Anaerobic Digestion – Production of biogas

Biogas is a renewable source of energy produced by the action of certain bacteria on organic matter in the absence of oxygen. Growing plants capture energy naturally from sunlight by photosynthesis and store it in the plant. A biogas plant (or anaerobic digester) releases and uses the stored energy from the plants and animal manures.

The process of anaerobic digestion is similar to the digestion process that occurs in the rumen of a cow. In the cow, the energy is utilised for milk production and body maintenance. In the biogas plant, the energy is harvested as biogas and used as a fuel for a biogas engine which produces electricity and heat. Alternatively the biogas could also be fed into a natural gas grid or be used as a vehicle fuel.

Anaerobic digestion has been used in the sewage treatment industry for many years. In Europe, on-farm biogas plants are common in a number of countries. While many on-farm plants were originally built to digest animal slurries, it was found that digesters running solely on animal slurry did not produce sufficient biogas to be economical. This is hardly surprising when you consider that around 70 percent of the energy has already been utilised by the animal. The co-digestion of animal manures with green crop silages greatly enhances the biogas yield.

In recent times, the biogas industry has expanded greatly in Europe due to attractive tariffs for producing renewable electricity and the development of co-digestion of slurry with green crop silages such as maize, wholecrop cereal and grass. The table below gives an indication of reported relative gas yields of different feedstuffs in Europe. (AD Portal http://www.biogas-info. co.uk/index.php/biogas-yields)

Research is on-going at AFBI Hillsborough to examine the potential biogas yields from co- digestion under conditions in the north of Ireland.

European biogas plants are commonly fed a ‘diet’ of 70 percent green crop silage and 30 percent slurry. It is possible to utilise many other organic wastes in biogas plants but this will involve extra regulation and licensing, and may require extra plant and storage facilities.

In Europe a typical on-farm biogas plant generally consists of a feed intake system, similar to a diet feeder, a slurry intake system, a primary digester, where around 70-80 percent of the gas will be produced in a 40 day period, a secondary digester where a further 20 - 30 percent of the biogas will be collected and a digestate store, similar to a slurry tank.

The gas is collected in the gas store, from where it is piped to the gas engine which generates electricity and heat. Some of these outputs are utilised to maintain the plant and the balance can be exported as electricity to the grid and heat for district heating use or to provide a drying service for grain or woodchip.

The digestate is a good quality fertiliser for land application. Pathogens and weed seeds are killed during digestion, while the nutrient content of the original slurry is maintained. At some biogas plants the digestate is separated with a slurry separator and some of the heat produced by the plant is used to dry the solid fraction. Following pelletisation it can then be sold as a garden or amenity fertiliser.

There are a number of key factors affecting the performance of a biogas plant.
Feedstock mixture
Plant size
Engine efficiency
Engine up-time Electricity tariff.

It is always advisable to utilise proven technology and a reputable technology supplier when choosing a make of plant. Visit a functioning plant, and talk to the operator. Ask about the performance of the plant and back-up service from the technology suppliers.

The plant size will be related to the availability of feedstock and finance available. For silage requirement in addition to slurry, many European technology suppliers quote a figure of silage from 40ha for each 100kW of electrical output. However, this will greatly depend on the management of the land, its location and the crop grown. It is better to assess the tonnage required and relate that to your current forage production.

Aim for an engine with an efficiency of over 40 percent for electrical output. Lower efficiency engines may be cheaper to purchase but the extra efficiency will normally pay for the extra cost many times over.

Currently, four Renewable Obligation Certificates (ROCs) are issued for each MWh of electricity produced by biogas plants up to 500kW in size and 3 ROCs for plants between 500kW and 5MW. The increase from two ROCs to four ROCs greatly enhanced the potential financial returns from biogas plants. This has resulted in a high level of interest in biogas production amongst farmers. However, raising the required level of finance has remained a major challenge and has encouraged a significant number of farmers to work together to try to obtain the substantial level of finance required to build an on-farm biogas plant in the north of Ireland.

Reliability is key to a profitable plant. At 4 ROCs and an export tariff of 4.5p/kWh, a 300kW plant would have the capacity to generate over £1.00 worth of electricity every minute. Downtime of the biogas engine costs money which cannot be clawed back.

A very useful source of information on biogas production is the AD Portal on the internet at www.biogas-info.co.uk There you will find information on planning permission, feedstocks, regulations, incentives, digestate quality, useful contacts and much more. This has recently been contextualised for the north of Ireland and will continue to be updated as information becomes available.

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