Implementation of diesel particulate filter technology in underground

metal and nonmetal mines

A.D. Bugarski, G.H. Schnakenberg, Jr. & L.D. Patts

National Institute for Occupational Safety and Health, Pittsburgh Research Laboratory, Pittsburgh,

Pennsylvania, USA

ABSTRACT: Achieving substantial reductions in the exposure of underground miners to diesel particulate

matter in a number of metal and nonmetal mines in the U.S. depends on the ability of the industry to widely

implement advanced diesel emissions control technologies, primarily diesel particulate filter (DPF) systems.

Recent field studies showed that diesel particulate filter systems with Cordierite and silicon carbide ceramic

filter elements are capable of reducing concentrations of diesel particulate matter and elemental carbon by

more than 70 and 90 percent, respectively. But those studies and several other attempts to implement DPF

systems in underground mines revealed a number of relatively unique technical and operational challenges that

are limiting industry-wide implementation of this technology. This paper provides detailed analysis of some of

those challenges and short overviews of several projects launched by NIOSH Pittsburgh Research Laboratory

in an attempt to provide the underground mining industry with more adequate DPF systems. These systems

employ advanced technologies to overcome the most pronounced challenge, the regeneration of DPF elements

installed on mining engines that generate relatively low exhaust temperature. This paper presents essential

findings obtained through a long-term evaluation of DPF system with diesel fuel burner technology installed

on a heavy-duty load-haul dump vehicle in an underground metal mine. In addition, the essential results of the

laboratory evaluation of a popular on-highway DPF system which was adapted and optimized for underground

mining applications will be summarized in this paper.

1 INTRODUCTION

Exposures of underground (UG) metal/nonmetal miners to diesel particulate matter (DPM) are significantly higher than those of any other occupation

(Schnakenberg and Bugarski 2002). Compliance sampling conducted by theMine Safety and HealthAdministration (MSHA) in 178 mines between 2002 and

2005 (Cash and Baughman 2005) showed that UG

miners in precious metal mines are exposed to DPM

concentrations as high as 3300μg/m3. The magnitude

of those exposures was found to be directly related

to the extent of use of diesel-powered equipment and

the ability of mines to provide fresh air in the work

zones. Ever since MSHA promulgated rule 30 CFR

57.5060 limiting exposures of UG metal and nonmetal miners to DPM, the mining community has

been working on identifying technically and economically feasible strategies and technologies to reduce

exposures to particulate matter and noxious gases

emitted by diesel powered equipment. The challenge

is designing, optimizing, and implementing technical and economical strategies and technologies that

can accommodate requirements unique to UG mining environments and operations. In the majority of

cases, solving these issues will require a multifaceted

approach, a coordinated effort from several mine

departments, and integral solutions.

The strategies available to the UG mining industry

to reduce the exposure of miners to DPM can be generally divided into two groups: (1) strategies that are

designed toward controlling the DPM once it has been

released in the atmosphere; and (2) strategies that rely

on controlling DPM emissions before they are released

from the tailpipe.

The leading strategies for improvements in controlling diesel emissions released in the atmosphere

are improvements in ventilation and more extensive

use of environmental cabs and personal protective

equipment (PPE). Ventilation is traditionally used to

control concentrations of various pollutants in UG

mines. The results of aforementioned MSHA compliance sampling indicate a need for improvements

in ventilation in a number of U.S. metal and nonmetal UG mines. The first step toward making a

safer work environment for UG miners with respect

to diesel emissions is to provide sufficient quantities

of fresh air to dilute the emitted carbon monoxide

(CO), carbon dioxide (CO2) and nitric oxide (NO),

to bellow their respective 1973 American Conference

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of Governmental Industrial Hygienists (ACGIH) timeweighted average (TWA) threshold limit values (TLV)

and the nitrogen dioxide (NO2) below its 1973 ACGIH

TWA TLV and short term exposure limit (STEL) values. The 1973 ACGIH TWA TLVs and STELs are

currently used by MSHA to regulate the exposure

of underground metal and nonmetal miners (30 CFR

57.5001 1995). Diluting DPM emissions sufficiently

to reduce ambient concentrations below interim and

eventually final U.S. standards (30 CFR 57.5060)

requires that substantial additional quantities of fresh

air are supplied to the work zones in underground

mines (Schnakenberg 2001). Unfortunately, increased

ventilation capacity is linked to a substantial increase

in capital and operational costs (McPherson 1993).

Some local improvements can be achieved with more

effective management and distribution of available air

quantities.

Environmental cabs are successfully used in UG

and surface mining operations to control operator’s

exposures to dust, noise and some other pollutants.

Nevertheless, MSHA surveys conducted in metal and

nonmetal UG mines (30 CFR 57.5060) showed that

adequately designed and maintained air conditioned

and filtered environmental cabs might reduce personal

exposures of underground miners to DPM by as high as

80%. Unfortunately environmental cabs do not provide

protection to the workers outside of those enclosures.

Costs associated with retrofitting existing vehicles and

ordering new vehicles with environmental cabs are

relatively high.

According to the current regulations (30 CFR

57.5060) the third available option to control DPM

exposures, PPE should be only used to supplement

engineering and administrative controls when those

are found unfeasible or insufficient to reduce exposures to DPM.

A number of in-mine studies (McGinn 2004,

Bugarski et al. 2005, Stachulak et al. 2005, Bugarski

et al. 2006a,b) showed that a viable alternative to controlling concentrations of diesel pollutants released

in UG environments is to reduce DPM and gaseous

emissions from their diesel-powered fleet. In general,

the concentrations of pollutants emitted by current

UG mining diesel fleets can be significantly reduced

by reducing engine-out emissions and implementing

various exhaust aftertreatment technologies. In a number of instances an integrated approach is needed to

achieve desired emissions reductions.

The adopted strategy should preferentially address

vehicles that are highest contributors to the miner’s

exposure to DPM and gases. Typically, the largest contributors are heavy-duty production vehicles such as

load-haul-dump (LHD) vehicles and haulage trucks

that are often operated in relatively poorly ventilated areas. The contribution of increasingly popular

light-duty vehicles, often powered by relatively high

polluting engines, to the total DPM and gaseous concentrations in mine air should not be neglected (Rubeli

et al. 2003).

Instituting a comprehensive emissions-based maintenance program for heavy-duty and light-duty dieselpowered vehicles should be the first step toward

reducing concentrations of particulates and gases

emitted by a diesel fleet (McGinn 2004). Establishing the in-use emissions from an existing diesel fleet

is essential to the selection of an adequate emissions

control strategy, the optimization of selected control

technologies, and is a prerequisite for implementation

of advance diesel emissions control technologies. Periodic emissions measurements should be also used to

evaluate the effectiveness of the implemented control

strategies and technologies.

Reductions in engine-out emissions can be also

achieved by the use of reformulated fuels. The effects

of water-fuel emulsions, blended biodiesel fuels, ultra

low sulfur diesel fuel, and #1 diesel on concentrations of particulate matter and gases in mine air were

evaluated by Bugarski et al. (2006b) during isolated

zone testing at Stillwater Mining Company Nye Mine.

This study showed that using the Lubrizol PuriNOx

cold-weather and warm-weather water-fuel emulsions

reduced EC concentrations by approximately 70%

and 85%, respectively. The tested 20% and 50% soy

biodiesel blends showed EC reductions of 49% and

66% respectively. The reductions were slightly less

pronounced for the 20% and 50% yellow grease

blends, 33% and 56% respectively. Several attempts to

implement alternative fuels in underground nonmetal

mines revealed problems related to compatibility of

certain fuels and engine components.

Although substantial reductions in exposures of

underground miners to DPM in a number of metal and

nonmetal mines in U.S. can be achieved implementing the aforementioned strategies and technologies,

the best results can be achieved by complementing

those strategies with a widespread implementation

of advanced aftertreatment diesel emissions control

technologies. Such control technologies include diesel

particulate filter (DPF) systems and filtration systems with high-temperature disposable filter elements

(DFE). Recent field studies (Bugarski et al. 2006a,b)

showed that diesel particulate filter systems with

Cordierite and silicon carbide wall flow monolith filter elements are capable of reducing concentrations

of DPM and EC in mine air by more than 70%

and 90%, respectively. The same study showed that

the disposable filter elements (DFE) from Donaldson

and Filter Services reduced the vehicle EC contribution by 92% and 70%, respectively. The same DFEs

reduced downstream TPM concentrations determined

using gravimetric analysis by 84 and 58%, respectively. Despite their very high effectiveness in removal

of EC and TPM, DPF systems have not been widely

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accepted by the UG mining industry. Various implementation issues were found to limit industry-wide

implementation of this technology.

2 DPF IMPLEMENTATION ISSUES

The high demand for aftertreatment systems for the

efficient control of particulate emissions from onhighway and off-road diesel-powered vehicles resulted

in a rapid development of DPF technology. Two comprehensive long-term studies sponsored by the Diesel

Emissions Evaluation Program (DEEP), one conducted at Noranda’s Brunswick Mining and Smelting

Mine (McGinn 2004) and one at the International

Nickel Company’s Stobie Mine (Stachulak 2005)

demonstrated the potential of modern DPF systems

for controlling DPM emissions in underground mines.

Those studies also offered insight into the problems associated with the use of those systems on

underground mining vehicles. Short-term studies conducted in SMC Nye mine (Bugarski et al. 2006a,b)

also revealed a number of technical and operational

challenges unique to UG mining applications.

The relatively large number of different types of

diesel-powered vehicles currently operated in UG

mines and the diversity of mining methods complicate

the selection and implementation of this technology.

Implementation is complicated by the fact that DPF

systems are currently available to UG mining industries only as aftermarket retrofits for existing and

newly introduced vehicles. Retrofitting aftermarket

DPF systems on a wide range of UG mining vehicles requires a relatively large level of optimization

and customization for individual applications.

One of the major technical difficulties associated

with DPF systems is filter medium regeneration,

which involves periodic oxidizing or burning of the

accumulated DPM on the filter. In general, the filter

can be regenerated using heat and temperature generated by the engine or by using heat and temperature

provided by various external sources of energy. Systems which are using the former concept are known

as passive systems. The latter concept is used in

active systems. Due to their relatively lower complexity and operational and logistics requirements, passive

DPF systems are the more desirable for underground

operations.

Surveys of duty cycles for UG mining vehicles

conducted across the mining industry revealed that

temperature generated by engines that power a majority of light-duty and large a number of heavy-duty

vehicles over regular duty cycles is insufficient to

support passive regeneration of currently available

uncatalyzed, and quite often catalyzed, filters. Data

indicates that mining vehicles spend a significant percentage of time at engine operating conditions which

do not favor passive regeneration. Data obtained at

Noranda’s Brunswick Mining and Smelting show that

their vehicles spend an average of over 30% of time

at low idle (McGinn 2004), engine operating mode

that produce DPM emissions but does not support

regeneration.

Several UG metal mines are currently operating

heavy-duty vehicles such as LHDs and haul trucks

with platinum coated passive DPF systems. Platinumbased catalyst formulations are found to be efficient

in facilitating passive regeneration of filters by lowering the regeneration temperature from approximately

550◦C (uncatalyzed DPF) to approximately 330◦C

(Jelles et al. 1999). Those formulations are also efficient in lowering CO and hydrocarbon emissions by

oxidizing them to CO2. Unfortunately they are also

promoting conversion of NO to NO2 (MSHA 2002).

Isolated zone studies conducted by Bugarski et al.

(2006a,b) showed a two or even three fold increase

in emissions of NO2 when certain types of platinum

catalyzed DPF and DPF/DOC systems were used.Theoretically, vehicles with platinum coated DPF systems

should be operated only in areas of UG mines where

sufficient quantities of fresh air are supplied to dilute

NO2 concentrations to or below a ceiling limit of 5

ppm as established in MSHA standards at 30 CFR

57.5001.

The alternative solution is to use active DPF systems

which are designed to collect DPM over a predetermined period of engine operation, e.g. one shift.

Depending on design, regeneration is performed with

the filter element remaining on-board of the vehicle or

with it removed from the vehicle. The on-board regeneration process commonly uses an electrical heater

but can use a diesel fuel burner. The regeneration of

such systems can only occur with the vehicle parked

with the engine off. Again depending on system

design, regeneration can take anywhere from a halfhour or less (fuel burner) to eight hours. The off-board

regeneration commonly uses electrical heaters or kilns

designed for the purpose.

Stachulak et al. (2005) found that active systems

with off-board and on-board electrically regenerated

systems can be successfully used to control DPM

emissions from light- and heavy-duty UG mining vehicles. The authors attributed difficulties with operating

on-board regenerated system in underground mine to

human error and stressed the importance of discipline

and education. A majority of other mine operators

found on- and off-board electrically regenerated systems to be too complex, delicate, and expensive to be

widely implemented in underground operations.

Therefore, there is an apparent need for efficient

DPF systems which will satisfy the following criteria:

(1) regeneration spontaneously regenerates at relatively low exhaust temperatures; (2) does not produce

secondary emissions of NO2 and other pollutants;

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(3) operates “transparently” to the operator; and (4)

is robust and reliable.

3 OPTIMIZATION OF DPF SYSTEMS

FOR UNDERGROUND MINING

APPLICATIONS

NIOSH PRL has partnered with selected manufacturers of control technologies and the mining industry to

develop and optimize several promising DPF concepts

for UG mining applications and therefore facilitate the

implementation of DPF technology in underground

mines. This paper describes two of those projects

which may provide mining industry with DPF systems

that satisfy the aforementioned criteria.

3.1 ArvinMeritor fuel burner DPF system

The objective of the first project, conducted in cooperation with ArvinMeritor (AM), Commercial Vehicle

Emissions, Troy, MI, was the field evaluation of a prototype DPF system that uses a diesel fuel burner to

periodically increase the temperature of the full flow

of diesel exhaust in order to initiate and support filter

element regeneration.

Under NIOSH contractAM designed and fabricated

the prototype fuel burner system for a heavy-duty production LHD from the underground mining fleet at

the Stillwater Mining Company’s (SMC’s) Nye Mine

located in Nye, MT. The vehicle was powered by a

modern electronically-controlled turbocharged Deutz

BF4M1013 FC MVS engine. The system shown in

Figure 1 consisted of a vertically mounted uncatalyzed ceramic wallflow monolith filter element, the

DuraTrap RC (Corning, NY), followed by a metal substrate diesel oxidation converter (DOC).The DOC was

designed to control CO and hydrocarbon emissions

during normal vehicle operation and the regeneration process. The original DOC had a platinum (Pt)

based formulation wash-coated onto a substrate. After

approximately 470 operating hours, the Pt DOC was

replaced with a similar DOC with a palladium (Pd)

based washcoat. The computer controlled diesel fuel

burner is integrated on the inlet side of the DPF

element.

The control box with fuel pump, air blower, and

controller is mounted in the engine bay (see Figure 2).

When activated, the burner ignites diesel fuel injected

into the combustion chamber to heat the engine

exhaust and filter element to about 650◦C, a temperature sufficient to support DPF regeneration. The

regeneration process was initiated at predetermined

time intervals.

The system was installed on the LHD in January

2004. After the installation the vehicles was returned

into production and DPF system was subjected to

Figure 1. ArvinMeritor DPF system installation.

Figure 2. ArvinMeritor DPF system control box.

long-term evaluation.The objective of the study was to

evaluate the suitability of the AM DPF system for controlling diesel particulate matter emissions generated

during heavy-duty underground mining applications.

The long-term field evaluation provided an opportunity to evaluate various aspects of implementation of

this DPF system, such as installation and regeneration, in an underground mining application. During

the evaluation the fuel burner, acting automatically

at preset intervals, successfully regenerated the filter

element.

After 470 hours in production the system was tested

in the isolated zone at the Nye Mine. The objective of

this testing was to establish the effects of two configurations of theAM DPF/DOC system on concentrations

of aerosols and selected gases in mine air. Three tests

were conducted, two with tested configurations (DPF

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with Pt- and Pd- DOCs) and a baseline in which the

host LHD was equipped with a muffler. For each of

the tests the vehicle was operated over the same simulated duty cycle and at a similar ventilation rate. The

vehicle was fueled with #1 diesel supplied by a local

refinery. This higher quality fuel was used by the mine

in underground operations as a part of their strategy to

reduce the exposure of miners to diesel emissions. The

effects on tailpipe emissions were estimated from the

results of aerosol and gas measurements performed at

the upstream and downstream end of the isolated zone

and at the vehicle. Several methods and instruments

were used to determine concentrations of the aerosols

in the incoming fresh air at the upstream station and

in the contaminated air at the downstream and vehicle

sampling stations.The gas monitors were used for realtime measurements and recording of concentrations of

CO, CO2, NO, and NO2 at all three sampling locations.

The details on the methodology and instrumentation

used during this test are available in Bugarski et al.

(2006b). The effects of each configuration were quantified by comparing the contaminant concentrations

observed for the test when the vehicle was operated

with the test configuration to those observed for the

baseline test.

With the exception of some difficulties in the development phase the operation of this system was found

to be relatively transparent to the vehicle operator.

Despite the complexity this prototype system was

found to be relatively reliable.The system required regular maintenance and couple of unscheduled repairs.

The isolated zone testing revealed undesired effects

of the Pt catalyzed DOC on the tailpipe emissions of

NO2. The test on the AM DPF system with the Pt

catalyzed DOC had to be terminated prematurely to

prevent an overexposure of the vehicle operator to

NO2. During the initial stages of that test, the gas

monitor at the vehicle sampling station recorded concentrations of NO2 that were close to, or exceeded, the

1973 ACGIH TLV of 5 ppm. This limit is currently

used by MSHA as a Ceiling Limit to regulate exposure of underground metal and nonmetal miners to

NO2 (30 CFR 57.5001). The Pt-catalyzed DOC was

then replaced with one with a Pd-based catalyst. The

AM DPF with the Pd DOC raised the average and

peak downstream NO2 concentrations approximately

30%, yet at the prevailing ventilation rate of 19 m3/s

(40,258 ft3/min) used in the tests, the concentrations of

NO2 were well below 1973 ACGIH TLVs and the test

was successfully completed.This test was extended for

an additional three full cycles to assess the effects on

emissions during the diesel fuel burner regeneration

process.

The tests showed that the AM fuel-burner DPF

system with a Pd catalyzed DOC reduced the EC concentrations by 92%. The same system reduced downstream concentrations of TPM by 69% (gravimetric

measurements) and by 72% (TEOM 1400a measurements). The Pd-catalyzed DOC increased the sulfate

concentration from 3.4 to 15.3μg/m3. Particle size distribution results indicated a 105% increase in the total

particle number, possibly corroborating the hypothesis of increased sulfate aerosol concentrations. During

the regeneration of the AM DPF system, the peak

aerosol number concentration increased from about

1,666,000/cm3, observed during the normal operation,

to approximately 3,177,000/cm3.

NO concentrations at the downstream and vehicle

sampling locations were found to be generally lower

during tests with the AM DPF having Pt and Pd DOCs.

The slight reduction in NO is partially attributable to

the conversion of NO to NO2 by the DOCs.

The net contributions to the concentration of CO at

the downstream and vehicle sampling stations of the

LHD, equipped with the AM DPF with either a Pt or

Pd DOC were found to be negligible. CO reductions

can be attributed to the DOC’s catalytic processes.

The system was successfully operated until March

2005. In fall 2004 ArvinMeritor decided to stop providing this system as a retrofit and to offer this system

only as part of OEM solutions.

The DPF system with somewhat similar fuel burner

concept is also offered by HUSS Umwelttechnik

GMBH, Nürnberg, Germany. This system is using

an on-board diesel fuel burner to regenerate a silicon

carbide monolith element. In contrast to the AM system, the regeneration is performed while the engine is

turned off. However, it requires less than 30 minutes to

perform and can be done anywhere there is normally

adequate ventilation (Rembor 2005). Thus regeneration can be performed at shift change or “lunch-time”

periods. It appears to be suitable for retrofit applications for light- and heavy-duty UG mining vehicles

and hence worthy of serious consideration.

3.2 Low NO2 Johnson Matthey Continuously

Regenerating Trap system

Johnson Matthey (JM) Royston, England developed

a Continuously Regenerating Trap (CRT) system to

control DPM emissions from on-highway trucks and

urban buses (O’Sullivan et al. 2004). The core of

the system is a Cordierite filter element. A platinum

catalyst upstream of the filter is used to promote conversion of NO to NO2. The exhaust gases, enriched

with NO2, pass to the filter element where they oxidize

the collected particulate matter, thereby regenerating

the filter. Since this reaction occurs at much lower temperatures than the reaction between particulate matter

and oxygen, it is capable of passively regenerating

when temperatures exceed approximately 250◦C for at

least 40% of the operation time. When the NO2 reacts

with the particulate matter, the products formed are

carbon dioxide and NO. Inevitably, some NO2 passes

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Figure 3. Johnson Matthey CRT system.

Figure 4. Johnson Matthey CRT system control box.

through the filter (Ayala et al. 2001, O’Sullivan et al.

2004). This process, commonly known as NO2 slip,

has prevented widespread application of the CRT system in underground mining, where the low exhaust

temperatures needed for regeneration would be greatly

appreciated.

JM engineers (O’Sullivan et al. 2004) proposed two

potential solutions to the problem. The first solution

uses an urea-based selective catalytic reduction (SRC)

system to control both NO and NO2 emissions. The

second solution uses hydrocarbon (HC) injection onto

a catalyst element after the filter which decomposes

the NO2.The former solution was deemed too complex

and delicate for UG mining applications. The latter

concept was adopted and pursued further.

NIOSH PRL sponsored construction of the prototype system shown in Figure 3. The NO2 decomposition catalyst is shown on the right-hand side of the

picture.

The operation of the system is controlled continuously using a HC injection controller that is designed

to be mounted on-board of the vehicle (see Figure 4).

The fuel is injected at the predetermined rates at exhaust temperatures above 250◦C. The controller box

contains the fuel pump, air compressor, and controller.

The test cell at University of Minnesota Center for

Diesel Research Laboratory was used to optimize and

evaluate this system. The system was tested using a

1999 Cummins ISM engine rated at 370 hp. An

eddy-current dynamometer was used to test the engine

using the ISO 8178 8-Mode Steady State Test and

Inco Transient Cycle. The engine was fueled with the

required ultra low sulfur fuel. This fuel, supplied by

Consumers’ Co-operative Refineries Ltd. Saskatoon,

Saskatchewan, had average sulfur content less than

15 ppm by weight.

The particulate matter and gaseous emissions were

established for three test configurations: (1) engine

only, (2) engine equipped with JM CRT system, and

(3) engine equipped with JM CRT system with HC

injection and NO2 decomposition catalyst. The emissions from the JM CRT system were used to optimize

the rate of hydrocarbon injection. After the system was

optimized the effects of hydrocarbon injection on NO2

slip from the CRT system were evaluated.

The effects of the three configurations on the concentrations of NO, NO2, CO, as well as non-methane

hydrocarbons (HC), and the effects on the mass emissions of diesel PM (gravimetric) and elemental carbon

(NIOSH 5040 analysis) were established using an

array of instrumentation.

The tests with the JM CRT system (Zarling et al.

2005) showed that the system reduced on average DPM

emissions by more then 90% and EC emissions by

95%.At the several test modes the system substantially

increased fraction of NO2 in total NOx emitted by the

test engines. When engaged the HC injection and NO2

decomposition catalyst reduced NO2 emissions out of

the CRT system below those produced by the engine

alone. The remaining issue is control of NO2 slip at

exhaust temperatures below 250◦C.

After completion of laboratory development and

evaluation, the JM CRT system with HC injection will

be subjected to long-term field evaluation. The system

will be installed on a heavy-duty underground mining

vehicle and evaluated in production for at least 1500

hours.

4 CONCLUSION

Implementation of advanced DPF technology plays

an important role in efforts to reduce the exposure

of UG miners to DPM in a number of U.S. metal

and nonmetal mines. Developing and optimizing DPF

systems for specific UG mining applications are essential for successful implementation of this relatively

novel technology in underground mines. Two concepts described in this paper showed that it is possible

to design DPF systems that will be able to operate

and regenerate at relatively low exhaust temperatures

without introducing the risk of exposing miners to

elevated NO2 concentrations. Field evaluation of the

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prototype AM system showed that engineering of a

rather complex DPF system with a fuel burner to a

reliable and dependable system transparent to a vehicle operator requires concerted efforts of both the

filter supplier and mine operator. If successfully completed, the JM CRT system with NO2 slip control will

provide the mining community with a much-needed

viable solution for controlling DPM emissions without

an adverse increase in NO2 production. The findings

from these two studies should facilitate the implementation of DPF technologies in underground mines

and ultimately lead to reducing miners’ exposure

to DPM.

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