Influence of water injection on the formation of CO emissions in oxyfuel combustion of low calorific gaseous fuels

Oxyfuel combustion is a promising approach to apply carbon capture and thus counteract global warming. The pure oxygen used as an oxidizer is usually diluted with CO₂, H₂O or recirculated flue gas to moderate the combustion temperature, which influences the combustion characteristics like temperature distribution and CO formation. This paper deals with how the injection of liquid water in the combustion chamber affects the characteristics of MILD oxyfuel combustion of a low calorific natural gas/CO₂ mixture, which is supposed to represent biogas. Experimental tests are carried out on a 50 kW pilot plant, where the amount of recirculated flue gas and the water injection rate are varied in order to determine whether the influences can also be measured in the cold carbon-capture-ready flue gas. Moreover, CFD simulations of the combustion process with and without water injection are conducted to better understand the influence of water injection on the combustion characteristics. Results show that the injection of water leads to lower CO emissions, measurable in the cold flue gas. Furthermore, the reaction CO + OH ⇌ CO₂ + H, which is mainly responsible for CO combustion, is shifted to the right side, indicating a promotion in CO burnout. In addition, it is found that water injection is an efficient way of controlling the combustion chamber temperature in oxyfuel combustion systems without causing significant temperature peaks or strong inhomogeneities in the temperature field.