Burning Timber
see also the Biomass Enegy Centre website
Arguably one of the greenest methods for heating a house in the UK (including domestic hot water) is a combination of solar thermal collectors (for summer) and wood pellet or chip. Logs come a close second if they are dry and burn cleanly. The UK undoubtedly has a considerable amount of timber which is under-utilized or goes to waste (see Woodfuel Resource in Britain - Main Report page 75).
However the question of long term sustainability is very complicated for a number of reasons. The whole subject has parallels with the debate about using vegetable oil as a bio-fuel (substitute for diesel oil). This initially seemed like an obvious way forward until the implications of using a food crop as an engine fuel on a global scale became apparent.
- How do you decide how much of the nation’s timber is only fit for burning? This leads to two further questions-
- How much timber is there anyway? There are big gaps in the statistics because a good deal of UK timber is grown in small, under managed woodlands for which figures are not reliably available. Also a large volume is disposed of in a small scale ad hoc manner in the form of local sales, factory offcuts, tree surgeons’ waste etc. Bear in mind also that a large proportion of timber waste comes from manufacturing offcuts. Information from the Biomass Energy Centre indicates that in England alone there are about 3 million tons p.a. of timber available (at 35% moisture content) from under managed woodlands and as this is not being collected at present it is not in competition with other existing markets. Assuming some very ‘ball park’ figures, this would equate to maybe 10-12 TWH of heat. Given that a fairly well insulated medium sized house (say 120 m²) might use 12,000 kWh of space and water heating per year then going on for a million extra houses could use wood for their main source of heating. This assumes that all the spare wood gets used which, of course would never be the case.
- What are the competing uses, such as manufacturing timber particle boards like MDF or OSB. It may be better to use timber waste for this rather than constantly importing plywood. Greenpeace are campaigning for a shift from ply to OSB in cases where the present supply of ply is unsustainable.
- Added to this there is the possibility of importing wood for use as a fuel and this could theoretically be a carbon neutral activity if ships could run on wood pellets or hydrolysis based fuels!
- How do you decide whether more or less agricultural land should be used for forestry in the future? There is the potential for considerably more woodland in the UK if meat production were reduced. This could help to lower nitrogen, methane and ethylene pollution at the same time. Ghent in Belgium has recently declared a ‘vegetarian day’ one day a week because of land use concerns.
- There is pressure from conservationists to do less ‘tidying up’ after forestry operations because fallen rotting timber is the very basis for much of a healthy woodland eco system. This conflicts with the idea of clearing everything up and burning it.
- How likely are the owners of the smaller woodlands to make more timber available for fuel?
There are a few useful web sites dealing with the issues of fuel wood supply -
- The Biomass Energy Centre who have a flow chart to help in decision making with fuel, delivery, supply etc
- The Forest Research Woodfuel Resource site is an academic study of the subject
- The Woodfuel Strategy and Implementation Plan for England by the Forestry Commission
There is also the question of what sort of timber species to plant in the UK in the future and to what extent woodland management should be geared partially towards fuel production. The last 150 years or so of UK planting policy has been an almost unmitigated disaster. Oak for sailing ships was planted just as iron ships arrived, larch and Douglas fir were planted for pit props just as steel started to take over, poplar was planted for matchsticks just as gas lighters were invented and sycamore was planted in large amounts everywhere for reasons no one can remember or understand. And then there was sitka spruce! At some point sensible management seemed to go out the window and what took over was management of woodland by keepers for pheasant shooting and sending all the bits of timber off to Sweden for paper making.
There is currently (August 2009) a public consultation exercise about the future role of forestry in the UK
Pellet and chip
Both pellet and chip are almost carbon neutral and can be burned in boilers in a controlled way so as to be efficient and very low in pollution. They can both be automated so that fuel is automatically fed in as required. Pellets can be delivered by tanker and blown into the storage area or come in tonne lots either as a 1 tonne bag or in 50kg bags. The ash is minimal and usually only needs cleaning out once a month or so.
Although wood pellet boilers (rather than stoves) tend to be expensive, the fuel costs have remained low compared with other fuels. For instance in April 2009 the cost per kWh of heat from an 85% efficient pellet stove was 5p (£235 per tonne including delivery and VAT). The price per kWh of gas from an 85% efficient boiler was about 5.9% at this time.
Recently there have appeared on the market pellet stoves which give out 15% of their heat through a glass door into the room and the rest into a water jacket. This sort of ratio becomes important in a well insulated house because rooms can easily overheat if a large proportion op the heat goes into the room rather than heat exchanger.
Unfortunately the UK has lagged behind in stove design and most are imported from Northern Europe, Italy and Austria. Fuel supplies are patchy here (but increasing quickly - e.g. see initiatives such as Future Energy Yorkshire) and so it is important to check this out for your area. A considerable amount of information is available on the Logpile web site. The Biomass Energy Centre web site (part of the Forestry Commission) gives some comparison of fuel costs per kWh including pellet, chip, gas, oil and electricity. (but note that these are not quite the same as delivered heat costs)
It is also important, for wood chip, to make sure that the fuel is available with the correct moisture content as there have been several cases of boilers not operating correctly due to the fuel being too damp and clogging in the auger feed.
The supply of both chip and pellet determines the amount of storage needed for minimum delivery quantities. The storage also needs to be adjacent to the boiler.
Logs
The burning of timber logs in the UK makes good sense for reasons similar to chip and pellet and modern stoves with air washed glazed doors and secondary and possibly tertiary burn can be extremely clean and efficient. However log burning does have certain peculiarities which the other two fuels don’t -
- the fuel is often inconsistent in terms of size of logs, calorific value and moisture content
- it is difficult to tell how much fuel you are getting in a load
- there is no convenient established method for delivery, handling and storage
- it really needs to be burnt in batches for clean burning
- it works best with a water heat accumulator because of the irregularity of the burning
There is an increasing market for both stoves and fuel suppliers such as the Champfleurie estate which has addressed most of the problems around delivery.
Chimneys
see also Chimneys
Burning timber can create considerable amounts of tar which can then coat the inside of a chimney and lead to chimney fires. It should be noted that the stainless steel twin wall flues are made in a special grade for burning timber. Wood burning stoves are class one appliances for Building Regulation purposes, just like open fires and there are quite a lot of restrictions about the size and type of hearth and the passage of flues through parts of the building’s structure.
The Building Regulations (part J) covers combustion appliances and fuel storage systems
