Nitroux oxide is almost 300 times as harmful as the greenhouse gas CO2 making it the most significant climate gas, at a modern wastewater treatment plant. A development project between Aarhus Vand and Nissen Energy demonstrates how nitrous oxide is converted into energy and eliminated through a gas engine.
In the auto racing motor sport, nitrous oxide boosts the perfomance of racing cars. At a wastewater treatment plant the gas is the biggest problem with regards to emissions of greenhouse gasses. These facts created an idea at Aarhus Vand and Nissen Energy A/S, a company producing gas engines for wastewater treatment plants.
"Initially we had the idea of uitilizing nitrous oxide, after reading an artickel from Aarhus Vand. Tt was about nitrous oxide being approximately 300 times as harmful as carbondioxide, and a big challenge at the wastewater treatment plants. If we could leverage the gas, as seen in the motor sport, and eliminate it, it would be really interesting" says Michael B. Nissen, CEO at Nissen Energy.
At Egå Wastewater Treatment Plant one of Nissen Energy's motors converts biogas from sludge to electricity and heat. Biogas forms in the tanks, and reject water is sent to a covered tank (anammox proces).
These are strong cases in terms of nitrogen - but also an opportunity.
The reject water has a high content of ammonia and emits high concentrations of nitrous oxide. This happens in a closed system, which is why, we found an opportunity to extract the nitrous oxide and utilize it.
For more than two years, Morten Bojsen has worked together with Nissen Energy on the development of a solution. The first step was to find out, whether nitrous oxide could be used.
"Initially, we made a pipe connection from the chimney on the tank, with the reject water, leading it outside and over to the intake of the gas engine. We saw that the particle emissions decreased significantly, but the air was getting too cold, condensed and water dripped into the gas engine, so we stopped the experiment." Morten Bojsen explains.
A more permanent solution was then found, in which the gas is essentially cooled in the ground and condenses in a well. The nitrous oxide is then sent through a filter and a new pipeline that ends at the gas engine’s intake. Today, the off‑gas (nitrous oxide and other air from the anammox process) makes up 10–20 percent of the combustion air supplied to the gas engine.
We quickly saw significant results. Nitrous oxide emissions were reduced to almost zero. At the same time, we have not observed any additional wear on the engine – on the contrary, it performs better than during traditional operation.
This is because nitrous oxide releases extra oxygen at high temperatures, resulting in cleaner and more efficient combustion.
At Egå Wastewater Treatment Plant, the new invention also provides a strong boost to the climate accounts. Greenhouse gases have been reduced by approximately 15% or 250 tons CO2‑e. Morten Bojsen explains that the setup uses minimal energy and has helped remove the largest single point source of nitrous oxide at the treatment plant.
Nissen Energy is in the process of securing a patent for the solution and sees opportunities for deployment both nationally and internationally. The goal is not to limit the technology to their own engines, but to create a model that can be broadly applied – under license or royalty.
The project demonstrates how close collaboration between utilities and suppliers can lead to concrete, scalable climate solutions.
“It’s rare to work on something where there are only benefits – environmentally, technically, and economically. But here, we’re very close,” Michael B. Nissen concludes.
Morten Bojsen, project manager, Aarhus Vand
Morten.bojsen@aarhusvand.dk
Phone: +45 2294 9851
Michael B. Nissen, CEO, Nissen Energy A/S
mbn@nissenenergy.com
Direct number: +45 2268 6507