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Imported wood combustion? Some say it’s fine..others disagree

By Dave Elliott

Many environmentalists are not keen on using imported wood pellets in old inefficient converted fossil-fueled plants. They say there are better ways to use biomass and better sources- local anaerobic digestion of biomass wastes and residues, along with Combined Heat and Power (CHP). Large-scale biomass conversion, and even co-firing with coal, is sometimes portrayed as an interim option, getting biomass use established, but not everyone is convinced that this helps build up support for local sourcing of biomass. It’s just a way to keep old power plants going, so as to avoid having to write off some sunk costs.  There is also the wider debate about the extent to which large-scale combustion of grown biomass, especially from forests, is net low carbon, given that it takes time for new growths to absorb emitted CO2. It’s even been claimed that using wood from trees might lead to more emissions net than from using coal, depending on the source of the wood: 

Some support for this view came from an authoritative EC JRC report on ‘Carbon accounting of forest bioenergy’, which said ‘The reviewed studies indicate that the use of stemwood from dedicated harvest for bioenergy would cause an actual increase in GHG emissions compared to those from fossil fuels in the short-and medium term (decades), while it may start to generate GHG savings only in the long-term (several decades to centuries).’ But it also said stem wood is not widely used, whereas, in the case of the more widely used  ‘forest residues, thinnings and salvage logging (if not otherwise used for other purposes)… GHG savings are achievable in the short term’.

So wood pellets might be fine, depending on which parts of the forest they came from. A new e-paper, published in Environmental Research Letters, ‘Potential greenhouse gas benefits of transatlantic wood pellet trade’ by Puneet Dwivedi, Madhu Khanna, Robert Bailis and Adrian Ghilardi, attempts to assess the net impacts of the use of wood pellets imported from the US to the UK, under a variety of growing/collection /transportation regimes, and used at the Selby plant in Yorkshire.  It’s not specified in the paper, but presumably this is not from stemwood. Certainly that’s what is usually claimed by the power companies that import and use wood pellets. The ERL paper says ‘GHG emissions arising from wood pellet production contributed most significantly towards total emissions (about 48%), followed by transatlantic transportation of wood pellets (about 31%) and burning of wood pellets (about 10%)’. 

The 48% and 31% figures seem reasonable, given that it is for emissions that are inevitably not abated by re-absorption i.e from the fuel used in the harvesting and long distance transport of a relatively low energy density material. The 10% combustion impact figure seems low, even assuming it is the net emissions based on replanting and re-absoption of CO2. The end result is that the paper concludes that ‘relative savings in GHG emissions with respect to a unit of electricity derived from fossil fuels in the United Kingdom range between 50% and 68% depending upon the capacity of power plant and rotation age. Relative savings in GHG emissions increase with higher power plant capacity …Overall, use of imported wood pellets for electricity generation could help in reducing the United Kingdom’s GHG emissions’..

How reliable is that conclusion? The ERL paper seem to take existing studies of GHG outputs as an unchallenged  starting point. It says they ‘demonstrate that the GHG intensity of a unit of electricity generated in European countries using imported wood pellets from the United States and Canada is about 65%–80% lower than the GHG intensity of a unit of grid electricity depending upon whether natural gas or wood residues were used to dry wood pellets in a wood pellet plant’. It adds ‘these studies typically assume that the feedstock needed for manufacturing of wood pellets was sourced from a nearby forest area or a wood processing facility which was located at a fixed distance from the wood pellet plant’. It then focuses on the implications of transporting wood longer distance from source to processing plant in the US and on the impacts of different harvesting cycles.

It’s a complex debate, and although it may be that, if these authors have done their sums right, you can get reasonable level of CO2 reduction from some sustainably managed but remote forestry sources, some uncertainties remain. For example, the paper does not seem to have taken the carbon debt/re-absorption delay issue on board.  Or the question of whether stemwood is in fact being used, as some greens fear:

In practice, it may come down to the relative commercial value of stemwood for energy versus other timber uses, and it’s usually argued that it is higher for the latter, so not much stemwood will be used for energy. Though that might change as demand rises. Wood pellet exports to Europe from the US nearly doubled last year, with almost 60% going to the UK.

On balance, many greens would prefer to avoid the problem, and certainly the bulk import of wood pellets, of uncertain source. Instead, they would prefer the use of  local fast growing plants (e.g short rotation coppice) to limit the carbon debt problem, as well as local wastes/ residues, with AD biogas production offering a storable medium, and in some locations CHP offering efficient links to heat and power end use requirements. The ERL paper does say the team would have liked to look next at the impact of CHP on imported wood pellet emissions/MWh, but it would also be good to have some comparisons with other approaches to biomass sourcing and use.

That is especially important given the claim by Manchester University’s Tyndall Centre for Climate Change that the UK could produce up to 44% of its energy from biomass by 2050, without the need to import and without impacting food systems, using indigenous biomass waste resources and energy crops. Their modeling suggests that residues from agriculture, forestry and industry could potentially provide up to 6.5% of primary energy demand by 2050. Waste resources were found to potentially provide up to 15.4% and specifically grown biomass and energy crops up to 22% of demand. Land-use issues have usually be thought to limit biomass from energy crop plantations in the UK, but using waste food and other bio-wastes along with farm wastes to make biogas via Anaerobic Digestion is seen as a major new option.

Of course some greens say that high value bio-material ought to be recycled into ecosystems (to support soil quality) rather than just being burnt for power production, but others argue strongly for biomass combustion with Carbon Capture and Storage- to get to negative net emissions. That’s a way off at present. For the moment, the Tyndall paper claims that the potential bioenergy generated from agricultural residues, particularly from household/food wastes, straws and slurry resources, offered significant opportunities for the bioenergy sector due to their abundance and current under-utilisation. Some of these bio-wastes should perhaps be recycled back to the land, but, if we want to avoid biomass imports, some could be used for biogas production

In my next post I will look at what is actually happening on the ground in the UK in terms of biomass projects. The simple message is that large biomass conversion (of old fossil plants) is being promoted strongly but selectively (DRAX has got CfD support but not Eggsborough), new dedicated biomass projects aren’t doing so well, but some biomass-fired CHP plants are going ahead, while AD projects are getting patchy support.

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  1. Vailhem

    Why wood? Why not self seeding perennial grasses like switchgrass or miscanthus? And why burn it? Why not gasification first through anaerobic digestion then pyrolysis such as to product a syngas that can be easily burned straight in natural gas turbines, or blended with n.gas and burned?

    The results are: you aren’t cutting down trees, you’re using a sell seeding fast growth grass that actually builds SOC (soil organic carbon) thus actually builds soil, is incredibly fast growth, energy dense, and native to many lands, as well the process would be extremely carbon negative. Imagine that! a carbon negative fuel! not carbon neutral like solar, wind, geothermal, hydroelectric, nuclear… incineration of biomass, definitely not carbon positive like coal, natural gas, petroleum, shale, etc… but actually carbon negative! It also doesn’t kill the soils or habitats, it actually helps to restore them!

    Just one of those crazy out there ideas.

  2. Let’s face the fact, some countries have trees, a lot of tree, the will cut them and trade them. Different countries have different conditions, right? As long as wood pellets are beneficial to people, you should definitely go support it.

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