Photo: Colourbox

Boosting the efficiency of biogas plants

Thursday 11 Apr 19
by Morten Andersen


Hariklia N. Gavala
Associate Professor
DTU Chemical Engineering
+45 45 25 61 96


Ioannis V Skiadas
Associate Professor
DTU Chemical Engineering
+45 45 25 27 29
Laboratory experiments have shown pretreatment with ammonia able to increase methane production from manure fibres by more than 200 per cent. Pilot Plant tests will take the method close to full-scale implementation.

The amount of methane produced from the solid fraction of swine manure has been increased three-fold under lab conditions at DTU Chemical Engineering. In collaboration with a range of Danish biogas industry operators and consultants, the cross departmental Pilot Plant facility at DTU Chemical Engineering is about to make the method operational.

“Upscaling the process from lab scale to full-scale is by no means straightforward. Ammonia is a pollutant. In the lab it is easy to handle ammonia, but in full scale with much larger amounts of ammonia involved, this is more challenging. Here, we see the value of having pilot plant facilities,” says Associate Professor Ioannis V. Skiadas, heading the new project DEMONIAGAS (Demonstration of enhanced methane production from ammonia-pretreated biomasses in biogas plants) for DTU Chemical Engineering. The project is funded by the Danish Energy Agency through EUDP (Energy Development and Demonstration Program).

While swine manure is available in large quantity in Denmark, the methane potential of the manure is too low for economically feasible biogas production. This has been addressed by mixing manure with other biomasses with a higher methane potential such as slaughterhouse wastes, energy crops, and fish oil. However, increased demand for biomass feedstock for other applications has raised prices of these types of biomass. Therefore, it is highly desirable to utilize the methane potential of the manure by making the fibre content already present in the manure available to the biogas producing microbes.

"In the lab, it is easy to handle your ammonia, but in full scale with much larger amounts of ammonia involved, this is more challenging. Here, we see the value of having pilot plant facilities."
Associate Professor Ioannis V. Skiadas

Preparing a meal for microbes
In the DEMONIAGAS project, the fibres are pretreated with ammonia.

“In analogy, if we as humans were to eat paper, we wouldn’t achieve any nutritional benefit. This is because, unlike cows for example, we lack the biological mechanisms necessary to digest cellulose fibres. But if the paper is suitably pretreated, sugars can be produced from the cellulose and these will have nutritional value for us. What we are doing in the project is basically the same thing, only are we pretreating the fibres to make them available for microbes to digest,” explains Associate Professor Hariklia N. Gavala, responsible for the bio-process engineering component.

The researchers have named the process Aqueous Ammonia Soaking (AAS). In a previous research project, the methane yield of manure fibres was increased almost 200 per cent in less than 20 days digestion at 37°C. The project also looked at adding extra sources of cellulose, primarily wheat straw. By subjecting the straw to ammonia treatment, its methane potential was raised by 50 per cent.

“So, this is very encouraging, as straw is a cheap feedstock. However, there is a limit to how much straw you can introduce. If one adds solid material in too large quantity, the slurry becomes difficult to pump,” says Ioannis V. Skiadas, noting that the project will also demonstrate a more economically feasible way to run the process. “Unlike in the lab, a full-scale process should preferably run continuously. Also, continuous production is more economical because stops between batches are avoided, just as are the start-up procedures for production of the next batch.”

A model to assist decisions by operators
Finally, the precise pretreatment needs to be adjusted for the type of biomass added to the sludge.

“Conditions such as temperature, duration, ammonia concentration, and solid/liquid ratio should be optimized for the biomass type. We plan to develop a model capable of telling the operators which parameter values to select,” Hariklia N. Gavala informs. “Further, the model will tell you what the maximum methane production could be in each case. This may assist business decisions by the plant operator.”

External partners in the DEMONIAGAS project are Madsen Bioenergi, Danish Gas Technology Center (coordinator), EnviDan, and Lundsby Biogas. The demonstration project follows up on a recently completed research project, AMMONOX, coordinated by DTU Chemical Engineering and funded by
27 MAY 2019