The effect of employing two staged recycle under oxy-fuel firing conditions has been studied under wet and dry recycle conditions firing a Russian high-volatile-bituminous coal. The impact on radiative and convective heat transfer and carbon-in-ash by varying the primary zone recycle ratio at constant overall recycle ratio has been studied. Results are compared to air firing.
Primary zone recycle ratio was adjusted between 65% and 72% for a constant overall recycle ratio of 72%. The overall recycle ratio was controlled by injecting recycled gases through overfire air ports located downstream of the primary zone in the furnace under two staged recycle operation. Conditions were investigated with and without recycled gas injection through the overfire air ports. The dry recycle data shows an air equivalent radiative heat flux of 72% (no recycled flue gas injection through the overfire air ports) and 74% (with recycled flue gas injection through the overfire air ports). The convective heat flux data shows that an air equivalent heat flux can be obtained with an overall recycle ratio of circa 80%. The wet recycle data shows that an air equivalent peak radiative heat flux can be obtained at overall recycle ratios between 70% and 72%, dependent on whether the recycled gas is introduced through the overfire air ports or not. The convective heat flux data for the wet recycle cases shows that an air equivalent heat flux can be obtained with an overall recycle ratio of circa 77%. The difference in the air equivalent radiative heat fluxes under wet and dry recycle conditions is commensurate with the higher specific heat of the flue gases under wet recycle due to the presence of H2O. This increases the heat content of the flue gas per unit of mass flow. For the majority of the oxy-fuel tests, the carbon-in-ash levels are below 1% and consistently lower than air firing for the same furnace exit O2. The primary zone recycle ratio has a positive impact on the burnout characteristics whilst the burnout is relatively insensitive to the overall recycle ratio and is primarily due to the oxygen enrichment in the primary zone on the coal’s overall oxidation rate.