Efficient anaerobic digestion plant design

Jon Lawson

Martin Grant explains how to maximise Renewable Heat Incentive payments

The future of the UK renewable energy sector is at a crossroads, with some renewable energy technologies being favoured more than others. Renewable heat generation in particular, is a primary focus for the UK Government. However, the favoured technology path ahead is still not clear, but whichever route is taken, cost effective operation will be a key factor of success.

In the recent autumn 2015 spending review the UK Government stated that it will increase funding for the Renewable Heat Incentive (RHI) scheme to £1.15 billion in 2021 to ensure that the UK continues to make progress towards its climate goals, while also reforming the scheme - delivering intended savings of almost £700 million by 2020-21.

In terms of anaerobic digestion (AD), the present RHI payments involve calculations that consider not only the overall heat and energy generated, but also the quantity of this that is re-cycled and used to sustain the anaerobic digestion process itself. Therefore payments made for energy generated are calculated on a net rather than gross basis. The heat used for sustaining the anaerobic digestion process is deducted from the overall heat output generated and the net surplus is used to calculate the final RHI payments. Clearly, on this basis, being able to maximise the efficiency of the anaerobic digestion process, using the least amount of energy to sustain efficient digestion and biogas release, will have a big impact on the net energy generation, and therefore payments received.

So what are some of the key factors that can affect this efficiency?

Overall thermal efficiency of the heat transfer required to support the anaerobic digestion process is a critical factor in efficient AD plant design and operation. Key technical considerations that affect this, in terms of effective thermal conductivity, and therefore operational efficiency, primarily include the materials used in tank design and the amount of surface area through which heat transfer takes place. For AD tanks these are steel, concrete and glass. In terms of these materials, steels are by far the most efficient for thermal conductivity. As a comparison, the thermal conductivity of stainless steel (~16 W/m.K) is considerably higher than either glass (~0.8 W/m.K) or concrete (~1.5 W/m.K), both of which have traditionally been used in UK AD plant design. Similarly, heat transfer will be both more consistent and more efficient through a large surface area than a small one. So in terms of optimising AD plant design, we can see that transferring heat to the digestion process through large surface area stainless steel tank walls is considerably more efficient than through small internal heating pipes, concrete sections or glass lined metal panels.

Lipp AD plants utilise a design involving spiral form tanks that give a rapid build time and very importantly, as we have seen, smooth stainless steel inner walls with an integrated external heating system – the walls being made from Lipp’s own stainless steel composite, Verinox. These carefully considered design features are integral to creating one of the most globally successful and thermally efficient AD systems available.

Lipp has installed several thousand AD systems in 80 different countries. With this wealth of experience we are very well equipped to deal with the future renewable energy challenges in the UK market, which this year has just celebrated the building of its 400th AD plant.

Martin Grant, is Lipp’s UK Marketing and Sales Manager