Exergy Analysis of Coal Energy Conversion With Carbon Sequestration Via Combustion in Supercritical Saline Aquifer Water

Decarbonization of electricity production is a vital component in meeting stringent emissions targets aimed at curbing the effects of global climate change. Most projected pathways toward meeting those targets include a large contribution from carbon capture and storage. Many capture technologies impose a large energy penalty to separate and compress carbon dioxide (CO₂). Also, injected neat CO₂ in a deep saline aquifer is buoyant compared to the aquifer brine and requires an impermeable seal to prevent it from escaping the aquifer. An alternative technology was recently proposed by Heberle and Edwards [1] that burns coal in supercritical water pumped from a saline aquifer. The entire effluent stream is sequestered, capturing all carbon and non-mineral coal combustion products in the process. This stream is denser than the aquifer brine and therefore offers a higher level of storage security, and can utilize aquifers without suitable structural trapping. This technology also increases energy security in the U.S., allowing for the use of its coal resources while avoiding atmospheric pollution. In this paper, a complete architecture employing supercritical water oxidation is proposed, including a liquid-oxygen-pumped air separation unit and regenerator system that heats and desalinates the incoming brine. A thermodynamic model calculates the overall thermal efficiency of the plant, including all separation and storage energy penalties. In addition, an exergy analysis gives insights into the least efficient parts of the proposed system. The details and assumptions of the model are discussed. Insights from the model and these analyses elucidate how the proposed system may be operated as a zero-emission electricity source and the technical challenges that must be addressed for deployment.