Biomass boilers burn
wood-derived fuel to produce heat for a central heating system in
much the same way as a gas boiler. However, it is difficult to
accurately regulate heat output and biomass boiler systems cannot be turned on and off at will, so 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. The fuel can be produced from purpose
grown crops, waste timber or forestry by-products
and can be supplied as chips, pellets or logs:
Wood pellet boilers
Wood pellets are manufactured from wood
products and as such is a consistent, high calorific (heating)
value, dry fuel. It is easier to handle and requires less
storage space than other biomass fuels. However, it is
also more expensive at around 2.5p/kWh and may be difficult to
Wood chip boilers
Wood chip fuel is somewhat inconsistent in
its calorific and moisture content and can cause severe sooting
of the heat exchangers, so boilers require constant maintenance.
It tends to be used for larger installations where the lower
cost (as little as 1-1.5p/kWh for bulk orders) offsets the
higher maintenance costs.
Dry logs can have a consistently high
calorific content, but require manual handling including at
least daily fuelling of the boiler. Depending on location,
fuel can be relatively inexpensive.
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 unviable
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.