TU researchers are testing ammonia from urine as a feed for fuel cells. The DHV-led project aims to make plants for wastewater treatment self-sufficient in energy.
A number of technologies have been developed to remove nitrogen and phosphates from sewage water as energy-efficiently as possible. The recently distinguished Anammox method developed by Professor Mark van Loosdrecht (AS) is one such technological application. DHV consultancy chose another tack: in an experimental plant near Groningen, they removed the nutrients chemically in order to optimise energy production. This DHV water innovation project was awarded the 2010 ‘Vernufteling’ award, as it saves energy and recycles phosphates at the same time.
The process involves adding magnesium hydroxide to the wastewater. The magnesium binds the phosphate and nitrate into an insoluble salt, which then precipitates. It is this salt, called ‘struvite’, which is transported from Groningen to Delft and stored in tanks for fuel tests.
“When you heat the salt, ammonia escapes,” explains Sander Tensen, who graduated on the overall system engineering. In fact, it’s a mixture of ammonia and water vapour that enters the system. When heated to over 750 ºC, ammonia will split up into its constituents: nitrogen and hydrogen. The high temperature fuel cell (solid oxide fuel cell) burns hydrogen with oxygen (from the air) while producing electrical power with an efficiency of about 50 to 60 percent. “And no nitrogen oxides or NOx’s are formed,” says project manager Dr P.V. Aravind, from the 3mE faculty’s process and energy department.
“Practice often proves to be a little more challenging,” says Tensen, as he explains that besides the ammonia, water vapour also enters the fuel cell. This not only dilutes the process but may reduce the performance of the fuel cell as well.
“The main thing is to get waste water plants self-sufficient in energy,” says Aravind. Especially in developing countries, sewage treatment is often interrupted by power cuts. Fuel cells fed with biogas from the sewage water treatment, as well as ammonia from urine, should be able to keep the plant going.