The phrase "harmless nitrogen" refers to nitrogen gas that does not cause any harm to humans, animals, or the environment. It is a way of emphasizing that nitrogen, in its pure form, is not dangerous or toxic. Full definition
«A bioreactor containing anaerobic methane and ammonium oxidizing microorganisms can be used to simultaneously convert ammonium, methane and oxidized nitrogen in wastewater into harmless nitrogen gas and carbon dioxide, which has much lower global warming potential.» (sciencedaily.com)
Periodically, during brief regeneration pulses, the nitrogen oxides are then desorbed, reacting with other exhaust gas constituents to form harmless nitrogen. (greencarcongress.com)
The method uses an aqueous urea solution bearing the tradename «AdBlue» to convert the NOx into harmless nitrogen via various chemical reactions in a special SCR (selective reduction) catalyst that has been optimized for NOx reduction. (sciencedaily.com)