Sulphur behaviour and capturing during a fixed-bed gasification process of coal
Skhonde, Madoda Pet
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During coal gasification, sulphur is liberated from the coal structure and released as H2S and COS. However, widespread concern about environmental emissions from coal utilization has started to limit the growth in use of coal as an energy source. The primary objective of the study reported in this thesis was to investigate the possibility of sulphur capturing through injection of S02 into a packed coal bed, under controlled conditions that simulates the zones of a fixed-bed gasifier. The secondary objective of the study was to investigate the sulphur behaviour in a commercial fixed-bed gasifier. In the study conducted, the behaviour of sulphur occurring within a fixed-bed gasification process was studied using a commercial Sasol-Lurgi fixed-bed gasifier, using the gasifier turnout sampling method. The investigation of sulphur capturing in a packed coal bed under controlled conditions was conducted in two different experimental set-ups, namely: a laboratory-scale furnace that simulates the combustion zone as well as a pilot-scale pipe reactor that simulates all the various zones of the gasifier. Sulphur behaviour across a commercial fixed bed gasifier was found to be influenced mainly by minerals-based sulphur such as sulphur in pyrite. Pyrite decomposition to pyrrhotite was found to be the predominant process in the top half of the gasifier, leading to the formation of H2S and COS. Pyrrhotite was found to be undergoing further transformation towards the formation of iron oxides, leading to formation of more H2S, or S02 and SO³, depending on whether reducing or oxidising conditions prevail. Sulphur was found to be retained in the ash in different forms, with one being sulphur oxides that reacted with the high temperature transformation product (CaO) of calcite or dolomite to form anhydrite. Findings from the investigation of sulphur capturing in a packed coal bed suggests that SO2² injection into a packed coal bed under controlled conditions leads to sulphur capturing in the coal minerals, particularly CaO from the decomposition of limestone and dolomite. Notable amounts of CaS were found to be the sulphur-capture product; this was associated with high carbon content that favours formation of CaS over CaS04. A linear relationship between carbon content and the amount of CaS formed was obtained.