Can we really have carbon negative power?
Future energy systems will need to capture more carbon dioxide than they emit, according to new Stanford University research. If we’re to have a fighting chance of doing so, the report says fitting power stations with carbon capture and storage technology (CCS) will be vital to stabilise warming below two degrees.
A lot about CCS is still unknown, and the report sheds light on how particular types of power stations fitted with CCS could work. It calls for technologies that take advantage of vegetation’s ability to absorb carbon dioxide to implement CCS on a global scale.
On closer look, it seems the technology is complex but workable on a small scale. The main obstacle to a larger scale deployment is policy, rather than science.
Negative emissions from biomass
The report describes generating electricity by burning natural materials such as wood and grass – known as biomass. The graphic below shows how adding CCS to a biomass power plant can lead to more carbon dioxide being extracted than is emitted – an outcome known as net negative emissions.
Source: Assessment Report from the Global Climate and Energy Project ( GCEP) workshop
There are three important parts to this process: conversion – where fuels are converted into usable energy, capture – where carbon dioxide is recaptured, and storage – where the carbon dioxide is locked away rather than being released to the atmosphere.
Step 1: Conversion
The current generation of plants need a lot of fuel to produce electricity. The 73 biomass plants currently operational in the US are only about half as efficient coal plants, the new report states.
But because biomass gets turned into electricity in a similar way to coal, existing coal plants can be adapted to burn biomass – a process known as co-firing. While this won’t provide negative emissions, it does mean that electricity can be generated fairly efficiently with nearly zero emissions.
But it’s not that simple. Biomass supply limits the potential for co-firing – currently only 10 per cent of a co-fired power plant’s fuel can be biomass, according to the research.
Step 2: Capture
The most advanced method of capturing carbon from co-firing power stations separates out the carbon dioxide using chemicals known as amines. But the process of amine scrubbing only captures about 90 per cent of flue gases and is hugely expensive, costing between $50 and $80 per ton of carbon dioxide produced, according to the report. The energy used in amine scrubbing also means the power plant is a third less efficient at producing useable energy.
A new and potential alternative to amine scrubbing is Cryogenic Carbon Capture (CCS), which involves cooling the flue gases until carbon dioxide can be removed as a solid. CCC is potentially twice as energy efficient as amine scrubbing and can capture more than 90 per cent of emissions.
Once carbon dioxide is captured, it needs to be locked away out of the atmosphere.
Step 3: Storage
Biomass can be buried underground or put back into the soil to act as a fertiliser in the form of biochar – a kind of biomass charcoal – potentially increasing crop yields. But more research is needed as, depending on the material used, biochar breaks down over decades or centuries releasing carbon dioxide back into the atmosphere.
Another alternative is to store carbon dioxide in the oceans. The oceans already absorb a large fraction of the carbon dioxide in the atmosphere, but as they do so they become more acidic. Scientists are concerned about even small shifts in ocean acidity, as it can affect how some marine algae grow their shells, which can be crucial to their survival.
The report discusses the possibility for combatting the acidity by adding alkaline material, such as magnesium carbonate. But so far a feasible way to sink excess carbon in the deep ocean remains elusive, says the report. What’s more, this level of interference may have unforeseen consequences for marine ecosystems and needs further research
Another possibility, not discussed in the report but which attracted some interest recently, is to mimic how sea urchins use the metal nickel to trap carbon dioxide from seawater and turn it into calcium carbonate. Storing large amounts of carbon dioxide in the form of calcium carbonate or magnesium carbonate – both of which are harmless in the environment – could be safe and viable alternatives to storing it underground or in the oceans, says the study.
And knowing whether or not CCS is working is a challenge too – but a group of Canadian researchers have built a tiny, super-sensitive nanosensor to monitor the amount of carbon dioxide emitted from power stations or storage facilities. The sensor can detect even a single molecule of the gas, making detection very precise.
Obstacles
So while biomass with CCS technology may be difficult, it is scientifically feasible. So why isn’t it being rolled out on a larger scale?
There are two main obstacles. First, the low carbon price means investors are more likely to continue building cheaper power plants with higher emissions than add expensive CCS to co-fired plants. Second, biomass with CCS cannot compete on price with other energy sources. DECC’s latest estimates suggests electricity from power plants built in 2018 and converted to use biomass will cost about £110 per megawatt hour – about £20 than gas with CCS, and £40 more than nuclear. And that’s without the additional cost of adding CCS.
At the moment it seems like the obstacles to large-scale CCS are to do with policy and finance more than the science behind the technology. But this is a dynamic area of research and the stakes are high. Following the GCEP workshop on CCS, the group launched an international request for proposals for negative emissions technologies. The winners – who will receive up to $6 million – will be announced later on this year.