Thermodynamic analysis of solar energy integrated underground coal gasification in the context of cleaner fossil power generation

• Integration of solar-UCG with supercritical CO2 turbine power generating system.
• Efficient utilization of low pressure UCG syn-gas in sCO2 turbine integrated system.
• Estimation of the net thermal efficiency of the proposed power generating systems.

Underground coal gasification (UCG) is an in-situ physico-chemical process for the conversion of deep coal resources into useful product gas. The UCG process is inherently a clean coal technology avoiding several difficulties of conventional coal mining process. However, it is a complex in-situ phenomenon depending on site-specific geological parameters. Energy loss occurs in a UCG process due to water influx, underground cavity pressure drop, gas loss to the surrounding strata and high temperature gasifying medium, which are major challenges of UCG operation. These losses can be compensated if UCG is integrated with solar energy, which is a renewable and a cleaner source of energy. In the present study, two types of low pressure UCG based power plants are conceptualised using solar energy – (i) a conventional steam turbine cycle consisting of a high pressure, a medium pressure and a low pressure turbine and (ii) a gas turbine cycle with supercritical carbon dioxide (sCO2) as a working fluid operating in a combined cycle power plant system. The scope of integration of solar power on the UCG based power plant systems is discussed for cleaner energy production. A detailed thermodynamic analysis is carried out to estimate the thermal efficiency of both cycles for cases with and without the integration of solar energy. A net thermal efficiency of the solar-UCG based steam turbine cycle is found as 28.2% with carbon capture and storage (CCS). The solar-UCG based sCO2 gas turbine cycle shows a high net thermal efficiency of 32.9% with CCS, which is 4% higher than the respective system without having solar energy utilization.