HCl removal from biogas for feeding MCFCs: Adsorption on microporous materials

Modified 13X zeolites and active carbons were studied for HCl adsorption from biogas employed as fuel of MCFCs. 13X zeolite was modified by addition of Cu or Zn by ion exchange or impregnation: commercial active carbon was modified by impregnation with KOH, NaOH or Na₂CO₃ solutions. The materials were tested for HCl adsorption in a laboratory apparatus designed to the scope. The tests were carried out under flow conditions with a 100 ppm HCl/N₂ mixture at T = 40 °C and atmospheric pressure. Breakthrough curves were obtained from potentiometric analysis of HCl in the stream effluent from the adsorption cell. The zeolites modified by ion exchange were more effective for HCl adsorption than those modified by impregnation. The active carbon modified with Na₂CO₃ solution showed the highest adsorption capacity of 3.3 × 10⁻⁴ mol HCl g⁻¹ at breakthrough concentration of 1 ppm.     

Thermally moderated hollow fiber sorbent modules in rapidly cycled pressure swing adsorption mode for hydrogen purification

We describe thermally moderated multi-layered pseudo-monolithic hollow fiber sorbents entities, which can be packed into compact modules to provide small-footprint, efficient H₂ purification/CO₂ removal systems for use in on-site steam methane reformer product gas separations. Dual-layer hollow fibers are created via dry-jet, wet-quench spinning with an inner “active” core of cellulose acetate (porous binder) and zeolite NaY (69 wt% zeolite NaY) and an external sheath layer of pure cellulose acetate. The co-spun sheath layer reduces the surface porosity of the fiber and was used as a smooth coating surface for a poly(vinyl-alcohol) post-treatment, which reduced the gas permeance through the fiber sorbent by at least 7 orders of magnitude, essentially creating an impermeable sheath layer. The interstitial volume between the individual fibers was filled with a thermally-moderating paraffin wax. CO₂ breakthrough experiments on the hollow fiber sorbent modules with and without paraffin wax revealed that the “passively” cooled paraffin wax module had 12.5% longer breakthrough times than the “non-isothermal” module. The latent heat of fusion/melting of the wax offsets the released latent heat of sorption/desorption of the zeolites. One-hundred rapidly cycled pressure swing adsorption cycles were performed on the “passively” cooled hollow fiber sorbents using 25 vol% CO₂/75 vol% He (H₂ surrogate) at 60 °C and 113 psia, resulting in a product purity of 99.2% and a product recovery of 88.1% thus achieving process conditions and product quality comparable to conventional pellet processes. Isothermal and non-isothermal dynamic modeling of the hollow fiber sorbent module and a traditional packed bed using gPROMS^® indicated that the fiber sorbents have sharper fronts (232% sharper) and longer adsorbate breakthrough times (66% longer), further confirming the applicability of the new fiber sorbent approach for H₂ purification.     

Electrochemical impedance study of the poisoning behaviour of Ni-based anodes at low concentrations of H2S in an MCFC

The effect was investigated of low H₂S concentrations, simulating biogas impurity, on the poisoning behaviour of a Ni-based, molten carbonate fuel cell anode. A conventional Ni–Cr anode was coated with ceria using dip coating to form a rare earth metal oxide thin layer on the surface of the anode. Electrochemical studies of the Ni-based samples were performed in symmetric cells under anode atmosphere (H₂, CO₂, H₂O and N₂ as balance) with 2, 6, 12, and 24 ppm of H₂S by means of electrochemical impedance spectroscopy.     

Hydrogen production from dimethyl ether over Cu/γ-Al2O3 catalyst with zeolites and its effects in the lean NOx trap performance

The purpose of this study is to investigate the effects of mixing three kinds of zeolites (MFI, MOR, and BEA) with the dimethyl ether steam reforming(DME-SR) Cu/γ-Al₂O₃ catalyst to improve H₂ yield at low temperatures, and to identify the de-NOx performance of a combined system of SR catalyst and Lean NOx Trap(LNT). The SR catalyst was prepared by the impregnation method, and a commercialized LNT catalyst was used. The SR reaction experiment was conducted to investigate the effect of the coexistence of CO₂, O₂, NO, and NO₂ among the exhaust gases of the DME engine on the H₂ yield. The study found that the proper mixing of Cu/γ-Al₂O₃ and zeolite increased the H₂ yield at low temperature improving DME hydrolysis. The variation in the H₂ yield according to the kinds of zeolite in the SR catalyst was due to the characteristics of zeolite. The Cu10/γ-Al₂O₃ catalyst mixed with 10% MOR showed the highest H₂ yield. A combined system of SR and LNT uses the H₂ generated mainly from the Cu-based catalyst using the DME-SR reaction for the LNT. When H₂ generated from the SR (Cu10/γ-Al₂O₃ + MOR10) catalyst was used as the reductant of LNT, the NOx conversion at 350 °C or below was improved up to 15% compared to when DME was used. This demonstrates that H₂ as the reductant of LNT is more beneficial than DME. The H₂ generated from the SR catalyst can be used as the reductant of LNT in an after-treatment system. Meanwhile, the SR catalyst that was mixed with zeolite caused the carbon deposition, but the combined system of SR + LNT caused no carbon deposition because the carbon deposited on the SR catalyst reacted with O₂ during the lean-operating period.     

Enhanced ethanol steam reforming by CO2 absorption using CaO,CaO*MgO or Na2ZrO3

This paper presents results of thermodynamic analysis and experimental evaluation of hydrogen production by steam reforming of ethanol (SRE) combined with CO₂ absorption using a mixture of a solid absorbent (CaO, CaO*MgO and Na₂ZrO₃) and a Ni/Al₂O₃ catalyst. Thermodynamic analysis results indicate that a maximum of 69.5% H₂ (dry basis) is feasible at 1 atm, H₂O/C₂H₅OH = 6 (molar ratio) and T = 600 °C. whereas, the addition of a CO₂ absorbent at 1 atm, T = 600 °C and H₂O/C₂H₅OH/Absorbent = 6:1:2.5, produced a H₂ concentration of 96.6, 94.1, and 92.2% using CaO, CaO*MgO, and Na₂ZrO₃, respectively. SRE experimental evaluation achieved a maximum of 60% H₂. While combining SRE and a CO₂ absorbent exhibited a concentration of 96, 94, and 90% employing CaO, CaO*MgO, and Na₂ZrO₃, respectively at 1 atm, T = 600 °C, SV = 414 h⁻¹ and H₂O/C₂H₅OH/absorbent = 6:1:2.5 (molar ratio).     

MSW landfill biogas desulfurization

Biogas utilization in MCFC systems requires a high level of gas purification in order to meet the stringent sulfur tolerance limits of both the fuel cells and the reformer catalysts. In this study, two commercial activated carbons (ACs) have been tested for H₂S removal from the biogas produced at the Montescarpino Municipal Solid Waste landfill in Genoa, Italy. The performed analyses show a low selectivity of activated carbon towards the adsorption of only sulfur species. This represents a drawback for the use of this type of system, however, the use of mixed beds of different ACs has demonstrated to be advantageous in improving the removal efficiency of H₂S. Thus, the adsorption treatments with AC can ensure the high level of gas desulfurization required for fuel cell application. Nevertheless, the low adsorption capacity observed using landfill biogas would lead to high operative costs that suggest the application of a preliminary gas-scrubbing stage.     

The effect of H2S and CO mixtures on PEMFC performance

Proton exchange membrane fuel cells (PEMFCs) most likely will use reformed fuel as the primary source for the anode feed which always contains carbon dioxide (CO) and hydrogen sulfide (H₂S). Trace amount of CO and H₂S can cause considerable cell performance losses. A comparison between the effect of CO and that of H₂S on PEMFC performance was made in this paper. Under the same conditions, the H₂S poisoning rate is much higher than CO because of different adsorption intensity. When the fuel stream contains the gas mixture (25 ppm CO and 25 ppm H₂S), the fuel cell performance deteriorates more quickly than 50 ppm CO but slowly than 50 ppm H₂S and can be only partially recovered by reintroducing neat H₂. The resulting effects of the mixtures can be divided into two parts roughly: during the inception phase, the cell voltage drops quickly and the actual values of anode overvoltage are bigger than the corresponding calculated values; then the deterioration rate of the cell performance decreases gradually.     

Influence of H2S concentration on the properties of Cu2ZnSnS4 thin films and solar cells prepared by sol-gel sulfurization

Cu₂ZnSnS₄ (CZTS) thin films prepared by a non-vacuum process based on the sulfurization of precursor coatings, consisting of a sol-gel solution of Cu, Zn, and Sn, under H₂S+N₂ atmosphere were investigated. The structure, microstructure, and electronic properties of the CZTS thin films as well as solar cell parameters were studied in dependence on the H₂S concentration. The sulfurization process was carried out at 500 °C for 1 h in an H₂S+N₂ mixed-gas atmosphere with H₂S concentrations of 3%, 5%, 10%, and 20%. As the H₂S concentration decreased from 20% to 5%, the S content of the CZTS thin films decreased. However, when the H₂S concentration was decreased below 3%, the S content of the films began to increase. A CZTS thin film prepared with an H₂S concentration of 3% had grains in the order of 1 μm in size, which were larger than those of films prepared at other H₂S concentrations. Furthermore, the most efficient solar cell, with a conversion efficiency of 2.23%, was obtained from a sample sulfurized at an H₂S concentration of 3%.     

Thermodynamic performance assessment and comparison of IGCC with solid cycling process for CO2 capture at high and medium temperatures

Solid sorbents can be used to capture CO₂ from pre-combustion sources at various temperatures. MgO and CaO are typical medium- and high-temperature CO₂ sorbents. However, pure MgO is not active toward CO₂. The addition of Na₂CO₃ increases the operating temperature and significantly increases the reactivity of sorbents to capture CO₂. Na₂CO₃-promoted MgO is a promising medium-temperature CO₂ sorbent. In this study, the thermodynamic performance of integrated gasification combined cycle (IGCC) systems with Na₂CO₃–MgO-based warm gas decarbonation (WGDC) and CaO-based hot gas decarbonation (HGDC) is evaluated and compared with that of an IGCC system with methyldiethanolamine (MDEA)-based cold gas decarbonation (CGDC). Assuming that the average CO₂ capture capacities of solid sorbents are one-third of their theoretical maxima, we reveal that the IGCC system undergoes approximately 2.8% and 3.6% improvement on net efficiency when switching from CGDC to WGDC and to HGDC, respectively. The net efficiency of the system is increased by improving the CO₂ capture capacity of the sorbent. The IGCC with Na₂CO₃–MgO experiences more significant increase in efficiency than that with CaO along with the improvement of sorbent average CO₂ capture capacity. The efficiency of the IGCC systems reaches the same value when the average CO₂ capture capacities of both sorbents are 53% of their theoretical levels. The effects of gas turbine combustor fuel gas inlet temperature on IGCC system performance are analyzed. Results show that the efficiency of the IGCC systems with HGDC and WGDC increases by 0.74% and 0.53% respectively as the fuel gas inlet temperature increases from 250 °C to 650 °C.     

Photocatalytic hydrogen production over CuGa2−xFexO4 spinel

Recovery of hydrogen from industrial H₂S waste using spinel photocatalyst was studied. Spinel metal oxide photocatalysts (CuGa_2−xFe_xO₄ for x = 0.8, 0.6 and 0.4) were synthesized by ceramic route. They were loaded with 0.5 and 1 wt% noble metal oxide, RuO_2. Their XRD pattern revealed a single phase cubic spinel crystalline structure for all the catalysts. SEM displayed small size cubic particles with the particle size decreasing with the decrease in iron content. 1 wt% RuO₂ loaded CuGa_1.6Fe_0.4O₄ decomposed H₂S in aqueous 0.5 M KOH solution under visible light (λ ≥ 420 nm) irradiation and generated H₂ to the tune of 10,045 μmol/h, giving rise to a high quantum efficiency of 21% at 510 nm.     

Potential of Ni supported on KH zeolite catalysts for carbon dioxide reforming of methane

The catalytic activity of Ni on a series of catalysts supported on the synthesized KH zeolite for the CO₂ reforming of methane has been investigated. The KH zeolite supports were previously synthesized via silatrane and alumatrane precursors using the sol–gel process and hydrothermal microwave treatment. Eight percent Ni was impregnated onto the synthesized KH zeolites, which have different morphologies: called dog-bone, flower, and disordered shapes. The prepared Ni/KH zeolites were tested for their catalytic activity at 700 °C, at atmospheric pressure, and at a CH₄/CO₂ ratio of 1. The results showed that Ni supported on dog-bone and flower-shaped KH zeolites provided better activity than that of disordered KH zeolite due to higher CH₄ and CO₂ conversions, a higher H₂ production, and a smaller amount of coke formation on the catalyst surface. Furthermore, the stability of the Ni/KH zeolite was greatly superior to that of Ni supported on alumina and clinoptiolite catalysts after 65 h on stream.     

Gas and liquid phase fuels desulphurization for hydrogen production via reforming processes

The present work focuses on the development of efficient desulphurization processes for multi-fuel reformers for hydrogen production. Two processes were studied: liquid hydrocarbon desulphurization and H₂S removal from reformate gases. For each process, materials with various chemical compositions and microporous structures were synthesized and characterized with respect to their physicochemical properties and desulphurization ability. In the case of liquid phase desulphurization, the adsorption of sulphur compounds contained in diesel fuel under ambient conditions was studied employing as sorbents, zeolite-based materials, i.e. NaY, HY and metal ion-exchanged NaY and HY, as well as a high-surface area activated carbon (AC), for three different diesel fuels with sulphur content varying between 5 and 180 ppmw. Among all sorbents studied, AC showed the best desulphurization performance followed by cerium ion-exchanged HY. The gas phase desulphurization experiments involved the evaluation of zinc-based mixed oxides, synthesized by non-conventional (combustion synthesis) techniques on high steam content reformate gas mixtures.     

Sorbent enhanced hydrogen production from steam gasification of coal integrated with CO2 capture

In this work, CO₂ capture and H₂ production during the steam gasification of coal integrated with CO₂ capture sorbent were investigated using a horizontal fixed bed reactor at atmospheric pressure. Four different temperatures (650, 675, 700, and 750 °C) and three sorbent-to-carbon ratios ([Ca]/[C] = 0, 1, 2) were studied. In the absence of sorbent, the maximum molar fraction of H₂ (64.6%) and conversion of coal (71.3%) were exhibited at the highest temperature (750 °C). The experimental results verified that the presence of sorbent in the steam gasification of coal enhanced the molar fraction of H₂ to more than 80%, with almost all CO₂ was fixed into the sorbent structure, and carbon monoxide (CO) was converted to H₂ and CO₂ through the water gas shift reaction. The steam gasification of coal integrated with CO₂ capture largely depended on the reaction temperature and exhibited optimal conditions at 675 °C. The maximum molar fraction of H₂ (81.7%) and minimum CO₂ concentration (almost 0%) were obtained at 675 °C and a sorbent-to-carbon ratio of 2.     

Effect of H2S on the performance of La0.7Ce0.2FeO3 perovskite catalyst for high temperature water–gas shift reaction

The effect of H₂S on the performance of La_0.7Ce_0.2FeO₃ perovskite catalyst was investigated for the production of hydrogen from simulated coal-derived syngas via the water–gas shift reaction at 600 °C and 1 atm. The results show that the catalyst activity decreases with increasing concentrations of H₂S up to 1100 ppm, but the negative effect of H₂S on its activity is reversible. However, even at the high H₂S concentrations catalyst activity is still greater than that measured with sour shift catalyst. Overall, the results indicate that La_0.7Ce_0.2FeO₃ perovskite catalyst has a high degree of H₂S tolerance, particularly in the low H₂S concentration regime.     

Comparative study on the effect of CO2 and H2O dilution on laminar burning characteristics of CO/H2/air mixtures

A study on the effect of CO₂ and H₂O dilution on the laminar burning characteristics of CO/H₂/air mixtures was conducted at elevated pressures using spherically expanding flames and CHEMKIN package. Experimental conditions for the CO₂ and H₂O diluted CO/H₂/air/mixtures of hydrogen fraction in syngas from 0.2 to 0.8 are the pressures from 0.1 to 0.3 MPa, initial temperature of 373 K, with CO₂ or H₂O dilution ratios from 0 to 0.15. Laminar burning velocities of the CO₂ and H₂O diluted CO/H₂/air/mixtures were measured and calculated using the mechanism of Davis et al. and the mechanism of Li et al. Results show that the discrepancy exists between the measured values and the simulated ones using both Davis and Li mechanisms. The discrepancy shows different trends under CO₂ and H₂O dilution. Chemical kinetics analysis indicates that the elementary reaction corresponding to peak ROP of OH consumption for mixtures with CO/H₂ ratio of 20/80 changes from reaction R3 (OH + H₂ = H + H₂O) to R16 (HO₂+H = OH + OH) when CO₂ and H₂O are added. Sensitivity analysis was conducted to find out the dominant reaction when CO₂ and H₂O are added. Laminar burning velocities and kinetics analysis indicate that CO₂ has a stronger chemical effect than H₂O. The intrinsic flame instability is promoted at atmospheric pressure and is suppressed at elevated pressure for the CO₂ and H₂O diluted mixtures. This phenomenon was interpreted with the parameters of the effective Lewis number, thermal expansion ratio, flame thickness and linear theory.     

Combustion characteristics of an SI engine fueled with H2–CO blended fuel and diluted by CO2

This paper presents further results of the study on fundamental combustion characteristics of gaseous fuels simulated for a biogas produced through a biomass gasification process with a catalyzer. The main work focuses on combustion characteristics of H₂–CO blended fuel and the effect of CO₂ dilution on it in a spark-ignition engine under the condition of WOT, MBT and a constant speed of 1500 rpm. Equivalence ratio were limited to lower than 0.8 in order to avoid excessive high combustion temperature to damage the engine, and lean conditions were maintained during the experiment to get acceptable economy and emissions. The results show that the BMEP decreases with an increase in dilution rate. The COV of IMEP is lower than 10% under most conditions, while H₂ and CO₂ have the opposite influence on brake thermal efficiency. CO₂ dilution combustion could induce to remarkable decreasing in NO_x emission with little decrease in brake thermal efficiency, which benefits for biomass gaseous fuel application. If 500 ppm of NO_x emission and 26% of brake thermal efficiency could be viewed as accepted level, the accepted operation range of H₂–CO mixture have been obtained.     

Durability of solid oxide fuel cells using sulfur containing fuels

The usability of hydrogen and also carbon containing fuels is one of the important advantages of solid oxide fuel cells (SOFCs), which opens the possibility to use fuels derived from conventional sources such as natural gas and from renewable sources such as biogas. Impurities like sulfur compounds are critical in this respect. State-of-the-art Ni/YSZ SOFC anodes suffer from being rather sensitive towards sulfur impurities. In the current study, anode supported SOFCs with Ni/YSZ or Ni/ScYSZ anodes were exposed to H₂S in the ppm range both for short periods of 24 h and for a few hundred hours. In a fuel containing significant shares of methane, the reforming activities of the Ni/YSZ and Ni/ScYSZ anodes were severely poisoned already at low H₂S concentrations of ∼2 ppm H₂S. The poisoning effect on the cell voltage was reversible only to a certain degree after exposure of 500 h in the state-of-the-art cell, due to a loss of percolation of Ni particles in the Ni/YSZ anode layers closest to the electrolyte. Using SOFCs with Ni/ScYSZ anodes improved the H₂S tolerance considerably, even at larger H₂S concentrations of 10 and 20 ppm over a few hundred hours.     

Fabrication and characterization of poly(phenylene oxide)/SBA-15/carbon molecule sieve multilayer mixed matrix membrane for gas separation

A novel multilayer mixed matrix membrane (MMM), consisting of poly(phenylene oxide) (PPO), large-pore mesoporous silica molecular sieve zeolite SBA-15, and a carbon molecular sieve (CMS)/Al₂O₃ substrate, was successfully fabricated using the procedure outlined in this paper. The membranes were cast by spin coating and exposed to different gases for the purpose of determining and comparing the permeability and selectivity of PPO/SBA-15 membranes to H₂, CO₂, N₂, and CH₄. PPO/SBA-15/CMS/Al₂O₃ MMMs with different loading weights of zeolite SBA-15 were also studied. This new class of PPO/SBA-15/CMS/Al₂O₃ multilayer MMMs showed higher levels of gas permeability compared to PPO/SBA-15 membranes. The permselectivity of H₂/N₂ and H₂/CH₄ combinations increased remarkably, with values at 38.9 and 50.9, respectively, at 10 wt% zeolite loading. Field emission scanning electron microscopy results showed that the interface between the polymer and the zeolite in MMMs was better at a 10 wt% loading than other loading levels. The increments of the glass transition temperature of MMMs with zeolite confirm that zeolite causes polymer chains to become rigid.     

Solar syngas production from CO2 and H2O in a two-step thermochemical cycle via Zn/ZnO redox reactions: Thermodynamic cycle analysis

Solar syngas production from CO₂ and H₂O is considered in a two-step thermochemical cycle via Zn/ZnO redox reactions, encompassing: 1) the ZnO thermolysis to Zn and O₂ using concentrated solar radiation as the source of process heat, and 2) Zn reacting with mixtures of H₂O and CO₂ yielding high-quality syngas (mainly H₂ and CO) and ZnO; the ZnO is recycled to the first, solar step, resulting in net reaction βCO₂ + (1 − β)H₂O → βCO + (1 − β)H₂. Syngas is further processed to liquid hydrocarbon fuels via Fischer-Tropsch or other catalytic processes. Second-law thermodynamic analysis is applied to determine the cycle efficiencies attainable with and without heat recuperation for varying molar fractions of CO₂:H₂O and solar reactor temperatures in the range 1900-2300 K. Considered is the energy penalty of using Ar dilution in the solar step below 2235 K for shifting the equilibrium to favor Zn production.     

Glass fiber entrapped sorbent for reformates desulfurization for logistic PEM fuel cell power systems

Glass fiber entrapped ZnO/SiO₂ sorbent (GFES) was developed to remove sulfur species (mainly hydrogen sulfide, H₂S) from reformates for logistic PEM fuel cell power systems. Due to the use of microfibrous media and nanosized ZnO grains on highly porous SiO₂ support, GFES demonstrated excellent desulfurization performance and potential to miniaturize the desulfurization reactors. In the thin bed test, GFES (2.5 mm bed thickness) attained a breakthrough time of 540 min with up to 75% ZnO utilization at 1 ppm breakthrough. At equivalent ZnO loading, GFES yielded a breakthrough time twice as long as the ZnO/SiO₂ sorbent; at equivalent bed volume, GFES provided a three times longer breakthrough time (with 67% reduction in ZnO loading) than packed beds of 1–2 mm commercial extrudates. GFES is highly regenerable compared with the commercial extrudates, and can easily be regenerated in situ in air at 500 °C. During 50 regeneration/desulfurization cycles, GFES maintained its desulfurization performance and structural integrity. A composite bed consisting of a packed bed of large extrudates followed by a polishing layer of GFES demonstrated a great extension in gas life and overall bed utilization. This approach synergistically combines the high volume loading of packed beds with the overall contacting efficiency of small particulates.