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How clean is wood burning?
Arguably one of the greenest methods for heating a rural house in the UK (including domestic hot water) is a combination of solar thermal collectors (for summer) and wood pellet or chip (for winter). Logs come a close second if they are dry and burn cleanly. They are also very low in CO2Carbon dioxide is a gas which is given off when carbon based materials such as fossil fuels (coal, oil, and natural gas) are burned. It is called a greenhouse gas because it works like the glazing of a greenhouse and causes global warming emissions; something better than a tenth of most other fuels. However burning wood in an urban setting will almost inevitably lead to dangerous levels of air pollution if it is practiced on a large scale rather than the occasional ‘romantic’ evening for lighting the stove. Already major air pollution problems are showing up in North American and German small towns where wood burning is reviving as a tradition. Denmark is getting rid of its older generation of wood burning stoves on the basis of pollution (interestingly they are turning up in the UK).
The Clean Air Act covers this problem in many urban areas. It is possible to get very clean burning stoves with secondary or even tertiary combustion and catalysts. Many of these are what is known as ‘exempt appliances’ and Defra have a web site covering which stoves and appliances are exempt for which types of fuels. In many cases stoves need a small modification to the air inlet before you can burn logs on them. Most manufacturers insist on logs being ‘seasoned’ or air dried. According to the graph on the Forestry Commission web site Wood as fuel: a guide to choosing and drying logs, ‘air dried’ means they have a moisture content of a 33% or lower ( compared with green wood which may be up around 60%). This is rather a peculiar statement because 33% is nowhere near 20% which is what proper air drying of timber achieves. Most of the approved stove manufacturers insist on 20% as a condition of achieving a clean burn. Well seasoned wood (at about 20% moisture) will give out about twice the energy of green timber when burnt.
Is there enough timber?
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. more +/-»
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 (particularly low in sulphur). 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.
With both chip and pellet it is important to be able to get local supplies of fuel of the right quality. With chip there have been a number of cases of the material being too moist when delivered and this can lead to poor combustion and also auger jams. With pellets it important that they do not break down and form dust while being delivered or pumped into a hopper or store. Dust can be an explosion hazard and any ductwork needs designing and fitting properly to avoid risk.
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) 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 should be adjacent to the boiler so that an auger feed can be used to transport the fuel over.
see also Timber as a fuel
The burning of dry timber logs in the UK can make 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. The cost of the fuel has so far remained very low. See Nottingham Energy Partnership’s ENERGY COST COMPARISON page which compares the costs of various fuels taking into account the efficiencies of the respective boilers. (column ‘Pence per kWh (after boiler efficiency)’). The CO2 output is also extremely low.
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 and it is difficult to tell how much fuel you are getting in a load unless you know both the weight and the moisture content. more +/-»
- There is no convenient established method for delivery, handling and storage (although a few suppliers such as the Champfleurie estate have addressed most of the issues)
- logs really need to be burnt in batches for clean burning. There can be problems with smoke emissions as has been experienced recently in Germany and the US where their widespread popularity has led to public debates on the subject. Legally in the UK the question of smoke emissions depends on whether you are in a smokeless zone. The legislation around the Clean Air Act does allow for dark smoke to be emitted providing ‘that the alleged emission was solely due to the lighting up of a furnace which was cold and that all practicable steps had been taken to prevent or minimise the emission of dark smoke’. The Log Pile Website has good information on emissions and also on the safety aspects of burning wood.
- log burning works best with a water heat accumulatorusually a large water tank used to store surplus heat (from say a wood fired boiler or thermal solar collector). see the page on Heat Stores because of the irregularity of the burning.
- If you are harvesting your own logs it is worth giving consideration to the storage of the timber. Ideally it should be allowed to dry under a cover or roof with good air circulation for several months, preferably over a summer. Kindling will dry in a week or two if there is a storage place near the stove.
- All combustion appliances require an air supply and wood burning stoves are no exception. As houses become more airtight, the correct design of air supply becomes more of an issue. Many stoves are now designed so that a combustion air supply pipe goes directly to the stove rather than the stove getting its air from the room. This also cuts down on cold drafts within the room.
The Biomass Energy Centre have recently placed a great deal of useful info about wood fuels on their web site
Also the Fotestry Commission has become more active in promoting wood burning and is beginning to publish information on the net. They have a web site which includes a ‘Directory for Woodfuel in the Yorkshire and Humber Region’. There is a graph which shows heat output in kWh per tonne of logs and compares green logs (newly felled) with seasoned ones with the percentage moisture contents.
In Wales there is Woodfuel Wales
HETAS is the official body recognised by Government to approve solid fuel domestic heating appliances, fuels and services including the registration of competent installers.
The Building RegulationsThese are the legal regulations which govern how a house is constructed. (not to be confused with Planning Permission which is about whether you are allowed to build the house at all or what it might look like) see Building Regulations) (part J) covers combustion appliances and fuel storage systems
A nifty little gadget is the Ecofan which sits on top of a stove and uses the stove’s heat to power itself in order to circulate the warm air coming off the stove.
With the burning of timber the issues are:
- 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 kWhkilowatt hour. This is a unit to measure an amount of energy. If you run your 30 kW gas boiler for 2 hours a day you use 60 kWh per day 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 MDFMedium Density Fibreboard or OSBOriented Strand Board. 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. Although most homegrown softwood timber in the UK is of fairly low quality (compared with say, Baltic timber which grows more slowly and to a higher quality) there is a strong possibility that modern computer controlled techniques for comb jointing and laminating timber will mean that lower quality timber will be in demand for joinery rather than for burning.
- 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! Transport by ship tends to be a relatively low energy method anyway.
- 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 concerns about the extremely intensive use of land in the country.
- 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 good deal of information on decision making with systems, 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 the matchstick industry 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.
At present it is difficult to find any woodland in the UK which is not basically managed for pheasant shooting. Try to find a wood without the blue plastic feeding bins for pheasants. Even if you are not against the wholesale slaughter of one of the most beautiful birds on the planet and even if you are not bothered that many of those birds are wounded rather than killed by pellets from badly aiming bankers then it is difficult to avoid the conclusion that most of the management of woodland is skewed to one end only.Powered by Hackadelic Sliding Notes 1.6.5
Weight for weight, all the timbers in the UK have very similar calorific values. See the Forestry Commission site on this. It works out at a bit over 5000kWh/oven dry Tonne. This means that softwoods are bulkier than hardwoods but give off the same amount of heat for a given weight. This is why you should always buy timber by weight. The other factor is the amount of moisture content in the timber. When timber is felled it may have 40% or 50% moisture content whereas after it has been seasoned it will go down to 20% or less. Damp wood at 40% moisture content only gives out half the heat of timber at 20% because so much energy is used in boiling off the water. More information here.
It is generally considered important to burn timber at 25% or less because the other factor which needs considering is tar production. If damp wood is burnt slowly and particularly if the fluepipe to conduct gas, typically ventilation air or boiler exhaust. see Flue pipe is not adequately insulated then wood tar and soot will condense out on the inside of the flue. Two things might then happen. You might get a blocked flue which can be dangerous because of smoke and carbon monoxide coming into the house, or the tar in the flue might set on fire. Fluespipe to conduct gas, typically ventilation air or boiler exhaust. see more on Flues for log stoves are normally double skinned with insulation between and made of materials able to withstand such a fire e.g. the Selkirk twin wall insulated flue.
The way many log merchants sell their fuel is deliberately confusing and it is something of a mystery why Trading Standards have not picked up on this. They frequently sell by volume rather than weight so there is no way of knowing the calorific value of the fuel which depends on weight. They claim that a set volume of timber at a certain moisture content will always give out the same heat, which is not true because different species have differing volumes for the same energy content.. They also use vague terms like ‘bulk bag’,'net bag’ ‘seasoned’ or ‘kiln dried’ without stating any figures for actual weight or moisture content. Imagine buying a ‘jug full’ of petrol that was ‘high quality’ or ‘very powerful’. There are actually standards for firewood produced by HETAS. The bottom line is that you need a certain weight of timber at a certain moisture content. For instance ’500kg at less than 20% moisture’. If you do have to buy it by volume then ask whether the logs are loose in the container or close stacked; it makes a considerable difference.Powered by Hackadelic Sliding Notes 1.6.5