In-depth numerical analysis on the determination of amount of CO2 recirculation in LNG/O2/CO2 combustion

The determination of proper amount of CO2 recirculation is one of the critical issues in oxy-fuel combustion technology for the reduction of CO2 emissions by the capture and sequestration of CO2 species in flue gas. The objective of this study is to determine the optimum value of O2 fraction in O2/CO2 mixture to obtain similar flame characteristics with LNG–air combustion. To this end, a systematic numerical investigation has been made in order to resolve the physical feature of LNG/O2/CO2 combustion. For this, SIMPLEC algorithm is used for the resolution of pressure velocity coupling. And for the Reynolds stresses and turbulent reaction the popular two-equation (k–ε) model by Launder and Spalding and eddy breakup model by Magnussen and Hjertager were incorporated, respectively. The radiative heat transfer is calculated from the volumetric energy loss rate from flame, considering absorption coefficient of H2O, CO2 and CO gases. A series of parametric investigation has been made as function of oxidizer type, O2 fraction and fuel type for the resolution of combustion characteristics such as flame temperature, turbulent mixing and species concentration. Further the increased effect of CO2 species on the flame temperature is carefully examined by the consideration of change of specific heat and radiation effect. Based on this study, it was observed that the same mass flow rate of CO2 with N2 appears as the most adequate value for the amount of CO2 recirculation for LNG fuel since the lower Cp value for the CO2 relative to N2 species at lower temperatures cancels the effect of the higher Cp value at higher temperatures over the range of flame temperatures present in this study. However, for the fuel with high C/H ratio, for example of coal, the reduced amount of CO2 recirculation is recommended in order to compensate the increased radiation heat loss. In general, the calculation results were physically acceptable and consistent with reported data in literature. Further work is strongly recommended for a large-scale combustor such as coal-fired power plant to figure out important parameters caused by the effect of increased combustor size and the presence of particle phase, etc.