Diffusion and hygroscopicity of particles from diesel and biodiesel combustion in an environmental chamber

Abstract

High humidity promotes the growth and agglomeration of particles, which accelerates particle deposition. The particles emitted from the natural combustion of diesel (B0), palm oil methyl ester (PME), soybean oil methyl ester (SME) and waste oil methyl ester (WME) in an environmental chamber were investigated. The particle size distributions of the four kinds of fuels were measured in the chamber at 50% and 75% relative humidity. In addition, the four kinds of combustion particles were microscopically analyzed, including SEM, contact angle and XPS to explain the hygroscopicity of particle surface. The results show that high humidity accelerates particle deposition, and coarse particles are easier to deposit than fine particles in most cases. The particle number density of the four fuels is mainly in the range of 0.3–1μm at different humidities. The peaks of the number density of 0.3-1μm particles corresponding to B0, PME, SME and WME are 3.79 × 106, 1.57 × 106, 1.20 × 106 and 0.76 × 106 #/m3, respectively. The number density of particles decreases with the increase of the humidity in most cases. It is noted that the particle number from B0 with the size of 0.3–0.5μm shows a secondary increase at about 330 min and the particle number with the size of 0.3μm at 75% humidity is higher than that at 50% humidity. The four kinds of particles possess superhydrophobic characteristics with contact angles of over 150°. The ratios of hydrophobic/hydrophilic functional groups of particles are PME, B0, SME and WME in descending order. It is found that the sulfur content and functional groups in the particles have an obvious influence on the hygroscopicity.