Wood biomass is an increasingly controversial energy source, but why is this? Here we explore what wood biomass actually is, some of the issues surrounding it and what role it should play in the future.
What is Wood Biomass?
Using wood to generate energy is perhaps the oldest form of man made energy production. Ever since we learnt to control fire humans have been burning wood biomass. The fuelwood stoves used across the world to cook food and warm homes and the massive biomass power plants that generate electricity for millions of families, display the breadth and importance of biomass in the lives of billions across the planet.
The form of wood biomass differs from place to place. Generally in industrial power stations it is in pellets or wood chips derived from roundwood or waste wood, but in fuelwood stoves it can be in many forms e.g. roundwood, charcoal or sawdust. There is also a geographic and social divide between these two main uses of wood biomass, with fuelwood being predominant in developing countries where access to electricity is poor and biomass power stations being more common in developed countries.
These power stations have become more popular in recent years due to their renewable status, this has meant many governments, especially in Europe, have used them to meet Paris climate agreement renewable energy targets. This has taken the form of increased investment in biomass, often via large subsidies to biomass companies, such the £10 billion given to Drax in the UK 1.
Currently, wood biomass is considered a renewable energy source because wood is an inherently renewable material. Recently however, this has been called into question by numerous environmental groups, scientists and even the British public, with a recent YouGov poll finding that 57% of those surveyed thought that burning wood to produce electricity should not be classified as renewable energy2.
The reasons for this bad press are varied with deforestation, carbon emissions and air pollution often cited. These are often countered by arguments from biomass companies referring to carbon neutrality and the use of waste wood. Each side has its facts, but both miss pieces of the puzzle. So what is the real environmental impact of wood biomass?
The imbalance between the carbon footprint of a particular country, group, person, etc. and any carbon offsetting that has been agreed or undertaken to counteract this 3. Depending on the scale of carbon debt, it is generally measured in months, years or decades.
In terms of wood biomass, the carbon footprint is equivalent to the amount of carbon released into the atmosphere when wood is burned, whilst carbon offsetting is the recapture of that carbon by the growth of wood in new trees. The carbon debt is the amount of time it takes for the carbon released to be recaptured by growing trees.
It seems like a simple calculation to make, however there are different perspectives that can be taken, each producing vastly different numbers for the carbon debt.
The simplest way to look at it may be at a tree by tree level. If a tree is burned, all the carbon captured by the tree in its lifetime is released into the atmosphere. If another tree is planted in its place, as the new tree grows it recaptures that carbon. The carbon debt in this scenario will depend on the species of tree and how old the burnt tree was, but generally it will be at least two decades or more.
From a forestry perspective, tree by tree level calculations do not reflect the real carbon debt produced. In this case it is necessary to use a whole forest perspective, taking into account the type of forest and forestry techniques used. For example, boreal forests are often subject to fire and pests, which clear large areas of forest in ‘natural clear cuts’, harvesting these trees for biomass could be seen as merely putting these natural clear cuts to industrial use.
The argument has also been made that sustainable harvesting of some forest results in the forest as a whole capturing carbon at a greater rate, reducing the carbon debt by some estimates to less than a year4. This is based on the assumption that younger trees, due to faster material growth, store more carbon more quickly, although the scientific research done in this area is inconclusive and incomplete.
It almost goes without saying that repaying carbon debt only happens if the trees regrow. This is a gamble, but the probability of regrowth can be improved by logging from sustainably managed forests, such as those in North America, which are actually expanding5. However, the problem will always remain; you can never be sure that the carbon debt will be repaid in full.
All these arguments around calculating carbon debt will not be resolved soon, with the elements of truth in both arguments making each side convinced of their validity. This makes it imperative to address a different, less abstract aspect in the biomass debate: should wood biomass be used at all?
When first introduced, industrial wood biomass was seen as a way for manufacturers of wood and paper products to use their waste as a source of energy, powering their own mills with their own offcuts. From a carbon perspective this is beneficial, as most of the tree is being converted into another carbon store – the wood product, and a smaller amount is being burnt, reducing the carbon debt. However, when scaled up to the same level as a coal fired power station, there simply isn’t enough waste wood to go around. Consequently, trees start being cut down and sent straight to the incinerator, with debatable carbon neutral outcomes.
There is certainly a case to be made for the use of waste wood as biomass, with it also being seen more positively by the general public. In a recent YouGov survey, 58% of UK adults supported the governmental promotion of waste wood biomass.
But what kind of waste wood is best?
The concept of cascaded use has long been touted as a stepping stone towards a circular economy and, for wood biomass to have a place in a ‘green energy mix’, this concept needs to be applied.
‘Cascading use is the efficient utilization of resources by using residues and recycled materials for material use to extend total biomass availability within a given system.'6
For waste wood biomass you can split the cascaded use into stages:
- Waste from logging activities
- Waste from manufacturing/processing activities
- Post-consumer waste
As you descend the list, the carbon debt becomes smaller as you are ‘extending the biomass availability’ by as much time as possible. When burning post-consumer waste, the carbon has already been stored for at least a few years in the wood product and there has been some time for the carbon debt to be paid back. Additionally, when this is done on a commercial scale strict regulations and flue gas abatement technology mean air quality is not severely impacted7.
Waste from logging and manufacturing processes come with another problem. Much of this waste is used in the manufacture of MDF and other composite board products. The massive subsidisation of biomass companies has meant that they are able to out compete these companies on price, thus diverting this waste wood away from more durable products that act as a carbon store, making the use of this waste wood less efficient. Small and medium sized sawmills rely on selling their waste (co-products) to make a profit, but from a carbon perspective it would be more beneficial if they could sell to MDF or other wood product manufacturers and only sell to biomass companies as a last resort.
Although a useful stepping stone away from fossil fuels such as coal, wood biomass has outgrown its role in the renewable energy sector in some countries, particularly in Europe. The massive subsidies it receives are out of proportion with its environmental benefits and are creating more problems than they solve. Diverting roundwood and wood waste away from becoming wood products and sending them straight to the incinerator is not the way to create a circular economy and does not make efficient, sustainable use of the material.
Governments would be better off, both environmentally and in the polls, if they subsidised energy solutions with surer renewable outcomes, while also supporting the development of infrastructure and markets for more durable sustainable wood products, particularly those based on waste fibre. Alongside this, they should fund research into exactly what are the environmental and economic impact of biomass and other renewable energy sources. In most cases the rhetoric has got ahead of the science which will inevitably result in unintended consequences.
Wood biomass does have a role to play, albeit on a much smaller scale. The burning of post-consumer wood waste for energy reduces pressure on landfills and produces renewable electricity. It is possible that recent negative publicity and public opinion have come at a good time, they may force governments to turn subsidies away from biomass, forcing the industry to ‘downsize’.
- McDonald, P., Moore, C., (2020). ‘The Burning Question: Should the UK end tax breaks on burning wood for power?’. Ember. June 2020. https://ember-climate.org/project/the-burning-question/
- YouGov / Lexington Communications. (2020). ‘Biomass Survey Results’ [Data set]. Retrieved from: https://docs.cdn.yougov.com/w02weplu5y/YG-Archive-23072020-Biomass.pdf
- Lexico. (2020). ‘Meaning of carbon debt in English’. https://www.lexico.com/definition/carbon_debt
- Ravilious, K., (2020). ‘Biomass energy: green or dirty?’. Physics World. January 2020. https://physicsworld.com/a/biomass-energy-green-or-dirty/
- Oswalt, S. N., Smith, W. B., Miles, P. D., Pugh, S. A., (2014). ‘Forest Resources of the United States, 2012: a technical document supporting the Forest Service 2015 update of the RPA Assessment.’ Gen. Tech. Rep. WO-91. Washington, DC: U.S. Department of Agriculture, Forest Service, Washington Office. 218 p. https://www.srs.fs.usda.gov/pubs/gtr/gtr_wo091.pdf
- Vis M., U. Mantau, B. Allen (Eds.). (2016). ‘Study on the optimised cascading use of wood.No 394/PP/ENT/RCH/14/7689’. Final report. Brussels 2016. 337 pages
- Hill, A., (2018). ‘Challenges lie ahead for the waste wood sector’. Recycling and Waste World. https://www.recyclingwasteworld.co.uk/in-depth-article/waste-wood-challenges-brexit-biomass-feedstock-public-perception/192620/