Particulate formation during black liquor char bed burning

grantor: University of TorontoFly-ash particles formed during black liquor combustion deposit on recovery boiler tube surfaces, reduce the thermal efficiency of the boiler, restrict the flue gas flow, and may lead to unscheduled boiler shutdowns due to plugging of the flue gas passages. Previous studies suggest that a major portion of these particulates form during the char burning stage of black liquor combustion. This work is the first to study the formation, characteristics, quantity, and composition of particulates generated during black liquor char bed burning. Particulates collected had two distinct size ranges: submicron and 1-to-100 micron. The submicron particles are consistent with fume, which is known to form by the condensation of volatised sodium and potassium compounds. Fume quantity and carbonate content increased with increasing bed temperature, while the sulphate, chloride, and potassium contents decreased. The 1-to-100 m m sized particles, called ejecta*, are suggested to form by a "mechanical" process, through which pieces of the molten material are ejected from the char bed surface. Ejecta formed in significant quantities: about 40 to 85% of total particulates collected was ejecta (the remainder was fume). The quantity of ejecta increased markedly with bed temperature up to 1000°C, but reached an almost constant value at higher temperatures. Consistent with the "mechanical" formation mechanism, the composition of ejecta was similar to that of an oxidised smelt. The rate of char bed burning increased with an increase in both the inlet gas O2 concentration and the char sulphate content. Data are found to be consistent with two paths for char burning. The major path is the combustion of carbon by reaction with oxygen containing compounds in the gas stream. The rate of this path is determined by the rate of O2 mass transfer to the bed surface. The minor path is the consumption of carbon mainly by reaction with the sulphate content of char. The finding of large quantities of 1-100 m m ejecta particles formed during char bed burning experiments introduces a new category of fly-ash particles which may form in kraft recovery boilers and contribute to the boiler fouling and plugging. *Following the terminology used by Verrill et al [11] for similar particles formed during liquor pyrolysis.Ph.D