AIR-FIRED AND OXY-FIRED COMBUSTION: RATE OF CHAR OXIDATION BY O2, CO2 AND H2O

Char conversion rates and mechanisms are investigated at oxy-fired and air-fired combustion conditions. Experimental char conversion data was obtained from the IFRF Isothermal Plug Flow Reactor within the RELCOM project. Char conversion rates were computed at the investigated conditions using a detailed single particle model. The model takes into account development of internal surface area, diffusion of gaseous species inside the particle, homogeneous chemistry outside the particle, the Stefan-flow, temperature gradients and diffusion inside and outside the particle and Langmuir-Hinshelwood type surface mechanisms. The modeled char burnout is in good agreement to the experimental char burnout. The results show that char oxidation can be more rapid at conventional combustion conditions than at oxy-combustion conditions, although the oxygen concentration is that same in both cases. This is surprising, considering that a significant part of the char is consumed by CO2 and H2O. This is mainly explained by additional consumption of O2 in the boundary layer of the particle, by decreased diffusion rates of O2 and by an increased coverage of occupied carbon sites in the oxy-combustion case. The results suggests that more than 50% of the char is consumed by gasification reactions both in conventional combustion and in oxy-combustion of pulverized coal.