Carbon Capture and Storage (CCS) – Energy Hurdles
Carbon dioxide Capture and Storage (CCS) is an evolving technology aiming to enable the continuing use of fossil fuels (coal, natural gas, crude oil) while mitigating the global warming potential of the carbon dioxide produced by fuel combustion.
The goal of CCS is to capture and sequester the CO2 from fossil fuel combustion while generating the smallest possible amount of additional CO2. However, CCS has thermodynamic challenges – the energy required by the unavoidable physics of the CCS process.
The thermodynamic challenges can be considered as series of hurdles to feasible CCS operation:
- Capturing CO2 from the exhaust stream
- Compressing or liquefying CO2 to a feasible density for transport
- Transporting CO2 to the storage site
- Securely injecting CO2 into the disposal site.
Hurdle 1: Capture
The more concentrated the CO2 is in the stream, the more efficient it is to capture and separate it from the other gas stream components (in terms of energy spent per kg CO2).
Consider an Acid Gas Removal Unit (AGRU) on a Gorgon or Browse reservoir-like gas composition with 12 to 18 mol% CO2 in natural gas. The other gas components are mostly methane, ethane and other hydrocarbons. Natural gas at the processing stage is very free of solids, dust and soot. The capture can take place at a pressure which increases efficiency for absorbing CO2 into an amine solvent or pushing it through a separation membrane. Typical processing conditions are 60 barg (~60 times atmospheric pressure or 60 atm) and approximately 20 to 40 deg C.
Contrast that with extracting CO2 from combustion exhausts where the CO2 is not only more dilute but at low pressure. The other gases include inert nitrogen, some unburnt fuel, as well as fumes and soot from the combustion process, making it a dirty stream. The condition of the stream is difficult for CO2 absorption – high temperatures and low pressures. With so much heat energy and room to move (that is what low pressure means), the CO2 molecules are much harder to convince to get into an absorbent or move through the molecule sized spaces in a membrane. Typical exhaust temperatures are 400 to 500 deg C, decreasing to 200 to 300 if there is waste heat recovery water cooling. And pressures are less than 1.5 barg under normal exhaust venting conditions (higher pressures would cause the engine producing the exhaust to lose power and must burn more fuel, resulting in more CO2 being produced.
Some select situations may offer ideal CO2 capture for efficiency – high CO2 concentration, large continuous volumes, access to extremely cold cooling water for example – but the great majority of fossil fuel combustion situations do not – gas and diesel fired power stations, diesel power ships, car, and truck exhausts.
Hurdle 2: Compression and/or Liquefaction
Once the CO2 is removed from the exhaust stream it must be compressed and or liquefied to make transport feasible. As a gas at atmospheric pressure, it is about 1000 times less dense than its liquid form. Compression takes energy. Above 5.1 bara (about 5 times atmospheric pressure), CO2 can be cooled until it becomes liquid at -55.6 deg C. CO2 is a little tricky to deal with because below 5.1 bar pressure, CO2 sublimes from dry ice to a gas, and vice versa – skipping the liquid phase completely.
Dehydration – carbon dioxide is very corrosive in water/ aqueous phase. Wet CO2 eats through carbon steel very quickly and is the main form of steel corrosion we see. Also, if you are going to liquefy CO2 it must be chilled to -55 degC as we mentioned above, so any water would form ice and clog the process equipment. So, to enable cheap transport by pipe or ship it must be dehydrated. The dehydration process will again require energy incurring additional CO2 emissions.
Hurdle 3: Transport
One of the great problems for CO2 disposal is that most of the small number of available disposal sites are deep below ground, and rarely near the (proposed) point of capture. Disposal sites are predominantly known pressured aquifers – extensive with a proven pressure containment seal, or depleted hydrocarbon reservoirs.
To get the CO2 from the point of compression/liquefaction to the disposal site will require energy, ie. to power a ship or push the CO2 through a pipeline. That energy will need to come from burning fuel for the ship or power generation equipment.
Hurdle 4: Storage – Sequestration
Sequestration is the process of pumping the CO2 down into the below-ground aquifer or depleted hydrocarbon reservoir system (i.e. porous rock) that will hold the CO2, hopefully for thousands if not millions of years.
The means to get it down there, it will need to be pushed down a well-hole – a hole of 2 to 30 inches diameter (50 to 525mm) lined with steel tubing of 1” up to a maximum of about 9 5/8″ diameter (that happens to be the largest diameter tubing used in natural gas wells).
Most sensibly proposed re-injection sites are at least 1000m deep – any shallower, and the risk that the CO2 will make it back to the surface is too high. The pressure at 1000m is normally around 100 bar (~100 times atmospheric pressure). Luckily, the reason the pressure is so high is a thing called “hydraulic gradient” or “static head”, meaning the CO2 travels down the well and puts weight on the CO2 below, helping rather than hindering injection.
However, the friction in the pipe, and the friction in squeezing the CO2 into the porous sands or rock will mean the surface pumping pressure is still considerable. And again, some CO2 will be generated to provide the energy for the pumping. Also, 1000m-deep injection wells are not cheap to drill, with prices starting at US$1million and can easily go to US$10million.
The best of industry has estimated that Coal fired power stations will need to raise power prices by 50% to make CCS viable. The price increase is for the capital and operating cost of the process equipment for CCS, and the largest part of the operating cost is the additional fuel required to power the CCS process. In broad terms, around 15% to 50% additional CO2 is generated by producing the energy to power the CCS process. The lower limit, 15% is where the situation is close to ideal – high pressure, high concentration CO2 close to the disposal site like the Gorgon project. The higher 50% is not a limit and can easily be exceeded if the process is poorly design and operated – to the point where the CCS process could even generate more CO2 than is disposed – yes, better to just not do it.
There are several factors that could significantly improve the viability of CCS opportunities:
- An efficient means to transport liquid CO2 to distant injection sites, ie. via pipeline or an efficient CO2 carrier ship. (Read more here: CO2-NH3 Carrier Design)
- Processes that avoid compression – such as conversion of CO2 to an insoluble solid such as Magnesium Carbonate. This replaces the compression step with a chemical reaction. BHP have been investigating using their spent high magnesium oxide tailings to capture CO2 in just this way.
- Use of renewables to power the CCS process. Obviously, this makes little sense for power generation (i.e. why not just use the renewable power directly and avoid the CO2 generation?), but may be more logical where (for example) the CO2 is coming from fertiliser production and CO2 generation is not avoidable.
The capture, compression, transport, and final storage of CO2 is typically an energy-intensive process that generates additional CO2. Capture of all vented CO2from existing infrastructure and equipment is never going to be feasible; however, many CCS projects will be feasible with thoughtful opportunity identification and system design.
Participants in CCS projects should be mindful of the overall goal and efficiencies of the overall CO2 capture-to-sequestration process, and challenge poor design and inefficiencies. The last thing anyone wants is a CCS process that fails to clear the hurdles and generates more CO2 than it sequesters.