Oxidation of low concentrations of hydrogen sulphide: Process optimization and kinetic studies

Low concentrations (e.g. < 3) of H₂ S in natural gas can be selectively oxidized over an “granular Hydrodarco” activated carbon catalyst to elemental sulphur, water and a small fraction of by-product sulphur dioxide, SO₂. To optimize the H₂ S catalytic oxidation process, the process was conducted in the temperature range 125—200 °C, at pressures 230—3200 kPa, with the O/H₂ S ratio being varied from 1.05 to 1.20 and using different types of sour and acid gases as feed. The optimum temperature was determined to be approximately 175 °C for high H₂ S conversion and low SO₂ production with an O/H₂ S ratio 1.05 times the stoichiometric ratio. The life of the activated carbon catalyst has been extended by removing heavy hydrocarbons from the feed gas. The process has been performed at elevated pressures to increase H₂ S conversion, to maintain it for a longer period and to minimize SO₂ production. The process is not impeded by water vapour up to 10 mol% in the feed gas containing low concentrations of CO₂ (< 1.0). A decrease in H₂ S conversion and an increase in SO₂ production were obtained with an increase in water vapour in the feed gas containing a high percentage of CO₂. The process works well with “sour natural gas” containing approximately 1% H₂ S and with “acid gas” containing both H₂ S and CO₂. It gives somewhat higher H₂ S conversion and low SO₂ production with feed gas containing low concentrations of CO₂. A kinetics study to determine the rate-controlling step for the H₂ S catalytic oxidation reaction over “granular Hydrodarco” activated carbon has been conducted. It was concluded that either adsorption of O₂ or H₂ S from the bulk phase onto the catalyst surface is the rate-controlling step of the H₂ S catalytic oxidation reaction.     

Sorption of sulphur dioxide by paper

Sulphur dioxide is sorbed by paper and slowly converted to sulphuric acid; the presence of this severely reduces the useful life of the material. The sorption (‘pick-up’) of sulphur dioxide has been investigated for a representative range of papers for exposures lasting many months at concentrations approaching those found in polluted urban atmospheres. It was found that the relative humidity does not significantly affect the long-term pick-up rate and that this rate remains constant for prolonged exposures, being proportional to the square root of the gas phase sulphur dioxide concentration. Rosin size increases markedly the long-term rate while certain other paper additives, e.g. Turkey Red Oxide and Anatase, have no such effect.     

Sorption of sulphur dioxide by indoor surfaces II. Wood

The sorption of sulphur dioxide by untreated wood samples has been investigated at a concentration of 90 μg/m³ by use of ³⁵SO₂. The softwoods sorbed less sulphur dioxide than the hardwoods and the sites of sorption were different in each class. Most of the sorbed sulphur dioxide was found in the outermost 0.05 mm of each sample and a large portion of it was water-soluble. The possible role of sulphur dioxide in the weathering of woods is discussed.     

Absorption of sulphur dioxide by electrosprayed droplets

This paper describes and discusses experimental results on the absorption of sulphur dioxide in electrified water sprays, either when the polluted gas is treated as is or when the gas is exposed to a corona source to ionize the sulphur dioxide. The experiments revealed that an electrified spray with a charge-to-mass ratio of 50 μC · kg⁻¹ enabled the absorption rate of droplets to double, regardless of their polarities. Corona charging gave rise to an increase in the SO₂ depletion rate over the scrubber wall, while negligible effects appeared on the actual droplets absorption rate. These findings suggested that faster absorption rates mostly, though not uniquely, depend on the modifications on the morphological and interfacial properties of the sprayed droplets induced by the free electric charge imposed on their surface. Conversely, the absorption rates were negligibly affected by the electrical interactions between droplets (either charged or uncharged) and the sulphur dioxide ions/radicals originating from the corona source.     

Oxidation of Aqueous Sulphur Dioxide Catalysed by Poly-4-vinylpyridine–Cu(II) Complex. Part 2: Combined Homogeneous and Heterogeneous Phase Oxidation

Aqueous phase oxidation of sulphur dioxide at low concentrations catalysed by a PVP–Cu complex in the solid phase and dissolved Cu(II) in the liquid phase is studied in a rotating catalyst basket reactor (RCBR). The equilibrium adsorption of Cu(II) and S(VI) on PVP particles is found to be of the Langmuir-type. The diffusional effects of S(IV) species in PVP–Cu resin are found to be insignificant whereas that of product S(VI) are found to be significant. The intraparticle diffusivity of S(VI) is obtained from independent tracer experiments. In the oxidation reaction HSO₃⁻ is the reactive species. Both the S(IV) species in the solution, namely SO₂(aq) and HSO₃⁻, get adsorbed onto the active PVP–Cu sites of the catalyst, but only HSO₃⁻ undergoes oxidation. A kinetic mechanism is proposed based on this feature which shows that SO₂(aq) has a deactivating effect on the catalyst. A rate model is developed for the three-phase reaction system incorporating these factors along with the effect of concentration of H₂SO₄ on the solubility of SO₂ in the dilute aqueous solutions of Cu(II). Transient oxidation experiments are conducted at different conditions of concentration of SO₂ and O₂ in the gas phase and catalyst concentration, and the rate parameters are estimated from the data. The observed and calculated profiles are in very good agreement. This confirms the deactivating effect of non-reactive SO₂(aq) on the heterogeneous catalysis. © 1997 SCI.     

Sorption of sulphur dioxide by indoor surfaces I. Wallpapers

The sorption of sulphur dioxide by PVC wallcoverings and conventional wallpaper samples has been measured at a maximum sulphur dioxide concentration of 150 μg/m³ by use of ³⁵SO₂. The sorption was found to be influenced by the surface finish and design pattern of the wallpapers. Sweat deposits on the samples also influenced the pattern of uptake. The conventional wallpapers showed a greater uptake than did PVC wallcoverings in all cases.     

Numerical analysis of particle clustering effects on desulphurization and NO emission in a circulating fluidized bed combustor

Desulphurization by a calcium oxide particle cluster and an isolated calcium oxide particle in a circulating fluidized bed (CFB) combustor was numerically analyzed. The gas flow field, the sulphur retention and the nitrogen oxide emission of the cluster were predicted. Computed results showed that the SO₂ capture rate by a calcium oxide particle in the cluster is less than that of the isolated calcium oxide particle out of the cluster. The captured SO₂ and NO emissions decrease with the decrease of the cluster porosity. The maximum SO₂ capture rate by the cluster is at a temperature between 1025 and 1055 K, whereas more NO emissions were found with the increase of the gas temperature. The sulphur removal and the NO emission increase with the increase of the inlet gas velocity.     

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.     

Oxidation of aqueous sulfur dioxide catalyzed by poly-4-vinylpyridine–Cu(II) complex. Part 1: Homogeneous phase oxidation

The oxidation of aqueous sulfur dioxide in the presence of polymer-supported copper(II) catalyst is also accompanied by homogeneous oxidation of aqueous sulfur dioxide catalyzed by leached copper(II) ions. Aqueous phase oxidation of sulfur dioxide of low concentrations by oxygen in the presence of dissolved copper(II) has therefore been studied. The solubility of SO₂ in aqueous solutions is not affected by the concentration of copper(II) in the solution. In the oxidation reaction, only HSO is the reactive S(IV) species. Based on this observation a rate model which also incorporates the effect of sulfuric acid on the solubility of SO₂ is developed. The rate model includes a power-law type term for the rate of homogeneous phase reaction obtained from a proposed free-radical chain mechanism for the oxidation. Experiments are conducted at various levels of concentrations of SO₂ and O₂ in the gas phase and Cu(II) in the liquid phase. The observed orders are one in each of O₂, Cu(II) and HSO. This suggests a first-order termination of the free radicals of bisulfite ions.     

Optimum Values of Process Parameters of the “Wet Limestone Flue Gas Desulfurization System”

In this article the method of cost optimization of the “Wet Limestone Flue Gas Desulfurization System” is presented. The optimization calculations include process and cost models. The process model describes the most important stage of SO₂ removal that runs in the absorber and in the holding tank. It includes absorption of sulfur dioxide, oxidation of SO^2–₃, dissolution of limestone, and crystallization of gypsum. The model was applied to calculate indispensable parameters for estimating costs and then to minimize capital and operating costs. Costs of all important equipment were estimated, such as SO₂ removal systems with the absorber and the holding tank, reagent feed system with the ball mill and dewatering gypsum slurry system. Optimum values of the process parameters for different conditions of running flue gas desulfurization system were found. The process and cost model can be useful when designing the wet limestone FGD systems and carrying out economic analysis of the flue gas desulfurization plants.     

Methods for measuring the concentrations of SO2, and of gaseous reduced sulphur compounds in the combustion chamber of a circulating fluidized bed boiler

The present work was aimed at developing and improving methods for measurement of gaseous sulphur compounds in the combustion chamber of a fluidized bed boiler (FBB). The sampling of SO₂ was improved by removing NH₃ and H₂O with a sorbent immediately after the probe. The concentration of reduced sulphur species was determined by means of two conventional SO₂ analyzers and an intermediate converter, where the reduced species are oxidized to SO₂. Gas phase sulphides were also sampled with a gas quenching probe by means of a basic solution which was subsequently analysed by wet chemistry. The methods were tested during coal combustion in a 12 MW circulating FBB without limestone for two cases of air‐staging.     

Platinum Oxidation and Sulphur Deactivation in NOx Storage Catalysts

Flow reactor experiments and X-ray photoelectron spectroscopy (XPS) measurements were used to investigate the importance of platinum oxide formation on Pt/BaO/Al₂O₃ NO _x storage catalysts during reactions conditions. The reaction studied was NO(g) + 1/2 O₂(g) NO₂(g). During NO₂ exposure of the catalyst the NO₂ dissociation rate decreased during the reaction. This activity decrease with time was also studied with XPS and it was found to be due to platinum oxide formation. The influence of sulphur exposure conditions on the performance of the NO _x storage catalysts was studied by exposing the samples to lean and/or rich gas mixtures, simulating the conditions in a mixed lean application, containing SO₂. The main results show that all samples are sensitive to sulphur and that the deactivation proceeds faster when SO₂ is present in the feed under rich conditions than under lean or continuous SO₂ exposure. Additionally, the influence of the noble metals present in the catalysts was investigated regarding sulphur sensitivity and it was found that a combination of platinum and rhodium seems to be preferable to retain high performance of the catalyst under SO₂ exposure and subsequent regeneration. Finally, the behaviour of micro-fabricated model NO _x storage catalysts was studied as a function of temperature and gas composition with area-resolved XPS. These model catalysts consisted of a thin film of Pt deposited on one-half of a BaCO₃ pellet. It was found that the combination of SO₂ and O₂ resulted in migration of Pt on the BaCO₃ support up to one mm away from the Pt/BaCO₃ interface.     

Absorption of SO<Subscript>2</Subscript> using PVDF hollow fiber membranes with PEG as an additive

In this study, removal of SO₂ from gas stream was carried out by using microporous polyvinylidene fluoride (PVDF) asymmetric hollow fiber membrane modules as gas-liquid contactor. The asymmetric hollow fiber membranes used in this study were prepared polyvinylidene fluoride by a wet phase inversion method. Water was used as an internal coagulant and external coagulation bath for all spinning runs. An aqueous solution containing 0.02 M NaOH was used as the absorbent. This study attempts to assess the influence of PEG additive, absorbent flow rate, SO₂ concentration, gas flow rate and gas flow direction on the SO₂ removal efficiency and overall mass transfer coefficient. The effect of liquid flow rate on SO₂ removal efficiency shows that at very low liquid flow rate, the NaOH available at the membrane surface for reacting with SO₂ is limited due to the liquid phase resistance. As liquid flow rate is above the minimum flow rate which overcomes the liquid phase resistance, the SO₂ absorption rate is controlled by resistance in the gas phase and the membrane. The SO₂ absorption rate with inlet SO₂ concentration was sharply increased by using hollow fiber membranes compared to a conventional wetted wall column because the former has higher gas liquid contacting area than the latter. The mass transfer coefficient is independent of pressure. When the gas mixture was fed in the shell side, the removal efficiency of SO₂ declined because of channeling problems on the shell side. Also, the addition of PEG in polymer dopes increased SO₂ removal efficiency. This work was presented at the 6 ^th Korea-China Workshop on Clean Energy Technology held at Busan, Korea, July 4–7, 2006.     

Chemical reduction of sulphur dioxide to free sulphur with lignite and coal. 1. Steady-state reaction chemistry and interaction of volatile components

The chemical reduction of SO₂ with North Dakota lignite has been discovered to be a facile reaction which occurs at a relatively low temperature of 600–650 °C. Under optimum conditions, the reaction chemistry can be controlled to allow 85–90% conversion of SO₂ to free sulphur in a single-stage reaction. Major by-products of the reaction are CO₂, H₂O and a free-flowing ash. The high sulphur yield from this reaction exceeds the calculated thermodynamic gas phase equilibrium value of 66–70%. The higher experimental yield was found to be due in part to a catalysed re-equilibration of the gaseous products in the exit line. With lignite and low-rank coals, the mechanism of SO₂ reduction appears to involve reaction of hydrocarbons within the pores structure and thus allows complete conversion of the volatile matter with no tar formation. Volatilization and tar formation successfully compete with SO₂ reduction in bituminous coals under the same reaction conditions.     

Investigation of sulphation behavior of two fly ash samples produced from combustion of different fuels in a 165 MWe CFB boiler

Emission of sulphur dioxide (SO₂) from combustion of fossil fuel is an important environmental issue. Circulating fluidized bed combustion (CFBC) technology can use limestone sorbent to achieve in situ SO₂ emissions control. This paper presents the chemical and physical analysis results of two fly ash samples derived from a 165 MWe CFBC boiler burning two different fuels with addition of limestone, as they pertain to sulphation behavior. One of the samples in this study was produced from combustion of a bituminous coal with high iron content, the other from firing of blended coal and petroleum coke fuel. The physical examination was conducted by scanning electron microscope (SEM) coupled with an energy dispersive X-ray (EDX) system for analysis of the surface structure or morphology of the sample, as well as the calcium and sulphur distribution. Some large particles derived from high-iron-content fuel were covered by dense iron shells; however, in general such a dense rim was found to not significantly impede the overall desulphurization performance in FBC in terms of the limestone utilization. The large particles (~ 100 μm in diameter) in both samples typically consisted of a CaSO₄ shell and an almost pure CaO core; however, numerous small particles of diameters of 10-20 μm consisted predominantly of CaO without sulphate shells. In particular, the emphasis of this investigation has been focused on the remaining capacity of the fly ash for reaction with sulphur dioxide and to clarify the effects of iron, both samples have been doped with additional iron content, and their sulphation behavior examined, and while both experienced a small reduction in sulphation capacity, the fly ash with the initial low iron content experienced the lowest reduction of sulphation capacity after doping, which is not supportive of the idea that iron has an important effect on sorbent capacity.     

Tentative modelling of spray-dry scrubbing of SO2

A 0.5 MW spray-dry scrubbing FGD pilot plant was used in the study of spray dryer performance over a wide range of operating conditions. Experimental findings were compared with a spray dryer model. During operation with large excesses of lime, the SO₂ absorption was limited by gas phase diffusion. At operation with a shortage of lime, the rate limiting step was the dissolution rate of lime. In addition, the flow regime in a spray dryer can be best described as well mixed. The SO₂ level in the flue gas was found to exert no direct effect on the efficiency of SO₂ removal. The observed effects are attributed solely to the changes in the drying process, due to the inter-dependence of slurry composition and SO₂ concentration.     

Preparation,characterization, and performance evaluation of coated PES polymer materials fabricated via dry/wet phase inversion technique

Sulfur dioxide (SO₂) is the major air pollutant which is emitted from the power plant. In this study, hollow fiber membrane (HFM) separation process is applied for the improvement of SO₂ removal efficiency in the post‐combustion gas. HFM was produced by dry/wet phase inversion method and then coated with Polydimethylsiloxane (PDMS). The membrane morphology and characterization were examined with help of scanning electron microscope (SEM), energy dispersion of X‐ray spectroscopy (EDX), Fourier transform infrared (FT‐IR) and atomic force microscopy (AFM). Polyethersulfone (PES) hollow fiber membranes were tested for the SO₂/N₂ binary mixed gas separation. Single gas permeance of SO₂, N₂, and binary mixture gas (200 ppm of SO₂) separation experiment was initiated to observe membrane behavior according to temperature and pressure difference and retentate flow rate. As a result, permeance of SO₂ was 24.9–47.4 GPU and selectivity of SO₂/N₂ was 1.6–4.2. From the mixture gas separation experiment, SO₂ removal efficiency increased according to stage cut and operating pressure. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39711.     

Fundamental studies on a new concept of flue gas desulphurization

A new process for removal of sulphur dioxide from waste gases is proposed consisting of both electrochemical and catalytic sulphur dioxide oxidation. In the catalytic step a part of the sulphur dioxide is oxidized by oxygen on copper producing sulphuric acid and copper sulphate. The other part is oxidized electrochemically on graphite. The cathodic reaction of this electrolysis is used for recovering the copper dissolved in the catalytic step. The basic reactions of this process have been studied experimentally in detail. It has been shown that sulphur dioxide can be electrochemically oxidized on carbon electrodes to sulphuric acid with high current efficiency. The reaction rate of the electrochemical copper deposition is increased by dissolved sulphur dioxide in the electrolyte. The catalytic oxidation of sulphur dioxide on copper has been investigated for different sulphur dioxide concentrations and temperatures. The ratio of the reaction products, sulphuric acid and copper sulphate, varies over a wide range depending on the experimental conditions.Nomenclature SO₂ concentration (gas phase) (vol % SO₂) - SO₂ concentration (electrolyte) (g l^–1) - E potential vs saturated calomel electrode (V) - E _s specific energy consumption (W g^–1 SO₂) - F Faraday constant (A s^–1 mol^–1) - i current density (mA cm^–2) - molecular weight (g mol^–1) - T temperature (° C) - U _c cell voltage (V) - v _e number of electrons being transferred - space-time yield of SO₂-oxidation (g SO₂ h^–1 dm^–3) - _cu space-time yield of Cu-corrosion (g Cu h^–1 dm^–3) - ratio - fractional conversion of SO₂ - current efficiency for SO₂ oxidation     

The activated electro-oxidation of sulphur dioxide on smooth platinum

The electrochemical behaviour of sulphur dioxide in sulphuric acid solutions at a platinum rde is characterized. Cycling the potential in these solutions between about ?0.10 and 1.2 V sce results in activation of the electrode so that diffusion-controlled SO₂ oxidation currents can be observed in the double layer region of platinum. Without activation, SO₂ oxidation proceeds noticeably only in the potenial region of surface oxide formation. Evidence is presented which indicates that activation results from formation of a catalytic layer of sulphur species. The catalytic activity of this layer decays with time in the course of SO₂ oxidation. The formation of sulphur oxides through oxidation of adsorbed sulphur and the formation of platinum oxides complicate the voltammetric behaviour of the system.     

Zum Stand der Schwefelsäure-Produktion

Sulphuric acid production – state of the art. Apart from pyrites and other metal sulfides, sulphur is mainly used today as raw material for the production of sulphuric acid. The oxidation of roaster gases or sulphur combustion gases is performed almost exclusively in multistage reactors over catalysts containing vanadium pentoxide. SO₂ conversions of more than 99.5% are achieved by intermediate absorption of SO₃ (“double catalysis”) and by using efficient catalysts. To further improve the economics of the manufacturing process, new technologies are being tried out, and attempts are being made to develop even more active catalysts. Although there is still some uncertainty regarding the kinetics of SO₂ oxidation, empirical findings can be explained quite satisfactorily with existing models.