Gasification Characteristics of Coal/Biomass Mixed Fuels

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A research project was undertaken that had the overall objective of developing the models needed to accurately predict conversion rates of coal/biomass mixtures to synthesis gas under conditions relevant to a commercially-available coal gasification system configured to co- produce electric power as well as chemicals and liquid fuels. In our efforts to accomplish this goal, experiments were performed in an entrained flow reactor in order to produce coal and biomass chars at high heating rates and temperatures, typical of the heating rates and temperatures fuel particles experience in real systems. Mixed chars derived from coal/biomass mixtures containing up to 50% biomass and the chars of the pure coal and biomass components were subjected to a matrix of reactivity tests in a pressurized thermogravimetric analyzer (TGA) in order to obtain data on mass loss rates as functions of gas temperature, pressure and composition as well as to obtain information on the variations in mass specific surface area during char conversion under kinetically-limited conditions. The experimental data were used as targets when determining the unknown parameters in the chemical reactivity and specific surface area models developed. These parameters included rate coefficients for the reactions in the reaction mechanism, enthalpies of formation andmore » absolute entropies of adsorbed species formed on the carbonaceous surfaces, and pore structure coefficients in the model used to describe how the mass specific surface area of the char varies with conversion. So that the reactivity models can be used at high temperatures when mass transport processes impact char conversion rates, Thiele modulus – effectiveness factor relations were also derived for the reaction mechanisms developed. In addition, the reactivity model and a mode of conversion model were combined in a char-particle gasification model that includes the effects of chemical reaction and diffusion of reactive gases through particle pores and energy exchange between the particle and its environment. This char-particle gasification model is capable of predicting the average mass loss rates, sizes, apparent densities, specific surface areas, and temperatures of the char particles produced when co-firing coal and biomass to the type environments established in entrained flow gasifiers operating at high temperatures and elevated pressures. A key result of this work is the finding that the reactivities of the mixed chars were not always in between the reactivities of the pure component chars at comparable gasification conditions. Mixed char reactivity to CO{sub 2} was lower than the reactivities of both the pure Wyodak coal and pure corn stover chars to CO{sub 2}. In contrast, mixed char reactivity to H{sub 2}O was higher than the reactivities of both the pure Wyodak coal and pure corn stover chars to H{sub 2}O. This was found to be in part, a consequence of the reduced mass specific surface areas of the coal char particles formed during devolatilization when the coal and biomass particles are co-fired. The biomass particles devolatilize prior to the coal particles, impacting the temperature and the composition of the environment in which the coal particles devolatilize. This situation results in coal char particles within the mixed char that differ in specific surface area and reactivity from the coal char particles produced in the absence of the devolatilizing biomass particles. Due to presence of this “affected” coal char, it was not possible to develop a mixed char reactivity model that uses linear mixing rules to determine the reactivity of a mixed char from only the reactivities of the pure mixture components. However, it was possible to predict both mixed char specific surface area and reactivity for a wide range of fuel mixture rat os provided the specific surface area and reactivity of the affected coal char particles are known. Using the kinetic parameters determined for the Wyodak coal and corn stover chars, the model was found to adequately predict the observed conversion times and off-gas compositions in gasification conditions established in a variety of commercial gasifiers. The model has the potential to provide insight on certain implications of co-firing coal and biomass in gasification and combustion application when kinetic parameters for the mixed chars are employed.« less