A combustion concept for oxyfuel processes with low recirculation rate - Experimental validation

Oxyfuel combustion is a technology for Carbon Capture & Storage from coal fired power plants. One drawback is the large necessary amount of recirculation of cold flue gases into the combustion chamber to avoid inadmissible high flame temperatures. The new concept of Controlled Staging with Non-stoichiometric Burners (CSNB) makes a reduction of the recirculation rate possible without inadmissible high flame temperatures. This reduction promises more compact boiler designs. We present in this paper experiments with the new combustion concept in a 3 × 70. kW natural gas combustion test rig with dry flue gas recirculation of 50% of the cold flue gases. The new concept was compared to a reference air combustion case and a reference oxyfuel combustion case with recirculation of 70% of the cold flue gases. FTIR emission spectroscopy measurements allowed the estimation of spectral radiative heat fluxes in the 2-5.5. μm range. The mixing of the gases in the furnace was good as the burnout and the emissions were comparable to the reference cases. The flame temperatures of the CSNB case could be controlled by the burner operation stoichiometry and were also similar to the reference cases. The heat flux in the furnace through radiation to the wall was higher compared to the oxyfuel reference case. This is an effect of the lowered recirculation rate as the mass flow out of the furnace and therefore the sensible heat leaving the furnace decreases. The higher oxygen consumption with lower recirculation rate could be compensated by a lower furnace stoichiometry. This was possible due to better burnout with increased oxygen concentrations in the burner. The results prove that a reduction of the flue gas recirculation rate in oxyfuel natural gas combustion from 70% down to 50% is possible while avoiding inadmissible high flame temperatures with the concept of Controlled Staging with Non-stoichiometric Burners.