Comenius

Biomass for heating

Biomass is the oldest and the most commonly used renewable energy source. Biomass is a collective term for plant material that can be burned to produce energy. Examples include wood, straw and energy crops such as willow and poplar.

Solar radiation falling on the earth produces light. This is converted by plants and trees into organic materials by photosynthesis enabling this biomass to grow. The infra red rays associated with sunlight provide suitable conditions for growth such that plants and crops can be harvested in the autumn. Trees take much longer to mature, up to 50 years or more; however short rotation crops can be grown specifically for providing biomass for space heating or hot water. Waste from forestry and farming can also be used.

The most common use of biomass is to replace oil or brown coal boilers.

Advantages

• it is a renewable energy source
• its widespread abundance
• its general local availability
• the management of production waste from forestry and farming
• the generation of local industry and employment

Disadvantages

• requires more space than a conventional gas boiler
• cannot be used in emission restricted zones
• pellets need a suitable storage space
• supply quality can be variable and may not always be available

Useful energy
Biomass can be burned in a conventional boiler that has been adapted for the type of biomass to be used. Generally biomass boilers are somewhat larger because of the lower calorific value of the fuel. This averages 15 MJ/kg compared with 48 MJ/kg for natural gas. It is essential to ensure complete combustion to minimise environmental emissions and residues. Some types of boiler will have automatic feeding of the biomass such as wood pellets, but other types will require supplying fuel at various intervals. The residues are generally beneficial for the soil.

It is generally possible to substitute a biomass boiler for a conventional boiler provided some extra space is available. A flue or chimney is required to disperse the combustion gases and draw fresh air into the boiler. If automatic feeding is required, then space is required for the feed pipe.

A volume of 2 cubic metres is generally required to store sufficient biomass for a period of 3 to 6 months.

Sources
Biomass sources include:
Wood

• firewood, logs or wood prepared for burning in a stove or fireplace,
• wastes and by-products of forest industry, bark, sawdust and shavings, wood chips, trimmings and other tree logging remains,
• energy crops: the common willow, poplar.