Thermodynamic limitations to direct CO2 utilisation within a small-scale integrated biomass power cycle

Partially recycling CO2-rich exhaust gases from a syngas fuelled internal combustion engine to a biomass gasifier has the capability to realise a new method for direct carbon dioxide utilisation (CDU) within a bioenergy system. Simulation of an integrated, air-blown biomass gasification power cycle was used to study thermodynamic as-pects of this emerging CDU technology. Analysis of the system model at varying gasifier air ratios and exhaust recycling ratios revealed the potential for modest system improvements under limited recycling ratios. Compared to a representative base thermodynamic case with overall system efficiency of 28.14 %, employing exhaust gas recycling (EGR) enhanced gasification system efficiency to 29.24 % and reduced the specific emissions by 46.2 g-CO2/kWh. Further investigation of the EGR-enhanced gasification system revealed the important coupling be-tween gasification equilibrium temperature and exhaust gas temperature through the syngas lower heating value (LHV). Major limitations to the thermodynamic conditions of EGR-enhanced gasification as a CDU strategy result from the increased dilution of the syngas fuel by N-2 and CO2 at high recycling ratios, restricting equilibrium temperatures and reducing gasification efficiency. N(2)dilution in the system reduces the efficiency by up to 2.5 % depending on the gasifier air ratio, causing a corresponding increase to specific CO2 emissions. Thermodynamic modelling indicates pre-combustion N2 removal from an EGR-gasification system could decrease specific CO2 emissions by 9.73 %, emitting 118.5 g/kWh less CO2 than the basic system.

Automated summary

Partially recycling CO2-rich exhaust gases can realize the direct utilization of carbon dioxide in a bioenergy system. Analysis shows the potential for system improvements under limited recycling ratios. Gasification system efficiency and specific emissions can be enhanced through exhaust gas recycling (EGR). The coupling between gasification equilibrium temperature and exhaust gas temperature is an important factor in the EGR-enhanced gasification system.