Reducing the water usage of post-combustion capture systems: The role of water condensation/evaporation in rotary regenerative gas/gas heat exchangers

Abstract

Water usage is expected to greatly increase when CO2 capture is added to thermal power plants. A major contribution is the reduction of flue gases temperatures from 100 to 150 °C to 30–50 °C. The majority of studies to date propose the use of direct contact cooling, combining a cold water loop with water cooling. This article expands on a previous study of the same authors (Herraiz et al., 2015) proposing dry air-cooled options with rotary regenerative gas/gas heat exchangers, relying on ambient air as the cooling fluid, to eliminate the use of process and cooling water prior to the carbon capture system. It proposes, for the first time, a new stand-alone model of a bi-sector air/gas rotary heat exchanger, which includes the contribution to heat transfer of condensation/evaporation when flue gases are cooled below the dew point. It shows that water condensation from the flue gases in one sector of the heat exchanger, enhances the total heat transfer rate, due to the diffusion of water through the non-condensable gases boundary layer. In order to maintain the cooling capacity of these rotary regenerative heat exchangers, initially designed to operate without condensation, this article shows that they should be designed with surface properties, gas velocities and heat transfer channel geometries with the aim of allowing water condensate to remain on the metal elements surface, and then evaporate into the air stream when the metal elements have rotated to the air side. The model also predicts the location of water condensation, so that enamelled elements can be incorporated to the cold-end tiers of metal elements and mitigate any possible long-term corrosion problems.