Ammonium nitrate nanoparticle emissions from ammonia-fueled internal combustion engines

Abstract

When synthesized from renewable energy, anhydrous ammonia (NH₃) is a zero-carbon fuel; therefore, by definition, its combustion produces no carbonaceous soot particles. However, this study reveals that ammonium nitrate ultrafine particles are emitted in significant quantities from ammonia combustion in engines. In the experimental work, a retrofitted single-cylinder Cummins ISB6.7 engine was operated at 1200 RPM and 1800 RPM at 50 % load. Gaseous emissions were measured using a Fourier transform infrared (FTIR) emissions bench, while particle size distribution was assessed using a scanning mobility particle sizer. Size-separated particles were collected using a MOUDI impactor and characterized to identify the chemical composition using attenuated total reflectance-FTIR, Raman spectroscopy, and X-ray photoelectron spectroscopy techniques. Additionally, particle morphology was studied using transmission electron microscopy, while energy-dispersive X-ray spectroscopy mapping was performed to confirm elemental composition. The particle size distribution indicated a monomodal lognormal distribution with diameters ranging from 6.15 nm to 224.7 nm. The geometric mean diameter was 30.6 nm at 1200 RPM and 28.9 nm at 1800 RPM. The corresponding total concentrations were 2.5 × 10⁶ and 3.4 × 10⁶ particles/cm³, respectively. Characterization results indicate that the measured particle emissions primarily consist of ammonium nitrate, potentially synthesized in combustion through the reaction of unburned ammonia with nitrogen dioxide. Elemental maps show nitrogen and oxygen, indicating the presence of nitrates, along with sulfur, magnesium, potassium, and calcium, likely originating from lubricant oil additives. One formation mechanism is believed to involve heterogeneous nucleation, during which particles are adsorbed onto existing ash particles containing inorganic metals originating from the entrainment and combustion of lubricant oil.