TECHNICAL SUBMISSION AUTOMOTIVE SULFATE EMISSIONS Submitted to: ENVIRONMENTAL PROTECTION AGENCY HEARINGS ON DELAY OF THE 197? CO AND HG AUTOMOTIVE EMISSION STANDARDS January 29, 1975 ------- TECHNICAL SUBMISSION AUTOMOTIVE SULFATE EMISSIONS Submitted to: ENVIRONMENTAL PROTECTION AGENCY HEARINGS ON DELAY OF THE 1977 CO AND HC AUTOMOTIVE EMISSION STANDARDS January 29, 197S PROPgRTV ©?; SwSS"Fua EW8s,ows 2030 TRAVgRWOOO DKIVE AWW ARB©R, Ml 4S1©§ ------- TABLE OF CONTENTS Page No. Summary . . 11 I. Introduction 1 II. Formation of Sulfate In Oxidation Catalyst Systems 2 III. Measurement Techniques Used to Study Sulfate Emissions. ... 3 A. Collection of Sulfate Particulate 3 B. Analysis of SOf on Filters 6 C. Measurement of SO2 7 IV. SO43 Emission Rates 8 A. S0^s Emissions from Non-Catalyst Cars 8 B. SOf Emissions from Cars Equipped with Oxidation Catalysts 10 1. Factors Affecting S0^= Formation - Laboratory Studies 10 a. Results for Monolithic Catalysts 12 b. Results for Pelleted Catalysts 18 c. Discussion of the Laboratory Data 22 2. Storage Phenomena .23 3. Vehicle S0^~ Emission 27 a. Effect of Catalyst Type 28 b. Effect of Gasoline Sulfur Level 41 c. Effect of Catalyst Age and Noble Metal Loading. . 41 V. Potential Methods for Controlling S0^= Emissions 41 A. Vehicle Tests of Limited Excess Air . 41 1. Vehicle Preparation and Base Line Testing 41 2. Tests of Limited Excess Air 44 B. Use of SO^9 Traps 47 1. Background 47 2. Vehicle Test of 85% Ca0/10% Si02/5% Na20 47 3. Work on Improved Sorbents 52 a. Calcium Compounds which Swell Less During Sulfation 52 b. CaO in a High Void Volume Shape 53 c. Materials which Sorb Less SOo 53 C. Other Information from EPA Contract 68-03-0497. . . . . . 55 1. Effect of Catalyst Age 55 2. Effect of Noble Metal Loading 56 References 59 ------- SUMMARY This document presents data generated by Exxon Research and Engineering Company on factors affecting the emission of sulfates from vehicles equipped with oxidation catalysts. Much of the data reported herein was developed as parts of two EPA contracts: Contract 68-03-0497, "An Assessment of Sulfate Emission Control Technology," and Contract 68-02-1297, "The Characterization of Particulate Emissions from Prototype Catalyst Vehicles." Our major findings are as follows: • Under FTP conditions monolithic catalysts emit about 10-15% of the sulfur in gasoline as sulfate. This is lower than the 25-351 emission rate previously reported by Exxon Research based on preliminary measurements on prototype catalysts, • Under FTP conditions pelleted catalysts emit about 5-10% of the sulfur in gasoline as sulfate. This value is in agree- ment with earlier findings, • Under high speed (60-70 mph) cruise conditions both types of catalyst emit about 25-351 of the'sulfur in gasoline as sul- fate, again in agreement with earlier findings. • Storage of SO2, S0^*» or both on catalyst surfaces occurs with both types of catalyst and accounts for many of the differences in SO^ emission rates observed. • Significant differences exist between the amount of S0^*° emitted from nominally similar monolithic catalysts. Cata- lysts from one manufacturer emitted less SO4" under certain conditions and also stored less sulfur oxide than did ------- - iii - nominally similar catalysts manufactured by another. The reasons for these differences are unknown. • Reducing the amount of excess air used over a catalyst sig- nificantly lowers SO4" emissions. Removing air pumps could lower S04" emissions by 50-75% in pelleted catalyst systems. We have yet to test this approach in vehicles using monolithic catalysts, but laboratory data Indicate that air pump removal _would also lower SOf emissions in these systems. This fact should be considered in the delay decision because the higher the CO and HC emissions allowed, the less the need for air pumps. • CaO was demonstrated to be effective in removing SOf from exhaust, but trapping SO2 and S0^= causes the sorbent to swell and unacceptable back pressure buildup occurs. Work is continuing to find better sorbents and to overcome this problem. We have made no estimates of SO40 emission factors for production 1975 vehicles because all of our work has been done rwith cars modified to meet emission levels of 3.4 g/mi CO and 0.41 g/mi HC. Similarly we have not commented on the impact of automotive sulfate emissions on either atmos- pheric SO^*4 levels or human health because we have no data of our own in either of these two areas. ------- I» Introduction To meet the 1975 interim emission standards for carbon monoxide (CO) and hydrocarbons (HC), most cars sold in the U. S. use emission control systems containing oxidation catalysts. In addition to meeting the statutory requirements for CO and HC control, these systems provide the additional benefits of lowering the reactivity, or smog-forming potential, of the hydrocarbons emitted and substantially reducing emissions of aldehydes f an(j polynuclear aromatics (3) # Oxidation catalyst systems do, however, create their own special concerns. They require the use of unleaded, phosphorus-free gasoline, which places a burden on the petroleum industry. They also convert some of the sulfur naturally present in gasoline to sulfate particulate. The purpose of this document is to present, in detail, the data generated by Exxon Research and Engineering Company on the factors affecting automotive sulfate emissions. Where relevant, we will also quote data developed by others. Much of the data which will be reported has been generated as parts of two EPA Contracts: Contract 68-03-0497, "An Assessment of Sulfate Emission Control Technology", and Contract 68- 02-1279, "The Characterization of Particulate Emissions from Prototype Catalyst Vehicles". Both of these contracts are currently in progress at Exxon Research and Engineering Company. * Numbers in parentheses refer to references listed at the end of this paper. ------- - 2 - The following topics will be discussed In this presentation: • formation of sulfate In oxidation catalyst systems, • measurement techniques used to study sulfate emissions, • . sulfate emission rates from oxidation catalyst-equipped vehicles, and • methods which could potentially lessen automotive sulfate emissions. II. Formation of Sulfate In Oxidation Catalyst Systems I Sulfur Is present In gasoline In trace quantities, usually less than 0.1 wt. %. The amount of sulfur In any given gasoline sample Is a function of the sulfur content of the crude from which the gasoline was made, and the refining processes used In making the gasoline. National average gasoline sulfur Is about 0.03 wt. %. The only area of the country having a significantly different gasoline sulfur content Is Southern Califomla.where the average Is 0.06 - 0.07 wt. %. When gasoline Is combusted In an Internal combustion engine, sulfur Is oxidized to sulfur dioxide (SO2): gasoline S + 02 engine SO2. In vehicles without an oxidation catalyst, sulfur Is emitted In that / form. When an oxidation catalyst Is present, some of the SO2 formed in the engine is oxidized to sulfur trioxide (SO3): S02 + 1/2 02 oxidation catalyst SO3. ------- - 3 - The fraction of SO2 converted to SO3 is a function of the type of oxidation catalyst used, its operating temperature, the amount of excess oxygen present, and the residence time during which the S02 is in contact with the catalyst. Each of these subjects will be discussed in detail later in the presentation. In the exhaust system, SO3 combines with the water present in the exhaust to form sulfuric acid (H2SO4): S03 + H20 * H2SO4, and is emitted as such. Since the analytical techniques used to measure the amount of H2SO4 in automotive exhaust are Incapable of distinguishing between SO3, H2SO4, and any products of reaction between H2SO4 and cations present in the exhaust, it is customary to refer to all of these materials as sulfate (SCty") emissions. III. Measurement Techniques Psed to Study Sulfate failssions A. Collection of Sulfate Particulate Sulfate is present in automotive exhaust as fine particulate. To correctly measure the amount of SO41* emitted, it Is necessary first to filter it from the exhaust. Exxon Research accomplishes this task with a device we call the Exhaust Particulate:Sampler, shown schematically in Figure 1. This device was designed to meet three basic criteria: 1. The equipment must be compatible with constant volume sampling (CVS) procedures for gaseous automotive emissions and must allow operation of the Federal Test Procedure (FTP) mandated for the measurement of these emissions. ------- Figure 1 EXHAUST PARTICULATE SAMPLER INTAKE DILUENT AIR DEHUMIDIFIER FILTER BOX FLOW DEVELOPMENT TUNNEL COUPLED MIXING BAFFLES TO CVS PUMP MIXING TURBULATORS ROTOMETER ISOKINETIC SAMPLING PROBE HEAT^T^- EXCHANGE FILTER HOUSING EXHAUST INJECTOR AIR COOLED COMPRESSOR ------- - 5 - 2. The sampling must be made under conditions In which a true proportional sample of the exhaust gas Is taken for measurement, I.e., Isokinetic sampling must be obtained. 3. The temperature at the point of sampling must be less than 90°F., to ensure the collection of all material that would be In particulate form in the atmosphere. The major features of this sampler include a 7.5 ft. flow development tunnel, which Is shorter than other devices developed for these measurements, and an advanced diluent air system. This latter point is important in that it allows low temperature (<90°F.) sampling without excessive dilution and/or long flow development tunnels. Diluent air is drawn into the particulate sampler In a manner analogous to that for a conventional CVS unit. This air is dehumidified, and filtered through a charcoal filter assembly. A portion of the dehumidified, filtered air is passed through an air-cooled heat exchanger which lowers its temperature to about 40°F. This chilled air is then blended with the remainder of the diluent air prior to being mixed with the exhaust gas. The amount of air passed through the heat exchanger is controlled by a signal from a thermocouple adjacent to the isokinetic sampling probe. When the probe temperature Increases, the position of the mixing baffles is changed to divert more air through the heat exchanger. This increase In chilled air assures the maintenance of probe temperatures less than 90°F during the FTP.. ------- - 6 - Data demonstrating the ability of this system to maintain 90°F filter temperature without condensation of water on the filter, and to prevent significant loss of particulate material in the flow development tunnel or probes have been presented by Beltzer, et al. in SAE Paper 740286, "Measurement of Vehicle Particulate Emissions". A copy of this paper appears as Attachment I. Exxon Research's dilution tunnel (10.9 cm diameter, 2.3 meters long) is smaller than those used by EPA (45.7 cm diameter, 4.5 meters long). However, in their soon to be published SAE Paper, "Sulfate Emissions from Catalyst Cars: A Review", Bradow and Moran of EPA, conclude that the two systems are equally effective. They state: "A comparison of the two systems has been conducted at EPA in Research Triangle Park where an Exxon-type, tunnel had been fabricated and Installed on an engine dynamometer test stand. Sulfate determinations with the GM catalyst have been found to be comparable with both systems. Further, experimental determinations *. -»¦- m . of wall losses Indicated comparable performance. Thus, non-catalyst organic aerosol wall losses were about 3% of the aerosol handled and sulfate losses about 1% with the Exxon tunnel. Thus, both the Exxon and EPA dilution systems appear to be effective sulfate aerosol samplers." B. Analysis of S0/t° on Filters The sulfate collected by the filter in the exhaust particulate sampler is leached from the filter with dilute nitric acid. The ------- - 7 - leach solution is heated to boiling to drive off excess nitric acid, filtered to remove insoluble material, passed through an ion-exchange column to remove interfering cations, and then buffered with methen- amine to a pH of 3-4. The resulting solution is titrated with barium perchlorate using Sulfanazo (III) as an indicator. This method has been found to be sensitive to levels of 2 yg S04*7cm^ of filter, a level equivalent to about 0.0005 g/mi. SO^" on the 1975 FTP. In their above mentioned paper, Bradow and Moran indicate that this method is one of several which, when correctly practiced, give comparable results. C. Measurement of S02 The measurement of SO2 is necessary to close the sulfur balance around a car, i.e., to account for all of the sulfur consumed with the fuel. Exxon Research uses a Thermo.Electron Corporation (TECO Model #40) SO2 Analyzer which operates on a pulsed-fluorescence UV absorption principle. This Instrument operates by exciting SO2 molecules with ultraviolet light, and measuring the fluorescent light emitted as the SO2 returns to ground state. The intensity of the fluorescence is directly proportional to the SO2 concentration. This Instrument is supposed to be specific for S02 and not affected by the other molecules typically found in auto exhaust. This, / however, is not the case. Water vapor interferes with the operation of the system, and it is necessary to completely dry the gas sample prior to its introduction into the unit. We have also found that CO2, CO, and O2 strongly quench the fluorescence. The Instrument is therefore sensitive to the composition of the background gas. To obtain accurate SO2 ------- - 8 - concentrations It Is necessary to calibrate the instrument with a background representative of the sample to be analyzed. For use with CVS system diluted exhaust, these problems can be circumvented by calibrating with dilute S02 In air. However, if the Instrument is to be used to analyze raw exhaust or a synthetic exhaust blend, recalibration is necessary. Further development of Instruments for measuring SO2 would be helpful. We also use the hydrogen peroxide (H2O2) bubbler technique to determine average SO2 concentrations in exhaust. In this technique, dilute or raw exhaust gas is filtered to remove SO^ particulate and passed through a bubbler containing 80 ml. of 3% H2O2 in high purity water. The SO2 present in the exhaust is quantitatively oxidized to SO^" in this solution. Tests with bubblers run in series have shown that collection efficiency, at flows up to about 5 liters/minute,is greater than 95%. After increasing solution volume to 100 ml., the collected SO40 is analyzed using the same method as is used to analyze for SO4" leached off the particulate filters, except that the step Involving Ion exchange to remove cations which might Interfere with the analysis is not necessary. IV. SOL" Emission Rates A. SO/j" Emissions from Non-Catalyst Cars Sufficient data have now been accumulated to demonstrate conclusively that non-catalyst cars emit very low levels of SO^". a Table 1 contains a summary of the SO^ emission rates measured on non- catalyst cars tested at Exxon Research, These data show conversions of gasoline sulfur to SO4™ of less than 1%, and are in general agreement with results published by GM^ and Ford ------- Table 1 SO2 and SO40 Emission data from Non-Catalyst Cars Vehicle Fuel Sulfur,% Mode SO, g/tai. Emissions % of Gasoline S so4 g/ml. % Emissions of Gasoline S % Sulfur Balance 1973 Chev. 0.040 1972 FTP A * - <0.007 <2.0 * II II 0.067 40 mph * * 0.004 0.1 * II If II 11 * * 0.004 0.1 * tf ft II ! 11 * * 0.009 0.2 * 11 II II It * * 0.0015 0.5 * 1969 Ply. 0.140 1972 FTP 0.647 107 0.018 2.0 109 11 it II If 0.660 103 0.012 1.3 104 ii it 0.056 II 0.268 115 0.007 2.1 117 ii tt II II 0.262 110 0.007 1.7 112 •1 11 0.032 II 0.178 135 * A * ii 11 tl II 0.166 120 0.006 2.9 123 1974 Chev. 0.019 1975 FTP 0.100 115 0.0014 1.07 116 ii 11 11 40 mph 0.040 71.4 0.0003 0.36 71.8 11 11 11 70 mph 0.056 93.3 0.0018 2.00 95.3 11 11 0.091 1975 FTP 0.498 112 0.0024 0.36 113 ii 11 If 40 mph 0.257 89.9 0.0006 0.14 90.0 11 11 II 70 mph 0.219 82.3 0.0027 0.66 83.0 it 11 0.110 1975 FTP 0.466 117 0.0024 0.40 117 11 •• ii *40 mph 0.325 79.3 0.0008 0.13 79.4 IT II 11 70 mph 0.269 83.5 0.0027 0.55 84.1 1974 Chev. 0.065 1975 FTP * * 0.002 0.4 * II tf II 60 mph *. * 0.002 0.6 * II II 0.032 1975 FTP * * 0.003 1.1 * II II II 60 mph * * 0.001 0.7 * 1974 Mazda 0.065 1975 FTP 0.40 126 0.002 0.4 126 II II If 60 mph 0.22 116 0.000 0.0 116 It M 0.032 1975 FTP 0.20 122 0.004 1.6 124 II II 11 60 mph * * 0.000 0.0 * 1974 Honda 0.065 1975 FTP 0.12 75 0.001 0.4 75 II II , II 60 mph 0.13 108 0.000 0.0 108 II ft 0.032 1975 FTP 0.06 120 0.000 0.0 120 II tf ti 60 mph 0.06 70 0.000 0.0 70 * Not measured ------- -lo- ll. S0&" Emissions from Cars Equipped with Oxidation Catalysts Data obtained by Exxon Research and others show wide variations in the amount of SO4" emitted by cars equipped with oxidation catalysts, and adjusted to control emissions to 3.4 g/mi. CO and 0.41 g/mi. HC. For example, under FTP conditions, vehicles equipped with pelleted oxidation B3 catalysts emit about 5% of the sulfur In gasoline as SO^ , while vehicles equipped with monolithic oxidation catalysts emit as much as 10-15% of the sulfur in gasoline as SO^". At high speed cruise conditions, S04*3 emissions from the two types of systems are comparable, at 25-35% conversion of the sulfur in gasoline. Before trying to explain these differences and comment on their meaning, the data obtained in laboratory studies of the factors affecting SO4 formation, and vehicle tests demonstrating storage of S04b on catalyst surfaces, will be presented. These two subjects provide the background necessary to resolve some of the differences observed In vehicle SO40 emission rates. It should be pointed out, however, that a complete explanation of these differences is not available, and many questions still remain. 1. Factors Affecting SQa" Formation - Laboratory Studies Thermodynamic equilibrium calculations for mixtures containing / less than 100 ppm SO2 and 1-5% O2 show that at temperatures above about 1500°F, equilibrium conversion to SO3 Is very low, while at temperatures below about 800°F, equilibrium conversion to SO3 Is essentially 100%. The results of these calculations are shown in Figure 2. The conditions under which SO3 concentration is a function of both temperature and oxygen content are exactly the conditions under which automotive oxidation catalysts operate. ------- EQUILIBRIUM CONVERSION SO2 + 1/2 O2 S 0 3 (1) - 5 % 0 2 100 (2) -2% 0 80 ( 3 ) - 60 (1) 20 (2 ) ( 3 ) TEMPERATURE, °F ------- - 12 - To learn more about the effect of operating variables on SO40 formation, a laboratory program to study the effects of catalyst type, O2 concentration, temperature, and residence time on SO413 formation was carried out. The equipment used In this study Is shown schematically In Figure 3. The procedure used was as follows: A synthetic exhaust containing the components shown In the figure was blended and passed over a sample of commercial oxidation catalyst contained In the reactor tube. Temperature of the catalyst sample could be varied between room temperature and 1500°F. Conversions of S02, CO, and HC were measured using the TECO SO2 analyzer described earlier,and conventional exhaust gas analytical Instrumentation. The use of the Goks^yr-Ross technique ^ for SO^™ determination was attempted, but because of the low flow rates of a sample available at that time, accurate values for SO4 could not be obtained. This problem has now been solved, but the unit is being used for the SO4" trap studies reported at the end of this paper. The results of this study show significant differences between the behavior of monolithic and pelleted oxidation catalysts. Many of these differences appear to be related to the different tendencies of these catalysts to store S(>2 and S0^°. The results for monoliths are presented first, followed by the results for pellets, followed by a discussion of what conclusions can be drawn from this study. Since the measurement made was SO2 In and out of the reactor, the results are reported in terms of SO2 disappearance,which is the sum of SO2 converted to SO4" and net SO2 stored, If any, on the catalyst. a. Results for Monolithic Catalysts Figure 4 shows SOo disappearance as a function of temperature \ over a monolithic oxidation catalyst operated at a space velocity of ------- HC Pulsed Fluorescence Sample Drying Strip Chart Record Gas Analyzers Temperature H20 In Bypass for Inlet Concentrations FIGURE 3 - LABORATORY REACTOR Goksoyr-Ross Technique for SOf Reactor Furnace Feed Gas f 20 ppm SO2 200 ppm C3H8 1% CO 1500 ppm NO / 0.5% H2 12% H20 12% C02 ^2% 02 Balance N2 to 1 yy ------- - 14 - 62,500 v/v/hr. Data were obtained by changing catalyst temperature In 50-100°F. Increments and maintaining that temperature until outlet SO2 concentration stabilized. At temperatures below 600°F, essentially no SO2 disappearance was found. Between 600 and 800°F, SO^ disappearance rose rapidly towards the value for equilibrium conversion to SO4". Above 800°F, SO2 disappearance rate was maintained at 75% or more of the equilibrium conversion to SO40. Figure 5 shows SO2 disappearance as a function of reactor exit oxygen concentration at 1000°F and a space velocity of 100,000 v/v/hr. for a mbnollthlc catalyst. Reactor exit oxygen concentration is roughly equivalent to excess oxygen since it represents what remains after reaction with CO, H2, and HC. SO2 disappearance is relatively independent of O2 concentration about 1% excess O2, but drops sharply below 1% excess O2. CO conversion, also shown In Figure 5, drops off much less than does SO2 disappearance. The same Is true of HC conversion, though these data are not shown. Figure 6 shows the effect of space velocity on SO2 disappearance over a monolithic catalyst at temperatures between 800 and 1100°F. Between 800 and 1000°F, the results show the expected decreases in disappearance with increased temperature and increased space velocity. The data obtained at 1100°F shows disappearance to be relatively independent of space velocity, which can be. explained by the fact that at this temperature, the oxidation of SO2 to SO 3 Is limited by thermodynamic equilibrium. At 1100°F, storage of SO2 or SO4" does not appear to be significant. ------- FIGURE 4 S02 DISAPPEARANCE VS. CATALYST TEMPERATURE ¦ OVER A MONOLITHIC CATALYST OXYGEN CONCENTRATION: SPACE VELOCITY - 2% EXCESS 62,500 V/V/HR. 100 w o 80 51 m 60 Q Csl o W 40 20 EQUILIBRIUM CONVERSION 2% EXCESS 02 h VJ 600 700 800 900 1000 1100 1200 _J 1300 1400 TEMPERATURE, °F ------- - 16 - FIGURE 5 S02 DISAPPEARANCE VS. EXIT OXYGEN CONCENTRATION FOR A MONOLITHIC CATALYST 100 SV «= 100,000 V/V/HR CATALYST TEMP. 1000°F 80 60 40 . 20 . CO CONVERSION S02 DISAPPEARANCE I 1.0 2.0 3.0 REACTOR EXIT OXYGEN CONCENTRATION, % ------- 80 70 60 w w P4 51 w cm 50 o w 40 30 ~ 800°F S 900 ~ 1000 m iioo 800 F 900 F 100 OF •si I llOO'F FIGURE 6 - S02 DISAPPEARANCE VS. SPACE VELOCITY FOR A MONOLITHIC CATALYST 20 10 20 30 40 50 SPACE VELOCITY, V/V/HR x 10 -3 60 70 80 ------- - 18 - b. Results for Pelleted Catalysts All of the data obtained In this study with pelleted catalysts show lower SO2 disappearance rates than were observed with monolithic catalysts. Figure 7 shows SO2 disappearance as a function of temperature at a space velocity of 28,500 v/v/hr. This is a lower space velocity than was used with the monolithic catalyst, but typical of that encountered In pelleted catalyst systems. S02 disappearance rates In this system do not approach the equilibrium for conversion to as closely as they did for the monolith. Figure 8 shows SO2 disappearance as a function of reactor exit oxygen concentration for a pelleted oxidation catalyst at a space velocity of 28,500 v/v/hr. These results are similar to those observed for monolithic catalysts except that instead of dropping sharply at O2 concentration below 1%, as was the case with the monolith, with pellets, S02 disappearance decreases with decreasing O2 concentration over the whole range of O2 concentrations studied. Figure 9 shows SO2 disappearance as a function of space velocity and temperature for pelleted catalysts. Up to 1000°F, this relationship is as expected with SO2 disappearance decreasing with increasing space velocity and temperature. As in the case of monoliths, the S02 disappearance data at 1100°F shows no space velocity effect, because at this temperature, the reaction is limited by thermodynamic equilibrium. ------- FIGURE 7 S02 DISAPPEARANCE VS. TEMPERATURE FOR A PELLETED CATALYST 100 OXYGEN CONCENTRATION: SPACE VELOCITY : 2.5% EXCESS 28,500 V/V/I 80 * w y fti 5! 3 60 Q CM O CO 40 EQUILIBRIUM CONVERSION OR 2% EXCESS 0. 20 X JL 600 700 800 900 1000 TEMPERATURE, °F 1100 1200 1300 1400 ------- FIGURE 8 SOo DISAPPEARANCE VS. EXIT 0» CONCENTRATION FOR A PELLETED CATALYST 100 80 o cl CO CONVERSION 60 40 20 SO, DISAPPEARANCE N> O SV - 28,500 V/V/HR TEMP. 1000°F 1.0 2.0 3.0 4.0 5.0 6.0 L. 7.0 REACTOR EXIT OXYGEN CONCENTRATION, % ------- 50 FIGURE 9 SO? DISAPPEARANCE VS. SPACE VELOCITY FOR A PELLETED CATALYST w u 51 in M O «M o CO 40 30 20 10 _ 800"F ¦ 900 ~ 1000 • 1100 -L. 10 20 30 AO 50 SPACE VELOCITY, V/V/HR x 10 60 -3 70 80 ------- - 22 - c. Discussion of the Laboratory Data The results obtained by varying O2 concentration, space velocity, and temperature are what would be predicted from simple equilibrium and kinetic considerations. The drop-off in SO2 disappearance with decreasing 02- concentration is of particular interest because It suggests a method of minimizing SO40 formation without significantly decreasing CO and HC conversion. More will be said on this subject in Section V on Control of SO4" Emissions. Space velocity effects probably do not offer a practical method of controlling SO4" emissions. It must be assumed that the auto manufacturers sized their catalyst systems to provide the degree of CO and HC control required. Decreasing catalyst volume, the only practical way of increasing space velocity on a vehicle, would probably result in unacceptable CO and HC emissions. The temperature effect data for pelleted catalysts could be interpreted as meaning that these catalysts are less active for SO2 oxidation than are monoliths. This explanation seems unlikely, however, because both catalyst types show equivalent performance for CO and HC oxidation. The laboratory data are not in agreement with vehicle SO4™ emission data, which will be presented later in this paper. At high speed cruise conditions, where vehicle results should compare most directly with laboratory results, vehicle tests show the emission of S04° to be similar for both types of catalysts. Until the sulfur balance can be closed for both vehicle and laboratory tests, and sulfur oxide storage phenomena understood, the reasons for the differences between laboratory and vehicle test results will remain unexplained. ------- - 23 - 2. Storage Phenomena Both monolithic and pelleted catalysts have a coating of high surface area alumina. It is well known that alumina can sorb both S(>2 and S0^B, the amounts being determined by temperature, and the structure of the alumina present. Alumina tends to store SO2 or SO4™ at lower temperatures, corresponding to lower operating speeds, and release them at higher temperatures corresponding to higher speeds. SO^" storage has been studied in some detail,, though SO2 storage has not received much attention. It is known, however, that alumina and other sorbents sorb SO^ more readily than S02- In early 1974, two sets of experiments were conducted to demonstrate SO4*3 storage on pelleted catalysts, one in which the catalyst was conditioned with 0.14 wt. % sulfur fuel, the other in which it was conditioned with 0.004 wt. % sulfur fuel. The conditioning procedure used involved 500 miles of operation on the Federal Durability Driving Schedule (AMA Cycle) followed by a cold start .1975 FTP. After this conditioning procedure, the vehicle was operated for two hours at 60 mph cruise. The particulate filter was changed every 20 minutes to allow an evaluation of SO^ emissions as a function of time. Data from these runs is summarized in Figures 10 and 11. ------- - 24 - Figure 10 shows the results of tests with 0.14 wt. % S fuel on a pelleted catalyst conditioned with 0.14 and 0.004 wt. % S fuel. Initial SO40 emissions from the run In which the catalyst was conditioned with 0.14 wt. % S fuel are much higher than Initial SO40 emissions from the run In which the catalyst was conditioned with 0.004 wt. % S fuel. After ^60 minutes both runs show the same SO^ emission rate. This Is strong evidence of storage. After the catalyst was conditioned with 0.14 wt. % S fuel, Its surface contained an excess of S04°, which was released at the start of the run. Conversely, after conditioning with 0.004 wt. % fuel, the catalyst sorbed S04® at the start of the run. Figure 11 shows the results of tests with 0.004 wt. % S fuel. The effect of conditioning is even more dramatic In this case. In the test with a catalyst conditioned with 0.14 wt. % S fuel, almost seven times as much S04e was emitted during the first 20 minute period as at steady state after 60 minutes of operation. After S04" storage on pelleted catalysts was verified, we conducted a similar set of experiments to determine whether SO4" was stored on monolithic catalysts. The major change in the experiments on monoliths was that 175 miles of Federal Durability Driving Cycle mileage accumulation was used in conditioning the catalyst. For the pelleted catalyst, 500 miles of conditioning had been used. Results of these experiments appear in Figures 12 and 13. These tests show some of the same type of behavior as was seen with the pellets, but the storage effect is not as large. ------- - 25 - Figure 10 •H £ o o* en cfl 4J o H 0.60 _ 0.45- 0.30- 0.15 _ Sulfate Emissions at 60 mph Cruise Pelleted Catalyst, 0.14% Sulfur Fuel T Conditioned on 0.14% Sulfur Fuel Conditioned on 0.004% Sulfur Fuel »0« •O 20 40 60 80 Time, Min. 100 120 Figure 11 Sulfate Emissions at 60 mph Cruise Pelleted Catalyst. 0.004% Sulfur Fuel Conditioned on 0.14% Sulfur Fuel 0.07 Conditioned on 0.004% Sulfur Fuel 0.05 o en 0.03 i-i fO 4J o H 0.01 o- 0 20 40 60 80 100 120 Time, Min. ------- - 26 - Figure_12 Sulfate Emissions at 60 MPH Cruise Monolith Catalyst, 0.14% Sulfur Fuel * 0.60 £ o d * 0.45 o CO « 0.30 ¦u o H 0.15 1 1 1 » Conditioned 1 on 1 0.14% 1 Sulfur Fuel \. _ Conditioned on 0.004% Sulfur Fuel • — 0 * 1 • ^ 0 0 1 1 9_ O 1 1 -6- i 0 20 40 60 80 100 120 Time, Min. Figure 13 Sulfate Emissions at 60 MPH Cruise Monolith Catalyst. 0.004% Sulfur Fuel Conditioned on 0.14% Sulfur Fuel 0.04 Conditioned on 0.004% Sulfur Fuel 0.03 0.02 0.01 0 20 40 60 100 120 Time, Min. ------- - 27 - Since this Initial set of experiments was conducted, we have confirmed these storage effects In other tests. These tests all show very strong storage effects with pelleted catalysts, and lesser, but definite, storage effects with monoliths. 3. Vehicle SOa" Emission Data In Its May 30, 1974 submission to EPA of data on automotive sulfate emissions, In response to a request which appeared In the March 8, 1974 Issue of the Federal Register, Exxon Research summarized Its data on SO40 emissions as follows: • Over both pelleted and monolithic catalysts actual conversion of gasoline sulfur to SO^0. and SO^" emission rate, can differ. Under FTP or low speed cruise conditions, some of the 804° formed Is stored on the catalyst, or possibly In the exhaust system. Stored S04~ can be emitted at high speed conditions. • In vehicles using monolithic oxidation catalysts, C3 25-35% of the sulfur In gasoline;Is emitted as SO4 under FTP, 40 mph, and 60 mph cruise conditions. • In vehicles using pelleted oxidation catalysts, only 5-10% of the sulfur In gasoline Is emitted as SO^" under FTP and 40 mph cruise conditions. With these catalysts, storage of SO^*3 is a major factor. At 60 mph cruise conditions, at least part of the SO4- stored ------- - 28 - at lower speeds Is released, and SO4 emissions are similar to those observed with monolithic oxidation catalysts. Recent data, obtained In the program described In Section III.B.3.a., suggest that over monolithic catalysts, no more than 10-15% of the sulfur In gasoline is emitted as SO^08 during the FTP. Furthermore, substantial differences in S04° emission rate may exist depending on which monolithic catalyst is used. Our earlier results were obtained on prototype catalysts using less controlled aging and conditioning techniques than were used in later programs. The 25-35% emission of gasoline sulfur as SO^" for monoliths at 40 and 60 mph cruise is also found In our later data, but again substantial differences are found depending on which catalyst is used. Our recent data on pelleted catalysts supports the estimate of no more than 10% emission of gasoline sulfur as SO40 under FTP conditions. However, at 40 mph cruise conditions, S0^s emission rates as high as 25% of gasoline sulfur were measured. At 60-70 mph.cruise 25-35% of gasoline sulfur was emitted as SO^". Details of the progran in which these data were generated are given below. a« Effect of Catalyst Type Under EPA Contract 68-02-1279, "The Characterization of Particulate Emissions from Prototype Catalyst Vehicles", Exxon Research has measured S0A° emissions from seven different oxidation catalysts, four monolithic and three pelleted catalysts. The following procedure was used. ------- - 29 - 1) Each catalyst was aged by operating for 2000 miles of AMA cycle on a fuel containing 0.004% sulfur. 2) The catalyst was removed from the car used for aging and mounted on a 1974 350 CID Chevrolet V-8, equipped with an air pump, and calibrated to control CO and HC to 3.4 g/ml. and 0.41 g/mi., respectively. 3) The vehicle was then operated through the following / series of tests on each of three fuels.. I a. 200 miles of conditioning on the AMA cycle followed by a 16 hour cold soak. b. 1975 FTP c. 1 hour idle d. 1 hour, 40 mph cruise e. 2 hour, 60 or 70 mph cruise f. overnight soak g. 1975 FTP S0A° emissions were measured for both 1975 FTP's, the Idle, 40 mph, and 60-70 mph cruise modes. The three fuels used were: 1) the EPA reference fuel,which contains 0.019 wt. % sulfur, 2) the EPA reference fuel doped with a 50% thiophene-50% t-butyl disulfide mixture to a sulfur content of 0.110%, and 3) a high aromatic content fuel doped with the thiophene-t-butyl disulfide mixture to a sulfur content of 0.091%. The fuels were always tested In the order listed above. ------- - 30 - S02 and SO40 measurements for all catalyst/fuel combinations are given In Table 2. In this table S02 and Stty*3 emissions In g/ml. and percent of gasoline sulfur are reported for the average of the two FTP's, and for the 40 mph, and the 60-70 mph cruise. The fraction of gasoline sulfur accounted for by the sum of the SO2 and S04" emitted Is also reported. These data are also presented graphically In Figures 14- 19. Figure 14 shows SO2 and SO4™ emissions for the monolithic catalysts for FTP conditions; Figure 15, for 40 mph cruise; and Figure 16, for 60- 70 mph cruise. Figure 17 shows SO2 and SO40 emissions for the pelleted catalysts for FTP conditions; Figure 18, for 40 mph cruise; and Figure 19 for 60-70 mph cruise. In Interpreting the data presented In Table 2, and In Figures 14-19, It should be remembered that, with the exception of Mono (III) monolith, only one sample of each catalyst was tested. Replicate testing should be carried out before any action Is taken based on these data. With that caution In mind, the following observations can be made. • Under FTP conditions, no more than 10-15% of the sulfur In gasoline Is emitted as SO4 when monolithic oxidation catalysts are used. This Is lower than the 25-35% reported earlier. • The Mono II catalyst showed lower SO^*3 emission rates at 40 mph than did the other two brands of monoliths tested. For the FTP and at 60-70 mph the S04° emissions from this catalyst were comparable to SO40 emission rates from the other two brands B of monoliths. The lower SO4 emission rates from ------- TABLE 2 Catalyst Fuel Sulfur, % Mono (I) 0.019 0.110 0.091 Mono (II) 0.019 0.110 0.091 Mono (III)-l 0.019 0.110 0.091 S02 AND SO4" EMISSION DATA FRtiM EPA CONTRACT NO. 68-02-1279 SO2 Emissions SO4" Emissions % Sulfur Mode g/ml % of Gasoline S g/mi % of Gasoline's Balance MONOLITHIC CATALYSTS FTP 0.00 0.00 0.005 2.1 2.1 40 mph 0.00 0.00 0.019 12.8 12.8 60-70 mph 0.00 0.00 0.016 13.2 13.2 FTP 0.220 37.3 0.091 10.5 47.8 40 mph 0.092 25.9 0.163 30.4 56.3 60-70 mph 0.014 2.9 0.088 12.2 15.1 FTP 0.143 29.1 0.110 14.8 43.1 40 mph 0.080 25.8 0.122 25.7 51.5 60-70 mph * * 0.092 15.7 * FTP 0.043 39.8 0.005 3.1 42.9 40 mph 0.060 85.7 0.010 9.4 95.1 60-70 mph 0.035 23.8 0.016 7.2 31.0 FTP 0.422 71.6 0.077 7.5 79.1 40 mph 0.317 83.9 0.088 15.3 99.2 60-70 mph 0.343 79.6 0.109 16.7 96.3 FTP 0.422 85.1 0.087 4.4 89.5 »¦ 40 mph 0.257 74.8 0.069 13.1 87.9 60-70 mph 0.312 79.6 0.093 15.8 95.4 FTP 0.072 67.9 0.003 1.9 69.8 40 mph 0.000 0.0 0.021 20.3 20.3 60-70 mph 0.050 30.9 0.018 7.4 38.3 FTP 0.302 50.0 0.040 4.3 54.3 40 mph 0.050 12.5 0.294 47.7 60.2 60-70 mph 0.188 21.8 0.105 8.1 29.9 FTP 0.063 21.9 0.032 4.2 26.1 40 mph 0.069 21.2 0.265 52.9 74.1 60-70 mph 0.172 20.3 0.098 7.8 28.1 ------- Catalyst Mono (III)-2 Pellet (I) Pellet (II) TABLE 2 (CONT.) SO2 Emissions S04~ Emissions % Sulfur Sulfur, % Mode g/mi % of Gasoline S E/ml % of Gasoline S Balance 0.019 FTP 0.021 18.4 0.011 6.5 24.9 40 mph 0.000 0.0 0.024 26.5 26.5 60-70 mph 0.029 36.7 0.039 32.9 69.6 0.110 FTP 0.154 26.6 0.098 11.0 37.6 40 mph 0.072 18.3 0.282 46.0 64.3 60-70 mph 0.177 44.9 0.182 31.0 75.9 0.091 FTP 0.226 47.0 0.108 14.6 61.6 40 mph 0.098 29.8 0.257 51.0 80.8 60-70 mph 0.008 2.4 0.177 35.7 38.1 PELLETED CATALYSTS 0.019 FTP 0.087 87.0 0.004 2.6 89.6 40 mph 0.000 0.0 0.002 1.5 1.5 60-70 mph * * 0.043 36.0 * 0.110 FTP 0.121 12.6 0.030 2.9 15.5 40 mph 0.000 0.0 0.167 27.2 27.2 60-70 mph 0.000 0.0 0.166 25.3 25.3 0.091 FTP 0.102 19.5 0.018 2.5 22.0 >. 40 mph 0.035 9.5 0.126 22.4 31.9 60-70 mph 0.208 50.5 0.074 12.0 62.5 0.019 FTP 0.034 32.7 0.007 4.5 37.2 40 mph 0.010 14.7 0.010 9.8 24.5 60-70 mph 0.035 46.1 0.031 27.1 73.2 0.110 FTP 0.161 27.1 0.037 3.8 30.9 40 mph 0.122 32.8 0.142 25.1 57.9 60-70 mph 0.211 49.5 0.230 36.1 85.6 0.091 FTP 0.123 25.7 0.031 4.6 30.3 40 mph 0.088 28.8 0.108 22.8 51.6 60-70 mph 0.008 2.1 0.154 27.2 29.3 ------- TABLE 2 (CONT.) Catalyst ¦ellet (III) Sulfur, % Mode SO2 Emissions S04= Emissions % Sulfur Balance g/mi % of Gasoline S g/mi % of Gasoline S 0.019 FTP 0.023 14.7 0.023 9.8 24.5 40 mph 0.003 4.2 0.019 17.4 21.6 60-70 mph 0.026 35.5 0.027 24.6 70.1 0.110 FTP 0.066 12.1 0.080 10.3 22.4 40 mph 0.048 11.3 0.156 24.0 35.3 60-70 mph 0.129 39.3 0.145 29.2 68.5 0.091 FTP 0.127 26.6 0.077 10.7 37.3 40 mph 0,071 21.8 0.159 32.7 54.5 60-70 mph 0.154 42.5 0.166 30.4 72.9 i LO I ------- 90 80 70 60 50 40 30 20 10 0 FIGURE 14 SO?.and S04° Emissions For Monolithic Catalysts For The 1975 FTP MONO (II) ~ S02 MONO (I) (A.V »» ABC Fuel mimm ABC MONO (III)-l ABC MONO (III)-2 ABC SO, SO2 not found Fuel Sulfur Content A - 0.019% B - 0.110 C - 0.091 u> ------- 90 80 70 60 50 40 30 20 10 0 FIGURE 15 S02 and SO&" Emissions For Monolithic Catalysts At 40 Mph Cruise MONO (II) MONO (I) ABC ABC MONO (III)-l Mi mi ABC MONO (III)-2 m tiV >~~~~; ~>»: ¦&$ »X» <&& ~>v ~%>; MM J222 ABC so- so, * S.02 not found Fuel Sulfur Content A - 0.019% B - 0.110 C «= 0.091 Fuel ------- 90 80 70 60 50 AO 30 20 10 0 FIGURE 16 SOp and SQ/f" Emissions For Monolithic Catalysts At 60-70 Mph Cruise MONO (I) ABC MONO (II) $ ABC MONO (III)-2 MONO (III)-l ABC ABC ~ * ** so. SO/ SO2 not found SO2 not measured Fuel Sulfur Content A - 0.019% B - 0.110 C - 0.091 u> On Fuel ------- .00 90 80 70 60 50 40 30 20 10 0 FIGURE 17 SOp•and SOa" Emissions For Pelleted Catalysts For The 1975 Frr PELLET (I) ~ S02 SO. Fuel Sulfur Content A - 0.019% B - 0.110 C - 0.091 LO PELLET (II) PELLET (III) ABC ABC ABC ------- 00 90 80 70 60 50 40 30 20 10 0 FIGURE 18 S02 and SOa" Emissions For Pelleted Catalysts At 40 Mph Cruise ~ SO, so. SO2 not found PELLET (II) PELLET (I) ABC ABC PELLET (III) ft »iV ft »X»ft VKft £%~, ft WW'S wJvv « 5% Cv K » w* % ABC Fuel Sulfur Content A = 0.019 B - 0.110 C - 0.091 to 00 Fuel ------- i 00 90 80 70 60 50 40 30 20 10 0 FIGURE 19 SOy and SOEmissions For Pelleted Catalysts At 60-70 Mph Cruise PELLET (I) PELLET (II) ABC MW. m ABC ~ S02 PELLET (III) so4 * S02 not found ** SO2 not measured Fuel Sulfur Content A - 0.019% B = 0.110 C - 0.091 ABC Fuel ------- - 40 - S04b emission rates from the Mono (II) catalyst are not the result of increased S0^s storage, since this catalyst showed the lowest tendency to store sulfur oxides of any of the catalysts tested. • The previously reported low (about 5-10%) emission rates for pelleted catalysts under FTP conditions were again observed. However, higher (no more than 25-35%) than previously reported S0^B emission rates were observed at AO mph cruise when using pelleted catalysts. This may be due S3 to release of stored SO^ , since the 40 mph cruise mode was the first run after the FTP. • S0AB emission rates at 60 or 70 mph cruise were similar for both monoliths and pellets and ranged up to 35%. The low SO^™ emission rates combined with low storage of sulfur oxides found with the Mono (II) catalyst is worthy of further study. This catalyst sample was supposedly representative of those manufactured for commercial use, and therefore should have been capable of good control of CO and HC. Data for the FTP runs with this catalyst, presented in Table 3, show that it did, in fact, control CO and HC near or below our targets of 3.4 and 0.41 g/mi., respectively. Table 3 - FTP Emissions From Monolithic Catalysts Catalyst CO HC Mono (I) 3.00 0.74 Mono (II) 2.45 0.40 Mono (III)-l 3.21 0.35 Mono (III)-2 1.49 0.22 ------- - 41 - b. Effect of Gasoline Sulfur Level In our previous submission to EPA on automotive sulfate emissions, Exxon Research reported that, in catalyst vehicles, these emissions in g/mi. were proportional to gasoline sulfur level. No further studies to Investigate this question have been carried out. The data used to reach this conclusion in our earlier report are reproduced In Tables 4 and 5. c. Effect of Catalyst Age and Noble Metal Loading Exxon Research has obtained limited data in these two areas under EPA Contract 68-03-0497. Since this contract is discussed in detail in Section V, presentation of these data will be delayed until Section V.C. V. Potential Methods for Controlling SO^" Emissions In Figures 5 and 8, laboratory data indicating that SO^** emissions can be limited by limiting excess air were presented. In this section, vehicle tests of this concept, as well as data obtained in a study of the feasibility of trapping SO40 in the exhaust system on a suitable sorbent, will be presented. Both of these studies were conducted as parts of EPA Contract 68-03-0497, "An Assessment of Sulfate Emission Control Technology". Work under this contract Is still in progress at Exxon Research, and the data reported herein are limited to those available as of December 19, 1974, the last monthly reporting period for which data have been submitted. A. Vehicle Tests of Limited Excess Air 1. Vehicle Preparation and Baseline Testing The vehicle tests of the effect of limited excess air on SO4 emission rate were conducted on a production model 1975 350 CID Chevrolet ------- - 42 - Table 4 Sulfate Emissions From Monolithic Oxidation Catalysts Catalyst No. of Tests Fuel Sulfur, % S0^D Emissions, g/mi. 1972 FTP Conversion S —» S0/B, Monolith A Monolith B 5 4 3 2 2 2 0.067 0.032 0.004 0.067 0.032 0.004 0.119 0.064 0.010 0.145 0.061 0.014 21 24 29 25 23 41 40 mph cruise Monolith A Monolith B 2 2 2 5 4 3 0.067 0.032 0.004 0.067 0.032 0.004 0.158 0.055 0.008 0.090 0.048 0.005 28 20 35 16 17 18 Monolith A 2 2 60 mph cruise 0.140 0.004 0.253 0.007 32 29 Table 5 Sulfate Emissions From A Pelleted Oxidation Catalyst Catalyst No. of Tests Fuel Sulfur, % SO^a Emissions, g/mt- 1975 FTP Conversion S SOa°. % Pelleted Pelleted Pelleted 3 2 3 2 3 2 2 6 5 6 5 0.140 0.065 0.056 0.034 0.004 0.111 0.036 0.015 0.011 0.003 40 mph cruise 0.065 0.034 0.049 0.009 60 mph cruise 0.140 0.056 0.032 0.004 0.313 0.113 0.063 0.007 10.6 5.8 3.2 4.2 7.7 12.8 4.7 35.6 31.4 27.7 26.0 ------- - 43 - V-8 modified to control CO and EC emissions to 3.4 and 0.41 g/ml. respectively. This modification consisted of adding an air pump to inject secondary air ahead of the oxidation catalyst. The catalyst used was the pelleted catalyst received with the vehicle. The unmodified test vehicle was first broken in with 2,000 miles of AHA cycle operation on an unleaded, low sulfur fuel. It's 1975 FTP emissions were then measured and found to be 3.3 g/ml. CO and 0.48 g/ml. HC, well below the standard of 15 g/ml. CO and 1.5 g/ml. HC for which the vehicle was designed, but above the 3.4 g/ml. CO, 0.41 g/ml. HC level at which the tests were to be conducted. Adding an air pump lowered these emissions to 3.5 g/mi. CO and 0.27 g/ml. HC. A series of baseline tests were then conducted using two fuels (0.032 and 0.012 wt. % sulfur) and two different modes of conditioning (500 miles of simulated turnpike driving and 500 miles of simulated city driving). Each test consisted of the following series of operating modes: 1. 500 miles of conditioning followed by an overnight cold soak 2. 1975 FTP 3. 20 minute idle 4. 2 hours at 60 mph during which time SO4" was measured for each 30 minute Interval 5. Overnight cold soak 6. 1975 FTP SO40 emission results for the baseline runs are reported in Table 6. ------- - 44 - The data in Table 6 show an average SO^" emissions equivalent to about 4% of the sulfur in the gasoline used under FTP condition. This is in good agreement with the FTP results for pelleted catalysts presented earlier in Tables 2 and 5. The 60 mph cruise runs were two hours in duration with separate samples taken for each half hour interval. S0^s emissions were highest during the first half hour of operation and gradually decreased with time. By the final half hour, SO^" emissions were down to the 25-35% of gasoline sulfur reported above. This initial high rate of SO^ emission is due to the release of stored sulfate. A similar pattern was observed with SO2 emissions. At the beginning of the 60 mph run, high levels of SO2 emission were recorded as the result of stored SO2. As the test proceeded, SO2 emission rates dropped to a steady state level comparable to the levels reported in Table 2. 2. Tests of Limited Air The effect of limited air was tested using the 0.032 wt. % sulfur fuel and both turnpike and city driving preconditioning. The operating sequence outlined above was followed with air injection used only for the first two minutes of each FTP and not at all under cruise conditions. This limited use of air injection raised FTP CO emissions / to an average of 5.4 g/mi. and HC emissions to an average of 0.31 g/mi., both well below the 1975 California standards. The effect of SO^° emissions was dramatic, about a 75% reduction in SO^" emissions under FTP conditions, and about 60% reduction in SO^" emissions at 60 mph ------- Table 6 SO2 and S04° Emissions from a 1975 Chevrolet with Air Pump During Baseline Testing SO2 Emissions SO4 Emissions % Sulfur Fuel Sulfur,% Mode g/mi. % of Gasoline S g/mi. % of Gasoline S Balance TURNPIKE DRIVING PRECONDITIONING 0.032 FTP * 0.044 24 0.010 4.2 28 60 mph-1 ** 0.19 144 0.168 84 228 2 0.13 94 0.099 47 141 3 0.11 80 0.076 37 117 4 0.08 60 0.061 30 90 0.012 FTP * 0.020 30 0.0025 3.6 34 60 mph-1** 0.066 122 0.084 103 225 2 0.059 107 0.054 65 172 3 0.059 105 0.050 59 164 4 0.083 145 0.052 61 206 CITY DRIVING PRECONDITIONING 0.032 FTP * 0.055 33 0.0088 3.6 37 60 mph-1 ** 0.15 106 0.15 72 178 2 0.09 64 0.080 38 102 3 0.10 70 0.081 38 108 4 "0.08 55 0.075 34 89 0.012 FTP * 0.055 82 0.0048 4.6 87 60 mph-1 ** 0.09 164 0.081 96 260 2 0.04 78 0.034 41 119 3 0.04 78 0.037 46 124 4 0.04 78 0.033 41 119 * Average of the Initial and final FTP tests. Numbers after the 60 mph indicates 1st, 2nd, etc. 30 minutes of operation at 60 mph cruise. ------- - 46 - cruise. Data for these tests is presented in Table 7. Similar tests on a monolithic catalyst system are expected to be completed within the next month. These vehicle tests, together with the laboratory data presented earlier, offer strong evidence that limiting excess air will reduce SO^9 emissions appreciably. This point should be considered by EPA in deciding whether to grant a delay in enforcement of the 1977 CO and HC standards for the following reason. The higher the CO and HC standards, the less need for air pumps. An appreciable fraction of the catalyst-equipped vehicles meeting 1975 Federal emission standards do not use air pumps. Many of these vehicles also meet 1975 California CO and HC emissions standards. If not required to meet CO and HC standards, the $27-33/car cost(7)of the air pump and its associated plumbing would likely be sufficient incentive for their removal. If 1975 Federal standards were extended through 1977, it is likely that an even greater number of cars which used catalysts would not use air pumps. If 1975 California CO and HC standards were imposed for the 1977 model year nationwide, it is still likely that a significant number of cars could be designed without air pumps. However, maintaining the statutory 3.4 g/mi. CO, 0.41 g/mi. HC standards in 1977 would make it very unlikely / that air pumps could be eliminated. ------- - 47 - B. Use of SO/;" Traps 1. Background On November 6, 1973, Exxon Research testified before the Committee on Public Works of the U. S. Senate on the subjects of gasoline desulfurlzatlon and automotive sulfate emissions. At that time we indicated that it might be possible to trap S0^= on a solid sorbent in the exhaust system. This position, which was based on work done at Exxon Research in the mid-1960's, was amplified in a November 16, 1973 letter to Senator Jennings Randolph, Chairman of the Committee on Public Works. This letter appears as Attachment II. A program to study the feasibility of SO^*5 traps was included as part of EPA Contract 68-03-0497, "An Assessment of Sulfate Emission Control Technology". This program Included both vehicle durability tests and a laboratory screening program to find new sorbents. The first vehicle durability test was carried out using 1/8" pellets of 85% Ca0/10% Si02/5% Na20 as the sorbent. . Results of this test are presented below. 2. Vehicle Test of 85% CaQ/10% Si02/5% Na20 as an SO4° Sorbent The test was conducted using a 1973 351 CID Ford V-8 equipped with an air pump and two Engelhard PTX-IIB oxidation catalysts in the post manifold position. Prior to testing the SO^" trap, the vehicle, without trap, was operated for 2,000 miles of AMA cycle on a fuel containing 0.048 wt. % sulfur. SO40 emissions were then measured at 40 mph cruise conditions, were 0.066 g/ml., equivalent to about 37% of the sulfur in the gasoline. ------- Table 7 SO2 and SO4 Emissions In Vehicle Test of Limited Excess Air Preconditioning Mode ,S02 Emissions g/mi. % of Gasoline S 804" Emissions % of Gasoline % Sulfur Balance All Tests with 0.032 wt.% S Fuel Turnpike City FTP * 0 • 14 130 0.0020 0.8 131 60 mph-1 ** 0.34 244 0.093 44 288 2 0.26 179 0.034 16 195 3 0.16 109 0.026 11 120 4 0.16 105 0.026 11 116 FTP * 0.19 121 0.0032 1.2 122 60 mph-1 ** 0.24 213 0.053 32 245 2 0.23 212 0.010 5.8 218 3 0.34 312 0.0016 1.0 313 4 0.11 99 0.0018 1.0 100 * Average of the initial and final FTP tests. ** Numbers after the 60 mph indicate 1st, 2nd, etc. 30 minutes of operation at 60 mph cruise. ------- - 49 - The car was then equipped with an S04 trap consisting of a GM toeboard catalyst reactor filled with 1/8" pellets of 85% CaO/10% SIO2/ 5% Na20. With fresh sorbent, S0^° emissions at 40 «ph were reduced to 0.003 g/mi., a reduction of 96%. The trap was tested for a total of 26,500 miles, during which time S(>4= removal generally remained above 95%. Data on S0^= emissions during this test are presented in Table 8. While CaO is a very active 6orbent, it does possess one inherent liability, in that its volume Increases significantly as it sulfates. Based on crystalline densities, the complete sulfation of CaO to CaSO^ would produce a three-fold Increase in volume. While the pellets are somewhat porous, they cannot accommodate such an expansion internally and must expand into the void volume of the bed. This expansion will cause the pressure drop across the bed to Increase as degree of sulfation increases. During the 26,500 miles described above, pressure drop across the sulfate trap Increased from an Initial value of 4" of H2O to a final value of 115-140" of H2O at 40 mph cruise conditions. Pressure drop data as a function of mileage for the trap are presented in Figure 20. Despite the swelling and high pressure drop encountered with the CaO sorbent, attrition was not a problem. Calcium emission rates were measured periodically through the run. The aaximum observed value was 3.7 x 10"^ g/mi., lower than the approximately 6.5 x 10~^ g/mi. observed on vehicles without traps. On vehicles witthout a CaO trap, calcium emissions occur as a result of the combustion of lube oil ------- Table 8 Summary of Results Obtained During Testing of 85% Ca0/10% SIO2/5Z Na£0 As A Sulfate Sorbent Trap Mileage Mode SO/™ Emissions, g/mi. % S0a= Removed Base Car * 40 mph 0.066 0 40 0.003 96 40 0.005 92 60 0.002 — 1,000 40 0.001 98 1,100 ** 40 0.002 97 2,000 40 0.002 97 3,000 40 0.002 97 40 0.004 94 6,000 40 0.002 97 8,000 40 0.002 97 11,000 40 0.002 97 40 0.003 96 1975 FTP 0.005 — 15,000 40 mph 0.001 98 40 0.001 98 1975 FTP 0.005 — 19,000 40 mph 0.0005 99 .. ao 0.0005 99 ;1975 FTP 0.001 — 22,000 40 mph 0.001 98 40 0.001 98 1975 FTP 0.003 — 26,500 40 mph 0.008 88 40 0.003 96 1975 FTP 0.003 '— * Fuel Sulfur Content = 0.048 vt.% ** Fuel Sulfur Content changed to 0.032 wt.% for the remainder of the test ------- FIGURE 20 Pressure Drop Across the CaO/SiO?/Na90 SO^" Trafl Va. Mileage 40 MPH CRUISE TYPICAL MUFFLER PRESSURE DROP AT 40 MPH THOUSANDS OF MILES ------- - 52 - which typically Includes calcium containing additives. Assuming the maximum calcium emission rate, slightly over 10 grams of calcium or 20 grams of sorbent was emitted during the entire durability test. This is less than 1% of the charge, and less calcium than would typically be emitted as lube ash in a non-trap car. 3. Work on Improved Sorbents While the test of Ca0/Si02/Na20 as sorbent showed that it ta is possible to trap SO^ in the exhaust, the high pressure drop encountered with this material made its use in pelleted form unattractive. Three approaches to improved sorbents have been considered. These are: • calcium compounds which swell less after sulfation, • CaO In a high void volume shape, and • materials which sorb less S07. This last approach is being taken because the Ca0/S102/Na20 material appeared to sorb about 50% of the SO2 passing through the trap. This reduces potential S04= sorption capacity and increases further swelling, and is therefore undesirable. The results to date in each of these areas are discussed below. a . Calcium Compounds Which Swell Less During Sulfation One such material was tested, CaCO-j. Converting CaC03 to C&SO4 Increases volume 1.4 times, much less than the three-fold increase which occurs when CaO is converted to CaSO^. A vehicle test was conducted on 4/17 mesh marble chips (marble is essentially pure CaCO-j) , but this material did not sorb SO^. We speculate that this is because of the very low surface area of the marble chips, which did not allow good gas-solid contacting. CaC03 will be reevaluated when pellets of compressed CaC03 powder are available. Attempts to form ------- - 53 - such pellets without binders were unsuccessful. Forming the pellets with binders will be attempted in the near future. b. CaO In A High Void Volume Shape Girdler Catalyst Company is currently fabricating CaO/SiO2/Na20 sorbent into 5/8"O.D X 3/8"I.D. X 1/4" high rings, a high void volume shape. We plan vehicle tests to determine whether this shape reduces pressure drop sufficiently, while maintaining SO^" sorption efficiency, to allow the use of this sorbent. c. Material Which Sorb Less SO? A laboratory program is now underway to screen new sorbent materials using the equipment shown in Figure 3. 15 ppm SO2 is blended into a synthetic exhaust and 5 ppm SO^ is added by controlled evaporation of dilute H2SO4. The first sorbent tested in the unit was 85% CaO/10% SiC>2/5% Na20, the material used in the vehicle durability test. It was tested to provide a base against which other material could be tested. In a three hour test at 900°F, 100,000 V/V/hr space velocity, a 13 ml sample of sorbent removed all S03 and >90% of S02. Of the new materials tested, a number can be eliminated from further consideration. Norton #4102 AI2O3 collected only 55% of the SO3. We plan to test other forms of AI2O3. A test of BaO as a sorbent failed when the material hydrated to form Ba(0H>2 and melted. A sample of commercially available MgO manufactured by Harshaw dropped from 100% SO4™removal to 17% SO4" removal in four hours. Marble chips dropped from 74% S0^°removal to 50% S0^= removal in four hours. ------- - 54 - The following materials were identified as being potentially useful sorbents: • zirconia • 80% CaO/20%SiO2, and • Micro-Cel, a commercially available calcium silicate_ A description of the tests of each of these materials is presented below. Harshaw zirconia, in the form of very strong pellets, gave, in sequential tests, 100 and 84% sulfate trapping efficiences. Since the test temperature is in the range of the zirconium sulfate decomposition I temperature, these results suggest that the sorbent may be reacting with the acid to form the sulfate, which then decomposes to sulfur dioxide and oxygen. Unfortunately, the sulfur dioxide results were not sufficiently accurate to determine if the outlet sulfur dioxide concentration increased. Further testing will be done with this material. An 80% Ca0/20% Si02 composition was prepared in an attempt to produce a stronger calcium containing pellet. In addition, this composition allowed the assessment of the effect of sodium oxide on the trapping efficiency, by comparison with the benchmark material. The 80% Ca0/20% Si02 removed all of the sulfuric acid, but only a small amount of the sulfur dioxide. Thps sodium oxide enhances trapping efficiency for the dioxide. However, sodium oxide acts as a binder, since its elimination decreased pellet strength. Development of a suitable binder material, which did not sorb SO2, would allow strong CaO pellets with increased capacity to be made. ------- - 55 - Micro-Cel, the commercial CaSiOj sorted 100% of the SO4" In the first hour, and 97% in the second hour of testing. SO2 trapping efficiency was 15% in the first hour and 7% in the second hour, forming this material into strong pellets is a problem. Finally MgO, in certain forms also shows promise. One sample trapped 100% of the SO43 in the feed but none of the SO2 in a 4.5 hour test. Further tests are planned on these and other materials, C. Other Information from EFA Contract 68-03-0497 I As parts of this contract, the effects of catalyst age and noble metal loading were also investigated. These results are reported below. 1' Effect of Catalyst Age A pelleted oxidation catalyst which had operated for 25,000 miles of AHA cycle on lead sterile (<0.01 gPb/gal) fuel, was mounted on the 350 CID Chevrolet used for the other work in this contract. Tests were conducted using the 0.032 wt.% sulfur fuel after preconditioning with 500 miles of city driving, and again after preconditioning with 500 miles of turnpike driving. Average CO and HC emissions for the four FTP tests involved in this sequence were 3.1 and 0.29g/mi respectively. SO4" emissions data are presented in Table 9. As might be expected, the aged catalyst gave lower SO4" emissions than a fresh catalyst (Table 6), but the reduction was not as great as when reduced air was used. These data indicate that SO4" emissions will not increase as catalysts age in customer use. 2. Effect of Noble Metal Loading In this test, a standard 260 in3 GM catalyst reactor was loaded with higher noble metal content catalyst normally used for the 160 in3 GM catalvsf, reactor. ------- Table 9 SO2 and SO4 Emissions With An Aged Catalyst Fuel Sulfur Content = 0.032 wt.% SO2 Emissions SO^" Emissions % Sulfur Preconditioning Mode g/mi % of Gasoline S g/mi. % of Gasoline S Balance Turnpike FTP * 0.027 17 0.0037 1.4 18 60 mph-1 ** 0.14 97 0.164 75 172 2 ^5. 3* 79 0.063 29 108 3 0.13 87 0.048 22 109 4 0.08 61 0.035 16 77 City FTP * 0.047 29 0.009 3.3 32 60 mph-1** *** *** 0.14 61 *** 2 *** *** 0.063 27 *** 3 *** *** 0.061 27 *** 4 *** ¦ *** 0.051 24 *** * Average of the Initial and final tests. ** Number after the 60 mph indicates 1st, 2nd, etc. 30 minutes of operation at 60 mph cruise. *** Accurate data not available due to air leak in SO2 detector. ------- - 57 - This resulted in about a 60% increase in the amount of Pt-Pd present in the catalyst bed. The high loading charge was tested with 0.032 wt.% of sulfur fuel after 500 miles preconditioning on turnpike operation. The SO^*3 measurements made (Table 10) showed no increase in S04° emissions compared with normal catalyst loading (Table 6). ------- Table 10 SO2 and SO^ Emissions With A High Noble Metal Loading Catalyst Fuel Sulfur Content = 0.032 wt.% so2 Emissions so4 Emissions % Sulfur Preconditioning Mode g/mi. % of Gasoline S g/mi. % of Gasoline S Balance Turnpike FTP * 0.052 35 0.004 1.9 37 60 mph-1** 0.14 114 0.16 88 202 2 0.13 95 0.11 58 153 3 0.10 75 0.076 38 113 4 0.064 50 0.069 36 86 1 <_r» 00 1 * Average of the Initial and final FTP tests. ** Number after the 60 mph Indicates 1st, 2nd, etc. 30 talnutes of operation at 60 mph cruise. ------- - 59 - References 1) E.G. Wlgg, "Fuel-Exhaust Compositional Relationships in Current and Advanced Emission Control Systems", API Paper 62-72, May-11, 1972. 2) Ibid. 3) G.P.Gross, "Automotive Emissions of Polynuclear Aromatic Hydrocarbons", SAE Paper 740564, Sept. 10-13, 1973. 4) "General Motors Response to the March 8, 1974 Federal Register Regarding Automotive Sulfate Emissions: A Status Report", May, 1974. 5) "Ford Response to EPA Request for Data On Automotive Sulfate Emissions.Federal Register Vol. 39, No. 47, Pg. 9229, March 8, 1974", Submitted May 7, 1974. 6) A. Gokstfyr and K. Ross, £. Inst. Fuel, 35:177-9 (1962). 7) "Report by the Committee on Motor Vehicle Emissions, Commission of Sociotechnical Systems, National Research Council", November, 1974. ------- |