Coal burnout and nitrogen pollutant formation processes during lignite slurry and pulverized lignite combustion

Detailed combustion tests of hot-water dried lignite slurry and the parent pulverized lignite were performed in a laboratory-scale combustor with local measurements obtained throughout the combustion zone. Operating parameters studied included secondary air swirl number, slurry mean droplet diameter, stoichiometric ratio, and primary air velocity. Greater than 99% coal burnout (daf) was achieved in an estimated reactor residence time of 1.4 s. Flame stability during slurry combustion was strongly affected by the atomized droplet size. A mass mean droplet diameter of 50 {dollar}mu{dollar}m or less was required to obtain a stable flame with high coal burnout. Rapid mixing of the fuel with the secondary air was obtained during the slurry combustion tests due to the high momentum provided to the slurry droplets from the atomizer. This rapid mixing prevented the formation of any fuel-rich regions within the reactor. Secondary air swirl only had a minor effect on the slurry combustion and pollutant formation processes. The pulverized lignite tests showed much slower mixing of the fuel with the secondary air by forming a strong fuel-rich region along the reactor centerline. Secondary air swirl aided flame stability and produced a minimum in NO emissions as swirl number was increased. Computer model predictions showed good agreement with the experimental burnout data.