Himansh Kumar,
Lakhya Jyoti Konwar,
Mohammad Aslam and
Anil Kumar Sarma*
Sardar Swaran Singh National Institute of Bio-Energy, (An Autonomous Institute of Ministry of New and Renewable Energy, Govt. of India), Kapurthala-144601, Punjab, India. E-mail: draksarma@gmail.com; aksarma@nire.res.in; Tel: +91 1822507414
First published on 11th April 2016
A Jatropha curcas oil (JCO) based hybrid microemulsion fuel (MHBF) comprising pretreated JCO–1-butanol–ethanol was prepared. The fuel characteristics of the hybrid microemulsion fuel (MHBF) were determined as per standard tests methods recommended by ASTM and compared with JCO B20, JCO B100 and petroleum diesel. The performance, combustion and emission characteristics of MHBF were studied for the first time in a variable compression ratio (VCR) direct injection CI engine. The fuel properties of the MHBF formulation were comparable to that of JCO B100 (viscosity of 5.9 mm2 s−1, density 0.872 g cm−3 and higher heating value of 38.25 MJ kg−1), while the cold flow properties were found to be superior than JCO B100 and JCO B20. In terms of engine performance, the brake specific fuel consumption (BSFC) of MHBF was comparable to B100 and B20 but higher than petrodiesel; while brake thermal efficiency (ηbte) was close to that of JCO B100 but slightly lower than petrodiesel at almost all load conditions. In terms of emission quality, MHBF was found to be superior to all other tested fuels under all conditions of loading. Most significantly it was observed that up to 80% NOx reduction as compared to petrodiesel was achieved when operated at full load with the MHBF at the maximum compression ratio of 17.5. Obviously, this is a significant finding and a factor of motivation and investigation for future biofuel applications.
Regardless of the above technical difficulties, the major impediments in large scale commercialization and application of vegetable oil based fuels like biodiesel originates due to the complicated and expensive conversion technology needed to reduce viscosity for CI engine applications. In addition, the role of biodiesel as renewable fuel has been criticised by many experts due to the increased NOx emissions resulting from higher combustion temperatures and with rigid environmental regulations aiming to virtually eliminate NOx emissions from CI engines; the intensity of the positive arguments for biodiesel as a diesel fuel substitute has become debatable.2,4–6
In sight of these difficulties, microemulsification technique has been regaining importance as an economically viable and ‘green’ approach for liquid biofuel production from vegetable oils.5,7–11 Especially, in the recent 2–3 years several research articles dealing with production, characterization and application of hybrid diesel fuels by microemulsification method were reported.7,8,10,12–14 The advantages of microemulsification approach over the chemical routes (transesterification, hydrocracking, hydroprocessing etc.) include lower production costs, short production time, simple and easy implementation (on the farm blending) and no by-product formation due to noninvolvement of chemical reactions. In addition, due to the presence of more volatile oxygenates the combustion temperature of the emulsion and microemulsion fuels are much lower, which results in a drastic reduction in emissions of NOx, CO, black smoke and particulate matter. Some other added benefit of such oxygenate fuels is to reduce ignition delay whilst engine efficiency also remain comparable to that of petrodiesel.7,8
Bora, Konwar and co-workers7 recently reported a novel method for the preparation of microemulsion based hybrid biofuels (MHBF) directly from low grade acidic oils (non-edible oil and waste cooking oils). Their method makes use of the naturally occurring monoglyceride, diglyceride and free fatty acids (or esters in case of pretreated oils) components present in these low grade oils as the surfactants to produce thermodynamically stable, isotropically clear microemulsion systems (Windsor Type IV) with anhydrous ethanol as dispersed phase and 1-butanol or 2-butanol as the co-surfactant.8–10
Keeping in view the importance of microemulsion systems like MHBF as renewable and low cost fuel alternative to reduce CI engine exhaust and maintain the engine efficiency, an investigation of the recently introduced MHBF was found necessary to confirm its potential utility as a fuel. Accordingly, the CI engine performance, combustion and emission analysis of Jatropha curcas oil (JCO) based MHBF were studied in a single cylinder direct injection CI engine and reported herewith. A comparative study was conducted with JCO B100, JCO B20 and petrodiesel with respect to engine efficiency, exhaust emission and combustion profile parameters such as brake specific fuel consumption (BSFC), brake thermal efficiency (ηbte) etc. All the attributes of the investigation have been compared with established biofuel systems.
Crude JCO (∼9.05 wt% FFA, acid value 18.12 mg KOH per g) was subjected to pretreatment with methanol to convert the FFA into FAME, which is an essential step to reduce the acid value (acidity) of oil to 1.1 mg KOH per g and improve fuel quality of MHBF.3–6 In a typical pretreatment reaction, 800 g preheated oil was added to 800 mL 2% (wt%) methanolic H2SO4, stirred and heated at 60 °C for 30 min in 2 L capacity jacketed reactor (Radleys, United Kingdom) equipped with mechanical stirring and reflux condenser. After completion of the reaction, excess methanol was removed with a rotary evaporator (Heidolph Hei-VAP Advantage, Germany). The resulting product was neutralized with 1 M NaHCO3 solution, followed by washing with deionised water. The oil layer was recovered in a separatory funnel and dried over anhydrous Na2SO4 prior to use. The composition (wt%) and physiochemical properties of the pretreated JCO (pretreated-JCO) are summarized in Table 1.
Fatty acid composition, name of the acid (wt%) | |
---|---|
Palmitic | 16.168 |
Stearic | 6.915 |
Palmitoleic | 0.856 |
Oleic | 39.103 |
Erucic | 0.261 |
Nervonic | 0.187 |
Linoleic | 35.305 |
Gamma-linolenic | 0.200 |
Alpha-linolenic | 0.279 |
cis-13,16-Decosadienoic | 0.202 |
cis-4,7,10,13,16,19-docosahexanoic | 0.228 |
Others | 0.296 |
Physicochemical properties | |
Density @ 15 °C (g cm−3) | 0.896 |
Viscosity @ 40 °C (mm2 s−1) | 19.230 |
Calorific value (MJ kg−1) | 38.680 |
Different components of MHBF (wt%) | |
FAME | 10.2 |
Monoglyceride | 9.9 |
Diglyceride | 6.7 |
Triglyceride | 73.1 |
The glyceride content in the oil samples, as well the conversion of fatty acids (FFA) to FAME (pretreatment), was also determined by GC according to ASTM D6584 method on an Agilent 7890A gas chromatograph equipped with FID detector and Agilent (Select Biodiesel for glycerides, 30 m length; 0.30 mm internal diameter; 0.10 μm film thickness) GC column.7,8 The column temperature was held at 50 °C for 1 min, 15 °C min−1 to 180 °C, 7 °C min−1 to 230 °C, 30 °C min−1 to 380 °C and finally held for 5 min. Helium was used as the carrier gas at a flow rate of 3 mL min−1.
The physicochemical fuel properties (density, viscosity, flash point, calorific value and pour point) of JCO, JCO B100, JCO B20, petrodiesel and MHBF were determined using standard test methods recommended by ASTM (list of test methods in Table S3, ESI†). The fatty acid composition, weight percentages of different components and major physicochemical properties of JCO are presented in Table 1.
The exhaust gas analysis was conducted through an AVL DI Gas 444 Analyzer (India). The resolution, accuracy, measuring range and technical specifications of the gas analyzer are presented in Table S2.† The exhaust gases namely CO, CO2, HC, O2, and NOx were analyzed under the test procedure approved by the Ministry of Road Transport and Highways, Government of India, and specified in MoRTH/CMVR/TAP-115/116, Issue no. 3, Part-VIII for 4-gas analyzer. In a typical analysis, initially the analyzer was allowed to suck in air, which in turn cleansed the passage joining the sensors through various filters and condensation trap. Then the gases from the exhaust were allowed to enter the analyzer through a probe during steady engine operation. The analyzer display showed the concentration of different components in exhaust gas and the results were manually recorded or printed for further analysis.
Oil (v) | 1-Butanol (v) | Ethanol (v) | Density @ 15 °C g cm−3 | Viscosity @ 40 °C mm2 s−1 | GCV MJ kg−1 |
---|---|---|---|---|---|
52 | 33 | 15 | 0.869 | 6.72 | 37.81 |
54 | 35 | 11 | 0.871 | 5.95 | 38.34 |
55 | 33 | 12 | 0.870 | 5.91 | 38.25 |
60 | 30 | 10 | 0.891 | 7.32 | n.d |
Table 3 presents a comparative summary of the main fuel characteristics of the optimum MHBF formulation with petrodiesel and JCO B100 and JCO B20. The data in Table 3 clearly indicated the superiority of MHBF over JCO B100 in terms of viscosity, density and cold flow properties. Nevertheless, the calorific value of MHBF was found to be somewhat lower than that of JCO B100. Further, to get an idea of the combustion characteristics of MHBF the activation energy of the optimum JCO MHBF was determined by thermogravimetric analysis under air flow (Coats and Redfern method) and compared with the activation energies reported for JCO B100, JCO B20 and petrodiesel.15 The values summarized in Table 3 indicated that the energy barrier for thermal combustion of MHBF were comparable to petrodiesel, but lower than that of JCO B100 and JCO B20. This suggested that the combustion characteristics of MHBF was similar to petrodiesel and validated its suitability as an alternative fuel for CI engine.15 The observation was also co-related with the relatively low flash point (37 °C) observed for MHBF, which resulted from the presence of 45% volatile light alcohols (ethanol and butanol) in the fuel system. In contrast, JCO B100 (the pure esters) exhibited the highest flash point (112 °C) and activation energy of combustion (Table 3).17
Density @ 15 °C g cm−3 | Viscosity @ 40 °C mm2 s−1 | GCV MJ kg−1 | Cetane no. | Flash point (°C) | Pour point (°C) | Activation energy kJ mol−1 | |
---|---|---|---|---|---|---|---|
a Values are reported for the optimum MHBF formation exhibiting highest HHV and viscosity within the ASTM D6751 limit: JCO oil (v):butanol (v):ethanol (v) = 55:33:12.b Values taken from ref. 11.c Stored in diesel tank for 60 days under normal environmental conditions. | |||||||
MHBFa | 0.870 | 5.91 | 38.25 | — | 37 | −7 | 13.72 |
MHBFa,c | 0.869 | 5.90 | 38.26 | — | 37 | −6 | — |
B100 | 0.894 | 6.09 | 39.45 | 57.35 | 112 | 1 | 30.05b |
B20 | 0.850 | 3.26 | 40.98 | n.d | 79 | n.d | 17.66b |
Petrodiesel | 0.813 | 2.55 | 42.83 | 45 | 65 | 12 | 16.47b |
Fig. 2 Variation in (a) BFSC (b) brake power (c) mechanical efficiency and (d) indicated thermal efficiency with respect to load (the engine was operating at constant speed of 1500 rpm). |
Fig. 2(b) shows the variations of ηbte of CI engine for petrodiesel, JCO B100, JCO B20 and MHBF under variable load conditions at CR 17.5 and 15. Here, the different trends observed for the different test fuel originated from the difference in their higher heating (calorific) values (refer to eqn (4), ESI†). Irrespective of the fuel type ηbte showed an increasing trend up to the full load beyond which it gradually decreased because of overload condition (which is the typical for constant speed CI engines).18 At no load condition, the time available for heat dissipation to the cylinder walls was relatively long as compared to part load and full load, and hence significant amount of heat loss occurred. When the load was increased the neat heat release rate also increased which resulted high combustion pressure that led to an equivalent increase in ηbte (Fig. 2). Here, the ηbte for CI engine fueled with MHBF was found to be higher than JCO B100 (for all load conditions at CR 17.5), which could be attributed to the presence of 45% light alcohol in MHBF system. Overall, under full load conditions at CR 17.5 the order of ηbte was found to be petrodiesel (31.75%) > JCO B20 (30.71%) > MHBF (29.8%) > JCO B100 (28.01%). The low ηbte was observed for MHBF tested at CR 15 (21.55%) which could be attributed to the incomplete combustion. Furthermore, at a low compression ratio (CR 15), combustion pressure and net heat release rate were also lowered which resulted low ηbte up to full engine load conditions. The slightly increased in ηbte of MHBF as compared to JCO B100 could also be attributed to the higher reaction activity of MHBF in the fuel-rich zone.5,12–14 Besides, due to the lower flame temperature of light alcohols (ethanol and 1-butanol) in comparison to the other test fuels (petrodiesel and biodiesel consisting long chain hydrocarbons and fatty esters) the heat losses in the engine cylinder also decreased when using MHBF (this is correlating with the lowest flash point for MHBF, Table 3).14,19,20
The trend of ηmech obtained with different test fuels under variable load conditions clearly showed that the mechanical efficiency of CI engine fueled with MHBF was comparable to that of petrodiesel at CR 17.5 and outperformed both petrodiesel and JCO B100 at CR 15 (Fig. 2(c)). The order of ηmech at full load at CR 17.5 is in the order JCO B20 ≥ MHBF = petrodiesel > JCO B100. This was because the 20% oxygenated biodiesel in JCO B20 (blend) facilitated improved combustion with high rate of brake power which in turn resulted higher mechanical efficiency at full load.20–22 Not surprisingly at CR 15, the ηmech value reduced further for all test fuels possibly due to incomplete combustion.14
ηite of CI engine is related to the indicated power developed in the combustion chamber during power stroke as defined by the ratio of indicated power to the total energy released from fuel air mixture. From the Fig. 2(d) it could be clearly seen that among all the tested biofuels (JCO B100, JCO B20 and MHBF) MHBF exhibited the highest ηite at 20% of load and in fact the value was found to be comparable with petrodiesel and higher than JCO B100 under certain load conditions at CR 17.5. This observation can be attributed to the high combustion pressure generated in CI engine cylinder with MHBF due to its superior atomization and combustion behavior.23–27 However, due to improper combustion at CR 15 the ηite value of MHBF lowest (also refer to Section 3.3).
Considering the calorific values of the test fuels, as expected petrodiesel produced highest BP while MHBF and JCO B20 were found to exhibit similar BP at all load conditions (particularly at CR 17.5, ESI†). However, the BP of JCO B100 (4.28 kW) was lowest to JCO B20 (4.33 kW), petrodiesel (4.33 kW) and MHBF (4.3 kW) on full load condition. It was also likely that in addition to the lower calorific value, higher density and viscosity of JCO B100 caused poor spray behavior which contributed to the reduce BP. On the other hand, due to the presence of light alcohols the BP of MHBF was significantly brought down under no load condition (1-butanol and ethanol causes a higher latent heat of evaporation which causes cooling effect in the combustion chamber and this negatively influences the thermal efficiency on no burden and at the commencement of the CI engine causing BP to decrease); the result was particularly prominent at CR 15 where upon MHBF combustion was incomplete (ESI data, Fig. S1†). However, this effect was overcome with increased engine load and at a high CR 17.5.28–30
Further, from Fig. 3(b) it was seen that for all test fuels, the net heat release (NHR) were increasing from compression stroke to power stroke and there was no observable difference in the trends of NHR at CR 17.5. However, as expected for MHBF tested at CR 15, the NHR pattern was significantly different and unveiled late combustion on crank angle 376° (also refer to Fig. 3(a)). Nonetheless, the maximum heat release rate (NHRmax) was found to be higher (45.48 J at CA 363°) for MHBF and lowest (33.24 J at CA 359°) for JCO B100. In fact, the ignition delay is dependent on latent heat of vaporization and auto ignition temperature of the fuel/air mixture; since, ethanol and 1-butanol have higher latent heat of vaporization as well as lower auto ignition temperature, this leads to premixed combustion and ignition delay in case of MHBF (the effect is more easily recognizable at CR 15, Fig. 3). Further, due to proper air fuel mixing, during this fraction of time, the net heat release as well as combustion pressure gets increased (Fig. 3).17 The nonlinear trend of combustion observed for MHBF could be ascribed to its multi component nature. Further, as the ignition delay increased, more fuel was required at a lower CR and the NHR was increased in combustion phase. This trend was clearly seen in the results where petrodiesel and biodiesel showed longer controlled combustion period and MHBF showed a longer premixed combustion period (Fig. 3(b)).22,35–37 Therefore, crank angle of 359–363° was optimum for combustion in CI engines at CR 17.5 with respect to no ignition delay.
Fig. 4 Variation of (a) CO (b) HC (c) CO2 and (d) NOx emission profiles with respect to load (at CR 15, the engine was operating at constant speed of 1500 rpm). |
Unburnt HC is another important parameter for determining the emission characteristics of a fuel, resulted from incomplete combustion in a CI engine due to inadequate combustion temperature and insufficient air supply. The solutions obtained in this study evidenced that, at CR 17.5, compared to the other test fuels, HC emission from MHBF was higher (at all load conditions). This observation could be accredited to the incomplete combustion of the heavier triglyceride component of MHBF at the end of combustion phase. Further, it was also observed from Fig. 4(b) that MHBF tested at CR 15 gave highest HC emission as combustion was incomplete (also refer to Section 3.3). The reason for the relatively higher HC of MHBF could be mainly attributed to the lower volatility of the high molecular weight triglyceride component of JCO as compared to the other carbon fuels as stated (petrodiesel, biodiesel and blends).27–30
CO2 is the byproduct of complete combustion and an increase of CO2 in the exhaust is an indication of proper combustion inside the CI engine combustion chamber. In case of incomplete combustion CO and HC emission always increases in the reciprocal rate of CO2.26,27 The variation of CO2 emissions with engine load have been illustrated in Fig. 4(c). For all the test fuels, the increasing trend of CO2 emission with increased engine load showed that the combustion was more effective at higher loads (also refer to Section 3.2). However, the CO2 emission for MHBF was fairly lower than all other test fuels (at all load conditions) as it was an oxygenated fuel, thus lesser amount of CO2 was produced as combustion byproduct. While the highest CO2 emission of petrodiesel at full load condition was at par with its complete combustion and non-oxygenated nature. Similarly, the other oxygenated fuels (JCO B100 and JCO B20) also exhibited reasonably reduced CO2 emissions. However, these values increased significantly and even outperformed petrodiesel at 60–80% load. Further, MHBF tested at CR 15 showed the similar CO2 emission trend, although it exhibited very high value of CO emission which may be again due to the incomplete combustion of this fuel at a lower compression ratio.27,29,30
The main sources of NOx emissions from IC engine are thermal NOx and fuel NOx. In fact, increased NOx emissions from traditional biofuels such as biodiesel have been one of the major bottlenecks in their large scale realization.4 As N content of all the tests fuels were insignificant, during our experiments thermal NOx (resulting from very high combustion temperature ≤ 1261 °C) was likely to be the most relevant source of emission.4,37 The rate of NOx formation is mainly dependent upon combustion (reaction) temperature, nitrogen residue in the air and the amount of oxygen content in the fuel.4,34 Here, similar trends of NOx emission were obtained for all the oxygenated fuels (JCO B100, JCO B20 and MHBF) that showed an increasing trend up to 80% load, after which it decreased slightly (at full load) (Fig. 4(d)). Conversely, the non-oxygenated petrodiesel was exceptional in this regard and showed an almost exponential increase of NOx emission at full engine load. The order of NOx emission can be related to the activation energy of the fuel, higher the activation energy higher is the NOx emission.17,33,34 The activation energy presented in Table 3 for the tested fuels and the NOx emission pattern as shown in Fig. 3 were in agreement with literature.12 The results obtained can be explained in terms of the reduced combustion temperatures for the oxygenated fuels and accordingly for MHBF (containing the highest amount oxygenate: ∼45% alcohol by volume) NOx emission was low (∼5 times lower than petrodiesel at full engine load).17,33,34 Further reduction of NOx was observed at CR 15, but this reduction was mainly due to incomplete combustion at low combustion pressure (also refer to Fig. 3 and Section 3.3).27–30
• The BSFC of MHBF was comparable with the other oxygenate fuels (JCO B100 and JCO B20) but slightly higher than petrodiesel under all engine load conditions.
• MHBF showed delayed and prolonged combustion at CR 15, but at CR 17.5 combustion of MHBF was sufficiently smooth and comparable to that of petrodiesel.
• The emissions levels (CO and CO2) of MHBF were comparable to that of petrodiesel for 0–80% load, while at full engine load the emissions reduced appreciably.
• HC emission for MHBF was higher than that for petrodiesel, JCO B100 and JCO B20, respectively, and was observed to increase with engine load at CR 17.5 and 15. The HC emission from MHBF could be attributed to the unburned triglyceride part of oil at the end of combustion. Nevertheless, HC emissions from MHBF could be reduced by operating the engine at a higher CR.
• A salient finding was that the NOx emissions from MHBF were considerably lower than all other test fuels (JCO B100, JCO B20 and petrodiesel) at all engine loads and CR. There was almost 5 fold reduction in NOx emissions observed for MHBF at full engine load and at CR 17.5 as compared to petrodiesel.
In summation, these results confirm the uniqueness and stability of MHBF as a fuel for CI engine application and can be recommended for application in engines with CR 17.5 without any modification of engine components.
MHBF | Microemulsion based hybrid fuel |
FFA | Free fatty acid |
JCO | Jatropha curcas oil |
FAME | Fatty acid methyl ester |
BP | Brake power |
MG | Monoglyceride |
CI | Compression ignition |
DG | Diglyceride |
BSFC | Brake specific fuel consumption |
TG | Triglyceride |
BTDC | Before top dead center |
CPmax | Maximum cylinder pressure |
FID | Flame ionization detector |
NHRmax | Maximum net heat release rate |
VCR | Variable compression ratio |
ηbte | Brake thermal efficiency |
CR | Compression ratio |
ηite | Indicated thermal efficiency |
CA | Crank angle |
ηmech | Mechanical efficiency |
DAD | Data acquisition device |
Cd | Coefficient of discharge of orifice |
rpm | Revolutions per minute |
Wden | Water density |
IP | Indicated power |
Aden | Air density |
HHV | High heating value |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra04795e |
This journal is © The Royal Society of Chemistry 2016 |