The world faces an enormous challenge to produce the energy we need without damaging the lives of our children and grandchildren. Capturing the carbon dioxide produced from combustion of fossil fuels such as coal, oil and gas before it gets into the atmosphere and placing it instead in secure storage deep underground is a key to meeting our responsibilities. Welcome to the world of Carbon Capture and Storage, or CCS.
Carbon dioxide can be captured from all types of modern power plants, conventional steam boilers and integrated gasifier combined cycle plants that are planned for future construction. ‘Post-combustion’ systems wash carbon dioxide out of waste combustion gases before they go to the plant’s chimney with a continuously-recycled solvent. State-of-the-art designs are a significant improvement over the small capture units that have been used for half a century to produce carbon dioxide for carbonated drinks, dry ice, fire extinguishers and other industrial uses.
In gasification-based power plants with ‘pre-combustion’ capture the coal gas is reacted with steam to make hydrogen, which can be burnt in a gas turbine to raise electricity without producing carbon dioxide. New power plants of either type are expected to have similar costs and performance with capture. In the longer term other types of capture system may be tried out to see if they can give better performance. Perhaps the best know of these is oxyfuel combustion; pure oxygen is produced and used to burn the coal, giving nearly pure CO 2 with little additional processing. But there are also a lot of ways in which the current post-combustion and pre-combustion systems can be improved. So, as with all other new technologies, there are plenty of opportunities for industries to compete to produce better products and for users to take advantage of a competitive market with multiple suppliers.
There is a catch: capturing carbon dioxide costs money. With current designs about 25% extra fuel has to be burnt and additional equipment must be purchased. This adds between 30 and 40% to the cost of electricity. This may seem like a lot, but dividing the extra cost by the amount of carbon dioxide that is not emitted to atmosphere has been estimated to give ‘abatement costs’ of 25-30 € per tonne of CO2 (250-300 yuan or US$30-38), a price already reached last winter in the EU Emissions Trading Scheme (ETS). Although carbon prices are now lower, in the ETS and the international Clean Development Mechanism (CDM), it does show that financing CCS in China could be quite an effective way to offset emissions elsewhere in the world, especially as technology improves and capture costs go down. The key is reaching international agreement to pursue sustained and significant emission reductions.
Another way to finance CCS, at least at first, is through ‘enhanced oil recovery’ (EOR). Carbon dioxide, compressed to a liquid, can be placed underground in old oil and gas wells. In oil wells, the carbon dioxide can help to wash out oil that is stuck in the pores of the rock and cannot be released by other means. Current prices in the USA for carbon dioxide for EOR are around $20/tonne CO 2. Petrochina and CNOOC are currently examining similar EOR schemes in China. But, while old oil and gas reservoirs offer proven leak-tight storage and EOR can give an extra source of revenue, most of the potential storage capacity for carbon dioxide in China (and globally) is in deep layers of porous rock a kilometre or more underground that contain only salty water, known as saline aquifers. One of these, under the North Sea, has already been used successfully to store a million tonnes of CO 2 a year from the Norwegian Sleipner gas platform, and requires only a single injection pipe.
While we are waiting for the necessary political progress on climate change mitigation to make CCS a marketable service, Western governments have offered to work with China to find out how much carbon dioxide can be stored underground in China and where the best storage sites might be and also to build the first CO 2 capture plants in China. Preliminary results from an Australian storage capacity project are shown in Figure 1; this work will be continuing with a team of Chinese and international geologists. While it held the EU Presidency in 2005 the UK set up the UK-EU-China Near-Zero Emissions Coal (NZEC) project, which is planned to lead to a jointly-designed and constructed power plant with carbon capture and storage starting operation by 2014. There are also other CCS-related research and capacity-building projects with the EU and, under bilateral agreements, with individual countries and the number of these is set to increase significantly.
China can also follow Western developments by seeing that new power plants are built to be ‘capture-ready’. This means that a few simple and inexpensive changes (principally space in the right places and access to carbon dioxide storage sites) are included at the design stage so that capture equipment can be added without prohibitive costs in the future. Utility companies building power plants in Europe and the USA are already doing this in their domestic markets to make sure they can use CCS to avoid large cost penalties for CO 2 emissions in the future.
Another energy-related development that prepares for CCS is introducing new ways to use ‘decarbonised’ energy, electricity and hydrogen. Even when the carbon dioxide produced in the production process is captured, making synthetic gasoline or diesel fuel from coal still results in half of the coal carbon being emitted to atmosphere. In contrast, using electricity made from coal with CCS in an electric vehicle or a new plug-in hybrid vehicle or using hydrogen made from coal, releases only about 10% of the carbon in the coal. Greater use of decarbonised energy reduces demand for expensive oil and natural gas in the short term and it can be produced from a wide range of non-fossil energy sources (nuclear, renewables, geothermal) as well as from fossil fuels with CCS. This allows the same motor vehicle technology to be sold into a wide range of markets and to be used unchanged while new energy technologies, like CCS, are introduced to tackle climate change.
In some respects the challenge of capturing billions of tonnes of carbon dioxide and pumping it deep underground sounds impossibly large. But this is because the world’s energy sector is itself so big. At the level of the power plant all that is required are some additional items of equipment: large but much less complex and costly than the existing steam or gas turbine generation machinery. Carbon dioxide capture and storage can similarly become a standard part of fossil fuel utilization – provided we act with urgency to use the irreplaceable opportunity that we have now to build up experience on CCS and to make all new power plants capture-ready, before growing awareness of climate change mandates widespread deployment.
The author: Dr Jon Gibbins is at the Energy Technology for Sustainable Development Group, Mechanical Engineering Department, Imperial College London. He is Principal Investigator for the UK Carbon Capture and Storage Consortium.
Homepage photo by Bret Arnett