561 / 2019-01-18 08:50:11

Numerical study on oxy-fuel co-combustion of semi-coke and coal in utility boiler: insight into NOx emission and combustion efficiency

semi-coke, oxy-fuel combustion, co-combustion, numerical study

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Oxy-fuel combustion is a clean and efficient combustion technology, which not only benefits the large-scale capture of CO2 but also reduces NOx emission, and then promotes the efficient and clean utilization of carbon-based solid fuels. The previous studies on semi-coke mainly focused on air case, while the co-combustion characteristics of semi-coke and coal in utility boiler under oxy-fuel condition have been seldom investigated and the correlations between NOx emission and combustion efficiency for oxy-fuel co-combustion is still unclear. This study mainly aimed at the oxy-fuel co-combustion of semi-coke and coal in utility boiler with special insight into NOx emission and combustion efficiency via numerical simulation approach. The simulation results showed that when the blending ratio of semi-coke was 45%, the NOx concentration at the furnace outlet under oxy-fuel condition was increased with the oxygen content. The flue gas circulation modes (“dry” versus “wet”) had insignificant impact on the NOx emission when the initial oxygen concentration was set as 24% and 27%. However, the dry flue gas circulation resulted in an increase in NOx concentration relative to “wet” mode when the initial oxygen concentration was 30%. When the blending ratio of semi-coke was 70%, the differences of NOx emission between “dry” and “wet” flue gas circulation modes were enlarged with a rise in oxygen content under oxy-fuel condition. The carbon content in fly ash was enhanced while the burnout ratio was reduced when the air condition was switched to oxy-fuel condition directly. For oxy-fuel cases, despite the growth of burnout ratio, the carbon content in fly ash first increased and then decreased with the rise of oxygen concentration. With the oxygen concentration increased to 30%, lower carbon content in fly ash and higher burnout ratio could be observed in comparison to the cases under air condition.