Carbon sequestration

Coal produces lots of carbon dioxide no matter how it is used. The currently hyped solution for large, fixed coal-powered plant, e.g. electricity generation, synthetic petrol or oil production, is carbon sequestration. This means that the carbon dioxide is separated from the plant's exhaust gases, compressed and pumped into exhausted coal mines or depleted oil and gas wells. It sometimes seems that supporters of carbon sequestration visualise a nice, shiny new power station with its chimney bent over and vanishing, together with the carbon dioxide, into the ground. Sorry, but it ain't so neat.

Some of the points that must be considered are:

• the economics of sequestration: apart from the capital costs there are energy costs for separating, compressing and transporting the carbon dioxide to the storage site. Estimates of the energy used for sequestration range from 10% to 40% of the plant's output.
• what proportion of the total emissions of the energy production cycle can be captured. A detailed estimate by Peter Viebahn of the German Aerospace Centre (DLR, Stuttgart) shows that, at best, carbon sequestration can capture 67% of the total emissions from coal burning energy production, which makes a coal burning plant with carbon sequestration at best comparable with a modern CCGT generator burning natural gas.
• the costs of building and maintaining the transport system. This is minimal if the consuming plant is close to the disposal site. However depleted oil wells are not usually close to coal mines, so either the fuel or the waste carbon will have to be shipped elsewhere. As a concrete example, Montana is currently promoting the idea that, by making synthetic oil from its vast coal seams, the USA will be freed from any dependence on Middle Eastern oil. Montana's coal is in shallow seams, so it would be extracted from open cast mines. You can't sequester anything in a worked-out open-cast mine for more than a few seconds, so sequestration would have to be off-site. I think the nearest depleted oil wells are in central California....
• the carbon dioxide will probably be piped away, so the pipeline needs to be duplicated unless the source plant is to be shut down whenever the pipeline needs maintenance. If the disposal is in undersea wells the pipeline maintenance gets harder and more expensive. These pipelines will not be small or cheap. Every tonne of coal burnt generates 3.67 tonnes of carbon dioxide, which must be removed just as fast as more coal is shipped in. In addition, carbon dioxide can only be liquified at room temperature at a pressure of 5.1 atmospheres so the pipes will either need to be large, to carry the gas at atmospheric pressure, or strong enough to be capable of handling the high pressure needed to keep the carbon dioxide in liquid form. Last but not least, the pipes and pumping equipment must be made of materials that are not attacked by carbon dioxide, even in the presence of water vapour.
• the sequestration site must guarantee to hold the compressed carbon dioxide for millions of years. It must be stored for far longer than nuclear waste has to be kept because, unlike nuclear waste, sequestrated carbon dioxide never loses its power to harm the ecosphere.
• some other questions that need attention are:
  • can a worked-out coal mine ever be made gas-tight, let alone for the forseeable future?
  • an oil or gas well was gas tight when it was tapped, but has it subsided and cracked its impermeable rock cap while it was being emptied?
  • did fracturing the oil-bearing rock, commonly done to improve extraction rates, cause the impermeable cap to crack too?
  • were the test and production bore holes all catalogued and mapped?
  • can they all be found?
  • can they all be sealed well enough to guarantee no leaks for millions of years?
  • how is a leak to be detected?
  • how can the leak be repaired without losing significant amounts of carbon dioxide?
  • if all carbon is to be sequestered, every fossil fuel carbon atom extracted must be replaced with a molecule of carbon dioxide. Carbon dioxide molecules weigh 3.67 times as much as as a carbon atom. It has 2.26 times the volume of a carbon atom or 2.07 time the volume of the CH₂ repeating unit in hydrocarbons (oil). What are the geological consequences of replacing carbon or CH₂ units with the equivalent number of carbon dioxide molecules?
  • undersea wells contain water. Carbon dioxide dissolves in water to form carbonic acid. Can we guarantee that this will not damage the impermeable rock cap or borehole seals over geologic ages? Indications are that we can't. See the Texan Gulf coast injection experiment below.
• the oil reservoirs may have been sealed since the Carboniferous era but can we guarantee that the refilled reservoirs remain sealed through future geological ages and tectonic shifts? Bear in mind that not all oil-bearing regions are geologically stable. California and Iran spring to mind here. Some projects appear to be ignoring this point. See the BP and the Edison Mission Group project below.

The most promising separation method is to gassify coal, extract the carbon dioxide from the resulting syngas and use this to drive combined cycle gas turbine generators. The extracted carbon dioxide would be dried, compressed and shipped off to the storage facility. This is not a zero carbon system: some carbon dioxide will remain in the turbine's fuel and this will be exhausted to the atmosphere. The energy cost of extracting the carbon dioxide is 15-20%, giving an overall thermal efficiency of 35-40% - about the same as current coal-fired plant. This information came the report of the Cycles for Low Carbon Dioxide Production conference, held at Cranfield University in March, 2003.

Carbon sequestration is future technology. Here's a sample showing the general state of play at July 2006. The projects and results were found in online EU reports and New Scientist.

Current projects

All the projects in this section are funded and active except where noted.

1. natural gas extracted from the Sleipner gas field in the North Sea, Norway, is 9% carbon dioxide. This component is removed from the extracted gas and re-injected into a deep saline formation consisting of a 200 m thick sandstone layer at a depth of 1040 m. Re-injection started in 1996. Approximately 1 million tonnes of carbon dioxide have been re-injected per year, replacing the 4 million tonnes of gas extracted.
2. BP, Sonatrach and Statoil started a similar injection project in 2004 at In Salah, Algeria, where the targeted injection rate is 1.1 million tonnes per year. So far it has stored 0.8 million tonnes of carbon dioxide.
3. Weyburn in Saskatchewan, Canada has a similar scheme to the one at In Salah, with a target injection rate of about 4.5 megatonnes per year. Both of these inject carbon dioxide into existing oil wells to help the extraction process.
4. Texan Gulf coast injection experiment. An old, brine-filled oil reservoir in the Upper Frio sandstone formation on the Texan Gulf coast has been the site of a carbon dioxide injection experiment that started in October 2004. This experiment is being run by the USGS^[8]. Liquid and gas samples have been periodically taken for analysis, starting before injection began. Recent samples taken in mid 2006, under two years since the experiment started, have a composition suggesting that minerals, including carbonates, are being dissolved out of the surrounding rock by the mixture of carbon dioxide and salt water in the reservoir. If this continues it may destroy the reservoir's seals, allowing the carbon dioxide to escape into the atmosphere. This is likely to be a general problem: many oil drilling sites have carbonate-filled cracks in the impermeable layer that prevents the oil and gas from escaping.
5. the world's largest carbon dioxide postcombustion capture was inaugurated on 15 March, 2006 at the Elsam coal-fired power station near Esbjerg, Denmark. This is a pilot project that doesn't seem to have done anything at all since inauguration.
6. CO2SINK: small-scale test trapping of CO₂ in a saline aquifer in Germany has started.
7. RECOPOL: small scale injection of CO₂ in deep seated coal layers in Poland has started.
8. Basalt sequestration is being investigated by the Battelle Memorial Institute. Lab-scale experiments show that if carbon dioxide is injected into volcanic rock, it combines with the minerals in basalt to form calcite. A larger field trial to inject 3000 tonnes of carbon dioxide, the daily emission from a 150 MW coal-fired power plant, into a Columbia River Baisin basalt body is being planned. Another feasibility study is being planned at the Wallula paper mill in Idaho. This is potentially a good sequestration method since calcite is a permanent, solid means of storing carbon dioxide and costs will be comparable with simple carbon dioxide storage. However, there may be unquantified consequences from forming large amounts of calcite within a stable rock stratum and the reaction that converts CO₂ into CaCO₃ (calcite) is 30-40 times slower than likely injection rates.

Only the first three of these are up and running, giving a total sequestration capacity of 6.5 megatonnes. The rest are at either either purely experimental or apparently moribund pilot projects (Esbjerg). Current emissions due to power stations and industrial plant total about 8 billion tonnes per year, so the much hyped sequestration currently handles 0.075% of this.

Planned projects

There is a lot of hot air being emitted about sequestration projects, so I've listed only projects which have projected operational dates and that indicate the technology to be used for carbon capture and storage, together with some indication that its transport has been considered.

• BP, ConocoPhillips, Shell and Scottish & Southern Energy have started planning the world's first industrial-scale project to generate low-carbon electricity from hydrogen. The Peterhead Hydrogen Power project in Scotland would "decarbonise" natural gas to produce hydrogen to power a 475MW generator. 90% of the carbon dioxide would be re-injected into the depleted Miller field in the North Sea. This should allow the enhanced recovery of oil that would otherwise have remained underground. It could be operational by the end of 2009.
• BP and the Edison Mission Group project. This project is planning to generate 500 Mw of low-carbon electricity in California by gasifying petroleum coke and avoiding 4 million tonnes/year CO₂ emissions. This plant could be operational by 2011. The carbon dioxide would be injected into depleted oil fields in the Los Angeles basin to recover oil that would otherwise remain trapped in the geological formations.
• FutureGen is a commercial scale project in Matoon, Illinois that promises near-zero emissions from the use of carbon sequestration. On 30 January 2008 the Bush administration cancelled funding for the project. It was subsequently restarted on 12 June 2009 by the Obama administration. On 3 October 2010 the domain name had lapsed and not been renewed.
• Kingsnorth, on the Medway in Kent started planning in 2006 for two replacement coal-fired generators with vague plans for retrofitting carbon capture systems at some unspecified time in the future. In March 2009 the UK Government postponed a decision on carbon capture indefinitely. The project is now on hold and all references to carbon capture have been removed from the web site.

Sequestration summary

Some ideas, such as extracting the carbon dioxide from the gasses emerging from gas wells and re-injecting it, look pretty reasonable.

However, I am not at all convinced that the current plot for burying all those megatonnes of carbon dioxide produced by burning coal in disused coal mines and oil wells is a safe or permanent solution. Some, such as the Montana scheme, look distinctly harebrained. The Texan Gulf Coast injection experiment may well show that many otherwise suitable brine-filled reservoirs can't be used.

There seems to be too much sequestration hype coming from the ignorant, the lazy-minded, politicians and oil companies who smell a revenue stream from dead oil wells. The fate of FutureGen and Kingsnorth tend to reinforce this view.

If the carbon dioxide cannot be gotten rid of coal may be usable for steam trains but not much else.