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Desulfurization of Digester Gas on Catalytic Carbonaceous Adsorbents: Complexity of Interactions between the Surface and Components of the Gaseous Mixture 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
Five carbonaceous materials exhibiting catalytic activity for hydrogen sulfide oxidation from moist air were used as H2S removal media from digester gas. The breakthrough capacity was measured at dynamic conditions with various amounts of water present in the system (either on carbon or in the challenging digester gas). The initial and exhausted materials after the breakthrough tests were characterized using sorption of nitrogen, thermal analysis, XRF, and surface pH measurements. The results obtained demonstrate the complex dependence of the capacity on surface chemistry, porosity (volume and sizes), and water content. In all cases, elemental sulfur is the predominant product of surface reactions. In the case of materials with potassium present in the ash, preadsorbed water enhances the adsorption capacity, likely contributing to dissociation of hydrogen sulfide, which is further oxidized to sulfur by oxygen chemisorbed on the carbon. When the inorganic phase contains alkaline earth oxides nonreactive with water, the moisture on carbons does not affect the removal process. On the other hand, water in the gas mixture impedes the adsorption capacity via deactivation of catalytic centers, which react with carbonic or sulfurous acids. When reactive adsorption takes place, for the efficient oxidation of hydrogen sulfide, the volume of small size micropores becomes very crucial. They act as microreactors where chemisorbed oxygen has a higher probability to be retained and elemental sulfur has a higher probability to be stored. Introduction Activated carbons are well-known as adsorbents of gases and vapors. Their high surface area provides space to store gases, and small pores help in their separation. While these two surface features are crucial for physical adsorption, in the case of adsorption which is enhanced by surface chemical reaction, reactive adsorption, the situation is much more complex. When adsorption/separation occurs at supercritical conditions and it is absolutely crucial to retain a pollutant on the surface, reactive adsorption has to be “provoked”. Examples are toxic gases which are removed on military filters, wehlerites, via complex reactive adsorption1 To impose a reaction, bases, acids, complexing agents, oxidizing/reducing agents, and metals are introduced to the surface via various techniques.1 The most common and the most efficient one is impregnation.2-4 Other techniques include introducing the catalytic phase as a separate component and then blending the materials.5 The reaction can be also imposed by the presence of active binder.6 In all these techniques, an important feature to ensure 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 on the size of the reagent molecule. Besides chemistry introduced to carbons with inorganic catalysts, the carbon surface also exhibits catalytic properties owing to the presence of ash or surface functional groups.7 The former can consist of transition metals such as iron, nickel, copper, zinc, or cobalt. On the other hand, the origin of the functional groups is in heteroatoms present in a carbon precursor. They can be also formed as a result of carbon surface * To whom correspondence should be addressed. Tel.: (212) 6506017. Fax: (212) 650-6107. E-mail:
[email protected].
oxidation.7 Basicity of functional groups, especially those nitrogen-containing ones, was indicated as a catalyst for an oxidation reaction via activation of oxygen.8 The above-mentioned methods for an enhancement of the catalytic properties are used to prepare adsorbents targeting removal of specific gases. An example is hydrogen sulfide. Since this malodorous and poisonous gas is a product of anaerobic digestion, it is present always in sewage treatment facilities, digester gas, or even in natural gas or other fossil-based fuels. Its removal from natural gas was studied at dynamic conditions by Tollefson and co-workers.9-15 They found that, with an increasing temperature, the rate of H2S conversion to elemental sulfur, which is the most common reaction product, increases. The undesired side effect is an increase in the rate of SO2 production when the temperature reaches 445 K. This limits the application temperature because of secondary air pollution problems. An increase in the content of oxygen added to the gas stream significantly increases the conversion of H2S, as expected based on the mechanism of the reaction. The flow rate and size of the bed also affect the conversion rate. Besides the experimental conditions for fuel gas desulfurization, the content of gas in terms of hydrocarbon specification also affects H2S removal.14 The steady-state conversion level is lower when heavier hydrocarbons are present in the challenge gas. They condense and are adsorbed in the pore system, limiting the pore space for sulfur deposition.16-18 Effects of experimental conditions on the desulfurization of digester gas with no heavier hydrocarbons present were studied by Bandosz and co-workers.19 They found that a decrease in the concentration of hydrogen sulfide increases the breakthrough capacity because of the slow kinetics of the process. No significant changes were observed with changing the oxygen content from 1% to 2% and the temperature from 38 to 60 °C.
10.1021/ie051388f CCC: $33.50 © 2006 American Chemical Society Published on Web 04/04/2006
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Besides the conditions of the process, the choice of an adsorbent seems to also be a very important task. Carbon manufacturers offer the catalytic carbons targeted toward hydrogen sulfide removal. It was found that the excellent H2S removal capacity (from air) of an expensive desulfurization catalyst, U.S. Filter’s carbon Midas,20 is linked to the presence of calcium and magnesium oxides dispersed within the microporous activated carbon. On this catalyst, hydrogen sulfide is oxidized on basic centers of alkali earth metal oxides and sulfur is formed. The capacity reaches 60 wt %. A similar mechanism was found on sewage-sludge-based adsorbents.20 On the other hand, catalytic carbon Centaur oxidizes H2S to sulfuric acid in the presence of air and moisture.21 Among virgin activated carbons, coconut-shell-based materials were found as the most suitable for desulfurization.21-22 Their performance is governed by the presence of potassium. It was also found that the existence of water film on the carbon surface is crucial for removal of hydrogen sulfide from air.22-25 The objective of this paper it to demonstrate the complexity of digester gas desulfurization on catalytic carbonaceous adsorbents. For this purpose, materials providing catalytically active basicity were chosen. Since digester gas also contains carbon dioxide, the effect on catalyst deactivation by the formation of carbonate should be investigated. The performance of the adsorbents is demonstrated and analyzed in conjunction with their surface features, which include the surface chemistry, porosity, and type and dispersion of the catalytic phase. The effect of water is also analyzed, since it was shown to be an important factor for the mechanism of H2S oxidation to elemental sulfur. Experimental Section Materials. Five carbonaceous adsorbents were chosen for this study. Two of them are catalytic commercial carbons, Midas OCM (U.S. Filters) and Darco H2S (Norit). Other adsorbents include a coconut-shell-based activated carbon, S208C supplied by Waterlink Barnabey and Sutcliffe; homemade sewage-sludgebased adsorbent, SC950; and activated carbon, CAS-26. The latter was obtained in the Moldavian Academy of Sciences from grape seeds via physical activation. The exact conditions of its technology are not revealed. Methods. (a) Evaluation of H2S Sorption Capacity from Digester Gas (DG). A custom-designed dynamic test was used to evaluate the performance of adsorbents for H2S adsorption from gas streams as described elsewhere.22,26,27 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 for dry DG and 3 cm3 for prehumidified carbon and dry DG and carbon with moist gas), and used as received or prehumidified with moist air (relative humidity 70% at 25 °C) for 2 h. The amount of water adsorbed was estimated from an increase in the sample weight. Simulated digester gas mixture (DG) (40% CO2, 60% CH4 at 25 °C) containing 0.5% (5000 ppm) of H2S (dry or with 70% moisture) was passed through the column of adsorbent with a flow rate between 0.060 and 0.5 L/min. The flow rate was controlled using Cole Palmer flow meters. The breakthrough of H2S was monitored using electrochemical sensors. The test was stopped at the breakthrough concentration of 350 ppm. The adsorption capacities of each adsorbent in terms of mg of hydrogen sulfide per a 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 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 a MicroMax (Max 8) electrochemical sensor. 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 digester gas, the samples are referred to as ED; after prehumidification, EPC; and after exposure to moist digester gas without prehumidification, EMG. (b) 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 minimized vaporization of elemental sulfur and weakly bonded sulfuric acid) to 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 relative pressure ) 0.99), volumes of micropores, Vmic (DR28), mesopore volume Vmes, along with pore-size distributions were calculated (DFT29,30). (c) 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 carbon 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. (d) Thermal Analysis. Thermal analysis was carried out using a TA Instrument thermal analyzer (SDT 2860). The instrument settings were a heating rate of 10 °C/min and a nitrogen atmosphere with 100 mL/min flow rate. For each measurement, ∼25 mg of a ground adsorbent sample was used. (e) XRF. X-ray fluorescence (XRF) analysis was applied to study the calcium and iron content in the samples. The SPECTRO model 300T benchtop analyzer from ASOMA Instruments, Inc., was used. The instrument has a titanium target X-ray tube and a high-resolution detector. The samples were studied in a solid phase after grounding and sieving in order to use the matrixes with similar physical properties. Results and Discussion Breakthrough curves for the adsorbents studied and SO2 emission curves are presented in Figures 1 and 2, respectively. It is clearly seen that the different experimental conditions result in the different performance of the materials. Although on either the dry or wet carbons with dry DG or moist DG the order of exhaustion is similar and carbon S208 lasts the longest time, the lasting times are different (longest after prehumidification of carbons) and the wet surface has various influences on the breakthrough time (Figure 3). Since for desulfurization for fuel cell applications a complete removal of sulfur-containing compounds is important, we also monitored the concentration of SO2, assuming that it can be the product of a surface reaction.26,27 From the point of view of SO2 emission in completely dry conditions, catalytic carbon Midas and sludgederived adsorbent, SC950, are the best materials. Less then 2 ppm of SO2 is emitted, and the emissions start at the same time when H2S breakthrough occurs. On S208 C and CAS-26, the emission of SO2 is detected much before H2S appears in the outlet gas. This indicates that the surface of these materials has strong oxidizing properties. When the carbons were exposed to moisture, no SO2 emissions were detected. This suggests that SO2, if formed, is converted either to sulfuric acid and or/and metal sulfates in the presence of water.31,32 When moist gas was used, the small emissions of SO2 were detected long before H2S appeared in the exhaust gas. This indicates that reaction in which sulfur dioxide is formed is favorable in this system.
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Figure 2. SO2 emission curves for the breakthrough capacity runs with dry (A) and moist (B) challenging gas.
Figure 1. H2S breakthrough curves for carbon exposed to dry digester gas (A), dry digester gas and prehumidified carbon (B), and moist digester gas (C).
Breakthrough capacity values calculated from the breakthrough curves along with the amounts of water adsorbed and the pH values of adsorbent surfaces are collected in Table 1. The values are in the range of those reported for commercial catalytic carbons. Both CAS-26 and S208 have a high capacity per unit mass, and between 12 and 15% of hydrogen sulfide can be adsorbed on their surface. It is interesting to note that the H2S removal capacity of Darco H2S carbon at dry conditions is very small. A significant improvement is noticed when this carbon is exposed to air. On the other hand, water adsorbed on the surface does not affect the performance of Midas and SC950. It is important to mention that, at these conditions, the H2S removal capacity of Midas is only 3-4× greater than that for SC950, whereas when the experiments are performed in air, the capacity of Midas is about 10× greater than that of SC950.20 Both Midas and SC950 adsorb similar quantities of water, which once again indicates the similarity of their surface chemistries.20 The huge amount of water adsorbed on S208 (12%) must be related to its microporosity and/or the reactivity of its ash. It is
Figure 3. Comparison of the H2S breakthrough capacities obtained at various conditions.
known that S208 contains potassium, so exposure to water may result in the formation of potassium hydroxide. The low capacities obtained when moist gas was in contact with the carbon adsorbents must be related to the different mechanisms of reactive adsorption and interactions between components of the mixture. This aspect is addressed later in the discussion of the results. The pH values of all our initial materials are very basic. They represent the average acidity/basicity of the materials’ surfaces. Exposure to dry digester gas decreases the pH values of CAS-
Ind. Eng. Chem. Res., Vol. 45, No. 10, 2006 3661 Table 1. Breakthrough Capacity, Amount of Water Adsorbed and the pH Values of the Carbon Surfaces sample
H2S B. cap (mg/g)
Darco Midas CAS-26 S208C SC950
9 84 100 106 23
Darco Midas CAS-26 S208C SC950
39 73 152 122 24
Darco Midas CAS-26 S208C SC950
7 24 31 27 17
H2S B. cap (mg/cm3)
water ads. (mg/g)
Dry DG 4 29 49 53 13 Prehumidified Carbons 17 37 27 19 47 64 61 121 14 17 Moist DG 3 10 9 14 10
pH
pHE
11.7 10.6 10.1 10.2 9.6
11.2 10.2 8.0 8.3 9.1
11.7 10.6 10.1 10.2 9.6
10.4 10.2 7.6 3.7 9.1
11.7 10.6 10.1 10.2 9.6
11.3 10.4 10.4 10.1 9.5
26 and S208 C ∼2 pH units, which suggests formation of a small amount of sulfuric acid.26,27 When the carbons were prehumidified, more drastic change occurred, especially for S208, whose pH became very acidic. Formation of that sulfuric acid may be related to a high water content. The product of H2S oxidation in small pores, SO2, can be further oxidized on the S208C surface to SO3, and after its reaction with water, H2SO4 is formed. The strong tendency for sulfuric acid formation on S208C carbon was also observed when H2S was removed from air.26 It was linked to the presence of small pores and, thus, high dispersion of sulfur radicals. Generally, for all carbons studied after contact with moist gas, the pH values remain practically unchanged, which may be the result of either the low capacity or formation of salts and/or elemental sulfur. Thermal analysis in nitrogen provides information about the quality and quantity of sulfur species deposited on the surface. Combined with pH measurements, this approach was proven to be very useful to balance the surface reaction products.21,26,27 DTG curves, on which the peaks represent the weight losses at various temperature ranges, are collected in Figures 4-6. For all adsorbents exposed to H2S adsorption, a new well-defined peak between 200 and 450 °C appears, and it is the main fingerprint of the presence of sulfur.26,27 The sulfuric acid peak (SO2 emission from decomposition of H2SO4) is represented by a shoulder centered at ∼250 °C.21 As expected, this shoulder is well-defined for the S208C and CAS-26 samples, especially after prehumidification (Figure 4). In the case of CAS-26 carbon after H2S adsorption, a decrease in the intensity of the peak located between 550 and 800 °C is noticed. This peak was also found for the initial mate, and it can be the result of gasification, if such metals as potassium or calcium are present. Indeed the initial pH of this carbon is high (Table 1), and ash content reaches 10%. After H2S adsorption, these species can be engaged in the reaction and new components decompose as a broad peak between 600 and 800 °C common for both CAS-26 and S208C carbons. The first peak centered at ∼120 °C represents the removal of physically adsorbed water and weakly adsorbed SO2 as either sulfur dioxide or sulfurous acid.33 A similar trend in the high-temperature DTG peaks to those observed for CAS-26 is also found for Midas (Figure 5), which suggests some degree of similarity in surface chemistry of these two adsorbents. For catalytic carbons tailored for H2S removal (Darco H2S and Midas), the sulfur peaks are almost at the same positions, although their intensities differ. When Darco H2S was
Figure 4. DTG curves for CAS-26 and S208C carbons.
Figure 5. DTG curves for Darco H2S and Midas carbons.
exposed to prehumidification and H2S adsorption, besides the appearance of the sulfur peak, the peak at ∼700 °C increased its intensity and a huge peak observed for the initial sample at a temperature >800 °C disappeared. The reason for the latter,
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Figure 6. DTG curves for SC950.
as indicated above, is due to the change in the chemistry of an inorganic phase, which does not further promote the gasification of carbon. It is likely that new inorganic salts containing sulfur, which decompose at ∼700 °C, are formed. For the SC950 run at two conditions (D and PC), the DTG curves look almost identical with an elemental sulfur peak between 200 and 400 °C (Figure 6). The shift of this peak to a lower temperature as compared to that of activated carbon34 is owing to the presence of larger pores in the sludge-derived material than those in carbons. Also for this peak, an offset of a huge peak at the end of the experimental window is noticed. This peak is likely related to the presence of sulfides.34 The common feature of the adsorbents exposed to moist digester gas is a lack of the elemental sulfur peak. An intense peak at ∼100 °C, despite a shorter total exposure to water than prehumidified carbons, suggests that either water is not engaged in the formations of sulfuric acid (no peak at ∼250 °C and unchanged pH) or weakly adsorbed SO2 is removed from the surface. The formation of the latter seems to be more plausible, since some capacity, however low, was measured. TheSO2 can have its origin in the formation of sulfurous acid and/or thermally unstable sulfites, such as potassium or magnesium sulfites. In the presence of water and some oxygen chemisorbed on the surface, hydrogen sulfide is oxidized at the first contact with carbon to sulfur dioxide and then sulfurous acid is formed. The acid reacts with alkali or alkaline earth metal oxides, forming sulfites. These sulfites do not contribute to the changes in the surface pH, and they decompose upon heat treatment at low temperature. Although carbonic acid is also formed in a gaseous mixture, the competition for the inorganic reaction centers is won by H2SO3, which is a stronger acid than H2CO3. The role of water is different when the carbon is exposed to moist digester gas compared to the situation when the carbon is prehumidifed and, thus, the water film is formed on the surface. That film is present in the small pores, and it enables solubility and dissociation of hydrogen sulfide. In the case of moist digester gas, before the film of water is formed and basic pH enabling dissociation of H2S is established by reactions of inorganic oxides with water, the catalytic centers providing this basic pH are probably already consumed in the reaction with sulfurous acid. This limits significantly the capacity. Another factor contributing to the limited capacity of the adsorbents in the presence of moist digester gas is formation of carbonic acid and its reaction with catalytic basic sites, leading to the formation of carbonates. However, this CO2 also exists in DG passing through the prehumidified carbon; it is not so reactive as carbonic acid. Its inhibition action is limited, since it needs
Figure 7. XRF spectra for carbons before and after H2S adsorption in dry run.
to be dissolved in water as a first step of the catalyst deactivation reaction. It is interesting that the above-mentioned processes do not deactivate the SC950 adsorbent (Figure 3). This is because the catalytically active phase of this material contains mineral-like structures based on iron and calcium, which do not participate in the neutralization reactions.35 Although methane present in the challenge gas is not expected to adsorb in noticeable quantity at our experimental conditions, it can compete with water for adsorption in very small pores if their surface is built entirely of carbonaceous phase. This may slow the kinetics of H2S adsorption and impede the oxidation process. The chemistries of the adsorbents chosen for this study were evaluated using XRF. With the knowledge the mechanism of the catalytic oxidation of hydrogen sulfide, such metals as potassium, calcium, and iron were targeted. The results for the initial carbons and those exposed to dry digester gas without the prehumidification (when the H2S adsorption was the highest) are presented in Figure 7. As expected in Midas,20 calcium and iron are present, whereas S208 has a prominent amount of potassium. Potassium is also detected in CAS-26, whereas Darco H2S and SC 950 have notable amounts of calcium and iron.34 After H2S adsorption, the amount of sulfur increased, especially for carbons with the high breakthrough capacities. A decrease in the intensity in the potassium peak for S208C-ED is only apparent because its relative amount decreased due to an increase in the content of sulfur. The balance of the sulfur content based on the weight lost in TA experiments for the samples exposed to dry digester gas is presented in Table 2. For carbons where a straightforward reaction of oxidation to elemental sulfur occurs, the agreement with the amount calculated from H2S breakthrough experiments is very good based on the sulfur content in hydrogen sulfide molecule). Small discrepancies,
