Diesel aftertreatment control technologies in underground mines: 

the NO2 issue 

E.G. Cauda, A.D. Bugarski, & L. Patts 

National Institute for Occupational Safety and Health (NIOSH), Office of Mine Safety and Health 

Research, Pittsburgh, Pennsylvania, USA 

ABSTRACT: Extensive use of diesel-powered equipment makes reduction of underground miner exposure to 

diesel particulate matter and gaseous emissions from diesel-powered equipment a major challenge for the mining 

industry in the U.S. and worldwide. Improvements in mine ventilation are a commonly used remedy to meet more 

stringent emission limit for all pollutants. However, mine operators have other options to reduce diesel particulate 

matter emissions including the adoption of more advanced engine technologies, aftertreatment control strategies, 

and the use of biodiesel fuels. These strategies have been found to be effective in reducing the mass concentration 

of diesel particulate matter (DPM) but, as a side effect, they can modify the tailpipe emission of another pollutant, 

nitrogen dioxide (NO2). If the concentration of NO2 exceeds the regulatory enforced limit, an increase in 

ventilation rate becomes necessary with related increased power demand and/or the cost of supplemental air 

shafts. This paper provides an overview of what effects the different exhaust aftertreatment technologies and 

control strategies available to the mining industry have on the NO2 emissions. Specific focus is given to their 

effect on the ventilation strategy in an underground mine environment. 

1 Introduction 

Diesel engines are the major source of exposure for 

underground miners to nitric oxide (NO) and nitrogen 

dioxide (NO2). NO and NO2 cause short-term (acute) and 

long-term (chronic) health problems, mostly pulmonary 

and cardiac related (Sunyer et al. 1997; Stieb et al. 2002). 

NO2 is a potent oxidizer and can potentially interact with 

other pollutants and induce the formation of ozone (O3) 

and secondary hazardous particles (Peters et al. 1999; 

WHO 2005). 

The exposure of underground miners to NO and NO2 

is regulated by the Mine Safety and Health 

Administration (MSHA). The enforced TLV limits for 

NO and NO2 are 25 ppm (TWA) and 5 ppm (ceiling) 

respectively for both metal/non metal mines (30CFR 

57.5001) and coal underground mines (30CFR 75.322). 

The TLV-Ceiling limit is by definition the concentration 

which may not be exceeded for any length of time. 

MSHA used the ACGIH's 1972 (coal) and 1973 (metal 

and nonmetal) TLVs to set these standards (OSHA 1988). 

In US coal mines, the exposure limits of underground 

miners to pollutant concentrations determine the 

ventilation rate specific for certified diesel engines. The 

ventilation rates (VR) are based on the amount of fresh air 

needed to dilute CO, CO2, NO, NO2 in the undiluted 

exhaust gas to the TLVs when the engine is operated at 

the worst emission condition for that specific pollutant. 

The prescribed nameplate VR (ventilation rate listed on 

the nameplate for the engine) pertains to the worst engine 

operating conditions from an emissions toxicity 

standpoint and therefore represents a minimum ventilation 

value applicable to the area where the engine is operated 

(30CFR 7.88). 

As engine exhaust emissions, NO and NO2 are 

generally referred to as nitrogen oxides (NOX) and 

together with the Diesel Particulate Matter (DPM), they 

represent the most difficult diesel engine emission 

products to control. NO is the main contributor to the 

NOX concentration, and several factors influence the 

partitioning between NO and NO2 in the diesel exhaust. 

The NOX and DPM emissions control technologies, in 

optional retrofit or in original equipment manufacturer 

(OEM) form, are gradually finding their way into 

underground mining. This manuscript will provide an 

overview of the effects of the various technologies used to 

control diesel emissions on NO, NO2, and NOX emissions. 

Emphasis will be placed on the technologies that are 

currently used by underground mining operators to 

control gaseous emissions and particulate matter 

emissions, such as diesel oxidation catalysts (DOCs) and 

diesel particulate filters (DPFs) systems. In addition, this 

manuscript will briefly examine the effects of other 

emission control technologies such as selective catalyst 

reduction (SCR), lean NOX trap (LNT), and lean NOX 

catalyst (LNC). 

2 The Ventilation rate 

Each specific engine model approved by MSHA and 

present in underground mines must have an approval plate 

that specifies the minimum volume of air required to 

ensure that the exhaust emission levels do not exceed the 

exposure limits for NO, NO2, CO, and CO2. The approval 

process involves emission testing on an engine 

dynamometer using the ISO 8178 C1 test cycle. Based on 

the test results, MSHA calculates the ventilation rates for 
each approved engine model (30CFR 7.88). The 

ventilation rates are computed as the quantity of 

additional ventilation air that has to be provided to the 

mine in order to dilute raw exhaust emissions to the 

respective TLV levels. This procedure ensures that mine 

operators increase the amount of ventilation as more 

engines are introduced underground. 

As described in the following sections, several 

aftertreatment control technologies can have an effect on 

the NO2 raw exhaust concentration. Periodic tailpipe NO2 

emission data can be collected while conducting the 

repeatable loaded engine test required for all heavy duty 

and permissible equipment under the weekly carbon 

monoxide (CO) check. This test measures the undiluted 

CO emissions of the engine operating under a loaded 

condition. This same test procedure can also be used to 

measure undiluted NO2 emitted from the engine (MSHA 

2002a). 

This procedure allows the mine company to determine 

the variation of the NO2 concentration caused by the 

aftertreatment control technology and to calculate a 

multiplier relative to this variation. In the case where NO2 

has increased, the mining company must obtain the 

MSHA ventilation rate for NO2 (MSHA 2002a) and 

increase this rate by the same multiplier. If this calculated 

value exceeds the ventilation rate listed on the nameplate 

for the engine, the new value should be considered the air 

quantity necessary to dilute NO2 emissions to safe 

concentrations. 

It is important to underline that the NO2 ventilation 

rate is different for each single approved engine, as is the 

prescribed nameplate ventilation rate, and that the same 

aftertreatment technology can have a different impact on 

the induced NO2 tailpipe concentration for each engine. 

For this reason the NO2 emission data collection needs to 

be performed for every single engine/control technology 

combination. 

3 Diesel Oxidation Catalyst 

Diesel Oxidation Catalysts (DOCs), are primarily 

designed to control carbon monoxide (CO), hydrocarbon 

(HC) and polycyclic aromatic hydrocarbons (PAH) 

emissions. Depending on catalyst formulation (described 

below), DOCs might have substantial effects on gas phase 

organics, but typically have limited effects on the organic 

fraction of particulate matter (Day et al. 2002). DOCs, 

initially introduced in underground mines to control the 

CO and HC emissions, became a widely used technology 

in mining applications. In recent years, DOCs have been 

often deployed to control emissions of hydrocarbons and 

the organic fraction of particulate matter emitted by 

mining vehicles fuelled with biodiesel. 

A catalyst is a compound applied to the surface of the 

ceramic or metal DOC substrate, and it has the role of 

oxidizing CO and HC to CO2 and H2O. Several types of 

catalyst formulations are available and the most common 

formulations are based on oxides of cerium, vanadium, 

platinum or palladium. The DOC activity is strictly 

dependent on exhaust temperature: a typical DOC shows 

low or no activity at low exhaust temperatures. The 

minimum temperature at which the DOC exhibits activity 

is called catalyst “light-off temperature”. To avoid the 

formation of sulphates and the consequent deactivation of 

the catalyst it is crucial to use fuel with low sulphur 

content. 

DOCs typically have minor effects on NOX emissions 

(Yamamoto et al. 2006). However, some of the catalysts 

used for DOC applications tend to promote the conversion 

of NO to NO2 and they can substantially change the 

partitioning between NO and NO2 emissions. The rate of 

oxidation of NO to NO2 depends on several factors: 

catalyst formulation, monolith geometry, and exhaust 

temperature (Ambs et al. 1993; Farrauto 1997). The 

catalysts with platinum formulations are known to have 

high NO to NO2 conversion potential. The catalysts with 

formulations based on vanadium and palladium were 

shown to suppress NO to NO2 conversion (Farrauto 

1997). 

For some applications, such as the regeneration of a 

Diesel Particulate Filter (DPF), an elevated concentration 

of NO2 in the exhaust is desired for the combustion of 

DPM. An 80% conversion of NO to NO2 can be reached 

with an optimized DOC system operated at an average 

exhaust temperature of approximately 300 °C (Layrer 

1994) A slight decrease of the NO2 concentration has 

been observed at low load engine operating conditions for 

the catalyst formulations with low platinum loading 

(Ayala et al. 2001). 

MSHA’s policy on this effect is that the increase in 

NO2 emissions due to DOC use is tolerable as long as the 

engine’s approved ventilation rate is not violated. In other 

words, if the NO2 ventilation rate is not higher than that 

listed on the engine approval, the aftertreatment 

technology should not degrade the mine environment 

significantly. 

The NO2 ventilation rate data for an Isuzu 4JG1T engine 

have been recently released by MSHA along with the 

results of a fuel testing campaign (MSHA 2009). The 

approved ventilation rate for this engine is 2.83 m3

/s 

(6000 cfm), based on the NO emissions. An increase in 

NO2 emissions between the bare engine (Figure 1a) and 

the DOC equipped engine (Figure 1b) is clearly noted. 

This increase, while large, does not approach the 

ventilation requirement of the engine. With adequate 

ventilation in the mine area where the engine is operating 

(6000 cfm and above) it does not seem likely for NO2 

emissions to rise above a miner’s TLV levels for NO2 

exposure. In mine environments with inadequate 

ventilation where local “dead spots” exist in work areas, it 

would be possible for NO2 levels to rise to problematic 

levels faster with use of a DOC than without. But if this 

event would occur, then other regulated gas levels would 

also be increasing and most likely exceeding their 

individual TLV limits as well. 
Figure 1 NO2 Ventilation Rates for Bare engine (top) and Engine with DOC (bottom) during a Isuzu 4JG1T engine test 

campaign by MSHA (2009). Each bar is representative of a different fuel formulation used during the campaign. 

The results are subdivided in the eight engine modes used by MSHA to certify any approved engine. 

It is important to underline that the DOC used for the 

MSHA testing has been found capable of converting at 

most 20% of NO to NO2. The average reduction of CO 

emissions with the use of a DOC was in the range of 

85%-100% for all the fuel tested. In this case, the 

reduction of the principal targeted pollutant, CO, by the 

DOC, does not induce an intolerable increase in NO2 and 

it should not affect a well managed mine ventilation 

strategy. 

4 Diesel Particulate Filter Systems 

Diesel Particulate Filter (DPF) systems are designed to 

physically remove DPM from the exhaust of diesel 

engines. The majority of DPF systems currently present in 

the mining industry utilize, as a filtration strategy, the 

wall-flow system: the particles are trapped in the walls of 

the filter while the gases pass through them (Figure 2) 

(Corning 1999). The wall-flow system can provide up to 
99% DPM mass filtration. The DPF must be periodically 

or continuously cleaned (regenerated) to avoid the 

occurrence of excessive engine back pressure. The 

regeneration is typically accomplished via oxidation of 

filtered DPM. Heat generated by exhaust temperatures in 

excess of the DPM ignition temperature is needed to 

initiate and support DPM oxidation. The DPM ignition 

temperature in a DPF is influenced by several parameters, 

but commonly it has to be higher than 250 °C (Cauda et 

al. 2007). Diesel engines operated over typical 

underground mining duty cycles rarely achieve exhaust 

temperatures sufficient to support regeneration of a DPF. 

However, the DPF regeneration can be promoted by the 

use of an active catalyst. The catalyst can be applied 

directly to the DPF wall using wash-coating technologies 

or it can be introduced in the system using fuel additives. 

 

 

Figure 2 A ceramic wall-flow diesel particulate filter 

(DPF). The dark arrows represent the inlet 

flow, with high concentration of DPM, whereas 

the outlet flow, light arrows, presents a low 

DPM concentration. The DPM has been 

filtered by the walls of the DPF. 

If an uncatalyzed DPF is utilized, the filtration process 

itself does not substantially affect the gaseous emissions. 

Under proper conditions the NO2 was found to react with 

the filtered DPM in a process known as chemisorption 

and a net reduction of NO2 is possible (Kalberer et al. 

1999). On the other hand, several wash-coated catalytic 

DPFs demonstrated a high activity in converting NO to 

NO2 (Chatterjee et al. 2004; Zhu et al. 2004; Czerwinski 

et al. 2007; Mogensen et al. 2009). 

MSHA advises against using platinum catalyzed DPF 

systems in underground mines due to the potential for an 

increase in NO2 emissions.(MSHA 2002b). 

Other catalyst formulations, such as the metal based 

fuel borne catalysts, were found to have minor effects on 

NO2 emissions (Richards et al. 2003). Nevertheless, a 

slight increase in NO2 emissions is possible for the fuel 

additive formulations using a precious metal such as 

platinum (Czerwinski et al. 2007). 

It is important to note that NO2 has a higher oxidative 

potential than oxygen at low exhaust temperatures 

(Cooper et al. 1989; Allansson et al. 2002) and it is useful 

in some applications to lower regeneration temperatures 

and enhance the regeneration of the DPF. The typical low 

concentration of NO2 in diesel exhaust is normally 

insufficient to support DPF regeneration. For this reason, 

a DOC is commonly employed ahead of the DPF to 

increase the concentration of NO2 in the exhaust gas. A 

DOC and an uncatalyzed DPF are the main components 

of the Continuously Regenerating Trap (CRT), patented 

by Johnson Matthey (Cooper et al. 1989; Cooper et al. 

1990). 

Balancing NO2 production with NO2 consumption by 

the DPM plays a critical role in supporting regeneration 

while preventing a potential increase in NO2 emissions 

from such a system, known as NO2 slip (Hawker 1998; 

Ayala et al. 2001). A proper choice of catalyst and 

settings of a DOC+DPF system theoretically can 

minimize the NO2 slip. NO2 slip can also be reduced by 

deploying the NO2 abatement strategies discussed in the 

next sections of this article. 

DPF systems actively regenerated using electrical 

energy off board or on board of the vehicle are popular in 

the mining industry. Actively regenerated DPFs are 

typically uncatalyzed and, as described in the previous 

paragraphs, they do not tend to increase NO2 emissions 

during their use. The regeneration of off board DPFs 

should be performed in well ventilated stations where 

emissions of NO2 and other gases should not be an issue. 

The challenges related to the regeneration of DPF 

systems led to the development of flow through filters 

(FTF). FTFs are devices that can capture and store 

carbonaceous PM material for a period of time sufficient 

for its catalytic oxidation, while having open flowthrough passages that allow exhaust gases to flow, even if 

the DPM holding capacity is saturated. The characteristic 

of this type of DPF is lower engine back pressure but only 

up to 50% DPM mass filtration (Okawara et al. 2005; 

Choi et al. 2007). Most FTF are wash-coated with a 

catalyst to continuously regenerate the DPM trapped and 

at the same time acting as a DOC by converting CO to 

CO2. 

From a mine ventilation perspective, the increase of 

NO2 emissions connected with the use of a DPF system is 

a serious threat: the use of DPFs has been indicated as one 

of the most effective strategies to reduce the emission of 

DPM and to meet the MSHA DPM requirements in both 

coal and metal/non metal mines. The introduction of 

DPFs represents a robust alternative strategy to the 

increase of the ventilation rate to meet the DPM 

requirements: a lower ventilation rate can translate to a 

lower energy consumption rate and relative economical 

positive effects for the mine company. Increased NO2 

emissions connected with the use of some DPFs can 

reverse these effects, if the ventilation rate has to be 

increased to meet the TLV levels for NO2 exposure. 

Recently, due to the NO2 issue in the mine 

environment, and to a much greater extent interest in NO2 

from the Air Resource Board in California, manufacturers 

of DPFs have begun working to reformulate these devices 

to minimize NO2 production. MSHA is constantly 

updating the list of DPM control technologies available 
on the market for underground mine applications (MSHA 

2007): the list is organized in order to provide valuable 

information about the risk of higher concentrations of 

NO2 connected with the use of some of these 

technologies. 

5 NOX control strategies. 

Different NOX emissions control technologies are 

gradually finding their way into underground mines, 

mostly in original equipment manufacturer (OEM) form. 

This introduction is strictly connected with the 

development of integrated OEM engine systems produced 

by the engine manufactures to meet the upcoming heavyduty off-road Tier 4 standard. 

The integrated OEM engine systems will assure lower 

NOX emissions for the equipment present in underground 

mines and for this reason the mining ventilation strategy 

should receive overall benefit by the introduction of these 

systems. The complexity of these integrated engine 

systems and the necessity of more fine tuning for the 

engines are the potential threats connected with their use 

underground. 

One of the most promising NOX control strategies is 

the employment of a Selective Catalytic Reduction (SCR) 

technology. The conversion of both nitrogen oxides, NO 

and NO2, in the SCR system follows a complex path of 

chemical reactions (Cho 1994). 

The combustion in a diesel engine is considered lean 

combustion, with a large excess of oxygen: for this 

reason, the reduction of NO and NO2 is only possible via 

the use of a specific reductant compound, such as 

ammonia or urea, injected in the exhaust ahead of the 

SCR system. A complete SCR system (Figure 3) includes 

an auxiliary oxidation catalyst (V), employed to modify 

the NOX ratio, a main SCR catalyst (S) where the NOX are 

converted, and a second oxidation catalyst (O) to prevent 

the leak of ammonia at the exit of the tailpipe, known as 

ammonia slip. 

Figure 3 The scheme of a generic Selective Catalytic Reduction (SCR) system for mobile source 

Due to toxicity and the difficulties in 

handling and storage, ammonia is generally found to be 

impractical for mobile applications. The aqueous solution 

of urea is currently the reductant of choice for mobile 

SCR systems: the urea is then converted to ammonia in 

the first part of the SCR system, the hydrolysis catalyst 

(H). The nitrogen oxides react with ammonia over the 

catalyst in the main monolith to form elemental nitrogen. 

Reduction of NOX on the SCR monolith surface is 

dependent on catalyst choice, temperature, and the 

NO2/NOX ratio. NO2 plays an important role in the SCR 

system. The optimum NO2/NOX ratio was reported to be 

about 50% (Gieshoff 2000; Koebel et al. 2002). The 

concentration of NO2 in diesel exhaust can be increased 

by using a proper auxiliary DOC, usually based on 

precious metals (e.g. platinum). 

The reported NOX conversion efficiencies in heavyduty engines range from 55% to 90%, depending on the 

application and test method (Havenith et al. 1997; Fritz 

1999; Miller et al. 2000). The SCR monolith activity is 

not selective towards NO and NO2, and the same 

reduction is attainable for both compounds. 

Overall the adoption of an SCR as an OEM 

integrating engine system will reduce the required 
ventilation rate for both NO and NO2. However, 

considering the increased NO2 concentration upstream of 

the main catalyst by the auxiliary DOC, the resulting 

reduction in required NO ventilation rate will be greater 

than the one for NO2. 

Two non-OEM aftertreatment technologies,

alternatives to SCR, are under development and may 

become available for underground equipments in the near 

future: NOX adsorber and LNC. 

The strategy of any NOX adsorber technology involves 

two distinct phases: the storage of NOX on a catalytic 

surface during an engine lean condition, with high oxygen 

content, and the desorption/reduction of NOX at a rich 

condition, with high content of hydrocarbons and low 

concentration of oxygen in the exhaust. The engine 

operating mode plays an important role in the shift from 

the adsorption to desorption/reduction phase (Epling et al. 

2004) and there is an extensive use of catalysts for both 

phases. It is still unclear whether the trapping materials 

adsorb NO2 more effectively than NO, or whether NO2 

may be the required intermediate compound for NO 

adsorption. In any case, the system promotes the 

oxidation of NO to NO2 on active oxidation catalyst sites 

in the first step of the adsorbing phase. In general, NOX 

adsorber catalysts can exhibit high NOX conversion 

efficiencies in excess of 80-90% (Brogan et al. 1998). 

The LNC system can be a valid alternative to the SCR 

system when the employment of urea and ammonia can 

be problematic. These systems were first commercialized 

for light-duty diesel engines but ongoing development 

efforts target the more challenging heavy-duty diesel 

engine applications. 

In the LNC system, the reduction in NOX is achieved 

by reacting NOX with hydrocarbons in the presence of a 

catalyst. In a few applications, unburned hydrocarbons 

from the engine chamber are sufficient to obtain the 

desired NOX conversion, otherwise a post-injection of 

hydrocarbon in the exhaust is necessary (Dorriah 1999). 

The conversion of both NO and NO2 depends mostly on 

the catalyst chemical formulation and configuration 

(Truex 1994; Kharas et al. 1998) and on the temperature 

window at which the catalyst has the highest activity. A 

maximum achievable NOX conversion by LNC systems 

for diesel engines operated over real duty cycles is 60% 

(Kharas et al. 1998). NO2 does not have a crucial role in 

the process and its conversion is proportional to the 

conversion of overall NOX. 

Both NOX adsorbers and LNC could be useful tools to 

reduce the NO and NO2 emissions from modern diesel 

engines in the mine environment. Unfortunately the high 

complexity of the chemical reactions employed by these 

systems and the requirement of a constant emissions 

monitoring and tuning, can delay the introduction of these 

technologies in mining equipment. While the reduction of 

required ventilation rate for both NO and NO2, a result of 

the reduced tailpipe emission, can be an advocate for their 

introduction, the dangerous misuse or malfunctioning of 

these technologies could potentially lead to an increase in 

NO2 emissions which could translate to a NO2 exposure 

level well above the miner’s TLV value. 

In general it is important to underline that SCR 

systems have been developed in particular for heavy duty 

applications, considering the temperature required and the 

space necessary for the system. On the other hand, LNC 

and NOX adsorbers, due to performance and durability 

requirements have been developed for passenger cars and 

light duty applications. 

6 Conclusions 

This paper provides an overview of the effects which 

different aftertreatment technologies have on NO2 

emissions and the relative effect on the mine ventilation 

strategy. Emphasis was given to the technologies viable 

for controlling the exposure of underground miners to 

diesel particulate matter and gases. Although a wealth of 

information is available in the scientific literature on the 

effects of control technologies on NOX emissions, limited 

information is available on the effect of those 

technologies on NO2 emissions. 

According to the literature, technologies such as 

uncatalyzed or base metal catalyzed filters have minor 

effects on NO2 emissions. On the contrary, DOC and 

noble metal catalyzed filters were shown to have the 

potential to substantially increase NO2 emissions. NO2 

slip was reported to be a particular problem for the system 

that depends on NO2 for DPF regeneration. The NO2 slip 

can be controlled with NOX abatement systems. The NOX 

abatement systems such as SCR and NOX adsorbers were 

reported to be very effective for controlling total NOX 

emissions. Those systems were shown to preferentially 

remove NO and therefore be less efficient in controlling 

NO2 emissions. 

The majority of the technologies described in this 

paper have been developed for on-highway and offhighway surface applications using design parameters 

different from those for underground mining applications. 

Due to the confined rugged environment and occupational 

rather than environmental regulations, the implementation 

of emission control technologies in an underground mine 

presents a unique challenge. 

The MSHA prescribed ventilation rate for NO2 can be 

influenced by the introduction of any of the aftertreatment 

control technologies described. If the modified NO2 

ventilation rate exceeds the MSHA nameplate ventilation 

rate, the adoption of the aftertreatment control technology 

can induce an increase in the air quantity necessary to 

dilute NO2 concentrations below the TLV level. This 

effect has been noticed especially with the use of highly 

catalyzed DOCs and catalyzed DPFs. A misuse of more 

complex NOX control strategies can have similar 

disadvantages. 

Substantial research is currently being conducted on 

new technologies designed to reduce diesel emissions 

including NO2 and on their effect on the ventilation 

requirements in underground mines. 
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