Numerical modeling and simulation of oxy-fuel combustion processes

Coal ignition and combustion under conventional and oxy-fuel conditions are numerically investigated to understand the interactions between chemical and transport processes and to support model development. Char burnout of coal particles is studied in highly resolved numerical simulations including a detailed description of the particle surface and the gas phase chemistry. At the solid-gas interface, heat and mass fluxes due to surface reactions involving carbon oxidation and gasification are considered. The model is validated with experimental results for char burnout in a flat flame burner. A comprehensive set of fully resolved reactive 2-D simulations has been performed by varying particle size, relative velocity, diluent, and oxygen composition in the surrounding gas. The simulation results are discussed regarding the CO2 and N2 content of the atmosphere highlighting the effects of oxy-fuel combustion. Increasing particle Reynolds number reduces gas temperature and results in a faster oxygen supply, which consequently yields a higher particle burning rate up to the point where the resulting temperature reduction leads to a lower burning rate. Combustion reduces the drag force of the reacting particle and thus a modified drag coefficient as a function of the dimensionless Stefan flow velocity is provided. In addition, ignition of coal particles is studied by using a fully coupled Euler- Lagrange approach. Devolatilization of coal particles is modeled with the chemical percolation devolatilization (CPD) method coupled with a detailed gas chemistry. Numerical simulations showed that an increase of ignition delay time in oxy-atmosphere compared to the air case is related to the depletion of radicals that react with the abundant CO2 of the oxy-atmosphere. Considering different particle streams showed that an increase in particle number density delays the onset of ignition and forms a more continuous and narrower flame front. Particle heating and ignition induce velocity variations in the gas phase. In the denser stream, velocity variations become significant and compromise the validity of a constant velocity assumption that is usually made in computing ignition delay time from the observed ignition location. It is also found that high particle slip velocities lead to a locally low volatile concentration and low temperature, which consequently increase ignition delay time