Experimental and numerical study on the effect of composition on laminar burning velocities of H2/CO/N2/CO2/air mixtures

Experimental and numerical study on laminar burning velocity of H₂/CO/N₂/CO₂/air mixtures was conducted by using a constant volume bomb and Chemkin package. Good agreement between experimental measurements and numerical calculations by using USCII Mech is achieved. Diffusional-thermal instability is enhanced but hydrodynamic instability is insensitive to the increase of hydrogen fraction in fuel mixtures. For mixtures with different hydrogen fractions, the adiabatic flame temperature is not the dominant influencing factor while high thermal diffusivity of hydrogen obviously enhances the laminar burning velocity. Laminar burning velocities increase with increasing hydrogen fraction and equivalence ratio (0.4–1.0). This is mainly due to the high reactivity of H₂ leading to high production rate of H and OH radicals. Reactions  and  play the dominant role in the production of H radical for mixtures with high hydrogen fraction, and reaction R31 plays the dominant role for mixtures with low hydrogen fraction.     

Adsorption of H2S on carbonaceous materials of different dimensionality

The effect of the dimension of carbonaceous systems, from two to zero, on the adsorption strength of H₂S is investigated by density functional theory based methods. To this end, a carbon nanocone (CNC), a (3, 3) carbon nanotube ((3, 3)-CNT), and graphene (G) are chosen as models for zero-, one- and two-dimensional systems, respectively. Pristine G and CNC have low tendency to adsorb H₂S but on the (3, 3)-CNT the molecule adsorbs dissociatively and deforms the surface. The effect of doping the surface of these materials with Ti has also been investigated. The presence of Ti modifies H₂S adsorption capability to the point that it is chemically adsorbed on the three decorated surfaces although H₂S adsorption on Ti decorated graphene appears to be different from two other doped surfaces. Only in this case, the H₂S molecule dissociates and releases hydrogen atoms which form H₂ molecule. The resulting H₂ molecule is physisorbed on the Ti-decorated graphene surface and the S atom adsorbs directly on the Ti atom. The density of states of pristine, Ti decorated and H₂S adsorbed nanostructures demonstrate that the systems change their conductivity and magnetic properties.     

A study on H2S permeability of CsHSO4 membranes

The gas permeability of H₂S gas at 150 °C through ultra-thin cesium hydrogen sulfate (CsHSO₄) membranes has been investigated. Gas chromatography–mass spectrometry analyses indicate that CsHSO₄ membrane is impermeable to H₂S gas under test conditions. The apparent micropore diameter of the membrane averaged between 9.5 and 11.5 Å with a maximum permeance of 0.09 Barrer (6.75 × 10⁻¹⁹ m² s⁻¹ Pa⁻¹). Atomic force microscope and X-ray diffraction analyses show respectively that the surface morphology and crystal structure of the membranes are preserved, with no adverse effect from prolonged exposure to H₂S gas. Electrochemical impedance spectroscopy analysis confirm over a 30% decrease in membrane resistance via an 80% reduction in membrane thickness.     

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.     

The effect of low concentrations of CO on H2 adsorption and activation on Pt/C. Part 1: In the absence of humidity

The presence of CO in the H₂-rich gas used as fuel for hydrogen fuel cells has a detrimental effect on PEMFC performance and durability at conventional operating conditions. This paper reports on an investigation of the effect of CO on H₂ activation on a fuel cell Pt/C catalyst close to typical PEMFC operating conditions using H₂-D₂ exchange as a probe reaction and to measure hydrogen surface coverage. While normally limited by equilibrium in the absence of impurities on Pt at typical fuel cell operating temperatures, the presence of ppm concentrations of CO increased the apparent activation energy (E_a) of H₂-D₂ exchange reaction (representing H₂ activation) from approximately 4.5-5.3 kcal mole⁻¹ (Bernasek and Somorjai (1975) [24], Montano et al. (2006) [25]) (in the absence of CO) to 19.3-19.7 kcal mole⁻¹ (in the presence of 10-70 ppm CO), similar to those reported by Montano et al. (2006) [25]. Calculations based on measurements indicate a CO surface coverage of approximately 0.55 ML at 80 °C in H₂ with 70 ppm CO, which coincide very well with surface science results reported by Longwitz et al. (2004) [5]. In addition, surface coverages of hydrogen in the presence of CO suggest a limiting effect on hydrogen spillover by CO. Regeneration of Pt/C at 80 °C in H₂ after CO exposure showed only a partial recovery of Pt sites. However, enough CO-free Pt sites existed to easily achieve equilibrium conversion for H₂-D₂ exchange. This paper establishes the baseline and methodology for a series of future studies where the additional effects of Nafion and humidity will be investigated.     

Deposition of sulfur from H2S on porous adsorbents and effect on their mercury adsorption capacity

The deposition of elemental sulfur on porous adsorbents (commercial pre-sulfurised activated carbons and alumina, an HY zeolite and a selenised adsorbent) has been observed when the adsorbents are exposed to a gaseous stream containing hydrogen sulfide and oxygen. The sulfur deposition from H₂S is more marked for the activated carbons and the alumina, it is substantially decreased when zeolite is used, and is practically insignificant for the selenium-based adsorbent. For the pre-sulfurised activated carbons, further sulfur deposition is initially beneficial in terms of increasing their mercury chemisorbing capacity but, subsequently, the structural properties are affected and this results in a drastic reduction in adsorption capacity. Further sulfur deposition on the pre-sulfurised alumina seems immediately to compromise its capacity to chemisorb mercury. The selenium-based adsorbent shows problems with detachment of the fine mercuric selenide powder from the matrix. Sulfur deposition on the initially unsulfurised zeolite improves its mercury adsorption capability. The initially sulfur-free zeolite could be a promising adsorbent for the treatment of this specific type of gas stream. In fact, after a moderate level of sulfur deposition from oxidation of hydrogen sulfide, it can simultaneously chemisorb the mercury vapours by reaction with the sulfur itself while, unlike the activated carbons and alumina, maintaining its required structural properties for a relatively long period.     

H2S removal from biogas for fuelling MCFCs: New adsorbing materials

Cu and Zn modified 13X zeolites prepared by ion exchange or impregnation and activated carbons (ACs) treated with KOH, NaOH or Na₂CO₃ solutions were studied as H₂S sorbents for biogas purification for fuelling molten carbonate fuel cells. H₂S sorption was studied in a new experimental set-up equipped with a high sensitivity potentiometric system for the analysis of H₂S. Breakthrough curves were obtained at 40 °C with a fixed bed of 20 mg of the samples under a stream (6 L h⁻¹) of 8 ppm H₂S/He mixture. The adsorption properties of 13X zeolite improved with addition of Cu or Zn:Cu exchanged zeolite showed the best performances with a breakthrough time of 580 min at 0.5 ppm H₂S, that is 12 times longer than the parent zeolite. In general, unmodified and modified ACs were more effective H₂S sorbents than zeolites. Treating ACs with NaOH, KOH, or Na₂CO₃ solutions improved the H₂S adsorption properties: AC treated with Na₂CO₃ was the most effective sorbent, showing a breakthrough time of 1222 min at 0.5 ppm, that is twice the time of the parent AC.     

Screening of catalysts for H2S abatement from biogas to feed molten carbonate fuel cells

Different catalysts vanadium-based supported on mixed oxide (CeO₂, TiO₂, CuFe₂O₄) were prepared, characterized and tested for the partial selective oxidation of H₂S at low temperature.     

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%.     

HDMR correlations for the laminar burning velocity of premixed CH4/H2/O2/N2 mixtures

Correlations for the laminar burning velocity of premixed CH₄/H₂/O₂/N₂ mixtures were developed using the method of High Dimensional Model Representation (HDMR). Based on experiment data over a wide range of conditions reported in the literature, two types of HDMR correlation (i.e. global and piecewise HDMR correlations) were obtained. The performance of these correlations was assessed through comparison with experimental results and the correlation reported in the literature. The laminar burning velocity predicted by the piecewise HDMR correlations was shown to agree very well with those from experiments. Therefore, the piecewise HDMR correlations can be used as an effective replacement for the full chemical mechanism when the prediction of the laminar burning velocity is needed in certain combustion modeling.     

Effect of H2O2 on Nafion properties and conductivity at fuel cell conditions

During PEM fuel cell operation, formation of H₂O₂ and material corrosion occurs, generating trace amounts of metal cations (i.e., Fe²⁺, Pt²⁺) and subsequently initiating the deterioration of cell components and, in particular, PFSA membranes (e.g., Nafion). However, most previous studies of this have been performed using conditions not relevant to fuel cell environments, and very few investigations have studied the effect of Nafion decomposition on conductivity, one of the most crucial factors governing PEMFC performance. In this study, a quantitative examination of properties and conductivities of degraded Nafion membranes at conditions relevant to fuel cell environments (30-100%RH and 80 °C) was performed. Nafion membranes were pre-ion-exchanged with small amounts of Fe²⁺ ions prior to H₂O₂ exposure. The degradation degree (defined as loss of ion-exchange capacity, weight, and fluoride content), water uptake, and conductivity of H₂O₂-exposed membranes were found to strongly depend on Fe content and H₂O₂ treatment time. SEM cross-sections showed that the degradation initially took place in the center of the membrane, while FTIR analysis revealed that Nafion degradation preferentially proceeds at the sulfonic end group and at the ether linkage located in the pendant side chain and that the H-bond of water is weakened after prolonged H₂O₂ exposure.     

The influence of the CO inhibition effect on the estimation of the H2 purification unit surface

The CO inhibition effect on H₂ permeance through commercial Pd-based membranes was analysed by means of permeation measurements at different CO compositions (0–30% molar) and temperatures (593–723 K) with the aim to determine the increase of the membrane area in order to compensate the H₂ flux reduction owing to the CO inhibition effect. The permeance of H₂ fed with carbon monoxide was observed to decrease with respect to the case of pure hydrogen. At 647 K the H₂ permeance of a pure feed of 316 μmol m⁻² s⁻¹ Pa^−0.5 reduces progressively until 275 μmol m⁻² s⁻¹ Pa^−0.5 when 15% or more of CO is present in the system, until it reaches a plateau at 20%. The inhibition effect occurring when CO is present in the feed stream reduces with the progressive temperature increase; the reduction of the permeance decreases exponentially by 23% at 593 K and by 3% at 723 K with 10% of CO. The inhibition effect is seen to be reversible. An H₂ flux profile in a Sieverts' plot shows the effect produced by the increase of the CO composition along the Pd-based membrane length. The H₂ flux profile allows the area of a Pd-based membrane to be evaluated in order to have the same permeate flow rate of H₂ when it is fed with CO or as a pure stream. Moreover, a qualitative comparison between the H₂ flux profiles and a previously proposed model has been carried out.     

CO2 and H2O gas conversion into CO and H2 using highly exothermic reactions induced by mixed industrial slags

This communication reports conversion phenomena in which CO₂ and H₂O gases are transformed into CO and H₂, respectively, when exposed to a mixture of molten CaO-rich metallurgical slag and V₂O₃-rich gasifier slag. On reaction, CO₂ and H₂O are thermodynamically driven to become CO and H₂, respectively, by giving up oxygen over the formation of calcium orthovanadate in the slag. The concept was experimentally investigated with a synthetic slag heated to 1500 °C (an assumed slag tap-out temperature in the metallurgical industry) in a CO₂ saturated atmosphere. On heating, a rapid drop in oxygen partial pressure occurred between 1405 °C and 1460 °C, where 97% of CO₂ transformed to CO. Potential industrial applications with the H₂O-to-H₂ conversion are then explored using detailed process computations. If the process is made economically viable, CO₂ and H₂O could be converted into products that are environmentally and industrially attractive and that have the potential for energy savings and greenhouse gas reduction in a process.     

Computed flammability limits of opposed-jet H2/CO syngas diffusion flames

Extensive computations were made to determine the flammability limits of opposed-jet H₂/CO syngas diffusion flames from high stretched blowoff to low stretched quenching. Results from the U-shape extinction boundaries indicate the minimum hydrogen concentrations for H₂/CO syngas to be combustible are larger towards both ends of high strain and low strain rates. The most flammable strain rate is near one s⁻¹ where syngas diffusion flames exist with minimum 0.002% hydrogen content. The critical oxygen percentage (or limiting oxygen index) below which no diffusion flames could exist for any strain rate was found to be 4.7% for the equal-molar syngas fuels (H₂/CO = 1), and the critical oxygen percentage is lower for syngas mixture with higher hydrogen content. The flammability maps were also constructed with strain rates and pressures or dilution gases percentages as the coordinates. By adding dilution gases such as CO₂, H₂O, and N₂ to make the syngas non-flammable, besides the inert effect from the diluents, the chemical effect of H₂O contributes to higher flame temperature, while the radiation effect of H₂O and CO₂ plays an important role in the flame extinction at low strain rates.     

Influence of coating conditions on the H2 separation performance from H2/CH4 gas mixtures by the PDMS/PEI composite membrane

In the present study, the composite polyetherimide (PEI) membrane coated with poly dimethyl siloxane (PDMS) was synthesized and optimum conditions of coating were obtained for separation of hydrogen from methane. Three coating techniques “pouring solution inclined by 45°”, “film casting” and “dip-coating” were used. The effect of sequential coating for different methods on permselectivity of the membranes was investigated. In addition, the influences of coating conditions including coating solution concentration, coating and curing temperatures were examined. The results showed that when the concentration of PDMS coating solution was increased; the permeance of H₂ was initially declined rapidly and was then gradually leveled off. The optimum concentration of coating solution was 15 wt.%. The examination of the curing and coating temperatures showed no significant effect on H₂ permeance and selectivity. In the “dip coating” method, two times coating showed superior permeance and selectivity and in “film casting”, the performance of triple coating was promising. Higher selectivities for the composite membrane prepared by “dip-coating” introduced this method as the best method. The sequential dip-coating with different PDMS concentrations was applied and the selectivity was enhanced significantly from 26 to 96 for pure gases and from 22 to 70 for the binary gas mixture. Finally, the influence of pressure on the separation performance of the fabricated membrane was investigated.     

Thermal efficiency evaluation of the thermochemical H2S splitting cycle for the hydrogen and sulfur production

A flowsheet of the thermochemical H₂S splitting cycle was designed and simulated for hydrogen and sulfur production. The heat and mass balance, as well as the thermal efficiency of the process, were calculated. A thermal efficiency of 40.865% for hydrogen production was obtained by optimizing the heat exchangers and the EED cell considering waste heat recovery. The effects of five calculation parameters, namely, the sulfuric acid concentration, hydrogen iodide (HI) conversion ratio, molar flow rate of HI_x phase, pressure, and reflux ratio at the distillation column, on thermal efficiency were evaluated. The results indicated that further research on the membrane reactor is needed. The optimized conditions for the over-azeotropic HI solution yield should be prioritized. Furthermore, an H₂SO₄ concentration system should be reasonably designed to reduce the complexity of the process and equipment settings, as well as to improve thermal efficiency.     

The effect of low concentrations of CO on H2 adsorption and activation on Pt/C: Part 2—In the presence of H2O vapor

CO affects H₂ activation on supported Pt in the catalyst layers of a PEMFC and significantly degrades overall fuel cell performance. This paper establishes a more fundamental understanding of the effect of humidity on CO poisoning of Pt/C at typical fuel cell conditions (80 °C, 2 atm). In this work, direct measurements of hydrogen surface concentration on Pt/C were performed utilizing an H₂-D₂ switch with Ar purge (HDSAP). The presence of water vapor decreased the rate of CO adsorption on Pt, but had very little effect on the resulting CO surface coverage on Pt_S (θ_CO) at steady-state. The steady-state θ_COs at 80 °C for Pt exposed to H₂ (PH₂=1 atm) and a mixture of H₂/H₂O (1 atm H₂, 10%RH) were 0.70 and 0.66 ML, respectively. Furthermore, total strongly bound surface hydrogen measured after exposure to H₂/H₂O was, surprisingly, the sum of the exchangeable surface hydrogen contributed by each component, even in the presence of CO. In the absence of any evidence for strong chemisorption of H₂O on the carbon support with/without Pt, this additive nature and seemingly lack of interaction from the co-adsorption of H₂ and H₂O on Pt may be explained by the repulsion of strongly adsorbed H₂O to the stepped-terrace interface at high coverages of surface hydrogen.     

Effects of H2S addition on hydrogen ignition behind reflected shock waves: Experiments and modeling

Hydrogen sulfide is a common impurity that can greatly change the combustion properties of fuels, even when present in small concentrations. However, the combustion chemistry of H₂S is still poorly understood, and this lack of understanding subsequently leads to difficulties in the design of emission-control and energy-production processes. During this study, ignition delay times were measured behind reflected shock waves for mixtures of 1% H₂/1% O₂ diluted in Ar and doped with various concentration of H₂S (100, 400, and 1600 ppm) over large pressure (around 1.6, 13, and 33 atm) and temperature (1045–1860 K) ranges. Results typically showed a significant increase in the ignition delay time due to the addition of H₂S, in some cases by a factor of 4 or more over the baseline mixtures with no H₂S. The magnitude of the increase is highly dependent on the temperature and pressure. A detailed chemical kinetics model was developed using recent, up-to-date detailed-kinetics mechanisms from the literature and by changing a few reaction rates within their reported error factor. This updated model predicts well the experimental data obtained during this study and from the shock-tube literature. However, flow reactor data from the literature were poorly predicted when H₂S was a reactant. This study highlights the need for a better estimation of several reaction rates to better predict H₂S oxidation chemistry and its effect on fuel combustion. Using the kinetics model for sensitivity analyses, it was determined that the decrease in reactivity in the presence of H₂S is because H₂S initially reacts before the H₂ fuel does, mainly through the reaction H₂S + H ? SH + H₂, thus taking H atoms away from the main branching reaction H + O₂ ? OH + O and inhibiting the ignition process.     

Economic assessment of biogas-to-electricity generation system with H2S removal by activated carbon in small pig farm

This study was conducted to assess the economic feasibility of electricity generation from biogas in small pig farms with and without the H₂S removal prior to biogas utilisation. The 2% potassium iodide (KI) impregnated activated carbon selected as H₂S adsorbent was introduced to a biogas-to-electricity generation system in a small pig farm in Thailand as a case study. With the average inlet H₂S concentration of about 2400 ppm to the adsorption unit, the H₂S removal efficiency could reach 100% with the adsorption capacity of 0.062 kg of H₂S/kg of adsorbent. Under the reference scenario (i.e., 45% subsidy on digester installation and fixed electricity price at 0.06 Euro/kWh) and based on an assumption that the biogas was fully utilised for electricity generation in the system, the payback period for the system without H₂S removal was about 4 years. With H₂S removal, the payback period was within the economic life of digester but almost twice that of the case without H₂S removal. The impact of electricity price could be clearly seen for the case of treated biogas. At the electricity price fixed at 0.07 Euro/kWh, the payback period for the case of treated biogas was reduced to about 5.5 years, with a trend to decrease at higher electricity prices. For both treated and untreated biogas, the governmental subsidy was the important factor determining the economics of the biogas-to-electricity systems. Without subsidy, the payback period increased to almost 7 years and about 11 years for the case of untreated and treated biogas, respectively, at the reference electricity price. Although the H₂S removal added high operation cost to the system, it is still highly recommended not only for preventing engine corrosion but also for the environment benefit in which air pollution by H₂S/SO₂ emission and impact on human health could be potentially reduced.     

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.