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Crops for energy: how to optimise AD yields

With the demand for renewable energy increasing, AD plant operators should be looking to get their plants to maximum efficiency. Alice Dyer finds out how to harness the most energy from feedstocks.

Crop-fed anaerobic digestion (AD) plants not only provide additional income streams to farming businesses, but give alternative crops a place in the rotation. But to improve return on investment, attention to detail in AD is important in gaining more control of the process and increasing biogas yields. Tim Elsome, general manager at FM Bio Energy offers his top tips for increasing AD plant productivity.


1. Tweak your feedstock

Certain feedstocks and materials are notoriously difficult to process such as grass and whole crops, so ensuring the crop is correct for the plant, and is of good quality, will make all the difference to a plant’s productivity.


This means looking at crops that are best suited to the AD plant, and crops that are easy to grow regionally, or easily accessible at a reasonable price.


“Plan future diets carefully around what the plant can process,” Mr Elsome advises. “It is probably too late to select the biogas plant around the crops you grow, but you still have the opportunity to select the crops around the type of plant you have got.


“One of our customers is a prime example of a badly designed project – the plant ran brilliantly on maize but being based in the North East where they can’t grow maize, the owner either had to buy it in or process grass and rye which is very difficult to digest.”


More fibrous and viscosity increasing crops are harder to break down. The grass and rye mixture created biological instability in the plant and mixing and heating issues, meaning it was running at just 40% efficiency.


“In this case we adapted the feedstock to a mix of equal parts maize, grass and whole crop cereals, with a micro/macro enzyme which helps to break them down in the digester, enabling the plant to better cope with these complex feedstocks,” says Mr Elsome.


To allow for the best quality crop to be fed into the plant, harvest should be carefully timed so the crop’s dry matter is in a good range, and the right chop length is met for those materials.


If AD owners are struggling to achieve 100% output due to biological instability or mixing issues such as high viscosity or crusting, they should speak to a consultant who can analyse the plant, provide guidance on the biological health within the digester and advise on different feed options to achieve full output, Mr Elsome says.

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2. Clamp management


Once the crop has been harvested, good storage management is important in preventing losses over winter, and utilising the total crop harvested.


Mr Elsome says: “Arable farmers are not always used to storing materials, but because they will be growing a harvest to last 12 months, they need to make sure storing practices are correct to reduce losses.”


Feed crops must be well compacted and sheeted down to ensure clamps are air tight and aerobically stable, and a biogas-specific silage additive should be included, he says.


“A biological agent to enhance and improve fermentation should be applied at the point of harvest to reduce storage losses and prevent clamps from heating up when you’re feeding it out. This means the clamp can be opened in as little as two weeks, and also prevents yeast and mould growth which are the causes of the clamp face reheating.”


Clamps should be fed out in a way that exposes the minimum amount of surface area to oxygen, because oxygen ingress will cause the face to heat up, translating to energy losses of 3% per day, says Mr Elsome.

3. Monitor the plant regularly


All biogas plants should be regularly analysed to ensure there is no biological instability and that the gas yield and quality are as expected for the feed input.


“For a crop fed plant, mixing is absolutely critical,” says Mr Elsome. “You can end up with floating or sinking layers, or balls of material floating round, so making sure it is a well homogenised tank is what you are trying to achieve.”


AD plant operators should look in the tank at least once a day to make sure feed is sufficiently mixed and temperatures are where they should be, adds Mr Elsome.


“AD plants are complex but often value-engineered. Feedstocks, digestate and gases can be corrosive and the conditions inside mean that sensors often fail or give false readings. These need to be manually checked and calibrated to ensure the operator has the best information to hand in order to take the correct decision.”


Getting the biological health of the AD plant analysed on a regular basis will flag up any impending problems with the biological aspect of the plant. Mr Elsome recommends this is done every two weeks, unless there is an instability issue.


“Then it should be every week. Normally once every two weeks gives us time to spot an issue before it becomes a problem. If the plant has bigger issues with performance, then samples should be sent to the lab twice a week until a solution has been found.


“It is also important that you have someone who can interpret those lab results to see where problems are potentially arising. Always make sure your AD consultant is suitably trained, with a knowledge of the biological process,” he adds.

4. Feed the feed


For crop fed plants, there is always a demand for trace element supplementation and any deficiencies will be picked up in the regular lab analyses.


A micro/macro nutrient additive supports the plant’s biology by creating optimal digester conditions for the bacteria to reproduce.


“However, it is very important to use a mix that is bespoke to your plant,” says Mr Elsome. “This ensures that not only are there no deficiencies, but also no excesses, as some nutrients can be toxic in high concentrations.


“Operators should certainly be supplementing the nutritional content of the digester with trace elements, but it is also worth considering other things like enzymes, but only as required.”


There are two reasons for using enzymes – to produce gas faster, which is ideal when the retention time is short or the organic loading rate is high; and when the digester gets very thick or viscous and hard to mix. An enzyme additive will help to help break that down and keep the tank flowing.

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