3864
Ind. Eng. Chem. Res. 1998, 37, 3864-3868
Oxidation of Diesel-Generated Volatile Organic Compounds in the Selective Catalytic Reduction Process M. Koebel* and M. Elsener Paul Scherrer Institute, Combustion Research, CH-5232 Villigen PSI, Switzerland
The main part of the VOCs (volatile organic compounds) contained in diesel exhaust (≈80%) is oxidized to CO and CO2 over an SCR (selective catalytic reduction) catalyst. CO is the major product of this oxidation, representing about 50-70% of the formed products (CO + CO2). This preferential formation of CO leads to a pronounced increase of CO emissions when an SCR process is added to a diesel engine. A small fraction of the VOCs is selectively oxidized to carboxylic acids over the SCR catalyst. This selectivity is due to the acidic properties of the catalyst causing the preferential desorption at the oxidation state of the acid. The main products of these oxidation reactions are the lower monocarboxylic acids and some dicarboxylic acids forming stable anhydrides, especially maleic and phthalic acid. The highest emissions of these acids are found at low temperatures; they decrease at higher temperatures. Formic acid is preferentially decomposed into carbon monoxide and water. It must therefore be assumed that the strong increase of CO mentioned above is due to a mechanism involving the thermal decomposition of formic acid formed from various primary VOCs. 1. Introduction The selective catalytic reduction (SCR) using urea instead of ammonia as the selective reducing agent is presently the most promising technique to reduce nitrogen oxides emitted by diesel engines. In Europe, several hundred commerical installations use urea-SCR to reduce the NOx emissions from stationary diesel engines used in the cogeneration of heat and electricity. This success has led to further research programs aiming at developing urea-SCR systems for mobile use, and in particular truck diesel engines. This task is especially challenging because trucks are contributing an ever-increasing percentage to the global NOx emission. In Switzerland, for example, this value presently amounts to about 28%. Early concerns about the possible formation of new byproducts caused by the use of urea instead of ammonia could be disproved in the meantime;1 there we have shown that no significant emission of isocyanic acid, urea, biuret, and cyanuric acid will occur in the treated exhaust under proper operating conditions. The latter implies that the stoichiometric ratio
R)
Nreducing 2‚urea ) NO NO
(1)
is small enough to obtain a low ammonia slip in the treated exhaust gas, typically 5-30 ppm of NH3. The maximum permissible value of R depends on the actual operating conditions of the catalyst, especially the gas hourly space velocity (GHSV) and the temperature. However, specialists dealing with such installations have remarked on pungent odors reminiscent of butyric acid at the start-up of the diesel engine. Kunz2 could detect various monocarbonic acids in the diesel exhaust treated by urea-SCR. In the work cited above1 we had * To whom correspondence should be addressed. Tel.: +4156-310 26 04. Fax: +41-56-310 21 99. E-mail: Manfred.
[email protected].
also found an increase of acetic acid in the treated exhaust at low loads/temperatures. The usual SCR catalyst is a bulk catalyst consisting of TiO2-WO3-V2O5 and has moderate oxidative properties. Similar catalysts are used technically for the selective oxidation of benzene, butane, or butene to maleic acid and of o-xylene or naphtalene to phthalic acid.3 In the following we report experiments performed on a diesel engine equipped with urea-SCR. The main objective of this study was to identify the products formed via the catalytic oxidation of diesel exhaust VOCs. To accomplish this goal, measurements of carbon monoxide, total VOCs by a FID (flame ionization detector), and organic acids by a HPLC (high-performance liquid chromatography) method were carried out. 2. Experimental Section 2.1. HPLC Method. The analytical method for the determination of the organic acids is based on highperformance liquid chromatography and is similar to the method used by Kunz.2 An ion-exclusion column (Interaction Ion Chromatography, type ORH-801, 300 × 6.5 mm) controlled to 35 °C was used for separation. A Waters ion chromatograph model ILC-1 was equipped with a conductivity detector (Waters model 430) and a UV-vis detector (Waters model 486) adjusted to a 210nm wavelength. The pump (Waters model 590) was operated at a flow rate of 0.5 mL/min. The eluent was 10-3 N sulfuric acid in ultrapure water and the injected sample volume amounted to 20 µL. Figure 1 shows a typical chromatogram for 10 organic acids. Other organic acids we checked included cyanuric (10.7), DL-lactic (11.3), isocyanic (16.6), phthalic (18.2), isobutyric (18.8), 2-methylbutyric (23.8), isovaleric (23.8), valeric (29.3), caproic (45.3), benzoic (87.8), and o-toluic acid (118). The values in parentheses give the approximate retention time. Alcohols gave only a weak negative peak with conductivity detection: glycerol (12.0), ethylene glycol (14.4), methanol (16.7), ethanol
S0888-5885(98)00110-9 CCC: $15.00 © 1998 American Chemical Society Published on Web 09/03/1998
Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998 3865
Figure 1. Typical chromatogram: (1) 10 ppm of oxalic acid. (2) 1 ppm of maleic acid. (3) 10 ppm of tartaric acid. (4) 10 ppm of malonic acid. (5) 1 ppm of fumaric acid. (6) 10 ppm of succinic acid. (7) 10 ppm of formic acid. (8) 10 ppm of acetic acid. (9) 10 ppm of propionic acid. (10) 10 ppm of butyric acid.
(18.5), and 2-propanol (19.8). Urea, formaldehyde, and acetaldehyde gave no signal with both UV and conductivity detection. The signals of NOx and SO2 absorbed in water both fell into the injection peak. CO2 absorbed in water gave weak signals at 7.1 and 11.4 min with UV detection. Biuret (21.2), maleimide (23.0), phthalimide (94), phthalodinitrile (122), o-tolualdehyde (>120), and phthalide (>120) gave strong signals in the UV. Conductivity detection is much more selective than UV detection in the case of acids. This is due to various interfering aromatic compounds in the exhaust showing very strong UV adsorption. Whenever possible, the acids were therefore quantified by conductivity. In particular, the determination of formic acid by conductivity is very selective and sensitive. 2.2. Sampling Method. Sampling of flue gas for the determination of organic acids by HPLC was made with the gas-sampling apparatus described previously.4 Its principle is the absorption of about 40 L of flue gas into 20 mL of absorption liquid by the use of glass frits, providing intimate contact between the gas and liquid phase. The absorption liquid was 10-3 N sulfuric acid like the HPLC eluent. HPLC samples were only taken after the catalyst. The emissions are referred to dry gas conditions. 2.3. Test Stand Conditions. The tests were performed at the diesel test stand “HARDI” of PSI (Figure 2). The engine is a six-cylinder, four-stroke, direct injection, turbocharged diesel engine with intercooler (MAN D 0826 LE) running at a fixed 1500 rpm. Details of the test stand have been given elsewhere.1 Table 1 lists relevant engine and catalyst parameters at the six operating points used in the experiments. A low sulfur diesel fuel (S < 0.05%) was burned, leading to very low levels of SO2 in the exhaust (10-20 ppm). The influence of SO2 on the oxidation of the VOCs is therefore expected to be negligible. Then 12.9 L of a commercial SCR DeNOx catalyst composed of TiO2-WO3-V2O5 with the internal designation “D35” was used. This catalyst has 200 cells/in.2 and a vanadium content of about 3%. The catalyst had been previously tested in a continuous mode for more than 1500 h and had shown only a small loss in catalytic activity. 2.4. Reducing Agents. The selective reducing agents were sprayed into the exhaust gas stream by means of an air atomizing nozzle about 3 m upstream
Figure 2. The diesel test stand. “HARDI” at PSI. Table 1. Operating Points of the Engine and Corresponding Mean Temperatures and Space Velocities in the Catalyst load [kWel.]
Tcat. [°C]
GHSV [h-1]
25 30 40 50 75 100
235 255 295 335 400 450
20 200 20 500 21 700 22 900 26 400 30 300
of the catalyst. The standard reducing agent was an aqueous 40% urea solution (freezing point ≈ 0 °C). Additional experiments were made with a mixture containing 35% urea, 10% ammonium formate, and 55% water. This mixture has about the same concentration of reducing nitrogen as the 40% urea solution, but a lower freezing point of ≈-12 °C. Such a mixture is believed to be useful for mobile applications during the cold season.5,6 2.5. Analysis of Gaseous Components. The concentrations of standard gaseous components were also monitored. CO, CO2, NO, NO2, and N2O were measured by a multicomponent IR-processphotometer (Perkin Elmer MCS 100). VOCs were determined as C1 by a FID analyzer (JUM VE7). Ammonia and isocyanic acid HNCO were measured with a continuous analyzer developed by the authors based on a gas-sensitive ammonia electrode (isocyanic acid may also be determined as ammonia after hydrolysis in acid medium). 3. Results 3.1. CO and FID Measurements. The measurement of carbon monoxide and VOCs by FID before and after the catalyst gives a rough impression about its oxidation behavior. Table 2 lists these results. The catalyst shows roughly an 80% conversion of the VOCs over the whole temperature range and a clear increase in CO levels. The CO formation being lower than the VOC reduction, it must be assumed that part of the VOCs are oxidized completely to CO2 and water. 3.2. HPLC Measurements Using Urea Solution. Table 3 lists the results obtained for various operating conditions of the diesel engine and the catalyst: The
3866 Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998 Table 2. Measurements of CO and CmHn before and after Catalyst VOC
CO
load/Tcat. [kW/°C]
before cat. [ppm C1]
after cat. [ppm C1]
conversion [%]
before cat. [ppm]
after cat. [ppm]
VOC reduction [ppm C1]
CO formation [ppm]
100/450 75/400 50/335 40/295 30/255 25/235
66 69 99 120 160 201
15 15 18 21 36 51
77 78 82 83 78 75
64 52 72 112 195 282
97 83 117 170 258 360
51 54 81 99 124 150
33 31 45 58 63 78
Table 3. Emissions of Organic Acids and Maleimide [mg/m3N] after Catalyst with Urea test no. load [kW] slip [ppm] C1 acid C2 acid C3 acid C4 acid maleic acid fumaric acid phthalic acid benzoic acida maleimide 1 2 4 6 7 8 9 10 11 a
0 0f25 25f75 25 25 25 75 75 75
0 0 0 0 5-10 40 0 5-10 40
185.6 12.9 2.1 1.5 3.2 4.5 0.1 0.0 0.0
36.5 167.9 10.9 17.8 15.9 13.6 0.0 0.0 0.0
8.0 30.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0
0.0 4.2 0.0 0.0 0.0 0.0 0.0 0.0 0.0
0.0 53.2 28.6 6.8 5.9 4.4 4.7 2.8 2.3
0.0 1.1 2.3 0.1 0.1 0.1 0.1 0.0 0.0
0.1 12.6 14.0 3.4 2.0 1.1 1.4 1.2 1.2
n.d. 1.2 n.d. 0.2 n.d. n.d. n.d. n.d. n.d.
0.0 0.7 0.1 0.0 1.5 2.8 0.0 0.2 0.1
n.d.: not determined.
Table 4. Comparing Urea and Urea-Ammonium Formate (u-af) as Reducing Agents test no.
power [kW]
reducing agent
1 2+3 4+5 6+7 8 9 + 10 11 + 12 13 + 14 15 16 + 17 18 + 19 20 + 21
25 25 25 25 50 50 50 50 100 100 100 100
blank urea u-af u-af blank urea u-af u-af blank urea u-af u-af
slip [ppm] 9 12 75 7 6 81.5 25 22 58.5
CO [ppm]
formic [mg/m3N]
acetic [mg/m3N]
maleic [mg/m3N]
phthalic [mg/m3N]
maleimide [mg/m3N]
340 332 414 402 112 108 319 330 107 99 275 344
1.8 3.7 24.4 62.1 0.0 0.0 0.5 0.6 0.0 0.0 0.6 0.9
21.8 15.5 16.4 13.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
5.3 3.8 4.5 0.2 3.9 3.2 3.3 0.5 2.1 1.4 1.4 1.7
2.6 1.7 1.9 1.2 1.8 1.4 1.6 1.0 1.0 0.9 0.9 0.9
0.0 1.5 2.0 3.6 0.0 0.3 0.3 0.1 0.0 0.1 0.1 0.1
maximum electrical load amounts to ≈100 kW corresponding to an average catalyst temperature of ≈450 °C (Table 1). An important criterion defining catalyst operation conditions is the amount of reducing agent added: According to our experience, it is best expressed by the total slip of NH3 + HNCO of the catalyst. The test series therefore report measurements at 0, 5-10, and ≈40 ppm of slip. The 0 ppm of slip corresponds to 0 dosing of urea. It is evident that at zero load and without urea dosing, only the lower acids, C1-C3, are emitted (test 1). In test 2, where the load was increased from 0 to 25 kW, huge amounts of higher acids are desorbed that have been formed previously at zero load. At this stage butyric acid with its strong odor is detected besides large amounts of propionic acid and maleic acid. Other components that could be detected include fumaric acid, phthalic acid, benzoic acid, and maleimide. The latter is probably due to traces of reducing nitrogen accumulated earlier in the exhaust tract or the catalyst. Much lower emissions are found during the load increase from 25 to 75 kW. In particular, the strongly smelling propionic and butyric acids could no longer be detected (test 4). During the following tests at constant loads of 25 and 75 kW, propionic and butyric acid are no longer observed. Formic and acetic acid are still detected at 25 kW, but at 75 kW these acids disappear. The most persistent acids are maleic and phthalic acid. The dosage of urea leads to the formation of maleimide. Its
formation is at the cost of disappearing maleic acid; at least this effect is clearly pronounced at 25 kW. Traces of fumaric and benzoic acid could also be detected. Other searched compounds could not be identified with certainty. There is some indication that oxalic and tartaric acid are formed, but their identification remains uncertain. On the other hand, some peaks of the chromatogram could not be attributed to known compounds. 3.3. Effect of Ammonium Formate on the Emissions. These tests included the chromatographic determination of the various acids, especially formic acid. It further turned out that the main part of the formic acid introduced by ammonium formate will lead to a substantial increase of the emission of carbon monoxide. Table 4 lists three test series performed at 2.5, 50, and 100 kW load. In each series the first test was a blank (no urea addition), followed by two tests with urea addition (ammonia slip 5-25 ppm), then by two tests with the addition of the urea-ammonium formate mixture (ammonia slip 5-25 ppm), and finally by two tests with an even higher addition of the urea-ammonium formate mixture (ammonia slip 60-80 ppm). This makes a total of seven tests for each series. In Table 4 average values are given for the double tests. The use of the urea-ammonium formate mixture instead of urea alone results in a pronounced increase of the emission of CO. At the lowest load (25 kW/235 °C) a clear increase of the formic acid emission is also observed. Propionic and butyric acid are not found in
Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998 3867
these tests, confirming the results found in Table 3. Maleic acid, phthalic acid, and maleimide are found again, and their dependence on load and slip is similar to the results with pure urea solution shown in Table 3. 4. Discussion The tests have shown that hydrocarbons contained in the diesel exhaust are oxidized in a complex way over the vanadium catalyst. According to the global measurements in Table 2, a substantial fraction of the hydrocarbons is oxidized to CO and CO2. In the present experiments this amount is about 80%. However, the fraction of CO formed is high: it represents about 5065% of the oxidized VOCs. This CO formation has been observed previously by various workers and is often mitigated by placing a strong oxidation catalyst containing platinum metals downstream of the SCR catalyst.7,8 Besides this deep oxidation up to CO and CO2, some VOCs are oxidized selectively up to the stage of the organic acids. This may be easily understood remembering the strong acidity of the catalyst mainly due to its WO3 and SO3 content. The VOCs will be chemisorbed by protonation at the Bro¨nsted acidity centers, where they are oxidized gradually: hydrocarbon f alcohol f aldehyde/ketone f carboxylic acid. At this stage, due to the acidity of both the product and the catalyst, the chance for the carboxylic acid to desorb increases dramatically. The ratio between oxidation and the desorption rate will therefore determine the fraction of VOCs that get deeply oxidized to CO/CO2. The VOCs (volatile organic compounds) contained in diesel exhaust consist mainly of hydrocarbons, aldehydes/ketones, and phenols.9 Paraffins and olefins up to about C3 represent the major part of the aliphatic hydrocarbons while benzene and toluene are the main aromatic hydrocarbons. Polycyclic aromatic hydrocarbons (PAHs) occur at much lower concentrations, about a factor of 103 lower than the other hydrocarbons. The important aldehydes and ketones range again up to C3 (formaldehyde, acetaldehyde, propionaldehyde, acrolein, and acetone), formaldehyde usually representing at least 70% of the total aldehyde emissions. The main aromatic aldehyde is benzaldehyde.10 Aliphatic VOCs yield mainly aliphatic monocarbonic acids. The C3 and the C4 monocarbonic acids are only detected at low temperatures, but the C1 and the C2 monocarbonic acids are stable up to high temperatures. Maleic and phthalic acid form volatile and very stable anhydrides and therefore do not diminish to the same extent as the C1 and the C2 monocarbonic acids with rising temperature. Maleic acid is produced commercially over similar V2O5-containing catalysts and at similar temperatures from benzene, butane, or butene and phthalic acid from o-xylene or naphthalene.3 Higher aliphatic VOCs are probably adsorbed, oxidized, and split into smaller molecules. In the presence of ammonia, products of ammoxidation may be formed.11 The experiments with urea-ammonium formate mixtures have revealed that formic acid behaves in a special way: the thermal decomposition competes effectively with oxidation. This leads to a strong increase of CO after the catalyst:
HCOOH S CO + H2O
(2)
Mass balance calculations have been performed comparing the formic acid introduced by the urea-ammonium formate mixture with the increase of (CO + formic acid) after the catalyst. They have shown that over the temperature range studied the major part of formic acid is thermolyzed according to reaction 2. Only at the lowest temperature (235 °C) does a substantial fraction of the formic acid (≈25%) pass the catalyst. At the highest temperature (450 °C) relevant amounts of formic acid are no longer detected after catalysis; about 80% of the formic acid input is thermolyzed to CO, the rest probably further oxidized to CO2. It seems therefore highly probable that the increase of CO mentioned before is due to a mechanism involving the previous formation of formic acid. Whenever formic acid is formed from an organic precursor compound, this will preferentially decompose on the catalyst according to reaction 2, yielding CO + H2O. As has been shown before, the precursor formaldehyde is emitted abundantly by diesel engines. Methane is also found in large amounts in the diesel exhaust but is thought to be too stable against oxidation at temperatures below 500 °C. Reactions between oxidized intermediates and ammonia must also be taken into consideration. Ammonia is formed rapidly by combined thermolysis/hydrolysis from urea over solids with acid or basic properties.12 Isocyanic acid and ammonia are the primary products of thermolysis; HNCO is subsequently hydrolyzed:
NH2-CO-NH2 f HNCO + NH3
(3)
HNCO + H2O f NH3 + CO2
(4)
Several additional peaks appear in the chromatograms when urea is dosed. Only one of these peaks could be identified with certainty (i.e., maleimide). Other aminated secondary products are expected to be formed when urea or ammonia is dosed. However, they could not be identified until now. Acknowledgment The financial support of the Swiss Federal Office of Energy (BEW) is gratefully acknowledged. Literature Cited (1) Koebel, M.; Elsener, M.; Marti, T. NOx-Reduction in Diesel Exhaust Gas with Urea and Selective Catalytic Reduction. Combust. Sci. Technol. 1996, 121, 85-102. (2) Kunz, D. Abgasanalytik bei Dieselmotoren mit Anlagen zur selektiven katalytischen Reduktion (SCR) von Stickoxiden. Ph.D. Thesis D 386, University of Kaiserslautern, 1996. (3) Satterfield, C. N. Heterogeneous Catalysis in Practice; McGraw-Hill: New York, 1980. (4) Koebel, M.; Elsener, M. Determination of urea and its thermal decomposition products by high-performance liquid chromatography. J. Chromatogr. A. 1995, 689, 164-169. (5) Weisweiler, W.; Wendler, M. Diesel-Entstickung - NOxEntfernung aus sauerstoffreichen Abgasen mittels NH3-abspaltender Reduktionsmittel. Berichtsband KfK-PEF 94. Projekt Europa¨isches Forschungszentrum fu¨r Massnahmen zur Luftreinhaltung (PEF), Kernforschungszentrum Karlsruhe, 1992; pp 535548. (6) Weisweiler, W.; Maurer, B.; Wendler, M. Diesel-EntstickungNOx-Entfernung aus sauerstoffreichen Abgasen mittels NH3abspaltender Reduktionsmittel. Forschungsbericht KfK-PEF 122. Projekt Europa¨isches Forschungszentrum fu¨r Massnahmen zur Luftreinhaltung (PEF), Kernforschungszentrum Karlsruhe, 1994; pp 1-53. (7) Hug, H. T.; Mayer, A.; Hartenstein, A. Off-highway exhaust gas after-treatment: Combining urea-SCR, oxidation catalysis and
3868 Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998 traps. International Congress and Exhibition, Detroit, MI, March 1-5, 1993; SAE technical paper series 930363, Warrendale, PA. (8) Havenith, C.; Verbeek, R. P.; Heaton, D. M.; Van Sloten, P. Development of a urea DeNOx catalyst concept for European ultralow emission heavy-duty diesel engines. International Truck & Bus Meeting & Exposition, Winston-Salem, Nov 13-15, 1995; SAE technical paper 952652, Warrendale, PA. (9) Metz, N. Trace gas emissions of passenger cars. ATZ, Automobiltechn Z. 1984, 86, 425-430. (10) Prescher, K.; Stieper, K.; Groth, K.; Stanev, A.; Lange, J.; Berndt, S. Die Aldehydemission von Dieselmotoren in Abha¨ngigkeit von der Kraftstoffqualita¨t. MTZ, Motortechn Z. 1997, 58, 318-325. (11) Busca, G. Selective and Nonselective Pathways in Oxidation and Ammoxidation of Methyl-Aromatic Compounds over
Vanadia-Titania Catalysts. In ACS Symposium Series; Oyama, S. T., Hightower, J. W., Eds.; American Chemical Society: Washington DC, 1992; Vol. 523, p 168. (12) Koebel, M.; Elsener, M. Ammonia production by combined pyrolysis/hydrolysis of urea. PSI Newsletter, Annex V, General Energy Technology: Switzerland, 1992; pp 50-52.
Received for review February 23, 1998 Revised manuscript received June 16, 1998 Accepted July 9, 1998 IE9801103
