Wood Pellet
Wood Pellets are a modern, extremely efficient and clean burning form of biomass fuel. Pellets come in two sizes, either 6mm or 8mm diameter. The European standard is 6mm, despite an early trend by the UK pellet industry towards production of 8mm pellets, it is now becoming the standard size here in the UK.
Larger boilers with rigid auger systems can be configured to burn both, but for flexible auger systems 8mm pellets should be avoided. Dust levels should be as low as possible; it is important to recognise that dusty pellets does not always signify a fault with the pellet manufacturer – excessive length or number of bends in the fuel delivery system can cause mechanical degradation of the pellet, as can the absence of a suitable pellet impact mat in the fuel store, or a poorly designed bunker or fuel feeding system. It should also be noted that while high dust levels can cause problems for certain types of boilers, clinker formation is in no way linked to the dust content of the fuel. Clinker formation is the most significant factor in wood pellet fuel quality, and can make operating the system a big headache for the user. The key factors are the melting temperature of ash residues, and the percentage of ash in the fuel. Commonly, pellets with a higher ash content than normal are contaminated with non-wood products; These contaminants have the effect of reducing melting temperature of the ash allowing clinkers to form on the grate, disabling the burner.
Wood Chip
Wood Pellets are also an extremely efficient and clean burning form of biomass fuel. With Wood chip fuel it is much easier to spot contamination that can result in clinker, combine this with the increased moisture content of the fuel which suppresses fire bed temperature and you can explain why severe clinker is less common with wood chip systems. Typically where clinker occurs with wood chip boilers, the boiler is burning contaminated recycled fuels with reduced ash melting temperatures. More important with wood chip fuel is the moisture content and size specification. The type of machine used to produce the chips has an impact on the shape of the ‘average’ piece of fuel. Roughly square chips flow better in augers and are less blown to blocking at restrictions such as rotary valves. The moisture content of the fuel can be difficult to estimate by hand, but consistency is key to successful operation of the boiler.
Wood Logs
Logs require considerably more work, and you will need a lot of logs to heat a building, however they can be cheaper than pellets if you have a good local supply.
Log-burning boilers have to be filled by hand compared with most pellet and chip burners which use automatic fuel feeders which refill them at regular intervals. The removal of the automatic feed is that the Boiler capital cost is likely to be cheaper.