Feedstock and Anaerobic Digestion Process
By Steve Last
The feedstock is everything to an Anaerobic Digestion Process, and the biggest successes will come from close attention to the volume, nature, composition and variability in composition, of the available feedstock, or range of feedstocks to be accepted, from the outset when planning an AD Plant.
This is where the crop fed biofuel AD plant designer starts with a huge advantage on the rest of the AD developers. There will be seasonal variations from the available crops of course, unless year-round storage can be provided for total feedstock inputs, which may be feasible for silage feedstock use, for example.
Sewage sludge AD and agricultural waste fed AD processes are possibly the next in security of feedstock provision. Where anaerobic digestion plants are built to process industrial wastes these are also reasonably secure in the knowledge of the characteristics of their feedstock materials.
Image: Feedstock Conveyor at the Greenfinch Plant, Ludlow, UK
However, things can get really difficult for AD Plants being built to process primarily food waste from householders within the council collection service. When these plants are built it is at present an act of faith to know to what extend the public will segregate their wastes to send them to the AD Plant rather than include these materials in the “black bag” mixed waste stream, or instead even send these materials for composting in the green/garden waste recycling bin.
Furthermore, how does the designer predict what proportion of high calorific value/high methane generating food waste will arrive within the material collected for Anaerobic Digestion? In time maybe it will be possible to predict but this is a doubtful art at present.
If the AD designer gets it wrong the result will be that methane production rates may be at a lower or higher rate than predicted, which (if low) can affect economic viability.
The solution for AD Plants which do not have the ability to directly control their own incoming feedstock quality, is to diversify if possible on feedstocks utilised, to provide the plant operator the flexibility to accept a number of different feedstocks available in season.
Feedstock choice is also then likely to vary over time, with the operator choosing to accept different feedstocks at economic prices as market forces act on their market.
We have discussed the range of different AD plant biogas feedstocks available and suitable for agricultural and waste anaerobic digestion plants elsewhere on this web site here.
If there is a specialist technology provider which specialises on a feedstock and you seek further information, you may continue to our anaerobic digestion technology supplier table.
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