Biodiesel Composition and Fuel
Properties
Gerhard Knothe
USDA / ARS / NCAUR
Peoria, IL 61604
U.S.A.
E-mail: gerhard.knothe@ars.usda.gov
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 Biodiesel
Bulletin 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
|
CH-OOCR2
|
CH2-OOCR3
Vegetable Oil
(Triacylglycerol)
R‫׳‬OOCR1
Catalyst
+
3 R‫׳‬OH
→
R‫׳‬OOCR2
+
R‫׳‬OOCR3
Alcohol
Vegetable Oil Alkyl Esters
(Biodiesel)
CH2OH
|
CHOH
|
CH2OH
Glycerol
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
|
CH-OOCR2
|
CH2-OOCR3
• Triacylglycerols
CH2OOCR1
|
CHOOCR2
|
CH2OH
• Diacylglycerols
• Glycerol
• Free Fatty Acids: R-COOH
• Alcohol
• Na, K, Ca, Mg, P, (S)
• Sterol glucosides
CH2OOCR
│
CHOH
│
CH2OH
• Monoacylglycerols
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
Flash point (closed cup)
D 93
Alcohol control. One of the following must be met:
1. Methanol content
EN 14110
2. Flash point
D 93
Water and sediment
D 2709
o
Kinematic viscosity, 40 C
D 445
Sulfated ash
D 874
Sulfur
D5453
Copper strip corrosion
D 130
Cetane number
D 613
Cloud point
D 2500
Carbon residue
D 4530
Acid number
D 664
Free glycerin
D 6584
Total glycerin
D 6584
Phosphorus content
D 4951
Distillation temperature,
D 1160
Atmospheric equivalent temperature, 90% recovered
Sodium and potassium, combined
EN 14538
Calcium and magnesium, comb.
EN 14538
Oxidation stability
EN 15751
Cold soak filterability
D7501
Limits
93 min
Units
oC
0.2 max
130 min
0.050 max
1.9-6.0
0.020 max
0.05 or 0.0015 max a)
No. 3 max
47 min
Report
0.050 max
0.50 max
0.020
0.240
0.001 max
360 max
% volume
130 min
% volume
mm2 / s
% mass
% mass
5 max
5 max
3 min
360 max
oC
% mass
mg KOH / g
% mass
% mass
% mass
oC
ppm (µg/g)
ppm (µg/g)
hours
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 14214
Property
Test method
Limits
Ester content
EN 14103
96.5 min
o
Density; 15 C
EN ISO 3675, 12185
860-900
o
Viscosity, 40 C
EN ISO 3104, ISO 3105 3.5-5.0
Flash point
EN ISO 2719, 3679
101 min
Sulfur content
EN ISO 20846, 20884
10.0 max
Carbon residue (10% dist. res.)
EN ISO 10370
0.30 max
Cetane number
EN ISO 5165
51 min
Sulfated ash
ISO 3987
0.02 max
Water content
EN ISO 12937
500 max
Total contamination
EN 12662
24 max
o
Copper strip corrosion (3h, 50 C)
EN ISO 2160
1
o
Oxidative stability, 110 C
EN 14112, 15751
6.0 min
Acid value
EN 14104
0.50 max
Iodine value
EN 14111
120 max
Linolenic acid content
EN 14103
12 max
Content of FAME with ≥ 4 double bonds
1 max
Methanol content
EN 14110
0.20 max
Monoglyceride content
EN 14105
0.80 max
Diglyceride content
EN 14105
0.20 max
Triglyceride content
EN 14105
0.20 max
Free glycerine
EN 14105, 14106
0.02 max
Total glycerine
EN 14105
0.25max
Alkali metals (Na + K)
EN 14108, 14109, 14538 5.0 max
Earth alkali metals (Ca + Mg)
prEN 14538
5.0 max
Phosphorus content
EN 14107
4.0 max
Units
% (m/m)
kg/m3
mm2/s
oC
mg/kg
% (m/m)
% (m/m)
mg/kg
mg/kg
h
mg KOH / g
g iodine /100g
%(m/m)
% (m/m)
% (m/m)
% (m/m)
% (m/m)
%(m/m)
%(m/m)
%(m/m)
mg/kg
mg/kg
mg/kg
Some Fatty Acid Profiles
Vegetable Oil
C18:0
C18:1
C18:2
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
Palm
C16:0
C18:3
Properties of Vegetable Oil Esters
Methyl Ester
Palm
Rapeseed / Canola
Cloud Point
(°C)
16
Cetane Number
68-70
Kin. Visc.
(40°C; mm2/s)
4.4
-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 Consider
Two 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
Saturated methyl esters (ME)
Saturated ethyl esters
Mono-, di-, and triunsaturated ME
Highly polyunsaturated ME
100
90
C18:0 / 101
C16:0 / 85.9
Cetane Number
80
70
60
C18:1 9c / 59.3
C10:0 / 51.6
50
EN 14214 =51 min
ASTM D6751 = 47 min
40
C18:2 9c,12c / 38.2
C20:4 / 29.6
30
20
C18:3 9c,12,15c / 22.7
C22:6 / 24.4
10
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Number of carbon atoms in the fatty acid chain
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 in
standards (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 regulated
emissions (Source: USEPA report 420-P-02-001):
Petrodiesel
Biodiesel
100
100
80
Relative Emissions
Relative emissions
Petrodiesel
B20
60
40
80
60
40
20
20
0
0
NOx
PM
CO
Pollutant
HC
NOx
PM
CO
Pollutant
HC
NOx and PM Exhaust Emissions of
Petrodiesel, Biodiesel, Their Components
2003 Engine; EPA Heavy Duty Test
2.5
2.0
NOx
PMx10
1.5
1.0
2007PM Standard
0.5
te
lL
B
au
as
e
ra
2
te
ita
m
al
et
M
hy
et
M
hy
lP
od
D
ad
H
ex
ec
ec
an
an
e
e
e
at
et
M
So
y
B
hy
io
lO
di
le
es
as
e
el
0.0
B
Brake-Specific Emission Rate, g/hp-hr
3.0
G. Knothe, C.A Sharp, T.W. Ryan III, Energy & Fuels 20, 403-408 (2006).
10
0
-10
-20
-30
-40
-50
-60
-70
-80
Hexadecane
Dodecane
Me soyate
Me oleate
Me palmitate
Me laurate
NOx
PM
Change in NOx and PM vs. petrodiesel
Change in Exhaust Emissions Relative to Base Fuel (%)
10
0
-10
-20
-30
-40
-50
-60
Hexadecane
Dodecane
Me soyate
Me oleate
Me palmitate
Me laurate
Hydrocarbons
CO
Change in HC and CO vs. petrodiesel
Change in Exhaust Emissions Relative to Reference Fuel (%)
Viscosity
Saturated methyl esters (ME)
Saturated ethyl esters
Mono-, di-, and triunsaturated ME
Highly polyunsaturated ME
o
2
Kinematic Viscosity (40 C; mm /s)
8
7
C22:1 13c / 7.33
ASTM D6751 upper limit
6
C18:0 / 5.85
EN 14214 upper limit
5
C16:0 / 4.38
4
C18:1 9c / 4.51
C18:2 9c,12c / 3.65
3
C12:0 / 2.43
EN 14214 lower limit
C18:3 9c,12c,15c / 3.14 C20:4 / 3.11 C22:6 / 2.97
2
ASTM D6751 lower limit
C10:0 / 1.72
1
8
9
10 11 12 13 14
15 16 17
18 19 20 21
Number of carbon atoms in the fatty acid chain
22
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
80
70
58.6
Saturated methyl ester
Saturated ethyl ester
60
53.2
46.4
o
Melting Point ( C)
50
37.7
40
48.6
41.3
28.5
30
33.0
18.5
20
55.9
23.2
10
4.3
11.8
0
-13.5
-10
22:1 13c / -3.1
-1.8
20:1 11c / -7.8
-20
-30
-20.4
16:1 9c / -34
-40
-50
18:1 9c / -20.2
-37.4
-44.7
18:2 9c,12c / -43.1
18:3 9c,12c,15c / <-50
-60
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Number of carbon atoms in the fatty acid chain
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 Lubricity
Lubricity 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
Methyl
Methyl
Methyl
Methyl
laurate 416, 408,
stearate 322, 277,
oleate 290, 342,
linoleate 236, 219,
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
Unchanged fatty
ester composition
Additives
A
Change
alcohol
B
Physical procedures C
Modified fatty ester
composition
Change fatty acid
profile
Genetic
D
modification
Inherently different
fatty acid profile
Alternative
feedstocks
G. Knothe; Energy & Environmental Science, 2, 759-766 (2009).
E
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 Moiety
Branched and longer-chain esters:
● Lower melting points, similar cetane numbers compared to methyl esters
Ester
C16:0
C16:0
C16:0
C16:0
Methyl
Ethyl
Propyl
iso-Propyl
C18:1
C18:1
C18:1
C18:1
Methyl
Ethyl
Propyl
iso-Propyl
M.P. (°C)
28.5
23.2
20.3
13-14
CN
85.9
93.1
85.0
82.6
Ester
C18:0
C18:0
C18:0
C18:0
-20.2
-20.3
-30.5
59.3
67.8
58.8
86.6
C18:2 Me
C18:2 Et
C18:2 Pr
Me
Et
Pr
i-Pr
M.P. (°C)
37.7
33.0
28.1
-43.1
-56.7
● Disadvantage: Higher costs of alcohols
Source: Handbook of Chemistry and Physics; The Lipid Handbook, various publications.
CN
101
97.7
90.0
96.5
38.2
39.6
44.0
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 Monounsaturates
Methyl 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 °C
but: 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
Methyl octanoate
Ethyl octanoate
Methyl decanoate
Ethyl decanoate
Methyl laurate
M.P.
(°C)
-37.3
-44.5
-13.1
-19.8
4.6
Cetane
number
39.7
42.2
51.6
54.5
66.7
Kin. Visc.
Heat of comb.
(40°C; mm2/s)
(kJ/kg)
1.20
34907
1.32
1.71
36674
1.87
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 (8085%); 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 Sunflower
PSR 23
C8:0
C10:0
C12:0
C14:0
C16:0
C18:0
C18:1
C18:2
C18:3
0.3
64.7
3.0
4.5
7.0
0.9
12.2
6.7
14.5
7.5
34-45
29-44
< 0.5
44.1
4.4
39.0
10.6
0.3
3.6
1.5
61.6
21.7
9.6
11
4
23.4
53.2
7.8
6.4
4.5
24.9
63.8
-
Shorter-Chain Saturates:
Cuphea Methyl Esters
Properties of cuphea PSR23 methyl esters (CuME):
Cetane number:
Kinematic viscosity (40°C):
Oxidative stability:
Cloud point:
55-56
2.38-2.40 mm2/s
3.1 – 3.5 h
-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
Number
Castor methyl esters
37.55
ASTM D6751
EN 14214
47 min
51 min
Kinematic Viscosity
(40°C; mm2/s)
14.82
5.87
1.9-6.0
3.5-5.0
3 min
6 min
Cold flow related properties:
• Melting point of methyl ricinoleate:
• Pour point of castor methyl esters:
C18:1 12-OH 37
-5
15.29
-5.8°C
-20°C
0.67
Oxidative
Stability (h)
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:
→
3 C19H36O2
+
C3H8O3
C57H104O6 + 3 CH3OH
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).