The conversion to biomass energy has played a key role in reducing our dependence on fossil fuels but is this renewable energy source really as green as we first thought?
Biomass power plants – a key renewable-energy source and one of the main replacements for coal-fired power – are emitting more carbon dioxide from their smokestacks than the coal plants they have replaced. In its haste to get rid of coal, the UK may have inadvertently made global warming worse.
The case for biomass
The logic behind biomass energy is simple. Trees and plants absorb carbon dioxide from the air, use photosynthesis to isolate the carbon, and then use it to build tree trunks, bark and leaves. But when the plant dies, it rots down and much of the carbon is released back into the atmosphere as carbon dioxide. “When we use biomass as an energy source, we are intercepting this carbon cycle, using that stored energy productively rather than it just being released into nature,” explains Samuel Stevenson, a policy analyst at the Renewable Energy Association in London.
Currently around half the EU’s renewable energy is based on biomass – a figure that is likely to rise. “The benefit of biomass is that it can be implemented rapidly and uses the current energy infrastructure,” says Niclas Scott Bentsen, an expert on energy systems based at the University of Copenhagen in Denmark.
However, those calculated savings rest on a few key assumptions: first, that the carbon released when wood pellets are burned is recaptured instantly by new growth; second, that the biomass being burned is waste that would have released carbon dioxide naturally when it rotted down. But are those assumptions right?
Advocates of biomass energy claim that when forests are harvested sustainably, and the timber industry thinnings are used as fuel, the smokestack emissions are cancelled out by the carbon absorbed by forest regrowth. However, some scientists say that this carbon accounting simply doesn’t add up. “Wood bioenergy can only reduce atmospheric CO2 gradually over time, and only if harvesting the wood to supply the biofuel induces additional growth of the forests that would not have occurred otherwise,” says John Sterman, an expert on complex systems at Massachusetts Institute of Technology (MIT) in the US. The time needed for the regrowth to mop up the additional CO2 is known as the “carbon debt payback” time, and it is this that is hotly disputed.
The case against: delayed payback
Using a lifecycle analysis model, Sterman and his colleagues calculated the payback time for forests in the eastern US – which supply a large share of the pellets used in the UK – and compared this figure to the emissions from burning coal. Under the best-case scenario, when all harvested land is allowed to regrow as forest, the researchers found that burning wood pellets creates a carbon debt, with a payback time of between 44 and 104 years (Environ. Res. Lett. 13 015007). “Because the combustion and processing efficiencies for wood are less than coal, the immediate impact of substituting wood for coal is an increase in atmospheric CO2 relative to coal,” Sterman explains. “This means that every megawatt-hour of electricity generated from wood produces more CO2 than if the power station had remained coal-fired.”
Sterman stresses that he is not advocating a return to burning coal. “Coal and other fossil-fuel use must fall as soon and as fast as possible to avoid the worst consequences of climate change. [But] there are many ways to do that, with improving energy efficiency being one of the cheapest and fastest.”
It had been assumed that young trees mop up more carbon than old ones because they are fast-growing, but recent studies have revealed that ancient woodland growing in temperate regions takes up more CO2 than young plantations. This is because in some cases, growth accelerates with age and CO2 absorption is approximately equivalent to biomass (Nature 507 90). “Far from plateauing in terms of carbon sequestration at a relatively young age as was long believed, older forests (for example over 200 years of age without intervention) contain a variety of habitats, typically continue to sequester additional carbon for many decades or even centuries, and sequester significantly more carbon than younger and managed stands,” researchers write in the journal Frontiers in Forests and Global Change (2 27).
Mary Booth, an ecosystem ecologist and director of the Partnership for Policy Integrity in Pelham, Massachusetts, shares Sterman’s concerns. In 2017 she used a model to calculate the net emissions impact – the difference between combustion emissions and decomposition emissions, divided by the combustion emissions – when forestry residues are burned for energy. “It is the percentage of combustion emissions you should count as being ‘additional’ to the CO2 the atmosphere would ‘see’ if the residues were just left to decompose,” she explains. Her calculations revealed that even if the pellets are made from forestry residues rather than whole trees, combustion produces a net emissions impact of 55–79% after 10 years (Environ. Res. Lett. 13 035001). Even after 40 years her model shows that net emissions are still 25–50% greater than direct emissions. Like Sterman, Booth concludes that it takes many decades to repay the carbon debt, and she concludes that biomass energy can’t be considered carbon neutral in a time frame that is meaningful for climate-change mitigation.
The importance of a specific supply chain
But even if biomass energy isn’t 100% carbon neutral, there may still be a place for it in the energy mix. Currently around two-thirds of renewable energy in Denmark is provided by biomass, and it plays a vital role in keeping district heating systems running, particularly when the wind fails to blow.
In 2018 Scott Bentsen in Copenhagen calculated the carbon debt and payback time for a combined heat and power generation plant in Denmark. His results suggested that the carbon debt was paid back after just one year, and that after 12 years greenhouse-gas emissions were halved relative to continued coal combustion (Energies 11 807). These numbers are vastly different to the 40-plus years of payback time estimated by Sterman.
Scott Bentsen explains that there are a number of key differences. In this Danish study, the plant burns wood chips rather than pellets, which reduces processing energy. Furthermore, the wood is sourced locally from mixed forests in a cold temperate region, which have different growing characteristics from trees in a warm temperate region. And the energy it produces is maximised, producing both heat for local houses and electricity. “Obviously we shouldn’t cut down all forests just to burn them for energy purposes, but as long as we can harvest biomass in a way that doesn’t permanently jeopardize the forest’s carbon storage and its ability to grow, then it makes scientific and climatic sense to use biomass to displace fossil-fuel resources,” says Scott Bentsen. He believes that calculating the carbon payback time for a specific supply chain can play a significant role in helping to fine-tune the management practices and minimize emissions from individual biomass energy plants (Renewable and Sustainable Energy Reviews 73 1211).
Sterman accepts that there are arguments for using timber industry waste as a biofuel. “It’s not wrong to use sawmill residues for energy, but these sources are already fully utilized. There is not enough timber industry waste to allow the biomass-energy industry to grow without using more roundwood,” he explains.
Guidance published in November 2018 by the UK Committee on Climate Change concluded that there is a limited supply of sustainable biomass and that “no further policy support (beyond current commitments) should be given to large scale biomass plants that are not deployed with carbon capture and storage technology”.
But even if living trees can claw back these carbon-dioxide emissions relatively quickly, there is a danger in front-loading our emissions in this way. “Regrowth is not certain,” says Sterman. “Forest land may be converted to other uses such as pasture, agricultural land or development. And even if it remains as forest, wildfire, insect damage, disease and other ecological stresses including climate change itself may limit or prevent regrowth, so that the carbon debt incurred by biomass energy is never repaid.”
This is a shortened version of an article by Kate Ravilious based on studies reported in the January 2020 issue of Physics World