Biomass & Woodfuel
Wood has been used as a fuel ever since the first caveman rubbed a stick with some flint. It follows, therefore, that a system for burning wood efficiently and cleanly would be produced. Current biomass boiler models operate at around 90% efficiency, and they are also cheap to run; wood chip comes in at about 3.1 pence per kWh, and wood pellets at around 4.4 pence per kWh. This is up to 10.6 pence cheaper than other, non-renewable, sources such as gas, oil or electricity.
There are so many different sizes of boiler, they can be installed in a huge variant of homes and businesses. From little 5kW hand-fed stove boilers that sit in your living room and help to heat your hot water, to huge 200+ kW automated boilers that are installed into a basement or outbuilding to provide heat and hot water to a holiday home complex, biomass systems are versatile and effective.
As well as saving money on fuel, you can also earn an income from the Renewable Heat Incentive Payments offered by the government, if you have an eligible system installed by an MCS accredited installer.
How does it work?
Wood produces the same amount of CO2 when it burns as it absorbed when it was living, so it is already classed as Carbon Neutral, and it is renewable; we can grow more trees without any trouble. The issue comes of course the efficiency; most of the heat is lost in the smoke.
The smaller stove boilers behave as traditional log burners; hand fed by logs, or from a bag of chips or pellets, they provide heat to the room they are installed in. Often they are also attached to a hot water cylinder to help heat hot water for the house.
Larger boilers tend to be installed in a basement or outbuilding. They have a separate fuel store, which can store up a years worth of chip or pellets (large log-fed boilers are fed by hand), and are fed automatically from this by a screw-system auger, or a silo-style store. They are integrated into the central heating system and hot water cylinder, and can provide up to 100% of the heat and hot water for a home or business.
When fuel is fed into the boiler, an electric probe ignites it. The temperature is monitored and controlled electronically, allowing conditions to be regulated; for instance, how fast fuel is fed into the boiler, or the fan speed, which controls how much air is blown into the boiler and therefore how fast the fuel burns and how complete the combustion is. Note: a lack of oxygen promotes incomplete combustion, which wastes fuel and produces ashy woodsmoke. After primary combustion, many systems recirculate the smoke to complete secondary combustion, which burns the non-combusted particles to reduce pollution.
The hot gasses that are released flow up the flue and are passes through a heat exchanger, which withdraws the heat from the gasses and uses it to heat water in a buffer vessel. This then moves to the main hot water cylinder and is used as it is, or in the central heating or underfloor heating system.
Which fuel is best?
Chip is generally cheapest, although it is not quite as efficient
as wood pellet since it isn’t of uniform size. It is also not as condensed and so you need more to produce the same amount of heat.
Pellets are more expensive but they produce the most heat and are most efficient, since they are man-made and of uniform, regular size.
Both of these fuels are supplied all across the country and are fairly easy to come by, and both can be used in automated systems as well as hand-fed boilers.
Logs can be fairly cheap, but you must have a good regular supplier, or be able to supply, cut and dry the logs yourself. They are also only used for hand-fed systems.
The drier your fuel is, the better it will burn, and the more efficient it will be. Woodchip is normally at about a 30% moisture level, and pellets are lower, at about 8%.
Is my house suitable for a biomass system?
You should have space for the system, and for storage of fuel. It is possible to buy regular small batches of fuel, but it will be more expensive. Often a new hot water cylinder will be fitted with your system, so space is needed for this too.
Some people build new buildings to house their systems, if they are large, or replace their current log burner or stove with their new boiler.
Delivery of chip or pellets is important to consider; can a lorry get close enough to the store to deliver your fuel?
Biomass systems require a flue to operate. This can be placed into an existing chimney, or on an outside wall. You need to ensure you have a place for this.
Some systems require a lot more maintenance than others, in cleaning and ash removal. Log-fuelled boilers need a lot more cleaning, at least every week. Chip and pellet boilers require a lot less cleaning, but it does also depend on whether your boiler is self cleaning and whether it has an ash-compressing system.
To be eligible for the RHI, your house needs to meet current Building Regulations. You will also need to have an EPC, to claim RHI. If the EPC suggests you have certain insulation work done to make your house more energy efficient, you will need to have this done before you can apply for payments.
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