DETAILED KINETICS OF SOLID FUEL GASIFIERS. COMPARISONS BETWEEN BIOMASS AND COAL PERFORMANCES

摘要

Gasification is a thermo-chemical process aiming at the production of high heating value syngas, starting from biomass, coal, or refuse derived fuels. Depending on the solid fuel and on the operating parameters of the process, the quality and the chemical composition of the produced syngas is differently affected. The final applications of syngas include the power generation and the production of chemicals, with special reference to methanol/ammonia synthesis and Gas-to-Liquid technologies. The aim of this work is to propose a comprehensive mathematical model of a biomass and coal gasifier. The first complexity relies in the characterization of the solid fuels and their pyrolysis and devolatilization process. The pyrolysis of the different solid fuels is characterized by a multistep kinetic model, with a detailed characterization of gas, tar and solid residue. The secondary gas phase reactions describe the successive evolution of the released gas and tar components, while heterogeneous gasification and combustion reactions allow to account for the evolution of the solid residue. Moreover, the mathematical model of the gasifier requires a comprehensive description of the coupled transport and kinetic processes, both at the particle and the reactor scale. The complexity of the resulting numerical problem is due both to the dimension of the differential algebraic system and to the stiffness of radical reactions. A validation example of the overall model with proper experimental data supports the reliability of the comprehensive approach here discussed. Comparisons between the performances of a countercurrent coal and biomass gasifier are proposed.