Effect of sulfur contaminants on MCFC performance

Molten carbonate fuel cells (MCFC) used as carbon dioxide separation units in integrated fuel cell and conventional power generation can potentially reduce carbon emission from fossil fuel power production. The MCFC can utilize CO₂ in combustion flue gas at the cathode as oxidant and concentrate it at the anode through the cell reaction and thereby simplifying capture and storage. However, combustion flue gas often contains sulfur dioxide which, if entering the cathode, causes performance degradation by corrosion and by poisoning of the fuel cell. The effect of contaminating an MCFC with low concentrations of both SO₂ at the cathode and H₂S at the anode was studied. The poisoning mechanism of SO₂ is believed to be that of sulfur transfer through the electrolyte and formation of H₂S at the anode. By using a small button cell setup in which the anode and cathode behavior can be studied separately, the anodic poisoning from SO₂ in oxidant gas can be directly compared to that of H₂S in fuel gas. Measurements were performed with SO₂ added to oxidant gas in concentrations up to 24 ppm, both for short-term (90 min) and for long-term (100 h) contaminant exposure. The poisoning effect of H₂S was studied for gas compositions with high- and low concentration of H₂ in fuel gas. The H₂S was added to the fuel gas stream in concentrations of 1, 2 and 4 ppm. Results show that the effect of SO₂ in oxidant gas was significant after 100 h exposure with 8 ppm, and for short-term exposure above 12 ppm. The effect of SO₂ was also seen on the anode side, supporting the theory of a sulfur transfer mechanism and H₂S poisoning. The effect on anode polarization of H₂S in fuel gas was equivalent to that of SO₂ in oxidant gas.     

H2SO4 poisoning of Ru-based and Ni-based catalysts for HI decomposition in SulfurIodine cycle for hydrogen production

The sulfur–iodine (SI) cycle is deemed to be one of the most promising alternative methods for large-scale hydrogen production by water splitting, free of CO₂ emissions. Decomposition of hydrogen iodide is a pivotal reaction that produces hydrogen. The homogeneous conversion of hydrogen iodide is only 2.2% even at 773 K [1]. A suitable catalyst should be selected to reduce the decomposition temperature of HI and attain reaction yields approaching to the thermodynamic equilibrium conversion. However, residual H₂SO₄ could not be avoided in the SI cycle because of incomplete purification. The H₂SO₄ present in the HI feeding stream may lead to the poisoning of HI decomposition catalysts. In this study, the activity and sulfur poisoning of Ru and Ni catalysts loaded on carbon and alumina, respectively, were investigated at 773 K. HI conversion efficiency markedly decreased from 21% to 10% with H₂SO₄ (3000 ppm) present, which was reversible when H₂SO₄ was withdrawn in the case of Ru/C. In the case of Ru/C and Ni/Al₂O₃, catalyst deactivation depends on the concentration of H₂SO₄; the higher the concentration of H₂SO_4, the greater the severity of deactivation. Catalysts before and after sulfur poisoning were characterized by transmission electron microscopy (TEM), energy-dispersive X-Ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). Experimental results and characterization of poisoned and fresh catalysts indicate that the catalyst deactivation could be ascribed to the competitive adsorption of sulfur species and change in its surface properties.     

Exergy analysis of hydrogen production from different sulfur-containing compounds based on H2S splitting cycle

There is a tremendous demand for hydrogen production worldwide but the current H₂ production routes from natural gas and other carbon fuels lead to large greenhouse gas emissions. Intentionally coupled with nuclear power, the sulfur–iodine (S–I) thermochemical water splitting cycle is one of the most widely studied cycles for the large-scale hydrogen production that has environmental benignity. Based on the inspiration of the S–I cycle, a novel chemical cycle called hydrogen sulfide splitting cycle has been proposed for hydrogen production. In addition to the SO₂ production from the reaction of H₂S and sulfuric acid, SO₂ can be produced from the burning (direct oxidation) of hydrogen sulfide or elemental sulfur. And it can also be provided by SO₂ capture from flue gas or other SO₂-containing waste gases. This paper performs exergy analysis on the various SO₂ provisions to the Bunsen reaction that make different routes for hydrogen production from waste sulfur-containing compounds as feedstock. It has been found that the route including SO₂ from direct H₂S oxidation potentially makes the best energy-efficient process of H₂ production. The heat that is generated from H₂S oxidation can be recovered and used to support the energy requirements for other steps of the cycle, making the entire hydrogen production cycle more energy-efficient.     

The evolution characteristic of sulfur-containing gases during coal thermal conversion

In this study, the evolution characteristics of sulfur-containing gases during thermal conversion of two coals under different atmospheres were studied through temperature-program decomposition (TPD) and rapid-heating decomposition (RHD) coupled with online mass spectrum (MS). The releasing profiles of H₂, CH₄ and CO were also measured. Results showed that the effect of atmosphere and heating rate on evolution of sulfur-containing gases was very significant. It was found that Ar atmosphere was more favorable to the formation of sulfur-containing gases than CO₂ atmosphere by using TPD-MS. In CO₂, the formation of H₂S and SO₂ was restrained in 260–650 °C, but was promoted in 880–980 °C; the formation of COS was promoted during the whole process. In Ar, high releasing intensity of H₂ and CH₄ could stabilize sulfur-containing radicals which led to high amount of H₂S and SO₂; while high releasing intensity of CO in CO₂ resulted in high amount of COS. By using RHD-MS, it was found that the steam atmosphere was highly favorable for the transformation of H₂S, SO₂ and COS during the entire reaction period. However, the CO₂ atmosphere was disadvantageous to the transformation of H₂S, SO₂ and COS at the initial stage, but slight favorable for the transformation of H₂S, SO₂ and COS during the later stage. These was resulted from the gasification reaction of steam/CO₂ with coal. The key factor was the releasing amount of H₂ and CO, which promoted the formation and transformation of H₂S, SO₂ and COS.     

Experimental investigation on H2S and SO2 sulphur poisoning and regeneration of a commercially available Ni-catalyst during methane tri-reforming

H₂S and SO₂ poisoning during methane tri-reforming are the main degradation sources for nickel catalysts, especially when exhaust gases are used as reactants. To prolong the lifetime of such applications, it is of primary importance to find strategies which reduce the poisoning effects of sulphur or allow regeneration of poisoned catalysts. The specific feature of tri-reforming, oxygen addition to the reactants, offers possibilities for both of these objectives. This experimental study, based on a specific thermochemical recuperation process, thus scrutinizes three aspects of sulphur poisoning and regeneration during methane tri-reforming focusing on oxygen as key influencing factor: (I) Oxygen addition to the reactants is investigated. (II) A comparison between H₂S and SO₂ poisoning is made. (III) Catalyst regeneration by oxygen treatment is performed. The conclusions derived from the results allow significant improvements in terms of catalyst stability and regeneration and thus contribute to expand application possibilities of reforming catalysts.     

The effect of low concentrations of tetrachloroethylene on H2 adsorption and activation on Pt in a fuel cell catalyst

The poisoning effect of tetrachloroethylene (TTCE) on the activity of a Pt fuel cell catalyst for the adsorption and activation of H₂ was investigated at 60 °C and 2 atm using hydrogen surface concentration measurements. The impurity was chosen as a model compound for chlorinated cleaning and degreasing agents that may be introduced into a fuel cell as a contaminant at a fueling station and/or during vehicle maintenance. In the presence of only H₂, introduction of up to 540 ppm TTCE in H₂ to Pt/C resulted in a reduction of available Pt surface atoms (measured by H₂ uptake) by ca. 30%, which was not enough to shift the H₂-D₂ exchange reaction away from being equilibrium limited. Exposure of TTCE to Pt/C in a mixed redox environment (hydrogen + oxygen), similar to that at the cathode of a fuel cell, resulted in a much more significant loss of Pt surface atom availability, suggesting a role in TTCE decomposition and/or Cl poisoning. Regeneration of catalyst activity of poisoned Pt/C showed the highest level of recovery when regenerated in only H₂, with much less recovery in H₂ + O₂ or O₂. The results from this study are in good agreement with those found in a fuel cell study by Martínez-Rodríguez et al. [2] and confirm that the majority of the poisoning from TTCE on fuel cell performance is most likely at the cathode, rather than the anode.     

Thermodynamic analyses of hydrogen production from sub-quality natural gas: Part I: Pyrolysis and autothermal pyrolysis

Sub-quality natural gas (SQNG) is defined as natural gas whose composition exceeds pipeline specifications of nitrogen, carbon dioxide (CO₂) and/or hydrogen sulfide (H₂S). Approximately one-third of the U.S. natural gas resource is sub-quality gas [1]. Due to the high cost of removing H₂S from hydrocarbons using current processing technologies, SQNG wells are often capped and the gas remains in the ground. We propose and analyze a two-step hydrogen production scheme using SQNG as feedstock. The first step of the process involves hydrocarbon processing (via steam–methane reformation, autothermal steam–methane reformation, pyrolysis and autothermal pyrolysis) in the presence of H₂S. Our analyses reveal that H₂S existing in SQNG is stable and can be considered as an inert gas. No sulfur dioxide (SO₂) and/or sulfur trioxide (SO₃) is formed from the introduction of oxygen to SQNG. In the second step, after the separation of hydrogen from the main stream, un-reacted H₂S is used to reform the remaining methane, generating more hydrogen and carbon disulfide (CS₂). Thermodynamic analyses on SQNG feedstock containing up to 10% (v/v) H₂S have shown that no H₂S separation is required in this process. The Part I of this paper includes only thermodynamic analyses for SQNG pyrolysis and autothermal pyrolysis.     

Simultaneous hydrogen production and decomposition of dissolved in alkaline water over composite photocatalysts under visible light irradiation

A process of simultaneous hydrogen production and H₂S removal has been investigated over a highly active composite photocatalyst made of bulk CdS decorated with nanoparticles of TiO₂, i.e. CdS(bulk)/TiO₂. The photocatalytic activity was evaluated for hydrogen production from aqueous electrolyte solution containing H₂S dissolved in water or alkaline solution under visible light irradiation. The rate of hydrogen production from the H₂S-containing alkaline solution was similar to the rate obtained from photocatalytic hydrogen production from water containing sacrificial reagents (Na₂S+Na₂SO₃) in the similar concentration. The isotope experiment was carried out with D₂O instead of H₂O to investigate the source of hydrogen from photocatalytic decomposition of H₂S dissolved in H₂O or alkali solution under visible light. Hydrogen originated from both H₂S and H₂O when the reaction solution contained H₂S absorbed in alkaline water.     

The effect of powder preparation method on the artificial photosynthesis activities of neodymium doped titania powders

The effects of nanostructure on the artificial photosynthesis activities of undoped and Nd doped titania (TiO₂) powders prepared by three different chemical co-precipitation methods were investigated. Substitutional/interstitial N and S doping was observed in powders due to the presence of high concentrations of HNO₃ (NP) and H₂SO₄ (SP) in the powder preparation media, respectively. Nd, N and S doping caused anatase/rutile phase transformation inhibition and crystallite size reduction in the nanostructure. Light absorption was significantly enhanced by Nd doping and the residual SO₄^2?/NO_x species in the nanostructure. Photocatalytic hydrogen production activity of Nd doped NP powder was 4 times greater than undoped NP powder at 700 °C and had a high purity (CO:H₂ ratio～0.00). CO was determined to be the main product in photocatalytic CO₂ reduction. NP powders had the highest CO yields and Nd doping enhanced CO production. The powders with high crystallite sizes and rutile weight fractions had the highest artificial photosynthesis activities.     

Can H2S poison the surface of yttria-stabilized zirconia?

The adsorption and dissociation of H₂S on the yttria-stabilized zirconia (YSZ) (111) surface are studied using the first-principles methods. It is found that H₂S and SH species are bound weakly on the YSZ(111) surface. Sulfur atom is essentially immobile both into the YSZ bulk and along the surface. Instead, it is stably anchored on the O atop of the YSZ surface with the formation of the SO²⁻ fragment. The nudged elastic band (NEB) calculations show that the formation of SH from H₂S (H₂S → SH + H) is very easy, while the presence of a co-adsorbed H would inhibit the further dissociation of SH. In contrast, the hydrogenation of the adsorbed sulfur is rather easy. It is concluded that H could inhibit the formation of sulfur, thus the sulfur poisoning of the YSZ surface would be prevented by hydrogen.     

Evaluation of bio-hydrogen production using rice straw hydrolysate extracted by acid and alkali hydrolysis

This study was conducted to investigate the properties of hydrolysates obtained from acid and alkali hydrolysis and to evaluate the feasibility of employing them for bio-hydrogen production. High sugar concentrations of 16.8 g/L and 13.3 g/L were present in 0.5% and 1.0% H₂SO₄ hydrolysates, respectively. However, H₂SO₄ hydrolysis resulted in large amounts of short-chain fatty acids (SCFAs) and furan derivatives, which were removed by detoxification. In bio-hydrogen production, 1.0% H₂SO₄ hydrolysate showed a 55.6 mL of highest hydrogen production and 1.14 mol-H₂/mol-hexose equivalent_added of hydrogen yield. In control and 1.0% NaOH hydrolysate, 29.7 mL and 36.9 mL of hydrogen were produced, respectively. Interestingly, relatively high acetate and butyrate production resulted in lactate reduction. Also, NH₄OH hydrolysate produced less than 10 mL of hydrogen. Thus, these results indicate that hydrogen production and metabolite distribution can vary depending on the sugars and by-product composition in the hydrolysate.     

Methane steam reforming in water-deficient conditions on a new Ni-exsolved Ruddlesden-Popper manganite: Coke formation and H2S poisoning

This research deals with the catalytic behavior of the methane steam reforming reaction over a new Ni-exsolved Ruddlesden-Popper manganite during prolonged reaction time (up to 100 h) with special focus on the possible carbon deposition and H₂S poisoning. La_1.5Sr_1.5Mn_1.5Ni_0.5O_7±δ material was synthesized and reduced in diluted hydrogen to induce Ni exsolution. Its catalytic behavior in long reaction times was compared to Ni-impregnated manganite and Ni/YSZ cermet. The catalytic measurements for the steam reforming reaction were carried out at 850 °C in low steam-to-carbon conditions. All materials are susceptible to H₂S poisoning (50 ppm), forming undesired sulfide compounds with damaging impact on their catalytic activity. In contrast, during tests without H₂S, the activity for cermet and impregnated materials drops at relatively short reaction time due to coking formation, as evidenced by TEM and TGA/MS analysis, while the behavior of new exsolved material remains stable throughout the test. This high stability of the new exsolved catalyst over a prolonged reaction time is a noticeable advantage due to its potential use as SOFC anode fed with natural gas free of H₂S.     

Hydrogen assisted diesel combustion

Hydrogen assisted diesel combustion was investigated on a DDC/VM Motori 2.5L, 4-cylinder, turbocharged, common rail, direct injection light-duty diesel engine, with a focus on exhaust emissions. Hydrogen was substituted for diesel fuel on an energy basis of 0%, 2.5%, 5%, 7.5%, 10% and 15% by aspiration of hydrogen into the engine's intake air. Four speed and load conditions were investigated (1800 rpm at 25% and 75% of maximum output and 3600 rpm at 25% and 75% of maximum output). A significant retarding of injection timing by the engine's electronic control unit (ECU) was observed during the increased aspiration of hydrogen. The retarding of injection timing resulted in significant NO_X emission reductions, however, the same emission reductions were achieved without aspirated hydrogen by manually retarding the injection timing. Subsequently, hydrogen assisted diesel combustion was examined, with the pilot and main injection timings locked, to study the effects caused directly by hydrogen addition. Hydrogen assisted diesel combustion resulted in a modest increase of NO_X emissions and a shift in NO/NO₂ ratio in which NO emissions decreased and NO₂ emissions increased, with NO₂ becoming the dominant NO_X component in some combustion modes. Computational fluid dynamics analysis (CFD) of the hydrogen assisted diesel combustion process captured this trend and reproduced the experimentally observed trends of hydrogen's effect on the composition of NO_X for some operating conditions. A model that explicitly accounts for turbulence–chemistry interactions using a transported probability density function (PDF) method was better able to reproduce the experimental trends, compared to a model that ignores the influence of turbulent fluctuations on mean chemical production rates, although the importance of the fluctuations is not as strong as has been reported in some other recent modeling studies. The CFD results confirm that temperature changes alone are not sufficient to explain the observed reduction in NO and increase in NO₂ with increasing H₂. The CFD results are consistent with the hypothesis that in-cylinder HO₂ levels increase with increasing hydrogen, and that the increase in HO₂ enhances the conversion of NO to NO₂. Increased aspiration of hydrogen resulted in PM, and HC emissions which were combustion mode dependent. Predominantly, CO and CO₂ decreased with the increase of hydrogen. The aspiration of hydrogen into the engine modestly decreased fuel economy due to reduced volumetric efficiency from the displacement of air in the cylinder by hydrogen.     

Performance degradation of solid oxide fuel cells due to sulfur poisoning of the electrochemical reaction and internal reforming reaction

Hydrogen sulfide is known to degrade the solid oxide fuel cell (SOFC) performance by adsorbing on the nickel anode catalyst. In this research, the mechanism underlying such SOFC degradation was evaluated based on a theoretical mathematical modeling approach and the sulfur coverage was calculated from a Temkin-like isotherm which is related to both temperature and hydrogen sulfide (H₂S) concentration. The influences of the cell temperature, H₂S concentration and electrochemical performance on both the sulfur coverage and cell polarization are studied in detail. Two specific models were considered to identify whether sulfur poisoning has a larger impact on cell performance through its effect on the electrochemical reaction or on the internal reforming reaction. It was found that sulfur poisoning has different effects on the hydrogen oxidation reaction and internal reforming reaction, leading to competing changes in cell performance with temperature and H₂S concentration.     

The sulfur tolerance mechanism of the Cu/CeO2 system

The interaction of H₂S with the Cu/CeO₂ system is investigated using the first-principles method. It is found that the formation energy of surface oxygen vacancies is lower than that of interface oxygen vacancies and the spillover of an oxygen ion from ceria to Cu strip is an exothermic process, suggesting that the oxygen ions in the substrate are extremely active. The dissociation of H₂S molecule forms atomic S, which is absorbed preferentially at the Cu strip on both unreduced and reduced Cu/CeO₂(110), instead of interacting with the ceria and diffusing into the ceria bulk, alleviating the deactivation of the ceria. On the other hand, the sulfur atom at the Cu strip could be removed by forming SO₂ at suitable partial pressure of water as suggested by our thermodynamics prediction. Therefore the accumulation of sulfur at the Cu strip and the sulfur poisoning to the Cu/CeO₂ system can be avoided.     

Perovskite-based catalysts for tar removal by steam reforming: Effect of the presence of hydrogen sulfide

One of the greatest problems in biomass gasification processes is the conditioning of the produced synthesis gas, which contains various contaminants, including tar and hydrogen sulfide. Nickel catalysts, designed for steam reforming of aliphatic hydrocarbons (natural gas and nafta), are usually deactivated by coke deposition and sulfur poisoning. In this work, nickel and/or manganese catalysts derived from perovskites were prepared by the citrate method and characterized by X-ray diffraction, N₂ physisorption and temperature programmed reduction. The catalysts were evaluated in the steam reforming of toluene, used as tar model compound, in the absence of H₂S at 700 °C and in the presence of 50 ppm H₂S at 800 °C. LaNi_0.5Mn_0.5O₃ catalyst showed higher activity and stability in the absence of H₂S. LaMnO₃ catalyst, although less active in the absence of H₂S, showed increased stability in the presence of H₂S, with conversion of about 60%. H₂ production was only observed in the absence of H₂S.     

PEFC-type impurity sensors for hydrogen fuels

Hydrogen purity sensor cells were newly developed with the principle of PEFC. By using the phenomena of PEFC's voltage drop seen in the presence of impurities and further minimizing the amount of Pt to make the cells more sensitive to impurities, the sensor cells were prepared. This unique sensing principle was applied to typical impurities in practical hydrogen gases, including CO, H₂S, and NH₃. Sensor responses were derived by analyzing various kinds of dependency on Pt loading, current density, impurity concentration, and operational temperature. Possibility of recovery from impurity poisoning was also studied by varying impurities' supply and potential charge. Consequently, our simple PEFC-type hydrogen purity sensors were verified to have ability to sense ppm-level impurities within 10 min.     

Optimization of batch dilute-acid hydrolysis for biohydrogen production from red algal biomass

Marine algae are promising alternative sources for bioenergy including hydrogen. Their polymeric structure, however, requires a pretreatment such as dilute-acid hydrolysis prior to fermentation. This study aimed to optimize the control variables of batch dilute-acid hydrolysis for dark hydrogen fermentation of algal biomass. The powder of Gelidium amansii was hydrolyzed at temperatures of 120–180 °C, solid/liquid (S/L) ratios of 5–15% (w/v), and H₂SO₄ concentrations of 0.5–1.5% (w/w), and then fed to batch hydrogen fermentation. Among the three control variables, hydrolysis temperature was the most significant for hydrogen production as well as for hydrolysis efficiency. The maximum hydrogen production performance of 0.51 L H₂/L/hr and 37.0 mL H₂/g dry biomass was found at 161–164 °C hydrolysis temperature, 12.7–14.1% S/L ratio, and 0.50% H₂SO₄. The optimized dilute-acid hydrolysis would enhance the feasibility of the red algal biomass as a suitable substrate for hydrogen fermentation.     

Recovery of hydrogen from hydrogen sulfide with metals or metal sulfides

The following two types of reactions were investigated for the recovery of hydrogen from hydrogen sulfide: Type 1 H₂S → H₂ + S⁰, Type 2 H₂S + O₂ → H₂ + SO₂ Each type of reaction is constructed by a two-step cycle, in which H₂S is reacted with metal or metal sulfide and then the resulting sulfide undergoes thermal decomposition or oxidation. Ag₂S, FeS, Co₉S₈, Ni₃S₂, and the double sulfide CuFeS₂ were examined in the former type of reaction, while Ag, Cu, Ni, liquid Pb, and liquid BiAg alloy were used as an intermediate in the latter.