Evaluation of anthracite combustion and NOx emissions under oxygen-staging, high-temperature conditions

- High-temperature reduction of NOx was promoted under high-concentration O2, sub-stoichiometric conditions.
- The negative effect of oxygen staging on burnout was weakened with oxygen concentration.
- Char-CO2 gasification was promoted in oxygen staging combustion at high temperatures.

Oxy-coal combustion could be an alternative to reduce the high NOx emissions and unburned carbon (UBC) loss in pulverized-coal furnaces burning anthracite. Experimental evaluation of anthracite burnout and NOx emissions under various oxygen-staging conditions was performed in a high-temperature vertical tandem-type drop-tube furnace (DTF) system, in which a temperature of as high as 1650 °C can be achieved. Relatively low NOx emissions and very high UBC in ash (i.e., levels of 1150 mg m−3 at 6% O2 and 18.47%, respectively) were observed in an air-firing atmosphere as over fire air (OFA) ratio was 20.8% at 1600 °C. NOx emissions were dramatically lower in oxy-coal combustion compared with those in an air-firing atmosphere and identical low NOx emissions (200 mg MJ−1) occurred at higher stoichiometric ratio in the fuel-rich zone (SR1) with increasing oxygen concentration. High burnout efficiency can be obtained concurrently under high-concentration oxygen conditions, and the effect of oxygen staging on burnout was weakened with increasing oxygen concentration. In the fuel-rich zone, char-CO2 gasification reaction was promoted under oxygen-staging, high-temperature conditions, which contributed to the char consumption and early release of char-N. In addition, it was found that burnout efficiency was improved as the mixture of O2/CO2 was substituted by pure oxygen for OFA, which was of weak effect on the NOx generation simultaneously. Accordingly, the feasibility and availability of the high-temperature, oxygen-staging combustion technology on anthracite combustion and low NOx emissions was further discussed.