Ash deposition prediction in biomass fired fluidised bed boilers – combination of CFD and advanced fuel analysis
by Christian Mueller, Bengt-Johan Skrifvars, Rainer Backman, Mikko Hupa
Progress in Computational Fluid Dynamics, An International Journal (PCFD), Vol. 3, No. 2/3/4, 2003

Abstract: Operational problems caused by ash such as slagging, fouling and corrosion of boiler surfaces continue to be the most usual single reason for unscheduled shut downs of these boilers. The amount and distribution of various ash forming elements in the fuel and the local operational conditions in a boiler strongly affect the type of fly ash formed during the firing process, which again will affect the deposition behaviour. To avoid these operational uncertainties, it will be very beneficial to be able to predict ash deposition behaviour based on advanced fuel analysis combined with computational fluid dynamic (CFD) calculations. This paper presents for the first time a concept combining CFD calculations and advanced fuel analysis to predict the ash deposition behaviour on heat exchanger surfaces and boiler walls in the freeboard of a 105 MW biomass fired bubbling fluidised bed boiler. Extensive experimental investigations of the untreated fuel deliver the composition of the ash forming elements in the fuel. These fuel specific data are used as input for advanced thermodynamic equilibrium analysis leading to a detailed description of the temperature dependent melting behaviour of the ash. Based on this melting behaviour, a fuel-specific stickiness criterion is determined. This stickiness criterion, experiences from field studies and the operational set up of the boiler serve as boundary conditions for the CFD calculations. In these calculations physical and chemical processes occurring in the freeboard region of a bubbling fluidised bed combustor - starting from the bed up to the second super heater - are predicted in the form of gas phase and ash particle trajectory simulations. The exact positions of ash particle impacts on the boiler surfaces are recorded and the particle temperatures at these locations are the linking parameter to the fuel-specific stickiness criterion. The predicted locations of high ash deposition probability on boiler walls and super heater surfaces are compared with observations made in the boiler and very good qualitative agreement is found.

Online publication date: Wed, 24-Dec-2003

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