biodieselcomposition gerhardknothe 2012 08

Properties to ConsiderTwo types of specifications in biodiesel standards ASTM D6751; EN 14214: Properties inherent to fatty esters: • Water and sediment, sulfated ash, carbon residue Not

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?

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

Technical Problems with Biodiesel

• Other fuel quality issues:

• Minor components influencing fuel properties

Biodiesel Standard ASTM D6751-(11a)

Flash point (closed cup) D 93 93 min o C

Alcohol control One of the following must be met:

Water and sediment D 2709 0.050 max % volume

Copper strip corrosion D 130 No 3 max

Distillation temperature, D 1160 360 max o C

Atmospheric equivalent temperature, 90% recovered

Calcium and magnesium, comb EN 14538 5 max ppm (µg/g)

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

Viscosity, 40 o C EN ISO 3104, ISO 3105 3.5-5.0 mm 2 /s

Sulfur content EN ISO 20846, 20884 10.0 max mg/kg

Cetane number EN ISO 5165 51 min

Water content EN ISO 12937 500 max mg/kg

Copper strip corrosion (3h, 50 o C) EN ISO 2160 1

Linolenic acid content EN 14103 12 max %(m/m)

Diglyceride content EN 14105 0.20 max % (m/m)

Free glycerine EN 14105, 14106 0.02 max %(m/m)

Alkali metals (Na + K) EN 14108, 14109, 14538 5.0 max mg/kg

Some Fatty Acid Profiles

Properties of Vegetable Oil Esters

Properties to Consider

Two types of specifications in biodiesel standards (ASTM D6751; EN 14214):

Properties inherent to fatty esters:

• 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

• CN can be correlated to NOx exhaust emissions

• Saturated compounds (higher CN) show reduced NOx exhaust emissions

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 / 24.4 C18:3 9c,12,15c / 22.7

C10:0 / 51.6

C18:2 9c,12c / 38.2

C18:1 9c / 59.3

EN 14214 =51 min ASTM D6751 = 47 min

C16:0 / 85.9

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):

0 20 40 60 80 100

NO x and PM Exhaust Emissions of

Petrodiesel, Biodiesel, Their Components

0.0 0.5 1.0 1.5 2.0 2.5 3.0

2007PM Standard

G Knothe, C.A Sharp, T.W Ryan III, Energy & Fuels 20, 403-408 (2006)

2003 Engine; EPA Heavy Duty Test

Number of carbon atoms in the fatty acid chain

Saturated methyl esters (ME)

Saturated ethyl esters Mono-, di-, and triunsaturated ME Highly polyunsaturated ME

C18:2 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 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

-60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80

Number of carbon atoms in the fatty acid chain

Saturated methyl ester Saturated ethyl ester

41.3

48.6

55.9

11.8 -1.8

-20.4 -37.4

• 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 ofyear 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:

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 hMethyl oleate 2.79 h

Methyl linoleate 0.94 hMethyl linolenate 0.00 h

Methyl eicosatetraenoate (C20:4) 0.09 hMethyl docosahexaenoate (C22:6) 0.07 h

• ASTM D6751 minimum specification 3h

• EN 14214 minimum specification 6h

• Almost always antioxidant additives required

• 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):

Biodiesel and Lubricity

• Higher lubricity with increasing number of double bonds and greater chain length:

• 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

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

● 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

Major advantage compared to methyl oleate:

• Improved cold flow, lower kinematic viscosity

G Knothe; Energy & Fuels 22, 1358-1364 (2008)

High oxidative stability: All > 24 h

Extrapolation of exhaust emissions for C10 esters:

NOx likely slightly reduced (ca -5%); PM significantly reduced 85%); CO reduced; HC increased

(80-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

-Shorter-Chain Saturates: Cuphea Methyl Esters

Properties of cuphea PSR23 methyl esters (CuME):

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

ASTM D6751 47 min 1.9-6.0 3 min

EN 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

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) acidsmost 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 inthe 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