ABSTRACT
The radiative heat transfer in oxy-fuel flames is compared to corresponding conditions in air-fuel flames during combustion of lignite in the Chalmers 100 kW oxy-fuel test facility. In the oxy-fuel cases the flue gas recycle rate was varied, so that, in principle, the same stoichiometry was kept in all cases, whereas the oxygen fraction in the recycled flue-gas mixture ranged from 25 to 29 vol.%. Radial profiles of gas concentration, temperature and total radiation intensity were measured in the furnace. The temperature, and thereby the total radiation intensity of the oxy-fuel flames, increases with decreasing flue-gas recycle rate. Comparison of gas and total radiation intensities shows that their ratio (gas to total) increases under oxy-fuel conditions compared to airfired conditions in all cases. The largest difference between air and oxy-fuel firing appears at the maximum intensities recorded, for which the ratio of gas to total radiation intensity was less than 30% for the air flame, whereas it was 40% or more in the oxy-fuel flames. A large fraction of the radiation in these lignite flames is hence emitted by particles. Furthermore, despite the short path lengths investigated, the total radiation intensities become similar during oxy-fuel and air-fuel conditions for a case with similar gas temperature distribution. Thus, the particle radiation does not seem to be influenced by the oxy-fuel atmosphere. As a consequence, in these lignite-fired flames, the increment of gas radiation becomes less evident because of the background of particle radiation and the total emissivity of the oxy-fuel flames does not seem to differ significantly from that of the air-fired flame.

Keywords: Oxy-fuel, Combustion, Heat transfer, Radiation, Particle, Gases