Pyrolysis, Gasification, and Combustion of Solid Fuels

The aim of this chapter is to discuss and summarize the research activities done at Politecnico di Milano in the field of the detailed kinetic modeling of pyrolysis, gasification, and combustion of solid fuels. Different critical steps are involved in this multicomponent, multiphase, and multiscale problem. The first complexity relies in the characterization of the solid fuels and their pyrolysis and devolatilization process. Detailed kinetic mechanisms, both in the solid and gas phase, involve a large number of species and reactions, which make the computations expensive and strongly reduce model applicability. For this reason, they need to be reduced and simplified, while still maintaining their description capability. Therefore, chemical lumping procedures are extensively applied to allow the development and validation of the overall mathematical model. Whereas the composition of plastics is usually well defined, coals, biomasses, and MSW (municipal solid waste) are typical fuels with a large composition variability and they require a characterization in terms of a few reference components. Multistep kinetic mechanisms with a lumped characterization of gas, tar, and residue are discussed, for the different solid fuels. Successive or secondary gas-phase reactions involve gas and tar components released during the devolatilization phase, while heterogeneous gasification or combustion reactions further modify the solid residue. Finally, the mathematical modeling of solid fuel gasification or combustion requires a comprehensive description of the coupled transport and kinetic processes, both at the particle and at the reactor scale. Several examples illustrate the capabilities and limitations of this model.