Removal of hydrogen sulfide from biogas on sludge-derived adsorbents
Introduction
Search for new sources of energy suggests digester gas, or biogas, as a medium to provide hydrogen for fuel cell operations [1]. Since it contains a significant amount of sulfur, mainly as H2S, the gas has to be desulfurized to the ppb levels before any further application is considered. The reason for this process is in a poisoning action of sulfur compounds on the catalytic conversion catalysts [2]. Such catalysts are expensive since they are based on noble metals.
Desulfurization of gaseous fuel can be done on various adsorbents depending on the temperature of the feed gas. In the case of a hot gas, inorganic adsorbents such as zinc oxide or new cerium-based materials were shown to be very efficient [3], [4]. When the process occurs at room temperature the catalytic reactions are less feasible and the combined factors of the porosity of adsorbents and their surface chemistry start to play an important role [5], [6].
One group of porous adsorbents, which are often used for desulfurization at room temperature, are activated carbons [6]. They have high surface area and developed porosity where small molecules of hydrogen sulfide or methyl mercaptan can be physically adsorbed [7]. Moreover, the carbon surface has catalytic properties owing to the presence of functional groups and free valences at the edges of graphene sheet [8], [9]. They take part in the oxidation of sulfur containing light gases to elemental sulfur or sulfuric acid [5], [6]. The latter is formed when water is present in the system [5], [6], [10], [11], [12]. Unfortunately, due to the weak catalytic nature of activated carbon centers, only a relatively small amount of hydrogen sulfide can be retained on virgin, unmodified carbon [5], [6].
To increase the catalytic performance of activated carbons, various surface treatments are applied as oxidation [13], incorporation of nitrogen containing species [14], [15], or impregnations with oxidants [16], caustics [17], or metals [18], [19]. This dramatically improves the performance but also increases the cost of the catalysts. Nevertheless, due to often occurring blocking of small pore entrances, the activated carbon surface is almost never used to its full extent [6]. Another negative aspect of activated carbon application is the formation of sulfuric acid as an oxidation product. Its presence, although may lead to an easy and efficient regeneration [20], converts the spent adsorbents into hazardous wastes.
All of these directed the attention of the researchers toward municipal and industrials sludges as sources of desulfurization adsorbents [21], [22], [23], [24], [25]. Although their surface is much smaller than that of activated carbons, they provide specific surface chemistry active in the oxidation of hydrogen sulfide to elemental sulfur. Lack of microporosity is not a significant draw back in their applications since a high volume of mesopores provides sufficient space for the deposition of sulfur without eliminating the active surface area [24], [25]. So far sewage sludge and industrial sludges from heavy industry have been used as adsorbent precursors for desulfurization of digester gas [26]. The capacity obtained was much higher than that on expensive catalytic activated carbons such as Midas® or DarcoH2S® [27]. In the case of the latter materials their catalytic centers based on oxides of calcium and magnesium were deactivated by carbonic acid or carbon dioxide in the early stage of the desulfurization process.
The objective of this paper is to evaluate the applicability of metal sludge from the galvanizing industry in conjunction with sewage sludge as sources of adsorbents for desulfurization of biogas. Such factors as the pyrolysis temperature, adsorbent composition and the degree of adsorbent humidification during desulfurization are analyzed. The possible synergy, chemical and physical, between the sludge components is discussed and its effects on the performance in the process of desulfurization emphasized.
Section snippets
Materials
The adsorbents were prepared from dewatered NYC sewage sludge (S), metal sludge from General Galvanizing, Bronx NY (T) and their homogenized mixtures. The mixtures consisted of 50:50, 70:30 and 90:10 weight percent ratio of sewage sludge to metal sludge. After homogenation the sludges were dried at 120 °C.
In all cases the pyrolysis was done in a horizontal furnace under the nitrogen atmosphere with a heating rate of 10 °/min. The final pyrolysis temperature was 650, 800 and 950 °C with holding
Results and discussion
Compositions of the initial sludges are presented in Table 1. As seen, sewage sludge contains various transition metals and some of them are considered as hazardous. The main components are silica (not determined in the analysis), and iron and calcium compounds. Ferric oxide and lime are added to sludge as a part of the wastewater treatment technology. On the other hand, zinc is the main component of metal sludge. The content of solid in sewage sludge is about 25% and in metal sludge – 54%.
The
Conclusions
The results presented in this paper show that the surface of sludge derived adsorbents can be used for desulfurization of digester gas. The capacity obtained is comparable to those obtained on catalytic activated carbons. The performance of materials depends on their composition and the pyrolysis temperature. In the case of adsorbents obtained at a high temperature there are some synergetic effects enhancing the catalytic properties of the surface due to the solid-state reactions. The
Acknowledgements
This work is supported by NYSERDA Grant No. 9405 (RF CUNY No. 55771-0001).The experimental help Ms Anna Kleyman, Dr. Mykola Seredych and Dr. Jaroslaw Drapala is appreciated. The authors are grateful Ms. Peter Scorziello of NYFCO, Mr. Tony Viscentin of General Galvanizing and Mr. Neil Schultz of VTEC for providing the raw materials.
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2020, Journal of Natural Gas Science and EngineeringCitation Excerpt :For E800, when H2S inlet concentration increases from 200 ppm to 1500 ppm, the sulfur capacity enhances from 0.58 mg/g to 0.65 mg/g. This phenomenon perhaps be ascribed to the increased partial pressure of H2S in the simulated gas (Yuan and Bandosz, 2007a). It can be seen that increase of H2S inlet concentration significantly shortens the breakthrough time, but causes a positive effect on the H2S adsorption capacity.
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Stuyvesant High School, NY, United States.