Biogas is a valuable renewable energy carrier. It can be exploited directly as a fuel or as a raw material for the production of synthesis gas and/or hydrogen. Methane (CH4) and carbon dioxide (CO2) are the main constituents, but biogases also contain significant quantities of undesirable compounds (contaminants), such as hydrogen sulfide (H2S), ammonia (NH3) and siloxanes. The existence and quantities of these contaminants depend on the biogas source (i.e., landfills, anaerobic fermentation of manure). Their presence constitutes a major problem because (i) they can be detrimental to any biogas thermal or thermocatalytic conversion device (e.g., corrosion, erosion, fouling); and (ii) they generate harmful environmental emissions. It is therefore important to include biogas purification steps upstream of its final use processes. This review is aimed at presenting the scientific and technical state-of-theart in biogas purification processes. Both mature, already-applied and promising, under-development technologies are reported and described here. © 2008 Society of Chemical Industry and John Wiley & Sons, Ltd

A review of biogas purification processes

Bituminous coal-based activated carbons modified with nitrogen as adsorbents of hydrogen sulfide

The most widely used catalysts and processes for H2S-selective catalytic oxidation are overviewed in this review. Two kinds of catalysts have been investigated intensively: carbon-based catalysts (active carbon catalyst, carbon nanotube catalyst, and carbon nanofiber catalyst), metal oxide-based catalysts (metal oxide catalyst, oxide-supported catalyst, and clay-supported catalyst). Among them, carbon-based catalysts are utilized mainly in discontinuous processes at relatively low temperatures, whereas metal oxide catalysts are the most widely used in practice. However, the reaction temperature is relatively high. Fortunately, a MgAlVO catalyst derived from LDH materials and intercalated clay-supported catalysts exhibit excellent catalytic activities at relatively lower temperatures. According to various studies, the catalytic behaviors mainly obey the Mars–van Krevelen mechanism; however, the catalyst deactivation mechanism differs, depending on the catalyst. In practice, the mobil direct oxidation proce...

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H2S-Selective Catalytic Oxidation: Catalysts and Processes

The process used to upgrade natural gas, biogas and refinery-off-gas directly influences the cost of producing the fuel and often requires complex separation strategies and operational systems to remove contaminants such as hydrogen sulfide (H2S) and carbon dioxide (CO2). Here we report a fluorinated metal–organic framework (MOF), AlFFIVE-1-Ni, that allows simultaneous and equally selective removal of CO2 and H2S from CH4-rich streams in a single adsorption step. The simultaneous removal is possible for a wide range of H2S and CO2 compositions and concentrations of the gas feed. Pure component and mixed gas adsorption, single-crystal X-ray diffraction and molecular simulation studies were carried out to elucidate the mechanism governing the simultaneous adsorption of H2S and CO2. The results suggest that concurrent removal of CO2 and H2S is achieved via the integrated favourable sites for H2S and CO2 adsorption in a confined pore system. This approach offers the prospect of simplifying the complex schemes for removal of acid gases. Contaminants such as CO2 and H2S present in natural gas and biogas streams must be removed before use; existing strategies to do so can be rather complex. Here, the authors use a fluorinated porous metal–organic framework to remove CO2 and H2S from CH4-rich feeds in a single step, potentially simplifying the process.

Natural gas upgrading using a fluorinated MOF with tuned H2S and CO2 adsorption selectivity

To understand the nature of H2S adsorption onto carbon surfaces under dry and anoxic conditions, the effects of carbon pore structure and surface chemistry were studied using activated carbon fibers (ACFs) with different pore structures and surface areas Surface pretreatments, including oxidation and heattreatment, were conducted before adsorption/desorption tests in a fixed-bed reactor Raw ACFs with higher surface area showed greater adsorption and retention of sulfur, and heat treatment further enhanced adsorption and retention of sulfur The retained amount of hydrogen sulfide correlated well with the amount of basic functional groups on the carbon surface, while the desorbed amount reflected the effect of pore structure Temperature-programmed desorption (TPD) and thermal gravimetric analysis (TGA) showed that the retained sulfurous compounds were strongly bonded to the carbon surface In addition, surface chemistry of the sorbent might determine the predominant form of adsorbate on the surface

Adsorption of hydrogen sulfide onto activated carbon fibers: effect of pore structure and surface chemistry.

Four activated carbons of various origins were impregnated with different concentrations of sodium hydroxide and used as hydrogen sulfide adsorbents in an accelerated test. The materials were characterized using nitrogen sorption, thermal analysis, and standard ASTM methods. The results showed that, with increasing loading of NaOH, the H2S breakthrough capacity increases 4−5 times until maximum capacity is reached at about 10% NaOH. This capacity per unit volume of the carbon bed is the same for all carbons and insensitive to their pore structures and surface areas. The specific capacity per unit surface area is also the same for all materials studied, including activated alumina. This indicates that the amount of NaOH present on the surface is a limiting factor for the capacity. By increasing the pH value of the carbon, sodium hydroxide causes an increase in the HS- ion concentration. These ions can be further oxidized to elemental sulfur or sulfuric acid, as suggested by changes in the surface pH values...

A Role of Sodium Hydroxide in the Process of Hydrogen Sulfide Adsorption/Oxidation on Caustic-Impregnated Activated Carbons

Adsorption of hydrogen sulfide on fifteen samples of activated carbons (unmodified and catalytic with introduced basic nitrogen groups) was studied using inverse gas chromatography (IGC) at infinite dilution. Porous structure of carbons was evaluated using sorption of nitrogen. From the retention volume measured in IGC experiments, heats of adsorption, free energy of adsorption, entropy, and Henry's constants were calculated. The results showed the dependence of these quantities on pore sizes of the adsorbents. The strong dependence of the isosteric heat of adsorption on the temperature of measurement and, following this, unusually high heat capacity suggest the strong effect of specific interactions and catalytic influence of carbon surface even unmodified. The calculated Henry's constants are in agreement with the literature data. The adsorption on carbons with incorporated basic nitrogen groups is characterized by stronger adsorbent−adsorbate interactions than on unmodified carbons.

Andrey Bagreev† and

Papers

A Role of Sodium Hydroxide in the Process of Hydrogen Sulfide Adsorption/Oxidation on Caustic-Impregnated Activated Carbons

Study of Hydrogen Sulfide Adsorption on Activated Carbons Using Inverse Gas Chromatography at Infinite Dilution