Wood pellets are usually made of highly compressed sawdust, a by-product of the wood processing industry. Pellets manufactured for industrial purposes (such as co-firing in a coal-fired power plant) can also be made from materials such as recycled wood and straw. No chemical additives are needed for pellets made from sawdust as the natural lignin in the wood acts as a binder.
The combination of compression and low water content means that wood pellets have a much higher energy content by volume than logs or wood chip. This makes them more cost effective to transport than other wood fuels and means that less storage space is required.
Wood pellet quality
Getting the right quality pellets for your pellet stove or boiler is vitally important. Poor quality pellets are likely to crumble and produce fines, blocking the feed mechanism. They might also contain traces of foreign materials that will damage your heating equipment. Failing to use the right quality fuel could also invalidate the manufacturer’s or installer’s warranty.
Before having a pellet stove or boiler installed, find out the quality of fuel required and whether it's available locally. Historically, many European countries have introduced national pellet quality standards. The best known is the Austrian ÖNORM M7 135 which many Austrian boilers require. The equivalent in Germany is DINplus.
In 2011, a European-wide pellet standard ENplus was introduced. ENplus Class A1 are premium pellets designed to be used in domestic boilers and stoves. A1 pellets produce the least amount of ash and have to comply with the highest quality requirements. Class A2 pellets are for use in larger installations and produce more ash.
An Renewable Heat Incentive biomass suppliers list will be introduced to coincide with the introduction of the domestic Renewable Heat Incentive (RHI) expected in Spring 2014. To qualify for RHI payments householders will have to purchase their wood fuel from suppliers on this list. The list will consist of suppliers of solid biomass fuel (which includes wood pellets) who have demonstrated that their stock complies with the RHI sustainability criteria.
Wood pellet stoves
A pellet stove requires a lot less attention than a log burning stove. An integral fuel hopper (into which bags of pellets are tipped) stores enough pellets for several days’ operation. This and the automatic feed system mean that there is no need for frequent refuelling. In addition, the large ash pan only needs emptying between once a month and once a year, depending on the level of use.
Other benefits of pellet stoves include:
- Temperature and timer controls – sometimes via remote control.
- A fan that distributes warm air around the room. This method of distributing heat means that the stove itself stays cooler than a wood burning stove.
- Automatic ignition.
- High efficiencies (up to 90%).
On the downside, a pellet stove does need an electrical connection to run the feed system, fan and automatic ignition – something to think about if you live in an area frequently affected by power cuts.
Wood pellet boilers
Wood pellet boilers are fully automatic just like oil and gas boilers. Pellet boilers use advanced microprocessors to control the amount of fuel and air being supplied to the combustion chamber. This ensures extremely high efficiencies (up to 90%) and ultra-low emissions.
No supervision of the equipment is required, although the pellet-burner will need cleaning about once a month. At the same time, the ash must be removed, although the quantity is small as the pellets burn very efficiently. Some pellet boilers have an integral pellet store which is manually filled, like a pellet stove. Others are designed to be fed via an augur from a separate pellet store.
Unlike log boilers, pellet boilers are well-suited to meet variable load demands without the need of a thermal store and can be operated on a timer. If space and ease of use are important issues, then a pellet boiler is the one to go for. If cost is a key issue and space and ease of use are less important then a log boiler might be preferred.
Wood pellet storage
The best solution for a pellet boiler, allowing for easy and convenient pellets handling, is to install a pellet store that is designed to receive pellets delivered by bulk tanker. Pellets are transported to the boiler via a special feed systems consisting of a flexible auger inside a tube. Pellets must always be stored dry. For most households a 6 - 10m2 store (with two deliveries a year) would be appropriate.
Care should also be taken to ensure that pellets are not damaged by rough handling as this causes them to crumble and produce fines. These can block the feed mechanism and cause hazardous dust. Find out more here.
Wood pellet delivery
Pellets can be delivered in:
- Small bags of either 10kg or 15kg in size. If bought in bulk they will usually be delivered on a wood pallet.
- Large, dumpy bags containing up to one tonne of pellets.
- Loose, from a specially designed bulk tanker that will use a hose to blow the pellets into the store.
Individual bags of bagged pellets are also available from some DIY stores and garden centres. This is the most cost-effective option for the purchase of small quantities of pellets. However, it’s still very expensive compared to the bulk purchase of loose pellets – worth bearing in mind if you are planning to have a pellet heating system installed and have room for a pellet store. Before installing a pellet boiler and store it’s always worth checking with local pellet suppliers the maximum distance that pellets can be transported via a hose from the delivery lorry and whether there are any restrictions on the number of bends.
Incentives for using wood pellets
Since April 2001, wood combustion equipment has qualified for Enhanced Capital Allowances and a number of UK suppliers of pellet equipment have registered with the Inland Revenue so that customers can benefit from this.
The Renewable Heat Incentive (RHI) pays organisations and individuals (from Spring 2014) for generating heat from renewable technologies (including wood pellets boilers and wood pellet stoves with a back boiler).
To benefit from the RHI, wood fuel systems installed from Spring 2014 must not exceed the maximum permitted emissions limits of 30g per gigajoule (g/GJ) net thermal input of particulate matter (PM) and 150g/GJ for oxides of nitrogen (NOx). Systems installed between 15 July 2009 and the start of the scheme do not need to meet this requirement.