Evaluation of a Disposable Diesel Exhaust Filter for Permissible Mining Machines

ABSTRACT

The U.S. Bureau of Mines (USBM) Diesel Research Program emphasizes the development and evaluation of emission control devices to reduce exposure of miners to diesel exhaust pollutants; Studies by the USBM have shown that diesel exhaust aerosol (DBA) contributes a substantial portion of the respirable aerosol in underground coal mines using diesel equipment not equipped, with emission controls.

The USBM and the Donaldson Co., Inc., Minneapolis, MN, have developed a low-temperature, disposable diesel exhaust filter (DDEF) for use on, permissible diesel haulage vehicles equipped with waterbath exhaust conditioners. These were evaluated in three underground mines to determine their effectiveness in reducing DEA concentrations. 

The DDEF reduced DEA concentrations from 70 to 90 pct at these mines. The usable life of the filter ranged from 10 to 32 h, depending on factors that affect DEA output, such as mine altitude, engine type, and duty-cycle. Cost per filter is approximately $40.

FIELD EVALUATION OF DISPOSABLE DIESEL EXHAUST FILTER

The collection efficiency and life expectancy of the DDEF system were evaluated during week-long field studies at three mines.

Aerosol data were collected in continuous miner sections of these underground coal mines while the diesel haulage vehicles were equipped with and without the DDEF installed. These mines are designated M, R, and S. Mine M was located in Utah at an altitude of approximately 2,400 m (8,000 ft). Mine R was located in Alabama and mine S in Kentucky. Each mine produces high volatile, bituminous coal with shift production levels varying from 272 to 604 kg/shift. Seam heights varied from 1.5 to 2.4 m. Mines M and R use continuous mining to develop longwall panels. Mine S is a room-and-pillar operation that uses a continuous miner.

Mines R and S operated three to four Jeffrey 4110 Ramcars in the test section. The Jeffrey 4110 Ramcars were equipped with Motorenwerke Mannheim (MWM) D916-6 engines, rated at 74.6 kW (100 hp). Mine Mused three to four Jeffrey 4114 Ramcars powered by Caterpillar 3306 PCNA engines. These engines were derated for high altitude operation from 111.9 to 82.1 kW (150 to 110 hp).

At the first mine, the Ramcars were operated for 4 days with the DDEF installed and for 1 day without the DDEF installed. At the other two mines, the Ramcars were operated for 3 days with the DDEF and for 2 days without

SAMPLING AND ANALYSIS METHODS

It was shown in the laboratory, and in underground mines, that inertial impaction, followed by gravi­metric analysis, can be used to separate and sample DEA and mineral dust aerosol fractions, and provide estimates of DEA concentrations. Two types of personal diesel exhaust aerosol samplers (PDEAS) were developed to achieve this result and are depicted in figure 4.

Both have three stages and employ inertial impaction to separate diesel and mineral dust fractions of the sampled respirable aerosol. The first stage is an inertial pre­ classifier, a 10-mm-Dorr-Oliver cyclone that separates and removes the larger, nonrespirable aerosol. 

The second stage is a four-nozzle impactor with a sharp 50 pct cut point of 0.8 ).Lm aerodynamic diameter. Most aerosol particles larger than 0.8 ).Lm, the respirable coal dust, are deposited on an impaction substrate in this stage.

The third stage is a filter which collects the remaining aerosol of less than 0.8 ).Lm aerodynamic diameter, the DEA. Both samplers operate at a flow rate of 2 L/min, which is compatible with both personal sampler pumps and the Dorr-Oliver cyclone.

Preliminary evaluations of the sampling technique indi­cate that these are accurate to within 25 pct, 95 pct of the time, for concentration levels above the estimated limit of detection of 0.3 mg/m3. Below this level, indications are that the 95 pct confidence interval can exceed 60 pct due to interferences caused by submicron mineral dust and background atmospheric aerosol.

Both types of PDEAS were used to evaluate the filtra­tion efficiency of the DDEF. Figure 5 shows the sampling stations used in the three entry longwall development sec­ ions of one of the mines in which the evaluation tests were performed. 

Up to 35 PDEAS samples were collect­ ed during each normal production shift in the ventilation intake entry (I), haulageway entry (H), on the diesel shut­tle cars (SC), in the return air entry (R) and, in a few instances, individuals. Ventilation in the section is indi­cated by arrows.

In addition to the PDEAS samples, aerosol size dis­tribution samples were collected in mines M and S using a lO-stage micro-orifice, uniform deposit impactor (MOUDI). The analyses of MOUDI-derived size dis­tributions provided accurate concentrations of DEA and respirable coal mine dust aerosol and were also used to evaluate the performance of the PDEAS.

RESULTS AND DISCUSSION

Table 1 summarizes the PDEAS results of aerosol measurements taken with and without filters installed. The table also indicates the reduction of DEA in the mine environment.

DEA concentrations with the DDEF installed were less than or equal to 0.3 mg/m3 at all locations. The concen­trations and standard deviations shown in table 1 are av­erage values uncorrected for intake air concentration or production and ventilation changes. However, the DEA reductions shown in the table were calculated by including correction factors for these parameters. 


The equation used to calculate the percent reductions, 6, is 6 =100 [1- C ~/o]. w Here, Cw and Cw/ o are the intake corrected average DEA concentrations measured with and without the DDEF in place; corrected for ventilation, section production, and aerosol background concentration in the intake air.

They are calculated using; Here, C is the intake corrected average concentration, Cm is the average measured concentration for day n, 5m is the correction factor for the measured concentration, and CIS I is the average aerosol concentration measured in the section intake and its correction factor, respectively. 

The correction factors are determined from production tonnage and measured ventilation rates by; 

Here, P and Pm are the average production tonnage for the days during which the measured condition pertains, i.e., with or without filter, and the production tonnage for the shift of the measured concentration Cm. Similarly, V and Vx are the average ventilation rate for the shift during which the measured condition pertains and the ventilation rate at the location and on the day for the measured con­centration Cx. The production correction only applies to the concentrations measured on the section.

Also, since daily measured concentrations are not given in the table, a net correction factor is determined from the average measured and corrected values for the diesel aerosol con­centrations and is reported in the table as an indication of the size of the corrections made. 

These analyses indicate that the DDEF reduced DEA concentrations in the mine atmosphere by 95 pct with a standard deviation of 6 pet at mine M, 72 pct with a stand­ard deviation of 4.5 pct at mine R, and 87 pct with a standard deviation of 4 pct at mine S.

Figures 6 and 7 show the size distribution of mine aerosol for mines M and S with and without the DDEF. These figures are based on data collected from the MOUDIs located at the haulage site. They effectively il­lustrate two points. (1) The DDEF is effective at remov­ing most of the submicron aerosol from the mine atmos­phere and (2) most submicron aerosol is attributable to diesel exhaust.

FACTORS AFFECTING FILTER LIFE

The useful service life of a DDEF is determined by the engine manufacturer's allowable exhaust backpressure lim­it. The maximum backpressure specified for the Caterpil­lar 3306 engine is 86 cm (34 in) of water. For the MWM 916-6 engine the maximum specified backpressure is 102 cm (40 in) of water. 

The exhaust backpressure is de­termined by summing the pressure drops across the water­ bath scrubber, exhaust piping, and the filter. Measure­ments on DDEF systems, which were used during this testing, showed that the maximum backpressure imposed by the waterbath scrubber and exhaust system was ap­proximately 25 cm (10 in) of water, so when the pressure drop across the filter reached 61 cm (24 in) of water, for the CAT 3306, or 76 cm (30 in) of water for the MWM 916-6, the filter required replacement.

The major factor affecting filter life on a waterbath scrubber equipped vehicle is the amount of DEA gener­ated by the engine, which is dependent on such factors as the vehicle's duty cycle, engine type and condition, and mine altitude. During the field evaluations, the ODEF lasted up to 10 h on the Jeffrey 4114 Ramcar, and up to 32 h on the 4110 Ramcar.

Another consideration affecting filter life is water saturation. Owing to space limitations, the water trap was not sized to handle the excessive amount of water expelled from an overfilled waterbath scrubber upon starting the engine. Thus, a significant amount of water may pass into the filter canister, saturating the filter when the system is overfilled. The simplest solutions to this problem are to avoid overfilling waterbath scrubber, or to postpone install­ ing the filter until after waterbath scrubber maintenance and initial engine startup.

SAFETY CONCERNS

Jeffrey received MSHA approval to use the retrofit DDEF system on both the 4110 and 4114 Ramcars. Mine operators have reported some problems with vehicles equipped with DDEF systems. Under certain circum­stances, the exhaust temperature will significantly exceed the recommended 95° C maximum for the filter. This condition results when the waterbath safety shutdown sys­tem malfunctions or is bypassed. The excessive tempera­ture may cause ignition of the filter and collected die­sel particulate. To avoid this problem, it is important to maintain the vehicle's safety systems in a permissible condition.

Direct contact with used DDEFs should be avoided if possible. Gloves should be worn during installation and removal to avoid contact with surfaces coated with soot and grease. The DDEFs should be bagged and brought to a disposal facility.
