Wood as Fuel
Wood has always been the traditional fuel in Britain, being replaced by coal, gas, electricity and oil over the last 300 years. Wood is a major source of renewable heat energy, and burnt efficiently, it produces virtually no smoke and no acid rain.
The woodland area in Britain is limited, but with efficient management, a substantial quantity of fuel wood is available from parts of trees which are not suitable for saw timber.
Use Dry Wood
Heat energy comes from the dry wood; some is used up converting the water into steam.
1 cubic metre of fresh felled timber weighs about 1 tonne, or 1000 kilogrammes, and contains up to 600 kg of water.
Effective Heat Energy
Wood is a poor conductor of heat, it burns best if logs are less than 10cm thick.
A full central heating system needs 50-100 gigajoules per year. This energy is produced by 8-16 m3 of air dry beech logs. One gigajoule (GJ) is equivalent to about 280 kWh or 10 therms or 1 million Btu/ hour.
How to Dry Wood
- Fell trees in winter for next winter’s fuel.
- Stack on a sunny site until autumn to become air dry.
- Store under cover – e.g. log port – it will not reabsorb water.
- Convert into logs under 10 cm thick and convenient length for stove.
- Bring logs indoors a few weeks before use to become house dry.
Aim always to use air dry wood (house dry is even better.) It is not practicable to achieve the oven dry condition before burning.
Burn Wood Efficiently
Under efficient controlled conditions of burning, wood breaks down with air into ash, carbon dioxide and steam.
Control of the air supply is critical. Burning occurs in three stages:
Pre-heating to 200ºC to oven dry wood and steam.
- With primary air, wood burns at 200 to 500øC to hydrocarbon gases and charcoal.
- With hot secondary air, gases burn at 600 to 700øC into carbon dioxide and steam.
- With primary air, charcoal burns at 300 to 500øC into carbon dioxide and ash.
If hydrocarbons are not all burnt they set as tar in the chimney.
Conifers and broadleaves produce similar amounts of hydrocarbon gases.
Stove design: Choose the smallest fire box for your heat requirement, with controlled hot secondary air, and ash retained in the base of fire. The use of fans for secondary air and ducted hot air and/or pumps with thermostats for water circulation provide improved control at a lower cost. Stoves with doors open provide extra radiant heat but are less efficient.
Open fires: Traditional fire places provide radiant heat and some convection heat. They need a solid base to retain an ash bed in the smallest practicable fire base. A coal grate is not suitable for wood – cover it with a metal base plate. Reduce the area of the lowest part of fire with sloping fire bricks to deepen the ash and charcoal bed but leaving more space for logs above.
Modern double-box fires provide radiant heat and more convection heat, circulating warm room air.
All open fires need a fine mesh spark guard.
Chimney design: Insulated chimneys are essential. Whenever steam is in the chimney the temperature at its outlet must be above 100øC to avoid water condensing.
Visible smoke emission from the chimney is a sign of inefficient burning.
Operating: After stoking, set to fast burn ensuring all gases are fully burned. Only set to slow burn when all wood has been reduced to charcoal and ash. Newly added wood set to burn slowly will create smoke and tarry deposits in the chimney.
Users of older stoves are advised to consult their stove centre about current recommendations to achieve the best results.
Open Fires: Maintain a fast burn with a few thin logs at a time until there is enough ash and charcoal for a slow burn.
Overnight: The stove should not be banked up with logs. A bright fire which has turned the wood into charcoal should be left with the day’s ash, no secondary air and minimal primary air. Charcoal briquettes could be added or smokeless solid fuel.
- Users with storage space: Air dry at home. Order logs for delivery in the spring ready for use next winter; stack in a log-port with sunny aspect, or order lengths of round timber of, say, 4-15 cm diameter; stack one diameter apart in a sunny position and convert to logs in the autumn for storage in a log-port or buy a stack of round timber at forest roadside.
- Users with limited storage: Buy wood as dry as possible. Order well in advance from a supplier who can organise and guarantee summer drying, or in winter accept delivery as far ahead as space permits and store in a warm location as long as possible before burning.
- Buy wood by volume: Buy by solid cubic metres. Merchants should sell logs or lengths by solid measure. A lorry with a capacity of I.8m3 holds about 1 solid cubic metre of logs. A wire netting cage 2m x im x O.9m high will hold one solid cubic metre of logs. Sales by weight can be checked for volume and cost per cubic metre in a cage. One solid cubic metre of broadleaved round wood lengths built into a compact stack also occupy about 1.8m3 of space. Cleanly trimmed, straight conifer lengths occupy about 15% less space.
- Suppliers: Use up-to-date lists of firewood merchants, woodburning stove centres and heating consultants covering the counties of Cornwall, Devon, Somerset, Dorset, Avon and Wiltshire are available from Forestry Commission offices and woodburning stove centres. Additional lists covering Isle of Wight, Oxfordshire, Buckinghamshire, Bedfordshire, Berkshire, Hertfordshire, Hampshire, Surrey,. West Sussex, East Sussex and Kent are being prepared and should be available from local offices of the Forestry Commission and Timber Growers UK by the end of 1986.
Comparative Value of Wood for Heating
Comparisons must take into account the fuel Calorific Value, the Burning Efficiency of stove, fire or boiler and Fuel Price.
Calorific value – a measure of the total heat energy (expressed in gigajoules GJ) per unit of fuel, e.g:
|Broadleaves||7 to 11 GJ per m3 of air dry wood (19 GJ per oven-dry tonne)|
|Conifer||7 to 9 GJ per m3 of air dry wood (20 GJ per oven-dry tonne)|
|Coal||27.2 GJ per tonne|
|Electricity||0.003 60 GJ per kWh|
|Mains||gas 0.0952 GJ per therm|
|Propane||0.0463 GJ per kg|
|Oil||0.03 57 GJ per litre|
Burning efficiency: The type of stove, fire or boiler, at winter loading, determines how much useful heat is obtained per unit of fuel.
|Coal or air dry wood burns at||70% efficiency in free-standing stoves||(15% less efficient|
|60% efficiency in built-in stoves||when doors are open)|
|40—50% efficiency in double fire-box fires|
|35% efficiency in fire places|
|60% efficiency in central heating boilers|
|Gas or oil bums at||65% efficiency in central heating boilers|
|Electricity operates at||90% efficiency in night storage heaters|
Effective heat energy=Caloriflc Value x Burning Efficiency %
Values for domestic central heating systems at winter loading are:
|Fuel||Unit of Supply||EffectiveHeat EnergyGJ/unit||Units of fuel for 1 GJ|
|Smokeless fuel & house coal||tonne||16.4||0.061|
|AirDryWood(m3)||EffectiveHeat Energy GJ/m3||Units of Fuelfor 1 Gj|
Comparison of Fuel Prices and Values
The cost of 1 gigajoule of effective heat energy can be calculated using the values in columns headed Effective Heat Energy GJ/unit and Units of Fuel for 1 GJ of the tables above and the following formula:
Units of fuel for 1 GJ x price per Fuel Unit = Cost per GJ Effective Heat Energy, eg:
- For coal: at £110 per tonne, 0.061 x £110 = £6.71 per GJ.
- For oil: at £0.19 per litre, 43.1 x £0.19 = £8.19 per GJ.
- Air dry beech logs, split and ready for burning, would need to be valued at £42 and £51 per cubic metre respectively to equal these costs of coal and oil. Lower wood values apply to unseasoned or partly seasoned wood and to wood which has to be collected or sawn up or split.
Other factors – In considering costs and values, coal apd-wood are directly comparable. However, for electricity, oil and gas systems, allowance must be made for the convenience of automatic operation and for differing capital costs. Some credit may be claimed for the aesthetics of wood fires – the flicker of flames and the tang of wood smoke.
This leaflet was written with the encouragement of the Forestry Commission and of Timber Growers UK Ltd., Agriculture House, Knightsbridge, London SWIX 7NJ, who represent private woodland owners.
Advice and information were provided by the Energy Technology Support Unit, AERE, Harwell.
© Crown copyright 1986
First published 1985
Second edition 1986