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.     

Gasification of lignite and hard coal with air and oxygen enriched air in a pilot scale ex situ reactor for underground gasification

The main goal of the study presented in the paper was an experimental comparison of the underground lignite and hard coal seams air gasification simulated in the ex situ reactor. In the study lignite and hard coal were gasified with oxygen, air and oxygen enriched air as gasification agents in the 50- and 30-h experiments, respectively, with an intrinsic coal and strata moisture content as a steam source. Application of air as a sole gasification agent was problematic for a resulting rapid decrease in temperatures, deterioration of gas quality and, finally, cessation of gasification reactions. Use of oxygen/air mixture of an optimum ratio led to valuable gas production. In lignite seam gasification with oxygen/air (of 4:2 volume ratio) the average H₂ and CO contents in product gas were 23.1 vol.% and 6.3 vol.%, respectively, and the calorific value was 4.18 MJ/m³, whereas in hard coal gasification with the oxygen/air ratio (of 2:3 volume ratio) the average H₂ and CO contents in produced gas were 18.7 vol.% and 17.3 vol.%, respectively, and product gas calorific value equaled 5.74 MJ/m³.     

In situ plasma diagnostics of the chemistry behind sulfur doping of CVD diamond films

Microwave plasma enhanced chemical vapour deposition (CVD) has been used to grow sulfur doped diamond films using a 1% CH₄/H₂ gas mixture with various levels of H₂S addition (100–5000 ppm), upon undoped Si substrates. X-Ray photoelectron spectroscopy has shown that S is incorporated into the diamond at number densities (≤0.2%) that are directly proportional to the H₂S concentration in the gas phase. Four-point probe measurements showed the resistivity of these S-doped films to be a factor of three lower than undoped diamond grown under similar conditions. Sulfur containing diamond film was also obtained using a 0.5% CS₂/H₂ gas mixture, although the high resistivity of the sample indicated that the sulfur had been incorporated into the diamond lattice in a different manner compared with the H₂S grown samples. Molecular beam mass spectrometry has been used to measure simultaneously the concentrations of the dominant gas phase species present during growth, for a wide range of H₂S doping levels (1000–10 000 ppm in the gas phase). CS and CS₂ have been detected in significant concentrations in the plasma region as a result of gas phase reactions. Additional measurements from a 1% CS₂/H₂ plasma gave similar species mole fractions except that no CS was detected. These results suggest that CS may be the first step toward CS bond formation in the film and thereby a pathway allowing S incorporation into diamond. Optical emission spectroscopy has shown the presence of S₂ in both gas mixtures, consistent with the observed deposition of sulfur on the cool chamber walls.     

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.     

煤快速热解固相和气相产物生成规律

利用能有效避免二次转化反应的高频炉热解装置对3种不同变质程度的煤进行了600～1200℃条件下的快速热解,考察了在煤热解最初阶段焦产率、焦-C产率、热解气产率、热解气4种主要组分H₂、CO、CH₄和CO₂的比例以及热解气热值随煤阶和热解温度的变化规律。结果表明,焦的产率和焦-C的产率均随煤阶的升高而升高,热解气的产率随煤阶的升高而降低;热解温度的提高能显著降低煤焦和焦-C的产率并提高热解气的产率。热解气组分以H₂相似文献   

Water–Gas Shift Kinetics Over FeCr-based Catalyst: Effect of Hydrogen Sulphide

Kinetics of the water–gas shift reaction over a FeCr-based catalyst is measured with high and low extremes for CO₂ and H₂ content and containing 11–35 ppmv of H₂S, relevant for separation-enhanced water–gas shift in IGCC. Kinetics is well described by a power rate law. H₂S negatively affects the reaction rate. Comparison with literature shows a more elaborate kinetic model is better suited to fully capture kinetics from sulphur-free to high-sulphur.     

Removal of H2S using a new Ce(IV) redox mediator by a mediated electrochemical oxidation process

BACKGROUND: Hydrogen sulfide (H₂S) from industrial activities and anaerobic manure decomposition in commercial livestock animal operations is an offensive malodorous and toxic gas even in small concentrations, causing serious discomfort and health and social problems. The objective of this study was to employ for the first time a novel, attractive, low cost, environmentally benign mediated electrochemical oxidation (MEO) process with Ce(IV) as the redox catalyst for H₂S gas removal from an H₂S–air feed mixture. RESULTS: The influence of liquid flow rate (Q_L) from 2–4 L min^?1, gas flow rate (Q_G) from 30–70 L min^?1, H₂S concentration in the H₂S–air feed mixture from 5–15 ppm, and Ce(III) pre‐mediator concentration in the electrochemical cell from 0.1–1 mol L^?1 on H₂S removal efficiency were investigated. Both liquid and gas flow rates influenced the removal efficiencies, but in opposite directions. Nearly 98% H₂S removal was achieved when the concentration of Ce(IV) mediator ion in the flowing scrubbing liquid reached 0.08 mol L^?1. CONCLUSIONS: The new MEO method proved promising for H₂S removal, achieving high removal efficiency. Integration of the electrochemical cell with the scrubber set‐up ensured continuous regeneration of the mediator and its repeated reuse for H₂S removal, avoiding use of additional chemicals. Since the process works at room temperature and atmospheric pressure utilizing conventional transition metal oxide electrodes more commonly used in industrial applications, it is also safe and economical. Copyright © 2008 Society of Chemical Industry     

Co-gasification of a lignite/waste-tyre mixture in a moving bed

The thermal treatment of waste-tyre by co-gasification with lignite was investigated on a commercial scale during the Lurgi gasification process. The experiments proved that this material can be treated in a mixture with lignite in the process of oxygen–steam pressure co-gasification in a moving bed, because a waste-tyre admixture improves the net calorific value of the raw gas obtained by 3% in comparison with that from the gasification of lignite alone. Further, it was found that the H₂S and CH₃SH contents in the raw gas are lower in the case of co-gasification than those from the gasification of lignite alone. Considering the very low reactivity of the char from waste-tyre and the resultant unburned carbon in the ash, the optimal content of the waste-tyre admixture in the gasified feed should not exceed 10 wt.%, whereas short-term increases of up to 20 wt.% will not cause any technological problems or significant economic losses.     

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.     

Spectroscopic investigation,cage occupancy,and gas storage capacity of hydroquinone clathrates formed with H<Subscript>2</Subscript>S-N<Subscript>2</Subscript> and COS-N<Subscript>2</Subscript> binary gas mixtures

The objective of this investigation was to determine whether hydroquinone (HQ) can form clathrate compounds with two sulfides (hydrogen sulfide (H₂S) and carbonyl sulfide (COS)) at their diluted concentrations. Hydroquinone samples obtained at ambient temperature and at two pressures (40 and 80 bar) for binary gas mixtures consisting of H₂S-N₂ and COS-N₂, were analyzed using solid-state ¹³C NMR and Raman spectroscopy. An elemental analyzer was also used to obtain quantitative information regarding the kind and amount of gas captured in the solid samples. Results show that H₂S can be concentrated within the solid clathrate from H₂S-containing gas, while COS is little captured after reaction with the COS-containing gas. This suggests that the HQ clathrate can be used to remove H₂S, and that selective separation can be achieved when two sulfides of H₂S and COS coexist. On the basis of the calculated cage occupancies of the gas components in the solid clathrate, the enclathration preference of the gas components used in this research was found to be the order of H₂S>N₂>COS.     

Laboratory-scale coal reactivity testing for entrained gasification

A relatively simple and rapid micro-gasification test has been developed for measuring gasification reactivities of carbonaceous materials under conditions which are more or less representative of an entrained gasification process, such as the Shell coal gasification process. Coal particles of < 100 μm are heated within a few seconds to a predetermined temperature level of 1000–2000 °C, which is subsequently maintained. Gasification is carried out with either CO₂ or H₂O. It is shown that gasification reactivity increases with decreasing coal rank. The CO₂ and H₂O gasification reactions of lignite, bituminous coal and fluid petroleum coke are probably controlled by diffusion at temperatures 1300–1400 °C. Below these temperatures, the CO₂ gasification reaction has an activation energy of about 100 kJ mol^?1 for lignite and 220–230 kJ mol^?1 for bituminous coals and fluid petroleum coke. The activation energies for H₂O gasification are about 100 kJ mol^?1 for lignite, 290–360 kJ mol^?1 for bituminous coals and about 200 kJ mol^?1 for fluid petroleum coke. Relative ranking of feedstocks with the micro-gasification test is in general agreement with 6 t/d plant results.     

H2S Reactive Absorption from Off‐Gas in a Spray Column: Insights from Experiments and Modeling

H₂S removal from an off‐gas stream was performed in a spray column by H₂S reactive absorption into a NaOH solution. The individual and interactive effects of three independent operating variables on the percentage of absorbed H₂S were investigated: the initial pH of the scrubbing solution, the initial scrubbing solution temperature, and the volumetric liquid‐to‐gas ratio. The optimum operating variables were determined by response surface methodology (RSM) attaining a percentage of absorbed H₂S of 98.7 ± 0.2 %. Additionally, the process performance was modeled by an artificial neural network (ANN) to predict the percentage of absorbed H₂S. The results showed that the experimental data agreed better with the ANN model than with the RSM results.     

Removal of minor concentration of H2S on MDEA-modified SBA-15 for gas purification

A promising adsorbent for H₂S removal of minor concentration for gas purification was prepared by synthesizing and modifying the mesoporous molecular silica of SBA-15 with methyl-diethyl-amine (MDEA). Removal performance of minor concentration of H₂S on the adsorbent was experimentally studied in a dynamic setup. The adsorbents showed good performance in removing H₂S from gas steams. The loaded amines did not change the ordered structure of SBA-15, but enhanced its removing H₂S. The adsorbents were characterized by X-ray powder diffraction (XRD) and N₂ adsorption/desorption. Effect of the R (the loading ratio) of MDEA, effect of initial H₂S concentration and effect of moisture on removal performance of H₂S were studied respectively. With increase of the R of MDEA, the H₂S removal performance of the adsorbent was improved obviously. When R of MDEA was 0.6, the removal performance was optimum. Initial H₂S concentration had a large effect on the removal performance. Both breakthrough capacity and saturation capacity increased as the initial H₂S concentration increased. In the presence of moisture, the experimental results showed the improvement in the removal performance of H₂S. In addition, not only was the adsorbent regenerable by purging with the purified gas, but also the removal performance was stable in removal adsorption cycles.     

Electrochemical Separation of Hydrogen Sulfide from Natural Gas

Abstract

An electrochemical process has been developed for the removal of H₂S from contaminated natural gas. Removals as high as 80.7% have been achieved from a simulated process gas (2000 ppm H₂S). H₂S is removed by reduction to the sulfide ion and hydrogen gas at the cathode. The sulfide ion migrates to the anode through a molten electrolyte suspended in an inert ceramic matrix. Once at the anode it is oxidized to elemental sulfur and swept away for condensation in an inert gas stream. No materials are required beyond initial electrolyte membrane installation; the H₂S is converted in one step to elemental sulfur making it an economically attractive process both from the lack of raw materials and the lack of any solvent regeneration.     

Removal of hydrogen sulfide from a steam-hydrogasifier product gas by zinc oxide sorbent: Effect of non-steam gas components

Removal of H₂S from a steam-hydrogasifier product gas was studied at 636 K and 1 atm using a commercially available zinc oxide sorbent in a packed-bed reactor. A mixture gas containing 22% CH₄, 18.7% H₂, 8.8% CO and 5.5% CO₂ (non-steam components subtotaling to 55%) balanced with steam was used to simulate the steam-hydrogasifier product gas. Sorbent particles of 150–250 μm size were used to eliminate the effect of intraparticle mass transfer limitation. Experiments were conducted to monitor H₂S breakthrough of reactor effluent stream for operation parameters such as space velocity and inlet H₂S concentration. With space velocity varied from 6000 to 8000 to 12,000 h^?1 for inlet H₂S concentration in the range of 100–800 ppmv, sulfur capture capacity of the sorbent (S_cap) for 2 ppmv H₂S breakthrough did not change notably, indicating that, for each inlet H₂S concentration tested, sorbent utilization for sulfur removal was not affected by the space velocity. Meanwhile, for each space velocity tested, S_cap increased monotonically as the inlet H₂S concentration increased from 100 to 500 to 800 ppmv, which is opposite to the result observed for the mixture gas devoid of CH₄, H₂, CO and CO₂. As the overall content of these non-steam components of the simulation gas was halved for each inlet H₂S concentration tested at 8000 h^?1 space velocity, S_cap for non-steam gas components of 27.5% content corresponded approximately to the median value of those for the non-steam gas components of 55% and 0% content, suggestive of linear dependency of S_cap upon the content of the non-steam components for the inlet H₂S concentration tested.     

Protic ionic liquids for the selective absorption of H2S from CO2: Thermodynamic analysis

The solubilities of H₂S and CO₂ in four protic ionic liquids (PILs)—methyldiethanolammonium acetate, methyldiethanolammonium formate, dimethylethanolammonium acetate, and dimethylethanolammonium formate were determined at 303.2–333.2 K and 0–1.2 bar. It is shown PILs have higher absorption capacity for H₂S than normal ionic liquids (ILs) and the Henry's law constants of H₂S in PILs (3.5–11.5 bar at 303.2 K) are much lower than those in normal ILs. In contrast, the solubility of CO₂ in PILs is found to be a magnitude lower than that of H₂S, implying these PILs have both higher absorption capacity for H₂S and higher ideal selectivity of H₂S/CO₂ (8.9–19.5 at 303.2 K) in comparison with normal ILs. The behavior of H₂S and CO₂ absorption in PILs is further demonstrated based on thermodynamic analysis. The results illustrate that PILs are a kind of promising absorbents for the selective separation of H₂S/CO₂ and believed to have potential use in gas sweetening. © 2014 American Institute of Chemical Engineers AIChE J 60: 4232–4240, 2014     

Hydrocracking mechanisms in molten zinc chloride. Isotope scrambling and pyrolysis studies

Bruceton coal was hydrocracked in molten zinc chloride using H₂D₂ mixtures. No HD was observed. The pyrolysis of Bruceton coal and a lignite in molten zinc chloride and an inert salt was carried out and the tetrahydrofuran and pyridine extractability of the products determined. In the absence of H₂, zinc chloride is not an effective cracking catalyst. It is tentatively concluded that the catalytically active species is formed from zinc chloride and something in the coal and H₂. The interactions between zinc chloride and the lignite appear to be significantly different than the interactions between zinc chloride and the bituminous coal.     

Kinetics of removal of impurities from gases by high pressure adsorption

Among other processes, adsorption is used for the removal of hydrogen sulphide from natural gases. Hereby, competitive adsorption of the different gas components plays an important role, e.g., that of carbon dioxide. Data of equilibrium loading and adsorption kinetics are required for the design of adsorbers, filled with molecular sieve. In order to obtain these data under the prevailing operating conditions, hydrogen sulphide was removed from gas mixtures H₂S/CH₄ and H₂S/CO₂/CH₄, in a pilot plant, by adsorption on molecular sieve 5A. The equilibrium loading, the height of transfer zone, and the length of unused bed were determined from the measured breakthrough curves of H₂S. With these data, the breakthrough time and the optimum process conditions were calculated for a practical example.     

Highly selective absorption separation of H2S and CO2 from CH4 by novel azole-based protic ionic liquids

Recently, the selective removal of H₂S and CO₂ has been highly desired in natural gas sweetening. Herein, four novel azole-based protic ionic liquids (PILs) were designed and prepared through one-step neutralization reaction. The solubility of H₂S (0–1.0 bar), CO₂ (0–1.0 bar), and CH₄ (0–5.0 bar) was systematically measured at temperatures from 298.2 to 333.2 K. NMR and theoretical calculation were used to investigate the reaction mechanism between these PILs and H₂S. Reaction equilibrium thermodynamic model (RETM) was screened to correlate the H₂S solubility. Impressively, 1,5-diazabicyclo[4,3,0] non-5-ene 1,2,4-1H-imidazolide ([DBNH][1,2,4-triaz]) shows the highest H₂S solubility (1.4 mol/mol or 7.3 mol/kg at 298.2 K and 1.0 bar) and superior H₂S/CH₄ (831) and CO₂/CH₄ (199) selectivities compared with literature results. Considering the excellent absorption capacity of H₂S, high H₂S/CH₄, and CO₂/CH₄ selectivity, acceptable reversibility, as well as facile preparation process, it is believed that azole-based PILs provide an attractive alternative in natural gas upgrading process.     

Hydrogen sulfide gas detection by Au-decorated ZnO nanotube: a computational study and comparison to experimental observations

Based on the experimental reports, Au-decoration on the ZnO nanostructures dramatically increases the electronic sensitivity to H₂S gas. In the current study, we computationally scrutinized the mechanism of Au-decoration on a ZnO nanotube (ZON) and the influence on its sensing behavior toward H₂S gas. The intrinsic ZON weakly interacted with the H₂S gas with an adsorption energy of ?11.2 kcal/mol. The interaction showed no effect on the HOMO–LUMO gap and conductivity of ZON. The predicted response of intrinsic ZON toward H₂S gas is 6.3, which increases to 78.1 by the Au-decoration at 298 K. The corresponding experimental values are about 5.0 and 80.0, indicating excellent agreement with our findings. We showed that the Au atom catalyzes the reaction 3O₂?+?2H₂S?→?2SO₂?+?2H₂O. Our calculated energy barrier (at 298 K) is about 12.3 kcal/mol for this reaction. The gap and electrical conductance Au-ZON largely changed by this reaction are attributed to the electron donation and back-donation processes. The obtained recovery time is about 1.35 ms for desorption of generated gases from the surface of the Au-ZON sensor.