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In a large-scale circulating fluidized bed furnace, the local feeding of fuel, air, and other input materials, and the limited mixing rate of different reactants produce spatially non-uniform process conditions. To simulate the real conditions, the furnace should be modelled three-dimensionally or the three-dimensional effects should be accounted for. The fluidized beds can be studied by different model approaches, ranging from micro-scale particle models and meso-scale two-fluid models to macro-scale engineering models. The fundamentals-oriented micro- and meso-scale models are not yet capable for practical comprehensive calculations of industrial scale circulating fluidized bed units, including modelling of reactions, attrition of particles, and heat transfer. The following paper introduces a three-dimensional semi-empirical steady state model for modelling combustion and gasification in circulating fluidized bed processes. The incorporated submodels include fluid dynamics of solids and gases, fuel combustion and limestone reactions, comminution of solid materials, homogeneous reactions, heat transfer within suspension and to surfaces, models for separators and external heat exchangers, and a model for nitrogen oxide chemistry. The model structure and the main features together with a sample calculation are described. A review of the currently used model approaches for fluidized bed systems at different scales is included to relate the presented model to other modelling field and to justify the need for semi-empirical modelling approach.

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Carbon capture and storage is a concept to reduce greenhouse gas emissions of energy production from fossil fuels. In oxy-fuel combustion, the fuel is burned in a mixture of oxygen and recycled flue gas. This generates CO 2 -rich flue gas from which the CO 2 is easily separated and compressed. Foster Wheeler Power Group is developing the existing design tools and process models of air-fired circulating fluidized bed boilers to implement specific features of oxycombustion. The validation data is produced from bench-scale and pilot-scale experiments at the VTT, Technical Research Centre of Finland. A three-dimensional circulating fluidized bed (CFB) furnace model is developed and applied by Lappeenranta University of Technology for predicting the effects of oxycombustion in full scale units. This paper presents concept studies and initial 3D modeling results based on a 460 MW e supercritical CFB power plant at Lagisza, and pilot-scale studies with flue gas recirculation demonstrating real oxygen combustion conditions.

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