CO2 reforming of CH4 by atmospheric pressure glow discharge plasma: A high conversion ability

CO₂ reforming of CH₄ to syngas has been investigated by a special designed plasma reactor of atmospheric pressure glow discharge. High conversion of CH₄, CO₂, and high selectivity of CO, H₂, as well as high conversion ability are carried out. The experiment is operated in wider parameter region, such as CH₄/CO₂ from 3/7 to 6/4, input power from 49.50 W to 88.40 W and total feed flux from 360 mL/min to 4000 mL/min. The highest conversion of CH₄ and CO₂ is 98.52% and 90.30%, respectively. Under the experimental conditions of CH₄/CO₂ rate at 4/6, input power at 69.85 W and total feed flux at 2200 mL/min, the conversion ability achieves a maximum of 12.21 mmol/kJ with the conversion of CH₄ and CO₂ is 60.97% and 49.91%, the selectivity of H₂ and CO is 89.30% and 72.58%, H₂/CO rate is 1.5, respectively. This process has advantages of relatively large treatment and high conversion ability, which is a benefit from a special designed plasma reactor.     

Optimized mixed reforming of biogas with O2 addition in spark-discharge plasma

Adding O₂ into biogas to achieve partial oxidation and CO₂ mixed reforming can not only increase H₂ + CO concentration, but also reduce energy cost for H₂ production. In this study, optimized mixed reforming of biogas with O₂ addition in spark-discharge plasma was pursued in combination with thermodynamic-equilibrium calculation. With respect to mixed reforming of biogas with O₂ addition in spark-discharge plasma, combination coefficients of independent reactions were given to quantitatively evaluate the mixed extent at various O₂/(CH₄–CO₂) ratios. Compared thermodynamic-equilibrium with experimental results, it can be concluded that the optimal O₂/(CH₄–CO₂) ratio for optimized mixed reforming of biogas in spark-discharge plasma was about 0.7. When total-carbon conversion was relatively high (>75%), H₂ + CO concentration on wet basis was the highest and energy cost for H₂ production was the lowest at O₂/(CH₄–CO₂) = 0.7, and their experimental results were closest to their thermodynamic-equilibrium values.     

Role of CO2 in the CH4 oxidation and H2 formation during fuel-rich combustion in O2/CO2 environments

The effect of CO₂ reactivity on CH₄ oxidation and H₂ formation in fuel-rich O₂/CO₂ combustion where the concentrations of reactants were high was studied by a CH₄ flat flame experiment, detailed chemical analysis, and a pulverized coal combustion experiment. In the CH₄ flat flame experiment, the residual CH₄ and formed H₂ in fuel-rich O₂/CO₂ combustion were significantly lower than those formed in air combustion, whereas the amount of CO formed in fuel-rich O₂/CO₂ combustion was noticeably higher than that in air. In addition to this experiment, calculations were performed using CHEMKIN-PRO. They generally agreed with the experimental results and showed that CO₂ reactivity, mainly expressed by the reaction CO₂ + H → CO + OH (R1), caused the differences between air and O₂/CO₂ combustion under fuel-rich condition. R1 was able to advance without oxygen. And, OH radicals were more active than H radicals in the hydrocarbon oxidation in the specific temperature range. It was shown that the role of CO₂ was to advance CH₄ oxidation during fuel-rich O₂/CO₂ combustion. Under fuel-rich combustion, H₂ was mainly produced when the hydrocarbon reacted with H radicals. However, the hydrocarbon also reacted with the OH radicals, leading to H₂O production. In fact, these hydrocarbon reactions were competitive. With increasing H/OH ratio, H₂ formed more easily; however, CO₂ reactivity reduced the H/OH ratio by converting H to OH. Moreover, the OH radicals reacted with H₂, whereas the H radicals did not reduce H₂. It was shown that OH radicals formed by CO₂ reactivity were not suitable for H₂ formation. As for pulverized coal combustion, the tendencies of CH₄, CO, and H₂ formation in pulverized coal combustion were almost the same as those in the CH₄ flat flame.     

Effect of substitution with Cr and addition of Ni on the physical and electrochemical properties of Ce0.9Sr0.1VO3 as a H2S-active anode for solid oxide fuel cells

Whereas Ce_0.9Sr_0.1Cr_0.5V_0.5O₃ is an active fuel cell anode catalyst for conversion of only the H₂S content of 0.5% H₂S-CH₄ at 850 °C, inclusion of 5 wt% NiO to form a composite catalyst enabled concurrent electrochemical conversion of CH₄. A fuel cell with a 0.3 mm thick YSZ membrane and Ce_0.9Sr_0.1Cr_0.5V_0.5O₃ as anode catalyst had a maximum power density of 85 mW cm⁻² in 0.5% H₂S-CH₄ at 850 °C, arising only from the electro-oxidation of H₂S. Using a same thick membrane, promotion of the anode with 5 wt% NiO increased the total anode electro-oxidation activity to afford maximum power density of 100 mW cm⁻² in 0.5% H₂S-CH₄. The same membrane provided 30 mW cm⁻² in pure CH₄, showing that the incremental improvement arose substantially from CH₄ conversion. Performance of each anode was stable for over 12 h at maximum power output. XPS and XRD analyses showed that an increase in conductivity of Ce_0.9Sr_0.1Cr_0.5V_0.5O₃ in H₂S-containing environments resulted from a change in composition and structure from the tetragonal oxide to monoclinic Ce_0.9Sr_0.1Cr_0.5V_0.5(O,S)₃.     

Direct CH4 fuel cell using Sr2FeMoO6 as an anode material

A double-perovskite Sr₂FeMoO₆ (SFMO) has been synthesized with a combined citrate-EDTA complexing method. The material shows a double-perovskite structure after reduction in 5% H₂/Ar at 1100 °C for 20 h. A single fuel cell using this material as anode is constructed with the configuration of SFMO?La_0.8Sr_0.2Ga_0.83Mg_0.17O₃?Ba_0.5Sr_0.5Co_0.8Fe_0.2O₃. The cell exhibits a remarkable electrochemical activity in both H₂ and dry CH₄, respectively. With Oxygen as oxidant, the maximum power density is 863.7 mW cm⁻² with H₂ as the fuel and 604.8 mW cm⁻² with dry CH₄ as the fuel at 850 °C, respectively. SFMO has an almost linear thermal expansion coefficient from 30 to 900 °C and is very close to that of La_0.8Sr_0.2Ga_0.83Mg_0.17O₃. A durability test of the single cell indicates that SFMO is stable in dry CH₄ operation. Therefore SFMO can be recommended as a promising anode material for LaGaO₃-based solid oxide fuel cells operating with both H₂ and dry CH₄.     

Sr2CoMoO6 anode for solid oxide fuel cell running on H2 and CH4 fuels

The double perovskite Sr₂CoMoO_6−δ was investigated as a candidate anode for a solid oxide fuel cell (SOFC). Thermogravimetric analysis (TGA) and powder X-ray diffraction (XRD) showed that the cation array is retained to 800 °C in H₂ atmosphere with the introduction of a limited concentration of oxide-ion vacancies. Stoichiometric Sr₂CoMoO₆ has an antiferromagnetic Néel temperature T_N ≈ 37 K, but after reduction in H₂ at 800 °C for 10 h, long-range magnetic order appears to set in above 300 K. In H₂, the electronic conductivity increases sharply with temperature in the interval 400 °C < T < 500 °C due to the onset of a loss of oxygen to make Sr₂CoMoO_6−δ a good mixed oxide-ion/electronic conductor (MIEC). With a 300-μm-thick La_0.8Sr_0.12Ga_0.83Mg_0.17O_2.815 (LSGM) as oxide-ion electrolyte and SrCo_0.8Fe_0.2O_3−δ as the cathode, the Sr₂CoMoO_6−δ anode gave a maximum power density of 1017 mW cm⁻² in H₂ and 634 mW cm⁻² in wet CH₄. A degradation of power in CH₄ was observed, which could be attributed to coke build up observed by energy dispersive spectroscopy (EDS).     

Reforming of bioethanol over Ni/Al2O3 and Ni/CeZrO2/Al2O3 catalysts in supercritical water for hydrogen production

Bioethanol was reformed in supercritical water (SCW) at 500 °C and 25 MPa on Ni/Al₂O₃ and Ni/CeZrO₂/Al₂O₃ catalysts to produce high-pressure hydrogen. The results were compared with non-catalytic reactions. Under supercritical water and in a non-catalytic environment, ethanol was reformed to H₂, CO₂ and CH₄ with small amounts of CO and C2 gas and liquid products. The presence of either Ni/Al₂O₃ or Ni/CeZrO₂/Al₂O₃ promoted reactions of ethanol reforming, dehydrogenation and decomposition. Acetaldehyde produced from the decomposition of ethanol was completely decomposed into CH₄ and CO, which underwent a further water-gas shift reaction in SCW. This led to great increases in ethanol conversion and H₂ yield on the catalysts of more than 3-4 times than that of the non-catalytic condition. For the catalytic operation, adding small amounts of oxygen at oxygen to ethanol molar ratio of 0.06 into the feed improved ethanol conversion, at the expense of some H₂ oxidized to water, resulting in a slightly lower H₂ yield. The ceria-zirconia promoted catalyst was more active than the unpromoted catalyst. On the promoted catalyst, complete ethanol conversion was achieved and no coke formation was found. The ceria-zirconia promoter has important roles in improving the decomposition of acetaldehyde, the enhancement of the water-gas shift as well as the methanation reactions to give an extremely low CO yield and a tremendously high H₂/CO ratio. The SCW environment for ethanol reforming caused the transformation of gamma-alumina towards the corundum phase of the alumina support in the Ni/Al₂O₃ catalyst, but this transformation was slowed down by the presence of the ceria-zirconia promoter.     

Catalytic effect of monoclinic WO3, hexagonal WO3 and H0.23WO3 on the hydrogen sorption properties of Mg

The H sorption properties of mixtures Mg + WO₃ (having various structures) and Mg + H_0.23WO₃ are reported. First, the higher conversion of Mg into MgH₂ during reactive mechanical grinding (under 1.1 MPa of H₂) for higher WO₃ content is due to the improvement of the milling efficiency. Then, it is shown that the hydrogen absorption properties are almost independent of the crystal structure of the catalyst and that only the particles' size and the specific surface play a major role. Finally, for the desorption process, it appears that the chemical composition and structure of the catalyst, together with the particle size and specific surface have an effect.     

Sr2Fe4/3Mo2/3O6 as anodes for solid oxide fuel cells

Sr₂Fe_4/3Mo_2/3O₆ has been synthesized by a combustion method in air. It shows a single cubic perovskite structure after being reduced in wet H₂ at 800 °C and demonstrates a metallic conducting behavior in reducing atmospheres at mediate temperatures. Its conductivity value at 800 °C in wet H₂ (3% H₂O) is about 16 S cm⁻¹. This material exhibits remarkable electrochemical activity and stability in H₂. Without a ceria interlayer, maximum power density (P_max) of 547 mW cm⁻² is achieved at 800 °C with wet H₂ (3% H₂O) as fuel and ambient air as oxidant in the single cell with the configuration of Sr₂Fe_4/3Mo_2/3O₆|La_0.8Sr_0.2Ga_0.83Mg_0.17O₃ (LSGM)| La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF). The P_max even increases to 595 mW cm⁻² when the cell is operated at a constant current load at 800 °C for additional 15 h. This anode material also shows carbon resistance and sulfur tolerance. The P_max is about 130 mW cm⁻² in wet CH₄ (3% H₂O) and 472 mW cm⁻² in H₂ with 100 ppm H₂S. The cell performance can be effectively recovered after changing the fuel gas back to H₂.     

Combined catalytic partial oxidation and CO2 reforming of methane over ZrO2-modified Ni/SiO2 catalysts using fluidized-bed reactor

Nickel on zirconium-modified silica was prepared and tested as a catalyst for reforming methane with CO₂ and O₂ in a fluidized-bed reactor. A conversion of CH₄ near thermodynamic equilibrium and low H₂/CO ratio (1<H₂/CO<2) were obtained without catalyst deactivation during 10 h, in a most energy efficient and safe manner. A weight loading of 5 wt% zirconium was found to be the optimum. The catalysts were characterized using X-ray diffraction (XRD), H₂-temperature reaction (H₂-TPR), CO₂-temperature desorption (CO₂-TPD) and transmission election microscope (TEM) techniques. Ni sintering was a major reason for the deactivation of pure Ni/SiO₂ catalysts, while Ni dispersed highly on a zirconium-promoted Ni/SiO₂ catalyst. The different kinds of surface Ni species formed on ZrO₂-promoted catalysts might be responsible for its high activity and good resistance to Ni sintering.     

CO2 reforming of CH4 by synergies of binode thermal plasma and catalysts

To reduce the energy consumption of the process of CO₂ reforming CH₄ by plasma, the experiments about synergies of thermal plasma and commercial Z107 Ni/Al₂O₃ catalysts are investigated in three elaborate modes: the binode plasma only, the combination of the plasma and catalysts (CPC), the CPC with part of feed gases introduced into plasma discharge region. The optimal specific energy of 193 kJ/mol and energy conversion efficiency of 66% are achieved under the conditions of CH₄/CO₂ of 4/6, input power at 14.4 kW, feed gases of 5 m³/h in mode 3, when the conversions of CH₄ and CO₂ are 77% and 62%, and the selectivities of H₂ and CO are 88% and 97%, respectively. The experimental results are very close to the industrial requirement compared with steam-reforming process. The excellent performance of this process is attributed to three different reaction mechanisms, which will be discussed in this paper.     

Performance of LiNi0.5Mn1.5O4 prepared by solid-state reaction

LiNi_0.5Mn_1.5O₄ was prepared through a solid-state reaction using various Ni precursors. The effect of precursors on the electrochemical performance of LiNi_0.5Mn_1.5O₄ was investigated. LiNi_0.5Mn_1.5O₄ made from Ni(NO₃)₂·6H₂O shows the best charge–discharge performance. The reversible capacity of LiNi_0.5Mn_1.5O₄ is about 145 mAh g⁻¹ and remained 143 mAh g⁻¹ after 10 cycles at 3.0–5.0 V. The XRD results showed that the precursors and the dispersion methods had significant effect on their phase purity. Pure spinel phase can be obtained with high energy ball-milling method and Ni(NO₃)₂·6H₂O as precursor. Trace amount of NiO and Li₂MnO₃ phase were detected in LiNi_0.5Mn_1.5O₄ with manual-mixture method and using Ni(CH₃COO)₂·6H₂O, NiO and Ni₂O₃ as precursors.     

Tri-reforming of methane over Ni@SiO2 catalyst

A nickel-silica core@shell catalyst was applied for a methane tri-reforming process in a fixed-bed reactor. To determine the optimal condition of the tri-reforming process for production of syngas appropriate for methanol synthesis the effect of reaction temperature (550–750 °C), CH₄:H₂O molar ratio (1:0–3.0) and CH₄:O₂ molar ratio (1:0–0.5) in the feedstock was investigated. CH₄ conversion rate and H₂/CO ratio in the produced syngas were influenced by the feedstock composition. Increasing the amount of steam above the proportion of CH₄:H₂O 1:0.5 reduced the H₂:CO molar ratio in produced syngas to ∼1.5. Increasing oxygen partial pressure improved methane conversion to 90% at 750 °C. At low ∼550 °C reaction temperature the tri-reforming process was not effective with low hydrogen production (H₂ yield ∼20%) and very low <5% CO₂ conversion. Increasing reaction temperature increased hydrogen yield to ∼85% at 750 °C. From all the tested reaction conditions the optimal for tri-reforming over the 11%Ni@SiO₂ catalyst was: feed composition with molar ratio CH₄:CO₂:H₂O:O₂:He 1:0.5:0.5:0.1:0.4 at T = 750 °C. The results were explained in the context of characterisation of the catalysts used. The obtained results showed that the tri-reforming process can be applied for production of syngas with composition suitable for methanol synthesis.     

Miniature NH3 cracker based on microfibrous entrapped Ni-CeO2/Al2O3 catalyst monolith for portable fuel cell power supplies

A miniature ammonia cracker, with an overall weight of ≈195 g and volume of ≈50 cm³, has been developed for portable fuel cell power supply. The cracker is composed of a SS-316L tube body, a heating rod and monolithic microfibrous CeO₂-promoted Ni/Al₂O₃ catalysts incorporated within the annular housings between the heating rod and the inner wall of the tubular body. The catalyst monolith is obtained by placing CeO₂ and Ni onto the microfibrous carrier consisting of 3.5 vol% 8 μm diameter nickel fibers and 38 vol% 100–200 μm Al₂O₃ particulates through stepwise incipient wetness impregnation method using cerium and nickel nitrate precursors. This cracker shows pleasing operability for high efficiency H₂ production via ammonia cracking with low pressure drop. Roughly 158 W equivalents of H₂ can be produced with ammonia conversion of >99.9% at 600 °C and 1100 standard cubic centimeter per minute (sccm) ammonia feed gas rate within this cracker through the entire 300 h test. Power density and energy density are estimated to be ≈3160 W/L and ≈2150 Wh/kg, respectively.     

Cu1−xPdx/CeO2-impregnated cermet anodes for direct oxidation of methane in LaGaO3-electrolyte solid oxide fuel cells

A series of ceramic-metal composite anodes containing 1.0 wt.% Cu_1−xPd_x alloys (where x = 0, 0.15, 0.25, 0.4, 0.5, 0.75 and 1.0) were prepared by impregnation of the respective metal salts and 5.0 wt.% CeO₂ into a porous La_0.4Ce_0.6O_2−σ anode skeleton. The performance of these anodes was evaluated in both dry H₂ and CH₄ in the temperature range of 700-800 °C using the 300-μm thick La_0.8Sr_0.2Ga_0.83Mg_0.17O_3−σ (LSGM) electrolyte-supported solid oxide fuel cells (SOFCs). The addition of Pd to Cu significantly increased the performance of the single cells in dry CH₄, with the cell maximum power density changed from 66 mW cm⁻² for Cu_1.0Pd_0.0 to 345 mW cm⁻² for Cu_0.0Pd_1.0 at 800 °C. In H₂, however, the performance improvement was not as significant compared to that in CH₄. In addition, carbon formation was greatly suppressed in the Cu-Pd alloy-impregnated anodes compared to that with pure Pd after exposure to dry CH₄ at 800 °C, which led to different performance stability behaviors for these cells operating with dry CH₄.     

The effect of impregnation strategy on methane dry reforming activity of Ce promoted Pt/ZrO2

Dry reforming of methane has been studied over Pt/ZrO₂ catalysts promoted with Ce for different temperatures and feed compositions. The influence of the impregnation strategy and the cerium amount on the activity and stability of the catalysts were investigated. The results have shown that introduction of 1 wt.% Ce to the Pt/ZrO₂ catalyst via coimpregnation method led to the highest catalytic activity and stability. 1 wt.%Ce–1 wt.%Pt/ZrO₂ catalyst prepared by sequential impregnation displayed inferior CH₄ and CO₂ conversion performances with lowest H₂/CO production ratios. 1 wt.%Ce–1 wt.%Pt/ZrO₂ catalyst prepared by coimpregnation showed the highest activity even for the feed with high CH₄/CO₂ ratio. The reason for high activity was explained by the intensive interaction between Pt and Ce phases for coimpregnated sample, which had been verified by X-ray photoelectron spectroscopy and Energy Dispersive X-Ray analyses. Strong and extensive Pt–Ce surface interaction results in an increase in the number of Ce³⁺ sites and enhances the dispersion of Pt.     

Catalytic behavior of Co3O4 in electroreduction of H2O2

Spinel structure Co₃O₄ nanoparticles with an average diameter of around 17 nm were prepared and evaluated as electrocatalysts for H₂O₂ reduction. Results revealed that Co₃O₄ exhibits considerable activity and good stability for electrocatalytic reduction of H₂O₂ in 3 M NaOH solution. The reduction occurs mainly via the direct pathway when H₂O₂ concentration is lower than 0.5 M. An Al-H₂O₂ semi fuel cell using Co₃O₄ as cathode catalyst was constructed and tested at room temperature. The fuel cell displayed an open circuit voltage of 1.45 V and a peak power density of 190 mW cm⁻² at a current density of 255 mA cm⁻² operating with a catholyte containing 1.5 M H₂O₂. This study demonstrated that Co₃O₄ nanoparticles are promising cathode catalysts, in place of precious metals, for fuel cells using H₂O₂ as oxidant.     

Design of a novel biohythane process with high H2 and CH4 production rates

A biohythane process based on wheat straw including: i) pretreatment, ii) H₂ production using Caldicellulosiruptor saccharolyticus, iii) CH₄ production using an undefined consortium, and iv) gas upgrading using an amine solution, was assessed through process modelling including cost and energy analysis. According to simulations, a biohythane gas with the composition 46–57% H₂, 43–54% CH₄ and 0.4% CO₂, could be produced at high production rates (2.8–6.1 L/L/d), with 93% chemical oxygen demand (COD) reduction, and a net energy yield of 7.4–7.7 kJ/g dry straw. The model was calibrated and verified using experimental data from dark fermentation (DF) of wheat straw hydrolysate, and anaerobic digestion of DF effluent. In addition, the effect of gas recirculation was investigated by both wet experiments and simulation. Sparging improved H₂ productivities and yields, but negatively affected the net energy gain and cost of the overall process.     

Ce0.9Sr0.1VOx (x = 3, 4) as anode materials for H2S-containing CH4 fueled solid oxide fuel cells

The stability and activity in 0.5% H₂S-CH₄ of Ce_0.9Sr_0.1VO₃ and Ce_0.9Sr_0.1VO₄ anode materials for H₂S-containing CH₄ fueled SOFCs have been determined. XRD showed that Ce_0.9Sr_0.1VO₄ was reduced when the fuel gas was 0.5% H₂S-CH₄, while Ce_0.9Sr_0.1VO₃ remained stable over 24 h at 950 °C. Electrochemical tests in 0.5% H₂S-CH₄ showed stable performance at 950 and 800 °C for cells comprising Ce_0.9Sr_0.1VO₃|YSZ|Pt. Comparison of fuel cell performances using 0.5% H₂S-CH₄, 0.5% H₂S-N₂ and 5% H₂S-N₂ as feeds showed that Ce_0.9Sr_0.1VO₃ was not active for oxidation of methane, but highly active for conversion of H₂S. Electrochemical impedance results were consistent with the finding that the anode was activated only in an environment that contained H₂S. Conductivity measurements showed there was an increase in conductivity in H₂S-containing environments, and that this increase resulted from a change in composition and structure from the oxide to monoclinic Ce_0.9Sr_0.1V(O,S)₃, as evidenced by XPS and XRD analyses.     

The effects of hydrogen addition on Fenimore NO formation in low-pressure,fuel-rich-premixed,burner-stabilized CH4/O2/N2 flames

We investigate the effects of hydrogen addition on Fenimore NO formation in fuel-rich, low-pressure burner-stabilized CH₄/O₂/N₂ flames. Towards this end, axial profiles of temperature and mole fractions of CH and NO are measured using laser-induced fluorescence (LIF). The experiments are performed at equivalent ratios of 1.3 and 1.5, using 0.25 mole fraction of hydrogen in the fuel, while varying the mass flux through the burner. The results are compared with those reported previously for burner-stabilized CH₄/O₂/N₂ flames. The increased burning velocity caused by hydrogen addition is seen to result in a lower flame temperature as compared to methane flame stabilized at the same mass flux. This increase in burner stabilization upon hydrogen addition results in significantly lower CH mole fractions at φ = 1.3, but appears to have little effect on the CH profile at φ = 1.5. In addition, the results show that not only the maximum flame temperature is reduced upon hydrogen addition, but the local gas temperature in the region of the CH profile is lowered as well. The measured NO mole fractions are seen to decrease substantially for both equivalence ratios. Analysis of the factors responsible for Fenimore NO formation shows the reduction in temperature in the flame front to be the major factor in the decrease in NO mole fraction, with a significant contribution from the decrease in CH mole fraction at φ = 1.3. At φ = 1.5, the results suggest that the lower flame temperature upon hydrogen addition further retards the conversion of residual fixed-nitrogen species to NO under these rich conditions as compared to the equivalent methane flames.