Review of oil shale semicoke and its combustion utilization

Semicoke, one of the final products formed after retorting oil shale, is a potentially harmful solid waste often containing some toxic organic compounds and heavy metals, disposal of which can result in very great environmental contamination. However, the organic compounds remaining in semicoke lead to it having a potential heat of combustion, and thus semicoke may be considered for combustion utilization as a fuel. This paper reviews the fundamental characteristics and combustion utilization possibilities for semicoke that would allow treating and utilizing semicoke efficiently and in an environmentally friendly manner. Although properties of semicoke vary widely with both the retorting processes and their operational parameters, it generally contains significant amounts of inorganic minerals from the oil shale matrix, some organic compounds and lesser amounts of trace elements. Its leaching elutes have shown acceptable limit values at a landfill for non-hazardous waste and thus the leaching of heavy metals is not necessarily a problem. However the leaching of organic compounds may often exceed the limits for dissolved organic carbon. Thus, the incineration of semicoke will often be desirable both to ensure relatively low harm to the environment as well as efficient resource recycling for energy. Based on the current status and probable future development of oil shale industries, and the combustion characteristics of semicoke, two technical routes have been recommended for utilizing semicoke. These might also be regarded as references or benchmarks for evaluating the new development of oil shale retorting processes. One method involves an integrated utilization system for oil shale in which semicoke is actively treated and utilized in a circulating fluidized bed boiler for providing heat for both retorting the oil shale and for generating steam. The other option is to employ circulating fluidized bed technology to burn semicoke which has been previously produced and might still be a byproduct of some retorting technologies. With regard to both options, industrial circulating fluidized bed combustion results are discussed, in an effort to establish the feasibility of burning semicoke in a circulating fluidized bed boiler.