Desulfurization of Digester Gas on Industrial-Sludge-Derived

858

Energy & Fuels 2007, 21, 858-866

Desulfurization of Digester Gas on Industrial-Sludge-Derived Adsorbents Mykola Seredych and Teresa J. Bandosz* Department of Chemistry, The City College of New York, City UniVersity of New York, 138th Street and ConVent AVenue, New York, New York 10031 ReceiVed September 27, 2006. ReVised Manuscript ReceiVed NoVember 20, 2006

Waste sludges from heavy industry (shipyard) consisting of metals and waste oil were pyrolyzed either alone or as the 50:50 weight percent mixtures at 500, 650, and 950 °C. Resulting adsorbents were used as H2S removal media from a simulated mixture of dry digester gas. The breakthrough capacity was measured at dynamic conditions. The initial and exhausted materials after the breakthrough tests were characterized using sorption of nitrogen, thermal analysis, and surface pH measurements. The results obtained demonstrate the complex dependence of the capacity on the surface chemistry, porosity (volume and sizes of pores), and water content. In all cases, elemental sulfur is the predominant product of surface reactions. Generally speaking, an exposure of adsorbents to water enhances the H2S removal capacity, especially for materials obtained at low temperature. This is the result of their chemical instability and surface reactivity. This reactivity is linked to the presence of calcium, magnesium, and iron, which are known as catalysts for hydrogen sulfide oxidation. When water is not present, CO2 quickly deactivates alkaline-earth-metal-based centers, and thus, smaller H2S removal capacity is revealed.

Introduction Although activated carbons are well-known as adsorbents of gases and vapors,1 there is a constant search for new materials that can outperform activated carbons from the point of view of their costs and pollutant removal capacity. Firm assets of activated carbon adsorbents are their high surface area and small pores, which help in gas separation. These features play a very important role when dispersive, nonspecific interactions are predominant forces of the adsorption/separation process. In the case of adsorption of small molecule gases at ambient conditions or at supercritical temperatures, the whole surface of activated carbons cannot be properly utilized because adsorption can occur only in small pores, similar in size to those molecules. Moreover, the adsorption forces at such conditions are rather weak. To enhance the removal/separation efficiency, the specific forces have to be employed. They involve hydrogen bonding or surface reactions in which new products are formed and retained on the surface. They are either harmless or strongly adsorbed via physical forces. Examples of reactions include the redox process, neutralization, polymerization, or complexation. The separation process based on these principles can be referred to as reactive adsorption. This kind of mechanism in engaged in the removal of toxic gases on military filters, wehlerites, via complex chemical mechanisms.2 To prepare carbons for reactive adsorption, complex treatments are often applied.3 The most common and most efficient ones are impregnation4-6 and oxidation.7,8 The catalytic phase * To whom correspondence should be addressed. Telephone: (212) 6506017. Fax: (212) 650-6107. E-mail: [email protected]. (1) Bansal, R. C.; Donnet, J. B.; Stoeckli, F. ActiVe Carbon; Marcel Dekker: New York, 1988. (2) Lodewyckx, P. Adsorption of Chemical Warfare Agents. In ActiVated Carbon Surfaces in EnVironmental Remediation; Bandosz, T. J., Ed.; Elsevier: Amsterdam, The Netherlands, 2006; pp 475-528. (3) Bandosz, T. J. ActiVated Carbon Surfaces in EnVironmental Remediation; Elsevier: Oxford, U.K., 2006; pp 1-571.

can be also introduced as a separate component, and then blending of the materials takes place.9 A similar effect is achieved when an active binder is used.10 In all of these techniques, an important feature providing a good performance is a dispersion of the catalytic phase. This includes not only a concentration on the surface but also a physical location in the specific sizes of pores, depending upon the size of the adsorbate/ reagent molecule. All of the methods mentioned above increase the cost of the adsorbents. Moreover, despite those treatments, the full capacity of the carbon surface from the point of view of the utilization of its whole surface area and pore volume is almost never achieved. This directed the attention of the research toward other sources/precursors of adsorbents. These precursors are municipal and industrial sludges and various industrial wastes.11-22 Their advantage is the presence of catalytic metals and not to mention (4) Bagreev, A.; Bandosz, T. J. Ind. Eng. Chem. Res. 2002, 41, 672679. (5) Przepiorski, J.; Oya, A. J. Mater. Sci. Lett. 1998, 17, 679-682. (6) Przepiorski, J.; Abe, Y.; Yoshida, S.; Oya, A. J. Mater. Sci. Lett. 1997, 16, 1312-1314. (7) Leon y Leon, C. A.; Radovic, L. R. Interfacial chemistry and electrochemistry of carbon surfaces. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1992; Vol. 24, pp 213310. (8) Bandosz, T. J.; Ania, C. O. Surface chemistry of activated carbon and its characterization. In ActiVated Carbon Surfaces in EnVironmental Remediation; Bandosz, T. J., Ed.; Elsevier: Amsterdam, The Netherlands, 2006; pp 159-229. (9) Graham, J. U.S. Patent 6,858,192, 2005. (10) Nguyen-Thanh, D.; Bandosz, T. J. Carbon 2005, 43, 359-367. (11) Bagreev, A.; Bandosz, T. J.; Locke, D. C. Carbon 2001, 39, 19711979. (12) Martin, M. J.; Serra, E.; Ros, A.; Balaguer, M. D.; Rigola, M. Carbon 2004, 42, 1389-1394. (13) Bandosz, T. J.; Block, K. EnViron. Sci. Technol. 2006, 40, 33783383. (14) Bandosz, T. J.; Block, K. Ind. Chem. Eng. Res. 2006, 45, 36663672. (15) Bandosz, T. J.; Block, K. Appl. Catal., B 2006, 67, 77-85.

10.1021/ef060482l CCC: $37.00 © 2007 American Chemical Society Published on Web 01/12/2007

Desulfurization of Digester Gas

the abundant quantity and waste recycling and minimization aspects. Conversion of sewage sludge to adsorbents via pyrolysis leading to the adsorbents with surface areas between 100 and 500 m2/g has been broadly addressed in the scientific literature.11-31 These materials were used for the removal of hydrogen sulfide from air, sulfur dioxide, basic or acidic dyes, phenol, or mercury with a comparable performance to that of activated carbons.16,25-31 Because desulfurization of air on the sludge-derived materials showed promising results,14-15 comparable to those obtained on catalytic carbons, such as Midas, Centaur, or DarcoH2S,32-34 desulfurization of digester gas (DG) seems to be the natural progression of the research. This process has already been studied in our laboratory on activated carbons,32-34 and it was found that the moisture present in the DG has a detrimental effect on the capacity of the catalytic carbons because of the deactivation of catalyst sites by carbonic acid. On the other hand, the presence of water on the surface of carbonaceous adsorbents increases the H2S removal capacity by enhancing the dissociation of hydrogen sulfide and thus its oxidation to elemental sulfur. The objective of this paper is to evaluate the DG desulfurization capacity on adsorbents obtained by the pyrolysis of industrial sludges. These sludges are the sources of catalytic metals for hydrogen sulfide oxidation.13-16 The most important of them are calcium, magnesium, and iron. Calcium and magnesium provide basicity needed for H2S dissociation, and iron is a redox catalyst for the oxidation of HS- ions to elemental sulfur. Because it was shown previously that the quality and quantity of the catalytic phase depends upon the pyrolysis conditions, such as temperature and holding time,15 the materials were obtained at three different temperatures. The performance of adsorbents is linked to their chemical composition and porous structure, which are the consequences of the preparation technology applied and the nature of the sludge precursor. Experimental Section Materials. Industrial oil sludge (WO) from Newport News Shipyard (Northrop Grumman) was mixed with metal sludge (MS) originated from the same facility in the 50:50 ratio based on the (16) Ros, A.; Montes-Moran, M. A.; Fuente, E.; Nevskaia, D. M.; Martin, M. J. EnViron. Sci. Technol. 2006, 40, 302-309. (17) Chiang, P. C.; You, J. H. Can. J. Chem. Eng. 1987, 65, 922-929. (18) Lewis, F. M. U.S. Patent 4,122,036, 1977. (19) Sutherland, J. U.S. Patent 3,998,757, 1976. (20) Nickerson, R. D.; Messman, H. C. U.S. Patent 3,887,461, 1975. (21) Lu, G. Q.; Low, J. C. F.; Liu, C. Y.; Lau, A. C. Fuel 1995, 74, 3444. (22) Khalili, N. R.; Arastoopour, H.; Walhof, L. K. U.S. Patent 6,030,922, 2000. (23) Bagreev, A.; Bandosz, T. J. Ind. Eng. Chem. Res. 2001, 40, 35023510. (24) Bagreev, A.; Bandosz, T. J. J. Colloid Interface Sci. 2002, 252, 188-194. (25) Bagreev, A.; Bashkova, S.; Locke, D. C.; Bandosz, T. J. EnViron. Sci. Technol. 2001, 35, 1537-1543. (26) Zang, F. S.; Itoh, H. J. Hazard. Mater. 2003, 101, 323-337. (27) Zang, F. S.; Nriangu, J. O.; Itoh, H. J. Photochem. Photobio., A 2004, 167, 223-228. (28) Rio, S.; Faur-Brasquet, C.; Le Coq, L.; Courcoux, P.; Le Cloirec, P. Chemosphere 2005, 58, 423-437. (29) Rio, S.; Faur-Brasquet, C.; Le Coq, L.; Le Cloirec, P. EnViron. Sci. Technol. 2005, 39, 4249-4257. (30) Ansari, A.; Bandosz, T. J. EnViron. Sci. Technol. 2005, 39, 62176224. (31) Sioukri, E.; Bandosz, T. J. EnViron. Sci. Technol. 2005, 39, 62256230. (32) Seredych, M.; Bandosz, T. J. Ind. Chem. Eng. Res. 2006, 45, 36583665.

Energy & Fuels, Vol. 21, No. 2, 2007 859 wet mass, homogenized, dried at 120 °C for 48 h, and then pyrolyzed at 500, 650, and 950 °C in a nitrogen atmosphere in a fixed bed (horizontal furnace). The heating rate was 10 °C/min with a holding time of 0.5 h. The same treatment was applied for single components of the mixture, metal sludge, and waste oil sludge, separately. The adsorbents are referred to as WO, MS, or MSWO followed by the temperature of their heat treatment. Thus, for example, MSWO650 represents a metal sludge/oil sludge mixture pyrolyzed at 650 °C. Methods. EValuation of H2S Sorption Capacity from DG. A custom-designed dynamic test was used to evaluate the performance of adsorbents for H2S adsorption from gas streams as described elsewhere.35-37 Adsorbent samples were ground (1-2 mm particle size), packed into a glass column (length, 370 mm; internal diameter, 9 mm; bed volume, 6 cm3), and used as received or prehumidified with moist air (relative humidity of 70% at 25 °C) for 2 h. The amount of water adsorbed was estimated from an increase in the sample weight. A simulated DG mixture (40% of CO2 and 60% CH4 at 25 °C) containing 0.1% (1000 ppm) of H2S (dry or with 70% moisture) was passed through the column of the adsorbent with a flow rate of 150 mL/min. The flow rate was controlled using Cole Palmer flowmeters. The breakthrough of H2S was monitored using a MultiRae photoionization detector. The test was stopped at the arbitrary chosen breakthrough concentration of 100 ppm. The adsorption capacities of each adsorbent in terms of milligrams of hydrogen sulfide per unit volume 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. For each sample, the test was repeated at least twice. Besides H2S, the content of SO2 in the outlet gas was also monitored using the MultiRae electrochemical detector. The adsorbents exhausted after H2S adsorption are designated by adding an additional letter E to their names. After the tests with as-received carbons and dry DG, the samples are referred to as ED, and after prehumidification, the samples are referred to as EP. Pore Structure Characterization. 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 the vaporization of elemental sulfur and weakly bonded sulfuric acid) to a constant vacuum (10-4 Torr). From the isotherms, the surface areas [Brunauer-Emmett-Teller (BET) method], total pore volumes, Vt (from the last point of the isotherm at a relative pressure equal to 0.99), volumes of micropores, Vmic (DR),38 and mesopore volume, Vmes, along with pore-size distributions, were calculated [density functional theory (DFT)]39,40. 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. Thermal Analysis. Thermal analysis was carried out using a TA Instrument thermal analyzer (SDT 2860). The instrument settings were at a heating rate of 10 °C/min and a nitrogen atmosphere with a 100 mL/min flow rate. For each measurement, about 25 mg of a ground adsorbent sample were used. (33) Bagreev, A.; Katikaneni, S.; Parab, S.; Bandosz, T. J. Catal. Today 2005, 99, 329-337. (34) Bandosz, T. J. J. Colloid Interface Sci. 2002, 246, 1-20. (35) Bagreev, A.; Adib, F.; Bandosz, T. J. Carbon 2001, 39, 19871905. (36) Adib, F.; Bagreev, A.; Bandosz, T. J. EnViron. Sci. Technol. 2000, 34, 686-692. (37) Adib, F.; Bagreev, A.; Bandosz, T. J. J. Colloid Interface Sci. 1999, 214, 407-415. (38) Dubinin, M. M. In Chemistry and Physics of Carbon; Walker, P. L., Ed.; Marcel Dekker: New York, 1966; pp 51-120. (39) Lastoskie, Ch. M.; Gubbins, K. E.; Quirke, N. J. Phys. Chem. 1993, 97, 4786-4796. (40) Olivier, J. P. J. Porous Mater. 1995, 2, 9.

860 Energy & Fuels, Vol. 21, No. 2, 2007

Seredych and Bandosz

Figure 1. H2S breakthrough curves for adsorbents exposed to dry DG.

Figure 3. H2S breakthrough curves for the prehumidified adsorbents exposed to dry DG.

Figure 2. SO2 emission curves for the breakthrough capacity runs in dry DG.

Elemental Analysis. The content of catalytic metals was determined using ICP in LSL Labs, Syracuse, NY.

Results and Discussion The H2S breakthrough curves along with SO2 emission curves for samples run in dry conditions, without prehumidification, are presented in Figures 1 and 2, respectively. As seen, the performance of materials differs. The best performance is obtained for samples pyrolyzed at 500 °C, and the breakthrough time decreases with an increase in the heating temperature. Mixing sludges does not improve the performance. The MS500 sample, besides showing the best performance for adsorption in dry conditions, does not show any SO2 emission until the H2S breakthrough occurs. This is an important aspect, because some other samples, such as MSWO500, emit SO2 at a constant level from the beginning of the breakthrough experiments. The same is observed for MS650. Although the emissions of SO2 in almost all cases are less than 1 ppm, the fact that sulfur is still present in the effluent gas is a negative factor for the application of these materials. On the other hand, the presence of sulfur dioxide in the effluent gas even though the DG does not have any air in its composition indicates a strong oxidizing power of the materials. Moreover, the sources of oxygen exist

on the surface, and those active centers are reduced in the reaction with hydrogen sulfide. The situation looks different when the adsorbents are exposed to water prior to desulfurization of dry DG (Figure 3). Here, SO2 is not detected at all, and the breakthrough times are about 2-fold longer compared to runs in totally dry conditions. The best performing materials are still those based on metal sludge; however, the sample obtained at 650 °C lasts much longer than that obtained at 500 °C. As before, increasing the pyrolysis temperature to 950 °C has a detrimental effect on the capacity of the adsorbents. The performance of adsorbents in the breakthrough tests is summarized in Tables 1 and 2, where besides the capacity expressed in milligrams per unit mass of an adsorbent or in milligrams per unit bed volume, the amount of water adsorbed during prehumidification, bed density, and pH before and after exposure to DG are listed. The low density of the lowtemperature pyrolyzed waste-oil-sludge-based adsorbents leads to their highest capacity expressed per unit mass of the material. When the tests are run in totally dry conditions, 4-9 wt % of hydrogen sulfide can be adsorbed, but only on the lowtemperature pyrolyzed materials. Heating to 950 °C not only decreases the capacity but also increases the pH from being close to neutral to strongly basic. This is the totally opposite relationship compared to that found for desulfurization from air, where the pH was crucial for hydrogen sulfide dissociation into HS- ions, which were then oxidized to elemental sulfur in the pore system.34,41 This suggests a different mechanism of desulfurization. The visible decrease in the pH, although not dramatic and not associated with the formation of sulfuric acid, is also an indication of the changes in surface chemistry, not only associated with the formation of elemental sulfur on the surface. Exposure to water, before the contact with a dry DG, dramatically changes the performance of the adsorbents. For both metal-sludge- and waste-oil-sludge-derived samples ob(41) Bandosz, T. J. Desulfurization on activated carbons. In ActiVated Carbon Surfaces in EnVironmental Remediation; Bandosz, T. J., Ed.; Elsevier: Oxford, U.K., 2006; pp 231-292.


Energy & Fuels, Vol. 21, No. 2, 2007 861

Table 1. H2S Breakthrough Capacities, Bed Densities, and pH Values for the Samples Run in Dry Conditions H2S breakthrough capacity sample

(mg/g of carbon)

WO500 WO650 WO950 MS500 MS650 MS950 MSWO500 MSWO650 MSWO950

92.2 1.5 1.1 71.8 38.5 0.7 72.2 50.5 7.3

(mg/cm3

pH

of carbon)

24.8 0.43 0.51 30.3 21.5 0.68 30.1 24.3 4.4

sample WO500 WO650 WO950 MS500 MS650 MS950 MSWO500 MSWO650 MSWO950

170.8 144.2 85.3 121.1 108.8 1.1 73.8 55.5 18.8

46.0 41.4 39.3 51.1 60.7 1.1 30.8 26.7 11.3

ash content (%)

initial

exhausted

81 86 96 91 97 99 87 91 98

8.76 9.50 9.78 7.74 8.54 10.54 8.45 9.52 10.20

7.87 9.33 9.52 7.36 8.30 10.50 7.36 9.10 9.12

0.27 0.29 0.46 0.42 0.56 0.97 0.42 0.48 0.60

Table 2. H2S Breakthrough Capacities, Amount of Water Preadsorbed, and pH Values for the Prehumidified Samples H2S breakthrough capacity (mg/g of (mg/cm3 of carbon) carbon)

bed density

(g/cm3)

pH

water adsorbed (mg/g)

initial

exhausted

104.3 85.1 39.7 15.0 11.7 1.2 45.7 45.5 5.6

8.76 9.50 9.78 7.74 8.54 10.54 8.45 9.52 10.20

7.48 8.92 9.25 7.36 7.80 10.21 7.78 8.50 8.92

tained at 500 and 650 °C, a significant increase in the capacity is observed. That enhancement is also noticed for WO950. It is interesting that the contact with water is not beneficial for the MSWO samples, whose capacity is similar to those run in dry conditions. The visible difference in the mechanism of adsorption between the dry and prehumidified samples is also seen in the magnitude of pH decreases, which are more pronounced for the latter samples. On the basis of the amount of water preadsorbed, the waste-oil-sludge-based sample has the highest reactivity with moisture. For both MS and WO series of samples, the reactivity toward water decreases with an increase in the pyrolysis temperature. This was expected on the basis of dehydroxylation, decomposition, reduction, and solid-state reactions being more pronounced with an increase in the heattreatment temperature.14-16 A positive effect of water on the H2S removal capacity from the DG is seen in Figure 4. A linear trend is found for all samples but those MS obtained at 500

Figure 4. Relationship between the H2S breakthrough capacity and the amount of water preadsorbed during prehumidification.

Table 3. Content of Catalytically Important Metals sample

Fe (%)

Ca (%)

Mg (%)

Cu (%)

Zn (%)

Cr (ppm)

WO950 MS950

3.7 2.3

5.1 14

8.4 0.46

0.25 0.77

0.51 0.16

280 6700

and 650 °C, whose high capacity must be linked to complex surface reactions involving chemical components present in the pore system. Water on the surface converts oxide to hydroxides, and it enables the dissociation of hydrogen sulfide. When water is not present, those sites can react with CO2 and thus surface deactivation occurs. According to the information supplied by the shipyard, the waste oil sludge was treated with CaCl2, Na3PO4, NaOH, and alum. The metal sludge treatment history includes the addition of sulfuric acid and sodium hydroxide for pH adjustments, Al2(SO4)3 for coagulation, anionic and cationic polymers, sodium bisulfide for chromium reduction, lime, and CaCl2. Thus, besides alkaline- or alkaline-earth-element-containing compounds and iron, the waste oil sludge also contains 0.4% Cu, 2% Zn, and between 200 and 1000 ppm of chromium, lead, and nickel. In metal sludge, there are less than 1% each of cadmium, chromium, copper, lead, manganese, selenium, vanadium, and zinc. The contents of catalytically important metals for H2S oxidation in adsorbents obtained by pyrolysis at 950 °C are listed in Table 3. They are arranged in various chemical configurations,14-16 depending upon the temperature of heat treatment.35 It was found that an increase in the pyrolysis temperature results in more well-defined crystalline phases with more metals in the reduced states, less oxygen, and more pure metallic phases, such as iron or copper.34,42 The changes in the chemistry of materials can also be observed on DTG curves for the initial sludges and their mixtures (Figure 5). Although, even at the lowest pyrolysis temperature, all volatile organic compounds are already removed from the samples, some changes still occur at higher temperatures. They are related to the decomposition of an inorganic phase (salts/oxides/hydroxides) and solid-state reactions. In the case of waste oil sludge, a broad complex peak with a shoulder at about 450 °C is found. The curve for MS in (initial metal sludge) looks complex, and pyrolyzing between 500 and 950 °C should lead to materials with different chemistries. The intensity of the peaks is much smaller that those for waste oil sludge because of the small content of an organic matter.14,15 Nevertheless, the weigh loss is seen in a whole experimental window as a result of the decomposition of inorganic salts. When the materials are mixed, a new feature found is an increase in the weight loss between 100 and 200 °C. Waste oil sludge when heated over 800 °C reveals a significant weight loss, which may have an effect on the surface chemistry of this adsorbent. (42) Seredych, M.; Bandosz, T. J. Chem. Eng. J., in press.



Figure 5. DTG curves for the initial sludges and their mixture.

On the materials obtained at 500 and 650 °C upon contact with water, hydroxides and hydrated salts are formed.43 They increase the pH of the surface, which enhances the dissociation of hydrogen sulfide.34,41 Formed HS- ions are further oxidized by iron-based catalysts to elemental sulfur. The following pathways are possible:

CaO + H2O f Ca(OH)2

(1)

Ca(OH)2 + 2H2S f Ca(HS)2 + 2H2O

(2)

Figure 6. DTG curves for the MS series of materials. MS950 is not presented because adsorption of H2S was negligible. Thin lines, initial materials; thick lines, ED; thick dashed lines, EP.

still exist without engagement into the new crystalline phases. It is also probable that in the slightly acidic environment, which may exist in the pore system (the pH represents the average acidity, and it is measured in water, after the formation of hydroxides) chromium(VI) can be one of the oxidants contributing to the formation of SO2.

4H+ + 3S + 2Cr2O7-2 f 3SO2 + 2Cr2O3 + 2H2O (9)

with chemisorbed oxygen present on the surface:

CaO + H2S f CaS + H2O

(7)

CaS + Fe2O3 f 2FeO + S + CaO

(8)

The presence of CO2 in the challenge gas and lack of oxygen negatively affect the performance of adsorbents in comparison with the adsorption of H2S from air in the presence of water on waste-oil-sludge- and metal-sludge-derived adsorbents. For all adsorbents obtained at 650 and 950 °C, except for MS650, at least twice smaller capacity was measured than those reported previously for air desulfurization.14,15 This general trend is likely caused by deactivation of basic centers by carbonic acid, in the case of prehumidified samples or CO2 in the case of dry samples. Those reactions, especially involving the formation of carbonates from dissolved H2CO3 are competitive to those forming sulfides, sulfites, and sulfates. Probably the most important negative effect can be linked to the engagement of magnesium and calcium oxides in the carbonate entities, which, besides lowering surface pH and thus the number of HS- ions formed, limits the extent of reactions 2 and 7. Nevertheless, the performance of our adsorbents in desulfurization of DG is better than that of catalytically activated carbons, such as Midas or DarcoH2S, for which 73 and 39 mg/g of H2S adsorbed, respectively, was reported.32 It is interesting that water does not affect the performance of MSWO500 and MSWO650. Because mixing two sludges results in the adsorbents, whose capacity is almost twice smaller than those of the individual components, the interactions between the chemical components of the sludges lead to the formation of surface chemistry, which is relatively stable at low temperatures and whose pH apparently is not affected by water. This happens despite the fact that some amount of water is adsorbed in these systems. That water is likely engaged in chemical formulas without the formation of any film on the surface.

This may explain why the high capacity is found on the materials pyrolyzed at 500 °C, where alkaline or alkali metal oxides can

(43) Seredych, M.; Bandosz, T. J. J. Colloid Interface Sci. 2006, 302, 379-388.

Ca(HS)2 + O2 f 2S + Ca(OH)2

(3)

or with the involvement of redox reactions:

Ca(HS)2 + 2Fe2O3 f CaO + 2S + 4FeO + H2O (4) FeO(OH) + HS- f FeO + S + H2O

(5)

Fe2O3 + 2HS- f 2FeO + H2O + 2S

(6)

Besides the formation of sulfur, some hydrogen sulfide is involved in the formation of sulfides, either alkali-, alkalineearth-, or heavy-metal-based. Moreover, oxidation proceeds further, and a part of hydrogen sulfide is oxidized to SO2 by various components of sludges, such as, for instance, chromium oxides, or by a small amount of chemisorbed oxygen. That SO2 in the presence of water and metals forms sulfites or even sulfates on the surface if SO3 is able to be formed. They are responsible for a decrease in the pH of 1 of 2 units as reported in Tables 1 and 2. When water is not present on the adsorbent surface, hydrogen sulfide cannot dissociate, despite the basicity of the surface. Thus, hydrogen sulfide retention on the surface occurs mainly via the formation of sulfides, which are further oxidized to sulfur or sulfur dioxide by iron compounds.


Energy & Fuels, Vol. 21, No. 2, 2007 863 Table 4. Parameters of the Porous Structure Calculated from Adsorption on Nitrogen

Figure 7. DTG curves for the WO series of materials. Thin lines, initial materials; thick lines, ED; thick dashed lines, EP.

Figure 8. DTG curves for the MSWO series of materials. Thin lines, initial materials; thick lines, ED; thick dashed lines, EP.

The presence of products of hydrogen sulfide oxidation on the surface can be seen on DTG curves presented in Figures 6-8. For MS samples, on which a significant adsorption of H2S was measured, the predominant feature is a peak between 200 and 350 °C (Figure 6). It represents the removal of elemental sulfur.25 Exposure to water results in an increase in the peak intensity because more sulfur is deposited on the surface. It is interesting that the samples do not exhibit any well-defined water/SO2 peak between 30 and 200 °C, despite the fact that some quantity of water was adsorbed or formed on the surface as a result of oxidation. That water is likely engaged in the chemical formulas or desorbs as a small shoulder seen for the MS500 sample. As seen from Figure 4, water there plays a different role than in the case of other samples. For this sample, a new peak between 400 and 550 °C is present after prehumidification and a peak at 900 °C disappears. These two features are likely related to the mechanism of adsorption. The first peak represents decomposition of iron sulfate,44 and the disappearance of the last peak can be explained by the engagement of calcium oxide in rather sulfur-containing salts (44) Handbook of Chemistry and Physics, 67th ed.; Weast, R. C., Ed.; CRC Press: Boca Roton, FL, 1986.

sample

SBET (m2/g)

Vt (cm3/g)

Vmes (cm3/g)

Vmic (cm3/g)

Vmic/Vt

WO500 WO500-ED WO500-EP WO650 WO650-ED WO650-EP WO950 WO950-ED WO950-EP MS500 MS500-ED MS500-EP MS650 MS650-ED MS650-EP MSWO500 MSWO500-ED MSWO500-EP MSWO650 MSWO650-ED MSWO650-EP MSWO950 MSWO950-ED MSWO950-EP

166 119 39 187 192 116 166 149 83 90 56 42 38 24 22 112 72 59 88 54 48 40 38 17

0.775 0.626 0.385 0.791 0.786 0.562 0.417 0.418 0.355 0.337 0.301 0.277 0.168 0.150 0.142 0.407 0.335 0.328 0.305 0.272 0.264 0.129 0.145 0.109

0.719 0.580 0.365 0.719 0.714 0.517 0.353 0.360 0.322 0.304 0.279 0.258 0.152 0.139 0.132 0.366 0.307 0.302 0.269 0.250 0.244 0.112 0.129 0.099

0.056 0.046 0.020 0.072 0.072 0.045 0.064 0.058 0.033 0.033 0.022 0.019 0.016 0.011 0.010 0.041 0.028 0.026 0.036 0.022 0.020 0.017 0.016 0.010

0.072 0.074 0.052 0.091 0.092 0.080 0.154 0.139 0.093 0.098 0.073 0.069 0.095 0.073 0.070 0.101 0.084 0.079 0.118 0.081 0.076 0.132 0.110 0.092

than carbonates. The peaks between 800 and 950 °C likely represent the decomposition of alkaline earth metal carbonates, which must be formed in larger quantity when no water is present and catalytic centers are deactivated by a direct reaction with CO2. It is interesting that the intensity of the sulfur peak is greater for MS650 than for MS500, despite the fact that more hydrogen sulfide is adsorbed on the latter sample. The only plausible explanation of this is the above-mentioned formation of transition-metal sulfides in the reaction of hydrogen sulfide with metal oxides. Those sulfides decompose at temperatures higher than 1000 °C.44 The DTG curves for waste-oil-sludge-based materials look more complex (Figure 7). Although significant changes are seen for the WO500 and WO950 run in dry conditions owing to their low adsorption, new peaks at 120 °C and between 200 and 300 °C appear. The latter, as mentioned above, represents the removal of elemental sulfur, and the former, because water was not supplied to the system, is related to SO2 formed in the oxidation process. That SO2 was emitted from this sample during the breakthrough capacity test on a relatively high level (Figure 2). A new well-fined peak at 800 °C likely represents the decomposition of carbonates.44 Those carbonates and sulfates (a peak at about 600 °C) also appear for WO650-ED. When these samples are exposed to water, the weight loss represented by new peaks is related to their performance in the desulfurization tests. While, in WO950, because of the chemical stability of this sample, predominantly sulfur is detected with a small peak from water/SO2, the complexity of the DTG curve pattern increases with an increase in sample reactivity (a decrease in the pyrolysis temperature). Thus, for WO650-EP, three welldefined peaks are found at 120, 300, and 600 °C, with a shoulder between 350 and 500 °C. They are assigned to the removal of water/sulfur from the decomposition of sulfites, elemental sulfur, and transition-metal sulfates, respectively. A shoulder close to 400 °C can be linked to the dehydroxylation of hydroxide formed when the surface was exposed to water.42 The most complex pattern was found for WO500-EP for which the most intense low-temperature peak indicates a significant quantity of sulfur bound in the forms of thermally unstable sulfites. Then, the multicomponent peak appears, whose first maximum



Figure 9. Pore-size distributions for the MS series of materials. MS950 is not presented because adsorption of H2S was negligible. Thin lines, initial materials; thick lines, ED; thick dashed lines, EP.

represents elemental sulfur and whose second maximum, wellpronounced at 350 °C, must be related to the decomposition of hydroxides.44 The peaks at 600 °C from the decomposition of sulfates also exist. It is interesting that after prehumidification the decomposition of carbonates is not noticed. This indicates that when water is present H2S wins the competition for the adsorption/catalytic sites of the surface. Mixing sludges changes the nature and distribution of surface oxidation products (Figure 8). On the basis of the intensity of the first peaks and narrow, well-defined second peaks representing sulfur, the composite samples resemble metal-sludge-based samples in their mechanism of hydrogen sulfide removal from DG. Once again, for the samples run in dry conditions, the intensity of the sulfur peaks does not represent their capacity, which suggests the formation of sulfides in the case of the MSWO500-DG sample. The decomposition of carbonates at temperatures higher than 700 °C is also seen for this sample. For MSWO650-ED, the decomposition of sulfates is seen as a peak at about 450 °C. Adding water to the system results in the formation of more elemental sulfur in the case of the MSWO500EP sample, with no increase in the amount of released water/ SO2. That water must have been involved in the formation of sulfates because a new peak at about 480 °C appears. On the other hand, sulfates and carbonates are formed in large quantity on the MWO650-EP sample, for which much less sulfur is found in comparison with the run in the dry conditions. This is consistent with a noticeable pH decrease for this sample. Such a large quantity of sulfur adsorbed in the system has to be stored in the pores of materials. The structural parameters

Figure 10. Pore-size distributions for the WO series of materials. Thin lines, initial materials; thick lines, ED; thick dashed lines, EP.

calculated from nitrogen adsorption isotherms are collected in Table 4. Besides reporting the parameters that are most commonly used to characterize adsorbents, such as the BET surface area and volume of micropores obtained from the DR method, the pore-size distributions calculated using the DFT approach are also discussed. Although the model of pores used for calculation does not exactly represent our materials from the point of view of shape or chemistry (slit-shape pores in carbons), we believe that this approach can be used to obtain comparative information for a series of samples that have the same origin. Generally speaking, the materials studied have low surface areas and small volumes of micropores. They can be considered as mesoporous media because, in the majority of cases, the degree of microporosity represented by the ratio of Vmic/Vt is


Energy & Fuels, Vol. 21, No. 2, 2007 865

Figure 12. Dependence of the breakthrough capacity on the total pore volume of adsorbents. The line represents the linear trend.

Figure 11. Pore-size distributions for the MSWO series of materials. Thin lines, initial materials; thick lines, ED; thick dashed lines, EP.

about 10%. An increase in the heating temperature generally results in a decrease in the surface area and volume of mesopores. That decrease is especially seen for the metal-sludgederived adsorbent, which totally losses its porosity after heating at 950 °C (because the material is nonporous, the structural parameters are not listed in Table 4). Those changes are caused by the decomposition of inorganic compounds, reduction of metals by a small amount of carbon, and sintering of the structure. It is interesting that also from the point of view of porosity the composite metal-sludge/waste-oil-sludge-derived adsorbents resemble more the MS series of samples than the WO series. This strong effect of metal sludge was also observed when the surface oxidation products were analyzed (Figure 8). The differences in pore-size distributions and changes in them after H2S adsorption are presented in Figures 9-11. In the case

of MS500, for both samples of dry or wet adsorbents, the volume of pores with sizes between 20 and 100 Å is affected by sulfur/sulfide deposition. When the experiment is run with the prehumidification, more sulfur is deposited in small pores, smaller than 70 Å, where water can form a film. Otherwise, the surface seems to be chemically active to the same degree. In MS650, sulfur is deposited only in micropores and large pores with sizes of about 300 Å. In fact, the total volume of mesopores for this sample increased after the hydrogen sulfide adsorption process, which suggests the formation of a secondary pore volume within the sulfur/sulfur-containing species. Apparently, the mechanisms of surface oxidation are different on these two samples as also seen from the analysis of DTG curves. Exposing WO500 to DG results in changes in porosity similar to those observed for MS500; the volume of pores smaller than 100 Å in their sizes decreases, and the extent of that decrease is related to the amount of hydrogen sulfide deposited on the surface. Similar changes are found for the composite samples; however, the decrease in the volume of pores is much less pronounced, owing to the smaller adsorption of H2S. For MSW950, although only about 2% of hydrogen sulfide is retained on the surface, resulting sulfur must block the pore entrances because, for the prehumidified sample, significant changes in the pore-size distribution are noticed. The importance of the pore volume for hydrogen sulfide adsorption is shown in Figure 12, where the dependence of the H2S breakthrough capacity is plotted versus the pore volume. The majority of samples, either run in dry or wet conditions, follow the linear trend. In fact, the full dependence of the capacity on the pore volume is not expected here because the mechanism of reactive adsorption changes with the chemical composition of the sample, which is related to the pyrolysis temperature. The points that do not obey the linear relationship are those for MS samples prehumidified before the breakthrough, whose capacities are higher than expected because of the enhanced reactivity of the surface. Too small of an adsorption with regard to the pore volume is found for the WO samples, WO650 and WO950, run in dry conditions, whose active centers are likely deactivated by carbon dioxide. Conclusions The results presented in this paper demonstrate the high capacity of the industrial-sludge-derived adsorbents for de-


sulfurizaton of DG. The capacities measured are 3-4 times higher than those obtained on commercial catalytically activated carbons. The performance increases when adsorbents are prehumidified before the exposure to dry DG. Water activates the surface basic groups, which, besides providing basic pH for H2S dissociation, become strong centers for H2S chemisorption. Sulfur is the main product of the surface reaction; however, depending upon the chemistry of the sample, sulfates, sulfites, and sulfides are also formed. When the materials are not prehumidified, CO2 quickly deactivates the basic catalytic


centers. Mixing sludges does not have a positive effect on the performance of materials. Both the distribution of surface reaction products and the porosity become dominated by the metal sludge components, and the activity of the surface of such adsorbents is not enhanced by the presence of water. Acknowledgment. This work was supported by NYC DEP (58518-007) and PSC CUNY Grant number 67284-0036. EF060482L

Desulfurization of Digester Gas on Industrial-Sludge-Derived

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