Biodiesel Composition and Fuel · PDF fileBiodiesel Composition and Fuel Properties Gerhard...
Transcript of Biodiesel Composition and Fuel · PDF fileBiodiesel Composition and Fuel Properties Gerhard...
Biodiesel Composition and Fuel Properties
Gerhard Knothe
USDA / ARS / NCAUR
Peoria, IL 61604
U.S.A.
E-mail: [email protected]
It All Began With…
… the Diesel Engine
Diesel’s Vision:
Develop an engine more efficient than the
steam engine, but …
...Rudolf Diesel did not originally
investigate vegetable oils as fuel.
Rather…
Diesel’s first engine
The Original Demonstration in the Words of Rudolf Diesel
“At the Paris Exhibition in 1900 there was shown by the Otto
Company a small Diesel engine, which, at the request of the French
Government, ran on Arachide (earth-nut or pea-nut) oil, and worked
so smoothly that only very few people were aware of it. The engine
was constructed for using mineral oil, and was then worked on
vegetable oil without any alterations being made.
R. Diesel, The Diesel Oil-Engine, Engineering 93:395–406 (1912). Chem. Abstr. 6:1984 (1912).
The Original Demonstration in the Words of Rudolf Diesel
The French Government at the time thought of testing the applicability to
power production of the Arachide, or earth-nut, which grows in
considerable quantities in their African colonies, and which can be easily
cultivated there, because in this way the colonies could be supplied with
power and industry from their own resources, without being compelled to
buy and import coal or liquid fuel.”
Diesel, R., The Diesel Oil-Engine, Engineering 93:395–406 (1912). Chem. Abstr. 6:1984 (1912).
Vegetable Oils as Alternative Fuel for Energy Independence: Not a New
Concept● 1920’s-1940’s: Many European countries interested in vegetable oils
as fuels for their African colonies in order to provide a local energy
source.
● Also interest in Brazil, China, India.
● A.W. Baker and R.L. Sweigert, Proc. Oil & Gas Power Meeting of the
ASME :40-48 (1947): “The United States is one of the countries in
the world fortunate enough to have large supplies of petroleum,
which its inhabitants have not always used wisely. With a possible
diminishing supply of oil accompanied by an increase in consumption,
the study of substitute fuels becomes of some importance. Vegetable
oils loom as a possibility for engines of the compression-ignition type.”
The First Report on Biodiesel
Belgian Patent 422,877 (1937): Procédé de transformation d’huiles
végétales en vue de leur utilisation comme carburants.
An Extensive Report on Biodiesel
“Old” Research:First Cetane Number
Determination for BiodieselBulletin Agricole du Congo Belge, Vol. 33, p. 3-90 (1942):
(Potential) Sources of Biodiesel• Vegetable oils
• Classical (edible) commodity oils (palm, rapeseed /
canola, soybean, etc.)
• “Alternative” (inedible) oils (jatropha, karanja, pennycress, etc.)
• Animal fats
• Used cooking oils
• “Alternative” feedstocks
• Algae
• Variety of feedstocks with considerably varying fatty acid profiles
• Fuel properties vary considerably
Why biodiesel and not the neat oil?
CH2-OOCR1 R ׳OOCR1 CH2OH
| Catalyst | CH-OOCR2 + 3 R ׳OH → R ׳OOCR2 + CHOH| |CH2-OOCR
3 R ׳OOCR3 CH2OH
Vegetable Oil Alcohol Vegetable Oil Alkyl Esters Glycerol (Triacylglycerol) (Biodiesel)
Viscosity!
27-35 mm2/sec 4-5 mm2/sec
Kinematic viscosity of petrodiesel fuels usually ≈ 1.8-3.0 mm2/sec.
Major Ester Components of Most Biodiesel Fuels
Fatty esters in from common vegetable oils (palm, soybean, canola/rapeseed, sunflower, etc):
• Methyl palmitate (C16:0): CH3OOC-(CH2)14-CH3
• Methyl stearate (C18:0): CH3OOC-(CH2)16-CH3
• Methyl oleate (C18:1, ∆9c): CH3OOC-(CH2)7-CH=CH-(CH2)7-CH3
• Methyl linoleate (C18:2; all cis): CH3OOC-(CH2)7-(CH=CH-CH2)2-(CH2)3-CH3
• Methyl linolenate (C18:3; all cis): CH3OOC-(CH2)7-(CH=CH-CH2-)3-CH3
From other oils: • Methyl laurate (C12:0): CH3OOC-(CH2)10-CH3
• Methyl ricinoleate (C18:1, 12-OH; cis): CH3OOC-(CH2)7-CH=CH-CH2-CHOH-(CH2)5-CH3
• Algal Oils: Methyl eicosapentaenoate (C20:5): CH3OOC-(CH2)3-(CH=CH-CH2-)5-CH3
Methyl docosahexaenoate (C22:6): CH3OOC-(CH2)2-(CH=CH-CH2-)6-CH3
Minor Constituents in Biodiesel• Can influence fuel properties
• Cold flow, oxidative stability, corrosion, combustion, catalyst poisons, lubricity
CH2-OOCR1 CH2OOCR1 CH2OOCR| | │CH-OOCR2 CHOOCR2 CHOH| | │CH2-OOCR3 CH2OH CH2OH
• Triacylglycerols • Diacylglycerols • Monoacylglycerols
• Glycerol
• Free Fatty Acids: R-COOH
• Alcohol• Na, K, Ca, Mg, P, (S) • Sterol glucosides
Technical Problems with Biodiesel
• Cold flow
• Oxidative stability
• NOx exhaust emissions
• May fade with time due to new exhaust emissions control
technologies.
• Other fuel quality issues:
• Minor components influencing fuel properties.
Biodiesel Standard ASTM D6751-(11a)Property Test method Limits Units Flash point (closed cup) D 93 93 min oC Alcohol control. One of the following must be met:
1. Methanol content EN 14110 0.2 max % volume 2. Flash point D 93 130 min 130 min
Water and sediment D 2709 0.050 max % volume Kinematic viscosity, 40 oC D 445 1.9-6.0 mm2 / s Sulfated ash D 874 0.020 max % mass Sulfur D5453 0.05 or 0.0015 max a) % massCopper strip corrosion D 130 No. 3 max Cetane number D 613 47 min Cloud point D 2500 Report oC Carbon residue D 4530 0.050 max % massAcid number D 664 0.50 max mg KOH / g Free glycerin D 6584 0.020 % massTotal glycerin D 6584 0.240 % massPhosphorus content D 4951 0.001 max % massDistillation temperature, D 1160 360 max oCAtmospheric equivalent temperature, 90% recovered
Sodium and potassium, combined EN 14538 5 max ppm (µg/g)Calcium and magnesium, comb. EN 14538 5 max ppm (µg/g)Oxidation stability EN 15751 3 min hours Cold soak filterability D7501 360 max sec
a) The limits are for Grade S15 and Grade S500 biodiesel, respectively. S15 and S500 refer to maximum sulfur specifications (ppm).
Biodiesel Standard EN 14214Property Test method Limits Units Ester content EN 14103 96.5 min % (m/m) Density; 15oC EN ISO 3675, 12185 860-900 kg/m3
Viscosity, 40 oC EN ISO 3104, ISO 3105 3.5-5.0 mm2/sFlash point EN ISO 2719, 3679 101 min oC Sulfur content EN ISO 20846, 20884 10.0 max mg/kg Carbon residue (10% dist. res.) EN ISO 10370 0.30 max % (m/m)Cetane number EN ISO 5165 51 min Sulfated ash ISO 3987 0.02 max % (m/m)Water content EN ISO 12937 500 max mg/kgTotal contamination EN 12662 24 max mg/kgCopper strip corrosion (3h, 50oC) EN ISO 2160 1 Oxidative stability, 110 oC EN 14112, 15751 6.0 min hAcid value EN 14104 0.50 max mg KOH / gIodine value EN 14111 120 max g iodine /100gLinolenic acid content EN 14103 12 max %(m/m)Content of FAME with ≥ 4 double bonds 1 max % (m/m) Methanol content EN 14110 0.20 max % (m/m)Monoglyceride content EN 14105 0.80 max % (m/m) Diglyceride content EN 14105 0.20 max % (m/m)Triglyceride content EN 14105 0.20 max %(m/m)Free glycerine EN 14105, 14106 0.02 max %(m/m)Total glycerine EN 14105 0.25max %(m/m)Alkali metals (Na + K) EN 14108, 14109, 14538 5.0 max mg/kgEarth alkali metals (Ca + Mg) prEN 14538 5.0 max mg/kgPhosphorus content EN 14107 4.0 max mg/kg
Some Fatty Acid Profiles
Vegetable Oil C16:0 C18:0 C18:1 C18:2 C18:3
Palm 45 4-5 38-40 10-11
Rapeseed / Canola 3-4 1-3 58-62 20-22 9-12
Soy 8-13 2-6 18-30 49-57 2-10
Sunflower 6-7 3-5 21-29 58-67
Jatropha 13-15 7-8 34-44 31-43
Properties of Vegetable Oil Esters
Methyl Ester Cloud Point Cetane Number Kin. Visc.
(°C) (40°C; mm2/s)
Palm 16 68-70 4.4
Rapeseed / Canola -3 52-55 4.5
Soy 0 48-52 4.1
Sunflower 0 ≈ 55 4.4
Jatropha 4-5
Oxidative stability: usually antioxidants required to meet standard specifications
Properties to ConsiderTwo types of specifications in biodiesel standards (ASTM D6751; EN 14214):
Properties inherent to fatty esters: • Cetane number • Cold flow • Viscosity • Oxidative stability (• Feedstock restrictions: Iodine value, viscosity, specific esters in EN
14214) (• Density only in EN 14214)
Parameters related to production, storage, etc. • Acid value • Free and total glycerol • Na, K, Mg, Ca, P, S • Water and sediment, sulfated ash, carbon residue
Not in biodiesel standards: Exhaust emissions, lubricity
Some General Observations on Fatty Ester Fuel Properties
Fuel properties of fatty esters depend on
• Chain length (number of CH2 moieties)
• Number and position of double bonds
Cetane Number
• Dimensionless descriptor related to the ignition delay time of a fuel in a cylinder
• Higher cetane numbers indicate reduced ignition delay time,
“better” combustion.
• Hexadecane is the high-quality reference compound with assigned
CN = 100.
• CN can be correlated to NOx exhaust emissions
• Saturated compounds (higher CN) show reduced NOx exhaust
emissions.
Cetane Numbers
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 2410
20
30
40
50
60
70
80
90
100
Ce
tan
e N
um
be
r
Number of carbon atoms in the fatty acid chain
Saturated methyl esters (ME)
Saturated ethyl esters
Mono-, di-, and triunsaturated ME
Highly polyunsaturated ME C18:0 / 101
C20:4 / 29.6
C22:6 / 24.4C18:3 9c,12,15c / 22.7
C10:0 / 51.6
C18:2 9c,12c / 38.2
C18:1 9c / 59.3EN 14214 =51 min
ASTM D6751 = 47 min
C16:0 / 85.9
Cetane Number
• Cetane numbers of mixtures:
CNmix = ∑ AC x CNC
(CNmix = CN of the mixture, AC = relative amount of an individual neat ester in the mixture, CNC = CN of the individual neat ester)
• Most biodiesel fuels from vegetable oils meet CN requirements instandards (ASTM D6751: 47 min; EN 14214: 51 min) as there are usually sufficient amounts of esters with higher CN
Why Triacylglycerol Feedstocks?
• Alkanes are “ideal” diesel fuels.
• Branched compounds and aromatics have low cetane numbers
• Structural similarity (long hydrocarbon chains) responsible for suitability
of fatty esters as diesel fuels.
• Compounds such as methyl palmitate and methyl stearate have CN
comparable to hexadecane and other long-chain alkanes
Exhaust Emissions Studies
Average effect of biodiesel and B20 vs. petrodiesel on regulatedemissions (Source: USEPA report 420-P-02-001):
NOx PM CO HC0
20
40
60
80
100
Rela
tive E
mis
sio
ns
Pollutant
Petrodiesel
B20
NOx PM CO HC0
20
40
60
80
100
Re
lative e
mis
sio
ns
Pollutant
Petrodiesel
Biodiesel
NOx and PM Exhaust Emissions of Petrodiesel, Biodiesel, Their Components
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Bas
e
Soy
Bio
diese
l
Met
hyl O
leat
e
Hex
adec
ane
Dodec
ane
Met
hyl P
alm
itate
Bas
e 2
Met
hyl L
aura
te
Bra
ke-S
pecif
ic E
mis
sio
n R
ate
, g
/hp
-hr
NOx
PMx10
2007PM Standard
G. Knothe, C.A Sharp, T.W. Ryan III, Energy & Fuels 20, 403-408 (2006).
2003 Engine; EPA Heavy Duty Test
Change in
NOxand PM vs. p
etro
diesel
-80
-70
-60
-50
-40
-30
-20
-10 0
10
Change in Exhaust Emissions Relative to Base Fuel (%) N
Ox
PM
Hexadecane
Dodecane
Me soyate
Me oleate
Me palmitate
Me laurate
Change in
HC and CO vs. p
etro
diesel
-60
-50
-40
-30
-20
-10 0
10
Change in Exhaust Emissions Relative to Reference Fuel (%) H
ydro
ca
rbo
ns
CO
Hexadecane
Dodecane
Me soyate
Me oleate
Me palmitate
Me laurate
Viscosity
88 99 1010 1111 1212 1313 1414 1515 1616 1717 1818 1919 2020 2121 2222
1
2
3
4
5
6
7
8
Kin
em
atic V
isco
sity (
40
oC
; m
m2/s
)
Number of carbon atoms in the fatty acid chain
Saturated methyl esters (ME)
Saturated ethyl esters
Mono-, di-, and triunsaturated ME
Highly polyunsaturated ME
C16:0 / 4.38
C18:0 / 5.85
C12:0 / 2.43
C10:0 / 1.72
C18:1 9c / 4.51
C20:4 / 3.11C22:6 / 2.97C18:3 9c,12c,15c / 3.14
C18:2 9c,12c / 3.65
C22:1 13c / 7.33
EN 14214 upper limit
EN 14214 lower limit
ASTM D6751 upper limit
ASTM D6751 lower limit
Viscosity
• Viscosity increases with chain length and increasing saturation.
• Kinematic viscosity of mixtures νmix
νmix = ∑ Ac x νc
• Virtually all biodiesel fuels meet ASTM D6751 specifications
• EN 14214 more restrictive
• Biodiesel fuels with greater amounts of lower-viscosity
components may not meet lower limit
Cold Flow: Melting Points of Fatty Acid Esters
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24-60
-50
-40
-30
-20
-10
0
10
20
30
40
50
60
70
80M
eltin
g P
oin
t (o
C)
Number of carbon atoms in the fatty acid chain
Saturated methyl ester
Saturated ethyl ester
-13.5
4.3
18.5
28.5
37.7
46.4
53.258.6
33.0
23.2
41.3
48.6
55.9
11.8
-1.8
-20.4-37.4
-44.7
16:1 9c / -34
18:1 9c / -20.2
20:1 11c / -7.8
22:1 13c / -3.1
18:2 9c,12c / -43.1
18:3 9c,12c,15c / <-50
Cold Flow
• Melting points of fatty acid esters depend on chain length and unsaturation
• Cold flow properties determined by nature and amount of saturated compounds
• Cloud point common and stringent test procedure
• “Soft” specification in biodiesel standards
• ASTM D6751: Cloud point by report, cold soak filtration
• EN 14214: Cold-filter plugging point, depending on time of
year and geographic location
Cold Flow
• Minor constituents such as monoacylglycerols and sterol glucosides
also influence cold flow.
• Melting points of monopalmitin and monostearin > 70°C
• Melting points of sterol glucosides ≈ 240°C
• Effects often noticeable upon storage
Oxidative Stability
• Oxidative stability is one of the major technical challenges facing biodiesel.
• Affected by presence of air, temperature, light, extraneous materials, container material, headspace volume
• Structural reason for the autoxidation of fatty compounds:
Allylic CH2 positions
↓ ↓
H3CO2C-(CH2)x-CH2-CH=CH-CH2-CH=CH-CH2-(CH2)y-CH3
↑
especially: bis -allylic CH2 positions
Oxidative Stability
Relative rates of oxidation (E.N. Frankel, Lipid Oxidation, 2005):
• Oleates = 1 (two allylic positions)
• Linoleates = 41 (two allylic positions, one bis-allylic position)
• Linolenates = 98 (two allylic positions, two bis-allylic positions)
• Chains with > 3 double bonds have even higher relative rates
Is the oxidative stability of mixtures (vegetable oil esters) directly proportional to the amount of unsaturated compounds or do small amounts of unsaturated compounds have greater influence than their amounts indicate?
Oxidative Stability• Rancimat test (110°C):
Saturated esters > 24 h
Methyl palmitoleate 2.11 h
Methyl oleate 2.79 h
Methyl linoleate 0.94 h
Methyl linolenate 0.00 h
Methyl eicosatetraenoate (C20:4) 0.09 h
Methyl docosahexaenoate (C22:6) 0.07 h
• ASTM D6751 minimum specification 3h
• EN 14214 minimum specification 6h
• Almost always antioxidant additives required
Density
• Only in EN 14214
• Range of 0.86 – 0.90 g/cm3 (15°C)
• Not a problem for most biodiesel fuels.
• Only highly polyunsaturated fatty esters may be problematic:
C20:4 0.9064 g/cm3
C22:6 0.9236 g/cm3
• Density of a mixture: ρmix = ∑ Ac x ρc
Biodiesel and Lubricity• Neat biodiesel has excellent lubricity as do neat methyl esters.
• Low-level blends (~ 2% biodiesel in petrodiesel = B2):
• Lubricity benefits through biodiesel with (ultra-)low sulfur
petrodiesel which do not possess inherent lubricity
compared to non-desulfurized petrodiesel.
• Marginal cost impact.
• Not included in biodiesel standards.
• High-frequency reciprocating rig (HFRR) tester (ASTM D6079;
ISO 12156) in ASTM and EN petrodiesel standards.
• Maximum wear scars of 520 (ASTM) and 460 µm (EN).
Biodiesel and LubricityLubricity of low-level blends of biodiesel with petrodiesel to a great extent determined by minor constituents, especially free fatty acids and monoacylglycerols.
• In the neat form, even better lubricity than methyl esters.
• Glycerol has limited effect (insolubility in petrodiesel).
Example (HFRR wear scars):
• ULSD: 651, 636 µm
• w. 1% methyl oleate: 597, 515 µm
• 1% oleic acid in methyl oleate, then 1% thereof in ULSD: 356,
344 µm.
• w. 2% methyl oleate: 384, 368 µm
G. Knothe, K.R. Steidley; Energy & Fuels 19, 1192-1200 (2005).
Biodiesel and Lubricity
• Higher lubricity with increasing number of double bonds and greater
chain length:
Methyl laurate 416, 408,
Methyl stearate 322, 277,
Methyl oleate 290, 342,
Methyl linoleate 236, 219,
Methyl linolenate 183, 185
• Effect of oxygenated functional groups:
COOH > CHO > OH > COOCH3 > C=O > C-O-C
G. Knothe, K.R. Steidley; Energy & Fuels 19, 1192-1200 (2005).
Property Trade-off
Increasing chain length:
• Higher melting point (-)
• Higher cetane number (+)
Increasing unsaturation:
• Lower melting point (+)
• Decreasing oxidative stability (-)
• Lower cetane number (-)
Five Approaches to Improving Biodiesel Fuel Properties
Additives
Physical procedures
Genetic modification
Alternative feedstocks
Modified fatty ester composition
Inherently different fatty acid profile
Change alcohol
Change fatty acid profile
Unchanged fatty ester composition
A
B
C
D
E
G. Knothe; Energy & Environmental Science, 2, 759-766 (2009).
Additives, physical procedures
Additives
• Cold flow improvers
Do not affect cloud point
• Antioxidants
Oxidation delayers
Physical procedures
• Winterization for removing saturates to improve cold
flow
Influence of Alcohol MoietyBranched and longer-chain esters: ● Lower melting points, similar cetane numbers compared to methyl esters
Ester M.P. (°C) CN Ester M.P. (°C) CNC16:0 Methyl 28.5 85.9 C18:0 Me 37.7 101 C16:0 Ethyl 23.2 93.1 C18:0 Et 33.0 97.7C16:0 Propyl 20.3 85.0 C18:0 Pr 28.1 90.0C16:0 iso-Propyl 13-14 82.6 C18:0 i-Pr 96.5
C18:1 Methyl -20.2 59.3 C18:2 Me -43.1 38.2C18:1 Ethyl -20.3 67.8 C18:2 Et -56.7 39.6C18:1 Propyl -30.5 58.8 C18:2 Pr 44.0C18:1 iso-Propyl 86.6
● Disadvantage: Higher costs of alcohols
Source: Handbook of Chemistry and Physics; The Lipid Handbook, various publications.
Fatty Acid Profile: Something “Better” Than Methyl Oleate?
• Positional Isomers
No major advantages compared to methyl oleate
• Geometric Isomers (cis /trans)
Higher melting points, higher viscosity of trans
• Hydroxylated Chains
High viscosity, low cetane number, low oxidative stability
• Shorter Saturated Chains
• Shorter Unsaturated Chains
Shorter-Chain MonounsaturatesMethyl palmitoleate (C16:1) • Melting point: -34°C • Cetane number: 51-56 (ASTM D6890) • Kinematic viscosity (40°C): 3.67 mm2/s • Oxidative stability: 2.11 h • Extrapolation of exhaust emissions: Effect likely similar to methyl
oleate (slight chain-length effect)
Methyl myristoleate (C14:1) • Melting point: -52°C • Kinematic viscosity (40°C): 2.73 mm2/s
Major advantage compared to methyl oleate: • Improved cold flow, lower kinematic viscosity
G. Knothe; Energy & Fuels 22, 1358-1364 (2008).
Shorter-Chain Monounsaturates: An Example
Macadamia nut oil methyl esters:
Two examples: • 16 and 20 % C16:1; • 59 and 55% C18:1 ∆9; 4% C18:1 ∆11.
• Cetane number: 57-59
• Oxidative stability: 2 h
• Kinematic Viscosity: 4.5 mm2/s
• Cloud Point: 7.0 / 4.5 °Cbut: C16:0 ≈8.5%; C18:0 ≈3.5%; C20:0 ≈ 2.5%; C22:0 ≈ 0.8%.
G. Knothe; Energy & Fuels 24, 2098–2103 (2010).
Shorter-Chain Saturates
M.P. Cetane Kin. Visc. Heat of comb.(°C) number (40°C; mm2/s) (kJ/kg)
Methyl octanoate -37.3 39.7 1.20 34907Ethyl octanoate -44.5 42.2 1.32Methyl decanoate -13.1 51.6 1.71 36674Ethyl decanoate -19.8 54.5 1.87Methyl laurate 4.6 66.7 2.43 37968
High oxidative stability: All > 24 h.
Extrapolation of exhaust emissions for C10 esters: NOx likely slightly reduced (ca. -5%); PM significantly reduced (80-85%); CO reduced; HC increased
Shorter-Chain Saturates: Cuphea Methyl Esters
Fatty Acid Profile of Cuphea PSR 23 (C. Viscosissima × C. Lanceolata):
Fatty acid Cuphea Jatropha Palm Rapeseed Soybean SunflowerPSR 23
C8:0 0.3C10:0 64.7C12:0 3.0C14:0 4.5C16:0 7.0 14.5 44.1 3.6 11 6.4C18:0 0.9 7.5 4.4 1.5 4 4.5 C18:1 12.2 34-45 39.0 61.6 23.4 24.9C18:2 6.7 29-44 10.6 21.7 53.2 63.8 C18:3 < 0.5 0.3 9.6 7.8 -
Shorter-Chain Saturates: Cuphea Methyl Esters
Properties of cuphea PSR23 methyl esters (CuME):
Cetane number: 55-56
Kinematic viscosity (40°C): 2.38-2.40 mm2/s
Oxidative stability: 3.1 – 3.5 h
Cloud point: -9 to -10°C
G. Knothe, S.C. Cermak, R.L. Evangelista; Energy & Fuels, 23, 1743-1747 (2009).
Distillation Curve:CuME vs SME and ULSD
B.T. Fisher, G. Knothe, C.J. Mueller, Energy Fuels, 24, 1563-1580 (2010).
Castor Oil Methyl Esters
Fatty acid profile of castor oil 85-90% ricinoleic acid
Cetane Kinematic Viscosity Oxidative Number (40°C; mm2/s) Stability (h)
Castor methyl esters 37.55 14.82 5.87
ASTM D6751 47 min 1.9-6.0 3 minEN 14214 51 min 3.5-5.0 6 min
Cold flow related properties: • Melting point of methyl ricinoleate: -5.8°C • Pour point of castor methyl esters: -20°C
C18:1 12-OH 37 -5 15.29 0.67
Biodiesel from Algae• Claimed high production potential
• Order of magnitude greater than highest-yielding vegetable oils?
• Avoids food vs. fuel issue.
• Problems with growth and harvesting of algae, oil extraction.
• High production costs.
• Little to no technical information on biodiesel derived from algal oils.
• Potential properties need to be estimated from fatty acid
profiles and data on other biodiesel and neat compounds.
Biodiesel from Algae: Fatty Acid Profiles
• Most profiles contain high amounts of saturated and / or
polyunsaturated fatty acid chains
• Eicosapentaenoic (C20:5) and docosahexaenoic (C22:6) acids
most common highly polyunsaturated fatty acids in algal oils
• Palmitic acid most common fatty acid (m.p. of methyl
ester 28.5°C) in algal oils (and palm oil!);
• Myristic (C14:0) acid also present in many algal oils (m.p.
methyl ester 18°C).
• Some exceptions
Biodiesel from Algae: Fuel Properties
• Cetane numbers of most algal biodiesel likely lower to mid 40’s.
• Not all will meet CN specification in ASTM D6751; most will not
meet CN specification in EN 14214
• Kinematic viscosity (40°C) of most algal biodiesel likely in the range 3.0 – 4.0 mm2/s
• Oxidative stability low due to highly polyunsaturated fatty acids.
• Cold flow:
• Cloud point of palm oil (44% C16:0; 4% C18:0) around 16°C.
• Cloud point of soybean oil (10% C16:0; 5% C18:0) around 0°C.
• Cloud points of most algal biodiesel fuels likely between these
values.
Biodiesel from Algae
• Claimed high production potential not (yet) realized → Uncertain future.
• Any algal biodiesel will need favorable properties to compete in
the marketplace.
• Conversely, algae delivering fuels with favorable properties will need
actual high production.
• Property trade-off likely missing due to relatively low amounts
of monounsaturated fatty acid chains
Fatty Acid Profiles of Algal Oils
• A different profile: Trichosporon capitatum
• 16:0 7.0%, 18:0 1.1%
• 16:1 1.0%, 18:1 / 79.8%, C18:2 / 8.0% (H. Wu et al., Appl. Energy 2011, 88, 138-142) i
• Usually greater number of components than vegetable oils
• Fatty acid profiles of a species depend on growing conditions such as
• Temperature
• Light
• Nutrients.
Renewable Diesel: Overview• Closer in composition and properties to (ultra-low sulfur) petrodiesel.
• No / low sulfur, aromatics
• Higher oxidative stability
• Cold flow varies • “Lighter” form: Aviation fuel
• Regulated exhaust emissions likely reduced compared to “regular” petrodiesel (but not necessarily biodiesel).
• Feedstock availability and cost issues similar to biodiesel
• Low lubricity
• Energy use / energy balance? Likely less favorable than biodiesel
Biodiesel vs. Renewable Diesel: Mass (Energy) Balance of Products
Biodiesel - Methyl oleate from triolein:
C57H104O6 + 3 CH3OH → 3 C19H36O2 + C3H8O3
885.45 3 x 296.495 =889.458 = 100.5% mass
≈ 40000 kJ/kg x 1.005 = 40200 kJ 39547 kJ/L
Renewable Diesel - Heptadecane from triolein:
C57H104O6 + 6 H2 → 3 C17H36 + 3 CO2 + C3H8
885.45 3 x 240.475 =721.425 = 81.5% mass
≈ 47500 kJ/kg x 0.815 = 38305 kJ 41310 kJ / L
Glycerol and propane not accounted for here.
Biodiesel / Renewable Diesel: An Evaluation
Use each fuel where most appropriate for its properties?
• Biodiesel for ground applications? • Utilize environmental and other benefits: Reduced exhaust
emissions, biodegradability, safer handling
• Renewable diesel (in “lighter” form) for aviation applications due to cold flow?
• Energy balance may be of less interest here: “Sacrifice” some other energy source(s) in order to have aviation fuel available?
• No other (realistic) alternative jet fuel.
Biodiesel / Renewable Diesel: An Evaluation
• Consider limited amount of feedstock available.
• Feedstocks with high yield not (yet) available in sufficient quantities (algae).
• Fuel property issues.
• Co-products: Renewable glycerol is preferable
• Complex issue: Advantages and disadvantages to both approaches.
Summary / Conclusions
• Biodiesel with improved properties needed to take advantage of its
benefits
• Legislative and regulatory incentives may/do not
suffice if properties do not meet market demands
• Feedstocks with high supply potential (algae!) will need to address
the issue of fuel properties.
Parting Thoughts:Rudolf Diesel (1912)
“The fact that fat oils from vegetable sources can be used may seem
insignificant to-day, but such oils may perhaps become in course of time of
the same importance as some natural mineral oils and the tar products are
now. ... In any case, they make it certain that motor-power can still be
produced from the heat of the sun, which is always available for
agricultural purposes, even when all our natural stores of solid and liquid
fuels are exhausted.”
R. Diesel, The Diesel Oil-Engine, Engineering 93:395–406 (1912). Chem. Abstr. 6:1984 (1912).