Staged Combustor Evaluation of Low Rank Coal Fuels

The combustion characteristics of fuels derived from low rank, coals have been evaluated at firing conditions representative of an industrial gas turbine engine. Data have been acquired for five fuels containing sub-bituminous coal and one using a lignite. The sub-bituminous fuels were coal-water mixtures differing in either the coal processing or coal loading. One slurry was based on minimally-processed coal which contained relatively high ash and internal moisture levels; the coal loading was limited to 42 pct to sustain acceptable handling. The other four slurries presented different loading of an improved-quality form of the same parent coal; slurry loadings up to 55 pct were achieved, providing nearly 50-pct greater heating value than the minimally-processed fuel. The lignite coal was also processed to produce an improved-quality slurry. Attempts to deliver and combust powdered, sub-bituminous coal were not successful. All tests were performed in a combustor configured to achieve geometrically separated zones of fuel-rich and fuel-lean combustion. Test results indicated a lower limit of fuel energy density as necessary to sustain stable combustion; efficiencies greater than 95 pct were only achieved for improved-quality fuels. The staged combustor approach again demonstrated its ability to control the conversion of fuel-bound nitrogen to NOx as concentrations down to 40 ppm (15 pct 02) were recorded.