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NO2 Emission Increases Associated with the Use of Certain Diesel Particulate Filters in Underground Mines

Categories: Chemicals, Engineering Control, Mining

Schematic of flow through diesel particulate filter

Figure 1: Schematic of flow through diesel particulate filter

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.

Inlet and outlet of a diesel particulate filter showing the accumulation of diesel particulate matter on the inlet side.

Figure 2: Inlet and outlet of a diesel particulate filter showing the accumulation of diesel particulate matter on the inlet side.

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.

A second regeneration process is termed passive regeneration. This type of regeneration occurs as the engine is operating and typically uses the heat generated by the engine to complete the regeneration. Often, this type of regeneration uses an oxidation catalyst, such as platinum, vanadium or similar compound, to reduce the temperature necessary to begin regeneration. These catalysts can be used as either a fuel additive or as a washcoating. The catalyst in the form of a fuel additive is added to the diesel fuel prior to combustion, whereas, a catalyst in the form of a washcoating is used to coat the walls of a filter during the manufacturing process, creating a highly catalyzed filter system. Although the presence of a catalyst, in either form, will increase the feasibility of the filter by allowing regeneration at lower exhaust temperatures, it may also result in an increase in the concentration of NO2 in the exhaust. NIOSH research has not found an NO2 emissions increase associated with the use of catalysts as fuel additives. However, recent NIOSH studies have shown a two- to three-fold increase in NO2 concentrations in mine air, resulting from the use of highly catalyzed filter systems. Therefore, the use of highly catalyzed filters on underground equipment must be closely monitored. Of course, NIOSH has not tested all catalysts used with filters. Consequently, mines should be aware of the chemical composition of any catalysts they choose to employ and the resulting impact on NO2 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.

Public Comments

Comments listed below are posted by individuals not associated with CDC, unless otherwise stated. These comments do not represent the official views of CDC, and CDC does not guarantee that any information posted by individuals on this site is correct, and disclaims any liability for any loss or damage resulting from reliance on any such information. Read more about our comment policy ».

  1. February 2, 2009 at 1:10 pm ET  -   Dr. Jacob Paz

    In 1990 I presented a paper at the annual AIHA. The paper showed that the addition of oxgenated fuel to diesel locomotive reduced NO2, CO, and particulate matter in mining operation by more than 30%.

    Link to this comment

  2. February 2, 2009 at 1:10 pm ET  -   Mike Wright

    The research is excellent and the paper describes it well. But the authors should have chosen a more accurate title. NO2 increases were seen with one type of diesel particulate filter—highly catalyzed filters with on-board regeneration. No problems were found with other filters. Yet the title would lead one to believe that NO2 increases were found with diesel particulate filters in general. This is not just a copy-editing issue.

    Small operators may read the title and conclude that they cannot use filters at all. And sadly, some in the mining industry continue to resist DPM controls, and will cite the title in their public relations.

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    • AUTHOR COMMENT February 4, 2009 at 2:04 pm ET  -   Steven Mischler and Emanuele Cauda

      You raise a good point. We have changed the title of the blog to better reflect our research findings that indicate a two- to three-fold increase in NO2 concentrations in mine air resulting from the use of highly catalyzed filter systems. The new title reads “NO2 Emission Increases Associated with the Use of Certain Diesel Particulate Filters in Underground Mines.”

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  3. February 3, 2009 at 11:49 pm ET  -   B Belle

    Dr. Paz,
    What type of filters were used in your work presented at AIHA? Did you quantify NO2 reduction/increase by Filters & Fuel? Would you kindly share your work?
    regards, bkb, South Africa

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  4. March 12, 2009 at 7:13 pm ET  -   Daniel Sussman

    I am a graduate student at the University of Miami in a Public Health course. I read with interest the passage on NO2 emissions with diesel filters in mines. As with any intervention, unexpected side effects may be encountered that must be addressed. There exists evidence in the literature of success with tocopherols in detoxifying NO2 within ex vivo cells. Has there been any thought to use of tocopherols or other detoxifying agents to convert the nitrogen dioxide to a less toxic form?

    In other words, would a chemical reaction within the exhaust system distal to the filters be helpful in removing the harmful NO2 from the air?

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    • AUTHOR COMMENT March 16, 2009 at 10:56 am ET  -   Emanuele Cauda

      We thank you for your comment. To my knowledge no research has evaluated the use of tocopherols, or other biological chemicals, for converting Nitrogen dioxide (NO2) to a less toxic form. However, chemical reactions are extensively used in aftertreatment technologies to control NO2 and NOx emissions from diesel-powered applications. These technologies convert NO2 to N2 via a reduction reaction over a catalytic surface. The reduction is performed by a reductant species injected into the exhaust gases. The Selective Catalytic Reduction (SCR) strategy employs ammonia or urea as the reductant species while the Lean NOx Catalyst (LNC) uses the injection of hydrocarbons to perform this conversion. Another possible control approach is to absorb NO2 onto a catalysts’ active sites and then convert it to N2 during fuel-rich engine operation conditions (NOx adsorbers). All of these technologies have been studied in the past and developed for on-highway and off-highway surface applications, however the implementation of these emission control technologies into an underground mine presents unique challenges including a demanding work environment, limited space on the equipment and equipment accessibility.

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  5. June 10, 2009 at 1:53 pm ET  -   James Celenza

    With the push to expand retrofits in construction shouldn’t we reccommend that DPFs @ highly catalyzed filters with on-board regeneration not be recomended as an approved retrofit?

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  6. October 27, 2009 at 7:14 am ET  -   David Hogard

    Small operators may read the title and conclude that they cannot use filters at all. And sadly, some in the mining industry continue to resist DPM controls, and will cite the title in their public relations.

    Link to this comment

    • AUTHOR COMMENT October 30, 2009 at 3:34 pm ET  -   Emanuele Cauda

      Thank you for the comment. We agree that the introduction and adoption of advanced DPM aftertreatment strategies by the mining industry is still fragmentary. NIOSH’s role is to publish correct and fair information in order to explain the benefit and the threats connected with the use of a technology. The title of this blog well describes the possible increase of NO2 connected with the use of only CERTAIN and not ALL Diesel Particulate Filters.

      Link to this comment

  7. February 2, 2010 at 8:24 am ET  -   Shakka

    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. I think you’ve made some great points here and I look forward to hearing more.

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  8. November 30, 2011 at 2:06 am ET  -   disaster recovery

    it is very problemative issue and unfortunatly we are not able to deal with it:(

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  9. February 6, 2012 at 12:49 am ET  -   Mathew

    Hopefully NIOSH will continue its research on control technologies that both reduce DPM and NO2 concentrations.

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  10. March 29, 2012 at 4:25 pm ET  -   Keith Heeres

    Here is a site full of reports about diesel emissions control. [http://www.dieselnet.com/links/control_.html]

    Link to this comment

  11. January 25, 2014 at 11:47 am ET  -   Lynwood Hanes

    I am hoping I can get some experienced feedback from the professionals who have been there and done it and are currently doing it in the workforce as of today. Or any great advice will be greatly appreciated!

    Link to this comment

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