Adsorptive removal of H2S in biogas conditions for high temperature fuel cell systems

Desulfurization represents a crucial step in fuel processing for high temperature fuel cells, because of catalysts stringent requirements. Moreover, when fuel cell stacks are used in micro-CHP applications, it is necessary to build an efficient and compact system. The use of biogas from anaerobic digestion could have a significant impact in terms of fossil fuels saving and environmental conservation. Biogas contains different impurities, among which H₂S represents one of the most harmful components.Adsorption tests for H₂S removal were carried out in biogas conditions, using commercial adsorbents, defining the materials characterized by the best performance and resulting in a predominance of impregnated activated carbons. The influence on adsorption capacity of operating parameters, such as gas hourly space velocity, gas matrix composition (N₂, CH₄ and CO₂), humidity, temperature (30–150 °C), H₂S concentration (50–1000 ppmv) and filter geometry, was investigated. The aim of the study was the functional parameters optimization to obtain a compact filter with high removal activity.     

Ordered mesoporous Fe-Al2O3 based-catalysts synthesized via a direct "one-pot" method for the dry reforming of a model biogas mixture

Biogas plays a vital role in the emerging renewable energy sector and its efficient utilization is attracting significant attention as an alternative energy carrier to non-renewable fossil fuel resources. Since biogas consists mainly of CH₄ and CO₂, dry reforming of methane arises as an appropriate process enabling its chemical conversion to high-quality synthesis gas (syngas: H₂ and CO mixtures). In this study, we synthesized via a direct "one-pot" method following an evaporation-induced self-assembly approach, ordered mesoporous Fe_10%, Ni_5% and Fe_x%Ni_(1-x) (x: 2.5, 5 or 7.5%) in Al₂O₃ as catalysts for syngas production via dry reforming of a model biogas mixture (CH₄/CO₂ = 1.8, at a temperature of 700 °C). Monometallic Fe_10%Al₂O₃ catalyst presented lower reactivity levels and slightly deactivated during catalysis compared to stable Ni_5%Al₂O₃. According to physico-chemical characterization techniques, the incomplete reduction of Fe₂O₃ into Fe₃O₄ rather than Fe⁰ nanoparticles (catalytically active) coupled with the segregation of Fe₃O₄ oxides were the main factors leading to the low performance of mesoporous Fe_10%Al₂O₃. These drawbacks were overcome upon the partial substitution of Fe by Ni (another transition metal) forming specifically bimetallic Fe_5%Ni_5%Al₂O₃ displaying reactivity levels close to thermodynamic expected ones. The formation of Fe-Ni alloys stabilized iron inside alumina matrix and protected it from segregation. Along with the confinement effect, spent catalyst characterizations showed high resistance towards coke deposition.     

Performance characterization of a novel Fe-based sorbent for anaerobic gas desulfurization finalized to high temperature fuel cell applications

A novel sorbent (SCX) composed of mixed iron oxides and hydroxides was studied to assess its suitability for H₂S removal from biogas finalized to high temperature fuel cell systems. From an industrial point of view, the potential usage of this product would have a beneficial impact on operational costs, since it is less expensive than activated carbons. Moreover, it is a non-hazardous landfilled product with a reduced environmental impact and it can be easily regenerated.Sorbent SCX was tested in different operative conditions, evaluating the influence on sorption capacity of gas hourly space velocity (GHSV), gas composition (inlet H₂S concentration, gas matrix, humidity), reactor temperature and filter geometry. As main outcomes, the experimentation highlighted: i) a hyperbolic increase in adsorption capacity as GHSV decreased, ii) a parabolic performance enhancement with increasing temperature and iii) a negative effect of humidity.Moreover, a comparison with commercial activated carbons typically used for the same purpose (Cu–Cr or KOH–KI treated) was performed, identifying the optimal operative parameters to obtain higher H₂S sorption capacity for SCX sorbent. Specifically, SCX showed its potential in terms of H₂S removal for GHSV lower than 330 h^?1, values of particular interest for many practical applications. In this case, an improved removal capability is obtained with respect to the ones exhibited by the considered carbons. Moreover, quantitative evaluations provided for unitary installed power (1 kW_e fuel cell-based systems) highlighted the possibility to have more compact (volume reduction of about 50%) and cheaper (cost reduction up to six times) filters.     

Techniques for transformation of biogas to biomethane

Biogas from anaerobic digestion and landfills consists primarily of CH₄ and CO₂. Trace components that are often present in biogas are water vapor, hydrogen sulfide, siloxanes, hydrocarbons, ammonia, oxygen, carbon monoxide and nitrogen. In order to transfer biogas into biomethane, two major steps are performed: (1) a cleaning process to remove the trace components and (2) an upgrading process to adjust the calorific value. Upgrading is generally performed in order to meet the standards for use as vehicle fuel or for injection in the natural gas grid.Different methods for biogas cleaning and upgrading are used. They differ in functioning, the necessary quality conditions of the incoming gas, the efficiency and their operational bottlenecks. Condensation methods (demisters, cyclone separators or moisture traps) and drying methods (adsorption or absorption) are used to remove water in combination with foam and dust.A number of techniques have been developed to remove H₂S from biogas. Air dosing to the biogas and addition of iron chloride into the digester tank are two procedures that remove H₂S during digestion. Techniques such as adsorption on iron oxide pellets and absorption in liquids remove H₂S after digestion.Subsequently, trace components like siloxanes, hydrocarbons, ammonia, oxygen, carbon monoxide and nitrogen can require extra removal steps, if not sufficiently removed by other treatment steps.Finally, CH₄ must be separated from CO₂ using pressure swing adsorption, membrane separation, physical or chemical CO₂-absorption.     

Membrane enrichment of biogas from two-stage pilot plant using agricultural waste as a substrate

The separation of methane from raw biogas was the main purpose of this study. A polymer membrane was used in order to obtain the high energy product, which can be utilized in cogeneration systems (CHP) or as a natural gas substitute. The study showed that using a polyimide hollow fiber module for biogas purification was an efficient method (low energy consumption, small-sized devise and a simple separation module). The satisfying results of laboratory tests caused scale up the installation. Different synthetic gas mixtures were used at the lab-scale, while in the field tests, raw biogas from a Polish two-stage agricultural biogas plant was processed. The plant used the following substrates: maize silage, grass silage and blends of these substrates with different supplements. The concentration of methane in the raw gas was up to 70% volume and contained up to 250 ppm of H₂S. In both cases (laboratory and field tests), the retentate after membrane treatment was characterized by high methane concentration (up to 90% volume) and was free of H₂S. The applied membrane demonstrated high selectivity for separating CH₄ from CO₂, H₂S and H₂O. The permeate stream contained less than 5% volume of CH₄, which ensured low losses of the desired biogas component. The influence of pressure (below 10 bars) and stage cut on the quality of the product were analyzed to develop optimal process conditions for mobile plant construction.     

MSW landfill biogas desulfurization

Biogas utilization in MCFC systems requires a high level of gas purification in order to meet the stringent sulfur tolerance limits of both the fuel cells and the reformer catalysts. In this study, two commercial activated carbons (ACs) have been tested for H₂S removal from the biogas produced at the Montescarpino Municipal Solid Waste landfill in Genoa, Italy. The performed analyses show a low selectivity of activated carbon towards the adsorption of only sulfur species. This represents a drawback for the use of this type of system, however, the use of mixed beds of different ACs has demonstrated to be advantageous in improving the removal efficiency of H₂S. Thus, the adsorption treatments with AC can ensure the high level of gas desulfurization required for fuel cell application. Nevertheless, the low adsorption capacity observed using landfill biogas would lead to high operative costs that suggest the application of a preliminary gas-scrubbing stage.     

Hydrogen sulfide removal from biogas by biotrickling filter inoculated with Halothiobacillus neapolitanus

Hydrogen sulfide (H₂S), a highly corrosive gas, is found in biogas due to the biodegradation of proteins and other sulfur containing organic compounds present in feed stock during anaerobic digestion. The presence of H₂S is one of the biggest factors limiting the use of biogas. It should be removed prior to application of biogas in an electric generator or industrial boiler. The present research evaluated the performance of biotrickling filter inoculated with Halothiobacillus neapolitanus NTV01 (HTN) on the H₂S removal from synthetic biogas. HTN, isolated and purified from activated sludge, is a sulfur oxidizing bacteria able to degrade H₂S and thiosulfate to elemental sulfur and sulfate, respectively. Operational parameters in a short term operation were varied as following; gas flow rate (0.5–0.75 LPM); EBRT (40–120 s); the inlet H₂S concentrations (0–1500 ppmv); liquid recirculation rate (3.6–4.8 L/h). EBRT showed a greater effect to the removal efficiency than increasing H₂S concentration. Longer EBRT resulted higher removal efficiency. The changes of liquid recirculation rates did not significantly affect the removal efficiency. In long term operation, the gas flow rate and liquid recirculation rate were fixed at 0.5 LPM (120 s EBRT) and 3.6 L/h; and H₂S concentrations were varied (0–2040 ppmv). The maximum elimination capacity was found as 78.57 g H₂S/m³ h, which had greater performance than the previous studies.     

Biogas quality upgrade by simultaneous removal of CO2 and H2S in a packed column reactor

Biogas from anaerobic digestion of biological wastes is a renewable energy resource. It has been used to provide heat, shaft power and electricity. Typical biogas contains 50–65% methane (CH₄), 30–45% carbon dioxide (CO₂), moisture and traces of hydrogen sulphide (H₂S). Presence of CO₂ and H₂S in biogas affects engine performance adversely. Reducing CO₂ and H₂S content will significantly improve quality of biogas. In this work, a method for biogas scrubbing and CH₄ enrichment is presented. Chemical absorption of CO₂ and H₂S by aqueous solutions in a packed column was experimentally investigated. The aqueous solutions employed were sodium hydroxide (NaOH), calcium hydroxide (Ca(OH)₂) and mono-ethanolamine (MEA). Liquid solvents were circulated through the column, contacting the biogas in countercurrent flow. Absorption characteristics were examined. Test results revealed that the aqueous solutions used were effective in reacting with CO₂ in biogas (over 90% removal efficiency), creating CH₄ enriched fuel. H₂S was removed to below the detection limit. Absorption capability was transient in nature. Saturation was reached in about 50 min for Ca(OH)₂, and 100 min for NaOH and MEA, respectively. With regular replacement or regeneration of used solutions, upgraded biogas can be maintained. This technique proved to be promising in upgrading biogas quality.     

Effects of EGR on performance of engines with spark gap projection and fueled by biogas–hydrogen blends

In this study, the effects of exhaust gas recirculation (EGR) on the behavior of a spark ignition engine fueled by hydrogen-blended low-calorific biogas were investigated, and its performance and emission characteristics were compared with those of the lean burn engine investigated in our previous work. The engine was operated at a constant rotational speed of 1800 rpm under a 60 kW power output condition, and a simulated biogas containing H₂ was used to realize a wide range of gas compositions. The engine test results demonstrate that when less than 20% H₂ was added to the biogas, the EGR operations had inferior fuel economy to the lean burn technique. However, when the H₂ blending ratio was increased, the EGR method achieved higher engine performance with lower NO_x emissions than the legal standard. Analyses of the O₂ fraction and thermal capacity variations of the inlet charge also indicated that a dilution (O₂ replacement) effect rather than a thermal effect was the dominant factor when EGR was introduced in a low-calorific biogas engine. Subsequently, in order to improve the engine efficiency as well as combustion characteristics, the spark gap was projected further into the combustion chamber with EGR engine operations. The engine test results show that repositioning the discharge location improved the thermal efficiency, and the maximum tolerable EGR rate increased because of spatial advantages such as relatively short flame propagation lengths and high electrode temperatures.     

Experimental study on superheated steam generation by the reaction of high humidity hydrogen and oxygen in a model internal combustion steam generator

Internal combustion steam cycle (ICSC) is a novel steam power cycle using hydrogen as an energy carrier to produce superheated steam. High humidity hydrogen produced during fast hydrogen production process is directly used to produce superheated steam by combusting with stoichiometric oxygen without hydrogen storage. The ICSC efficiency is greatly affected by the content of non-condensable gas in superheated steam. In the present study, superheated steam generation by high humidity hydrogen was investigated in a model internal combustion steam generator. Effects of H₂O/H₂ molar ratio of humid hydrogen and velocity ratio of humid hydrogen to oxygen on non-condensable gas content, combustion efficiency, and mixing rate were evaluated. The results showed that the critical H₂O/H₂ ratio for the humid hydrogen humidity limit was 2.8. With increasing velocity ratio, mixing rate and combustion efficiency increased under the same H₂O/H₂ ratio. The H₂O/H₂ reaction rate monotonously decreased as the H₂O/H₂ ratio increased from 1.0 to 2.5, while the mixing rate increased along with the velocity ratio. The combustion efficiency initially increased and subsequently decreased, and the peak value was reached at a H₂O/H₂ ratio of 1.75. This result indicated that the humid H₂-O₂ combustion was controlled by diffusion under H₂O/H₂ ratios of 1.0 to 1.75, but turned to be controlled by chemical kinetics when the H₂O/H₂ ratio ranged between 1.75 and 2.5.     

Overview of hydrogen production from biogas reforming: Technological advancement

In this article, possibilities of biogas reforming techniques for hydrogen production are discussed. The consideration of biogas reforming to produce H₂ and fuel cell application from membrane technology is presented. In steam reforming process, methane requires a high temperature for reaction, but a suitable catalyst can manage a higher temperature. The ratio of H₂/CO is close to 3, which means higher H₂ yield (above 70%). The ratio of H₂/CO to nearly 2 and H₂ yield almost 67% and also reduces the soot formation for partial oxidation process. In Auto thermal reforming, higher yield of H₂ is around 74% with the ratio of H₂/CO close to 2.8. The dry reforming process leads to a molar ratio H₂/CO of nearly one and H₂ yield of approximately 50%. The ratio of H₂/CO correspondingly improves and generates H₂ yield of approximately 60% for dry oxidation reforming process. For sustainable decentralized power generation in remote and rural areas, large-scale development of H₂ energy technology is required. Biogas reforming is an auspicious process for the production of green hydrogen gas as well as for reducing overburden on natural gas. The main benefit of using biogas for H₂ production as a renewable energy source is reducing excessive burden on natural gas and greenhouse gas emissions. Nowadays, the importance of renewable H₂ production has increased due to many reasons such as depletion of fossil fuel reserves, global environmental issues, energy issues, and demand for pure H₂.     

The diffusion overpotential increase and appearance of overlapping arcs on the Nyquist plots in the low humidity temperature test conditions of polymer electrolyte fuel cell

We have investigated cell voltage characteristics and AC impedance characteristics of polymer electrolyte fuel cells (PEFCs) at various humidity temperatures for H₂/O₂ and H₂/air test conditions (current density: 200, 400, and 600 mA cm⁻², cell temperature: 80 °C, humidity temperature at respective electrodes: 40, 50, 60, and 70 °C). The diffusion overpotential increases with decreasing humidity in the low humidity temperature region such as 40 and 50 °C and the Nyquist plots obtained from AC impedance measurements show a small arc superposed on an elliptic arc in the low frequency region. The diameter of this small arc increases with decreasing humidity temperature from 50 to 40 °C or with increasing current density. These results suggest that oxygen transport across the ionomer film in the catalyst layer is significantly reduced in the low humidity condition, which causes a decrease in cell voltage, increase in diffusion overpotential, the appearance of overlapping arcs (two separate arcs) in the lower frequency region on the Nyquist plots, and the increase of mass transport resistance from Nyquist plots.     

Experimental investigation on biogas enrichment with hydrogen for improving the combustion in diesel engine operating under dual fuel mode

Biogas valorization as fuel for internal combustion engines is one of the alternative fuels, which could be an interesting way to cope the fossil fuel depletion and the current environmental degradation. In this circumstance, an experimental investigation is achieved on a single cylinder DI diesel engine running under dual fuel mode with a focus on the improvement of biogas/diesel fuel combustion by hydrogen enrichment. In the present investigation, the mixture of biogas, containing 70% CH₄ and 30% CO₂, is blended with the desired amount of H₂ (up to 10, 15 and 20% by volume) by using MTI 200 analytical instrument gas chromatograph, which flow thereafter towards the engine intake manifold and mix with the intake air. Depending on engine load conditions, the volumetric composition of the inducted gaseous fraction is 20–50% biogas, 2–10% H₂ and 45–78% air. Near the end of the compression stroke, a small amount of diesel pilot fuel is injected to initiate the combustion of the gas–air mixture. Firstly, the engine was tested on conventional diesel mode (baseline case) and then under dual fuel mode using the biogas. Consequently, hydrogen has partially enriched the biogas. Combustion characteristics, performance parameters and pollutant emissions were investigated in-depth and compared. The results have shown that biogas enriched with 20% H₂ leads to 20% decrease of methane content in the overall exhaust emissions, associated with an improvement in engine performance. The emission levels of unburned hydrocarbon (UHC) and carbon monoxide (CO) are decreased up to 25% and 30% respectively. When the equivalence ratio is increased, a supplement decrease in UHC and CO emissions is achieved up to 28% and 30% respectively when loading the engine at 60%.     

Effects of temperature and humidity on the cell performance and resistance of a phosphoric acid doped polybenzimidazole fuel cell

Performance and electrochemical impedance spectroscopy (EIS) tests were performed at different temperatures and humidity levels to understand the effects of temperature and humidity on the performance and resistance of a PBI/H₃PO₄ fuel cell.The results of the performance tests indicated that increasing the temperature significantly improved the cell performance. In contrast, no improvement was observed when the gas humidity was increased. On the other hand, the EIS results showed that the membrane resistance was reduced for elevated temperatures. This development can be interpreted by the increase in membrane conductivity, as reflected by the Arrhenius equation. As the formation of H₄P₂O₇ and the self-dehydration of H₃PO₄ start around 130-140 °C, in PBI, they increase the membrane resistance at temperatures that are higher than 130 °C. In addition, the membrane resistance was reduced for elevated gas humidity levels. This is because an increase in humidity leads to an increase of the membrane hydration level.The resistance of the catalyst kinetics mainly contributes to the charge transfer resistance. However, under certain conditions, the interfacial charge transfer resistance is also important. It was concluded that the gas diffusion is the main contributor to the mass transfer resistance under dry conditions while it is the gas concentration under humid conditions.     

Hydrogen and syngas production through dynamic chemical looping reforming-decomposition of methane

This work introduces CeZr_0.5GdO₄ spinel particles as novel oxygen carriers for use in the reforming and decomposition of methane into H₂ and CO via the chemical looping technique. These particles were prepared by a modified sol-gel combustion method to increase their reactivity by increasing the surface area and consequently more accessibility of the gas feed to the solid phase. The performance of the synthesized materials was dynamically evaluated in terms of activity and stability at different operating temperatures (800–900 °C). The air used in the oxidation step eliminates almost all of the deposited solid carbon and converts it to CO, while providing the oxygen consumed in the reduction step. Oxygen carrier particles showed a conversion of more than 90% in all cycles after about 30 min of reduction operations. By the optimal operating path proposed in this research, more than 90% of the reactor exhaust gas is allocated to the production of H₂ and CO with almost complete elimination of CO₂ and H₂O in a shorter period of time. This will also reduce the time required for coke gasification and lattice oxygen replenishment of the spinel. Finally, the CeZr_0.5GdO₄ proved to be a successful oxygen carrier for the continuous production of hydrogen and carbon monoxide with almost no remarkable reduction in activity during successive redox cycles.     

Simultaneous production of hydrogen and carbon nanotubes from biogas: On the effect of Ce addition to CoMo/MgO catalyst

In this study, hydrogen and carbon nanotubes (CNTs) are simultaneously produced via a synergistic combined process of CO₂ methanation (METH) and chemical vapor deposition (CVD) processes using biogas as a feedstock. METH process could upgrade CO₂ containing biogas into CH₄-rich gas which then decomposed into H₂ and forming CNTs over CoMo/MgO catalyst by CVD process. The effects of Ce addition to CoMo/MgO were investigated. Comprehensive characterization confirms that all as-synthesized samples composed of well-aligned multi-walled carbon nanotubes (MWCNTs) with a narrow size distribution. The Ce addition improved CoMo dispersion on MgO, resulting in smaller and uniform CNTs. The small addition of Ce into CoMo/MgO catalyst could enhance the production CNTs yield. The higher Ce addition would, however, result in the CNTs yield decreased, attributed to a high basicity of CeO₂ surface and a large coverage of CeO₂ on the catalyst surface. The I_G/I_D increased with increased Ce addition, while the surface area monotonically decreased, attributed to a decrease in defects of nanotubes. In addition, this wisely combined process could result in a remarkable 100%CO₂ elimination, while high CH₄ conversion of 90% was obtained. The H₂ production yield could gain more than 30 vol% with respect to H₂ in the feed stream. The H₂ yield and purity in the effluent gas stream were approximately 90%.     

Laminar burning velocity and interchangeability analysis of biogas/C3H8/H2 with normal and oxygen-enriched air

Numerical and experimental measurements of the laminar burning velocities of biogas (66% CH₄ – 34% CO₂) and a biogas/propane/hydrogen mixture (50% biogas – 40% C₃H₈ – 10% H₂) were made with normal and oxygen-enriched air while varying the air/fuel ratio. GRI-Mech 3.0 and C₁–C₃ reaction mechanisms were used to perform numerical simulations. Schlieren images of laminar premixed flames were used to determine laminar burning velocities at 25 °C and 849 mbar. The mixture's laminar burning velocity was found to be higher to that of pure biogas due to the addition of propane and hydrogen. An increase in the laminar burning velocities of both fuels is reported by enriching air with oxygen, a phenomenon that is explained by the increased reactivity of the mixture. Additionally, an analysis of interchangeability based on both the Wobbe Index and the laminar burning velocity between methane and a biogas/propane/hydrogen mixture is presented in order to consider this mixture as a substitute for natural gas. It was found that the variations of these properties between the fuels did not exceed 10%, enabling interchangeability.     

Overview of hydrogen production technologies from biogas and the applications in fuel cells

Traditionally, H₂ is a large-scale production by the reforming process of light hydrocarbons, mainly natural gas, used by the chemical industry. However, the reforming technologies currently used encounter numerous technical/scientific challenges, which depend on the quality of raw materials, the conversion efficiency and security needs for the integration of H₂ production, purification and use, among others. Biogas is a high-potential versatile raw material for reforming processes, which can be used as an alternative CH₄ source. The production of H₂ from renewable sources, such as biogas, helps to largely reduce greenhouse gas emissions. Within this context, the integration of biogas reforming processes and the activation of fuel cell using H₂ represent an important route for generating clean energy, with added high-energy efficiency. This work expounds a literature review of the biogas reforming technologies, emphasizing the types of fuel cells available, the advantages offered by each route and the main problems faced.     

Hydrogen production from biogas: Process optimization using ASPEN Plus®

This work is part of the VABHYOGAZ (valorization of biogas into hydrogen) program, which targeted the industrial deployment of hydrogen production from biogas in France. To-date, different processes of methane reforming, such as steam reforming of methane (SRM), dry reforming of methane (DRM) and tri-reforming of methane (TRM), have been studied in the literature, but only SRM is applied at industrial scale. Since SRM is an energy-intensive process, a critical analysis of these routes for hydrogen production from biogas is indispensable for process optimization. This has been addressed in this work, by using ASPEN Plus® simulation. Different global processes of hydrogen production from biogas, via DRM, SRM, or TRM, with or without tail gas recycling, have been studied. Among them, hydrogen production using TRM technique (H₂-TRM0.3C process) with a partial recycling of tail gas (30%) was found to be the best option, leading to the highest hydrogen production rate and the best energy yield. H₂-TRM0.3C process was also found to be more efficient than the actual industrial process (H₂-REF), which is based on SRM technique. Under the same conditions, H₂-TRM0.3C process led to a higher H₂ production (8.7% more), a lower total energy consumption (18.6% less), and a lower waste heat generation (15.4% less), in comparison with the actual industrial process (H₂-REF).     

Simultaneous production of hydrogen and carbon nanotubes from biogas: On the design of combined process

We introduced a novel combined process of CO₂ methanation (METH) and catalytic decomposition of methane (CDM) for simultaneous production of hydrogen (H₂) and carbon nanotubes (CNTs) from biogas. In this process, biogas is catalytically upgraded into CH₄-rich gas in METH reactor using Ni/CeO₂ catalyst, and the obtained CH₄-rich gas is subsequently decomposed into H₂ and CNTs in CDM reactor over CoMo/MgO catalyst. Among the three different process scenarios proposed, the combined process with a steam condenser equipped between METH and CDM reactors could greatly improve a CNTs productivity. The CNTs production yield increased by more than 2.5-fold, maximizing at 9.08 gCNTs/gCat with a CNTs purity of 90%. The deposited carbon product was characterized as multi-walled carbon nanotubes (MWCNTs) with a surface area of 136.0 m²/g, comparable with commercial CNTs of 199.8 m²/g. The remarkable I_G/I_D ratio of 2.18 confirms a superior portion of graphitic carbon in the synthesized CNTs upon the commercial CNTs with I_G/I_D = 0.74. Notably, the CH₄ conversion reached 94.5%, while the CO₂ conversion achieved 100%, resulting in the H₂ yield and H₂ purity higher than 90%. This combined process demonstrates a promising route for production of high quality CNTs and high purity H₂ with complete CO₂ conversion using biogas as abundant renewable energy resources. In addition, the test of raw biogas showed no deactivation of catalyst, justifying the implementation of the developed process for real biogas without purification.