Experimental study on NOx formation and burnout characteristics of pulverized coal in oxygen enriched and deep-staging combustion

Abstract

Deep-staging combustion is an effective technology for reduction of NOx, but is often accompanied by the combustion worsening. Oxygen enriched and deep-staging combustion is a promising technology to realize both low NOx formation and high burnout rate. NOx reduction and burnout characteristics of pulverized coal in oxygen enriched combustion were experimentally investigated in this study with a drop-tube reactor. Results show that the NOx reduction is significantly promoted by elevated O2 concentration under deep-staging condition. The NOx formation of coal under 40% oxygen concentration condition is 40.7% lower than those in air combustion as the stoichiometric ratio in the fuel-rich zone (SR1) is 0.5. The thermal and fuel NOx formations were separately investigated. The fuel NOx reduction with increase of O2 concentration makes a major contribution to the total NOx reduction under deep-staging conditions. The elevated O2 concentration facilitates the release of fuel nitrogen in the fuel-rich zone, avoiding its oxidation in the fuel-lean zone. Moreover, the negative correlation between NOx formation and unburned carbon (UBC) in coal ash is evidently weakened with the increase of O2 concentration. Ultra-low NOx formation and high combustion efficiency could be concurrently achieved in oxygen enriched combustion under deep-staging condition. Furthermore, the comprehensive comparison of the oxygen enriched combustion, air combustion and oxy-fuel combustion was conducted. Low NOx formation can be reached in oxy-fuel combustion under non-staging condition, while oxygen enriched combustion performs much better in NOx reduction under deep-staging conditions.