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.     

Kinetics and reaction mechanism of catalytic oxidation of low concentrations of hydrogen sulfide in natural gas over activated carbon

The kinetics of the hydrogen sulfide oxidation process, producing mostly sulfur and water, was studied using 0.25 to 1.0 g Hydrodarco activated carbon catalyst and varying the O₂/H₂S ratio (molar basis) in the feed gas between 0.5 to 0.6 in the temperature, and pressure ranges from 125 to 200°C and 225 to 780 kPa. SO₂ was obtained as an undesirable by-product during H₂S oxidation reaction or as a product during regeneration of the catalyst. The feed gas contained 0.9 — 1.3 mol% H₂S with approximately 80 mol% CH_4. In this paper, the factors affecting the H₂S conversion and SO₂ formation are presented. The rate expressions for (a) H₂S conversion and (b) SO₂ formation were developed from the Langmuir-Hinshelwood surface control reaction model. The experimental data were well correlated by the rate equations. Also, the rate parameters were evaluated and correlated with temperature. The activation energies for H₂S oxidation and SO₂ production reactions were calculated to be 34.2 and 62.5 kJ/mol, respectively. Partial pressures of oxygen and H₂S were found to influence H₂S conversion whereas, the presence of water in the feed gas up to 10.5 mol% did not affect H₂S conversion significantly. Heats of adsorption for various species on the active sites were calculated. SO₂ production was, as expected, enhanced at higher temperature, and its rate was much smaller than the oxidation rate of H₂S under the reaction conditions used.     

Effect of H2S on the hydrogenation of carbon dioxide over supported Rh catalysts

The hydrogenation of CO₂ was studied on supported noble metal catalysts in the presence of H₂S. In the reaction gas mixture containing 22 ppm H₂S the reaction rate increased on TiO₂ and on CeO₂ supported metals (Ru, Rh, Pd), but on all other supported catalysts or when the H₂S content was higher (116 ppm) the reaction was poisoned. FTIR measurements revealed that in the surface interaction of H₂ + CO₂ on Rh/TiO₂ Rh carbonyl hydride, surface formate, carbonates and surface formyl were formed. On the H₂S pretreated catalyst surface formyl species were missing. TPD measurements showed that adsorbed H₂S desorbed as SO₂, both from TiO₂-supported metals and from the support. IR, XP spectroscopy and TPD measurements demonstrated that the metal became apparently more positive when the catalysts were treated with H₂S and when the sulfur was built into the support. The promotion effect of H₂S was explained by the formation of new centers at the metal/support interface.     

THERMODYNAMIC CONSIDERATIONS ON THE PURIFICATION OF H2SO4 AND HIX PHASES IN THE IODINE-SULFUR HYDROGEN PRODUCTION PROCESS

The reaction equilibrium and phase equilibrium in H₂SO₄ and HIx phases produced by the Bunsen reaction of the iodine-sulfur thermochemical hydrogen production process were examined using a chemical process simulator, ESP, with a thermodynamic database based on the mixed solvent electrolyte model. At temperatures lower than ca. 110°C, the reaction of HI and H₂SO₄ produced elemental sulfur in both phases. At higher temperatures, the reverse Bunsen reaction occurred, and SO₂ was produced in the H₂SO₄ phase. In the HIx phase, conversely, SO₂ formation predominated in a narrow temperature range and H₂S was produced with the increase in temperature. The presence of N₂ gas lowered the temperature of the predominant reaction change. A feed of O₂ for purification was proposed to suppress the consumption of objective components in the H₂SO₄ phase purification, and an O₂ feed to the HIx phase for the suppression of H₂S and S impurities was proposed by the simulation.     

Water electrolysis on carbon electrodes enhanced by surfactant

In this paper, we present the water electrolysis on the carbon cloth electrode (CC) enhanced by a cationic surfactant, namely, hexadecyltrimethylammonium bromide (HTMAB). The influence of HTMAB on the two processes in water electrolysis, i.e., hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) was investigated. The results showed that an opposite effect of HTMAB on the HER and OER occurs with carbon-based electrodes, i.e., inhibiting the HER indicated as a small negative-potential shift, but enhancing OER as a large negative-potential shift. The improvement on the OER due to the introduction of HTMAB into H₂SO₄ is large enough to offset the declination of the HER in the water electrolysis. The enhancement of HTMAB on the OER is mainly due to accelerating H₂O₂ production from water oxidation at a relatively negative potential. The intermediate H₂O₂ in water electrolysis was detected and confirmed by an enzymatic method. The enhancement of HTMAB on the OER, however, was not observed on systems of Pt/H₂SO₄, CC/KOH and Ni/KOH. It shows that the mechanism of the water electrolysis on the different electrodes in the different electrolyte decides whether the HTMAB plays an enhanced role in the water electrolysis or not. All plausible reasons causing current increase in water electrolysis in H₂SO₄ apart from the catalytic effect of the HTMAB on enhancement of H₂O₂ production have been clearly eliminated. We conceive that there may be a possibility to obtain H₂O₂ rather than O₂ besides H₂ from the water electrolysis just using the carbon-based electrode with introduction of the cationic surfactant HTMAB as the electrolysis voltage is controlled within a certain range.     

Direct oxidation of hydrogen sulphide to sulphur using impregnated activated carbon catalysts

Low concentrations of H₂S were directly oxidized to sulphur and small quantities of SO₂, over seven different activated carbons with or without impregnation. The effectiveness of virgin activated carbon was tested at 175°C, 700 kPa, and O₂/H₂S ratio with 5% greater than stoichiometry. The conversion of H₂S was 99.9 mol% with SO₂ production of 3–6%, for 360 min runtime for Fisher coconut shell activated carbon and 648 min for Envirotrol bituminous (EB) activated carbon. Then the activated carbons became deactivated due to deposition of sulphur on the surface. Under these conditions mesoporous activated carbons such as EB and Hydrodarco had the longest breakthrough time. The addition of 5.5 wt% ammonium iodide, potassium iodide and potassium carbonate individually to EB decreased the production of SO₂ while having minimal effect on the overall H₂S conversion. The addition of 5.5 wt% NH₄I decreased the average SO₂ production from 2.5% to 0.9%. The activation energy for the H₂S oxidation on the 5.5 wt% NH₄I on EB activated carbon was determined to be 40 kJ/mol.     

Elemental sulfur production through regeneration of a SO2-adsorbed V2O5-CoO/AC in H2

H₂ regeneration of an activated carbon supported vanadium and cobalt oxides (V₂O₅-CoO/AC) catalyst–sorbent used for flue gas SO₂ removal is studied in this paper. Elemental sulfur is produced during the H₂-regeneration when effluent gas of the regeneration is recycled back to the reactor. The regeneration conditions affect the regeneration efficiency and the elemental sulfur yield. The regeneration efficiency is the highest at 330 °C, with SO₂ as the product. The production of elemental sulfur occurs at 350 °C and higher with the highest elemental sulfur yield of 9.8 mg-S/g-Cat. at 380 °C. A lower effluent gas recycle rate is beneficial to elemental sulfur production. Intermittent H₂ feeding strategy can be used to control H₂S concentration in the gas phase and increase the elemental sulfur yield. Two types of reactions occur in the regeneration, reduction of sulfuric acid to SO₂ by AC and reduction of SO₂ to elemental sulfur through Claus reaction. H₂S is an intermediate, which is important for elemental sulfur formation and for conversion of CoO to CoS that catalyzes the Claus reaction. The catalyst–sorbent exhibits good stability in SO₂ removal capacity and in elemental sulfur yield.     

Mass Spectrometric Studies at High Temperatures: XXIX, Thermal Decomposition and Sublimation of Alkali Metal Sulfates

Mass spectrometric investigations of the high temperature equilibrium vapor species over the solids Li₂SO₄, Na₂SO₄, K₂SO₄, Rb₂SO₄, and Cs₂SO₄ have shown that the products of thermal decomposition are M(g), where M = Li, Na, K, Rb, or Cs, SO₂(g), and O₂(g). In addition, molecules like K₂SO₄(g), Rb₂SO₄(g), and Cs₂SO₄(g) were observed, and the results show that sublimation as molecules becomes increasingly important for the heavier alkali metal sulfates. The equilibrium constants at several temperatures and heats for the decomposition reaction were determined for Li₂SO₄, Na₂SO₄, and K₂SO₄. Calculated values of the heats of decomposition for Na₂SO₄ and K₂SO₄, for which thermodynamic data are available, were in agreement with the measured quantities. The heats and entropies of sublimation to monomeric molecules for K₂SO₄(s), Rb₂SO₄(s), and Cs₂SO₄(s) were ΔH⁰₂, _1154°K= 72.8 ± 2.0 kcal mole^-1 and ΔS⁰_S1554°K= 29.2 ± 1.8 eu; ΔH⁰_S, _1123°K= 69.7 ± 1.0 kcal mole^-1 and ΔS⁰_{S, 1123°K}= 27.4 ± 0.8 eu; and ΔH⁰_{S, 1081°K}= 63.9 ± 1.7 kcal mole^-1 and ΔS⁰_{S, 1081°K}= 26.2 ± 1.6 eu, respectively.     

A study on the reactivity of Ce-based Claus catalysts and the mechanism of its catalysis for removal of H2S contained in coal gas

In this study, Claus reaction was applied for the selective removal of H₂S contained in the gasified coal gas, and the characteristics of Claus reaction over the Ce-based catalysts were investigated to propose the reaction mechanism. The Ce-based catalysts showed a high activity on Claus reaction. Specially, Ce_0.8Zr_0.2O₂ catalyst had a higher activity than CeO. On the basis of our experimental results, it was proposed that the selective oxidation of H₂S was carried out by the lattice oxygen in the Ce-based catalysts and that the reduction of SO₂ was performed by the lattice oxygen vacancy in the reduced catalyst. Since the mobility of the lattice oxygen in Ce_0.8Zr_0.2O₂ composite catalyst was better than the one in CeO₂, Ce_0.8Zr_0.2O₂ provided more lattice oxygen for the selective oxidation of H₂S. It was presumed that the reaction mechanism to convert H₂S and SO₂ into elemental sulphur over our prepared catalysts was different from the mechanism over the solid-acid catalysts. It is believed that Claus reaction over the Ce-based catalysts was carried out by the redox mechanism. Since the moisture was contained in the major components, CO and H, of the gasified fuel gas, the effects of CO and H₂O on the catalytic reaction were investigated over a Ce-based catalyst. The conversion of H₂S and SO₂ was decreased in Claus reaction over the Ce-based catalysts as the concentration of either H₂O or CO in the gasified coal gas was increased. Under the circumstances of the coexistence of both moisture and CO, however, the conversion was increased as the concentration of CO was increased. The reactivity of Claus reaction was varied in terms of the concentration ratio of CO to H₂O. The maximum conversion of H₂S and SO₂ was achieved in the condition of that the concentration of CO contained in the reacting gas was higher than the one of H₂O. The conversions of H₂S and SO₂ did not match to the stoichiometric ratios of Claus reaction. The higher conversion of H₂S was obtained in the higher concentration of H₂O, while the higher conversion of SO₂ was achieved in the higher concentration of CO. It was another evidence to indicate that the Claus reaction over the Ce-based catalysts was carried out by the redox mechanism.     

Selective oxidation of h2s in the presence of ammonia and water using co3o4/sio2 catalyst

The catalytic performance of some metal oxides in the selective oxidation of H₂S in the stream containing water vapor and ammonia was investigated in this study. Among the catalysts tested, Co₃O₄SiO₂ was the most promising catalyst for practical application. It showed very small amount of SO₂ emission even in the presence of excess water and ammonia. The solid product formed in the reaction was a mixture of elemental sulfur, ammonium thiosulfate and ammonium sulfate.     

Coproduction of sulphuric acid and hydrogen by sulphur-assisted water electrolysis process

The addition of sulphur powder to the anode compartment of the sulphuric acid-water electrolysis cell resulted in the suppression of oxygen evolution rate; at a cell voltage of 2 V, there was an acceleration of the hydrogen production rate, and the anode reaction led to the production of sulphate ion (SO ₄ ^2? ) by the oxidation of the sulphur powder. The net equation for this sulphur-assisted water electrolysis system may be written as S + 4H₂O → H₂SO₄ + 3H₂ whose stoichiometry was proved experimentally using phosphoric acid as the electrolyte. The whole process can be rationalized if the anodic oxidation of sulphur occurs at a lower potential than oxygen evolution and hence the energy consumption is lower. Thus the addition of sulphur to the anolyte helps to improve the energy efficiency of the acidic water electrolysis system and generates, simultaneously, sulphuric acid and hydrogen.     

Impurity control in the production of boric acid from colemanite in the presence of propionic acid

The most important problem in boric acid production from colemanite ores with H₂SO₄ is the formation of MgSO₄ impurity due to the partial decomposition of clay minerals in the reaction media. Increase of MgSO₄ concentration in solution may be balanced by the discharge of mother liquor which leads to decrease the efficiency of the process. Therefore, the intake rate of MgSO₄ should be lowered for obtaining high purity product in a high yield process. In order to control the intake rate of MgSO₄ impurity, propionic acid, which does not decompose the clay minerals, is used in an acid mixture with H₂SO₄. Batch wise laboratory experiments showed that the higher the propionic acid in reacting acid mixture, the lower the magnesium intake rate. When 10% of required H₂SO₄ replaced by propionic acid, magnesium intake concentration decreased to approximately half of the value obtained in the reaction with H₂SO₄.     

Scrubbing of SO2 with trona solution in a spray drier

Effect of operating variables on the scrubbing efficiency of SO₂ in a spray drier was investigated using trona (Na₂ CO₃ · NaHCO₃ · 2H₂ O) solutions of different compositions. Results indicated that for a Na/S mole ratio of two (stoichiometric ratio) about 80% of the SO₂ was removed. For higher Na/S ratios, SO₂ removal efficiency increased by up to 100%. Increased SO₂ concentration in the feed gas and decreased outlet temperature caused an increase in the overall conversion. At low temperatures and low SO₂ concentration, the reaction takes place mainly between SO₂ and Na₂ CO₃ As the temperature is increased, decomposition of NaHCO₃ to Na₂ CO₃ and sorption reactions take place simultaneously.     

The combined effect of H2O and SO2 on the simultaneous calcination/sulfation reaction in CFBs

The combined effect of H₂O and SO₂ on the reaction kinetics and pore structure of limestone during simultaneous calcination/sulfation reactions under circulating fluidized bed (CFB) conditions was first studied in a constant-temperature reactor. H₂O can accelerate the sulfation reaction rate in the slow-sulfation stage significantly but has a smaller effect in the fast-sulfation stage. H₂O can also accelerate the calcination of CaCO₃, and should be considered as a catalyst, as the activation energy for the calcination reaction was lower in the presence of H₂O. When the limestone particles are calcining, SO₂ in the flue gas can react with CaO on the outer particle layer and the resulting CaSO₄ blocks the CaO pores, increases the diffusion resistance of CO₂, and, in consequence, decreases the calcination rate of CaCO₃. Here, gases containing 15% H₂O and 0.3% SO₂ are shown to increase the calcination rate. This means that the accelerating effect of 15% H₂O on CaCO₃ decomposition is stronger than the impeding effect caused by 0.3% SO₂. The calcination rate of limestone particles was controlled by both the intrinsic reaction and the CO₂ diffusion rate in the pores, but the intrinsic reaction rate played a major role as indicated by the effectiveness factors determined in this work. This may explain the synergic effect of H₂O and SO₂ on CaCO₃ decomposition observed here. Finally, the effect of H₂O and SO₂ on sulfur capture in a 600 MWe CFB boiler burning petroleum coke is also analyzed. The sulfation performance of limestone evaluated by simultaneous calcination/sulfation is shown to be much higher than that by sulfation of CaO. Based on our calculations, a novel use of the wet flue gas recycle method was put forward to improve the sulfur capture performance for high-sulfur low-moisture fuels such as petroleum coke. © 2019 American Institute of Chemical Engineers AIChE J, 65: 1256–1268, 2019     

Adiabatic Calorimetric Decomposition Studies of 35 Mass% Hydroxylamine Hydrochloride/water

The HH (35 mass% hydroxylamine hydrochloride/water solution) and HH‐ind (stabilized 35 mass% hydroxylamine hydrochloride/water solution for industrial use) decomposition reaction has an onset temperature of ～145°C. The measured gas phase decomposition products are 0.25 mol N₂ and 0.06 mol O₂ per mol of hydroxylamine hydrochloride with small quantities of N₂O and H₂, and a trace amount of NO for a total noncondensable gas production of ～0.39 mol/mol of hydroxylamine hydrochloride. The reaction system is tempered by the evaporation of the solvent (water) and the reaction is drastically catalyzed by stainless and carbon steel.     

Auto-thermal chemical looping combustion of sulphur with Fe2O3 oxygen carriers for sulphuric acid production: Thermodynamics

Chemical looping is a novel fuel conversion and material separation technology. It can be applied to obtain sulphur through selective oxidation of H₂S. Further, chemical looping combustion (CLC) of sulphur can generate SO₂ with a high concentration without NO_x formation. The high SO₂ concentration is adjustable and facilitates large-scale H₂SO₄ production. In this study, we examined the thermodynamics of the CLC of sulphur for H₂SO₄ production, which has not been reported previously. We analyzed the effects of reactor temperature and sulphur to Fe₂O₃ oxygen carrier (OC) ratios on sulphur allotrope transformations and on the distributions of reaction products. Moreover, the reactors were operated auto-thermally. Based on this design, we examined the effects of fuel reactor (FR) and air reactor temperatures on the minimum recirculation of the OC, as well as the gas and solid products and heat released from the air reactor. Our results showed that the CLC of sulphur with Fe₂O₃ OC could occur through an auto-thermal process. The FR in a sulphur CLC system should be operated over a temperature range of 800–950°C, with an Fe₂O₃ OC recirculation between 45 and 143 kg/kg_S(s). Furthermore, when the FR was operated in the auto-thermal mode, we achieved 100% SO₂ conversion. The findings of this study may be applied to reactor design for large-scale H₂SO₄ production through CLC of sulphur.     

Direct Oxidation of H2 to H2O2 and Decomposition of H2O2 Over Oxidized and Reduced Pd-Containing Zeolite Catalysts in Acidic Medium

Both the conversion and H₂O₂ selectivity (or yield) in direct oxidation of H₂-to-H₂O₂ (using 1.7 mol% H₂ in O₂ as a feed) and also the H₂O₂ decomposition over zeolite (viz. H-ZSM-5, H-GaAlMFI and H- ) supported palladium catalysts (at 22 °C and atmospheric pressure) are strongly influenced by the zeolite support and its fluorination, the reaction medium (viz. pure water, 0.016 M or 1.0 M NaCl solution or 0.016 M H₂SO₄, HCl, HNO₃, H₃PO₄ and HClO₄), and also by the form of palladium (Pd⁰ or PdO). The oxidized (PdO-containing) catalysts are active for the H₂-to-H₂O₂ conversion and show very poor activity for the H₂O₂ decomposition. However, the reduced (Pd⁰-containing) catalysts show higher H₂ conversion activity but with no selectivity for H₂O₂, and also show much higher H₂O₂ decomposition activity. No direct correlation is observed between the H₂-to-H₂O₂ conversion activity (or H₂O₂ selectivity) and the Pd dispersion or surface acidity of the catalysts. Higher H₂O₂ yield and lower H₂O₂ decomposition activity are, however, obtained when the non-acidic reaction medium (water with or without NaCl) is replaced by the acidic one.     

Mechanism of the action of Ag and As on the anodic corrosion of lead and oxygen evolution at the Pb/PbO(2−x)/H2O/O2/H2SO4 electrode system

When a Pb electrode, immersed in H₂SO₄ solution, is polarized anodically in the PbO₂ potential range the Pb/PbO(_2−x)/H₂O/O₂/H₂SO₄ electrode system is established. Oxygen is evolved at the oxide—solution interface. The oxygen atoms formed as intermediates diffuse into the anodic layer and oxidize the metal. Through a solid-state reaction, the metal is oxidized first to tet-PbO and then to PbO₂. By studying the changes in the rate—potential relations of the above reactions, as well as the phase and chemical composition of the anodic layer, it was possible to elucidate the effect of Ag and As on these processes. The additives were introduced into the electrode system either by alloying with lead or by dissolving them in the H₂SO₄ solution. When added to the solution, both Ag and As lower the overvoltage of the oxygen evolution reaction. They have practically no effect on the corrosion reaction under galvanostatic polarization conditions. If alloyed in the metal, Ag reduces the oxidation rate of Pb significantly, while As enhances it. Both additives lower the stoichiometric number of the anodic oxide layer, ie they retard the oxidation of PbO to PbO₂. The results of these investigations were used to develop further the model of the mechanism of the reactions proceeding during the anodic oxidation of lead in H₂SO₄ solutions.     

Activation of Supported Pd Metal Catalysts for Selective Oxidation of Hydrogen to Hydrogen Peroxide

Catalytic activity of supported Pd metal catalysts (Pd metal deposited on carbon, alumina, gallia, ceria or thoria) showing almost no activity in the liquid-phase direct oxidation of H₂ to H₂O₂ (at 295 K) in acidic medium (0.02 M H₂SO₄) can be increased drastically by oxidizing them using different oxidizing agents, such as perchloric acid, H₂O₂, N₂O and air. In the case of the Pd/carbon (or alumina) catalyst, perchloric acid was found to be the most effective oxidizing agent. The order of the H₂-to-H₂O₂ conversion activity for the perchloric-acid-oxidized Pd/carbon (or alumina) and air-oxidized other metal oxide supported Pd catalysts is as follows: Pd/alumina < Pd/carbon < Pd/CeO₂ < Pd/ThO₂ < Pd/Ga₂O₃. The H₂ oxidation involves lattice oxygen from the oxidized catalysts. The catalyst activation results mostly from the oxidation of Pd metal from the catalyst producing bulk or sub-surface PdO. It also caused a drastic reduction in the H₂O₂ decomposition activity of the catalysts. There exists a close relationship between the H₂-to-H₂O₂ conversion activity and/or H₂O₂ selectivity in the oxidation process and the H₂O₂ decomposition activity of the catalysts; the higher the H₂O₂ decomposition activity, the lower the H₂-to-H₂O₂ conversion activity and/or H₂O₂ selectivity.     

Reduction of phosphogypsum for SO2 production: Insights into the release characteristics of SO2 following the solid–solid reaction of CaS and CaSO4 under different atmospheres

Phosphogypsum (PG) severely pollutes the environment and is difficult to recycle. PG is primarily composed of CaSO₄ · 2H₂O. In this study, the characteristics of SO₂ released from the solid–solid reaction between calcium sulphide (CaS) and CaSO₄ were thoroughly investigated using thermodynamic calculations. Experiments were performed by tuning the molar ratio of CaS to CaSO₄, reaction atmosphere (S, CH₄, N₂, and air), and the heating rate. As shown by the phase diagram, high reaction temperatures favour CaO stability, and the corresponding maximum SO₂ equilibrium partial pressure increases. The total SO₂ production significantly increased with increasing molar ratio and slightly increased when the ratio exceeded 1:3. The SO₂ productions were ranked from highest to lowest as follows: S, CH₄, N₂, CO, and air. The total SO₂ production decreased with increasing heating rate. For the reaction between CaS and CaSO₄, a higher molar ratio of CaS to CaSO₄ no less than 1:3, both S and CH₄ reductive atmospheres, and a lower heating rate (2°C/min) favour the total SO₂ emission.