Modeling of Combustion, Mixing and Heat Transfer in Oxy-Fuel CFB Power Plants

Licentiatavhandling, 2013

An important contribution to anthropogenic emissions of CO2 is from electricity generation, i.e. emissions from large fossil fuelled power plants. Carbon Capture and Storage technologies offer the possibility for continued use of fossil fuels while almost eliminating the CO2 emissions. Oxy-fuel combustion is a promising CO2 capture technology which can be applied to electricity generation. Oxy-fuel combustion is currently being developed for power plants with pulverized coal combustion and for plants with circulating fluidized bed (CFB) combustion, the latter being the focus of this thesis. The large flexibility in heat extraction that CFB combustion offers through heat extraction possibilities within and outside the furnace enables combustion at low temperature (around 850C) even at high inlet oxygen concentrations. This potential to operate at high O2 inlet concentrations implies a possibility for a considerable reduction in the furnace dimensions. Furthermore, the high thermal capacity of the bed particles allows combustion with a relatively even temperature profile in the unit. Based on experimental data and previous modeling experience from air fired CFB combustion, this work develops modeling tools for assessing oxy-fuel CFB combustion across a wide range of operational conditions. Firstly, the additional requirements of oxy-fuel CFB boilers in comparison to air-fired CFB boilers are identified by means of modeling, closing the heat balance under different oxygen inlet concentrations for a 278 MWth CFB. Second, a generic model for combustion, valid for both air- and oxy-fired CFB combustion is proposed, including heterogeneous and homogeneous reactions and accounting for axial and lateral gas and solids mixing. The model results are compared with experimental data from two oxy-fuel-fired CFB units of 100 kW and 4 MW, yielding satisfactory results. Model simulations are carried out for a broad range of CFB-furnace operational conditions, investigating the progress of combustion using CO as an indicator. The model predicts that in the 278 MWth unit, heat extraction from the return leg is required at inlet O2 concentrations above 29% and the net external solids flux must increase when applying inlet O2 concentrations beyond approximately 34%. The model and experimental results imply that for similar inlet concentrations of O2 (i.e. around 21%) oxy-firing results in a higher CO peak concentration, as compared to the air-fired condition. In addition, the modeling shows that both the CO peak concentration and lateral gas mal-distribution increase with increasing O2 inlet concentration.