Freezing of coal in the underground storage of a power plant

A coal freezing incident in the underground coal storage of the Salmisaari power plant in Helsinki represents a unique risk factor in terms of the contributing mechanisms. Filling with subzero temperature coal in winter froze silo drains with seepage water, resulting in its leakage into the silo to form large lumps of icy coal that blocked the fuel discharge system. Compared to any previous experience with freezing of coal in transport or storage, including closed storage silos, the unexpected direction of heat transfer from storage wall to coal means an additional challenge in its mitigation, for example, by application of freeze conditioning agents. To assess the accompanying risk and options, the strength of the frozen coal has been modelled from the expected behaviour bounded by the extremes of moisture content. Based on this strength model and realistic distributions of the input variables, a Monte Carlo analysis has been used to assess the expected strength of frozen coal under the local winter climatic conditions. The results suggest that the strength of frozen coal in the discharge hoppers is typically strong to render immediate remedial action tedious and thus explain the challenge to the plant operation.