Biomass - Using Anaerobic Digestion

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Biomass Types

Biomass can be split into two distinct categories:

  • Waste biomass
  • Energy Crops

Waste Biomass

Forestry Residue

Forestry residues include biomass not harvested or removed from logging sites in commercial forests, as well as material resulting from forest management operations such as pre-commercial thinnings and removal of dead and dying trees.

Wood is the most commonly used biomass fuel for heat and power, and unwanted woody waste is a by-product of many forestry operations. Using these materials for electricity generation recovers their energy value while avoiding landfill disposal.

picture: forestry cut down

Forest Residue

Forestry waste includes logging residues, imperfect commercial trees, dead wood, and other non-commercial trees that need to be thinned from crowded, unhealthy, fire-prone forests. Forest thinning is necessary to help some forests regain their natural health, but for smaller woodlands the cost of removing the wood cannot be recovered through timber sales due to their poor quality.

About 12% of UK land is covered by trees, with about 47% of this area in Scotland. Commercial amounts of wood for fuel can be produced as a by-product of forestry management. Wood has a relatively low calorific value of around 19GJ/dry tonne. When harvested, wood has moisture content of around 55% by weight.

In Scotland there are over 1,318,000 hectares of woodland and forest, and is estimated to generate sustainable wood fuel for nearly 250,000 homes.

Scottish Executive Building and Planning

Substantial areas of forest are needed to support a wood-fired power station. For example, a 6MW station with an efficiency of 20% would need between 430 and 2,150 hectares a year of sustainably managed forest at harvest. This exceptionally wide range shows the variable yield of forest residues. The yield itself depends on various factors such as terrain, accessibility, tree species and age, and end use of the timber - as opposed to the residues - will be put. However, the feasibility of this will also depend on other issues, such as the conservation status of the woodland, accessibility and countryside policies.

Animal Farming

Farm Slurries

Farm slurry is a watery animal sewage containing a high concentration of suspended solids. Farm slurries are obtained mainly from pig farming and cattle farming. It can be used as a fuel source for Anaerobic Digestion

  • Poultry litter
  • Farm slurries

These are used primarily because they are easy to collect and can be utilised with basic equipment.

Poultry Litter

Poultry litter is the bedding material collected from broiler sheds. Itís usually made up of wood shavings, shredded paper or straw, mixed with the chicken droppings. It has a calorific value of 9-15GJ/tonne, which is slightly lower than that for wood. It has a moisture content of between 20-50% depending on the methods of husbandry used by farmers.

picture: poultry Litter

An example of using poultry litter as a biomass fuel source can be found in Fife. The Westfield biomass plant uses a combustion system to burn poultry litter to generate electricity and produce fertiliser. It has a net electricity output of 10MW and has been fully operational since October 2000. Scottish Executive Building and Planning

The practice of spreading large quantities of poultry litter on the land is no longer considered acceptable because it can cause serious environmental problems by polluting watercourses and producing odours if not correctly managed. The poultry farming industry has come under increasing pressure from environmental agencies to adopt a more environmentally acceptable method of disposal. Westfield was chosen to deal with the poultry waste because of its central location in the poultry farming industry, its high quality road links and close proximity to the electricity grid system.

Cattle Farming

Cattle farming techniques significantly affect the quantity and quality of manure that may be delivered to the anaerobic digestion system. The number of cows, the housing, transport, and bedding systems used by the farms determines the amount of slurry that must be used and therefore the amount of energy produced.

Cattle farming may be housed using a variety of methods. The most commonly used systems include free stalls, corrals with paved feed lanes, and open lot systems. The type of housing used determines the quantity and quality of manure that can be econom ically collected.

picture: Free stall barn cows

Free Stall Barn - Dairy Waste Handbook

The commonly used methods of collecting the manure are flush, scrape, vacuum, and loader systems. In free stall barns the manure can be flushed, scraped, or vacuum collected.

Pig Farming

Several options for collecting and storing swine manure are available, depending on the manure form. Common storage methods include underfloor pits, outdoor (above or below ground) structures, earthen pits, lagoons and holding ponds. Flushing gutters and scraper systems are among the methods used to collect and transport manure to appropriate storage facilities.

Pig Farming Management

picture: Swine in stalls

Intensive Pig Farming

As with cattle farming, the manure can be used as a fuel source for anaerobic digestion Anaerobic Digestion

Slaughterhouse & Fishery Waste

At a slaughterhouse or a fish processing plant, there is a huge amount of organic waste. This has the possibility of being a danger to the environment and human or animal health. The EU Animal By-Products Regulation (2003) specifies these animal wastes may be disposed of safely. This can be an extremely costly process, but this type of waste can also be used as a feed stream for Anaerobic Digestion

Organic Municipal Solid Waste (MSW)

Organic MSW is any matter collected from commercial or residential properties such as food waste, paper etc.

picture: Municipal waste in a large bin

US waste website

Organic waste, whether from commercial or residential properties, makes up a substantial amount of waste that is landfilled. As with other wastes, it can converted into energy by various methods. One is direct combustion (incinerator), or by anaerobic digestion in a landfill or in a process plant.

Sewage Waste

Sewage waste is a source of biomass that is comparable to the other animal wastes previously mentioned. Energy can be extracted from sewage using anaerobic digestion, pyrolysis or drying and incineration. All these technologies can be found here.

Energy Crops

Energy crops are produced specifically for their fuel value. There are several crops that can be readily utilised as energy sources. Both Short Rotation Coppice (SRC) and Miscanthus are covered under the Energy Crops Scheme (ECS), which aims to increase the area set aside for energy crops in England by using grants. Because of this grant system, these are the most common energy crops.

Short Rotation Coppice (SRC)

SRC is a densely planted, high-yielding varieties of either willow or poplar, harvested on a 2 to 5 year cycle, although commonly every 3 years. SRC is a woody, perennial crop, the rootstock or stools remaining in the ground after harvest with new shoots emerging the following spring. A plantation could last up to 30 years before re-planting becomes necessary, although this depends on the productivity of the stools.

picture: Field of Coppice

Short rotation crop field

In the UK, yields achievable from willow SRC at first harvest are expected to be in the range 7 to 12 oven dry tonnes per hectare per year depending on site and efficiency of establishment. New varieties are expected to greatly increase yields. It would take roughly 3,000 ha of SRC to sustain a 5 MWe gasifier. Defra Energy Crops
North Devon Energy crops


Miscanthus crops are woody, perennial, rhizomatous grasses, which can be harvested annually for at least 15 years. By the third year harvestable yields are between 10-13 tonnes per hectare. Peak harvestable yields of 20 tonnes per hectare have been recorded.Defra Energy Crops 22,000 tonnes can provide enough electricity to power 2,000 homes.


Pellets are a refined, solid fuel biomass with a low moisture content, which makes it easy to transport, store and convert into energy. It is manufactured from saw dust, wood chips, shaving or bark. Pellets are typically 6-8mm in diameter and 5-30 mm long. The maximum water content is 8%.

picture:wood pellets

The following list contains the main advantages of using pellets:

  • Pellets burns almost without any smoke development. The dust in the flue gas is very basic.
  • The ash produced is basic.
  • Less carcinogens are produced in the high temperature combustion of pellets compared with unrefined fuel.
  • It has a low heavy metal content.
  • Only small quantities of NOX oxides are formed.

Biomass is usually processed to pellets in plants using piston or roller presses. The raw material used may be sawdust, shavings or chips. The raw material is heated, to make the fuel dry and at the same time transforming the lignin acting like the glue that holds the raw material together, which makes it possible to shape the pellets in the desired form.

To use pellets for heating, a specialised pellets burner is needed. The ash left over from the burning is rich in minerals, and can be used as a soil conditioner.


The term woodchips refers to mechanically processed wood particles, ranging in size from 1 to 100 mm. The criteria used for woodchip quality are as follows:

  • Chip size: only the "fine" (smaller than 30 mm) and "medium" grades (below 50 mm) are suitable for small-scale installations;
  • Water content: this determines the energy content of the fuel on the one hand and its storability on the other;
  • bulk density: this indicates the weight per cubic metre (bulk volume) and depends on wood type, particle shape, degree of compaction and water content.

picture: Woodchips


The price of woodchips largely depends on their water content, as high water content reduces the energy content. Moisture is therefore just as important as chip size in determining the price of woodchips.

Other biogenous fuels

In addition to the above fuels, the following other plant products are also classed as biomass:

  • straw;
  • wheat;
  • potatoes;
  • sugar beet;
  • residues from fruit processing (e.g. stones, husks, Ö);
  • wood wastes from the woodworking industry (saw dust, sanding dust);
  • residues from the woodworking industry (e.g. from the pulp industry);
  • biogenous fuels (biodiesel from oil seeds such as oilseed rape, methanol from cereals).