Environ. Sci. Technol. 2007, 41, 5070-5076
Reduction of Particulate Matter Emissions from Diesel Backup Generators Equipped with Four Different Exhaust Aftertreatment Devices S A N D I P D . S H A H , †,‡,| D A V I D R . C O C K E R I I I , * ,†,‡ KENT C. JOHNSON,† JOHN M. LEE,§ BONNIE L. SORIANO,§ AND J. WAYNE MILLER† Department of Chemical and Environmental Engineering, University of California, Riverside, California 92521, Center for Environmental Research and Technology (CE-CERT), Bourns College of Engineering, University of California, 1084 Columbia Avenue, Riverside, California 92507, and California Air Resources Board, 1001 “I” Street, P.O. Box 2815, Sacramento, California 95812
Diesel particulate matter (PM) reduction efficiencies for backup generators (BUGs) (>300 kW) equipped with a diesel oxidation catalyst (DOC), DOC+fuel-borne catalyst additive combination (DOC+FBC), passive diesel particulate filter (DPF), and an active DPF were measured. Overall, the DOC and DOC+FBC technologies were found to be effective in reducing mainly organic carbon (OC) emissions (56-77%) while both DPFs showed excellent performance in reducing both elemental carbon (EC) and OC emissions (>90%). These findings demonstrate the potential for applying DOCs to older engines where PM is dominated by the OC fraction. In most modern engine applications, where the PM consists of mainly EC, the DOC will be largely ineffective. Alternatively, passive and active DPFs are expected to be efficient for most engine technologies. Measurements of particle size distributions provided evidence of the high temperature formation of sulfate nanoparticles across the control technologies despite the use of ultralow sulfur diesel. Changes in the particle size distribution and the organic fraction of PM indicate that the OC component of PM is primarily found in the smaller sized particles.
Introduction The importance of heavy-duty diesel (HDD) engine contributions to airborne particulate matter (PM) and ozone precursors has been documented in many studies (1). Aside from environmental effects, evidence of its health effects * Corresponding author phone: (951) 781-5695; fax: (951) 7815790; e-mail:
[email protected]. † Department of Chemical and Environmental Engineering, University of California. ‡ Center for Environmental Research and Technology (CE-CERT), Bourns College of Engineering, University of California. § California Air Resources Board. | Currently at Research & Advanced Engineering, Ford Motor Company, P.O. Box 2053, MD 3179, Dearborn, Michigan 48121. 5070
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has led to the classification of diesel exhaust as a Toxic Air Contaminant in California (2-4). In 2000, the California Air Resources Board (CARB) reported that there were approximately 11,000 portable or stationary HDD engines operating in the state (1). Stationary/portable engines are predominately used in constant load applications such as pumps and backup power generators (BUGs). Recent efforts to reduce the impact of stationary sources have led to EPA’s addition of regulations for controlling non-road diesel emission sources and their fuels (6). Additionally, CARB has published a number of air toxic control regulations with significant emphasis on PM control, the main thrust of this research (4). The means for controlling PM emissions include diesel oxidation catalysts (DOCs), diesel particulate filters (DPFs), fuel-borne catalysts (FBCs), and engine modifications. A number of studies have been conducted to quantify the emissions reductions achievable by these PM control technologies when applied to mobile sources (7-12). The effectiveness of aftertreatment technologies is also dependent on the makeup of the PM. Diesel PM primarily consists of elemental and organic carbon (EC and OC) (13). EC forms during fuel pyrolysis, while OC originates from incomplete combustion of fuel and slip of lubricating oil past the engine seals (14-15). OC has been found to contain many mutagenic and carcinogenic compounds (2, 16). The composition of diesel PM (with respect to EC and OC) varies with many factors including engine age, engine type, load, and mode of operation (17-19). DOCs have been reported to primarily reduce OC emissions, while DPFs have been found to be effective against both EC and OC (20-21). Reductions of diesel PM emissions through the use of DOCs have been reported to be in the range of 20-65% (8, 22-26), while DPFs are reported to have emissions reductions as high as 98% (11, 21, 27-28). The reductions for both DOCs and DPFs are highly dependent on exhaust temperature, fuel sulfur content, engine operating mode, engine age, and engine technology (9, 26, 28-29). Sulfate formation across catalysts, low catalyst activity due to low exhaust temperatures, and transient operations have been identified as causes for reduced aftertreatment effectiveness (7-11, 20-21). Currently, in the United States, regulations mandating the availability of ultralow sulfur diesel (ULSD) (300 hp). The study utilizes BUGs as representative examples of stationary diesel engines, however, the results can be applied to any stationary 10.1021/es0614161 CCC: $37.00
2007 American Chemical Society Published on Web 06/15/2007
diesel engine. Consideration is given to the effects of control technologies on fuel consumption and particle size distributions.
Experimental Procedures Emissions Measurements. Engine emissions were measured using CE-CERT’s Mobile Emissions Laboratory (MEL). The MEL consists of a total capture dilution tunnel, analyzers for gaseous emissions, and a secondary dilution system for particle sampling (33-34). The dilution tunnel provides primary dilution in the range of 6:1 to 3:1 depending on the engine operating load, while the secondary dilution system provides a dilution of 2.7:1. Prior to mixing with the sample stream, secondary dilution air is dried to a dewpoint of -73 °C and temperature controlled such that the diluted sample is maintained at 47 ( 5°C. The exhaust from each test engine was plumbed directly to the MEL with a 316 stainless steel, insulated, flexible tube. PM mass was collected on Pall Gelman (Ann Arbor, MI) 47 mm PTFE Teflo filters. Filter preparation and handling met the requirements of the Code of Federal Regulations (CFR) (35-36). Filter weights were measured with a Cahn (Madison, WI) C-35 microbalance. EC and OC were collected on Pall Gelman (Ann Arbor, MI) 47 mm Tissuquartz fiber filters. Each quartz filter was pretreated in a furnace at 600 °C for 5 h. Each filter was stored in a separate sealed petri dish at 10 °C prior to and after sampling. A Sunset Labs (Forest Grove, OR) Thermal/ Optical Carbon Aerosol Analyzer analyzed a 1.5 cm2 quartz filter punch for EC and OC following the temperature program outlined in the NIOSH 5040 method (37). OC values were corrected by a factor of 1.2 to account for hydrogen and oxygen present in the OC and 35% to account for the gas adsorption artifact (16, 19, 38). Gas adsorption artifacts were previously measured for similar samples using annular denuders (17). Particle size distributions were measured at each load point using a Scanning Mobility Particle Spectrometer (SMPS) connected to the secondary dilution system (39). In its current configuration, the instrument is capable of classifying and counting particles from 28 to 425 nm electrical mobility diameter. Test Cycle. Emissions were determined for engines operated over the CFR specified five-mode test cycle for nonroad compression ignition engines (40). This cycle consists of initially operating the engine at full power for a minimum of 30 min to allow emissions to stabilize. Following this, the engine is sequentially operated at 100, 75, 50, 25, and 10% load at steady engine speed. Engine loading was provided by a load-bank, which dissipates generated electricity as heat. Emissions were measured at each load point with a weighted average emissions factor determined by
EFo )
∑ WF × g ∑ WF × P i
i
i
i
(1)
where EFo is the weighted average emission factor in g kW-hr-1, WFi is the weighting factor for mode i, gi is the emission rate for mode i in g hr-1, and Pi is the power output at mode i. Weighting factors for each load point are 0.05, 0.25, 0.30, 0.30, and 0.10 for 10, 25, 50, 75, and 100% loads, respectively (40). These weighting factors are specified in the CFR and are based on the fraction of time that the engines are expected to operate at these load points. Test Fuels. The two fuels used in this study were CARB diesel fuel and Arco ultralow sulfur diesel fuel (ULSD). Fuel properties for each are listed in Table 1. The manufacturer of each control technology specified the test fuel. The primary difference between test fuels is weight % S. Test Protocol. Triplicate emissions tests were performed for each BUG prior to installation of the control technology.
TABLE 1. Properties of CARB Diesel Fuel and ULSD Fuel property
test method
CARB limit
ULSD limit
cetane number, typical Cu strip corr., 3 h @ 122 F, max distillation T 90%, F flash point, F, min gravity, API, typical sulfur, ppm viscosity, cSt @ 40 C
D-613 D-130 D-86 D-56 D-287 D-5453 D-482
53 3 540-640 125 36.9 500 2.0-4.1
53.5 3 540-640 125 38 15 1.9-4.1
TABLE 2. Summary of Control Technologies Tested, Engines and Fuels fuel engine
control
uncontrolled
controlled
1985 DDC V92 1985 DDC V92 2000 CAT 3406C 2000 CAT 3406C 2000 CAT 3406C
DOC1 DOC2+FBC DOC1 passive DPF active DPF
CARB CARB CARB CARB CARB
CARB CARB CARB ULSD ULSD
Following baseline testing, control technologies were mounted to the exhaust in place of the unit’s original muffler. After installation, the control technology underwent a degreening process consisting of operating the engine at 50% load for 24 h. Triplicate emissions tests were repeated for each BUG equipped with the control technology. These tests are referred to as “zero-hour” emissions tests. This was followed by a durability phase in which the engine was operated for 48 cold-starts, followed by operation at 25, 65, and 80% load for 48 h each. Cold-starts were performed at 12 h intervals. This protocol was modeled after CARB’s Verification Program (41). Following the durability phase, final triplicate emissions tests were performed over the same five-mode cycle as previously described. These tests are referred to as “post-durability” emissions tests. Engines Tested. Two engine models were selected for testing: 1985 Detroit Diesel (DDC) V92 two-stroke diesel engine installed in a 300 kW generator and 2000 Caterpillar (CAT) 3406C four-stroke diesel engine installed in a 350 kW generator. Typically, two-stroke engines have significant slip of unburned fuel and oil from the engine when compared to four-stroke technology engines. This results in excess emissions of hydrocarbons and OC (42). Each emission control technology was tested on the engine/fuel combination selected by the manufacturer of the control technology. Not all technologies were tested on both engines. Table 2 summarizes the entire test matrix. Technologies Tested. At the initiation of this work, CARB and the California Energy Commission issued a request for the submission of commercial and near-commercial control technologies. From the responses, four control technologies, representing a wide range of available technologies, were selected for study. A brief description of the technologies is provided here, however details of the precious metal content, washcoat, catalyzed volume, substrate composition/geometry, etc., were considered proprietary information and not provided by the manufacturers. DOC1 consists of a honeycomb-structured substrate impregnated with active metals. DOC2+FBC consists of a catalyst made by a different manufacturer than DOC1. The FBC contains 4-8 ppm of cerium and platinum in a petroleum distillate solution. Testing followed the manufacturer’s recommendation of initially doping the first tank of fuel with a volumetric ratio of 300:1 for fuel:FBC ratio. Following initial doping, the engine was operated with a fuel:FBC ratio of 1500:1. The fuel and VOL. 41, NO. 14, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 3. Average Percent Change and Standard Deviation of Power Specific Fuel Consumption for Each Load Point (Fuel Consumption was Measured as CO2 Production) 2000 CAT 3406C
1985 DDC V92
load (%)
DOC
passive DPF
active DPF
DOC
DOC+FBC
10 25 50 75 100
-3.43 ( 0.93 -1.25 ( 1.08 -0.44 ( 0.41 -0.16 ( 0.72 -0.9 ( 0.11
4.03 ( 2.62 1.94 ( 0.64 1.67 ( 0.95 2.01 ( 0.71 2.49 ( 0.88
0.17 ( 1.29 1.69 ( 0.74 1.38 ( 0.88 2.01 ( 0.64 0.04 ( 0.03
2.52 ( 2.02 -3.19 ( 0.69 -1.87 ( 0.61 -2.62 ( 0.44 -2.75 ( 1.81
3.62 ( 0.14 -2.89 ( 1.02 -1.41 ( 0.69 -1.83 ( 0.68 -1.21 ( 0.52
TABLE 4. Average and Standard Deviation of PM Mass Emission Rates (in g kW-hr)-1 for Triplicate Testing of Each Engine and Control Technology engine
control
10%
25%
50%
75%
100%
weighted average
2000 CAT 3406C
baseline DOC % difference baseline passive DPF % difference baseline active DPF % difference
0.769 ( 0.021 0.730 ( 0.042 5.17 0.863 ( 0.045 0.095 ( 0.009 88.97 0.732 ( 0.012 0.009 ( 0.005 98.81
0.281 ( 0.009 0.254 ( 0.011 9.57 0.264 ( 0.009 0.023 ( 0.001 91.19 0.276 ( 0.009 0.004 ( 0.001 98.61
0.238 ( 0.005 0.216 ( 0.011 9.05 0.193 ( 0.013 0.017 ( 0.001 91.03 0.238 ( 0.005 0.007 ( 0.001 97.11
0.241 ( 0.007 0.216 ( 0.020 10.38 0.167 ( 0.011 0.013 ( 0.000 91.94 0.242 ( 0.008 0.007 ( 0.002 97.12
0.181 ( 0.024 0.212 ( 0.043 -17.17 0.125 ( 0.008 0.014 ( 0.002 88.82 0.181 ( 0.024 0.005 ( 0.002 97.37
0.302 ( 0.009 0.279 ( 0.018 7.68 0.272 ( 0.014 0.026 ( 0.002 90.51 0.297 ( 0.008 0.006 ( 0.002 97.96
1985 DDC V92
baseline DOC % difference baseline DOC+FBC % difference
0.683 ( 0.027 0.516 ( 0.041 24.52 0.683 ( 0.027 0.684 ( 0.094 -0.17
0.433 ( 0.022 0.201 ( 0.048 53.57 0.433 ( 0.022 0.253 ( 0.004 41.67
0.295 ( 0.003 0.193 ( 0.005 34.64 0.295 ( 0.003 0.201 ( 0.007 31.78
0.203 ( 0.021 0.104 ( 0.003 48.78 0.203 ( 0.021 0.114 ( 0.005 43.85
0.214 ( 0.020 0.078 ( 0.005 63.60 0.214 ( 0.020 0.082 ( 0.004 61.85
0.348 ( 0.016 0.200 ( 0.021 42.67 0.348 ( 0.016 0.237 ( 0.014 31.88
FBC were splash blended and agitated in drums prior to the start of testing. After testing with the FBC was completed, the fuel tank was purged and the engine was operated with undoped fuel for a period of time. The passive DPF is a continuously regenerating filter consisting of a catalyst unit followed by an uncatalyzed cordierite filter. The catalyst serves to oxidize NO to NO2. The NO2 then oxidizes PM trapped in the filter unit. NO2 reduces the required PM combustion temperature from approximately 500 °C in the presence of 5% O2 to approximately 200 °C in the presence of 1% NO2 (29). The active DPF consists of three parallel units, each with an uncatalyzed silicon carbide filter and an electrical heating element. Each unit cycles through a sooting, regeneration, and cooling mode. During sooting mode, all three units filter DPM from the exhaust stream. During regeneration mode, a valve in one filter unit diverts the engine exhaust to the other two units and bypass air is routed to the regenerating unit. The electrical heating element raises the temperature of the bypass air to allow for combustion of particulate collected on the filter. Regeneration is followed by a cooling step that returns the DPF to normal operating temperature before engine exhaust is allowed to pass through it. The cycle is repeated for each parallel unit as they become loaded.
Results and Discussion Fuel Consumption. For both the passive and active DPF, it is expected that the backpressure the engine encounters will increase as PM accumulates inside the DPFs. The increase in backpressure will be accompanied by an increase in fuel consumption. For DOC1 and DOC2+FBC, it is expected that backpressure will be primarily affected by the substitution of the catalyst for the original muffler. Therefore, there will be a change in fuel consumption associated with these control technologies, as well. Table 3 shows the % change in power specific fuel consumption over each load point for all of the 5072
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control technologies relative to the baseline conditions with no aftertreatment. These calculations assume that the efficiency of combustion remains consistent for each engine regardless of the control technology installed. For the passive and active DPFs, it is assumed that the carbon content of the two fuels is the same. In most instances, DOC1 and DOC2+FBC had lower fuel consumption than the original muffler on the engines. At the lowest load point (10%), the 1985 DDC V92 showed an increase in fuel consumption for both DOC1 and DOC2+FBC. Based on the variability of the measurement, at 25, 50, and 75% load the passive and active DPFs have similar changes in fuel consumption. It should be noted that the fuel consumption values for the active DPF do not account for the energy needed to operate the electrical heating elements during regeneration. The increase in fuel consumption reported here for the passive DPF is within the range reported by others for a smaller engine operating on a similar test cycle (43) and smaller (2-4% versus 1-8%) than that reported for an on-road HDD vehicle after several years of operation (44). Earlier work in the DESCE program found an average increase in fuel consumption of 1.4% with a similar passive DPF (21). Particulate Emissions. Table 4 summarizes PM mass emission rates (measured gravimetrically) for each load point and the weighted average. Emission rates were measured for both zero-hour emissions tests and post-durability emissions tests. Only one technology (active DPF) showed differences in emission rates between zero-hour and post-durability emissions tests. Zero-hour PM emission rates for the active DPF were below detection limits of the laboratory while postdurability tests were detectable. The results presented here are for post-durability tests for all technologies. Within the range of the uncertainty shown in Table 4, DOC1 was ineffective in reducing PM mass emissions from the 2000 CAT 3406C engine. At 100% load, the PM mass emission rate increased by 17% for this control technology/engine com-
FIGURE 1. PM, EC, and OC emission rates for each control technology on the 1985 DDC V92 and 2000 CAT 3406C engines. Note the change in scale of the y-axis for the 2000 CAT 3406C. bination. This increase will be discussed further in the following sections. The same control technology was effective in reducing the PM mass emissions from the 1985 DDC V92. On the 1985 DDC V92 engine, the DOC2+FBC had comparable reductions in PM mass as DOC1 for all but the 10% load point. Both the active and passive DPF showed reductions of 90% or greater for PM mass emissions from the 2000 CAT 3406C. Figure 1 shows baseline and controlled PM, EC, and OC emissions for each of the emission control technologies on their respective engines. All of the data from Figure 1 are presented in Table S1 of the Supporting Information. These data suggest an explanation of the discrepancy for the DOC at reducing PM emissions from the two different engine types. Table 4 shows that weighted average PM reductions were 7.6 and 42.6% for the CAT 3406C and DDC V92 engines, respectively. The discrepancy between control efficiency for the two combinations is attributed to the disparity between EC and OC emission rates for the two sources. As previously stated, two-stroke engines generally emit more OC than fourstroke engines. The CAT 3406C engine is a four-stroke technology engine, having 32-53% (depending on load) lower OC emissions compared to the DDC V92. Further investigation of the control efficiency for EC and OC demonstrates that DOC1 has slight reductions in EC, but reduced OC by 56-77% (depending on load) for both engines. Johnson et al. have shown that EC reduction in a DOC is enabled through the deposition of particles on the catalyst surface and can be a small contributor to total PM reduction (45). The reduction in OC is similar to that reported by others (23, 32). As seen here and by other investigators, the effectiveness of the DOC control is dependent on the fraction of OC present in the engine exhaust, with total PM reduction efficiency increasing with increasing OC content (20, 32, 46, 47). The formation of sulfate PM through DOC1 may also contribute to the discrepancy. Evidence for sulfate formation through DOC1 is seen in the smaller fraction of carbonaceous PM for
the controlled (88% total PM attributed to carbon) versus the uncontrolled emissions from the CAT 3406C (78% total PM attributed to carbon). Carbonaceous material contributed to 79% of the total PM mass for the DDC V92 for both uncontrolled and controlled conditions. At 100% load although EC and OC reductions are on the order of 13.9 and 54.7%, respectively, total PM mass increased by 17% (Figure 1c). At this load point, the gains made by the reduction of EC and OC by the control technology may be masked by the potential increase in sulfate particulate. The increase in sulfate particulates at high load operation (i.e., high temperature) has been reported by other researchers as well (20, 32). A comparison of Figure 1a and b shows the DOC2+FBC combination showed similar reductions to the stand-alone DOC1 tested on the same engine (1985 DDC V92). DOC2+FBC reduced weighted average PM mass emissions by 31.8% with -4.4% and 55.9% reductions for EC and OC, respectively. Based on the weighted emission factor 88 and 87% of the total PM mass was attributable to carbonaceous material for the uncontrolled and controlled emissions, respectively. One caveat is that DOC2 is from a different manufacturer than DOC1 and therefore, may have different catalyst and substrate properties. Other researchers have reported significant reductions in PM mass emissions through the use of a DOC+FBC, where the DOC+FBC combination has improved PM mass reductions by 10-20% over a standalone DOC with engines operated on the FTP cycle (22, 48). The passive DPF tested on the 2000 CAT 3406C engine reduced weighted average PM mass emissions by 90.5%, with significant reductions for both EC (89.7%) and OC (93.6%) fractions (Figure 1d). The reduction of EC and OC was consistent for all 5 engine load points. PM trapping performance of the passive DPF was not affected by variations of PM composition, exhaust temperature, or engine operating load. Average exhaust temperatures for each load point were 228, 293, 371, 434, and 490 °C for 10, 25, 50, 75, and 100% load, respectively. These results contrast with those of Vertin VOL. 41, NO. 14, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Percent reduction of PM number for each control technology on the 1985 DDC V92 and 2000 CAT 3406C engines. et al. (27), who reported a minimum temperature of 250 °C is required for proper filter regeneration. Unlike the DOCs, the passive DPF is expected to be efficient for both EC and OC dominated sources. A comparison of the carbonaceous fraction of total PM mass, for the uncontrolled and controlled tests, showed little evidence of additional sulfate PM formation across the passive DPF. EC and OC consisted of approximately 93 and 91% of the PM mass for the uncontrolled and controlled tests, respectively. This is contradictory to results found under similar testing conditions with a 30 ppm S fuel, where the carbonaceous fraction of total PM was approximately 99 and 63-70% of the total PM mass for uncontrolled and controlled tests, respectively (21). The active DPF reduced PM mass by 97.9%. This control technology was effective in reducing both the EC (97.1%) and OC (92.1%) fractions (Figure 1e). The active DPF achieved 5074
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the greatest reduction in PM mass. Performance of the active DPF was not affected by variations of PM composition, exhaust temperature, or engine operating load. At 10 and 25% load, OC emission rates were greater than total PM (measured gravimetrically). The explanation for this may be due to errors in the gravimetric measurement of PM at these low levels. This is evidenced by the high variation in the gravimetrically determined mass emission rates for the active DPF (14-63% COV) in Table 4. For this technology, emissions were measured during both sooting and regenerating phases with no significant difference between operating modes. It must be noted that at zero-hour testing the post-control emissions were below instrument detection limits, but after the durability phase, the emissions were significant enough to measure. This suggests that a long-term durability test for the active DPF may be needed. Post-mortem examinations
of the DPF material showed a slight discoloration (chalky color) of the filter material. This type of discoloration has previously been attributed to oxidation of the DPF material, although in this case, an extensive study of the change in material characteristics of the DPF was not undertaken (49). Particle Size Distribution. Figure 2 shows the particle size based percent reduction (PR) of each control technology/ engine combination examined in this work. PR was calculated by the following equation:
(
PRi ) 1 -
)
Nci × 100 Nui
where PRi is the percent reduction of the number of particles of size i; Nci is the number of particles of size i emitted with the control technology installed, and Nui is the number of particles of size i emitted without the control technology. In Figure 2a, we see the PR of DOC1 while installed on the CAT 3406C. Since DOCs are only effective at reducing OC, for 10, 25, 50, and 75% loads on the CAT 3406C, the trend of decreasing PR with increasing particle size indicates that the small particles consist primarily of volatile materials, while as particle size increases, solid particles consisting of EC are prevalent. The negative values for PR at 100% load indicate the formation of new nanoparticles. Liu et al., have previously shown that there is a tendency to produce sulfate nanoparticles at elevated exhaust temperatures over catalytic control technologies (30). In their work, sulfate nanoparticles were observed using fuel with >300 ppm sulfur for exhaust temperatures in the range of 325-525 °C. Further testing with fuel sulfur
