The answer may be a new material that is a thousand times more efficient at capturing carbon dioxide than trees. This substance, a synthetic resin, is a part of diverse attempts to make carbon capture and storage (CCS) practical.
Mercedes Maroto-Valer, professor of sustainable energy engineering at Heriot-Watt University, defines CCS as “a portfolio of technologies that aim to separate carbon dioxide from other gases, then capture and store them in a permanent situation”.
CCS is a pragmatic solution, recognising that we will continue to emit CO2, and so need to remove the gas from the atmosphere and store it away where it can do no harm.
There are two primary strategies for capturing carbon dioxide. The natural mechanism is absorption by plants, which use CO2 to build their carbon-based structures, emitting oxygen as waste.
Trees absorb a considerable amount of carbon and lock it away for much longer than smaller plants. However, trees take decades to reach a state when they absorb significant quantities of carbon.
Trees, like the new resin, take carbon dioxide from the atmosphere wherever they are located. This makes for easy deployment, but limits the rate at which the gas can be removed.
The alternative strategy is to take CO2 directly from the exhausts of power stations and carbon-intensive industries such as steel and concrete production, where the gas is much more concentrated.
Maroto-Valer notes “these provide percentage levels rather than parts per million levels. A gas-fired power station might produce around 5% carbon dioxide and a coal fired station 10 to 12%.”
Whatever the source, when the CO2 has been extracted, the most common storage technique is to pump it along a pipeline to be injected into an underground reservoir, often formed by layers of porous sandstone lying underneath shale, which helps keep the heavier-than-air gas in place.
This process, called sequestration, which has undergone wide field studies, experimental verification and modelling should store carbon for millions of years.In other places, though, the suggestion is that the CO2 could be used to extract oil from dwindling reserves by pumping the gas into the well to maintain pressure. Unsurprisingly, this is considered a backward step by green groups.
Mike Childs, head of policy for Friends of the Earth, said: “Places where [CCS] is being trialled are talking about taking the carbon dioxide captured and then injecting it into oil wells to extract more oil. It’s pretty nonsensical to try to address climate change by taking carbon out the atmosphere to try to get more oil out of the ground.”
Capture processes come in three forms, pre- and post-combustion, and oxy-fuel. Pre-combustion converts fuel into hydrogen and CO2 before it is used. In principle it could produce highly efficient turbines, but as yet is the least tested option.
Post-combustion is easiest to use with existing power plants and the only approach appropriate for collecting CO2 from industrial sources, while oxy-fuel is attractive for new purpose-built coal power plants, as it produces a particularly pure CO2 output and is flexible in operation, able to store energy by producing liquid oxygen during periods of low demand.
In post-combustion, the exhaust gas is taken through a scrubber, where chemicals react with the carbon dioxide to produce a compound that can be moved as a solid or liquid before heating it to extract the pure CO2 for storage.
However the new resin, the best so far of a number of such possible materials, is much simpler to use, as it is a passive absorber that sits in a flow of air, directly extracting the gas.The resin, developed at Arizona State University’s centre for negative carbon emissions, is an “ion-exchange” substance where positively charged ions in the resin attract and hold on to carbon dioxide molecules.
Rather than requiring chemical reagents, the process is simply reversed by humidity. If the air is dry, the resin absorbs carbon; in humid air, CO2 is released. The downside is that the system will operate best in dry climates, though this doesn’t totally rule out use in the UK.
Dr Klaus Lackner, director of the Arizona centre says: “My rule of thumb is that it will work if you can dry a towel on a clothes line. It’s a process that just like wind and solar energy will be affected by weather. So it would work in the UK, but not as well as Australia. But keep in mind that the air mixes well, so CO2 emitted in the UK can be collected in Australia.”
A panel made up of layers of the material would collect between 10 and 50% of the carbon dioxide that passes through it – around a thousand times more efficient than the equivalent amount of CO2 collected by a tree in its lifetime.
Each panel becomes saturated with carbon in about an hour, so a system based on this material would need continuous recycling of collectors, moving them to a humid location where the gas can be pumped away, then returning them to collect more.
Like all CCS techniques there is an energy cost, which Lackner suggests will be 20-30% – similar to existing CCS technologies.
While such imaginative schemes are at the early stages of development, conventional capture from power stations is further advanced. In the UK, three demonstration projections are planned – White Rose, a new coal-fired power plant in Yorkshire, Peterhead, retrofitting an existing gas-fired power plant with CCS in Aberdeenshire and a possible third plant at Grangemouth near Edinburgh.
Each is a full-scale power plant intended to produce 380- 440 megawatts, able to power 500,000 to 650,000 homes while capturing around 90% of the carbon.
These projects should come online around the end of the decade, but they have been beaten into action by Boundary Dam power station in Saskatchewan, Canada, the first major CCS installation worldwide, which went online in October 2014. Although smaller in scale, this rebuilt coal-fired station has already shown that it is practical to capture over 90% of carbon and is producing valuable data for future developments.
The amount of energy used in the capture process reduces the efficiency of the station. The expectation had been that around 50MW of the 160MW total output would be used, but in practice only 40MW were lost, leaving 75% of output for consumers.
Not even the most enthusiastic supporter of carbon capture and storage suggests that it should replace renewable sources like solar and wind, but renewables alone aren’t enough.
Mike Childs says: “If we want to keep global warming to below two degrees we need to make very drastic reductions in carbon emissions. Given the situation at the moment it probably looks as though we will have to find some way of taking carbon out of the air and storing it.
Now that can be tree planting, but it can also involving trying to capture carbon from the air and storing it underground. In terms of reducing emissions quickly, in some areas we probably need carbon capture and storage technology.
Particularly, for example, steel or cement production. In energy, even with carbon capture and storage, emissions from coal-fired or gas-fired power plants would still be much higher than they are for renewable energy.”
Dr Sarah Mander, research fellow at the Tyndall climate change research centre in the University of Manchester emphasises the need for CCS and the existence of a significant barrier to success: “If you think globally, coal and other fossil fuels will continue to be used for a long time… but political will has to be matched up by a policy regime to drive it… The world of CCS is full of a lot of people who want to make it happen, but ultimately there has to be money there.”
Capturing carbon increases the cost of energy production, and someone has to pick up the bill. CCS has its detractors. Organisations that doubt the existence of manmade climate change regard it as a waste of money.
Meanwhile, some green groups, while expressing support for the removal of carbon dioxide from the atmosphere, feel that it provides an excuse to continue using energy sources they consider untenable.
As Mike Childs puts it: “Whichever government had been elected, they would still see carbon capture and storage as an opportunity for UK economic development. Both parties want to keep fossil fuels as part of the energy mix.”
The three British demonstration power plants are arguably required because fossil fuels will not be removed from the equation in the immediate future, whilethe potential to install similar collectors at industrial sites like steel plants reflects that even with carbon-free generation, CO2 would still be pumped into the atmosphere elsewhere.
Meanwhile carbon reduction projects, perhaps using the new resin collectors, could enable us to deal with carbon from sources unsuited to direct CCS such as transport, and to address the reality that even if we stopped CO2 production tomorrow, levels are already too high. Carbon capture and storage has broad support, and is integral to the UK’s strategy to meet the 2050 emission reduction target. Progress is being made.
As Mercedes Maroto-Valer remarks: “Compared to other technologies CCS is a relatively new kid on the block, and the others have subsidies. We are asking CCS to mature very quickly, and it has come a long way in only 10 to 15 years. And CCS is something we learn by doing.”
Energy saver: Yorkshire’s new Rose
The Drax power station in North Yorkshire was once the target of environmental demonstrations, but is now the planned home for a demonstrator CCS power station, White Rose, built from scratch to use the oxy-fuel process, capturing carbon dioxide to be pumped through a 90km National Grid pipeline for storage a kilometre beneath the seabed.
The White Rose plant, operated by Capture Power, is expected to produce up to 448MW, which would support 650,000 households, with 90% of CO2 emissions captured, based on evidence from existing projects like Boundary Dam, a total of 2m tonnes of carbon dioxide a year.
Although the plant could run on coal alone, the intention is to use around 15% biomass, material such as willow grown locally for burning, which would counter remaining emissions with the CO2 taken out of the atmosphere during growth.
As well as providing energy and valuable data, White Rose would be the “anchor project” for the CO2 pipeline, which could carry a further 15m tonnes of carbon dioxide a year.
Dr Leigh Hackett, CEO of Capture Power, said: “The Humber region is widely recognised, alongside Rotterdam, as representing the best opportunity for CCS development in the whole of Europe, given the proximity of secure North Sea storage sites and the high concentration of both power generation and industrial emitters in the region.”
National Grid estimate that the planned site should hold more than 20 years of emissions. Begun in 2013, the project is in a final consultation and risk assessment phase. Assuming planning consent and funding, it is hoped that construction will begin in the first half of 2016, with the plant coming online in 2020.
theguardian.com
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