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Sedimentation in biodiesel and Ultra Low Sulfur Diesel Fuel blends
Fuel 90 (2011) 951–957
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Sedimentation in biodiesel and Ultra Low Sulfur Diesel Fuel blends M. Farahani a, D.J.Y.S. Pagé a,⇑, M.P. Turingia b a b
Department of Chemistry and Chemical Engineering, Royal Military College of Canada, Kingston, Ontario, Canada K7K 7B4 Director General Land Equipment Program Management, National Defence Headquarters, Ottawa, Ontario, Canada
a r t i c l e
i n f o
Article history: Received 14 August 2009 Received in revised form 21 October 2010 Accepted 22 October 2010 Available online 4 November 2010 Keywords: Biodiesel Ultra Low Sulfur Diesel Fuel Methyl esters Storage Free water and sediment
a b s t r a c t A biodiesel storage stability study was conducted on Ultra Low Sulfur Diesel Fuel (ULSDF) and three biodiesel fuels (B100), including a Tallow-based Methyl Ester (TME), a Canola-based Methyl Ester (CME), and, a Yellow Grease Methyl Ester (YGME), and fuel blends (B5 and B20). The stability study was conducted over ten months (Aug 07–Jun 08) and consisted of storing fuel samples in quarter filled plastic and steel 20 L cans, in an unheated shed. Fuel cans were vented twice a week to ensure exposure of the fuel to air. Changes in Acid Number, kinematic viscosity and free water and sediments levels were monitored over the storage period. Temperature and relative humidity ranged from 25 °C to 35 °C and 25% to 100%, respectively. Acid Number and the viscosity did not increase beyond the uncertainty of the method used. Reference samples kept at 40 °C were used for comparison. Free water and sediment levels were barely above the detection limit of 0.003 mL/100 mL of fuel for CME and YGME and their blends. TME and its B20 blend displayed free water and sediment levels up to 0.05 mL/100 mL of fuel in February, after six months of storage. These values decreased considerably after the warm Summer months back to below 0.01 mL/100 mL. All free water and sediment levels were measured after fuel samples came to equilibrium with room temperature. The TME B5 free water and sediment levels remained low throughout the storage period. Proton Nuclear Magnetic Resonance (NMR) spectroscopy performed on particulates from the sediments revealed a similar composition but a slightly lower concentration for protons in alkenyl groups. The presence of these sediments was attributed to more saturated molecules coming out of solution in colder weather and very slowly going back to solution at room temperature. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Increasing worldwide attention and planning have been focused on the promotion of renewable energy as replacement for more conventional sources such as natural gas, petroleum and coal. The reasons are multiple and include: reduction of greenhouse gas (GHG) emissions associated with climate change, poor air quality in large cities, rising oil prices and uncertainties shrouding the security of future petroleum supplies. Biofuels from indigenous sources, such as ethanol and biodiesel, have gained a substantial market share and lead the way in partial replacement or diluents of gasoline and petroleum diesel fuels, respectively, for a variety of engine applications. More specifically, the benefits in reduced emissions include lower levels of hydrocarbons, carbon monoxide and particulate matter. Unlike fossil fuels, CO2 emissions from renewable biofuels are considered neutral as long as the CO2 emitted, by producing and burning the biofuel, is balanced by a new plantation capable to recapture that CO2 through photosynthesis.
⇑ Corresponding author. Tel.: +1 613 541 6000x6353. E-mail address: [email protected] (D.J.Y.S. Pagé). 0016-2361/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2010.10.046
In an effort to reduce the environmental impact related to the use of petroleum products, the Canadian government announced in 2006 its intention to develop regulations leading to the addition of a renewable fuel component into petrodiesel [1]. This effort will translate into putting in place a requirement for an average 2% renewable fuel content in diesel fuel and heating oil by 2012. This planned measure is similar to the ones taken or planned by many other countries including China, India, Brazil, Japan, the United States and the European Union. Biodiesel, a long chain fatty acid alkyl ester derived from basecatalyzed transesterification of vegetable oils and animal fats, has assumed the leading supplementary role to the conventional diesel fuel in many parts of the world. Biodiesel shares many common physical and chemical features with conventional diesel fuel, and as such, the two are compatible at any mixture ratio. Many different biosources are considered around the world to produce biodiesel [2]. The most common form of biodiesel in Asia originates from Palm, in the European Union it originates from rapeseed oil, while soybean oil is the predominant feedstock in the US. Production of biodiesel from waste cooking oils and animal fats has also come to the forefront as a way to reduce raw material costs and alleviate waste oil disposal [3,4]. A comprehensive review spanning all the
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aspect of biodiesel from production to combustion has been compiled [5]. The biodiesel fuels mostly supplied currently in Canada are methyl esters of Canola, Soybean, Tallow and Yellow Grease. Based on the 2009 price of petrodiesel, biodiesel is still not a cost effective replacement for petrodiesel, but makes sense as a fuel additive or blend component with petrodiesel in order to reduce GHG emissions, improve the lubricity of Ultra Low Sulfur Diesel Fuel (ULSDF), and reduce fossil fuel dependency. Biodiesel supply and sustainability is also an issue. Based on a study done for the federal government [6], biodiesel production in Canada could only amount to 2% of the diesel consumption of the country. When comparing feed cost, it is much more effective to produce a biodiesel from waste vegetable oils and fats, which is about half the cost of a biodiesel produced from an edible crop like Canola [7]. The advantage of Canola methyl ester (CME) is its cloud point that is below freezing, which is better suited to the Winter temperatures of North America. The spectrum of advantages inherent to biodiesel, ranges from environmental issues to economic and strategic benefits. The chemical and physical properties such as cetane number, lubricity, absence of sulfur and aromatic components, somewhat offset some long term stability issues. Biodiesel is more susceptible to oxidative attack than diesel fuel [8,9] and in the advanced stages, this process may render the biodiesel unsuitable for use in modern diesel engines. The rate of oxidation is largely determined by the degree of unsaturation in the fatty acid alkyl ester chain [10], water content and environmental factors such as temperature, nature of the storage container [11] and air exposure [12] during storage. The end product of oxidation includes short chain carboxylic acids and polymeric sediments, resulting in an increase in total acid number and viscosity of biodiesel under storage. These negative impacts on the chemical and physical properties of biodiesel can be accelerated if the initial oils and fats used in its production originated from cooking waste [13]. The addition of synthetic antioxidants to biodiesel has proven effective in countering the oxidative degradation and in increasing the stability of the fuel [10,13,14]. In response to environmental concerns, several countries have imposed dates for lower sulfur content levels in diesel fuels. The maximum sulfur content in North America for ULSDF is currently 15 ppm. However, a limit of 50 ppm is also in effect or being implemented in many parts of the world [15]. The hydrodesulfurization process of diesel fuels leads to lower lubricity [16], which affects fuel pumps and injectors. Biodiesel has been proposed as a potential component to ULSDF to alleviate low lubricity performance [17]. The instability of biodiesel in blends with ULSDF is also a matter of concern [10], which has not been fully explored due to the recent introduction of ULSDF. A particular challenge to the implementation of biodiesel in Canada is the cold temperatures leading to the formation of sediments in biodiesel blends with petrodiesel. This problem is further compounded by the fact that some sediments appear at tempera-
tures above the cloud point of the fuel. The compounds responsible for those sediments have low solubility in biodiesel and petrodiesel components and they have been identified for the most part as Saturated Mono-Glycerides (SMG) and Sterol Glucosides [18–20]. These compounds tend to form hydrogen bonds with water and glycerol and come out of solution to form particles in the fuel at temperatures above the cloud point of the fuel. These particles can clog fuel filters, which can cause problems in the fuel supply system and premature pump failures. Also, those particles tend to not burn cleanly, resulting in deposits on injectors or in the engine cylinders, thereby reducing the effectiveness of the system. The aim of this investigation was to study the stability of methyl ester biodiesels from three sources: Canola oil, Tallow and a Yellow Grease. B5 and B20 blends with ULSDF were also investigated. Acid number (ASTM D664-04), kinematic viscosity (ASTM D445) and free water and sediment (ASTM D2709), were monitored over a ten month period, with samples stored in quarter filled 20 L steel and plastic jerry cans. Storage was done in a secured unheated fuel shed. Jerry cans were vented twice per week to ensure a regular exposure to fresh air, and fuel sampling was performed every two months for analysis. Temperature and relative humidity were recorded electronically every 4 h throughout the storage period. Studied fuels were also submitted to Nuclear Magnetic Resonance (NMR) spectroscopy in order to detect potential changes in the chemical structure of the fuels. 2. Experimental 2.1. Fuels and blends ULSDF was obtained from Shell Canada and blended with biodiesels from three different sources: Tallow-based Methyl Ester fuel (TME); Yellow Grease-based Methyl Ester fuel (YGME); and, a Canola-based Methyl Ester fuel (CME). The biodiesels were obtained from Canadian suppliers but their names were withheld due to confidentiality agreements. All fuels were studied as received without modification. The blends with ULSDF were comprised of 5% (B5) and 20% (B20) of all three biodiesels (v/v). All biodiesel fuels and blends with ULSDF were initially tested against specification requirements (ASTM D6751-07, CAN/CGSB 3.520-2005 and CID A-A-59693A January 15, 2004) as listed in Table 1. Some fuel testing was contracted out to Quality Engineering Test Establishment (QETE) in Ottawa and included Cloud point (ASTM D5773), Oxidation Stability (D2274) and Sulfur Content (D5453). All biodiesel fuels and blends fell within stated specification limits. 2.2. Storage conditions The three biodiesels (B100), ULSDF and their blends (B20 and B5) were each stored both in a quarter filled steel can and a plastic polyethylene (PE) fuel can. The 20 L steel cans were supplied by Briggs and Stratton (Boucherville, QC, Canada) and the 20 L plastic
Table 1 Specification requirements for selected properties of biodiesel and blends.
*
Property
ASTM
Units
ASTM D6751-07
CID A-A-59693
CAN/CGSB-3.520
Fuel type Acid number Cloud point Kinematic viscosity Oxidation stability* Sulfur content Water and sediment
D664 D974 D5773 D445 D2274 D5453 D2709
mg KOH g1 °C cSt (mm2 s1) mg/100 mL % by mass mL/100 mL
B100 0.5 max Report 1.9–6.0 – 0.0015 max 0.05 max
B20 0.2 max Report 1.3–4.1 – 0.0015 max 0.05 max
B1–B5 0.1 max – 1.7–4.1 – 0.0015 max 0.05 max
Caterpillar recommends a maximum of 15 mg/100 mL for their diesel engines.
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cans were supplied by Scepter (Scarborough, Ontario, Canada). The steel and plastic cans, containing the fuel samples, were stored in an unheated shed on the grounds of the Royal Military College of Canada (RMC/CMR) in the vicinity of Lake Ontario for a period of ten months starting in August 2007. In addition, TME, CME, YGME and ULSDF were also stored in Brown QorpakÒ Bottles (Fisher Scientific Co., Whitby, Ontario, Canada) and maintained at 40 °C in a laboratory oven. The steel and plastic fuel cans with all three B100s and their B20 and B5 blends, in addition to ULSDF were subjected to the environmental temperatures and humidity experienced in the period August 2007 to June 2008 in the fuel shed. The temperature and relative humidity in this time period were recorded every 4 h (24/7) using a VeriteqÒ temperature and humidity logger (Veriteq Instruments Inc., Richmond, British Colombia, Canada, model SP2000-20R, calibration certificate No. 0123554). The steel and plastic fuel cans undergoing environmental study and those stored at laboratory temperature were vented for a period of 5 min twice weekly to balance oxygen and humidity levels with their storage environment. Cans were turned upside down and vigorously shaken before each fuel sample collection.
fuged at a relative centrifugal force of 800 for 10 min at 21–25 °C in a centrifuge tube readable to 0.005 mL and measurable to 0.01 mL. After centrifugation, the volume of free water and sediment which has settled into the tip of the centrifuge tube is read to the nearest 0.005 mL. A Benchmark 2000 centrifuge (L-K Industries Inc., Houston, TX, USA, model Lab-C115C) was employed to generate the 800 relative centrifugal force at 1710 rpm. Trace sediment tubes allowed the measurement of free water and sediment to 0.0025 mL level, well below the ASTM standard requirement of 0.005 mL.
2.3. Acid number
The period for the fuel storage stability in an oxidative environment extended for ten months from Aug 07 to Jun 08. The temperature and the humidity in the outdoor storage facility, was recorded every four hours using an electronic device. Temperature and relative humidity ranged from 15 °C to 35 °C and 25% to 100%, respectively. In Fig. 1, relative humidity, absolute humidity and temperature was reported as a function of a full calendar year. The relative humidity was generally high due to the fact that the storage facility was 60 feet from the shore line of Lake Ontario. The relative humidity also appeared to be highest during the cooler months of Winter. The corresponding absolute humidity varied from 5 mg of water per liter of air during the Winter to 15 mg/L during the Summer.
The ASTM D 664-04 standard test method was used in the determination of the Acid Number (AN). This test method covers procedures for the potentiometric determination of acidic constituents in petroleum products soluble or nearly soluble in mixtures of toluene and propan-2-ol with alcoholic potassium hydroxide (KOH). The test method may be used to indicate relative changes that occur in oil during use under oxidizing conditions. The range of acid numbers included in the precision statement is 0.1– 150 mg g1 KOH. High purity grade toluene and propan-2-ol were obtained from Caledon Laboratories Ltd. (Georgetown, Ontario, Canada). The electrode system (ROSS Sure-Flow combination glass body pH electrode, model 81-72) and bench top pH meter (model 420Aplus) were obtained from Thermo Orion (Beverly, MA, USA). Buffer solutions (pH 4, 7, and 10) and electrode storage solutions were all obtained from Caledon Laboratories Ltd. (Georgetown, Ontario, Canada). For each AN determination, approximately 20 g of fuel or fuel blend was utilized. The uncertainty for acid numbers in this study ranges from 3% to 13% for Bio1 (Tallow)-B100 and ULSDF, respectively. 2.4. Kinematic viscosity The ASTM D 445-04 standard test method was used in the determination of the kinematic viscosity. This test method provides a procedure for the determination of kinematic viscosity of distillate fuels in a capillary viscometer. Three ubbelode viscometers with capillary constant of K = 0.01 (Schott Instruments, GmbH, Mainz, Germany) were used to determine the kinematic viscosities of Biodiesels and their blends with Diesel fuel at 40.0 ± 0.1 °C. These viscometers were calibrated with S6 viscosity reference standard (ISO 17025; certificate number, KA0598, Koehler Instrument Co. Inc., Bohemia, NY, USA). The viscosity measurements were carried out in a Lauda PVS1 automatic viscometer system, comprised of Lauda Clear-View thermostat (D 15 KP) with Edition 2000 temperature controller (Lauda, Germany). The uncertainty for kinematic viscosity values in this study remained below 0.4%.
2.6. Nuclear Magnetic Resonance (NMR) The 1H NMR spectra were measured in deuterated chloroform (CDCl3; 0.05% TMS, CDN Isotopes, Pointe-Claire, QC, Canada) on a Varian 300-MHz spectrometer. The uncertainty for NMR peak integration values, in this study, is below 1%. 3. Results and discussion 3.1. Temperature and humidity
3.2. Initial fuel characterization Characterization data for the three biodiesel fuels is provided in Table 2. The B100 data presented in Table 2 meets the standard for biodiesel stipulated under ASTM D6751-07 and provide a basis for comparison of the properties for the base fuels studied. Although cloud points for TME and YGME B100s were 15 °C and 12 °C higher than for CME respectively, the cloud points for all B20s were within
100
80
60
40
20
0
-20 06/08/2007
04/11/2007
02/02/2008
02/05/2008
31/07/2008
2.5. Free water and sediment Month
The ASTM D 2709-96 standard test method was used in the determination of the volume of free water and sediment in middle distillate fuels by centrifuge. A 100-mL sample of the fuel is centri-
0
3
6
9
12
Fig. 1. Temperature in °C (gray diamonds), absolute humidity in mg/L (black line) and relative humidity in% (black circles) measured in the storage fuel facility from August 2007 to August 2008 (Month 12). All values are reported on the same scale.
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Table 2 Characterization of the base fuels and blends used in this study. Kinematic viscosity (mm2 s1) ASTM D445
Sulfur content (wt.%) ASTM D5453
Oxidation stability (mg/100 mL) ASTM D2274
12.9 2.2 4.5 10.6 3.9 4.8 2.2 4.1 5.5 26
4.65 3.11 2.85 4.39 3.06 2.84 4.41 3.06 2.85 2.82
0.0016 0.0006 0.0004 0.0008 0.0004 0.0004 0.0005 0.0004 0.0003 0.0003
0.12 0.10 0.02 0.10 0.08 0.01 0.09 0.22 0.01 0.01
Acid No (/mg KOH.g -1)
Cloud point (°C) ASTM D5773
0.4
0.3
0.2
0.1
0 0
1 °C of each other. Cloud points for B5s were also within 1 °C of each other. Sulfur content of all fuels and blends was consistent with levels expected in ULSDF. Viscosity measurements were performed on fuel samples stored in the fuel shed throughout the study period. All tests on fuels (including ULSDF, B100s-TME, CME, and YGME, and their blends) were conducted at 40 °C. No significant changes in viscosity were observed as shown in Fig. 2, indicating that this fuel property is very stable under the studied conditions. 3.3. Acid number Figs. 3–5 show the AN throughout the storage period for all biodiesel fuels, namely TME, YGME and CME and their B20 and B5 blends with ULSDF respectively. With respect to B100s, YGME displayed the highest Acid Number on arrival at 0.48 mg KOH.g1, followed by TME at 0.28 mg KOH g1, CME at 0.20 mg KOH g1, and ULSDF at 0.05 mg KOH g1. The AN remained stable throughout the study for all fuels samples stored in the fuel shed. No significant differences were observed between samples stored in plastic or steel cans. Biodiesel samples stored in glass bottles in the oven at 40 °C, displayed an increase in AN, which is consistent with a previous study [21]. TME displayed the highest increase in AN reaching 0.50 mg KOH g1 after 10 months. This corresponded to a change in AN of 0.2 mg KOH g1 in comparison to a change of only 0.1 mg KOH g1 for CME and YGME.
2
4
6 Period (/month)
8
10
12
Fig. 3. Acid Number of TME-ULSDF blends as a function of storage period (Aug 07– Jun 08) for B100 in steel (j), in plastic (h), for B20 in steel (), in plastic (}), for B5 in steel (d), and plastic (s), and for B100 stored in glass bottles at 40 °C (N). Lines indicate trends only.
0.6
0.5
Acid No (/mg KOH.g -1 )
TME-B100 TME-B20 TME-B5 YGME-B100 YGME-B20 YGME-B5 CME-B100 CME-B20 CME-B5 ULSDF
0.5
0.4 0.3 0.2
0.1 0 0
2
4
6 Period (/month)
8
10
12
Fig. 4. Acid Number of YGME-ULSDF blends as a function of storage period (Aug 07–Jun 08) for B100 in steel (j), in plastic (h), for B20 in steel (), in plastic (}), for B5 in steel (d), and plastic (s), and for B100 stored in glass bottles at 40 °C (N). Lines indicate trends only.
0.5
Acid No (/mg KOH.g -1 )
4.7
2
-1
Kinematic Viscosity(/mm .s )
B100 4.3
3.9
3.5
0.4
0.3
0.2
0.1
0
B20
3.1
0
2
4
6 Period (/month)
8
10
12
B5 2.7 0
2
4
6
8
10
Storage Period (/month) Fig. 2. Kinematic viscosity measured at 40 °C as a function of the storage period (Aug 07–Jun 08) in the fuel shed for TME and B20, B5 blends (h), YGME and B20, B5 blends (N), CME and B20, B5 blends (). The average between samples taken from plastic and steel cans is reported.
Fig. 5. Acid Number of CME-ULSDF blends as a function of storage period (Aug 07– Jun 08) for B100 in steel (j), in plastic (h), for B20 in steel (), in plastic (}), for B5 in steel (d), and plastic (s), and for B100 stored in glass bottles at 40 °C (N). Lines indicate trends only.
With respect to B20 blends (20% biodiesel in ULSDF), B20-YGME displayed the highest initial AN with a value of 0.17 mg KOH g1,
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Figs. 6–8 show the free water and sediment levels measured in TME, YGME, CME respectively and their B20 and B5 blends throughout the storage period and against the averaged weekly storage temperature. The free water and sediment values were taken from both steel and plastic fuel cans. Free water and sediment values were measured after the fuel came to equilibrium with the room temperature, which occurred after several hours in the Winter months. Free water and sediment values were recorded in the fuel following storage in the cooler months, a process that reversed itself with the arrival of the warmer Summer months. Generally,
30
Temperature (oC)
25
0.05
20 0.04
15 10
0.03
5
0.02
0 0.01
-5 -10
Free Water & Sediment (mL/100mL)
0.06
0 2
4
25 20 0.02
15 10 5
0.01
0 -5 -10
Free Water & Sediment (mL/100mL)
0.03
0 0
2
4 6 8 Storage Period (Months)
10
12
Fig. 8. Free water and sediment measured at room temperature for CME-ULSDF blends as a function of storage period (Aug 07–Jun 08) and the storage temperature (thick line) for B100 (), B20 (h) and B5 (N). Other lines indicate trends only.
3.4. Free water and sediment
0
30
Temperature (°°C)
followed by B20-TME at 0.12 mg KOH g1 and B20-CME at 0.11 mg KOH g1. No significant differences were observed between samples stored in plastic or steel cans. The AN remained stable throughout the study for all fuels samples. With respect to B5 blends, B5-YGME displayed the highest initial AN with a value of 0.10 mg KOH g1, followed by B5-CME at 0.07 mg KOH g1 and B5-TME at 0.06 mg KOH.g1. No significant differences were observed between samples stored in plastic or steel cans. The AN remained stable over the reported period for all fuel samples. Although YGME displayed the highest AN, all biodiesels met the 0.5 mg KOH g1 limit stipulated in ASTM D6751-07 and all B20 blends met the 0.2 mg KOH g1 limit stated under A-A-59693A. All B5 blends displayed AN values comparable to ULSDF, which met the 0.1 mg KOH g1 limit stated under CAN/CGSB 3.520.
6
8
10
12
Storage Period (Months) Fig. 6. Free water and sediment measured at room temperature for TME-ULSDF blends as a function of storage period (Aug 07–Aug 08) and the storage temperature (thick line) for B100 (), B20 (h) and B5 (N). Other lines indicate trends only.
the free water and sediment profile was reversed from the storage temperature profile. No free water and sediment values were observed for any of the fuels or blends aged in the oven at 40 °C. For fuels aged in the shed, ULSDF displayed no free water and sediment value, whereas CME and YGME displayed values just above the detection limit. TME displayed the most free water and sediment values up to 0.05 mL/100 mL after six months, which corresponded to the cooler months as indicated in Fig. 1. The free water and sediment level decreased to below 0.01 mL/100 mL after the warm Summer months suggesting that the increase of free water and sediment values is a reversible process for the most part. B20-CME and B20-YGME displayed free water and sediment values just at the detection limit. B20-TME displayed the highest values up to 0.05 mL/100 mL after 6 months which corresponded to the cooler months. Interestingly, the B20 of TME produced just as much free water and sediment as the B100 under the same storage conditions. The free water and sediment level decreased to below 0.01 mL/100 mL after the warm Summer months. Similar to B100s, the appearance and disappearance of free water and sediment levels indicate a very slow process that can occur over long periods of time, such as several hours and days. Work is underway to determine the kinetics of formation of these sediments and their composition. Note that free water and sediment levels for all B5s were measured just above the detection limit. The reversible free water and sediment formation behavior described above with B100s and B20s was not observed for the B5s.
30 25
Temperature (°°C)
20 0.02
15 10 5
0.01
0 -5 -10
0 0
2
4
6
8
10
Free Water & Sediment (mL/100mL)
0.03
12
Storage Period (Months) Fig. 7. Free water and sediment measured at room temperature for YGME-ULSDF blends as a function of storage period (Aug 07–Jun 08) and the storage temperature (thick line) for B100 (), B20 (h) and B5 (N). Other lines indicate trends only.
Free Water and Sediment (mL/100mL)
0.06 B100(TME)
0.05
B20(TME) B5(TME)
0.04
B100(YGME) B100(CME)
0.03
0.02
0.01
0 -20
-10
0
10
20
30
T Storage - Cloud Point (°°C) Fig. 9. Free water and sediment as a function of the (storage temperature – cloud point (CP)) for the B100, B20 and B5 for TME, B100 for YGME and B100 for CME (CPTME = 12.9 °C, CPYGME = 10.6 °C, CPCME = 2.2 °C).
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It should be noted that the free water and sediment levels in B20s started to appear later in the cooler season than for B100s and started to disappear earlier than for the B100s. Overall, the period with higher free water and sediment levels was longer for the B100s than for the B20s. This is consistent with the cloud points for the B100, B20 and B5 for TME, which are 12.9 °C, 2.2 °C and 4.5 °C, respectively. In Fig. 9, free water and sediment is reported as a function of the (storage temperature – the cloud point) for the respective B100 fuels. Free water and sediment levels are highest for the B100 and B20 from TME when the storage temperature was around or below the cloud point for the respective fuel. These two fuels were the only ones that showed significant amount of free water and sediment during storage when temperatures, up to ten degrees above their cloud points, were encountered. Free water and sediment levels for the B5 remained low even when temperatures were around the cloud point. Free water and sediment levels were not a concern for the other fuels and their blends as indicated in Figs. 7 and 8 for B100s of YGME and CME, which had cloud points of 10.6 °C and 2.2 °C, respectively. 3.5. Proton Nuclear Magnetic Resonance (NMR) Proton NMR spectra taken both from the fuel and particulate, collected from the free water and sediment testing, showed a similar chemical group composition. However, differences between NMR spectra indicate the presence of more saturated molecules in the sediments when compared with their respective fuels. Proton NMR spectra were taken for the three B100s. A summary of the results are presented in Table 3. The information can be read in the table columns from left to right as: the position of every peak on the ppm scale of the spectrum, the position of the protons (in bold) in the chemical structure associated with each peak, and the integration value under each peak. The integration provides the relative chemical group composition of the three biodiesel fuels. For example, the level of saturation of each fuel, associated with lower levels of alkenyl groups (carbon double bonds), is obtained from the last row. TME is the most saturated fuel having the lowest integration value of 4.05 for protons involved in carbon double bonds. In comparison, CME is the least saturated and has twice as many double bonds in its average fatty acid carbon chain than for TME, while YGME is slightly less saturated than TME. The NMR results imply that CME should be more sensitive to oxidation leading to acidifying reactions in accordance with accelerated oxidation studies that found that less saturated fatty acid chains are richer in allylic and bis-allylic group and more prone to react with oxygen [22,23]. However, in this study, CME stored under normal environmental conditions did not display a lower stability than the other two biodiesel fuels. No significant changes in chemical composition were reported for any of the fuels throughout any of the storage conditions. This result supports the observation on the stability of the fuels’ kinematic viscosities
Table 3 Summary of Proton NMR results for TME, CME and YGME. The columns read, from left to right as: the peak position in chemical shifts d (ppm) relative to the internal protons of Tetramethylsilane (TMS), the corresponding proton position in the chemical structure and the integration values under each peak. d (ppm)
Functionality
TME
CME
YGME
0.85 1.30 1.60 2.05 2.25 2.75 3.65 5.35
–CH3 –(CH2)–n –CH2–CH2–CO2R –CH2–CH@CH–CH2–CH@CH–CH2– –CH2–CO2R –CH@CH–CH2–CH@CH– –O–CH3 –CH@CH–
7.68 60.88 5.79 6.62 5.84 0.72 8.43 4.05
7.08 52.15 6.03 10.80 5.86 2.22 8.46 7.41
7.84 59.20 5.77 7.22 5.85 0.89 8.63 4.59
Table 4 Proton NMR integration for the peak at 5.35 ppm, representative of the level of unsaturation per fatty acid chain for the biodiesel fuels and their sediments.
TME-B100 TME-Sediment YGME-B100 YGME-Sediment CME-B100 CME-Sediment
Integration at 5.35 ppm
Change in%
4.05 3.93 4.59 3.98 7.41 7.18
7 14 12
but does not contradict the observed changes in AN after storage at 40 °C, which was previously observed [21]. The precision of the integration under the NMR peaks permits quantification of the concentration of chemical changes in the order of 102–103. On the other hand, AN permits quantification of the concentration of chemical changes down to the order of 104–105. The concentration of the chemical changes observed with the AN were in the order of 103–104, therefore falling just under what can be detected with NMR. Particulate from the sediments found in the fuels were dissolved in deuterated chloroform (CDCl3) and characterized using Proton NMR. The results were compared with those of the base fuels, which are summarized in Table 4. At a first glance, the integration values show that there are no fundamental differences between the molecules composing each sediment from those of its respective base fuel. However, a closer look at some peaks does point towards a key difference. The peaks at 5.35 ppm are directly proportional to the number of unsaturated bonds in the structure as described in Table 3. The integrations at 5.35 ppm provided a basis to compare the level of saturation of the molecules in the base fuel and the sediment. As seen in Table 4 those values are all smaller for the sediments found in the fuels. These results indicate that the sediments are formed by a higher number of the more saturated ester chains.
4. Conclusion A biodiesel storage stability study was conducted at RMC on ULSDF petrodiesel and three B100s and fuel blends (B5 and B20). The storage stability study consisted of storing fuel samples in quarter filled plastic and steel Jerry cans, in a fuel shed from August 2007 to June 2008. Temperature and relative humidity in the outdoor storage facility ranged from 25 °C to 35 °C and 25% to 100%, respectively. Three biodiesel fuels were studied, a Tallow-based Methyl Ester (TME), a Canola-based Methyl Ester (CME), and, a Yellow Grease Methyl Ester (YGME). All fuels and blends proved to be very stable throughout the storage period. Acid Number and the viscosity did not increase beyond the uncertainty of the method used. Reference samples kept at 40 °C in the same conditions as the previous study, displayed an increase in AN comparable to what was previously reported [21]. Free water and sediment levels were barely above the detection limit of 0.003 mL/100 mL of fuel for CME and YGME and their blends. TME and its B20 blend displayed free water and sediment levels up to 0.05 mL/100 mL of fuel in February, after six months. Free water and sediment levels for the B20 were comparable to B100 levels when the storage temperature was below the cloud point of the fuel. Those levels decreased considerably after the warm Summer months and fell back to below 0.01 mL/100 mL. B5 free water and sediment levels remained low even when temperatures were below the cloud point. For the most part the B5s studied are stable over long term storage.
M. Farahani et al. / Fuel 90 (2011) 951–957
Proton NMR spectroscopy was used to identify chemical structure differences namely the levels of unsaturation among the three biodiesel base fuels. CME was the least saturated and TME was the most saturated. No significant changes were detected in the chemical structure of the fuels and blends throughout storage. NMR performed on the sediments revealed a similar composition but a lower concentration of alkenyl groups than for the base fuels. The presence of those sediments was attributed to more saturated molecules coming out of solution in colder weather and very slowly going back to solution at room temperature. Acknowledgements The research work was funded by the Director General Land Equipment Program Management of National Defence Headquarters, Ottawa. Authors are thankful for the technical support provided by Dr. Neda Bavarian of RMC and Mrs. Anelia Krasteva from the Quality Engineering Test Establishment. References [1] Government of Canada, Department of the Environment. Notice of intent to develop a federal regulation requiring renewable fuels, Canada gazette, part I, vol.140, no.52; 30 December 2006. [2] Binfield J et al. US and world agricultural outlook, food and agricultural policy research institute. Iowa, USA: Ames; 2009. p. 223–265. [3] Ma F, Hanna MA. Biodiesel production, a review. Bioresour Technol 1999;70:1–15. [4] Knothe G. Some aspects of biodiesel oxidative stability. Fuel Process Technol 2007;88:669–77. [5] Knothe G, Dunn RO. Biofuels derived from vegetable oils and fats. In: Gunstone FD, Hamilton RJ, editors. Oleochemical manufacture and applications. Sheffield, UK: Academic Press; 2001. p. 106–63. [6] Praksash CB. A critical review of biodiesel as a transportation fuel in Canada report prepared by global Change Strategies International Inc. for Environment Canada; March 25th, 1998. p. 15–16.
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[7] Zhang Y, Dubé MA, McLean DD, Kates M. Biodiesel production from waste cooking oil: 2. Economic assessment and sensitivity analysis. Bioresour Technol 2003;90:229–40. [8] Waynick JA. Characterization of biodiesel oxidation and oxidation products, technical literature review, southwest research institute project no. 08-10721, National Renewable Energy Laboratory, US Department of Energy, (TP-54039096); 2005. [9] Yamane K, Kawasaki K, Sone K, Hara T, Prakoso T. Oxidation stability of biodiesel and its effect on diesel combustion and emission characteristics. Int J Engine Res 2007;8:307–19. [10] Stauffer EC. Fats and oils. St. Paul, Minnesota: Eagen Press; 1996. [11] Bondioli P, Gasparoli A, Lanzani A, Fedeli E, Veronese S, Sala M. Storage stability of biodiesel. J Am Oil Chem Soc 1995;72:699–702. [12] Leung DYC, Koo BCP, Guo Y. Degradation of biodiesel under different storage conditions. Bioresour Technol 2006;97:250–6. [13] Loh S-K, Chew S-M, Choo Y-M. Oxidative stability and storage behaviour of fatty acid methyl esters derived from used palm oil. J Am Oil Chem Soc 2006;83:947–52. [14] Polavka J, Paligova J, Cvengros J, Simon P. Oxidation stability of methyl esters studied by differential thermal analysis and rancimat. J Am Oil Chem Soc 2005;82:519–24. [15] Peckam J. South America begins ULSD March, World Refining; July, 2004. [16] Anastopoulos G, Lois E, Karonis D, Kalligeros S, Zannikos F. Impact of oxygen and nitrogen compounds on the lubrication properties of low sulfur diesel fuels. Energy 2005;30:415–26. [17] Wain KS, Perez JM, Chapman E, Boehman AL. Alternative and low sulfur fuel options: boundary lubrication performance and potential problems. Tribol Int 2005;38:313–9. [18] Lee I, Pfalzgraf LM, Poppe GB, Powers E, Haines T. The role of sterol glucosides on filter plugging, Biodiesel Magazine; April 2007. [19] Pfalzgraf L, Lee I, Foster J, Poppe G. The effect of minor components on cloud point and filterability, Biodiesel Magazine; November 2007. [20] Tang H, Salley SO, Ng KYS. Fuel properties and precipitate formation at low temperature in soy-, cottonseed-, and poultry fat-based biodiesel blends. Fuel 2008;87:3006–17. [21] Farahani M, Pagé DJYS, Turingia MP, Tucker BD. Storage stability of biodiesel and ultra low sulfur diesel fuel blends. J Energy Resour Technol 2009;131: 1–6. [22] Sendzikienne E, Makareviciene V, Janulus P. Oxidation stability of biodiesel fuel produced from fatty wastes. Pol J Environ Stud 2005;14:335–9. [23] Knothe G. Structure indices in FA chemistry. How relevant is the iodine value? J Am Oil Chem Soc 2002;79:847–54.
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Sedimentation in biodiesel and Ultra Low Sulfur Diesel Fuel blends M. Farahani a, D.J.Y.S. Pagé a,⇑, M.P. Turingia b a b
Department of Chemistry and Chemical Engineering, Royal Military College of Canada, Kingston, Ontario, Canada K7K 7B4 Director General Land Equipment Program Management, National Defence Headquarters, Ottawa, Ontario, Canada
a r t i c l e
i n f o
Article history: Received 14 August 2009 Received in revised form 21 October 2010 Accepted 22 October 2010 Available online 4 November 2010 Keywords: Biodiesel Ultra Low Sulfur Diesel Fuel Methyl esters Storage Free water and sediment
a b s t r a c t A biodiesel storage stability study was conducted on Ultra Low Sulfur Diesel Fuel (ULSDF) and three biodiesel fuels (B100), including a Tallow-based Methyl Ester (TME), a Canola-based Methyl Ester (CME), and, a Yellow Grease Methyl Ester (YGME), and fuel blends (B5 and B20). The stability study was conducted over ten months (Aug 07–Jun 08) and consisted of storing fuel samples in quarter filled plastic and steel 20 L cans, in an unheated shed. Fuel cans were vented twice a week to ensure exposure of the fuel to air. Changes in Acid Number, kinematic viscosity and free water and sediments levels were monitored over the storage period. Temperature and relative humidity ranged from 25 °C to 35 °C and 25% to 100%, respectively. Acid Number and the viscosity did not increase beyond the uncertainty of the method used. Reference samples kept at 40 °C were used for comparison. Free water and sediment levels were barely above the detection limit of 0.003 mL/100 mL of fuel for CME and YGME and their blends. TME and its B20 blend displayed free water and sediment levels up to 0.05 mL/100 mL of fuel in February, after six months of storage. These values decreased considerably after the warm Summer months back to below 0.01 mL/100 mL. All free water and sediment levels were measured after fuel samples came to equilibrium with room temperature. The TME B5 free water and sediment levels remained low throughout the storage period. Proton Nuclear Magnetic Resonance (NMR) spectroscopy performed on particulates from the sediments revealed a similar composition but a slightly lower concentration for protons in alkenyl groups. The presence of these sediments was attributed to more saturated molecules coming out of solution in colder weather and very slowly going back to solution at room temperature. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Increasing worldwide attention and planning have been focused on the promotion of renewable energy as replacement for more conventional sources such as natural gas, petroleum and coal. The reasons are multiple and include: reduction of greenhouse gas (GHG) emissions associated with climate change, poor air quality in large cities, rising oil prices and uncertainties shrouding the security of future petroleum supplies. Biofuels from indigenous sources, such as ethanol and biodiesel, have gained a substantial market share and lead the way in partial replacement or diluents of gasoline and petroleum diesel fuels, respectively, for a variety of engine applications. More specifically, the benefits in reduced emissions include lower levels of hydrocarbons, carbon monoxide and particulate matter. Unlike fossil fuels, CO2 emissions from renewable biofuels are considered neutral as long as the CO2 emitted, by producing and burning the biofuel, is balanced by a new plantation capable to recapture that CO2 through photosynthesis.
⇑ Corresponding author. Tel.: +1 613 541 6000x6353. E-mail address: [email protected] (D.J.Y.S. Pagé). 0016-2361/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2010.10.046
In an effort to reduce the environmental impact related to the use of petroleum products, the Canadian government announced in 2006 its intention to develop regulations leading to the addition of a renewable fuel component into petrodiesel [1]. This effort will translate into putting in place a requirement for an average 2% renewable fuel content in diesel fuel and heating oil by 2012. This planned measure is similar to the ones taken or planned by many other countries including China, India, Brazil, Japan, the United States and the European Union. Biodiesel, a long chain fatty acid alkyl ester derived from basecatalyzed transesterification of vegetable oils and animal fats, has assumed the leading supplementary role to the conventional diesel fuel in many parts of the world. Biodiesel shares many common physical and chemical features with conventional diesel fuel, and as such, the two are compatible at any mixture ratio. Many different biosources are considered around the world to produce biodiesel [2]. The most common form of biodiesel in Asia originates from Palm, in the European Union it originates from rapeseed oil, while soybean oil is the predominant feedstock in the US. Production of biodiesel from waste cooking oils and animal fats has also come to the forefront as a way to reduce raw material costs and alleviate waste oil disposal [3,4]. A comprehensive review spanning all the
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aspect of biodiesel from production to combustion has been compiled [5]. The biodiesel fuels mostly supplied currently in Canada are methyl esters of Canola, Soybean, Tallow and Yellow Grease. Based on the 2009 price of petrodiesel, biodiesel is still not a cost effective replacement for petrodiesel, but makes sense as a fuel additive or blend component with petrodiesel in order to reduce GHG emissions, improve the lubricity of Ultra Low Sulfur Diesel Fuel (ULSDF), and reduce fossil fuel dependency. Biodiesel supply and sustainability is also an issue. Based on a study done for the federal government [6], biodiesel production in Canada could only amount to 2% of the diesel consumption of the country. When comparing feed cost, it is much more effective to produce a biodiesel from waste vegetable oils and fats, which is about half the cost of a biodiesel produced from an edible crop like Canola [7]. The advantage of Canola methyl ester (CME) is its cloud point that is below freezing, which is better suited to the Winter temperatures of North America. The spectrum of advantages inherent to biodiesel, ranges from environmental issues to economic and strategic benefits. The chemical and physical properties such as cetane number, lubricity, absence of sulfur and aromatic components, somewhat offset some long term stability issues. Biodiesel is more susceptible to oxidative attack than diesel fuel [8,9] and in the advanced stages, this process may render the biodiesel unsuitable for use in modern diesel engines. The rate of oxidation is largely determined by the degree of unsaturation in the fatty acid alkyl ester chain [10], water content and environmental factors such as temperature, nature of the storage container [11] and air exposure [12] during storage. The end product of oxidation includes short chain carboxylic acids and polymeric sediments, resulting in an increase in total acid number and viscosity of biodiesel under storage. These negative impacts on the chemical and physical properties of biodiesel can be accelerated if the initial oils and fats used in its production originated from cooking waste [13]. The addition of synthetic antioxidants to biodiesel has proven effective in countering the oxidative degradation and in increasing the stability of the fuel [10,13,14]. In response to environmental concerns, several countries have imposed dates for lower sulfur content levels in diesel fuels. The maximum sulfur content in North America for ULSDF is currently 15 ppm. However, a limit of 50 ppm is also in effect or being implemented in many parts of the world [15]. The hydrodesulfurization process of diesel fuels leads to lower lubricity [16], which affects fuel pumps and injectors. Biodiesel has been proposed as a potential component to ULSDF to alleviate low lubricity performance [17]. The instability of biodiesel in blends with ULSDF is also a matter of concern [10], which has not been fully explored due to the recent introduction of ULSDF. A particular challenge to the implementation of biodiesel in Canada is the cold temperatures leading to the formation of sediments in biodiesel blends with petrodiesel. This problem is further compounded by the fact that some sediments appear at tempera-
tures above the cloud point of the fuel. The compounds responsible for those sediments have low solubility in biodiesel and petrodiesel components and they have been identified for the most part as Saturated Mono-Glycerides (SMG) and Sterol Glucosides [18–20]. These compounds tend to form hydrogen bonds with water and glycerol and come out of solution to form particles in the fuel at temperatures above the cloud point of the fuel. These particles can clog fuel filters, which can cause problems in the fuel supply system and premature pump failures. Also, those particles tend to not burn cleanly, resulting in deposits on injectors or in the engine cylinders, thereby reducing the effectiveness of the system. The aim of this investigation was to study the stability of methyl ester biodiesels from three sources: Canola oil, Tallow and a Yellow Grease. B5 and B20 blends with ULSDF were also investigated. Acid number (ASTM D664-04), kinematic viscosity (ASTM D445) and free water and sediment (ASTM D2709), were monitored over a ten month period, with samples stored in quarter filled 20 L steel and plastic jerry cans. Storage was done in a secured unheated fuel shed. Jerry cans were vented twice per week to ensure a regular exposure to fresh air, and fuel sampling was performed every two months for analysis. Temperature and relative humidity were recorded electronically every 4 h throughout the storage period. Studied fuels were also submitted to Nuclear Magnetic Resonance (NMR) spectroscopy in order to detect potential changes in the chemical structure of the fuels. 2. Experimental 2.1. Fuels and blends ULSDF was obtained from Shell Canada and blended with biodiesels from three different sources: Tallow-based Methyl Ester fuel (TME); Yellow Grease-based Methyl Ester fuel (YGME); and, a Canola-based Methyl Ester fuel (CME). The biodiesels were obtained from Canadian suppliers but their names were withheld due to confidentiality agreements. All fuels were studied as received without modification. The blends with ULSDF were comprised of 5% (B5) and 20% (B20) of all three biodiesels (v/v). All biodiesel fuels and blends with ULSDF were initially tested against specification requirements (ASTM D6751-07, CAN/CGSB 3.520-2005 and CID A-A-59693A January 15, 2004) as listed in Table 1. Some fuel testing was contracted out to Quality Engineering Test Establishment (QETE) in Ottawa and included Cloud point (ASTM D5773), Oxidation Stability (D2274) and Sulfur Content (D5453). All biodiesel fuels and blends fell within stated specification limits. 2.2. Storage conditions The three biodiesels (B100), ULSDF and their blends (B20 and B5) were each stored both in a quarter filled steel can and a plastic polyethylene (PE) fuel can. The 20 L steel cans were supplied by Briggs and Stratton (Boucherville, QC, Canada) and the 20 L plastic
Table 1 Specification requirements for selected properties of biodiesel and blends.
*
Property
ASTM
Units
ASTM D6751-07
CID A-A-59693
CAN/CGSB-3.520
Fuel type Acid number Cloud point Kinematic viscosity Oxidation stability* Sulfur content Water and sediment
D664 D974 D5773 D445 D2274 D5453 D2709
mg KOH g1 °C cSt (mm2 s1) mg/100 mL % by mass mL/100 mL
B100 0.5 max Report 1.9–6.0 – 0.0015 max 0.05 max
B20 0.2 max Report 1.3–4.1 – 0.0015 max 0.05 max
B1–B5 0.1 max – 1.7–4.1 – 0.0015 max 0.05 max
Caterpillar recommends a maximum of 15 mg/100 mL for their diesel engines.
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cans were supplied by Scepter (Scarborough, Ontario, Canada). The steel and plastic cans, containing the fuel samples, were stored in an unheated shed on the grounds of the Royal Military College of Canada (RMC/CMR) in the vicinity of Lake Ontario for a period of ten months starting in August 2007. In addition, TME, CME, YGME and ULSDF were also stored in Brown QorpakÒ Bottles (Fisher Scientific Co., Whitby, Ontario, Canada) and maintained at 40 °C in a laboratory oven. The steel and plastic fuel cans with all three B100s and their B20 and B5 blends, in addition to ULSDF were subjected to the environmental temperatures and humidity experienced in the period August 2007 to June 2008 in the fuel shed. The temperature and relative humidity in this time period were recorded every 4 h (24/7) using a VeriteqÒ temperature and humidity logger (Veriteq Instruments Inc., Richmond, British Colombia, Canada, model SP2000-20R, calibration certificate No. 0123554). The steel and plastic fuel cans undergoing environmental study and those stored at laboratory temperature were vented for a period of 5 min twice weekly to balance oxygen and humidity levels with their storage environment. Cans were turned upside down and vigorously shaken before each fuel sample collection.
fuged at a relative centrifugal force of 800 for 10 min at 21–25 °C in a centrifuge tube readable to 0.005 mL and measurable to 0.01 mL. After centrifugation, the volume of free water and sediment which has settled into the tip of the centrifuge tube is read to the nearest 0.005 mL. A Benchmark 2000 centrifuge (L-K Industries Inc., Houston, TX, USA, model Lab-C115C) was employed to generate the 800 relative centrifugal force at 1710 rpm. Trace sediment tubes allowed the measurement of free water and sediment to 0.0025 mL level, well below the ASTM standard requirement of 0.005 mL.
2.3. Acid number
The period for the fuel storage stability in an oxidative environment extended for ten months from Aug 07 to Jun 08. The temperature and the humidity in the outdoor storage facility, was recorded every four hours using an electronic device. Temperature and relative humidity ranged from 15 °C to 35 °C and 25% to 100%, respectively. In Fig. 1, relative humidity, absolute humidity and temperature was reported as a function of a full calendar year. The relative humidity was generally high due to the fact that the storage facility was 60 feet from the shore line of Lake Ontario. The relative humidity also appeared to be highest during the cooler months of Winter. The corresponding absolute humidity varied from 5 mg of water per liter of air during the Winter to 15 mg/L during the Summer.
The ASTM D 664-04 standard test method was used in the determination of the Acid Number (AN). This test method covers procedures for the potentiometric determination of acidic constituents in petroleum products soluble or nearly soluble in mixtures of toluene and propan-2-ol with alcoholic potassium hydroxide (KOH). The test method may be used to indicate relative changes that occur in oil during use under oxidizing conditions. The range of acid numbers included in the precision statement is 0.1– 150 mg g1 KOH. High purity grade toluene and propan-2-ol were obtained from Caledon Laboratories Ltd. (Georgetown, Ontario, Canada). The electrode system (ROSS Sure-Flow combination glass body pH electrode, model 81-72) and bench top pH meter (model 420Aplus) were obtained from Thermo Orion (Beverly, MA, USA). Buffer solutions (pH 4, 7, and 10) and electrode storage solutions were all obtained from Caledon Laboratories Ltd. (Georgetown, Ontario, Canada). For each AN determination, approximately 20 g of fuel or fuel blend was utilized. The uncertainty for acid numbers in this study ranges from 3% to 13% for Bio1 (Tallow)-B100 and ULSDF, respectively. 2.4. Kinematic viscosity The ASTM D 445-04 standard test method was used in the determination of the kinematic viscosity. This test method provides a procedure for the determination of kinematic viscosity of distillate fuels in a capillary viscometer. Three ubbelode viscometers with capillary constant of K = 0.01 (Schott Instruments, GmbH, Mainz, Germany) were used to determine the kinematic viscosities of Biodiesels and their blends with Diesel fuel at 40.0 ± 0.1 °C. These viscometers were calibrated with S6 viscosity reference standard (ISO 17025; certificate number, KA0598, Koehler Instrument Co. Inc., Bohemia, NY, USA). The viscosity measurements were carried out in a Lauda PVS1 automatic viscometer system, comprised of Lauda Clear-View thermostat (D 15 KP) with Edition 2000 temperature controller (Lauda, Germany). The uncertainty for kinematic viscosity values in this study remained below 0.4%.
2.6. Nuclear Magnetic Resonance (NMR) The 1H NMR spectra were measured in deuterated chloroform (CDCl3; 0.05% TMS, CDN Isotopes, Pointe-Claire, QC, Canada) on a Varian 300-MHz spectrometer. The uncertainty for NMR peak integration values, in this study, is below 1%. 3. Results and discussion 3.1. Temperature and humidity
3.2. Initial fuel characterization Characterization data for the three biodiesel fuels is provided in Table 2. The B100 data presented in Table 2 meets the standard for biodiesel stipulated under ASTM D6751-07 and provide a basis for comparison of the properties for the base fuels studied. Although cloud points for TME and YGME B100s were 15 °C and 12 °C higher than for CME respectively, the cloud points for all B20s were within
100
80
60
40
20
0
-20 06/08/2007
04/11/2007
02/02/2008
02/05/2008
31/07/2008
2.5. Free water and sediment Month
The ASTM D 2709-96 standard test method was used in the determination of the volume of free water and sediment in middle distillate fuels by centrifuge. A 100-mL sample of the fuel is centri-
0
3
6
9
12
Fig. 1. Temperature in °C (gray diamonds), absolute humidity in mg/L (black line) and relative humidity in% (black circles) measured in the storage fuel facility from August 2007 to August 2008 (Month 12). All values are reported on the same scale.
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Table 2 Characterization of the base fuels and blends used in this study. Kinematic viscosity (mm2 s1) ASTM D445
Sulfur content (wt.%) ASTM D5453
Oxidation stability (mg/100 mL) ASTM D2274
12.9 2.2 4.5 10.6 3.9 4.8 2.2 4.1 5.5 26
4.65 3.11 2.85 4.39 3.06 2.84 4.41 3.06 2.85 2.82
0.0016 0.0006 0.0004 0.0008 0.0004 0.0004 0.0005 0.0004 0.0003 0.0003
0.12 0.10 0.02 0.10 0.08 0.01 0.09 0.22 0.01 0.01
Acid No (/mg KOH.g -1)
Cloud point (°C) ASTM D5773
0.4
0.3
0.2
0.1
0 0
1 °C of each other. Cloud points for B5s were also within 1 °C of each other. Sulfur content of all fuels and blends was consistent with levels expected in ULSDF. Viscosity measurements were performed on fuel samples stored in the fuel shed throughout the study period. All tests on fuels (including ULSDF, B100s-TME, CME, and YGME, and their blends) were conducted at 40 °C. No significant changes in viscosity were observed as shown in Fig. 2, indicating that this fuel property is very stable under the studied conditions. 3.3. Acid number Figs. 3–5 show the AN throughout the storage period for all biodiesel fuels, namely TME, YGME and CME and their B20 and B5 blends with ULSDF respectively. With respect to B100s, YGME displayed the highest Acid Number on arrival at 0.48 mg KOH.g1, followed by TME at 0.28 mg KOH g1, CME at 0.20 mg KOH g1, and ULSDF at 0.05 mg KOH g1. The AN remained stable throughout the study for all fuels samples stored in the fuel shed. No significant differences were observed between samples stored in plastic or steel cans. Biodiesel samples stored in glass bottles in the oven at 40 °C, displayed an increase in AN, which is consistent with a previous study [21]. TME displayed the highest increase in AN reaching 0.50 mg KOH g1 after 10 months. This corresponded to a change in AN of 0.2 mg KOH g1 in comparison to a change of only 0.1 mg KOH g1 for CME and YGME.
2
4
6 Period (/month)
8
10
12
Fig. 3. Acid Number of TME-ULSDF blends as a function of storage period (Aug 07– Jun 08) for B100 in steel (j), in plastic (h), for B20 in steel (), in plastic (}), for B5 in steel (d), and plastic (s), and for B100 stored in glass bottles at 40 °C (N). Lines indicate trends only.
0.6
0.5
Acid No (/mg KOH.g -1 )
TME-B100 TME-B20 TME-B5 YGME-B100 YGME-B20 YGME-B5 CME-B100 CME-B20 CME-B5 ULSDF
0.5
0.4 0.3 0.2
0.1 0 0
2
4
6 Period (/month)
8
10
12
Fig. 4. Acid Number of YGME-ULSDF blends as a function of storage period (Aug 07–Jun 08) for B100 in steel (j), in plastic (h), for B20 in steel (), in plastic (}), for B5 in steel (d), and plastic (s), and for B100 stored in glass bottles at 40 °C (N). Lines indicate trends only.
0.5
Acid No (/mg KOH.g -1 )
4.7
2
-1
Kinematic Viscosity(/mm .s )
B100 4.3
3.9
3.5
0.4
0.3
0.2
0.1
0
B20
3.1
0
2
4
6 Period (/month)
8
10
12
B5 2.7 0
2
4
6
8
10
Storage Period (/month) Fig. 2. Kinematic viscosity measured at 40 °C as a function of the storage period (Aug 07–Jun 08) in the fuel shed for TME and B20, B5 blends (h), YGME and B20, B5 blends (N), CME and B20, B5 blends (). The average between samples taken from plastic and steel cans is reported.
Fig. 5. Acid Number of CME-ULSDF blends as a function of storage period (Aug 07– Jun 08) for B100 in steel (j), in plastic (h), for B20 in steel (), in plastic (}), for B5 in steel (d), and plastic (s), and for B100 stored in glass bottles at 40 °C (N). Lines indicate trends only.
With respect to B20 blends (20% biodiesel in ULSDF), B20-YGME displayed the highest initial AN with a value of 0.17 mg KOH g1,
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Figs. 6–8 show the free water and sediment levels measured in TME, YGME, CME respectively and their B20 and B5 blends throughout the storage period and against the averaged weekly storage temperature. The free water and sediment values were taken from both steel and plastic fuel cans. Free water and sediment values were measured after the fuel came to equilibrium with the room temperature, which occurred after several hours in the Winter months. Free water and sediment values were recorded in the fuel following storage in the cooler months, a process that reversed itself with the arrival of the warmer Summer months. Generally,
30
Temperature (oC)
25
0.05
20 0.04
15 10
0.03
5
0.02
0 0.01
-5 -10
Free Water & Sediment (mL/100mL)
0.06
0 2
4
25 20 0.02
15 10 5
0.01
0 -5 -10
Free Water & Sediment (mL/100mL)
0.03
0 0
2
4 6 8 Storage Period (Months)
10
12
Fig. 8. Free water and sediment measured at room temperature for CME-ULSDF blends as a function of storage period (Aug 07–Jun 08) and the storage temperature (thick line) for B100 (), B20 (h) and B5 (N). Other lines indicate trends only.
3.4. Free water and sediment
0
30
Temperature (°°C)
followed by B20-TME at 0.12 mg KOH g1 and B20-CME at 0.11 mg KOH g1. No significant differences were observed between samples stored in plastic or steel cans. The AN remained stable throughout the study for all fuels samples. With respect to B5 blends, B5-YGME displayed the highest initial AN with a value of 0.10 mg KOH g1, followed by B5-CME at 0.07 mg KOH g1 and B5-TME at 0.06 mg KOH.g1. No significant differences were observed between samples stored in plastic or steel cans. The AN remained stable over the reported period for all fuel samples. Although YGME displayed the highest AN, all biodiesels met the 0.5 mg KOH g1 limit stipulated in ASTM D6751-07 and all B20 blends met the 0.2 mg KOH g1 limit stated under A-A-59693A. All B5 blends displayed AN values comparable to ULSDF, which met the 0.1 mg KOH g1 limit stated under CAN/CGSB 3.520.
6
8
10
12
Storage Period (Months) Fig. 6. Free water and sediment measured at room temperature for TME-ULSDF blends as a function of storage period (Aug 07–Aug 08) and the storage temperature (thick line) for B100 (), B20 (h) and B5 (N). Other lines indicate trends only.
the free water and sediment profile was reversed from the storage temperature profile. No free water and sediment values were observed for any of the fuels or blends aged in the oven at 40 °C. For fuels aged in the shed, ULSDF displayed no free water and sediment value, whereas CME and YGME displayed values just above the detection limit. TME displayed the most free water and sediment values up to 0.05 mL/100 mL after six months, which corresponded to the cooler months as indicated in Fig. 1. The free water and sediment level decreased to below 0.01 mL/100 mL after the warm Summer months suggesting that the increase of free water and sediment values is a reversible process for the most part. B20-CME and B20-YGME displayed free water and sediment values just at the detection limit. B20-TME displayed the highest values up to 0.05 mL/100 mL after 6 months which corresponded to the cooler months. Interestingly, the B20 of TME produced just as much free water and sediment as the B100 under the same storage conditions. The free water and sediment level decreased to below 0.01 mL/100 mL after the warm Summer months. Similar to B100s, the appearance and disappearance of free water and sediment levels indicate a very slow process that can occur over long periods of time, such as several hours and days. Work is underway to determine the kinetics of formation of these sediments and their composition. Note that free water and sediment levels for all B5s were measured just above the detection limit. The reversible free water and sediment formation behavior described above with B100s and B20s was not observed for the B5s.
30 25
Temperature (°°C)
20 0.02
15 10 5
0.01
0 -5 -10
0 0
2
4
6
8
10
Free Water & Sediment (mL/100mL)
0.03
12
Storage Period (Months) Fig. 7. Free water and sediment measured at room temperature for YGME-ULSDF blends as a function of storage period (Aug 07–Jun 08) and the storage temperature (thick line) for B100 (), B20 (h) and B5 (N). Other lines indicate trends only.
Free Water and Sediment (mL/100mL)
0.06 B100(TME)
0.05
B20(TME) B5(TME)
0.04
B100(YGME) B100(CME)
0.03
0.02
0.01
0 -20
-10
0
10
20
30
T Storage - Cloud Point (°°C) Fig. 9. Free water and sediment as a function of the (storage temperature – cloud point (CP)) for the B100, B20 and B5 for TME, B100 for YGME and B100 for CME (CPTME = 12.9 °C, CPYGME = 10.6 °C, CPCME = 2.2 °C).
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M. Farahani et al. / Fuel 90 (2011) 951–957
It should be noted that the free water and sediment levels in B20s started to appear later in the cooler season than for B100s and started to disappear earlier than for the B100s. Overall, the period with higher free water and sediment levels was longer for the B100s than for the B20s. This is consistent with the cloud points for the B100, B20 and B5 for TME, which are 12.9 °C, 2.2 °C and 4.5 °C, respectively. In Fig. 9, free water and sediment is reported as a function of the (storage temperature – the cloud point) for the respective B100 fuels. Free water and sediment levels are highest for the B100 and B20 from TME when the storage temperature was around or below the cloud point for the respective fuel. These two fuels were the only ones that showed significant amount of free water and sediment during storage when temperatures, up to ten degrees above their cloud points, were encountered. Free water and sediment levels for the B5 remained low even when temperatures were around the cloud point. Free water and sediment levels were not a concern for the other fuels and their blends as indicated in Figs. 7 and 8 for B100s of YGME and CME, which had cloud points of 10.6 °C and 2.2 °C, respectively. 3.5. Proton Nuclear Magnetic Resonance (NMR) Proton NMR spectra taken both from the fuel and particulate, collected from the free water and sediment testing, showed a similar chemical group composition. However, differences between NMR spectra indicate the presence of more saturated molecules in the sediments when compared with their respective fuels. Proton NMR spectra were taken for the three B100s. A summary of the results are presented in Table 3. The information can be read in the table columns from left to right as: the position of every peak on the ppm scale of the spectrum, the position of the protons (in bold) in the chemical structure associated with each peak, and the integration value under each peak. The integration provides the relative chemical group composition of the three biodiesel fuels. For example, the level of saturation of each fuel, associated with lower levels of alkenyl groups (carbon double bonds), is obtained from the last row. TME is the most saturated fuel having the lowest integration value of 4.05 for protons involved in carbon double bonds. In comparison, CME is the least saturated and has twice as many double bonds in its average fatty acid carbon chain than for TME, while YGME is slightly less saturated than TME. The NMR results imply that CME should be more sensitive to oxidation leading to acidifying reactions in accordance with accelerated oxidation studies that found that less saturated fatty acid chains are richer in allylic and bis-allylic group and more prone to react with oxygen [22,23]. However, in this study, CME stored under normal environmental conditions did not display a lower stability than the other two biodiesel fuels. No significant changes in chemical composition were reported for any of the fuels throughout any of the storage conditions. This result supports the observation on the stability of the fuels’ kinematic viscosities
Table 3 Summary of Proton NMR results for TME, CME and YGME. The columns read, from left to right as: the peak position in chemical shifts d (ppm) relative to the internal protons of Tetramethylsilane (TMS), the corresponding proton position in the chemical structure and the integration values under each peak. d (ppm)
Functionality
TME
CME
YGME
0.85 1.30 1.60 2.05 2.25 2.75 3.65 5.35
–CH3 –(CH2)–n –CH2–CH2–CO2R –CH2–CH@CH–CH2–CH@CH–CH2– –CH2–CO2R –CH@CH–CH2–CH@CH– –O–CH3 –CH@CH–
7.68 60.88 5.79 6.62 5.84 0.72 8.43 4.05
7.08 52.15 6.03 10.80 5.86 2.22 8.46 7.41
7.84 59.20 5.77 7.22 5.85 0.89 8.63 4.59
Table 4 Proton NMR integration for the peak at 5.35 ppm, representative of the level of unsaturation per fatty acid chain for the biodiesel fuels and their sediments.
TME-B100 TME-Sediment YGME-B100 YGME-Sediment CME-B100 CME-Sediment
Integration at 5.35 ppm
Change in%
4.05 3.93 4.59 3.98 7.41 7.18
7 14 12
but does not contradict the observed changes in AN after storage at 40 °C, which was previously observed [21]. The precision of the integration under the NMR peaks permits quantification of the concentration of chemical changes in the order of 102–103. On the other hand, AN permits quantification of the concentration of chemical changes down to the order of 104–105. The concentration of the chemical changes observed with the AN were in the order of 103–104, therefore falling just under what can be detected with NMR. Particulate from the sediments found in the fuels were dissolved in deuterated chloroform (CDCl3) and characterized using Proton NMR. The results were compared with those of the base fuels, which are summarized in Table 4. At a first glance, the integration values show that there are no fundamental differences between the molecules composing each sediment from those of its respective base fuel. However, a closer look at some peaks does point towards a key difference. The peaks at 5.35 ppm are directly proportional to the number of unsaturated bonds in the structure as described in Table 3. The integrations at 5.35 ppm provided a basis to compare the level of saturation of the molecules in the base fuel and the sediment. As seen in Table 4 those values are all smaller for the sediments found in the fuels. These results indicate that the sediments are formed by a higher number of the more saturated ester chains.
4. Conclusion A biodiesel storage stability study was conducted at RMC on ULSDF petrodiesel and three B100s and fuel blends (B5 and B20). The storage stability study consisted of storing fuel samples in quarter filled plastic and steel Jerry cans, in a fuel shed from August 2007 to June 2008. Temperature and relative humidity in the outdoor storage facility ranged from 25 °C to 35 °C and 25% to 100%, respectively. Three biodiesel fuels were studied, a Tallow-based Methyl Ester (TME), a Canola-based Methyl Ester (CME), and, a Yellow Grease Methyl Ester (YGME). All fuels and blends proved to be very stable throughout the storage period. Acid Number and the viscosity did not increase beyond the uncertainty of the method used. Reference samples kept at 40 °C in the same conditions as the previous study, displayed an increase in AN comparable to what was previously reported [21]. Free water and sediment levels were barely above the detection limit of 0.003 mL/100 mL of fuel for CME and YGME and their blends. TME and its B20 blend displayed free water and sediment levels up to 0.05 mL/100 mL of fuel in February, after six months. Free water and sediment levels for the B20 were comparable to B100 levels when the storage temperature was below the cloud point of the fuel. Those levels decreased considerably after the warm Summer months and fell back to below 0.01 mL/100 mL. B5 free water and sediment levels remained low even when temperatures were below the cloud point. For the most part the B5s studied are stable over long term storage.
M. Farahani et al. / Fuel 90 (2011) 951–957
Proton NMR spectroscopy was used to identify chemical structure differences namely the levels of unsaturation among the three biodiesel base fuels. CME was the least saturated and TME was the most saturated. No significant changes were detected in the chemical structure of the fuels and blends throughout storage. NMR performed on the sediments revealed a similar composition but a lower concentration of alkenyl groups than for the base fuels. The presence of those sediments was attributed to more saturated molecules coming out of solution in colder weather and very slowly going back to solution at room temperature. Acknowledgements The research work was funded by the Director General Land Equipment Program Management of National Defence Headquarters, Ottawa. Authors are thankful for the technical support provided by Dr. Neda Bavarian of RMC and Mrs. Anelia Krasteva from the Quality Engineering Test Establishment. References [1] Government of Canada, Department of the Environment. Notice of intent to develop a federal regulation requiring renewable fuels, Canada gazette, part I, vol.140, no.52; 30 December 2006. [2] Binfield J et al. US and world agricultural outlook, food and agricultural policy research institute. Iowa, USA: Ames; 2009. p. 223–265. [3] Ma F, Hanna MA. Biodiesel production, a review. Bioresour Technol 1999;70:1–15. [4] Knothe G. Some aspects of biodiesel oxidative stability. Fuel Process Technol 2007;88:669–77. [5] Knothe G, Dunn RO. Biofuels derived from vegetable oils and fats. In: Gunstone FD, Hamilton RJ, editors. Oleochemical manufacture and applications. Sheffield, UK: Academic Press; 2001. p. 106–63. [6] Praksash CB. A critical review of biodiesel as a transportation fuel in Canada report prepared by global Change Strategies International Inc. for Environment Canada; March 25th, 1998. p. 15–16.
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