Coppiced Wood as an Energy Source
There’s nothing new about burning wood for energy – nor about coppicing trees to produce it, for that matter – but over the last 50 years or so, ‘short rotation coppicing’ (SRC) has gone from a largely unknown rural activity, to forming an important part of the overall energy debate.
Wood itself makes a good fuel for many heating applications; it has an energy value of around 6,000 mega-joules per cubic metre (MJ/m3) and although this is only around a quarter that of coal, it has one significant advantage in these environmentally conscious times. Wood is carbon neutral; the CO2 it releases when it is burnt comes from the current atmosphere in which it grew – it’s not 300 million year-old carbon from the Carboniferous period!
CoppicingThe basic idea behind SRC is very simple, and it depends on the ability of some kinds of trees to be coppiced – a method of management that has been used for hundreds of years to produce healthy new shoots for a variety of purposes. This involves first allowing a new young tree to become established and grow for a few years, before harvesting it by cutting it off close to the ground – but leaving the root system intact. New growth will shortly begin to appear from around the cut stump, and after a few years the process can be repeated once more. It is the ultimate in sustainable production.
The trees most commonly used as energy crops are usually varieties of either willow or poplar. Although both have their own particular characteristics, as a rule willow tends to be the one that is most often used, since it is tolerant of almost all kinds of soil conditions, roots more shallowly than poplar and has a high resistance to wind and salt, making it suitable to a wide range of sites. Various kinds are used, with the likes of Salix viminalis and S. caprea being two of the most frequently encountered.
Growing the Energy CropEstablishing a new commercial coppice involves planting long rows of ‘whips’. These tall, thin cuttings have themselves been harvested from another coppice during the winter when the trees are naturally lying dormant. Willows are a particularly un-fussy crop, so the only real care necessary is to ensure that weed competition is kept to a minimum, at least until the newly planted whips become properly established.
The key to the whole thing lies in the enormous growth potential of both poplars and willows; under fairly normal conditions, they can grow up to 6 feet (1.8 metres) in a year, and even in thin Pennine soil 1000 feet or more above sea level, they routinely put on around half of this. After only three years, the new coppice is ready for its first harvest. This is done in the winter, using specially converted machines, which cut the shoots into short ‘billets’ and any new whips needed for planting can also be taken at this time.
The wood produced then needs to dry before it can be used, but at the end of this fairly lengthy process, the typical harvest would work out to be around five dry tonnes per acre – or some 70,000MJ of energy!
Using the EnergyMuch of the wood produced in this manner tends to go to heating schemes of one sort or another, usually to warm homes, small factories or industrial estates. Since wood is a bulky material relative to its energy value, transport costs can often be a limit on the use of this kind of fuel, which often means that it is only viable within fairly close proximity to where it is grown.
There are some notable exceptions to this small-is-beautiful approach, and perhaps the best known of these is the huge Drax power station in Selby, North Yorkshire, which has been using coppiced wood alongside coal to generate electricity since 2004. As an experiment in truly large-scale energy cropping, it has obviously been a success; at the end of 2010, ambitious plans were announced that will see the plant dramatically increase its use of wood. One of the 660MW units, currently fired by coal, is to be converted to run entirely on coppice fuel, a new range of dedicated biomass-burning units are to be built and the existing joint coal/wood fired generator is also expected to be used more. Where Drax goes, other power stations must surely soon follow.
As a way of managing trees, coppicing may have a long history, but it seems it has a pretty bright future too.