The Impact of CO2 Capture on Water Requirements of Power Plants

Implementing CO₂ capture in an existing power station has some effect on its water consumption. In fact a CO₂ capture system in general requires additional water for cooling and make-up, increasing the existing requirements, which can be of concern in areas where water is scarce. For post combustion capture in Pulverized Coal (PC) and Natural Gas Combined Cycle (NGCC) power plants and for pre combustion capture in Integrated Gasifier Combined Cycle (IGCC) facilities, water use estimations reported by the U.S. Department of Energy’s National Energy Technology Laboratory (DOE NETL) show increases in water consumption per MWh of power generated of around 87% for supercritical PC, 76% for NGCC and 61% for slurry fed IGCC.

Oxy-combustion would require lower water volumes amongst currently available CO₂ capture methods as the water requirements associated with oxy-combustion would be only for the CO₂ compression cooling system whereas in post- and pre- combustion systems water is also needed for cooling other process streams (e.g. lean solvent cooler). The increase in water use for an oxy-combustion retrofit calculated by DOE-NETL is around 56% per MWh of net power production.

The studies available in the literature provide quantification of these requirements, indicating that they are significant, however, alternative solutions exist to mitigate the problem, like using less water-intensive capture technologies or optimizing the integration with the capture plant. 

Some of the new-generation technologies for CO₂ capture currently under development can bring improvements in terms of water demands, although it is not the primary target of developers. Membranes, solid sorbents and calcium looping are examples of new technologies whose supplemental water requirements are limited to cooling water for CO₂ compression.  

Freshwater withdrawal and usage can also be reduced through the employment of waste heat integration as shown by a recent study on the implementation of a new 250 MWe demonstration carbon capture unit of the Rotterdam Opslag en Afvang Demonstratieproject (ROAD) with a recently-built 1070 MWe coal-fired unit. Estimates indicate that cooling water usage would increase by 6% in absolute terms, or 12% more water consumption per MWh of power production. Extrapolation of the result for a full scale plant with similar characteristics and integration philosophy would result in a 25% absolute increase or 63% more water consumption per MWh of power production. In addition, water can be recovered in the flue gas cooler of the capture system and reused as make-up water in the power plant after proper treatment, achieving a reduction in freshwater use up by 94%, therefore nearly eliminating the need for increased freshwater supply.

These literature values, however, should not be used as universal indicators. The actual increase in water consumption must be estimated on a case by case basis, carefully accounting for the capture technology (type and size), the cooling systems adopted and the level of integration. The aim is to provide a review of the reported water use information and present options to reduce water consumption associated with CO₂ capture.