Bioenergy is produced from organic material known as biomass. Biomass contains carbon absorbed by trees and plants through photosynthesis. When used for energy, it comes in a range of different forms, from waste wood to compressed wood pellets, agricultural and energy crops, like willow, miscanthus, and straw, to biogas and biofuels.

Biomass power is the second largest producer of low carbon energy in the UK. It provides firm consistent power meaning it can provide energy seven days a week, every week of the year. This makes it a really vital part of the UK’s energy mix, complementing other forms of renewable energy, like wind and solar.

While this all sounds positive, critics aren’t always convinced. Concerns about carbon accounting, carbon payback periods, and sustainability continue to play out in the media. So, let’s take each of those in turn.

Carbon accounting for biomass generation

Industry accounts for CO₂ emissions in accordance with internationally agreed and peer reviewed carbon accounting methodologies, set out by the UN IPCC. The UN IPCC counts CO₂ emissions from the combustion of biomass in the Agriculture, Forestry and Other Land-Use (AFOLU) sector which recognises the change in carbon stocks when the biomass is harvested.

So, CO₂ emissions are recorded within the land use sector rather than within the energy sector emissions total. This prevents double counting while still ensuring the full life cycle emissions – production, cultivation, harvesting, collection, transportation, and processing of biomass – are accounted for.

The IPCC’s methodology has been reviewed and reaffirmed multiple times since its development in 1995, most recently in 2019. Importantly, it is set independently of industry, by the scientists of the IPCC and governments.

This also explains why Bioenergy Carbon Capture and Storage (BECCS) projects in the UK that deliver Greenhouse Gas removals or GGRs will have those counted in the UK, at the point of capture, not harvest (because changes in carbon stocks will already have been counted). This method of carbon accounting works, particularly for GGR projects utilising things like waste wood in which the tree would’ve been harvested for use in the building or furniture industry, before becoming a waste wood product many years later. That’s why counting the GGRs at any point other than at capture simply wouldn’t make sense.

Carbon payback periods

When people talk about carbon payback periods, they’re referring to the amount of time it takes a tree to grow back and start capturing CO₂ again, after being harvested.

Working forests – like those from where the UK sources woody biomass – are managed on a landscape level. They contain thousands of trees of different ages, harvested at different times, to produce a constant supply of wood products. As such, when small numbers of trees are removed, others continue to grow and sequester carbon and when new trees are planted to replenish the forestry stocks, the whole process starts again, maintaining a constant cycle of plant, grow, harvest across the forest. Moreover, as per requirements set out in UK land criteria, tree growth – which is monitored and measured annually – always remains level or exceeds harvest rate. This ensures there is no net reduction in forest carbon.

It’s also important to recognise the impact of bioenergy within the forestry sector. Bioenergy makes use of low value residues and by-products already produced by the forestry sector to supply wood for other industries like construction, furniture, and paper. To put it in context, in 2022, wood pellets accounted for 4% of total harvest removals and just .08% of total forest inventory across the US South. Since its foundation, the US pellet export industry, which supplies many of the UK’s biomass generators, has had less than 0.1% impact on the proportion of wood harvested compared to forest inventory (number of trees).

Biomass sustainability

The UK’s bioenergy sustainability governance arrangements are comprehensive. They include land use criteria and GHG emission savings criteria, meaning that life-cycle emissions associated with biomass must meet certain thresholds to ensure they’re delivering significant savings compared to fossil fuels.

Voluntary, independent certification schemes like the Sustainable Biomass Program (SBP) are also used by industry to demonstrate sustainability. The SBP includes over 30 forestry specific indicators to ensure that forests are maintained or increased, biodiversity is preserved, and that forests of high conversion value are protected. Air and water quality, conservation, and biodiversity are also measured.

The Cross Sectoral Sustainability Framework, promised in the Biomass Strategy, should also provide even greater confidence that biomass, when done correctly, is sustainable. Hopefully 2025 is the year we see that Framework published!

Written by Samantha Smith, Head of Heat and Biomass UK, REA