NOx emissions and burnout characteristics of bituminous coal, lignite, and their blends in a pulverized coal-fired furnace

• Combustion and NOx emission characteristic were studied in a 15 kWth pulverized coal fired furnace.
• The staged-air level and primary zone air ratio affected the NOx reduction and particle burnout.
• There is a critical blending ratio of lignite influence on NOx reduction and combustion efficiency.

For coal-fired power plants, the application of low-rank coals like brown coal and lignite steadily has been a great issue due to the shortage and rising prices of high-rank coal. In this research, therefore, co-firing experiments with low-rank coal, lignite, were performed in a 15 kWth pulverized coal-fired furnace, and combustion and emission characteristics of bituminous, lignite, and their blends were investigated. Influences of staged-air injection, primary zone air ratio, coal types, and blending ratios of bituminous coal and lignite on NOx emission and particle burnout were evaluated. For the bituminous coal, NOx emissions decreased slightly with a decreasing primary zone air ratio and increasing staged-air injector level. At a given value of the primary zone air ratio and staged-air injector level, NOx emissions were lower for coal with lower nitrogen content. However, in the case of the lignite, the NOx concentration was the highest, because of maintenance of a constant thermal output. For a multi-staged combustion, increasing the distance between the staged-air injectors has a positive effect on NOx emission but a negative effect on particle burnout. Furthermore, in co-combustion with bituminous coal and lignite, the blends with a 10% addition of lignite show a minimum value for NOx concentration and a similar temperature distribution to the bituminous coal case. This is because of the higher devolatilization rate of lignite and the formation of a strong reducing environment at high temperatures from the reaction between oxygen and the combustible gas species. Such gas species were released from lignite devolatilization in the primary combustion zone that is conducive to NOx destruction.