A Laboratory Investigation of Abatement of Airborne Diesel Particulate Matter Using Water Droplets

Abstract

The term diesel particulate matter (DPM) is used to refer to the solid phase of diesel exhaust, which is mainly composed of elemental carbon and organic carbon. DPM is generally in the nano-size range (i.e., 10-1,000 nm). Occupational exposure is a health concern, with effects ranging from minor eye and respiratory system irritation to major cardiovascular and pulmonary diseases. Significant progress has been made in reducing DPM emissions by improving fuels, engines and after-treatment technologies. However, the mining industry, in particular, remains challenged to curb exposures in some operations where relatively many diesel engines are working in confined environments with relatively low airflow.

Basic theory and a limited amount of prior research reported in the literature suggest that water sprays may be able to scavenge airborne DPM. The goals of the work presented in this thesis were to build an appropriate laboratory set up and to test the efficacy of micron-scale water (or fog) droplets to remove DPM from an air stream. The general experimental approach was to direct diesel exhaust through a chamber where fog drops are generated, and to measure DPM up- and down-stream of the treatment.

Initially, fundamental experiments were conducted to explore the effect of the fog drops on the removal of (electrically neutralized) DPM from a dry exhaust stream. Compared to no treatment (i.e., control) and with the use of a diffusion dryer downstream of the fog treatment, the fog improved DPM removal by about 57% by mass and 45% by number density (versus no treatment). Without the use of the diffusion dryer, improvement in DPM removal was about 19% by mass. Analysis of the results suggests that a likely mechanism for the DPM removal in this experimental system is thermal coagulation between DPM and fog droplets, followed by gravitational settling and/or impaction of the droplets with system components.

Further tests using raw exhaust (i.e., neither dried nor neutralized) having a higher DPM number density; shorter residence times; additional fogging devices; and no diffusion dryer downstream of the fog treatment were also carried out. These yielded an average overall improvement in DPM mass removal of about 45% attributed to the fog treatment (versus no treatment). The significant increase in DPM removal in these tests compared to the initial test (i.e., 19% removal by mass) cannot be fully explained by differences in residence time or DPM and fog droplet densities. Increased humidity in the system (due to the undried exhaust) may have allowed for a larger mean droplet size, and therefore might explain more rapid settling of DPM-laden droplets. Another possible contributing factor is ambient surface charge of the DPM, which might perhaps result in more efficient attachment between DPM and fog drops and/or increased deposition loses in the system.