(113e) Application of a Multi-Grain Model: Single Large Coal Particle Oxy-Combustion

In this paper, we present a mathematical framework for a multi-grain solid-gas reaction diffusion model. Typically, the application of single particle models is limited due to their assumptions. The shrinking-core model is the most frequently used model. It is a simplified model for gas-solid reactions, which assumes a sharp boundary between the un-reacted core and the formed product layer. It is often valid for non-porous particles but not for porous particles, unless the overall rate is kinetically controlled. However, very few general gas-solid reaction-diffusion models have been implemented for generalized gas-solid particle cases.  Our mathematical framework is based on “multiple grains,” instead of a “product layer.” Our particle model is a general formulation that couples pore gas diffusion, external mass convection in the bulk gas, internal heat conduction, external heat convection, radiation with chemical reaction. It is based on the governing conservation equations including one energy equation and several mass equations whose numbers are determined by the number of species in the system (gas and solid). All of the mass conservation equations and the energy equation are coupled with reaction and heat generation. The solution of the governing equations is accomplished using Matlab. In addition, some numerical solution techniques to solve the stiff problem were used such as the implicit method and dynamic time stepping.

One application of our mathematical framework is single coal particle combustion under oxy-fired conditions in fluidized beds. Fuel particles in fluidized beds are significantly larger than in pulverized coal combustion, and thus the assumptions typically invoked for coal particle combustion models may not be applicable for all fuel particle sizes or over the entire lifetime of the particle. Initial model results illustrate the effect of particle size on combustion evolution: smaller particles favor kinetic control, while diffusion control occurs in the larger particles, as expected. For large particles, the O₂ penetration into the particle was limited to a limited region near the particle surface as would be expected for diffusion control. The evolution of CO/CO₂ was also examined and as the simulation results show, the process of CO converting to CO₂tendsto happen inside the particle for larger fluid-bed-sized particles (d=3000µm). However, the process of CO converting to CO₂ is more likely to occur outside the coal particle (gas phase) for pulverized coal-sized particles (d=50µm). The results of the simulation support the applicability of a shrinking core particle model for large particle coal combustion. The model framework presented here, however, provides a broader flexibility to address a range of particle sizes and temperature conditions.