Flame temperatures and species concentrations in non-stoichiometric oxycoal flames

The oxyfuel technology offers the possibility for CO2 sequestration from coal fired power plants. One drawback is the need for a high external flue gas recirculation to avoid inadmissible high flame temperatures. The concept of controlled staging with non-stoichiometric burners (CSNB) allows a significant reduction of the commonly proposed flue gas recirculation rate while fulfilling all requirements on temperature limitations. The concept aims at a more efficient oxyfuel process with a higher degree of freedom for heat-flux adjustments suitable for a new generation of oxyfuel boilers. The steam generator size could be reduced and in this way a more cost effective steam generator concept is possible. Additionally the energy demand for the flue gas recirculation is lowered.This paper presents the experimental investigations of non-stoichiometric oxycoal flames. Temperature and gas profiles were taken to analyze the combustion behavior of coal with high oxygen concentrations in the oxidizer under oxygen deficiency and accordingly oxygen excess. In addition an optical flame monitoring system allowed a comparison of ignition, flame shape and stability. In the test rig lignite was burned under different stoichiometries ranging from 0.5 to 2.5 and different oxygen concentrations in the oxidant ranging from 30 to 40 vol.%. The thermal input of the burner was 70 kW at a total thermal input of 140 kW and a dry flue gas recirculation was used. The results were compared to a conventional air-blown combustion and showed that similar temperature ranges can be reached even with oxygen concentrations in the oxidizer as high as 40 vol.%.

► Stoichiometric air and air-like oxyfuel flames were compared to non-stoichiometric oxyfuel flames with 40 vol.% oxygen in the oxidant.
► Leaving stoichiometric combustion reduces flame temperatures significantly.
► The commonly proposed flue gas recirculation rate could be reduced with the concept of controlled staging with non-stoichiometric burners.
► Adjustments in oxygen concentration and stoichiometry leads to significant differences in gas concentrations, especially for CO and NOx.