Numerical investigation of oxy-coal combustion in a large-scale furnace: Non-gray effect of gas and role of particle radiation

摘要

Computational fluid dynamics (CFD) simulations of the oxy-coal combustion in a large-scale furnace were performed by applying different weighted sum of gray-gases models (WSGGMs). Before simulation of the furnace, the different WSGGMs were tested in an enclosed rectangular box containing only gases inside. In both cases of air and oxy-fuel combustion, the calculated radiative sources and wall heat fluxes obtained with gray and non-gray WSGGMs were completely different. Only with the non-gray WSGGMs (i.e., non-gray implementation), the calculated results could well agree with the benchmark data. In the simulation of furnace, three WSGGMs were adopted: Smith's gray WSGGM (i.e., gray implementation), Johansson's non-gray-A WSGGM with uniform distribution of radiative energy of particles in each gray band, and Johansson's non-gray-B WSGGM, in which the radiative energy of particles was distributed using weighting factors. The weighting factors for the particles can be calculated with the same function as that used for gas but using the particle temperature instead of gas temperature, i.e., w_(p,i)= w_(g,i) (T_p). Being different from the rectangular box without particle radiation, in the real furnace, the gas temperature and wall heat flux calculated by gray and non-gray-A WSGGMs almost show no obvious difference. This is attributed to the fact that particle radiation plays a more significant role than gas radiation in the real furnace. However, using the non-gray-B model instead of Smith's gray model or non-gray-A model can obviously influence the distribution of wall heat flux and total amount of heat absorbed by the walls. This indicates that in the numerical simulation of oxy-fuel combustion, the non-gray WSGGMs should be used by combining with the weighting factors of w_(p,i)= w_(g,i) (T_p) for particles, in order to rigorously describe both the gas and particle radiation during oxy-fuel combustion of coal in real furnace.