LASER-INDUCED EMISSION OF ULTRAFINE PARTICULATES EVOLVED BY PULVERIZED COAL PYROLYSIS

The importance of on line measurement of ultrafine particulates in pulverized coal flames is mainly due to the detection of ultrafine particulate in the effluent for pollution control, and the quantification of fuel burnout in real time within a boiler for improved understanding of the flame heat transfer soot modeling as well. A method has been investigated using laser-heated emission within an O-2-free flame which provides a continuous in situ measurement of ultrafine particles during high-temperature pulverized coal pyrolysis. Bituminous coal particles are entrained by nitrogen along the centerline of a laminar flow flat flame burner, where a hydrogen-air flame under fuel-rich condition is used as a heat source. The temperatures of the hydrogen flame were measured by a finite-wire silica-coated Platinum-Rhodium type B Thermocouple. Volatiles released during the coal pyrolysis form a cloud of ultrafine particles at high temperature. A pulse laser sheet introduced to the flame heats the ultrafine particles to incandescent temperatures. The time-resolved laser-induced emission signals with different incident laser-pulse fluences were evaluated. The volume faction of ultrafine particles was associated with the peak value of the signals, and the mean particle size characterized by a time constant of the exponential signal decay. A strong dependence of the characteristic peak value and emission time constant during laser-heated particle cooling from the measured coal particle class could be determined. Specialties in signal evaluation due to residence time in the hydrogen flame for two sizes of coal particles are discussed.