Effects of a diesel particulate filter regeneration process on aerosols in an underground mine

This study was conducted to evaluate the effects of a diesel particulate filter (DPF) regeneration process on the concentrations and size distributions of aerosols in an underground mine. Two active regeneration strategies were examined for an electrically regenerated sintered metal fiber filtration system, and the effects were compared with those observed for a muffler and the same system operated in a passive configuration. The effects were assessed for three steady-state engine operating conditions. The number and mass concentrations and size distributions of aerosols in mine air were found to be strongly affected by the exhaust configuration, engine operating conditions and the regeneration process. When the system was operated in active configurations, the concentrations exhibited a transient and cyclic nature. At light load conditions, the distributions of aerosols in mine air were dominated by accumulation mode aerosols for all exhaust configurations and regeneration strategies. High concentrations of nucleation aerosols were observed when the DPF system was used and the engine was operated at high load conditions. A positive correlation between accumulation of particulate matter in the filter and the efficiency of the filter in removing accumulation mode aerosols was observed for all conditions. In the case of high engine load, reductions in accumulation mode aerosol concentrations were linked to a substantial increase in number concentrations of nucleation mode aerosol. Overall, the filtration system was found to be very effective in reducing total aerosol mass and elemental carbon concentrations for all engine operating conditions. However, the system was substantially more effective in passive than in active regeneration configurations. Introduction The curtailment of diesel particulate matter (DPM) and toxic gaseous emissions using various exhaust aftertreatment technologies is considered to be an important component of an integrated strategy implemented by the underground mining industry to meet Mine Safety and Health Administration (MSHA) regulations limiting exposure of underground coal (66 Fed. Reg. 27864, 2001) and metal and nonmetal (71 Fed. Reg. 28924, 2006) miners to DPM. These technology-forcing regulations promoted the implementation of diesel particulate filter (DPF) systems in underground mines. DPF systems with wall-flow monolith and sintered metal elements are recognized to be effective in trapping DPM (Mayer et al., 2000; Vaaraslahti et al., 2004; Kittelson et al., 2006) and could be used for reducing exposure of underground miners to DPM (Bugarski et al., 2006a; Bugarski et al., 2006b; Bugarski et al., 2009). A rugged environment, a wide variety of specialized heavyand lightduty applications, confined space with limited ventilation and NO2-specific occupational exposure limits are some of the underground mining-specific technical challenges that need to be addressed prior to wide implementation of DPF systems in underground mines. A DPF element requires periodic removal of the collected DPM (regeneration) to prevent excessive engine backpressure buildup. Active DPF systems, similar to the one evaluated in this study, that are regenerated using electrical energy available on board the vehicle are a potential solution for a number of underground mining applications where equipment has been operated over medium-duty and lightduty cycles and exhaust gas temperatures are often below those needed for passive catalytic regeneration. These systems are also perceived as an alternative to the DPF systems with filter, prefilter and diesel oxidation catalyst elements wash-coated with platinum-based catalyst formulations, which are found to promote NO to NO2 conversion (Mayer et al., 2003) and have the potential for an undesirable increase in NO2 concentrations in an underground mining environment (Bugarski et al., 2006a; Bugarski et al., 2006b). The diesel particulates are, in general, distributed in accumulation and nucleation modes (Kittelson, 1998). The particle mass is controlled by accumulation mode agglomerates. In the presence of a pronounced nucleation mode, particle total number count is dominated by these particles. The solid core of accumulation mode particles is primarily made of elemental carbon, while semivolatile fractions are primarily made of organic carbon. Nucleation mode particles primarily consist of liquid particles (Vaaraslahti et al., 2004) that can be evaporated considerably between 150o and 230o C (Biswas et al., 2008) and might have a nonvolatile core with volatile species condensed on it (Rönkö et al., 2007). The nucleation mode particles downstream of catalyzed DPFs were found to consist mainly of sulfates (Kittelson et al., 2006; Vaaraslahti et al., 2004; Grose et al., 2006). Without a DPF, the formation of the nucleation mode was observed only at low engine loads, and those particles were found to be made primarily of hydrocarbon compounds (Vaaraslahti et al., 2004). Due to the very short duration of the regeneration process for a continuously regenerated DPF, the increases in emissions of regulated and nonregulated pollutants during that process are typically considered negligible compared to those produced during normal operating conditions. The DPF regeneration process was found to increase hydrocarbon, CO and particulate mass emissions (Bikas and Zervas, 2007). Information on the effects of electrical DPF regeneration on emissions is not readily available in the literature. The objective of this study was to evaluate the effects of an electrically regenerated sintered metal fiber DPF system on the concentration, size distribution and carbon composition of nano and ultrafine aerosols in mine air downwind of the exhaust discharge. Two active regeneration strategies were examined for an electrically regenerated sintered metal fiber filtration system, and the effects were compared with those observed for a muffler and the same system operated in passive configuration. Figure 1 NIOSH Diesel Laboratory in the D-drift of Lake Lynn Experimental Mine. Experimental methodology The DPF system with sintered metal fiber filtration elements (Model RH305-M) was supplied by Rypos Inc., Holliston, MA. This type of DPF is certified by the California Air Resources Board as a Level 3 Plus device for stationary emergency standby generator and pump applications and verified by MSHA (MSHA, 2009). The evaluated system had three stacks of active sintered metal fiber filter elements. Two of those stacks were equipped for active (electrical current) regeneration (RA). The third stack was not equipped with active components. Two solenoid actuated valves were used to control exhaust flow through the system. A diesel oxidation catalyst (DOC) with proprietary formulation was installed at the outlet of the DPF system. The system was originally designed to be operated on 12 VDC power vehicle’s system. In this study, the system was powered via a transformer from 220 VAC grid power. When power was not supplied to the system, both solenoids remained open and the system was operated in passive regeneration (RP) configuration. The system was evaluated in the RP configuration and in two RA configurations. For the first active configuration (RA1), the system was programmed to initiate two-minute regeneration in regular 14-minute time intervals. In the case of the second active configuration (RA2), the DPF system was programmed to initiate five-minute regeneration every time engine backpressure reached approximately 7.5 kPa (30 in. H2O). Prior to the start of the first evaluation run, the DPF system was conditioned in RA1 mode with the engine operating at intermediate speed and full load for approximately 30 hours. A baseline was established for each operating condition using a generic muffler installed in place of the DPF system. Those tests were noted as being conducted in the muffler configuration, as compared to the DPF configurations. Because of the sensitivity of diesel aerosols to sampling and dilution conditions (Khalek et al., 2000), the measurements were performed directly in a simulated occupational setting using the National Institute for Occupational Safety and Health (NIOSH) Diesel Laboratory (Bugarski et al., 2009). This laboratory has been developed in the D–drift of the NIOSH Lake Lynn Experimental Mine (LLEM) and is designed to allow evaluation of control technologies in an underground environment. The major components of the laboratory (Fig. 1) are an engine/dynamometer system, three sampling and measurement stations, and a ventilation measurement and control system. A mechanically controlled, naturally aspirated, directly injected Isuzu C240 diesel engine was coupled to the watercooled eddy current dynamometer. The engine was operated over three steady-state conditions (Table 1). Table 1 Engine operating conditions.