Biomass fuels encompass a wide variety of products but, not surprisingly, most come from wood.
Clean wood is a wonderful fuel, producing up to 5.5kWh per kg dry matter (dm). Wood pellets produce slightly less, at 4.7 to 5kWh per kg. Most suppliers guarantee moisture content to be less than 10 per cent (often 8 per cent in real life) and 4.7kWh per kg (again this is often nearer 5kWh per kg). The lower levels are stated as varieties of timber vary slightly.
The difference in energy values of softwood and hardwood is also small, with softwood usually having a slightly higher value. Because hardwood is denser, hence heavier, it is often assumed it has more energy, but per kg this is not the case (tonne of feathers, tonne of lead!). Wood also has one of the lowest ash contents of any biomass fuel, as little as 0.5 per cent.
Modern biomass heating systems can burn wood pellets, chip or logs at efficiency levels of up at 95 per cent, which is at least comparable to the best fossil fuel boilers.
So which fuel is most appropriate? And are there concerns about supply?
Obviously the least processed, and hence the most readily available fuel is logs. Woodland produces in the region of seven tonnes of wood dry matter per hectare per year, and normal forestry management ensures a ready supply of suitable fuel. Logs are available in almost all areas of the UK, and of course many people have access to their own. The cost per kWh is likely to be half that of oil, if the logs are purchased.
Wood chip is only slightly more expensive than logs and the boilers which burn chip can be fully automated. The problems associated with wood chip are the cost of the boiler and the space that the chip storage requires. As wood chip is simply logs which have been chipped by a fuel grade chipper (chip quality is very important and arboreal chippers such as used by tree surgeons do not produce consistent length chip), supplies are readily available. Wood chip is the best solution where the heat demand is high and storage space is not a problem.
Wood pellets have been used throughout Europe for many years, with millions of tonnes produced and sold per year. Most is produced from waste sawdust, a by-product of sawmills. This sawdust is then dried and pelleted to produce wood pellet fuel.
Previously most wood pellets were imported, some from Northern Ireland where there is a plant producing over 50,000 tonnes per year, or Austria and Germany. Recently many small to medium plants have opened in the UK, with a great deal more coming on stream in the next couple of years. Wood pellets cost almost as much as oil at current prices, however after the ‘credit crunch’, as fossil fuel prices rise and with increases in supply, the differential will increase.
Wood pellet boilers are the closest in terms of convenience to fossil fuel, being physically smaller and requiring less storage space for fuel, as per cubic metre pellets contain 3 times the energy of chip. Pellet boilers are cheaper than chip boilers and more appropriate for average size dwellings, or where storage space is an issue.
So the choice between log, chip or pellet? We at Treco supply all three system, all have advantages and disadvantages. Sometimes biomass is not the appropriate technology and we would recommend an efficient conventional boiler, perhaps supplemented by PV and or solar thermal panels, but alas that’s not for me to talk about!