The carbon footprint of wood pellets

Wood and the carbon cycle

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.

The embodied fossil carbon of wood pellets

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.

Wood pellets vs other technologies

The key point is that 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. Even wood pellets from a supply involving significant transport distances or that is produced with fossil-fired electricity will avoid:

  • 90% of the carbon emissions attributable to equivalent natural-gas heating,
  • 93% compared to oil-fired heating,
  • 96% compared to direct electric heating, and even
  • 90% compared to heat-pumps.

A low-carbon-footprint source of wood pellets will avoid:

  • 97% of the carbon emissions from natural gas,
  • 97.5% compared to oil-fired heating,
  • 99% compared to direct electric heating, and
  • 97% compared to heat-pumps.

What matters is the 90+% saving from wood pellets from almost any source compared to non-renewable heat, not the few additional percentage points that can be saved if you have access to pellets that have been transported efficiently from a high-efficiency wood-pellet plant.

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 very small carbon footprint compared to fossil-fired and electric heating. If they were produced using biomass CHP before travelling thousands of miles, they may well 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 to use wood pellets, rather than obsess about the source of the pellets.

Heating a hypothetical property near Okehampton

For example, let us compare the carbon footprint of various alternative ways of heating a property 27 miles from Okehampton. As a remote location in the South-West, with potentially long haulage distances, this is chosen as a difficult test for wood pellets. Okehampton is referenced because it is the location of one of our depots in the South-West.

The wood pellets will be delivered from Okehampton by specialist blower lorry in all the wood-pellet scenarios. But, as well as the mainstream heating fuels, we consider a number of different sources and routes for the pellets at Okehampton. They could be:

The following table compares the carbon footprint of these options with the carbon footprint of other heating energy-supplies.

Energy source kg of CO2 equiv. released per MWhth
Wood pellets (Swedish CHP by boat) 19.98 
Wood pellets (Scottish CHP by boat)  8.39 
Wood pellets (Scottish CHP by road)19.09
Wood pellets (local, dried by wood)20.57
Wood pellets (local, dried by gas)67.56
Oil 307.86 
LPG 259.07 
Natural gas 225.54 
Coal 413.42 
Electricity  617.07  

Even the least efficient wood-pellet scenario is significantly better than the most efficient fossil-fired option. In practice, carbon footprint of wood-pellet production will not normally be anywhere near as bad as the worst-case scenario above, where all the drying is done by burning natural gas. Nor will it usually be as good as the best-case scenario where the pellets are produced using CHP and shipped a short distance by boat. Mostly, the carbon footprint of wood pellets will fall in the range 10-25 kg of CO2 per MWhth.

Allowing for conversion efficiencies

The kg of CO2 equivalent released per MWhth in the table above refer to emissions from burning a MWhth of the material (or producing a MWhe in the case of electricity). This does not take account of differences in the efficiencies of the technologies that convert the fuels into usable heat. The output MWhth is arguably more important than the input MWhth.

There are a range of efficiencies amongst the technologies that use each energy source, but typically the efficiency of conversion of natural gas, LPG and electricity will be higher (maybe 90% for the gases, more for electricity) than the efficiency of conversion of wood pellets, oil and coal (maybe 80-85%, less in the case of old oil- and coal-fired boilers).

In the case of electricity, efficiencies of 130 - 360% (with an average Coefficient Of Performance, or COP, of around 240%) can be achieved with Ground-Source Heat Pumps (GSHPs), and 120 - 330% (with an average COP of around 220%) can be achieved with Air-Source Heat Pumps (ASHPs), according to a recent report by the Energy Saving Trust.

The final table, recalculated for output MWhth according to these efficiencies, looks like:

Energy source kg of CO2 equiv. released per MWhth
Wood pellets (Swedish CHP by boat) 23.51 
Wood pellets (Scottish CHP by boat)  9.87
Wood pellets (Scottish CHP by road)22.46
Wood pellets (local, dried by wood) 24.20
Wood pellets (local, dried by gas) 79.48
Oil 362.19 
LPG 287.86 
Natural gas 250.60 
Coal 516.78 
Electricity (direct heating)  623.30  
GSHP (average COP) 257.11 
ASHP (average COP) 280.49 

 

Notes

  1. No allowance is made for the carbon cost of cutting and transporting the wood from the forest to the factory. It is assumed that the carbon cost of felling and haulage is de minimis. In the case of the Swedish plants, no allowance is made either for the carbon displaced by the district heating systems fuelled from the waste heat of the biomass CHP scheme that powers the factory. The carbon cost of cutting and transporting the wood to the factory is more than counterbalanced by the benefit of the green CHP.
  2. No allowance is made for the energy cost of blowing pellets into the customer's store. Again, this is assumed to be de minimis.
  3. The kg of CO2 released per MWhth for the other energy sources take account only of the direct emissions from combustion of the fossil fuels (the weighted average of emissions from the UK's mix of power stations, including nuclear and renewables, in the case of electricity). The carbon cost of extracting and transporting these fuels from their source to the customer are ignored in most calculations of carbon footprints, even though the figure for wood pellets relates entirely to this factor. To compare like with like, this carbon cost should be included for the fossil fuels and electricity as well, which would further increase their carbon footprint relative to wood pellets.

Sources

Emission and conversion factors from DEFRA publication:

2010 Guidelines to Defra / DECC's GHG Conversions Factors for Company Reporting