Methane conversion to C2 hydrocarbons and hydrogen in atmospheric non-thermal plasmas generated by different electric discharge techniques

Methane conversion to C₂ hydrocarbons and hydrogen has been investigated in a needle-to-plate reactor by pulsed streamer and pulsed spark discharges and in a wire-to-cylinder dielectric barrier discharge (DBD) reactor by pulsed DC DBD and AC DBD at atmospheric pressure and ambient temperature. In the former two electric discharge processes, acetylene is the dominating C₂ products. Pulsed spark discharges gives the highest acetylene yield (54%) and H₂ yield (51%) with 69% of methane conversion in a pure methane system and at 10 SCCM of flow rate and 12 W of discharge power. In the two DBD processes, ethane is the major C₂ products and pulsed DC DBD provides the highest ethane yield. Of the four electric discharge techniques, ethylene yield is less than 2%. Energy costs for methane conversion, acetylene or ethane (for DBD processes) formation, and H₂ formation increase with methane conversion percentage, and were found to be: in pulsed spark discharges (methane conversion 18–69%), 14–25, 35–65 and 10–17 eV/molecule; in pulsed streamer discharges (methane conversion 19–41%), 17–21, 38–59, and 12–19 eV/molecule; in pulsed DBD (methane conversion 6–13%), 38–57, 137–227 and 47–75 eV/molecule; in AC DBD (methane conversion 5–8%), 116–175, 446–637, and 151–205 eV/molecule, respectively. The immersion of the γ-Al₂O₃ pellets in the pulsed streamer discharges, or in the pulsed DC DBD, or in the AC DBD has a positive effect on increasing methane conversion and C₂ yield.     

Hydrogen production from methanol through dielectric barrier discharge

The hydrogen fuel cell is a promising option as a future energy resource and the production of hydrogen is mainly depended on fossil fuels now. In this paper, methanol reforming to produce H₂ through dielectric-barrier discharge (DBD) plasma reaction was studied. Effects of the power supply parameters, reactor parameters and process conditions on conversion of methanol and distribution of products were investigated. The best reaction conditions were following: input power (45 W), material of inner electrode (stainless steel), discharge gap (3.40 mm), length of reaction zone (90.00 mm), dielectric thickness (1.25 mm), and methanol content (37.65%). The highest conversion of methanol and the yield of H₂ were 82.38% and 27.43%, respectively.     

介质阻挡放电微等离子体分解二氧化碳研究

二氧化碳既是主要的温室气体之一,也是包含碳和氧的资源,把相对惰性的CO₂转化为易于利用的CO是其利用的方法之一。采用介质阻挡微等离子体反应器通过单变量和正交实验探究了反应器参数(放电区长度、放电间距、介质厚度)和工艺参数(输入功率、放电频率和停留时间)对CO₂分解为CO的转化率和能量效率的影响规律。研究结果表明,影响CO₂转化率的大小顺序依次为：放电间距>放电长度>输入功率≈停留时间>介质厚度>放电频率;输入功率60.0 W、放电频率9.0 kHz和停留时间1.5 s、放电区长度60 mm、放电间距0.5 m、介质厚度1.6 mm时,CO₂的转化率为10.6%,能量效率为4.1%。     

Tri-reforming of methane: a novel concept for catalytic production of industrially useful synthesis gas with desired H2/CO ratios

A novel process concept called tri-reforming of methane has been proposed in our laboratory using CO₂ in the flue gases from fossil fuel-based power plants without CO₂ separation [C. Song, Chemical Innovation 31 (2001) 21–26]. The proposed tri-reforming process is a synergetic combination of CO₂ reforming, steam reforming, and partial oxidation of methane in a single reactor for effective production of industrially useful synthesis gas (syngas). Both experimental testing and computational analysis show that tri-reforming can not only produce synthesis gas (CO + H₂) with desired H₂/CO ratios (1.5–2.0), but also could eliminate carbon formation which is usually a serious problem in the CO₂ reforming of methane. These two advantages have been demonstrated by tri-reforming of CH₄ in a fixed-bed flow reactor at 850 °C with supported nickel catalysts. Over 95% CH₄ conversion and about 80% CO₂ conversion can be achieved in tri-reforming over Ni catalysts supported on an oxide substrate. The type and nature of catalysts have a significant impact on CO₂ conversion in the presence of H₂O and O₂ in tri-reforming in the temperature range of 700–850 °C. Among all the catalysts tested for tri-reforming, their ability to enhance the conversion of CO₂ follows the order of Ni/MgO > Ni/MgO/CeZrO > Ni/CeO₂ ≈ Ni/ZrO₂ ≈ Ni/Al₂O₃ > Ni/CeZrO. The higher CO₂ conversion over Ni/MgO and Ni/MgO/CeZrO in tri-reforming may be related to the interaction of CO₂ with MgO and more interface between Ni and MgO resulting from the formation of NiO/MgO solid solution. Results of catalytic performance tests over Ni/MgO/CeZrO catalysts at 850 °C and 1 atm with different feed compositions confirm the predicted equilibrium conversions based on the thermodynamic analysis for tri-reforming of methane. Kinetics of tri-reforming were also examined. The reaction orders with respect to partial pressures of CO₂ and H₂O are different over Ni/MgO, Ni/MgO/CeZrO, and Ni/Al₂O₃ catalysts for tri-reforming.     

Partial oxidation of alkanes to oxygenates in supercritical carbon dioxide

The catalytic partial oxidation of propane in supercritical carbon dioxide has been investigated in a stirred batch reactor. Various metals (oxides) have been used as supported catalysts with respect to their activity and selectivity for the formation of oxygenates. The reactions run with a 1:2.3–2.9:68–108 molar ratio of propane:synthetic air:CO₂ at 453–573 K and 80–100 bar. Using a precipitated 2.4 wt.% Co₃O₄–SiO₂ catalyst at 573 K, a total oxygenate (i.e. acetic acid, acetone, acetaldehyde, methanol) selectivity of 59% and a propene selectivity of 21% were obtained at a propane conversion of 12 mol%. The same catalyst has been used to investigate the influence of the supercritical conditions and initial feed composition on the reaction, varying the density of CO₂ and the concentration of synthetic air, respectively.     

Conversion of methane through dielectric-barrier discharge plasma

Methane coupling to produce C₂ hydrocarbons through a dielectric-barrier discharge (DBD) plasma reaction was studied in four DBD reactors. The effects of high voltage electrode position, different discharge gap, types of inner electrode, volume ratio of hydrogen to methane and air cooling method on the conversion of methane and distribution of products were investigated. Conversion of methane is obviously lower when a high voltage electrode acts as an outer electrode than when it acts as an inner electrode. The lifting of reaction temperature becomes slow due to cooling of outer electrode and the temperature can be controlled in the expected range of 60°C–150°C for ensuring better methane conversion and safe operation. The parameters of reactors have obvious effects on methane conversion, but it only slightly affects distribution of the products. The main products are ethylene, ethane and propane. The selectivity of C₂ hydrocarbons can reach 74.50% when volume ratio of hydrogen to methane is 1.50.     

Dielectric barrier micro-plasma reactor with segmented outer electrode for decomposition of pure CO2

Four coaxial cylinder dielectric barrier discharge micro-plasma reactors were designed for the non-catalytic decomposition of pure CO₂ into CO and O₂ at low temperature and ambient pressure. The influence of segmented outer electrodes on the electrical characteristics and the reaction performance was investigated. Experimental results indicated that the introduction of segmented outer electrodes can significantly promote the decomposition of CO₂. Encouragingly, the highest conversion of 13.1% was obtained at an applied voltage of 18 kV, which was a substantial increase of 39.4% compared to the traditional device. Compared with other types of dielectric barrier discharge plasma reactors, the proposed segmented outer electrode micro-plasma reactor can give a higher CO₂ conversion and acceptable energy efficiency. The increase in conversion can be attributed mainly to the enhanced corona discharge caused by the fringe effect at electrode edges, the increase in energy density and the increase in the number of micro-discharges. In addition, detailed electrical characterization was performed to reveal some trends in the electrical behavior of proposed reactors.     

Investigation on the partial oxidation of methane to syngas in a tubular Ba0.5Sr0.5Co0.8Fe0.2O3−δ membrane reactor

A perovskite material of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF), with both electronic and ionic conductivity, was synthesized by a combined citrate–EDTA complexing method. The dense membrane tube made of BSCF was fabricated using the plastic extrusion method. The partial oxidation of methane (POM) to syngas was performed in the tubular BSCF membrane reactor packed with a LiLaNiO/γ–Al₂O₃ catalyst. The reaction performance of the membrane reactor was investigated as functions of temperature, air flow rate in the shell side and methane concentration in the tube side. The mechanism of POM in the membrane reactor was discussed in detail. It was found that in the tubular membrane reactor, combustion reaction of methane with permeated oxygen took place in the reaction zone close to the surface of the membrane, then followed by steam and CO₂ reforming of methane in the middle zone of the tube side. The membrane tube can be operated steadily for 500 h in pure methane with 94% methane conversion and higher than 95% CO selectivity, and higher than 8.0 ml/cm² min oxygen permeation flux.     

A photocatalytic membrane reactor for gas-phase reactions using porous titanium oxide membranes

Photocatalytic membrane reactors using porous titanium oxide membranes having pore sizes of several nanometers were utilized for a gas-phase reaction of methanol. Air mixed with methanol (MeOH) vapor, the concentration of which was controlled in the range of 500–6000 ppm, was fed to the photocatalytic membrane reactor in the range of 50–500 cm³/min using several types of flow patterns. Photocatalysis with membrane permeation resulted in a large decomposition rate, compared to photocatalysis without membrane permeation. The characteristics of the reaction such as decomposition ratio of MeOH, the conversion of the decomposed MeOH to CO₂ and H₂O were found to be a function of the residence time in the reactor. The photocatalytic reaction was analyzed based on pseudo-first-order kinetics to ascertain its simplicity, and the fitted curves were found to be in a relatively good agreement with the experimental data. Apparent rate constants with and without membrane permeation were 2.5 and 1.5×10⁻⁶ m s⁻¹, respectively, indicating that the performance of the photocatalytic reaction system with membrane permeation was enhanced.     

Cu/γ-Al2O3 catalyst for the combustion of methane in a fluidized bed reactor

The applicability of a catalyst based on copper dispersed on γ-Al₂O₃ spheres (1 mm diameter) for fluidized bed catalytic combustion of methane has been assessed. Catalyst properties have been determined by physico-chemical characterization techniques and fixed bed activity tests revealing the presence of a surface CuAl₂O₄ spinel phase, still active and stable in methane combustion after repeated thermal ageing treatments at 800 °C. Methane catalytic combustion experiments have been performed in a 100 mm premixed fluidized bed reactor under lean conditions (0.15–3% inlet methane concentration), showing that complete CH₄ conversion can be attained below 700 °C in a fluidized bed of 1 mm solids with a gas superficial velocity about twice the incipient fluidization velocity.     

Pure hydrogen production by methane steam reforming with hydrogen-permeable membrane reactor

Low temperature steam reforming of methane mainly to hydrogen and carbon dioxide (CH₄ + 2H₂O → 4H₂ + CO₂) has been performed at 773 and 823 K over a commercial nickel catalyst in an equilibrium-shift reactor with an 11-μm thick palladium membrane (Mem-L) on a stainless steel porous metal filter. The methane conversion with the reactor is significantly higher than its equilibrium value without membrane due to the equilibrium-shift combined with separation of pure hydrogen through the membrane. The methane conversion in a reactor with an 8-μm membrane (Mem-H) is similar to that with Mem-L, although the hydrogen permeance through Mem-H is almost double of that through Mem-L. The amount of hydrogen separated in the reaction with Mem-H is significantly large, showing that the hydrogen separation overwhelms the hydrogen production because of the insufficient catalytic activity.     

Influence of cycle parameters on periodically operated fluidised bed reactor for CH4 autoreforming

Autothermal reforming of CH₄ has been studied under both periodic and steady state conditions. The investigation was conducted over Co–NiO in a fluidised bed reactor at 873 K and 101.32 kPa. Cycle periods of 1–40 min were used whilst the cycle split, S_ox (with respect to the O₂-rich cycle) was varied from 0.1 to 0.9. Generally, CH₄ oxidation stimulated CO formation, however, steam reforming yielded predominantly CO₂ and H₂. Although O₂-rich cycling (S_ox≥0.5) was detrimental to H₂ formation, H₂O-rich cycling resulted in a 15% improvement in steady state H₂ formation. Theoretical as well as experimental investigations pointed to a resonant frequency of about 6.7 mHz for CH₄ oxidation to produce super steady state H₂ yields. By periodic operation, it is possible to tune H₂/CO ratios over the range 2.5–7 for the same feed composition. Interestingly, S_ox=0.1 yielded the highest ratios, whereas the lowest ratios were attained at S_ox=0.9. Periodic composition cycling introduces a more flexible approach to reactor operation — H₂/CO can be easily modulated by varying the cycle parameters — compared to steady state operation.     

Nitrous oxide processing by a combination of gliding and microwave discharges

Non-thermal plasma of microwave discharge coupled with gliding discharge was applied to convert nitrous oxide. The experiments were carried out using air or oxygen as carrier gases for N₂O (5%). The overall rates of nitrous oxide conversion determined for the N₂O + air mixture were slightly higher than those for N₂O + oxygen. No significant effect of the carrier gas (air or oxygen) on the rate of N₂O → NO conversion was observed. The effect of the power of the microwave discharge and gas flow rate (air) on the overall rate of nitrous oxide conversion and rate of N₂O conversion to NO was studied. The increase of the gas flow rate from 200 to 400 N l/h resulted in an increase of the N₂O conversion rates both overall (r) and to NO (r_NO). For 400 N l/h, both rates were higher by about 80–100% than those determined in the experiments performed with 200 N l/h.     

La2NiO4 tubular membrane reactor for conversion of methane to syngas

La₂NiO₄ tubular membranes of relative density over 92% were used to separate oxygen from air and facilitate the partial oxidation of methane to H₂ and CO at 900 °C. When methane was fed into a tube of inner surface area 5.11 cm² at a rate of 10.5 ml/min, methane throughput conversion was 89%, CO selectivity 96%, H₂/CO ratio 1.5, and the equivalent oxygen flux was 6.8 ml/min. The surface of the La₂NiO₄ membrane exposed to CH₄ decomposed into La₂O₃ and Ni, while the surface in contact with air remained almost unchanged. It is suggested that the conversion of methane in the membrane reactor involves the reforming of methane by the H₂O and CO₂ catalyzed by nickel.     

H2 production by ethanol decomposition with a gliding arc discharge plasma reactor

A gliding arc discharge (GRD) reactor was used to decompose ethanol into primarily H₂ and CO with small amounts of CH₄, C₂H₂, C₂H₄, and C₂H₆. The ethanol concentration, electrode gap, input voltage and Ar flow rate all affected the conversion of ethanol with results ranging from 40.7% to 58.0%. Interestingly, for all experimental conditions the S_H2/S_CO selectivity ratio was quite stable at around 1.03. The mechanism for the decomposition of ethanol is also described.     

Activity and product distribution of alumina supported platinum and palladium catalysts in the gas-phase oxidative decomposition of chlorinated hydrocarbons

The complete catalytic oxidation of 1,2-dichloroethane (DCE) and trichloroethylene (TCE) over alumina supported noble metal catalysts (Pt and Pd) was evaluated. Experiments were performed at conditions of lean hydrocarbon concentration (around 1000 ppm) in air, between 250°C and 550°C in a conventional fixed bed reactor. The catalysts were prepared in a range of metal contents from 0.1 to 1 wt%. Palladium catalysts resulted to be more active than platinum catalysts in the oxidation of both chlorinated volatile organic compounds. DCE was completely destructed at 375°C, whereas TCE required 550°C. HCl was the only chlorine-containing product in the oxidation of DCE in the range of 250–400°C. Tetrachloroethylene was observed as an intermediate in the oxidation of TCE, being formed to a significant extent between 400°C and 525°C. CO was also detected in the oxidation of both DCE and TCE over Pd catalysts, though at temperatures of complete destruction, CO₂ was the only carbon-containing product. The Pt catalysts were selective to CO₂ at the studied conditions.     

二氧化碳电化学还原反应器的研究进展

电化学还原二氧化碳（CO₂）可实现CO₂的资源化转化，是实现自然界“碳循环”、缓解因CO₂过度排放所引起诸多环境问题的关键技术。本文综述了CO₂电还原反应器的研究发展现状，并依据电解质的不同对比分析了各反应器的结构、传质特征及与之匹配阴极的CO₂转化活性与选择性。研究指出膜电极构型反应器是当前CO₂电还原反应器发展的主要方向，其电解质材料的优选不仅决定于其离子选择性与电导率，还需考虑电催化材料的性质与膜电极效率。最后，提出膜电极构型反应器内的过程强化技术与自支撑结构的阴极设计将成为CO₂还原研究发展的新方向。     

Kinetic studies of CH4 oxidation over Pt–NiO/δ-Al2O3 in a fluidised bed reactor

The oxidation of CH₄ over Pt–NiO/δ-Al₂O₃ has been studied in a fluidised bed reactor as part of a major project on an autothermal (combined oxidation–steam reforming) system for CH₄ conversion. The kinetic data were collected between 773 and 893 K and 101 kPa total pressure using CH₄ and O₂ compositions of 10–35% and 8–30%, respectively. Rate–temperature data were also obtained over alumina-supported monometallic catalysts, Pt and NiO. The bimetallic Pt–NiO system has a lower activation energy (80.8 kJ mol⁻¹) than either Pt (86.45 kJ mol⁻¹) and NiO (103.73 kJ mol⁻¹). The superior performance of the bimetallic catalyst was attributed to chemical synergy. The reaction rate over the Pt–NiO catalyst increased monotonically with CH₄ partial pressure but was inhibited by O₂. At low partial pressures (<30 kPa), H₂O has a detrimental effect on CH₄ conversion, whilst above 30 kPa, the rate increased dramatically with water content.     

Combined plasma-catalytic processing of nitrous oxide

The gliding arc discharge, combined with a catalytic bed, has been applied for nitrous oxide processing in oxygen containing gases. It has been found that under conditions of the gliding arc, nitrous oxide in mixtures with oxygen or air not only decomposes to oxygen and nitrogen, but is also oxidised to nitric oxide. The overall conversion of nitrous oxide, as well as the degree of N₂O oxidation to NO were studied as a function of its initial concentration, flow rate, and discharge power. The overall N₂O conversion and degree of oxidation to NO decreased with increasing flow rate and initial N₂O concentration, and increased with increasing discharge power. The degree of N₂O oxidation to NO varied within 20–37%. The overall conversion and degree of N₂O oxidation increased when granular dielectric materials (TiO₂, SiO₂ (quartz glass), and γ-Al₂O₃) were introduced into the reaction zone. The energy efficiency and the overall conversion of N₂O were still further increased due to catalytic effects of a number of metal oxides (CuO, NiO, MnO₂, Fe₂O₃, Co₃O₄, ZrO₂) deposited on γ-Al₂O₃. The activity of the oxide catalysts within the active power range of 300–360 W decreased in the order: CuO>Fe₂O₃>NiO>MnO₂>Co₃O₄>ZrO₂. It has been concluded that the combined plasma-catalytic processing may be an efficient way for the reduction of N₂O emissions.