CFD Based Ash Deposition Prediction in a BFB Firing Mixtures of Peat and Forest Residue Available to Purchase

Fuels currently used for energy production in thermal power plants are characterized by their huge variety ranging from fossil fuels to biomass and waste. This multitude of fuels offers opportunities to the energy industry and nowadays many power plants do not fire either of these fuels but mixtures of them are burnt. While this procedure may lead to overall economic and environmental advantages it is very demanding for the boiler operators to still meet expectations concerning boiler performance, boiler availability and emission regulations. In the course of this latest trend in boiler operation, ash related operational problems such as slagging, fouling and corrosion are ranking very high on the list of reasons leading to significant reduction of boiler availability. Ash related problems strongly dependent on fuel specific aspects, such as the mineral matter distribution in the fuel, aspects specific to the used combustion technique as well as design aspects unique for the combustion chamber of any operating power plant. The overall goal in combustion related research is therefore the prediction of potential operational problems originating from fuel streams entering the combustion chamber as well as those originating from the design of individual furnaces. In our earlier work we have strongly focused on developing an advanced ash behavior prediction tool for biomass combustion combining computational fluid dynamic calculations (CFD) and advanced fuel analysis. In this paper the tool is applied to analyze the slagging and fouling tendency in a 295 MW bubbling fluidized bed boiler fired with mixtures of peat and forest residue. In addition to the overall deposition prediction this work focuses on details of the models used in the computational fluid dynamic calculations. These include a study on the importance of the accurate description of the fuel feeding system and related to this aspect the advanced description of the bubbling bed with regard to release of primary gas and ash particles from its surface to the freeboard. Evaluation of the predictions comparing simulation results with deposits on the furnace walls show good agreement.