Evaluation and Control of Employee Exposure to Diesel Particulate at Several Australian Coal Mines

ABSTRACT

Employee exposures to diesel particulate (DP) were measured under normal and abnormal mining operations at BHP Steel Collieries Tower Colliery and other underground coal mines in New South
Wales coal fields. DP was measured using submicron samplers developed by the U.S.Bureau of Mines, and the results were verified using quantitative scanning electron microscopy. Personal monitoring (n = 480 full-shift samples) conducted at nine underground coal mines has indicated that the exposure of the work force ranges from less than 0.1 to 2.2 mg/m3 of DP, depending onj ob type and mining
operation. Operations such as longwall moves and stonedusting, which generate elevated levels of dust, cause severe interference to submicron DP measurements. Based on previous knowledge that engine tune and condition affect the balance between gaseous and particulate emissions, five technologies for controlling DP were investigated in a combination of studies conducted in an above-ground simulated tunnel and in a special controlled section of underground mine roadway. Potential improvements were validated by application in standard coal mining operations at Tower Colliery. Tests conducted under controlled conditions indicate that, depending on fuel quality, levels of DP in return airways can be reduced by up to 50 percent, and in actual mining situations a reduction of 20 percent can be achieved in exposure of the work force by the use of low sulfur fuels.  In addition, subjective responses from the work force indicate that exhaust emissions from low sulfur fuels provide lower irritation and a more pleasant aroma. The use of water-filled scrubber tanks reduces the level of DP emissions by 25 percent. In addition, it was found that the cleanliness of these tanks in general does not affect their scrubbing properties. Chemical decoking of engines resulted in a reduction of 20 percent in DP in return airways. However, the long-term benefits and cost effectiveness of this treatment in mining operations have yet to be established. A nonflammable, disposable dry exhaust filter constructed f b m synthetic organic fibers with an operational lifetime in excess of 20 hours was found to reduce DP exhaust emissions by 80 percent. Investigations have indicated that the use of increased ventilation to control DP levels does not follow a simple
dilution factor, and in some instances compliance with current regulatory requirements may not produce the required reduced exposure levels. The results from single-component control strategies provide considerable reduction in exposure to DP. However, the most efficient and cost-effective control methodology is the use of a combination of individual systems modeled to operations conducted at each mine.

Materials and Methods

Tower Colliery Operations

Tower is typical of contemporary operations in underground coal mining in NSW. With a work force of 370 operating a longwall over three production shifts, it achieves an annual production of 1.85 million tonnes of high grade coking coal, which is used both domestically and for export. Heavy methane drainage is used to supplement ventilation control of gaseous emissions. Thirty-two trackless, rubber-tired, diesel-powered units are used throughout the mine mainly for transport of manpower and materials (70 kW) with heavy-duty, diesel-powered equipment (120 kW) being used for longwall moves.

Diesel Particulafe Measurements

All sampling for diesel aerosol particulates was conducted using the University of Minnesota/U.S. Bureau of Mines sampling device. This device uses a combination of a cyclone and an impaction plate to separate the submicron fraction of aerosols. Transmission electron microscopy studies indicated that more than 85 percent of the DP was captured using this technique, with the remainder being excluded since it is attached to larger dust particles. Quantitative analytical scanning electron microscopy indicated that the amount of positive interference from submicron nondiesel particulates such as coal dust was less than 10 percent, provided that the respirable dust levels are kept below the statutory limit of 3.0 mg/m3.This technique was used to verify samples collected during tunnel tests and to investigate all high mass DP samples collected in mining operations to determine interferences from coal and roadway dust that occur when the impaction plates are overloaded in very dusty situations.

Above-Ground Simulated Tunnel Testing

Initial test work was carried out in a test tunnel (45 m long, 5 m wide, and 2.5 m high) constructed on pit top from steel formwork, reinforced plastic sheeting, and brattice to create a simulated section of mine roadway. An auxiliary fan was fitted at one end to provide ventilation inside the tunnel in accordance with the NSW Coal Mines Regulation Act. The test vehicle chosen was a Domino Minesmobile fitted with a Perkins direct injection engine. A stringent driving schedule incorporating loaded and unloaded cycles was used that represented as much as possible the normal driving cycle of the vehicle if it were being used underground (total time for the cycle was 5 hours). Personal DP samples were collected on the driver during the various test regimes. The raw exhaust gases (carbon monoxide, oxides of nitrogen, and carbon dioxide) were continually sampled using a probe inserted into the exhaust manifold with flexible tubing connected to direct-reading instruments with data stored directly into a portable computer. This above-ground system allowed the use of test equipment that did not meet the underground intrinsic safety requirements of the Coal Mines Regulations.

Controlled Section UndergroundMine Roadway Testing

A suitable section of mine roadway approximately 110 m long was identified and pneumatically operated steel doors were installed at one end to provide variable control of ventilation to deliver the exact legislative ventilation requirements for various types of machinery. The layouts of the test station and DP air sampling sites are schematically represented in Figure 1. To reduce undue influences with respect to extraneous airborne dust, a large section of the ribs was meshed and sprayed with concrete while the floor was graded and filled with road ballast. A Noyes multi-purpose vehicle (MPV) was selected as the most appropriate vehicle for testing because of the number of these vehicles in the colliery fleet and previous research which had demonstrated that these vehicles produced diesel aerosol particulates at a level that minimized the analytical errors associated with the selected sampling device. A driving schedule including various load cycles was developed that reflected as much as possible the normal activities of this type of vehicle in transporting supplies around the colliery over a normal shift. The vehicle was driven backward and forward along the test section of roadway for a number of shifts. Diesel aerosol particulate samplers were placed on the MPV operator; the machine itself adjacent to the driver’s cabin; inbye of the MPV between the vehicle stopping barrier and the ventilation doors; and outbye of the MPV near the entrance to the test station roadway. A field blank was used for every sampling exercise.

Results and Discussion

Employee Exposure Profile

Some 203 personal DP samples were collected over an 18-month period to profile the exposures for all job types at Tower Colliery. Under normal operating conditions, exposures ranged from 0.05 to 0.4 mg/m3. This increased during extreme load conditions such as longwall change to between 0.4 and 0.6 mg/m3.One sample of 2.2 mg/m3 was subject to scanning electron microscopy and found to be contaminated with roadway dust blown over the driver by the force of the exhaust gases exiting the vehicle. For tasks linked to diesel engine activity a linear relationship was found between exposure and number of hours of driving.


The monitoring program was extended to a number of operations within the NSW underground coal mining industry, with diesel activities sufficiently different from that of Tower Colliery. Railtrack and diesel locomotive haulage were identified as two such areas that needed to be monitored. A group of eight mines was selected that met all the criteria. Each colliery was contacted to seek its assistance and followed up with a site visit by the project coordinator to outline the research and to seek its commitment to the project. During this presampling site visit, considerable time was allocated to the selection of those activities that would provide the most useful data in terms of differences from Tower Colliery. At least one operation similar to that at Tower Colliery was included in the tasks to be sampled as a control. Sampling was then undertaken at the eight collieries in the period ffom July to December 1994, with the aim of obtaining a minimum of six full shifts of sampling per mine. All samples collected were on a full-shift personal basis with a minimum of 4 hours’ sampling duration. In all, 134 personal samples were collected at the eight collieries (Table 1).

While direct comparisons between exposures found at individual collieries is not possible because of varying ventilation and duty cycle requirements, colliery C appeared to have consistently lower results relative to other operations, including Tower Colliery. Discussions with colliery management indicated the use of a low sulfur diesel fuel, good road conditions throughout the mine which minimized engine revving, an intensive scheduled maintenance program, a system for limiting the number of vehicles in ventilation splits, a computerized weekly exhaust emission testing program, and a policy of replacing older design engines led to the improved conditions.

Fuel Quality

Three commercially available diesel fuels that are commonly used in the mining industry-one commercial low emission fuel and one experimental low emission fuel-were tested under controlled conditions over a series of runs in the above-ground tunnel. Full chemical and physical analysis was carried out on all the fuels by a petroleum industry laboratory using standardized analytical methods. Both aromatic hydrocarbon and sulfur content affected DP exposures, with increased sulfur content in particular indicating a linear increase in driver exposure to DP, carbon monoxide, and oxides of nitrogen (Figure 2). Sulfur in diesel fuel results in the creation of particulate sulfates during combustion, which catalyzes the formation and agglomeration of carbon nuclei.

Operational field trials were conducted by introducing the low emission test fuel into a small underground lead/zinc mine that traditionally used high sulfur imported fuel.  A 40 to 50 percent reduction in DP measurements was found in the general work areas and return airways in this mine, which was similar to the results predicted in the above-ground tunnel tests. Trials conducted at Tower Colliery, where commercial diesel fuel that complied with the coal mines regulations was substituted with the experimental fuel, resulted in reductions in DP exposure to the work force of around 10 to 15percent, which is slightly lower than the 25 percent reduction predicted by the tunnel tests. In both mines the work force commented favorably on the use of the low emission fuels, reporting an immediate reduction in irritation and an improvement in the aroma of the emissions. The low emission test fuel has now been incorporated into all BHP underground coal mines in NSW. In designing a fuel specification to meet low emission standards, other parameters in addition to sulphur have to be considered, such as cetane number (which affects cold start, emissions, and diesel knock), density (which affects power and hence emission levels), cloud point (which creates problems in cold climates), flash point (which indicates flammability), and distillation range (which indicates higher ends that result in the creation of additional DP).

Scrubber Tank and Air Filter Cleanliness

A series of tunnel runs were conducted separately on two machines to determine the extent of operator exposure to DP consistent with both normal and abnormal maintenance procedures. The raw exhaust scrubber tank was trialed dry, wet, and after extensive cleaning using chemical agents and a high pressure water jet. The NSW legislative requirements are that this trap be filled with water so as to act as a flame trap. Water in the scrubber tank resulted in a 20 to 30 percent reduction in DP exposures. However, the additional degree of cleaning instigated in the tests did not increase efficiency of capture above that of normal flushing procedures. It was concluded that the cleanliness of the internal surfaces of water-filled scrubber tanks is not a major factor in reducing diesel aerosol particulate from engines. It is more likely that the presence of an impaction barrier such as water is the major influence in achieving the levels of reductions observed with vehicles fitted with scrubber tanks, compared with those without.

Testing using a series of new and very dirty air intake filters failed to find any significant effect on DP exposures. This suggests that intake air filters would need to be in an extremely poor condition before any deterioration in DP levels may occur.

Engine Chemical Decoking

Mechanical decoking of engines has long been known to have a beneficial effect on engine performance, and the emergence of chemical decoking systems has the potential for similar benefits. Short-term tests previously conducted at Tower Colliery have shown a significant reduction in the generation of DP after chemical decoking. Further tests were conducted to determine the long-term effectiveness of procedures for the chemical decoking of engines and to evaluate the effects on component wear. Sufficient baseline DP data were obtained in the underground tunnel on a Noyes MPV, which was then removed to the underground workshop and chemically decoked. After decoking, the vehicle was returned to the test station and retested over a number of successive days until the DP indicated a downward trend. The vehicle was returned to the vehicle fleet to resume normal duties, from which it was extracted at regular intervals over a 9-month period for retesting. A significant downward trend of DP levels was obtained soon after treatment and maintained over the 9-month period of sampling (Figure 3). Apart from routine maintenance, no other work has been performed on the engine over the sampling period and no adverse mechanical effects were noted up to the present, which is 18 months after treatment.

A second Noyes MPV was decoked some 3 months after the first vehicle. Decreased DP emissions were also found, although the amount of reduction was less due to the better condition of the engine prior to the deco-king. During the decoking process and for a number of hours thereafter, extensive quantities of soot were observed in the vehicle exhaust and water-filled scrubber tank. This release of built-up carbon requires the careful management of exhaust emissions to ensure that workers are not inadvertently exposed to a range of noxious products.

Disposable Exhaust Filter

Considerable work has been undertaken by the U.S.Bureau of Mines, Donaldson Inc., and Jeffrey Inc. to develop a disposable exhaust filter system. These filter types have a proven record in reducing DP exposures in underground vehicles, but unfortunately they have contributed to several unacceptable situations under operational conditions in U.S.mines where hot exhaust has carbonized and even ignited the paper filters. A polypropylene filter material was obtained which had enhanced flame-resistant characteristics and good filtration characteristics, was not adversely affected by water mist, and was obtainable at reasonable cost.

A mobile generator (P. J. Berriman Pty Ltd Power Tram) fitted with and without filters to the scrubber tank exhaust was tested under full power load in the underground test tunnel for durations of up to four shifts. A pressure gauge was placed between the exhaust manifold and the filter to measure changes in back pressure that are known to affect engine performance and exhaust emission levels. Significant reductions in diesel aerosol particulates were recorded using the filter for periods in excess of three consecutive shifts (Figure 4), and even though back pressure increased to 20 kPa, no effect was detected on engine performance. Without the filter visibility in the heading was poor, with significant levels of water vapor and soot present, while with the filter visibility was improved significantly.

A prototype filter unit was also fitted to a front end loader vehicle (Eimco 913). Similar reductions in DP levels were achieved; however, only a maximum two shifts’ duration was obtained from the filters as the poorer engine condition generated very high DP levels (nonfilter, 1.2 mg/m3 operator exposure).

Such exhaust filter devices provide the best short-term means of controlling DP from low use, heavy haulage vehicles in the underground coal mining industry. It is necessary that appropriate safety systems be installed with the unit to ensure that the filters are never exposed to temperatures above their design characteristics and that they are changed out on a regular basis (e.g., after each 24 hours of use).

Mine Ventilation

Considerable emphasis is placed on ventilation as a viable means of controlling diesel exhaust emissions in underground coal mines by regulatory authorities. A series of tests were conducted with a mobile generator (PJB Power Tram) and a transport vehicle (Noyes MPV) over a range of ventilation rates of 5 to 15 and 5 to 10 m3/s, respectively. In addition, tests were carried out when both machines were in combined operations in the same heading with flow rates of 5 to 15 m3/s.

A linear decrease in operator DP levels was found for increased air flow when machines were operating alone. When machines were used in combination, the actual results were different from the calculated additive values, being less in the low flow conditions and higher in the air flows greater than 12 m3/s (Figure 5). Subsequent ventilation testing indicated that velocity pressure was not a significant factor in affecting engine performance or emission levels and that thermal stratification in the tunnel was observed only at flow rates of less than 6 m3/s. The reason for the difference in additive effects of multiple machinery remains unclear. At some flow rates thermal stratification may result in higher exposures to some members of the work force.
