Types of Biomass Suitable for Direct Combustion

Types of Biomass Suitable for Direct Combustion.

Burning biomass is one of the oldest ways of obtaining useful energy. The use of wood as a fuel dates back nearly to the beginnings of civilisation and continues to the present day. Around 2.1 billion people worldwide still cook using open fires or inefficient stoves fuelled by biomass and other polluting fuels, while modern biomass combustion systems can use these fuels much more efficiently and with substantially lower emissions. Biomass can also be converted into gaseous fuels by gasification and subsequently used for heat and power generation.

The main sources of combustible biomass include residues from forestry and the woodworking industry, agricultural plant residues and crops grown specifically for energy production.

Residues from Forestry and the Woodworking Industry

Wood waste from sawmills and wood-processing plants is suitable for direct combustion.

Forest harvesting produces substantial quantities of residues such as branches, treetops, bark and other material unsuitable for conventional wood products. These residues can be used as fuel for heat and electricity production. Further wood-processing residues are generated during the primary processing of logs at sawmills and during subsequent manufacturing in woodworking facilities.

Wood residues in the form of bark, sawdust, shavings, chips or cut-offs can be burned separately in biomass boilers or, after suitable processing, co-fired with coal in conventional coal-fired boilers.

Another important energy source is black liquor from the pulp and paper industry. During chemical pulping, lignin is dissolved to separate the cellulose fibres used for paper production. The resulting lignin-rich black liquor is concentrated and burned, usually in recovery boilers, to generate energy and recover process chemicals.

Agricultural Residues

Only part of annual straw production can be used for energy purposes because substantial quantities are required for other agricultural uses.

Agricultural residues include cereal straw, maize stover, rapeseed straw and low-grade grain. Cereal straw, with a heating value of around 14 GJ/t, can be a useful fuel. However, not all straw is available for energy production because significant quantities are used for animal feed and bedding or are returned to the soil to maintain fertility. A significant disadvantage of straw as a fuel is its low bulk density, which increases transport and storage requirements. Fuel-handling equipment and combustion systems also need to be adapted to its specific properties.

Rapeseed straw can also be used as a fuel and typically has a heating value of around 16 GJ/t. Its availability depends on crop production and on the proportion of residues that must remain in agricultural use or be returned to the soil.

Processing sugar cane produces a fibrous residue called bagasse. Dry bagasse has a heating value of around 18—19 GJ/t, but freshly produced bagasse typically contains about 50% moisture, reducing its heating value to roughly 8—10 GJ/t. Most bagasse is used directly in sugar mills as a fuel for producing process heat and electricity, while smaller amounts may be used for paper production, composting or, after suitable treatment, as animal feed.

Energy Crops and Woody Plants Grown for Fuel

The low bulk density of straw complicates transport, storage and combustion.

Biomass can also be grown specifically for energy production. Energy crops include fast-growing herbaceous plants and woody crops cultivated for use as solid fuels or for the production of biofuels. Such crops may be grown on agricultural land, marginal land or reclaimed areas, depending on local conditions and land-use policies.

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10 pictures

Anaerobic Digestion of Biomass

Flotation tanks are used in wastewater treatment plants to capture fats and other floating materials.

Biomass suitable for anaerobic digestion includes liquid and solid manure, food-processing waste, sewage sludge and other biodegradable organic waste.

Organic material from animal husbandry and food production, such as liquid manure, solid manure and processing waste, can be treated in digesters at biogas plants. Anaerobic digestion converts part of the organic matter into biogas, consisting mainly of methane and carbon dioxide, while the remaining digestate can be used as an organic fertiliser.

Biogas is also produced during sewage-sludge digestion at wastewater treatment plants. Gas generated by anaerobic decomposition of organic waste in landfills is known as landfill gas.

Biogas can be used for heating or for combined heat and power generation.

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6 pictures

Mechanical Biomass Processing

Wood chips produced from woody biomass residues.

Woody biomass produced during logging and wood processing can be further processed into wood chips, pellets or briquettes. Chips are produced by chipping or shredding woody material, while pellets and briquettes are produced by compression. These processes make the biomass easier to transport, store, dry and feed into combustion systems, while also improving the consistency of the fuel.

Biomass often requires mechanical processing before use in energy installations. Wood chippers and shredders use rotating cutting elements to reduce branches, logging residues and other woody material to relatively uniform chips, typically around 2.5—5 cm in size. The resulting wood chips can be transported and stored more easily and can be fed automatically into suitable boilers. Another common form of processing is pelletising or briquetting, in which finely divided biomass is compressed into dense, regularly shaped fuel. Wood-processing residues are commonly used for this purpose, although other suitable plant-based materials can also be processed in this way.

Pellets

Pellets are produced by compressing finely ground biomass in pellet mills. Heating during pressing softens the lignin naturally present in wood, helping the particles bind without the need for additional glue. Standard wood pellets are typically 6 or 8 mm in diameter and up to about 40 mm long. Pelletising substantially increases the bulk density of the material, making storage, transport and automatic fuel feeding more efficient. Wood pellets have a relatively low moisture content and a heating value typically around 15—20 MJ/kg. They are widely used in automatic boilers and stoves as a solid biofuel.

Briquettes

Briquettes are produced in a similar way and can be used in stoves or fireplaces instead of logs. Their shape depends on the type of pressing equipment used. Some briquettes are produced by screw extrusion, in which the material is pushed through a pressing chamber by a rotating screw. This technology can be used for straw, wood residues and other suitable biomass materials.

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9 pictures