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The Carbon Trust鈥檚 recent practical guide for biomass heating concluded that small and large-scale heat-generating biomass systems with an output between 100kW and 3MW are the best options to take if the cost of carbon is the currency of evaluation. The best investment scenario was to replace oil-fired boilers with small-scale biomass heating plant.

There are a range of biomass heating systems on the market. Smaller-scale heating boilers for solid fuel (heat output to 300kW) may be specified to BS EN 303-5. This standard gives assurance about operational safety, but does not demonstrate compliance with clean air legislation.

Biomass heating plant may be classified by the type of grate which controls fuel combustion:

• In a moving grate, the fuel combusts as it is moved down the grate to ash collection. This type tends to be used for higher output plant (300kW-1MW). The system can be used for a wide range of fuels with up to 60% moisture content. Cheaper fuels and highly efficient combustion can offset higher capital costs.
• A plane grate feeds fuel onto or up to a plate where combustion takes place. Ash is displaced by fresh fuel. These systems rely on fuel with low moisture content 鈥� below 35-40%, typically wood pellets or wood chips. Plane grates are usual for units with an output between 25kW and 300kW. They are cheaper than moving grates.
• Stoker burner systems allow fuel to be burned as it is transferred into the burner head. Ash drops into a pan and is usually removed manually. These systems are typically found in lower-cost biomass plant with an output of 30-500kW. They rely on fuel being less than 30% moisture content and the fuel size must be consistent.
• Batch-fired or manual systems rely on the human loading of fuel into the combustion chamber. Typically one load a day provides a single burn period. These are a low cost option where fuel is cheap and labour availability is good.

Fuel

The key cost issues relate to fuel availability, efficiency, delivery, storage, feed and extraction. The most commonly used biomass fuels in the UK include logs, bales, shredded or chipped wood, wood pellets, wood waste and cereals or grains. One cubic metre of wood chips at 30% moisture content has an energy output of 690-870kWh, compared with wood pellets with an energy output of 2,800-3,300kWh. Wood pellets may give four times the energy.

Energy output is reduced as the moisture content increases; wood chips at 85% moisture content have no net energy value.

A biomass heating system requires more space than a conventional mains gas boiler and the optimum design must be chosen for a given set of site and energy requirements. The high capital cost of designing the biomass system to accommodate high volume deliveries and storage with automatic feed and extraction should be balanced by minimal operational costs.

Design and installation issues

The biomass space heating system energy requirements and efficiencies may be calculated to BS EN 15316-4-7 to determine optimal energy performance.

Biomass systems are most cost effective where there is constant demand, such as in swimming pools, hospitals and factories, as opposed to purely residential buildings. Biomass systems do not function well with variable or low loads. One solution is to use a biomass system to supply the base load and install a more gas boiler to meet peak loads.

Biomass boilers require regular maintenance, and operational costs are generally higher than for fossil fuel systems. Biomass boilers also tend to operate at lower efficiencies than gas boilers, and the parasitic electricity costs for running them are higher.

Further guidance on planning, selecting, installing and running biomass energy systems is published by Carbon Trust, Energy Savings Trust, CIBSE and the HVCA.

There are a host of government-funded financial incentives that, if they apply, can reduce the capital cost of setting up a biomass energy system. These include an interest-free energy-efficiency loan from The Carbon Trust or the low carbon building programme run by Department of Energy and Climate Change. The Biomass Energy Centre is a good source of information.