Low-rank coal-water fuel combustion in a laboratory-scale furnace

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A detailed study of hot-water dried lignite slurry combustion and the formation of nitrogen-containing pollutants was performed in a vertical, laboratory-scale combustor. Space-resolved local measurements of solid and gaseous combustion products were obtained from throughout the combustion zone using a stainless steel, water-quenched sample probe. Coal burnout (daf) of greater than 99% was achieved without supplementary fuel support, in an estimated residence time of 1.4 s. Flame stability was strongly affected by the atomized droplet size, which is controlled by the atomizing air to slurry mass ratio ( A S ). For A S greater than 0.7, coal burnout was relatively insensitive to further increases in A S , yet burnout decreased rapidly as A S was decreased. Nitric oxide (NO) emissions were not affected greatly by changes in A S . Decreasing stoichiometric ratio (SR) to about 0.8 caused coal burnout to decrease from about 98% to 94% and NO emissions to decrease from around 600 ppm to less than 100 ppm. Changes in secondary air swirl number from 0 to 4.25 had little or no effect on coal burnout or NO emissions for a SR of 1.1 and an A S of 0.75. At low A S (0.24), high secondary air swirl was required to stabilize the slurry flame. Reactor mapping tests showed rapid mixing between the slurry and the combustion air. CO was found only near the slurry inlet at a maximum concentration of 0.3%. No other fuel-rich species were detected in measurable quantities.