The Forth Energy biomass power schemes
Critical appraisal of the carbon balance,  from Dr Ulrich Loening

Renewable energy has finally become government policy. However the proposal by Forth Energy to construct four new biomass power plants cannot, in all probability, amount to a renewable energy project. The arguments why not, are set out below.

  1. The core problem is that wood from forests can be renewable but equally is depletable. In this respect it differs fundamentally from solar, wind, wave and hydro power, which are perpetual and cannot be depleted however much they are harvested. Harvesting this energy of course can lead to various environmental problems; but depletion of the resource is not one of them.
  2. For biomass, a forest can be thought of as a solar factory that produces wood fuel. The carbon dioxide emitted by burning is then re-absorbed by the re-growth of the trees.  This is simple and obvious.
  3. Calculation also shows that over the medium term of decades, more carbon will be emitted by a biomass wood fired plant than if fossil fuel had continued to be used to obtain the same amount of energy.  It is not easy to appreciate why this is so, but the argument below describes the situation as simply as possible.
  4. When the new power plant starts up the first batch of wood that is harvested and burnt will emit carbon into the atmosphere which amounts to some 40% more than would have been emitted by coal burning, because wood is a less efficient fuel than coal in terms of energy gained per unit of carbon, and much less efficient than oil or gas.
  5. In any case, even if wood fuel were as efficient as coal, it will take at least 25 to 50 years for that first batch of wood to be replaced by new growth of the forest. A study of the woods of Massachusetts, USA, showed that it will take longer, a century or more, because of the lower efficiency of wood.
  6. Therefore the new plant starts with what we can call a “carbon debt” of excess emissions.  While new growth will gradually re-pay this debt, a further debt is incurred each year by that year’s harvest.  Each year there will be new rapid emissions and new slow forest growth, and the balance between these results in a sort of zig-zag reduction of net carbon emissions. An extensive study of this was published by the JOANNEUM RESEARCH Institute in Austria in May 2010.  The eventual time it takes to reach a sustainable equilibrium depends on details of forest management, on the rates of re-growth (which are improved by the space and light resulting from harvesting), and on the proportion of each years growth that is harvested.
  7. It should also be noted that with any level of harvesting the average age of the trees as well as the carbon stored will be lower than if a forest left to grow.  In addition, felling trees affects the soil: spruce forests in Nova Scotia lost about 60% as much carbon from their deep soils as was in the harvested crop itself.   This took a century to restore.
  8. Short rotation tree harvesting, as would be used to supply imports of biomass wood, is not sustainable in the longer term because of soil losses. It is ecologically damaging in other ways, for example to water control

    and biodiversity.

  9. The above means that if Forth Energy plan to import the bulk of the biomass timber needed, they will in effect

    be purloining other countries’ land and its sustainability.  This has serious international implications.

  10. How then can one make use of the advantages that renewable biomass offers over fossil fuels?  First, of course is the need for extensive re-forestation, preferably of long-term forests. Forth Energy would actually have needed to plant forests some 20 to 50 years before they were required! One might then be able to start harvesting biomass from these new growing forests without incurring a carbon debt.  But even this is best done on a small scale, so that the forest can be managed sustainably, by thinning without disturbing the soil and by encouraging re-growth at a faster rate than in the untouched forest.  I am left to doubt that any massive plantation, which involves heavy machinery for large scale felling, can ever be ecologically sound or sustainable;  one would not want that at home nor impose it on other countries.  Scotland could do much to re-forest large areas with mostly native species for long term ecological improvements as well as high quality timber and biomass for local and national uses.
  11. We personally endeavour to make our own contribution by heating our house with wood from a young sycamore woodland, communally owned, and which is growing much faster than the timber being harvested and is improved in the process.
  12. No power plant, least of all a biomass plant, should be built without full a Combined Heat and Power facility.
  13. In conclusion, The Scottish Government is obliged to assess the total net changes in carbon emissions that would result if Forth Energy were to be given consent to build their power plants. Such assessment which includes all aspects of the carbon balance would show that in the medium term, net carbon emission would increase, running counter to the government’s Climate Act.


The upfront carbon debt  of bioenergy,  Giuliana Zanchi Naomi Pena Neil Bird

Bioenergy, Land Use Change and Climate Change Mitigation
Göran Berndes (Chalmers University of Technology),  2010

Biomass Sustainability and Carbon Policy Study  Executive Summary
Prepared for: Commonwealth of Massachusetts Department of Energy Resources100 Cambridge StreetBoston, Massachusetts 02114
Manomet Center for Conservation Sciences81 Stage Point Road P.O. Box 1770 Manomet, Massachusetts  June 2010

Review of the Manomet Biomass
Sustainability and Carbon Policy Study
Prepared by Mary S. Booth, PhD
For the Clean Air Task Force
July, 2010

Looking deeper: An investigation of soil carbon losses following
harvesting from a managed northeastern red spruce (Picea rubens Sarg.) forest chronosequence

A. Diochon a,b,*, L. Kellman a, H. Beltrami a
Environmental Sciences Research Centre, Nova Scotia,
Forest Ecology and Management 257 (2009) 413–420