Structural Changes of Sewage Sludge Char during Fixed-Bed Pyrolysis

Ind. Eng. Chem. Res. 2009, 48, 3211–3221

3211

Structural Changes of Sewage Sludge Char during Fixed-Bed Pyrolysis ´ brego,*,† Jesu´s Arauzo,† Jose´ Luis Sańchez,† Alberto Gonzalo,† Toma`s Cordero,‡ and Javier A Jose´ Rodrı´guez-Mirasol‡ Thermo-chemical Processes Group, Aragoń Institute for Engineering Research (I3A), Marıá de Luna 3, 50018 Zaragoza (Spain), and Chemical Engineering Department, School of Industrial Engineering, UniVersity of Ma´laga, Campus de El Ejido s/n, 29013 Ma´laga (Spain)

Undigested dried sewage sludge from a wastewater treatment plant was pyrolyzed at temperatures between 300 and 900 °C, with an additional hold time at the highest temperature. A fixed-bed reactor was used with a heating rate of 20 °C/min under an atmosphere of nitrogen. Pyrolysis product distribution, FTIR, XRD, BET, SEM, and ultimate and proximate analyses were used to gain a better understanding of the structural changes occurring during pyrolysis. At low to medium pyrolysis temperatures, major mass loss occurs, and most of the gaseous and liquid products are released with little porous development, whereas at temperatures between 700 and 900 °C, structural changes seem to be triggered by calcium carbonate decomposition. This leads to a second stage of gas evolution, as CaO promotes gasification of the char in the presence of iron sulfides. The subsequent release of CO runs parallel with an increase in the BET surface area. In addition, the aromatic character of the char increases with temperature, and nanotube-like tubular structures can be detected by SEM. 1. Introduction Sewage sludge disposal is one of the major problems for wastewater treatment plants in the European Union.1 The constant increase in its production is caused by the higher constraints imposed on effluent treatments, so that disposal itself has become an environmental problem of the first magnitude. Conventional disposal routes such as landfilling, dumping, or farmland application are subject to strict legislative constraints or are even banned in several countries, and therefore, alternative options need to be considered. One of the main characteristics of sewage sludge is the presence of high amounts of inorganic ash and low carbon contents when compared with other materials, such as wood or lignocellulosic char from agriculture. As a result, sewage sludge has a relatively low energy value, but is sufficient for some kind of waste-to-energy process to be considered as feasible. In addition, large amounts of sewage sludge are generated in every wastewater treatment plant, and as mentioned before, appropriate disposal methods need to be found. Thermochemical processes seem to be a promising alternative pathway for this sewage sludge treatment.2,3 Pyrolysis is a thermochemical process that consists of the thermal decomposition of a material in the absence of air. Pyrolytic char can be considered as an adsorbent precursor if prepared under controlled pyrolysis conditions,4,5 so a large reduction in volume is achieved, together with the preparation of a valuable product. Thus, if one also considers energy recovery from the gaseous and liquid products of pyrolysis, an integral sewage sludge valorization can be performed using this technology. Pyrolytic char from sewage sludge has less surface area than commercial activated carbons. Reported Brunauer-EmmettTeller (BET) surface area values found in the literature are no greater than 400 m2/g, whereas commercial activated carbon can exhibit much higher surface areas. Nevertheless, adsorbents * To whom correspondence should be addressed. E-mail: javabr@ unizar.es. † Aragoń Institute for Engineering Research (I3A). ‡ University of Ma´laga.

from sewage sludge can play a very effective role in the removal of some contaminants generated in pyrolysis or gasification, such as hydrogen sulfide and sulfur or nitric oxides.6-15 It can also be used for the effective removal of metals, dyes, phenol, and chemical oxygen demand (COD) in aqueous solutions.16-22 If necessary, adsorption properties can be improved by chemical or physical activation stages,10,14-24 as well as by mixing with other materials.6,8,9,13,19 To investigate the use of sewage sludge char as an adsorbent, the specific goal of this study was to investigate the structural changes and the evolution of the char during low-heating-rate pyrolysis on a fixed-bed reactor. Extensive analysis of the char will provide useful knowledge about the processes that occur during pyrolysis and will contribute to a better understanding of the thermochemical transformation pathways of sewage sludge. 2. Experimental Section 2.1. Sewage Sludge Analysis. A sample of dry sewage sludge was supplied as a granulated product from an urban wastewater treatment plant. The sludge was undigested and had previously undergone various physicochemical processes and thermal drying. Table 1 lists the results of proximate, ultimate, and ash analyses and and measurements of the low heating value of the received sewage sludge, performed by the Instituto de Carboquı´mica (CSIC, Zaragoza, Spain). The dried sewage sludge was crushed and sieved to provide a feed sample in the size range of 150-250 µm and kept at 105 °C for 24 h in an oven to remove the moisture prior to each experiment. 2.2. Laboratory-Scale Fixed-Bed Pyrolysis Installation. Pyrolysis of sewage sludge was carried out on a fixed-bed reactor under an inert atmosphere. In each experiment, a sample of 2.5 g of raw sewage sludge was pyrolyzed at a heating rate of 20 °C/min. The small sample amount minimized temperature profiles during the experiments. Figure 1 shows a diagram of the pyrolysis experimental system. The reactor was cylindrical with a diameter of 90 mm and a height of 320 mm and was inside a 900-W furnace.

10.1021/ie801366t CCC: $40.75  2009 American Chemical Society Published on Web 02/05/2009

3212 Ind. Eng. Chem. Res., Vol. 48, No. 6, 2009 Table 1. Analysis of Dried Sewage Sludge Sample

analysis proximate analysis moisture ash volatiles fixed carbon ultimate analysis (organic fraction) carbon hydrogen nitrogen sulfur oxygen ash analysis Al Ca Fe K Mg Na Si Ti heating value LHVa HHVb a

analytical standard/ instrument ISO-589-1981 ISO-1171-1976 ISO-5623-1974 by difference Carlo Erba 1108 Carlo Erba 1108 Carlo Erba 1108 Carlo Erba 1108 by difference Horiba Horiba Horiba Horiba Horiba Horiba Horiba Horiba

JY-2000 JY-2000 JY-2000 JY-2000 JY-2000 JY-2000 JY-2000 JY-2000

ASTM D-3286-96 ASTM D-3286-96

values wt % 0.7 26.0 64.6 8.7 wt % (dry) 41.7 5.8 3.0 0.8 22.7 wt % in the ash 2.3 13.6 8.3 0.6 1.2 0.3 9.7 0.4 kJ/kg 16400 17670

Lower heating value. b Higher heating value.

Sewage sludge pyrolysis took place in a steel basket hanging from the upper part of the reactor. Nitrogen, used as an inert carrier gas, entered the reactor at the top at a flow rate of 100 mL/min. This value for the nitrogen flow rate was selected after preliminary gas analysis using the same experimental system. The sample temperature was measured tith four type K thermocouples placed inside the steel basket. Outside the reactor, the pyrolysis gases and nitrogen flowed through a gas cleaning system consisting of a condenser (cooled by a continuous flow of a mixture of water and ethylene glycol at -2 °C) and a cotton filter. During the tests, gas composition was measured semicontinuously by an Agilent 3000A gas chromatograph with a thermal conductivity detector (GC-TCD) connected online after the gas cleaning system. The chromatograph used a Porapak N column and a molecular sieve. The compounds analyzed were H2, N2, CH4, CO, CO2, C2H6, C2H4, C2H2, and H2S. Gas mass yield was measured as a result of mass balances from chromatographic data, taking nitrogen flow as the reference. Char and condensate mass yields were measured gravimetrically. In the following discussion, the char is referred to as C followed by a number representing the pyrolysis temperature (e.g., C700). In the case of char pyrolyzed at 900 °C with a holding time of 2 h, the sample is referred to as C900B. Raw sewage sludge is referred to as SS. 2.3. Char Analysis. Proximate and ultimate analyses of the char were conducted using the same equipment and procedures as for the raw sewage sludge sample, detailed in Table 1. Fourier transform infrared (FTIR) spectra were obtained in a Bruker Vertex 70 spectrometer, with KBr pellets and a resolution of 2 cm-1. X-ray diffraction (XRD) patterns were recorded in a Bruker D8 Advance Series 2 diffractometer. Nitrogen adsorption isotherms at 77 K were measured using a Micromeritics ASAP 2020 analyzer. Samples were degassed at 250 °C and 5 × 10-3 torr vacuum. Surface area was obtained by the BET method, and t-plot external area, micropore area, and micropore volume were also calculated.

Scanning electron microscopy (SEM) images were recorded using a JEOL JSM 6400 electronic microscope equipped with an energy-dispersive X-ray (EDX) detector (eXL10 from Link Analytical). 3. Results and Discussion 3.1. Product Distribution. Figure 2 shows the weight percentage evolution of solid, liquid, and gaseous products with pyrolysis temperature (percentages related to initial weight of sewage sludge). As can be observed, the sewage sludge weight loss for 300 °C is low (6.5%), and thus, small quantities of liquid and gaseous fractions are generated. In contrast, the next temperature range (up to 400 °C) exhibits a dramatic solid fraction weight loss. Consequently, the major liquid and gaseous fractions are then generated. For higher temperatures, the weight loss slows to what seems like a small and almost constant descent from samples C700 to C900B. Gaseous and liquid products show opposite trends from 400 °C. Whereas liquids increase linearly up to 700 °C, gases show very little generation in this range. At 700 °C, liquid release virtually stops, and an increase in gaseous products is detected. The evolution of the solid yield is very similar to that observed in previous thermogravimetric studies carried out with sewage sludges from various sources.25-30 The slight, initial weight loss under 300 °C is thought to be caused mainly by dehydration.31 The maximum rate of weight loss is in accordance with the literature. At high temperatures, a reduction in devolatilization rates has also been reported for sewage sludge.2 Regarding the liquid fraction, some authors have observed very similar increases up to 650 °C, with more or less constant yields above this temperature.2 As a consequence, gases evolve from char at high temperatures at approximately the same rate as occurs with devolatilization. High-temperature weight loss has been attributed mainly to decomposition of inorganic materials such as carbonates.26,27,32 The sum of fixed carbon and ash (34.69% on a sludge basis, as given by the proximate analysis) is higher than the final solid yield measured for C900B (32.4%). This suggests a loss of either mineral matter or fixed carbon during pyrolysis. 3.2. Gas Fraction. Evolution of the gaseous products was measured by means of a chromatograph for all experiments. As no relevant differences were found between them, gas evolution is presented for the longest experiment, which includes overall gas formation through all pyrolysis temperatures. Figure 3 presents the mass flow of released gases as a function of the extent of pyrolysis. The mass flow for each gas was calculated from chromatographic measurements. As can be seen, the predominant gases formed are CO and CO2. CO evolves in the first stage, with a maximum rate at 450 °C, and is released in higher amounts from 700 °C until the final temperature. CO2 forms mainly at temperatures around 425 °C, with a smaller second stage near 700 °C. The rest of the gases detected were present in much lower quantities, as shown in Figure 3b. H2 evolves significantly over a wide temperature range (500-900 °C), with a maximum near 625 °C. C2H4 and C2H6 show identical trends with a maximum at 550 °C, whereas CH4 is released in greater amounts with a maximum at 600 °C and continuing to 900 °C. Inguanzo et al.2 found almost identical trends for CO, CH4, C2H4, and C2H6, with H2 evolution increasing even beyond 600 °C and CO2 without a second formation stage. Thipkhunthod et al.28 observed an additional intermediate step in CO2 formation at 500 °C and retarded H2 evolution (with a maximum at 750 °C). CO was not determined, and C2H4 and CH4 showed similar behaviors. Biagini et al.27

Ind. Eng. Chem. Res., Vol. 48, No. 6, 2009 3213

Figure 1. Schematic diagram of the laboratory-scale pyrolysis reactor.

Figure 4. Ultimate analysis of sewage sludge and chars. Figure 2. Product distribution as a function of temperature or hold time.

Figure 5. Ultimate analysis of sewage sludge and chars (based on the initial weight of the sewage sludge).

Figure 3. Evolution of released gases as a function of pyrolysis temperature and isothermal time.

also found a three-stage release of CO2. CO and H2 were not reported, and the rest of the compounds showed similar temperature dependencies. Conesa et al.33 described methane formation in two stages, H2 evolution at lower temperatures, and a greater second-stage CO2 formation at somewhat lower temperatures (500-600 °C). Obviously, comparisons with other

works do not take into account differences in heating rates, sewage sludge composition, reactor type, and several other factors. According to the release rates (in grams per minute) given by Figure 3, it is clear that most of the mass of the gas fraction (more than 90 wt %) corresponds to carbon oxides. CO is practically the only gas detected at the isothermal 900 °C stage of the experiment. Even though it is being released at very low rates at this point, it is generated continuously for 120 min and thus contributes to a substantial percentage of the gas fraction.

3214 Ind. Eng. Chem. Res., Vol. 48, No. 6, 2009

Figure 6. FTIR spectrum of raw sewage sludge (SS). Table 2. Comparison between Characteristic Absorptions (cm-1) from Cellulose and SS Sample band

this work

Liu et al.44

Yang et al.45

Carrillo46

CsO stretch

1034 1059 1109 1161

1030 1064 1114 1161

1060 1080 1108 1170

1035 1055 1111 1155

3.3. Char Fraction. 3.3.1. Ultimate Analysis. Evolution of the ultimate analysis of the char can provide useful information about the transformations of pyrolytic char. Figure 4 shows elementary compositions of chars. It can be observed that the relative mass proportions of each element follow different trends with pyrolysis. The relative percentage of carbon in the chars shows little variation, with slight decreases between C400 and C700. In contrast, the amounts of hydrogen and nitrogen in the sample decrease at high temperatures. The sulfur percentage shows a slight and constant increase, suggesting that most of this element remains within the char regardless of the temperature or hold time.

Figure 8. Apparent aromaticity of chars.

Figure 9. XRD spectra of the chars.

Figure 10. Comparison of BET surface area, micropore surface area, and evolved CO. Figure 7. Detailed evolution of FTIR spectra for selected wavenumber intervals. Table 3. Wavenumber Shift of Initial 1642 cm-1 Band with Temperature band positiona (cm-1) 1646 1646 1616 1601 1578 a

Initially assigned to amide I.

sample SS C300 C400 C500 C600

Figure 5 shows compositions calculated on a sewage sludge basis. This seems a more convenient way to show mass loss for each main char constituent, as it refers to the initial weight of the sewage sludge and thus provides a more consistent comparison. The total carbon content of the sewage sludge decreases from more than 40% to about 10% in the final experimental stage C900B. Thus, the carbon mass loss represents more than 50% of the weight loss of the initial sludge. Hydrogen is found in minimal amounts for this final sample, indicating nearly 100% weight loss during pyrolysis. As mentioned before, sulfur does not show any significant loss apart from a slight initial descent up to C400. Nitrogen exhibits a behavior similar


Figure 11. SEM image for C900 and EDX analyses for selected points. Table 4. Crystallographic Structures Identified by XRD C300 cellulose quartz SiO2

calcite CaCO3 anhydrite CaSO4

C400

C500

C600

C700

C900

C900B

quartz SiO2 feldspars: albite, anorthite

quartz SiO2 feldspars: albite, anorthite silicates: almandine

quartz SiO2 feldspars: albite, anorthite silicates: almandine

calcium oxide CaO

calcium oxide CaO

calcium oxide CaO

oldhamite CaS

oldhamite CaS

oldhamite CaS

calcium oxide CaO Portlandite Ca(OH)2 oldhamite CaS

troilite FeS pyrrhotite Fe1-xS (x ) 0-0.2)

troilite FeS pyrrhotite Fe1-xS (x ) 0-0.2)

troilite FeS pyrrhotite Fe1-xS (x ) 0-0.2) barringerite Fe2P

barringerite Fe2P

quartz SiO2

quartz SiO2

quartz SiO2

quartz SiO2

calcite CaCO3

calcite CaCO3

calcite CaCO3

calcite CaCO3

oldhamite CaS

C800

iron sulfates FeSO4, Fe2(SO4)3 FeCl3

to that of hydrogen, leading to minimal nitrogen content in C900B. Nevertheless, the weight loss for nitrogen takes place continuously from 400 to 900 °C, in contrast to hydrogen, which has pronounced weight loss in the interval between C300 and C400. Finally, the weight loss curves for carbon and hydrogen show equivalent evolutions with temperature, which might be indicative of a close relationship regarding evolved or decomposed hydrocarbon products. Comparing these results with gas evolution, it can be seen that the maximum hydrogen weight loss from char is located between 300 and 400 °C. This does not correspond with the maximum release of hydrogen-containing gases (H2, C2H6, C2H4, and CH4). Thus, the generated liquid fraction at this temperature interval must contain most of this hydrogen. Such observation might indicate the formation of aliphatic compounds, which collect in the condensed fraction. Light hydrocarbons have been identified as the major constituents of the condensate yield34 in

fixed-bed sewage sludge pyrolysis. As the temperature increases, these aliphatic compounds undergo further cracking, thus leading to lower-molar-weight aliphatic compounds as detected in the chromatographic analyses. From 600 °C, hydrogen weight loss is slower, indicating that the maximum formation of hydrogencontaining gaseous species has finished. At the same time, this relative decrease of the hydrogen release could indicate a predominance of aromatic compounds (rather than aliphatic ones) in the liquid fraction at high temperatures, as has been reported in several studies.2,35 As previously mentioned, nitrogen suffers its maximum weight loss between 300 and 400 °C, which can be attributed to the decomposition of the less stable amino acid structures, leading to ammonia formation. Further observations regarding the amino acid content of sewage sludge will be discussed in the next sections. According to Tian et al.,36 the low-heatingrate pyrolysis of sewage sludge yields significant amounts of


Figure 12. SEM images of (a) sewage sludge and (b) C900 char.

Figure 13. Detailed SEM micrograph of C900 char.

ammonia between 300 and 500 °C and low yields of HCN at higher temperatures (up to 900 °C), which is in good agreement with the nitrogen loss in our study. Nevertheless, nitrogen compounds are probably not limited to these two species. Some minor amounts of other nitrogen compounds, such as cyano compounds and those with heteroaromatic rings, are probably incorporated in the liquid fraction throughout a wide temperature range.34 Unfortunately, no measurements for any nitrogen compounds were carried out in the present work. Sulfur is assumed to be present mainly in organic forms in sewage sludge, as will be discussed later. Only a minor weight loss occurs before 400 °C, which corresponds to H2S formation, as detected by chromatographic analysis. Further variations are

negligible, suggesting that sulfur persists in chars, probably incorporated in inorganic structures. 3.3.2. FTIR Spectra. Spectral interpretation was done according to the IR spectroscopy literature.37,38 3.3.2.1. FTIR Spectrum of Raw Sewage Sludge. Taking into account its origin and undigested character, the sludge is likely to be composed mainly of proteins and its constituents; a cellulosic fraction, which includes cellulose, hemicellulose, and lignin; and fatty acids.39 The sewage sludge should then include most of the functional groups of these constituents: alcoholic, carboxylic, amide, amine, aromatic, and methylene groups. In fact, FTIR analysis confirms the predominance of these functionalities, as shown in Figure 6. A broad and intense band at 3200-3600 cm-1 corresponds to OsH and NsH stretching vibrations. These functional groups might indicate compounds such as alcohols, carboxylic acids, and amides/amines present in the sample. Moving toward lower wavenumbers, two peaks appear at 2926 and 2853 cm-1 corresponding to asymmetrical and symmetrical stretching, respectively, of methylene groups forming predominantly aliphatic structures. Although very weak, a band at 2516 cm-1 provides the only indication of organic sulfur in the sample, corresponding to SsH stretching vibration. Other organic sulfur bands may be obscured by stronger absorptions. The peak at 1642 cm-1 is due to the amide I band.40 However, other contributions cannot be excluded, such as the CdO stretching vibrations from other functionalities (suggested by a distinguishable shoulder at the left of the 1720 cm-1 peak, assignable to fatty acid carboxylic groups). The band at 1545 cm-1 might be due to NsH bending, which is characteristic of the amide II band. CdC aromatic skeletal vibrations can also be found between 1600 and 1500 cm-1 and might be present in the sample because of lignin structures.41 In the interval between 1300 and 1500 cm-1, vibration of CH2 groups and OsH bending vibrations for alcohols and carboxylic acids give rise to a variety of peaks. As a result, a broad band around 1430 cm-1 is visible. This wavenumber has been said to correspond to typical positions for CH2 scissoring for cellulose42 and saturated fatty acids.43 In the fingerprint region of the spectrum (900-1300 cm-1), the most remarkable feature is a combination of bands in the interval between 1150 and 1000 cm-1 that has been assigned by several authors to polysaccharides.39 More precisely, this region of the spectrum consists of a broad band with several shoulders, and it is almost identical to results from several reported cellulose FTIR studies,44-46 as summarized in Table 2. Some inorganic structures in the sludge might also contribute to the spectra. In fact, SisO structures generate bands around 1080 cm-1 that probably contribute to the mentioned band combination above 1000 cm-1. Si has been identified in the ash analysis (Table 1) as one of the major inorganic constituents of sewage sludge. Also, a small but sharp peak at 873 cm-1 could be due to inorganic carbonates, especially calcium carbonate (calcium is another major inorganic compound in the ashes). A very sharp band at 1383 cm-1 can be attributed to nitrates or phosphates. Finally, the last portion of the spectrum below 900 cm-1 is less representative for group determination purposes and could be due to aromatic structures, skeletal vibrations, and amine and amide groups. The main absorption bands found for this particular sludge donotdifferqualitativelyfromthosereportedpreviously,5,24,39,47-50


although marked discrepancies can be found among spectra interpretation in the existing literature. Commonly, FTIR analysis has been used as an additional technique to support previous experimental observations. In the present work, an attempt was made to detail the FTIR spectra of sewage sludge and chars regarding the changes experienced by the solid fraction during pyrolysis within the entire temperature range, while being aware of its obvious limitations. 3.3.2.2. Changes in FTIR Spectrum during Pyrolysis. The FTIR spectra exhibit visible changes during pyrolysis that are related to the surface structure. Figure 7 shows detailed spectral evolution with pyrolysis for selected wavenumber ranges. In general, the organic functional groups found in sewage sludge tend to decrease or even disappear as pyrolysis takes place. Upon closer examination, some structural changes in the solid matrix can be discerned. For instance, pyrolytic decomposition of polysaccharides has a marked effect in the strong decrease of the 1000-1200 cm-1 band.51 This band combination (as described in Table 2) is clearly visible for the SS and C300 samples but is not present at higher temperatures, thus confirming that the decomposition of the cellulose fraction is complete at 400 °C. Surprisingly, an increase of methylene stretching vibrations for C300 shows that this functionality increases its concentration compared to that found in sewage sludge. The partial decomposition of cellulose, fatty acids, and polypeptides and subsequent formation of long aliphatic chains could explain this trend. Also, Green et al.52 stated that cellulose depolymerization takes place at low temperatures without significant mass loss or generation of volatiles, as suggested by the negligible variation of the 1000-1200 cm-1 band at 300 °C. The weight loss of C300, compared to sewage sludge, is only 6.5%. The spectra of the two samples show few differences, which seems to fit with the low weight loss. Apart from an increase in the methylene stretching peak, a slight decrease in the OsH stretch absorption can be observed. This could be related to the dehydration of mineral matter. However, the most prominent difference is found in the band previously assigned to nitrates and/or phosphates at 1383 cm-1. This band is much smaller at 300 °C and, thus, might indicate nitrate decomposition. The most likely predominant compound seems to be ammonium nitrate, because of its low decomposition temperature.53 Such decomposition would give NO2, H2O, and NH3 as the main products up to 300 °C.54 Persistence of other nitrates and phosphates is suggested by the small remaining peak at higher temperatures (C300-C500 samples). Also, minerals absorbing at the same wavenumbers (such as quartz and albite) might be present in the samples obtained at high temperatures. A distinguishable change can be seen in what was previously assigned to the amide I band (1642 cm-1). As the temperature increases, this band gradually broadens and shifts toward lower wavenumbers, although the intensity of the absorption remains fairly constant up to 400 °C. This can be explained by a decrease in amide groups and a simultaneous increase of amino acid functionalities, due to hydrolysis of the polypeptides or protein peptide bonds. Amine groups absorb near 1600 cm-1, overlapping with the decreasing amide I band, thus causing a shift to lower wavenumbers (Table 3). Amine functionalities also manifest themselves between 1000 and 1200 cm-1, as might be indicated by the broad shoulder at these wavenumbers for C400. At the same time, carboxylic groups show increased absorptions at the 1710 cm-1 band for the mentioned sample. Additional formation of amino acid salts due to the sewage sludge content of metals or halides cannot be ruled out. For

instance, carboxylate anions absorb strongly near 1600 and 1400 cm-1, which is in agreement with previous observations.43 Although it might seem unusual for amino acids to exist under such conditions, a significant persistence of such structures at these temperatures has been reported in previous pyrolysis studies, without volatilization or sublimation,55 Furthermore, interactions within the char matrix might enhance its stability. The amide II band found at 1545 cm-1 disappears almost completely at 400 °C. The band at 1430 cm-1 increases in intensity from 300 to 400 °C and shows little change up to 500 °C. This could be due to several factors. One could be the previously mentioned carboxylate salts formation. Aromatic compounds might also be contributing. For wavenumbers above 2800 cm-1, two visible changes occur. First, the mentioned maximum of methylene stretching decreases with increasing temperatures until it is almost undetectable beyond 500 °C, thus indicating a loss of aliphatic structures. Second, the OsH and NsH stretching bands become broader with increasing temperatures, and their absorbance decreases gradually from 300 to 500 °C. At higher carbonization temperatures, thess bands undergo little change, suggesting unchanged inorganic links with OsH.24 The spectral region below 900 cm-1 shows two main bands that grow sharper up to a maximum at 500 °C. For the C600 and C700 chars, both bands are still significant. As the temperature rises to 800 and 900 °C, the peaks gradually fade, but still show some minor bands. This behavior could be related to a gradual increase in the aromatic nature of the char, as reported in some works.56,57 The maximum at 500 °C might correspond to the maximum aromatic formation, and the subsequent decrease at higher temperatures might be due to the sequential release of aromatics that can be later found in the condensate fraction. At 600 and 700 °C, these bands are still prominent. Persistence of the band around 1420 cm-1 to 700 °C is also likely to be related to aromatics, as the carboxylic/ carboxylate groups seem to have disappeared earlier (as reflected by the shoulder at 1710 cm-1, which is not present at temperatures higher than 500 °C), and is in accordance with the aforementioned spectral trends below 900 cm-1. As can be seen, the main recognizable changes in the spectra occur up to 500 °C and correspond to the breakdown of polymeric structures and a huge range of reactions involving the released products, which seem to lead to the gradual formation of aromatic structures as the temperature increases. This is in accordance with the maximum weight loss, observed between 300 and 500 °C. Two sharp but small peaks appearing near 876 and 712 cm-1 for dried sewage sludge remain almost unaltered until 700 °C and suddenly disappear beyond 700 °C. This could indicate calcium carbonate decomposition, as this compound has recognizable peaks at these wavenumbers. Another characteristic band for calcium carbonate is located near 1420 cm-1. At low temperatures, it is probably overlapped by other functionalities, but as mentioned before, it disappears beyond 700 °C. XRD analysis should confirm the presence of only calcium carbonate for chars pyrolyzed up to 700 °C. The decomposition temperature of calcium carbonate, as found in the literature, seems to confirm this.58 At high temperatures, few spectral details are visible at first glance. This is probably due to the strong background absorption of amorphous carbon in the samples. To provide further insight into functionality changes, apparent aromaticity was calculated using the procedure proposed elsewhere for coal chars.59-62 The apparent aromaticity, fa, is


obtained by calculating the aliphatic and aromatic hydrogen mole fractions (determined from FTIR integrated areas for the aliphatic CsH region, 2700-2900 cm-1, and the aromatic CsH region, 700-900 cm-1) and the atomic hydrogen/carbon ratio. The results are shown in Figure 8. It is worth noting that this calculated aromaticity cannot be considered an accurate measurement of the aromatic content of chars. Nevertheless, it can serve as a useful parameter for comparative purposes between chars. As was expected, a sharp increase in the aromatic character of the chars is observed up to 500 °C. At higher temperatures, the aromaticity continues its increase at a lower rate. This is consistent with the FTIR interpretation above. Several authors have also correlated an increasing pyrolysis temperature to an increase in the degree of aromatization of chars, based mainly on FTIR observations.5,47,56,63 3.3.3. XRD Spectra. XRD spectra of the chars are shown in Figure 9. As can be seen at first glance, an increase in the pyrolysis time and temperature results in higher amounts of crystallographic phases, with sharper peaks appearing in the samples pyrolyzed at 900 °C. This could be a consequence of the disappearance of amorphous organic phases and an increase in the ash percentage during pyrolysis, as well as the formation of new crystal structures by a combination of species released from decomposing material, under extreme temperature and reduced atmosphere conditions. As with the infrared spectra, detailed analysis of the XRD patterns becomes difficult because of the heterogeneous composition of the chars. XRD spectra seem to be built up from a huge range of either crystal or amorphous structures. Thus, only well-defined structures are identified in this section. Quartz is the most recognizable crystallographic structure for all temperatures. Thus, XRD spectra confirm FTIR observations regarding SisO structures. Calcite is detected for chars pyrolyzed up to 700 °C. Beyond this temperature, carbonates undergo decomposition and therefore are not present in high-temperature samples (C800 and C900), as indicated by FTIR spectroscopy. Table 4 summarizes the crystallographic structures detected by XRD analysis. For the C300 sample, the strongest peak at 2θ ) 22.6° might correspond to cellulose.64 It seems to have a low degree of crystallinity, probably because, at this point, partial depolymerization has taken place. As mentioned earlier, cellulose decomposition in pyrolysis takes place between 300 and 400 °C,45 and thus, cellulose could not be detected at significant levels in the C400 sample. The FTIR spectra are in agreement with these observations. Phase identification allows carbonate decomposition to be confirmed, as concluded from the FTIR analysis discussed earlier. As seen in the gas evolution plots, near 700 °C, there is a second stage of generation of CO2, which is one of the decomposition products of carbonates. The other compound, CaO, is detected the in XRD patterns for temperatures higher than 700 °C. For high-temperature chars, new phases become distinguishable, as mentioned before. Silicates, feldspars, and sulfides appear in these patterns. It is not only the temperature that causes changes in the surface chemistry; the long isothermal pyrolysis time (2 h for C900B) produces significant differences in the diffraction pattern compared with that of the C900 char. For instance, relatively high amounts of barringerite (iron phosphide) are detected in the C900B char, whereas this phase is less important in C900 and is not present in the lower-temperature chars. Also, more oldhamite (calcium sulfide) is present for

C900B. Oldhamite is a typical mineral phase formed under extremely reducing conditions.65 The total sulfur loss from the solid matrix at the final stage of pyrolysis is 35%, as derived from ultimate analysis of C900B. Hydrogen sulfide release in the gas product is below 6 wt % (based on the initial sulfur weight) and corresponds to the interval between 300 and 400 °C. Consequently, some of the sulfur might also be forming tarry compounds in the condensate fraction. For high-temperature chars, the XRD spectra suggest that sulfur from sewage sludge appears to be found mainly in calcium and iron sulfates. However, an occurrence of calcium and iron sulfides at high temperatures appears to be much more significant than the initial sulfates detected by XRD for C300. This suggests that most of the organic sulfur in the sewage sludge is reacting with inorganic cations to form sulfides during pyrolysis. Although not quantified, the phosphorus content in the sewage sludge seems to follow a similar trend, forming metal phosphides that remain in the chars (especially barringerite) and tar compounds. Regarding iron phosphides and sulfides (detected only at the highest temperatures), sample C300 contains iron chloride (FeCl3), which is one of the reagents used during the physicochemical treatment of the original sludge. Thus, this compound is bound to exist in sufficient amounts to provide most of the iron sulfides and phosphides at high temperature. Further information about iron transformations could not be inferred from XRD spectra, as iron compounds were not detected at significant levels at intermediate temperatures. In general, inorganic compounds in chars appear to be affected significantly by both pyrolysis temperature and time. Some inorganic phases were detected by XRD (especially those corresponding to the most abundant metals in sewage sludge ashes, as showed in Table 1). Other dispersed phases might be present in minor amounts not detectable by this experimental technique, but they might play an important role when considering these materials as potential adsorbent precursors.56 3.3.4. BET Surface Area. The evolution of the BET surface area of pyrolysis chars can be observed in Figure 10. As can be seen, the BET surface area is very small for chars pyrolyzed under 600 °C, despite the major solid weight loss below this temperature. This fact could be due to the blocking of pores caused by volatile compounds from the char. As the temperature increases, these volatiles are further cracked and released from the solid. As confirmed by chromatographic analyses, products likely to result from volatile cracking (CH4, C2H6, and C2H4) reach their maximum generation rate between 500 and 600 °C. Cracking and subsequent release of gaseous species might partly explain surface area development from these temperatures. Lu et al.4 found the lowest surface area for sewage sludge chars pyrolyzed at 650 °C, and this fact was attributed to an intermediate melt formation in addition to volatiles being evolved. A high increase in BET surface area was observed after gases were released. In this work, the minimum BET surface area was observed for sample C500. For higher temperatures, the surface area increased with increasing temperature, and thus, the maximum BET surface area (124 m2/g) corresponds to char C900. Isothermal pyrolysis for 2 h at 900 °C (C900B) causes only a very slight reduction in BET surface area. Nevertheless, the largest area of micropores is found at lower stages of pyrolysis, near 800 °C. Beyond this temperature, micropore area production decreases. Thus, the observed increase in BET surface area might be due to mesopore


formation from widening micropores. This effect has been observed in previous works for sewage sludge.4 An attempt was made to correlate the changes in BET surface area with the observations derived from various analyses in the present work. It was found that the BET surface area growth at high temperatures seems to be closely linked with the CO generation rate. The maximum generation rate for CO is located at 900 °C, and 120 min of holding time produces a considerable decrease in its evolution. This fact is also shown in Figure 10 and suggests that CO is released in proportion to newly forming or widening pores. As previously mentioned, the micropore surface area reaches its maximum value near 800 °C and then starts to decrease. This could be due to the high rates achieved at this temperature for evolved CO. CO2 evolved at this temperature is thought to be a product of direct carbonate decomposition, unlike CO formation, which could be a consequence of carbon removal from chars, i.e., char gasification. For coal and biomass, it has been found that the lower the rate of char gasification, the greater the micropore development.66,67 Increasing rates of char gasification would cause microporosity development to decelerate. Further decreases of microporosity can be attributed to pore widening, as mentioned earlier. According to these results, attention should be focused on CO formation, as it seems to be directly linked with surface area evolution. The temperature range between 700 and 900 °C is the region of interest for BET surface area evolution. In this interval, the following facts were derived from experimental observations and analyses made throughout this work: (1) The CO2 flow rate decreases continually in the temperature range, whereas CO exhibits the opposite trend. (2) Calcium carbonate decomposition takes place at these temperatures and might explain the secondstage CO2 formation. Consequently, CaO is formed, as revealed by XRD. (3) The majority of sulfur is still forming part of the char as a constituent of inorganic sulfides. Within this range, some sulfur transfer reactions between metallic cations are observed by XRD. More precisely, sulfur tends to move from Fe compounds to Ca ions. (4) The latter is consistent with the observed CaO decrease forming CaS. (5) At these temperatures, both char and carbon weight loss are still taking place, as shown by elementary analyses of chars and product distribution. (6) The CO/CO2 ratio for this range does not fit with theoretical Boudouard equilibrium relationships. (7) The BET surface area increases continuously with increasing temperature. (8) Taking into account these facts, this temperature region seems to be the optimal for the development of adsorptive properties of chars, especially regarding sulfur compound adsorption. Taking into account all of these factors, the following hypotheses can be formulated: (1) Decomposition of carbonates (predominantly CaCO3) starts at 700 °C and produces a temperature-decreasing flow of CO2 (proportional to the carbonate content). Simultaneously, CaO is formed, which promotes char gasification and sulfur transfer within the inorganic constituents of char ash. Calcium compounds have been extensively studied as catalysts for char gasification of coal and desulfurization.68,69 (2) The aforementioned char gasification occurs in the presence of iron sulfides. It is known that calcium oxide can promote the decomposition of ferrous sulfide to form calcium sulfide.68 The proposed main reaction70 can be written as FeS + C + CaO f Fe + CaS + CO In accordance with this mechanism, calcium and iron have been identified as main factors governing the catalytic activity of chars

toward desulfurization.13 However, no evidence has been found for the presence of reduced iron. This is thought to be caused by the immediate formation of barringerite (iron phosphide), as detected clearly in XRD spectra. Probably, phosphide ions have their origin in the high-temperature decomposition of phosphates, which were identified by FTIR spectra. In fact, barringerite and oldhamite follow similarly increasing trends in the XRD patterns. (3) As a result of this reaction, CO is evolved from chars. BET surface area is closely related to evolved CO because it is proportional to char gasification. The micropore surface area reduction at high temperatures could be related to pore widening as CO continues being released. 3.3.5. SEM/EDX Analysis. To confirm earlier observations, a series of SEM images were taken for raw sewage sludge and the C900 char. Figure 11 shows an SEM image of C900. The diverse char particle shapes are a consequence of sewage sludge heterogeneity. At some selected points, an EDX analysis was made to identify the predominant elements and their dispersion. As can be seen, the main elements identified are C, O, Si, Ca, P, and Fe, and their relative amounts differ significantly from one point to another. This indicates the heterogeneous composition of char. A comparison of the SEM images of sewage sludge and the C900 char is shown in Figure 12a,b. The images indicate a significant reduction in volume for pyrolyzed particles and show the presence of fibrous structures formed as a result of pyrolysis. More detailed SEM micrographs show structures resembling nanotube bundles or fibers, together with dispersed particles of inorganic phases (Figure 13). Similar observations with evidence of such carbon structures have been reported for sewage sludge pyrolysis chars.7 The tubular structures found in our study seem to be somewhat greater than those reported by Bandosz et al. 4. Conclusions At low pyrolysis temperatures, CO2, CO, H2O, and tars are evolved by means of devolatilization of sewage sludge fractions such as cellulose, long-chain fatty acids, and less stable proteins. NH3 is thought to be evolved as well. Most of the condensate fraction and char weight loss is observed in the temperature range between 300 and 500 °C. At intermediate temperatures (400-700 °C), light hydrocarbons (CH4, C2H6, C2H4) are evolved as a consequence of thermal cracking of those volatiles remaining in the char as a product of low-temperature devolatilization. This causes the development of a preliminary pore structure for chars, although the BET surface area development is small. H2 evolution is prolonged even at 900 °C. Decomposition of carbonate (predominantly CaCO3) starts at 700 °C and produces a temperature-decreasing flow of CO2 (proportional to the remaining carbonate content). CaO is formed at the same time, which promotes char gasification and sulfur transfer within the inorganic constituents of the char ash (specially Fe species). As a result, an increasing rate of CO production is observed. BET surface area is closely related to the CO released and thus increases substantially. Over the entire temperature range, the aromatic character of the chars increases as pyrolysis advances. Finally, the formation of filament-like carbon structures was detected for high-temperature pyrolysis samples. Acknowledgment The authors thank the Spanish Ministry of Science and Innovation (MICINN) for providing frame support for this work (Project CTQ2007-66885).


Abbreviations BET ) Brunauer-Emmett-Teller COD ) chemical oxygen demand EDX ) energy-dispersive X-ray FTIR ) Fourier transform infrared GC ) gas chromatograph HHV ) higher heating value LHV ) lower heating value SEM ) scanning electron microscopy TCD ) thermal conductivity detector XRD ) X-ray diffraction

Literature Cited ´ brego, J.; Gonzalo, A.; Arauzo, J. (1) Manya`, J. J.; Sańchez, J. L.; A Influence of gas residence time and air ratio on the air gasification of dried sewage sludge in a bubbling fluidised bed. Fuel 2006, 85, 2027. (2) Inguanzo, M.; Domıńguez, A.; Meneńdez, J. A.; Blanco, C. G.; Pis, J. J. On the pyrolysis of sewage sludge: The influence of pyrolysis conditions on solid, liquid and gas fractions. J. Anal. Appl. Pyrol. 2004, 63, 209. (3) Paterson, I.; Zhuo, Y.; Reed, G. P.; Dugwell, D. R.; Kandiyoti, R. Pyrolysis and gasification of sewage. Water EnViron. J 2004, 18, 90. (4) Lu, G. Q.; Low, J. C. F.; Liu, C. Y.; Lua, A. C. Surface area development of sewage sludge during pyrolysis. Fuel 1995, 74, 344. (5) Bagreev, A.; Bandosz, T. J.; Locke, D. C. Pore structure and surface chemistry of adsorbents obtained by pyrolysis of sewage sludge-derived fertilizer. Carbon 2001, 39, 1971. (6) Ansari, A.; Bagreev, A.; Bandosz, T. J. Effect of adsorbent composition on H2S removal on sewage sludge-based materials enriched with carbonaceous phase. Carbon 2005, 43, 1039. (7) Seredych, M.; Bandosz, T. J. Sewage sludge as a single precursor for development of composite adsorbents/catalysts. Chem. Eng. J. 2007, 128, 59. (8) Yuan, W.; Bandosz, T. J. Removal of hydrogen sulfide from biogas on sludge-derived adsorbents. Fuel 2007, 86, 2736. (9) Seredych, M.; Strydom, C.; Bandosz, T. J. Effect of fly ash addition on the removal of hydrogen sulfide from biogas and air on sewage sludgebased composite adsorbents. Waste Manage. 2008, 28, 1983. (10) Bagreev, A.; Bandosz, T. J. H2S Adsorption/Oxidation on Materials Obtained Using Sulfuric Acid Activation of Sewage Sludge-Derived Fertilizer. J. Colloid Interface Sci. 2002, 252, 188. (11) Bagreev, A.; Bashkova, S.; Reznik, B.; Zibat, V.; Bandosz, T. J. Heterogeneity of sewage sludge derived materials as a factor governing their performance as adsorbents of acidic gases. Stud. Surf. Sci. Catal. 2002, 144, 217. (12) Pietrzak, R.; Bandosz, T. J. Interactions of NO2 with sewage sludgebased composite adsorbents. J. Hazard. Mater. 2008, 154, 946. (13) Bandosz, T. J.; Block, K. Effect of pyrolysis temperature and time on catalytic performance of sewage sludge/industrial sludge-based composite adsorbents. Appl. Catal. B: EnViron. 2006, 67, 77. (14) Lu, G. Q.; Lau, D. D. Characterisation of sewage sludge-derived adsorbents for H2S removal. Part 2: Surface and pore structural evolution in chemical activation. Gas Sep. Purif. 1996, 10, 103. (15) Jeyaseelan, S.; Lu, G. Q. Development of adsorbent/catalyst from municipal wastewater sludge. Water Sci. Technol. 1996, 34, 499. (16) Martıń, M. J.; Artola, A.; Balaguer, M. D.; Rigola, M. Activated carbons developed from surplus sewage sludge for the removal of dyes from dilute aqueous solutions. Chem. Eng. J. 2003, 94, 231. (17) Rozada, F.; Otero, M.; Morań, A.; Garcıá, A. I. Adsorption of heavy metals onto sewage sludge-derived materials. Bioresour. Technol. 2008, 99, 6332. (18) Otero, M.; Rozada, F.; Calvo, L. F.; Garcıá, A. I.; Morań, A. Elimination of organic water pollutants using adsorbents obtained from sewage sludge. Dyes Pigm. 2003, 57, 55. (19) Seredych, M.; Bandosz, T. J. Removal of copper on composite sewage sludge/industrial sludge-based adsorbents: The role of surface chemistry. J. Colloid Interface Sci. 2006, 302, 379. (20) Gasco´, G.; Meńdez, A.; Gasco´, J. M. Preparation of carbon-based adsorbents from sewage sludge pyrolysis to remove metals from water. Desalination 2005, 180, 245. (21) Meńdez, A.; Gasco´, G.; Freitas, M. M. A.; Siebielec, G.; Stuczynski, T.; Figueiredo, J. L. Preparation of carbon-based adsorbents from pyrolysis and air activation of sewage sludges. Chem. Eng. J. 2005, 108, 169. (22) Zhai, Y.; Wei, X.; Zeng, G.; Zhang, D.; Chu, K. Study of adsorbent derived from sewage sludge for the removal of Cd2+, Ni2+ in aqueous solutions. Sep. Purif. Technol. 2004, 38, 191.

(23) Martıń, M. J.; Balaguer, M. D.; Rigola, M. Feasibility of activated carbons from biological sludge by chemical activation with ZnCl2 and H2SO4. EnViron. Technol. 1996, 17, 667. (24) Ros, A.; Lillo-Ro´denas, M. A.; Fuente, E.; Montes-Morań, M. A.; Martıń, M. J.; Linares-Solano, A. High surface area materials prepared from sewage sludge-based precursors. Chemosphere 2006, 65, 132. (25) Karayildirim, T.; Yanik, J.; Yuksel, M.; Bockhorn, H. Characterisation of products from pyrolysis of waste sludges. Fuel 2006, 85, 1498. (26) Scott, S. A.; Dennis, J. S.; Davidson, J. F.; Hayhurst, A. N. Thermogravimetric measurements of the kinetics of pyrolysis of dried sewage sludge. Fuel 2006, 85, 1248. (27) Biagini, E.; Lippi, F.; Petarca, L.; Tognotti, L. Devolatilization rate of biomasses and coal-biomass blends: An experimental investigation. Fuel 2002, 81, 1041. (28) Thipkhunthod, P.; Meeyoo, V.; Rangsunvigit, P.; Kitiyanan, B.; Siemanond, K.; Rirksomboon, T. Pyrolytic characteristics of sewage sludge. Chemosphere 2006, 64, 955. (29) Go´mez-Rico, M. F.; Font, R.; Fullana, A.; Martıń-Gulloń, I. Thermogravimetric study of different sewage sludges and their relationship with the nitrogen content. J. Anal. Appl. Pyrol. 2005, 74, 421. (30) Stolarek, P.; Ledakowicz, S. Thermal Processing of sewage sludge by drying, pyrolysis, gasification and combustion. Water Sci. Technol. 2001, 44, 333. (31) Chen, X.; Jeyaseelan, S. Study of sewage sludge pyrolysis mechanism and mathematical modelling. J. EnViron. Eng. 2001, 127, 585. (32) Folgueras, M. B.; Dıáz, R. M.; Xiberta, J. Pyrolysis of blends of different types of sewage sludge with one bituminous coal. Energy 2005, 30, 1079. (33) Conesa, J. A.; Marcilla, A.; Moral, R.; Moreno-Caselles, J. M.; Perez-Espinosa, A. Evolution of gases in the primary pyrolysis of different sewage sludges. Thermochim. Acta 1998, 313, 63. (34) Fullana, A.; Conesa, J. A.; Font, R.; Martıı`n-Gulloń, I. Pyrolysis of sewage sludge: Nitrogenated compounds and pre-treatment effects. J. Anal. Appl. Pyrol 2003, 68, 561. (35) Domıńguez, A.; Meneńdez, J. A.; Inguanzo, M.; Pis, J. J. Investigations into the characteristics of oils produced from microwave pyrolysis of sewage sludge. Fuel Process. Technol. 2005, 86, 1007. (36) Tian, F. J.; Li, B. Q.; Chen, Y.; Li, C. Z. Formation of NOx precursors during the pyrolysis of coal and biomass. Part V. Pyrolysis of a sewage sludge. Fuel 2002, 81, 2203. (37) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric Identification of Organic Compounds; Wiley: New York, 1981. (38) Patnaik, P. Dean’s Analytical Chemistry Handbook, 2nd ed.; McGraw-Hill: New York, 2004. (39) Thipkhunthod, P.; Meeyoo, V.; Rangsunvigit, P.; Rirksomboon, T. Describing sewage sludge pyrolysis kinetics by a combination of biomass fractions decomposition. J. Anal. Appl. Pyrol. 2007, 79, 78. (40) Barth, A. Infrared spectroscopy of proteins. Biochim. Biophys. Acta: Bioenergetics 2007, 1767, 1073. (41) Shen, Q.; Zhang, T.; Zhu, M. F. A comparison of the surface properties of lignin and sulfonated lignins by FTIR spectroscopy and wicking technique. Colloid. Surf. A: Physicochem. Eng. Aspects. 2008, 320, 57. (42) Boylston, E. K.; Morris, N. M. Preparation of Thin Sections for Fourier Transform-Infrared Microscopy: A New Embedding Medium for Cellulosic Samples. Biotech. Histochem. 1997, 72, 213. (43) Kochkar, H.; Morawietz, M.; Ho¨lderich, W. F. Oxidation of potato starch with NO2: Characterization of the carboxylic acid salts. Appl. Catal. A: Gen. 2001, 210, 325. (44) Liu, C. F.; Xu, F.; Sun, J. X.; Ren, J. L.; Curling, S.; Sun, R. C. Physicochemical characterization of cellulose from perennial rye grass leaves (Lolium perenne). Carbohyd. Res. 2006, 341, 2677. (45) Yang, H.; Yan, R.; Chen, H.; Lee, D. H.; Zheng, C. Characteristics of hemicellulose, cellulose and lignin pyrolysis. Fuel 2007, 86, 1781. (46) Carrillo-Navarrete, F. Caracterizacioń estructural de fibras lyocell y su comportamiento frente a procesos de degradacioń. Ph.D. Thesis, Universidad Politećnica de Catalunã, Catalunã, Spain, 2002. (47) Gasco, G.; Cueto, M. J.; Meńdez, A. The effect of acid treatment on the pyrolysis behavior of sewage sludges. J. Anal. Appl. Pyrol. 2007, 80, 496. (48) Won, S. W.; Choi, S. B.; Yun, Y. S. Performance and mechanism in binding of Reactive Orange 16 to various types of sludge. Biochem. Eng. J. 2006, 28, 208. (49) Amir, S.; Hafidi, M.; Merlina, G.; Revel, J. C. Sequential extraction of heavy metals during composting of sewage sludge. Chemosphere 2005, 59, 801.

Ind. Eng. Chem. Res., Vol. 48, No. 6, 2009 3221 (50) Grube, M.; Lin, J. G.; Lee, P. H.; Kokorevicha, S. Evaluation of sewage sludge-based compost by FT-IR spectroscopy. Geoderma 2006, 130, 324. (51) Sharma, R. K.; Wooten, J. B.; Baliga, V. L.; Lin, X.; Chan, W. G.; Hajaligol, M. R. Characterisation of chars from the pyrolysis of lignin. Fuel 2004, 83, 1469. (52) Green, A. E. S.; Zanardi, M. A.; Mullin, J. P. Phenomenological models of cellulose pyrolysis. Biomass Bioenergy 1997, 13, 15. (53) Sun, J. H.; Sun, Z. H.; Wang, Q. S.; Ding, H.; Wang, T.; Jiang, C. S. Catalytic effects of inorganic acids on the decomposition of ammonium nitrate. J. Hazard. Mater. 2005, 127, 204. (54) Ettarh, C.; Galwey, A. K. A kinetic and mechanistic study of the thermal decomposition of calcium nitrate. Thermochim. Acta 1996, 288, 203. (55) Basiuk, V. A.; Douda, J. Pyrolysis of simple amino acids and nucleobases: Survivability limits and implications for extraterrestrial delivery. Planet Space. Sci. 1999, 47, 577. (56) Kante, K.; Qiu, J.; Zhao, Z.; Cheng, Y.; Bandosz, T. J. Development of surface porosity and catalytic activity in metal sludge/waste oil derived adsorbents: Effect of heat treatment. Chem. Eng. J. 2007, 138, 155. (57) Kante, K.; Qiu, J.; Zhao, Z.; Cheng, Y.; Bandosz, T. J. Role of oil-derived carbonaceous phase in the performance of sewage sludge-based materials as media for desulfurisaton of digester gas. Appl. Surf. Sci. 2007, 254, 2385. (58) Sanders, J. P.; Gallagher, P. K. Kinetic analyses using simultaneous TG/DSC measurements Part I: Decomposition of calcium carbonate in argon. Thermochim. Acta 2002, 288, 115. (59) Jones, J. M.; Pourkashanian, M.; Rena, C. D.; Williams, A. Modelling the relationship of coal structure to char porosity. Fuel 1999, 78, 1737. (60) Solomon, P. R.; Carangelo, R. M. FT-i.r. analysis of coal: 2. Aliphatic and aromatic hydrogen concentration. Fuel 1988, 67, 949.

(61) Solomon, P. R.; Carangelo, R. M. FTIR analaysis of coal. 1. Techniques and determination of hydroxyl concentrations. Fuel 1982, 61, 663. (62) Ibarra, J. V.; Moliner, R.; Bonet, A. J. FT-i.r. investigation on char formation during the early stages of coal pyrolysis. Fuel 1994, 73, 918. (63) Miikki, V.; Ha¨nninen, K.; Knuutinen, J.; Hyo¨tylaïnen, J. Pyrolysis of humic acids from digested and composted sewage sludge. Chemosphere 1999, 38, 247. (64) Zhao, H. B.; Kwak, J. H.; Zhang, Z. C.; Brown, H. M.; Arey, B. W.; Holladay, J. E. Studying cellulose fiber structure by SEM, XRD, NMR and acid hydrolysis. Carbohydr. Polym. 2007, 68, 235. (65) Rubio, B.; Izquierdo, M. T. Influence of low-rank coal char properties on their SO2 removal capacity from flue gases: I. Non-activated chars. Carbon 1997, 35, 1005. (66) Miura, K.; Aimi, M.; Naito, T.; Hashimoto, K. Steam gasification of carbon: Effect of several metals on the rate of gasification and the rates of CO and CO2 formation. Fuel 1986, 65, 407. (67) Raveendran, K.; Ganesh, A. Adsorption characteristics and pore development of biomass-pyrolysis char. Fuel 1998, 77, 769. (68) Guan, R.; Li, W.; Li, B. Effects of Ca-based additives on desulfurisation during coal pyrolysis. Fuel 2003, 82, 1961. (69) Ohtsuka, Y.; Asami, K. Highly active catalysts from inexpensive raw materials for coal gasification. Catal. Today 1997, 39, 111. (70) Tao, D.; Chen, S.; Parekh, B. K.; Hepworth, M. T. An investigation of a thermochemical process for conversion of gypsum and pyrite wastes into useful products. AdV. EnViron. Res. 2001, 5, 277.

ReceiVed for reView September 10, 2008 ReVised manuscript receiVed December 16, 2008 Accepted December 18, 2008 IE801366T

Structural Changes of Sewage Sludge Char during Fixed-Bed Pyrolysis

Recommend Documents