Two issues are the focus of considerations on the environmental sustainability of wood pellets:
- Carbon emissions
- Land use
(2) relates closely to the main consideration for economic sustainability, which is the sustainable rate of production of forest products for energy, in relation to competing uses for the forest biomass.
Carbon emissions from wood pellets
Trees are part of the biosphere's carbon cycle. Simplistically, the carbon that is emitted when wood is burnt is absorbed by the new trees as they regrow, resulting in no net increase in the carbon dioxide in the atmosphere. For this reason, energy from wood has historically been treated as carbon neutral.
Three refinements of that view are gaining greater attention:
- Fossil fuels are used in the production and distribution of the wood pellets. The extent varies significantly. The details of the production and supply chain have a significant effect on the true carbon footprint of wood pellets.
- Taking an area of land in isolation, it takes many years for the trees that regrow on that land to absorb the carbon that is emitted when the wood from that land is harvested and burnt. This is a simplistic statement of what has come to be known as the "carbon debt" question. This simplistic statement does not stand up to close scrutiny, but there are more complex scenarios in which the proportion of the carbon stock in the atmosphere may be increased and the land-based carbon stock may be reduced in the short- to medium-term. It depends how we go about producing and using the wood fuel.
- The net efficiency of the conversion of the wood pellets into useful energy is critical. If you halve the amount of energy produced from the wood, you halve the amount of fossil fuels that are avoided by using wood energy.
Areas with substantial sustainable wood resources are often distant from areas with substantial potential to displace fossil fuels with wood. Pelletisation significantly improves the efficiency with which wood energy can be distributed and used. The energy for the pelletisation process can be produced from low-grade wood that is not suitable to be pelletised.
Where there is a suitable, efficient use for wood chip or log close to a sustainable forest, this can have the lowest carbon footprint of any form of wood energy use. For the majority of opportunities to displace fossil fuels, however, wood pellets that have been efficiently distributed and produced from sustainable forests will represent the most efficient way of using the wood for energy.
Using the traditional method of calculating carbon footprints (i.e. treating the wood itself as carbon neutral), and assuming reasonably efficient distribution paths, the following is a reasonable approximate comparison of the carbon content of various fuels in various circumstances:
|Energy source||kg of CO2 equiv. released per MWhth|
|Wood pellets (produced with biomass CHP) ||10-20|
|Wood pellets (dried with biomass, powered by grid electricity) ||20-30|
|Wood pellets (dried with gas, powered by grid electricity) ||65-75|
To make efficient use of the wood pellets, a substantial proportion of the heat must be utilised. Conversion of wood pellets into power (without using the heat) wastes 60 to 80 per cent of the energy up the chimney. If the wood pellets are used to produce heat or heat-led Combined Heat and Power (CHP), less than 10 per cent of the energy may be lost up the chimney. The use of wood pellets for heat increases the amount of energy that is recovered (and therefore the amount of fossil fuels that are displaced) by 100 to 300 per cent.
|Energy source and use||kg of CO2 equiv. released per MWh output|
|Heat||Heat-led CHP||Power-led CHP||Power only|
|Wood pellets ||11-25||12-25||20-40||29-100|
|Wood pellets ||22-38||25-38||40-60||57-150|
|Wood pellets ||74-94||81-94||130-150||186-375|
|Electricity (heat pump)||150-250|
Land use impact of wood pellets
The use of land for biomass can have a number of impacts on the environment, e.g.:
• Water consumption
• Changes in the carbon stock
• Impacts on the availability of wood for other products, and on the demand for and production of alternatives to wood products.
The effect can be direct or indirect. Indirect effects may occur (for example) where land is converted from producing food to producing energy. Besides the direct impact on the original land, other land may be converted to produce food, to replace the lost agricultural area. This conversion to agriculture may release a substantial carbon stock (e.g. primary forest) into the atmosphere, or damage a significant ecosystem. These are considered to be the indirect effects of the original change of land use.
The environmental impact of land use change varies widely according to the specific circumstances in which biomass fuel is produced. The most significant effects may be felt with energy crops (e.g. willow, poplar, or miscanthus for solid biomass, or maize for biogas), as they are often planted on agricultural land, and may indirectly cause virgin land to be brought into cultivation to replace the lost food production.
Wood energy tends to have less impact than energy crops, because wood energy tends to exploit established forest resources, and therefore usually does not displace agricultural production.
Whereas wood chips may be produced from softwood or hardwood (including ancient native forests), wood pellets tend to be produced from softwood, which is grown on long-established commercial plantations in many parts of the world. The ecosystem of these plantation forests is maintained by harvesting only a small fraction of these plantations each year. Plantation forests are rarely irrigated. The softwoods tend to regrow significantly quicker than hardwoods (which is why they are planted), and reach maturity (where their growth slows and they are in danger of dying if not harvested) in decades rather than in centuries.