Energy & Fuels 2008, 22, 1335–1340
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Recovery of Oils With High Caloric Value and Low Contaminant Content By Pyrolysis of Digested and Dried Sewage Sludge Containing Polymer Flocculants Eun-Seuk Park, Bo-Sung Kang, and Joo-Sik Kim* Faculty of EnVironmental Engineering, UniVersity of Seoul, 90 Jeonnong-Dong, Dongdaemun-Gu, Seoul 130-743, Republic of Korea ReceiVed October 5, 2007. ReVised Manuscript ReceiVed December 6, 2007
The production of sewage sludge in Korea amounts to 2 million ton/y. It is disposed mainly by ocean dumping. At the London Convention, however, there was a strong demand for this practice to be stopped. Therefore, new treatment methods for sewage sludge should be found. One of the strong candidates for these new methods is the pyrolysis technique, which obtains oil from the organic part of sewage sludge. The pyrolysis oil, however, usually shows bad characteristics, mainly due to its low heating value and high contaminant content. In this study, digested and dried sewage sludge containing polymer flocculants was pyrolyzed in a temperature range of 446–720 °C in a pyrolysis plant equipped with a fluidized bed reactor. The mass balance was established in each experiment, and the produced gas and oil were analyzed with the aid of GCs and a GC-MS system. In the experiments, above 50 wt % pyrolysis oil was obtained with a maximum caloric value of 33 MJ/kg. In addition, the authors tried to reduce contaminants in the pyrolysis oil by using a char separation system composed of a cyclone and a hot filter and calcium oxide as an adsorbent, to capture the HCl liberated during the pyrolysis. The pyrolysis oil was almost free of hazardous metals, and its chlorine content was significantly reduced.
Introduction
Table 1. Analysis of the Digested and Dried Sewage Sludge Proximate Analysis
Because of the increase in the number of sewage treatment facilities and the emergence of advanced treatment plants, the production of sewage sludge in the country is continually increasing and now amounts to 2,500 ton/d. Agricultural application of sewage sludge, incineration, and landfills are recognized as the most common treatment methods for the sewage sludge.1 These methods are, however, not free of contamination. The use of sewage sludge in agriculture leads to an increase in the concentration of heavy metals in the soils.2 To prevent the release of gases and solid pollutants, incineration plants must be provided with expensive equipment. In landfills, the soil has to be sealed adequately to prevent the leaching of harmful compounds. Up to now, more than 77% of sewage sludge in Korea is disposed of through ocean dumping and landfills. Not only does the London Convention strongly limits ocean dumping, but also the Korean government prohibits the direct landfill and the utilization of sewage sludge in agricultural lands; new approaches to the disposal of sewage sludge must be found. One of the stronger candidates is the pyrolysis technique. Pyrolysis has some advantages over incineration. Pyrolysis usually releases less pollutants, and the heavy metals present in the carbonaceous matrix that pyrolysis produces are relatively resistant to natural lixiviation.3 Moreover, pyrolysis produces gases and oils that could be used as potential fuels. * Corresponding author: Tel.: +822-2210-5621. Fax: +822-2244-2245. E-mail:
[email protected]. (1) Werther, J.; Ogada, T. Prog. Energ. Comb. Sci. 1999, 25, 55–116. (2) Lu, G. Q.; Low, J. C. F.; Liu, C. Y.; Lua, A. C. Fuel 1995, 74, 344–348. (3) Koch, J.; Kaminsky, W. Petrochemistry 1993, 46, 323–325.
component moisture volatile matter fixed carbon ash
content (wt
Elemental Analysis %)a
5.56 66.78 0.84 26.81
Higher Heating Value (MJ/kg) HHV 15.22 a
component
content (wt %)b
C H N Oc S
55.45 8.20 7.42 27.82 1.11
Chlorine Content (ppm) Cl 860
Dry basis. b Dry and ash-free basis. c Calculated by difference.
Some researchers have pyrolyzed sewage sludge at various reaction conditions to study the pyrolysis mechanism.4,5 Other researchers focused on the production of a product fraction, such as, oil, gas, and char and on the analysis of such product.6–9 In relation to the application of pyrolysis products, the utilization of the char has been investigated.10,11 Much attention is being given, however, to the production of oil. Pyrolysis oil usually shows undesirable characteristics, though, largely due to its low heating value, high heavy metal content, and the content of (4) Urban, D. L.; Antal, M. J. Fuel 1982, 61, 799–806. (5) Conesa, J. A.; Marcilla, A.; Prats, D.; Rodrg´uez-Pastor, M. Waste Manage. Res. 1997, 15, 293–305. (6) Kaminsky, W.; Kummer, A. B. J. Anal. Appl. Pyrol. 1989, 16, 27– 35. (7) Piskorz, J.; Scott, D. S.; Westerberg, I. B. Ind. Eng. Chem. Process Des. 1986, 25, 265–270. (8) Boroson, M. L.; Howard, J. B.; Longwell, J. P.; Peters, W. A. Energy Fuels 1991, 3, 735–740. (9) Conesa, J. A.; Marcilla, A.; Moral, R.; Moreno-Caselles, J.; PerezEspirola, A. Thermochim. Acta 1998, 313, 63–73. (10) Martin, M. J.; Artola, A.; Balaguer, M. D.; Rigola, M. Chem. Eng. J. 2003, 94, 231–239. (11) Mendez, A.; Gasco, G. Desalination 2005, 183, 765–771.
10.1021/ef700586d CCC: $40.75 2008 American Chemical Society Published on Web 01/31/2008
1336 Energy & Fuels, Vol. 22, No. 2, 2008
Park et al.
Figure 1. Schematic diagram of a simplified pyrolysis plant. Table 2. Reaction Parameters of the Experiments temp (°C) flow rate (N m3/h) duration (min) feed rate (g/min) feed size (mm)
R1
R2
R3
R4
R5
R6
R7
R8
R9
R10
R11
446 1.9 95 11
470 1.9 95 11
500 1.8 100 10
560 1.7 105 10
