Sulfur Form in Coal and SO₂ Emission Behavior at AFBC Plant

Abstract

This paper reports the results of an investigation on SO₂ formation from sulfur in fuel (Fuel-S) and SO₂ 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 SO₂ from fuel-S in a fluidized bed consisted of sand, which was inert for SO₂ capture, indicated that SO₂ was formed from char-S and volatile-S and the contribution of ash-S played only minor role in SO₂ formation.
Under limestone feed conditions, the emission of SO₂ was strongly dependent on conversion of fuel-S to char-S during pyrolysis, whereas conversion to volatile-S had little influence on SO₂ emission. This implies that the sulfur in the volatile matter is oxidized at the bottom of the bed and most part of SO₂ from volatile-S is captured in the dense bed. In contrast, char-S is considered to be oxidized to SO₂ throughout the bed, thus the SO₂ 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 SO₂ emission from bubbling fluidized bed combustors under limestone feed conditions.