There are many different types of bioenergy fuels which can be converted into sustainable energy, categorized as woody biomass, energy crops, agricultural residues and industrial residues.
Woody Biomass is a renewable energy source from the roots, wood, bark and leaves of living and dead woody trees and shrubs used in bioenergy generation. This can originate from hard and soft woods. The different types of woody biomass are;
Wood Logs – Split logs, of up to 500mm can be used for bioenergy generation. Ideal moisture content for logs is less than 20%, however some boilers can manage a moisture content of 35%. The lower the moisture content of the logs, the more calorific value is generated. Logs are best sourced from local suppliers (to reduce transportation costs).
Wood Chips – Originate from forestry logging residues, energy crops from e.g. willow, or as a co-product from industrial wood processing. The quality of chips depends on the quality of the raw material and the chipping process. Chips are typically 5-50mm long. When wood is freshly cut, it can have a moisture content of up to 60%. Usually wood is seasoned prior to chipping or otherwise dried after chipping to reach the ideal moisture content of less than 30%. Large chips or those with high moisture content can be used in large boilers, and generally smaller, drier chips are preferred for smaller boilers. Good quality wood chips (e.g. with moisture content of 30%) will have a c.3,700 kWh/tonne calorific value.
Wood Pellets – are refined wood products originating from wood shavings and sawdust. They have a low moisture content (generally less than 10%) and burn very efficiently. They are compact, easy to store and transport, and are popular in domestic heating systems. Good quality wood pellets (e.g. with moisture content of less than 8%), will have a c.4,700 Kwh/tonne calorific value.
Forest Residues – are the residues after the quality timber has been harvested. This includes roots, branches and tree tops. Whilst this may be left to decompose in the forest, or line the route of the forestry machinery, it may also be used as fuel source.
Short Rotation Coppice (SRC) – is an energy crop, usually willow or poplar, grown at close spacing of plants on short time rotations. Fast growing species are cut to a very low stump when they are dormant in winter and then produce new stems in the next growing season.
Reed Canary Grass – is a perennial 2 metre high grass with a stiff and robust straw. It is commonly found in wetlands and flooded areas and thrives in humus-rich soils, but can also grow on mineral soils. The crop grows well in all of Europe, but is more suitable for northern Europe due to competition from other types of energy crops with higher energy yield further south. The roots are very strong and bind through underground rhizomes when cultivation is established properly. This makes the grass grow in large clumps and is green well into fall. The yield is about 4.2MWh per tonne or about 20MWh per hectare, roughly enough to provide a house with heat for one year.
Agricultural wastes in bioenergy include residues from animals and sewage sludge.
Residues from animals – this includes animal slurry and manure, and chicken litter. Wet wastes such as animal manure are suitable for anaerobic digestion. Chicken litter, due to its lower moisture content can be combusted.
Other agricultural waste – this includes spent mushroom compost, straw, grass cuttings etc. Theses residues can be converted into bioenergy by combustion.
Sewage sludge – municipal solid waste, sewage sludge from wastewater treatment plants and food processing waste can be converted to energy / biogas by anaerobic digestion. Due to increased EU regulations regarding the treatment of waste, increased amounts are becoming available as a biomass fuel.
Food / Canteen Waste – due to increased EU regulations and the rising costs of waste disposal, food and canteen waste and now being viewed as biomass fuel, which can be converted into biogas by anaerobic digestion.
Waste vegetable oils – similar to food waste, vegetable oils are subject to increased EU regulations. They can be reprocessed to produce biodiesel for transport, and can also be used to produce bioenergy in an anaerobic digester.