Modeling of a Single Char Particle Burning in Oxygen-enriched O2/N2 and O2/CO2 Environment

The combustion characteristics of pulverized coal in O-2/CO2 were reported considerably different from those in conventional air combustion. Properly describing the combustion behaviors of coal particles at elevated O-2 levels in both oxy-firing and air firing is essential for the design and simulation of the advance combustion systems. The objective of the present work is to describe the rate-limit step of coal conversion, i.e., the combustion process of char particles with modeling approach. In order to understand the reasons to cause the differences when replacing N-2 with CO2 in the oxidant stream, a fundamental model has been developed to describe in detail the combustion of a single pulverized char particle at elevated O-2 mole fractions in either N-2 or CO2 balance gas. The model includes char oxidation and a kinetic scheme which is coupled with the diffusive properties within the particle and in the boundary layer. The main features of the model include: 1) The properties e.g., thermal conductivity and diffusivities, of the gaseous species in the gas mixture are modeled in detail; 2) The model considers the distributions of carbon reactivity and structure, temperature and gas concentrations within the particle and their evolutions during the conversion process; 3) Thermal annealing and its impact on the evolution of carbon reactivity are considered; 4) The model describes the intra-particle reactions and diffusion as well as the reactions, particularly CO oxidation, and diffusion outside the particle. The model is validated with the comparisons of particle surface temperature between the calculation results and the experimental data of char particle combustion reported in the literature. The oxygen concentration involved covers from normal concentration to up to 100% for both O-2/CO2 and O-2/N-2 combustion. Model results represent that coal particles burned at lower mean temperatures in O-2/CO2 than in O-2/N-2 environments at analogous oxygen mole fractions. Additionally, the model has also been applied to numerically investigate the effects of char property and combustion condition parameters on the combustion process so as to explore the applicability of model in prediction char conversion process in practice.