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Sewage Sludge/Metal Sludge/Waste Oil Composites as Catalysts for Desulfurization of Digester Gas Karifala Kante and Teresa J. Bandosz* Department of Chemistry, The City College of New York, 160 ConVent AVe, New York, New York 10031, USA ReceiVed August 23, 2007. ReVised Manuscript ReceiVed October 13, 2007
Composite adsorbents based in mixtures of sewage sludge/metal sludge and waste oil were prepared by pyrolysis at 650 and 950 °C. The materials were characterized using adsorption of nitrogen, Fourier transform infrared (FTIR), X-ray diffraction (XRD), inductively couple plasma (ICP), scanning electron microscopy (SEM), pH measurements, and thermal analysis. As-received materials were used as adsorbents of hydrogen sulfide from digester gas. To evaluate the importance of waste oil addition, the results were compared to those obtained on sewage sludge/metal sludge composites. The results clearly showed the importance of the carbonaceous phase from the oil precursor for enhancing the catalytic properties. Besides providing porosity, necessary for a dispersion of inorganic sludge base catalyst, an addition of a carbon phase alters the surface chemistry via providing a more reducing environment during the pyrolysis and via providing the carbon for more efficient formation of such compounds as carbides. The results indicated that the adsorbents obtained at 950 °C are much more active in the process of hydrogen sulfide oxidation than those obtained at 650 °C. In the case of the high temperature of heat treatment, longer treatment is beneficial for the development of surface catalytic properties. As a result of this, the carbon phase was stabilized via increasing its degree of aromatization and it became more porous owing to the release of activation agents from the decomposition and rearrangement of an inorganic phase. In very small pores, oxygen important for oxidation can be chemisorbed.
Introduction Contemporary societies produce large quantities of various sludges, which need to be efficiently disposed, reused, or recycled.1 Examples of sludges are sewage sludge, metal sludge, and waste oil sludge. The production of sewage sludge is 10 million dry tons in the United States per year1 and can reach 20 millions dry tones per year in China.2 It is well-known that often the disposal of sludge creates environmental hazards linked to metal leaching and to changes in metal oxidation states. The most popular way of sewage sludge utilization is landfilling, road building, and application as a fertilizer or as cement additive. Besides this, the vast majority of sludge is incinerated, especially in Europe, to reduce the volume of wastes. On the other hand, metal sludges, often considered as a hazardous waste, are buried underground on special waste disposal sites. This requires the constant monitoring. Moreover, the fate of the sludge is the liability of its originator. A way to effectively utilize sludge, which is gaining more and more attention, is to convert them into adsorbents, reactive adsorbents, or catalysts.3–29 The abundance of transition metals in sludges makes them very attractive materials for this purpose. * To whom correspondence should be addressed. Tel.: (212) 650-6017. Fax: (212) 650-6107. E-mail:
[email protected]. (1) Biosolids regeneration, Use, and Disposal in the United States; EPA530-R-99-009, U.S. EPA: Washington, D.C., Septemeber 1999. (2) Deng, B. H.; Tao, Q. Nonferrous Met. Eng. Res. 2003, 24, 91–93. (3) Chiang, P. C.; You, J. H. Can. J. Chem. Eng. 1987, 65, 922–927. (4) Lewis, F. M. US Patent 4,122,036, 1977. (5) Sutherland, J. US patent 3,998,757, 1976. (6) Nickerson, R. D.; Messman, H. C. US patent 3,887,461, 1975. (7) Abe, H.; Kondoh, T.; Fukuda, H.; Takahashi, M.; Aoyama, T.; Miyake, M. US Patent 5,338,462, 1994. (8) Khalili, N. R.; Arastoopour, H.; Walhof, L. K. US Patent 6,030,922, 2000.
Moreover, organic coagulants and dead bacteria, added at various steps of treatment technology, provide the source of the carbonaceous phase, if pyrolysis is a choice of conversion approach. Applying heat treatment in an inert atmosphere, (9) Lu, G. Q.; Low, J. C. F.; Liu, C. Y.; Lau, A. C. Fuel 1995, 74, 3444–3451. (10) Bagreev, A.; Bandosz, T. J. Ind. Eng. Chem. Res. 2001, 40, 3502– 3510. (11) Lu, G. Q.; Lau, D,D Gas Sep. Purif. 1996, 10, 103–110. (12) Bagreev, A.; Bandosz, T. J.; Locke, D. C. Carbon 2001, 39, 1971– 1979. (13) Bagreev, A.; Bandosz, T. J. J. Colloid Interface Sci. 2002, 252, 188–194. (14) Bagreev, A.; Bashkova, S.; Locke, D. C.; Bandosz, T. J. EnViron. Sci. Technol. 2001, 35, 1537–1542. (15) Bagreev, A; Bandosz, T. J. EnViron. Sci. Technol 2004, 38, 345– 351. (16) Martin, M. J.; Artola, A.; Dolors Balaguer, M.; Rigola, M. Chem. Eng. J. 2002, 94, 231–239. (17) Zang, F. S.; Toh, H. J. Hazard. Mater. B. 2003, 101, 323–330. (18) Zang, F. S.; Nriangu, J. O.; Itoh, H. J. Photochem. Photobiol. A: Chem. 2004, 167, 223–228. (19) Rio, S.; Faur-Brasquet, C.; Le Coq, L.; Courcoux, P.; Le Cloirec, P. Chemosphere 2005, 58, 423–437. (20) Rio, S.; Faur-Brasquet, C.; Le Coq, L.; Le Cloirec, P. EnViron. Sci. Technol. 2005, 39, 4249–4257. (21) Ansari, A.; Bagreev, A.; Bandosz, T. J. Carbon 2005, 43, 359– 367. (22) Ansari, A.; Bandosz, T. J. EnViron. Sci. Technol. 2005, 39, 6217– 6224. (23) Sioukri, E.; Bandosz, T. J. EnViron. Sci. Technol. l 2005, 39, 6225– 6230. (24) Martin, M. J.; Serra, E.; Ros, A.; Balaguer, M. D.; Rigola, M. Carbon 2004, 42, 1389–1394. (25) Ros, A.; Montes-Moran, M. A.; Fuente, E.; Nevskaia, D. M.; Marin, M. J. EnViron. Sci. Technol. 2006, 40, 3102–3110. (26) Bandosz, T. J.; Block, K. Ind. Chem. Eng. Res. 2006, 45, 3666– 36672. (27) Bandosz, T. J.; Block, K. EnViron. Sci. Technol. 2006, 40, 3378– 3383.
10.1021/ef700507g CCC: $40.75 2008 American Chemical Society Published on Web 11/27/2007
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besides producing a carbon phase, also results in the decomposition of inorganic salts and reduction of oxides, their solid-state reactions. These changes in the inorganic phase, via a release of gases, contribute to development of the porous structure, mainly in the range of mesopores.12,26–28 This porous structure provides the surface area for the dispersion of a catalytically active inorganic phase.13,25–28 An important feature of the sludge-derived materials is their basicity which originates mainly in the calcium or magnesium added during the sludge treatment process.25 The basicity can be also provided by some metal components of sludges if they are present in the form of oxides. The basic nature of sludgederived adsorbents governs their efficient applications for removal of hydrogen sulfide.12–15,21–27 Moreover, in this process, iron species are also important. They are usually present in metal sludges or they are added as flocculants during treatment. Iron oxides accept electrons from HS- causing its oxidation to elemental sulfur. Those HS- ions are formed as a result of the basic environment provided by alkaline and alkali earth oxides. Examples of other environmentally important applications of the sludge-derived materials are their usefulness for the adsorption of heavy metals17–19 or dyes.24,27,29 Here, the surface activity is governed by the presence cation exchange centers, which come from inorganic mineral-like phases or complex-formation sites. On the basis of the numerous reports, which have recently appeared in the literature, the sludge-derived materials are comparable in their adsorption capacity to activated carbons and even to catalytic carbons manufactured using patented technologies.17–29 Although sludge-derived adsorbents were demonstrated as media for various environmental remediation processes, there is always a possibility to improve their properties by further modifications. This can be done by utilizations of sludge mixtures, which lead to the synergy between the sludge components resulting in new surface chemistry,26–28,32 or by addition other wastes, such as waste oil.31,33,34 In each case, unique adsorbents are formed, and owing to the complexity of surface chemistry, there is no clear indication of which surface component provides the most catalytically important sites.14,25–28,30,33,34 The objective of this paper is to investigate the performance of sewage sludge/metal sludge/ waste oil-derived adsorbents in the desulfurization of digester gas. Our previous studies showed that two-component mixtures have promising surface properties important for oxidation of hydrogen sulfide.33–35 The good performance was linked to the synergy between the sludge and the presence of a new carbonaceous phase. The latter provided porosity important for oxidation reactions to occur.31,33–35 Moreover, this phase also interferes/interacts during carbonization and affects the formation of new species via solid-state reactions. The important aspect to investigate, related to the presence of this carbonaceous phase, is the role of this phase in preserving the activity of inorganic alkali or alkaline earth metalbased centers. These centers, when introduced to such catalytic carbons as Midas get deactivated by carbon dioxide present in the digester gas.32,33 As variables influencing the catalytic (28) Bandosz, T. J.; Block, K. Appl.Catal. EnViron. 2006, 67, 77–85. (29) Seredych, M.; Bandosz, T. J. Ind. Chem. Eng. Res. 2006, 45, 3658– 3665. (30) (a) Bagreev, A.; Bandosz, T. J. Ind. Chem. Eng. Res 2005, 44, 530. (b) Bandosz, T. J. In ActiVated Carbon Surfaces in EnVironmental Remediation; Bandosz, T. J., Ed.; Elsevier: Oxford, 2006, 231–292. (31) Bandosz, T. J.; Seredych, M.; Allen, J.; Wood, J.; Rosenberg, E. Chem. Mater. 2007, 19, 2500–2511. (32) Seredych, M.; Bandosz, T. J. Energy Fuels 2007, 21, 858–866.
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Figure 1. H2S breakthrough curves for the samples studied. Table 1. H2S Breakthrough Capacity and the pH Values for the Initial and Exhausted Samples pH
H2S breakthrough capacity sample
mg/g
mg/cm3
CST-30-650 CST-120-650 CST-30-950 CST-120-950 CSTO-30-650 CSTO-60-650 CSTO-120-650 CSTO-30-950 CSTO-60-950 CSTO-120-950
10.4 14.7 16.6 15.2 17.3 17.4 15.0 24.3 34.2 40.9
5.91 8.62 8.61 8.78 8.96 9.34 7.99 12.46 16.80 19.66
initial
exhausted
9.89 10.21 10.68 10.45 10.60 10.32 10.33 10.03 10.33 10.57
10.43 9.94 10.26 10.09 10.22 10.01 10.07 9.56 9.66 9.73
Table 2. Content of Catalytic Metals in Original Sludges sample
Fe [%]
Ca [%]
Cu [%]
Mg [%]
Zn [%]
metal sludge (T) sewage sludge (S)
4.8 2.2
17 3.1
0.45 0.08
0.05 0.62
13 0.3
performance, the holding time and final heating temperature are applied and analyzed. Experimental Details Materials. The adsorbents were prepared from the mixture of sewage sludge from New York City,34 metal sludge from General Galvanizer,33 and spent car oil. First, the homogeneous mixture of sludges (50/50 was dried, crushed, and sieved to 1–2 mm in size). The particles of dry sludge were mixed with the spent car oil with the ratio of mass of dry sludge (mg) to volume of oil (mL) of 7. The materials were left at this stage for 3 days, and then, the pyrolysis was carried out. In all cases, the pyrolysis was performed in a horizontal furnace in a nitrogen atmosphere with a heating rate of 10 °C/min. The final pyrolysis temperatures were 650 and 950 °C with holding times of either 30, 60, or 120 min The adsorbents are referred to as CSTO. The name is followed by the numbers representing holding time and heating temperature, respectively. Thus, CSTO-30-950 represents the adsorbent derived from the mixture of sewage sludge and metal sludge impregnated with oil which was heated at 950 °C for 30 min. For comparison, the adsorbents were also prepared from the mixture of sludges without oil, and they are referred to as CST followed with the numbers indicating their pyrolysis conditions, as for instance CST30-950. (33) Kante, K.; Qiu, J.; Zhao, Z.; Chang, Y.; Bandosz, T. J. Chem. Eng. J.,in press. (34) Kante, K.; Qiu, J.; Zhao, Z.; Chang, Y.; Bandosz, T. J. Appl. Surf. Sci., in press. (35) Yuan, W.; Bandosz, T. J. Fuel 2007, 86, 2736–2746.
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Figure 2. X-Ray diffraction patterns for the samples pyrolyzed at 650 °C.
Figure 4. FTIR spectra for the series of samples pyrolyzed at 650 °C.
Figure 3. X-Ray diffraction patterns for the samples pyrolyzed at 950 °C.
Methods. Characterization of Pore Structure of Adsorbents. On the materials obtained, sorption of nitrogen at its boiling point was carried out using ASAP 2010 (Micromeritics). Before the experiments, the samples were outgassed at 120 °C (the exhausted samples were outgassed at 100 °C to minimize vaporization of elemental sulfur and weakly bonded sulfuric acid) to a constant vacuum (10-4 torr). From the isotherms, the surface areas (BET method), total pore volumes, Vt (from the last point of the isotherm at relative pressure equal to 0.99), volumes of micropores, Vmic (DR36), and
mesopore volume, Vmes, along with pore size distributions, were calculated (DFT37,38). Thermal Analysis. Thermal analysis was carried out using TA Instrument thermal analyzer. The instrument settings were as follows: heating rate 10 °C/min in a nitrogen atmosphere with a 100 mL/min flow rate. For each measurement, about 25 mg of a ground adsorbent sample was used. The carbonaceous phase content was evaluated by heating the samples in air either at 650 or 950 °C, depending on the pyrolysis temperature of the particular sample based on the weight loss during this process. Surface pH. The pH of a carbonaceous sample suspension provides information about the acidity and basicity of the surface. A sample of 0.4 g of dry adsorbent powder was added to 20 mL of distilled water, and the suspension was stirred overnight to reach equilibrium. Then, the pH of the suspension was measured. Scanning Electron Microscopy. Scanning electron microscopy (SEM) was performed on a DSM 940 cold field emission instrument. The accelerating voltage was 2000 V. Scanning was performed in situ on a sample powder. Fourier Transform Infrared Spectroscopy. Fourier transform infrared (FTIR) spectroscopy was carried out using a Nicolet Magna-IR 830 spectrometer using the attenuated total reflectance method (ATR). The spectrum was collected 16 times and corrected for the background noise. The experiments were done on the powdered samples, without KBr addition. X-ray Diffraction. X-ray diffraction (XRD) measurements were conducted using a standard powder diffraction procedure. Adsor(36) Dubinin, M. M., In Chemistry and Physics of Carbon; Walker, P. L., Ed.; M. Dekker: New York, 1966; Vol. 2, pp 51–120. (37) Lastoskie, Ch. M.; Gubbins, K. E.; Quirke, N J. Phys. Chem. 1993, 97, 4786–4796. (38) Olivier, J. P. J. Pourous Mater. 1995, 2, 9–15.
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Figure 6. DTG curves for the series of samples pyrolyzed at 650 °C with (A) and without oil (B).
Figure 5. FTIR spectra for the series of samples pyrolyzed at 950 °C.
the performance in desulfurization measured. On the other hand, the samples obtained at 650 °C behave similarly without the
bents were ground with methanol in a small agate mortar. The mixture was smear-mounted onto the zero-background quartz window of a Phillips specimen holder and allowed to air-dry. Samples were analyzed by Cu KR radiation generated in a Phillips XRG 300 X-ray diffractometer. A quartz standard slide was run to check for instrument wander and to obtain the accurate location of 2θ peaks. EValuation of H2S Sorption Capacity. A custom-designed dynamic test was used to evaluate the performance of adsorbents for H2S adsorption from gas streams as described in the technical literature.32 Adsorbent samples were ground (1–2 mm particle size) and packed into a glass column (length 370 mm, internal diameter 9 mm, bed volume 3 cm3). A digester gas mixture (60% CH4, 40% CO2) containing 0.1% (1000 ppm) H2S was passed through the column of adsorbent at 0.100 L/min. The flow rate was controlled using Cole Palmer flow meters. The breakthrough of H2S was monitored using electrochemical sensors. As a breakthrough concentration, 100 ppm was arbitrarily chosen. The adsorption capacities of each sorbent in terms of milligrams of sulfur containing gases per gram of adsorbent were calculated by integration of the area above the breakthrough curves and from the H2S concentration in the inlet gas, flow rate, breakthrough time, and mass of the sorbent. The experiments were run on dry samples.
Results and Discusion The H2S breakthrough curves are collected in Figure 1. Their shapes suggest fast kinetics and no diffusion limitations. The longest breakthrough time is measured for the samples obtained at 950 °C. For them, also, a clear dependence on the time of treatment is visible; the longer the pyrolysis time, the better
Figure 7. DTG curves for the series of samples pyrolyzed at 950 °C with (A) and without oil (B).
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Table 3. Structural Parameters Calculated From Adsorption of Nitrogen sample
SBET [m2/g]
Vtot [cm3/g]
Vmic [cm3/g]
Vmeso [cm3/g]
Vmic/ Vtot [%]
CSTO-30-650 CSTO-60-650 CSTO-120-650 CSTO-30-950 CSTO-60-950 CSTO-120-950 CSTO-30-650ED CSTO-60-650ED CSTO-120-650ED CSTO-30-950ED CSTO-60-950ED CSTO-120-950ED
67 68 67 113 106 105 30 29 31 73 67 66
0.138 0.146 0.145 0.199 0.197 0.206 0.124 0.121 0.128 0.173 0.176 0.181
0.031 0.031 0.030 0.047 0.044 0.043 0.012 0.012 0.013 0.029 0.026 0.026
0.107 0.115 0.115 0.152 0.153 0.163 0.112 0.109 0.115 0.144 0.150 0.155
22 21 21 24 22 21 10 10 10 17 15 14
direct effect of the holding time on the breakthrough performance. In all cases, no SO2 emissions were detected, which is an indication that the secondary pollutants were not formed on the adsorbents/catalysts surface.26–28 The breakthrough times are longer than those for the samples obtained previously without oil addition.33,34 From the breakthrough curves, the H2S breakthrough capacities were calculated. They are summarized in Table 1 along with the pH values for the initial and exhausted samples. The values listed are consistent with the observations based on the breakthrough time. For comparison, we also measured the capacity of the samples obtained without oil pyrolyzed for 30 or 120 min at 650 or 950 °C. The results show obvious improvement in the capacity after the addition of oil, especially for the samples obtained at 950 °C for which up to 150% improvement is found. The samples obtained at 650 °C have about 30% less capacity than the worst sample pyrolyzed at 950 °C, CSTO-30-950, for which about 24 mg H2S/g adsorbent
Figure 8. Pore size distribution for the series of samples pyrolyzed at 650 °C with (A) and without oil (B).
was measured. Then, with an increase in the holding time, the capacity increases about 90% from 30 to 60 min and about 30% from 60 to 120 min The most dramatic effect of an increase in the capacity is seen between 30 and 60 min of pyrolysis time, which must be related to the changes in the surface of the materials.26–28 The effect of the sludge mixture and oil addition is very strong here since for only metal sludge mixed with oil and for sewage sludge mixed with oil 22 and 29 mg/g were measured for the best-performing samples obtained at 950 °C with a holding time of 120 min, respectively.33,34 These capacities are also much better than those for the mixture of only two sludges. In Table 1, also, the capacities per unit volume of the bed are listed since they reflect the real-life performance of the adsorber where a volume of the bed is one of the limiting factors of the performance. Differences in the capacity between samples studied must be caused by differences in their surface features. As established based on the previous studies,26–28,30 the surface chemistry and porosity are important factors for desulfurization. The latter becomes crucial when catalytic reaction takes place on the surface. The first obvious indication of the surface chemical character is its pH value. As seen from Table 1, the pH values of our samples are very similar. One has to remember that the pH represents here the average number and strength of surface groups, so even with the same pH, the differences in the surface acidity/basicity are likely to exist. After exposure to dry digester gas, the pH values for the samples obtained at 650 °C only slightly decrease, whereas visible changes are seen for the samples obtained at 950 °C (almost 1 pH unit). Since the pH remains basic, we do not expect the formation of sulfuric acid on the surface. That decrease is likely caused by the formation of salts (mainly sulfides) with active components of the surface. On the other hand, the pH of samples obtained without oil at 950 °C does not change after reaction with H2S, which might be caused by the small adsorption and conversion to sulfur.31 That formation of salts must be linked to the sample’s chemical composition. Table 2 lists the content of catalytic metals in the initial sludges (supplied by the producers). These metals were found as important for the reactive adsorption of hydrogen sulfide.26–28,30 Both sludges have a noticeable content of iron, which can participate in redox surface reactions. Metal sludge has strikingly high contents of calcium and zinc. While zinc is expected to decrease significantly after pyrolysis even at 650 °C owing to the low boiling point of its salts,39 the content of other metals in the final products will increase taking into account the fact that the yield of materials in the pyrolysis is about 45%. The high contents of calcium and magnesium are responsible for the high pH values of our materials. The oxides of those metals were found to enhance the adsorption of hydrogen sulfide from air on Midas catalytic carbon.30 When hydrogen sulfide was adsorbed from digester gas on Midas, the fast deactivation of these centers occurred due to their reaction with CO2 and carbonic acid.40 Although the content of metals in the sludges/samples can help to roughly estimate the suitable precursor/adsorbent for the desulfurization process, it is quite obvious that the chemical arrangement of those metals should play and, in fact, plays a role in the hydrogen sulfide removal. This is seen based on the differences in the H2S capacities measured on the samples where the general chemical compositions are expected to be more or (39) Handbook of Chemistry and Physics, 67th ed.; Weast, Ed.; CRC Press: Boca Raton, FL, 1986. (40) Seredych, M.; Bandosz, T. J. Ind. Chem. Eng. Res. 2006, 45, 3658– 3665.
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Figure 9. Pore size distribution for the series of samples pyrolyzed at 950 °C with (A) and without oil (B).
less the same (as those obtained at 950 °C). Some information about the chemical composition of the samples can be obtained from X-ray diffraction analysis. The results are presented in Figures 2 and 3. Certainly, between the series obtained at 650 and 950 °C, noticeable differences exist in crystallographic phases. For the latter series, the crystallographic phases are more developed with more peaks revealed. This is a general pattern observed for this kind of material, and the exact analysis of the result is very difficult owing to the chemical complexity of the surface. Generally, heating at higher temperature leads to the more advanced solid-state reactions in the reducing environment and more species are present as spinel-like structures with metal at the lower oxidation level (or zero-valent) with less oxygen containing salts and more carbides.26–28 For the samples obtained at 650 °C, with an increase in the holding time between 30 and 60 min, the spectra show sharper peaks indicting an increase in the degree of crystallinity. Then, between 60 and 120 min, difference are not so obvious indicating that the surface reactions may have reached their completness. Although showing all the phases in Figures 2 and 3 is an impossible task, the analysis indicated that heating to 650 °C results in silica, sulfides of zinc, copper and calcium, iron carbide (cohenite, Fe3C)and ferrisilicite (FeSi), and moissanite (SiC). The most severe pyrolysis conditions lead to formation of barringerite (Fe2P), fersilicite (FeSi), xifengite (Fe5Si3), bornite (Cu5FeS4) and digenite (Cu1.8S), moissanite (SiC), lime (CaO), pyrrhotite (Fe1-xS), hematite (Fe2O3), tenorite (CuO), and other mixedvalency copper oxides. Besides them, various aluminosilicates with calcium, magnesium, or iron substitutions are present, mainly in low temperature pyrolyzed samples.26–28 Changes in surface chemistry with an increase in the temperature of pyrolysis can be also clearly seen on FTIR spectra presented in Figures 4 and 5. For the samples obtained
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at 650 °C (Figure 4), intense bands between 800 and 1200 cm-1 represent Si-O (Si) and SiO (Al) vibrations from tetrahedral or alumino and silico-oxygen bridges in aluminosilicates.41 Those mineral-like structures were previously detected after pyrolysis at low temperature.26–28 Moreover, the sewage sludge has a significant content of silicon and aluminum oxides (8.35% of SiO2 and 3.43% of Al2O3).42 In the range between 3200 and 3600 cm-1 OH- groups are detected. This band is much less intense for the samples obtained at 950 °C. For those samples, also, the bands representing alumino-silicates are less pronounced which is in agreement with the previous studies, a reduction of alumina, and the hypothesized involvement of silicon in the formation of carbides. Nevertheless, the band at 1050 cm-1 representing alumina43 can still be seen. Another difference between the samples is the intensity of the band located at about 1620 cm-1. Since thermal analysis indicated about 20% of carbonaceous phase in our materials, this band can be assigned to the oxygen C-O stretching vibration.30 Higher level of carbon aromatization results in a decrease in its intensity (Figure 5). The bands between 750 and 800 cm-1 may indicate an Si-C bond in silicon carbide.44 This compound was found to catalyze selective oxidation of hydrogen sulfide, especially when some porosity can be developed.45,46 Differences in surface activity of the samples obtained at 650 and 950 °C can be also analyzed based on TA results. The DTG curves for the initial and exhausted samples are presented in Figures 6 and 7. The peaks represent weight loss as a result of decomposition of surface species. For the samples obtained at 650 °C, a quite typical pattern is revealed (Figure 6). The small weight loss between 300 and 650 °C for the initial samples must be related to their exposure to atmospheric moisture. Their surface, when obtained without oil addition, was found as very water reactive and the peaks can be linked to the decomposition of hydroxides and hydrated salts.29 Moreover, oxygen groups from the low degree aromatized carbon surface can also decompose at this temperature range.30 The first peak at about 100 °C represents the removal of physically adsorbed water. After exposure to hydrogen sulfide in digester gas, new peaks appear between 200 and 350 °C and 350 and 500 °C. While the former peak represents the removal of elemental sulfur, the second one can be linked either to the removal of sulfur from very small pores, if those are present, or decomposition of sulfur containing salts such as alkaline earth metal sulfides and/or carbonates39 formed on the surface as a result of exposure to the challenge gas. The DTG patterns for the samples obtained at 950 °C look totally different (Figure 7). As observed before, for the initial samples, the weight gain (negative peaks) between 200 and 400 °C and between 400 and 800 °C is noticed even though the experiments are run in Ultrahigh purity (UHP) nitrogen. The only plausible explanation at this stage of our study is the reactivity of the surface toward nitrogen and its incorporation into the structure, maybe as nitrides.47,48 Nevertheless, the surface of these samples is expected to be less reactive toward (41) Li, L.; Liu, X.; Ge, Y.; Xu, R.; Rocha, J.; Klinowski, J. J. Phys. Chem. 1993, 97, 10389–10393. (42) Serdych, M. Strydom, Ch.; Bandosz, T. J. Waste Manage., in press. (43) Costa, T. M. H.; Gallas, M. R.; Benvenutti, E. V.; da Jornada, J. A. H. J. Phys. Chem. B 1999, 103, 4278–4284. (44) Li, J. P.; Steckl, A.; Golecki, L.; Reidinger, F.; Wang, L.; Ning, X. J.; Pirauz, P. Appl. Phys. Lett. 1993, 62, 3135–3137. (45) Ledoux, M. J.; Pham-Huu, CATTECH 2001, 5, 226–246. (46) Keller, N.; Viero, R.; Nhut, J.-M.; Phan-Huu, C.; Ledoux, M. L. J. Braz. Chem. Soc 2005, 16, 202–209. (47) Yamamoto, O.; Ishida, M.; Saitoh, Y.; Sasamoto, T.; Shimada, S. Int. J. Inorg. Mat. 2001, 3, 715. (48) Koc, R.; Kaza, S. J. Eur. Ceram. Soc. 1998, 18, 1471.
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Figure 10. Examples of SEM images for the samples studied.
water than that of their 650 °C counterparts. After exposure to digester gas, a new peak appears between 200 and 400 °C representing elemental sulfur. Its intensity/surface area is in agreement with the order in the H2S breakthrough capacity. Compared to the samples obtained without oil (Figures 6B and 7B), where more peaks are revealed after exposure to digester
gas, the surfaces of three-component adsorbents show less reactivity from the point of view of heterogeneity of the centers and more selectivity toward the formation of sulfur. Since our analysis of surface chemistry, besides indicating more crystallographic phases with metals at lower levels of oxidation,26–28 changes in the chemical environment of silica
396 Energy & Fuels, Vol. 22, No. 1, 2008
Figure 11. Comparison of capacities for H2S removal on adsorbents derived various sludge/oil mixtures an on catalytic Midas and DarcoH2S carbons. Data was taken from refs 33, 34, and 40.
(silicon carbide) in the high temperature treated sample, and differences in surface reactivity, cannot provide an unambiguous answer regarding the differences in the performance of materials obtained at various conditions from various compositions,33,34 the porosity should be analyzed in detail. Structural parameters calculated from nitrogen adsorption isotherms are collected in Table 3 for the initial samples and for those after exposure to digester gas. The adsorbents obtained at 950 °C have surface area values that are twice as high as their low-temperature counterparts. The differences are also seen in the volume of pores, especially micropores, which increased about 50% by heating from 650 to 950 °C. Those differences may contribute to the performance. Even though the catalytic surface is present, the pore volume has to be developed to some extent to provide space for storage of sulfur. After exposure to digester gas for all samples, a 50% decrease in the surface area and volume of micropores is noticed with rather small changes in the volume of mesopores. This indicates that micropores in these materials are active centers in the process of hydrogen sulfide oxidation. On the other hand, comparison with the structural parameters obtained for samples without oil shows that their volumes of micropores are comparable and that they are even slightly higher than those for the three-component samples. The striking difference in the pore structure is in the volume of mesopores, which increases 20–30% when oil is the precursor component. This indicates that the carbonaceous phase may prevent sintering between inorganic particles leaving the structure more open. This increases the accessibility of the active centers. The more subtle differences in porosity are seen on the pore size distributions presented in Figures 8 and 9. As seen, the materials obtained at two temperatures differ mainly in the volume of pores with sizes between 20 and 100 Å, which are not present for the samples pyrolyzed at low temperature. These pores must be formed as a result of a high temperature decomposition of the inorganic matter and solid-state reactions. Gases evolved must be released from the bulk of the materials forming pores. Moreover, those gases such as water or SO2 are the activation agents for the carbon deposit. Support for this is a greater volume of small micropores in the sample obtained at 950 °C, less than 10 Å in diameter, which are likely within the carbon deposit originated form the oil phase. For the materials obtained at 650 °C after exposure to digester gas, the intensities of both peaks representing the micropores with diameters less than 20 Å decreased with the total disappearance of the pores with diameters less than 10 Å. On the other hand, for the
Kante et al.
samples pyrolyzed at 950 °C, even though the volume in the pores less than 10 Å also disappears, the pores with sizes between 10 and 20 Å apparently are left intact. The only plausible explanation of this is that those small pores, which were filled with sulfur, have different chemistry or different chemical surroundings than others. Taking into account the difference in mesoporosity between the sample with and without oil, those pores are accessible from lager mesopores and are likely within the carbon deposit in a close vicinity of adsorption centers on which catalytic oxidation of hydrogen sulfide occurs.30 Sulfur formed in the reaction immediately migrates there and fills their volume. Then, the activity of the catalyst vanishes due to the deposition of sulfur on catalytic centers. Since the oil-derived carbonaceous phase seems to play a crucial role in hydrogen sulfide retention, the texture of the materials obtained without and with the addition of oil is compared in Figure 10. Although the similarities in the SEM images for the samples without and with oil exit, the texture of the latter seems fluffier which is consistent with the higher volume of larger pores. This can be important for reactive adsorption. Moreover, when the temperature of pyrolysis is higher, the samples look more spongy/fluffy which is consistent with the proposed mechanism of their activation. It is interesting that the surface of the adsorbents obtained from only sewage or metal sludges looked much more “dense”33,34 than that for the mixture of those two components. That combination of waste materials must result in unique surface chemistry, which provides a high capacity for hydrogen sulfide removal from digester gas without surface deactivation by CO2. Figure 11 compares the capacities for various combination of the precursors used in this research to those of catalytic activated carbons. Clearly, addition of a small amount of the carbonaceous phase (based on the content of oil, yield of the carbon phase obtained in its carbonization,15,33 and the yields of adsorbents from sludges, we estimated that oil may provide a maximum 2% increase in the content of carbon) increases the performance. As seen, mixing two sludges and adding oil definitely results in a synergetic effect, especially for samples heated for more than 30 min This indicates that reactions between the components of mixtures need some time for their completeness and porosity development. As compared to the commercial catalytic activated carbons, the capacity reached on our materials is twice that obtained on DacroH2S and only twice as small as that on Midas.40 This is of paramount importance since the catalysts for H2S oxidation are expensive materials. In our case, the adsorbents are obtained from wastes. Silicon carbide can be an important component since it should not get deactivated by carbon dioxide as occurs with alkali or alkaline earth metal oxides.40 The important aspect of the catalytic performance of our materials is the lack of oxygen supply to the system. Only oxygen that is chemisorbed on the surface, oxygen involved in some oxides such as in iron oxides or with elements in the reduced forms, can accept electrons from S.2–30 This can shed some light on the activity of those small pores where the higher probability for chemisorbed oxygen to exist occurs. Conclusion The results presented in this paper show that mixing three wastes brings some valuable assets for H2S removal such as the contents of catalytic oxides or the organic phase. Especially good performance is found for samples obtained at 950 °C where an increase in pyrolysis time from 30 to 120 min results in an approximately 70% increase in the H2S removal capacity. The
Catalysts for Desulfurization of Digester Gas
capacity on these materials is comparable to those on the catalytic activated carbons. It is proposed that solid-state reactions between the components of the sludge lead to the formation of the active catalyst. There is an indication that silicon carbide plays a catalytic role. More carbon phase may result in more silicon carbide formed. Moreover, pyrolysis at 950 °C results in the release of activation agents for the carbonaceous phase providing porosity where sulfur can be stored after its oxidation on the neighborhood catalytic center.
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That porosity seems to be crucial for the performance of adsorbents. It is likely that it attracts oxygen important for oxidation reactions. Acknowledgment. This work was supported by NYSERDA agreement no. 9405 (RF CUNY no. 55771-0001 and PSC CUNY grant no. 68445-00-37). The authors are grateful to Ms. Anna Kleyman and Dr. Mykola Seredych for experimental help. EF700507G
