Understanding Net Energy Yields of Anaerobic Digestion Plants

Understanding anaerobic digestion energy yeild

By Francis Flinch (Own work) [CC BY 3.0],
via Wikimedia Commons

┬áThe net energy yield of anaerobic digestion plants, which is the sale-able energy left after all the sacrificial energy demands of running an anaerobic digestion plant are taken into account, is a subject of huge concern for all AD plant operators. Not only is the sale of the net energy the main income for most such biogas plants, but a plant’s claim to legitimacy as a low-carbon energy producer is also at stake.

Strictly, also the plant operator should be able to quantify the energy used in the construction and maintenance of his plant and carbon dioxide emissions caused, as that will have a bearing on not only lifetime biogas plant costs, but also be part of the whole-life plant energy (and carbon footprint) balance. Ultimately, it is this essential to understand how environmentally sustainable any anaerobic digestion plant truly is.

The net energy yield from anaerobic digestion is calculated from the methane output achieved under operational conditions, less the energy losses incurred during feedstock transport and handling, before the digestion process starts.

In addition, digester heating and mixing, and the energy costs of digestate handling and disposal need to be factored-in. Anaerobic digestion typically yields methane in quantities ranging from 0.2 Nm3 methane/ kg VSrem for low calorific value feedstocks, for example farmyard manures/ slurries, to more than 0.5Nm3 methane/kg VSrem for high strength feedstocks (e.g. food waste).

The key to optimising this yield is to optimise the volatile solids VS removal. In other words, to make sure that as much as possible of the organic content within the feed material is digested, and does not simply avoid being further decomposed in its passage through the process, and flow out in the digestate. Achieving optimisation requires that close attention is paid to operation and maintenance 24/7, and a healthy micro-organism population in the digester.

The internal costs of feedstock processing and digestion typically account for 15 to 20 percent of the energy yield. This cost is relatively consistent and unchanging for the life of any particular plant, as long as the feedstock type and digestate disposal routes remain unchanged.

The same cannot be said for the energy used for all activities in feedstock delivery and digest recycling, because this is solely dependent on the transportation distances travelled.

To illustrate this point, consider the degree of change should the source of the feedstock alters, or a digestate disposal route ceases to exist. In such circumstances, the net available energy may fall-off to unsustainable levels.

To avoid this biogas plant operators must see it as of the highest priority to negotiate secure sources of feedstock, and fall-back recycling routes for the digestate. Without this, anaerobic digestion plant owners/ operators might find themselves, at some future date, unable to prevent their biogas project descending into an energy deficit.

It is clear that an energy deficit at a biogas plant, if it was to occur, would render the project concerned unsustainable, and that coupled with a likely loss of business goodwill, is a result which it is all-important to guard against.

This in implies that biogas plant operators, who wish to safeguard their investment, must sign-up the producers of the organic waste they use as their feed material in-advance. The need is for long-term contracts, set up with terms compiled from professional advice, for stated quantities and costs of feedstock set-in advance, and the with the assured availability of farmland suitable for recycling the digestate.

Centralised Anaerobic Digestion Plants
On-Farm Anaerobic Digestion Plants

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