Feedstocks

Feedstocks Print E-mail

The material that is used in anaerobic digestion is called feedstock.  What goes into a digester determines what comes out, so careful choice of feedstocks is essential.  Securing a reliable feedstock supply is fundamental to profitable AD and if feedstocks are to be bought from an third party securing long-term contract on acceptable terms is critical.

The feedstock doesn't have to be waste, any biodegradable plant or animal matter that is not woody is a suitable feedstock for a digester.  However, anaerobic micro-organisms cannot break down lignin, the complex polymer that gives plants their strength, which means that wood products, paper and straw will slow the digester.  Some examples of feedstock streams are:

  • Slurry or manure
  • Domestic or commercial food waste
  • Food processing (industrial) waste
  • Crops grown specifically for AD, as silage

A list of biodegradable wastes that can be used in AD can be found in the new Quality Protocol for anaerobic digestate.


Wastes

It is estimated that the UK generates over 16 million tonnes of food waste per year that, if sent to landfill, will emit methane, a powerful greenhouse gas. There are several options for diverting this wet organic waste from landfill: home composting; large-scale composting, either in-vessel or windrow; thermophilic aerobic digestion; and AD.

Approach Takes all food waste Takes garden waste Reduces emissions Saleable product Energy Recovery
Home composting - + + - -
Industrial composting + + + + -
Thermophilic aerobic digestion + - + + -
Anaerobic digestion + - + + +


AD is the only one of the four options that produces renewable energy as well as recovering the nutrients in the waste in the form of digestate. AD costs will fall as the industry expands and new incentives are introduced. Combined with the escalating cost of landfill, this could make AD of wastes more attractive.

A list of biowaste types suitable for AD can be found in Appendix B of the Quality Protocol for Anaerobic Digestate.

The Organic Waste and Recycling Market in the UK

WRAP have several reports and case studies on the treatment, use and availability of waste including;

 

Crops

Purpose grown high biomas crops can be grown specifically for ananerobic digestion, for stabilising or supplementing other feedstockssuch as low yielding slurries or variable quality food waste. Such crops include maize and grass silage for example, both of which can be incorporated into existing crop rotations or management regimes.

Cereals and rape meal can also be used as AD feedstocks, giving high biogas yields, but as they are expensive commodities they are generally not grown solely for digestion. They can be used to provide a short term yield boost or where they would achieve a low return elsewhere, e.g. poor quality grain.

A database of reported values for methane potentials for a range of European crop materials and residues is available from the University of Southampton Bio-energy and Organic Resources research group website. Here you will also find a useful on-farm energy and emissions calculator which can be used for calculating the energy requirements and GHGs for crop production and potential energy generation using an on-farm AD plant.

From October 2013, the electricity generated by any UK anaerobic digester above 1MWe capacity that uses crops as a feedstock, must achieve a minimum greenhouse gas (GHG) saving of 60 per cent relative to fossil fuel to receive Renewables Obligation Certificates ( ROCs) under the Renewable Obligation. To support this requirement, Ofgem has launched a GHG reporting tool for use by generators. A complementary approach is planned for the use of biomass under the Renewable Heat Incentive.


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