Wood pellets have a very small carbon footprint. Wood is a form of solar energy – converting sunlight and atmospheric carbon into carbohydrates whose energy can be released when we need it rather than when it hits us (like conventional solar energy).
Wood absorbs as much carbon when it is growing as is released when it is burnt. Provided that the wood is harvested sustainably, so at least as much is grown back as is cut, wood is assumed to be a carbon-neutral energy source.
To address concerns about the sustainability of wood fuels, the UK government introduced a system called the Biomass Suppliers List (BSL) in 2015. Fuels certified on the BSL are guaranteed to achieve at least 60% carbon savings relative to the average emissions from heating in the EU. The wood pellets offered by Forever Fuels achieve certified savings between 70% and 94%.
The main fossil-carbon releases associated with wood pellets are due to the fossil fuels used to produce and transport them. The amounts of fossil fuels used for this purpose differ from one pellet factory to another, and according to the distance and manner of transport of the pellets.
Because bulk transport by sea is so efficient compared to road transport, it cannot be assumed that wood pellets from abroad and from remoter parts of Britain necessarily have a higher carbon footprint than local pellets - it depends how they are produced and transported. Unless the factory uses a very inefficient fossil-fired process, and the pellets are transported a very long distance by road, the fossil-carbon embodied in wood pellets is a small fraction of the fossil-carbon released by most other ways of producing heat.
The differences between wood pellets from one source and another (and indeed between wood pellets, chip and logs) are generally insignificant compared to the differences between wood pellets and alternative energy sources.
According to the BSL calculations, wood pellets from the UK and from continental Europe typically have a carbon footprint of 5 - 15 gCO2e/MJ. Wood pellets from further afield, such as Siberia, will typically have a higher carbon footprint in the range 20 - 30 gCO2e/MJ.
The average emissions from heating fuels in the EU is 87 gCO2e/MJ. The BSL's requirement to save at least 60% equates to a limit of 34.8 gCO2e/MJ.
According to DEFRA's greenhouse gas figures, the average carbon emissions from other technologies are:
|Fuel||Emissions /input (gCO2e/MJ)||Assumed Conversion Efficiency||Emissions /output (gCO2e/MJ)|
|Heat Pump @ SPF 3.5 (e.g. Ground-Source)||32.7||100%||32.7|
|Heat Pump @ SPF 2.5 (e.g. Air-Source)||45.8||100%||45.8|
One point of wood pellets is to make wood a portable, merchantable fuel, so that the benefits of wood energy are available where people need the heat, and not just where the trees are. The carbon footprint calculations demonstrate that, because the fuel is relatively dense and easy to transport, the embodied carbon due to transport emissions is relatively insignificant unless the pellets travel a very long way indeed by road.
Even wood pellets that have travelled thousands of miles have a small carbon footprint compared to fossil-fired and electric heating. If they were produced using biomass CHP before travelling thousands of miles, they may have a smaller carbon footprint than wood pellets produced locally without biomass CHP. But both are so much better than fossil fuels and electricity that the important point is that you can make a significant reduction in your carbon emissions whatever the source of your wood pellets.
Modes of transport
Two popular false assumptions about the relationship between carbon emissions and transport distance are:
The emissions from transporting wood fuel use up most of the carbon benefit and make it a much "dirtier" renewable fuel than other forms.
The emissions from transporting wood fuel are directly proportionate to the distance travelled, so you should only buy wood fuel produced in the local area.
DEFRA's Greenhouse Gas Conversion Factors give the following figures for the emissions from various modes of transport:
|Mode||gCO2e /tonne.km||MJ /tonne||gCO2e /MJ.km||Embodied carbon per 100 km|
As of late 2016, the UK wood pellet heating market is too small to support ships of over 10,000 tonnes. Most wood pellet sea shipments are done in "Coasters" with capacity of around 2,000 - 5,000 tonnes.
Bearing in mind that the total emissions from wood pellets are in the range 5-30 gCO2e/MJ, and from fossil fuels is in the range 62-125 gCO2e/MJ, we can draw a number of conclusions from these figures.
Transport only becomes a significant component of the embodied carbon in wood pellets if they are transported long distances by road.
Our depots are typically around 200 km from our hubs (i.e. a 400 km round-trip). By moving the wood pellets this distance by artic rather than on a rigid delivery vehicle, we save around 2 gCO2e/MJ.
Wood pellets can travel 1000 km by sea with lower emissions than if they travelled 200 km by road. Wood pellets hauled by road from Scotland to the south of England probably have higher embodied carbon from their transport than wood pellets hauled by sea from the Baltics to Britain. (Of course, wood pellets could also be transported by sea from Scotland to the south of England, but that rarely happens in practice.)
If the wood heat market were an order of magnitude larger, it would support larger ships, and this would dramatically reduce the emissions per tonne.km from shipping. The emissions from transporting wood pellets from the Baltics to the UK by Coaster are 50% higher than the emissions from transporting wood pellets from the USA to the UK by Handysize vessels, even though the distance is more than twice as far.
Energy density and transport emissions
The energy density of wood fuel is quite low compared to oil and coal. The amount of energy that can be transported is therefore constrained more by volume than by mass (i.e. you run out of space on the truck before you reach the legal weight limit).
In the case of wood pellet, this is finely balanced. At normal densities, you can just about get the maximum permitted road weight onto a tanker. If the density is at the bottom of the permitted range (say 620 kg/m3), then some vehicles cannot quite accommodate their maximum payload.
The energy density of wood pellet is roughly 3 - 4 times higher than the energy density of wood chip. Wood chip transporters run out of space long before they hit the legal weight limit. Each vehicle movement of wood chip therefore moves a lot less energy than an equivalent vehicle movement of wood pellets.
The carbon emissions per km vary only marginally according to the weight carried. Most of the energy is expended in moving the weight of the truck itself and pushing the air in front of the truck. If the energy density of wood chip is 3 - 4 times lower than wood pellet, the carbon emissions of transporting wood chip are around 3 times higher per km than for wood pellet, for an equivalent vehicle.
That puts the emissions from transporting wood chip 100km by a rigid delivery vehicle at around 3 gCO2e/MJ. That is roughly 60 miles one way. For a round trip 100 miles each way, the carbon emissions are approaching 10 gCO2e/MJ.
That is why wood chip is rightly seen as a fuel that should be produced and delivered locally. Wood pellets are the solution to minimising energy costs and emissions for longer transport distances. It is nonsensical to invest in a pellet boiler and yet insist on local fuel, particularly as there is no local pellet factory in most parts of the UK.
In general, the areas that are densely forested are not the areas that are densely populated. Wood pellets make it practical and environmentally beneficial to bring the renewable fuel from where it is produced to where it can be used by as many people as possible.
Decarbonising the UK's heat
Heating represents more than twice as much of our final energy consumption as electricity. It is responsible for more carbon emissions than electricity generation.
Wood heat is much the most practical and cost-effective form of renewable heat to replace fossil-fired heating in most buildings. That is why wood heat has been a central component in the energy mix in the large number of EU countries that have been more successful than the UK at deploying renewable energy and reducing carbon emissions.
If the UK is serious about reducing its carbon emissions and dependence on fossil fuels, it needs to lose its myopic focus on electricity, and accept that wood heat will be a critical option for displacing fossil fuels in the heat sector.
And it will recognise that heat is a naturally diffuse sector that cannot practically be centralised and distributed like electricity. But the fuel can be. A decarbonised heat sector will consist of millions of renewable-energy heating systems. Wood pellets will play a key role in such a system, because of its distribution advantages, similarity of heat quality and dispatchibility to fossil-fired systems, and the huge resource that is made accessible and sustainable through densification.