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Effect of aromatic amine antioxidants on NOx emissions from a soybean biodiesel powered DI diesel engine
Fuel Processing Technology 106 (2013) 526–532
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Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
Effect of aromatic amine antioxidants on NOx emissions from a soybean biodiesel powered DI diesel engine K. Varatharajan a,⁎, M. Cheralathan b a b
Department of Mechanical Engineering, Velammal Engineering College, Chennai 600 066, India Department of Mechanical Engineering, SRM University, Chennai 603 203, India
a r t i c l e
i n f o
Article history: Received 7 November 2011 Received in revised form 27 May 2012 Accepted 6 September 2012 Available online 23 October 2012 Keywords: NOx reduction Antioxidants DPPD Soybean biodiesel Prompt NO
a b s t r a c t It is an overwhelming argument that the use of biodiesel instead of petrodiesel causes a reduction in harmful exhaust emissions from engines. A number of studies, however, indicate substantial increases in engine out NOx emissions with biodiesel fuel. Some studies have pointed out that the increased formation of prompt NOx is responsible for biodiesel NOx effect. Treatment of biodiesel with antioxidants is a promising approach because it reduces the formation of hydrocarbon free radicals, which are responsible for prompt NOx production in combustion process. Aromatic amine antioxidants are known as to be efficient inhibitors of free radicals. This study examines the use of p-phenylenediamine derived aromatic amine antioxidants for NOx reduction in a soybean biodiesel fuelled DI diesel engine. The antioxidant additives, N,N′-diphenyl-1,4-phenylenediamine (DPPD) and N-phenyl-1,4-phenylenediamine (NPPD) were tested on a computerised Kirloskar-make 4 stroke water cooled single cylinder diesel engine of 4.4 kW rated power. Results show that significant reduction of NOx could be achieved by the addition of antioxidants but smoke, CO and HC emissions were found to have increased. © 2012 Elsevier B.V. All rights reserved.
1. Introduction There is considerable interest worldwide in the replacement of petrodiesel by biodiesel in order to reduce the harmful diesel exhaust emissions from engines. However, the use of biodiesel results in a noticeable increase (about 10%) in NOx emissions when compared to conventional diesel [1]. The biodiesel market in the US is expected to reach 6,453 million litres in 2020 and 45,291 million litres globally [2]. As a consequence, the increase in NOx emissions could become a significant barrier to biodiesel market expansion. The nitrogen oxide compounds not only have direct effects on human health, but also affect the environment and have ground level ozone-forming potential. When compared to diesel NOx, a relatively small amount of research has been conducted on biodiesel NOx emissions. Thermal, prompt and N2O pathway are the dominant NO formation mechanisms in biodiesel combustion. In general, biodiesel does not contain fuel-bound nitrogen, so NO formation by the fuel NO mechanism can be considered negligible [3]. Thermal NO is formed by oxidation of nitrogen at elevated temperatures (above1700 K). Prompt NO is produced by the formation of free radicals in the flame front of hydrocarbon flames. Generally, the prompt NO contribution to total NO from combustion process is considered less important when compared to thermal NO. On the other hand, in biodiesel combustion, significant quantities of NO are ⁎ Corresponding author at: Department of Mechanical Engineering, Velammal Engineering College, Surapet, Chennai 600066, India. Tel.: + 91 9677039183; fax: + 91 4426591771. E-mail address: [email protected] (K. Varatharajan). 0378-3820/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fuproc.2012.09.023
formed by prompt mechanism [4]. N2O pathway mechanism is significant under elevated pressure and lean air fuel ratio conditions. Numerous reasons have been proposed for biodiesel NOx effect; however, the exact cause is still unclear. Increased fuel mass delivery due to advancement of injection timing resulting from higher bulk modulus of biodiesel fuel, electronic injection advance due to reduced heating value, higher cetane number of biodiesel, increased premixed-burn fraction, change in chemical kinetic pathways, increased adiabatic flame temperature and heat release rate, more stoichiometric burning and lessened radiative heat transfer from in-cylinder soot are stated as the reasons for increased thermal NO formation in biodiesel fuel [5]. Zhang and Boehman [6] found much higher NOx emissions with common rail system, and concluded that injection timing shift alone could not be the reason for biodiesel NOx effect. Monyem et al. [7] have shown that biodiesel has lower adiabatic flame temperature than petrodiesel. Moreover, Szybist et al. [8] observe a lower rate of heat release for biodiesel fuel at all loads. McCormick and co-workers of US National Renewable Energy Laboratory (NREL) reported that the increased formation of prompt NO as one of the primary reasons for biodiesel NOx effect [4,9] and also they suggested that the antioxidants can be used to control prompt NO generation. Hess et al. [10] studied the effect of different antioxidant fuel additives with B20 (soy) fuel on NOx emissions. They observed significant reduction in NOx emissions with butylated hydroxytoluene and butylated hydroxyanisole antioxidants. Brezinsky et al. conducted experiments to analyse the biodiesel combustion using high pressure single pulse shock tube (HPST) [11] and jet stirred reactor (JSR) [12]. In both the tests they have observed increased formation of prompt NO from the unsaturated methyl ester compared to the saturated methyl
K. Varatharajan, M. Cheralathan / Fuel Processing Technology 106 (2013) 526–532
ester. They predicted the prompt NOx by measuring the rate of acetylene formation. In general, biodiesel feedstocks containing higher degree of unsaturated components like soybean, sunflower and jatropha emits more NOx compared to their saturated counterparts. Knothe et al. [13] found more NOx emissions with biodiesel fuelled engine fitted with EGR (a method to reduce thermal NO) when compared to conventional petro-diesel fuelled engine. This shows that thermal NO has less effect on biodiesel NOx emissions. Furthermore, Mueller et al. [5] concluded that changes in prompt NO formation may play an important role in biodiesel NOx effect. Recently, Som and Longman [14] of Argonne National Laboratory found higher prompt NO formation in biodiesel combustion. CH and OH radicals are continuously formed during combustion reactions. The formation of CH radicals is an indicator of low temperature pre-combustion reactions and the formation of prompt NO. And the presence OH radicals indicate high temperature reactions and thermal NO [15]. Love et al. [16] observed lesser concentration of OH radicals and higher populations of CH radicals (especially unsaturated biodiesel fuels) in biodiesel flames. The reactions of fuel with oxygen from the air are the basis of energy production, but they are also the major cause of pollutant formation. A molecule that has an unpaired electron is called a free radical that determines the oxidation reaction rate. The most important reactive radicals formed during combustion reactions are hydroperoxyl (•OOH), hydroxyl (HO•), alkoxyl (RO•) and peroxyl (ROO•) radicals. These radicals react with N2 and N2O forming nitrogen oxides. Free radicals from the ester can be formed by four routes: the bimolecular reaction with hydrogen atom abstraction by dioxygen from the weakest CH bond, the bimolecular addition of dioxygen to the double bond of nonsaturated ester, the trimolecular reaction of dioxygen with the weakest CH bonds of two nonsaturated esters and the trimolecular reaction of dioxygen with double bonds of two molecules of nonsaturated esters [17]. Additions of small amounts of antioxidants into the fuel suppress free radical formation by reacting with peroxyl radicals to form new inactive radicals so interrupting the propagation step. The hydrogen donating ability of a chain breaking antioxidant has a very important effect on its antioxidant activity. The hydrogen is released from the weak OH (phenols, hydroquinones) and NH (aromatic amines, diamines) bonds of antioxidants. In general phenolic antioxidants (TBHQ, BHT, BHA etc.) are added to biodiesel to prevent degradation. They are so effective in controlling free radicals at room temperature but their antioxidant activity decreases rapidly with increasing temperature. The quantum-chemical study of an aromatic amine N, N′-diphenyl-p-phenylenediamine (DPPD) indicate that it retains its antioxidant activity even at increased temperatures [18]. Amines possess a pair of p-electrons on the nitrogen atom. The nitrogen atom has a low electron affinity in comparison with oxygen. Therefore, amine can be the electron donor reactant in a charge-transfer complex (association of two or more molecules) in association with oxygencontaining molecules and radicals. Moreover, the hydrogen atom from the NH bond of aromatic amine can be separated more easily than from the OH bond of phenols since the N―H hydrogen bond is not as strong as the O―H hydrogen bond [17]. The purpose of this study was to evaluate the effects of aromatic amine antioxidants on NOx and other emissions of a DI diesel engine powered by soybean methyl ester. In this study p-phenylenediamine
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Table 2 Specifications of test antioxidants. Antioxidant
Specifications
N,N′-diphenyl-1,4-phenylenediamine (DPPD)
CAS number Molecular weight Chemical formula Melting point CAS number Molecular weight Chemical formula Melting point
N-phenyl-1,4-phenylenediamine (NPPD)
74-31-7 260.34 C18H16N2 144 °C 101-54-2 184.24 C12H12N2 68 °C
based antioxidants N,N′-diphenyl-1,4-phenylenediamine (DPPD) and N-phenyl-1,4-phenylenediamine (NPPD) were selected as test antioxidants. The antioxidant-free non commercial soybean biodiesel was purchased from Chandrasekar biodiesel consulting firm, Hyderabad, India. The antioxidant additives used in this study were purchased from Sigma-Aldrich India. The specifications of the test fuels and antioxidants and biodiesel are presented in Tables 1 and 2 respectively. The chemical structures of the antioxidants are given in Fig. 1. The antioxidant DPPD contains NH substituent and NPPD contains both NH and NH2 substituents. 2. Experiment details The engine used in the present study is the computerised Kirloskar-make 4 stroke water cooled single cylinder diesel engine of 4.4 kW rated power. The schematic diagram of the experimental setup is shown in Fig. 2. The engine was directly coupled to an eddy-current dynamometer equipped with a load controller. The fuel flow rate, speed, load, exhaust gas temperature and gas flow rate were displayed on a personal computer. The specifications of the engine are given in Table 3. The cylinder pressure was measured by a Piezo sensor of PCB Piezotronics Model M111A22 and the signal of the cylinder pressure was acquired for every 1°CA. Exhaust emissions were measured with an AVL DiGas 444 five gas analyser. The analyser provided a NO range of 0 to 5000 ppm with a resolution of 10 ppm, CO measurement range of 0% to 20% by volume with a resolution of 0.01%, and HC range of 0 to 20,000 ppm with a resolution of 10 ppm. The accuracy of the instrument is 10%, 5% and 0.5% of the indicated value for the measurement of NO, HC and CO respectively. As for smoke measurement, the automatic NETEL NPM CH 1 smoke meter was employed. The smoke intensity was measured as light absorption coefficient (m−1). The display range, scale resolution, repeatability,
Table 1 Specifications of test fuels. Properties 3
Density at 15 °C kg/m Viscosity at 40 °C mm/s2 Flash point °C Pour point °C Calorific value kJ/kg Cetane number
Soy methyl ester
Diesel Indian oil
850 4.05 95.28 2 39,600 48
830 2.5 66 12 45,300 43
Fig. 1. Chemical structures of selected additives. a) DPPD (N,N′-diphenyl-1,4phenylenediamine), b) NPPD (N-phenyl-1,4-phenylenediamine).
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Fig. 2. Schematic diagram of experimental setup.
response time and warm up time of smoke meter are 0–9.99 m−1, 0.01 m −1, 0.1 m −1, 0.3 s and 3 minutes respectively. Experiments were conducted with different antioxidant concentrations of biodiesel fuel mixtures (50, 100, 250, 500, 750, 1000, 1500, 2000 ppm) in addition to neat biodiesel and diesel fuel. To make 100 ppm mixture, 100 mg of antioxidant was added to 1 litre of biodiesel. In each load levels, the measurements of exhaust gas temperature, fuel consumption, fuel pressure, coolant temperature, exhaust gas flow rate, combustion pressure, crank angle, NO, HC, CO and smoke emissions were carried out and recorded.
3. Results and discussions The effect of aromatic amine antioxidant additives on NO, CO, HC and smoke emissions of soybean methyl ester fuelled diesel engine were systematically investigated in this study. The NO measurements were repeatable within each engine run, with replicate measurements varying by 3–6 ppm. The exhaust emissions of engine are greatly influenced by the addition of antioxidants with biodiesel. The results of performances and emissions of test antioxidant mixtures are compared with neat biodiesel and discussed in this section, as follows.
Table 3 Specifications of test engine. Parameter
Specification
Model Type
Kirloskar TAF-1 Single cylinder, four stroke, direct injection, bowl-in-piston, constant speed, diesel engine. 661 cc 87.5 mm × 110 mm 17.5:1 1500 rpm 4.4 kW Eddy current dynamometer 200 bar
Capacity Bore and stroke Compression ratio Speed (constant) Rated power Loading type Injection pressure
3.1. Effects on NO emissions The NO emissions during combustion of test antioxidant mixtures were compared to neat biodiesel and diesel. The changes in NO emissions that resulted from the antioxidant addition were found to be significant. Fig. 3 shows the NO reduction percent of different antioxidant mixtures relative to neat biodiesel and B20 fuel at full load (4.326 kW), 75% load (3.357 kW), 50% (2.238 kW) load and 25% load (1.119 kW) respectively. From the figure it can be seen that, antioxidant addion with B20 fuel reduce the NO emission up to a certain concentration and beyond the limit emission of NO increase with antioxidant loadings. At 75% load, the maximum NO reduction percent of DPPD and NPPD additives are 9.35 and 4.06 respectively. As shown in Fig. 3 for B100 fuel, the NO emission reduction increased linearly with the concentration of DPPD additive. In the case of NPPD additive, the reduction in NO emission increases with the increase of concentration up to a certain limit and starts to decrease beyond it. Similar trends were obtained by Dunn [19] and he observed increased antioxidant activity at lower loadings (less than 1000 ppm) and constant or reduced activity at higher loadings. The possible reason for the inverse relationship between treatment rate and amount of NO reduction is that all the p-phenylenediamine based antioxidants contain nitrogen in its chemical structure and at higher loading, the excess antioxidant reacts with oxygen and forms additional NO. The antioxidant efficiency is defined by the ratio F/ [In H]. Where F is the antioxidant activity and In H is the acceptor reacting with alkoxyl and peroxyl radicals. This ratio does not depend on the antioxidant concentration [17]. For B100 fuel, we found 28.36% and 20.96% reductions in NO emission, respectively, when the fuel was loaded with DPPD and NPPD additives as compared with neat biodiesel fuel (75% load). DPPD was the most effective of the antioxidants studied, giving more than 25% decrease in measured NO emissions at all engine loads. The comparison of specific NO emission of soy methyl ester with the best antioxidant additive to B20 and B100 fuels at various loads is shown in Fig. 4. For B20 fuel, the NO produced by DPPD additive and base fuel at 75% load was 2.38 and 2.49 g/kWh respectively. The corresponding NO emissions for B100 fuel were 2.12 and 2.72 g/kWh respectively. Fig. 4 shows that across nearly the full spectrum of engine load examined the NO emission
K. Varatharajan, M. Cheralathan / Fuel Processing Technology 106 (2013) 526–532
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Fig. 3. Effect antioxidant additives on NOx emissions for B20 and B100 fuels. a) 25% load (1.12 kW), b) 50% load (2.24 kW), c) 75% load (3.36 kW), d) 100% load (4.33 kW).
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5
Brake specific CO emissions (g/kWh)
Brake Specific NO emission (g/kWh)
530
Diesel B100 B100 + 2000 ppm DPPD B20 B20 + 1000 ppm DPPD
4 3 2 1 0 0
1
2
3
4
24 Diesel B100 B100 + 2000 ppm DPPD B20 B20 + 1000 ppm DPPD
20 16 12 8 4 0 0
5
1
2
3
4
5
Brake power (kW)
Brake power (kW)
Fig. 6. Effect of antioxidant additives with biodiesel fuels on CO emissions. Fig. 4. Variation of NO emission with brake power.
of biodiesel fuels containing antioxidants are below those not only of biodiesel lacking antioxidants but also below those of petroleum diesel. As shown in Fig. 4, the biodiesel fuel emits more NO than conventional diesel fuel. The main reason for the NO increase with the use of biodiesel is discussed as follows. Fenimore [20] proposed that NO formation was initiated by reactions of hydrocarbon radicals (CH, CH2, C2, C, and C2H) with molecular nitrogen. CH þ N2 ↔HCN þ N
ð1Þ
N þ O2 ↔NO þ O
ð2Þ
HCN þ OH↔CN þ H2 O
ð3Þ
CN þ O2 ↔NO þ CO
ð4Þ
The production rates of HCN, N and NO increases with the concentration of CH radicals. Brezinsky et al. [11,12] observed increased formation of CH radicals during biodiesel combustion. This could be the reason for increased NOx formation for biodiesel fuel. Fig. 5 shows the effect of antioxidant additives on NO emission on diesel fuel. The addition of aromatic amine antioxidant with diesel increases the engine out NO emission in proportion with the increase in concentrations. Presence of nitrogen in the antioxidant additive has NO reducing effect with biodiesel but has a NO increasing effect with diesel. Furthermore, the nitrogen containing cetane improver additive 2- EHN (2-ethylhexyl nitrate) with diesel fuel reduces NO emissions considerably [21]. Peroxyl radical (HO2) produced by ester is nearly
five times more active than the alkylperoxyl radical produced by hydrocarbons. The highly reactive ester peroxy radicals are consumed by antioxidants and alkylperoxyl radicals are not consumed by them [17]. From the results, it appears that more free radicals (peroxyl radicals) are formed during biodiesel combustion than diesel combustion and prompt NO could play an important role in biodiesel NOx emissions. Moreover, Lin et al. [22] found increased production of peroxyl radicals and reduced formation of OH radicals in biodiesel combustion. As mentioned earlier, the decreased formation of OH radicals indicate low temperature reactions and the factors such as heat release rate, adiabatic flame temperature and stoichiometric burning may not be the major reason for biodiesel NOx effect. An ester molecule has two different types of weak C―H bonds: α-C―H bonds of the alcohol substituent and the α-C―H bonds of the acid substituent. The oxidation proceeds in ester via the reaction of the ester peroxyl radical with weakest C―H bond of the ester. The reduction of NOx emission from antioxidant fuel mixtures is mainly due to the suppression of peroxyl free radical formations by reaction with aromatic amines. p-Phenylenediamines reacts with peroxyl radical to form primary amine radical which is very reactive. The peroxyl radical further reacts with amine radical and produce benzoquinonediimine and nitroxyl radical. These reaction products efficiently trap free radicals [23].
3.2. Effects on CO and HC emissions Fig. 6 shows the influence of the DPPD antioxidant additive on the brake-specific CO emissions at various loads for B20 and B100 fuels. It can be seen that CO emissions increase with the addition of the
40
NO emissions (g/kWh)
Brake specific HC emission (g/kWh)
B100 + DPPD B100 + NPPD
35 30 25 20 15 10 5 0 -5
50
100
250
500
750
1000
1500
2000
Antioxidant concentrations (ppm) Fig. 5. Effect of antioxidants on diesel NO emission at part load conditions (75% load).
0.7 Diesel B100 B100 + 2000 ppm DPPD B20 B20 + 1000 ppm DPPD
0.6 0.5 0.4 0.3 0.2 0.1 0 0
1
2
3
4
Brake power (kW) Fig. 7. Effect of antioxidant additives with biodiesel fuels on HC emissions.
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H2 O2 →2OH
ð5Þ
HO2 →OH þ O
ð6Þ
CO þ OH→CO2 þ H
ð7Þ
HC þ OH→HCHO
ð8Þ
HCHO þ OH→H2 O þ HCO
ð9Þ
HCO þ O2 →CO2 þ HO
ð10Þ
3.3. Effects on smoke emissions Fig. 8 shows the characteristics of the exhaust smoke emissions of biodiesel fuel and its blend containing the DPPD additive. The DPPD fuel mixture increased the smoke density by 12.5 % and 6.6% when compared with the B20 and B100 fuels respectively for 75% load conditions. It is important to note that the increase in smoke emissions were still below the level of diesel. Several factors may contribute to the increase of smoke opacity with antioxidant addition. The possible reasons for the increase of smoke are reduction of oxygen availability, increase of C―C bonds and increase of aromatic content due to the addition of antioxidants with fuels. 3.4. Effects on brake thermal efficiency The variation of brake thermal efficiency with loads for the antioxidant fuel mixtures is shown in Fig. 9. At part loads change in brake
36 Diesel B100 B100 + 2000 ppm DPPD B20 B20 + 1000 ppm DPPD
Smoke intensity (HSU)
32 28 24 20 16 12
36
Brake thermal efficiency (%)
antioxidants. At 75% load, the DPPD additive had about 9.09% and 14.28% more CO emissions than the neat B20 fuel and neat biodiesel fuel respectively. The variation of brake-specific HC emissions with load is shown in Fig. 7. In the same manner as the CO emissions, the antioxidant addition was found to significantly increase HC emissions. The increase in HC emissions for B20 and B100 fuels were 10.52% and 16.92% respectively at 75% load conditions. However, the levels of CO and HC emissions with the addition of antioxidants were still below those for petro-diesel. The reason for the increase in CO and HC emissions is explained as follows: Peroxyl (HO2) and hydrogen peroxide (H2O2) radicals are continuously produced during oxidation. These radicals are further converted into hydroxyl (OH) radicals by absorbing heat (Eqs. (5) and (6)). The OH radicals are responsible for the conversion of CO into CO2 and HC into H2O and CO2 (Eqs. (7), (8), (9) and (10)). Treating biodiesel with antioxidants reduce the concentration of peroxyl and hydrogen peroxide radicals. This reduction in free radicals may have a significant effect on the formation of OH radicals and oxidation of CO and HC.
531
32 28 24 20 16 Diesel B100 B100 + 2000 ppm DPPD B20 B20 + 1000 ppm DPPD
12 8 4 0
0
1
2
3
4
5
Brake power (kW) Fig. 9. Effect of antioxidant additives with biodiesel fuels on brake thermal efficiency.
thermal efficiencies due to antioxidants addition are insignificant but at full load, efficiencies were slightly lower than the neat biodiesel. At 75% load, the brake thermal efficiency produced by the DPPD and B20 fuel mixture was 24.85%, while the base B20 fuel had 25.20%. For B100 fuel, there is no significant change in brake thermal efficiency at 75% load but at full load, 0.80% loss in BTE was observed. The reason for the reduction in thermal efficiency is possibly due to slight cylinder pressure reduction with the addition of antioxidants. 4. Conclusions The objective of this experimental work was to investigate the effect of p-phenylenediamine derived antioxidants on engine out NOx emissions from a biodiesel fuelled DI diesel engine. Based on the experimental results, the following conclusions can be drawn. 1. The test antioxidants and biodiesel mixtures produced lower emissions of nitrogen oxides. Among the tested antioxidants, the additive N,N′-diphenyl-1,4-phenylenediamine (DPPD) showed the highest activity in reducing NO in both B20 and B100 fuels. At 75% load, the maximum NO reduction percent relative to B20 fuel for DPPD and NPPD additives were 9.35% and 4.06% respectively and the corresponding reductions for neat biodiesel were 28.36% and 20.96% respectively. Moreover, the emission results show that the studied aromatic amine antioxidants additives reduced the NO emission below the level observed with petro-diesel combustion. 2. Increased formation of prompt NO could be the major reason for biodiesel NOx effect. 3. The antioxidant biodiesel mixtures produced slightly higher CO and HC emissions when compared with neat biodiesel. The DPPD additive increased the CO emissions over 9.09% with B20 fuel and 14.28% with B100 fuel at 75% load conditions. Use of DPPD additive with biodiesel fuels leads to a significant increase in HC emissions by about 10.52% and 16.92% for B20 and B100 fuels respectively. Smoke density also slightly increased with the addition of antioxidants with biodiesel. It should be noted however that these emissions are well below the diesel emission levels. The addition of antioxidants with biodiesel on engine brake thermal efficiency was also found to be insignificant. However, slight reduction in efficiency (0.8%) was observed with antioxidant fuel mixtures at full load.
8
Acknowledgement
4 0 0
1
2
3
4
5
Brake power (kW) Fig. 8. Effect of antioxidant additives with biodiesel fuels on smoke emissions.
The authors would like to thank the Chairman (Mr. M.V. Muthuramalingam) and CEO (Mr. M.V.M. Velmurugan) of Velammal engineering college, Chennai, India for providing laboratory facilities to our experimental work. The authors also express their thanks to
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Mr. K. Chandrasekar, biodiesel consultant, Hyderabad for providing soybean biodiesel for this work.
References [1] EPA, A Comprehensive Analysis of Biodiesel Impacts on Exhaust Emissions. Accessed May 26, 2012 from US Environmental Protection Agency http://www. epa.gov/otaq/models/analysis/biodsl/p02001.pdf EPA420-P-02-001. [2] GlobalData, Global Biodiesel Market Analysis and Forecasts to 2020. Accessed May 26, 2012 from market-study.de http://www.markt-studie.de/168/d/2010/ 03/30/globaldata-global-biodiesel-market-analysis-and-forecasts-to-2020/. [3] B. Moser, A. Williams, M. Haas, R. McCormick, Exhaust emissions and fuel properties of partially hydrogenated soybean oil methyl esters blended with ultra low sulfur diesel fuel, Fuel Processing Technology 90 (2009) 1122–1128. [4] NREL, NOx Solutions for biodiesel. Accessed May 26, 2012 from US National Renewable Energy Laboratory http://www.nrel.gov/docs/fy03osti/31465.pdf. [5] C. Mueller, A. Boehman, G. Martin, An experimental investigation of the origin of increased NOx emissions when fueling a heavy-duty compression-ignition engine with soy biodiesel, SAE International Journal of Fuels and Lubricants 2 (1) (2009) 789–816 (2009-01-1792). [6] Y. Zhang, A. Boehman, Impact of biodiesel on NOx emissions in a common rail direct injection diesel engine, Energy and Fuels 21 (2007) 2003–2012. [7] A. Monyem, J. Van Gerpen, M. Canakci, The effect of timing and oxidation on emissions from biodiesel-fueled engines, Transactions of the ASAE 44 (2001) 35–42. [8] J. Szybist, S. Kirby, A. Boehman, NOx emissions of alternative diesel fuels: a comparative analysis of biodiesel and FT diesel, Energy and Fuels 19 (2005) 1484–1492. [9] NREL, Effects of Biodiesel Blends on Vehicle Emissions. Accessed May 26, 2012 from US National Renewable Energy Laboratory http://www.nrel.gov/vehiclesandfuels/ npbf/pdfs/40554.pdf. [10] M.A. Hess, M.J. Haas, T.A. Foglia, W.N. Marmer, The effect of antioxidant addition on NOx emissions from biodiesel, Energy and Fuels 19 (2005) 1749–1754.
[11] S. Garner, K. Brezinsky, Biologically derived diesel fuel and NO formation: an experimental and chemical kinetic study, Part 1, Combustion and Flame 158 (2011) 2289–2301. [12] S. Garner, T. Dubois, N. Chaumeix, P. Dagaut, K. Brezinsky, Biologically derived diesel fuel and NO formation. Part 2: model development and extended validation, Combustion and Flame 158 (2011) 2302–2313. [13] G. Knothe, C. Sharp, T. Ryan, Exhaust emissions of biodiesel, petrodiesel, neat methyl esters, and alkanes in a new technology engine, Energy and Fuels 20 (1) (2006) 403–408. [14] S. Som, C. Longman, Numerical study comparing the combustion and emission characteristics of biodiesel to petrodiesel, Energy and Fuels 25 (2011) 1373–1386. [15] F. Salvador, J. Gimeno, J. Morena, Effects of nozzle geometry on direct injection diesel engine combustion process, Applied Thermal Engineering 29 (2009) 2051–2060. [16] N. Love, N. Parthasarathy, R. Gollahallia, Concentration measurements of CH and OH radicals in laminar biofuel flames, International Journal of Green Energy 8 (1) (2011) 113–120. [17] E. Denisov, I. Afanas'ev, Oxidation and Antioxidants in Organic Chemistry and Biology, Taylor & Francis, Boca Raton, 2005. [18] G. Gatial, J. Polovkováa, M. Breza, Quantum-chemical study of N, N′-diphenylp-phenylenediamine (DPPD) dehydrogenation, Acta Chimica Slovaca 1 (2008) 72–84. [19] R. Dunn, Effect of antioxidants on the oxidative stability of methyl soyate (biodiesel), Fuel Processing Technology 86 (2005) 1071–1085. [20] C. Fenimore, Formation of nitric oxide in premixed hydrocarbon flames, in: 13th Symp. on Combustion, 13, The Combustion Institute, 1975, pp. 373–380. [21] EPA, The Effect of Cetane Number Increase due to Additives on NOx Emissions from Heavy-duty Highway Engines. Retrieved May 26, 2012 from US Environmental Protection Agency http://www.epa.gov/otaq/models/analysis/r03002.pdf. [22] K. Lin, J. Lai, A. Violi, The role of the methyl ester moiety in biodiesel combustion: a kinetic modeling comparison of methyl butanoate and n-butane, Fuel 92 (2012) 16–26. [23] M. Breza, New Research on Antioxidants, Nova Science Publishers, Inc., New York, 2008. Chapter 7.
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Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
Effect of aromatic amine antioxidants on NOx emissions from a soybean biodiesel powered DI diesel engine K. Varatharajan a,⁎, M. Cheralathan b a b
Department of Mechanical Engineering, Velammal Engineering College, Chennai 600 066, India Department of Mechanical Engineering, SRM University, Chennai 603 203, India
a r t i c l e
i n f o
Article history: Received 7 November 2011 Received in revised form 27 May 2012 Accepted 6 September 2012 Available online 23 October 2012 Keywords: NOx reduction Antioxidants DPPD Soybean biodiesel Prompt NO
a b s t r a c t It is an overwhelming argument that the use of biodiesel instead of petrodiesel causes a reduction in harmful exhaust emissions from engines. A number of studies, however, indicate substantial increases in engine out NOx emissions with biodiesel fuel. Some studies have pointed out that the increased formation of prompt NOx is responsible for biodiesel NOx effect. Treatment of biodiesel with antioxidants is a promising approach because it reduces the formation of hydrocarbon free radicals, which are responsible for prompt NOx production in combustion process. Aromatic amine antioxidants are known as to be efficient inhibitors of free radicals. This study examines the use of p-phenylenediamine derived aromatic amine antioxidants for NOx reduction in a soybean biodiesel fuelled DI diesel engine. The antioxidant additives, N,N′-diphenyl-1,4-phenylenediamine (DPPD) and N-phenyl-1,4-phenylenediamine (NPPD) were tested on a computerised Kirloskar-make 4 stroke water cooled single cylinder diesel engine of 4.4 kW rated power. Results show that significant reduction of NOx could be achieved by the addition of antioxidants but smoke, CO and HC emissions were found to have increased. © 2012 Elsevier B.V. All rights reserved.
1. Introduction There is considerable interest worldwide in the replacement of petrodiesel by biodiesel in order to reduce the harmful diesel exhaust emissions from engines. However, the use of biodiesel results in a noticeable increase (about 10%) in NOx emissions when compared to conventional diesel [1]. The biodiesel market in the US is expected to reach 6,453 million litres in 2020 and 45,291 million litres globally [2]. As a consequence, the increase in NOx emissions could become a significant barrier to biodiesel market expansion. The nitrogen oxide compounds not only have direct effects on human health, but also affect the environment and have ground level ozone-forming potential. When compared to diesel NOx, a relatively small amount of research has been conducted on biodiesel NOx emissions. Thermal, prompt and N2O pathway are the dominant NO formation mechanisms in biodiesel combustion. In general, biodiesel does not contain fuel-bound nitrogen, so NO formation by the fuel NO mechanism can be considered negligible [3]. Thermal NO is formed by oxidation of nitrogen at elevated temperatures (above1700 K). Prompt NO is produced by the formation of free radicals in the flame front of hydrocarbon flames. Generally, the prompt NO contribution to total NO from combustion process is considered less important when compared to thermal NO. On the other hand, in biodiesel combustion, significant quantities of NO are ⁎ Corresponding author at: Department of Mechanical Engineering, Velammal Engineering College, Surapet, Chennai 600066, India. Tel.: + 91 9677039183; fax: + 91 4426591771. E-mail address: [email protected] (K. Varatharajan). 0378-3820/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fuproc.2012.09.023
formed by prompt mechanism [4]. N2O pathway mechanism is significant under elevated pressure and lean air fuel ratio conditions. Numerous reasons have been proposed for biodiesel NOx effect; however, the exact cause is still unclear. Increased fuel mass delivery due to advancement of injection timing resulting from higher bulk modulus of biodiesel fuel, electronic injection advance due to reduced heating value, higher cetane number of biodiesel, increased premixed-burn fraction, change in chemical kinetic pathways, increased adiabatic flame temperature and heat release rate, more stoichiometric burning and lessened radiative heat transfer from in-cylinder soot are stated as the reasons for increased thermal NO formation in biodiesel fuel [5]. Zhang and Boehman [6] found much higher NOx emissions with common rail system, and concluded that injection timing shift alone could not be the reason for biodiesel NOx effect. Monyem et al. [7] have shown that biodiesel has lower adiabatic flame temperature than petrodiesel. Moreover, Szybist et al. [8] observe a lower rate of heat release for biodiesel fuel at all loads. McCormick and co-workers of US National Renewable Energy Laboratory (NREL) reported that the increased formation of prompt NO as one of the primary reasons for biodiesel NOx effect [4,9] and also they suggested that the antioxidants can be used to control prompt NO generation. Hess et al. [10] studied the effect of different antioxidant fuel additives with B20 (soy) fuel on NOx emissions. They observed significant reduction in NOx emissions with butylated hydroxytoluene and butylated hydroxyanisole antioxidants. Brezinsky et al. conducted experiments to analyse the biodiesel combustion using high pressure single pulse shock tube (HPST) [11] and jet stirred reactor (JSR) [12]. In both the tests they have observed increased formation of prompt NO from the unsaturated methyl ester compared to the saturated methyl
K. Varatharajan, M. Cheralathan / Fuel Processing Technology 106 (2013) 526–532
ester. They predicted the prompt NOx by measuring the rate of acetylene formation. In general, biodiesel feedstocks containing higher degree of unsaturated components like soybean, sunflower and jatropha emits more NOx compared to their saturated counterparts. Knothe et al. [13] found more NOx emissions with biodiesel fuelled engine fitted with EGR (a method to reduce thermal NO) when compared to conventional petro-diesel fuelled engine. This shows that thermal NO has less effect on biodiesel NOx emissions. Furthermore, Mueller et al. [5] concluded that changes in prompt NO formation may play an important role in biodiesel NOx effect. Recently, Som and Longman [14] of Argonne National Laboratory found higher prompt NO formation in biodiesel combustion. CH and OH radicals are continuously formed during combustion reactions. The formation of CH radicals is an indicator of low temperature pre-combustion reactions and the formation of prompt NO. And the presence OH radicals indicate high temperature reactions and thermal NO [15]. Love et al. [16] observed lesser concentration of OH radicals and higher populations of CH radicals (especially unsaturated biodiesel fuels) in biodiesel flames. The reactions of fuel with oxygen from the air are the basis of energy production, but they are also the major cause of pollutant formation. A molecule that has an unpaired electron is called a free radical that determines the oxidation reaction rate. The most important reactive radicals formed during combustion reactions are hydroperoxyl (•OOH), hydroxyl (HO•), alkoxyl (RO•) and peroxyl (ROO•) radicals. These radicals react with N2 and N2O forming nitrogen oxides. Free radicals from the ester can be formed by four routes: the bimolecular reaction with hydrogen atom abstraction by dioxygen from the weakest CH bond, the bimolecular addition of dioxygen to the double bond of nonsaturated ester, the trimolecular reaction of dioxygen with the weakest CH bonds of two nonsaturated esters and the trimolecular reaction of dioxygen with double bonds of two molecules of nonsaturated esters [17]. Additions of small amounts of antioxidants into the fuel suppress free radical formation by reacting with peroxyl radicals to form new inactive radicals so interrupting the propagation step. The hydrogen donating ability of a chain breaking antioxidant has a very important effect on its antioxidant activity. The hydrogen is released from the weak OH (phenols, hydroquinones) and NH (aromatic amines, diamines) bonds of antioxidants. In general phenolic antioxidants (TBHQ, BHT, BHA etc.) are added to biodiesel to prevent degradation. They are so effective in controlling free radicals at room temperature but their antioxidant activity decreases rapidly with increasing temperature. The quantum-chemical study of an aromatic amine N, N′-diphenyl-p-phenylenediamine (DPPD) indicate that it retains its antioxidant activity even at increased temperatures [18]. Amines possess a pair of p-electrons on the nitrogen atom. The nitrogen atom has a low electron affinity in comparison with oxygen. Therefore, amine can be the electron donor reactant in a charge-transfer complex (association of two or more molecules) in association with oxygencontaining molecules and radicals. Moreover, the hydrogen atom from the NH bond of aromatic amine can be separated more easily than from the OH bond of phenols since the N―H hydrogen bond is not as strong as the O―H hydrogen bond [17]. The purpose of this study was to evaluate the effects of aromatic amine antioxidants on NOx and other emissions of a DI diesel engine powered by soybean methyl ester. In this study p-phenylenediamine
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Table 2 Specifications of test antioxidants. Antioxidant
Specifications
N,N′-diphenyl-1,4-phenylenediamine (DPPD)
CAS number Molecular weight Chemical formula Melting point CAS number Molecular weight Chemical formula Melting point
N-phenyl-1,4-phenylenediamine (NPPD)
74-31-7 260.34 C18H16N2 144 °C 101-54-2 184.24 C12H12N2 68 °C
based antioxidants N,N′-diphenyl-1,4-phenylenediamine (DPPD) and N-phenyl-1,4-phenylenediamine (NPPD) were selected as test antioxidants. The antioxidant-free non commercial soybean biodiesel was purchased from Chandrasekar biodiesel consulting firm, Hyderabad, India. The antioxidant additives used in this study were purchased from Sigma-Aldrich India. The specifications of the test fuels and antioxidants and biodiesel are presented in Tables 1 and 2 respectively. The chemical structures of the antioxidants are given in Fig. 1. The antioxidant DPPD contains NH substituent and NPPD contains both NH and NH2 substituents. 2. Experiment details The engine used in the present study is the computerised Kirloskar-make 4 stroke water cooled single cylinder diesel engine of 4.4 kW rated power. The schematic diagram of the experimental setup is shown in Fig. 2. The engine was directly coupled to an eddy-current dynamometer equipped with a load controller. The fuel flow rate, speed, load, exhaust gas temperature and gas flow rate were displayed on a personal computer. The specifications of the engine are given in Table 3. The cylinder pressure was measured by a Piezo sensor of PCB Piezotronics Model M111A22 and the signal of the cylinder pressure was acquired for every 1°CA. Exhaust emissions were measured with an AVL DiGas 444 five gas analyser. The analyser provided a NO range of 0 to 5000 ppm with a resolution of 10 ppm, CO measurement range of 0% to 20% by volume with a resolution of 0.01%, and HC range of 0 to 20,000 ppm with a resolution of 10 ppm. The accuracy of the instrument is 10%, 5% and 0.5% of the indicated value for the measurement of NO, HC and CO respectively. As for smoke measurement, the automatic NETEL NPM CH 1 smoke meter was employed. The smoke intensity was measured as light absorption coefficient (m−1). The display range, scale resolution, repeatability,
Table 1 Specifications of test fuels. Properties 3
Density at 15 °C kg/m Viscosity at 40 °C mm/s2 Flash point °C Pour point °C Calorific value kJ/kg Cetane number
Soy methyl ester
Diesel Indian oil
850 4.05 95.28 2 39,600 48
830 2.5 66 12 45,300 43
Fig. 1. Chemical structures of selected additives. a) DPPD (N,N′-diphenyl-1,4phenylenediamine), b) NPPD (N-phenyl-1,4-phenylenediamine).
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Fig. 2. Schematic diagram of experimental setup.
response time and warm up time of smoke meter are 0–9.99 m−1, 0.01 m −1, 0.1 m −1, 0.3 s and 3 minutes respectively. Experiments were conducted with different antioxidant concentrations of biodiesel fuel mixtures (50, 100, 250, 500, 750, 1000, 1500, 2000 ppm) in addition to neat biodiesel and diesel fuel. To make 100 ppm mixture, 100 mg of antioxidant was added to 1 litre of biodiesel. In each load levels, the measurements of exhaust gas temperature, fuel consumption, fuel pressure, coolant temperature, exhaust gas flow rate, combustion pressure, crank angle, NO, HC, CO and smoke emissions were carried out and recorded.
3. Results and discussions The effect of aromatic amine antioxidant additives on NO, CO, HC and smoke emissions of soybean methyl ester fuelled diesel engine were systematically investigated in this study. The NO measurements were repeatable within each engine run, with replicate measurements varying by 3–6 ppm. The exhaust emissions of engine are greatly influenced by the addition of antioxidants with biodiesel. The results of performances and emissions of test antioxidant mixtures are compared with neat biodiesel and discussed in this section, as follows.
Table 3 Specifications of test engine. Parameter
Specification
Model Type
Kirloskar TAF-1 Single cylinder, four stroke, direct injection, bowl-in-piston, constant speed, diesel engine. 661 cc 87.5 mm × 110 mm 17.5:1 1500 rpm 4.4 kW Eddy current dynamometer 200 bar
Capacity Bore and stroke Compression ratio Speed (constant) Rated power Loading type Injection pressure
3.1. Effects on NO emissions The NO emissions during combustion of test antioxidant mixtures were compared to neat biodiesel and diesel. The changes in NO emissions that resulted from the antioxidant addition were found to be significant. Fig. 3 shows the NO reduction percent of different antioxidant mixtures relative to neat biodiesel and B20 fuel at full load (4.326 kW), 75% load (3.357 kW), 50% (2.238 kW) load and 25% load (1.119 kW) respectively. From the figure it can be seen that, antioxidant addion with B20 fuel reduce the NO emission up to a certain concentration and beyond the limit emission of NO increase with antioxidant loadings. At 75% load, the maximum NO reduction percent of DPPD and NPPD additives are 9.35 and 4.06 respectively. As shown in Fig. 3 for B100 fuel, the NO emission reduction increased linearly with the concentration of DPPD additive. In the case of NPPD additive, the reduction in NO emission increases with the increase of concentration up to a certain limit and starts to decrease beyond it. Similar trends were obtained by Dunn [19] and he observed increased antioxidant activity at lower loadings (less than 1000 ppm) and constant or reduced activity at higher loadings. The possible reason for the inverse relationship between treatment rate and amount of NO reduction is that all the p-phenylenediamine based antioxidants contain nitrogen in its chemical structure and at higher loading, the excess antioxidant reacts with oxygen and forms additional NO. The antioxidant efficiency is defined by the ratio F/ [In H]. Where F is the antioxidant activity and In H is the acceptor reacting with alkoxyl and peroxyl radicals. This ratio does not depend on the antioxidant concentration [17]. For B100 fuel, we found 28.36% and 20.96% reductions in NO emission, respectively, when the fuel was loaded with DPPD and NPPD additives as compared with neat biodiesel fuel (75% load). DPPD was the most effective of the antioxidants studied, giving more than 25% decrease in measured NO emissions at all engine loads. The comparison of specific NO emission of soy methyl ester with the best antioxidant additive to B20 and B100 fuels at various loads is shown in Fig. 4. For B20 fuel, the NO produced by DPPD additive and base fuel at 75% load was 2.38 and 2.49 g/kWh respectively. The corresponding NO emissions for B100 fuel were 2.12 and 2.72 g/kWh respectively. Fig. 4 shows that across nearly the full spectrum of engine load examined the NO emission
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Fig. 3. Effect antioxidant additives on NOx emissions for B20 and B100 fuels. a) 25% load (1.12 kW), b) 50% load (2.24 kW), c) 75% load (3.36 kW), d) 100% load (4.33 kW).
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5
Brake specific CO emissions (g/kWh)
Brake Specific NO emission (g/kWh)
530
Diesel B100 B100 + 2000 ppm DPPD B20 B20 + 1000 ppm DPPD
4 3 2 1 0 0
1
2
3
4
24 Diesel B100 B100 + 2000 ppm DPPD B20 B20 + 1000 ppm DPPD
20 16 12 8 4 0 0
5
1
2
3
4
5
Brake power (kW)
Brake power (kW)
Fig. 6. Effect of antioxidant additives with biodiesel fuels on CO emissions. Fig. 4. Variation of NO emission with brake power.
of biodiesel fuels containing antioxidants are below those not only of biodiesel lacking antioxidants but also below those of petroleum diesel. As shown in Fig. 4, the biodiesel fuel emits more NO than conventional diesel fuel. The main reason for the NO increase with the use of biodiesel is discussed as follows. Fenimore [20] proposed that NO formation was initiated by reactions of hydrocarbon radicals (CH, CH2, C2, C, and C2H) with molecular nitrogen. CH þ N2 ↔HCN þ N
ð1Þ
N þ O2 ↔NO þ O
ð2Þ
HCN þ OH↔CN þ H2 O
ð3Þ
CN þ O2 ↔NO þ CO
ð4Þ
The production rates of HCN, N and NO increases with the concentration of CH radicals. Brezinsky et al. [11,12] observed increased formation of CH radicals during biodiesel combustion. This could be the reason for increased NOx formation for biodiesel fuel. Fig. 5 shows the effect of antioxidant additives on NO emission on diesel fuel. The addition of aromatic amine antioxidant with diesel increases the engine out NO emission in proportion with the increase in concentrations. Presence of nitrogen in the antioxidant additive has NO reducing effect with biodiesel but has a NO increasing effect with diesel. Furthermore, the nitrogen containing cetane improver additive 2- EHN (2-ethylhexyl nitrate) with diesel fuel reduces NO emissions considerably [21]. Peroxyl radical (HO2) produced by ester is nearly
five times more active than the alkylperoxyl radical produced by hydrocarbons. The highly reactive ester peroxy radicals are consumed by antioxidants and alkylperoxyl radicals are not consumed by them [17]. From the results, it appears that more free radicals (peroxyl radicals) are formed during biodiesel combustion than diesel combustion and prompt NO could play an important role in biodiesel NOx emissions. Moreover, Lin et al. [22] found increased production of peroxyl radicals and reduced formation of OH radicals in biodiesel combustion. As mentioned earlier, the decreased formation of OH radicals indicate low temperature reactions and the factors such as heat release rate, adiabatic flame temperature and stoichiometric burning may not be the major reason for biodiesel NOx effect. An ester molecule has two different types of weak C―H bonds: α-C―H bonds of the alcohol substituent and the α-C―H bonds of the acid substituent. The oxidation proceeds in ester via the reaction of the ester peroxyl radical with weakest C―H bond of the ester. The reduction of NOx emission from antioxidant fuel mixtures is mainly due to the suppression of peroxyl free radical formations by reaction with aromatic amines. p-Phenylenediamines reacts with peroxyl radical to form primary amine radical which is very reactive. The peroxyl radical further reacts with amine radical and produce benzoquinonediimine and nitroxyl radical. These reaction products efficiently trap free radicals [23].
3.2. Effects on CO and HC emissions Fig. 6 shows the influence of the DPPD antioxidant additive on the brake-specific CO emissions at various loads for B20 and B100 fuels. It can be seen that CO emissions increase with the addition of the
40
NO emissions (g/kWh)
Brake specific HC emission (g/kWh)
B100 + DPPD B100 + NPPD
35 30 25 20 15 10 5 0 -5
50
100
250
500
750
1000
1500
2000
Antioxidant concentrations (ppm) Fig. 5. Effect of antioxidants on diesel NO emission at part load conditions (75% load).
0.7 Diesel B100 B100 + 2000 ppm DPPD B20 B20 + 1000 ppm DPPD
0.6 0.5 0.4 0.3 0.2 0.1 0 0
1
2
3
4
Brake power (kW) Fig. 7. Effect of antioxidant additives with biodiesel fuels on HC emissions.
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H2 O2 →2OH
ð5Þ
HO2 →OH þ O
ð6Þ
CO þ OH→CO2 þ H
ð7Þ
HC þ OH→HCHO
ð8Þ
HCHO þ OH→H2 O þ HCO
ð9Þ
HCO þ O2 →CO2 þ HO
ð10Þ
3.3. Effects on smoke emissions Fig. 8 shows the characteristics of the exhaust smoke emissions of biodiesel fuel and its blend containing the DPPD additive. The DPPD fuel mixture increased the smoke density by 12.5 % and 6.6% when compared with the B20 and B100 fuels respectively for 75% load conditions. It is important to note that the increase in smoke emissions were still below the level of diesel. Several factors may contribute to the increase of smoke opacity with antioxidant addition. The possible reasons for the increase of smoke are reduction of oxygen availability, increase of C―C bonds and increase of aromatic content due to the addition of antioxidants with fuels. 3.4. Effects on brake thermal efficiency The variation of brake thermal efficiency with loads for the antioxidant fuel mixtures is shown in Fig. 9. At part loads change in brake
36 Diesel B100 B100 + 2000 ppm DPPD B20 B20 + 1000 ppm DPPD
Smoke intensity (HSU)
32 28 24 20 16 12
36
Brake thermal efficiency (%)
antioxidants. At 75% load, the DPPD additive had about 9.09% and 14.28% more CO emissions than the neat B20 fuel and neat biodiesel fuel respectively. The variation of brake-specific HC emissions with load is shown in Fig. 7. In the same manner as the CO emissions, the antioxidant addition was found to significantly increase HC emissions. The increase in HC emissions for B20 and B100 fuels were 10.52% and 16.92% respectively at 75% load conditions. However, the levels of CO and HC emissions with the addition of antioxidants were still below those for petro-diesel. The reason for the increase in CO and HC emissions is explained as follows: Peroxyl (HO2) and hydrogen peroxide (H2O2) radicals are continuously produced during oxidation. These radicals are further converted into hydroxyl (OH) radicals by absorbing heat (Eqs. (5) and (6)). The OH radicals are responsible for the conversion of CO into CO2 and HC into H2O and CO2 (Eqs. (7), (8), (9) and (10)). Treating biodiesel with antioxidants reduce the concentration of peroxyl and hydrogen peroxide radicals. This reduction in free radicals may have a significant effect on the formation of OH radicals and oxidation of CO and HC.
531
32 28 24 20 16 Diesel B100 B100 + 2000 ppm DPPD B20 B20 + 1000 ppm DPPD
12 8 4 0
0
1
2
3
4
5
Brake power (kW) Fig. 9. Effect of antioxidant additives with biodiesel fuels on brake thermal efficiency.
thermal efficiencies due to antioxidants addition are insignificant but at full load, efficiencies were slightly lower than the neat biodiesel. At 75% load, the brake thermal efficiency produced by the DPPD and B20 fuel mixture was 24.85%, while the base B20 fuel had 25.20%. For B100 fuel, there is no significant change in brake thermal efficiency at 75% load but at full load, 0.80% loss in BTE was observed. The reason for the reduction in thermal efficiency is possibly due to slight cylinder pressure reduction with the addition of antioxidants. 4. Conclusions The objective of this experimental work was to investigate the effect of p-phenylenediamine derived antioxidants on engine out NOx emissions from a biodiesel fuelled DI diesel engine. Based on the experimental results, the following conclusions can be drawn. 1. The test antioxidants and biodiesel mixtures produced lower emissions of nitrogen oxides. Among the tested antioxidants, the additive N,N′-diphenyl-1,4-phenylenediamine (DPPD) showed the highest activity in reducing NO in both B20 and B100 fuels. At 75% load, the maximum NO reduction percent relative to B20 fuel for DPPD and NPPD additives were 9.35% and 4.06% respectively and the corresponding reductions for neat biodiesel were 28.36% and 20.96% respectively. Moreover, the emission results show that the studied aromatic amine antioxidants additives reduced the NO emission below the level observed with petro-diesel combustion. 2. Increased formation of prompt NO could be the major reason for biodiesel NOx effect. 3. The antioxidant biodiesel mixtures produced slightly higher CO and HC emissions when compared with neat biodiesel. The DPPD additive increased the CO emissions over 9.09% with B20 fuel and 14.28% with B100 fuel at 75% load conditions. Use of DPPD additive with biodiesel fuels leads to a significant increase in HC emissions by about 10.52% and 16.92% for B20 and B100 fuels respectively. Smoke density also slightly increased with the addition of antioxidants with biodiesel. It should be noted however that these emissions are well below the diesel emission levels. The addition of antioxidants with biodiesel on engine brake thermal efficiency was also found to be insignificant. However, slight reduction in efficiency (0.8%) was observed with antioxidant fuel mixtures at full load.
8
Acknowledgement
4 0 0
1
2
3
4
5
Brake power (kW) Fig. 8. Effect of antioxidant additives with biodiesel fuels on smoke emissions.
The authors would like to thank the Chairman (Mr. M.V. Muthuramalingam) and CEO (Mr. M.V.M. Velmurugan) of Velammal engineering college, Chennai, India for providing laboratory facilities to our experimental work. The authors also express their thanks to
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Mr. K. Chandrasekar, biodiesel consultant, Hyderabad for providing soybean biodiesel for this work.
References [1] EPA, A Comprehensive Analysis of Biodiesel Impacts on Exhaust Emissions. Accessed May 26, 2012 from US Environmental Protection Agency http://www. epa.gov/otaq/models/analysis/biodsl/p02001.pdf EPA420-P-02-001. [2] GlobalData, Global Biodiesel Market Analysis and Forecasts to 2020. Accessed May 26, 2012 from market-study.de http://www.markt-studie.de/168/d/2010/ 03/30/globaldata-global-biodiesel-market-analysis-and-forecasts-to-2020/. [3] B. Moser, A. Williams, M. Haas, R. McCormick, Exhaust emissions and fuel properties of partially hydrogenated soybean oil methyl esters blended with ultra low sulfur diesel fuel, Fuel Processing Technology 90 (2009) 1122–1128. [4] NREL, NOx Solutions for biodiesel. Accessed May 26, 2012 from US National Renewable Energy Laboratory http://www.nrel.gov/docs/fy03osti/31465.pdf. [5] C. Mueller, A. Boehman, G. Martin, An experimental investigation of the origin of increased NOx emissions when fueling a heavy-duty compression-ignition engine with soy biodiesel, SAE International Journal of Fuels and Lubricants 2 (1) (2009) 789–816 (2009-01-1792). [6] Y. Zhang, A. Boehman, Impact of biodiesel on NOx emissions in a common rail direct injection diesel engine, Energy and Fuels 21 (2007) 2003–2012. [7] A. Monyem, J. Van Gerpen, M. Canakci, The effect of timing and oxidation on emissions from biodiesel-fueled engines, Transactions of the ASAE 44 (2001) 35–42. [8] J. Szybist, S. Kirby, A. Boehman, NOx emissions of alternative diesel fuels: a comparative analysis of biodiesel and FT diesel, Energy and Fuels 19 (2005) 1484–1492. [9] NREL, Effects of Biodiesel Blends on Vehicle Emissions. Accessed May 26, 2012 from US National Renewable Energy Laboratory http://www.nrel.gov/vehiclesandfuels/ npbf/pdfs/40554.pdf. [10] M.A. Hess, M.J. Haas, T.A. Foglia, W.N. Marmer, The effect of antioxidant addition on NOx emissions from biodiesel, Energy and Fuels 19 (2005) 1749–1754.
[11] S. Garner, K. Brezinsky, Biologically derived diesel fuel and NO formation: an experimental and chemical kinetic study, Part 1, Combustion and Flame 158 (2011) 2289–2301. [12] S. Garner, T. Dubois, N. Chaumeix, P. Dagaut, K. Brezinsky, Biologically derived diesel fuel and NO formation. Part 2: model development and extended validation, Combustion and Flame 158 (2011) 2302–2313. [13] G. Knothe, C. Sharp, T. Ryan, Exhaust emissions of biodiesel, petrodiesel, neat methyl esters, and alkanes in a new technology engine, Energy and Fuels 20 (1) (2006) 403–408. [14] S. Som, C. Longman, Numerical study comparing the combustion and emission characteristics of biodiesel to petrodiesel, Energy and Fuels 25 (2011) 1373–1386. [15] F. Salvador, J. Gimeno, J. Morena, Effects of nozzle geometry on direct injection diesel engine combustion process, Applied Thermal Engineering 29 (2009) 2051–2060. [16] N. Love, N. Parthasarathy, R. Gollahallia, Concentration measurements of CH and OH radicals in laminar biofuel flames, International Journal of Green Energy 8 (1) (2011) 113–120. [17] E. Denisov, I. Afanas'ev, Oxidation and Antioxidants in Organic Chemistry and Biology, Taylor & Francis, Boca Raton, 2005. [18] G. Gatial, J. Polovkováa, M. Breza, Quantum-chemical study of N, N′-diphenylp-phenylenediamine (DPPD) dehydrogenation, Acta Chimica Slovaca 1 (2008) 72–84. [19] R. Dunn, Effect of antioxidants on the oxidative stability of methyl soyate (biodiesel), Fuel Processing Technology 86 (2005) 1071–1085. [20] C. Fenimore, Formation of nitric oxide in premixed hydrocarbon flames, in: 13th Symp. on Combustion, 13, The Combustion Institute, 1975, pp. 373–380. [21] EPA, The Effect of Cetane Number Increase due to Additives on NOx Emissions from Heavy-duty Highway Engines. Retrieved May 26, 2012 from US Environmental Protection Agency http://www.epa.gov/otaq/models/analysis/r03002.pdf. [22] K. Lin, J. Lai, A. Violi, The role of the methyl ester moiety in biodiesel combustion: a kinetic modeling comparison of methyl butanoate and n-butane, Fuel 92 (2012) 16–26. [23] M. Breza, New Research on Antioxidants, Nova Science Publishers, Inc., New York, 2008. Chapter 7.