Experimental evaluation of rich oxyfuel combustion characteristics in varying over-stoichiometric conditions

Cement production is a highly CO2-intensive process, where two-thirds of the emissions are unavoidable process emissions and the rest come from the combustion process. Oxyfuel technology is an innovative way to apply carbon capture and allow the industry to reduce CO2 emissions, even reaching net zero CO2 emissions. The oxyfuel combustion process can be retrofitted to existing plants and designed for new-build plants, where the plant can be designed with no flue gas recirculation (FGR). A down-scaled kiln burner is tested in oxyfuel conditions with different oxygen-to-fuel ratios in technical and pilot-scale facilities at the University of Stuttgart. In the technical-scale facility, experiments are conducted to compare a case with synthetic FGR at near-stoichiometric conditions (OXY32) and over-stoichiometric conditions (with lambda*3.4), lambda* being the oxygen-to-fuel ratio. Experiments in the pilot-scale facility are conducted at varying stoichiometric conditions, lambda*2, lambda*3 and lambda*4. In both facilities, a reference case with air combustion is conducted. The highest measured temperature in the air, lambda*3 and lambda*4 cases were 1020 degrees C, 1321 degrees C and 1116 degrees C, respectively. In the oxyfuel cases, after the peak temperature is reached, the temperature profiles stabilize to similar temperatures as measured in the air case. The inlet oxidizer gas concentration and stoichiometry highly affect the CO and NO formation. For all oxyfuel cases, the CO emission rate in the flue gas measurements is below 20 mg/MJ indicating high burnout efficiency. In the technical-scale tests, the NO emission rate at 2.5 m from the burner is lower in the OXY32 case compared to the air case, with 138 and 247 mg/MJ, respectively. The NO emission rate of the lambda*3.4 case is 461 mg/MJ, a consequence of no reducing or reburning zone. The experiments show that the increased over-stoichiometric conditions have a desirable effect on the temperature profile and oxygen can be used as a suitable diluent in comparison to N-2 and CO2, but NO formation is increased. In the cement production process, this can be solved by designing a reducing zone in the calciner to properly reduce NO.