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The critical difference between biomass fuels and fossil fuel, is that of fossil and contemporary carbon. Burning fossil fuels results in converting stable carbon sequestered millions of years ago into atmospheric carbon dioxide (when the global environment has adapted to current levels). Burning biomass fuels however, returns to the atmosphere contemporary carbon recently taken up by the growing plant, and currently being taken up by replacement growth.
If woodfuel is sourced from well managed woodlands, then carbon released from the wood during combustion will be removed from the atmosphere as the remaining trees and seedlings photosynthesize.
Imagine a 15 ha block of chestnut coppice managed on a 15 year rotation and producing 4 oven dry tonnes per hectare per year. Assuming that dry wood has a carbon content of 50%. To ensure a steady supply of fuel is produced, 1ha of coppice is harvested each year. This means each year 60 oven dry tonnes of wood is harvested from the site (15 years x 4 oven dry tonnes per year) and used as fuel.
When it is burned this releases roughly 30 tonnes of carbon to the atmosphere. After harvest, the coppice stools will produce new shoots and as a whole, each year, the coppice woodland will produce 60 tonnes of new wood (15 ha x 4 oven dry tonnes per year), removing 30 tonnes of carbon from the atmosphere.
As a result, the net carbon flow between the coppice and the atmosphere is zero. If you imagine this principle expanded to take into account woodland management at a landscape scale, with individual hectares of coppice being replaced by different woodland areas being harvested or thinned, then the carbon flow between woodland and atmosphere remains zero. This is sometimes referred to by researchers involved in greenhouse gas and energy balance studies as a ‘normal forest’.
While in practice a small amount of fossil fuel will be burned during felling, extraction and processing operations; overall this still results in very low net carbon emissions to the atmosphere.
If deforestation operations are used to produce fuel and no new growth is encouraged then carbon emissions will approach those of conventional fossil fuel systems – direct carbon dioxide emissions from producing 1MWh of heat energy from wood are roughly the same as for coal and significantly more than for oil and gas. If carbon stored in the soil of these forests is also burned as part of these clearance operations then higher emissions still would result. However, if forests were managed in this way, woodfuel suppliers and their customers would very soon run out of the raw material they are selling or use.
Although burning biomass can often be beneficial in climate and carbon terms, this is not always the case, as outlined above. Although some general principles can be given for when it is likely to be beneficial, or when counterproductive, in order to truly assess the total climate impact of burning biomass for energy detailed lifecycle assessment (LCA) must be undertaken. However LCA is not a single, universal process. It must be performed to answer a specific, stated question with a stated set of boundary conditions and assumptions. It is also necessary to distinguish between attributional LCA (ALCA), which calculates the lifecycle impact of an individual product or process (such as a biofuel), and consequential LCA (CLCA) which calculates the total lifecycle impact of a course of action, such as increasing use of bioenergy, compared to the counterfactual situation where the course of action is not followed. CLCA therefore does not attempt to allocate impacts (such as carbon emissions) to an individual product or process, but calculates the overall impact of the course of action, such as a policy decision, with all its ramifications.
Forest Research and North Energy Associates have undertaken detailed CLCA calculations on behalf of the European Commission to calculate the total carbon impacts of biomass consumed in the EU. Calculations have been performed for different types and sources of biomass, both produced domestically within the EU and imported, to identify under what circumstances genuine lifecycle carbon reductions can be achieved, and when the use of bioenergy may be counterproductive. The results are published and available in the final report: Carbon impacts of biomass consumed in the EU: quantitative assessment.
On a national scale, activities such as changing rotation length to influence the amount of biomass available for fuel, and changing the mix of products from timber harvesting, including what is used for fuel, can change the overall climate impact. It is important that national and international policy, and the mechanisms in place to implement it, ensure that support for biomass energy produces positive climate benefits and prevents negative ones, and CLCA is the tool to allow the impact of different behaviour to be assessed.
The UK Forestry Standard defines the principles of sustainability for UK forestry, including conservation of carbon in forests and soils. The UK Government is working with the Scottish and Welsh administration, industry stakeholders and NGOs to develop ‘sustainability criteria’ which will include a minimum GHG emissions reduction threshold. This threshold will take into account include the carbon lifecycle of the fuel, including land use.
When using coal, oil or gas, unless carbon capture and storage in used, carbon released during combustion remains in the atmosphere. Fuel is also burned as these materials are mined, refined, transported and stored. If wood from a well managed forest is used to displace these fuels, significant carbon savings can be made. Looking to the future, biomass fired combined heat and power stations fitted with carbon capture and storage equipment could offer a possible mechanism for removing carbon from the atmosphere permanently. Overall lifecycle figures for carbon released by different fuel types are shown in a table here.
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