Burning Wood
The current accepted figure for CO2e (carbon dioxide equivalent) emissions due to burning wood is 0.02 kg CO2/kWh.
See Guidelines to Defra / DECC's GHG Conversion Factors for details (p42)
This includes:
This does not include:
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More details on the excluded items:
1. The CO2 emitted by the actual burning of the wood
CO2 from burning the wood is excluded because the CO2 emitted by burning the wood is equal to the CO2 absorbed by the tree whilst it was growing.
This is over simplistic. It takes no account of the changes in carbon due to different land managements, what the wood would do otherwise and the timescales of these changes.
This is over simplistic. It takes no account of the changes in carbon due to different land managements, what the wood would do otherwise and the timescales of these changes.
CO2 moves in cycles (the carbon cycle) between the atmosphere, plants, other biomass, dead biomass, soil, oceans and sediments (including fossil fuels).
The amount of carbon in each phase of the cycle is affected by how much is stored in all the other carbon reservoirs. If we weren't releasing long term carbon stores into the atmosphere (fossil fuels), then we'd be in a fairly steady state with carbon cycling between the carbon reservoirs. Land with no biomass or soil (desert) does not store any carbon, and land with lots of biomass and soil stores a lot of carbon. If land use changes from forest to desert, then the carbon that was held in storage in the biomass and soil, is released into the atmosphere. In the same way, if forestry is managed a certain way, it has a specific amount of carbon stored in its living biomass, dead biomass and soil. If that changes and there is less carbon stored in any of these reservoirs, then more carbon ends up in the atmosphere. |
Drax is the UK coal burning power station in the process of switching to Biomass. It uses (mostly US) waste wood, forest residues and thinnings to produce it's wood pellets. This is labelled by Drax as "good biomass". But if this "waste wood" had been previously left to replenish the soil and provide a layer of dead and rotting wood, then burning it will reduce the carbon stored in the forest and the difference will be added to the atmosphere. If this continues for the length of time for the forest to reach a steady state, then no net carbon will be emitted to the atmosphere but this will be at a higher steady state level of atmospheric CO2. Conversely, if forests are managed (or left undisturbed) to store more carbon in their living and dead biomass and soils, then the atmospheric CO2 will eventually reach a lower steady state.
Domestic Biomass burning doesn't always come from "waste wood" (detracting from soil carbon). There are a number of things that the wood would have done otherwise. Construction timber for example stores carbon for long periods of time out of the atmosphere. The paper in 2010 "A burning issue" looked at this issue.
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We are changing the size of carbon reservoirs by changes to land management because of biomass burning. So the effect of this change on atmospheric CO2 should be accounted for in the emissions due to biomass burning.
DECC (Department for Energy and Climate Change) produced a calculator under David Mackay which quantified this effect and found it to be very significant.
The BEaC (Biomass Emissions and Counterfactual) Calculator found that wood burning emissions can be up to 3 times worse than burning coal. |
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However, if a government is more interested in seeming to meet their climate commitments than actually meeting them, the best response to research which shows you are moving rapidly in the wrong direction, is to ignore it. Which is what has happened unfortunately. If we as individuals want to avoid falling into the same trap, then taking full account of the impacts is important.
To me the most sensible way of allocating an emissions factor to wood is as a function of the amount of wood burnt per person. That to me seems more sensible than just looking at where it's coming from. If we can get enough biomass globally from sawmill residues that aren't already used for particleboard etc, then it is low carbon as the BEAC calculator shows. If we are trying to meet over 50% of our renewables target (we are!!) with it, then no matter how sustainable one person/company is in sourcing, the global emissions from biomass will be far higher per kWh provided.
To me the most sensible way of allocating an emissions factor to wood is as a function of the amount of wood burnt per person. That to me seems more sensible than just looking at where it's coming from. If we can get enough biomass globally from sawmill residues that aren't already used for particleboard etc, then it is low carbon as the BEAC calculator shows. If we are trying to meet over 50% of our renewables target (we are!!) with it, then no matter how sustainable one person/company is in sourcing, the global emissions from biomass will be far higher per kWh provided.
2. Other Greenhouse Gases (GHG) emitted by the burning of the wood
Other greenhouse gases, Defra (p42) states: "Direct emissions of methane (CH4) and nitrous oxide (N2O), which are not absorbed in the biomass growth phase are not currently available".
However, the DECC BEAC calculator does include these emissions in its life cycle analysis.
However, the DECC BEAC calculator does include these emissions in its life cycle analysis.
3. The Soot and Particulates emitted by the burning of the wood.
The AEA advisory report to Defra states that: "Given the complex nature of the relationship between emissions of NOx and PM10 (particulate matter including soot) and climate change, the uncertainties involved in estimating their contribution to global warming, and the lack of an accepted GWP for emissions of these pollutants, it does not seem appropriate, particularly for a regulatory scheme such as the supplier obligation to convert the estimates of NOx and PM10 emissions to kg CO2 eq and to include them in the total carbon factor for wood fuels." i.e. we don't know the answer, so count it as zero.
The BEAC calculator also does not account for soot. |
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The report "Emission factors for black carbon" estimated that:
Black Carbon (soot) Emissions from burning wood are estimated to be between 0.05 and 0.35 kg CO2e/kWh. Even the lowest estimate would more than triple estimated Greenhouse gas emissions from burning wood |
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In summary
To sum it up, the official line on emissions of CO2 and of other GHGs and soot by burning wood is:
As we don't know how much of an impact the actual burning of biomass has, we are going to ignore most of the impact completely.
To me that seems like a very unsound basis for an emissions figure driving major policy changes.
Photo used under Creative Commons from Bhavishya Goel