Energy & Fuels 1995,9, 248-256
248
Liquid Hydrocarbons from Catalytic Pyrolysis of Sewage Sludge Lipid and Canola Oil: Evaluation of Fuel Properties Nisheeth P. Bahadur, David G. B. Boocock,* and Samir K. Konar Department of Chemical Engineering, 200 College Street, University of Toronto, Toronto, Canada M5S lA4
Received September 8, 1994. Revised Manuscript Received December 22, 1994@
The physical fuel properties which are routinely used to define fuel quality were measured for the unrefined hydrocarbon liquids produced by the pyrolysis of sewage sludge lipid, a carboxylic acid-rich substrate and canola oil (a triglyceride) over activated alumina at 450 "Cand atmospheric pressure; comparisons were made with the properties of diesel. The measured properties included cloud point, pour point, specific gravity, viscosity, distillation range, cetane index, flash point, heat of combustion, water and sediment, ash, carbon residue, and elemental analysis. Pyrolysis products were also analyzed by gas chromatography and infrared and 13C NMR spectroscopy. Specific gravity, viscosity, flash point, ash content, carbon residue, and heating values of both products were well within the ASTM prescribed andlor typical range of diesel fuel. The liquid products from sewage sludge lipid and canola oil pyrolysis had cetane values of 48.9 and 47.5, respectively, both of which are above the ASTM specified minimum limit (40). The cloud points of both liquid products (-3 and -12 "C, respectively) were higher than that of diesel fuel. However, removal of the upper 20% distillate lowered the cloud point of both products quite significantly (-23 and -30 "C, respectively). The products satisfied the average volatility requirements of diesel fuel except that the front-end temperatures were low (%60"C)compared to diesel ( ~ 1 7 "C). 0 The carbon, hydrogen, nitrogen, and sulfur contents as well as the infrared and proton-decoupled 13CNMR spectra of both products were similar to those of diesel fuel. Gas chromatograms of the liquid products showed a uniform concentration across the C6 to C17 mass range, whereas for diesel fuel a distribution across the C10 to C19 mass range is common. These results suggest that the major fractions of these liquid products could be viable diesel fuel substitutes or cetane enhancers.
Introduction Recently reviewed supply and demand scenarios suggest that the demand for transportation fuels will continue t o rise. The production of alternative fuels derived from biological materials has already received much attention. One of the possible convenient routes is thermochemical liquefaction by which biomass such as vegetable oils and dry sewage sludge can be converted to fuels. Sewage sludge is an inevitable major byproduct of wastewater treatment and its disposal is a great problem. Sludge treatment costs can account for 50%of the capital and operating costs of wastewater treatment faci1ity.l The major fates of sewage sludge are agricultural utilization, incineration, landfill, and ocean disposal. Sludges of industrial origin often have high heavy metal content, ruling out land application. The availability of landfill sites is extremely limited for most municipalities so the major form of sludge disposal is incineration. One of the ways to reduce the cost of sewage sludge disposal is to produce hydrocarbon liquids. * To whom correspondence should be addressed.
Abstract published in Advance ACS Abstracts, February 1, 1995. (1)Boocock, D. G . B.; Agblevor, F.; Chirigoni, T.; Crimi, T.; Khelawan, A,; Campbell, H. In Research in Thermochemical Biomass Conuersion; Bridgwater, A. V., Kuester, J. K., Eds.; Elsevier Applied
Direct liquefaction of sewage sludge results in a foulsmelling viscous liquid which has high nitrogen and sulfur contents. Boocock et a1.l confirmed that nitrogen and sulfur arise from the protein fraction in the sludge. They also showed that the lipid fraction is the major source of useful hydrocarbons. The lipid fraction (18 w t %) of a typical raw sludge was found to contain 65% straight chain carboxylic acids, 28% unsaponifiables, and 7% triglycerides.2 When triolein, representative of an unsaturated triglycerides, was added to the sewage sludge it did not survive the pyrolysis. However, stearic acid, a straight chain carboxylicacid, for the most part, did survive and resulted in an increase in viscosity of the oil. Since triglycerides did not survive the sewage sludge pyrolysis and are in themselves a potentially important substrate in the form of vegetable oils, their fate during pyrolysis is important. Therefore, subsequent studies were done on the pyrolysis of vegetable oils3 such as canola oil (a representative mixture of unsaturated and saturated triglycerides). The liquid products after pyrolysis were hydrocarbon mixtures of alkanes and alkenes and comparable to diesel fuel. Boocock et aL4 investigated the pyrolysis of tolueneextracted lipid from dry raw sewage sludge over activated alumina. The pyrolysis yielded very mobile liquid
@
Science: Essex, UK, 1988; pp 816-826.
0887-0624/95/2509-0248$09.00/0
(2) Boocock, D. G. B.; Konar, S. K.; Leung, A.; Ly, L. D. Fuel 1992,
71, 1283-1289.
(3) Boocock, D. G. B.; Konar, S. K.; Mackay, A,; Cheung, P. T. C.; Liu, J. Fuel 1992, 7 1 , 1291-1297.
0 1995 American Chemical Society
Pyrolysis of Sewage Sludge Lipid
products containing straight-chain alkanes and alkenes with low nitrogen and sulfur content; these products appear to have potential for use as high-grade transportation fuels. Data on the fuel properties of the catalytic pyrolysis products are lacking. The primary objective of the current work has been to measure the relevant physical properties of the above liquid hydrocarbons from which fuel characteristics can be predicted and compared to those of diesel fuel.
Experimental Section Materials. Dry Atlanta raw sludge was supplied by the Wastewater Technology Centre of Environment Canada (Burlington, Ontario, Canada L7R 4A6). Technical grade toluene was obtained from Canada Colour and Chemicals, Toronto, Canada. Benzoic acid (purity 98%) was purchased from BDH Chemicals, Toronto, Canada. Activated alumina (Alcan AA 200,8 x 14 mesh; a mixture of chi, eta, and some boehimites; BET surface area, 280-300 m2/g;average pore size, 40 A) was supplied by Alcan Chemicals, Cleveland, OH. Canola oil was prepared by Spectrum Naturals, 133 Copeland street, Petaluma, CA 94952. Diesel oil (type 2, summer grade) was obtained from a gas station in Toronto. Infrared spectra were recorded on a Perkin-Elmer Model 1310 infrared spectrophotometer. Proton-decoupled 13CNMR spectra were obtained on a Gemini Varian 200 MHz spectrometer. Gas chromatographic analyses for the liquid products were performed on a Hewlett Packard 5880A series gas chromatograph equipped with a flame ionization detector and a DB 17 (30 m x 0.58 mm) fused silica column. The operating parameters were as follows: detector temperature 250 "C; injector temperature 250 "C; temperature program, 5 min at 50 "C; heated a t a rate of 50 W m i n to 210 "C; hold for 30 min. The carrier gas (He) flow rate was 8 mumin. Analytical standards were supplied by Polyscience Corp., IL. Carbon, hydrogen, nitrogen, and sulfur analyses of the liquid pyrolysis products were performed by Schwarzkopf Microanalytical Laboratory, New York. The carbon residue test (ASTM D 189) was performed by Petro Laboratory, 140 Advance Blvd., Unit 8, Brampton, Ontario L6T 454. Pyrolysis Apparatus and Procedures. Both the sewage sludge lipid and the canola oil were pyrolyzed to produce liquid hydrocarbon products. The lipid fraction of a dried raw Atlanta sludge was extracted with toluene using a Soxhlet extraction method. The detailed extraction and pyrolysis methods have been discussed elsewhere.2 However, a brief description of apparatus and procedure is included here for convenience. The pyrolysis unit consisted of a 316 stainless steel feed preheater tube (1.3 cm i.d. x 46 cm length), a block heater containing a 316 stainless steel fixed reactor tube (2.5 cm i.d. x 46 cm length), a chromel-alumel thermocouple probe, a temperature controller, a syringe pump, a condenser, a gas trap, a gas collection vessel, and a nitrogen cylinder. For each run, the reactor was packed with 40 g of activated alumina catalyst. The feed preheater and the reactor tubes were operated at 200 and 450 "C, respectively, both a t normal atmospheric pressure. During the heating, the system was flushed with nitrogen to remove air and to ensure t h a t all moisture was driven from the catalyst. A final purge was conducted just prior to liquefaction to establish a nitrogen atmosphere in the reactor vessel. The toluene extracted lipid was injected at approximately 0.46 weight hourly space velocity (WHSV) into the preheater tube using the 40 mL of sample in 50 mL syringe. Normal vaporization, accompanied by reactions, drove the material through the reactor, Le., no inert gas was used. Since toluene extracted lipid is semisolid a t room temperature, it was melted and maintained in liquid (4) Boocock, D. G. B.; Konar, S. K.; Leung, A.; Liu, J.; Ly, L. D. In Advances in Thermochemical Biomass Conversion;Bridgewater,A. V., Ed.; Chapman and Hall: Glasgow, UK, 1994; pp 986-999.
Energy & Fuels, Vol. 9,No. 2, 1995 249 Table 1. List of Fuel Properties Investigated property
ASTM method
specific/API gravity kinematic viscosity distillation cetane index cloud point pour point flash point heat of combustion water and sediments ash carbon residue
D 1298 D 445 D 86 D 976 D 2500 D 97 D 56 D 240 D 1796 D 482 D 189
form by wrapping a heating coil around the syringe. Canola oil was also pyrolyzed in the same manner. The pyrolyzed products exiting the reactor tube were condensed (in watercooled condenser) and collected in a cooled receiving flask. The liquid hydrocarbon yield from sewage sludge lipid and canola oil pyrolysis were 74.9 and 70.8%. The products from several runs were combined to provide sufficient hydrocarbon product for testing. After each run, the products were examined by IR and 13C NMR to provide information on their chemical composition, in particular the absence or presence of carbonyl groups. Fuel Testing Procedures. Fuel tests were performed on both sewage sludge lipid pyrolysis (SSLP) and canola oil pyrolysis (COP) liquid products in accordance with ASTM procedure^^^^ (as listed in Table 1). The same tests were also performed on diesel fuel for comparative purposes. Unfortunately, the current apparatus did not allow us to prepare the large quantity of samples required for engine testing.
Results and Discussion Table 2 summarizes the overall results of the tests conducted on the unrefined hydrocarbon liquids produced from sewage sludge lipid and canola oil pyrolysis, as well as the diesel fuel. Table 3 lists the ASTM D 975 requirements for the No. 1-D and No. 2-D grades of diesel fuels. Average properties of diesel fuel reported in a U.S.Department of Energy conducted survey,7and Japan and typical range of values found in the U.S., Germany during a worldwide product quality survef carried out in 1988, are shown in Tables 4 and 5, respectively. Fuel Properties and Their Probable Influence on Performance. Fuel properties are closely interrelated and individual properties are frequently influenced by more than one fuel parameter. For example, cetane number is a function of density, volatility, and composition. It is very difflcult to design test fuel matrices that break these intrinsic intercorrelations and, therefore, the greatest source of difficulty in the fuel performance debate lies in the interpretation of test data. Therefore, the measured properties only provide a reference point for quality. From national or guide standard specifications for automotive diesel fuels, background information, and comparison of results from published research, some prediction can be made about general performance such as combustion, engine per(5) Annual Book ofASTM Standards, section 5, Vol05.01, Petroleum Products, Lubricants and Fossil Fuels; American Society for Testing and Materials: Philadelphia, 1992. (6) Bahadur, N. P. M.A.Sc. Thesis, Department of Chemical Engineering, University of Toronto, Toronto, Canada, 1994. (7) Shelton, E. M. Diesel Fuel Oils, 1982; Report No. DOE/BETC/ PPS-82/5, Bartlesville Energy Technology Centre, U.S. Department of Energy, Bartlesville, OK, 1982. (8)Keith, 0.; Coley, T. Automotive Fuels Handbook; Society o f Automotive Engineers, Inc.: Warrendale, PA, 1990.
Bahadur et al.
250 Energy & Fuels, Vol. 9,No. 2, 1995
Table 2. Fuel Properties of Pyrolyzed Hydrocarbon Liquid Products Compared with Those of Diesel Fuel property ASTM method SSLP hydrocarbon liquid COP hydrocarbon liquid diesel fuel Specific gravity, at 15.6 "C (60 "F) D 1298 0.8325 0.8310 0.8520 API gravity kinematic viscosity, cSt at 40 "C distillation, "C IBP
D 445 D 86
10% 20% 50% 90% 95%
cetane index cloud point, "C" pour point, "C flash point, "C heat of combustion kJlg kJlmL water and sediment, vol % ash, mass % sulfur, mass%b Carbon residue, mass % Conradson Ramsbottom a
D 976
D 2500 D 97 D 56 D 240 D 1796 D 482 D 189 D 524
38.5 1.88
38.8 2.07
34.6 2.20
62 91 155 250 330 335 48.9 -3 (-23) -30 55
60 90 130 243 322 325 47.5 -12 (-30) -33 53
165 203 217 251 306 308 42.8 -18 -27 58
46.18 38.38 0.06 0.01 0.22
46.00 38.19 0.05 0.01
n.a.c
45.89 39.06 0 0.02 0.22
0.05 0.10
0.13 0.17
0.10 0.15
Cloud point values shown in the parentheses are those for the refined samples (see text). Not by ASTM method. n.a. = not applicable.
Table 3. Specification for Diesel Fuels, ASTM D 975 property
ASTM method
kinematic viscosity, cSt, 40 "C min max cloud point, "C flash point, "C (min) cetane number (min) distillation, "C (90 vol % recovered) min max sulfur, mass % (max) ash, mass % (max) water and sediment, vol % (max) carbon residue on 10%bottoms, mass %
D 445
grade No. 1-D
grade No. 2-D
1.3 2.4
1.9 4.1
D 2500
a
a
D 93
38 40
52 40
288 0.50 0.01 0.05 0.15
282 338 0.50 0.01 0.05 0.35
D 613 D 86 D 129 D 482 D 1796 D 524
Cloud point is specified for area and season.
Table 4. Average Properties of Diesel Fuelsa property
grade No. 1-D
grade No. 2-D
specific gravity gravity, "API viscosity, cSt at 40 "C distillation, "C IBP
0.8132 42.5 1.70
0.8524 34.5 2.85
175 197 220 251 272 4.8.0 46.9 59 0.003 0.086
195 225 265 313 340 44.4 45.8 76 0.002 0.272
10% 50% 90%
end point cetane number cetane index flash point, "C ash, mass % sulfur, mass %
Data available from U. S. Department of Energy survey report.6
formance, fuel handling system, and low temperature and emission characteristics. Specific GravityIAPI Gravity. As evident from the Table 2, the specific gravities of the liquid hydrocarbon products are a little lower than that of a diesel fuel and the respective API gravities are higher. Of the two hydrocarbon products, COP liquid has a slightly higher specific gravity than the SSLP liquid. Specific gravity or API gravity is not featured in standard specifications of diesel fuel, ASTM D 975, but a typical range of
0.8132-0.8524 for specific gravity has been reported by the US. Department of Energy conducted survey7 (Table 4). The density has an importance for various aspect of diesel engine performance. Diesel injection equipment meters by volume, so changes in density will influence engine output due to a different mass of fuel injected. The slightly lower specific gravities of pyrolyzed liquid products compared with the diesel fuel result in only 1.74% decrease in heat of combustion by volume, which is probably not significant. Viscosity. The viscosity of a fuel has a strong influence on fuel injector and engine performance. Therefore, it can adversely effect engine performance if it falls outside the accepted range.g According t o ASTM specifications (D 975) (Table 31, the prescribed range of kinematic viscosity for diesel fuel (grade No. 1-D and 2-D) is 1.3-4.1 cSt at 40 "C. An upper limit has been imposed to ensure that fuel will flow readily during cold starting, and a lower minimum limit is often specified to avoid the possibility of serious power loss at high temperature. The kinematic viscosities of the liquid hydrocarbons are lower than that of the diesel fuel tested (Table 2) whereas the kinematic viscosity of the COP hydrocarbon liquid is slightly higher than that (9) Criteria for Quality of Petroleum Products; Allison, J. P., Ed.;
Applied Science: Barking, UK, 1973.
Energy &Fuels, Vol. 9, No. 2, 1995 251
Pyrolysis of Sewage Sludge Lipid
Table 6. Typical Diesel Fuel Characteristicsa property density, g/mL, at 15 "C viscosity, cSt, at 20 "C distillation ( " C ) IBP 20% 50% 90% FBP cetane number cetane index cloud point, "C pour point, "C sulfur, mass % a
USA (21 samples mean values)
Japan (20 samples mean values)
Germany (16 samples mean values)
0.8556 4.56
0.8350 4.73
0.8319 3.54
180 239 269 316 346 45.5 45.8 -11 -24 0.21
168 237 275 326 348 54.5 53.7 -6 -16 0.38
162 214 254 322 360 49.5 50.2 -7 -26 0.17
Average values found in the U. S., Japan, and Germany during a worldwide product quality survey.6
of the SSLP hydrocarbon liquid. However, these viscosities lie toward the middle of prescribed ASTM range. They also lie well within the average values of viscosity (1.70-2.85 cSt) for diesel fuel as listed in Table 4. Distillation. The distillation ranges of the hydrocarbon liquids and diesel fuel are reported as temperatures corresponding to initial boiling point (IBP) 10, 20, 50, 90, and 95% volume recovered. It is clear from Table 2 that the IBP of both the hydrocarbon liquid is much lower than that of diesel fuel. The 10 and 20% points are also low compared with the diesel fuel. This clearly indicates that pyrolysis liquid contains some low molar mass material. The COP liquid has a little more of this low boiling material than that the SSLP liquid. The 50% point of SSLP hydrocarbon liquid is nearly the same as diesel fuel whereas that of the COP hydrocarbon liquid is a little lower. Both hydrocarbon liquids have higher 90 and 95% points compared with the diesel fuel. SSLP and COP hydrocarbon liquids have a boiling range covering a temperature spread of around 268 and 262 "C, respectively, from IBP to 90% point. In the case of diesel fuel this range is only 141 "C. From Tables 4 and 5 the overall distillation temperature spread for typical diesel fuels found in the U.S. are around 145 and 136 "C, respectively, from initial boiling t o 90% point. The shapes of the distillation curves (Figure lA,B) for the hydrocarbon liquids are almost identical. However, they differ from diesel fuel at the low- and hightemperature ends where distillation volumes per "C rise are lower. As evident from Figure 2A, diesel fuel contains components that boil within a relative narrow range. It also shows a uniform distillation volume per "C rise across the temperature range. Front-end volatility is not usually specified in ASTM D 975; however, the 90% distillation point is limited to the range of 282338 "C. At certain times, it may be necessary to lower the temperature at the back end. For example, cold property specifications, which reflect the climate of the country of use, do impose, to a certain extent, constraints on distillation range particularly the back-end temperature. As can be seen from the Table 2, the 90% distillation temperatures of both the hydrocarbon liquids falls in this range. Comparison of distillation ranges and volatility curves of pyrolyzed hydrocarbon liquids with those of a diesel fuel indicates that hydrocarbon liquids fulfill the average volatility requirements of a fuel. It may, however, be necessary to raise the front-end temperature by
Temperature O C 350 1
300
50
FInp
0' 0
10
I 20
30
40
50 80 K Evaporated
70
80
90
100
30
40 60 60 K Evaporated
70
80
90
100
Temperature OC 350 I
260 200
Y
0
10
20
Figure 1. Distillation curve of (A) SSLP hydrocarbon liquid and (B) COP hydrocarbon liquid.
removing some of the low boiling fractions from the distillate. The presence of low boiling compounds might make the fuel volatile enough to cause preignition in the engine resulting in poor combustion, high levels of smoke, and carbon residuals. There could also be vapor lock in the fuel injection system causing engine misfiring or failure to restart after a brief shutdown in hot weather. How much low boiling cut is needed t o be removed will be determined by selection of suitable front-end volatility (IBP andlor 10%point). In order to match distillation characteristics of the diesel fuel more closely, the IBP's of the products (SSLP and COP) were set at 165 "C. The low boiling fractions which therefore had to be excluded theoretically from the above liquid
Bahadur et al.
252 Energy & Fuels, Vol. 9, No. 2, 1995
i
A
100
1
I
I
0'
,
0
10
20
30
40 50 60 K Evaporated
,
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4- SSLP product COP Droduct
0
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io
20
30
40
50
eo
70
80
90
100
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Figure 2. (A) Distillation curve of diesel fuel and (B) comparison of distillation curves: SSLP/COP hydrocarbon liquids (the fractions remaining after theoretically excluding the material boiling below 165 "C (extended on a 0-100 scale)) and diesel fuel.
products amounted to approximately 24 and 28 vol %, respectively. The remaining sections of the boiling point curves (marked in Figure lA,B) were extended on a 0-100 scale. As shown in Figure 2B, the distillation curves of both the hydrocarbon liquids now look similar to that of diesel fuel. The mid-boiling points shift to 268 and 274 "C for SSLP and COP hydrocarbon liquids,respectively. However, there might be a slight increase in specific gravity, viscosity, flash point, and cloud and pour point after removal of low boiling fractions. The cetane index is not expected to change much from its original value as increase in specific gravity would be counterbalanced by increase in midboiling point in nomograph (ASTM D 976).5 Cetane Index. The calculated cetane indices of the hydrocarbon liquids were higher than that of the diesel fuel. Also, the cetane index for the SSLP hydrocarbon was a little higher than that of the COP hydrocarbon liquid (see Table 2). The cetane index is a useful tool for estimating ASTM cetane number where either a test engine is not available or quantities of samples are too small for an engine rating. The calculated cetane index values are usually similar t o the cetane number kt2 units) especially in the range of 30-60 cetane as mentioned in ASTM D976.5 Lowest cetane levels are found in North America where the specification minimum is 40, while in most other parts of the world the
minimum is at least 45. A typical average value found in U S . for diesel fuel is around 45 (refer Tables 4 and 5). The cetane numberhndex is a measure of ignition quality of a fuel and indicates the readiness of fuel t o ignite spontaneously under temperature and pressure conditions in the combustion chamber of the engine. The higher the cetane index, the shorter the ignition delay and the easier it is to ignite, Le., the better the ignition quality of fuel. Thus, the higher cetane values of pyrolyzed hydrocarbon liquids over the specified minimum limit make them attractive as alternative diesel fuels or cetane enhancers, where there is continuing stress on raising the cetane number of diesel fuel. This property is of greatest concern to most diesel fuel users as difficulties exist in meeting current specifications. This situation has developed as a result of the large amount of cracking operations needed in North American refineries to meet high demand for gasoline, leaving relatively low cetane blend components for absorption into diesel fuel. Cloud Point and Pour Point. The cloud and pour points affect the low-temperature fuel handling characteristics. As the fuel is cooled, the larger normal paraffin compounds solidify. At the cloud point temperature they begin to precipitate and form visible wax crystals. The crystals become large enough to plug most unheated fuel filters just below the cloud point. As the temperature is lowered further, the wax crystals grow and the fuel becomes more viscous and takes on the properties of a slurry. Eventually the wax crystals grow together and form a gel, which will not flow. The temperature just before this occurs is called the pour point. The cloud points of the hydrocarbon liquids were higher compared with the diesel fuel. The higher cloud points, particularly of the SSLP hydrocarbon liquid could be predicted from the distillation curves which show the presence of more higher-molar mass paraffinic hydrocarbons. However, the COP liquid hydrocarbon shows a lower cloud point (-12 "C) than the SSLP hydrocarbon liquid (-3 "C) as it contains lesser amounts of the back-end hydrocarbons. The cloud point measurements were repeated on "refined liquid samples from which the upper 20% distillate fraction was removed. A reduction of 20 and 18 "C in cloud point to -23 and -30 "C was found for the SSLP and the COP hydrocarbon liquids, respectively. This shows that less of the heavy distillate could have been rejected to attain the same cloud point (-18 "C) of the diesel fuel. The other properties such as viscosity, specific gravity, and flash point would go down slightly. The cold property specifications depend on the temperature of usage; therefore, cloud point is specified for area and season. For instance, the Canadian Government's specificationlo for diesel fuel recognizes five grades of diesel fuel differing mainly in their cloud points ranging from 0 to -34 "C. They are intended for distribution throughout different areas of the country at different seasons of the year. In ASTM specification (D 975), cloud point is specified for different U.S. (10) Steer, D. E.; Nunn, T. J. Diesel Fuel Quality Trends in Canada. SAE Paper No. 790922; Society of Automotive Engineers, Inc.: War-
rendale, PA, 1979.
Pyrolysis of Sewage Sludge Lipid
locations and seasons. However, typical average values of cloud point found in CanadalO are -15 "C (Great Lakes and Eastern Regions) and -35 "C (Western Regions) and in the U.S. it is -11 "C (Table 5). Thus, it can be concluded that COP hydrocarbon liquid as such may be suitable for most parts of United States and Canadian great lakes and eastern regions. Since refining process lowers the cloud point of both the hydrocarbon liquids quite significantly, these may be particularly suited to some cold climatic regions. The SSLP hydrocarbon liquid might be used without refining in regions where low cloud point requirements are not severe or at least during the summer season. The pour point of both the hydrocarbon liquids was lower than that of diesel fuel. However, the SSLP liquid has a higher pour point compared with the COP liquid. There is no ASTM specification,however, the Canadian Government's specificationloshows a range of -6 to -39 "C for different seasons and areas of the country and typical average value pour point found in CanadalO are -21 "C (Great Lakes and Eastern Regions) and -39 "C (Western Regions) and in U.S., it is -24 "C (Table 5). Thus, it is clear from the Table 2 that both the hydrocarbon liquids fulfil the pour point requirement of the Canadian specification for most grades of diesel fuel as well as matching the average U.S. values. Flash Point. The flash points of both the hydrocarbon liquids are a little lower than that of the diesel fuel. The flash point of the SSLP hydrocarbon liquid, however, is higher than that of the COP hydrocarbon liquid. The importance of flash point is primarily related to safe handling of the product. If the flash point is too low, there could be fire hazard and, for this reason, mandatory minimum limits on flash point have been set and storage criteria established by insurance companies and government agencies. In the U.S., the permitted minimum flash point for No. 1-D and No. 2-D grade diesel fuel are 38 and 52 "C, respectively, and in Canada it is minimum of 40 "C. However, typical minimum values for automotive diesel fuels range from 38 "C in the U.S. (ASTM D 975, No. 1-Dgrade) to 56 "C in some European countries.8 From the above data, it clear that pyrolyzed hydrocarbon liquids meet the specified minimum limits of U.S. and Canada and may be suited to some other countries as well. The flash point relates to the front-end volatility of the fuel; consequently, it will have an influence on the amount of light fractions present in a fuel sample. Thus by raising the front-end temperature (initial boiling point and 10% distillation temperature) sufficiently high, the flash point can also be raised such that it is at or above the specified legal minimum. Heat of Combustion. As is evident from Table 2, the heating values of the pyrolysed products are similar and comparable to that of diesel fuel, both on a mass and volume basis. Heat of combustion is an important fuel property. It is a measure of the energy available from a fuel when it is burned and is the basis for calculating the thermal efficiency of an engine using the fuel. The power available from an engine under constant running conditions and with the constant rate of fuel supply is governed by the calorific value of the fuel. A fuel of low calorific value yields less heat on combustion and, therefore, less power than the same amount of a fuel with higher calorific value. Consequently, to
Energy & Fuels, Vol. 9, No. 2, 1995 263 Table 6. Elemental Analyses of Pyrolyzed Liquid Products and Diesel Fuel component %C % H %Oa % N % S
SSLP liquid hydrocarbon
86.57 13.51 b 0.08 COP liquid hydrocarbon 84.67 13.87 1.46 diesel fuel 86.07 12.80 0.86 0.05 a By difference. Negligible. n.a. = not applicable.
0.22 n.a: 0.22
maintain power output with low calorific value fuel, more of it would have to be used." Water and Sediment. Water and sediment content of the SSLP hydrocarbon liquid was a little higher than that of the COP hydrocarbon liquid. ASTM specifications impose limits for water and sediment at 0.05 vol %. Thus, the COP hydrocarbon liquid meets the prescribed ASTM requirement. However, water sediment content (0.06%)in SSLP hydrocarbon liquid just exceeds this value (Table 2). The water found in both the hydrocarbon liquids is probably pyrolytic water, which forms during the catalytic process. Much of this water is removed by decanting the upper hydrocarbon liquid layer. More could be removed using a stripping unit. Therefore, any excess water and sediment in these products, relative to the ASTM requirements, would be removed in the necessary refining process. Ash. Ash content of both hydrocarbon liquid was 0.01 mass %which is the same as the maximum limit set by ASTM specification. At present, we have no information on the chemical and physical (including morphological) nature of the ash. Sulfur. As shown in Table 2, it is quite obvious that the COP hydrocarbon liquid does not contain any sulfur whereas SSLP hydrocarbon liquid and diesel fuel both have the same sulfur content (0.22 mass %). Sulfurcontaining emissions is controlled by means of the fuel sulfur specifications. The current maximum sulfur limit for automotive diesel fuel in the U S . is specified (in ASTM D 975) at 0.5 mass %, but the EPA has proposed a reduction to 0.05% maximum. Carbon Residue. Carbon residues of both the hydrocarbon liquids are nearly the same as diesel fuel and meeting the ASTM requirement which is 0.15 and 0.35 for diesel fuel of grades No. 1-D and 2-D, respectively (Tables 2 and 3). Although the COP hydrocarbon liquid has a higher carbon residue than that of SSLP hydrocarbon liquid, the value is not high enough to indicate a problem with carbonaceous deposits. Elemental Analysis. Elemental analyses for carbon, hydrogen, nitrogen, and sulfur for the pyrolyzed liquid hydrocarbons (SSLP and COP) as well as diesel fuel are shown in Table 6. As can be seen from this table, the percentages of nitrogen and sulfur are quite low in the case of SSLP hydrocarbon liquid and very much comparable to that of diesel fuel. This also indicates that sewage sludge lipid was completely converted to hydrocarbons; the carbon and hydrogen contents were close to the values for diesel fuel. As reported by previous workers in our laboratory2, the removal of lipid by Soxhlet extraction of dry raw sludge with toluene rejected 99.5%nitrogen and 94%of sulfur. The catalytic pyrolysis of the lipid rejected little, if any, further nitrogen but rejected as much as 62% of the lipid sulfur. (11)Manual on Significance of Tests for Petroleum Products, 5th ed.; Dyroff, G. V., Ed.; ASTM Manual Series: MNL1; American Society for Testing and Materials: Philadelphia, 1989.
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254 Energy & Fuels, Vol. 9, No. 2, 1995
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20 25 30 35 Time, min Figure 3. Gas chromatogram of (A) SSLP hydrocarbon liquid and (B) COP hydrocarbon liquid. were completely converted to mixtures of straight-chain Results from COP hydrocarbon liquids showed that it alkanes and alkenes. contained very little oxygen (ca. 1%) and hence conGas Chromatography. The gas chromatograms of firmed essentially complete conversion of the oil to a hydrocarbon mixture. the SSLP and COP hydrocarbon liquids (Figure 3,A and Evaluation of IR and I3C NMR Spectra. The B) showed a fairly uniform hydrocarbon chain length infrared spectra of the SSLP and the COP hydrocarbon distribution across the C7 to C17 and C6 t o C17 mass liquids showed only a very weak carbonyl peak at 1720 range, respectively. Gas chromatography showed, in cm-l which was assigned to a carboxylic acid ~ a r b o n y l . ~ , ~ particular for the COP product, the cracking of the The peaks appearing at 907 and 965 cm-l were identihydrocarbon chains with a preference for C6 and C7 fied as the CH out-of-plane bending which may be products. Multiple peaks near the retention times of standard hydrocarbon peaks also revealed the possibildue to typical alkene (RHC=CH2) and trans-alkene (RHC=CHR) products. Proton-decoupled 13C NMR ity of presence of double bond isomers in the pyrolysis products. It was confirmed that a small part of the peak spectra of both products exhibited various sharp signals at 8 min in case of SSLP product was due to residual (14.6-33.2ppm) associated with straight chain hydrocarbons; no carbonyl signal was observed in the C=O toluene and combination of C9 aliphatic hydrocarbon region of the ~ p e c t r a . The ~ , ~ five peaks at 137.8,129.3, isomers. Diesel fuel was also analyzed by gas chromatography 128.5,125.7,and 21.4ppm are from a small amount of residual toluene in case of SSLP product. Therefore, and the spectrum is shown in Figure 4. As expected, it indicates the uniform hydrocarbon chain length distri13C NMR spectroscopy confirmed that both products 0
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Energy & Fuels, Vol. 9, No. 2, 1995 255
Pyrolysis of Sewage Sludge Lipid C14
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Time, Figure 4. Gas chromatogram of diesel fuel.
bution across the C10 to C19 mass range as well as multiple peaks showing the possibility of some branched chain and double bond isomers. The pattern of this spectrum is quite similar to those found with SSLP and COP hydrocarbon liquids, except that it does not contain the C6-C9 hydrocarbons. This was also consistent with the distillation curve (Figure 2A) and high IBP and 10% point (Table 2) compared with those of SSLP and COP hydrocarbon liquids. However, the chromatograph of diesel also shows the presence of some C18 and C19 products in the sample. Environmental Considerations. There is no doubt that the quality of automotive fuel has an influence on exhaust emissions. Density, cetane number, and volatility are some of the very important fuel properties that influence the emission performance of a fuel. Research conducted by the French Petroleum Institute12 has shown that it is necessary to keep the density below 0.850 and the cetane number above 50 in order to have lower emissions. High density may cause black smoke and excessive particulate e m i s ~ i o n . ~ Increasing ,~~ cetane number results in decreased hydrocarbon emission.14 Cetane number has also been shown to influence smoke emissions, the level increasing with lower cetane fuels.l5 Fortunately, densities of the hydrocarbon liquids are below 0.850 and the cetane value is also close to 50. So these may have favorable effect on emission performance. Numerous papers have suggested that mid-boiling point and back-end volatility influence particulate e m i s s i ~ n s . ~AJ 50% ~ recovery temperature (12) Advenier, P.; Marez. P. In Fuels for Automotive and Industrial Diesel Engines; Mechanical Engineering, Publications Limited for The Institution of Mechanical Engineers: London, 1990; p 143. (13) Hutcheson, R. C. In Fuels for Automotive and Industrial Diesel Engines; Mechanical Engineering, Publications Limited for The Insti-
tution of Mechanical Engineers: London, 1990; p 151. (14) Hills, F. J.; Schleyerbach, C. G. Diesel Fuel Properties and Engine Performance; SAE Paper No. 770316, Society of Automotive Engineers, Inc.: Warrendale, PA, 1977. (15) Cole, R. D.; Taylor, M. G.; Rossi, F. Additive Solution to Diesel Combustion Problems; Paper No. C310/86; Institution of Mechanical Engineers International Conference on Petroleum Based Fuels and Automotive Applications: London, UK, 1986.
in the range of 232-280 "C has been found most desirable in reducing smoke and particulate emission^.^ The 50% recovery temperature of SSLP and COP hydrocarbon liquids falls within this range. Another very important feature which strongly favors the use of pyrolyzed hydrocarbon liquids as alternative to diesel fuel is their low sulfur content (see Table 2). The COP product, in particular, could be blended with diesel or SSLP to bring the latter into compliance with more stringent EPA sulfur standards. Sulfur content was the first fuel property t o be controlled as a means of limiting harmful exhaust emissions. During combustion, sulfur content burns to form sulfur oxides (SO2 and SO3) and also solid compounds, such as sulfates. The gases will have an influence on the exhaust odour, while the sulfates will contribute t o the particulates burden in the exhaust.16
Conclusions Many of the usual physical properties which are used as fuel indicators suggest that the unrefined hydrocarbon liquids which are produced by the catalytic pyrolysis of sewage sludge lipid (SSLP) and canola oil (COP)could serve as diesel fuel substitutes. The specific gravities, viscosities, ash contents, carbon residues, flash points, and higher heating values all fall within the allowable ranges. The calculated cetane indices of 48.9 (SSLP) and 47.5 (COP) are above the ASTM specified minimum limit of 40. However, both products differ from a typical diesel fuel in having wider boiling ranges (60-335 "C vs 165-308 "C) but still give acceptable cetane index values. The presence of the higher boiling material results in the cloud points of both products being higher than most diesel fuels (-3 and -12 "C for SSLP and COP, respectively). Removal of the light and heavy (16) Barry, E. G.; McCabe, L. J.; Gerke, D. H.; Perez, J. M. Heavy Duty Diesel Engine lFuel Combustion Performance and Emissions-A Cooperative Research Program; SAE No. 852078, Society of Automotive Engineers, Inc.: Warrendale, PA, 1985.
256 Energy & Fuels, Vol. 9,No. 2, 1995
ends t o match the initial and 95%boiling points of a typical diesel fuel results in boiling point curves for both products which are similar to those of diesel. The removal of the heavy ends solves any cloud point problems whereas the removal of the light ends would prevent otherwise potential vapour lock in the fuel injection system. Spectral information shows that the major chemical components in the two products are n-alkanes and alkenes. The evaluation of the two products as possible diesel fuels derives from this observation because the values of the physical properties really only depend on the molar mass distribution. Recently it has been shown that the mono-ene content of the canola oil
Bahadur et al.
product may be particularly high. This may lower slightly the actual performance of this product in diesel engines. Work is continuing on the evaluation of other liquids produced by the catalytic pyrolysis of lipids. In addition, the major pathways for lipid deoxygenation are being studied.
Acknowledgment. This research was supported by a Natural Sciences and Engineering Research Council (Canada) Strategic Grant. A Canadian Commonwealth Scholarship (to N.P.B) is gratefully acknowledged. EF9401740
