Plant and system-level performance of combined heat and power plants equipped with different carbon capture technologies

Installing carbon capture and storage (BECCS) capability at existing biomass-fired combined heat and power (bio-CHP) plants with substantial emissions of biogenic CO2 could achieve significant quantities of the negative CO2 emissions required to meet climate targets. However, it is unclear which CO2 capture technology is optimal for extensive BECCS deployment in bio-CHP plants operating in district heating (DH) systems. This is in part due to inconsistent views regarding the perceived value of high-exergy energy carriers at the plant level and the extended energy system to which it belongs. This work evaluates how a bio-CHP plant in a DH system performs when equipped with CO2 capture systems with inherently different exergy requirements per unit of CO2 captured from the flue gases. The analysis is based upon steady-state process models of the steam cycle of an existing biomass-fired CHP plant as well as two chemical absorption-based CO2 capture technologies that use hot potassium carbonate (HPC) and amine-based (monoethanolamine or MEA) solvents. The models were developed to quantify the plant energy and exergy performances, both at the plant and system levels. In addition, heat recovery from the CO2 capture and conditioning units was considered, as well as the possibility of integrating large-scale heat pumps into the plant or using domestic heat pumps within the local DH system. The results show that the HPC process has more recoverable excess heat (∼0.99 MJ/kgCO2,captured) than the MEA process (0.58 MJ/kgCO2,captured) at temperature levels suitable for district heating, which is consistent with values reported in previous similar comparative studies. However, using energy performance within the plant boundary as a figure of merit is biased in favor of the HPC process. Considering heat and power, the energy efficiency of the bio-CHP plant fitted with HPC and MEA are estimated to be 90% and 76%, respectively. Whereas considering exergy performance within the plant boundary, the analysis emphasizes the significant advantage the amine-based capture process has over the HPC process. Higher exergy efficiency for the CHP plant with the MEA capture process (∼35%) compared to the plant with the HPC process (∼26%) implies a relatively superior ability of the plant to adapt its product output, i.e., heat and power production, and negative-CO2 emissions. Furthermore, advanced amine solvents allow the BECCS plant to capture well beyond 90% of its total CO2 emissions with relatively low increased specific heat demand.