Biomass refers to the use of a wide variety of organic material such as wood, straw, dedicated energy crops, sewage sludge and animal litter for the generation of heat, electricity or motive power.
It can be viewed as a form of stored solar energy. The sun’s energy is captured and stored via the process of photosynthesis in growing material. This energy is released by processes of conversion such as combustion (burning) or fermentation and distillation (to produce liquid transport fuels).
Biomass is a low carbon fuel source because the carbon dioxide released when biomass is converted for energy purposes is largely offset by that absorbed by the organic material during its growth. With the appropriate management this can be recaptured with new growth.
However, other energy inputs may affect this carbon balance, for example via the energy used by vehicles harvesting or transporting the biomass to its point of use. Combusting biomass fuels such as wood, straw or energy crops (for example, willow coppice or specific types of grasses) to raise heat or steam for space or process heating is one of the most cost-effective applications for biomass from a cost-of-carbon point of view at the present time.
Biomass heating plant can come in a wide range of sizes from a few kWs to many MW of heat. For biomass CHP (combined heat and power), sizes tend to range from around 1MW to many MW of electrical generation capacity. At the smaller sizes, fuel is usually supplied as wood pellets or wood chips. Hand-fed, log-based systems are rare outside the domestic sector. At the larger scale, wood chip is one of the most common fuels at present.
Wood pellets are a from of biomass material, dried and processed into easily combustible pellet form. Pellets can be made form various biomass materials; one of the most common is excess wood form the joinery industry in the form of sawdust, or recycled pallets. Pellets are also made from specially grown crops such as willow coppice or even from straw.
What about the wood smoke?
A traditional fire will sometimes produce thick smoke, however pellet burning appliances are up to 92% efficient and burn at a much higher combustion temperature. This means that most of the particulates that cause dark smoke are burnt off in the combustion chamber. The pellets themselves are also low in moisture content (below 10%) - this also helps to reduce smoke levels. Observing the flue of a pellet appliance, it is difficult to see any smoke emissions at all.
There are currently two main types of pellet appliance available, these are smaller-scale space heaters and larger scale water heaters suitable for a range of uses form top-up space heating to complete space and water heating.
Providing top-up space heating with a pellet stove
Small-scale pellet stoves are suitable for top-up space heating in houses or large rooms and provide an elegant feature for any room, typically providing 2 to 12Kw of heating energy. They can take the place of an open fire; wood burning, coal, gas or electric stove and use an attractive genuine flame for heating. They are more efficient, easier to use and require far less maintenance than wood burners or coal stoves.
Most pellet stoves include a convection fan - this means that heat is more evenly distributed around a room then other types of space heating. Pellet stoves can be controlled by thermostats and usually have a variable output, allowing for easy heat output adjustment.
How does a pellet stove work?
A pellet stove consists of a hopper, screw feed, heating element, electronic control, suction & convection fans and a combustion chamber. Heat output is controlled by a thermostat, which regulates how much fuel (how many pellets) are fed into the heating chamber. Unlike a wood burning stove, heat output can be adjusted according to requirement. The hopper typically takes 25Kg of pellets and, depending on heat demand will last 2-5 days.
When the stove is fired up some fuel is released from the hopper into the combustion chamber via a worm screw, and ignited by way of an electric heating element. The worm screw delivers the correct amount of fuel to the combustion chamber according to the temperature set on the control panel.
Cold air is drawn from outside, through the fire chamber by a fan and then directed back outside through a low temperature flue.
A separate convection fan draws in cooler air from the room at the bottom of the appliance past the firebox, warming it as it goes and blowing it out at the top of the appliance. The convection fan distributes the warmed air throughout the room, rather like an electric fan heater.
Ash drops down into an ash pan, which, thanks to the high efficiency and combustion temperatures, only requires occasional emptying (less than 5 times a year). Ash deposits are normally less than 2% of the total fuel volume of fuel burnt.
Providing space and water heating with a pellet boiler
Water heating pellet boilers work much like a pellet burning stove - but on a bigger scale, some are 90%+ efficient and nearly all are microprocessor controlled. Like oil or gas systems, they are totally automatic but do require some occasional maintenance typically:• Emptying of ash pan every 3 months• Cleaning of the burner once a year
The fuel supplier will usually agree to carry out this work as part of a maintenance contract. These appliances are suitable for variable heat loads and can be timer controlled. They range in sizes from 15kW - 500kW in output and are suitable for both domestic and commercial applications e.g.: houses, schools, offices and factories. There are some appliances available that are capable of running on both wood pellets and wood chips, offering the consumer more choice of fuel source.
How does a pellet boiler work?Pellet fuel is blown through a pipe from the delivery truck into a fuel bunker, which is often hidden underground (e.g.: under the lawn or driveway). Pellet fuel is delivered directly from the bunker into to the boiler’s combustion chamber using an auger feed screw. A microprocessor control unit is used to determine the feed rate of the pellet fuel into the combustion chamber, which is regulated according to the desired heat output.
Just like an equivalent oil or gas system, heat exchangers above the combustion chamber are used to heat the water, which is then piped throughout the building. Ash collects in a bin below the combustion chamber and is taken away by the pellet fuel supplier.
From a user's perspective, a pellet boiler is no different to an oil fired boiler.Central heating~capital costs:Oil Fired: Entire System Fitted: 3-bed house (23 kW)• New Combi-Boiler, New Oil tank,• Installation & connection to existing hot water network• Electrical connectionsApprox cost £ 3500 + vatEquivalent wood pellet fired system (23kW):• New pellet boiler, Pellet storage tank (3m, Insulated Flue)• Installation & connection to existing hot water tank• Electrical connectionsGrant available ~ £50 per kW @ 23 kW = £1150Approx cost £6000 - £1150 = £4850 + vat•£1350 more expensive to install pellet-heating equipment. Stoves~capital costs:Pellet stoves range in price from £1400 to £2500, some models do not require expensive insulated flues, this can make their capital cost comparable with wood high quality burning stoves.