Pressurised Oxy-Coal Combustion Rankine-Cycle for Future Zero Emission Power Plants: Process Design and Energy Analysis

This paper presents the process design and the energy analysis for a coal-fired power plant based on pressurised oxycoal combustion and including carbon capture technologies. A combustion technology performing a pressurised combustion of coal in an atmosphere of O₂/CO₂/H₂O and including flue gases recycling has been selected. Combustion and steam production occur in separated equipments and the combustor’s design allows achieving high ash removal efficiency. The Rankine cycle has been chosen as the most viable thermodynamic cycle in a short-term scenario. Oxygen required by the combustion process is supplied by a cryogenic Air Separation Unit (ASU) and a double-reheat ultrasupercritical cycle is employed with main steam conditions of 250bar/605°C and reheat steam temperatures of 605°C/620°C. All choices related to thermal cycle selection and process design have been conducted upon the principle of feasibility and reliability. In order to increase net plant efficiency both sensible and latent heat is recovered from the flue gas stream before entering the purification and compression section. By operating in pressure it becomes possible to recover a larger amount of heat than in the atmospheric case. As a result, all low pressure steam bleedings and the corresponding regenerative heat exchangers can be eliminated. Process simulation is carried out in the paper and the expected efficiency is evaluated, as well as other cycle performance parameters. Since a relevant benefit may arise from the combustion of cheap coals, the impact of burning high-ash content and low ash-fusion-temperature coals is assessed. The impact of energy penalties associated to oxygen production and the benefit arising from high heat-transfer coefficients due to the increased pressure of the flue gas are deeply investigated.