Fuels and chemicals from sewage sludge: 3. Hydrocarbon liquids from the catalytic pyrolysis of sewage sludge lipids over activated alumina Samir
G. B. Boocock,
Mao and Jinan
Department of Chemical Engineering and Applied Chemistry, Toronto, Ontario, Canada M5S ?A4 (Received 7 September 7992; revised 77 December 7992)
Toluene-extracted lipids (obtained by extraction with boiling toluene) from a dried raw Atlanta sewage sludge were pyrolysed over activated alumina at 450°C and atmospheric pressure. Pyrolysis yielded Iowviscosity liquids (10.7-67.5 wt %), non-condensable gases (12.1-15.6 wt%), semi-solids (only at higher weight-hourly space velocity) and water. The liquid products were hydrocarbon mixtures which contained predominantly alkanes. Infrared spectra, as well as proton-decoupled ’ 3C nuclear magnetic resonance, confirmed the absence of carbonyl groups in the pyrolysed liquid products; showing that even the carboxylic acids, the major component of the separated lipid fraction, did not survive the pyrolysis reaction. Gas chromatography of the liquid products showed a uniform hydrocarbon distribution across the C&C20 mass range. Non-condensable gases consisted of carbon dioxide, carbon monoxide, methane, ethane, ethene and C3-C5 compounds. This process resulted in rejection of both nitrogen and sulfur. The lipid extract only contained 1 wt% of the nitrogen and 16 wt% of the sulfur in the sludge. But these elements were reduced by a further 84% in the liquid product with respect to the lipid. The pyrolysed liquid contained 0.08 wt% nitrogen and 0.22 wt% sulfur. This liquid has a great potential to be used as diesel fuel, heating fuel and chemical feedstocks. (Keywords:
sewage sludge lipids; pyrolysis;
Sewage sludge is produced in significant quantities when municipal and industrial wastewater effluents are treated by biological processes. The cost of disposal of these sludges has increased greatly in recent years, and it has been estimated that the disposal costs are in the order of 50% of the total wastewater treatment costsl. The major fates of sewage sludge are agricultural utilization, incineration, landfill and ocean disposal. Sludges of industrial origin are often high in heavy metals, thereby ruling out land application. The availability of landfill sites is extremely limited for most municipalities so the major form of sludge disposal is incineration, which is expensive. The increased costs of waste disposal, and growing environmental concern about it, have prompted investigation of alternate technologies for sludge disposal. One of these technologies is thermal conversion to yield liquid fuel products and chemical feedstocks. The conversion of sewage sludge to fuel products has been known for more than five decades’. Recently German work3s4 has demonstrated that a ‘synthetic crude oil’ could be produced from a sewage sludge by heating it at 30&35O”C in an oxygen free environment for about 30 min. Campbell et ~1.~ have developed technology for the thermal liquefaction of sewage sludge based on these German results. Piskorz et ~1.~ have reported the flash pyrolysis of sewage sludge, in which a dried mixture of raw and activated sludge was pyrolysed in a bench-scale fluidized-bed reactor at residence times of less than 1 s over a temperature range of 0016-2361/94/05/0642-05 $2 1994 Butterworth-Heinemann
The direct thermomechanical liquefaction of sewage sludge without a reducing agent has been investigated by Molton et ~1.~. They developed a process in which a dewatered sewage sludge was heated with sodium carbonate above 250°C in a nitrogen environment in a continuous flow mode. Recently, Suzuki et ~1.~ have reported an advanced treatment of sewage sludge by direct thermochemical liquefaction. All these pyrolysis processes, although performed under different experimental conditions, yielded liquid products which contained significant amounts of oxygenated compounds and unacceptable amounts of nitrogen and possibly sulfur. Recent research’ showed that the major mechanism by which oil is produced from the thermal treatment of sewage sludge is one involving distillation of lipids. The lipids were concluded to be the major precursor of useful oil. Oil is also derived from protein. However, this protein gives rise to high amounts of nitrogen and sulfur in the pyrolysis oils and is, therefore, a nuisance. Hence selective separation and pyrolysis of the lipid fraction would lower the nitrogen and sulfur content in the oils. The lipid fraction contains hydrocarbons, triglycerides and straight chain carboxylic acidsI’*“. Raw sludges are high in carboxylic acid content, whereas in digested sludges most of the carboxylic acids have been biodegraded. Therefore, the oil yields from raw sludges are higher than those from digested sludges. Furthermore, the viscosity of oils from raw sludges are higher than those of the oils produced from digested sludges. When sewage sludge was spiked 400-700°C.
Fuels and chemicals
from sewage sludge: S. K. Konar et al.
with triolein, representative of unsaturated triglycerides, this compound did not survive the pyrolysis. However, stearic acid, a straight chain carboxylic acid, for the most part, did survive the pyrolysis and resulted in an increase in viscosity of the oil. This shows that pyrolysis alone will not produce hydrocarbons as the major product from lipids. In addition, the catalytic hydrogenation of carboxylic acids to yield hydrocarbons is virtually impossible. Based on these above observations, it was decided to investigate the catalytic pyrolysis of sewage sludge lipids. This paper reports the pyrolysis of toluene-extracted lipids from raw sewage sludge over activated alumina. EXPERIMENTAL Dry Atlanta raw sludge [supplied by the Wastewater Technology Centre (WWTC) of Environment Canada (Burlington, Ontario)] and activated alumina (Alcan AA 200, particle diameter, 1.168 x lop3 m-2.362 x lop3 m; a mixture of chi, eta and some boehimites; BET surface area, 28&300 m2 g- ‘; pore size distribution, 4 nm) were used. Infrared (ir.) spectra were recorded and protondecoupled 13C nuclear magnetic resonance (nmr.) spectra were obtained on a 200 MHz spectrometer. Deuterated chloroform was used as solvent during analysis. Carbon, hydrogen, nitrogen and sulfur analyses of liquid pyrolysis products were performed. Gas chromatographic analyses for liquid products were performed on a 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 programme, 5 min at 50°C; heated at a rate of 5°C min - 1 to 210°C; hold for 30 min. Carrier gas (He) flow rate was 25 ml min- ‘. Gas chromatographic analyses for non-condensable gases were carried out on a similar gas chromatograph, equipped with a flame ionization detector and a 316 stainless steel porapak (SO%Q + 20%T; 8G-100 mesh) column (2.44 m x 3.18 mm). The operating conditions were as follows: detector temperature, 120°C; injector temperature, 160°C; temperature programme, 3 min at 50°C; heated at a rate of 30°C min- 1 to 160°C; hold for 15 min. Carrier gas (Ar) flow rate was 30 ml min- I. Extraction of lipids
Lipids were extracted in an 18 wt% yield from Atlanta dry raw sludge by a boiling extraction method using toluene as the extracting solvent. The extraction procedure has been reported elsewhere”. Pyrolysis apparatus and procedure
The pyrolysis unit consisted of a 3 16 stainless steel feed preheater tube (1.3 cm i.d. x 46 cm length), a block heater containing a 316 stainless steel fixed bed 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 containing brine solution and a nitrogen cylinder, as shown in Figure 1. The pyrolysis unit used was almost the same as that reported earlier”. The only difference was the use of a syringe pump instead of a peristaltic pump to deliver the subtrate to the reactor and the addition of a gas collection device. The volume of gas
Figure 1 Apparatus for pyrolysis of sewage sludge lipids: 1, 316 stainless steel feed preheater tube (1.3 cm i.d. x 46 cm length); 2, block heater containing a 316 stainless steel fixed bed reactor tube (2.5 cm i.d. x 46 cm length); 3, catalyst bed; 4, chromel-alumel thermocouple probe; 5, temperature controller; 6, syringe pump; 7, condenser; 8, receiving flask; 9, gas trap; 10, gas collection vessel; and 11, nitrogen cylinder
collected was determined by measuring the volume of brine displacement. The procedure used was exactly the same as that reported earlier”. For each run, the reactor was packed with 40 g of activated alumina. The reactant was semi-solid at room temperature and, therefore, it was melted and maintained in liquid form by wrapping a heating coil around the syringe. RESULTS The moisture contents of the original dried sludge and the extracted sludge, as determined by the Karl-Fisher titration method, were 2 and 1.5 wt%, respectively. This method could not be used for extracted lipids because negative values were obtained. The lipid, in its free form appears to interfere with the chemistry of the Karl-Fisher reaction. The moisture content of the lipids was determined by an oven-drying method and was found to be 5 wt%. The lipid fraction consisted of 65 wt% free fatty acid, 28 wt% unsaponifiables and 7 wt% triglycerides. The composition of the extracted lipids has been discussed elsewhere”. The elemental composition of the extracted lipids were as follows: C, 72.2 wt%; H, 10.7 wt%; 0, 14.2 wt%; N, 0.35 wt%; S, 0.85 wt% and ash, 0.6 wt%. 1.r. and 13C n.m.r. of the extracted lipids” have confirmed the presence of carboxylic and ester carbonyl functional groups, as well as unsaturation within the long hydrocarbon chain of both the free fatty acids and the triglycerides. Pyrolysis of sewage sludge lipids over activated alumina yielded low viscosity liquids, small amounts of solids [only at higher weight-hourly space velocity (WHSV)], non-condensable gases, pyrolytic water and a carbonaceous residue remaining on the catalyst (coke). The percentage yield of the pyrolysis products at different WHSV are shown in Table I. The liquid product densities are listed in Table 2. The effect of WHSV on liquid yield is shown in Figure 2. A typical elemental composition (Run No. 2, see Table I for conditions) of the pyrolysed liquid products was as follows: C, 86.6 wt%; H, 13.5 wt%; N, 0.08 wt% and S, 0.22 wt%. Since the total of the analysed elements is 100.4%, the liquid products contained little, if any, oxygen.
Fuels and chemicals
from sewage sludge: S. K. Konar et al.
Experimental conditions and pyrolysis of sewage sludge lipids (boiling extracted)
Run No. I 2 3 4 5
Original mass (g)
0.24 0.46 0.17 0.91 1.60
33.6 35.5 33.3 36.4 33.5
n.d. 2.71 n.d. 3.70 n.d.
n.d. 4.32 n.d. 5.70 n.d.
22.1 24.0 21.0 22.0 3.6
65.7 67.5 63.0 60.3 10.7
0.0 0.0 0.0 trace 18.0
2.9 2.2 2.7 2.9 2.7
“Carbon absorbed on catalyst n.d., not determined
Table 2 Density measurements of the liquid lipids at various weight-hourly space velocities
evident as well as peaks assigned to trace amounts of residual toluene. A typical gas chromatogram of the liquid pyrolysis products is depicted in Figure 3. The gas chromatographic retention times for the standard hydrocarbons are presented in Table 3. Runs No. 2 (0.46 WHSV) and 4 (0.91 WHSV) were subjected to gas analysis by gas chromatography and the results are shown in Table 4. Elemental compositions of the original dried sludge and the extracted lipids are presented in Table 5. Elemental balances (C, H and 0) were made for the pyrolysis products from Runs No. 2 and 4 and are shown in Table 6.
DISCUSSION Product mass WHSV
Product volume (ml)
Product density (gml-‘)
Average density @ml-‘)
3.9 6.1 9.6
4.8 7.4 11.9
0.81 0.82 0.81
3.0 7.6 11.4
3.8 9.3 14.1
0.79 0.82 0.81
4.6 8.8 9.7
5.5 10.7 11.8
0.84 0.82 0.82
2.7 7.6 15.8
3.2 8.9 18.6
0.84 0.85 0.85
4.9 5.1 8.8
5.9 6.0 10.5
0.83 0.85 0.84
The i.r. spectrum of the liquid product, obtained by the pyrolysis of toluene-extracted lipids over glass beads, was identical to that of the substrate. Hence under these
70 4 60. .. 50.
-. -. . . \. ~.
TIME, min \\
Effect of weight-hourly
Each i.r. spectrum of the liquid pyrolysis products (at different WHSV) showed only a very weak carbonyl peak at 1720 cm-’ which was assigned to carboxylic acids. The peaks appearing at 907 and 965 cm-’ were identified as the CH out of plane bending, which may be due to general alkene and trans-alkene products, respectively. Since the i.r. spectra of all the liquid pyrolysis products were almost identical, it was decided to run only one 13C n.m.r. spectrum of the liquid proton-decoupled products (from Run No. 2). It showed various sharp signals (14.6-33.2 ppm) associated with straight chain hydrocarbons. No carbonyl signal was observed in the C=O region of the spectrum. Peaks (124-132 ppm) associated with the olefinic double bonds were also
of the liquid pyrolysis alumina
Figure 2 yield
Figure 3 Typical gas chromatogram of sewage sludge lipids over activated
Table 3 Gas hydrocarbons
n-hexane n-heptane n-octane n-nonane n-decane n-undecane n-dodecane n-tridecane n-tetradecane n-pentadecane n-hexadecane n-heptadecane n-octadecane n-nonadecane n-eicosane
0.48 0.64 1.oo 1.88 3.79 7.30 10.48 13.44 16.13 18.62 20.95 23.20 25.16 21.29 29.27
Fuels and chemicals
In general, the colour of the liquid pyrolysis products ranged from light yellow to black. The products often had a gasoline-type odour. The average density of the liquid products obtained at 0.24,0.46,0.77,0.91 and 1.60 WHSV were 0.81, 0.81, 0.83, 0.85 and 0.84, respectively. These values are similar to those of liquid hydrocarbons. For the most part, pyrolytic water and low viscosity liquid products existed as distinct layers when the pyrolyses were performed at 0.24, 0.46, 0.77 and 0.91 WHSV. A small amount of emulsion was collected near the end of the run. No significant amount of semi-solids was obtained. However, in the case of the products obtained at 1.60 WHSV, large amounts of emulsions were formed together with the high viscosity liquids. It is believed that the long chain carboxylic acids survive the pyrolysis and these act as surfactants leading to fairly stable emulsions. Large amounts of semi-solids were also obtained.
Evaluation 0fi.r. and 13C n.m.r. spectra
All the i.r. spectra of the liquid pyrolysis products, obtained over activated alumina, were essentially the same. The very weak carbonyl peak (1720 cm-‘) associated with the pyrolysis products confirms complete decarboxylation of the free fatty acids, as well as cleavage of the glycerol moieties and removal of the ester carbonyl groups in the triglycerides. Decarboxylation of the carboxylic acids was indeed very surprising because this reaction is notoriously difficult; in the WWTC process long chain carboxylic acids survive the pyrolysis resulting in high viscosity oils’. The size of the carboxylic carbonyl peak increased slightly as the WHSV was increased. In other words, at a lower WHSV, the lipids have more time to react with the catalyst and resulted in more conversion. The absence of carbonyl signals in the C=O region of the i3C n.m.r. spectrum of the liquid pyrolysis products supports the observation deduced from the i.r. spectra.
Run No. (WHSV)
2 (0.46) 4 (0.91)
Wt% 7.8 14.1
2 (0.46) 4 (0.91)
Mol.% 10.4 20.1
S. K. Konar et al.
As can be seen from Table 1, the highest liquid yield of 67 wt% was obtained at 0.46 WHSV. It is obvious from the results that as the WHSV was increased the percentage yield of low viscosity material was decreased. Figure 2 shows the effect of WHSV on liquid yield. If throughput is to be maximized, then a WHSV up to 0.9 can be used without dramatically lowering the liquid yield. The exact trend beyond the 0.91 WHSV is not known, except that at a WHSV of 1.60 the liquid yield is very low. No semi-solids were observed at 0.77 WHSV, but a very small amount of semi-solids (< 1 wt%) was formed at 0.91 WHSV. Hence, within the range 0.77-0.91 WHSV there is a space rate at which solid starts to form; above 0.91 WHSV a significant amount of solid will be formed. The particularly low yield at higher WHSV (1.60) is believed to be a result of low substrate residence time. The percentage yield of coke was in the range of 6.2-8.0. Gas yields ranged from 12.2 to 15.7 wt% of the total pyrolysis yield.
Characteristics of the pyrolysis products
experimental conditions, the pyrolysis had little or no effect on the lipids. Thus, a catalyst was necessary if the sewage sludge lipids were to undergo pyrolysis. Several alternatives” were considered before activated alumina was chosen as the catalyst. It was found to be very effective for partial removal of nitrogen and sulfur from pyrolysis oilsi3. In addition it is relatively inexpensive.
Table 4 Gas analysis of the gaseous sludge lipids (Runs No. 2 and 4 only)
nd., not determined
Gas chromatograms Table 5 Elemental composition of the original lipids and pyrolysed liquid products
dried sludge, extracted
Original sludge Extracted lipids Liquid products
39.5 72.2 86.6
6.0 10.7 13.5
26.1 14.2 0.0
6.00 0.35 0.08
0.70 0.85 0.22
20.5 0.6 0.0”
From a comparison of the liquid product peaks with those obtained from analytical alkane standards in the gas chromatogram (Figure 3), it was apparent that catalytic cracking of the hydrocarbon side chain had occurred. It showed a fairly uniform hydrocarbon chain length distribution across the C6-C20 mass range. Multiple peaks near the retention times of standard hydrocarbon peaks also revealed the presence of double
a By difference
of Runs No. 2 and 4
Run No. 2 (0.46 WHSV)
Component Liquid Water Gas Char Total Lipid output
20.8 2.7 2.2 25.7 25.6 100.4%
Run No. 4 (0.91 WHSV)
24.0 1.8 4.3 2.2 32.3 34.4 93.9%
3.2 0.2 0.5 3.9 3.8 102.6%
_ 1.6 1.1 2.7 5.0 54.0%
4.1 2.9 26.1 26.3 99.2%
3.0 0.2 0.8
3.9 3.9 100.0%
2.6 5.2 50.0%
Total (g) 22.1 1.7 6.0 2.9 32.6 35.4 92.1%
Fuels and chemicals
from sewage sludge: S. K. Konar et al
bond isomers in the pyrolysis products. The presence of some branched chain compounds is not ruled out. The peak at 1.62 min is due in part to residual toluene in the lipid extract. Gas chromatographic analyses of the non-condensable gases (Runs No. 2 and 4) (Table 4) showed the presence of carbon monoxide, carbon dioxide, methane, ethane, ethene and C3-C5 compounds, the latter escaping the liquid collection system because of their high vapour pressure. The non-detection of methane in Run No. 4 is not explainable at this time. It also showed that as the WHSV was increased, the percentage of hydrocarbons from C2-C4 was also increased. At lower WHSV, carbon dioxide appeared to be the predominant component in the pyrolysed gaseous mixture. The gas phase contained about 11 wt% of the total carbon present in the sewage sludge lipids (see Table 6). Elemental analyses and elemental balances
Elemental analyses of the liquid pyrolysis products were made in order to determine the degree of nitrogen and sulfur rejected after the pyrolysis, and to allow calculation of elemental balances. It can be seen from Table 5, that the amount of nitrogen and sulfur in the liquid products was very low compared to the original dried sludge and extracted lipids. It was reported earlier”, that boiling extraction of the sludge resulted in 99 wt% rejection of nitrogen and 84 wt% rejection of sulfur. A further rejection of 84 wt% of both nitrogen and sulfur was accomplished by the pyrolysis of the extracted lipids. It is also evident from elemental analyses that the liquid products contained virtually no oxygen. Since gas chromatographic analyses of the noncondensable gases were performed for only two of the WHSVs studied, elemental balances were only made for these two runs. It is obvious from Table 6, that carbon and hydrogen balances were very close to 100 wt% in both cases. The oxygen balances were very poor. The total amount of pyrolytic water produced in the liquefaction process is not exactly known, as some water vapour is undoubtedly carried to the brine displacement vessel. Balancing the missing oxygen with water would not seriously upset the hydrogen balance. Uses of the process and products Production of light liquid hydrocarbons with virtually no nitrogen nor sulfur content appears to be unique to this process. This product is storable, transportable and potentially saleable. The liquid products contained hydrocarbons ranging from C6 to C20 mass range and, therefore, the potential end-uses of this liquid include diesel fuel, heating fuel and chemical feedstocks. The sulfur content currently is less than the US allowable content14 of 0.5 wt% for diesel fuel, although the Environmental Protection Agency (EPA) has proposed
0.05 wt%, the limit currently set in California. However,
this value is likely to be set in all cities in the US by 1993. The European standard is currently 0.3 wt%, but some countries have already moved to a level of 0.2 wt %. CONCLUSIONS Pyrolysis of sewage sludge lipids over activated alumina produced liquid hydrocarbons (which contained mostly alkanes) and gases. Even the carboxylic acids of the separated lipid fraction were completely converted to hydrocarbons. This is in contrast to direct sewage sludge pyrolysis where long chain carboxylic acids survive and result in high viscosity oils. In addition the pyrolysed liquid products from lipids contained virtually no nitrogen nor sulfur. The liquid has potential for use as a high quality fuel. ACKNOWLEDGEMENT This research was supported by a Natural Sciences and Engineering Research Council of Canada (NSERC) Strategic Grant. REFERENCES
10 11 12 13
Proctor and Redfern Ltd, Final report (DSS-UP-205) to Environment Canada, 1987 Shibata, S. French patent 838 063, 1939 Bayer, E. and Kutubuddin, M. ‘Proceedings of the International Recycling Congress, Berlin, West Germany’, 1982, pp. 314-319 Baver. E. and Kutubuddin. M. ‘Prepr. Verfahrenstechnik Klarsdhlammverwertung’, Baden-Baden,.West Germany, 1984, pp. 141-156 Campbell, H. W. and Bridle, T. R. Water Sci. Technol. 1989, 21, 1467 Piskorz, J., Scott, D. S. and Westerberg, 1. B. Ind. Eny. Chem. Process Des. Den 1986, 25, 265 Molton, P. M., Fassbender, A. G. and Brown, M. D. EPA Cooperative Agreement CR-810690, EPA/600/D-85/193, NTIS PB 85247534, WERC, Cincinnati, 1985 Suzuki, A., Nakamura, T., Yokoyama, S., Ogi, T. and Koguchi, K. in ‘Research in Thermochemical Biomass Conversion’ (Eds A. V. Bridgwater and J. L. Kuester), Elsevier Applied Science, London,.1988, pp. 816-826 Boocock. D. G. B.. Aeblevor. F.. Chirigoni, F.. Crimi. T., Khelawan, A. and CampI&, H. W. in ‘Research in Thermochemical Biomass Conversion’ (Eds A. V. Bridgwater and J. L. Kuester), Elsevier Applied Science, London, 1988, pp. 497-507 Boocock, D. G. B., Konar, S. K., Leung, A. and Ly, L. D. Fuel 1992, 71, 1283 Higgins, A. J., Kaplovsky, A. J. and Hunter, J. V. J. Water Poll. Control Fed. 1982, 54, 466 Boocock, D. G. B., Konar, S. K., Mackay, A., Cheung, P. T. C. and Liu, J. Fuel 1992, 71, 1291 Boocock, D. G. B., Agblevor, F. A., Fruchtl, R. A., Chirigoni, F. and Khelawan, A. ‘Final Report on the Denitrogenation of WWTC Sewage Sludge Oils’ prepared for Wastewater Technology Centre, Canada Centre for Inland Waters (Environmental Protection Service), May, 1988 Owen, K. and Coley, T. ‘Automotive Fuels Handbook’, Society of Automotive Engineers, Inc., Warrendale, PA, 1990, p. 536