CO2 Extraction

Current CO2 extraction methods are variants of gas separation—isolation of one gaseous component from a blend of other gases or vapors. If it were easy to separate CO2 from exhaust gas products, and if the market price of the gas justified the separation, a large industrial market for the product would probably already exist. But as it turns out, it is not easy and industrial processes attempting to extract CO2 don't survive without some unique set of financial conditions or incentives. The largest CO2 capture plant in the world is only 2,000 tonnes per day, and most of the plants are extracting CO2 from a relatively clean natural gas exhaust flow. In contrast, a 500 MWe fossil plant can generate 500 tonnes of CO2 per hour and any new plant will come equipped with SO2, NOx, and PM controls already placed in the exhaust flow path.

Overcoming the energy penalties associated with gas separation could be considered a monumental technology prize. Extracting CO2 from a fossil steam plant may exact a penalty as high as 400 kW h per tonne of CO2. For coal, each MW h produces a tonne of CO2 [13], thus, this level of energy penalty could result in efficiency decrease as high as 40%. Optimizing plant performance and selecting solvents more appropriate for the process conditions (e.g. physical solvents for pressurized systems, chemical solvents for atmospheric systems) could reduce that energy penalty, but it would probably still require 20-25% of plant output. Such a large reduction in performance is completely counter to the first principle of carbon reduction—improve plant efficiency to reduce the fuel consumption (which on its own merits results in a significant CO2 emissions reduction).

Gas separation, as a technology improvement, is not limited to extracting CO2. Oxygen production is critical to some of the most novel technologies being explored—gasification and oxy-fuel. Gasification, one of the most important steps in an IGCC facility, consumes so much additional power that the overall net efficiency may even be less than that of a modern supercritical fossil steam plant—in the range of 38-41% net efficiency. Reducing that load, or changing the cycle in such a way that oxygen separation is achieved in an entirely different way, could recover a significant amount of that loss.

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