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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 100-150 ºC to 30-50 ºC. 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 [1] 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.

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This work is a first-of-a-kind feasibility study investigating technology options with gas/gas rotary heat exchangers for the water management in the integration of Natural Gas Combined Cycle (NGCC) plants with post-combustion carbon capture, with and without exhaust gas recirculation (EGR). A range of configurations are examined for wet and dry cooling of the flue gas entering a postcombustion capture (PCC) absorption system, and regenerative heating of the CO2-depleted flue gas prior to the power plant stack. First, this work examines the addition of a gas/gas rotary heat exchanger to transfer heat from the exhaust gas entering the absorber into the CO2-depleted gas stream leaving the absorber. It then investigates the performance of a configuration with an additional air/gas rotary heater to further reduce exhaust flue gas temperature and water consumption, and, eventually, a more compact arrangement which combined the two heaters into a single gas/gas/air heater with a trisector configuration. A thermal performance analysis was conducted for each of the previous configurations, in order to evaluate the dimensions and the operational parameters of the heaters. By replacing the direct contact cooler traditionally used in PCC technology by a dry-cooling system, a significant reduction in the overall process water usage and cooling water consumption associated to the capture plant can be achieved. The second part of this work examines the use of a similar system for NGCC plant with EGR. This strategy increases CO2 concentration in gas turbine exhaust gases and reduce O2 induced solvent degradation. In addition to the heat and water balance around the absorber column of the PCC process, an important aspect of EGR is that recirculated gas stream temperature should be as low as possible so that the gas turbine performance is not compromised. The performance of the rotary heat exchanger configurations is analysed at different recirculation ratios.

No abstract