This paper reports the results of an investigation on SO
2 formation from sulfur in fuel (Fuel-S) and SO
2 capture by limestone in a bench-scale bubbling fluidized bed combustor. Conversion of fuel-S to S in ash (ash-S), S in char (Char-S) and S in volatile matter (Volatile-S) during pyrolysis and combustion was experimentally evaluated under fluidized bed combustion condition. The formation of SO
2 from fuel-S in a fluidized bed consisted of sand, which was inert for SO
2 capture, indicated that SO
2 was formed from char-S and volatile-S and the contribution of ash-S played only minor role in SO
2 formation.
Under limestone feed conditions, the emission of SO
2 was strongly dependent on conversion of fuel-S to char-S during pyrolysis, whereas conversion to volatile-S had little influence on SO
2 emission. This implies that the sulfur in the volatile matter is oxidized at the bottom of the bed and most part of SO
2 from volatile-S is captured in the dense bed. In contrast, char-S is considered to be oxidized to SO
2 throughout the bed, thus the SO
2 from char formed in the vicinity of the bed surface is considered to escape from the bed without being captured by limestone since the contact time is short. These results suggest that the conversion of combustible sulfur (char-S+volatile-S) to char-S is very important to predict the SO
2 emission from bubbling fluidized bed combustors under limestone feed conditions.
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