Regeneration of Fe2O3-based high-temperature coal gas desulfurization sorbent in atmosphere with sulfur dioxide

Regeneration of a high-temperature coal gas desulfurization sorbent is a key technology in its industrial applications. A Fe₂O₃-based high-temperature coal gas desulfurizer was prepared using red mud from steel factory. The influences of regeneration temperature, space velocity and regeneration gas concentration in SO₂ atmosphere on regeneration performances of the desulfurization sorbent were tested in a fixed bed reactor. The changes of phase and the composition of the Fe₂O₃-based high-temperature coal gas desulfurization sorbent before and after regeneration were examined by X-ray diffraction (XRD) and X-ray Photoelectron spectroscopy(XPS), and the changes of pore structure were characterized by the mercury intrusion method. The results show that the major products are Fe₃O₄ and elemental sulfur; the influences of regeneration temperature, space velocity and SO₂ concentration in inlet on regeneration performances and the changes of pore structure of the desulfurization sorbent before and after regeneration are visible. The desulfurization sorbent cannot be regenerated at 500°C in SO₂ atmosphere. Within the range of 600°C–800°C, the time of regeneration becomes shorter, and the regeneration conversion increases as the temperature rises. The time of regeneration also becomes shorter, and the elemental sulfur content of tail gas increases as the SO₂ concentration in inlet is increased. The increase in space velocity enhances the reactive course; the best VSP is 6000 h⁻¹ for regeneration conversion. At 800°C, 20 vol-% SO₂ and 6000 h⁻¹, the regeneration conversion can reach nearly to 90%.     

Metal-catalyzed etching of graphene governed by metal–carbon interactions: A comparison of Fe and Cu

We present a comparative investigation on the etching of graphene catalyzed by Fe and Cu. When Fe or Cu thin film deposited on graphene is rapidly annealed in either N₂ or forming gas (10% H₂/90% N₂), particles are produced due to the dewetting of thin films. Low-voltage scanning electron microscopy reveals different morphology for Fe and Cu particles and their strikingly different catalytic etching behaviors. For the Fe thin film on graphene annealed at 950 °C in either gas environment, graphene is severely damaged, suggesting that the etching could occur through catalytic carbon hydrogenation or carbon dissolution into Fe due to the strong Fe–C interactions. In contrast, while no etching takes place for Cu particles on graphene at 1050 °C in N₂, Cu particles catalytically etch channels in graphene in forming gas through carbon hydrogenation, and the width of the channel is much narrower than the diameter of Cu particle due to the non-wetting behavior of Cu on graphene. The weak interactions between Cu and graphene, along with the low solubility of carbon in Cu, make Cu particles ideal for tracking their etching paths on graphene. This work provides new insights into the metal-catalyzed etching of graphene.     

Mo and Co doped V2O5/AC catalyst-sorbents for flue gas SO2 removal and elemental sulfur production

Flue gas SO₂ removal at 200 °C over Mo and Co doped V₂O₅/AC catalyst-sorbents and regeneration of the used catalyst-sorbents in H₂ at 380 °C in the same reactor are studied in this paper. Compared with V₂O₅/AC, the catalyst-sorbents containing Co show higher SO₂ uptake while the one containing Mo shows a slightly lower SO₂ uptake. Elemental sulfur is produced during H₂-regeneration of the used catalyst-sorbents when effluent gas of the regeneration is recycled back to the reactor. H₂-regeneration of the used V₂O₅/AC produces little elemental sulfur, but the Mo and Co doped ones show high elemental sulfur yields with an elemental sulfur selectivity of 50% for a catalyst-sorbent containing 2% V₂O₅, 0.5% MoO₃ and 0.5% CoO, V2Mo0.5 Co0.5/AC. Molybdenum and cobalt sulfides are likely formed in the regeneration, which catalyze the elemental sulfur formation but reduce the SO₂ uptake of the catalyst-sorbents in the subsequent SO₂ removal stage.     

Regeneration of Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>-based high-temperature coal gas desulfurization sorbent in atmosphere with sulfur dioxide

Regeneration of a high-temperature coal gas desulfurization sorbent is a key technology in its industrial applications. A Fe₂O₃-based high-temperature coal gas desulfurizer was prepared using red mud from steel factory. The influences of regeneration temperature, space velocity and regeneration gas concentration in SO₂ atmosphere on regeneration performances of the desulfurization sorbent were tested in a fixed bed reactor. The changes of phase and the composition of the Fe₂O₃-based high-temperature coal gas desulfurization sorbent before and after regeneration were examined by X-ray diffraction (XRD) and X-ray Photoelectron spectroscopy(XPS), and the changes of pore structure were characterized by the mercury intrusion method. The results show that the major products are Fe₃O₄ and elemental sulfur; the influences of regeneration temperature, space velocity and SO₂ concentration in inlet on regeneration performances and the changes of pore structure of the desulfurization sorbent before and after regeneration are visible. The desulfurization sorbent cannot be regenerated at 500°C in SO₂ atmosphere. Within the range of 600°C–800°C, the time of regeneration becomes shorter, and the regeneration conversion increases as the temperature rises. The time of regeneration also becomes shorter, and the elemental sulfur content of tail gas increases as the SO₂ concentration in inlet is increased. The increase in space velocity enhances the reactive course; the best VSP is 6000 h^?1 for regeneration conversion. At 800°C, 20 vol-% SO₂ and 6000 h^?1, the regeneration conversion can reach nearly to 90%.     

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.     

SO2 retention on CaO/activated carbon sorbents. Part III. Study of the retention and regeneration conditions

The retention of SO₂ on CaO/activated carbon sorbents is studied. The effect of several variables such as the reaction temperature, partial pressure of SO₂ for different calcium loads, and O₂ presence are analysed. Additionally, the regeneration and reutilization of spent sorbents is investigated. In all cases presence of well-dispersed CaO in the sorbents improves SO₂ retention in comparison with the activated carbon. In absence of O₂ in the gas mixture, the amount of SO₂ retained does not depend on the SO₂ partial pressure in the range of partial pressures studied and, as expected, SO₂ physisorption on the activated carbon support occurs at room temperature. SO₂ retention occurs in surface CaO between 100 °C and 250 °C, and in bulk CaO above 300 °C. The total calcium conversion is reached at 500 °C. Above 550 °C calcium-catalysed carbon gasification by SO₂ occurs. In presence of O₂ in the gas mixture, the studied sorbents are very effective for SO₂ removal. However, the SO₂ retention process in presence of oxygen must be carried out at temperatures lower than 300 °C to avoid carbon gasification by O₂. The thermal regeneration of the spent sorbents can be done under inert atmosphere (880 °C) with only 20% activity loss after the first regeneration cycle due to sintering and formation of CaS. No additional activity loss is detected in the subsequent cycles.     

Catalytic conversion of CO,NO and SO2 on the supported sulfide catalyst: I. Catalytic reduction of SO2 by CO

Catalytic reduction of SO₂ to elemental sulfur by CO has been systematically investigated over γ-Al₂O₃-supported sulfide catalysts of transition metals including Co, Mo, Fe, CoMo and FeMo with different loadings of the metals. The sulfided CoMo/Al₂O₃ exhibited outstanding activity: a complete conversion of SO₂ was achieved at a temperature of 300°C. The reaction proceeds catalytically and consistently over time and most efficiently at a molar feed ratio CO/SO₂ = 2. A precursor CoMo/Al₂O₃ oxide which experienced sulfurization through the CO–SO₂ reaction yielded a working sulfide catalyst having a yet lower activity than the CoMo catalyst sulfided before reaction (pre-sulfiding). The catalytic activity of various metal sulfides decreased in order of 4% Co 16% Mo > 4% Fe15% Mo > 16% Mo ≥ 25% Mo > 14% Co ≥ 4% Co > 4% Fe. A DRIFT study showed that CO adsorbs exclusively on CoMo phase and that SO₂ predominantly on γ-Al₂O₃. It is suggested that the Co–Mo–S structure is more adequate than the other metal-sulfur structures for the formation of a carbonyl sulfide (COS) intermediate because of the proper strength of metal–sulfur bond, and catalytically works with γ-Al₂O₃ for the COS–SO₂ reaction.     

Improved performance of the direct methanol redox fuel cell

Advancements in the performance of the direct methanol redox fuel cell (DMRFC) were made through anolyte/catholyte composition and cell temperature studies. Catholytes prepared with different iron salts were considered for use in the DMRFC in order to improve the catholyte charge density (i.e., iron salt solubility) and fuel cell performance. Following an initial screening of different iron salts, catholytes prepared with FeNH₄(SO₄)₂, Fe(ClO₄)₃ or Fe(NO₃)₃ were selected and evaluated using electrolyte conductivity measurements, cyclic voltammetry and fuel cell testing. Solubility limits at 25 °C were observed to be much higher for the Fe(ClO₄)₃ (>2.5 M) and Fe(NO₃)₃ (>3 M) salts than FeNH₄(SO₄)₂ (~1 M). The Fe(ClO₄)₃ catholyte was identified as a suitable candidate due to its high electrochemical activity, electrochemical reversibility, observed half-cell potential (0.83 V vs. SHE at 90 °C) and solubility. DMRFC testing at 90 °C demonstrated a substantial improvement in the non-optimized power density for the perchlorate system (79 mW cm⁻²) relative to that obtained for the sulfate system (25 mW cm⁻²). Separate fuel cell tests showed that increasing the cell temperature to 90 °C and increasing the methanol concentration in the anolyte to 16.7 M (i.e., equimolar H₂O/CH₃OH) yield significant DMRFC performance improvements. Stable DMRFC performance was demonstrated in short-term durability tests.     

Breakthrough and Sulfate Conversion Analysis during Removal of Sulfur Dioxide by Calcium Oxide Sorbents

Sorption of sulfur dioxide (SO₂) was carried out on calcium‐based sorbents under dynamic conditions in a fixed bed. The experimental conditions were reaction temperature (700 to 1000°C), SO₂ concentration (1000‐10 000 ppm), sorbent particles size (1 to 2 mm) and the types of sorbents (hydroxide or carbonate). The sorption process was found to be effective at low concentration levels (less than 10 000 ppm) as the breakthrough time significantly decreased with increase in concentration. The maximum removal of SO₂ was observed at a reaction temperature of 950°C. The hydroxide‐based sorbents of relatively smaller particle size were found to exhibit superior sorption performance in terms of longer breakthrough time and higher sulfate conversion. A mathematical model developed, assuming a porous structure of the sorbent materials, attributed the low sulfation conversion during SO₂ sorption due to a pore diffusion mechanism.     

Study of SO2 adsorption and thermal regeneration over activated carbon-supported copper oxide catalysts

The mechanisms of SO₂ adsorption and regeneration over activated carbon-supported copper oxide sorbent/catalysts were analyzed. Studies were carried out in a fixed-bed reactor equipped with a non-dispersive infrared gas analyzer to detect the reaction products and by using X-ray powder diffraction (XRPD) and temperature-programmed desorption (TPD) experiments to characterize the nature of the sulfate species and surface oxygen complexes. The results indicate that SO₂ was catalytically oxidized to SO₃ over a copper phase in the presence of gaseous oxygen, and then reacted with a copper site to form a sulfate linked to copper without desorption into the gas phase. The activated carbon support did not participate in this sulfation reaction. After the adsorption of SO₂, the exhausted sorbent/catalysts could be regenerated by direct heat treatment in inert gas at temperatures between 260 and 480 °C, while the neighboring surface oxygen complexes on the carbon surface were acting as the reducing agents to reduce CuSO₄ to Cu. During the subsequent adsorption process, the copper is rapidly oxidized by oxygen in the flue gas.     

Water-gas shift reaction over Cu–Zn,Cu–Fe,and Cu–Zn–Fe composite-oxide catalysts prepared by urea-nitrate combustion

Water-gas shift reaction was investigated over Cu–Zn, Cu–Fe and Cu–Zn–Fe composite-oxide catalysts at atmospheric pressure from 200 to 375 °C in terms of reducing the CO content with maximal H₂ yield. The Cu_0.15ZnFe₂ spinel catalyst expressed a higher CO conversion level and H₂ yield at a lower temperature compared to the Cu_0.15Zn and Cu_0.15Fe catalysts. Adding H₂O to the feed up to 30% (v/v), but not above, increased the CO reduction level, presumably by increasing the hydroxyl species to react with the adsorbed CO. Increasing the W/F ratio to 0.24 g s cm^?3 increased the CO conversion level to 0.76 at 275 °C with the Cu_0.15ZnFe₂ catalyst, and could be further increased to 0.86 at 350 °C by increasing the Cu molar ratio to 0.30 (Cu_0.30ZnFe₂). Nevertheless, increasing the Cu molar content to 0.50 reduced the CO conversion level. No requirement for adding O₂ when using the Cu_0.30ZnFe₂ catalyst at >260 °C was observed. Increasing the CO content in the reactant decreased its conversion level. The performance of the Cu_0.30ZnFe₂ catalyst was stable over a test period in a CO-rich condition. No undesired product was detected, suggesting a higher selectivity for hydrogen production with a low CO content.     

Microstructure,mechanical properties,and corrosion resistance of Mo2(Fe,Ni)B2-based cermets prepared by reaction boronizing sintering

Mo₂(Fe,Ni)B₂-based cermets were successfully prepared by reaction boronizing sintering strategy, and their phase transformation, microstructure evolution, mechanical properties, and corrosion resistance were investigated. Mo₂(Fe,Ni)B₂ was formed by the reaction between (Fe,Ni)₂B and Mo during solid-phase sintering. In the temperature range of 1010–1270 °C, extremely rapid densification occurred, and nearly full densification was obtained at the sintering temperature of 1270 °C. Mo₂(Fe,Ni)B₂-based cermets demonstrated superior mechanical properties with transverse rupture strength of 2140 ± 35 MPa, Rockwell hardness of 83.9 ± 0.1 HRA, and fracture toughness of 22.5 ± 0.6 MPa m^1/2. Moreover, corrosion current density of Mo₂(Fe,Ni)B₂-based cermets was about four orders of magnitude lower than that of Mo₂FeB₂-based cermets, which indicates excellent corrosion resistance.     

NO reduction over nanostructure M-Cu/ZSM-5 (M: Cr,Mn, Co and Fe) bimetallic catalysts and optimization of catalyst preparation by RSM

The selective catalytic reduction (SCR) of NO with NH₃ in the presence of oxygen over a series of H-ZSM-5 supported transition metal oxides (Co, Mn, Cr, Cu and Fe) was investigated. Among them, Cu/ZSM-5 nanocatalyst was found to be the most promising catalyst based on activity. The modification of Cu/ZSM-5 by adding different transition metals (Co, Mn, Cr and Fe) to improve the efficiency of NO conversion was studied. The results indicated that the Fe–Cu/ZSM-5 bimetallic nanocatalyst was the highest active catalyst for NO conversion (67% at 250 °C and 93% at 300 °C). Response surface methodology (RSM) involving central composite design (CCD) was employed to evaluate and optimize Fe–Cu/ZSM-5 preparation parameters (Fe loading, calcinations temperature, and impregnation temperature) in SCR of NO at 250 °C. The optimum condition for maximum NO conversion was estimated at 4.2 wt.% Fe loading, calcinations temperature of 577 °C and impregnation temperature of 43.5 °C. Under these condition, experimental NO conversion efficiency was 78.8%, which was close with the predicted value (79.4%).     

Methane Combustion in the Presence of Zirconia-Based Catalysts

The influence of “superacidic” modification has been shown to enhance the methane combustion activity of ZrO₂ at 800 °C. Modification with SO₄ ²⁻, Fe/Mn/SO₄ ²⁻ and MoO₃ has been investigated, with the latter showing the most marked effect although this is a critical function of loading. Ceria–zirconia catalysts are also active, although the compositions studied are not as effective as 5 wt% MoO₃/ZrO₂ which shows a greater mass normalised activity than Fe₂O₃.     

Oxidative regeneration of sulfided sorbent by H2S without emission of SO2

Much SO₂, another perilous air pollutant, was emitted during the oxidative regeneration of sulfided sorbent by H₂S. In order to prevent emission of SO₂, we carried out oxidative regeneration with the physical mixture of CaO and sulfided sorbent and investigated the effect of regeneration temperature and oxygen concentration on the reactivity of CaO with S0₂. The effluent gases were analyzed by G.C. and the properties of sorbent were characterized by XRD. SEM, TG/DTA and EPMA. Deterioration of reactivity of CaO with S0₂ resulted in increment of emission of SO₁₂ due to the structural changes of CaO above 750°C and that at 850°C was more severe. Furthermore EPMA and XRD analysis revealed that product layer diffusion through the solid product, CaSO₄, was the rate limiting step for CaO sulfidation. The reaction of CaO w:.th SO₂ was first order approximately and that was accelerated by high O₂ concentration.     

Enhanced insulation and piezoelectric properties of 0.57(Bi0.8La0.2)FeO3-0.43PbTiO3 solid solutions with Fe addition

0.57(Bi_0.8La_0.2)FeO₃-0.43PbTiO₃-x mol%Fe₂O₃ ceramics (BLF-PT-xFe, x = 0, 0.025, 0.05, 0.125, and 0.25) were prepared by the conventional solid-state reaction method. X-ray diffraction (XRD) reveals that all samples display the perovskite structure with a coexistence of tetragonal (T) phase and rhombohedral (R) phase, while the incorporation of Fe promotes the phase transition from T to R. Scanning electron microscopy (SEM) images show that all samples are well crystallized and their grain size increases noticeably with the increase of Fe content. X-ray photoelectron spectroscopy (XPS) results indicate that Fe doping significantly inhibits the formation of oxygen vacancies, thereby improving insulation of BLF-PT-xFe ceramics. Interestingly, the Curie temperature of BLF-PT-xFe is around 330°C, little changing with the variation of Fe content. However, the depolarization temperatures of BLF-PT ceramics with Fe are 50°C higher than that of the sample without Fe doping. The hopping of second ionized oxygen vacancies are the major carriers in the temperature range of 200°C–500°C. The optimal component of BLF-PT-xFe ceramics appear at x = 0.05, where the dielectric loss tanδ, AC resistivity (200°C), and piezoelectric coefficient d₃₃ could be 0.015, 7 × 10⁶ Ω cm, and 245 pC/N, respectively. All these results indicate that the Fe addition is an effective method to enhance dielectric and piezoelectric properties.     

High temperature interfacial phase stability of a Mo/Ti3SiC2 laminated composite

A Mo/Ti₃SiC₂ laminated composite is prepared by spark plasma sintering at 1300 °C under a pressure of 50 MPa. Al powder is used as sintering aid to assist the formation of Ti₃SiC₂. The fabricated composites were annealed at 800, 1000 and 1150 °C under vacuum for 5, 10, 20 and 40 h to study the composite's interfacial phase stability at high temperature. Three interfacial layers, namely Mo₂C layer, AlMoSi layer and Ti₅Si₃ solid solution layer are formed during sintering. Experimental results show that the Mo/Ti₃SiC₂ layered composite prepared in this study has good interfacial phase stability up to at least 1000 °C and the growth of the interfacial layer does not show strong dependence on annealing time. However, after being exposed to 1150 °C for 10 h, cracks formed at the interface.     

Nucleation and growth of graphene/Mo2C heterostructures on Cu through CVD

We investigated the chemical vapor deposition synthesis of Mo₂C/graphene heterostructures on a partially wetted liquid copper surface, studied the morphology of resulting phases using electron and optical microscopy, and determined the rate-limiting step for the growth of Mo₂C on graphene. The morphology of the Mo₂C crystals varied from the center to the edge of the copper substrate because of the change in the Mo diffusion pathways owing to the variation in the thickness of the Cu substrate. Thin, hexagonal-shaped crystals of Mo₂C were found in the central region, where Cu is the thickest. In addition, the growth pressure substantially affects the nucleation and growth kinetics of both Mo₂C and graphene. At high pressures (750 Torr), the graphene layer fully covered the Cu surface and Mo₂C crystals formed with a regular shape, while at low pressures (5 Torr), the nucleation of both domains was suppressed, leading to the evolution of Mo₂C crystals with irregular shapes. The activation energy for the growth of Mo₂C on graphene was calculated to be 3.76 ± 0.3 eV, and the diffusion of Mo to the Cu surface through uncovered Cu or graphene vacancies/defects was determined to be the rate-limiting step.     

Lower temperature aluminizing and its effect on improving corrosion resistance of iron treated by surface mechanical attrition treatment

An ultrafine-grained surface layer with the average grain size of about 28 nm in the surface layer was fabricated on a pure Fe plate by the surface mechanical attrition treatment (SMAT). Lower temperature aluminizing treatments of the SMAT samples were investigated by scanning electron microscope and X-ray energy dispersive spectroscope. The electrochemical corrosion behavior of the aluminized SMAT sample was studied in 0.05 mol/L Na₂SO₄ + 0.05 mol/L H₂SO₄ solution, in comparison with the original SMAT and the coarse-grained sample. The results showed that SMAT had a negative effect on the corrosion resistance of Fe. An aluminized surface layer was formed on SMAT sample by aluminizing treatment at 400°C, which was much lower than that of the conventional aluminizing treatment. A successive lower temperature aluminizing process made the aluminized layer thicker and continuous. The SMAT sample treated by a successive lower temperature aluminizing had much higher corrosion resistance and exhibited passive behavior, which was due to the formation of a protective passive film.     

Oxidation-induced crack healing and erosion life assessment of Ni–Mo–Al–Cr7C3–Al2O3 composite coating

The present investigation focuses on the effect of Cr₂AlC MAX phase addition on erosion and oxidation-induced crack healing behavior of Ni–Mo–Al alloy. For this, Ni–Mo–Al and 20 wt% Cr₂AlC-blended Ni–Mo–Al powders were coated by Air Plasma Spray (APS). For oxidation-induced crack healing studies, the samples were heat treated at 500, 800, and 1100°C in the air for 5 hours. The heat-treated samples were analyzed by X-Ray Diffraction (XRD) analysis, Scanning Electron Microscopy (SEM), and Energy Dispersive Spectroscopy (EDS) for the phases, morphology, and composition. Erosion behavior studies were carried out at 30, 250, 500, 800, and 1000°C temperatures. The average hardness was obtained to be 400 ± 10 HV for Ni–Mo–Al coating and 580 ± 10 HV for 20 wt% Cr₂AlC-blended Ni–Mo–Al coating. The addition of Cr₂AlC MAX into Ni–Mo–Al matrix reduces the overall erosion rate and improved the crack healing ability. This was attributed to the presence of in-situ-formed Cr₇C₃ and Al₂O₃ phases.