Fire exit staircase

It is no secret that in recent years the anaerobic digestion industry has been undergoing a period of change. There is a distinct sentiment that development has slowed and something needs to be done to bring new life back into the sector.

This fact is most clear in the German market which used to boast over 1,000 new plants a year; this metric has since dropped to below 150 in 2016. We see the effects of this trend in the UK as well with the recent receivership of big AD developer Clearfleau, and no end of reports that existing plants are under performing on a technical and economic level.

The government’s response to this, at least in the UK, was to bump up RHI (renewable heat incentive) figures last May. This was great short-term news for the industry and has certainly improved the situation; but it should not be forgotten that government subsidies like this have shown themselves to be inherently variable and they are generally reducing. And even more importantly; this will not last forever. Industry has also responded positively with European average plant performance increasing to over 89% in recent years.

At a recent Biovale AD group meeting I heard someone say, “The key to success in AD, regardless of feedstock, is a good plant manager; a good engineer that understands the proper mixing, feedstocks and retention times.” Indeed, it seems the industry has heeded this advice as plant performance continues to improve. However, this approach will only go so far. Many plants are designed on marginal economics and even a 10% deviation from planned performance can be enough to raise payback times above the 3-4 year ideal. To me, these solutions although great for the industry, treat the symptoms not the infection. A significant proportion of AD processes are not productive enough to drive the economics and the industry is suffering because of it. A technical solution is needed to increase the viability and stability of the industry.

The slowest step in an AD process is the hydrolysis stage where water reacts with the feedstock breaking it down into a more digestible component. In any process to speed up generation of products you need to focus on increasing the rate of slowest step; and indeed for AD there are many technologies that are able to achieve this. Enzymes can be added to catalyse the hydrolysis, steam can be injected to provide the energy for this reaction, ultrasonic devices work by creating cavitation in the fluid, separate material with specific nutritional components can be added to optimise the activity of the anaerobic microbes, thermophilic microbes can be used to run the reactor at a hotter temperature increasing gas generation, and the list goes on. Indeed, my own work is in developing industrial microwave technology for this purpose.

What is most exciting to me is that for the vast majority of existing AD sites there will be at least one technology that can be retrofitted. And with most technologies boasting a 25%-60% increase in performance compared to a standard process (be that gas output, reduction in required retention time or something else) there is huge potential to push existing AD assets beyond their original designs.

However, technology development is a tough business and only a few of the above systems have been successfully commercially deployed in the water/sludge industry and beyond. Of course, part of this is down to the “race to be second”. Plant operators and investors can have much more confidence in a technology when then know it isn’t first of a kind. Also, the benefits of speeding up an AD process are not always clear: you could run the reaction to a greater completion generating more methane, you could achieve the same gas yield in a much shorter time (this is particularly important for new plants), an existing AD system could be adapted to handle increased feedstock flowrates, less feedstock could be used to achieve the same gas output, and again the list goes on.

All these benefits mean different things to different people. As an example, I spoke to a farmer recently running a small CHP facility; he wanted to reduce his energy crop usage to free up his land for more productive use. The fact that the industry has a huge variety in customer needs on top of variations in plant design, scale of process and feedstocks makes this a very complex problem to solve. Despite this I remain positive that these solutions will find their place in the market. It is my hope that in 10 years it becomes standard practice for any AD site to come with a form of pre-treatment or process intensification. And with this; the industry can take a step forwards in the right direction towards improved profitability and sustainability.

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