Burning biomass releases CO2. How can this be sustainable?
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 15ha 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 (15ha 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.
Where it doesn't work
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.
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 lifecyle 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 life cycle figures for carbon released by different fuel types are shown in a table here.