AbstractNatural gas (NG), which consists of mostly methane, can be co-combusted with diesel fuel in existing compression ignition engines through dual fuel technology with reasonable engine modifications. The removal of short-chain alkanes (e.g. CH4, C2H6 and C3H8) of a dual fuel (natural gas and diesel) engine raises a distinctive topic to the exhaust aftertreatment system (ATS). However, there are few studies reported based on tests with real engine exhaust. This present study focuses on the conversion of short-chain alkanes by Co, Ni and Cu/ZSM-5 catalysts, which are commonly used for oxidation/partial oxidation and reforming. These catalysts are tested with exhaust of a dual-fuel (natural gas and diesel) engine. The complicated and dynamic exhaust composition, determined by the engine loading and natural gas substitution, can result in different components in the exhaust and various conversions for species. Co-Ni-Cu/ZSM-5 had the highest conversion of CH4 and non-methane hydrocarbon (NMHC) at 30000 h-1 space velocity (SV) when the engine was operated at 1000 RPM, 25% loading, 70% natural gas substitution. Significant amounts of CO and formaldehyde were generated by the dual fuel engine, but can be mostly converted starting at 450 °C. Methane conversion of up to 42% was attainable at a higher temperature of 500 °C by using Co-Ni-Cu/ZSM-5 in the aftertreatment system.

IntroductionIndustry worldwide has been considering the potential of natural gas to diminish dependence on conventional diesel fuel and to reduce pollutant emissions. Natural gas, which consists of mostly methane, can be added in existing compression ignition (CI) engines through dual fuel technology with moderate engine modifications [1]. Generally, natural gas is injected in the intake manifold and mixed with air. This blended air/natural gas mixture goes into the cylinder of CI engine, supplanting a portion of the energy that would otherwise be provided by the diesel fuel. This portion is determined by the “substitution rate”, which is the amount of natural gas substituted for diesel fuel and tied with the flow rate of natural gas injected. The diesel fuel is injected through the conventional diesel injection equipment and compressed rapidly to meet the autoignition

conditions of the fuel-air mixture. This advantage of CI engine over spark ignited gas engine entitles the dual fuel engine to switch over entirely to the operation by conventional liquid diesel fuel without break to the power product.

Dual fuel technology exhibits advantages by utilizing widely available, low cost natural gas as a supplementary fuel in the CI diesel engine, offering economic and environmental benefits [2]. Natural gas is available in oil gas fields to power operations near the point of production. This avoids the need to convey, refine and transport diesel fuel to power oilfield operations, reducing energy inputs and truck traffic. It presents the lowest carbon-to-hydrogen ratio among all the fossil fuels, which can reduce CO2 emission. However, non-combusted methane (NCM), sometimes referred to as “methane slip”, is a recognized problem with most current dual fuel technology. Natural gas that is not consumed in the combustion process exits the engine along with other exhaust gases. Methane is estimated to have a Global Warming Potential (GWP) of 25. This means that as compared with CO2 on a mass basis, methane is 25 times more potent as a greenhouse gas (GHG) and thus is of environmental concern [3]. Aftertreatment catalysts have the potential to remove methane as well as other short-chain alkanes to reduce these emissions.

It is well known that costly Pd and Pt-based catalysts are the most practical solutions to completely oxidize short-chain alkanes in exhaust emissions [4, 5]. Researchers have been looking for lower-cost alternative catalysts for alkane oxidation that are also resistant to degradation by sulfur, water, etc. For example, a recent study reported using a Cu/zeolite catalyst to actively convert CO and CH4 at 600 °C [6]. This finding was unexpected according to the authors’ statement, because they did not find data showing oxidation of short chain alkanes with Cu/zeolites. This research drew attention to explore possible solutions to replace or supplement the consumption of noble metal catalyst for oxidation of short chain alkanes. Notably, this experiment was carried out with a synthesized mixture of gases instead of real engine exhaust of dual fuel engine. Therefore, it is of interest to test aftertreatment catalysts with the actual exhaust of a test engine. Real exhaust composition is more complex and dynamic because the engine is operated under varied conditions of RPM, loading and natural gas substitution rate.

Conversion of Exhaust Gases from Dual-Fuel (Natural Gas-Diesel) Engine under Ni-Co-Cu/ZSM-5 Catalysts

2017-01-0908

Published 03/28/2017

Fanxu Meng, Asanga Wijesinghe, John Colvin, Carolyn LaFleur, and Richard HautHouston Advanced Research Center

CITATION: Meng, F., Wijesinghe, A., Colvin, J., LaFleur, C. et al., "Conversion of Exhaust Gases from Dual-Fuel (Natural Gas-Diesel) Engine under Ni-Co-Cu/ZSM-5 Catalysts," SAE Technical Paper 2017-01-0908, 2017, doi:10.4271/2017-01-0908.

Copyright © 2017 SAE International

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There are many studies of catalysts used for the oxidation of methane in other industries [7]. For example, Torniainen, et.al. reported the investigation of partial oxidation of CH4 in O2 near atmospheric pressure on monolith-supported metals [8]. Dissanayake reported partial oxidation of methane by reaction of a CH4/O2 mixture at 1 atm over a 25 wt% Ni/Al2O3 catalyst [9]. However, most oxidation catalysts were used at high temperature and low SV, neither of which is practical in engine exhaust aftertreatment systems. Among them, Ni and Co were mostly reported [10, 11] either singly or in combination with other metals. Again, these tests were carried out with only methane and oxygen, and consequently excluded potential effects of other gases present in dual fuel engine exhaust. Methane is a significant constituent of concern in the exhaust, but actual engine exhaust is more complicated. Thus, the investigation of how these catalysts perform in eliminating methane from dual fuel engine exhaust is a topic of great interest.

This study evaluated the activity of Co- Ni- and Cu-based ZSM-5 catalyst for short-chain alkane conversions. Real engine exhausts from a lean-burn dual fuel (natural gas and diesel) engine were fed to a bench reactor in these experiments. Catalytic activity is presented by the conversions of short-chain alkanes (CH4, C2H6, C3H8), NMHC, CO, formaldehyde and NO2. Experimentation revealed that the conversion of these species varied as the engine was operated at different RPMs, loadings and natural gas substitution rates. We Also discussed are the effects of temperature in the conversions of short-chain alkanes, especially methane and other components. As compared with other works that utilized a synthetic mixture of gases to simulate engine exhaust, these experiments were conducted using actual exhaust from a dual fuel diesel engine on a dynamometer. Testing with actual exhaust better informs the investigation of new, novel catalyst formulations.

Experiments

Test Dual Fuel EngineTo generate the dual fuel exhaust for these experiments, a CI diesel engine was modified to introduce natural gas into the air intake system, as is typical of most dual fuel engines on the market. Specifications of the CI diesel engine are listed in Table 1. Figure 1 shows the schematic diagram of the test engine setup which included a natural gas injection system, a compression ignition engine (Cummins) loaded by a dynamometer and the data acquisition/control systems. The natural gas injection system consisted of a pressurized fuel tank, filter, pressure regulator (from 3000 to 150 psi), and a flow controller (Alicat) to further reduce gas pressure before injection into the air intake. Natural gas is injected at the intake manifold and mixed with air supplied to the cylinder. The gas fuel provides energy to substitute for some portion of diesel fuel. Under fixed loading the diesel flow decreased as more natural gas was injected. A LABVIEW-based system Developed by Houston Advanced Research Center (HARC) was used to automatically control the test engine and acquire data. Loading was adjusted by dynamometer and the natural gas substitution was adjusted by injection rate of natural gas. The low sulfur diesel and natural gas used are commercially available, respectively purchased from Valero and Freedom CNG, in Houston, TX.

Table 1. CI engine specifications

Figure 1. Schematic diagram of the test engine setup

CatalystA gas-assisted washcoat method [12, 13] was used to coat catalysts onto a monolithic substrate of honeycombed structure. The ceramic monolith featured 100 cells per square inch (CPSI) and the dimensions of 1” diameter by 1” long. This monolith was fixed into a stainless steel pipe, into which a prepared water-based slurry was injected, as shown in Figure 2. This slurry contained metal precursors, HZSM-5 zeolites and silica/alumina binders. The slurry continuously accumulated on the top cross-section of honeycomb monolith. After all the cross-section of honeycomb structure was covered by injected slurry, the pipe was capped and pressurized nitrogen gas was released through the orifice in the cap into the pipe. By this process, the nitrogen flow pushed slurry through the channels of monolith, displaced excess slurry from the inner surfaces of the channels, and dried the attached slurry coating. This process was repeated until the target loading was achieved. A series of test catalysts coated onto monolith all had similar loadings of 1 g/in3.

Figure 2. Schematic diagram of washcoat

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The catalyst slurry formulations were prepared by a co-precipitation method. Salts of cobalt nitrate hexahydrate (Sigma Aldrich), nickel nitrate hexahydrate (Alfa Aesar) and copper acetate (Sigma Aldrich) were respectively dissolved in water. Each was then mixed together with silica/alumina binders, stirred into ZSM-5 water-based slurry in order to deposit mixed oxides on the zeolite surfaces. After the gas-assisted washcoat process, the test catalyst monoliths were dried overnight followed by calcination at 600 °C for 2 hrs.

Bench Catalytic ReactorA bench-scale catalytic reactor was used for all experimental tests in this study. The schematic of the system is presented in Figure 3. A MKS Fourier Transform Infrared (FTIR) spectrometer was used to analyze composition of the gas streams into and out of reactor to determine the treatment effectiveness of each catalytic formulation. All the sampling lines were heated to 191 °C to reduce condensation of water vapor and hydrocarbons. Conversions were calculated by comparing the gas analysis before and after catalysts. Software used by the FTIR to analyze the components was 191 °C Dual Fuel method provided by MKS, manufacturer of the FTIR. Stainless steel tubes used in this study were 1-inch ID. The monolith blocks were stacked in the section near the outlet of the reactors. The reactor section between the inlet and the test catalysts was filled with ceramic structure to assure that incoming exhaust gases were uniformly heated. This reactor was directly connected with the engine-out exhaust pipe of the dual fuel engine. A ball-valve and Alicat flow meter were used for gas metering.

As a comparison of catalytic effectiveness in converting methane in dual fuel exhaust, additional experiments were conducted using a mixture of methane and oxygen; methane mixed in air (2% volume) instead of engine exhaust was injected into the reactor mounted with Co/ZSM-5 and Co-Ni-Cu/ZSM-5 catalysts to test the oxidation of methane. The SV was manipulated at 50000 h-1. However, at up to 650 °C, no notable methane conversion was observed.

There are two limitations in our tests with the bench catalytic reactor:

1. The emission data was collected after catalysts were exposed in the emission stream for 10 minutes. No aging treatment was applied to the tested catalysts. The data is not suitable to describe the stability of the catalysts.

2. We used commercial diesel and natural gas. Sulfur resistance was not investigated in our tests

Figure 3. Schematic diagram of bench reactor

Results and Discussion

Co/ZSM-5 CatalystsCo is one of the most popular metals used for catalytic conversion of CH4, usually deposited by a co-precipitation method on zeolites [14]. The tests with a bench scale catalytic reactor and Co/ZSM-5 demonstrate that increased temperature can improve the conversion of CH4 and other HC. Figure 4 shows the results of Co/ZSM-5 catalysts tested at 400 °C, 100000 h-1 SV when the dual fuel engine is operated at 2000 RPM, 50% loading, 50% NG substitution and at 550 °C, 140000 h-1 when the dual fuel engine is operated at 75% loading, 25% NG substitution, respectively. The increased temperature greatly promoted the conversion for C2H6, C3H8 and formaldehyde by 80.6%, 62.5% and 62.2% respectively. Notably, CH4 conversion increases from 2.5% to 12.1%. This conversion of CH4 at 550 °C is comparable with the result reported by Ottinger, et al [6], though their conversions were obtained by oxidation of simulated gases. The conversions of C2H6 and C3H8 are much higher in our study, but the conversion of CO is much less. This trend indicates Co/ZSM-5 have great selectivity for oxidation of alkanes at 550 °C, though activity of our catalyst was minor at 400 °C.

Conversion of NO2 increases from 61% to 93% when temperature was increased from 400 to 550 °C. NO2 is partially eliminated by the reactions of HC and CO oxidation with NO2[15]. However, the concentration of NOx is in the same because NO is produced from NO2 conversion. This result is not desirable, since NOx is not effectively reduced by short-chain alkanes.

CO conversion was as little as 11.3% even when the test temperature is set to 550 °C. The list of constituents present after treatment with Co/ZSM-5 is shown in Table 2. These emissions contain significant CO which is produced by partial oxidation of CH4. When the dual fuel engine was operated at a higher loading and lower NG substitution, the concentration of CO decreases after catalytic treatment as does CH4. The decrease in CO and CH4 are likely due to conversion to CO2 related to the more completed combustion in the engine. This is reasonable since more fuel is used to meet higher power requirement while less NG is injected.

Figure 4. Conversion of gases in exhaust by Co/ZSM-5 catalysts

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It is noted that when methane was mixed with air for the bench reactor test, no notable methane conversion was observed at up to 650 °C. This indicates that methane oxidation is not the main reaction mechanism. It is very likely that steam facilitates the reforming and oxidation of CH4. However, we cannot rule out the possibility that O2 and NOx (or some combination of these) promote the oxidation of CH4. This is possible because of the complicated nature of dual fuel exhaust composition that may instigate varied reactions. Though not addressed in this work, this concept warrants investigation in future research, that would include detailed study on characterization of catalysts and mechanisms of reactions.

Table 2. The list of components after the bench reaction with Co/ZSM-5 or Co-Ni/ZSM-5 catalysts

Co-Ni/ZSM-5 CatalystsNi is another active metal catalyst widely used to promote conversion of CH4. Reforming [16] and partial oxidation [11] can occur with Ni catalysts. Figure 5 shows the conversion of gases in exhaust by Co-Ni/ZSM-5 formulation, tested at high temperature and low SV, which is conducive to conversion of short-chain alkanes. More than 98% of NO2, 94% of formaldehyde and 43% of CO was converted. The conversion of C2H6 and C3H8 are more than 28% and 63%, respectively, for both tests. However, this catalyst is apparently not active for CH4 conversion, since less than 4% CH4 is converted. All of these conversions of short-chain alkanes are unexpectedly less than those of Co/ZSM-5. Adding Ni is not an effective way to promote conversion of these short-chain substances. At low temperature and high SV, which is not suitable for this catalytic reaction, the CH4 conversion is less than 2%. The other exhaust gas constituents had much lower conversion as well. Co-Ni/ZSM-5 is not active at 400 °C in a SV comparable with real operating conditions of a dual fuel engine.

The list of components after Co-Ni/ZSM-5 is given in Table 2. The trends of decreasing of CH4 and increasing of CO indicate the increased consumption of CH4 when the dual fuel engine is operated at a higher loading but fixed NG substitution. The consumed CH4 generates CO and increases its level of emission.

a.

b.

Figure 5. Conversion of gases in exhaust by Co-Ni/ZSM-5 catalysts at low SV (a) and high SV (b)

Co-Ni-Cu/ZSM-5Figure 6 shows the results of exhaust gases after the monolith catalysts. Co-Ni-Cu/ZSM-5 is used at 550 C, 80000 h-1 SV. The dual fuel engine is operated at 2000 RPM, 75% loading and 50% natural gas substitution. The activity of NO2, formaldehyde and diesel C1 is more than 85%. Consistent with the results previously reported, the Cu deposited on zeolites promotes the activity for short-chain alkane (CH4, C2H6 and C3H8) and CO, whose conversion is comparable with previous study. Notably, the CH4 conversion was enhanced after Cu is added in the catalyst. The total HC consists of mostly CH4, so the conversion is similar to that of CH4. The non-methane HC has a conversion of 35% in this experiment. The list of components after Co-Ni-Cu/ZSM-5 is given in Table 3.

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Figure 6. Conversion of gases in exhaust by Co-Ni-Cu/ZSM-5 catalysts

Table 3. The list of components after the bench reaction with Co-Ni-Cu/Z catalysts

Figure 7 shows the Co-Ni-Cu/ZSM-5 was tested at 500 °C and 30000 h-1 SV, when the loading of dual fuel engine is increased from 25% to 75% with a fixed 1000 rpm and 25% natural gas substitution. The change of loading determines exhaust composition. Consequently, the varied concentrations of components in exhaust may affect the equilibrium and rates of the heterogeneous reactions between gasous components and surfaces of solid catalysts. In this way, the activity of aftertreatment catalysts is influenced, though the reaction order for methane is the same. A detailed study of the mechanism by which methane conversion occurs with these catalysts is worthy of further investigation. Conversion of NO2 and formaldehyde is high for both loading conditions. However, NOx is still not converted because the converted NO2 mostly becomes NO. Additionally, considering the small ratio of NOx to CH4 in actual dual fuel exhaust, the potential for oxidation of NCM by NOx appears limited in this study. Selective Catalytic Reduction (SCR) uses NH3 and would be worthy of further study. Though reduction with NH3 is not included in this study.

There was little C3H8 and diesel present at 75% loading and 25% substitution, since these were not detected in this study. As such their conversion could not be calculated nor shown in Figure 7a. CH4 and

C2H6 had higher conversion at greater engine loading while CO conversion was low. This can be explained by differing concentrations of constituents in the exhaust. As seen in Table 3, there is much more CO and less CH4 and C2H6 when the engine is operated at 75% loading and 25% substitution. A high dose of a specific component will result in saturation of the surface reaction of heterogeneous catalysts, which can slow the conversion of that component. Thus the conversion of CO decreases because more CO is generated. Conversely, the conversion of CH4 and C2H6 increases because lower concentrations are present.

a.

b.

Figure 7. Engine operation and temperature affect conversion of gases in exhaust by Co-Ni-Cu/ZSM-5 catalysts at 25% NG substitution (a) and 70% NG substitution (b)

Further investigation of temperature effects yields more information on performance of these catalyst formulations. The catalysts were tested at 30000 h-1 SV, 25% loading and 70% natural gas substitution at 1000 rpm. Test temperature was increased from 450 to 475 and finally to 500 °C. All constituents were similarly converted in the temperature range of 450-500 °C, except CH4 and HC. The conversion of CH4 increases significantly to 42% when temperature is increased from 475 to 500 °C. Conversion of C2H6, C3H8 and NMHC also increase with temperature. However, because these compounds already exhibit more than 45% conversion at 450 °C, the increase in conversion with temperature is not as great as CH4. All these conversions observed in this study are higher than the results reported by Ottinger et al [6] with simulated gases.

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ConclusionsThe activity of Co- Ni- and Cu-based ZSM-5 catalyst for short-chain alkane conversions was evaluated by testing in a bench-scale catalytic reactor with the actual exhaust of a lean-burn dual fuel (natural gas and diesel) engine. Conversions of short-chain alkanes (CH4, C2H6, C3H8), NMHC, CO, formaldehyde and NO2 are measured using a FTIR gas analyzer. Formaldehyde and NO2 were more amenable to treatment. NO2 was largely converted even at 400 °C, though NOx levels were not effectively reduced because the converted NO2 became NO. Conversions of short-chain alkanes increased as the catalyst was tested under increased temperature. The enhancement of methane conversion was more obvious when Co-Ni-Cu/ZSM-5 catalyst was used. Co and Ni-based catalysts did not exhibit significant performance at low temperature and high SV as compared with the results of methane oxidation tests in the literature [10, 11]. After Cu was added in this metal/zeolites catalyst, the performance of methane conversion was considerable as compared with reported results [6]. It is quite likely that steam facilitates the reforming of CH4, and O2 and NOx (or some combination of these) aid in the oxidation of CH4. Concentrations of constituents in dual fuel engine exhaust after catalytic treatment varied with engine operating parameters such as RPM, NG substitution rate and loading. High concentration of some specific component in exhaust emissions will result in saturation of the surface reaction of heterogeneous catalysts, which can slow the conversion of that component. These interesting results indicate the potential for application of catalytic aftertreatment to reduce emissions from dual fuel engines, though further experimentation is needed to refine performance characteristics. Especially as compared with a diesel-only engines, dual fuel combustion dynamics and exhaust gas composition are considerably more complex. The variability of gas substitution rate over a range of load and RPM contributes to this complexity and the variability of constituents presented to the catalytic aftertreatment system. As part of this characteristic, constituents in dual fuel exhaust can react with each other in the presence of the appropriate catalysts. This insight provides perspectives on how improved catalytic aftertreatment systems can be applied to reduce emission from dual fuel engines.

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Contact InformationCorrespondence should be addressed to:

Dr. Fanxu Meng at Houston Advanced Research Center (HARC):4800 Research Forest Dr., The Woodlands, TX 77381, USAfmeng@harcresearch.org

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AcknowledgmentsThe authors would like to thank the Houston Advanced Research Center (HARC) and the Environmentally Friendly Drilling (EFD) Program for their commitment to further the understanding of dual fuel engines and research aimed at reducing emissions through improved aftertreatment technology.

Definitions/AbbreviationsCI - Compression ignition

CPSI - Cells per square inch

EGR - Exhaust gas recirculation

FTIR - Fourier Transform Infrared

GWP - Global Warming Potential

HC - Hydrocarbon

NCM - Non-Combusted Methane

NG - Natural gas

NMHC - Non-methane hydrocarbon

RPM - Revolutions per minute

SCR - Selective catalytic reduction

SV - Space velocity

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