Energy & Fuels 2005, 19, 1749-1754
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Effect of Antioxidant Addition on NOx Emissions from Biodiesel Melissa A. Hess,* Michael J. Haas, Thomas A. Foglia, and William N. Marmer United States Department of Agriculture, Agricultural Research Service, Eastern Regional Research Center, 600 E. Mermaid Lane, Wyndmoor, Pennsylvania 19038 Received December 3, 2004. Revised Manuscript Received April 8, 2005
Biodiesel is a renewable, domestically produced fuel that has been shown to reduce particulate, hydrocarbon, and carbon monoxide emissions from diesel engines. Biodiesel produced from certain feedstocks, however, has been shown to cause an increase in nitrogen oxides (NOx), which is of particular concern in urban areas that are subject to strict environmental regulations. There are several pathways proposed that try to account for NOx formation during the combustion process, one of which is the Fenimore mechanism. In the Fenimore mechanism, it is postulated that fuel radicals formed during the combustion process react with nitrogen from the air to form NOx. We proposed that if these radical reactions could be terminated, NOx production from biodiesel combustion would decrease. To test this hypothesis, we investigated the ability of antioxidants, which are capable of terminating these kinds of radical reactions, to reduce NOx levels in biodiesel exhaust. Several antioxidants added to a 20% soy biodiesel/80% diesel fuel blend (B20) at a concentration of 1000 ppm were screened using a small, minimally instrumented diesel engine to test their ability to reduce NOx emissions. The engine used for these studies was a single cylinder, direct-injection, air-cooled, naturally aspirated Yanmar engine. The NO and NO2 in the exhaust stream were quantified using electrochemical sensors, and differences in NOx emissions from the combustion of B20 with and without antioxidant were compared. The addition of butylated hydroxyanisole or butylated hydroxytoluene reduced NOx emissions, but the other antioxidants tested did not have this effect.
Introduction Biodiesel is an alternative diesel fuel consisting of the alkyl monoesters of fatty acids. The use of biodiesel offers many environmental advantages over petrodiesel. Biodiesel is made from animal and vegetable fats and oils and is therefore a renewable resource. It is less toxic and more biodegradable than petrodiesel, and its combustion results in a decrease in particulate, hydrocarbon, and carbon monoxide emissions compared to petrodiesel. However, an increase in nitrogen oxide (NOx) emissions from biodiesel combustion, relative to levels observed from petrodiesel combustion, has been reported by several researchers.1-3 This increase is of concern in areas that are subject to strict environmental regulations, such as air quality low-attainment areas, inner cities, and national parks. For universal acceptance of biodiesel, it is desirable to reduce NOx emissions at least to levels observed with petrodiesel. NOx can be formed by two major pathways during diesel fuel combustion: the Zeldovich (thermal) mechanism and the Fenimore (“prompt”) mechanism.4 In the Zeldovich mechanism, NO is formed by the oxidation * Author to whom correspondence should be addressed. Phone: 215-836-3753. Fax: 215-233-6559. E-Mail:
[email protected]. (1) Graboski, M. S.; McCormick, R. L. Prog. Energy Comb. Sci. 1998, 24, 125-164. (2) Choi, C. Y.; Reitz, R. D. Fuel 1999, 78, 1303-1317. (3) Song, J.; Cheenkachorn, K.; Want, J.; Perez, J, Boehman, A. L.; Young, P. J.; Waller, F. J. Energy Fuels 2002, 16, 294-301.
of atmospheric nitrogen (N2) and atmospheric oxygen (O2). The principal reactions that make up the extended Zeldovich mechanism are
O• + N2 f NO + N•
(1)
N• + O2 f NO + O•
(2)
N• + •OH f NO + H•
(3)
The rate of these reactions is largely temperaturedependent, with a higher flame temperature resulting in higher NO emissions. The activation energy required for the reaction depicted in eq 1 is the highest of the three, making the reaction of nitrogen with the oxygen radical the rate-limiting step. Increased temperature will accelerate thermal NO production as the ratelimiting step is temperature-dependent and the higher temperatures increase the rate of dissociation of O2. Prompt NO is formed through a more-complex pathway. Essentially, fuel radicals formed in the flame react with molecular nitrogen, leading to the formation of compounds that subsequently react with oxygen to form NO. Although no one species has been pinpointed as being solely responsible for NO formation in hydrocar(4) Miller, J. A.; Bowman, C. T. Mechanism and Modeling of Nitrogen Chemistry in Combustion. Prog. Energy Comb. Sci. 1989, 15, 287-338.
10.1021/ef049682s CCC: $30.25 © 2005 American Chemical Society Published on Web 05/12/2005
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bon flames, the following reactions are considered plausible: •
CH + N2 f HCN + N•
(4)
CH2 + N2 f HCN + NH
(5)
•
•
CH2 + N2 f H2CN + N• •
C + N2 f CN + N•
(6) (7)
These reactions depict the species •CH, •CH2, and •C radical reactions with nitrogen to form amines and cyano compounds, as well as nitrogen radicals. The cyano compounds can react with oxygen to form cyanate and amine compounds. The nitrogen radicals and amines can then go on to react with OH, H•, or O• radicals formed within the flame or molecular oxygen to form NO:4
HCN + O• f NCO + H•
(8)
NCO + H• f NH + CO
(9)
NH + H• f N•+ H2
(10)
N•+ OH f NO + H•
(11)
The above pathways are simplified but are considered representative of the prompt NO formation mechanism. Nitrogen dioxide (NO2), the other component of NOx, makes up as much as 10-30% of NOx emissions. It is produced in the flame zone from the reaction of NO with peroxy radicals:
NO + HO2• f NO2 + •OH
(12)
NO2 can be converted back to NO by reaction with oxygen radicals:
NO2 + O• f NO + O2
(13)
Higher amounts of NO2 formation occurs at light loads, where the cooler flame temperatures lead to quenching of the reaction shown in eq 13.5 Several explanations for the increase in NOx accompanying the use of biodiesel have been postulated. Tat et al.6 and Boehman et al.7 both demonstrated that physical property differences are observed for biodiesel as compared to conventional diesel fuel. These differences stem from variation in chemical structures of the fuels. Most notably, biodiesel has a higher isentropic bulk modulus than conventional petrodiesel, which leads to an earlier injection time in a pump-line-nozzle fuel injector. The advance of injection timing leads to an increase in NOx emissions. 5 The work done by Boehman et al.7 also demonstrates that bulk modulus increases as iodine value increases, and these results suggest that the increase in bulk modulus and the (5) Heywood, J. R. Internal Combustion Engine Fundamentals; McGraw-Hill: New York, 1988. (6) Tat, M. E.; Van Gerpen, J. H.; Soylu, S.; Canakci, M.; Monyem, A.; Wormley, S. J. Am. Oil Chem. Soc. 2000, 77, 285-289. (7) Boheman, A. L.; Morris, D.; Szybist, J. Energy Fuels 2004, 18, 1877-1882.
resultant increase in NOx emissions explain why fuels with higher iodine values have higher NOx emissions. Cheng et al.8 retarded the injection time of biodiesel to match the injection time of conventional diesel. While NOx emissions from biodiesel were reduced, they were still higher than those observed from conventional petroleum diesel, even though the injection timing was the same. This indicates that the differences in injection timing that result from differences in bulk modulus cannot fully explain the increase in NOx emissions associated with biodiesel as compared to conventional diesel fuel. Some data have indicated that the chemical makeup of biodiesel can influence the amount of NOx formed during its combustion. McCormick et al.9 published data that correlated the properties of various biodiesels to NOx emissions. One notable result showed a strong positive correlation between the iodine value of the fuel (a measure of the number of double bonds in the fuel) and NOx: as iodine value increased, so did the NOx emissions. These data suggest a role of double bonds in elevating NOx output. Sites of unsaturation (double bonds) are known to have a greater tendency to promote radical formation than do carbon-carbon single bonds.10 It is plausible that an increase in free-radical generation is one cause of the increase in NOx output by some biodiesels. If so, a quenching of free radicals during combustion may reduce or eliminate this NOx elevation. Antioxidants are known to be free-radical quenching agents.11 Acting on this postulate, McCormick et al.12 added a common antioxidant, tert-butyl hydroquinone (TBHQ), to biodiesel as a possible method of NOx reduction. They observed a slight reduction in NOx and suggested that the use of antioxidants as a NOx mitigation strategy deserved more study. Using an antioxidant additive is an attractive strategy for NOx mitigation if it proves successful. The use of a commercially available additive is not only straightforward but also has a lower economic impact than other strategies such as reformulating the fuel. Furthermore, antioxidant addition could stabilize the fuel against oxidation during storage. There are several types of antioxidants, classified by their mechanism of action.11 The most likely candidates for reduction of NOx generation were judged to be chainbreaking antioxidants. These antioxidants inhibit oxidation by interfering with the radical propagation reactions by donating hydrogen atoms to force radical termination. Although these hydrogens are generally donated to peroxy radicals, it is possible that they may serve to terminate radical reactions in NO formation as well. Many classic antioxidants used in the food industry such as butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), and TBHQ fall into this category. Another class of antioxidants is preventive antioxidants, which include metal inactivators. These (8) Cheng., A. S.; Upatnieks, A.; Mueller, C. Presented at the Biodiesel Technical Workshop, January, 2005. (9) McCormick, R. L.; Graboski, M. S.; Alleman, T. L.; Herring, A. M.; Tyson, K. S. Environ. Sci. Technol. 2001, 35, 1742-1747. (10) McMurray, J. Organic Chemistry; Brooks/Cole Publishing Co.: Pacific Grove, CA, 1992 (11) Frankel, E. N. Lipid Oxidation; The Oily Press: Dundee, Scotland, 1998. (12) McCormick, R. L.; Alvarez, J. R.; Grabowski, M. S. Report NREL/SR-510-31465, 2003.
Effect of Antioxidant Addition on NOx Emissions
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Figure 1. Structures of selected additives: (I) 2-ethylhexyl nitrate, (II) 2,2′-methylenebis(6-tert-butyl-4-methylphenol), (III) R-tocopherol, (IV) tert-butyl hydroquinone, (V) propyl gallate, (VI) diphenylamine, (VII) butylated hydroxyanisole, (VIII) butylated hydroxytoluene, (IX) ascorbic acid 6-palmitate, and (X) citric acid.
deactivate the metal ions that can promote the initiation and decomposition of hydroperoxides. The final two categories are hydroperoxide destroyers and ultraviolet light deactivators, but these are less relevant to this work and to the combustion mechanism.11 Additionally, synergism between antioxidants is possible, and the combined use of two different antioxidants can increase the impact as opposed to using each singly.11,13 Antioxidants serve an additional purpose as well. Biodiesel has been observed to oxidize more quickly than petrodiesel,14 and storage stability is a concern. Antioxidants are likely to be added to biodiesel blends to improve storage stability. Any new additive has a potential to impact emissions from biodiesel combustion. Therefore, it is important to assess the effect of antioxidant use on biodiesel combustion and emissions. Experimental Section Materials. Diesel fuel was obtained from Chevron Phillips Chemical Company (Houston, TX). This was a low sulfur (365 ppm) emissions certification grade petroleum fuel with a cetane number of 46.8 (data provided by supplier, cetane improvers were not added to achieve cetane number). The biodiesel used was Soygold brand, a methyl soyate biodiesel purchased from Ag Environmental Products (Omaha, NE) in 1-gal units to protect the fuel from extensive oxidation. Although the biodiesel was packaged in individual gallon units, the units were all from the same lot. An antioxidant-free soy biodiesel was ordered from Peter Cremer, Inc. (Cincinnati, OH). The fuel additives 2-ethylhexyl nitrate (2-EHN), 2,2′-methylenebis(6-tert-butyl-4-methylphenol), R-tocopherol, TBHQ, propyl gallate, diphenylamine, BHA, BHT, citric acid, and ascorbic acid-6-palmitate were purchased from SigmaAldrich Chemical Co. (St. Louis, MO). Structures of the additives are shown in Figure 1. (13) Dunn, R. O. Annual Meeting and Expo of the American Oil Chemists' Society; American Oil Chemists’ Society: Champaign, IL, 2004; p 77. (14) Mittlebach, M.; Schober, S. J. Am. Oil Chem. Soc. 2003, 80, 817.
Table 1. Engine Parameters brand model stroke cylinders injection displacement cooling system compression ratio engine output fuel injection timing fuel injection pressure
Yanmar L100 4 cycle 1 vertical direct 0.418 L air 20:1 6.6 kW (9.0 hp) 17 ( 0.5 bTDC degrees 19.6 MPa
The biodiesel blend used had a composition of 80% petrodiesel and 20% biodiesel (B20). The biodiesel was mixed with the diesel fuel, halved, and then antioxidants were dissolved in one-half of the fuel to form a B20 blend containing antioxidant at a final concentration of 1000 ppm. For each test, three fuel blends were used. Petrodiesel was used to bring the engine up to steady state (as defined by a constant exhaust temperature) and to confirm the increase in NOx emissions observed upon the addition of biodiesel. Additives were tested at a level of 1000 ppm in the final fuel, except for the ascorbic acid 6-palmitate, which was tested at a level of 500 ppm due to limited solubility in the fuel. Method. A Yanmar L100 single cylinder, four stroke, naturally aspirated, air cooled, direct-injection diesel engine (Bowers Power Systems, Kent, WA) was installed and instrumented for NOx measurements. Details of the engine are shown in Table 1. The engine was connected by the manufacturer to an electrical generator. Load was added to the engine by using the generator to power work lights. The results discussed here were obtained at a load of 5 kW (82% maximum load) at an engine speed of 3200 rpm. These conditions were chosen to generate NOx levels high enough to allow small changes in NOx emission levels to be detected. NO and NO2 were measured using single-gas monitors (model CA-CALC) manufactured by TSI Instruments (Shoreview, MI). These monitors have electrochemical sensors and were chosen for simplicity and affordability. Monitors were calibrated weekly using calibration gases from TSI Instruments. Exhaust temperature was monitored using a thermocouple from Omega Instruments (Stamford, CO) inserted into the exhaust gas stream.
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Table 2. Effect of Additive Addition on NOx Emissions
a
fuel
change in NOx from B20 levels (%)
change in NOx from certified diesel levels (%)
diesel fuel (B0) B20 + 2-ethylhexyl nitrate B20 + 2,2′-methylenebis(6-tert-butyl-4-methylphenol) B20 + citric acid B20 + R-tocopherol B20 + ascorbic acid 6-palmitate B20 + tert-butyl hydroquinone B20 + propyl gallate B20 + diphenylamine B20 + butylated hydroxytoluene B20 + butylated hydroxyanisole
-6.6 ( 1.7a -4.5 ( 1.0 +0.2 ( 1.0 -0.7 ( 0.5 +0.3 ( 0.2 -1.3 ( 0.9 -0.3 ( 1.6 -0.4 ( 2.8 +0.7 ( 1.3 -2.9 ( 1.5 -4.4 ( 1.0
N/A +1.7 ( 1.0 +6.0 ( 1.0 +5.5 ( 0.5 +6.5 ( 0.2 +5.9 ( 1.6 +5.8 ( 2.8 +6.9 ( 1.3 +3.3 ( 1.5 +1.8 ( 1.0
Data expressed as average value ( 95% confidence limits.
Since the engine was minimally instrumented with no provisions for the control of such variables as inlet air temperature and humidity, exhaust emissions measurements were generally not repeatable from day to day. Therefore, for each test, NOx emission levels from petrodiesel, B20, and B20 with additive were measured in the same day. An experiment began by bringing the engine to steady state using certified diesel fuel. The speed of the engine was monitored using a tachometer (Cole Parmer, Vernon Hills, IL). The engine was held at a constant speed and load for the duration of the test, and the NOx emissions of the petrodiesel were measured. A minimum of triplicate determinations were made to ensure an accurate NOx measurement. The fuel was then switched to B20 without disrupting the operation of the engine. The engine was run for a time sufficient to ensure that the petrodiesel was flushed from the system and that steady state with the B20 fuel had been achieved. The NOx emissions from the B20 fuel were then measured. The fuel was then switched to B20 blend containing 1000 ppm of the antioxidant additive, again without disrupting engine operation. The engine was allowed to come to steady state with the additized B20 fuel, and the NOx levels were measured again. Experiments were performed to ensure that when the fuel was switched back to unadditized B20, the same NOx levels as prior to running the additized blend were achieved. The NOx emission levels for the additized fuel were compared to the NOx levels measured for B20 in that experiment only. The percent changes in NOx emissions from experiment to experiment were compared. With such day-to-day variation a consideration, results for unamended biodiesel are expressed as a percent change relative to the average NOx emissions obtained on that day with petrodiesel. A reported decrease/increase upon addition of antioxidant in NOx emissions refers to a decrease/increase from the emissions observed from biodiesel, not those observed from conventional diesel fuel. The data are presented as the percentage by which the NOx emissions are reduced, not the percentage by which the NOx increase associated with biodiesel is reduced. Results are expressed as average values (95% confidence limits. Natural tocopherols in the biodiesel were analyzed by highpressure liquid chromatography coupled with fluorescence detection by Dr. Robert Moreau of the Eastern Regional Research Center.
Results and Discussion The NOx measurements during each test were highly repeatable within that test series. Triplicate measurements of NO and NO2 values were taken during each run of the engine and only varied by 1-3 ppm (
