Is Wood Pellet Energy Carbon Neutral (or Negative)?

A few years ago in the days of the Code for Sustainable Homes and Ecotowns strategic schemes had a problem. Residual emissions from heating required a power source and wood pellet energy centres – often fed from local sources, were proposed.

Since three giant power stations in the UK have switched to wood pellet burning, notably from the Drax group. This is very controversial and very complex. Clearly there are huge exonomies of scale from large power stations and it might make sense to heat from the grid rather than use a local energy centre. Few Energy centres have been built and the jury is still out on their viability and usefulness.

The claim from pellet promoters is that they are at least carbon neutral – carbon is absorbed during tree growth and released when burnt. If you have carbon capture it could be carbon negative.

In reality its more complex. It all depends on the rate of burn and the rate of regrowth. If the rate of felling increases and trees regrowth is shortened as a result then it could be carbon positive. However if you just use sawmill clippings for example this has zero carbon cost, other then transport, then there is no carbon cost as it would decompose, and release methane, anyway.

Such is the complexity of the issue that the DECC then Chief scientist Professor David MacKay created the Biomass Emissions and Counterfactual model, or BEaC for short. The model has been used on Drax group by Friends of the Earth who claim that they are massively carbon positive. Drax group don’t use and dispute the BEaC model. They contest that the model doesn’t properly account for the changes to the management of the forestry inventory in the North America (their primary source of stock), and that regeneration is increasing not decreasing in US forests. Again its complex. Drax use they mainly use ‘thinnings’ mishapen or suboptimal trees that would be cut anyway (many could still be used for pulp or chipboard), but campaign group claim this is cover for cutting down whole trees. You can see a summary of their arguments in a letter in the Guardian today.

“The UK’s plan to burn more trees to generate “renewable” electricity has come under fire from green groups and sustainable investment campaigners over the controversial claim that biomass energy is carbon-neutral.

A letter to the government signed by more than a dozen green groups including Greenpeace and Friends of the Earth warns ministers against relying too heavily on plans to capture carbon emissions to help tackle the climate crisis.

The letter warned that burning more imported wood pellets could accelerate the climate crisis, increase the company’s contribution to biodiversity loss, and the potential for Indigenous people’s land rights violations.”

Whether it is carbon positive or not depends on the change in the rate at which forests regenerate. If as a result of conversion to pellets that reduces the age at which trees are felled that increases emissions.

This was firmly denied by Drax in a must read investigation by Carbon Brief.

[Drax] insists harvest rotation lengths are unaffected by Drax’s demand for biomass. He tells Carbon Brief: “You don’t change rotation for biomassâ?¦ the lowest value product doesn’t dictate the actions of a forester. Ring up one of the big foresters and ask if biomass would affect rotation rates. That would be an insane proposition.”

Nonsense. What is missing here is an understanding of forestry economics. The optimum point at which you fell a tree is different and shorter in financial terms then its point of maximum point of growth. Also what would previously have been thrown away is now what economists call a ‘joint product’ the classic example being wool and meat from sheep. A new high value joint product could certainly change rotation.

Given the complexity of the issue it is important to take a pragmatic approach and work with the industry to minimise emissions. Certainly the shift from coal to wood has led to Britain (if considered alone) having the fastest rate of reduction of carbon emissions in the world. However if campaigns groups are right if whole life cycle emissions are considered Wood pellets emit three times more carbon than coal, making the reduction an illusion.

Drax needs to improve transparency and publicise a model of its forestry management, building on BEaC at least.

Monitoring global supply chains and forest floors around the world will be intrinsically difficult and so encouraging more local sources, through growth of fast growing feedstocks in the UK, such as willow or even bamboo, should be encouraged on marginal land not needed for agriculture. Rewilding and wood pellets can go hand in hand with the right forest management (though this is a very long term solution).

Perhaps the method with the best potential is to replace burn and exhaust with anaerobic techniques, such as Pyrolysis, where the charcoal created in the process is used as an agricultural feedstock created a carbon negative cycle – called biochar. Perhaps the simplest thing the government could do regulate so that mass burn power stations (after a certain date) switch from wood pellet burning to carbon pellet burning – this would immediately create a mass market for zero carbon biochar pyrolysis plants in forests – as below.