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Sulphate Chemistry under Pressurised Oxidising, Reducing and Fluctuating Conditions (LIEKKI 2-217)

Content: In atmospheric fluidised bed combustion it has been found that sulphur capture with limestone or dolomite reaches a maximum at about 850°C.This has been concluded to be due to fluctuating oxidising/reducing conditions present in an atmospheric combustor.The fluctuating conditions result in a partial decomposition of CaSO4 to CaO and SO2.The CaSO4 decomposition rate increases with tempeature and above 850°C the rate gets higher than the sulphur capture reaction rate.In this project the decomposition of CaSO4 has been studied also under pressurised conditions.In this case the CO2 partial pressure is high enough to prevent calcination and therefore the CaSO4 will not form CaO but CaCO3.In 1996 both atmospheric and pressurised experiments were made under conditions representing Reactions 1-6.This means that the conditions fluctuated between oxidising and slightly reducing conditions.The results confirmed that there exists a sulphur capture maximum at about 850°C under atmospheric conditions due to both sintering effects and fluctuating gas atmosphere.No optimum temperature was observed under pressurised conditions and the sulphur capture rate increased with temperature in the interval 850-950°C.The objective for 1997 is to study the effects of strongly reducing conditions, i.e. when CaS is the stable sulphur compound (Reaction 7).Atmospheric: CaCO3 <=> CaO + CO2 (1), CaO + SO2 + 1/2O2 <=> CaSO4 (2), CaSO4 + CO <=> CaO + SO2 + CO2 (3).Pressurised: CaCO3 + SO2 + 1/2 O2 <=> CaSO4 + CO2 (4), CaSO4 + CO <=> CaCO3 + SO2 (5), CaSO4 + 4 CO <=> CaS + 4 CO2 (6), CaSO4 + 4 CO <=> CaS + 4 CO2 (7).

Publication(s)

1.10.1996 / 18.6.1998