August 2006

Meeting the Final Diesel Particulate Matter Limit
As the DPM limit continues to be reduced, it is important for mine operators to continue to sample for DPM at their operations.

by Robert A. Haney, P.E.

The 350 micrograms of total carbon per cubic meter limit becomes effective on Jan. 20, 2007, while the 160 micrograms of total carbon per cubic meter limit becomes effective on May 20, 2008. MSHA will undertake another rule-making activity to establish an elemental carbon (EC) equivalent for the 350 and 160 micrograms of total carbon per cubic meter limits and it will update its compliance assistance guide. The complete rule and the compliance assistance guide (when updated) are available at www.msha.gov .

If the equivalent EC is not established prior to the Jan. 20, 2007 date, a conversion of 1.3 will be used. This will establish a 270 micrograms of elemental carbon per cubic meter limit. The 160 micrograms of total carbon per cubic meter limit becomes effective in two years, almost three years sooner than the original 2011 proposed date (see Figure 1). While the 350 micrograms of total carbon per cubic meter limit should not have a significant impact on most underground stone mines, the 160 micrograms of total carbon per cubic meter limit could pose some challenges, if upgrades to DPM controls are left to the last minute.

In the June 2006 issue of Aggregates Manager, the DPM controls that have generally been adopted by the underground stone mining industry were discussed. These controls include the introduction of modern engines (EPA Tier 1 and Tier 2); providing a mine airflow of four times the total engine Particulate Index (PI); and improving airflow distribution, using environmental cabs on production loaders, and adopting work practices that keep drilling and blasting workers up wind of production equipment. In this article, the controls that are available to reduce workers exposure to the 160 limit will be discussed.

As the DPM limit continues to be reduced, it is important for mine operators to continue to sample for DPM at their operations. The sample results can proactively provide informational on the effectiveness of existing controls and show where and what types of additional controls might be needed.

Tier 4 engines for emissions reduction

With the phase-in of EPA Tier 2 engines complete, the next step in clean engine technology will be the phase-in of Tier 3 and Tier 4 engines. Phase-in of Tier 3 engines began in 2006. Phase-in of Tier 4, on road engines, will begin in 2007, and phase-in of Tier 4, off-road engines, will begin in 2011. Tier 3 engines have reduced NOx emissions but not reduced in DPM emissions. Tier 4 will reduce the engine DPM emissions to 0.015 gm/hp-hr (grams per horsepower-hour).

This 90-percent additional reduction in DPM engine emissions will occur as the engine manufacturers install diesel particulate filters on the engines. Tier 4 engines will not be available for all equipment when the 160 limit takes effect. As a result, mine operators may need to upgrade the other DPM controls available. These controls include improved ventilation, environmental cabs, work practices, alternative fuels, and aftermarket diesel particulate filters.

Extending and maintaining ventilation systems

Ventilation continues to be an important control for the reduction of DPM exposure. As mine workings advance, the airflow distribution system — whether brattice cloth or long-barrier pillars — also needs to be advanced. Air walls damaged from blasting need to be promptly repaired. Many mines are incorporating long-barrier pillars into the mine design to reduce the number of air walls that need to be built and maintained.

Additional upgrades to the ventilation system include airshafts or cutting openings to the surface and additional fan installations. To minimize airflow resistance, air shafts less than 200 feet deep should be at least 10 feet in diameter. Airshafts or other openings to the surface should be placed to improve airflow distribution. Large-volume, low-pressure fans are well suited to the conditions in underground stone mines.

Environmental cab improvements

Over-the-road trucks (10 to 30 tons) typically have pressurizing systems but often do not have filtration systems. As a result, unfiltered air is drawn into the truck cab. To improve the effectiveness of these cabs, the existing air vents should be blocked, and an after-market pressurizing/filtration system should be installed. A number of manufacturers supply aftermarket filtration systems that can be installed on truck cabs.

Similarly, the effectiveness of cabs on drills needs to be evaluated. Cab air-filtration systems should provide one air change per minute. Drill cabs are typically 200 cubic feet in volume (loader, haul truck, and scaler cabs are less than 100 cubic feet). Drill cabs often contain holes for cables and hose conduits. Pressurizers supplied by the equipment manufacturer typically have an airflow capacity of 75 cubic feet per minute. As a result, there is not enough airflow to pressurize the cab, resulting in cab pressures of 0.00. Additional pressurizing/filtration capacity should be added to large-volume cabs, and cab openings need to be sealed.

Work practices for DPM reduction

Companies have implemented most available work practices to meet the 308 micrograms of elemental carbon per cubic meter limit. Work practices to meet the final DPM limit will continue to focus on scheduling drillers and powder crews to work upwind of the loaders and haul trucks or on off-shifts when stone is not loaded or hauled. This is one of the most straightforward and proven means of lowering worker DPM exposure.

Several mines have eliminated haul trucks in favor of conveyor haulage. This removes a significant amount of horsepower (and DPM emissions) from the mine. The replacement of haulage vehicles with belts is especially effective in reducing DPM emissions as haulage distances increase and there becomes a need to add additional haul trucks to maintain the load frequency. It should be noted that underground salt mines have almost universally eliminated the use of haul trucks in favor of LHD operations.

Alternative fuels to reduce emissions

The use of alternative fuels offers a relatively easy solution to reducing diesel particulate emissions. Unlike an engine or diesel exhaust filter that only reduces emissions for the specific vehicle, alternative fuels reduce emissions for all vehicles in a mine. Additionally, due to filter size limitations, vehicles greater than 500 horsepower would require a dual diesel particulate filtration system.

Bio-diesel blends greater than 50 percent or diesel-water emulsion fuels can reduce emissions by 50 to 70 percent. The downside for both fuels is the supply. Although bio-diesel fuel is available nationally, mines will have to make long-term arrangements with fuel distributors to assure availability in the quantities needed to provide bio-diesel blends greater than 50 percent.

A note of caution is needed when changing from standard fuel to bio-diesel fuel. The latter acts as a solvent and cleans the fuel system. As a result, the fuel filters will plug. Extra fuel filters need to be available at the mine to avoid prolonged equipment downtime. A complete guide for handling and use of bio-diesel fuel can be found on the National Biodiesel Board Web site, www.biodiesel.org.

Water emulsion fuels have proven effective in reducing diesel particulate emissions. However, future availability of water emulsion fuels is uncertain.

Diesel particulate filters reduce emissions

While diesel particulate filters have not been widely used in stone mines, they can be an important factor in meeting the final DPM limit. There are three categories of diesel particulate filters: passive regeneration, active regeneration, and high-temperature disposable particulate filters (HTDPF). The DPF replaces the muffler in the engine exhaust system. For any of the filter choices, an engine temperature profile, during normal operation, is needed to determine the type of filter that can be used. Passive filters can be used at engine temperatures above 626oF (330oC) 300oC. Active systems can be used at temperatures below 626oF (330oC), and HTDPF can be used at engine temperatures below 650oF (243oC).

Typical candidates for passive regeneration are production loaders and haul trucks. While passive regeneration is desirable, a mine should have an active regeneration alternative in the event that the engine duty-cycle changes and the filter does not passively regenerate. HTDPF are ideal for light-duty vehicles or vehicles that get intermittent use.

Exhaust back pressure gauges are an essential part of a DPF retrofit. Monitoring the increase in back pressure indicates if passive systems are working properly, when active systems need to be regenerated and when HTDPF’s need to be replaced.

High-horsepower, off-road Tier 4 engines, will be available starting in 2011. These engines will be factory equipped with diesel particulate filters.

Understanding impacts before installation

Because there are a lot of variations on the DPM controls that can be installed, it is helpful to get an indication of their potential impact on DPM prior to their installation. MSHA has developed a simulator that will approximate the effect of control changes on DPM levels. To conduct the simulation requires DPM sampling of existing conditions, ventilation measurements, the diesel equipment inventory, and an estimate of engine emissions and equipment usage. The impact of various control strategies can be simulated before they are implemented.

The final DPM limit of 160 micrograms of total carbon per cubic meter will become effective on May 20, 2008. To meet the limit, many underground stone mines will have to use bio-diesel fuel or install diesel particulate filters. Additionally, ventilation systems will have to be optimized and environmental cabs will have to be maintained. Installation of these controls may take some lead-time. As a result, mines should develop and implement a DPM control strategy in the near future.

This is the second part of a two-part Special Report on the impact of the final diesel particulate matter rule and how to deal with it in your operation. The first part was printed in the June 2006 issue of Aggregates Manager.

Robert A. Haney is president of Haney Environmental Consulting, LLC, and a professional engineer. He formerly served as chief of the dust division in the Mine Safety and Health Administration’s Office of Technical Support before his retirement. Questions or comments on this article can be sent to him at bob.haney@verizon.net.

References

1. Federal Register:  “Diesel Particulate Matter Exposure of Underground Metal and Nonmetal Mines, Final Rule,” Vol. 71, No. 96, Page 28924-29012, May 18, 2006.

2. Haney, R. A., Schultz, M. J., Rude, R., and Tomko, D. M., “Controls Being Used to Reduce Diesel Particulate Matter Exposures in U.S. Underground Metal and Nonmetal Mines,” Proceedings of the 8th International Mine Ventilation Congress, Brisbane, Queensland, Australia, July 6-8, 2005.

3. National Bio-Diesel Web site, www.biodiesel.org.

Reprinted from Aggregates Manager Magazine
August 2006

The use of alternative fuels, such as soy bio-diesel fuel, offers a relatively easy solution to reducing diesel particulate emissions.

The muffler on this haulage vehicle could be replaced with a disposable particulate filter, or DPF.