DOCs are commonly used in mining applications primarily to reduce CO and hydrocarbon (HC) emissions. DOC manufacturers offer various catalyst formulations, which are commonly based on oxides of cerium and vanadium or precious metals such as platinum or palladium. The catalyst is typically wash-coated on the surface of a metal substrate or ceramic monolith within the DOC housing and oxidizes the CO and HC to CO2 and water (H2O). DOCs require low-sulfur fuel to limit the formation of sulfates that can “poison” the catalyst. The ability of the DOC to control CO and HC emissions depends upon the engine exhaust stream temperature, which should be higher than the activation temperature for CO and HC in order to maximize their conversion.

DOCs used in mining applications typically do not alter nitrogen oxide (NOx) emissions significantly, but some catalysts can alter the percentage of NO that is converted to NO2, which can substantially increase NO2, making NO2 an undesirable byproduct of the DOC.

This report compares two commercially available DOCs used by the underground mining industry to control CO and NOx emissions. Both DOCs were tested under several different engine operating conditions to mimic those encountered by machines performing in underground mining operations.

Laboratory testing was used to evaluate the capability of the DOCs to modify the concentration of gaseous pollutants in the tailpipe of a Mercedes-Benz 904 Tier 2 engine. The testing protocol was designed to find answers to two questions about the performance of the DOCs:
• Is the CO concentration reduction similar for both DOCs at each operating condition tested?
• Is the NO2 concentration similar for the DOCs at each operating condition tested?

Test Facility & Equipment
The laboratory evaluation of the two DOCs was performed at the Diesel Engine Emissions Laboratory (DEEL) at the National Institute for Occupational Safety and Health (NIOSH) Office of Mine Safety and Health Research (OMSHR) near Pittsburgh, Pa. The DEEL is a well-equipped, state-of-the-art diesel engine/ dynamometer emission test facility capable of testing the performance of diesel engines, alternative fuels, exhaust after-treatment devices, and other control technologies used to reduce the exposure of workers to diesel aerosols and gases. The diesel engine used for this DOC evaluation was a 2004 Mercedes-Benz OM904 LA certified to Tier 2 emissions standards. An electronically controlled turbo-charged unit displacing 4.3 l, the engine had a rated output of 174 HP (130kW) at 2,200 RPM-rated speed. It is a type of engine used in underground mining for light-duty applications.

The engine intake air is routed through a Meriam laminar flow element (LFE), which measures the intake air flow rate, as well as the intake temperature and relative humidity. The engine exhaust system is configured to allow a change-out of after-treatment devices with minimal modification to the exhaust piping. The exhaust is fabricated from a 4-in. steel tube with two stainless steel test sections at 10 pipe diameters (40 in.) in length before and after the tested device. These test sections are fitted with welded bungs and thermocouples, which allow for exhaust gas temperature measurement as well as exhaust gas sampling.

The engine dynamometer (dyno) has a power rating of 400 kW at 1900 RPM with a torque rating of 475 lb/ft at 700 RPM. The dyno is coupled to the engine with a driveshaft, and the torque is measured by a strain gage load cell with an accuracy of ±5 Nm (±3.7 lb/ft). The baseplate provides a machined, flat, stable mounting surface for the engine and dyno. When mounted on a floor, the interior base plate can be filled with steel shot or sand to reduce vibration and enhance stability.

The dyno is mounted on a steerable wheeled chassis, which allows it to be moved and taken underground for emissions testing in situ. In addition to the dyno chassis, a mobile dyno cooling unit, fuel and 480 Vac power are required for operation. The dyno system is electronically controlled by a single computer; it can also be manually controlled, except during data acquisition tasks. The computer provides instructions for dyno control related to load, throttle or speed regulation as well as engine data collection and storage. The Mercedes OM904 utilizes fly-by-wire technology requiring no mechanical throttle linkage.

The engine fuel system consists of a mobile 70-gallon storage tank fabricated with a “hull within a hull” for added safety and containment, and it is protected with automatic fire suppression. The storage tank is coupled to a Max Machinery fuel metering system. The metering system can provide instantaneous information related to fuel rate and usage. The fuel system also provides an output, which is recorded with the other engine data channels.

The dyno cooling system as configured in the lab is capable of exchanging 250 kW of heat from the dyno at less than 140°F. The cooling system is completely self-contained, requiring only 480 Vac power and a waterline connection to the dyno. The cooling system pump, heat exchangers and reservoir are wheel-mounted.

In addition to the engine/dyno, the following exhaust gas analyzers were employed to record data both before and after the DOCs were tested:
• ECOM-KL was used to confirm baseline engine tailpipe emissions. It is a portable gas analyzer based on electrochemical gas sensors. It uses heated lines and pre-treatment filters. It is capable of measuring oxygen, CO, NO, NO2, sulfur dioxide and combustibles in the tailpipe.
• The Gasmet DX-4000 was used for the collection of data on CO2, CO, NO and NO2. It is a portable Fourier Transform Infrared (FTIR) gas analyzer, which provides accurate measurement of multiple gas compounds in hot and humid sample gas.
• A hot flame ionization detector (HFID) was used to measure concentrations of total HCs in the diesel exhaust. This measurement method is based on detection of a current generated during the combustion of HCs in an ionized hydrogen flame. This current is proportional to the number of carbon atoms in the molecule of HC.
• A non-dispersive infrared (NDIR) technique was used to measure CO and CO2 concentrations in the raw exhaust. The NDIR technology uses the interaction of infrared light and gaseous compounds. The adsorption of the IR light of a certain wavelength by targeted gas is a function of the gas concentration and the adsorption is measured by a detector.

Test Procedure
Identical test procedures were used to evaluate the two DOCs, and each received the same placement in the dyno exhaust system. The following seven engine operating conditions were used to evaluate each DOC:
• Four standard MSHA 8-mode test conditions (I50, I100, R50, R100);
• Low idle and high idle; and
• Transient cycle.

The transient cycle was generated by adjusting a mine transient duty cycle data. The relative load and speed data in the original cycle were applied to the maximum speed and load specification for the engine used in this study.

For all DOC evaluation tests, the dyno was fueled with ultra-low sulfur fuel (ULSF) <15 ppm. Prior to each test session, the engine was warmed up at one operating condition (I50) for 30 minutes, allowing the engine, dyno and other systems to achieve stable conditions. When tailpipe exhaust gas concentrations and temperature were stable, seven alternating 10-minute samples — four before the DOC and three after — were collected and recorded for each test operating condition. The data were analyzed to obtain the average and standard deviation for the concentration of the targeted gases at each operating condition. The same procedure was used to process the temperature data (engine exhaust manifold and device inlet) and engine-dyno operating conditions (speed, torque and fuel consumption).

DOCs Tested
The two DOCs compared during this evaluation were an AirFlow Catalyst Systems Inc. EZDoc and a CleanAIR Systems ASSURE.

The AirFlow Catalyst Systems EZDoc (AirFlow Catalyst) is packaged as a single unit for use with diesel engines. Designed to meet engine and performance requirements for the Mercedes 904 diesel engine at the NIOSH DEEL, the AirFlow Catalyst mimics the design intended for use on light-duty underground mining equipment. The AirFlow Catalyst as tested is 14.5-in. long and has a 6.15-in. body diameter. It is advertised to be a flow-through substrate able to achieve a 90%-plus conversion of CO to CO2 with a gaseous hydrocarbon conversion to CO2 and H2O vapor. The AirFlow Catalyst is claimed to effectively control CO and reduce NO2 emissions at varied engine loads and speeds typical of those found in mining applications. The CleanAIR Systems ASSURE DOC (CleanAIR Catalyst) for diesel engines is a flow-through catalyst designed to reduce CO, HC and DPM. The unit is advertised as a high-performance, durable oxidation catalyst and is housed within a 304-l stainless steel, corrosion-resistant package designed to withstand the harsh mining environment. The CleanAIR Catalyst is advertised to reduce HC up to 95%, CO up to 95% and PM by 20%, and it can be retrofitted as a direct muffler or silencer replacement. Depending on exhaust gas temperature and fuel sulfur content, the CleanAIR Catalyst may reduce emissions at temperatures as low as 180°C (356°F), with reductions increasing rapidly as exhaust temperature increases.

Both the CleanAIR and AirFlow Catalysts were fitted with three-bolt 4-in. inlet and outlet flanged pipes to accommodate the 904 engine/dyno exhaust system. One-quarter-inch National Pipe Thread (NPT) pipe plugs were welded into each end cap of the tested device for use as exhaust gas sampling ports.

Results
The average concentrations and standard deviations of CO, HC, NO and NO2 at different engine operating conditions are summarized in Figure 1 for both DOCs. For the AirFlow Catalyst, with the exception of low idle condition, the CO concentration is greatly reduced by the DOC. The effect is similar for HC, even though the emissions of HC at high load (R100 and I100) are relatively low for this engine. The effect on NO and NO2 is an inverse relationship; the increase in NO at idle conditions is matched with a small decrease in NO2. For all other modes, the NO concentration is reduced and NO2 increased. The transient cycle test showed a substantial reduction in CO and NO and the highest increase in NO2. The charts for CO, HC, NO and NO2 for the CleanAIR Catalyst show only small changes after the DOC. Both CO and HC are reduced slightly at non-idle modes. Neither NO nor NO2 show substantial change because of the DOC.

The ratios of the concentrations — after and before the DOC — for the four gases are plotted in Figure 2. The dark line indicates a ratio of 1, which implies no change caused by the DOC. The engine operating conditions are plotted from left to right based on the increasing DOC temperature. The CO ratio (Figure 2a) for the AirFlow Catalyst is well below 1 for all the modes; the DOC was capable of reducing the CO concentration to a very low level for most operating conditions. The NO2 ratio was below 1 for the idle modes and then gradually increased to more than 10 before decreasing again at higher temperatures (see Figure 2c). The portion of NO2 in NOx (NO+NO2) after the DOC reached its maximum at 350°C and then decreased at higher temperatures. This plot is typically used by DOC developers to tune and characterize the catalyst to optimize or reduce NO2 generation.

The Airflow Catalyst at transient cycle yielded the temperature at which the NO2 ratio was the highest, which implies that a similar engine retrofitted with a similar DOC and employed in a similar transient cycle will demonstrate a more than tenfold increase in emitted NO2, and a reduction in CO of 98%. However, the charts in Figure 2 also show that if the same engine and DOC combination are employed in a cycle characterized by an average temperature below 300°C, the increase of NO2 might only double, with an 87% reduction in CO. At a temperature of 200°C, there is a simultaneous reduction of CO and NO2 of 90% and 84%, respectively. The temperature of the device is crucial to CO reduction and NO2 conversion.

The ratio data for the CleanAIR catalyst confirmed the results presented in Figure 1; the effect of the DOC was minimal for all gases. The CO ratio was consistently lower than 1 for all the modes except for idle modes, but the ratio was never below 0.65. Similarly, the NO2 ratio also was consistently below 1 except in low idle mode. Figure 2d illustrates how the portion of NO2 in NOx after the DOC decreased with the increase of the device inlet temperature — a trend similar to one demonstrated by an untreated exhaust. The data presented in Figure 1 and Figure 2 describe the CleanAIR Catalyst DOC as a low catalyzed device that provides a relatively small reduction in CO and no increase in NO2 at any temperature.

Conclusion
The test results from this study confirm that commercially available DOCs formulated for the same or similar mining applications can yield substantially different gaseous emissions when tested under the same engine operating conditions. Although both units tested operated within the manufacturers’ claims, one DOC had a minimal effect on all gases and displayed a similar trend to one for an untreated exhaust. Based on the test results, this unit could be described as a low catalyzed device that provides a relatively small reduction in CO and no increase in NO2 at the temperatures encountered during testing. The second DOC proved to be extremely efficient at low exhaust temperatures in reducing CO but also in increasing NO2. The increase in NO2 was strongly affected by temperature.

Disclaimer
The mention of any company or product does not constitute an endorsement by the National Institute for Occupational Safety and Health. The findings and conclusions in this paper are those of the authors and do not necessarily represent the views of the National Institute for Occupational Safety and Health.

Larry Patts is a research physical scientist for NIOSH (412)-386-6852; E-mail: lpatts@CDC.gov, Emanuele Cauda is a senior service fellow for NIOSH, and Jon Hummer is an engineering technician for NIOSH.