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Biomass burning – worse than coal

By Dave Elliott

The Climate Change Committee’s report on bioenergy last year argued that, at best, the UK might only get 10% of its energy from bio sources by 2050. The subsequent DECC/DEFRA/DfT Bioenergy Strategy was a lot more positive, as was the parallel DECC Heat Strategy. It claimed that biomass could supply up to 21% of the UK’s energy by 2050: http://www.decc.gov.uk/assets/decc/11/meeting-energy-demand/bio-energy/5142-bioenergy-strategy-.pdf

Certainly it has attractions. Consultants Deloitte say “As the amount of intermittent generation technologies in the UK’s energy mix increases, flexible fuel sources that can provide stable and predictable electricity will become increasingly more valuable. Sustainably sourced biomass could provide this stability.”

The big issue is whether biomass can in fact be used sustainably. DECC have proposed a limit on life cycle greenhouse gas emissions from biomass power that ranges from 200 to 285g CO2/kWh, but a report from the RSPB, FoE and Greenpeace says the standard is fundamentally flawed since it doesn’t count significant emissions to the atmosphere when electricity is generated from wood harvested from forests. It claims they are actually much worse than from coal combustion.

The report argues that the usual rationale, that biomass combustion is carbon neutral since emissions are balanced by the carbon that was absorbed when the biomass was growing, is faulty. It is usually accepted that it wont quite be 100% balanced, since there will be emissions resulting from energy used in harvesting, transport and processing, and indeed that is included in DECCs emission standard, but the NGO report goes much further and opens up a fundamental issue. Focusing on trees, it claims that in fact, even with replanting, there will be significant excess net carbon dioxide added to the atmosphere since it will be emitted rapidly when they are burnt, but only absorbed slowly when the new plants start growing.

There is some truth in this- it’s a dynamic system. It is clear that there will be a delay after mature trees have been harvested and burnt, before new plantings will start absorbing carbon dioxide. The report says the delay can be ‘many decades’. It claims that over a 20-year period, emissions from power generation using wood from conifer plantations are 1879 g/kWh- 80% greater than coal power. It adds ‘Over a 40-year period emissions are lower because the trees have had longer to re-capture carbon, but even then biomass emissions would be 49% greater than coal power. Only after 100 years does electricity generation from conifer trees perform better than coal. And, regardless of the time period, it’s never better than the current grid average and never meets DECC’s proposed maximum emission limit for Biomass’ .

This may be overstated. If you look at the complete carbon life cycle, there may be mitigating factors. Trees absorb carbon dioxide most rapidly when growing and slow down later in life -so although mature trees and their roots will still absorb some extra CO2, they are less active and basically just carbon stores. Moreover they are not permanent carbon stores- they will eventually release it (as CO2 or methane) when they die, rot or burn. So if carbon sequestration is the only issue, it might be best to grow biomass and chop it down regularly before it’s fully grown, and then grow more, while using it for fuel- and so avoiding the use of fossil fuels and their emissions. An extreme version of this approach is represented by the development of Generically Modified versions of fast-growing plants like eucalyptus, which, it is claimed, can grow 40% faster, so speeding up CO2 absorption and compensating for any short delay in absorption: gu.com/p/3bd4y/tw

However, this approach highlights some of the problems with using trees for energy. Trees play key complex roles in ecosystems, not just as carbon absorbers, so we need to understand what we might be doing if we use them for energy, not least in terms of water use. For example, fast growing eucalyptus sucks water out of the soil rapidly, which could be disastrous in some locations, as has already been seen in Asia (http://www.hindu.com/2011/04/28/stories/2011042865310400.htm), although not everyone agrees (http://etff.org/Articles/Eucalyptus.html).

But GM-enhanced biomass could also have many other negative environmental and climate impacts: http://www.globaljusticeecology.org/files/biofuels-ppt-web2.pdf

These issues, and issues relating to biodiversity and changes in land use, may turn out to be as important as the absorption delay issue raised in the aforementioned NGO report. For example, the report notes that the use of trees for combustion can divert timber from other uses, which means that more will have to be imported. That does seem to be unwise. So of course does the destruction of forests anywhere. That’s why many environmentalists would prefer to stick with just occasional wood fellings, forestry and farm wastes, along with domestic biowastes, as a source of biomass for energy use, and many would also prefer AD biogas production to direct combustion. Certainly burning biomass, instead of coal, in old modified low efficiency coal plant is not much of a step forward- we need to think about CHP and district heating.

There is no question that the use of some types of biomass for energy is likely to be a poor choice- some energy crops have very low calorific value and mono-cultural plantations can be very bad for biodiversity as well as requiring a lot of water and undermining local ecosystems. Working conditions in some biofuel plantations in Asia and elsewhere can be abysmal. And they can divert land from food production. But there may still be a role for some high yield energy crops on marginal land, and for less invasive approaches, such as short rotation coppicing. Forests however, well that’s a different matter. It seems clear that we should avoid deforestation and unsustainable imports, and look to other less damaging approaches to biomass sourcing and use, but we need to understand a lot more about the carbon life cycle in varying climates and locations before we can pronounce finally on the net impact of managed wood use.

(Friends of the Earth, Greenpeace and the RSPB 2012 Dirtier Than Coal? Why Government plans to subsidise burning trees are bad news for the planet)

While some greens see it as the only sensible use for CCS, the idea of capturing CO2 from biomass combustion, so making it carbon negative, has also come under attack A recent Biofuelwatch report says Biomass energy with carbon capture (BECC), like CCS in general, will not be economic, or carbon neutral (due to delayed re-absoprtion) much less negative, and it worries about ‘underground land-grabs’ for storage space. Given their concerns about the environmental implications of biofuel production, Biofuelwatch also worry about ethanol production involving fermentation, which results in a pure stream of CO2 that can be directly captured and biodiesel production using the Fischer-Tropsch method, which involves production of syngas as an intermediary step, and offers opportunity for CO2 capture. Overall, even with high yield biomass crops , they don’t think there will be enough land to allow for significant sustainable biomass production, so BECC is unlikely anyway to be much of an option. (www.biofuelwatch.org.uk/2012/beccs_report)

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One comment to Biomass burning – worse than coal

  1. Asking questions are truly good thing if you are not understanding something fully, but this article offers pleasant understanding even.

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