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 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).