Reaching Net Zero requires more than just reducing emissions. To account for processes that will be exceptionally difficult to decarbonise completely (such as steel or cement making), we actually have to remove greenhouse gases from the atmosphere, thereby balancing out at ‘net’ zero.
‘Negative emissions’ technologies (NETs), also known as Greenhouse Gas Removal (GGR) technologies, allow us to do that. They remove greenhouse gases – usually carbon dioxide – from the atmosphere and they are needed to prevent the worst effects of climate change.
There is some disagreement over the exact level of negative emissions that will be needed (Figure 1), but it is generally agreed that NETs are needed to ‘mop up’ residual emissions from sectors that cannot be completely decarbonised.
Figure 1. The UK will need to take out between 10% and 37% of its annual emissions by 2050.
Models generally agree that to keep warming in line with the Paris Agreement (1.5oC by the end of this century), around 6 gigatons of CO2 a year (GtCO2/yr) will need to be captured globally by 2050 – the equivalent of seventeen times the UK’s current annual emissions being taken out of the atmosphere, every year. However, current global capacity to capture CO2 and store it for the long-term through negative emissions projects is estimated to be around 0.7% of this figure.
These technologies have had a rocky history. In 2008 and 2011, Policy Exchange argued that carbon capture and storage (CCS – a key component among some NETs) should be placed more centrally in the UK’s decarbonisation policy. But in 2015 the Treasury shut down a £1bn competition to deliver CCS at scale, which led to the collapse of two major development projects in Scotland and North Yorkshire. At the time, officials saw it as an expensive project that the UK did not need to pioneer. This time around, the Prime Minister and many senior advisers see it as a key plank in the ‘green industrial revolution’.
The Government is actively forming policy to support the development of NETs, recently announcing up to £100 million of innovation funding to support them as well as £1bn to establish carbon capture, usage and storage (CCUS) in at least four of the UK’s industrial clusters by 2030. Within its consultation on a Carbon Emissions Tax, it also sought views on the best ways to support different NETs.
Here is an outline of 4 promising NETs that could form part of portfolio of technologies needed to meet Net Zero:
1. Direct Air Capture with Carbon Storage (DACCS) describes technologies that capture carbon dioxide straight out of the air through two methods.
The first is ‘liquid’ based, and involves passing air over a chemical solution which absorbs CO2. The resulting air has less CO2 in, and is returned to the atmosphere. The liquid can then be put in permanent storage, such as in deep geological formations.
The second is ‘solid’ direct air capture, which involves passing air through a filter that binds with CO2 from the air. When heated, the filter releases this captured CO2, which can then be captured more easily and stored.
DACCS technologies tend to have a small spatial and environmental footprint, such as through not consuming much water. However, DACCS is energy intensive and needs to be powered by renewable energy to have emissions below zero. It is also very expensive at the moment, with recent cost estimates ranging as high as £750/tonne, far above the current total carbon price in the UK or even the projected price for 2050. DACCs technology therefore needs to become more energy efficient and cheaper per tonne of CO2 captured to be commercially scalable. Costs are falling however: a Canadian firm recently estimated that its technology’s costs could range between £71 – £174 / tonne of CO2, depending on the financial, energy and technology assumptions used.
An innovative example of a DACCS project is Climeworks, which uses renewable geothermal energy and waste heat from a nearby powerplant as its energy source. The direct air capture technology is made of ‘stackable’ units, meaning the plant can be scaled easily if costs fall or if the incentives to capture carbon increase. The captured CO2 is mixed with water and pumped underground. Here, it reacts with basalt rocks and eventually turns into stone, locking away carbon in the process.
One option for lowering costs is to sell the captured CO2 for use as a fuel or in other industrial processes, but this does not always result in negative emissions as the onward processes often result in emissions of their own. For DAC to find a sustainable revenue stream, the technology cost will likely need to drop until it matches an acceptable carbon price, at which point it can be paired with industries seeking to offset their emissions by paying for its carbon capture properties.
2. Enhanced weathering is a technique that aims to speed up natural weathering processes that absorb CO2. Rocks break down when exposed to the wind, rain and other natural processes, and they release charged particles which react with and bind to CO2 in rainwater and the air. In theory, we can speed this up and so increase this binding process. For that reason, it’s also known as ‘mineral carbonation’.
One approach to enhanced weathering is therefore to spread crushed silicate minerals on soils. This increases the surface area of rock exposed to the atmosphere, increasing rates of weathering and the binding of CO2 by the rocks themselves.
It is estimated that natural rock weathering currently absorbs around 0.3% of global emissions annually, although evidence is limited. Although promising, there are uncertainties to using enhanced weathering to create negative emissions. There are risks that the CO2 drawn into soils could be easily re-released in the future if land management practices change.
There is also ambiguity in current methods of measuring and verifying how much carbon is absorbed through the process. In common with other geo-engineering solutions to climate change, there are unknown environmental risks to the widespread application of enhanced weathering, such as impacts on soil quality.
These issues do not change the fact that, theoretically, enhanced weathering shows significant potential as a negative emissions technology (theoretically up to 430 billion tCO2e in the UK alone), but it needs to be piloted more widely. Different projects are working to test biochar (see below) and overcome these barriers, such as a £2 million research programme, led by the University of Oxford, to inform methods for measuring the carbon effects of enhanced weathering.
The other question is, again, cost. Whilst enhanced weathering is relatively low cost and could fall under the UK’s new farm subsidy scheme, which will pay land owners to capture carbon, there is currently no obvious revenue stream to pay for it.
3. Biochar is charcoal produced by the decomposition of biomass (organic materials such as wood, human wastes or food production by-products) at high temperatures in the absence of oxygen – a process known as ‘pyrolysis’. Pyrolysis also produces useful gases that can be used for energy.
The process produces some carbon emissions through combustion, but the biochar it produces ‘locks’ carbon into a stable form. The biochar ash is then mixed with soils where it is stored for long periods, effectively reducing the concentration of carbon in the atmosphere by transferring it to the soil. Biochar’s main competitive edge over other negative emissions technologies is that it could be integrated with existing agricultural practices, making it easily scalable.
However, considerable uncertainty exists around the negative emissions potential of biochar, with the Intergovernmental Panel on Climate Change (IPCC) estimating its potential sequestration range as 1 to 35 GtCO2/yr (between 3% and 95% of global emissions). A lack of large scale pilots of biochar underpins this uncertainty, as well as a number of concerns over biochar’s potential side effects, such as evidence that applying biochar to soils can reduce soil quality. Based on these risks, the CCC decided to act with caution and excludebiochar from its modelling scenarios for the UK’s Net Zero target, as is common practice in most decarbonisation models. At high levels of deployment, biochar would also directly compete with other negative emissions technologies for biomass as a feedstock, most notably BECCS.
4. Bio-energy with carbon, capture and storage (BECCS) describes a group of technologies that produce energy from plant-based materials (‘biomass’) and capture and store any CO2 produced in the process.
‘Biomass’ describes any organic material, but certain types are normally used as a feedstock for energy production, such as wood and agricultural by-products, landfill waste and fuels derived from plants like bioethanol.
BECCS is fundamentally considered to be a negative emissions technology because biomass (plants) absorbs CO2 as it grows. Around 50% of a tree’s wood, for example, is carbon absorbed from the atmosphere.
A feedstock is either combusted to produce electricity, or converted to a biofuel using a digestion or fermentation process to produce a gas or liquid fuel (see ‘pyrolysis’ in the biochar section above). The CO2 produced through either process can be captured before being released into the atmosphere using similar methods to direct air capture technologies (either ‘liquid’ or ‘solid’ carbon capture methods). The captured CO2 is then stored in a geological aquifer, such as former gas fields under the North Sea.
BECCS is considered an integral part of limiting warming to below 2oC (the goal of the 2015 Paris Agreement) in national and international decarbonisation assessments, but it faces two important hurdles to overcome as it scales up. The first is cost. In the UK, a pilot BECCS project currently captures a tonne of CO2 each day. But to scale it up to the level needed to make a difference will need a clear revenue stream, most likely a subsidy and then later a market income. However, there is a fully operational BECCS project operating at a commercial scale – a plant in Illinois in the US that produces ethanol from corn – which provides a ‘proof of concept’ that BECCS can be done commercially and at scale.
The second is sustainability: regulations must be in place to ensure biomass is not depleting resources, especially forests. Currently, most biomass is sourced from the low-grade by-products of the forestry industries in well-developed economies like the USA and EU, and regulated through supply chains. This means it complements sustainable timber supplies, which is the leading forest product and a good way to capture and store carbon in buildings. As Policy Exchange has argued, revenues from such forest products are a key route to protecting woodlands from land use change and increasing tree cover. High-value, sustainable timber supply should remain the driving force in commercial forestry, not bioenergy supply. Ensuring this is still the case when BECCS plays a much bigger role in the world’s energy markets will be a key regulatory challenge, though cross-border supply chain rules (such as those used by the UK and EU) can play an important role, as is the case with the drive for other sustainable commodity supply chains.