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Wood Chip Boilers

What are wood chips?

Wood Chips are now used instead of logs – once wood is in chip form it can be fed into the boiler in a controlled and measured quantity and not by the client in their pyjamas at three in the morning throwing logs into a boiler. It also means that none of the heat has to be wasted or lost as it might be in a log boiler.

It is important for the customer to understand that replacing your fuel from oil or gas to wood you will need to know and understand the process. It is also vital to understand about the quality of the fuel. The efficiency of the fuel can change dramatically and that will alter the quantity burnt and heat produced. The more water in the chip the greater the quantity of fuel that will be needed to be burned to get to the same temperature as a dryer chip. Around 25% moisture content is ideal.

For smaller systems the chip store is known as a hopper or silo. For a larger system a fuel store room separated from the boiler room by a fire-proof partition is used. The majority of wood chip is fed into the boiler with an auger screw. Most wood chips are delivered using a tipper truck or tipping farm trailer, particularly for larger loads. ‘Big bags’ similar to those used in the building and agricultural trades would suit some systems with appropriate hoppers. Large bins delivered by ‘hook’ lorries (called Ro-Ro Bins) can act as a delivery container and fuel feed hopper if fitted with a scraper or moving floor out-feeder system.

If your wood contains pieces of plastic or nails (large or small), they will put more pressure on the auger screw and will either stop your boiler working completely or greatly reduce its life expectancy.

When ordering your chip you need to make sure that the quality of chip is of good quality and the correct size. Different boilers will demand a different chip, however most like a chip that is between three to five inches long.

What moisture content should a wood chip have?

The efficiency of wood fuel is closely linked to the moisture content of the wood when it is burnt.

Therefore the key question regarding using wood as a fuel is “how is the moisture content of the raw wood to be reduced?” There are three main approaches:

• Traditionally logs are ‘seasoned’ by being left out or undercover for a summer and dry naturally. Moisture contents of about 25% are typically achieved, less if the logs are well stacked and covered.

• An alternative approach is simply to burn the ‘green’ wood (i.e. freshly cut) directly. In this case the moisture is reduced in the boiler or fire. However, this greatly reduces the calorific value of the fuel, making good combustion difficult unless the boiler is specifically designed for this fuel, such as the Binder Step-Grate machines (USRF), which is used to avoid greater accumulation of tar deposits in the flue/chimney.

• Reduce the moisture content close to where the wood is produced by forced drying. This happens in a pelleting plant where the raw material is finely divided, force-dried and compressed. The end product is a dense, low moisture content fuel (typically less than 10%), which is convenient to transport and handle.

The process is:
• Cut down tree
• Remove smaller branches and leaves (this can be mulched and used in the garden).
• Cut the tree into logs (the size that would be used in a fire place).
• Store the wood undercover – this can be a little as a tarpaulin. We advise no less than two years.
• Chip the logs.
• Transport to chip store.

There are three different types of chipper, however the main thing is to make sure that the wood chips are not dusty. This is for the reason that dust does not have a controlled burn and creates more waists than is necessary.

Calculation of moisture content (MC)

There are two different ways of specifying the moisture content of wood; on a ‘wet’ or ‘dry’ basis. The moisture content of a single piece of wood will be different using the two methods and so care must be taken to be clear which method is being used.

The moisture content of a piece of wood is given by the mass of water contained in the wood divided by the total mass of the piece of wood as found.

Example: A sample of wood chip has a mass of 10kg. It is dried to an oven-dried condition, and then it has a mass of 8kg. The mass of ‘wet’ wood (10kg) minus the mass of oven-dried wood (8kg) = mass of water (2kg).
2kg water / 10kg wet wood x 100 = 20% moisture content.
It is very important that you find a good supply of fuel. The majority of biomass breakdowns are caused because one buys poor quality fuel. The different type of wood obviously makes a difference.

Navigate our website for more information on wood chip suppliers, or call us pn 0208 964 6716 today for a free quote.