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Using Wood as a Fuel: carbon neutral and sustainable.

The year on year increasing cost of gas and oil, our growing awareness of the earth’s finite resources, and soc-political concerns of fuel security has re-ignited an interest in wood as a sustainable, renewable, low or carbon neutral alternative way to heat our homes



Carbon Neutral and Sustainable

The laws of physics state Energy cannot be created or destroyed; it can only be changed from one form to another. When we burn wood, we are in effect releasing the sun’s energy stored in the wood as heat. The process of photosynthesis converts the sun’s energy, alongside water and carbon dioxide into oxygen and the organic molecules which make up the tree, 50% of which by weight is carbon in the form of carbohydrates of one type or another. When wood is burnt the energy is released in the form of heat and the carbon oxidises and is released back into the atmosphere as carbon dioxide. If wood fuel is harvested from sustainable woodland, where replacement trees are planted or existing trees are pollard, the carbon dioxide released back into the atmosphere by the burnt fuel is reabsorbed by the new growth or the replacement tree. The carbon dioxide is reabsorbed via the trees respiration process and oxygen released back into the atmosphere. This creates a neutrally closed carbon cycle. Furthermore, if the tree or wood is left to decompose on the ground it would still release similar amounts of carbon dioxide into the atmosphere as if it is burnt.


Types of Wood Fuel

In the context of fuels within the UK domestic market the term biomass fuels predominantly refer to wood in one form or another. Wood as a fuel is available in the following forms.


  • Wood Chip: A lower grade fuel consisting of literally chipped wood typically being less than 45mm in size. Wood chip by its very nature is a low energy density fuel and often has a moisture content of approximately 30%. Special stoves are required to burn wood chip as a fuel.


  • Wood briquettes: These are usually made from the waste from wood mills and other wood processing plants. The wood waste is forced at extremely high pressure through a die or mould. The pressure causes the lignin in the sawdust to soften and bind together producing a round briquette of between 50-75 mm diameter and 130-150mm in length. Briquettes are available in two forms one with a hole through the middle, and as a solid block. The briquettes with a hole in the middle are made using a screw press, the hole being where the screw has passed through the material. Solid briquettes are manufactured using a piston press to squeeze the sawdust together. The briquettes with the hole running through the middle have the advantage that the cavity increases the surface area of the burn. Wood briquettes can be used in conventional stoves or open fires as an alternative to logs or fossil fuels.


  • Wood Pellets: Again, constituted out of the waste products from sawmills and other wood processing plants. Pellets are manufactured in a similar way to briquettes and share similar characteristics but are much smaller in size typically being 6mm in diameter and 40 mm long. You will need a pellet wood burning stove to use this type of fuel, as the fuel is automatically delivered to the fire box via a hopper. It is possible to obtain specialist kits to convert some former oil fuelled appliances to wood pellet fuels.

The quality of wood briquettes and pellets differs from one manufacturer to another in terms of the woods used, whether they are made of sawdust, or chips and shavings, if and what binding agents are used, and to what density they are compressed. Moves are being made at a European level to develop industry standards covering the manufacture, the properties, and the evaluation of biomass fuels, which will include wood briquettes and pellets CEN/TC 335 whilst specific fuel specifications and classes are already defined in CEN/TS 14961.


Although, briquettes and pellets are commercially available made from other biomass materials such as straw these are not suitable for domestic use due to their formation of corrosive clinkers and ashes. Wood briquettes and pellets have low moisture content typically ranging between 8% - 10% and due to their compression (approximately, 1,000 Kg per cubic meter) a high energy density that is energy or heat content by volume

Wood Logs: Produced from trees cut and split into pieces approximately 40 -150mm round and 200-500 mm in length.


When Buying Wood Fuels

Before purchasing logs there are several issues you will need to consider, and they can broadly speaking be broken down into to two themes. First the logistics which underpin using logs as your chosen fuel and secondly the quality of the fuel itself. Taking the former first you need to ascertain how and where you going to obtain your logs and how much time you are willing to invest in your fuel. You can track down your local tree surgeons, whom may for a small remittance be quite happy to drop off trailer loads of logs, but they are likely to be in the round. That is not split or cut to your required size which can be quite hard, if satisfying work. Also keep in mind the wood will be green and require stacking and seasoning. If the wood is ‘green’ and unseasoned the new fuel regulations state, the load must be two metres cubed or more and be accompanied with directions on how to season wood.


If you would rather miss out the hard work of manhandling cutting, splitting, and stacking your logs then you need to find a log merchant. Try and secure a local log supplier as this cut down on the environmental cost of transporting the logs and therefore the financial cost. It is not a good idea to transport logs from one area to another due to the spread of invasive tree killing bugs or diseases. Scientists have found many of the tree killing or damaging bugs and diseases travel relatively small distances if left to their own devices but if infected firewood is moved great distances, then so are the bugs and diseases. Buying locally will also give your local economy and woodland a much-needed boost. It is a good idea to ask friends and neighbours whom they might recommend. Always, if possible, go and have a look at the wood you are buying. Do not be afraid to go and visit the wood yard and build up a relationship with your supplier, it will pay dividends later. Have a look at the logs you are buying are they seasoned, stacked well, what woods are there, hard, or soft. Is the wood clean, not covered in mud or worse? Are the logs cut to the size you require; it is recommended that a log is approximately two to three inches smaller than your stove hearth and three to five inches in diameter. This not only helps to maximise heat output but makes handling the logs much easier. It is not a lot of fun attempting to fit an oversized log into the log burner. If possible and if you have the storage spaces buy your logs in the spring so you can stack and season the logs yourself over the summer. If you are purchasing logs or wood-fuel in volumes less than two metre cube they must be certified as Ready to Burn with a moisture content of twenty percent or less.


If you opt to buy kiln dried logs, please be aware it can take a quarter of a tonne of wood fuel to dry one tonne of logs this automatically adds a green tariff of twenty five percent to kiln dried logs before even considering the green mile cost of the transportation of logs from as far away as Eastern Europe and the American Continent


See below to download infographic


 

Green fact about log burners Infographic
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