Carbon Capture and Storage is the process of absorbing carbon dioxide (CO2) and storing it. The purpose is to either prevent CO2 from being released or remove it from the atmosphere. The process involves three steps: (a) CO2 capture, (b) storage of the gas, and finally (c) regeneration of the capture agent.
CCS facilities are grouped into two main classes: (a) CO2 capture from a point source (power station, smelter, brewery, etc) and (b) direct CO2 removal from the atmosphere. The second is considerably more costly because the atmospheric concentration of CO2 is very low – only 0.04%, where a power station exhaust gases may be 10% CO2. Capture from the atmosphere means processing a volume of gas which is about 400 times greater!
Operation begins by choosing a CO2 capture agent. This is usually an alkaline substance which reacts reversibly with the acid formed when CO2 dissolves in water. (Soda pop and sparkling wine are based on dissolving CO2 in water to produce a tingling acidic flavour.) The key is to react with the CO2 concentration present, but not to hold on to it so firmly that it becomes hard (costly) to restore the solution to its original alkaline condition.
One commonly used alkali is ammonia (NH3) dissolved in water. Exposed to CO2, this yields ammonium bicarbonate. In simple terms, the ammonium bicarbonate is heated to yield ammonia and CO2. Heat is used to separate the ammonia and CO2, the former returns to the reactor and the latter pumped away to be stored. Many industrial processes have an abundance of waste heat which can be used for this purpose.
A major issue with CCS is finding a suitable site to which the CO2 can be sent. Ideally, the gas would react chemically with local alkaline rock, becoming a solid which can be expected to remain solid and intact for millions of years. More likely, the CO2 will end up in a played out petroleum or gas well. Since these rock formations have held their pressurised hydrocarbons securely for millions of years, it is not unreasonable to expect that they will now hold their pressurised CO2 for similarly long periods of time. Salt caverns (formerly mines) are yet another type of site which, once tested, can be expected to provide safe long-term CO2 storage.
Capturing CO2 from a point source emitting the gas in concentrated form is relatively simple. The volume of exhaust gas to be treated is relatively small. However, some people suggest that combatting climate change will require removing CO2 from the air itself. Since the concentration of CO2 in air is so small, the volume of air which needs to be “cleaned” is hundreds of times larger. That means the equipment needed to do the job have to be far larger (simply to handle the volumes involved), as is the power needed to move the air, hence more costly. The choice of CO2 capturing medium must be carefully selected to do the job effectively.
CCS is frequently installed at partly depleted petroleum wells. CO2 produced from refining operations is then injected into a part-depleted well. When compressed to above 31oC and 73 bar (73 times atmospheric) pressure, CO2 forms a supercritical fluid. This has the properties of both a gas and a liquid. In particular, supercritical CO2 is a non-polar solvent, which means it is a good solvent for hydrocarbons (which are also non-polar liquids). It is able to sweep up residual petroleum (can be up to 50% of the oily matter present) from the rock formation, and bring that to the surface.
The additional hydrocarbons liberated by CO2 injection easily pay for installing and operating the required equipment. Unfortunately, burning these hydrocarbons liberates more CO2 than were originally buried in the rock formation. So paying for CCS in this way cannot ever be a sensible way to capture carbon!
In August 1986, Cameroon’s Lake Nyos “erupted” during the night. The “eruption” was of CO2 gas which had accumulated in the cold, deep water of the lake. “Something” caused the sudden release of millions of tonnes of CO2. Heavier than air, the gas moved outwards from the lake at ground level, killing almost every living creature within 25 kilometres of the lake, including over 1700 people and twice that number of livestock!
The CO2 appears to have been of volcanic origin. It may have been disturbed by a small volcanic eruption, an earthquake or an onshore rock slide, causing the CO2 supersaturated deep water to rise to the surface. No longer under the pressure of exerted by the depth of water, the gas came out of solution very rapidly. Although nobody saw the event, it is thought that this caused a 100 metre water column in the lake.
Since this event, an off-gassing system has been installed in the lake which releases accumulating CO2 gradually. Similar systems have been installed in other lakes which also have supersaturated solutions of CO2 gas in their deep water to prevent their populations from suffering such a tragedy.
The Lake Nyos event indicates that volcanic CO2 is being released from deep underground continuously. It is unlikely, but not impossible for CO2 stored underground to be released like this as a result of an earthquake.
CCS is a costly enterprise. Equipment must be installed along with pipelines to transport it to a suitable storage site. The storage site must be prepared to receive the gas by drilling injection well, and equipment to securely cap them – for centuries – when the reservoir is full.
Once this equipment is installed, it must be operated. Manpower must be hired and trained. The equipment will require energy to operate, particularly to compress and move the gas through pipelines.
An example of such an installation exists in West Virginia. In 2009, American Electric Power began to install CCS equipment at its Mountaineer power plant near New Haven West Virginia (completed 2011). The installation was designed to treat 20% of the flue gases produced by the plant, and to remove 90% of the CO2 from these gases. In fact, it exceeded expectations and removed nearly 100% of the CO2!
The CO2 was injected into a sandstone formation over 2km deep. The US Department of Energy provided 50% of the development, installation and operating costs of the facility. Approximately 30% of the electricity generated with the coal whose CO2 was captured was consumed in operating the system – an unsustainable burden for a generating plant which had to earn its keep and compete with other electricity sources! This is particularly true of a private company (Few American electricity generators are publically owned.) which must produce profits for its shareholders . . .
American Electric Power applied to the West Virginia government for permission to increase the price of its electricity to cover the extra costs created by CCS, but was turned down. In 2011, they cancelled a planned expansion of the project and in 2016, having proved that it operated as planned, the project was terminated.
CCS costs money to set up and to operate. Without the ability to increase electricity prices to cover these costs it simply will not be done. A suitably high carbon tax, which would be rebated to coal-fired generation with CCS, would make it viable. Good intentions on their own will not work!
As explained above, the cost of operating a CCS facility to remove CO2 from air would be even higher because the size of the equipment required is larger and the energy cost to operate it would be higher too.
The bottom line is that CCS technology is not economically viable without carbon taxation. And that carbon price would need to be far higher than we are contemplating today. It is likely that CCS will not be economically viable unless the price on carbon emissions rises to well over $100 per tonne, perhaps as high as $200 per tonne.