Operability and Reliability Constraints

Many of the technical proposals for carbon capture target treatment of 100% of the exhaust gases at each source (although limiting carbon capture to something less than 90%). Because the CO2 extraction system typically incurs high-pressure drop, a large number of fans and pumps must be employed to move the gas and liquid flow streams (typically in counter-current flow). Pressure imbalances, flow disruptions, system controls, even small leaks, have the potential to cause a unit trip or malfunction of the emission control system. If the environmental control is tightly integrated to the overall plant control, there is increased risk of tripping the entire generating unit off-line. These risks are compounded by the size of the unit. Losing 50 MWe of electrical generation in a small community could easily be corrected by transferring additional power to the district from a much larger grid (assuming it is fully interconnected to this grid). However, if the plant is large, say 500 MWe, the impact has the potential to quickly propagate through the grid, possibly beyond region to other reliability council regions. System integration, connecting the carbon capture to the power generation and carbon removal has the potential to reduce plant availability and overall system reliability. Larger, more complex plants will likely be predisposed to experience faults owing to the significant increase in plant complexity. The degree of integration will affect overall system reliability. Doubling the overall system complexity has the distinct potential of degrading overall system reliability.

From a power generation perspective, the United States is sub-divided into a series of smaller regulatory council regions, each overseeing requirements related to transmission, distribution, interconnection, and system reliability (see Fig. 10.8). However, the country is also segmented electrically into three asynchronous

Fig. 10.8 North American reliability council regions

regions: the far west, WECC; the East, and effectively the state of Texas (ERCOT). These regions are effectively out-of-phase with one another, a barrier that makes it difficult to "ship" electrons from East-to-West, or North-to-South. Some of the regulatory issues in these regions are state controlled, through their separate regulatory commission, and some regulated at the federal level.

In a vast network designed to extract carbon dioxide from exhaust gases, the problem of handling mechanical upsets that affect power system reliability must be addressed early on. During process upsets, and they will happen, a number of issues would need to be addressed quickly. At a minimum these should include;

• What to do with the exhaust gases that were supposed to be processed by the carbon capture system,

• How to handle the enormous quantities of solvent cycling through the system,

• Does this malfunction propagate back towards the primary generator, the power plant,

• Will an environmental control malfunction transition into a larger grid disturbance? If the fans and dampers are not able to quickly redirect the gas flow and the process safely corrected, the potential for a complete unit trip increases, as well as upsets to the larger grid

A large carbon capture and sequestration system added to a block of major power generating sources is suggestive of placing another network layered on top of the existing electrical grid. As with electricity, there is likely to be little capacity to store captured CO2 at the point of generation. The volumes would be enormous, and the most convenient form of storage (as a supercritical liquid) would demand significant quantities of high-pressure storage capacity. Because of this, when CO2 is extracted, it will be necessary to move it off-site as fast as the recovery process extracts it, or on-site accumulation would quickly overwhelm any local above-ground storage. Similar to the electricity, the CO2 and must be moved onto a separate grid (in this case, a CO2 pipeline, or possibly a gas pipeline converted to CO2 transmission). If there is a failure in the CO2 transport network, this event will backup very quickly to the operating plant. Since the CO2 is likely to be supercritical, it will act more like a liquid and not a gas, with substantially reduced compressibility. Thus, the viability of the carbon capture process is now tied to the capacity to withdraw the product stream as quickly as it is generated. A significant failure in any one component may require operator action that entails: (1) venting of CO2 where it is generated, (2) shutdown of the capture system, or (3) shutdown of the generator system. These complex system issues have not been factored into the grid reliability assessments and must be part of any long-range strategy to deploy the technology.

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