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Simon Evans

07.10.2014 | 4:50pm
RenewablesIs burning wood for energy worse for the climate than coal?
RENEWABLES | October 7. 2014. 16:50
Is burning wood for energy worse for the climate than coal?

An article in today’s Daily Mail  says it is “lunacy” to run Drax power station on biomass instead of coal. Converting the plant to burn wood destroys forests and emits more carbon, it says. The paper calls this a “living, humming, forest-destroying symbol of the shameful absurdity of European energy policies”.

Drax and the Department of Energy and Climate Change (DECC)  do not agree. DECC is giving money to conversions at Drax and other power plants because they are helping the UK meet an EU target to get 15 per cent of its energy from renewable sources by 2020.

DECC says  this will cut carbon too, if the right kind of biomass is used. But what does the right kind of biomass look like? Let’s try to unpack things a little.

Sums with wood

To understand whether burning wood at Drax is a smart move for the climate, we need to understand the emissions this will produce. Coal-fired power stations emit on average 1,018 kilograms of carbon dioxide per megawatt hour of electricity. Coal is usually considered the dirtiest form of power generation, with sources like gas coming in at less than half that, at 437 kilograms per megawatt hour.

So are biomass emissions higher or lower than coal?

When trees grow they absorb carbon dioxide and lock it away in wood. If you chop down a tree and burn it, you can’t possibly emit more than was absorbed in the first place, so the whole thing is effectively carbon neutral, or zero kilograms per megawatt hour.

At least that’s the theory. In practice there will be some emissions related to the energy needed to drive the truck hauling the log, the power needed to turn that into wood pellets and the heat required to dry it.

Then we need to ask what type of wood is being burnt. Is it whole logs or just branches, twigs and sawdust that can’t be used to make furniture? Does it come from the cold northern forests of Canada or the warm, moist forests of the southern US? Is it from a plantation or a natural forest?

These are all pretty simple questions. But what about demand for wood? Do imports to places like Drax change the amount of wood needed? Is supply diverted away from paper or other products? If it is, then is it replaced with new plantations? Or more regular harvest? Or will additional natural forests be converted to plantations?

All of these questions matter. That’s why DECC combined them into a massive biomass calculator. This is a big spreadsheet that tries to account for all the factors above and many more besides.

Last year we wrote about this DECC biomass calculator, before it had been formally published, but we weren’t able to reach any firm conclusions on the climate impacts of burning wood.

Good wood

Now the final calculator is out, can we say what climate-friendly biomass power look like?

If Drax is burning wood residues like twigs, small branches or sawdust that would otherwise have been burnt as waste, then the emissions will be below 100 kilograms per megawatt hour, or at least ten times lower than burning coal.

Burning wood residues instead of leaving them in the forest to rot is also generally low carbon. Increasing plantation yields with fertiliser or better management also yields climate-friendly biomass, as long as the rate of harvest does not increase.

If Drax burns wood from plantations that would otherwise have reverted to natural forest, or from regrowth forest converted to plantations then the carbon benefits are also significant, and emissions will come in at under 100 kg/MWh.

Bad (worse than coal) wood

Not all types of biomass are good for the climate. In the cold northern forests of Canada it is usually lower carbon to leave coarse forest residue or trees killed by beetles to rot, because this happens slowly. Biomass from sources like this produces emissions higher than natural gas, though still lower than coal.

Reducing the rotation rate of trees to boost forest output is also generally a bad idea for the climate and can lead to the production of biomass that’s higher carbon than coal. Several other DECC scenarios also show that burning wood in UK power plants can be worse than coal. This includes harvesting wood from naturally regenerating forest, with emissions of up to 5,174 kilograms per megawatt hour – a staggering five times that of coal.

Nevertheless, it’s clear that some types of biomass can replace coal with positive effects for the climate.

Would a climate-smart UK burn wood?

The UK is burning nearly four million tonnes of wood a year to generate power, and Drax is the single largest user. Most of its supplies come from north America.

DECC says it has tough standards that mean only good wood is burnt in the UK, and that its biomass calculator confirms this can be good for the climate. It says generators like Drax must report compliance, and that their emissions savings must exceed 68 per cent compared to coal, rising to 72 per cent by 2020.

Drax says it uses wood from thinnings and off-cuts, and that this reduces emissions by 80 per cent compared to burning coal. The saving is calculated with a less complete method than that used by DECC’s calculator, however. According to Greenpeace, who can usually be relied on for a well-turned quote, this leaves a hole in the methodology “big enough to drive a logging truck through”.

The emissions saving from burning wood at Drax might still be as high as 80 per cent using the more complete DECC biomass calculator method. Or it might not. A Drax spokesperson tells Carbon Brief there’s no way to directly compare the two methodologies because the calculator is theoretical and “does not model real situations”.

The spokesperson adds:

“We’ve always said there’s a right and a wrong way to source biomass. We only source sustainable biomass that is low carbon and does not deplete carbon stocks, in line with a recommendation made by the government’s Committee on Climate Change in July.”

It’s all immensely complicated to model, let alone keep track of under DECC’s reporting rules. It probably is good for the climate to burn biomass at Drax. But just in case, maybe keep your fingers crossed too. Or as they say in north America – knock on wood.

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