Coal combustion at high reynolds number

Abstract

Measurements of the burning rate and characteristic combustion times of a bituminous Kentucky coal, a sub-bituminous Montana coal and a Texas lignite were conducted in a jet of heated gas and in a small externally heated fluidized bed. Individual coal particles from 2 to 10 mm were tested at 871, 815 and 705°C at 21% and 10.5% oxygen. The Reynolds number varied from 19 to 50 in the fluidized bed tests and up to 1000 in the jet tests. A volatile flame ignited after 2–12 sec and lasted for 10 to 60 sec. Char burnout occurred after 100 to 700 sec. The reactivity (burning rate per unit surface area) of the volatiles varied from 0.83 to 2.3 mg/cm 2 -s in the fluid bed tests and from 1.9 to 3.6 in the jet tests. The char reactivity varied from 0.3 to 1.3 mg/cm 2 -s in the fluid bed tests and from 0.4 to 1.0 in the jet tests. Reaction rate constants for the coal char were calculated and compared to theoretical diffusion and kinetic rate constants. Temperature rather than diffusion is the main rate limiting factor although diffusion is also important.