Thermodynamic analyses of hydrogen production from sub-quality natural gas: Part II: Steam reforming and autothermal steam reforming

Part I of this paper analyzed sub-quality natural gas (SQNG) pyrolysis and autothermal pyrolysis. Production of hydrogen via direct thermolysis of SQNGs produces only 2 mol of hydrogen and 1 mol of carbon per mole of methane (CH₄). Steam reforming of SQNG (SRSQNG) could become a more effective approach because the processes produce two more moles of hydrogen via water splitting. A Gibbs reactor unit operation in the AspenPlus™ chemical process simulator was employed to accomplish equilibrium calculations for the SQNG + H₂O and SQNG + H₂O + O₂ systems. The results indicate that water and oxygen inlet flow rates do not significantly affect the decomposition of hydrogen sulfide (H₂S) at temperatures lower than 1000 °C. The major co-product of the processes is carbonyl sulfide (COS) while sulfur dimer (S₂) and carbon disulfide (CS₂) are minor by-products within this temperature range. At higher temperatures (>1300 °C), CS₂ and S₂ become major co-products. No sulfur dioxide (SO₂) or sulfur trioxide (SO₃) is formed during either SRSQNG or autothermal SRSQNG processes, indicating that no environmentally harmful acidic gases are generated.     

Thermodynamic analyses of hydrogen production from sub-quality natural gas: Part I: Pyrolysis and autothermal pyrolysis

Sub-quality natural gas (SQNG) is defined as natural gas whose composition exceeds pipeline specifications of nitrogen, carbon dioxide (CO₂) and/or hydrogen sulfide (H₂S). Approximately one-third of the U.S. natural gas resource is sub-quality gas [1]. Due to the high cost of removing H₂S from hydrocarbons using current processing technologies, SQNG wells are often capped and the gas remains in the ground. We propose and analyze a two-step hydrogen production scheme using SQNG as feedstock. The first step of the process involves hydrocarbon processing (via steam–methane reformation, autothermal steam–methane reformation, pyrolysis and autothermal pyrolysis) in the presence of H₂S. Our analyses reveal that H₂S existing in SQNG is stable and can be considered as an inert gas. No sulfur dioxide (SO₂) and/or sulfur trioxide (SO₃) is formed from the introduction of oxygen to SQNG. In the second step, after the separation of hydrogen from the main stream, un-reacted H₂S is used to reform the remaining methane, generating more hydrogen and carbon disulfide (CS₂). Thermodynamic analyses on SQNG feedstock containing up to 10% (v/v) H₂S have shown that no H₂S separation is required in this process. The Part I of this paper includes only thermodynamic analyses for SQNG pyrolysis and autothermal pyrolysis.     

Methane steam reforming for producing hydrogen in an atmospheric-pressure microwave plasma reactor

A methane steam reforming process for producing mainly hydrogen in an atmospheric-pressure microwave plasma reactor is demonstrated. Nano carbon powders, CO_x, C₂H₂, C₂H₄, and HCN were also formed. Intermediates such as OH, NH, CH, and active N₂ were identified using optical emission spectroscopy. The selectivity of H₂ was greater than 92.7% at inlet H₂O/CH₄ molar ratio (R) ≧ 0.5, and was higher than that obtained using methane plasmalysis because steam inhibited the formation of C₂H₂. The highest methane conversion was obtained at R = 1, reaching 91.6%, with the lowest specific energy consumption of H₂ formation at [CH₄]_in = 5%, 1.0 kW, and 12 slpm. The plasma-assisted catalysis process, which packed Ni/Al₂O₃ catalysts in the discharge zone and supplied heat using hot effluents, was used to elevate the methane conversion and hydrogen selectivity. However, large amounts of 40–70 nm carbon powder, which is electrically conductive, were produced, resulting in rapid catalyst deactivation due to carbon being deposited on the surface and in the pores of catalysts.     

SOFC anodes for direct oxidation of hydrogen and methane fuels containing H2S

Single cell solid oxide fuel cells using Ni-YSZ and Co-YSZ anodes were tested in H₂, CH₄, H₂S/H₂ and H₂S/CH₄ fuel mixtures. Their performance was found to quickly degrade in dry CH₄ due to carbon deposition and lifting of the anode from the electrolyte. In contrast, hydrogen or methane containing H₂S showed an increase in exchange current densities when compared to H₂,M/YSZ/LSM,air systems (M = Ni or Co) despite having less optimal anode microstructure. Conversion from metal to metal-sulfide in the presence of H₂S produced large, dense metal-sulfide particles surrounded by YSZ, thus decreasing the triple-phase boundary. Furthermore, CoS-based anodes showed phase segregation and densification toward the electrolyte. Despite this, long-term testing at η_a = 0.5 V of the H₂S/CH₄,CoS-YSZ/YSZ/LSM,air system showed no signs of degradation of the anode over a 6-day period. Only after removal of H₂S from the H₂S/CH₄ stream did the CoS-anode reduce back to Co, signifying H₂S is required to maintain the metal-sulfide active anode.     

Production of hydrogen and nano carbon powders from direct plasmalysis of methane

In this study, a single-stage, non-catalytic, dry methane plasmalysis process for producing mainly hydrogen and carbon powders using an atmospheric-pressure microwave plasma reactor is demonstrated. A high selectivity of H₂ and carbon powders (C₁), and a low required energy consumption of H₂ could be obtained simultaneously at a low-applied power and a high inlet concentration of CH₄ ([CH₄]_in), reaching 86.0% (selectivity of H₂), 50.7% (selectivity of C₁), and 6.7 eV/molecule–H₂, respectively, at 0.8 kW, [CH₄]_in = 20%, and 12 slpm when CH₄ gas was fed from the downstream of the cavity resonator. In addition, large amounts of nano carbon powders, which consisted of C atoms and had a graphite–rhombohedral structure with a particle size of about 50 nm, were produced. These particles had potential for using as a support for platinum catalysts in fuel cells.     

A modified steam-methane-reformation reaction for hydrogen production

Hydrogen is mostly produced by the Steam Methane Reforming (SMR) reaction which adds many tonnes of carbon emissions to the environment for each tonne of hydrogen. A modified scheme for carbon-emission free production of hydrogen, which involves sodium hydroxide, methane and steam, has been explored here. The modification of the SMR reaction isCH₄ + 2NaOH + H₂O = Na₂CO₃ + 4H₂The modified reaction has several advantages: it does not require catalysis, the temperature of reaction is considerably reduced and the products are industrially important. By this process, we can produce hydrogen without any carbon dioxide emission as shown in this theoretical and experimental study. The reaction has been studied in the temperature range of 873-1073 K in an open configuration for 30 min and at various methane and constant water vapor flow. It is determined that at a methane flow rate of 25 ml/min the reaction is 98% complete at 873 K.     

Life cycle assessment of hydrogen production by methane decomposition using carbonaceous catalysts

Methane decomposition to yield hydrogen and carbon (CH₄ ? 2H₂ + C) is one of the cleanest alternatives, free of CO₂ emissions, for producing hydrogen from fossil fuels. This reaction can be catalyzed by metals, although they suffer a fast deactivation process, or by carbonaceous materials, which present the advantage of producing the catalyst from the carbon obtained in the reaction. In this work, the environmental performance of methane decomposition catalyzed by carbonaceous catalysts has been evaluated through Life Cycle Assessment tools, comparing it to other decomposition processes and steam methane reforming coupled to carbon capture systems. The results obtained showed that the decomposition using the autogenerated carbonaceous as catalyst is the best option when reaction conversions higher than 65% are attained. These were confirmed by 2015 and 2030 forecastings. Moreover, its environmental performance is highly increased when the produced carbon is used in other commercial applications. Thus, for a methane conversion of 70%, the application of 50% of the produced carbon would lead to a virtually zero-emissions process.     

Sorption enhanced reaction process for direct production of fuel-cell grade hydrogen by low temperature catalytic steam-methane reforming

New experimental data are reported to demonstrate that a sorption enhanced reaction (SER) concept can be used to directly produce fuel-cell grade H₂ (<20 ppm CO) by carrying out the catalytic, endothermic, steam-methane reforming (SMR) reaction (CH₄ + 2H₂O ↔ CO₂ + 4H₂) in presence of a CO₂ selective chemisorbent such as K₂CO₃ promoted hydrotalcite at reaction temperatures of 520 and 550 °C, which are substantially lower than the conventional SMR reaction temperatures of 700-800 °C. The H₂ productivity of the sorption enhanced reactor can be large, and the conversion of CH₄ to H₂ can be very high circumventing the thermodynamic limitations of the SMR reaction due to the application of the Le Chetalier's principle in the SER concept. Mathematical simulations of a cyclic two-step SER concept showed that the H₂ productivity of the process (moles of essentially pure H₂ produced per kg of catalyst-chemisorbent admixture in the reactor per cycle) is much higher at a reaction temperature of 590 °C than that at 550 or 520 °C. On the other hand, the conversion of feed CH₄ to high purity H₂ product is relatively high (>99+%) at all three temperatures. The conversion is much higher than that in a conventional catalyst-alone reactor at these temperatures, and it increases only moderately (<1%) as the reaction temperature is increased from 520 to 590 °C. These results are caused by complex interactions of four phenomena. They are (a) favorable thermodynamic equilibrium of the highly endothermic SMR reaction at the higher reaction temperature, (b) faster kinetics of SMR reaction at higher temperatures, (c) favorable removal of CO₂ from the reaction zone at lower temperatures, and (d) higher cyclic working capacity for CO₂ chemisorption at higher temperature.     

Hydrogen production from biogas reforming and the effect of H2S on CH4 conversion

Biogas produced during anaerobic decomposition of plant and animal wastes consists of high concentrations of methane (CH₄), carbon dioxide (CO₂) and traces of hydrogen sulfide (H₂S). The primary focus of this research was on investigating the effect of a major impurity (i.e., H₂S) on a commercial methane reforming catalyst during hydrogen production. The effect of temperature on CH₄ and CO₂ conversions was studied at three temperatures (650, 750 and 850 °C) during catalytic biogas reforming. The experimental CH₄ and CO₂ conversions thus obtained were found to follow a trend similar to the simulated conversions predicted using ASPEN plus. The gas compositions at thermodynamic equilibrium were estimated as a function of temperature to understand the intermediate reactions taking place during biogas dry reforming. The exit gas concentrations as a function of temperature during catalytic reforming also followed a trend similar to that predicted by the model. Finally, catalytic reforming experiments were carried out using three different H₂S concentrations (0.5, 1.0 and 1.5 mol%). The study found that even with the introduction of small amount of H₂S (0.5 mol%), the CH₄ and CO₂ conversions dropped to about 20% each as compared to 65% and 85%, respectively in the absence of H₂S.     

Technoeconomic analysis for hydrogen and carbon Co-Production via catalytic pyrolysis of methane

Catalytic Methane Pyrolysis (CMP) is an innovative method to convert gaseous methane into valuable H₂ and carbon products. The catalytic approach to methane pyrolysis has the potential to decrease the required operating temperature for methane decomposition from >1000 °C to under 700 °C. In this work, a novel inexpensive catalyst is discussed that displays low operating temperatures, while still maintaining high reactivity and long proven lifetimes. The kinetics associated with the catalyst's performance are modeled and a correlation was developed for use with practical simulation tools. A techno-economic assessment was conducted applying experimentally determined kinetics for the CMP reaction with the specific catalyst. Two process concepts that utilize CMP using the novel catalyst are presented in this work. Optimizations were considered in these processes and the CO₂ emissions and cost of hydrogen production of the two optimized cases, CMP with H₂ combustion (CMP-H₂) and CMP with CH₄ Combustion (CMP-CH₄), are compared to that of the current industrial standard for hydrogen production, Steam Methane Reforming with carbon capture and sequestration (SMR-CCS). Both of the proposed concepts convert methane into gaseous hydrogen and valuable carbon products, graphitic carbon to carbon Nano fibers. The carbon price was treated as a variable to determine the sensitivity of hydrogen production cost to the carbon price. The analysis indicates that cost of hydrogen production is highly dependent on the recovery and sale of carbon byproducts. Based on Aspen modeling of these two concepts for large scale hydrogen production (216 tons/day), the cost of hydrogen production, without considering carbon sales, was estimated to be $<3.25/kg, assuming a natural gas price of $7/MMBTU and conservative catalyst cost of $8/kg. Assuming 100% recovery of carbon, the price can be reduced to $0/kg by selling the carbon at <$1/kg. A market assessment suggests that values of graphitic carbon and carbon fibers range from ~$10/kg and ~$25–113/kg, respectively. The cost of H₂ production via conventional SMR is ~$2.2/kg when accounting for the cost of CO₂ sequestration. The proposed processes produce a maximum of 0–2 kg CO₂/kg H₂ in contrast to the 10 kg CO₂/kg H₂ produced via conventional SMR-CCS. The process displays an enormous potential for competitive economics accompanied by reduced greenhouse gas emissions.     

Investigation of combustion characteristics and emissions in a spark-ignition engine fuelled with natural gas–hydrogen blends

In this study, an experimental study on the performance and exhaust emissions of a spark-ignition engine fuelled with methane–hydrogen mixtures (100% CH₄, 10% H₂ + 90% CH₄, 20% H₂ + 80% CH₄, and 30% H₂ + 70% CH₄) were performed at different engine speeds and different excessive air ratios. This present work was carried out on a Ford engine. This is a four-stroke cycle four-cylinder spark-ignition engine with a bore of 80.6 mm, a stroke of 88 mm and a compression ratio of 10:1. Experiments were performed at 1500, 2000, 2500 and 3000 rpm and at wide open throttle (WOT). CO, CO₂ and HC emission values and cylinder pressure were measured. The results showed that while the speed and excessive air ratio increase, CO emission values decrease. The reduction of HC and CO emissions could be obtained by adding hydrogen into the natural gas when operating on the lean mixture condition. Increasing the excessive air ratio also decreases the maximum peak cylinder pressure.     

Technical assessment of cryo-compressed hydrogen storage tank systems for automotive applications

On-board and off-board performance and cost of cryo-compressed hydrogen storage are assessed and compared to the targets for automotive applications. The on-board performance of the system and high-volume manufacturing cost were determined for liquid hydrogen refueling with a single-flow nozzle and a pump that delivers liquid H₂ to the insulated cryogenic tank capable of being pressurized to 272 atm. The off-board performance and cost of delivering liquid hydrogen were determined for two scenarios in which hydrogen is produced by central steam methane reforming (SMR) or by central electrolysis. The main conclusions are that the cryo-compressed storage system has the potential of meeting the ultimate target for system gravimetric capacity, mid-term target for system volumetric capacity, and the target for hydrogen loss during dormancy under certain conditions of minimum daily driving. However, the high-volume manufacturing cost and the fuel cost for the SMR hydrogen production scenario are, respectively, 2–4 and 1.6–2.4 times the current targets, and the well-to-tank efficiency is well short of the 60% target specified for off-board regenerable materials.     

Hydrogen from hydrogen sulfide: towards a more sustainable hydrogen economy

The decomposition of hydrogen sulfide (H₂S) with simultaneous hydrogen (H₂) generation offers a sustainable energy production option and an environmental pollution abatement strategy. H₂S is both naturally occurring and human-made. In the future, H₂S production is expected to increase due to increased heavy oil refining. Currently, H₂S is largely converted to sulfur and water using industrial processes such as the Claus process, however, it would be more useful and economical to convert H₂S to sulfur and H₂ instead. H₂ currently comes from the steam reforming of natural gas, which is an energy-intensive process. Because H₂ is a valued commodity and global consumption is expected to increase, alternative sources of H₂ and hydrogen conservation have become topics of active research. Alberta is an especially large consumer of H₂ due to its oil sands processing. H₂ from petroleum-based H₂S sources could be reused in petroleum upgrading, as a partial replacement of steam methane reforming. This review paper highlights some of the methods of H₂S utilization, such as partial oxidation, reformation and decomposition techniques and approaches that convert H₂S to sulfur, water and, more importantly, H₂. To date, almost no technologies exist that are suitable for converting H₂S to sulfur and H₂ for industrial-scale applications. Here, we survey the literature to identify the most promising approach.     

Effect of hydrogen additive on methane decomposition to hydrogen and carbon over activated carbon catalyst

The effect of H₂ addition on CH₄ decomposition over activated carbon (AC) catalyst was investigated. The results show that the addition of H₂ to CH₄ changes both methane conversion over AC and the properties of carbon deposits produced from methane decomposition. The initial methane conversion declines from 6.6% to 3.3% with the increasing H₂ flowrate from 0 to 25 mL/min, while the methane conversion in steady stage increases first and then decreases with the flowrate of H₂, and when the H₂ flowrate is 10 mL/min, i.e. quarter flowrate of methane, the methane conversion over AC in steady stage is four times more than that without hydrogen addition. It seems that the activity and stability of catalyst are improved by the introduction of H₂ to CH₄ and the catalyst deactivation is restrained. Filamentous carbon is obtained when H₂ is introduced into CH₄ reaction gas compared with the agglomerate carbon without H₂ addition. The formation of filamentous carbon on the surface of AC and slower decrease rate of surface area and pores volume may cause the stable activity of AC during methane decomposition.     

Batch slurry photocatalytic reactors for the generation of hydrogen from sulfide and sulfite waste streams under solar irradiation

In this study, two solar slurry photocatalytic reactors i.e., batch reactor (BR) and batch recycle reactor with continuous supply of inert gas (BRRwCG) were developed for comparing their performance. The performance of the photocatalytic reactors were evaluated based on the generation of hydrogen (H₂) from water containing sodium sulfide (Na₂S) and sodium sulfite (Na₂SO₃) ions. The photoreactor of capacity 300 mL was developed with UV-vis transparent walls. The catalytic powders ((CdS/ZnS)/Ag₂S + (RuO₂/TiO₂)) were kept suspended by means of magnetic stirrer in the BR and gas bubbling and recycling of the suspension in the BRRwCG. The rate constant was found to be 120.86 (einstein⁻¹) for the BRRwCG whereas, for the BR it was found to be only 10.92 (einstein⁻¹). The higher rate constant was due to the fast desorption of products and suppression of e⁻/h⁺ recombination.     

Prediction of the liftoff,blowout and blowoff stability limits of pure hydrogen and hydrogen/hydrocarbon mixture jet flames

The paper presents experimental studies of the liftoff and blowout stability parameters of pure hydrogen, hydrogen/propane and hydrogen/methane jet flames using a 2 mm burner. Carbon dioxide and Argon gas were also used in the study for the comparison with hydrocarbon fuel. Comparisons of the stability of H₂/C₃H₈, H₂/CH₄ and H₂/CO₂ flames showed that H₂/C₃H₈ produced the highest liftoff height and H₂/CH₄ required highest liftoff, blowoff and blowout velocities. The non-dimensional analysis of liftoff height was used to correlate liftoff data of H₂, H₂/C₃H₈, H₂/CO₂, C₃H₈ and H₂/Ar jet flames tested in the 2 mm burner. The suitability of extending the empirical correlations based on hydrocarbon flames to both hydrogen and hydrogen/hydrocarbon flames was examined.     

Effect of H2S on carbon-catalyzed methane decomposition and CO2 reforming reactions

The effect of H₂S on catalytic processing of methane is of a great practical importance. In this work, the effect of small quantities (0.5–1.0 vol.%) of H₂S present in the feedstock on the methane decomposition and CO₂ reforming reactions over carbon and metal based catalysts was investigated. Activated carbon (FY5), an in-house prepared alumina-supported Ni catalyst (NiA) and the mixture of both (FY5 + NiA) were used as catalysts in this study. It was found that CH₄ and CO₂ conversions were noticeably increased when H₂S was added to the reacting mixture, which points to (i) the tolerance of carbon catalyst to H₂S and (ii) the catalytic effect of H₂S on carbon-catalyzed decomposition and dry reforming of methane. In contrast, NiA catalyst and the mixture FY5 + NiA were deactivated in the presence of H₂S in both reactions. The effect of the heating system (i.e., conventional electric resistance vs microwave heating) on the products yield of the dry reforming reaction in the presence of H₂S is also discussed in this paper.     

Autothermal reforming of methane for producing high-purity hydrogen in a Pd/Ag membrane reactor

Autothermal reforming of methane includes steam reforming and partial oxidizing methane. Theoretically, the required endothermic heat of steam reforming of methane could be provided by adding oxygen to partially oxidize the methane. Therefore, combining the steam reforming of methane with partial oxidation may help in achieving a heat balance that can obtain better heat efficacy. Membrane reactors offer the possibility of overcoming the equilibrium conversion through selectively removing one of the products from the reaction zone. For instance, only can hydrogen products permeate through a palladium membrane, which shifts the equilibrium toward conversions that are higher than the thermodynamic equilibrium. In this study, autothermal reforming of methane was carried out in a traditional reactor and a Pd/Ag membrane reactor, which were packed with an appropriate amount of commercial Ni/MgO/Al₂O₃ catalyst. A power analyzer was employed to measure the power consumption and to check the autothermicity. The average dense Pd/Ag membrane thickness is 24.3 μm, which was coated on a porous stainless steel tube via the electroless palladium/silver plating procedure. The experimental operating conditions had temperatures that were between 350 °C and 470 °C, pressures that were between 3 atm and 7 atm, and O₂/CH₄ = 0–0.5. The effects of the operating conditions on methane conversion, permeance of hydrogen, H₂/CO, selectivities of CO_x, amount of power supply, and the carbon deposition of the catalyst after the reaction is thoroughly discussed in this paper. The experimental results indicate that an optimum methane conversion of 95%, with a hydrogen production rate of 0.093 mol/m². S, can be obtained from the autothermal reforming of methane at H₂O/CH₄ = 1.3 and O₂/CH₄ near 0.4, at which the reaction does not consume power, and the catalysts are not subject to any carbon deposition.     

Hydrogen production by radio frequency plasma stimulation in methane hydrate at atmospheric pressure

Methane hydrate, formed by injecting methane into 100 g of shaved ice at a pressure of 7 MPa and reactor temperature of 0 °C, was decomposed by applying 27.12 MHz radio frequency plasma in order to produce hydrogen. The process involved the stimulation of plasma in the methane hydrate with a variable input power at atmospheric pressure. It was observed that production of CH₄ is optimal at a slow rate of CH₄ release from the methane hydrate, as analyzed by in light of the steam methane reforming (SMR) and the methane cracking reaction (MCR) processes in accordance with the content of gas production. In comparison with the steam methane reforming (SMR), it was found that methane-cracking reaction (MCR) was dominant in conversion of CH₄ into hydrogen. An H₂ content of 55% in gas production was obtained from conversion of 40% of CH₄ at an input power of 150 W. The results clearly show that hydrogen can be directly produced from methane hydrate by the in-liquid plasma method.     

Durability of a Ni based monolithic catalyst in the autothermal reforming of biogas

A Ni based catalyst supported on a cordierite monolithic substrate was applied to the autothermal reforming (ATR) of biogas to produce hydrogen. When the feed rates of oxygen and steam were constant, the Steam/CH₄ (S/CH₄) and O₂/CH₄ ratios changed because of an increase or decrease in the methane concentration of the biogas. The concentration of methane in the biogas fluctuates roughly between 35% and 65% according to factors such as the properties or amount of the waste. Therefore, the effect of S/CH₄ and O₂/CH₄ ratios on catalyst durability was confirmed by using actual biogas, which was produced by anaerobic fermentation of biomass at the biogasification bench-scale plant in Kyoto. Reforming reactions were carried out at ratios of S/CH₄ = 0–4, O₂/CH₄ = 0.5 and at S/CH₄ = 2, O₂/CH₄ = 0.6. The S/CH₄ range of 0–2.0 and the O₂/CH₄ range of 0.5–0.6 had no effect on the catalyst durability and a S/CH₄ ratio of more than 3.0 led to decreased catalytic performance.