Following up on its ambitions, Yara Porsgrunn has managed a successful reduction in both laughing gas - nitrous oxide N2O - emissions and operating costs.
"Laughing gas - nitrous oxide (N2O) - was found during measurements in the stack of Nitro phosphate plant #2 in Porsgrunn," explains principal engineer Olav Lier in Yara Innovation and R&D. "N2O is a greenhouse gas, 310 times more powerful than CO2. Further measurements also found similar emissions in Nitro phosphate plant #3."
Further tests and a search in the archive showed that the N2O emission is mainly caused by adding urea to the digestion tanks. "Urea has been added since 1973, to reduce NOx emissions," Olav says. "Urea reacts with NOx, forming nitrogen gas. Unfortunately, urea also reacts with nitric acid in a similar mechanism, forming N2O."
Continuous stack measurements of NOx and N2O were established in both plants. Statistical analyses of NOx and N2O emission data, combined with a wide range of process data, confirmed the importance of the urea feed rate on these emissions. Decreased urea addition reduced N2O emissions - but increased NOx emissions.
"Previously the urea feed was manually set, based on manual observation, to avoid yellow (from NO2) ventilation air from the stack," Porsgrunn plant Technical Manager Johanne Theie explains. "Overdosage of urea was common, and periodical sampling and analyses confirmed the NOx emission was well below limits."
Based on this information, a control loop was established, dosing urea to keep the NOx emission at a constant level, well below limits. The result - a reduction in N2O emissions of 100,000 tons CO2 equivalents. Lower urea addition levels also reduced operational costs by 2 MNOK per year.
This was made possible by fine teamwork between Yara Porsgrunn and Yara Innovation and R&D, throughout the stages of technical analysis that led to the improvements.
"We gained important knowledge through this excellent collaboration with Innovation and R&D and Yara Porsgrunn," Yara plant manager Per Knudsen says. "The installation of on-line analyzers, enabling us to use instrumentation/process control systems to minimize emissions from an operational point of view, was a key success factor."
“This is a great example of how we Create Impact while doing business. Our company’s aim is to combine business goals and at the same time tackle global challenges," says Per. "This solution adds value for Yara and reduces stress on the environment - and shows how we can use our know-how to differentiate us from our competitors."