Nongray EWB and WSGG Radiation Modeling in Oxy-Fuel Environments

Abstract

According to a recent U.S. Greenhouse Gas Emissions Inventory (1), about 42% of 2008 CO2 (a greenhouse gas) emissions in the U.S were from burning fossil fuels (especially coal) to generate electricity. The 2010 U.S. International Energy Outlook (2) predicts that the world energy generation using coal and natural gas will continue to increase steadily in the future. This results in increased concentrations of atmospheric CO2, and calls for serious efforts to control its emissions from power plants through carbon capture technologies. Oxy-fuel combustion is a carbon capture technology in which the fossil fuel is burned in an atmosphere free from nitrogen, thereby reducing significantly the relative amount of N2 in the flue-gas and increasing the mole fractions of H2O and CO2. This low concentration of N2 facilitates the capture of CO2. The dramatic change in the flue composition results in changes in its thermal, chemical, and radiative properties. From the modeling point of view, existing transport, combustion, and radiation models that have parameters tuned for air-fuel combustion (where N2 is the dominant gaseous species in the flue) may need revision to improve the predictions of numerical simulations of oxy-fuel combustion. In this chapter, we consider recent efforts done to revise radiation modeling for oxy-fuel combustion, where five new radiative-property models were proposed to be used in oxy-fuel environments. All these models use the weighted-sum-of-gray-gases model (WSGGM). We apply and compare their performance in two oxy-fuel environments. Both environments consist of only H2O and CO2 as mixture species, and thus there is no N2 dilution, but the environments vary in the mole fractions of these two species. The first case has a CO2 mole fraction of 65%, whereas the second has a CO2 mole fraction of 90%. The former case is more relevant to what is referred to as wet flue gas recycle (wet FGR) where some flue gas is still recirculated into the furnace, but after to act as coal carrier or diluent (to temper the flame temperature). On the other hand, the second case is more relevant to what is referred to as dry flue gas recycle (dry FGR) where some flue gas is still recirculated into the furnace but after a stage of H2O condensation. This increases the CO2 fraction in the recycled flue gas (RFG) and consequently in the final flue gas leaving the furnace and the boiler of the plant. To highlight the influence of using an air-fuel WSGGM (a model with parameters were developed for use in air-fuel combustion) in oxy-fuel environments, the air-fuel WSGGM of Nongray EWB and WSGG Radiation Modeling in Oxy-Fuel Environments 23