Nongray-gas Effects in Modeling of Large-scale Oxy-fuel Combustion Processes

Quite some studies have been conducted in order to implement oxy-fuel combustion with flue gas recycle in conventional utility boilers as an effective effort of carbon capture and storage. However, combustion under oxy-fuel conditions is significantly different from conventional air-fuel firing, among which radiative heat transfer under oxy-fuel conditions is one of the fundamental issues. This paper demonstrates the nongray-gas effects in modeling of large-scale oxy-fuel combustion processes. Oxy-fuel combustion of natural gas in a large-scale utility boiler is numerically investigated, in which a recently refined weighted-sum-of-gray-gases model (WSGGM) applicable to oxy-fuel conditions is used to perform non-gray and gray calculations, respectively, and a widely used air-fuel WSGGM is also employed for gray calculation. This makes the only difference among the different computational cases. The simulation results show that the gray and non-gray calculations of the same oxy-fuel WSGGM make distinctly different predictions in the wall radiative heat transfer, incident radiative flux, radiative source, gas temperature and species profiles. In relative to the non-gray implementation, the gray calculation of the oxy-fuel WSGGM remarkably over-predicts the radiative heat transfer to the furnace walls and under-predicts the gas temperature at the furnace exit plane, which also result in a higher incomplete combustion in the gray calculation. Moreover, the gray and non-gray calculations of the same WSGGM make much more pronounced difference in the results than the gray implementation of different WSGGMs does (i.e., the oxy-fuel and air-fuel WSGGMs). Even though particle radiation also has an important impact and will compromise the demonstrated nongray-gas effects to some extent in large-scale oxy-coal combustion, the non-gray formulation of an oxy-fuel WSGGM is still highly recommended for a reliable oxy-fuel combustion modeling.