The Commons’ Environmental Audit Committee announced an inquiry yesterday into the role of negative emissions technologies (NETs), with a particular focus on bioenergy with carbon capture and storage (BECCS) and direct air carbon capture and sequestration (DACCS). The government expects both to play a role in meeting the UK’s climate goals.
“Achieving net zero will only be possible if industries across the economy decarbonise. But for some of our energy-intensive industries, this will be harder to achieve than other sectors,” said committee chair Philip Dunne.
There are a number of ways that humanity could start to reduce the concentration of greenhouse gases in the atmosphere, on top of limiting emissions. Some are more practical propositions than others, here’s what you need to know.
Bioenergy with carbon capture and storage (BECCS)
Until recently, carbon capture and storage (CCS) has been generally associated with burning fossil fuels, processing flue gases into purified CO2, then piping it away for permanent geological storage – a way for heavy industry to adapt to the net zero era with relatively minimal change.
But with BECCS, the CO2 is ultimately derived from the atmosphere, having been absorbed by trees or other plantlife while they grew. So, indirectly, the gas is taken from the air and buried, while also producing power.
In a 2018 report on the subject, the Climate Change Committee (CCC) said that, “Effective, low-cost CCS that can be applied at large scale and with high capture rates is vital to getting the largest possible carbon reduction from scarce biomass supplies.”
So far, the only BECCS scheme underway in the UK is at Drax’s vast power station in North Yorkshire, confirmed earlier this year and preceded by demonstration-scale capture projects at the site and similar ones in the US. Thus far, it has not been conducted at full commercial scale anywhere. Its CO2 would be piped under the North Sea.
But, like burning any form of biomass, the key concern is whether the technology will have net negative emissions. Earlier this year, a coalition of 19 environmental NGOs, including WWF and Greenpeace, warned that such projects would be expensive and would not account for emissions released from the soil of harvested forests, or for the CO2 absorption foregone by tree felling. Back in 2014, a commentary in the journal Nature Climate Change blasted it as “unproven” and a “dangerous distraction” from other efforts to limit climate change.
Nevertheless, BECCS is expected to play a major role in the forthcoming Biomass Strategy, due next year.
Direct air carbon capture and sequestration (DACCS)
CO2 is already being sucked out of the atmosphere through technology, the largest example of which is in operation in Iceland, where hydropower and geothermal power make electricity cheap. It has the ability to capture and store about 4,000t of CO2 per year – equivalent to about 0.000012% of annual anthropogenic emissions. It is the world’s first commercially operating system.
The captured CO2 is dissolved in water and injected into rock, where it mineralises as carbonate within the fractures in about two years. The firms behind it, Climeworks and Carbfix, plan to reach megaton removal capacity by the second part of the decade.
But there are concerns about its practicality. Though rising, the concentration of CO2 in the air is hundreds of times less than in an industrial stack, making it an expensive option. Delivering other negative emissions technologies may be significantly cheaper. There are also concerns about how practical it would be to scale up its use, the amount of electricity needed to operate it and available storage capacity.
In 2011, the American Physical Society estimated that capturing a billion tonnes of CO2 per year – 2.5% of anthropogenic emissions – would require 16 gigawatts of power as a theoretical minimum. The amount is equivalent to four Draxs running continuously. But practical issues may put the figure closer to a terawatt.
Tree planting and natural regeneration
The simplest method of achieving negative emissions is simply to let the land naturally regenerate into woodland, or accelerate the process by planting trees in unwooded locations. The United Nations’ Clean Development Mechanism, part of the Kyoto Protocol, has long provided a financial incentive for poorer countries to do so.
While cheap, the major obstacle to widespread afforestation and reforestation is the availability of suitable land – a particular issue in densely populated countries such as the UK.
Building with wood
Using wood rather than burning it is also a form of carbon storage.
The CCC’s 2018 report said that there is “significant potential for this to scale-up in the future both by increasing timber frame construction and by expanding the use of engineered wood, particularly in the non-residential sector. Such a scaling up aligns with government priorities.” But it identified some problems, including how to build wooden homes that are also energy efficient, and a lack of skills needed for construction in timber.
Some forms of rock naturally absorb CO2, dissolved in rainwater as carbonic acid, as they weather, forming bicarbonate that is eventually transported to the sea floor. In principle, this process could be hugely accelerated, while also improving the fertility of farmland soil.
Grinding rock increases the surface area available for the process. Spread on fields, microbes would make it even faster, while increasing mineral concentrations in the soil. Alternatively, mineral dust could be dumped directly in the ocean – which would also help reduce acidification from excess CO2.
According to a study published in Nature last year, China, India, the USA and Brazil alone could achieve removals of 0.5-2 billion tonnes of CO2 per year, at a relatively moderate cost of US$80–180 per tonne. “However, success will depend upon overcoming political and social inertia to develop regulatory and incentive frameworks,” it said.
Another paper, published in 2012, said that the UK’s “silicate resources are large and could theoretically capture 430 billion tonnes” of CO2 over all, at a cost that could be as low as £15/t for rocks with a very low silica content (known as ultrabasic or ultramafic).
Blue carbon is carbon stored in salt marshes, seagrass beds and other coastal and marine ecosystems, which has been reduced over the course of history due to human activity. Restoring such habitats could play a small but significant role in global carbon storage efforts.
Soil carbon sequestration
Ploughing, crop burning and industrial fertilisers have reduced the amount of carbon in farm soils – but it should be possible to return them to being carbon sinks by making some changes to farming methods and using different species of grass, for example.
Biochar is simply charcoal, added to soils rather than used as fuel. It can take hundreds or even thousands of years to break down – as demonstrated in the manufactured, highly fertile terra preta soil of the Amazon - making it a form of carbon storage.
The University of Edinburgh has had biochar unit operating for more than a decade.
There is little dissolved iron in the ocean, posing a limiting factor in the growth of phytoplankton, and hence the rate at which they photosynthesise CO2 into biomass. Increasing the availability of iron by injecting it into the sea could therefore boost their productivity, storing CO2 as they die and sink to the ocean floor.
Using nitrogen or pumping up more nutrient-rich water from deeper waters has also been suggested to do the same job. But, like other forms of geoengineering, there are ethical issues about disturbing the carbon cycle at vast scale and legal concerns about what could be interpreted as a breach of the London Convention on marine pollution, alongside its cost.
The ability of water to dissolve CO2 is increased if it is alkaline. Therefore, adding an alkali to clouds or the sea should help take the gas out of the atmosphere, where (as described earlier) it would eventually reach the sea floor.
One such idea would be to add lime (presumably manufactured using CCS, as heating limestone produces CO2) to the sea, which would also reduce ocean acidification. But every tonne of CO2 sequestered would require about 1.5t of limestone, at a cost of $72–159, according to a 2013 paper led by Oxford University researchers.