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Electrochemical Enhancement of Nitric Oxide Removal from Simulated Lean-Burn Engine Exhaust via Solid Oxide Fuel Cells Ta-Jen Huang,* Chung-Ying Wu, and Yu-Hsien Lin Department of Chemical Engineering, National Tsing Hua University Hsinchu 30013, Taiwan, ROC ABSTRACT: A solid oxide fuel cell (SOFC) unit is constructed with Ni-YSZ as the anode, YSZ as the electrolyte, and La0.6Sr0.4CoO3-Ce0.9Gd0.1O1.95 as the cathode. The SOFC operation is performed at 600 °C with a cathode gas simulating the lean-burn engine exhaust and at various fixed voltage, at open-circuit voltage, and with an inert gas flowing over the anode side, respectively. Electrochemical enhancement of NO decomposition occurs when an operating voltage is generated; higher O2 concentration leads to higher enhancement. Smaller NO concentration results in larger NO conversion. Higher operating voltage and higher O2 concentration can lead to both higher NO conversion and lower fuel consumption. The molar rate of the consumption of the anode fuel can be very much smaller than that of NO to N2 conversion. This makes the anode fuel consumed in the SOFC-DeNOx process to be much less than the equivalent amount of ammonia consumed in the urea-based selective catalytic reduction process. Additionally, the NO conversion increases with the addition of propylene and SO2 into the cathode gas. These are beneficial for the application of the SOFC-DeNOx technology on treating diesel and other lean-burn engine exhausts.
1. INTRODUCTION The lean-burn engines, such as the diesel engines, can offer superior fuel efficiency and contributes to greenhouse gas reduction. However, the exhaust of the lean-burn engines contains excessive O2 and thus the three-way catalytic system for the stoichiometric-burn engine can no longer function to reduce nitric oxide (NO). Currently, urea-based ammonia selective catalytic reduction (SCR) is one of the most promising technologies for NO removal from the lean-burn engine exhaust. Nevertheless, the urea SCR aftertreatment system is quite complex and has concerns including the urea intrastructure and the potential freezing of the urea solution. For reducing NO, electrochemical reduction of NO without any reducing agent has been studied extensively.1-6 However, this process of electrochemical NO reduction is based on oxygen pumping and thus performed with an applied current; the current efficiency is generally only a few percent since simultaneous O2 reduction consumes substantial amount of electrical current.6 Thus, electrochemical NO reduction with an applied current is not efficient for treating an exhaust with excessive O2. On the other hand, simultaneous NO reduction and power generation has been shown to be feasible via solid oxide fuel cell (SOFCs);7-10 this is an SOFC-DeNOx technology. For the SOFC-DeNOx technology to compete with the ureabased NH3-SCR technology for NO removal from the leanburn engine exhaust, one important consideration is the consumption rate of the reactant, ammonia or SOFC anode fuel, which has to be stored onboard the automobiles and be refueling frequently. For the urea-based NH3-SCR technology, the overall urea reaction is11 ðNH2 Þ2 CO þ H2 O f 2NH3 þ CO2
ð1Þ
For the SOFC-DeNOx technology, the usage of the reformate from the methanol solution as the anode fuel is equivalent to that of ammonia from the urea solution as the reactant for the urear 2011 American Chemical Society
based NH3-SCR technology. The overall reaction of methanol steam reforming is12 CH3 OH þ yH2 O f ð2 þ yÞH2 þ yCO2 þ ð1 - yÞCO ð2Þ When the feed has a molar CH3OH/H2O ratio of 1 and the reaction temperature is 600 °C, almost 100% methanol conversion is obtained.12 Noteworthy, for SOFC operation, CO is an effective anode fuel.13 A comparison of reactions 1 and 2 indicates that the methanol solution can offer more fuel (reactant) than the urea solution for NO removal. For example, on a per weight basis, 50 g of methanol/water equimolar solution can generate 3 mols H2, whereas the corresponding urea solution (ca. 30 wt.% urea) can generate only 0.5 mol NH3. Notably, when hydrogen is used as the anode fuel, the overall reaction for NO removal is similar to that of H2-SCR:14 NO þ H2 f 1=2N2 þ H2 O
ð3Þ
For the urea-based NH3-SCR process, the standard reaction is11 4NH3 þ 4NO þ O2 f 4N2 þ 6H2 O
ð4Þ
Thus, the fuel (reactant) consumption of the SOFC-DeNOx process can be less than that of the NH3-SCR process. Therefore, the usage of the methanol solution for the SOFC-DeNOx process would need less storage space or fewer refueling than that of the urea solution for the NH3-SCR process. This can be an advantage of the SOFC-DeNOx technology. The SOFC-DeNOx technology is based on electrochemistry.10 Over the SOFC cathode, the oxygen molecule dissociates to produce the O species: O2 f 2O
ð5Þ
Received: October 12, 2010 Accepted: January 27, 2011 Revised: January 17, 2011 Published: June 13, 2011 5683
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Then, the charge transfer reaction of the O species occurs to form the oxygen ion: O þ 2e- f O-2
ð6Þ
Note that the O species can be transformed to the oxygen ion indiscriminating its source. Thus, the O species produced by NO decomposition: NO f N þ O
ð7Þ
has exactly the same role in the electrochemical process, that is, to generate a voltage, as that produced by O2 dissociation. This is why the generated voltage increases with increasing oxygen and NO concentrations.15 Then, the oxygen ion is transported from the cathode via the electrolyte to the anode and consumed by reaction with the anode fuel; restated, the O species is electrochemically pumped from the cathode to the anode. Electrochemical oxidation over the anode generates an electrical current; the produced electron flows through the outer circuit to the cathode to be used in the charge transfer reaction. Notably, the anode fuel is consumed only when an electrical current is generated.10 In this work, an SOFC unit cell is constructed with Ni-YSZ as the anode, YSZ as the electrolyte, and La0.6Sr0.4CoO3-Ce0.9Gd0.1O1.95 as the cathode, noting that La0.6Sr0.4CoO3 is the wellknown cathode material and Ce0.9Gd0.1O1.95 is the well-known electrode material for intermediate-temperature SOFCs. Electrochemical enhancement of NO removal from simulated leanburn engine exhaust occurs in the SOFC-DeNOx process. This can reduce the fuel consumption of the SOFC-DeNOx process to a very low level so that the storage space for the anode fuel (reactant) can be very much smaller or the refueling frequency can be very much lower than that of the NH3-SCR process. Thus, the SOFC-DeNOx technology can compete with the urea-based NH3-SCR technology for application onboard automobiles.
2. EXPERIMENTAL SECTION 2.1. Preparation of Cathode Materials. La0.6Sr0.4CoO3-δ (LSC) was prepared by the glycine-nitrate process. Appropriate amounts of reagent-grade metal nitrates La(NO3)3 3 6H2O (Stream chemicals, Inc., Newburyport, MA), Sr(NO3)2 (Sigma, USA) and Co(NO3)2 3 6H2O (Stream Chemicals) were dissolved in deionized water. Glycine (Sigma, St. Louis, MO) was also dissolved in deionized water. Then, these two solutions were mixed together with a glycine to NO3- ratio of 0.8:1. The mixture was then heated under stirring at 110 °C until combustion occurred. The product was ground to powders. Then, the powders were calcined by heating to 500 °C and held for 2 h, then to 900 °C and held for 4 h, and then slowly cooled down to room temperature. In this work, the heating of the powders was always done in air at a rate of 5 °C min-1. Gadolinia-doped ceria (GDC) with a composition of Ce0.9Gd0.1O2 was prepared by coprecipitation. The details of the method have been described elsewhere.16 The GDC powders were calcined by heating to 900 °C and held for 4 h. LSC-GDC composite was prepared by mixing the aboveprepared LSC and GDC powders at LSC:GDC = 1:1 in weight. The mixture was ground for 24 h, then calcined by heating to 500 °C and held for 2 h, and then to 800 °C, held for 4 h. 2.2. Construction of SOFC Unit Cell. A disk of anodesupported bilayers of yttria-stabilized zirconia (YSZ) and Ni-YSZ
(NexTech, Lewis Center, OH) was used to make an anodesupported cell. The cathode side of the bilayers was spin-coated with a thin interlayer of LSC-GDC to enhance adhesion; the coated cell was dried at 50 °C for 6 h, then heated in air at 10 °C min-1 to 500 °C and held for 2 h, and then heated at 5 °C min-1 to 1250 °C and held for 2 h before cooling down. Then, the cell was spin-coated with LSC-GDC as the cathode functional layer; the thus-coated cell was dried at 50 °C for 6 h, then heated at 10 °C min-1 to 500 °C and held for 2 h, and then heated at 5 °C min-1 to 900 °C and held for 2 h. And then, the cell was spin-coated with LSC as the current collecting layer; the heat treatment was the same as that for the cathode functional layer. The SOFC unit cell thus prepared had a configuration of Ni-YSZ/YSZ/LSC-GDC/LSC-GDC/LSC. 2.3. Fixed-Voltage Test. The fixed-voltage test was performed at 600 °C with the generated voltage, during power generation, kept constant at the designated one. The anode gas was pure hydrogen. The inlet cathode gas was simulated leanburn engine exhaust with a composition containing 10% H2O and 10% CO2 always, plus 6-14% O2 and various concentrations of NO, and balanced by helium. The composition of O2 and NO is designated in the figure captions. This composition of the gas mixture is similar to that as reported in refs 17,18. For designated tests, 267 ppm C3H6 and/or 25 ppm SO2 were added into the above cathode gas mixture. The overall flow rate of either anode or cathode gas was always 150 mL min-1. The tests were conducted with introducing a designated gas mixture to the cathode side of the SOFC unit cell for 30 min. During the test, the electrical current, the voltage and the outlet gas composition were always measured. The NO content was measured by NO analyzer (NGA 2000, Emerson, Germany). The N2 content was measured by a gas chromatograph equipped with a thermal conductivity detector (China Chromatography 8900, Taiwan). The C3H6 content was measured by a gas chromatograph equipped with a flame ionization detector (China Chromatography 8900, Taiwan). For the test with the operating condition designated as “OCV”, the designated cathode gas was flowing through the cathode side under open-circuit condition; thus, an open-circuit voltage (OCV) was measured during this type of test. For that designated as “inert gas”, helium (an inert gas) was flowing through the anode side instead of hydrogen or any other fuel; this type of test was also under open-circuit condition but no OCV was measured. However, a very low voltage was measured during the “inert gas” test; this voltage could be related to a leakage current.19
3. RESULTS AND DISCUSSION 3.1. The Effect of Operating Condition. Figure 1 shows the concentration profiles of formed N2 under various operating conditions. It is noted that the N2 formation behavior cannot reach a steady state under the fixed-voltage operation at 1.0 V; this is attributed to the fact that the fixed voltage of 1.0 V is very close to the OCV of 1.1 V and thus this operation results in very small current density, as also shown in Figure 1; this made the control of the fixed voltage not easy during our practice. In this work, the presented result is the averaged value of the data obtained during each measurement period of 30 min. Figure 2 shows the variation of the NO conversion, which is calculated from the amount of N2 formed via NO decomposition:
2NO f N2 þ O2 ; ΔH298 ¼ - 21:6kcal=mole NO 5684
ð8Þ
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Figure 1. Outlet N2 concentration and current density profiles at various operating conditions with 1820 ppm NO, 10% H2O, 10% CO2 and 6% O2.
Figure 2. Variation of NO to N2 conversion with operating condition at 1820 ppm NO, 10% H2O, 10% CO2 and various O2 concentrations.
The NO conversion generally increases with increasing O2 concentration from 6 to 14%. The highest NO conversion appears under the fixed-voltage operation at 1.0-0.8 V. With the operating voltage decreasing from 0.8 to 0.1 V, the NO conversion decreases. The NO conversion also decreases when the operation is shifted from the fixed-voltage of 1.0 V to the OCV (at 1.1 V) and further decreases when an inert gas is flowing through the anode side. Noteworthy, the NO conversion under the operation at the OCV has been reported with NO in helium but not in a simulated leanburn engine exhaust.10 It is noted that the NO conversion at a fixed voltage or at the OCV is larger than that under the inert gas operation. This indicates the well-known effect of electrochemical promotion of catalysis (EPOC).20 However, the EPOC effect is one under an applied voltage while the equivalent one in the SOFC-DeNOx process is under a self-generated voltage, noting that the OCV is also a generated voltage but has a different effect on the NO conversion from that at the fixed voltage. The different effect between the fixed voltage and the OCV may be attributed to the existence of an electrical current in the former case; notably, the generated electrical current should have an effect on the
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enhancement of the NO conversion as that of electrochemical NO reduction with an applied current.1-6 Therefore, the promotion effect of this self-generated voltage is termed “electrochemical enhancement” in this work to distinguish it from the EPOC effect. Figure 2 also shows that the NO conversion increases with increasing O2 concentration even under the inert gas operation. This indicates a promotion effect of oxygen. This promotion effect for catalytic NO decomposition has also been observed at 400 °C but without the presence of H2O and CO2.21 The promotion effect at the OCV may be attributed to the fact that the OCV increases with increasing O2 concentration;15 thus, this promotion effect is considered to be part of the effect of electrochemical enhancement. Notably, the OCV is a generated voltage at open circuit; thus, other generated voltages are associated with the OCV and can thus increase with increasing O2 concentration,15 this explains the increase of the NO conversion and thus the increase of the promotion effect with increasing O2 concentration—that is, higher O2 concentration leads to higher enhancement. Additionally, the promotion effect at a fixed voltage may be attributed to a so-called “oxygen carrying capability”, as will be described in Section 3.2. For NO removal with power generation, Table 1 shows that an intermediate voltage can result in the largest power density without sacrificing much of the NO conversion, in terms of the N2 formation rate. Additionally, an intermediate operating voltage can be easier to result in a steady state than an operating voltage close to the OCV, as discussed in the above. 3.2. The Effect of NO Concentration. Figure 3 shows that the inlet NO concentration affects the NO conversion and also the N2 formation rate. Figure 3A shows that the NO conversion increases with decreasing NO concentration and this increase becomes dramatic when the NO concentration decreases from 3000 to 1820 ppm. The phenomenon of the increase of the NO conversion with decreasing NO concentration has been reported for electrochemical NOx reduction at 800 °C without the presence of H2O and CO2.8 It has also been observed for catalytic NO decomposition at 400 °C, also without the presence of H2O and CO2.21 Figure 3B shows that the N2 formation rate decreases when the NO concentration decreases from 9000 to 3000 ppm but increases when the NO concentration further decreases to 1820 ppm. This decrease of the N2 formation rate with decreasing NO concentration is a kinetic behavior of reaction 8, that is, the rate decreases with decreasing concentration, while its increase with further decreasing NO concentration is a result of the dramatic increase of the NO conversion. This phenomenon of the dramatic increase of the NO conversion may be attributed to the “oxygen carrying capability”, as described in the following. First, it is noted that electrochemical O2 reduction occurs with the first step being O2 dissociation to produce the O species, that is, reaction 5, followed by charge transfer reaction to produce the oxygen ion, that is, reaction 6. The transport of the oxygen ion from the cathode to the anode generates an electrical current via electrochemical oxidation with the anode fuel; this completes the process of electrochemical O2 reduction. Notably, the occurrence of reaction 6 does not discriminate the source of the O species. Thus, the O species produced by NO decomposition, that is, reaction 7, can also be transformed to the oxygen ion. When the NO concentration becomes smaller in a gas mixture with constant O2 concentration, the amount of the O species produced via NO decomposition also becomes smaller. Then, it 5685
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Table 1. Effect of Voltage on Current Density, Power Density, N2 Formation Rate and Fuel Consumption Factor (ψ) with 9000 ppm NO, 10% H2O, 10% CO2, and 14% O2 voltage (V)
a
current density (mA cm-2)
power density (mW cm-2)
N2 formation rate (μmol cm-2 min-1)
ψa
1.0
3.56
3.56
5.55
0.1
0.8 0.6
21.20 37.33
16.96 22.40
5.42 5.25
0.61 1.11
0.4
51.62
20.65
5.17
1.55
0.2
66.36
13.27
5.15
2.00
0.1
70.29
7.03
5.11
2.14
Fuel consumption factor as defined by eq 90 .
Figure 4. Variation of NO conversion with reaction condition under inert gas operation. Base case: 1820 ppm NO, 10% H2O, 10% CO2, and various O2 concentrations; C3H6: 267 ppm; SO2: 25 ppm.
Figure 3. Variation of (A) NO conversion and (B) N2 formation rate with inlet NO concentration at 0.6 V, 10% H2O, 10% CO2, and various O2 concentrations.
would be easier for the flux of oxygen ion from the cathode to the anode to carry away these O species, after their transformation via reaction 6. This capability to carry away the O species is termed the “oxygen carrying capability”. Figure 3 also shows that the extent of the dramatic increase of the NO conversion can increase with increasing O2 concentration, which increases the amount of the oxygen ions and thus increases the oxygen carrying capability; this may confirm the existence of the oxygen carrying capability. Restated, the oxygen carrying capability increases with increasing O2 concentration. Since the oxygen carrying capability occurs in the electrochemical process, it is also considered to be part of the effect of
electrochemical enhancement. This also explains the above claim that higher O2 concentration leads to higher enhancement. Noteworthy, the oxygen carrying capability is associated with the amount of the oxygen ions and thus associated with the current density. Nevertheless, Table 1 shows that the N2 formation rate decreases with decreasing voltage which results in substantially increasing current density; thus, the major source for the effect of electrochemical enhancement should be the generated voltage instead of the current density. Figure 3B also shows that the N2 formation rate always increases when the O2 concentration increases. This increase may be attributed to the increase of the generated voltage with increasing O2 concentration.15 However, this may also be attributed to the increase of the oxygen carrying capability with increasing O2 concentration. It is noted in Figure 2 that, at a fixed voltage, the NO conversion increases with increasing O2 concentration. Since the generated voltage is fixed, this increase should be due to an increase in the current density, noting that the power density increases with increasing O2 concentration 16 and the current density increases with increasing power density at a fixed voltage. It is noted that the increase of the curent density is associated with an increases of the oxygen carrying capability according to the above discussion. 3.3. The Effect of the Presence of Propylene and SO2. Figure 4 shows the variation of the NO conversion with the reaction condition under the inert gas operation. It is seen that adding 267 ppm C3H6 into the cathode gas increases the NO conversion, noting that the reaction activity of C3H6 is usually 5686
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used to represent that of hydrocarbons in the engine exhaust. Also note that C3H6 was completely consumed in the test of this work. Adding 25 ppm SO2 into the C3H6-added cathode gas further increases the NO conversion. The increase of the NO conversion in the presence of C3H6 is attributed to hydrocarbon reduction of NO similar to that in C3H6-SCR;22 that in the presence of SO2 may be due to a sulphated surface.23 This indicates that both C3H6 and SO2 are beneficial for the conversion of NO in the exhaust and that the above-described NO removal technology should be practical for treating the lean-burn engine exhaust. It should be noted that, based on the current SOFC manufacturing technology, the SOFC-DeNOx technology is better realized by a stack of tubular SOFCs with the exhaust passing the outer cathode side of an SOFC stack for NO conversion—that is, the exhaust flows through the space between the tubes in the stack. The SOFC tubes can be heated electrically from the inside, that is, at the center of the tube that is on the anode side.24 Note that, for microtubular SOFCs, heating from the outside can result in a startup time less than 10 s.25 With such an SOFC-DeNOx device, an issue is that the particulate matter (soot) in the exhaust may affect the SOFC performance. In fact, the SOFC tubes with the ceramic surface may not even be able to capture the diesel soot, due to the loose contact.26 Noteworthy, the space between the tubes in a tubular SOFC stack is usually quite large and thus there is almost no pressure drop for the exhaust flow through the SOFC stack. 3.4. Fuel Consumption. An SOFC needs the anode fuel for its operation. Although the fuel processing technology has gained impressive progress recently, such that an SOFC-based auxiliary power unit can use the diesel vapor as the fuel onboard the diesel trucks,27,28 fuel saving for the device of NO emission control may still be desirable. If the SOFC-DeNOx device needs only very small amount of the anode fuel, a micro reformer can put this device for application onboard every gasoline and diesel cars with fuel processing of the gasoline and diesel vapors, respectively. For evaluating the consumption of the anode fuel in this SOFC-based NO removal process, a fuel consumption factor (ψ) is defined as ψ ¼ ðmolar rate of anode fuel consumedÞ= ðmolar rate of NO convertedÞ
ð9Þ
It is noted that the transfer rate of oxygen ion transported from the cathode to the anode, that is, the O2,electro rate in terms of the O2 concentration, equals the rate of electrochemical oxidation of the anode fuel. Thus, the O2,electro rate is equivalent to the molar rate of anode fuel consumed. On the other hand, the molar rate of NO converted is equivalent to the NO conversion rate. When NO has been completely converted to N2, the molar rate of NO converted is equivalent to the N2 formation rate. Therefore, the expression 9 becomes ψ ¼ ðO2 , electro rateÞ=ðN2 formation rateÞ
ð9'Þ
where the O2,electro rate is calculated from the current density. Notably, when hydrogen is used as the anode fuel, the overall reaction can be expressed as reaction 3. Therefore, an operation with ψ = 1 means that the molar rate of converted NO equals that of consumed hydrogen while that with ψ < 1 would consume less fuel, in terms of the equivalent molar rate, than NO converted. Figure 5 shows that higher operating voltage results in lower fuel consumption; it may also result in higher NO conversion. Additionally, higher O2 concentration can also result in lower
Figure 5. Variation of fuel consumption factor (ψ) with operating voltage at 1820 ppm NO, 10% H2O, 10% CO2, and various O2 concentrations.
fuel consumption. In fact, higher O2 concentration is beneficial in every aspect considered in this work. Thus, it is beneficial to introduce secondary air into the exhaust. For the urea-based NH3-SCR process, reaction 4 reveals that the molar rate of ammonia consumption equals that of NO conversion. Nevertheless, the rate of ammonia consumption cannot be smaller than that of NO conversion. On the other hand, in the SOFC-DeNOx process, the rate of fuel consumption can be much smaller than that of NO conversion, as indicted in both Figure 5 and Table 1. This means that the amount of the fuel (reactant) in the SOFC-DeNOx process can be much less than that in the urea-based NH3-SCR process. Therefore, in the SOFC-DeNOx process, the storage space for the anode fuel (reactant) can be very much smaller or the refueling frequency can be very much lower than that of the NH3-SCR process. This would allow the SOFC-DeNOx technology to compete with the urea-based NH3-SCR technology for application onboard automobiles. Notably, operation at the OCV would consume no fuel in the SOFC-DeNOx process; however, the feasibility of the OCV operation for practical application needs further studies for clarification. Additionally, the progress in the SOFC manufacturing technology has increasingly lowered the cost of the SOFC stack for energy use; this is beneficial for the practical application of the SOFC-DeNOx technology for environmental use.
’ AUTHOR INFORMATION Corresponding Author
*Phone: þ886 3 5716260; fax: þ886 3 5715408; e-mail:
[email protected].
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