Oxy-coal combustion and its integration with power systems for CO2 capture

Abstract: Oxy-fuel combustion is one of the technologies for carbon dioxide (CO2) capture and storage (CCS) in fossil fuel based power systems to mitigate global greenhouse gases (GHGs) emissions. When introducing oxy-fuel combustion into the power systems, energy penalty for CCS has significant impacts on the system performance. The heat and mass balance of the oxy-fuel combustion power system need to be investigated due to the change of combustion environment. This study investigated oxy-fuel combustion with coal as the fuel, so called oxy-coal combustion, and its integration with power systems for CO2 capture. First, mass balance was formulated for the oxy-coal combustion considering flue gas recycle (FGR). Then, computational fluid dynamic (CFD) modelling was conducted on the oxy-coal combustion to identify its characteristics in terms of flame profile and radiation heat transfer. Finally, process simulation was performed on the oxy-coal combustion power system to evaluate its technical and economic performance including the subsystems of air separation unit (ASU), furnace/boiler, and cryogenic CO2 purification. In addition, a new peak and off-peak (POP) operation mode of ASU to shift the energy penalty for CCS and improve the performance of the whole system was addressed and analysed by net present value method. The results show that oxy-coal combustion can match well to conventional (air-coal) combustion under specific operating conditions, and results in a minimal change of existing boilers under conventional technology. The increase of moisture content in the flue gas has little impact on the flame temperature, but results in a higher surface incident radiation on boiler side walls. Compared with air-coal combustion power systems, oxy-coal combustion power systems have much lower flow rate of flue gas, lower NO and SO2 emissions, higher boiler efficiency, but a higher flue gas dew point. Furthermore, various FGR options in the oxy-coal combustion power system have no clear effect on recycle ratio, flow rate of flue gas, and electrical efficiency of the whole system, but cause much different flue gas compositions at the exit of the boiler. Energy penalty for ASU in the oxy-coal combustion power system accounts for about 7% based on low heating value. Comparatively, ASU has a larger effect than cryogenic CO2 purification on energy consumption in the oxy-coal combustion power system. The new POP operation mode of ASU is technically and economically feasible for shifting the energy use of ASU in the peak and off-peak periods, and more electricity could be generated at a higher price.