Catalytic decomposition of biogas to produce hydrogen rich fuels for SI engines and valuable nanocarbons

Catalytic Decomposition of Biogas (CDB), producing simultaneously syngases (SG), with high hydrogen contents, for spark ignition (SI) engines and bio-carbon nanofibers (BCNFs) to be further used as precursor of synthetic graphite, is presented as an alternative to the usual direct combustion. Synthetic biogas mixtures were decomposed in the presence of a Ni catalyst at different temperatures and the SG thus produced were further tested as fuel in a specifically designed SI engine, whereas the BCNFs were subjected to heat treatment to graphitize. The influence of CDB process conditions on product yields and properties, the effect of SG composition/quality in SI engine performance and emissions, as compared with the use of raw biogas and the influence of BCNFs characteristics on the structural and textural properties of the graphitic materials have been studied. The syngases presented better combustion characteristics than biogas resulting in higher engine brake thermal efficiencies and lower exhaust emissions. Furthermore, high added value graphite-like materials, with a crystalline structure similar to that of oil-derived graphite which is currently commercialized to be used as anode in rechargeable lithium-ion batteries, were prepared.     

Catalytic transformation of H2S for H2 production

Hydrogen sulfide (H₂S) gas is a by-product from natural gas refining, hydrodesulfurization of various fossil fuels, and syngas cleaning from pyrolysis and gasification. Catalytic pyrolysis of H₂S provides an alternative and effective pathway to recover both H₂ and sulfur. Catalysts from hydrotalcite of ZnAl, ZnNiAl, and ZnFeAl were employed for H₂S pyrolysis and compared with TiO₂ and MoS₂ at atmospheric pressure and temperatures in the range of 923–1123 K. Kinetic analysis was carried out in a packed bed reactor which revealed the effect of H₂S partial pressures to be of the order of 0.8–1 with respect to H₂S. The developed novel catalysts showed improved performance with significantly reduced activation energy compared to TiO₂ by 30 kJ/mol as well as higher H₂S conversion during pyrolysis (17% at 1173 K) than with MoS₂ catalyst, even at high H₂S partial pressure which is necessary for viable hydrogen production. The new approach showed an alternate economical and efficient pathway of catalyst design to obtain high activity and stability for simultaneous H₂ energy and pure sulfur recovery from unwanted H₂S resources.     

H2 production by methane decomposition: Catalytic and technological aspects

Ni and Co supported on SiO₂ and Al₂O₃ silica cloth thin layer catalysts have been investigated in the catalytic decomposition of natural gas (CDNG) reaction. The influence of carrier nature and reaction temperature was evaluated with the aim to individuate the key factors affecting coke formation. Both Ni and Co silica supported catalysts, due to the low metal support interaction (MSI), promotes the formation of carbon filament with particles at tip. On the contrary, in case alumina was used as support, metals strongly interact with surface thus depressing both the metal sintering and the detachment of particles from catalyst surface. In such cases, carbon grows on metal particle with a “base mechanism” while particles remain well anchored on the catalyst surface. This allowed to realize a cyclic dual-step process based on methane decomposition and catalyst oxygen regeneration without deactivation of catalyst. Technological considerations have led to conclude that the implement of a process based on decomposition and regeneration of catalyst by oxidation requires the development of a robust catalytic system characterized by both a strong MSI and a well defined particle size distribution. In particular, the catalyst should be able to operate at high temperature, necessary to reach high methane conversion values (> 90%), avoiding at the same time the formation of both the carbon filaments with metal at tip or the encapsulating carbon which drastically deactivate the catalyst.     

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.     

Thermocatalytic decomposition of methane for hydrogen production using activated carbon catalyst: Regeneration and characterization studies

A series of experiments was conducted to study the deactivation and regeneration of activated carbon catalyst used for methane thermocatalytic decomposition to produce hydrogen. The catalyst becomes deactivated due to carbon deposition and six decomposition cycles of methane at temperatures of 850 and 950 °C, and five cycles of regeneration by using CO₂ at temperatures of 900, 950 and 1000 °C were carried out to evaluate the stability of the catalyst. The experiment was conducted by using a thermobalance by monitoring the mass gain during decomposition or the mass lost during the regeneration with time. The initial activity and the ultimate mass gain of the catalyst decreased after each regeneration cycle at both reaction temperatures of 850 and 950 °C, but the amount is smaller under the more severe regenerating conditions. For the reaction at 950 °C, comparison between the first and sixth reaction cycles shows that the initial activity decreased by 69, 51 and 42%, while the ultimate mass gain decreased by 62%, 36% and 16% when CO₂ gasification carried out at 900, 950 and 1000 °C respectively. Temperature -programmed oxidation profiles for the deactivated catalyst at reaction temperature of 950 °C and after several cycles showed two peaks which are attributed to different carbon characteristics, while one peak was obtained when the experiment was carried out at 850 °C. In conclusion, conducting methane decomposition at 950 °C and regeneration at 1000 °C showed the lowest decrease in the mass gain with reaction cycles.     

Screening of catalysts for H2S abatement from biogas to feed molten carbonate fuel cells

Different catalysts vanadium-based supported on mixed oxide (CeO₂, TiO₂, CuFe₂O₄) were prepared, characterized and tested for the partial selective oxidation of H₂S at low temperature.     

Production of hydrogen and carbon nanofibers through the decomposition of methane over activated carbon supported Pd catalysts

Hydrogen has been produced by decomposing methane thermocatalytically at 1123 K in the presence of activated carbon supported Pd catalysts (Samples coded as Pd5 and Pd10 respectively) procured from SRL Chemicals, India. The studies indicated that the Pd10 catalyst has higher catalytic activity and life for methane decomposition reaction at 1123 K and volume hourly space velocity (VHSV) of 1.62 L/hr?g. An average methane conversion of 50 mol % has been obtained for Pd10 catalyst at the above reaction conditions. SEM and TEM-EDXA images of Pd10 catalyst after methane decomposition showed formation of carbon nanofibers. XRD of the above catalyst revealed, moderately crystalline peaks of Pd which may be responsible for the increase in the catalytic life and the formation of carbon nanofibers.     

Biogas upgrade to syn-gas (H2-CO) via dry and oxidative reforming

Fuel reforming processes are primarily used to generate hydrogen for fuel cells and in automotive internal combustion engines to improve combustion characteristics and emissions. In this study, biogas is used as the fuel source for the reforming process as it has desirable properties of being both renewable and clean. Two reforming processes (dry reforming and combined dry/oxidative reforming) are studied. Both processes are affected by the gas stream temperature and reactor space velocity with the second process being affected by O₂/CH₄ ratio as well. Our results imply that oxidative reforming is the dominant process at low exhaust temperatures. This provides heat for the dry reforming of biogas and the overall reforming is exothermic. Increase in O₂/CH₄ ratio at low temperature promotes hydrogen production. At high exhaust temperatures (>600 °C), dry reforming of biogas is dominant and the overall reaction is net endothermic.     

An experimental investigation into the CO2 gasification of deactivated activated-carbon catalyst used for methane decomposition to produce hydrogen

A series of experiments was conducted to study the CO₂ gasification of a deactivated palm-shell-based activated-carbon (ACPS) catalyst used for the thermocatalytic decomposition of methane to produce hydrogen. This catalyst becomes deactivated due to the accumulation of carbon deposits during the methane-decomposition process. The CO₂ gasification was carried out at 850, 900, 950 or 1000 °C to study the deactivated ACPS, which was used at methane-decomposition temperatures of 850 or 950 °C. A series of six methane-decomposition cycles at 950 °C alternating with five gasification cycles using CO₂ at 900, 950 and 1000 °C was also carried out to evaluate the stability of the catalyst. The experiments were conducted using a thermobalance by monitoring the change in mass of the catalyst with time, i.e., the mass gain during methane decomposition or the mass loss during CO₂ gasification. Gasification of the virgin and deactivated ACPS showed strong temperature dependence, with the half and complete gasification times having an exponential dependence on temperature. The gasification reactivity at different conversions was higher for the virgin ACPS and increased with increases in the decomposition temperatures used for deactivation of the ACPS. The activation energies of virgin ACPS and ACPS deactivated at a decomposition temperature of 850 °C decreased with an increase in conversion, while they increased for the ACPS deactivated at a decomposition temperature of 950 °C; the activation energies varied between 81 and 163 kJ/mol. The gasification reactivity changed with methane conversion, showing maximum values for both the virgin and deactivated ACPS at a decomposition temperature of 950 °C. The initial gasification reactivity of the catalyst decreased after three gasification cycles at 1000 °C, while no significant change was observed with gasification cycles at 950 or 900 °C.     

Tolerance tests of H2S-laden biogas fuel on solid oxide fuel cells

Biogas is a variable mixture of methane, carbon dioxide and other gases. It is a renewable resource which comes from numerous sources of plant and animal matter. Ni-YSZ anode-supported solid oxide fuel cell (SOFC) can directly use clean synthesized biogas as fuel. However, trace impurities, such as H₂S, Cl₂ and F₂ in real biogas can cause degradation in cell performance. In this research, both uncoated and coated Ni-YSZ anode-supported cells were exposed to three different compositions of synthesized biogases (syn-biogas) with 20 ppm H₂S under a constant current load at 750-850 °C and their performance was evaluated periodically using standard electrochemical methods. Postmortem analysis of the SOFC anode was performed using X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The results show that H₂S causes severe electrochemical degradation of the cell when operating on biogas, leading to both complete electrochemical and mechanical failure. The Ni-CeO₂ coated cell showed excellent stability during CH₄ reforming and some tolerance to H₂S contamination.     

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.     

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.     

Catalytic decomposition of methane over enteromorpha prolifera-based hierarchical porous biochar

Biochar is a potential catalyst for methane decomposition (CMD) owing to its environmental-friendly and application prospects. In this work, the hierarchical porous biochar was prepared by carbonization and H₃PO₄ activation using Enteromorpha prolifera (EP) as precursor, respectively. The results show that when the ratio of H₃PO₄/EP is 1.5, the maximum CH₄ conversion is 46%, along with hydrogen output of 396 mmol/g_cat, which is 5.8 times as that of the unactivated biochar. The characterization results by XPS, Raman, SEM and HRTEM indicate that P element is inserted in carbon layer in the form of C–O–P, resulting in lattice distortion of carbon layer and larger defect density, and C–O–P plays a dominant role in initial CH₄ conversion. The mesopores formed by H₃PO₄ activation alleviate the influence of the deposited carbon on the catalyst and decrease the deactivation rate, thereby exhibiting better performance in CMD.     

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.     

The study of Au/MoS2 anode catalyst for solid oxide fuel cell (SOFC) using H2S-containing syngas fuel

Au/MoS₂ is a promising anode catalyst for conversion of all components of H₂S-containing syngas in solid oxide fuel cell (SOFC). MoS₂-supported nano-Au particles have catalytic activity for conversion of CO when syngas is used as fuel in SOFC systems, thus preventing poisoning of MoS₂ active sites by CO. In contrast to use of MoS₂ as anode catalyst, performance of Au/MoS₂ anode catalyst improves when CO is present in the feed. Current density over 600 mA cm⁻² and maximum power density over 70 mW cm⁻² were obtained at 900 °C, showing that Au/MoS₂ could be potentially used as sulfur-tolerant catalyst in fuel cell applications.     

The effect of H2S on the performance of Ni-YSZ anodes in solid oxide fuel cells

Biomass-derived fuel, e.g. biogas, is a potential fuel for solid oxide fuel cells (SOFCs). At operating temperature (∼850 °C) reforming of the carbon-containing biogas takes place over the Ni-containing anode. However, impurities in the biogas, e.g. H₂S, can poison both the reforming and the electrochemical activity of the anode.Tests of single anode-supported planar SOFCs were carried out in the presence of H₂S under current load at 850 °C. The cell voltage dropped as we periodically added 2-100 ppm H₂S to an H₂-containing fuel in 24 h intervals, but it regenerated to the initial value after we turned off the H₂S. Evaluation of the changes of the cell voltage suggests that saturation coverage was reached at approximately 40 ppm H₂S. A front-like movement of S-poisoning over the anode was seen by monitoring the in-plane voltage in the anode. Furthermore, impedance spectra showed that mainly the polarization resistance increased when adding H₂S. These changes in resistance were found to happen at 1212 Hz, which is related to reactions at the anode-electrolyte interface. These findings can be used to identify S-related effects on the performance, when an SOFC is fuelled with biogas or other fuels with H₂S impurities and thus help in the development of more sulfur tolerant SOFCs.     

Towards H2-rich gas production from unmixed steam reforming of methane: Thermodynamic modeling

In this work, the Gibbs energy minimization method is applied to investigate the unmixed steam reforming (USR) of methane to generate hydrogen for fuel cell application. The USR process is an advanced reforming technology that relies on the use of separate air and fuel/steam feeds to create a cyclic process. Under air flow (first half of the cycle), a bed of Ni-based material is oxidized, providing the heat necessary for the steam reforming that occurs subsequently during fuel/steam feed stage (second half of the cycle). In the presence of CaO sorbent, high purity hydrogen can be produced in a single reactor. In the first part of this work, it is demonstrated that thermodynamic predictions are consistent with experimental results from USR isothermal tests under fuel/steam feed. From this, it is also verified that the reacted NiO to CH₄ (NiO_reacted/CH₄) molar ratio is a very important parameter that affects the product gas composition and decreases with time. At the end of fuel/steam flow, the reforming reaction is the most important chemical mechanism, with H₂ production reaching ∼75 mol%. On the other hand, at the beginning of fuel/steam feed stage, NiO reduction reactions dominate the equilibrium system, resulting in high CO₂ selectivity, negative steam conversion and low concentrations of H₂. In the second part of this paper, the effect of NiO_reacted/CH₄ molar ratio on the product gas composition and enthalpy change during fuel flow is investigated at different temperatures for inlet H₂O/CH₄ molar ratios in the range of 1.2-4, considering the USR process operated with and without CaO sorbent. During fuel/steam feed stage, the energy demand increases as time passes, because endothermic reforming reaction becomes increasingly important as this stage nears its end. Thus, the duration of the second half of the cycle is limited by the conditions under which auto-thermal operation can be achieved. In absence of CaO, H₂ at concentrations of approximately 73 mol% can be produced under thermo-neutral conditions (H₂O/CH₄ molar ratio of 4, with NiO_reacted/CH₄ molar ratio at the end of fuel flow of ∼0.8, in temperature range of 873-1073 K). In the presence of CaO sorbent, using an inlet H₂O/CH₄ molar ratio of 4 at 873 K, H₂ at concentrations over 98 mol% can be obtained all through fuel/steam feed stage. At 873 K, carbonation reaction provides all the heat necessary for H₂ production when NiO_reacted/CH₄ molar ratio reached at the end of fuel/steam feed is greater or equal to1. In this way, the heat released during air flow due to Ni oxidation can be entirely used to decompose CaCO₃ into CaO. In this case, a calcite-to-nickel molar ratio of 1.4 (maximum possible value) can be used during air flow. For longer durations of fuel/steam feed, corresponding to lower NiO_reacted/CH₄ molar ratios, some heat is necessary for steam reforming, and a calcite-to-nickel molar ratio of about 0.7 is more suitable. With the USR technology, CaO can be regenerated under air feeds, and an economically feasible process can be achieved.     

Preparation and characterization of Ni based catalysts for the catalytic partial oxidation of methane: Effect of support basicity on H2/CO ratio and carbon deposition

The catalytic partial oxidation of methane (CPOM) was studied on Ni based catalysts. Catalysts were prepared by wet impregnation method and characterized by using AAS, BET, XRD, HRTEM, TPR, TPO, Raman Spectroscopy and TPSR techniques. The prepared catalysts showed nearly 95% CH₄ conversion and nearly 96% H₂ selectivity under the flow of 157,500 (L kg⁻¹ h⁻¹) with the ratio of CH₄/O₂ = 2 by using air as an oxidant at 1 atm and 800 °C. Support basicity greatly influenced the H₂/CO ratio and carbon deposition. It was found that the lowest carbon deposition occurred on Ni impregnated MgO catalyst. Considering the results, it was found that Ni/MgO catalyst with 10% Ni content would be the best catalyst amongst Ni/Al₂O₃, Ni/MgO/Al₂O₃, Ni/MgAl₂O₄ and Ni/Sorbacid for the CPOM only under more reductive conditions. Under optimum conditions, Ni/MgO showed poor performance and therefore Ni/Sorbacid would be the ideal catalyst because of its greater carbon resistance than the other catalysts.     

Electrolysis of coal slurries to produce hydrogen gas: Relationship between CO2 and H2 formation

The purpose of this study was to produce hydrogen gas by electrolysis of coal slurries and to investigate the relation between hydrogen (H₂) and carbon dioxide (CO₂) formation. Electrolysis of coal slurries was evaluated at 40 °C and 1.0 V cell potential to examine H₂ and CO₂relationship. When electrolysis was performed after the coal slurry was mixed with Fe(II)/Fe(III) ions and stirred overnight (>12 h), no CO₂ gas was observed at the anode compartment. The results of total organic carbon (TOC) indicated that after electrolysis, few organic compounds were transformed into the solution and these organic compounds did not convert into CO₂. GC analysis, on the other hand, revealed that the H₂ collected at the cathode was pure and did not require any further purification process. Hydrogen generation or electrolysis efficiency of coal slurries cannot be calculated or estimated by examining CO₂ generation as reported in the literature. Low temperature and low cell potential were not sufficient to oxidize coal quantitatively.     

Effect of equivalence ratio on knocking tendency in spark ignition engines fueled with fuel blends of biogas,natural gas,propane and hydrogen

This research evaluates the effect of the equivalence ratio on knocking tendency in two Spark Ignition (SI) engines fueled with gaseous fuels. A Lister Petter TR2 Diesel engine (TR2) converted to SI was used to evaluate the equivalence ratio effect when the engine was fueled with fuel blends of biogas, natural gas, propane, and hydrogen. A Cooperative Fuel Research (CFR) engine was used to study the effect of equivalence ratio on the Critical Compression Ratio (CCR) which is a metric to evaluate the knocking tendency of gaseous fuels. In both engines, the tests were conducted using the knocking threshold as the engine limit operation to quantify the effect of the equivalence ratio on knocking tendency. Experimental results in the CFR engine revealed that a lean mixture reduces the knocking tendency allowing to operate the CFR engine at higher CCR. In contrast, the effect of the equivalence ratio on the knocking tendency in the TR2 engine was different since leaner mixtures increased the engine knocking tendency. This tendency was caused by the increase in the % throttle which increased the mixture pressure at the end of the compression stroke. The high knocking tendency to lean mixtures forces to reduce the output power to find the knocking threshold for all fuel blends.