Wood used to be the fuel of the past, necessary for heating and cooking before the large-scale exploitation of coal, gas and oil, and is still the only source of energy for many of the rural poor in developing countries. More recently, however, it has once again become one of the fuels of the future. The reason, of course, is climate change policy, but does it make sense?
There is plenty of ‘spare’ biomass available as a genuinely renewable raw material. For example, only about half of the wheat plant is grain; the remaining straw has some added-value uses such as for livestock bedding, but represents a low-cost feedstock for further processing. Given the general interest in moving away from coal and oil as feedstocks and boosting a transition from chemical to biological processing, it is not surprising that this has received quite a lot of attention.
The most attractive uses for biomass are in principle those that create the highest value end products. Straw, wood or any other harvested material is a source of carbon, and can be processed either by pyrolysis (to create syngas) or used to supply a source of fermentable carbon. The problem with the latter route is that the lignocellulosic skeletons of these materials are not easy to break down, and usually require use of an expensive blend of enzymes and batch processing.
For these reasons, most of biomass is used for its energy value. Quite a lot of harvested material is turned into biofuels – mainly ethanol and bio-diesel – to supply an artificial market created by public subsidy and currently dominated by the use of readily fermented starch and sugars (plus edible oils) which are straightforward to process but use materials which would otherwise go into the food chain. Much time and effort continues to be put in to developing an economic process to produce ‘second generation’ fuels from more intractable materials such as straw.
Although harvested materials can be regrown, they have the disadvantage of low energy density compared with fossil fuels. This makes transport a more significant part of the overall cost and so puts an effective limit on the economic scale of the plant where it is to be used. For example, the Elean power station, near Ely in Cambridgeshire was built to burn straw from the surrounding catchment area. Its rated output is just 38MW and it produces around 270 GW annually from 200,000 tonnes of straw.
Nor can biofuels ever replace oil as the prime energy source for motor transport (unless, that is, an economic process can be developed for the large-scale exploitation of highly productive algae). Already, a large proportion of the corn (maize) harvest in the USA is used for bioethanol, but it is reckoned that using the entire acreage of American corn and soy could only fulfil 12% of current demand for gasoline and 6% of diesel demand. Ten per cent of global transport fuel in 2030 could require something in the order of a quarter to a third of total cropland, which is inconceivable in a world where the food supply must continue to grow.
And to put this in perspective, transport fuel is a small part of total energy use. According to the latest REN21 report (Renewable Energy Policy Network for the 21st Century), biofuels represented just 0.8% of global energy use in 2011.
On the other hand, much more biomass is simply used directly as fuel. Taking figures again from REN21, 19% of global energy use in 2011 was delivered from renewables. However, nearly half of that – 9.3% – was classified as ‘traditional biomass’. That is, it was mainly firewood burnt as the sole source of heat in the developing world (largely on inefficient indoor fires which are a major contributor to respiratory disease).
Of the ‘modern renewables’, 4.1% was biomass/solar/geothermal heat and hot water, 3.7% hydropower, 1.1% wind/solar/biomass/geothermal power generation and 0.8% biofuels, as already mentioned. But this is set to grow: the UK renewable energy roadmap is a good illustration of how the use of biomass is likely to be extended. The target for 2020 is for 15% of total energy to come from renewables (lower than the overall 20% target for the EU because it started from a relatively low baseline).
This represents an annual total of 234TWh, about half of which is planned to come from biomass: 32-50TWh of electricity, 36-50TWh of (non-domestic) heat and up to 48TWh for transport biofuels. We may be very aware of the growing intrusion of solar panels and wind turbines, but they are set to make a smaller contribution than biomass.
Most of this will be in the form of wood rather than straw, and there is little enough of this available on a large scale, other than the waste from forestry or dedicated short-rotation coppice. Already RWE has converted its coal-fired plant at Tilbury to burn wood chips (at 750 MW output, the world’s largest biomass station) and Drax, the largest coal-fired plant in Europe, is converting more of its units to wood burning. By 2016, Drax alone will require seven million tonnes annually.
Most of the wood is grown and processed in the USA and shipped across the Atlantic. The theory is that the overall process is effectively carbon neutral, because the felled trees are replaced by new ones and the carbon dioxide emitted is recaptured during the time to reach maturity. While this may be true, burning wood still increases emissions in the short- to medium-term. It is by no means obvious that there is a significant overall reduction in CO2 emissions using this source of supply.
Recently, this issue has been directly addressed in a study published by a team from the University of Georgia (Potential greenhouse gas benefits of transatlantic wood pellet trade). Their conclusion, based on a range of scenarios, is that typical savings of between 50 and 68% are made, even allowing for the significant energy used in pelletisation and transport.
Which is all fine as far as it goes, but this does illustrate the enormous problems of trying to radically reduce the use of fossil fuels using today’s technology. It’s not just a question of availability, but biomass and other renewable energy technologies which require subsidy (I don’t include most hydroelectric schemes in this) suffer from lower energy density, resource limitations or intermittency. Biomass may be one of the better ways of delivering quite large amounts of energy but, until some major steps are made in electricity generation and storage technologies, radical decarbonisation remains an aspiration rather than a realisable ambition.
Martin Livermore
The Scientific Alliance
St John’s Innovation Centre
Cowley Road
Cambridge CB4 0WS
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