Kinetic and thermodynamic analysis of the fate of sulphur compounds in gasification products

Synthesis gas produced from gasification of coal contains sulphur compounds that need to be removed before the gas can be further processed. For the sulphur removal process, it is normally assumed that all sulphur is present as hydrogen sulphide (H₂S) with minor amounts of carbonyl sulphide (OCS). This paper examines the equilibrium composition of a raw synthesis gas from a Texaco gasifier, and how the composition changes kinetically as the gas is cooled. It is confirmed that H₂S should always be the dominant sulphur species in cold syngas, with about 5% of sulphur appearing as OCS. Higher temperatures, and very high cooling rates can cause significant conversion of the sulphur to elemental S₂.     

Sulphur potential measurements with a two-phase sulphideoxide electrolyte

The open circuit potentials of the galvanic cell,Pt (or Au)¦(Ar + H₂S + H₂)′∥CaS + ZrO₂(CaO)∥ (Ar + H₂S+ H₂)″£t (or Au) has been measured in the temperature range 1000 to 1660 K and P_H2S:P_{H 2} ratios from 1.73×10^?5 to 2.65×10^?1. The solid electrolyte consists of a dispersion of calcium sulphide in a matrix of calcia-stabilized zirconia. The surface of the electrolyte is coated with a thin layer of calcium sulphide to prevent the formation of water vapour by reaction of hydrogen sulphide with calcium oxide or zirconia present in the electrolyte. The use of a ‘point electrode’ with a catalytically active tip was necessary to obtain steady emfs. At low temperatures and high sulphur potentials the emfs agreed with the Nernst equation. Deviations were observed at high temperatures and low sulphur potentials, probably due to the onset of significant electronic conduction in the oxide matrix of the electrolyte. The values of oxygen and sulphur potentials at which the electronic conductivity is equal to ionic conductivity in the two-phase electrolyte have been evaluated from the emf response of the cell. The sulphide-oxide electrolyte is unsuitable for sulphur potential measurements in atmospheres with high oxygen potentials, where oxidation of calcium sulphide may be expected.     

Shift catalysts in biomass generated synthesis gas

One of the CHRISGAS project objectives is to study the shift catalysts in biomass-generated synthesis gas. The water gas shift reaction is ruled by equilibrium, and the state of the gas can for a given H₂/CO ratio be shifted by addition/removal of water, CO₂ and/or by a change in the temperature. Stability area in respect to gas composition, sulphur content, pressure and temperature for FeCr shift catalyst has been investigated by thermodynamic equilibrium calculations. The calculations show that carbide formation is favourable in the “Normal water” case without sulphur in the gas. If sulphur is present in the gas, the situation improves due to sulphide formation.     

Thermodynamic analysis of the thermochemical decomposition of H2S in the presence of iron sulphide

The decomposition of H₂S by a two-step closed loop thermochemical decomposition process was investigated. In the first step, H₂S decomposes by reacting with iron sulphide to produce hydrogen and a solid solution consisting of FeS and S₂. In the second step, the solid solution is decomposed at high temperature to recover sulphur and the iron sulphide which is reused in the first step. The thermodynamics of the first reaction were studied in detail by taking into account 12 gas phase chemical reactions and the phase equilibrium that is established between the sulphur in the gas and in the solid solution. The non-linear system of 13 equations, which relate the fugacities of the components at equilibrium to the equilibrium constants, were numerically solved by the Newton-Raphson iteration technique. The calculation results showed that the gas phase at equilibrium contains predominantly H₂S, H₂, and H₂S₂. The equilibrium conversion was found to decrease with increasing temperature; however, when the sulphur content of the solid solution was small, high hydrogen yields were possible even at high temperatures.     

Activated carbon catalyst for selective oxidation of hydrogen sulphide: On the influence of pore structure, surface characteristics, and catalytically-active nitrogen

The catalytic oxidation of hydrogen sulphide (H₂S) on various activated carbon materials was studied. The effects of pore structure, surface characteristics, and nitrogen content on the activity and selectivity of the carbons towards oxidation of H₂S were investigated. It was found that a high volume of both micropores and small mesopores, in combination with a relatively narrow pore size distribution, were crucial for the retention of sulphur dioxide (SO₂), a by-product of H₂S oxidation. For the retention of carbonyl sulphide (COS), another H₂S oxidation by-product, high surface reactivity with a significant amount of basic groups were found to be important. The only carbon with all these characteristics, and consequently the carbon that was able to retain both H₂S and COS for an extended period of time, was an experimental product, “WSC”. This carbon was found to be superior to the other carbons studied, exhibiting high activity and selectivity for oxidation of H₂S to sulphur. H₂S breakthrough capacities and selectivity values of the carbons were found to be dependent on the nitrogen content of the carbons. In a hydrogen stream, carbons possessing the highest nitrogen contents exhibited the greatest H₂S breakthrough capacities but, at the same time, the lowest selectivity with respect to sulphur formation. In reformate streams, the maximum breakthrough capacity and greatest selectivity were exhibited by carbons with a nitrogen content of about 1-1.5 wt%.     

Monitoring and control of biogas desulphurization using oxidation reduction potential under denitrifiying conditions

BACKGROUND: Hydrogen sulphide (H₂S) present in biogas can be oxidized to elemental sulphur (S₀) or sulphate (SO₄^2?) using nitrate and nitrite. Both nitrate and nitrite are normally available in most wastewater treatment plants and could be used to oxidize H₂S depending on the molar loading ratio of wastewater and biogas. A control approach is required in order to minimize the fluctuations in inlet and outlet H₂S concentrations in biogas, and the oxidation potential of the wastewater used. RESULTS: A control scheme has been developed for biogas desulphurization using oxidation reduction potential under industrial conditions. The redox potential was maintained at about + 50 to + 100 mV in the activated sludge plant to monitor the performance of the nitrification process. The redox potential in the bioscrubber was related to sulphide removal from biogas. More than 90% of the hydrogen sulphide was removed from the biogas. CONCLUSION: The oxidation reduction potential can be used as a key parameter for monitoring and controlling biogas cleaning. Fluctuations of the inlet H₂S concentration in biogas can be compensated by manipulating the flowrates of wastewater used in order to achieve consistent and desired H₂S concentrations in treated biogas. Copyright © 2011 Society of Chemical Industry     

Application of wavefront analysis for kinetic investigations of watergas shif reaction

It has been demonstrated that during dynamic operation of the low temperature watergas shift reaction on the wavefront the CO₂—production is faster than the H₂—production. For a catalyst pretreated for several hours with a mixture of CO/N₂ its re-oxidation by a mixture of H₂O/CO/N₂ is a slow process, whereas on the wavefront the H₂—concentration is twofold the CO₂—concentration. The quantitative analysis of CO₂—wavefront data for a catalyst pretreated for several hours with a H₂O/N₂—Mixture has given a reaction order n_co = 0.625 and an activation energy E = 46.27 kJ/mole for the CO oxidation. It is shown that H₂O/N₂inhibits the CO oxidation. For a catalyst pretreated for a long time period with a H₂O/N₂—mixture and then reduced in a short time period by a H₂O/CO/N₂—mixture the reoxidation by a H₂O/N₂—mixture is a slow process. The wavefront analysis has also been applied to clear-up the H₂O-sorption on the low temperature catalyst, suggesting that a physisorption of Langmuir type is combined with a second mechanism saturating the H₂O—capacitance for low values of P_H2O.     

Kinetics of dissolution of copper(II) sulphide in aqueous sulphuric acid solutions

The rate of dissolution of synthetic cupric sulphide (CuS) which is the analogue of the naturally occurring sulphide mineral covellite, in sulphuric acid solutions of concentration range 5 × 10^?3 to 3 mole/l over the temperature range 20–65°c at atmospheric pressure was studied. Thermodynamic considerations suggested that the primary reaction taking place during the leaching is: CuS+1/2O₂ + H₂SO₄ → CuSO₄ + S+H₂O. It is proposed that up to an initial sulphuric acid concentration of 1 mole/l, the reaction rate is controlled by the diffusion of H⁺ ions through the sulphur film to the CuS/S interface. For higher sulphuric acid concentrations the oxygen dissolved in the acid decreases and the rate-determining step is believed to be diffusion of dissolved oxygen through the sulphur film to the CuS/S interface. A parabolic model is suggested for the dissolution reaction, and this model is supported by the experimental data. The activation energy for the dissolution reaction was found to be 8000±2000 cal/mole and this was independent of the H⁺ concentration and hence the nature of the diffusing species.     

Hydrogen sulphide — air equilibria under claus furnace conditions

Equilibrium compositions of mixtures resulting from reactions between hydrogen sulphide and air at atmospheric pressure were calculated for temperatures and O₂/H₂S ratios ranging from 600 to 2000°K and 0.05 to 1.0, respectively. Forty-four compounds containing nitrogen, hydrogen, oxygen and sulphur were assumed to be formed but only 25 had concentrations exceeding 0.1 ppm. These compounds should not, as in previous studies, be omitted from equilibrium calculations. Sulphur yields are shown to be increased by approximately 10% if O₂/H₂S ratios less than stoichiometric are used. Reasons for this and implications for Claus plant operation are provided.     

X-ray photoelectron spectroscopic investigation of coal

X-ray photoelectron spectroscopy (XPS) has been used to study the carbon, oxygen and sulphur content of several coking coals, a series of sink—float fractions from a high-volatile coking coal, and several inorganic minerals that are commonly found in coals. Single carbon and oxygen peaks were obtained that corresponded to carbon 1s orbital and oxygen 1s orbital electron binding energies which are expressed in units of electron volts (eV). Two sulphur 2p (S 2p) peaks were found. The 169–171 eV S 2p peak corresponded to the sulphates resulting from the oxidation of pyrite (FeS₂), while the 163–164 eV S 2p peak was assigned to the iron sulphide compounds such as pyrite or marcasite. No separate organic sulphur peaks were found for coal, because the majority of the organic sulphur peaks are probably overlapped by the inorganic sulphide line at 163–164 eV. The total carbon and sulphur determined using the XPS peak height correlated with the chemical analysis, although the accuracy of the XPS determinations seems to be lower. A possible direct quantitative method for determining the organic sulphur in coal by XPS is discussed.     

The Effect of SO2 and H2O on the Activity of Pd/CeO2 and Pd/Zr–CeO2 Diesel Oxidation Catalysts

Pd/CeO₂ and Pd/Zr-CeO₂ diesel oxidation catalysts were treated with sulphur, sulphur–water and water. According to the BET, TEM-EDS, XPS, ICP-OES analyses and catalytic activity tests, both studied catalysts were deactivated by sulphur due to formation of sulphates. Water treatment was found to have a promoting effect on the oxidation of CO and C₃H₆.     

Inhibiting the sp2 carbon deposition by adjunction of sulphurous species in refractory ceramics subjected to CO and H2 reducing atmosphere

The formation of sp² carbon by the Boudouard reaction significantly damages the refractory ceramics. Sulphur is an efficient way to prevent the carbon deposition catalysed by Fe₃C, in the presence of H₂. Thermogravimetric analysis was carried out on Fe₂O₃ samples exposed to a CO/H₂ gas mixture at 600 °C. Solid sulphur was mixed with Fe₂O₃ powder or continually added in the form of gas into the CO/H₂ reducing gas. The samples were characterised by X-ray diffraction, Raman spectroscopy, SEM and TEM. The addition of 100 ppm of sulphur species in the gas prevents the formation of carbon. The mechanism that governs the inhibition of the reaction is proposed, in which the formation of a thin protective FeS layer (0.5–1 nm) is involved. This study paves the way to an effective solution to inhibit the sp² carbon deposition in the refractories by poisoning the Fe₃C catalyst with sulphur.     

Mechanisms leading to process improvements in lignite liquefaction using CO and H2S

Optimum distillate yields from US lignites can be as high on a dry, ash-free basis as those obtained from bituminous coals, but only if the vacuum bottoms are recycled. Lignites are more readily liquefied if the reducing gas contains some carbon monoxide and water, which together with bottoms recycle has proven to yield the highest conversions and the best bench-unit operability. The recycle solvent in the reported tests consisted of unseparated product slurry, including coal mineral constituents. Variability in coal minerals among nine widely representative US low-rank coals did not appear to correlate with liquefaction behaviour. Addition of iron pyrite did, however, improve yields and product quality, as measured by hydrogen-to-carbon ratio. Future improvements in liquefaction processes for lignite must maintain high liquid yields at reduced levels of temperature, pressure, and reaction time whilst using less reductant, preferably in the form of synthesis gas (CO + H₂) and water instead of the more expensive pure hydrogen. Understanding the process chemistry of carbon monoxide and sulphur (including H₂S) during lignite liquefaction is a key factor in accomplishing these improvements. This Paper reviews proposed mechanisms for such reactions from the viewpoint of their relative importance in affecting process improvements. The alkali formate mechanism first proposed to explain the reduction by CO does not adequately explain its role in lignite liquefaction. Other possible mechanisms include an isoformate intermediate, a formic acid intermediate, a carbon monoxide radical anion, direct reaction with lignite, and the activation of CO by alkali and alkaline earth cations and by hydrogen sulphide. Hydrogen sulphide reacts with model compounds which represent key bond types in low-rank coal in the following ways: (1) hydrocracking; (2) hydrogen donor; (3) insertion reactions in aromatic rings; (4) hydrogen abstraction, with elemental sulphur as a reaction intermediate; and (5) catalysis of the water-gas shift reaction. It appears that all of these reaction pathways may be operative when catalytic amounts of H₂S are added during liquefaction of lignite. In bench recycle tests, the addition of H₂S as a homogeneous catalyst reduced reductant consumption as much as three-fold whilst maintaining high yield levels when the reaction temperature was reduced by 60°C. Attainment of the high distillate yield at 400°C was accompanied by a marked decrease in the production of hydrocarbon gases, which normally is a major cause of unproductive hydrogen consumption and solvent degradation via hydrocracking. Processing with synthesis gas and inherent coal moisture using bottoms recycle and H₂S as a catalyst appears to be the most promising alternative combination of conditions for producing liquids from lignite at reduced cost.     

An investigation of the deposition and reactions of sulphur on gold electrodes

The oxidation of sulphide species in solution has been investigated on a gold electrode in borate solutions of pH 6.8 and 9.2 and in 1 mol dm^?3 NaOH. Sub-monolayer coverage by sulphur occurs at underpotentials. At more positive potentials, multilayers of sulphur can be deposited. The reaction producing sulphur, and the reverse process, involve the formation of poly sulphides as intermediates. The predominant polysulphide intermediate is S ₂ ^2? . Oxidation of sulphide to sulphate occurs to some extent in parallel with sulphur deposition. Both reactions are inhibited by the presence of a surface layer of sulphur. At high potentials, sulphur is oxidized to sulphate.     

Dealkylation of N,N-dimethylaniline with sulphur and hydrogen sulphide

N,N-Dimethylaniline successively demethylates at 425 °C in the presence of hydrogen and hydrogen sulphide or sulphur to give successively N-methylaniline and aniline. For the dealkylation reaction, the rank order of promotors is S >SH₂ >H₂S >H₂SH₂ >H₂.     

Catalytic oxidation of hydrogen sulphide on activated carbons

Activated carbon is a suitable adsorbent for removal of hydrogen sulphide from natural, synthesis or other product gases. The process depends predominantly on physical adsorption, though catalytic oxidation is also involved. During catalytic oxidation the H₂S is converted in the presence of oxygen to elemental sulphur, which is adsorbed onto the internal surface of the activated carbon, thus leading to a sulphur load of up to 120% by weight. The oxidation rate depends on the partial pressure of both reactants, H₂S and O₂ and is largely controlled by the characteristics of the activated carbon. The activity of the catalyst can be improved by impregnating the activated carbon with promoters such as iron and iodine. The regeneration of spent carbon is currently carried out using hot gas desorption methods at temperatures around 450 °C.     

Extraction of metals from sulphides using sulphur trioxide in fluidised bed

The kinetics of the reaction of chalcopyrite, iron sulphide, copper sulphide, and nickel sulphide with sulphur trioxide gas were studied using a fluidised bed technique. O₂, N₂, or air was used as a carrier gas for the sulphur trioxide in fluidisation. Binary mixtures of finely ground (0.37–75 μm) samples were reacted with the sulphur trioxide in a Pyrex column at 373–673 K. The reaction products were leached with water and the soluble metals in the solution were determined by atomic absorption spectrometry. The total soluble reaction products were determined gravimetrically. The results obtained showed that a higher yield of soluble salts was obtained when O₂ or air was used as a carrier gas for sulphur trioxide than when an inert gas was used. Higher yields of soluble salts were obtained when the samples were most finely ground. Increase of copper sulphide content in binary mixtures with iron or nickel sulphide led to an increase in the yield of soluble salts. For iron sulphide/nickel sulphide mixtures, the yield of soluble salts increased with the nickel sulphide content. There were maximum values for the soluble metal ratios Ni/Fe and Cu/Ni in the corresponding sulphide binary mixtures and this maximum was at about 50% weight. The soluble Cu/Fe ratio increased with copper sulphide content in mixtures with iron sulphide.     

Removal of organic sulphur from coal gas

Organic sulphur compounds present in coal gas containing 15-20% CO were effectively converted into H₂S over a “Nimox” (nickel-molybdenum) conversion catalyst. H₂S was effectively removed by “Luxmasse”, a prepared iron oxide. The overall removal of organic sulphur depended upon the concentration of thiophene present. With only 10 ppm thiophene in the gas, the conversion of organic sulphur was 97% at 350°C after a single treatment. With six-stage treatment at 350 psig, the final gas contained only 0.2 ppm total organic sulphur in the presence of 4-5% water vapor.     

Biocatalytic detoxification of 2-chloroethyl ethyl sulfide

Rhodococcus rhodochrous IGTS8 (ATCC 53968) was shown to be capable of utilizing 2-chloroethyl ethyl sulphide (CEES) as the sole source of sulphur for microbial growth. 2-Chloroethanol and a compound tentatively identified as 2-chloroethanesulfinic acid have been detected as metabolites. This demonstrates that carbon—sulphur bonds were cleaved in CEES prior to hydrolysis of the chlorine atom. These data indicate that Rhodococcus rhodochrous IGTS8 may be useful for the biodetoxification of the chemical warfare agent mustard (2,2′ dichlorodiethyl sulphide).