Transformations of sulfur compounds in oil shale ash suspension

Abstract

Oil shale (sulfur content <2%) is mainly used for the production of electricity and heat in Estonia. Calcium-rich waste formed in the process of oil shale combustion can be used as sorbents in the CO2 mineralisation process. CaSO4 is the main sulfur compound in the ashes of oil shale, but traces of sulfides are also found. This means that transformations of sulfur compounds in the water environment must be considered while using hydrotransport and open air deposition of oil shale ash as well as treating alkaline ash transportation waters. The transformations of sulfur compounds in model systems (ash - water and CaS - water) were studied in anaerobic and aerobic conditions as well as in the process of aqueous carbonization. The separation of poisonous H2S(g) was continuously monitored. The results revealed that the leachates of oil shale ash contained mainly sulfates in both aerobic and anaerobic conditions but sulfides and sulfites were also present. Small amount of H2S(g) was emitted already in the first minutes of carbonation process at high pH region (~12). The alkalinity decreased during carbonation of ash - water suspensions, triggering substantial H2S release at pH<9 (the content of H2S(g) in off-gas reached to 110 ppm). Taking environmental issues into consideration, the critical pH value of the aqueous carbonation system of oil shale ash must be optimized in order to minimize the separation of gaseous H2S emissions. © 201 WIT Press.