Biomass boilers burn wood-derived fuel to produce heat for a central heating system in much the same way as a gas boiler. They tend to incorporate a large thermal storage tank (hot water cylinder) to act as a buffer storing heat when demand is low and providing additional capacity when demand is highest. Efficient modern biomass boilers do not directly burn the fuel, but heat it to release combustion gas which is then burned in a second stage of the boiler. This results in much cleaner and more effective combustion so that boilers can be installed in locations where the Clean Air Act would otherwise prohibit their use.
In all cases, there is a need for dry storage of substantial volumes of fuel which can add significantly to the cost and size of an installation. A typical family home (20,000kWh per year heat demand) would, for example, require a total of 27m3 for a year’s fuel (that’s a lot of space: a 3 metre cube – a double bedroom). The fuel can be produced from purpose grown crops, waste timber or forestry by-products and can be supplied as chips, pellets or logs.
Carbon emissions from biomass
Although biomass boilers emit CO2 when burning the fuel, it is the same amount as the original wood has absorbed during its lifetime. Biomass can thus be considered effectively carbon neutral, the only net emissions being those resulting from the processing and transport of the fuel. There is a widely held, but incorrect, view that if biomass fuel is transported for significant distances, it quickly becomes un-viable in CO2 terms; the rule of thumb generally quoted is 25 miles for break-even. In fact, even transporting a truckload of woodchips (22 tonnes-enough for 5 homes for a year) would only produce 400kgCO2 for a trip of 250 miles; even shipping woodchips from the other side of the planet works out at no more than 120kgCO2 per tonne of woodchip transported. When compared with the annual CO2 savings for a biomass heating system of 4 tonnes against gas, 6 tonnes against oil and 10 tonnes against electric heating, it should be clear that the CO2 economics are not substantially reduced by transport of fuel, regardless of its source.
It also appeals to us in that it could use up some of the timber that would otherwise end up as landfill.