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Increasing biogas yield from manure

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Posted November 6, 2018

We have plenty of manure in Denmark, but we are having a hard time meeting the energy policy goals of utilizing the manure as a raw material in the production of biogas.

In collaboration with a number of businesses, researchers at DTU Chemical Engineering  have now developed a method that makes the animal manure significantly more fit for purpose.

Image credit: EnviDan via DTU

“In colloquial terms, we use chemistry to ’chew’ the parts of the fertilizer that is hard to reach for the microbes, so they can use all the organic content to produce biogas,” says MSc Kim Paamand, Head of Development of Biogas at EnviDan, which is one of the largest consulting engineering companies in Denmark within the areas of the environment and energy.

In 2009, the then government set the goal that half of the manure in the country should be used as raw materials for the production of biogas by 2020. But according to Foreningen Biogasbranchen, the proportion is less than 10 per cent today.

“Fifty per cent wasn’t very ambitious, but as it turns out, we’re very far from achieving that goal. But the extra gas exploitation yielded by the new method is really significant,” says Kim Paamand.

Problem turned into benefit

There are several reasons why the process of using manure for biogas is slow. One of them is that the manure contains significant amounts of materials which are difficult to digest for the microorganisms in biogas plants. These are partly undigested plant material from the animal’s food and partly deep litter—the straw strewn on the stable floors.

Straw and similar plant material contain the matter lignocellulose, which microorganisms have a hard time breaking down.

“As long as you only feed the microbes easily digestible organic waste such as fat, alcohol and sugar, they will practically eat it all up. But for lignocellulosic biomass, they only use 60-70 per cent. This results in a smaller biogas production, and you’re also left with by-products. Both of these hurt the operating economy,” explains Kim Paamand.

It is also a problem that manure contains large amounts of ammonia—with the chemical formula NH3—from the animals’ urine. Ammonia is unwanted in a biogas plants because microorganisms are sensitive to the compound.

However, researchers at DTU Chemical Engineering have turned this problem into an advantage. They use the ammonia to soften the lignocellulose in the fertilizer. This technique has been named Aqueous Ammonia Soaking (AAS).

Link to old farmer’s trick

After the pretreatment with ammonia, the laboratory has succeeded in tripling the production of gas from lignocellulose in the fertilizer. In other words: Two-hundred per cent extra output.

”The existing Danish biogas plants are built in a different way than our laboratory set-up,” says the head of the project, Associate Professor Ioannis Skiadas, DTU Chemical Engineering.

He continues:

“Therefore, we cannot go straight out and get as large an output on the plants, but a 25-30 per cent larger output is definitely realistic. In that case, it will mean a significant improvement of the operating economy. This will, in turn, contribute to a larger part of the manure being used for biogas, which is the aim of Danish energy policies.”

Even though it has required extensive research to develop the method, Ionnis Skiadas explains that a similar practice was used in Danish agriculture back in the middle of the last century:

“Until 50 years ago, it was common for the farmer to treat stacks of straw with ammonia. Back then, the purpose wasn’t to produce biogas but to make the straw more easily digestible for the cattle. After all, straw was a cheap form of feed. The pretreatment we’re doing now is basically the same. We’re simply preparing a meal for microorganisms instead of cattle.”

The old practice got a bad name because ammonia would steam out into the surroundings. It caused environmental and health problems, so the method was prohibited.

“There’s, of course, no ammonia leaks in our method. We capture the steam and reuse the ammonia to soak plant materials,” stresses Ioannis Skiadas.

Let’s close the ring

EnviDan has designed and managed the construction of a number of biogas plants. Here, Kim Paamand sees a great need for the new method:

“The method is attractive because the entire infrastructure in the form of biogas plants and the transport of fertilizer and other raw materials has already been established. In other words, you have already paid for your ingredients but now you can make more out of them. Of course, there will be some expenses because we first need to add ammonia only to extract it later, so it doesn’t interfere with the microbes. But based on the laboratory results, we expect the overall operating economy to be good. We now need to demonstrate this when we take the method a step further in pilot projects in the industry.”

Kim Paamand adds that the method extends beyond the utilization of manure:

“The method can also be used to soak straw, grass, and other plant materials that are hard to digest in ammonia. Additionally, it’s not only a question of producing renewable energy. It’s also about closing the ring, so we’ll activate the nutrients and energy in the by-products instead of viewing them as a waste problem.”

Source: DTU

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