Highly active Nd-promoted Ni@Al2O3 core-shell catalyst for steam reforming of acetic acid: Insights of the remarkable coke resistance

Aiming at enhancing the coke resistance of Ni-based catalysts, the Nd-doped Ni@A₂O₃ core-shell catalysts were prepared and their catalytic performance was evaluated in the steam reforming of acetic acid (SRAA). The catalysts were characterized by BET, XRD, XRF, HRTEM, H₂-TPR, NH₃-TPD, and DTG. The catalytic performance was greatly enhanced by the incorporation of Nd, with decreased yields of CO, CH₄, and acetone, increased yield of CO₂, and remarkable carbon resistance. The decoking behavior of the spent catalyst was elucidated by H₂O¹⁸-TSD. A low reaction temperature facilitates the formation of amorphous carbon, leading to catalyst deactivation. The decoking ability of the catalyst is greatly improved by the Nd incorporation but is also catalyzed by the exposed Ni surface. The Ni0.01Nd@Al catalyst greatly balanced the exposed Ni surface and the mobile lattice oxygen, showing the highest catalytic activity, lowest coke deposition, and superb decoking ability.     

Effect of small quantities of potassium promoter and steam on the catalytic properties of nickel catalysts in dry/combined methane reforming

Carbon dioxide and methane are two of the principal greenhouse gases. Reduction of their content in the atmosphere is currently the subject of much worldwide research. Dry and combined reforming of methane are effective methods of CO₂ and CH₄ utilization and production of synthesis gas (syngas) in chemical technology. Testing of catalysts that provide the desired H₂/CO ratio and long operation time is one of the critical aspects of syngas production and the focus of much study. In this study, K-promoted Ni/MgAl₂O₄ catalysts prepared using a co-precipitation-impregnation method with different K/Ni ratios (range of 0–0.15) were examined in dry reforming of methane (DRM). The obtained catalysts were characterized using X-ray diffractometry (XRD), atomic emission spectrometry (MP-AES), Brunauer–Emmett–Teller (BET) specific surface area, BJH pore size distribution, TEM imaging, analysis of reducibility H₂-TPR, infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), Hammet basicity analysis, thermogravimetric analysis (TG) and elemental carbon-hydrogen-nitrogen analysis (CHN). Promotion of nickel catalysts with potassium led to changes in nickel distribution, metal-support interactions and deceleration of carbon deposition while enhancing sorption of carbon dioxide and reduction of n_H2:CO to 0.5 for 0.7 K–Ni/MgAl₂O₄ catalyst. To obtain the required H₂:CO ratio close to unity a study on the effect of steam in inlet stream was performed. It was found that maintaining inlet stream composition equal CH₄:CO₂:H₂O = 1.0:1.0:0.1 obtained an H₂:CO ratio close to unity.     

Steam reforming of acetic acid in the presence of Ni coated with SiO2 microsphere catalysts

Steam reforming of acetic acid was investigated in the presence of Ni@SiO₂ microsphere catalysts. The effects of Ni loading, H₂O/AcOH ratio, and temperature on hydrogen selectivity and acetic acid conversion were determined via statistical analysis. Results indicated the dependence of parameters on hydrogen selectivity.The stable activity observed for the reaction conducted at 2.5H₂O/AcOH ratio and 750 °C implied future utilization potential of the catalyst for time on stream experiments despite the inevitable coke formation. Boudard reaction and methane decomposition, known as possible carbon sources, was ruled out due to the opposite trends between CO₂ and CO selectivities and mitigation of methanation reactions. The ambiguous pattern of conversions observed for varying H₂O/AcOH implied the presence of a different reaction path leading to consecutive ketonozation of acetic acid and aldol-condensation of acetone as the primary sources of carbon deposition.     

Dry reforming of model biomass pyrolysis products to syngas by dielectric barrier discharge plasma

Biomass is frequently used to produce CO and H₂ together with undesirable by-products containing CO₂ and liquid tar by pyrolysis and gasification. This leads to decreased energy efficiency and increased maintenance costs. This study investigated the reforming of biogas and tar, respectively, using non-thermal plasma featuring dielectric barrier discharge (DBD). The gas surrogates studied were CH₄ and CO₂, and toluene was used as a substitute for tar. During reforming of biogas, CO or H₂ was added to the CH₄ and CO₂ to investigate their effects on CH₄ and CO₂ conversion. Both the discharge power and gas components influenced the conversion of CH₄ and CO₂. The conversion efficiency of CH₄ and CO₂ and the selectivity of H₂ and CO both increased with the discharge power while reforming the mixture of CH₄ and CO₂. The maximum conversion efficiency of CH₄ and CO₂ and selectivity of CO and H₂ were obtained with a CH₄:CO₂ ratio of 1:2. During reforming of toluene, the conversion efficiency of toluene reached a maximum value of 90% and the production yields of H₂, CO, and CO₂ reached respective maximums of 0.79, 2.24, and 1.51 mol/mol-toluene at a discharge power of 90 W and temperature of 300 °C. Higher temperatures of 400–500 °C did not favour toluene destruction due to the thermal breakdown of the quartz dielectric and the rapid decrease in the discharging intensity. In addition, reaction mechanism for reforming of both biogas and toluene was proposed to improve our understanding of the reforming process in DBD plasma.     

Effect of preparation method on the performance of the Ni/Al2O3 catalysts for aqueous-phase reforming of ethanol: Part II-characterization

In the first part of this paper [1], we have discussed the effect of preparation method on the performance of Ni/Al₂O₃ catalysts for aqueous-phase reforming of ethanol (EtOH). One catalyst was synthesized using a sol–gel method (SG). The other was synthesized by adding nickel nitrate to a solution combustion synthesized alumina support (SCS). Based on the product distribution, we proposed the reaction pathway as a mixture of dehydrogenation of EtOH to acetaldehyde followed by C–C bond breaking to produce CO and CH₄ and oxidation of acetaldehyde to acetic acid followed by decarbonylation to CO₂ and CH₄. CH₄ (C₂H₆ and C₃H₈ also) can form via Fischer–Tropsch reactions of CO/CO₂ with H₂. The CH₄ (C₂H₆ and C₃H₈) reacts to form hydrogen and carbon monoxide through steam reforming, while CO converts to CO₂ mostly through the water-gas shift reaction (WGSR). The SG catalysts showed poorer WGSR activity than the SCS catalysts. The difference of the metal particle size distribution governed by preparation method appeared to be the key factor of controlling catalytic efficiency, but some contradictory results could not be explained.     

Yields of pyrolysis products from refuse-derived fuel

Pyrolysis yields and gas characteristics must be studied to control pollution caused by waste-energy recycling and to develop a refuse-derived fuel technology. In this study, refuse-derived fuel pyrolysis experiments were performed in a high-temperature tube furnace. The effects of the final pyrolysis temperature, material mixture ratio, and pyrolysis rate on the yields of pyrolysis products, including gas, tar, and semi-coke, were studied. The volume fractions of the pyrolysis gas components (H₂, CO, CH₄, and CO₂) of the samples were also detected. Results showed that with increased final temperature, the tar and gas yields increased but the semi-coke yield decreased. The volume fractions of the components had the following trends: H₂ increased, CO initially decreased and then increased, CH₄ initially increased and then decreased, and CO₂ decreased. With decreased biomass, the tar yield decreased and then increased, whereas the semi-coke and gas yields increased and then decreased. Compared with slow pyrolysis, fast pyrolysis decreased the tar yield by 9.13%, increased the gas yield by 7.45%, increased the CO and CH₄ volume fractions, and decreased the CO₂ volume fraction.     

Preparation of gas standards for quality assurance of hydrogen fuel

This study has developed traceable standards for evaluating impurities in hydrogen fuel according to ISO 14687. Impurities in raw H₂, including sub μmol/mol levels of CO, CO₂, and CH₄, were analyzed using multiple detectors while avoiding contamination. The gravimetric standards prepared included mixtures of the following nominal concentrations: 1, 2, 3–5, 8–11, 17–23, and 47–65 μmol/mol for CO₂, CH₄ and CO, O₂, N₂, Ar, and He, respectively. The expanded uncertainty ranges were 0.8% for Ar, N₂, and He, 1% for CH₄ and CO, and 2% for CO₂ and O₂. These standards were stable, while that for CO varied by only 0.5% during a time span of three years. The prepared standards are useful for evaluating the compliance of H₂ fuel in service stations with ISO 14687 quality requirements.     

Thermodynamic analysis of aqueous phase reforming of three model compounds in bio-oil for hydrogen production

Thermodynamic analysis with Gibbs free energy minimization was performed for aqueous phase reforming of methanol, acetic acid, and ethylene glycol as model compounds for hydrogen production from bio-oil. The effects of the temperature (340-660 K) and pressure ratio Psys/PH₂O (0.1-2.0) on the selectivity of H₂ and CH₄, formation of solid carbon, and conversion of model compounds were analyzed. The influences of CaO and O₂ addition on the formation of H₂, CH₄, and CO₂ in the gas phase and solid phase carbon, CaCO₃, and Ca(OH)₂ were also investigated. With methanation and carbon formation, the conversion of the model compounds was >99.99% with no carbon formation, and methanation was thermodynamically favored over hydrogen production. H₂ selectivity was greatly improved when methanation was suppressed, but most of the inlet model compounds formed solid carbon. After suppressing both methanation and carbon formation, aqueous phase reforming of methanol, acetic acid and ethylene glycol at 500 K and with Psys/PH₂O = 1.1 gave H₂ selectivity of 74.98%, 66.64% and 71.38%, respectively. These were similar to the maximum stoichiometric hydrogen selectivity of 75.00% (methanol), 66.67% (acetic acid), and 71.43% (ethylene glycol). At 500 K and 2.90 MPa, as the molar ratio of CaO/BMCs increased, the normalized variation in H₂ increased and that for CH₄ decreased. Formation of solid carbon was effectively suppressed by addition of O₂, but this was at the expense of H₂ formation. With the O₂/BMCs molar ratio regulated at 1.0, oxidation and CO₂ capture increased the normalized variation in H₂ to 33.33% (methanol), 50.00% (acetic acid), and 60.00% (ethylene glycol), and the formation of solid carbon decreased to zero.     

H2 PSA purifier for CO removal from hydrogen mixtures

A two-bed PSA purifier was developed to produce high purity hydrogen for fuel cell applications. Two types of hydrogen-rich mixtures produced from coal off-gas were used. Feed 1 consisted of a 99% H₂ mixture (H₂:CO:CO₂:N₂ = 99:0.1:0.05:0.85 vol.%) containing 0.1% CO while Feed 2 was a 95% H₂ mixture (H₂:CO:CO₂:CH₄:N₂ = 95:0.3:0.1:0.05:4.55 vol.%) containing 0.3% CO. An increase in the P/F ratio and adsorption pressure led to an almost linear decrease in H₂ recovery with increasing purity. However, a sharp drop in CO concentration occurred at a specific operating range in both feeds. The feed was purified to 1.1 ppm CO with 99.99+% H₂ purity and 80.0% recovery under 6.5 bar and 0.15 P/F ratio while CO in Feed 2 could be reduced to 6.7 ppm with 99.96% H₂ purity and 78.4% recovery. The PVSA process, which combined vacuum and purge steps, could improve recovery by about 10% compared to the PSA process.     

Syngas production through CH4-assisted co-electrolysis of H2O and CO2 in La0.8Sr0.2Cr0.5Fe0.5O3-δ-Zr0.84Y0.16O2-δ electrode-supported solid oxide electrolysis cells

High temperature co-electrolysis of H₂O/CO₂ allows for clean production of syngas using renewable energy, and the novel fuel-assisted electrolysis can effectively reduce consumption of electricity. Here, we report on symmetric cells YSZ-LSCrF | YSZ | YSZ-LSCrF, impregnated with Ni-SDC catalysts, for CH₄-assisted co-electrolysis of H₂O/CO₂. The required voltages to achieve an electrolysis current density of ?400 mA·cm^?2 at 850 °C are 1.0 V for the conventional co-electrolysis and 0.3 V for the CH₄-assisted co-electrolysis, indicative of a 70% reduction in the electricity consumption. For an inlet of H₂O/CO₂ (50/50 vol), syngas with a H₂:CO ratio of ≈2 can be always produced from the cathode under different current densities. In contrast, the anode effluent strongly depends upon the electrolysis current density and the operating temperature, with syngas favorably produced under moderate current densities at higher temperatures. It is demonstrated that syngas with a H₂:CO ratio of ≈2 can be produced from the anode at a formation rate of 6.5·mL min^?1·cm^?2 when operated at 850 °C with an electrolysis current density of ?450 mA·cm^?2.     

Carbon dioxide reforming of methane in a SrCe0.7Zr0.2Eu0.1O3−δ proton conducting membrane reactor

Utilizing CO₂ for fuel production holds the promise for reduced carbon energy cycles. In this paper we demonstrate a membrane reactor, integrating catalytic CO₂ reforming of methane with in-situ H₂ separation, that results in increased CO₂ and CH₄ conversion and H₂ production compared to a Ni catalyst alone. The tubular proton-conducting SrCe_0.7Zr_0.2Eu_0.1O_3−δ membrane reactor demonstrates that the addition of the membrane improves CO₂ conversion, due to in-situ H₂ removal, by 10% and 30% at 900 °C for CH₄/CO₂ = 1/1 and CH₄/CO₂/H₂O = 2/1/1 feed ratios, respectively. It also improves total H₂ production at 900 °C by 15% and 18% for CH₄/CO₂ = 1/1 and CH₄/CO₂/H₂O = 2/1/1, respectively. Further, the H₂/CO in the reactor side effluent can be adjusted for subsequent desired Fischer-Tropsch products by combining CO₂ reforming and steam reforming of methane.     

Effects of CO addition on laminar flame characteristics and chemical reactions of H2 and CH4 in oxy-fuel (O2/CO2) atmosphere

An experimental study is conducted to investigate the effect of CO addition on the laminar flame characteristics of H₂ and CH₄ flames in a constant-volume combustion system. In addition, one-dimensional laminar premixed flame propagation processes at the same conditions are simulated with the update mechanisms. Results show that all mechanisms could well predict the laminar flame speeds of CH₄/CO/O₂/CO₂ mixtures, when Z_CO is large. For mixtures with lower CO, the experimental laminar flame speeds are always smaller than the calculated ones with Han mechanism. For mixtures with larger or smaller Z_CO2, GRI 3.0, San diego and USC 2.0 mechanisms all overvalue or undervalue the laminar flame speeds. When CO ratio in the CH₄/CO blended fuels increases, laminar flame speed firstly increases and then decreases for the CH₄/CO/O₂/CO₂ mixtures. For H₂/CO/O₂/CO₂ mixtures, San diego, Davis and Li mechanisms all undervalue the laminar flame speeds of H₂/CO/CO₂/CO₂ mixtures. Existing models could not well predict the nonlinear trend of the laminar flame speeds, due to complex chemical effects of CO on CH₄/CO or H₂/CO flames. Then, the detailed thermal, kinetic and diffusive effects of CO addition on the laminar flame speeds are discussed. Kinetic sensitivity coefficient is far larger than thermal and diffusive ones and this indicates CO addition influences laminar flame speeds mainly by the kinetic effect. Based on this, radical pool and sensitivity analysis are conducted for CH₄/CO/O₂/CO₂ and H₂/CO/O₂/CO₂ mixtures. For CH₄/CO/O₂/CO₂ mixtures, elementary reaction R38H + O₂ ↔ O + OH and R99 OH + CO ↔ H + CO₂ are the most important branching reactions with positive sensitivity coefficients when CO ratio is relative low. As CO content increases in the CH₄/CO blended fuel, the oxidation of CO plays a more and more important role. When CO ratio is larger than 0.9, the importance of R99 OH + CO ↔ H + CO₂ is far larger than that of R38H + O₂ ↔ O + OH. The oxidation of CO dominates the combustion process of CH₄/CO/O₂/CO₂ mixtures. For H₂/CO/O₂/CO₂ mixtures, the most important elementary reaction with positive and negative sensitivity coefficients are R29 CO + OH ↔ CO₂ + H and R13H + O₂(+M) ↔ HO₂(+M) respectively. The sensitivity coefficient of R29 CO + OH ↔ CO₂ + H is increasing and then decreasing with the addition of CO in the mixture. Chemical kinetic analysis shows that the chemical effect of CO on the laminar flame propagation of CH₄/CO/O₂/CO₂ and H₂/CO/O₂/CO₂ mixtures could be divided into two stages and the critical CO mole fraction is 0.9.     

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.     

Hydrogen production by steam reforming of ethanol over an Ir/CeO2 catalyst: Reaction mechanism and stability of the catalyst

Steam reforming of ethanol over an Ir/CeO₂ catalyst has been studied with regard to the reaction mechanism and the stability of the catalyst. It was found that ethanol dehydrogenation to acetaldehyde was the primary reaction, and acetaldehyde was then decomposed to methane and CO and/or converted to acetone at low temperatures. Methane was further reformed to H₂ and CO, and acetone was directly converted into H₂ and CO₂. Addition of CO, CO₂, and CH₄ to the water/ethanol mixture proved that steam reforming of methane and the water gas shift were the major reactions at high temperatures. The Ir/CeO₂ catalyst displayed rather stable performance in the steam reforming of ethanol at 650 °C even with a stoichiometric feed composition of water/ethanol, and the effluent gas composition remained constant for 300 h on-stream. The CeO₂ in the catalyst prevented the highly dispersed Ir particles from sintering and facilitated coke gasification through strong Ir–CeO₂ interaction.     

Solar-driven pyrolysis and gasification of low-grade carbonaceous materials

Three low-grade carbonaceous materials from biomass (Scenedesmus algae and wheat straw) and waste treatment (sewage sludge) have been selected as feedstock for solar-driven thermochemical processes. Solar-driven pyrolysis and gasification measurements were conducted directly irradiating the samples in a 7 kW_e high flux solar simulator and the released gases H₂, CO, CO₂ and CH₄ and the sample temperature were continuously monitored.Solar-driven experiments showed that H₂ and CO evolved as important product gases demonstrating the high quality of syngas production for the three feedstocks. Straw is the more suitable feedstock for solar-driven processes due to the high gas production yields. Comparing the solar-driven experiments, gasification generates higher percentage of syngas (mix of CO and H₂) respect to total gas produced (sum of H₂, CO, CO₂ and CH₄) than pyrolysis. Thus, solar-driven gasification generates better quality of syngas production than pyrolysis.     

Bi-reforming of methane on Ni/SBA-15 catalyst for syngas production: Influence of feed composition

Bi-reforming of methane (BRM) was evaluated for Ni catalyst dispersed on SBA-15 support prepared by hydrothermal technique. BRM reactions were conducted under atmospheric condition with varying reactant partial pressure in the range of 10–45 kPa and 1073 K in fixed-bed reactor. The ordered hexagonal mesoporous SBA-15 support possessing large specific surface area of 669.5 m² g^?1 was well preserved with NiO addition during incipient wetness impregnation. Additionally, NiO species with mean crystallite dimension of 14.5 nm were randomly distributed over SBA-15 support surface and inside its mesoporous channels. Thus, these particles were reduced at various temperatures depending on different degrees of metal-support interaction. At stoichiometric condition and 1073 K, CH₄ and CO₂ conversions were about 61.6% and 58.9%, respectively whilst H₂/CO ratio of 2.14 slightly superior to theoretical value for BRM would suggest the predominance of methane steam reforming. H₂ and CO yields were significantly enhanced with increasing CO₂/(CH₄ + H₂O) ratio due to growing CO₂ gasification rate of partially dehydrogenated species from CH₄ decomposition. Additionally, a considerable decline of H₂ to CO ratio from 2.14 to 1.83 was detected with reducing H₂O/(CH₄ + CO₂) ratio due to dominant reverse water-gas shift side reaction at H₂O-deficient feedstock. Interestingly, 10%Ni/SBA-15 catalyst was resistant to graphitic carbon formation in the co-occurrence of H₂O and CO₂ oxidizing agents and the mesoporous catalyst structure was still maintained after BRM. A strong correlation between formation of carbonaceous species and catalytic activity was observed.     

Formation of two kinds of carbon with different properties by acetone decomposition using in-liquid plasma method

Most waste solvent treatment methods involve combustion; however, such methods emit carbon dioxide and are also problematic relative to energy costs. As well as reducing carbon dioxide generate, in-liquid plasma has been used to generate valuable substances and hydrogen energy from the waste solvent. As a waste solve treatment model, decomposition of acetone using 27.12 MHz in-liquid plasma was performed at atmospheric pressure. The produced gases were H₂, CO, CH₄, C₂H₂, C₂H₄, C₂H₆, and CO_2, where proportion of CO₂ was less than 0.25%. Two types of carbons with different properties were obtained as byproducts during the acetone decomposition. These carbons analyzed using elemental analysis and Raman spectroscopy. The analysis results revealed that the crystallinity of the carbons different significantly. Undesirable organic matter, such as benzene, was also produced during acetone decomposition. The acetone decomposition efficiency at 407 W discharge power was 0.9 μmol J⁻¹.     

An experimental and kinetic modeling study of premixed NH3/CH4/O2/Ar flames at low pressure

An experimental and modeling study of 11 premixed NH₃/CH₄/O₂/Ar flames at low pressure (4.0 kPa) with the same equivalence ratio of 1.0 is reported. Combustion intermediates and products are identified using tunable synchrotron vacuum ultraviolet (VUV) photoionization and molecular-beam mass spectrometry. Mole fraction profiles of the flame species including reactants, intermediates and products are determined by scanning burner position at some selected photon energies near ionization thresholds. Temperature profiles are measured by a Pt/Pt-13%Rh thermocouple. A comprehensive kinetic mechanism has been proposed. On the basis of the new observations, some intermediates are introduced. The flames with different mole ratios (R) of NH₃/CH₄ (R0.0, R0.1, R0.5, R0.9 and R1.0) are modeled using an updated detailed reaction mechanism for oxidation of CH₄/NH₃ mixtures. With R increasing, the reaction zone is widened, and the mole fractions of H₂O, NO and N₂ increase while those of H₂, CO, CO₂ and NO₂ have reverse tendencies. The structural features by the modeling results are in good agreement with experimental measurements. Sensitivity and flow rate analyses have been performed to determine the main reaction pathways of CH₄ and NH₃ oxidation and their mutual interaction.     

Thermodynamic equilibrium analysis of combined carbon dioxide reforming with partial oxidation of methane to syngas

The chemical equilibrium analysis on combined CH₄-reforming with CO₂ and O₂ (combined CORM–POM) has been conducted by total Gibbs energy minimization using Lagrange's undetermined multiplier method. The equilibrium compositions of the combined CORM–POM process were considerably influenced by CH₄:CO₂:O₂ feed ratios and operating temperatures. Methane oxidation reaction occurred predominantly at low temperatures, while the CO₂ conversion was strongly influenced by the O₂/CH₄ feed ratio. The addition of O₂ to the CORM process improved the CH₄ conversion, H₂ and H₂O yields and also the H₂/CO product ratio at the expense of CO₂ conversion and CO yield. Accordingly, the optimal equilibrium conditions for the CH₄:CO₂:O₂ ratio were within the range of 1:0.8:0.2–1:1:0.2 and a minimum requirement temperature of 1000 K.     

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.