|By Robynn Andracsek, P.E., and Jeffrey Binder, P.G., Burns & McDonnell|
By now most utilities have probably heard the details of EPA’s latest greenhouse gas (GHG) proposal for New Source Performance Standards (NSPS) from new fossil fuel-fired electric generation units. The rule depends strongly on future coal- and gas-fired power plants controlling greenhouse gas (GHG) emissions through carbon capture and storage (CCS). How exactly does the storage/sequestration element of CCS work and where can it occur?
CCS is not a new technology. It is has been used for over 40 years in the oil and gas industry for enhanced oil recovery. Expanding CCS into the power generation industry is a new application of an old technology. There is an economic benefit to injection CO2 into depleted oil and gas field, collapsed coal mines, or un-minable coal beds.
The key to CCS is the right geology. By injecting CO2 under pressure into suitable rock formations, the gas absorbs into the pores of the rock. Picture the type of holes and cavities found in a reef. These are the types of spaces, on a much smaller scale, that the CO2 fills in the rock. Over time the pressure releases as the gas reacts with fluids in the rock and the CO2 gas becomes a solid precipitate. The pores fill up and the rock is “full.”
The volume of gases produced and the amount of storage capacity in the U.S. are important to consider. If a 750 MW supercritical pulverized coal-fired boiler is considered to be the reference plant, the annual CO2 output is around 5 million tons. Assume that this plant is ideally located over a 300-foot sandstone formation. The 50 year output of this plant would need a 442 square mile area (mi2) for storage. Eight plants of this size would require a 3,450 mi2 storage reservoir which is equivalent to a Class 18 oil field, of which there are only 20 in the U.S. There are currently over 200 coal-fired power plants in the U.S. of the size of this reference plant. Therefore, there is not enough storage capacity in the U.S. for the CO2 output from existing plants.
The Department of Energy (DOE) and the EPA, 1 however, would disagree. They estimate that the storage capacity in the U.S. is equivalent to 600 to 6,700 years of current level emissions from large stationary sources in the United States. The difference is that DOE and EPA are taking into account all potential geologic settings without screening ones that meet suitability criteria and have the highest potential for success from a practical standpoint.
One issue is that these appropriate rock formations are not located everywhere. Deep sedimentary basins in North Dakota, Illinois, Louisiana, and Texas, for example, have the right geology to make CCS theoretically feasible. A new power plant would ideally be sited over one of these locations. An existing plant would need pipeline infrastructure to move the captured gas to a suitable formation.
Safety is often a public concern whenever CCS development is proposed. Carbon dioxide is a low hanging gas, denser than air. It is corrosive, odorless, and colorless. However, when properly sited, CCS in enhanced oil recovery (EOR) operations has been safely operated. Intensive monitoring of the CCS injection wells and storage reservoirs will help to maintain safe operation of these facilities to protect the safety of the public. As in all perceived new ventures, public perceptions as well as a NIMBY (not in my backyard) mentality could prevent and delay proposed large scale CCS operations.
Currently there are five large scale CCS projects at U.S. power plants, all in the construction or planning stages.11 Likewise in other countries, power plant CCS projects are not yet in full-scale operation.
At the power plant, on-site space issues can be an issue given that the CO2 capture and injection infrastructure can have a foot print as large as the power block. This will become a fatal flaw for many utilities if, as expected, the proposed CCS requirement is expanded to existing coal-fired boilers.
When all of these issues are taken into account, it is clear that the proposed GHG NSPS was not written with the intent of encouraging CCS as a solution to CO2 emissions from modern power plants. Rather, the CCS requirement is meant to serve as a deterrent to construction of new power plants.
Given the proper geology, storage and sequestration of CO2 might be technically feasible but it is hardly a practical solution.
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