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Optimal CO2 capture and storage to reduce the energy costs of CCS

Figure 1: Heat, power and mass flows in integrated CCS system. Note that in this case intermittent power (curved lines) is smoothed by intermittent capture and compression of CO2.

Carbon dioxide capture and storage (CCS) has the potential to significantly reduce greenhouse gas emissions from electric power production, especially from coal-fired power generation.  For this reason, it is seen as an important “bridge” technology to reduce emissions in the coming decades before renewable and low-carbon technologies can supply all electricity demand.

Unfortunately, capturing CO2 via CCS is energy intensive due to thermal energy requirements for capturing CO2 from power plant fuel gas, as well as the need to compress captured CO2 for subsurface storage.  This energy demand can reduce net output from power plants by more than 20%. 

Another key problem facing the electricity grid in coming decades is the intermittency of renewable electricity generation.  Wind and solar power are produced when the natural resource is available, not when the power is demanded.  Historically, bulk storage of electricity has been expensive, causing problems in some areas of where a weakly connected grid has significant fractions of renewable generation (e.g., western Texas).

Our project aims to address these problems simultaneously through optimal capture of CO2 in response to electricity prices.  Our system consists of an existing baseload coal-fired power plant, a CO2 capture system, a natural gas turbine, and external wind power inputs to the system (see Figure 1).  Our optimization algorithm operates the gas turbine and CO2 capture and compression equipment in response to power prices and the availability of wind power so as to capture CO2 at the lowest cost.

For example, if excess wind generation occurs at a time of low demand and low power prices (e.g., in the night), it may be optimal to capture and compress excess CO2, compared to the average level of capture required to meet regulatory standards.  Conversely, when power prices are high, it may be optimal to avoid capturing and compressing CO2 so that all power from the coal-fired power plant and gas turbine can be sold.  As long as the average level of capture meets regulatory requirements, the operating costs of capturing CO2 could be significantly reduced.