Air-Staged Combustion Characteristics of Pulverized Coal under High Temperature and Strong Reducing Atmosphere Conditions

Abstract

It is of great importance to clarify the effect of the combustion temperature in actual boilers on air-staging combustion of coals. In this work, a novel vertical tandem-type drop-tube furnace system was set up, in which a temperature of as high as 1600 °C can be achieved. Experimental investigations were performed thoroughly with particular attention to NOx formation and unburned carbon in ash. The results indicate that, to maximize NOx removal efficiency by air-staged combustion, the reducing zone should be under high temperature and strong reducing atmosphere (HT&SRA) conditions. A remarkably low NOx emission and high coal burnout can be attained concurrently by creating HT&SRA conditions in the fuel-rich zone. The NOx emission for high-volatile coals can be below 150 mg/m3. The special role of the temperature is discussed in detail in terms of the difference of NOx formation between the reducing zone exit and burnout zone exit. For comparison, the NOx emission in Ar/O2- and oxy-fuel-staged combustion was also studied. Staged combustion is as effective for NOx reduction in Ar/O2 and oxy-fuel as in air combustion. However, for oxy-fuel-staged combustion, the effect of HT&SRA conditions presents some differences, which are closely related with the oxygen proportion in the gas atmosphere. In addition, it is also found that the temperature in the burnout zone (800–1200 °C) has a weak effect on NOx generation, but CO emission would drastically increase when it is below 1100 °C.