Characteristics and formation of natural gas engine exhaust nanoparticles

Natural gas is an attracting alternative for diesel in piston engines due to smaller carbon dioxide and particulate mass emissions. However, also natural gas engines need techniques that reduce their emissions. Particle mass emissions of natural gas engines are small in comparison with diesel engines (Jayaratne et al., 2009). Yet, particle number emissions can be of the same order of magnitude as those of diesel engines without particulate filters (Holmén et al., 2002). Knowing the generation process and origin of the emissions helps in reducing them. Measurements (Alanen et al. 2014) were steady-state tests performed at an engine dynamometer with a passenger car petrol engine. Test engine was retrofitted to run with natural gas without exhaust after-treatment and with a selective catalytic reactor (SCR). The engine was not equipped with a turbocharger. Particle measurements were made using EEPS, Nano-SMPS and PSM with CPC. A thermodenuder (TD, maximum temperature 265 °C) was used to study particle volatility characteristics and Nano-SMPS without the neutralizer was used to study the electric charge state of particles. Particle emission sampling system consisted of a porous tube diluter, a residence time tunnel and an ejector diluter (Dekati Ltd). Residence time in the dilution system was 3 s. The dilution ratio over the porous tube diluter (PDR) during the measurements was six but also larger dilutions ratios were tested. Results indicate that natural gas engine exhaust particles are initially formed in engine cylinders and they increase in size during sample dilution and cooling process. The growth occurs by condensation of gaseous compounds in exhaust gas if the conditions during primary dilution process are favourable. A small fraction of the particles carry an electric charge. Particles carrying electric charge have been charged most probably in the high temperatures of the engine cylinders. Thermodenuder volatility measurements suggest that the particles have a non-volatile core but a notable part of their volume consists of volatile matter. SCR influences particles by reducing their number concentration. This can result from oxidative reactions of hydrocarbons taking place inside the SCR or from diffusion of particles on its walls. Particles emitted from the natural gas engine were extremely small with the particle size distribution peak at about 4 nm. Number of particles with a diameter larger than 50 nm was very low (Figure 1). This was the first time the electrical charge of the particle emission of a natural gas engine power plant was investigated or particles under 4 nm in the particle emission of a natural gas engine were measured. Also the observation of particle size reduction at 265 °C rather than complete evaporation was made for the first time in this work. Figure 1. Natural gas engine exhaust nanoparticle size distribution measured by PSM, Nano-SMPS and EEPS This work was supported by the The Finnish Funding Agency for Technology and Innovation (TEKES), Neste Oil Corporation, Wärtsilä Finland Oy, Dinex Ecocat Oy, AGCO Power, Dekati Ltd, Viking Line, Suomi Analytics Oy and Gasum Gas Fund. Alanen, J. et al. Formation and characteristics of natural gas engine exhaust nanoparticles. Manuscript in prep. Holmén, B. A, & Ayala, A. (2002). Environ. Sci. Technol., 36, 5041-5050 Jayaratne, E. R.., Ristovski, Z. D. Meyer, N. and Morawska, L. (2009). Science of the Total Environment, 407, 2845-2852.