Study of cracking of methane for hydrogen production using concentrated solar energy

In this study, the cracking phenomenon of methane taking place in a cylindrical cavity of 16 cm in diameter and 40 cm in length under the heat of concentrated solar radiation without any catalyst is analysed. Three cases have been chosen; in all cases the primary phase contains methane and hydrogen gases. In the first case, we consider two phases; the secondary phase is a homogeneous carbon black powder with 50 nm of diameter; in the second case we have three phases where the two secondary phases are a particles powder with two diameters 20 and 80 nm and finally, a third case of five phases with a powder of four different diameters 20, 40, 60 and 80 nm. The low Reynolds K-ε turbulence model was applied. A calculation code "ANSYS FLUENT" is used to simulate the cracking phenomena where an Eulerian – Eulerian model is applied. The choice of several diameters greatly increases the calculation time but it approaches more of the physical reality of the radiation by these particles during the cracking. Results have shown that increasing the number of diameters gives higher cracking rates; the case of the powder of 4 different diameters gives the highest cracking rate. A parametric study as a function of the inlet velocity, carbon particle diameters and the intensity of solar radiation is realized. For the cracking heat, provided by the choice of the two concentrators of 5 and 16 MW/m² used in this simulation, the CH₄ inlet velocity is a decisive parameter for the cracking rate. Any increase in the inlet velocity requires more heat and this leads to a decrease in the cracking rate. For a velocity not exceeding 0.177 m/s (i.e. 0.3 L/min), both solar concentrations give the same amount of hydrogen produced. These quantities of hydrogen obtained reach maximum values for an inlet flow rate of CH₄ between 0.58 L/min (i.e. 0.34 m/s) and 0.62 L/min (i.e. 0.3655 m/s) for both reactors. The results are interpreted and compared with experimental work.     

Kinetic modeling of hydrogen production by thermal decomposition of methane

A kinetic model for the thermal decomposition of methane was developed by modifying a model for soot formation in combustion. The modifications consisted of adding five reactions representing surface chemistry on carbon particles. One kinetic parameter for the new surface reactions was fit to experimental data and the other parameters were determined by analogy to known gas-phase reactions. The modified model reproduces well the hydrogen concentrations at the beginning and end of recent experiments on the thermal decomposition of methane in the presence of carbon catalysts. The model predicts that increasing the pressure from 0.1 to 3.0 MPa will decrease hydrogen production with carbon catalysts by 48–60%. This prediction is robust to variations in the rate constants for the surface reactions and the rate constant for a key gas-phase reaction.     

Optimizing the operational strategy of a solar-driven reactor for thermochemical hydrogen production

In this paper the operational strategy of a pilot plant for regenerative hydrogen production based on two-step thermochemical redox cycles is investigated with focus on optimal operational parameters for highest solar-to-fuel efficiency. The current plant consists of a solar driven large-scale 250 kW thermochemical inert gas reactor using ceria as reactive material for water splitting and an efficient fluid heat recovery system.Here we analyse the most important process conditions, which are operating temperatures, mass flow rates and duration times for both steps in the cycle. A highly accurate and detailed simulation model combined with well-suited optimization routines reveals new insights in most efficient operational strategies. Within the optimization material and technical limits of the used components are considered, thereby yielding reliable practical results.Optimal operational parameters are found by using a temperature swing strategy with corresponding solar-to-fuel plant efficiency determined by up to 1.1%.     

Hydrogen production from thermo-catalytic decomposition of methane using carbon black catalysts in an indirectly-irradiated tubular packed-bed solar reactor

The solar thermo-catalytic decomposition of methane using carbon black catalysts for CO₂-free hydrogen production is studied in a packed-bed reactor. The indirectly-irradiated reactor is based on a cavity receiver and a tube-type absorber in which a given load of particle catalyst is injected during on-sun operation, while enabling multiple refilling for catalyst replacement. Concentrated solar power is used as an external radiative source for supplying the high temperature process heat and for driving the endothermic reaction. The indirect irradiation via the intermediate opaque tubular absorber results in a more uniform heating of the whole reacting bed volume and thus an easier reaction temperature control and determination. Carbon particles are used for enhancing the rate of the heterogeneous decomposition reaction and the coupling of the reactor with a particle injection system is implemented to operate in semi-continuous mode with possibility of catalyst load renewal after deactivation.     

One-dimensional model of solar thermal reactors for the co-production of hydrogen and carbon black from methane decomposition

The solar thermal decomposition of methane is a promising route for the large scale production of hydrogen and carbon black with zero CO₂ emissions, however careful control of the reactor is required to ensure product particles of specific sizes. A one-dimensional model employing a sectional method is developed to simulate the evolution of polydisperse fresh and seed particle populations in an indirectly heated solar reactor. The model accounts for the homogeneous nucleation of fresh particles, the heterogeneous growth of the fresh and seed particles, particle coagulation, and the growth of carbon on the walls of the reactor from heterogeneous reaction and particle deposition. The heat transport mechanisms modelled include wall-gas convection, wall-particle radiation exchange, particle-gas convection and heat release from chemical reaction. The model is validated in terms of methane conversion against a 10 kW experimental solar reactor and used to extract kinetic parameters for the homogeneous and heterogeneous reaction paths. The model shows promise as a quick and simple tool for the design and control of industrial scale solar reactors.     

Solar hydrogen production from the thermal splitting of methane in a high temperature solar chemical reactor

This study addresses the single-step thermal decomposition (pyrolysis) of methane without catalysts. The process co-produces hydrogen-rich gas and high-grade carbon black (CB) from concentrated solar energy and methane. It is an unconventional route for potentially cost effective hydrogen production from solar energy without emitting carbon dioxide since solid carbon is sequestered.A high temperature solar chemical reactor has been designed to study the thermal splitting of methane for hydrogen generation. It features a nozzle-type graphite receiver which absorbs the solar power and transfers the heat to the flow of reactant at a temperature that allows dissociation. Theoretical and experimental investigations have been performed to study the performances of the solar reactor. The experimental set-up and effect of operating conditions are described in this paper. In addition, simulation results are presented to interpret the experimental results and to improve the solar reactor concept. The temperature, geometry of the graphite nozzle, gas flow rates, and CH₄ mole fraction have a strong effect on the final chemical conversion of methane. Numerical simulations have shown that a simple tubular receiver is not enough efficient to heat the bulk gas in the central zone, thus limiting the chemical conversion. In that case, the reaction takes place only within a thin region located near the hot graphite wall. The maximum CH₄ conversion (98%) was obtained with an improved nozzle, which allows a more efficient gas heating due to its higher heat exchange area.     

Co-production of hydrogen and carbon black from solar thermal methane splitting in a tubular reactor prototype

This study addresses the solar thermal decomposition of natural gas for the co-production of hydrogen and carbon black (CB) as a high-value nano-material with the bonus of zero CO₂ emission. The work focused on the development of a medium-scale solar reactor (10 kW) based on the indirect heating concept. The solar reactor is composed of a cubic cavity receiver (20 cm-side), which absorbs concentrated solar irradiation through a quartz window by a 9 cm-diameter aperture. The reacting gas flows inside four graphite tubular reaction zones that are settled vertically inside the cavity. Experimental results in the temperature range 1740-2070 K are presented: acetylene (C₂H₂) was the most important by-product with a mole fraction of up to about 7%, depending on the gas residence time. C₂H₂ content in the off-gas affects drastically the carbon yield of the process. The effects of temperature and residence time are analyzed. A preliminary process study concerning a 55 MW solar chemical plant is proposed on the basis of a process flow sheet. Results show that 1.7 t/h of hydrogen and 5 t/h of CB could be produced with an hydrogen cost competitive to conventional steam methane reforming.     

Semi-continuous hydrogen production from catalytic methane decomposition using a fluidized-bed reactor

Non-oxidative, catalytic decomposition of hydrocarbons is an alternative, one-step process to produce pure hydrogen with no production of carbon oxides or higher hydrocarbons. Carbon produced from the decomposition reaction, in the form of potentially valuable carbon nanotubes, remains anchored to the active catalyst sites in a fixed bed. To facilitate periodical removal of this carbon from the reactor and to make hydrogen production continuous, a fluidized-bed reactor was envisioned. The hypothesis that the tumbling and inter-particle collisions of bed material would efficiently separate nanotubes anchored to the active catalyst sites of the bed particles was tested and shown to be invalid. However, a switching mode reaction system for the semi-continuous production of hydrogen and carbon nanotubes by periodic removal and replenishment of the catalytic bed material has been successfully demonstrated.     

Low carbon hydrogen production in Canada via natural gas pyrolysis

Large scale, low cost, and low carbon intensity hydrogen production is needed to reduce emissions in the energy and transportation sectors. We present a techno-economic analysis and life cycle assessment of natural gas pyrolysis technologies for hydrogen production, with carbon black (CB) as a co-product. Four designs were considered based on the source of heat to the pyrolysis system, the combustion medium, and use of carbon capture (CC) technology. The oxygen-fired-CB design with CC is the most attractive from financial and environmental perspectives, superior to a conventional steam methane reformer (SMR) process with CC. The estimated pre-tax minimum hydrogen selling prices for the pyrolysis technologies range between $1.08/kg and $2.43/kg when natural gas (NG) costs $3.76/GJ. Key advantages include near-zero onsite GHG emissions of the oxygen-fired-CB design with CC and up to 41% lower GHG emissions compared to the SMR + CC process. The results indicate that natural gas pyrolysis may be a feasible pathway for hydrogen production.     

Microwave-assisted methane decomposition over pyrolysis residue of sewage sludge for hydrogen production

The microwave-assisted methane decomposition over a pyrolysis residue of sewage sludge (PRSS), which acted as a microwave receptor and a low-cost catalyst without further activation, was investigated in a multimode microwave reactor. For comparison purpose, methane conversion (MC) over an activated carbon (AC) was also studied. The results indicate that PRSS is a better microwave receptor than AC. Under the same microwave power (MWP), MC over PRSS is markedly higher than that over AC, due to the remarkably higher Microwave heating (MWH) performance of PRSS. MWH of PRSS and AC is heavily influenced by atmosphere. Under the same MWP, the stable temperatures of the catalysts in hydrogen, nitrogen and methane atmosphere follow the sequence: T_nitrogen > T_hydrogen > T_methane. On the other hand, it was observed that nitrogen showed different effect on MC over PRSS and AC under MWH. Specifically, under the microwave-assisted methane decomposition reactions, the effect of nitrogen on MC over PRSS is not obvious, but it has remarkable effect on MC over AC. Additionally, a large number of molten beads were formed on the surface of the used PRSS by microwave irradiation. The composition and formation mechanism of the molten beads were also reported.     

Design,simulation and experimental study of a directly-irradiated solar chemical reactor for hydrogen and syngas production from continuous solar-driven wood biomass gasification

The use of concentrated solar energy as the high-temperature heat source for the thermochemical gasification of biomass is a promising prospect for producing CO₂-neutral chemical fuels (syngas). The solar process saves biomass resource because partial combustion of the feedstock is avoided, it increases the energy conversion efficiency because the calorific value of the feedstock is upgraded by the solar power input, and it also reduces the need for downstream gas cleaning and separation because the gas products are not contaminated by combustion by-products. A new concept of solar spouted bed reactor with continuous biomass injection was designed in order to enhance heat transfer in the reactor, to improve the gasification rates and gas yields by providing constant stirring of the particles, and to enable continuous operation. Thermal simulations of the prototype were performed to calculate temperature distributions and validate the reactor design at 1.5 kW scale. The reliable operation of the solar reactor based on this new design was also experimentally demonstrated under real solar irradiation using a parabolic dish concentrator. Wood particles were continuously gasified at temperatures ranging from 1100 °C to 1300 °C using either CO₂ or steam as oxidizing agent. Carbon conversion rates over 94% and gas productions over 70 mmol/g_biomass were achieved. The energy contained in the biomass was upgraded thanks to the solar energy input by a factor of up to 1.21.     

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.     

Microwave-assisted catalytic decomposition of methane over activated carbon for -free hydrogen production

The aim of this work was to combine microwave heating with the use of low-cost granular activated carbon as a catalyst for the production of CO₂-free hydrogen by methane decomposition in a fixed bed quartz-tube flow reactor. In order to compare the results achieved, conventional heating was also applied to the catalytic decomposition reaction of methane over the activated carbon. It was found that methane conversions were higher under microwave conditions than with conventional heating when the temperature measured was lower than or equal to . However, when the temperature was increased, the difference between the conversions under microwave and conventional heating was reduced. The influence of volumetric hourly space velocity (VHSV) on the conversion tests using both microwave and conventional heating was also studied. In general, there is a substantial initial conversion, which declines sharply during the first stages of the reaction but tends to stabilise with time. An increase in the VHSV has a negative effect on CH₄ conversion, and even more so in the case of microwave heating. Nevertheless, the conversions obtained in the microwave device at the beginning of the experiments are, in general, better than the conversions reported in other works which also use a carbonaceous-based catalyst. Additionally, the formation of carbon nanofibres in one of the microwave experiments is also reported.     

Ni doped carbons for hydrogen production by catalytic methane decomposition

Ni doped carbons were prepared from raw coal and direct coal liquefaction residue (CLR) by KOH activation with addition of Ni(NO₃)₂, and used for catalytic methane decomposition (CMD) to produce hydrogen. The catalytic activity of the Ni doped carbon for CMD was compared with those of metal catalysts (Ni/SiO₂ and Ni/Al₂O₃), coal- and CLR-based carbons, and Ni-carbon catalysts prepared by traditional impregnation and precipitation methods. The results show that the Ni doped carbon has higher and more stable activity than the metal and carbon catalysts at 850 °C. The preparation method for Ni doped carbons can make full use of the reducibility of the carbon composition and simplify the traditional synthesis process. The Ni content and the morphology of carbon deposits produced during CMD have a great effect on the catalytic activity of the Ni doped carbon.     

Effect of seeding on hydrogen and carbon particle production in a 10 MW solar thermal reactor for methane decomposition

Solar methane decomposition reactors are a novel technology for the production of carbon neutral hydrogen; however, the impact of this technology depends greatly on the ability to co-produce carbon black particles of commercial grade in order to offset the cost of hydrogen production and, therefore, the control of the reactor is very important. To this end, the seeding of indirect heating concept reactors using the product particles themselves could be used to control heat transfer inside the reactor. In this work, a previously developed one-dimensional reactor – particle population model was used to simulate the effect of seeding on the hydrogen and carbon particle production rates in the absorber tubes of a 10 MW indirect heating concept solar reactor. It was found that seed particle feed rates less than 10% of the methane-contained carbon feed rate allowed the hydrogen and fresh particle production rates to be doubled while keeping the rate of carbon growth on the tube walls constant. It was also found that similar seed fee rates could be used to maintain the hydrogen and particle production rates constant, given variations in the absorber tube wall temperature within a 100 °C range, for example due to cloud passage. Furthermore, it was found that the size characteristics of the freshly produced particles were not affected at these seed feed rates. Thus, seeding could be an effective means for increasing and controlling the hydrogen and carbon particle production rates in industrial scale indirect heating concept solar methane decomposition reactors, while also reducing carbon growth on the walls of the absorber tubes.     

Thermodynamic evaluation of hydrogen production from methane

Exergetic and energetic analysis has been utilized to estimate the effect of process design and conditions on the hydrogen purity and yield, exergetic efficiencies and CO₂ avoided. Methane was chosen as a model compound for evaluating single stage separation. Simple steam reforming was considered as the base – case system. The other chemical processes that were considered were steam reforming with CO₂ capture with and without chemical looping of a reactive carbon dioxide removal agent, and steam gasification with both the Boudouard reaction catalyst and the reactive carbon dioxide removal agent with and without the solids regeneration. The information presented clearly demonstrates the differences in efficiencies between the various chemical looping processes for hydrogen generation. The incremental changes in efficiencies as a function of process parameters such as temperature, steam amount, chemical type and amount were estimated. Energy and exergy losses associated with generation of syngas, separation of hydrogen from CO_x as well as exergetic loss associated with emissions are presented. The optimal conditions for each process by minimizing these losses are presented. The majority of the exergy destruction occurs due to the high irreversibility of chemical reactions. The results of this investigation demonstrate the utility of exergy analysis. The paper provides a procedure for the comparison of various technologies for the production of hydrogen from carbon based materials based on First and Second Law Analysis. In addition, two figures of merit, namely the comparative advantage factor and the sustainable advantage factor have been proposed to compare the various hydrogen production methods using carbonaceous fuels.     

Solar thermochemical gasification of wood biomass for syngas production in a high-temperature continuously-fed tubular reactor

Biomass gasification is an attractive process to produce high-value syngas. Utilization of concentrated solar energy as the heat source for driving reactions increases the energy conversion efficiency, saves biomass resource, and eliminates the needs for gas cleaning and separation. A high-temperature tubular solar reactor combining drop tube and packed bed concepts was used for continuous solar-driven gasification of biomass. This 1 kW reactor was experimentally tested with biomass feeding under real solar irradiation conditions at the focus of a 2 m-diameter parabolic solar concentrator. Experiments were conducted at temperatures ranging from 1000 °C to 1400 °C using wood composed of a mix of pine and spruce (bark included) as biomass feedstock. This biomass was used under its non-altered pristine form but also dried or torrefied. The aim of this study was to demonstrate the feasibility of syngas production in this reactor concept and to prove the reliability of continuous biomass gasification processing using solar energy. The study first consisted of a parametric study of the gasification conditions to obtain an optimal gas yield. The influence of temperature, oxidizing agent (H₂O or CO₂) or type of biomass feedstock on the product gas composition was investigated. The study then focused on solar gasification during continuous biomass particle injection for demonstrating the feasibility of a continuous process. Regarding the energy conversion efficiency of the lab scale reactor, energy upgrade factor of 1.21 and solar-to-fuel thermochemical efficiency up to 28% were achieved using wood heated up to 1400 °C.     

Thermochemical hydrogen production from a two-step solar-driven water-splitting cycle based on cerium oxides

A new thermochemical cycle for H₂ production based on CeO₂/Ce₂O₃ oxides has been successfully demonstrated. It consists of two chemical steps: (1) reduction, 2CeO₂ → Ce₂O₃ + 0.5O₂; (2) hydrolysis, Ce₂O₃ + H₂O → 2CeO₂ + H₂. The thermal reduction of Ce(IV) to Ce(III) (endothermic step) is performed in a solar reactor featuring a controlled inert atmosphere. The feasibility of this first step has been demonstrated and the operating conditions have been defined (T = 2000 °C, P = 100–200 mbar). The hydrogen generation step (water-splitting with Ce(III) oxide) is studied in a fixed bed reactor and the reaction is complete with a fast kinetic in the studied temperature range 400–600 °C. The recovered Ce(IV) oxide is then recycled in first step. In this process, water is the only material input and heat is the only energy input. The only outputs are hydrogen and oxygen, and these two gases are obtained in different steps avoiding a high temperature energy consuming gas-phase separation. Furthermore, pure hydrogen is produced (it is not contaminated by carbon products like CO, CO₂), thus it can be used directly in fuel cells. The results have shown that the cerium oxide two-step thermochemical cycle is a promising process for hydrogen production.     

Solar-driven novel methane reforming with carbon looping for hydrogen production

As a renewable source, solar energy plays an important role in meeting energy demand for human beings and in relieving global warming. In this paper, the concentrated solar heat is utilized to drive the high-endothermic methane reforming with carbon looping. In so doing, the process increases the utilization of CO₂ and reduces the carbon emissions as well as saves the extra fuel consumption for combustion, leading to high efficiency of energy utilization. By optimizing this proposed system, the energy efficiency can reach approximate 67.13% with simultaneously reducing CO₂ emissions by 34.98% compared with SMR process. Exergy analysis is used to assess the location of irreversibility within process. The maximal part of exergy destroyer was localized in reformer with a contribution of 68%. In addition, the effects of hourly variation of direct normal irradiation on thermodynamic performance and methane conversion on the four typical days (spring equinox, summer solstice, autumn equinox, winter solstice) were analyzed in this work. The current work might be insightful for solar-hydrogen production field.     

High temperature iron-based catalysts for hydrogen and nanostructured carbon production by methane decomposition

The production of hydrogen and filamentous carbon by means of methane decomposition was investigated in a fixed-bed reactor using iron-based catalysts. The effect of the textural promoter and the addition of Mo as a dopant affects the catalysts performance substantially: iron catalyst prepared with Al₂O₃ showed slightly higher catalytic performance as compared to those prepared with MgO; Mo addition was found to improve the catalytic performance of the catalyst prepared with MgO, whereas in the catalyst prepared with Al₂O₃ displayed similar or slightly poorer results. Additionally, the influence of the catalyst reduction temperature, the reaction temperature and the space velocity on the hydrogen yield was thoroughly investigated. The study reveals that iron catalysts allow achieving high methane conversions at operating temperatures higher than 800 °C, yielding simultaneously carbon nanofilaments with interesting properties. Thus, at 900 °C reaction temperature and 1 l g⁻¹_cat h⁻¹ space velocity, ca. 93 vol% hydrogen concentration was obtained, which corresponds to a methane conversion of 87%. Additionally, it was found that at temperatures higher than 700 °C, carbon co-product is deposited mainly as multi walled carbon nanotubes. The textural and structural properties of the carbonaceous structures obtained are also presented.