The problem with burning wood for heat and power

There are manifold problems with treating forest bioenergy (wood) as a carbon-neutral fuel. Yet, a whole industry has emerged based on subsidies for "Green" energy.
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What is Biomass?

Plant biomass, meaning all organic (carbon-based) vegetation, plays a critical role in regulating the Earth’s climate through the carbon cycle. Billions of tonnes of carbon dioxide are absorbed (or “sequestered”) from the atmosphere as plants and other biological matter photosynthesise, much of which is gradually released back again when they decay or burn (soils, peatlands, ice and oceans are natural “carbon-sinks”, storing many multiples of global annual emissions of carbon in the Earth’s surface).

The balance between the carbon stored in the Earth’s land and ocean surface and the carbon in the atmosphere is important, as it affects global average temperatures. In order to stay below a 1.5 degree average temperature rise (on pre-industrial temperature levels) by the end of this century, the Intergovernmental Panel on Climate Change (IPCC) has set a target of no more than 350 parts per million of CO2 in the atmosphere. Today’s level is over 415 parts per million. Hitting this target forms the basis of the Paris Agreement on climate change, forged in 2015 and coming fully into force in 2021. In 2019, the UN Environment Programme (UNEP) estimated that this would require global cuts to greenhouse gas emissions of over 7 per cent per year between now and 2030 [1].

For centuries, forests have been harvested to serve human demands and – in more recent centuries – many have been “managed” at different degrees of intensity in order to increase specific harvests; global primary forest area is around 4 billion hectares according to the UN Food and Agriculture Organisation, an area that has decreased by 80 million hectares in the last thirty years[2].

The majority of forest clearances are due to agriculture, but they have also been thinned or felled for timber. Major customers for timber and “woody biomass” include the construction, furnishing, and paper and packaging industries. These are the “higher value” uses of forest biomass. But in little more than a decade, a new global industry has grown up – financed very largely through public subsidies. This is the forest bioenergy sector.

Burning wood emits CO2 faster than new tree growth can absorb it

The rationale behind the forest bioenergy sector is to substitute the burning of wood (usually in the form of processed wood pellets) for energy derived from fossil fuels thereby – so the argument goes – reducing emissions from fossil fuels. This is mainly to convert energy for heat (heating our homes and buildings) and power (providing electricity for the grid). As well as the development of specialist biomass burning facilities, this can also take place in coal-fired power plant conversions – often in old plants that were due for retirement[3]. In 2009, the EU’s Renewable Energy Directive (RED) explicitly designated biomass burning as a renewable technology and the update to the same directive in 2018 extended that designation, adding additional “sustainability criteria”.

This is deeply counter-intuitive. Wood, as a fuel, is much less energy-dense than coal (the worst-polluting fossil fuel). That means you need more wood – by weight – to generate the same amount of power in MWs (for example) than you would with coal. This is a major part of the reason why economic historians argue that the switch from wood to coal underpinned many of the advances of the first Industrial Revolution.[4]

“The IPCC itself – based on hundreds of scientific studies – classifies wood burning as being around 18 per cent more emissions-intensive than bituminous coal (the most common type of coal used for power generation) in terms of kg of CO2 per TJ.”

The IPCC itself – based on hundreds of scientific studies – classifies wood burning as being around 18 per cent more emissions-intensive than bituminous coal (the most common type of coal used for power generation) in terms of kg of CO2 per TJ. [5]

And combustion isn’t the only part of the process that generates carbon emissions: carbon is released from the soil when forests are harvested, the drying and processing of wood into pellets is energy-intensive and so is the transportation of the wood pellets from forests to the power plants or other facilities where they are burned. This is on top of the carbon sequestration potential lost to the Earth when biomass is removed from forests for burning; rather than (for example) for use as timber in buildings where it should stay “locked up” for decades – or just being left in the forest to photosynthesise [6].

The whole argument for burning biomass as a substitute for fossil fuels is that trees and plants can re-absorb carbon from the atmosphere as they re-grow. And this is why the international carbon accounting rules devised as part of the Kyoto Protocol designate wood burning to be zero emissions at the point of combustion; provided the loss of carbon from forests is accounted for as emissions from land use (which it often isn’t).  

The problem with this argument in the context of increasingly tight deadlines to reduce emissions in line with the Paris Agreement (recall UNEP’s estimate that the world needs to cut its emissions by 7.6 per cent every year between now and 2030) is well summarized in a recent paper by the European Academies Science Advisory Council (EASAC) [7]:

The classification of forest biomass as ‘renewable’ is based on the reasoning that, since biomass carbon came from atmospheric CO2 and regrowth absorbs CO2 over time, it can be regarded as ‘carbon neutral’ with net emissions over the harvesting/regrowth cycle of zero. The ‘carbon neutrality’ concept is, however, a gross misrepresentation of the atmosphere’s CO2 balance since it ignores the slowness of the photosynthesis process which takes several decades for trees to reach maturity. [8]

Put even more simply, woody biomass from forests is being burned faster than any new growth can reabsorb the CO2 released. This is sometimes referred to as a “carbon debt” or a “carbon payback period…which range from decades to centuries” according to many papers collated by EASAC. If it takes more than a decade, and potentially up to a century, for the CO2 released in a single year of wood burning to be compensated by sequestration from new forest growth, this is no good for the Paris Agreement targets. If 2019 emissions levels are not reduced, the 1.5 degree temperature target will probably be breached in little over a decade. [9] So a technology that emits more carbon over that period than it sequesters or displaces is not helpful. Utility scale wind power arrays – for comparison – have a lifecycle carbon payback period of around two years.

Three further arguments are often advanced in support of burning wood from forests:

  1. New forest growth can absorb carbon at a faster rate than older trees and plants
  2. The forest bioenergy sector is providing an economic incentive for landowners to expand forest coverage
  3. Power from biomass generation is needed on electricity grids as (“baseload”) backup to ‘intermittent’ renewables like wind and solar.

The remainder of this essay assesses each point in turn:

Old growth forests and old trees store more carbon than plantations and young trees 

In the case of new forest growth outstripping old growth in terms of carbon sequestration, this is both counterintuitive (as photosynthesis takes place through leaves and older trees generally have denser leaf-coverage) and not borne out by scientific evidence. As an influential 2014 paper in the journal Nature put it:

…for most species, mass growth rate increases continuously with tree size. Thus, large, old trees do not act simply as senescent [biologically incapable of  growth] carbon reservoirs but actively fix large amounts of carbon compared to smaller trees; at the extreme, a single big tree can add the same amount of carbon to the forest within a year as is contained in an entire mid-sized tree [10].

Most researchers and foresters agree that the carbon storage potential of unmanaged, “natural” or “old growth” forests are significantly greater than those for managed forests or plantations, not to mention their huge advantages in terms of biodiversity and other ecosystem services. Even if you are aiming to restore “marginal” or degraded land (such as land with little economic value, at the edge of deserts or other desolate areas), a natural forest will end up being able to store much greater amounts of carbon over a period of decades than a fast growing plantation of energy crops; and there are numerous other benefits natural forests offer.

There are better ways to incentivise afforestation than creating markets for wood burning

The second argument, (that forest bioenergy is providing an economic incentive to increase forest coverage) requires two responses. The first – connected to the argument above – is to bear in mind that forest coverage per hectare depends on the type, quality and age of species and not simply coverage in hectares – although the two are often conflated. If you “thin” a forest, you are generally removing mature trees and they would have been sequestering carbon unless they are dead and slowly decaying (and decaying organic material is crucial for soil quality).

The second response is to highlight that there are numerous ways to incentivize afforestation and that – in many ways – forest growth would have continued in the absence of the forest bioenergy industry – but without any burning of wood and associated CO2 emissions. It is, anyway, somewhat perverse to encourage additional forest growth by expanding the market for wood burning, if you have environmental goals in mind. As Duncan Brack and Richard King argue in a 2020 paper for Chatham House:

Left to themselves, trees continue to grow and sequester carbon. If trees or energy crops are harvested specifically for energy, not only is the stored biomass converted into carbon dioxide, but the future carbon sequestration potential of the vegetation is lost…tree plantations are much poorer at storing carbon than are natural forests. [11]

It should also be emphasised that the evidence that forest coverage is increasing in areas where the biomass industry operates is meagre. Most of the time evidence suggests that the opposite is true. [12]

A zero-carbon power sector doesn’t need wood burning  

The final argument in support of forest bioenergy concerns developments in electricity networks and the increasing use of intermittent renewables. It is argued by advocates of fossil fuels that the electricity networks of the future will continue to rely on “baseload” power provided by fossil fuels like coal and natural gas to keep grids stable when “the sun doesn’t shine and the wind doesn’t blow” and that this justifies additional investments in these fuels. A decade or so ago, this argument had some salience but as we look towards a completely zero carbon heat and power sector by 2050 – in the Global North at least – it ignores the breakthroughs there have been both in terms of renewable electricity generation – which is hugely less expensive and more efficient than it was even ten years ago – as well as the leaps in demand management and energy storage (both in batteries and e-fuels) and interconnection between regions and nations. There are developments in certain advanced biofuels but there are many problems with these and – even though they may be a very small part of the solution – they are certainly not forest-based and are best reserved for highly specialised uses like next-generation aviation fuel or for heavy industry. Advanced biofuels should certainly not be wasted on generating heat and power, where the genuinely clean substitutes for fossil fuels abound. Most of the academic literature that has compared energy crops with natural forests on a range of criteria – but mainly focussing on long term carbon sequestration and storage potential – has come down on the side of natural forests.[13]

A 2018 study by Vivid Economics and Imperial College London[14], showed that “the United Kingdom can decarbonise emissions from its electricity system by 2030 relying almost entirely on new investments in wind, solar, and smart resources such as battery storage, demand response, and interconnection with Europe”. Existing gas and nuclear assets would easily take up the slack and allow a further 10-20 years for the grid to be completely decarbonised. The authors also argue that a high renewables pathway would be significantly cheaper than continuing to subsidise industrial-scale wood burning.

These findings chime with the results of several other research and modelling exercises, looking at a very high levels of variable renewables integration onto European grids by 2030 – none of which requires back up from biomass power.[15]   

The forest bioenergy industry will grow even faster, unless challenged

The Forest bioenergy sector is not an abstract problem but a serious (albeit well-intentioned) threat to the aims of the Paris Agreement. As EASAC has noted, currently around half of the EU’s ‘renewable’ energy comes from solid biomass.

One of the largest facilities in the world is Drax power plant in the UK. Between 2013 and 2018, Drax converted four of its six units to burn biomass pellets instead of coal. The company now imports 34 TWh of primary bioenergy to the UK, mostly from feedstocks in North America, generating around 13 TWh electricity per annum, around 40% of all UK bioelectricity. Currently Drax receives government subsidies under various renewable energy schemes of over £600m and £1 billion per year [16].

 Despite the risk, utilities in the EU are currently considering copying the UK model by converting a large number of old coal plants to burning wood. Analysis by researchers at the NGO Ember found 67 coal-to-biomass projects under various stages of development in Europe, which would triple wood pellet consumption, creating an additional demand of 36 million tonnes per year [17]. This would require the felling of around 2,700 km2 of forest each year (around half the size of Germany’s black forest). For comparison, the global wood pellet trade, which did not exist twenty years ago, reached 24 million tons in 2015, mostly supplied by North American forests.

If loopholes in carbon accounting are not closed and subsidy for wood burning ended, the “dash to biomass” in a bid to keep alive old coal assets could seriously risk both the energy transition, global biodiversity targets and the Paris Agreement.

References

1 UN Environment Programme (UNEP), Emissions Gap Report 2019

2 UN FAO – State of the World’s Forests 2020: https://doi.org/10.4060/ca8642en

3 You can see a map of European wood burning facilities hosted by the Environmental Paper Network, here.

4 See, for example, E.A Wrigley “Energy and the English Revolution” (2010), Cambridge University Press

5 Intergovernmental Panel on Climate Change (2006). “Table 2,” Guidelines for National Greenhouse Gas Inventories, Vol. 2 (Energy), pp 2.16-2.17. Available online at: https://bit.ly/2AlqsR5 

6 Recent research for the UK’s Royal Society for the Protection of Birds (RSPB) has found that “commercial tree plantations in Britain do not store carbon to help the climate crisis because more than half of the harvested timber is used for less than 15 years and a quarter is burned” – Guardian 10/03/2020

7 Norton, M, Baldi, A, Buda, V, et al. Serious mismatches continue between science and policy in forest bioenergy. GCB Bioenergy. 2019; 11: 1256– 1263. https://doi.org/10.1111/gcbb.12643

8 Ibid.

9 In 2018, the IPCC warned that releasing any more than a total of 570 gigatons of CO2 into the atmosphere would make it less than “likely” the world could stay under 1.5 degrees of additional average warming. In 2019, over 43 gigatons of CO2 were added to the atmosphere. You can read a briefing on “carbon budgets” by Carbon Tracker here. The IEA statistics on energy-related emissions are here. The Global Carbon Project’s estimate of all 2019 emissions – including from land use – are available here:

10 Stephenson, N., Das, A., Condit, R. et al. Rate of tree carbon accumulation increases continuously with tree size. Nature 507, 90–93 (2014). https://doi.org/10.1038/nature12914

11 Duncan Brack and Richard King, “Net Zero and Beyond What Role for Bioenergy with Carbon Capture and Storage?” Chatham House (2020)

12 Dogwood Alliance 2020

13 See, for example, UK Energy Research Centre (UKERC) 2020, “Bioenergy and the road to net-zero”

14 Reality Check:  Biomass Is Unnecessary For The Reliability Of UK Electricity Supply, NRDC (2018)

15 See, for example, Energy Union Choices (2017) and Cambridge Econometrics and Element Energy (2019)

16 Committee on Climate Change, (2018), “Biomass in a low-carbon economy” and Ember (2019) “The Burning Question”

17 Ember, 2019: Playing with Fire – An assessment of company plans to burn biomass in EU power stations