Wood & Space Heating
In Ireland, our mild temperatures and plentiful precipitation mean that we have the most suitable land in Europe for growing trees. The strange thing is that we also have the lowest percentage of all European countries under forestry. Of further significance is the fact that wood imports for the European community are second only to oil.In global terms, wood has been the most important fuel used by humans for thousands of years. With the harnessing of fossil and nuclear fuels, the use of wood declined. However, wood is still a major source of energy worldwide, both in “richer” and “poorer” countries. Technological advancements over the last five decades in the conversion of wood into both electrical and heat energy have removed many of the barriers to the greater use of wood as a fuel source.
The renewed interest in wood fuel is being driven largely by economic and environmental concerns – wood fuel is increasingly price competitive with fossil fuel alternatives and the environmental benefits of wood fuel are now recognized and valued. The production and use of wood fuel also provides social benefits too, including the creation of additional employment.
Things to know about wood fuel
Moisture
and Energy Content
Wetness or moisture content is very
important when talking about wood. This is especially true, if buying
wood fuel by weight. A freshly cut or green log might weigh a tonne
when it falls on the ground from where it was growing as a tree. It
would be a safe assumption that this green log consists of ½ a
tonne of water and ½ tonne of dry wood. A seasoned log of the
same weight would consist of ¾ tonne of dry wood and ¼
tonne of water.
In a laboratory, a sample of wood can be dried in an oven until it has zero moisture content. Outside of a laboratory, wood is generally left somewhere that allows it to lose as much of its moisture content as is possible, before burning. This is why it is possible to see piles of cut logs stacked under the eaves of some houses in parts of Europe. They are allowing air to dry the wood and keeping it as dry as possible by piling it up under the roof edge. You get more heat energy from wood that has less moisture content and it makes the machine that is burning that wood, more efficient.
The same is true of buying wood chips or wood pellets. The buyer should not focus on the overall weight but on the “oven dried” weight, which is often given in “oven dried tonnes”. Well-seasoned, air-dried wood will typically have a moisture content of around 20%. A rough approximation of the effect of moisture content on heating value is for every percent increase in moisture, there is a 1% increase in heating value. Wood pellets are made from sawdust and typically have a moisture content of around 10%.
For the simpler wood burning technologies (e.g. a solid fuel range) the wetness or moisture content of the wood will have little effect on the technology itself. It will merely mean that you will have to clean more ashes or you will not get as much heat efficiency if using wood with high moisture content. For the more technologically advanced wood burning devices, burning wood fuel with a high moisture content will mean a significantly higher heat energy bill or could even damage costly equipment, depending on the fuel type being used.
Firewood should be stored under cover, preferably under a roof. Ideally it should be cut and split at least 6-9 months before use, preferably longer. Wood should be split for firewood as soon as possible after the trees are felled, to facilitate drying. Wood ready for the fireplace should be as dry as possible, preferably around 20% moisture content. Firewood with higher moisture content causes a buildup of tar and soot in the chimney-vent and it burns with much lower efficiency and increases smoke pollution. Firewood should be stored under cover, to allow it to dry out and prevent it re-wetting. The cover could consist of a strip of tarpaulin or plastic but it is best to store it under a roof, in a well-ventilated shed, shelter lean-to or outhouse.
Species
of tree
The species of tree is also important.
Hardwoods for example are generally denser than softwoods so a
hardwood log will give more heat. Hardwoods generally have natural
moisture content of between 40% and 50%, while softwoods have a
moisture content of between 50% and 60%. A list of Irish hardwoods
would include Ash, Beech, Oak, Birch, Poplar and Sycamore. Many
hardwoods come from abroad and burning local wood that is grown in an
ecologically sound manner is a better idea for several reasons than
growing imported wood. The reasons might include;
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The costs of carbon emissions will hit the tax payer’s pocket. Even though burning wood releases carbon, if you are using local wood, you are using trees that are taking carbon from the atmosphere while they are growing. This will reduce the carbon bill for your own country and pocket. Wood for burning can also regarded as being carbon neutral.
Creating employment, boosting the local economy and traceability.
Environmental benefits; burning locally grown, ecologically-managed wood is better than using wood that may have originated thousands of miles away, perhaps from a sensitive jungle habitat.
Firewood from hardwood has a higher heating value per m³ than softwood, due to the higher density, but on a dry weight-for-weight basis softwood is just as good a fuel. More cubic metres of softwood is needed to achieve the same energy output as hardwood, so the price (per cubic metre) for softwood firewood should be lower than that of hardwood.
The heating value of a tonne of dry hardwood and softwood is very similar – in fact, the heating value of softwoods is slightly higher because of the resin content. However, firewood is sold by weight in Ireland and this complicates the situation. The table below shows the amount of dry matter per cubic metre solid wood for a number of species. The amount of dry matter determines the energy content of the wood. Thus firewood from beech has a much higher energy content than that from spruce, at the same moisture content.
Physical
shape and size of the material
If a machine is automatically fed, logs
and chips should be the right size for your stove and boiler. Wood
logs of the wrong size might not fit your equipment. Wood chips must
pass through the fuel handling machinery. Chips that are too big can
get stuck in the auger on
the way from the wood chip silo and can cause the machinery to stop
functioning until it is unblocked (See photo of auger on right).
Firewood can be bought by weight or by volume (cubic metre loose, stacked or solid). The main difference between the three volume measurements is the price per unit. The price for a cubic metre solid should be much higher than a cubic metre stacked and very much higher than for the same volume of loose firewood. A splitting hammer is recommended when manually cutting small amounts of firewood. Machines are available for commercial production of firewood. Bark insulates the stem against loss of moisture, so roundwood dries much slower than wood that has been cut and split. The golden rule is to prepare firewood in the winter prior to the one during which it will be used – even better would be to leave the firewood until the second winter. There are three ways to split wood:
By hand (using a split hammer with a thick head is much more effective than using a regular axe)
By a splitting machine
By a combined cutting and splitting machine
How
energy comes from wood
Wood can be converted to energy through
several processes;
Combustion is burning wood
Gasification is vaporizing wood. The gas is then used in an engine or turbine.
Pyrolysis is where oil is made from wood and then used as an energy source.
At the moment, processes that produce biofuels from wood are being heavily researched. An enzyme based process for making biofuel is popular (enzymes are “disorganized” chemicals that differ from living ferments such as yeast).
Most of the wood burning technologies that are looked at on this webpage, simply combust the wood in an efficient manner. Some of the technologies may also gasify the wood.
Other
things to consider.
The following is a list of other things
that you should consider if considering using wood as fuel;
Fuel quality (even size and no foreign bodies e.g. metal)
Access for delivery
Room for unloading
Pneumatic delivery systems – air extraction
Design of storage area – ensure safety
Fuel transfer system
Wood-fuel
burning Technologies
Some of the wood burning devices listed
below do not require a chimney. The process is so efficient at using
the energy from the wood that no smoke is created. For this reason,
many of these wood-fuel machines can be located anywhere in a room
without any concerns. Examples of machinery that convert wood energy
into heat through some of the processes listed above are;
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Solid Fuel Range; most people will have seen one of these. Many rural houses burn turf in them but they can also be used for other fuels such as wood.
Free standing enclosed stove (with or without a glass front); these have no side touching a wall and only legs touching the floor. There is plenty of space between the floor and bottom of the stove. Such stoves allow heat to radiate from all sides
Built – in stoves; these would be built into the side wall of a room.
Built-in Masonry Oven; the heat from these ovens is transferred to the masonry (heat resistant material) that surrounds them which acts as a heat sink. The masonry then realizes the heat slowly.
Log-burner; these are automated but can take up quite a significant amount of space.
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Wood chip burners. These require a storage space for the chips and a mechanism for transferring the chips to the chip burner. This can be an auger or air can be used for suction or blowing of the chips. Some companies offer to store the chips in a specially designed container in your attic but most companies in Ireland suggest a storage area for chips. This silo can be underground or in a shed, for example. For this reason, depending on the storage area, wood chips are sometimes better suited to rural than urban areas.
Wood pellet stoves or burners. These use pellets which are made from sawdust, compressed and stuck together by the wood’s natural glue. They look similar to large, rounded pieces of “all-bran. For rural dwellers; they look similar to calf-nuts. They are heavier than they look because of their density. Their density means that they sink in water and relatively few are required to keep a house warm. Pellets can be purchased in bags or in bulk. Pellets delivered in bulk require a “dry storage area”.
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Wood-energy CHP (Combined Heat & Power) In Ireland CHP is largely used by companies or energy consumers that use more energy than a domestic household. Hotels and others in the services sector are a typical example of a CHP user. The energy from the CHP machine provides both electricity and heating. In 2004, 145MW of energy were generated by CHP for the Service (77%), Industrial (21%) and Other (2%) sectors from a total of 139 units. Most CHP plants in Ireland have traditionally been run on gas. With increases in gas prices, wood energy is now becoming a more popular option for economic and environmental reasons. CHP’s often use gasification as a means of converting wood energy to heat. A gasification boiler can run on a variety of wood fuels but specially shaped logs are a common fuel type. Ireland’s first biomass CHP plant was built in Graingers Sawmills in Cork. It uses wood by-products such as sawdust, bark, peelings and forest thinnings.
Finding
the right wood-fuel technology to suit your residence
After looking at various designs, you
should then work out what your heat demand is. To calculate your heat
demand, you should be able to answer some of the following questions;
What is the area of the house (total floor area including upstairs and downstairs)?
How may people living in the house?
What is the average height of ceilings in the house?
What quantity and type of fuel is being used by existing system
Does the house have underfloor heating or radiators?
Is it possible to get a copy of the construction plans?
Where is the building located
What are my wood fuel supply options? (e.g. do you live on a farm that has a wood supply?)
At BEO, we are happy to provide you with further advice relating to wood-fuel technology.
Click here for a historical look at wood & traditional heating in Ireland.