Has carbon capture’s time finally come?

The world’s two biggest polluters have cut a deal to curb their carbon habit.  Carbon capture and storage at coal-fired power stations is a key pillar. James Mitchell Crow reports.

China has committed to using CCS at coal-fired power stations such as this one in Qinghai province. – China Photos/Getty Images

At last, leadership on climate change. On 11 November the US and China, the world’s two largest polluters, jointly announced a deal. The US will cut its emissions to at least 26% below 2005 levels by 2025. And China pledged to start reducing its total carbon emissions by 2030, or sooner if possible. This is reason to celebrate – and also puts a new spin on carbon capture and storage (CCS), a technical solution which until now has often been dismissed.

CCS is a suite of technologies that allows power plants to continue burning fossil fuels. Rather than releasing their CO2 emissions into the atmosphere the plants capture and store them deep underground in the pores of rocks. The technology is a pillar of the US-China agreement – both countries have committed to pursuing large-scale CCS projects.

“Without CCS there’s no realistic way China can achieve its ambitious new emissions target”, argues geologist Peter Cook from the University of Melbourne. He has pioneered the technology in Australia for the past 15 years. “We can pretend that all China’s energy needs can be met by moving to renewables, but that’s not going to happen. They’re not going to junk [coal-fired] power stations they’ve only just built.”

CCS is no silver bullet, he says, but is an essential part of the transition to low-carbon energy, alongside renewables and nuclear.

The commitment to implement CCS caps a transformative year for the field. For more than a decade the world has dithered over the technology. Sceptics rejected the technique on the grounds that it propped up the fossil fuel industry at a time when we should be investing in renewables. There were also concerns that not enough of the right types of underground rocks would be found in close enough proximity to power stations. Finally, CCS had never been demonstrated on a full-scale power station. On 1 October that changed.

Canada’s Boundary Dam plant is the world’s first commercial-scale coal-fired power station to capture 90% of its carbon dioxide. – Saskpower CCS

Canada switched on the Boundary Dam plant. It is the world’s first commercial-scale coal-fired power station to capture 90% of its CO2 – one million tonnes per year – and pipe it underground. “Boundary Dam is a very significant step forward,” says Dianne Wiley, who heads the “capture” program of Australia’s Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC). Clare Penrose, general manager for Asia Pacific at the Global CCS Institute, which has its headquarters in Melbourne, agrees. “It’s playing a really critical role in convincing decision makers and the general public CCS is do-able, efficient and effective and we should be doing more of it.”

Boundary Dam is not a one-off. In its annual report, the Global CCS Institute recently concluded that 2014 and 2015 are “watershed years for CCS”. By the end of 2015 or early 2016 two plants in the US, now at a stage of advanced construction, should also be up and running. One, the Kemper County project in Mississippi, will use an ambitious new carbon-capture technology called “pre-combustion”, and will dwarf Boundary Dam by capturing three million tonnes of CO2 per annum.

Three other North American CCS plants are making good progress towards start-up by the end of the decade. The Global CCS Institute counts 19 other large-scale projects in advanced planning or under construction across the US, Europe, the Middle East and China. In Australia the Chevron-operated Gorgon LNG project is expected to start operating in 2016. More than three million tonnes a year of CO2 will be geologically stored under Barrow Island, making it the largest storage project in the world.

The take-up of CCS has been a long time coming. Early optimism that the technology could be rolled out quickly proved unfounded. “There’s now realistic optimism”, says Cook. Although investment in solar and wind power has soared, fossil fuel use is still rising globally. “Everybody agrees it’s not a good thing, but nobody seems to know what to do about it – apart from CCS,” says Cook

Boundary Dam’s storage facility, dubbed Aquistore. The plant uses the “post-combustion carbon capture” approach. – Saskpower CCS

Even the Intergovernmental Panel on Climate Change (IPCC), which has historically been cool on the technology, is now flagging its importance. In November, while launching its Synthesis Report, which summarises the reports released over the previous 18 months by its three working groups, IPCC chairman Rajendra Pachauri singled out CCS as an essential technology in the transition to low-carbon power generation. “With carbon capture and storage it’s entirely possible fossil fuels can continue to be used on a large scale,” he said.

But the price of capturing carbon must come down if the technology is to realise its full potential. Building the 120 MW Boundary Dam power plant cost $1.5 billion. ($500 million was spent on refurbishing an old boiler, so the capture part of the project cost was actually closer to $1 billion). “With every large-scale project that gets built, the lessons learned will bring down those costs,” says Penrose. “Boundary Dam’s operators have already said the next project will cost about 30% less.”

Nevertheless the running costs of a CCS plant remain a challenge (see How carbon capture and storage works, below). Early adopters of CCS, including Boundary Dam, are offsetting some of this cost by selling some of the CO2 they capture to oil companies. These companies inject CO2 into ageing oil wells where it has a detergent-like effect and makes the last drips of oil flow out more easily.

From lab scale to pilot scale, several promising innovations are being developed that could slash the cost of carbon capture. The US-China climate agreement could play a vital role in bringing those ideas to fruition, says Cook. The agreement promises to combine US CCS experience with China’s can-do attitude. “The Chinese have been very good at developing technologies at half the price of everybody else,” he says. “If they could develop CCS at half the price that would be a wonderful breakthrough.”

The carbon dioxide absorbers at Mississippi Power's Kemper County energy facililty. – Mississippi Power

How carbon capture and storage works

When coal, oil or gas are burned they release water vapour and carbon dioxide. Rather than releasing that carbon dioxide into the atmosphere, it needs to be “captured”, followed by a process that compresses the gas and stores it underground in the same kinds of geological formations that have kept natural gas trapped for millennia. The CO2 can be compressed 350-fold to a “super-critical state” which makes it so dense it is liquid-like.

The trick is doing all that in a cost-efficient way. The first challenge is separating the CO2 from the other gases that come out of a power plant’s flue. That’s necessary to save the energy and cost of compressing large volumes of gas. Most of the gas in the exhaust of a conventional coal-fired power station is nitrogen – only 10% or so is CO2. Extracting that CO2 from the power station’s emissions is the most expensive part of CCS. Three main strategies are being tried.

“Post-combustion carbon capture” is the approach used at Canada’s Boundary Dam plant. Here the flue gas mixture is bubbled through an amine (a nitrogen-based solvent) that traps the CO2 while nitrogen passes straight through. Once saturated with CO2 the solvent is cycled into a separate reactor and heated to drive out a pure stream of CO2. That typically uses about one fifth of the power station’s entire energy output – which is why a carbon capture plant is expensive to run – and why a lot of effort is being expended into bringing down those costs.

Night view of the carbon dioxide and hydrogen sulphide absorbers at Mississippi Power’s Kemper County energy facililty. – Mississippi Power

A potential variation on the theme of post-combustion capture is to replace the solvent with a membrane that sieves out CO2. Membranes are already used on a huge scale to strip CO2 from newly extracted natural gas before shipping it to be burned in power plants. The hunt is on to find materials that can do the same job for the mixture of gases from burning fossil fuels. Dianne Wiley of CO2CRC is pilot testing some promising membranes. “They look like they might be in the right ballpark, eventually, to halve the costs of carbon capture,” she says. “If we are going to meet our 2050 climate targets we need really significant breakthroughs and really significant reductions in cost.”

The second strategy, known as “oxycombustion”, burns the fossil fuel not in air, but in pure oxygen. The flue gas from this process is a mixture of pure CO2 and water vapour. Simply allowing the water vapour to condense produces a pure stream of CO2. The cost here lies in producing the necessarily large quantities of pure oxygen in the first place. This process has recently been successfully tested with a retrofit at the Callide power station in Queensland.

The third alternative that will be trialled at the Kemper County plant in Mississippi is called “pre-combustion” carbon capture. Somewhat akin to the time-honoured method of charring, this process heats the coal to temperatures of 700°C in the absence of air, but in the presence of steam. Under those conditions the coal does not burn – it disintegrates into a mixture of gases, mainly carbon dioxide, carbon monoxide and hydrogen. In other words solid coal “gasifies”. From this mixture, known as “syngas”, the CO2 can be separated and piped off for storage while the hydrogen is burned to produce energy. Overall this process uses about the same amount of energy as post-combustion carbon capture.

  1. https://cosmosmagazine.com/physical-sciences/crystals-can-clean-planet
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