Climate change is becoming harder to ignore. This summer whilst Britain has been underwater, struggling against the worst flooding in the island's history, crippling heat has left 35 dead on European mainland, driven 19,000 Romanians to contact emergency services and caused wild fires in Italy, Greece and Croatia.
Most scientists agree that climate change is caused by an increase in levels of greenhouse gases caused by human activity. The main offender in this is carbon dioxide, responsible for 80% of emissions in industrialised countries. The UK alone emits more than 500 million tonnes of CO2 each year.
What can be done? For eco-friendly individuals, the challenge is to alter their behaviour to try and reduce their carbon footprint. For industry, the answer being proposed is carbon capture and storage (CCS).
WHAT IS CCS?
CCS does what it says on the tin – it refers to the capture of carbon dioxide as it is released by power stations, which is then transferred to a place where it can be safely stored.
It is believed that CCS, if applied to a modern conventional power plant, could reduce emissions of CO2 by up to 90%. The process so captivated current British Prime Minister Gordon Brown that, when he was chancellor of the exchequer in 2005 he said CCS was, "likely to prove a critical technology in global carbon reduction strategies, particularly for countries with fast-growing economies and rapidly growing fossil fuel consumption."
So, is CCS the answer to all our prayers? Is it cost effective enough to be widely accepted by the big companies? And, most important of all, is it safe?
Technology for capturing of CO2 is already commercially available for large CO2 emitters, such as power plants. There are three different types of technology available: post-combustion, pre-combustion and oxyfuel combustion.
As the name might suggest, in post-combustion the CO2 is removed after combustion of the fossil fuel. This is the technology that would be used by conventional power plants, where the CO2 would be captured from the flue gases. In the case of coal, this is sometimes known as clean coal.
The technology for pre-combustion is widely applied in fertiliser, chemical, gaseous fuel and power production. In these cases, the fossil fuel is gasified and the resulting CO2 can be captured from a relatively pure exhaust stream.
In oxyfuel combustion, sometimes referred to as 'zero emission', the lignite is burned in oxygen instead of air. This produces a flue gas consisting of only carbon dioxide and water vapour, which is cooled and condensed, resulting in an almost pure CO2 stream.
A fourth, more complex method of carbon capture is currently under development. Chemical looping combustion (CMC) uses a metal oxide as a solid oxygen carrier. Metal oxide particles react with a solid, liquid or gaseous fuel in a fluidised bed combustor, producing solid metal particles and a mixture of carbon dioxide and water vapour.
The water vapour is condensed, leaving pure carbon dioxide. The solid metal particles are circulated to another fluidised bed where they react with air, producing heat and regenerating metal oxide particles that are recirculated to the fluidised bed combustor.
Before any captured CO2 can be stored it does, of course, have to be transported.
This is done by pipeline, which is generally the cheapest form of transport, or by ship when no pipelines are available. Both methods are already in use for projects where CO2 has to be transported.
Once it has been captured and transported, CO2 can be stored in a number of ways: as a gas in a deep geological formation; as a liquid in the ocean or as a solid by making it react with metal oxides so it can be stored as a carbonate.
Geological storage, also known as geo-sequestration, involves injecting CO2 directly into underground geological formations such as oil fields, gas fields, saline formations and unminable coal seams. Physical and geometrical trapping mechanisms prevent CO2 escaping to the surface.
IPCC, the Intergovernmental Panel on Climate Change, estimates that these sites are likely to retain over 99% of the injected CO2 over 1,000 years.
There are two types of ocean storage. Dissolution involves injecting CO2 by ship or pipeline 1,000m or more under the water, where it simply dissolves. During the lake method CO2 is injected 3,000m under the water, where it is denser than water and therefore forms a lake that remains trapped at the bottom of the sea.
Mineral storage, where the CO2 is turned via reactions with metal oxides into a stable carbonate, is the most natural of the three processes. This reaction is responsible for most of the world's surface limestone.
However, it is a slow process. Although it can be speeded up by temperature and pressure, the IPCC estimates that a power plant equipped with CSS using mineral storage will use 60% to 180% more power than a non-CSS-using plant.
CCS does not present any problems in terms of the technology used, according to the experts.
"We see huge potential for carbon capture and storage in power generation projects where there are large dirty fuel resources and large energy demand," says a BP spokesman.
"We have been working on three CCS projects around the world and the technology is fairly standard. The difference is putting the technology together on a commercial scale. The challenge is to set about using existing technology in new ways."
Capturing and compressing CO2 will require plants to expend more energy and may also involve added investment or capital costs. The IPCC estimates that CSS-equipped plants will need 10% to 40% more energy, raising costs to 30% to 60% above what they are now. Although this is not an attractive business suggestion, governments in the US, Australia and the UK have all committed to subsidising plants willing to switch to CSS.
"If you're asking a plant to do more than it does currently, then it is clearly going to cost more," says the BP spokesman. "CCS needs government support. We'd like to see it treated as a renewable energy like wind power. The real challenge for CCS is an economic one – to keep the costs down."
There are fears that CO2 storage does not offer a permanent solution to the emission problem, particularly the ocean storage method.
The IPCC estimates that only 39% to 85% of CO2 would be retained after 500 years at depths of 1,000m to 3,000m.
It is feared that large build-ups of CO2 underwater will make the sea more acidic and kill ocean organisms, whilst leakage through soils and pipelines could have a damaging impact on human life and ecosystems, particularly if it were to infect drinking water. Scientists are currently researching reservoir geology to help negate these risks.
A final accusation levelled at CCS is that it may delay the world's uptake of renewable energy sources by continuing our dependence on fossil fuels.
"The notion is that the sooner we wean ourselves off fossil fuels, the sooner we'll be able to tackle the climate problem," says Sally Benson, executive director of the Global Climate and Energy Project (GCEP). "But the idea that we can take fossil fuels out of the mix very quickly is unrealistic. We're reliant on fossil fuels, and a good pathway is to find ways to use them that don't create a problem for the climate."
WHO'S DOING IT?
The world's oldest CCS project is Sleipner. Since 1996 Norwegian state-controlled company Statoil has been injecting CO2 into saline aquifers 1,000m down into the North Sea. They store around one million tonnes of CO2 every year.
There are two other fully operational CCS projects.
The Weyburn Project, which started in 2000 and is funded by the US Department of Energy, captures CO2 from a coal gasification project in North Dakota and transports it over 200 miles to Weyburn, Canada for sequestration. The project uses the CO2 during the process of advanced oil recovery, at the rate of 1.5m tonnes a year.
In 2004 BP launched a CO2 capture and storage project at the In Salah gas field in the Algeria desert. In Salah is a joint venture between Sonatrach, the Algeria national energy company, BP and Statoil.
Approximately 10% of the gas in the reservoir is made up of CO2, which is captured, compressed and injected into wells 1,800m deep, where the reservoir is filled with water. Around one million tonnes of CO2 are injected into the reservoir every year.
THE FUTURE OF CCS
Although there are only three plants currently using CCS, there are plans afoot to increase that number drastically.
The US Department of Energy is ploughing over £300m into the FutureGen project, which aims to sequester one million metric tonnes of CO2 each year from 2012.
The Australian government is investing £25m into Chevron's Gorgon Project in Western Australia. They hope that up to 25% of Australia's carbon dioxide emissions could be stored in underground reservoirs each year.
The EU has proposed the development of 12 showcase carbon capture and storage power plants by 2015, with the envisaged goal of all new power plants within the EU being built with mandatory carbon storage technology by 2020.
It was thought that one of these would be based at Peterhead in Scotland, although BP has recently pulled the plug on the £500m project, citing government feet-dragging over the issue of subsidies.
However, the UK could still get a CCS-equipped power plant, as the government is launching a competition in November, the winner of which will receive funds towards making themselves capable of capturing and storing CO2.