Power Plant Ash Composition Transformations During Load Cycling

Abstract

Mainly due to the introduction of new renewable non-dispatchable capacities to the electric grid the load cycling of power plants is increasing. This includes condensing power plants using fossil fuels. In case of power plants working on fossil fuels, their environmental footprint is reduced by using their combustion ash in construction material production, agriculture, etc. Chemical composition of each type of ash is crucial for each specific use and the constant unpredictable variation in the ash content is a problem for power plants. We can presume that ramping up and ramping down of power plants influences this composition. The focus of this study are the effects of load cycling on power plant output ash chemical composition. The results of industrial tests on a power unit are summarized and analyzed. Bottom ash samples were taken at different loads in 3 repeated experiments. The experiments were carried out on one of the two 320 t/h boilers of a 200 MWel power unit. As primary fuel the boiler uses oil shale, a low-grade solid fossil fuel with similar properties to brown coals. 6 Mt of oil shale ash is generated from 13 Mt of oil shale annually in Estonia. The utilization of this ash is still a major challenge as is the utilization of other fossil fuel ashes in other countries. The results of the study show that load cycling has a consistent impact on ash composition making it possible to anticipate certain changes. At half load the share of carbonate Carbon in the ash on the average increased twice and Sulphur components by 30 %. During ramping the according increases were 40 % and 10 %. This method of determining these trends demonstrated in the study is applicable for all solid fuel power plants. This information can then be used to produce ash by batches and by that increase its utilization.