A consistent reaction mechanism and model for the combustion and gasification of coal and biomass, and the co-firing thereof
Keywords: Mechanical Engineering.

This work seeks to provide a complete and thermodynamically-consistent reaction mechanism for the combustion and gasification of coal and biomass chars. In pursuit of this, mass loss data obtained via thermogravimetric analysis as well as data from temperature programmed desorptions were used to determine kinetic and thermochemical parameters. These parameters were determined via their adjustment in an intrinsic chemical reactivity model, which was developed to predict char conversion rates and off-gas compositions when pulverized coal and biomass char particles are exposed to reactive gases within our pressurized thermogravimetric analyzer (PTGA). Char reactivity tests were performed in H2O/H2/N2 environments in order to obtain data to determine kinetic parameters for char reactivity to H2O, in CO2/CO/N2 environments in order to obtain data to determine kinetic parameters for char reactivity to CO2, and in O2/N2 environments in order to obtain data to determine kinetic parameters for char reactivity to O2. This work then seeks to assess whether the reactive properties of the two individual fuels are altered when they are co-fired together. The two fuels were co-fired together at various mixture fractions, and subjected to similar reactivity tests as the pure fuels. Results indicate that kinetic parameters of the pure fuels alone cannot be used to predict reactive properties when co-fired, as properties of the coal char are indeed affected when devolatilizing in close proximity to biomass. To explain and account for these changes, this work proposes two models, a mixed-char surface area model, and a mixed-char reactivity model. The underlying principle is the same: observed changes can largely be predicted by employing a combination of the properties if three types of chars instead of the originally assumed two types. A coal particle that devolatilizes in an area that had just immediately been affected by a prior biomass particle devolatilization possesses different properties than a coal particle that devolatilizes in an unaffected area. Thus, the mixed-char models include properties of the original coal char, the original biomass char, and a third "affected" coal char. Finally, this work then seeks to extend the results beyond the kinetically-controlled regimes in which the data were obtained. In many conditions of interest, diffusion phenomena begins to impact the particle conversion rates, so predicting intrinsic reactivity alone is not enough to predict particle conversion rates. A fully-resolved DNS model was created that jointly simulates the internal pore diffusion of the particle as well as the location-resolved char reactivity in order to assess methods for accounting for the effects of diffusion. The main method this work investigates is Thiele's method, which makes use of an analytical solution that relates the Effectiveness Factor (η) to the Thiele Modulus (φ). This work reveals that the analytical η − φ relationship set forth by Thiele does not accurately apply to C-H2O and C-CO2 reactions, due primarily to the fact that the elementary reactions are no longer irreversible and 1st-order (but rather are reversible and inhibited by their products). However, Thiele's method can still be used with slight changes proposed in this work.