Field evaluation of a passive diesel 


particulate filter at a limestone mine
 

J.D. NOLL AND L. PATTS 

J. D. Noll and L. Patts are lead research scientists with the 

Pittsburgh Research Laboratory, National Institute for Occupational Safety and Health (NIOSH), Pittsburgh, PA. 

Abstract

The Mine Safety and Health Administration (MSHA) has 

promulgated rules to limit the exposure of metal/nonmetal 

underground miners to diesel particulate matter (DPM). 

These rules have resulted in many types of control technologies being implemented including diesel particulate 

fi lters (DPF). The passive type DPF is the most desirable 

because it requires little maintenance and it regenerates 

while the vehicle is operating. One problem with passive 

DPFs is that many vehicles cannot develop the sustained 

exhaust temperatures needed to initiate DPF regeneration, and thus it is required that the DPF be catalyzed to 

decrease the temperature needed for regeneration. In some 

past studies, such catalyzed filters have been shown to cause 

an increase in the nitrogen dioxide concentrations in the 

exhaust, resulting in a potential health hazard. In this study, 

a lightly catalyzed DPF along with a fuel-born catalyst were 

employed to help reduce the DPM emissions from a 980F 

loader used for cleanup in a stone mine. After 30 days of 

use, the filter continued to regenerate successfully at the 

exhaust temperatures generated by this loader, substantially 

reduced DPM concentrations emitted in the exhaust, and 

did not cause a measurable increase in nitrogen dioxide 

levels that could be attributed to the device. 

Introduction

The Mine Safety and Health Administration (MSHA) has promulgated rules to limit the exposure of 

metal/nonmetal underground miners 

to diesel particulate matter (DPM) 

(MSHA, 2001, 2005). In May 2008, 

the personal exposure limit was 

lowered to 160 μg/m3

 total carbon 

(TC) (MSHA, 2005). In an effort to 

comply with this rule, mines are attempting to implement control technologies to decrease 

DPM concentrations in the mines. One type of control 

technology being tried is passive diesel particulate fi lters 

(DPFs). 

DPFs employed on equipment in underground metal/ 

nonmetal mines are usually a ceramic or silicon carbide 

wall-flow monolith type. The soot is collected and stored 

in the filter, and if the filter is not periodically cleaned, it 

will fill up and plug. Such plugging causes a high backpressure on the engine, which could 

damage the engine and possibly result in a filter fire. The filter either 

has to be replaced or the soot needs 

to be removed from the fi lter before 

the backpressure gets too high. 

The soot can be removed from 

the filter by regenerating it, which 

entails oxidization of the soot and 

the formation of CO2

. Uncatalyzed 

filters must reach a temperature 

above 500°C (930°F) to regenerate, while a catalyzed 

filter can regenerate at temperatures between 250° and 

300°C (480° and 570°F). 

Two filter regeneration schemes are in practice today. 

An “active” filter relies on an outside source to generate 

the heat to clean the filter. In this case, the filter has to 

be removed from the machine and be regenerated at a 

regeneration station or, if not removed, the filter has to 

be plugged into a power source at a regeneration station 

for a certain time period. A “passive” fi lter regenerates 

while the vehicle is operating, with the heat needed to 

initiate the regeneration process generated by the engine 

exhaust. The “passive” filter is obviously the preferred 

choice because the vehicle is not taken out of service and 

little maintenance is necessary. Passive fi lters, however, 

usually have to be catalyzed for regeneration to occur at 

the temperatures produced by most vehicles, and this catalyzation can cause an increase in NO2

 emissions, which 

could result in a health hazard. Even if a catalyzed fi lter is 

employed, not all vehicles can generate exhaust temperatures high enough to initiate fi lter regeneration. 

Several studies (laboratory and field) have shown that 

the ceramic filters can reduce DPM effi ciently.They have 

reported more than a 75% reduction in total DPM and 

more than a 90% reduction in elemental carbon when using ceramic filters for a short period (Roegner et al., 2002; 

Bugarski et al., 2003; Bugarski et al., 2005; MSHA, 2005). 

These studies have also shown an increase in nitrogen 

dioxide when using certain catalyzed filters, but they gave 

little information on the implementation and regeneration characteristics of these filters when operating in real 

mining conditions. Limited information also exists on 

the conditions under which passive filters regenerate, on 

the filter backpressure after extended use or on other 

implementation issues (such as a decrease in machine 
operator visibility) when installing and operating these 

DPFs under real mining conditions. 

In this study, the operators of a limestone mine retrofitted a loader used for clean-up operations with a 

passive DPF that was lightly catalyzed (lightly catalyzed 

filters are loaded with 0.11 to 0.53 kg/m3

 (3 to 15 g/cu 

ft) of platinum compared to around 1.76 to 2.82 kg/m3 

(50 to 80 g/cu ft) of platinum for catalyzed and heavily catalyzed filters) and used a fuel additive to initiate 

regeneration at low exhaust temperatures.This fi lter was 

selected because studies have shown that it will regenerate at regeneration temperatures are representative of a 

heavily catalyzed fi lter. This same fi lter was shown in an 

isolated zone study to reduce elemental carbon by more 

than 90% and yet not substantially increase atmospheric 

NO2 concentrations (Bugarski et al., 2003). In this study, 

the filter’s performance was monitored for more than one 

month under actual mining conditions. 

Methods 

Installation of DPF. A CleanAir Systems DPF system 

(described in the next section) was installed on a 980F 

Cat Loader (CAT 3406 engine, 310 hp,Tier 0).Two 305 by 

381 mm (12 by 15 in.) DPF filters were used in the system. 

The mine mechanics disconnected the OEM muffl er and 

installed the DPF. The DPF was affixed to the top of the 

loader as seen in Fig. 1. 

FIGURE 1 

The passive dpf installed on a loader. 

The engine exhaust temperature and backpressure 

were logged every 15 seconds.The temperature and backpressure were recorded with a CleanAir HiBack exhaust 

monitoring system and were downloaded every day for 

the first week to ensure that the filter was regenerating 

and that excessive backpressure would not pose the potential for engine damage.After two weeks, the backpressure (collected every 15 seconds) was downloaded for 

those two weeks and finally was again downloaded at the 

end of 30 days.Although data were downloaded at select 

intervals, backpressure and temperature data were collected every 15 seconds of equipment operation for the 

entire 30-day test. 

CleanAir Systems DPF system. The CleanAir Systems DPF system (CleanAir Systems, Santa Fe, NM) uses 

a Corning cordierite wall-flow monolith fi lter element 

wash coated with a proprietary platinum-based catalyst. 

The system is used in conjunction with a fuel additive 

(Clean Diesel Technology, Stamford, CT, Platinum Plus 

DFX-DPF), which is a bimetallic catalyst that contains 

both platinum and cerium allowing it to be used effectively at a dosage level substantially lower than other 

fuel-borne catalysts. Theoretically, the DPF system, with 

this fuel-borne catalyst, should passively regenerate during a duty cycle, which results in exhaust temperatures 

over 300°C (570°F) for extended periods (at least 30% of 

the operating time) of the cycle. According to the manufacturer, this catalyzed filter/fuel additive system does not 

promote conversion of NO to NO2

. 

Tailpipe measurements. After installation of the fi lter, 

the loader was tested while loading material from an embankment outside the mine.This loading operation simulated a high load condition for the engine. The vehicle 

was run at this high load for 1 minute for each sampling 

(one sample for gases and one sample for particulate). 

During the simulated loading operation, an ECOM KL 

exhaust emissions analyzer was used to measure tailpipe 

exhaust gasses and exhaust opacity. The ECOM tailpipe 

probe was placed in the exhaust upstream of the DPF and 

a smoke dot was collected. (A smoke dot is an opacitymeasuring method whereby the exhaust is collected on a 

paper filter and the color of the dot formed is correlated 

to a number between 0 and 9, with 9 being the darkest 

or having the most soot.) The ECOM probe was next 

inserted upstream of the DPF, and the exhaust was measured for nitrogen dioxide. Both the smoke dot and nitrogen dioxide measurements were repeated downstream 

of the DPF. 

After the DPF was operated for 30 days, the exhaust 

downstream of the filter was measured again to see if 

there would be a difference after 30 days of fi lter use. 

Vehicle. The loader used in this study was used to 

clean up muck piles in the mine. It would typically be 

used for 4 to 10 hours per day, fi ve to six days per week. 

During the month of testing, it was run for approximately 

125 hours. 

NIOSH researchers were at the mine every day for 

the first week and then about once per week after that. 

The data were downloaded and reviewed, and the vehicle 

operators were asked about any problems while using the 

dpf (e.g., visibility issues). 

Results and discussion 

Visibility. The filter was installed on top of the loader as seen in Fig. 1, and this location could decrease the 

miner’s visibility. In this case, the operators (there were 

two different operators) of this vehicle did not have a 

problem performing their operational duties, and they 

recorded no significant problems with visibility. There 

is no guarantee that this dpf will not affect a machine 

operator’s visibility when retrofitted on other vehicles 

or if this vehicle were used by another operator (e.g., a 

smaller person). Visibility is a concern when retrofi tting 

some dpfs, and this needs to be addressed any time a fi lter 

is retrofitted to a particular vehicle. 

Tailpipe measurements. On the first day of installation, the Bacharach smoke dot number was 9 upstream 
 

of the filter and 0 downstream of the 

fi lter, as seen in Fig. 2. 

FIGURE 2 

The results from the Bosch smoke test before and after the dpf installation. The Bacharach 

smoke number measures the color of 

the exhaust collected on a paper fi lter. 

A smoke dot number of 9 would be 

a dark black. The smoke dot number 

of 0 would indicate no color. Because 

elemental carbon (EC) is black and no 

color was seen after the filter but dark 

black was observed before the fi lter, 

the fi lter seemed to eliminate most of 

the EC. EC has also been shown to 

correlate to blackness on a filter in 

other studies. In a study in an isolated 

zone, when the smoke dot number 

was reduced significantly, it was shown 

that EC was also reduced signifi cantly 

when sampling before and after dpfs 

(Bugarski et al., 2003). One technique 

for determining EC concentrations 

measures the blackness of the fi lter by 

adsorption (Noll and Janisko, 2007). 

The reduction in EC by the dpf is not 

surprising because this fi lter has been 

tested and found to be 99% effi cient in 

filtering EC (Bugarski et al., 2003). 

The tailpipe nitrogen dioxide 

increased from 20 to 25 ppm. This 

amount of increase (5 ppm) could not 

be attributed to the filter device because it could be due to engine variation. For example, in a previous study, 

when tailpipe measurements were taken from a LHD mining vehicle in an 

isolated zone of a metal mine, the NO2 

concentrations ranged from 12 to 17 

ppm (a difference of 5 ppm) between 

three runs from the same vehicle under the same conditions (engine under 

torque converter stall and sampling 

upstream of control technology) (Bugarski et al., 2003). In fact, when the 

authors measured NO2 downstream 

of the dpf in this study after 30 days, it 

was 19 ppm (see below), which is less 

than the measurement before the dpf. 

The increase from 20 to 25 ppm NO2 is 

likely attributed to engine fl uctuation. 

After 30 days of use, the smoke 

dot number downstream of the fi lter 

remained 0 (as seen in Fig. 2) and the tailpipe nitrogen dioxide was 19 ppm. EC continued to be effectively fi ltered 

and nitrogen dioxide was not higher than the reading 

before the filter even after 30 days of use. 

Backpressure and exhaust temperature. Figure 3 

shows the backpressure and temperature trace of the 

exhaust near the filter. 

FIGURE 3 

(a) The backpressure vs. point (a point was taken every 15 seconds) and (b) 

temperature of the dpf vs. point. 

The backpressure would build 

up and then decrease, indicating that regeneration was 

occurring at the exhaust temperatures produced by this 

loader.When the filter was green (unseasoned), the backpressure occasionally rose to 17 kPa (70 in. of water) (but 

did not remain long at this pressure); however, after a 

week of use, the filter regenerated when the backpressure reached approximately 7.5 kPa (30 inches of water). 

The filter manufacturer recommends that the machine 

fuel be treated with twice the dosage of the fuel additive 

when the filter is first installed.This recommendation was 

not followed for this test because there was not enough 

fuel additive delivered at the start of the fi lter trial (this 

could be the reason for the backpressure rising to 17 kPa 

[70 inches of water] during the first week of fi lter use). 

The filter should initially be degreened to achieve optimal performance without subjecting the engine to higher 

backpressures. 

Figure 3 also shows the temperature trace of the fi lter. This loader filter temperature was above or at 300°C 

(570°F) for about 65% of the time (calculated from the 
temperature data). These temperatures are above those 

recommended to initiate the regeneration process with 

this filter and were reached by a cleanup loader. The engine of a cleanup loader is not worked as hard as that of a 

production loader and will not produce exhaust temperatures as high as those of a production loader. Therefore, 

the production loaders in this mine should be able to 

produce the exhaust temperatures necessary to regenerate this DPF. 

Cleaning or replacing the fi lter. NIOSH recorded data 

from this filter for 30 days of loader operation. The fi lter 

remained on the vehicle for several weeks after this 30­

day test period.The mine reported that after six to seven 

weeks of use, the tailpipe downstream of the fi lter began 

to show signs of a light blackness indicating that the fi lter 

probably needed cleaning as a result of ash accumulation, 

which is a result of the filter regeneration process. 

Conclusion 

This passive DPF performed well on this loader and 

probably would perform well for most of the loaders in 

this mine (based on their projected duty cycle).The DPM 

concentration generated by this loader was substantially 

reduced and nitrogen dioxide increase could not be attributed to the dpf. Little maintenance was required for 

this filter because filter regeneration was accomplished at 

the exhaust temperatures generated by the loader. 

A fuel additive is necessary for this fi lter to regenerate at its inherent operational exhaust temperatures, and 

occasionally the fi lter will need to be cleaned to remove 

the ash buildup resulting from filter regeneration. Before 

retrofitting this DPF on a mining vehicle, visibility, regeneration (exhaust temperature profiles), backpressure and 

filter capacity should be considered. Exhaust temperature 

profiles are important because the dpf may not work 

properly on a vehicle without the appropriate temperature profi le. The fi lter needs to be sized large enough so 

that excessive backpressure is not created. Operator visibility could be different for each vehicle fitted with such 

a filter and should be considered on a case-by-case basis. 

Economic issues associated with the use of this fi lter were 

not examined during this study. 

Disclaimer 

The mention of any company or product does not 

constitute an endorsement by the Centers for Disease 

Control and Prevention. The findings and conclusions in 

this report are those of the authors and do not necessarily 

represent the views of the National Institute for Occupational Safety and Health. 

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