Biodiesel Composition and Fuel Properties

Transcription

1 Biodiesel Composition and Fuel Properties Gerhard Knothe USDA / ARS / NCAUR Peoria, IL U.S.A. gerhard.knothe@ars.usda.gov

2 the Diesel Engine It All Began With Diesel s Vision: Develop an engine more efficient than the steam engine, but...rudolf Diesel did not originally investigate vegetable oils as fuel. Diesel s first engine Rather

3 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: (1912). Chem. Abstr. 6:1984 (1912).

4 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: (1912). Chem. Abstr. 6:1984 (1912).

5 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.

6 The First Report on Biodiesel Belgian Patent 422,877 (1937): Procédé de transformation d huiles végétales en vue de leur utilisation comme carburants.

7 An Extensive Report on Biodiesel

8 Old Research: First Cetane Number Determination for Biodiesel Bulletin Agricole du Congo Belge, Vol. 33, p (1942):

9 (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

10 Why biodiesel and not the neat oil? CH 2 -OOCR 1 OOCR R 1 CH 2 OH Catalyst CH-OOCR OH R OOCR R 2 + CHOH CH 2 -OOCR 3 OOCR R 3 CH 2 OH Vegetable Oil Alcohol Vegetable Oil Alkyl Esters Glycerol (Triacylglycerol) (Biodiesel) Viscosity! mm 2 /sec 4-5 mm 2 /sec Kinematic viscosity of petrodiesel fuels usually mm 2 /sec.

11 Major Ester Components of Most Biodiesel Fuels Fatty esters in from common vegetable oils (palm, soybean, canola/rapeseed, sunflower, etc): Methyl palmitate (C16:0): CH 3 OOC-(CH 2 ) 14 -CH 3 Methyl stearate (C18:0): CH 3 OOC-(CH 2 ) 16 -CH 3 Methyl oleate (C18:1, 9c): CH 3 OOC-(CH 2 ) 7 -CH=CH-(CH 2 ) 7 -CH 3 Methyl linoleate (C18:2; all cis): CH 3 OOC-(CH 2 ) 7 -(CH=CH-CH 2 ) 2 -(CH 2 ) 3 -CH 3 Methyl linolenate (C18:3; all cis): CH 3 OOC-(CH 2 ) 7 -(CH=CH-CH 2 -) 3 -CH 3 From other oils: Methyl laurate (C12:0): CH 3 OOC-(CH 2 ) 10 -CH 3 Methyl ricinoleate (C18:1, 12-OH; cis): CH 3 OOC-(CH 2 ) 7 -CH=CH-CH 2 -CHOH-(CH 2 ) 5 -CH 3 Algal Oils: Methyl eicosapentaenoate (C20:5): CH 3 OOC-(CH 2 ) 3 -(CH=CH-CH 2 -) 5 -CH 3 Methyl docosahexaenoate (C22:6): CH 3 OOC-(CH 2 ) 2 -(CH=CH-CH 2 -) 6 -CH 3

12 Minor Constituents in Biodiesel Can influence fuel properties Cold flow, oxidative stability, corrosion, combustion, catalyst poisons, lubricity CH 2 -OOCR 1 CH 2 OOCR 1 CH 2 OOCR CH-OOCR 2 CHOOCR 2 CHOH CH 2 -OOCR 3 CH 2 OH CH 2 OH Triacylglycerols Diacylglycerols Monoacylglycerols Glycerol Free Fatty Acids: R-COOH Alcohol Na, K, Ca, Mg, P, (S) Sterol glucosides

13 Technical Problems with Biodiesel Cold flow Oxidative stability NO x exhaust emissions May fade with time due to new exhaust emissions control technologies. Other fuel quality issues: Minor components influencing fuel properties.

14 Biodiesel Standard ASTM D6751-(11a) Property Test method Limits Units Flash point (closed cup) D min o C Alcohol control. One of the following must be met: 1. Methanol content EN max % volume 2. Flash point D min 130 min Water and sediment D max % volume Kinematic viscosity, 40 o C D mm 2 / s Sulfated ash D max % mass Sulfur D or max a) % mass Copper strip corrosion D 130 No. 3 max Cetane number D min Cloud point D 2500 Report o C Carbon residue D max % mass Acid number D max mg KOH / g Free glycerin D % mass Total glycerin D % mass Phosphorus content D max % mass Distillation temperature, D max o C Atmospheric equivalent temperature, 90% recovered Sodium and potassium, combined EN max ppm (µg/g) Calcium and magnesium, comb. EN max ppm (µg/g) Oxidation stability EN min hours Cold soak filterability D max sec a) The limits are for Grade S15 and Grade S500 biodiesel, respectively. S15 and S500 refer to maximum sulfur specifications (ppm).

15 Biodiesel Standard EN Property Test method Limits Units Ester content EN min % (m/m) Density; 15 o C EN ISO 3675, kg/m 3 Viscosity, 40 o C EN ISO 3104, ISO mm 2 /s Flash point EN ISO 2719, min o C Sulfur content EN ISO 20846, max mg/kg Carbon residue (10% dist. res.) EN ISO max % (m/m) Cetane number EN ISO min Sulfated ash ISO max % (m/m) Water content EN ISO max mg/kg Total contamination EN max mg/kg Copper strip corrosion (3h, 50 o C) EN ISO Oxidative stability, 110 o C EN 14112, min h Acid value EN max mg KOH / g Iodine value EN max g iodine /100g Linolenic acid content EN max %(m/m) Content of FAME with 4 double bonds 1 max % (m/m) Methanol content EN max % (m/m) Monoglyceride content EN max % (m/m) Diglyceride content EN max % (m/m) Triglyceride content EN max %(m/m) Free glycerine EN 14105, max %(m/m) Total glycerine EN max %(m/m) Alkali metals (Na + K) EN 14108, 14109, max mg/kg Earth alkali metals (Ca + Mg) pren max mg/kg Phosphorus content EN max mg/kg

16 Some Fatty Acid Profiles Vegetable Oil C16:0 C18:0 C18:1 C18:2 C18:3 Palm Rapeseed / Canola Soy Sunflower Jatropha

17 Properties of Vegetable Oil Esters Methyl Ester Cloud Point Cetane Number Kin. Visc. ( C) (40 C; mm 2 /s) Palm Rapeseed / Canola Soy Sunflower Jatropha 4-5 Oxidative stability: usually antioxidants required to meet standard specifications

18 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

19 Some General Observations on Fatty Ester Fuel Properties Fuel properties of fatty esters depend on Chain length (number of CH 2 moieties) Number and position of double bonds

20 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.

21 Cetane Numbers Saturated methyl esters (ME) Saturated ethyl esters Mono-, di-, and triunsaturated ME Highly polyunsaturated ME C16:0 / 85.9 C18:0 / 101 Cetane Number C10:0 / 51.6 C18:1 9c / 59.3 EN =51 min ASTM D6751 = 47 min C18:2 9c,12c / 38.2 C20:4 / C18:3 9c,12,15c / 22.7 C22:6 / Number of carbon atoms in the fatty acid chain

22 Cetane Number Cetane numbers of mixtures: CN mix = A C x CN C (CN mix = CN of the mixture, A C = relative amount of an individual neat ester in the mixture, CN C = 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

23 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

24 Exhaust Emissions Studies Average effect of biodiesel and B20 vs. petrodiesel on regulated emissions (Source: USEPA report 420-P ): 100 Petrodiesel Biodiesel 100 Petrodiesel B20 Relative emissions Relative Emissions NOx PM CO HC Pollutant 0 NOx PM CO HC Pollutant

25 NO x and PM Exhaust Emissions of Petrodiesel, Biodiesel, Their Components 2003 Engine; EPA Heavy Duty Test PM Standard NOx PMx10 Brake-Specific Emission Rate, g/hp-hr 0.0 Base Soy Biodiesel Methyl Oleate Hexadecane Dodecane Methyl Palmitate Base 2 Methyl Laurate G. Knothe, C.A Sharp, T.W. Ryan III, Energy & Fuels 20, (2006).

26 Change in NO x and PM vs. petrodiesel Change in Exhaust Emissions Relative to Base Fuel (%) NOx PM Hexadecane Dodecane Me soyate Me oleate Me palmitate Me laurate

27 Change in HC and CO vs. petrodiesel Change in Exhaust Emissions Relative to Reference Fuel (%) Hydrocarbons CO Me oleate Hexadecane Me soyate Me palmitate Dodecane Me laurate

28 Viscosity Kinematic Viscosity (40 o C; mm 2 /s) Saturated methyl esters (ME) Saturated ethyl esters Mono-, di-, and triunsaturated ME Highly polyunsaturated ME C12:0 / 2.43 C10:0 / 1.72 C16:0 / 4.38 C18:0 / 5.85 C18:3 9c,12c,15c / 3.14 C18:1 9c / 4.51 C22:1 13c / 7.33 C18:2 9c,12c / 3.65 ASTM D6751 upper limit EN upper limit EN lower limit C22:6 / 2.97 C20:4 / 3.11 ASTM D6751 lower limit Number of carbon atoms in the fatty acid chain

29 Viscosity Viscosity increases with chain length and increasing saturation. Kinematic viscosity of mixtures ν mix ν mix = A c x ν c Virtually all biodiesel fuels meet ASTM D6751 specifications EN more restrictive Biodiesel fuels with greater amounts of lower-viscosity components may not meet lower limit

30 Cold Flow: Melting Points of Fatty 80 Acid Esters Melting Point ( o C) Saturated methyl ester Saturated ethyl ester :1 9c / :1 9c / :1 11c / :2 9c,12c / Number of carbon atoms in the fatty acid chain :3 9c,12c,15c / < :1 13c / -3.1

31 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

32 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

33 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 CH 2 positions H 3 CO 2 C-(CH 2 ) x -CH 2 -CH=CH-CH 2 -CH=CH-CH 2 -(CH 2 ) y -CH 3 especially: bis -allylic CH 2 positions

34 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?

35 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 Oxidative Stability Methyl eicosatetraenoate (C20:4) 0.09 h Methyl docosahexaenoate (C22:6) 0.07 h ASTM D6751 minimum specification 3h EN minimum specification 6h Almost always antioxidant additives required

36 Density Only in EN Range of g/cm 3 (15 C) Not a problem for most biodiesel fuels. Only highly polyunsaturated fatty esters may be problematic: C20: g/cm 3 C22: g/cm 3 Density of a mixture: ρ mix = A c x ρ c

37 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).

38 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, (2005).

39 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, (2005).

40 Property Trade-off Increasing chain length: Higher melting point (-) Higher cetane number (+) Increasing unsaturation: Lower melting point (+) Decreasing oxidative stability (-) Lower cetane number (-)

41 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 modification D Inherently different fatty acid profile Alternative feedstocks E G. Knothe; Energy & Environmental Science, 2, (2009).

42 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

43 Influence of Alcohol Moiety Branched and longer-chain esters: Lower melting points, similar cetane numbers compared to methyl esters Ester M.P. ( C) CN Ester M.P. ( C) CN C16:0 Methyl C18:0 Me C16:0 Ethyl C18:0 Et C16:0 Propyl C18:0 Pr C16:0 iso-propyl C18:0 i-pr 96.5 C18:1 Methyl C18:2 Me C18:1 Ethyl C18:2 Et C18:1 Propyl C18:2 Pr 44.0 C18:1 iso-propyl 86.6 Disadvantage: Higher costs of alcohols Source: Handbook of Chemistry and Physics; The Lipid Handbook, various publications.

44 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

45 Shorter-Chain Monounsaturates Methyl palmitoleate (C16:1) Melting point: -34 C Cetane number: (ASTM D6890) Kinematic viscosity (40 C): 3.67 mm 2 /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 mm 2 /s Major advantage compared to methyl oleate: Improved cold flow, lower kinematic viscosity G. Knothe; Energy & Fuels 22, (2008).

46 Shorter-Chain Monounsaturates: Macadamia nut oil methyl esters: An Example Two examples: 16 and 20 % C16:1; 59 and 55% C18:1 9; 4% C18:1 11. Cetane number: Oxidative stability: 2 h Kinematic Viscosity: 4.5 mm 2 /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, (2010).

47 Shorter-Chain Saturates M.P. Cetane Kin. Visc. Heat of comb. ( C) number (40 C; mm 2 /s) (kj/kg) Methyl octanoate Ethyl octanoate Methyl decanoate Ethyl decanoate Methyl laurate 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

48 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 0.3 C10: C12:0 3.0 C14:0 4.5 C16: C18: C18: C18: C18:3 <

49 Shorter-Chain Saturates: Cuphea Methyl Esters Properties of cuphea PSR23 methyl esters (CuME): Cetane number: Kinematic viscosity (40 C): mm 2 /s Oxidative stability: h Cloud point: -9 to -10 C G. Knothe, S.C. Cermak, R.L. Evangelista; Energy & Fuels, 23, (2009).

50 Distillation Curve: CuME vs SME and ULSD B.T. Fisher, G. Knothe, C.J. Mueller, Energy Fuels, 24, (2010).

51 Castor Oil Methyl Esters Fatty acid profile of castor oil 85-90% ricinoleic acid Cetane Kinematic Viscosity Oxidative Number (40 C; mm 2 /s) Stability (h) Castor methyl esters ASTM D min min EN min min Cold flow related properties: Melting point of methyl ricinoleate: Pour point of castor methyl esters: -5.8 C -20 C C18:1 12-OH

52 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.

53 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

54 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 Kinematic viscosity (40 C) of most algal biodiesel likely in the range mm 2 /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.

55 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

56 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, ) i Usually greater number of components than vegetable oils Fatty acid profiles of a species depend on growing conditions such as Temperature Light Nutrients.

57 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

58 Biodiesel vs. Renewable Diesel: Mass (Energy) Balance of Products Biodiesel - Methyl oleate from triolein: C 57 H 104 O CH 3 OH 3 C 19 H 36 O 2 + C 3 H 8 O x = = 100.5% mass kj/kg x = kj kj/l Renewable Diesel - Heptadecane from triolein: C 57 H 104 O H 2 3 C 17 H CO 2 + C 3 H x = = 81.5% mass kj/kg x = kj kj / L Glycerol and propane not accounted for here.

59 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.

60 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.

61 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.

62 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: (1912). Chem. Abstr. 6:1984 (1912).