Selective flue gas recirculation using membranes in coal-fired power plants

Abstract

A simplified, conceptual process model is presented for coal-fired power plants with membrane-based selective recirculation of flue gas components (CO2, H2O). Selective circulation is aimed at increasing CO2-concentrations in the flue gas, which will benefit the process of CO2 capture. The use of membrane technology for selective recirculation has been proposed for this purpose by Membrane Technology Research (MTR) in the USA. The process model involves the assessment of heat balances and mass flows as determined by the combustion process, the capture process, and the mass transfer unit for selective recirculation. It is applied to two types of Australian coal: Hunter Valley black coal and Latrobe Valley brown coal, a low-rank coal that is locally utilized. The process model provides data on the Adiabatic Flame Temperature (AFT) with and without recirculation and the gas composition and flow rate at various points in the integrated process. The process model results clearly indicate the benefit of selective recirculation in raising CO2 concentrations in the flue gas and the disadvantages of reduced combustion temperatures (quantified through the AFT) and increased mass flow rate. An opportunity for application of the selective circulation concept is identified for the case of combustion of dried brown coal in a retrofit application to an existing boiler, originally designed for the combustion of the same wet brown coal. Here selective circulation of CO2, replacing water vapor generated from wet coal combustion, will enable the combustion of dried brown coal with ensuing benefits for the power plant efficiency. Furthermore, the AFT can be higher than for combustion of the wet coal and efficiency losses may not occur. More targeted modeling is needed to provide a detailed quantification of the benefits, which would then be used for an economic assessment and estimation on the impact on the power station efficiency. This would also need a realistic assessment of the membrane process using representative values for the membrane permeance and selectivity. © 2017 Society of Chemical Industry and John Wiley & Sons, Ltd.