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June 2006
by Robert A. Haney, P.E. |
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As the Mine Safety and Health Administration (MSHA) moves forward
with final diesel particulate regulation, underground stone mines
will have to evaluate their DPM controls and control strategies and
determine if they are sufficient to meet the final DPM exposure
limits. In January 2006, MSHA held hearings
on a final DPM regulation that would phase in the 160 micrograms of
total carbon per cubic meter limit during a five-year period, with
the final regulation being fully implemented by Jan. 20, 2011. This
regulation should be finalized sometime in the summer of 2006. While
the regulation would extend the implementation date of the final DPM
exposure limit, it does not appear that it will change the actual
limit. The main remaining question will be the conversion factor the
agency chooses to obtain the elemental carbon equivalent to the 160
micrograms of total carbon per cubic meter total carbon value (see
Developing control strategies Prior to the 2001 diesel particulate regulation, typical DPM exposures in underground stone mines ranged from 700 to 1,000 micrograms per cubic meter. Some mines had exposures ranging from 1,500 micrograms per cubic meter to above 2,000 micrograms per cubic meter. The current exposures |
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in underground stone mines are typically below the 308 micrograms of elemental carbon per cubic meter limit with an average exposure around 200 to 250 micrograms of elemental carbon per cubic meter. Currently, the higher exposures are on loader operators who don’t have environmental cabs and powder crews who work outside a cab or downwind from other production equipment. To meet the current and future DPM exposure limits, mines have to develop and implement a DPM control strategy. The value of an air-quality and control-monitoring program should not be overlooked when establishing the control strategy. Typically, air-quality monitoring has only been considered a compliance check. However, when combined with control monitoring, it can be used to determine what controls are successful and what controls need to be upgraded. For a company that has multiple mines, it can be used to compare the effectiveness of controls among mines and to determine the most effective means of making improvements. Monitoring for DPM can be expensive; the number, frequency, and location of samples should be carefully chosen. The DPM regulation gives the mine operators the ability to pick and implement the DPM controls or combination of controls that are best suited to their operations. These controls fall into the following six broad categories: clean engines, ventilation, environmental cabs, alternative fuels, work practices, and diesel particulate filters. Impact of clean engine technology Of these controls, the most significant change has occurred in engines emissions. In 1990, EPA passed the Clean Air Act. This legislation mandated lower diesel emissions and required manufacturers to conduct engine emission testing. Prior to this Act, the only engine emission testing was preformed by MSHA. DPM emissions for direct injection engines manufactured before 1993 could range from 0.5 to 0.7 gm/hp-hr. Off-highway, phase in of Tier 1 requirements began in 1994. Tier 2 requirements began in 1997. Since the 2001 MSHA DPM regulation, almost all high-horsepower production equipment has been replaced with equipment having Tier 1 or 2 engines. These engines have emissions ranging from 0.1 to 0.2 gm/hp-hr. This has resulted in a 60- to 80-percent reduction in DPM emissions (and exposure). A few operations are still using older indirect injection engines. These engines are approved by MSHA, however the emissions can be as high as 0.5 gm/hp-hr. If operations are still using these engines in high-horsepower, high-use equipment, they should consider replacing them with cleaner engines. Understanding ventilation significance Ventilation is a significant part of a control strategy for DPM. With the engine testing mandated by EPA, the amount of airflow required to dilute diesel particulate matter to various levels can be easily calculated. The Particulate Index (PI) is defined as the amount of air that is required to dilute whole diesel particulate (total engine exhaust) to 1,000 micrograms per cubic meter. A 1,000 micrograms carbon per cubic meter whole diesel particulate value is approximately equal to 800 micrograms of total carbon per cubic meter or 616 micrograms of elemental carbon per cubic meter. Airflow of double the PI will dilute the diesel particulate emissions to 308 micrograms of elemental carbon per cubic meter. For a 0.10 gm/hp-hr engine, the PI is 32 cfm/hp. Therefore, the airflow to dilute the diesel particulate emissions to 308 micrograms of elemental carbon per cubic meter, would be 64 cfm/hp. Multiplying the total mine horsepower by 64 approximates the airflow needed in the production area to dilute the diesel emissions. For example, if a mine has equipment with diesel engines totaling 1,500 horsepower, the requisite airflow would be approximately 100,000 cfm. If half the airflow going into the mine gets to the production area, the total mine airflow would be approximately 200,000 cfm. Fortunately, for underground stone mines, the cost of moving airflow is relatively inexpensive due to the low mine resistance, resulting from the large openings. It takes about 60 horsepower to move airflow of 300,000 cfm against 1 inch of water gauge pressure. The historic rule-of thumb for underground ventilation for diesel equipment was airflow of 100 to 200 cfm/hp to dilute gaseous emissions. For modern engines, Tier 1 and Tier 2 (0.1 gm/hp-hr), the rule of thumb for dilution of diesel particulate is a total mine airflow of approximately 125 cfm/hp. Because the ventilation requirements for clean engines are so much less than for the old engines, the mines have been able to optimize current mine ventilation systems rather than make major upgrades in airflow. The optimization measures included: installation of brattice or waste rock stoppings, repairing brattice lines, incorporating long pillars into mine design to reduce air leakage and improved fan placement. All of these measures more effectively distribute air flow throughout the mine. Mines that were ventilated by natural ventilation do need to upgrade ventilation systems. Prior to 2001, approximately 25 percent of the underground stone mines were ventilated by natural ventilation. In the spring and fall, when there was no temperature difference between inside and outside air, there would be no airflow in the mine. As a result, mines that were solely ventilated with natural ventilation would not be able to consistently comply with the diesel particulate exposure limit. These mines needed to upgrade their ventilation system by installing fans.
Environmental cabs for exposure reduction Most of the production loaders used in underground stone mines come with factory-equipped environmental cabs. As a result, the loader operators who may work in the poorest ventilated areas of the mine are protected from diesel particulate. An environmental cab is equipped with a pressurizing/filtration system. In order for the cab pressurizing/filtration systems to be effective, cab doors and windows must be closed and sealed. Both the intake and re-circulation filters should be changed on a regular basis. Environmental cabs are an effective means to reduce exposure not only to DPM but also to dust and noise. They can provide 60- to 80-percent reduction in DPM exposure for miners who remain inside a cab. Environmental cabs do not provide protection for downwind workers, whose work position is outside of an environmental cab. The DPM benefits of a cab can be fully realized only if the cab is sealed and pressurized to prevent inflow of contaminants. A simple pressure test can be used to assure the pressurizing/filtration system is working. Work practices to reduce diesel particulate exposure In addition to the introduction of clean engines, optimization of ventilation systems, and use of environmental cabs, stone mines have implemented various administrative controls to reduce worker exposure to DPM. Work practices that are currently being used to reduce exposures include the following: hauling in exhaust airflow (environmental cabs on trucks), drilling and shooting on separate shifts, keeping drillers and blasters upwind of production equipment, and limiting idling time. Hauling in exhaust air prevents the truck exhaust from traveling over up wind drilling and powder crews and from going to the production face. Drilling and shooting on separate shifts allows the drillers and blasters, who often work outside a cab, from being exposed to emissions from production equipment. Similarly, keeping drillers and blasters upwind of production equipment keeps these workers from being exposed to emissions from production equipment. Limiting idling time reduces emissions that would have to be controlled by other methods. Alternative fuels for diesel emissions reduction The primary fuel in use by the underground mining industry is low sulfur, No. 2 diesel fuel (D2). Several types of alternative fuels are available to reduce diesel engine emissions. These include No. 1 diesel fuel (D1), bio-diesel blend fuels (either virgin soy oil or recycled vegetable oil), and diesel-water emulsion fuels. All of these fuels are EPA approved fuels (not fuel additives). No. 1 diesel fuel is not in general use because it does not result in a significant reduction in DPM emissions (12 percent). Diesel-water emulsion fuels have demonstrated potential to reduce DPM emissions, however, they are currently not in significant use due to distribution problems. Bio-diesel fuels are gaining in popularity due to high cost of No. 2 diesel fuel and favorable tax incentives for their use. Figure 2 shows a summary of the results of the alternative fuel tests conducted by MSHA. While in-mine tests on bio-diesel fuels have been limited to blends of 50 percent and less, laboratory testing indicates that the higher the bio-diesel content the more reduction in DPM emissions. Bio-diesel blends of less than 20 percent do not give significant reductions in DPM emissions. Tests conducted on fuel additives, fuel-oxygenating systems and magnetic fuel systems have not indicated any significant DPM reductions from their use (see Figure 2). Diesel particulate filters Diesel particulate filters are a proven means of reducing DPM exposures; simply stated — filters filter. They can reduce diesel particulate emissions by more than 90 percent. To date, underground stone mines have avoided their uses and elected to install other controls. While stone mines have been successful in meeting the 308 micrograms of elemental carbon per cubic meter limit without the use of diesel particulate filters, their use may be necessary to meet the final DPM limit. During the last five years, stone mines have done a remarkable job of reducing DPM exposures. While some mines are currently meeting the future 160 micrograms of total carbon per cubic meter limit, many mines will need to continue upgrading controls to meet the limit. Since there are no new or novel controls being developed, the control upgrades will have to come from additional application of the six categories of controls discussed in this article. Because upgrading controls can be expensive, a long-range control strategy should be developed. Watch Aggregates Manager for the second installment of this report. It will be published after the final DPM rulemaking. |
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References
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Reprinted from Aggregates Manager Magazine |
