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Production of bio-fuels by high temperature pyrolysis of sewage sludge using conventional and microwave heating
Bioresource Technology 97 (2006) 1185–1193
Production of bio-fuels by high temperature pyrolysis of sewage sludge using conventional and microwave heating A. Domı´nguez, J.A. Mene´ndez *, M. Inguanzo, J.J. Pı´s Instituto Nacional del Carbo´n (INCAR) C.S.I.C., Apartado 73, 33080 Oviedo, Spain Received 29 March 2004; received in revised form 21 March 2005; accepted 20 May 2005 Available online 10 February 2006
Abstract The pyrolysis of sewage sludge was investigated using microwave and electrical ovens as the sources of heat, and graphite and char as microwave absorbers. The main objective of this work was to maximize the gas yield and to assess its quality as a fuel and as a source of hydrogen or syngas (H2 + CO). Both gases were produced in a higher proportion by microwave pyrolysis than by conventional pyrolysis, with a maximum value of 38% for H2 and 66% for H2 + CO. The oils obtained were also characterized using FTIR and GC–MS. The use of conventional electrical heating in the pyrolysis of sewage sludge produced an oil that could have a significant environmental and toxicological impact. Conversely, microwave pyrolysis still preserved some of the functional groups of the initial sludge such as aliphatic and oxygenated compounds, whereas no heavy PACs were detected. 2005 Elsevier Ltd. All rights reserved. Keywords: Bio-gas; Hydrogen; Syngas; Sewage sludge; Pyrolysis; Gasification; Microwave
1. Introduction The increase in the production of sewage sludge from municipal and industrial wastewater plants is a matter of growing concern because it represents a potential risk for human health and the environment. In recent years, methods formerly used for the disposal of sewage sludge (Werther and Ogada, 1999), including landfill, ocean dumping and disposal on agricultural land, have become much less acceptable. There is an urgent need for alternative solutions to the sludge disposal problem. Sewage sludge and other organic solid wastes such as biomass are examples of renewable energy resources. They can be used in a variety of ways to provide energy via: direct combustion (Werther and Ogada, 1999), gasification (McAuley et al., 2001; Pakdel and Roy, 1991) *
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62. E-mail address: [email protected] (J.A. Mene´ndez). 0960-8524/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2005.05.011
and pyrolysis (Evans and Milne, 1987; Bridgwater et al., 1999). Of these three thermochemical processes, pyrolysis has attracted most attention in recent years (Conesa et al., 1998; Caballero et al., 1997) since the process conditions can be optimised to maximize the production of chars, oils or gases depending on the specific interest (Horne and Williams, 1996; Inguanzo et al., 2002; Sharypov et al., 2001). The char can then be burnt as fuel or disposed of—since the heavy metals are fixed inside the carbonaceous matrix—or even be upgraded to activated carbon (Lu et al., 1995; Lu and Lau, 1996). The gas can be used as fuel, whereas the oil can either serve as fuel or as raw material for chemicals. Many works in the literature focus on the production of liquids from the pyrolysis of organic wastes at low temperature (Cunliffe and Williams, 1988; Piskorz et al., 1986; Shen and Zhang, 2003). However, the use of these waste materials to produce a hydrogen rich gas or synthesis gas has also aroused considerable interest recently. The reasons for this are that hydrogen is an environmentally clean
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energy source and that syngas can be converted to liquid fuels, which would help to reduce environmental pollution (Asadullah et al., 2002; Mills, 1994). The gasification and pyrolysis of biomass has been investigated for the production of H2 or syngas (Chen et al., 2003; Iwasaki, 2003; Kim, 2003). The majority of these works have focused on using fixed and fluidised bed reactors and thermal energy. However, nowadays, the use of microwave heating is applied not only in the field of analytical, organic and environmental chemistry (Zlotorzynski, 1995), but also for the pyrolysis of materials such as biomass (Kriegerbrockett, 1994), coal (Monsef-Mirzai et al., 1995), oil shales (Chanaa et al., 1994; El harfi et al., 2000) and various organic wastes (Kenneth, 1995). It is known that these organic wastes are poor receptors of microwave energy so that with this method it is impossible to achieve the temperature necessary to carry out the pyrolysis. Nevertheless, microwave-induced pyrolysis is possible, if the raw material is mixed with an effective receptor (Mene´ndez et al., 2002). In the present work, the production of solids, oils and gas from sewage sludge pyrolysis at high temperatures using microwave energy was examined. The results were compared with those obtained with conventional heating. The sludge was analysed by FTIR whereas the pyrolysed oils were characterized by FTIR and GC– MS. The main objective was to obtain a high yield of gas and to evaluate the quality of the gas produced on the basis of the proportions of H2 or syngas (H2 + CO).
The high H/C ratio value indicates a high content of aliphatic hydrogen in these wastes, while the large amounts of oxygen show that a high number of polar compounds were present in the sludge. The calorific value observed for dried sewage sludge is similar to that of other conventional and non-conventional fuels such as low rank coal, paper, wood or black liquor (Perry, 1984). It is well known that the presence of inorganic matter plays an important role in the thermal decomposition process. Table 1 shows that there were considerable amounts of Ca, Al and Si, whereas other metals such as Mg, Fe, K and Na were found in lower proportions. 2.2. Microwave pyrolysis The drying and pyrolysis of the sewage sludge was carried out in a single process using both multimode (M) and single mode (S) microwave cavity ovens. Experiments were carried out by placing samples of wet sludge (ca. 15 g) in a quartz reactor (40 cm height · 3 cm i.d.). In order to maintain an inert atmosphere during the treatments, a He flow rate of 100 mL/min was passed through the sample bed for 10 min prior to the commencement of the experiment, this being reduced during the experiment to 10 mL/min. A constant input power of 1000 W was used and the microwave frequency was 2450 MHz. A sketch of the experimental device is shown in Fig. 1. Either small strips of graphite (G) of about 3 · 3 mm, or of a carbonaceous solid residue (char) (C) from the pyrolysis of sewage sludge were employed as microwave receptors (Mene´ndez et al., 2002). Thus, graphite or char (ca. 1 g) were homogenously blended with 15 g of wet sewage sludge, and the mixture was subjected to microwave treatment. The temperature of the sample during the experiments was monitored by an infrared optical pyrometer. A plot of the evolution of temperature with time is shown in Fig. 2. There was a rapid increase in temperature up to around 135 C, the sample remaining at this temperature for about 2 min.
2. Methods 2.1. Sewage sludge characteristics A sewage sludge from an urban waste water treatment plant subjected to aerobic digestion was used. Table 1 summarizes its main chemical characteristics. Table 1 Chemical characteristics of the sewage sludge Ultimate analysisa,b (wt%)
Proximate analysis (wt%) a
M
A
71
31.2
Na2O
a
FC
C
62.3
6.5
52.3
V
a,c
MgO
Al2O3
CV (kJ/kg)
H
N
S
8.0
6.7
0.7
CaO
Fe2O3
Oc
H/C
H/O
32.3
1.83
4.0
H/N 16.7
16,682
K2O
MnO
SiO2
TiO2
0.76
0.04
15.7
0.25
a
Composition of main inorganic elements (expressed as wt% of metal oxides ) 0.28 1.15 4.71 5.97 1.75 Co (ppm)
Cr (ppm)
Trace elements 3.5 121
Pb (ppm)
Mn (ppm)
Ni (ppm)
Cu (ppm)
Zn (ppm)
Fe (ppm)
Cd (ppb)
Hg (ppb)
246
214
13.2
143
662
9900
2906
919
M: moisture content; A: ash content; V: volatile matter content; CV: calorific value; FC: fixed carbon. a Dry base. b Ash free basis. c Calculated by difference.
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He 3
S
2
5 4
8
1
6
M
2
7
He
1 3 4
Fig. 1. Schematic diagram of the microwave pyrolysis reactors: S, single mode and M, multimode ovens: (1) magnetron; (2) waveguide; (3) quartz reactor and sample; (4) optical pyrometer; (5) condenser with dichloromethane; (6) ice bath; (7) gas container and (8) gas sampling.
1200
Temperature (°C)
1000 800 600 400
S(g)
S(c)
1187
tor, sludge amounts and He flow rate conditions as in the case of microwave pyrolysis. The temperature was set at 1040 C and samples were heated at maximum power in order to get the maximum heating rate possible from the equipment. The time needed to complete the pyrolysis was 24 min, the pyrolysis temperature being reached after 14 min. Thus, the heating rate with the electric furnace (74.3 C/min) was much lower than the heating rate obtainable in the microwave (200 C/min). The reason for this is that the microwave is more efficient at removing the high moisture content of sludge, making it possible to obtain the pyrolysis temperature in a shorter time than with an electrical furnace. 2.4. Recovery of the pyrolysis fractions Fig. 1 shows a sketch of the experimental set-up used for collecting the different pyrolysis fractions (a similar collecting set-up was used in the case of the electric furnace pyrolysis). The aqueous fraction recovered in the condensers was separated from the organic fraction by decantation, while the organic fraction dissolved in the dichloromethane was first dried, using anhydrous sodium sulphate, next filtered and then concentrated by solvent evaporation. The solid, aqueous and oil fraction yields were calculated from the weight of each fraction, while the gas yield was evaluated by difference (see Fig. 3).
200
2.5. Gas chromatography with TCD detector 0 0
2
4
6
8
10
12
14
16
Time (min)
Fig. 2. Microwave heating profile of the sludge with graphite (G) and char (C).
During this period of time the wet sludge was completely dried and when all the moisture was removed from the sample another rapid increase in temperature up to 1040 C took place. The temperature remained more or less stable until the end of the pyrolysis process. The evolution of the temperature was followed by means of preliminary experiments, on the basis of which it was possible to select the appropriate pyrolysis time. The volatile matter content of the carbonaceous residues (chars) obtained in these experiments was determined, and the pyrolysis time was selected according to the minimum time necessary for obtaining a char with no significant volatile matter content. The pyrolysis time was about 10 min for all microwave experiments, maximum temperature being reached after 5 min.
The non-condensable gases were collected in a propylene container completely filled to the top with water so that the gases could be collected without their coming into contact with atmospheric air. Samples of 150 lL of these gases were then analysed in an HP 5890 gas chromatograph fitted with a TCD detector. An HP Porapak N 80/100 (3.07 m · 3.2 mm o.d. · 2.1 mm i.d.) and an HP Molecular Sieve 13X, 45/60 (0.91 m · 3.2 mm
Aqueous
80
For comparative purposes sludge was also pyrolysed in an electric furnace (EF) using the same quartz reac-
Char
Oil
70 60 50 40 30 20 10 0 MG
2.3. Electric furnace pyrolysis
Gas
SG
MC
SC
EF
Fig. 3. Product yield (wt%) from the pyrolysis of sewage sludge using different ovens and microwave absorbers. M, multimode microwave cavity oven; S, single mode microwave cavity oven; EF, electrical furnace; G, graphite and C, char.
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o.d. · 2.1 mm i.d.) column were used. The oven temperature was set at 50 C. The carrier gas flow rate (He) was 20 mL/min. The injector and detector temperatures were 80 and 220 C, respectively. The TCD was calibrated with a standard gas mixture at regular intervals. 2.6. Infrared spectroscopy (FTIR) The FTIR spectra were collected in a Nicolet MagnaIR 560 spectrometer, taking 128 scans at a resolution of 4 cm1. The raw sewage sludge was dried and analysed using KBr pellets at a ratio of 1:100 (sample/KBr). The pellets were dried at 100 C overnight. In the case of the oils a small amount of each sample was deposited between two NaCl disks, allowing the dichloromethane to evaporate before the spectra were collected. 2.7. Gas chromatography–mass spectrometry (GC–MS) The oils were dissolved in dichloromethane and analysed by GC–MS using a Hewlett-Packard HP 6890 gas chromatograph coupled to an HP 5973 quadrupole detector. The gas chromatograph was equipped with a 60 m · 0.25 mm capillary column coated with a 0.25 lm thick film of 5% phenylmethylpolysiloxane (HP-5). Helium was employed as a carrier gas at a constant flow of 0.9 mL/min. The initial oven temperature was 40 C held for 5 min and then programmed from 40 to 300 C at 5 C/min with an isothermal hold for 30 min. Splitless injection was carried out at 300 C and the purge valve was switched on after 1 min. The ion source and transfer line temperatures were 230 and 325 C, respectively. Data were acquired in the full-scan mode between m/z 33–533 and a solvent delay of 6 min was used. The compounds were identified by comparing their mass spectra with those from the NIST mass spectral data library, interpreting the observed fragmentation and evaluating the retention times in comparison with standard compounds. Calibration was not carried out due to the large number of compounds and functionalities present in the fractions analysed. A semiquantitative evaluation of the samples was performed by integrating the total ion chromatogram (TIC) and calculating the area percentage of each peak identified. These data, therefore are not absolute and only serve for purposes of comparison.
3. Results and discussion 3.1. Fraction yields The yields of the pyrolysis products obtained from sludge using microwave and conventional heating (EF) are shown in Fig. 3. For microwave heating two microwave ovens, one multimode (M) and one single mode
(S), and two absorbers, graphite (G) and char (C), were employed. The char yield for all the pyrolysis experiments was around 10–12 wt%, the maximum value being achieved with the single mode microwave oven and graphite (SG). Due to the high moisture content of the sludge (71 wt%), the aqueous fraction yield was found to be higher than that obtained for the other fractions. In this work the highest value corresponded to the MC system and the lowest to EF. In all cases the aqueous fraction content was found to be lower than the initial moisture content of the sludge. Most of the initial water (free water) never reaches high temperatures. However, a low percentage of this water is contained inside the particles of the sludge and more energy is required to remove it. Thus, some of the water may remain even at high temperatures. It therefore reacts mainly with the carbonaceous solid (gasification reaction) although it also reacts with other gas components (water gas shift reaction). The high temperatures (1040 C) used in the present work would favour these reactions, giving rise to water yields lower than the initial moisture of the sludge. On the other hand, the water gas shift reaction also occurs with the water formed during pyrolysis at temperatures around 600 C (Piskorz et al., 1986; Morf et al., 2002) due to condensation reactions of functional groups of the volatile compounds. The high pyrolysis temperature used gave rise to high gas fraction yields. At maximum gas yield, the oil production is relatively low due to gasification and secondary cracking reactions of the pyrolysis vapours. The lowest and the highest values for the oil and gas fractions, respectively, were obtained with the EF. 3.2. Solids Table 2 shows the ultimate and proximate analyses of the chars (Ch) obtained in the pyrolysis of sewage sludge using the different devices. The ash content increased up to values of 83.5% due to the high degree of devolatilization produced by pyrolysis. Due to the high ash content, the char should be classed as a low-grade fuel with a calorific value between 5000 and 7000 kJ/kg. The char from the SG experiment (Ch-SG, Table 2) contained a higher amount of volatile organic material (not yet degraded) than the rest of the chars. For all the chars the percentage of organic matter (volatiles and fixed C) was found to be lower than 24%. The very high carbon content of the organic fraction indicates a char with a highly aromatic organic fraction in which the amount of sulphur had increased, whereas the amounts of nitrogen and oxygen had become depleted with respect to the values of the raw sludge (Table 1). The char from the SG device exhibited the highest oxygen content, which is in agreement with the high amount of organic volatile matter.
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Table 2 Analysis data of the char (Ch) obtained from the pyrolysis of sewage sludge Ultimate analysisa,b (wt%)
Proximate analysis (wt%)
Ch-MG Ch-SG Ch-MC Ch-SC Ch-EF
CV (kJ/kg) c
M
A
V
FC
C
H
N
S
O
0.4 1.8 1.5 1.2 0.6
82.5 74.6 78.5 83.5 80.0
3.6 14.1 5.8 2.8 5.0
13.5 9.5 14.2 12.5 14.4
92.4 83.9 92.0 94.8 88.1
2.3 0.0 0.0 0.0 3.1
3.5 4.7 3.5 2.6 4.6
1.9 1.6 2.0 2.6 2.0
0.0 9.8 2.5 0.0 2.2
5576 7223 6713 5839 6207
Ch-MG and Ch-MC: char from the multimode microwave oven using graphite and char as absorbers, respectively; Ch-SG and Ch-SC: char from the single mode microwave oven using graphite and char as absorbers, respectively; Ch-EF: char from the electrical furnace; M: moisture content; A: ash content; V: volatile matter content; FC: fixed carbon and; CV: calorific value. a,b Moisture and ash free. c By difference.
3.3. Oils The ultimate analysis, H/C, H/O and H/N atomic ratios, and calorific values of the pyrolysis oils are given in Table 3. The severity of the thermal treatment gives rise to oils that become considerably more deoxygenated than the parent sludge (Table 1), indicating that a large number of functional groups were lost during pyrolysis, especially when the EF device was used. Thus, the H/O atomic ratio values were much higher in the pyrolysis oils than in the initial sludge. The same trend is observed for the H/N atomic ratio, with the exception of the EF oil which presented the lowest H/N value. However, the differences from the initial sludge are not so large as those observed for the H/O values. The H/C atomic ratio of the oils from the microwave suggests the presence of compounds with a high aliphatic content. Nevertheless, these H/C values are lower than those for sludge (see Table 1), indicating that aromatisation reactions have occurred to some extent. Conversely, the low H/ C value for the EF oil is typical of highly aromatic materials. As mentioned before, the characterization of the oils was carried out by means of infrared spectroscopy (FTIR) and gas chromatography–mass spectrometry (GC–MS). The FTIR spectra of the dried sludge and oils obtained by microwave and conventional pyrolysis are
shown in Fig. 4. The interpretation of the main bands is based on the literature (Socrates, 1994; Mansuy et al., 2001). The spectrum for the dried sludge shows a very broad and strong absorption band at 3300 cm1, which can be assigned to H-bonded O–H and N–H groups, bands in the region between 2880 and 3000 cm1 being due to aliphatic C–H stretching. Two partially overlapping bands at 1720 and 1660 cm1 were observed in all cases. The former was assigned to C=O of esters and acids, and the latter to amides. Peaks at 1532 and at 1241 cm1 could also be due to secondary amides. The region between 1506 and 1328 cm1 is due to the vibrations of CH2 and CH3 groups. A strong band was also observed at 1064 cm1 which could have been caused by the stretching vibrations of the C–O and C–O–C groups. Below 700 cm1, skeletal vibrations occurred, which are often difficult to interpret. A comparison of the spectra in Fig. 4 reveals significant differences between the dried sludge and the pyrolysis oils. Thus, the band at 1064 cm1 which was the largest in sludge and corresponds mainly to oxygen compounds such as alcohols, carboxylic acids, ethers and esters, was either totally absent or present only at a very low intensity in the pyrolysis oils. The relative intensity of the bands in the oil spectra points to both an increase in the concentration of aliphatic hydrogen and a decrease in the concentration of heteroatoms, with respect to the initial sludge. This result can be explained
Table 3 Ultimate analysis, H/C, H/O and H/N atomic ratios and calorific value (CV) of the pyrolysis oils from sewage sludge
O-MG O-SG O-MC O-SC O-EF
Ca (wt%)
Ha (wt%)
Na (wt%)
Sa (wt%)
Oa,b (wt%)
H/C
H/O
H/N
CV (kJ/kg)
73.5 71.8 72.3 72.6 86.5
8.5 9.1 9.1 9.3 3.6
6.2 5.6 5.8 5.7 4.9
– 0.6 0.9 0.4 –
11.8 12.9 11.9 12.0 5.0
1.4 1.5 1.5 1.5 0.5
11.5 11.3 12.2 12.4 11.5
19.2 22.7 22.0 22.8 10.3
36,813 35,718 35,668 35,818 36,429
O-MG and O-MC: oil from the multimode microwave cavity oven using graphite (G) and char (C) as absorbers, respectively. O-SG and O-SC: oil from the single mode microwave cavity oven using graphite and char as absorbers, respectively. O-EF: oil from the electrical furnace. a Dry and ash free base. b Calculated by difference.
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Table 5 Distribution (% area) of the different classes of compounds in the chromatographied fraction of the pyrolysis oils
C-O C=O C-Haliphatic Raw sewage sludge O-H, N-H
-CONH-CH2-, -CH3-
O-MG
Groups
O-MG
O-SG
O-MC
O-SC
O-EF
Monoaromatics Aliphatics Methyl aliphatics Carboxylic acids Esters Amides Nitriles PACs Terpenoids and steroids
22.0 10.8 1.0 14.0 2.8 2.1 0.9 6.2 4.2
24.8 10.9 0.5 12.0 3.7 3.0 0.8 6.0 5.2
11.1 10.6 0.8 30.0 3.3 2.1 0.5 5.4 5.7
13.9 16.1 1.7 18.6 4.7 2.7 0.5 7.5 8.3
n.d.a n.d. n.d. n.d. n.d. n.d. n.d. 78.0b n.d.
a b
O-MC
O-UG
O-UC C-Haromatic
C-N
O-EF C-Haromatic 3500
3000
2500
2000
1500
1000
600
Wavenumbers (cm-1) Fig. 4. Fourier-transform infrared spectra of the raw sewage sludge and of its pyrolysis oils obtained with microwave and electrical ovens.
by the absorbance ratios shown in Table 4. The ratio between the absorbance of aliphatic hydrogen (A2956 + A2923) and the absorbance of the band at 3330 cm1 (A3300) which is assigned to compounds with O–H and N–H groups, shows an increase in the microwave pyrolysis oils. These results are in agreement with the values of the H/O and H/N ratio for the oils given in Table 3. This trend is also observed for the (A2956 + A2923)/A1662 ratio, although the differences between microwave pyrolysis oils and the initial sludge are not so great as for the (A2956 + A2923)/A3330 ratio. In relation to the bands at 1720 and 1660 cm1, higher values of the ratio A1720/ A1660 were observed for the microwave oils than for
n.d.: not detected. See Domı´nguez et al. (2003) for detailed composition.
the initial sludge, which reflects an increase in the concentration of esters and a decrease of the amide compounds in the oils. As regards the oil obtained by conventional heating the main functional group was aromatic hydrogen (band at 3050 cm1 and between 700 and 900 cm1), although aliphatic and carbonyl compounds were also detected by FTIR. In this oil the band at 1662 cm1 was totally absent, however a new band at 2225 cm1 appeared which may be assigned to nitrile groups (–CN). The composition of pyrolysis oils from different sewage sludges was also examined by GC–MS. The compounds identified in the fractions analysed have been grouped into eight main classes: monoaromatics, aliphatics, acids, esters, amides, nitriles, steroids and polycyclic aromatic compounds. Table 5 shows the distribution of the different classes of compounds. The monoaromatic compounds include benzene, benzene alkyl derivatives, benzeneacetonitrile, pyridine, methylpyridine and phenols. Among the aliphatics, 1-alkenes, n-alkanes and their methyl derivatives, with a number of carbons ranging between C10 and C18, were found. The carboxylic acids (RCOOH), esters (RCOOR 0 ), nitriles (RCN) and amides (RCONH2) were heavy compounds, where R and R 0 are long aliphatic chains with 12, 14, 16 and 18 carbon atoms. The following steroids were also identified: cholestene, cholestadiene, alkylcholestene and cholestenone and a series of polycyclic aromatic compounds (PACs) including quinoline, 1Hindene, methyl-1H-indene, naphthalene, methylnaphthalenes, acenaphthylene and phenanthrene.
Table 4 Absorbance ratios for the dried sludge and for its microwave pyrolysis oils
(A2956 + A2923)/A3330 (A2956 + A2923)/A1662 A1710/A1662
Dried sewage sludge
O-MG
O-SG
O-MC
O-SC
2.1 1.2 0.65
6.2 2.7 1.2
5.3 1.8 1.2
10.0 2.9 1.3
10.0 2.9 1.2
O-MG and O-MC: oil from the multimode microwave cavity oven using graphite (G) and char (C) as absorbers, respectively. O-SG and O-SC: oil from the single mode microwave cavity oven using graphite and char as absorbers, respectively.
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The GC–MS analysis of the oil obtained by pyrolysis with conventional heating (O-EF) demonstrated that its chromatographied fraction was very different to that of the microwave oils. Thus, O-EF oil was composed basically of highly condensed polyaromatic compounds (up to six rings). Compounds such as aliphatics, carboxylic acids, esters, nitriles, amides and steroids, probably of biological origin and present in the microwave pyrolysis oils (see Table 5), were not detected in this case. Heterocyclic compounds with sulphur and nitrogen were found, i.e., benzothiophenes, quinoline, pyridine, benzonitriles and naphthalencarbonitrile, although the dominant compounds were polycyclic aromatic hydrocarbons (PAHs). Among the PAHs the main compounds were naphthalene (11%), phenanthrene (6%), fluoranthene (5%), pyrene (6%) and benzo(a)anthracene (5%). However, heavy hydrocarbons with a high level of mutagenic activity such as benzofluoranthenes (3%), benzo(a)pyrene (3%), perylene (1%), benzo(ghi)perylene (1%) and anthanthrene (1%) were also identified (Domı´nguez et al., 2003). From a comparison of the FTIR and GC–MS results, it can be concluded that both techniques need to be used in oil characterization because of the complementary information provided by each one. Thus, in the microwave oils, mono aromatics such as benzene, toluene or styrene and polycyclic aromatic hydrocarbons were detected by GC–MS. However, in the FTIR it was difficult to detect these compounds, probably due to the low concentration of aromatic hydrogen with respect to aliphatic hydrogen. On the other hand, in the O-EF oil only polycyclic aromatic compounds were observed by GC–MS, although the FTIR spectra of this oil indicate that aliphatic, carbonyl and nitrogen compounds were also present. These compounds, which have a high molecular weight, are the components of the non-chromatographied fraction and for this reason they cannot be assessed by GC–MS. 3.4. Gases Table 6 shows the yields of non-condensable gases (H2, O2, N2, CO, CO2, CH4, CO2, C2H4 and C2H6) from the microwave ovens with graphite and char as absorbers and for the electrical furnace. Hydrogen was the main gas produced in all the experiments, its yield being higher in microwave pyrolysis than in conventional pyrolysis. CO and CH4 were the second most important components of the gases from microwave and conventional pyrolysis, respectively. Independently of the absorber used, the highest values for CO were reported for the multimode microwave oven (G-MG and G-MC). In accordance with these results the values for the H2 + CO mixture (synthesis gas) were much higher in microwave than in conventional pyrolysis, the gases from the multimode microwave oven having a higher
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Table 6 Composition (vol%) of the gases (G) from the pyrolysis of sewage sludge
H2 O2 N2 CO CH4 CO2 C2H4 C2H6 H P2 + CO CxHy CV (kJ/m3)
G-MG
G-SG
G-MC
G-SC
G-EF
32.7 2.5 16.9 30.1 6.4 8.0 3.1 0.3 62.3 9.8 8591
34.6 2.3 16.4 24.9 7.0 10.6 3.7 0.7 59.4 11.4 9517
33.6 2.4 17.6 32.8 4.7 6.7 1.9 0.2 66.4 6.9 7427
38.0 2.7 19.5 22.3 4.4 9.7 3.0 0.5 60.3 7.8 8361
29.0 3.0 15.4 16.2 18.4 11.8 6.2 0.1 45.1 24.7 13,856
G-MG and G-MC: Gas from the multimode microwave oven using graphite and char as absorbers, respectively. G-SG and G-SC: Gas from the single mode microwave oven using graphite and char as absorbers, respectively. G-EF: Gas from the electrical furnace.
proportion of this mixture than those from the single mode. It can also be deduced from the results in Table 6 that char had a more significant influence than graphite on the formation of H2 + CO. Accordingly, the highest values were obtained in the case of G-MC (66.4% v/v). These results, along with the low proportion of CO2, indicate that the endothermic gasification reactions of an organic material (reactions 1–4) would be favoured in a microwave oven: C þ H2 O $ CO þ H2 C þ CO2 $ 2CO
ð1Þ ð2Þ
ð–CH2 –Þ þ H2 O $ CO þ 2H2
ð3Þ
ð–CH2 –Þ þ CO2 $ 2CO þ H2
ð4Þ
Table 6 alsoPshows the values of the light hydrocarbon yields ( CxHy = CH4 + C2H4 + C2H6) in the pyrolysis gases. The values of the hydrocarbons in the gas obtained from conventional pyrolysis (24.7% v/v) were higher than those for the microwave experiments (7–11% v/v). The effect of the graphite and the single mode microwave oven was more pronounced than that of the char and the multimode oven, respectively. The high CH4 and CO2 and the low H2 and CO contents of the gas from the electrical furnace could have been due to the fact that exothermic reactions 5 and 6 were favoured CO þ H2 O $ CO2 þ H2
ð5Þ
CO þ 3H2 ! CH4 þ H2 O
ð6Þ
The composition of the gases from conventional and microwave pyrolysis with graphite and char as absorbers can be explained by the different systems of heating. In the microwave-assisted pyrolysis the sample is heated directly, so that it reaches a high temperature in a very short time, while the reactor walls remain at a lower
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temperature than the bulk sample. On the other hand, the heat generated inside the sample by the microwaves causes the internal water to vaporize before it leaves the sample bed. The high temperatures reached (1040 C) and the contact between the pyrolysis vapours (water included), the organic material and the hot mineral matter (see Table 1) may have favoured the gasification reactions. These reactions would be promoted with the addition of char as an absorber. Piskorz et al. (1986) reported the catalytic effect of the char in a fluidisedbed reactor at 450 C. They concluded that the use of char as fluidised material increases the gas yield while decreasing the oil yield. The same effect was observed when the sewage sludge, which had previously been acid extracted, was pyrolysed. In conventional heating the heat flow goes from the walls of the reactor to the sample, so the temperature is higher in the reactor than inside the bulk sample. The volatile compounds generated have more possibilities to react in the gas phase, undergoing secondary reactions. This gives rise to more favourable conditions than in the microwave oven for reactions between noncondensable gaseous products and for the cracking and condensation reactions of the oil components, which in turn would increase the hydrocarbon content of the pyrolysis gas. On the other hand, the heterogeneous gasification reactions are less favoured in conventional than in microwave heating. The calorific values of the gases (Table 6) from conventional pyrolysis (14,000 kJ/m3) were comparable to those of synthetic coal gas while the heating values of the gases from microwave pyrolysis ranging from 7000 to 9500 kJ/m3 were similar to those of coal hydrogasification (Perry, 1984).
4. Conclusions The results from the analysis of the microwave pyrolysis oils by means of FTIR showed that they were made up of aliphatic hydrogen together with ester, carboxylic or amide groups. Oil from conventional heating contained a high proportion of aromatic hydrogen, although aliphatic hydrogen, esters and nitriles were also observed. The characterization of the oils by GC–MS indicated that the use of electrical heating in the pyrolysis of sewage sludge produced an oil that could have a significant environmental and toxicological impact. Conversely, microwave ovens still preserved some of the functional groups of the initial sludge such as aliphatic and oxygenated compounds, whereas no heavy PACs were detected. The FTIR and GC–MS results suggest that both techniques must be used for an optimal characterization of the oil because of the complementary information provided by each method.
The high pyrolysis temperature used in this work favours the gasification and the secondary cracking reactions of the pyrolysis vapours and therefore leads to both a high gas yield and a low oil production. Syngas (H2 + CO) can be produced in a high yield from the microwave pyrolysis of sewage sludge with the assistance of a microwave absorber. This yield was higher when char was used as microwave absorber rather than graphite and higher in a multimode oven than in a single mode oven. Thus, the combination of the char plus the multimode oven increased the production of syngas by up to 66%. In contrast, the production of hydrocarbons in the gas increased when graphite and the single mode oven were used. The gas from the conventional oven was much richer in hydrocarbons (25%) than that from the microwave ovens (6–11%), giving rise to a gas with a high calorific value. Acknowledgements The authors thank the Spanish Ministry of Science and Technology (Research Project PPQ2001-2083-C0201) and FEDER for financial support. A. Domı´nguez is also grateful to FICYT (Asturias-Spain) for financial assistance. References Asadullah, M., Ito, S., Kunimori, K., Yamada, M., Tomishige, K., 2002. Biomass gasification to hydrogen and syngas at low temperature: novel catalytic system using fluidized bed reactor. J. Catal. 208, 255–259. Bridgwater, A.V., Meier, D., Radlein, D., 1999. An overview of fast pyrolysis of biomass. Org. Geochem. 30, 1479–1493. Caballero, J.A., Font, R., Marcilla, A., Conesa, J.A., 1997. Characterization of sewage sludges by primary and secondary pyrolysis. J. Anal. Appl. Pyrol. 40, 433–450. Chanaa, M.B., Lallemant, M., Mokhlisse, A., 1994. Pyrolysis of Timahdit morocco oil shales under microwave field. Fuel 73 (10), 1643–1649. Chen, G., Andries, J., Spliethoff, H., 2003. Catalytic pyrolysis of biomass for hydrogen rich fuel gas production. Energy Convers. Manage. 44, 2289–2296. Conesa, J.A., Morcilla, A., Moral, R., Moreno-Caselles, J., PerezEspinosa, A., 1998. Evolution of gases in the primary pyrolysis of different sewage sludges. Thermochim. Acta 313, 63–73. Cunliffe, A.M., Williams, P.T., 1988. Composition of oils derived from the batch pyrolysis of tyres. J. Anal. Appl. Pyrol. 44, 131–152. Domı´nguez, A., Mene´ndez, J.A., Inguanzo, M., Bernard, P.L., Pı´s, J.J., 2003. Gas chromatographic-mass spectrometric study of the oil fractions produced by microwave-assisted pyrolysis of different sewage sludges. J. Chromatogr. A 1012, 193–206. El harfi, K., Mokhlisse, A., Chanaa, M.B., Outzourhit, A., 2000. Pyrolysis of the Moroccan (Tarfaya) oil shales under microwave irradiation. Fuel 79, 733–742. Evans, R.J., Milne, T.A., 1987. Molecular characterization of the pyrolysis of biomass. 1. Fundam. Energy Fuels 1 (2), 123–137. Horne, P.A., Williams, P.T., 1996. Influence of temperature on the products from the flash pyrolysis of biomass. Fuel 75 (9), 1051– 1059.
A. Domı´nguez et al. / Bioresource Technology 97 (2006) 1185–1193 Inguanzo, M., Domı´nguez, A., Mene´ndez, J.A., Blanco, C.G., Pı´s, J.J., 2002. On the pyrolysis of sewage sludge: the influence of pyrolysis conditions on solid, liquid and gas fractions. J. Anal. Appl. Pyrol. 63, 209–222. Iwasaki, W., 2003. A consideration of the economic efficiency of hydrogen production from biomass. Int. J. Hydrogen Energy 28, 939–944. Kenneth, H., 1995. Apparatus for waste pyrolysis. Patent US 5387321. Kim, H.Y., 2003. A low cost production of hydrogen from carbonaceous wastes. Int. J. Hydrogen Energy 28, 1179–1186. Kriegerbrockett, B., 1994. Microwave pyrolysis of biomass. Res. Chem. Intermed 20 (1), 39–49. Lu, G.Q., Low, J.C.F., Liu, C.Y., Lua, A.C., 1995. Surface area development of sewage sludge during pyrolysis. Fuel 74 (3), 344– 348. Lu, G.Q., Lau, D.D., 1996. Characterization of sewage sludge derived adsorbent for H2S removal. Part II: surface and pore structural evolution in chemical activation. Gas Sep. Purif. 10, 103–111. Mansuy, L., Bourezgui, Y., Garnier-Zarli, E., Jarde´, E., Re´veille´, V., 2001. Characterization of humid substances in highly polluted river sediments by pyrolysis methylation-gas chromatography– mass spectrometry. Org. Geochem. 32, 223–231. McAuley, B., Kunkel, J., Manahan, S.E., 2001. A new process for the drying and gasification of sewage sludge. Water Eng. Manage. 148 (5), 18–20. Mene´ndez, J.A., Inguanzo, M., Pı´s, J.J., 2002. Microwave-induced pyrolysis of sewage sludge. Water Res. 36, 3261–3264. Mills, G.A., 1994. Status and future opportunities for conversion of synthesis gas to liquid fuels. Fuel 73, 1243–1279.
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Monsef-Mirzai, P., Ravindran, M., McWhinnie, W.R., Burchill, P., 1995. Rapid microwave pyrolysis of coal. Fuel 74 (1), 20– 27. Morf, P., Hasler, P., Nussbaumer, T., 2002. Mechanisms and kinetics of homogenous secondary reactions of tar from continuous pyrolysis of wood chips. Fuel 81, 843–853. Pakdel, H., Roy, C., 1991. Hydrocarbon content of liquid products and tar from pyrolysis and gasification of wood. Energy Fuels 5, 427–436. Perry, R.H., 1984. Perry’s Chemical Engineers Handbook. McGraw Hill, New York, USA. Piskorz, J., Scott, D.S., Westerberg, I.B., 1986. Flash pyrolysis of sewage sludge. Ind. Eng. Chem. Process Des. Dev. 25, 265– 270. Sharypov, V.I., Marı´n, N., Beregovtsova, N.G., Baryshnikov, S.V., Kutnetzov, B.N., Cebolla, V.L., Weber, J.V., 2001. Co-pyrolysis of wood biomass and synthetic polymer mixture. Part I: influence of experimental conditions on the evolution of solid liquids and gases. J. Anal. Appl. Pyrol. 64, 15–28. Shen, L., Zhang, D.K., 2003. An experimental study of oil recovery from sewage sludge by low temperature pyrolysis in a fluidized bed. Fuel 82, 465–472. Socrates, G. (Ed.), 1994. Infrared Characteristic Group Frequencies, second ed. John Wiley & Sons, England, UK. Werther, J., Ogada, T., 1999. Sewage sludge combustion. Prog. Energy Combust. 25, 55–116. Zlotorzynski, A., 1995. The application of microwave radiation to analytical and environmental chemistry. Crit. Rev. Anal. Chem. 25 (1), 43–76.
Production of bio-fuels by high temperature pyrolysis of sewage sludge using conventional and microwave heating A. Domı´nguez, J.A. Mene´ndez *, M. Inguanzo, J.J. Pı´s Instituto Nacional del Carbo´n (INCAR) C.S.I.C., Apartado 73, 33080 Oviedo, Spain Received 29 March 2004; received in revised form 21 March 2005; accepted 20 May 2005 Available online 10 February 2006
Abstract The pyrolysis of sewage sludge was investigated using microwave and electrical ovens as the sources of heat, and graphite and char as microwave absorbers. The main objective of this work was to maximize the gas yield and to assess its quality as a fuel and as a source of hydrogen or syngas (H2 + CO). Both gases were produced in a higher proportion by microwave pyrolysis than by conventional pyrolysis, with a maximum value of 38% for H2 and 66% for H2 + CO. The oils obtained were also characterized using FTIR and GC–MS. The use of conventional electrical heating in the pyrolysis of sewage sludge produced an oil that could have a significant environmental and toxicological impact. Conversely, microwave pyrolysis still preserved some of the functional groups of the initial sludge such as aliphatic and oxygenated compounds, whereas no heavy PACs were detected. 2005 Elsevier Ltd. All rights reserved. Keywords: Bio-gas; Hydrogen; Syngas; Sewage sludge; Pyrolysis; Gasification; Microwave
1. Introduction The increase in the production of sewage sludge from municipal and industrial wastewater plants is a matter of growing concern because it represents a potential risk for human health and the environment. In recent years, methods formerly used for the disposal of sewage sludge (Werther and Ogada, 1999), including landfill, ocean dumping and disposal on agricultural land, have become much less acceptable. There is an urgent need for alternative solutions to the sludge disposal problem. Sewage sludge and other organic solid wastes such as biomass are examples of renewable energy resources. They can be used in a variety of ways to provide energy via: direct combustion (Werther and Ogada, 1999), gasification (McAuley et al., 2001; Pakdel and Roy, 1991) *
Corresponding author. Tel.: +34 985 11 89 72; fax: +34 985 29 76
62. E-mail address: [email protected] (J.A. Mene´ndez). 0960-8524/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2005.05.011
and pyrolysis (Evans and Milne, 1987; Bridgwater et al., 1999). Of these three thermochemical processes, pyrolysis has attracted most attention in recent years (Conesa et al., 1998; Caballero et al., 1997) since the process conditions can be optimised to maximize the production of chars, oils or gases depending on the specific interest (Horne and Williams, 1996; Inguanzo et al., 2002; Sharypov et al., 2001). The char can then be burnt as fuel or disposed of—since the heavy metals are fixed inside the carbonaceous matrix—or even be upgraded to activated carbon (Lu et al., 1995; Lu and Lau, 1996). The gas can be used as fuel, whereas the oil can either serve as fuel or as raw material for chemicals. Many works in the literature focus on the production of liquids from the pyrolysis of organic wastes at low temperature (Cunliffe and Williams, 1988; Piskorz et al., 1986; Shen and Zhang, 2003). However, the use of these waste materials to produce a hydrogen rich gas or synthesis gas has also aroused considerable interest recently. The reasons for this are that hydrogen is an environmentally clean
A. Domı´nguez et al. / Bioresource Technology 97 (2006) 1185–1193
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energy source and that syngas can be converted to liquid fuels, which would help to reduce environmental pollution (Asadullah et al., 2002; Mills, 1994). The gasification and pyrolysis of biomass has been investigated for the production of H2 or syngas (Chen et al., 2003; Iwasaki, 2003; Kim, 2003). The majority of these works have focused on using fixed and fluidised bed reactors and thermal energy. However, nowadays, the use of microwave heating is applied not only in the field of analytical, organic and environmental chemistry (Zlotorzynski, 1995), but also for the pyrolysis of materials such as biomass (Kriegerbrockett, 1994), coal (Monsef-Mirzai et al., 1995), oil shales (Chanaa et al., 1994; El harfi et al., 2000) and various organic wastes (Kenneth, 1995). It is known that these organic wastes are poor receptors of microwave energy so that with this method it is impossible to achieve the temperature necessary to carry out the pyrolysis. Nevertheless, microwave-induced pyrolysis is possible, if the raw material is mixed with an effective receptor (Mene´ndez et al., 2002). In the present work, the production of solids, oils and gas from sewage sludge pyrolysis at high temperatures using microwave energy was examined. The results were compared with those obtained with conventional heating. The sludge was analysed by FTIR whereas the pyrolysed oils were characterized by FTIR and GC– MS. The main objective was to obtain a high yield of gas and to evaluate the quality of the gas produced on the basis of the proportions of H2 or syngas (H2 + CO).
The high H/C ratio value indicates a high content of aliphatic hydrogen in these wastes, while the large amounts of oxygen show that a high number of polar compounds were present in the sludge. The calorific value observed for dried sewage sludge is similar to that of other conventional and non-conventional fuels such as low rank coal, paper, wood or black liquor (Perry, 1984). It is well known that the presence of inorganic matter plays an important role in the thermal decomposition process. Table 1 shows that there were considerable amounts of Ca, Al and Si, whereas other metals such as Mg, Fe, K and Na were found in lower proportions. 2.2. Microwave pyrolysis The drying and pyrolysis of the sewage sludge was carried out in a single process using both multimode (M) and single mode (S) microwave cavity ovens. Experiments were carried out by placing samples of wet sludge (ca. 15 g) in a quartz reactor (40 cm height · 3 cm i.d.). In order to maintain an inert atmosphere during the treatments, a He flow rate of 100 mL/min was passed through the sample bed for 10 min prior to the commencement of the experiment, this being reduced during the experiment to 10 mL/min. A constant input power of 1000 W was used and the microwave frequency was 2450 MHz. A sketch of the experimental device is shown in Fig. 1. Either small strips of graphite (G) of about 3 · 3 mm, or of a carbonaceous solid residue (char) (C) from the pyrolysis of sewage sludge were employed as microwave receptors (Mene´ndez et al., 2002). Thus, graphite or char (ca. 1 g) were homogenously blended with 15 g of wet sewage sludge, and the mixture was subjected to microwave treatment. The temperature of the sample during the experiments was monitored by an infrared optical pyrometer. A plot of the evolution of temperature with time is shown in Fig. 2. There was a rapid increase in temperature up to around 135 C, the sample remaining at this temperature for about 2 min.
2. Methods 2.1. Sewage sludge characteristics A sewage sludge from an urban waste water treatment plant subjected to aerobic digestion was used. Table 1 summarizes its main chemical characteristics. Table 1 Chemical characteristics of the sewage sludge Ultimate analysisa,b (wt%)
Proximate analysis (wt%) a
M
A
71
31.2
Na2O
a
FC
C
62.3
6.5
52.3
V
a,c
MgO
Al2O3
CV (kJ/kg)
H
N
S
8.0
6.7
0.7
CaO
Fe2O3
Oc
H/C
H/O
32.3
1.83
4.0
H/N 16.7
16,682
K2O
MnO
SiO2
TiO2
0.76
0.04
15.7
0.25
a
Composition of main inorganic elements (expressed as wt% of metal oxides ) 0.28 1.15 4.71 5.97 1.75 Co (ppm)
Cr (ppm)
Trace elements 3.5 121
Pb (ppm)
Mn (ppm)
Ni (ppm)
Cu (ppm)
Zn (ppm)
Fe (ppm)
Cd (ppb)
Hg (ppb)
246
214
13.2
143
662
9900
2906
919
M: moisture content; A: ash content; V: volatile matter content; CV: calorific value; FC: fixed carbon. a Dry base. b Ash free basis. c Calculated by difference.
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He 3
S
2
5 4
8
1
6
M
2
7
He
1 3 4
Fig. 1. Schematic diagram of the microwave pyrolysis reactors: S, single mode and M, multimode ovens: (1) magnetron; (2) waveguide; (3) quartz reactor and sample; (4) optical pyrometer; (5) condenser with dichloromethane; (6) ice bath; (7) gas container and (8) gas sampling.
1200
Temperature (°C)
1000 800 600 400
S(g)
S(c)
1187
tor, sludge amounts and He flow rate conditions as in the case of microwave pyrolysis. The temperature was set at 1040 C and samples were heated at maximum power in order to get the maximum heating rate possible from the equipment. The time needed to complete the pyrolysis was 24 min, the pyrolysis temperature being reached after 14 min. Thus, the heating rate with the electric furnace (74.3 C/min) was much lower than the heating rate obtainable in the microwave (200 C/min). The reason for this is that the microwave is more efficient at removing the high moisture content of sludge, making it possible to obtain the pyrolysis temperature in a shorter time than with an electrical furnace. 2.4. Recovery of the pyrolysis fractions Fig. 1 shows a sketch of the experimental set-up used for collecting the different pyrolysis fractions (a similar collecting set-up was used in the case of the electric furnace pyrolysis). The aqueous fraction recovered in the condensers was separated from the organic fraction by decantation, while the organic fraction dissolved in the dichloromethane was first dried, using anhydrous sodium sulphate, next filtered and then concentrated by solvent evaporation. The solid, aqueous and oil fraction yields were calculated from the weight of each fraction, while the gas yield was evaluated by difference (see Fig. 3).
200
2.5. Gas chromatography with TCD detector 0 0
2
4
6
8
10
12
14
16
Time (min)
Fig. 2. Microwave heating profile of the sludge with graphite (G) and char (C).
During this period of time the wet sludge was completely dried and when all the moisture was removed from the sample another rapid increase in temperature up to 1040 C took place. The temperature remained more or less stable until the end of the pyrolysis process. The evolution of the temperature was followed by means of preliminary experiments, on the basis of which it was possible to select the appropriate pyrolysis time. The volatile matter content of the carbonaceous residues (chars) obtained in these experiments was determined, and the pyrolysis time was selected according to the minimum time necessary for obtaining a char with no significant volatile matter content. The pyrolysis time was about 10 min for all microwave experiments, maximum temperature being reached after 5 min.
The non-condensable gases were collected in a propylene container completely filled to the top with water so that the gases could be collected without their coming into contact with atmospheric air. Samples of 150 lL of these gases were then analysed in an HP 5890 gas chromatograph fitted with a TCD detector. An HP Porapak N 80/100 (3.07 m · 3.2 mm o.d. · 2.1 mm i.d.) and an HP Molecular Sieve 13X, 45/60 (0.91 m · 3.2 mm
Aqueous
80
For comparative purposes sludge was also pyrolysed in an electric furnace (EF) using the same quartz reac-
Char
Oil
70 60 50 40 30 20 10 0 MG
2.3. Electric furnace pyrolysis
Gas
SG
MC
SC
EF
Fig. 3. Product yield (wt%) from the pyrolysis of sewage sludge using different ovens and microwave absorbers. M, multimode microwave cavity oven; S, single mode microwave cavity oven; EF, electrical furnace; G, graphite and C, char.
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o.d. · 2.1 mm i.d.) column were used. The oven temperature was set at 50 C. The carrier gas flow rate (He) was 20 mL/min. The injector and detector temperatures were 80 and 220 C, respectively. The TCD was calibrated with a standard gas mixture at regular intervals. 2.6. Infrared spectroscopy (FTIR) The FTIR spectra were collected in a Nicolet MagnaIR 560 spectrometer, taking 128 scans at a resolution of 4 cm1. The raw sewage sludge was dried and analysed using KBr pellets at a ratio of 1:100 (sample/KBr). The pellets were dried at 100 C overnight. In the case of the oils a small amount of each sample was deposited between two NaCl disks, allowing the dichloromethane to evaporate before the spectra were collected. 2.7. Gas chromatography–mass spectrometry (GC–MS) The oils were dissolved in dichloromethane and analysed by GC–MS using a Hewlett-Packard HP 6890 gas chromatograph coupled to an HP 5973 quadrupole detector. The gas chromatograph was equipped with a 60 m · 0.25 mm capillary column coated with a 0.25 lm thick film of 5% phenylmethylpolysiloxane (HP-5). Helium was employed as a carrier gas at a constant flow of 0.9 mL/min. The initial oven temperature was 40 C held for 5 min and then programmed from 40 to 300 C at 5 C/min with an isothermal hold for 30 min. Splitless injection was carried out at 300 C and the purge valve was switched on after 1 min. The ion source and transfer line temperatures were 230 and 325 C, respectively. Data were acquired in the full-scan mode between m/z 33–533 and a solvent delay of 6 min was used. The compounds were identified by comparing their mass spectra with those from the NIST mass spectral data library, interpreting the observed fragmentation and evaluating the retention times in comparison with standard compounds. Calibration was not carried out due to the large number of compounds and functionalities present in the fractions analysed. A semiquantitative evaluation of the samples was performed by integrating the total ion chromatogram (TIC) and calculating the area percentage of each peak identified. These data, therefore are not absolute and only serve for purposes of comparison.
3. Results and discussion 3.1. Fraction yields The yields of the pyrolysis products obtained from sludge using microwave and conventional heating (EF) are shown in Fig. 3. For microwave heating two microwave ovens, one multimode (M) and one single mode
(S), and two absorbers, graphite (G) and char (C), were employed. The char yield for all the pyrolysis experiments was around 10–12 wt%, the maximum value being achieved with the single mode microwave oven and graphite (SG). Due to the high moisture content of the sludge (71 wt%), the aqueous fraction yield was found to be higher than that obtained for the other fractions. In this work the highest value corresponded to the MC system and the lowest to EF. In all cases the aqueous fraction content was found to be lower than the initial moisture content of the sludge. Most of the initial water (free water) never reaches high temperatures. However, a low percentage of this water is contained inside the particles of the sludge and more energy is required to remove it. Thus, some of the water may remain even at high temperatures. It therefore reacts mainly with the carbonaceous solid (gasification reaction) although it also reacts with other gas components (water gas shift reaction). The high temperatures (1040 C) used in the present work would favour these reactions, giving rise to water yields lower than the initial moisture of the sludge. On the other hand, the water gas shift reaction also occurs with the water formed during pyrolysis at temperatures around 600 C (Piskorz et al., 1986; Morf et al., 2002) due to condensation reactions of functional groups of the volatile compounds. The high pyrolysis temperature used gave rise to high gas fraction yields. At maximum gas yield, the oil production is relatively low due to gasification and secondary cracking reactions of the pyrolysis vapours. The lowest and the highest values for the oil and gas fractions, respectively, were obtained with the EF. 3.2. Solids Table 2 shows the ultimate and proximate analyses of the chars (Ch) obtained in the pyrolysis of sewage sludge using the different devices. The ash content increased up to values of 83.5% due to the high degree of devolatilization produced by pyrolysis. Due to the high ash content, the char should be classed as a low-grade fuel with a calorific value between 5000 and 7000 kJ/kg. The char from the SG experiment (Ch-SG, Table 2) contained a higher amount of volatile organic material (not yet degraded) than the rest of the chars. For all the chars the percentage of organic matter (volatiles and fixed C) was found to be lower than 24%. The very high carbon content of the organic fraction indicates a char with a highly aromatic organic fraction in which the amount of sulphur had increased, whereas the amounts of nitrogen and oxygen had become depleted with respect to the values of the raw sludge (Table 1). The char from the SG device exhibited the highest oxygen content, which is in agreement with the high amount of organic volatile matter.
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Table 2 Analysis data of the char (Ch) obtained from the pyrolysis of sewage sludge Ultimate analysisa,b (wt%)
Proximate analysis (wt%)
Ch-MG Ch-SG Ch-MC Ch-SC Ch-EF
CV (kJ/kg) c
M
A
V
FC
C
H
N
S
O
0.4 1.8 1.5 1.2 0.6
82.5 74.6 78.5 83.5 80.0
3.6 14.1 5.8 2.8 5.0
13.5 9.5 14.2 12.5 14.4
92.4 83.9 92.0 94.8 88.1
2.3 0.0 0.0 0.0 3.1
3.5 4.7 3.5 2.6 4.6
1.9 1.6 2.0 2.6 2.0
0.0 9.8 2.5 0.0 2.2
5576 7223 6713 5839 6207
Ch-MG and Ch-MC: char from the multimode microwave oven using graphite and char as absorbers, respectively; Ch-SG and Ch-SC: char from the single mode microwave oven using graphite and char as absorbers, respectively; Ch-EF: char from the electrical furnace; M: moisture content; A: ash content; V: volatile matter content; FC: fixed carbon and; CV: calorific value. a,b Moisture and ash free. c By difference.
3.3. Oils The ultimate analysis, H/C, H/O and H/N atomic ratios, and calorific values of the pyrolysis oils are given in Table 3. The severity of the thermal treatment gives rise to oils that become considerably more deoxygenated than the parent sludge (Table 1), indicating that a large number of functional groups were lost during pyrolysis, especially when the EF device was used. Thus, the H/O atomic ratio values were much higher in the pyrolysis oils than in the initial sludge. The same trend is observed for the H/N atomic ratio, with the exception of the EF oil which presented the lowest H/N value. However, the differences from the initial sludge are not so large as those observed for the H/O values. The H/C atomic ratio of the oils from the microwave suggests the presence of compounds with a high aliphatic content. Nevertheless, these H/C values are lower than those for sludge (see Table 1), indicating that aromatisation reactions have occurred to some extent. Conversely, the low H/ C value for the EF oil is typical of highly aromatic materials. As mentioned before, the characterization of the oils was carried out by means of infrared spectroscopy (FTIR) and gas chromatography–mass spectrometry (GC–MS). The FTIR spectra of the dried sludge and oils obtained by microwave and conventional pyrolysis are
shown in Fig. 4. The interpretation of the main bands is based on the literature (Socrates, 1994; Mansuy et al., 2001). The spectrum for the dried sludge shows a very broad and strong absorption band at 3300 cm1, which can be assigned to H-bonded O–H and N–H groups, bands in the region between 2880 and 3000 cm1 being due to aliphatic C–H stretching. Two partially overlapping bands at 1720 and 1660 cm1 were observed in all cases. The former was assigned to C=O of esters and acids, and the latter to amides. Peaks at 1532 and at 1241 cm1 could also be due to secondary amides. The region between 1506 and 1328 cm1 is due to the vibrations of CH2 and CH3 groups. A strong band was also observed at 1064 cm1 which could have been caused by the stretching vibrations of the C–O and C–O–C groups. Below 700 cm1, skeletal vibrations occurred, which are often difficult to interpret. A comparison of the spectra in Fig. 4 reveals significant differences between the dried sludge and the pyrolysis oils. Thus, the band at 1064 cm1 which was the largest in sludge and corresponds mainly to oxygen compounds such as alcohols, carboxylic acids, ethers and esters, was either totally absent or present only at a very low intensity in the pyrolysis oils. The relative intensity of the bands in the oil spectra points to both an increase in the concentration of aliphatic hydrogen and a decrease in the concentration of heteroatoms, with respect to the initial sludge. This result can be explained
Table 3 Ultimate analysis, H/C, H/O and H/N atomic ratios and calorific value (CV) of the pyrolysis oils from sewage sludge
O-MG O-SG O-MC O-SC O-EF
Ca (wt%)
Ha (wt%)
Na (wt%)
Sa (wt%)
Oa,b (wt%)
H/C
H/O
H/N
CV (kJ/kg)
73.5 71.8 72.3 72.6 86.5
8.5 9.1 9.1 9.3 3.6
6.2 5.6 5.8 5.7 4.9
– 0.6 0.9 0.4 –
11.8 12.9 11.9 12.0 5.0
1.4 1.5 1.5 1.5 0.5
11.5 11.3 12.2 12.4 11.5
19.2 22.7 22.0 22.8 10.3
36,813 35,718 35,668 35,818 36,429
O-MG and O-MC: oil from the multimode microwave cavity oven using graphite (G) and char (C) as absorbers, respectively. O-SG and O-SC: oil from the single mode microwave cavity oven using graphite and char as absorbers, respectively. O-EF: oil from the electrical furnace. a Dry and ash free base. b Calculated by difference.
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Table 5 Distribution (% area) of the different classes of compounds in the chromatographied fraction of the pyrolysis oils
C-O C=O C-Haliphatic Raw sewage sludge O-H, N-H
-CONH-CH2-, -CH3-
O-MG
Groups
O-MG
O-SG
O-MC
O-SC
O-EF
Monoaromatics Aliphatics Methyl aliphatics Carboxylic acids Esters Amides Nitriles PACs Terpenoids and steroids
22.0 10.8 1.0 14.0 2.8 2.1 0.9 6.2 4.2
24.8 10.9 0.5 12.0 3.7 3.0 0.8 6.0 5.2
11.1 10.6 0.8 30.0 3.3 2.1 0.5 5.4 5.7
13.9 16.1 1.7 18.6 4.7 2.7 0.5 7.5 8.3
n.d.a n.d. n.d. n.d. n.d. n.d. n.d. 78.0b n.d.
a b
O-MC
O-UG
O-UC C-Haromatic
C-N
O-EF C-Haromatic 3500
3000
2500
2000
1500
1000
600
Wavenumbers (cm-1) Fig. 4. Fourier-transform infrared spectra of the raw sewage sludge and of its pyrolysis oils obtained with microwave and electrical ovens.
by the absorbance ratios shown in Table 4. The ratio between the absorbance of aliphatic hydrogen (A2956 + A2923) and the absorbance of the band at 3330 cm1 (A3300) which is assigned to compounds with O–H and N–H groups, shows an increase in the microwave pyrolysis oils. These results are in agreement with the values of the H/O and H/N ratio for the oils given in Table 3. This trend is also observed for the (A2956 + A2923)/A1662 ratio, although the differences between microwave pyrolysis oils and the initial sludge are not so great as for the (A2956 + A2923)/A3330 ratio. In relation to the bands at 1720 and 1660 cm1, higher values of the ratio A1720/ A1660 were observed for the microwave oils than for
n.d.: not detected. See Domı´nguez et al. (2003) for detailed composition.
the initial sludge, which reflects an increase in the concentration of esters and a decrease of the amide compounds in the oils. As regards the oil obtained by conventional heating the main functional group was aromatic hydrogen (band at 3050 cm1 and between 700 and 900 cm1), although aliphatic and carbonyl compounds were also detected by FTIR. In this oil the band at 1662 cm1 was totally absent, however a new band at 2225 cm1 appeared which may be assigned to nitrile groups (–CN). The composition of pyrolysis oils from different sewage sludges was also examined by GC–MS. The compounds identified in the fractions analysed have been grouped into eight main classes: monoaromatics, aliphatics, acids, esters, amides, nitriles, steroids and polycyclic aromatic compounds. Table 5 shows the distribution of the different classes of compounds. The monoaromatic compounds include benzene, benzene alkyl derivatives, benzeneacetonitrile, pyridine, methylpyridine and phenols. Among the aliphatics, 1-alkenes, n-alkanes and their methyl derivatives, with a number of carbons ranging between C10 and C18, were found. The carboxylic acids (RCOOH), esters (RCOOR 0 ), nitriles (RCN) and amides (RCONH2) were heavy compounds, where R and R 0 are long aliphatic chains with 12, 14, 16 and 18 carbon atoms. The following steroids were also identified: cholestene, cholestadiene, alkylcholestene and cholestenone and a series of polycyclic aromatic compounds (PACs) including quinoline, 1Hindene, methyl-1H-indene, naphthalene, methylnaphthalenes, acenaphthylene and phenanthrene.
Table 4 Absorbance ratios for the dried sludge and for its microwave pyrolysis oils
(A2956 + A2923)/A3330 (A2956 + A2923)/A1662 A1710/A1662
Dried sewage sludge
O-MG
O-SG
O-MC
O-SC
2.1 1.2 0.65
6.2 2.7 1.2
5.3 1.8 1.2
10.0 2.9 1.3
10.0 2.9 1.2
O-MG and O-MC: oil from the multimode microwave cavity oven using graphite (G) and char (C) as absorbers, respectively. O-SG and O-SC: oil from the single mode microwave cavity oven using graphite and char as absorbers, respectively.
A. Domı´nguez et al. / Bioresource Technology 97 (2006) 1185–1193
The GC–MS analysis of the oil obtained by pyrolysis with conventional heating (O-EF) demonstrated that its chromatographied fraction was very different to that of the microwave oils. Thus, O-EF oil was composed basically of highly condensed polyaromatic compounds (up to six rings). Compounds such as aliphatics, carboxylic acids, esters, nitriles, amides and steroids, probably of biological origin and present in the microwave pyrolysis oils (see Table 5), were not detected in this case. Heterocyclic compounds with sulphur and nitrogen were found, i.e., benzothiophenes, quinoline, pyridine, benzonitriles and naphthalencarbonitrile, although the dominant compounds were polycyclic aromatic hydrocarbons (PAHs). Among the PAHs the main compounds were naphthalene (11%), phenanthrene (6%), fluoranthene (5%), pyrene (6%) and benzo(a)anthracene (5%). However, heavy hydrocarbons with a high level of mutagenic activity such as benzofluoranthenes (3%), benzo(a)pyrene (3%), perylene (1%), benzo(ghi)perylene (1%) and anthanthrene (1%) were also identified (Domı´nguez et al., 2003). From a comparison of the FTIR and GC–MS results, it can be concluded that both techniques need to be used in oil characterization because of the complementary information provided by each one. Thus, in the microwave oils, mono aromatics such as benzene, toluene or styrene and polycyclic aromatic hydrocarbons were detected by GC–MS. However, in the FTIR it was difficult to detect these compounds, probably due to the low concentration of aromatic hydrogen with respect to aliphatic hydrogen. On the other hand, in the O-EF oil only polycyclic aromatic compounds were observed by GC–MS, although the FTIR spectra of this oil indicate that aliphatic, carbonyl and nitrogen compounds were also present. These compounds, which have a high molecular weight, are the components of the non-chromatographied fraction and for this reason they cannot be assessed by GC–MS. 3.4. Gases Table 6 shows the yields of non-condensable gases (H2, O2, N2, CO, CO2, CH4, CO2, C2H4 and C2H6) from the microwave ovens with graphite and char as absorbers and for the electrical furnace. Hydrogen was the main gas produced in all the experiments, its yield being higher in microwave pyrolysis than in conventional pyrolysis. CO and CH4 were the second most important components of the gases from microwave and conventional pyrolysis, respectively. Independently of the absorber used, the highest values for CO were reported for the multimode microwave oven (G-MG and G-MC). In accordance with these results the values for the H2 + CO mixture (synthesis gas) were much higher in microwave than in conventional pyrolysis, the gases from the multimode microwave oven having a higher
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Table 6 Composition (vol%) of the gases (G) from the pyrolysis of sewage sludge
H2 O2 N2 CO CH4 CO2 C2H4 C2H6 H P2 + CO CxHy CV (kJ/m3)
G-MG
G-SG
G-MC
G-SC
G-EF
32.7 2.5 16.9 30.1 6.4 8.0 3.1 0.3 62.3 9.8 8591
34.6 2.3 16.4 24.9 7.0 10.6 3.7 0.7 59.4 11.4 9517
33.6 2.4 17.6 32.8 4.7 6.7 1.9 0.2 66.4 6.9 7427
38.0 2.7 19.5 22.3 4.4 9.7 3.0 0.5 60.3 7.8 8361
29.0 3.0 15.4 16.2 18.4 11.8 6.2 0.1 45.1 24.7 13,856
G-MG and G-MC: Gas from the multimode microwave oven using graphite and char as absorbers, respectively. G-SG and G-SC: Gas from the single mode microwave oven using graphite and char as absorbers, respectively. G-EF: Gas from the electrical furnace.
proportion of this mixture than those from the single mode. It can also be deduced from the results in Table 6 that char had a more significant influence than graphite on the formation of H2 + CO. Accordingly, the highest values were obtained in the case of G-MC (66.4% v/v). These results, along with the low proportion of CO2, indicate that the endothermic gasification reactions of an organic material (reactions 1–4) would be favoured in a microwave oven: C þ H2 O $ CO þ H2 C þ CO2 $ 2CO
ð1Þ ð2Þ
ð–CH2 –Þ þ H2 O $ CO þ 2H2
ð3Þ
ð–CH2 –Þ þ CO2 $ 2CO þ H2
ð4Þ
Table 6 alsoPshows the values of the light hydrocarbon yields ( CxHy = CH4 + C2H4 + C2H6) in the pyrolysis gases. The values of the hydrocarbons in the gas obtained from conventional pyrolysis (24.7% v/v) were higher than those for the microwave experiments (7–11% v/v). The effect of the graphite and the single mode microwave oven was more pronounced than that of the char and the multimode oven, respectively. The high CH4 and CO2 and the low H2 and CO contents of the gas from the electrical furnace could have been due to the fact that exothermic reactions 5 and 6 were favoured CO þ H2 O $ CO2 þ H2
ð5Þ
CO þ 3H2 ! CH4 þ H2 O
ð6Þ
The composition of the gases from conventional and microwave pyrolysis with graphite and char as absorbers can be explained by the different systems of heating. In the microwave-assisted pyrolysis the sample is heated directly, so that it reaches a high temperature in a very short time, while the reactor walls remain at a lower
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temperature than the bulk sample. On the other hand, the heat generated inside the sample by the microwaves causes the internal water to vaporize before it leaves the sample bed. The high temperatures reached (1040 C) and the contact between the pyrolysis vapours (water included), the organic material and the hot mineral matter (see Table 1) may have favoured the gasification reactions. These reactions would be promoted with the addition of char as an absorber. Piskorz et al. (1986) reported the catalytic effect of the char in a fluidisedbed reactor at 450 C. They concluded that the use of char as fluidised material increases the gas yield while decreasing the oil yield. The same effect was observed when the sewage sludge, which had previously been acid extracted, was pyrolysed. In conventional heating the heat flow goes from the walls of the reactor to the sample, so the temperature is higher in the reactor than inside the bulk sample. The volatile compounds generated have more possibilities to react in the gas phase, undergoing secondary reactions. This gives rise to more favourable conditions than in the microwave oven for reactions between noncondensable gaseous products and for the cracking and condensation reactions of the oil components, which in turn would increase the hydrocarbon content of the pyrolysis gas. On the other hand, the heterogeneous gasification reactions are less favoured in conventional than in microwave heating. The calorific values of the gases (Table 6) from conventional pyrolysis (14,000 kJ/m3) were comparable to those of synthetic coal gas while the heating values of the gases from microwave pyrolysis ranging from 7000 to 9500 kJ/m3 were similar to those of coal hydrogasification (Perry, 1984).
4. Conclusions The results from the analysis of the microwave pyrolysis oils by means of FTIR showed that they were made up of aliphatic hydrogen together with ester, carboxylic or amide groups. Oil from conventional heating contained a high proportion of aromatic hydrogen, although aliphatic hydrogen, esters and nitriles were also observed. The characterization of the oils by GC–MS indicated that the use of electrical heating in the pyrolysis of sewage sludge produced an oil that could have a significant environmental and toxicological impact. Conversely, microwave ovens still preserved some of the functional groups of the initial sludge such as aliphatic and oxygenated compounds, whereas no heavy PACs were detected. The FTIR and GC–MS results suggest that both techniques must be used for an optimal characterization of the oil because of the complementary information provided by each method.
The high pyrolysis temperature used in this work favours the gasification and the secondary cracking reactions of the pyrolysis vapours and therefore leads to both a high gas yield and a low oil production. Syngas (H2 + CO) can be produced in a high yield from the microwave pyrolysis of sewage sludge with the assistance of a microwave absorber. This yield was higher when char was used as microwave absorber rather than graphite and higher in a multimode oven than in a single mode oven. Thus, the combination of the char plus the multimode oven increased the production of syngas by up to 66%. In contrast, the production of hydrocarbons in the gas increased when graphite and the single mode oven were used. The gas from the conventional oven was much richer in hydrocarbons (25%) than that from the microwave ovens (6–11%), giving rise to a gas with a high calorific value. Acknowledgements The authors thank the Spanish Ministry of Science and Technology (Research Project PPQ2001-2083-C0201) and FEDER for financial support. A. Domı´nguez is also grateful to FICYT (Asturias-Spain) for financial assistance. References Asadullah, M., Ito, S., Kunimori, K., Yamada, M., Tomishige, K., 2002. Biomass gasification to hydrogen and syngas at low temperature: novel catalytic system using fluidized bed reactor. J. Catal. 208, 255–259. Bridgwater, A.V., Meier, D., Radlein, D., 1999. An overview of fast pyrolysis of biomass. Org. Geochem. 30, 1479–1493. Caballero, J.A., Font, R., Marcilla, A., Conesa, J.A., 1997. Characterization of sewage sludges by primary and secondary pyrolysis. J. Anal. Appl. Pyrol. 40, 433–450. Chanaa, M.B., Lallemant, M., Mokhlisse, A., 1994. Pyrolysis of Timahdit morocco oil shales under microwave field. Fuel 73 (10), 1643–1649. Chen, G., Andries, J., Spliethoff, H., 2003. Catalytic pyrolysis of biomass for hydrogen rich fuel gas production. Energy Convers. Manage. 44, 2289–2296. Conesa, J.A., Morcilla, A., Moral, R., Moreno-Caselles, J., PerezEspinosa, A., 1998. Evolution of gases in the primary pyrolysis of different sewage sludges. Thermochim. Acta 313, 63–73. Cunliffe, A.M., Williams, P.T., 1988. Composition of oils derived from the batch pyrolysis of tyres. J. Anal. Appl. Pyrol. 44, 131–152. Domı´nguez, A., Mene´ndez, J.A., Inguanzo, M., Bernard, P.L., Pı´s, J.J., 2003. Gas chromatographic-mass spectrometric study of the oil fractions produced by microwave-assisted pyrolysis of different sewage sludges. J. Chromatogr. A 1012, 193–206. El harfi, K., Mokhlisse, A., Chanaa, M.B., Outzourhit, A., 2000. Pyrolysis of the Moroccan (Tarfaya) oil shales under microwave irradiation. Fuel 79, 733–742. Evans, R.J., Milne, T.A., 1987. Molecular characterization of the pyrolysis of biomass. 1. Fundam. Energy Fuels 1 (2), 123–137. Horne, P.A., Williams, P.T., 1996. Influence of temperature on the products from the flash pyrolysis of biomass. Fuel 75 (9), 1051– 1059.
A. Domı´nguez et al. / Bioresource Technology 97 (2006) 1185–1193 Inguanzo, M., Domı´nguez, A., Mene´ndez, J.A., Blanco, C.G., Pı´s, J.J., 2002. On the pyrolysis of sewage sludge: the influence of pyrolysis conditions on solid, liquid and gas fractions. J. Anal. Appl. Pyrol. 63, 209–222. Iwasaki, W., 2003. A consideration of the economic efficiency of hydrogen production from biomass. Int. J. Hydrogen Energy 28, 939–944. Kenneth, H., 1995. Apparatus for waste pyrolysis. Patent US 5387321. Kim, H.Y., 2003. A low cost production of hydrogen from carbonaceous wastes. Int. J. Hydrogen Energy 28, 1179–1186. Kriegerbrockett, B., 1994. Microwave pyrolysis of biomass. Res. Chem. Intermed 20 (1), 39–49. Lu, G.Q., Low, J.C.F., Liu, C.Y., Lua, A.C., 1995. Surface area development of sewage sludge during pyrolysis. Fuel 74 (3), 344– 348. Lu, G.Q., Lau, D.D., 1996. Characterization of sewage sludge derived adsorbent for H2S removal. Part II: surface and pore structural evolution in chemical activation. Gas Sep. Purif. 10, 103–111. Mansuy, L., Bourezgui, Y., Garnier-Zarli, E., Jarde´, E., Re´veille´, V., 2001. Characterization of humid substances in highly polluted river sediments by pyrolysis methylation-gas chromatography– mass spectrometry. Org. Geochem. 32, 223–231. McAuley, B., Kunkel, J., Manahan, S.E., 2001. A new process for the drying and gasification of sewage sludge. Water Eng. Manage. 148 (5), 18–20. Mene´ndez, J.A., Inguanzo, M., Pı´s, J.J., 2002. Microwave-induced pyrolysis of sewage sludge. Water Res. 36, 3261–3264. Mills, G.A., 1994. Status and future opportunities for conversion of synthesis gas to liquid fuels. Fuel 73, 1243–1279.
1193
Monsef-Mirzai, P., Ravindran, M., McWhinnie, W.R., Burchill, P., 1995. Rapid microwave pyrolysis of coal. Fuel 74 (1), 20– 27. Morf, P., Hasler, P., Nussbaumer, T., 2002. Mechanisms and kinetics of homogenous secondary reactions of tar from continuous pyrolysis of wood chips. Fuel 81, 843–853. Pakdel, H., Roy, C., 1991. Hydrocarbon content of liquid products and tar from pyrolysis and gasification of wood. Energy Fuels 5, 427–436. Perry, R.H., 1984. Perry’s Chemical Engineers Handbook. McGraw Hill, New York, USA. Piskorz, J., Scott, D.S., Westerberg, I.B., 1986. Flash pyrolysis of sewage sludge. Ind. Eng. Chem. Process Des. Dev. 25, 265– 270. Sharypov, V.I., Marı´n, N., Beregovtsova, N.G., Baryshnikov, S.V., Kutnetzov, B.N., Cebolla, V.L., Weber, J.V., 2001. Co-pyrolysis of wood biomass and synthetic polymer mixture. Part I: influence of experimental conditions on the evolution of solid liquids and gases. J. Anal. Appl. Pyrol. 64, 15–28. Shen, L., Zhang, D.K., 2003. An experimental study of oil recovery from sewage sludge by low temperature pyrolysis in a fluidized bed. Fuel 82, 465–472. Socrates, G. (Ed.), 1994. Infrared Characteristic Group Frequencies, second ed. John Wiley & Sons, England, UK. Werther, J., Ogada, T., 1999. Sewage sludge combustion. Prog. Energy Combust. 25, 55–116. Zlotorzynski, A., 1995. The application of microwave radiation to analytical and environmental chemistry. Crit. Rev. Anal. Chem. 25 (1), 43–76.