(587k) Critical CO2 Carnot Engine for Industrial Waste Heat Recovery and Utilization

Industrial processes regularly discard large volumes of low-grade waste heat to the environment. Harnessing this heat and transforming it into usable power would be environmentally and economically advantageous for industries. An overlooked solution to utilizing this waste heat is the use of a Carnot cycle to generate work with supercritical CO₂ as the working fluid. Supercritical CO₂is a novel working fluid for a Carnot engine as its critical temperature (31.1C) is much lower than the boiling point of water. This allows for the lower isotherm of the Carnot cycle to be constructed at the critical temperature, resulting in a sizable Carnot cycle with a near flat lower isotherm and upper isotherm just below the temperature of the low-grade heat. Thus, this type of Carnot cycle encompasses a larger area on a P-V diagram than any other thermodynamic conditions, thereby optimizing the amount of work that can be converted to power using a relatively small engine.We describe this approach of waste heat harnessing and illustrate ideas for utilizing the waste heat from an industrial toluene process. The impact of compression ratio, upper isotherm temperature, and pressure in the engine are investigated. Resulting savings amount to hundreds of thousands of kWh/year, thereby reducing cooler loads and electricity bills for the process, as well as reducing environmental emissions.

References:

[1] J.T. Banchero, B.D. Smith (Ed.), R.J. Hengstebeck. Disproportionation of toluene, 1969.

[2] Eldred H. Chimowitz Madeleine R. Laitz, F. Douglas Kelley. Critical CO₂Carnot Cycle for Waste Heat Utilization, 2017.