Application of different diffusion approaches in oxy-fuel combustion of single coal char particles

摘要

The paper elaborates the influence of the non-equimolar and equimolar counterdiffusion on combustion of single coal char particles in the oxy-fuel combustion conditions. The phenomenon of the non-equimolar counterdiffusion is usually neglected and superseded by the equimolar counterdiffusion. Such a replacement can lead to results, e.g. to char burnout, particle temperature and species concentrations, which do not agree with the real process. The paper presents the numerical results of single coal char particle combustion in the oxy-fuel combustion conditions at which the effect of the equimolar and non-equimolar counterdiffusion has been taken into consideration. It has been assumed that the char carbon heterogeneously reacts with O_2, CO_2 and H_2O forming CO or CO_2 and H_2 depending on the combustion conditions. The reactions in the gas phase have been neglected to achieve the state of pure diffusion. The combustion of the particle is described by the mass and energy conservation equations commonly used in Euler-Lagrange computations of pulverized coal combustion. Numerical simulations performed for various values of the particle diameter and reagent concentrations clearly show that the use of the equimolar counterdiffusion model always overpredicts the non-equimolar one. Since the molar fluxes of the equimolar counterdiffusion are not coupled each other, the mass transfer towards the particle burning is higher which gives particle temperature and reaction rates too high compared to the non-equimolar counterdiffusion. Occurring discrepancy further develops during combustion also for other quantities describing the process, i.e. char burnout and gas concentrations at the particle surface. A simple correction which is proposed consists in reducing the value of the equimolar mass transfer coefficient that decreases the mass transfer and consequently letting the equimolar counterdiffusion model to be effortlessly used and get results which well follow the non-equimolar model. Such a simplified treatment of complex non-equimolar counterdiffusion can be easily implemented into numerical codes and needs no numerical solution of coupled non-linear equations describing the non-equimolar counterdiffusion.