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The new report by the Intergovernmental Panel on Climate Change makes them the latest authoritative voice to highlight the necessity for negative emissions technologies (NETs) to be developed if the world is to avert catastrophic climate change in the second half of the 21st Century. A NET is any technology that has the overall effect of reducing the concentration of carbon dioxide in the atmosphere, through extracting it in some way from the air, thereby helping to slow or reverse global warming.
A recent study by the Royal Academy of Engineers and Royal Society, ‘Greenhouse gas removal’, outlined the vast range of potential methods that could be deployed. One low tech and relatively inexpensive way of doing this is simply to plant more trees. Although this sounds easy, globally in a time of increasing deforestation for agriculture, building, and power generation, it could be far more challenging than it sounds – according to the WWF we are losing 27 football fields of forest coverage every minute. There are many other ways of achieving negative emissions and the technology that currently gets the most attention is bioenergy with carbon capture and storage, known as BECCS.
Bioenergy is any fuel made from recently living organic matter. Examples of modern bioenergy include woody biomass from trees or bio-methane produced from food waste. Bioenergy is already the largest contributor to renewable sources of heat and power, providing 50% of total consumption. This is more that all hydro, wind and solar combined. Some of this is in the developing world where it is used for things like cooking in homes, but it also provides a significant proportion of Europe’s energy supply. Incentivised by clean energy subsidies and the EU’s Renewable Energy Directive, which set percentage targets for renewable energy for member states, power plants fuelled by woody biomass have proliferated, particular in Germany. The UK also has a large-scale biomass fuelled power plant operated by Drax in Yorkshire, who converted part of their coal-fired power station to biomass units. In addition, there are small-scale biomass power and heat facilities dotted around the country.
Using woody biomass as a fuel source is one of the most controversial ideas in energy policy. The reason for this is that the life-cycle effect on greenhouse gas emissions is devilishly difficult to ascertain. In the best case scenario, demand for wood pellets in power stations creates a market that encourages more tree planting which leads to an overall increase in forest coverage and therefore negative life-cycle emissions. In the worst case scenario, virgin forests are cut down for fuel and not replanted, resulting in emissions equivalent or larger than an equivalent unabated coal plant.
However, regardless of the life cycle emissions associated with sourcing the wood pellets, the emissions associated with power from bioenergy are considerably reduced if combined with CCS. There are different ways to capture carbon from a power station, but the most technically mature is post-combustion CCS. This method passes the exhaust gases from a conventional biomass power station through amine solvents to separate CO2 from the other combustion products. If the wood pellets are sourced sustainably then the whole life-cycle emissions become negative. This means that carbon dioxide is, over time, removed from the atmosphere and stored, either in a dedicated geological storage site or in the form of increased tree coverage. If such technology were to become widespread it could make a significant contribution to reducing the concentration of carbon dioxide in the atmosphere in the second half of this century. This is why it is considered by many to be a key technology in global efforts to limit the dangerous effects of climate change. However, at the present time NETs are not widely deployed and would require a radical increase in use to meet the targets presented by the Paris Agreement and IPPC report.
BECCS could also make a contribution to significantly reducing emissions from the heat sector. Decarbonisation of heat is one of the greatest economic, political and engineering challenges we face in tackling climate change. This is especially the case in the UK, where we have an extensive natural gas network that provides efficient and relative cheap heating to most homes – there is no easy low carbon substitute. As the gas network is primarily fed with methane extracted from oil and gas reservoirs, this results in high carbon emissions. In order to reduce emissions, methane produced from organic waste is blended into the gas network. Methane is a much more potent greenhouse gas than carbon dioxide, so it is better to burn it as a fuel than to allow it to be released into the atmosphere as the organic waste breaks down.
One option to completely decarbonise heat is to use hydrogen in our natural gas network, rather than methane. The primary way of producing hydrogen is currently through the steam methane reformation. If the feedstock for this process is methane from organic waste and the carbon emissions from the steam methane reformation process are captured instead of being released, this could lead to negative emissions. Methane released through the breaking down of food is a major source of greenhouse gas emissions, with one estimate suggesting that if food waste were a country it would be the third largest source of emissions worldwide. Making better use of this as a fuel source should be a priority, as well as reducing the amount of food wasted in the first place, a problem studied by the IMechE in our 2013 report 'Global Food: Waste Not, Want Not'.
As the name suggests, BECCS is the combination of two technologies: bioenergy and carbon capture. BECCS is not likely to be widely deployed in the short term as there is no CCS infrastructure yet in the UK and there are doubts over the sustainability of biomass for energy.
In climate policy, there are two main priorities in the short term. The first is to develop rigorous sustainability standards for biomass feedstocks to ensure they deliver genuine emissions reductions compared with fossil fuels.
The second, for CCS, is to develop regional clean energy hubs where multiple industries (e.g. power, cement, steel) can share the same CO2 transport and storage infrastructure in order to bring down costs through collaboration, technological learning and economies of scale.
Earlier this year the IMechE’s Engineering Policy Unit collaborated with the Power and Process Industry Divisions to co-ordinate a response to a parliamentary inquiry into CCUS. The evidence will soon be published soon.