The cheapest type of wood biomass fuel at around 2.5p per kilowatt hour (kWh) of heat energy produced, best suited to medium and large scale fully and semi-automated boilers (50kW). If a uniform size, wood chips also can be used in smaller automated systems (uniform wood chips also cost more). Wood chips are predominantly pretty dry at around 30% moisture content; because of the large amount of moisture stored in the wood they should be used relatively quickly and not stored for long periods of time as they may begin to degrade. An unwanted by-product of this degradation process is the germination of fungal spores which (in large quantities) can cause a condition in humans that is called ‘farmers lung’ which is an incurable repertory disease. If the wood chips are to be stored for large periods of time then it is advisable to agitate them to keep airflow and aid the drying process, preventing the microbial activity.
Slightly more expensive than wood chip at around 4p/kWh of heat energy produced. Wood pellets are made from waste and excess material from wood manufacturing. Pellets are the densest form of biomass, and therefore the most efficient and compact. Given their uniform size, they are also best suited to fully automated systems. On a larger scale, safety issues can arise from the dust that is produced from these pellets whilst being transferred to the hopper via pneumatic conveyance. It is advisable to minimise abrasion and impact to the pellets upon delivery by using large radius, smooth delivery pipes with an impact baffle inside the hopper to prevent the pellets shattering upon entry (some hoppers have these pre-installed). Dust and outgas (CO) that can be present in hoppers is extremely flammable and highly toxic, so no open electrics should be present near the hopper and any person entering stores should do under strict supervision.
Suitable if you have access to large wood resources and enough space in which to store them. Logs can be expensive to buy if you don’t have free access to wood. Given their high moisture content, logs need to be seasoned in a dry place for at least a year in order to reduce their moisture content, thus allowing maximum efficiency. Wood bark and sawdust can also be used.
Waste agricultural materials such as straw and wheat husk can be used in biomass boilers. Some types of industrial waste, such as paper pulp, may also be suitable, though this will depend on the type of boiler that you choose.