Aerosol Particle Emissions from Natural Gas Engines

Fine particle pollution emitted from anthropogenic sources such as traffic and energy production causes globally millions of premature deaths annually and, for example, cardiovascular and lung diseases. Airborne particles also contribute to the greenhouse effect either by absorbing or reflecting radiation and by serving as cloud condensation nuclei. In this thesis, the particle emissions from two retrofitted natural gas engines – one the size of a passenger car engine and the other the size of a ship’s engine – from the exhaust line to the aged exhaust in the atmosphere are presented.

The size distribution measurement of tiny particles (1–5 nm) proved to be very important: With the smaller-scale engine, the particle emission was dominated by particles with a number size distribution peak diameter of 1–5 nm. With the larger engine, a high mode of nonvolatile core particles below 3 nm was found. All natural gas exhaust particles did not evaporate in thermal treatment, but only decreased in size, forming a trimodal size distribution that comprised a fuel-originated core mode with
A flow-through reactor simulating the atmospheric photo-oxidation of several days was utilized to study the secondary aerosol formation potential of the exhaust. The measured formation potential of secondary aerosol mass was substantial, 1-2 magnitudes larger than the fresh exhaust particle mass and at a similar level to the secondary aerosol formation potential of clean diesel or gasoline engines. It was concluded that the urea used in the exhaust after-treatment system was the main source for secondary inorganic (mainly ammonium sulfate and ammonium nitrate) and the lubricating oil for the secondary organic aerosol mass.

This thesis produced a novel understanding of the number concentration, size distribution, composition, volatility, morphology, electric charge, and origin of particle emissions from natural gas engines and the effect of catalysts on the emissions. Also, new knowledge on the secondary aerosol formation potential of natural gas engine exhaust and its composition and volatility characteristics was obtained.