Biogas generation

What is AD?

AD is the process where plant and animal material (biomass) is converted into useful products by micro-organisms in the absence of air. Biomass is put inside sealed tanks and naturally occurring micro-organisms digest it, releasing methane that can be used to provide clean renewable energy. The material left over at the end of the process is rich in nutrients and can be used as fertiliser. The process reduces emissions of methane and produces a product that provides more nitrogen to crops than slurry. 

Outputs

Biogas is a mixture of 60% methane, 40% carbon dioxide and can be combusted to provide heat, electricity or both. Alternatively, the biogas can be cleaned up and the pure methane injected into the mains gas grid or used as a road fuel.

Digestate comprises of left over indigestible material and dead micro-organisms. It contains valuable plant nutrients like nitrogen and potassium and can be used as a fertiliser and soil conditioner.

What can AD process?

Almost any biomass can be processed by AD including food waste, energy crops, crop residues, slurry and manure. Cereals and rape meal can be used as AD feedstocks, giving high biogas yields, but are generally expensive. Dedicated energy crops with high biomass such as maize, can be grown specifically for anaerobic digestion.

What can't AD process?

Woody biomass cannot be used in AD because the micro-organisms can't breakdown the lignin, the compound that gives wood its strength.

Yield

The yield of biogas from a particular feedstock will vary according to:

• Dry matter content. Food wastes in particular will vary greatly.
• The energy left in the feedstock. If ithasundergone prolonged storage it may already have begun to break down.
• Length of time in the digester .
• The type of AD plant and the conditions in the digester.
• The purity of the feedstock.

The table below shows data on the potential biogas yields of the feedstock commonly used in anaerobic digestion (from a report by the Andersons Centre).

Permissions

• Planning permission is generally required, apart from small scale digesters using only on-farm waste, these may only require an Agricultural Notification.
• Depending on the size of the digester, either an environmental permit or exemption from the Environment Agency will be required.
• If you wish to spread the digestate onto land, it will either have to be PAS110 verified (it is then a product and not a waste) or a permit/exemption will be required.
• Animal By-Products permits may be required depending on the feedstock.

Income

Payment for power
Where excess power is diverted to the National Grid, a payment will be provided from 3p per unit. This is negotiable with the licensed supply (electricity company)

Use of the power generated at the farm or business will save around 10p/unit. In other words instead of buying electricity from a power company it can be used directly from the AD plant. 

Feed-in-Tariffs (FiTs)
AD facilities are eligible for the FITs. Facilities up to 250kW are entitled to 14.0 p/kWh and facilities of 250kW to 500kW are entitled to 13.0 p/kWh. The lifetime of the tariff is 20 years. These bands and tariffs take effect from 1 August 2011.

Renewables Obligation
The Renewables Obligation (RO) is the main support scheme for large-scale (>5MW) renewable electricity projects in the UK. A Renewables Obligation Certificate (ROC) is a green certificate issued to an accredited generator for eligible renewable electricity generated within the United Kingdom and supplied to customers within the United Kingdom by a licensed electricity supplier.
Anaerobic digestion is among the technologies that receive additional support in the form of multiple ROCs. Anaerobic digestion can receive 2 ROCs/ MWh.

How can we help?

If you need help to considered the above we can help. Once you have an idea about the type of operation you want you may then start to look at the technology available. We can help you identify reputable suppliers of biogas equipment. 

E4environment can carry out the following:

• Feasibility/business plan
• Funding and grant applications
• Planning applications including Environmental Impact Assessment
• Environmental permit applications
• Environmental Management System

Who we have worked with...

• South Shropshire District Council
• Biocycle South Shropshire
• Greenwold Energy, Driffield
• Harper Adams University College
• The Barrett's Mill Anaerobic Digester (BMAD) biogas plant

For further information please call Gemma on 01743 343403 or email gsf@e4environment.co.uk.