In response to new exposure standards to lower miners’ exposure to diesel particulate matter (DPM), the National Institute for Occupational Safety and Health (NIOSH) and others have conducted research into control technologies to reduce DPM emissions. The mining industry—and specifically dieselized mines—also continue to work toward finding feasible controls to implement in their mines. Although emissions of and exposure to DPM can sometimes be controlled through the use of newer diesel engines, better engine maintenance, use of alternative fuels, or ventilation upgrades, some mines may need to use diesel particulate filters. However, this has created concern about potential exposure to nitrogen dioxide (NO2) resulting from their use.
NO2 is a deep lung irritant and the Mine Safety and Health Administration (MSHA) has set a ceiling value (a concentration that shall not be exceeded even instantaneously during a shift) of 5 parts per million (ppm). NO2 is emitted from a naturally aspirated diesel engine at a concentration of between 50 and 100 ppm. These concentrations are subsequently diluted by mine air resulting in lower concentrations in the workplace air. However, the use of diesel particulate filters can cause up to a three-fold increase in the concentrations of NO2 emitted from the tailpipe, with a corresponding increase in the concentration of NO2 in the work area.
Diesel particulate filters have been shown to be highly efficient in filtering DPM, removing up to 99% of DPM from diesel engine emissions. Figure 1 shows a schematic of the basic concept of filtering where engine emissions (blue arrows) enter at one end of the filter and then must physically flow through the filter wall, which removes the DPM, before exiting the filter. Figure 2 presents a picture of both the inlet and outlet of a filter showing that the black diesel soot is only seen on the inlet side of the filter. Disposable and nondisposable filters have been successful in efficiently removing DPM on mining equipment. After use, the disposable filters are simply removed and thrown away. Disposable filters have not been shown to cause an increase in NO2 tailpipe emissions.
However, nondisposable filters must have a way of removing the DPM that has accumulated on the filter, a process called regeneration. Active regeneration uses an external heat source to burn off accumulated DPM. For example, the filter can be physically removed from the equipment and the DPM burned off in an oven. Once this process is complete the filter can be reinstalled on the equipment. Diesel particulate filters relying on this off-line regeneration have not been shown to cause an increase in NO2 tailpipe emissions.
The increase in NO2 emissions resulting from the use of highly catalyzed filters on engines of model year 2007 or newer on-highway pick-up trucks is also a concern in the mining industry. Western coal mines use these trucks as transportation and utility vehicles inside the mines. NIOSH recommends the use of newer modern engines as part of the solution to controlling DPM in underground mines. Environmental Protection Agency (EPA) data for these trucks show a significant reduction in tailpipe emissions of DPM and nitrogen oxide (NOx) when compared to previous year models. However, NIOSH recently cooperated in a study that showed if these pick-up trucks are tested at several engine modes from the MSHA 8-mode test, the NOx emissions actually increase over previous model year levels. An increase in NOx emissions along with the use of a highly catalyzed filter will result in a correspondingly greater increase in NO2 emissions. These data suggest that using pick-up trucks with a model year later than 2007 in underground mines may create elevated NO2 concentrations in the workplace. NO2 concentrations should be continuously monitored when these trucks are used to ensure a safe workplace.
As the mining industry continues to look for ways to reduce the concentration of diesel particulate matter in underground mines, they also need to consider the effects that these controls may have on NO2 concentrations. Although NIOSH is aware of many DPM control strategies, the discussion presented here focuses only on NO2 increases associated with the use of highly catalyzed filters in underground mines. Mines implementing any control strategies must be mindful of their potential to increase NO2 emissions.
NIOSH will continue its research on control technologies that both reduce DPM and NO2 concentrations and will present these results to the industry. As a means of improving this research and technology transfer, NIOSH requests the assistance of the mining community by informing us of their successes and failures related to DPM control strategies.
More information on NIOSH diesel research can be found at the NIOSH mining webpage.
Steven Mischler, MS, Emanuele Cauda, PhD
Mr. Mischler is the Acting Manager of Diesel and Dust Monitoring in the NIOSH Respiratory Hazards Control Branch in the Pittsburgh Research Laboratory.
Dr. Cauda is a Research Fellow in the NIOSH Respiratory Hazards Control Branch in the Pittsburgh Research Laboratory.