its mixture [$(Na,K)_2CO_3$] were impregnated into the chars in order to determine the effects of catalyst type and amount of the catalyst loading (1 - 5 wt %) at different reaction temperatures (700-800℃) on the rate of steam gasification in a 5.5 cm-ID thermobalance reactor with larger samples up to 1.0 g. The reaction orders with respect to carbon are found to be 2/3 in the case of non-catalytic and $Li_2CO_3$-catalytic reactions. Whereas, the reaction with $Li_2CO_3$ catalyst reveals a zeroth reaction order. In the $Na_2CO_3$ and mixed $(Na,K)_2CO_3$ catalytic steam gasification reactions, the reaction order is found to be zero and the reaction rate constant can not be described as an unique value. The reaction rate increase with the reaction temperature and the amount of catalyst loading. Under the same experimental conditions with 3 wt % catalyst loading, the catalytic activities are found to be ranked as $Na_2CO_3>$ mixed $(Na,K)_2CO_3>K_2CO_3>K_2CO_3>Li_2CO_3$. Reactivity enhancement factors for the mixed $(Na,K)_2CO_3$ catalysts are found to be the arithmetic mean values of the both $Na_2CO_3$ and $K_2CO_3$ catalysts. Activation energies and frequency factors of the catalytic reactions are smaller than those of the comparable non-catalytic steam gasification reactions which may give an evidence of the compensation effect in the reaction.
Kinetic parameters such as activation energy and frequency factor for lignite char-$O_2$ reaction are determined using temperature-programmed TGA method under the non-isothermal condition with variation of heating rate (2-20 K/min).
Gibbs free energy minimization technique is employed to calculate the equilibrium composition of complex gasification products. Nonlinear optimization problem is converted into a standard linear program. The equilibrium composition with variation of temperature (1000-1500 K), pressure (1-10 atm), and feed condition are calculated using a conventional SIMPLEX subroutine to guide in selecting appropriate process conditions for the coal gasification. Both equilibrium carbon conversion and mole fraction of CO and $H_2$ gradually increase with an increase in the temperature, whereas the amounts of $CH_4$ and $CO_2$ which can be produced by the exothermic reactions decrease. As the pressure is increased, mole fractions of $H_2$ and CO decrease, while those of $CH_4$ and $CO_2$ increase significantly. For C-$H_2O$-$O_2$ reaction system, equilibrium carbon conversion increases with an increase in the amount of $O_2$. Yields of $H_2$ and CO increase with an increase in the amount of $O_2$ up to a point at which thermally balanced reaction can be carried out, thereafter, excess $O_2$ beyond the point generates large amount of $H_2O$ and $CO_2$.
A cyclone gasifier model is developed to estimate the product gas composition and the carbon conversion with the variation of operating conditions such as temperature (1000-1500 K) and mole ratio of oxygen to steam(0-2) for a 12.5 cm I.D. cyclone reactor which was devised in order to gasify Australian lignite char with the mixture of oxygen and steam. The model is based on the information of the kinetic parameters of various char-gas reactions (C-$H_2O$, C-$CO_2$ and C-$O_2$) obtained thermogravimetrically and the char residence time obtained from the cold tests of the cyclone gasifier. Cyclone pressure drop increases with gas inlet velocity and decreases substantially with the solid loading in the gas stream. Under the experimental range of gas inlet velocity (6-17 m/s), overall collection efficiency increases with gas inlet velocity up to 8 m/s, thereafter, it decreases with the higher gas inlet velocity. Fractional collection efficiency for a particle size 10-15 μm is approximately 50%, and the collection efficiency of the char particle size larger than 25 μm is more than 99% in the cyclone. For the simulated cyclone gasifier system, carbon conversion increases significantly with an increase in the temperature and the amount of oxygen feeding. In the case of ($O_2/H_2O$) mole ratio of 0.6 to maintain the energy balance, carbon conversions at the reactor exit are 21% and 40% at 1000 K and 1500 K, respectively. Heating values of the moisture-free product gas increases with an increase of the temperature since yields of $H_2$ and CO are increased. In the case of ($O_2/H_2O$) ratio of 0.6 at 1500 K, heating value of product gas is determined to be 4.65 MJ/$m^3$, which corresponds to the low-Btu gas.
