Laboratory demonstration of DPM mass removal from an exhaust stream by fog drops

Abstract

Introduction Diesel engines have seen widespread use for well over a century due to their relatively high thermal efficiency and fuel economy (Heywood, 1988). Recently, however, the adverse health risks of diesel exhaust have become increasingly clear. The term diesel particulate matter (DPM) is used to refer to the solid components of diesel exhaust, which are an ultrafine mixture of elemental and organic carbon (EC and OC) and minor constituents including sulfates and metal ash (Kittelson, 1997). DPM is generally considered to occur almost entirely in the submicrometer range. It is classified as a carcinogen (Occupational Safety and Health Administration, 2013), and epidemiological studies have demonstrated a positive correlation between long-term exposure to DPM and other combustionrelated fine particulates and increased cardiovascular and pulmonary diseases (Pope et al., 2002; McDonald et al., 2011). Many of the risks of diesel exhaust are associated with the physical and chemical properties of exhaust components (Heywood, 1988; El-Shobokshy, 1994; Kittelson, 1997). Exposures are generally measured and regulated on the basis of mass concentration. However, DPM number density and particle size are increasingly recognized as critical factors in terms of health outcomes (Bugarski et al., 2012; Kittelson, 1997; Occupational Safety and Health Administration, 2013; Pope et al., 2002). Diesel engines operate in relatively fuel-rich/oxygen-lean conditions and are characterized by high emissions of particulates relative to those from spark-ignition engines (El-Shobokshy, 1994; Kittelson, 1997; Fiebig et al., 2014). Emissions from large equipment such as that used in mining applications typically range from 10 to 10 DPM particles per cubic centimeter (Kittelson, 1997). The physical and chemical properties of DPM vary with the type of engine, fuel and operating conditions such as loading, which is a function of torque and rotational speed (El-Shobokshy, 1994; Kittelson, 1997; McDonald et al., 2011; Bugarski et al., 2010; Huang et al., 2015). Loading is a particularly important factor with respect to DPM toxicity (McDonald et al., 2011; Stevens et al., 2009; McDonald et al., 2004) and the effectiveness of after-treatment technologies (ElShobokshy, 1994; Kittelson, 1997; An et al., 2012). Engine load alone can affect the EC/OC ratio, and the size distribution and number density of DPM. Light loads generally favor the formation of OC and small particles. As load is increased, the volatiles are oxidized, leading to larger soot particles (EC) but lower total number density of DPM. With further loading, the formation of soot offsets the decrease in volatiles, resulting in increased DPM mean size and number density (Kittelson, 1997). A significant body of work has been devoted to the development of DPMreducing technologies, including aftertreatments like oxidation catalysis and Laboratory demonstration of DPM mass removal from an exhaust stream by fog drops