Methane

Rice husk (RH) co-combustion with natural gas in highly oxygen-enriched concentrations presents a net carbon-negative energy production opportunity while minimizing flue gas recycling. However, recent experiments have shown enhanced ash deposition rates in oxygen-enriched conditions, with deposition/shedding also being dependent on the particle size distribution (PSD) of the parent RH fuel. To uncover the causative mechanisms behind these observations, add-on models for ash deposition/shedding and radiative properties were employed in computational fluid dynamics simulations. The combustion scenarios investigated encompassed two types of RH (US RH, Chinese RH) with widely varying ash contents (by % mass) and inlet fuel PSD with air and O₂/CO₂ (70/30 vol %, OXY70) as oxidizers. Utilizing the measured fly-ash PSDs near the deposit surface and modeling the particle viscosity accurately, particle kinetic-energy (PKE)-based capture and shedding criteria were identified as the keys to accurate deposition/shedding rate predictions. The OXY70 scenarios showed higher ash-capturing propensities due to their lower PKE. Conversely, higher erosion rates were predicted in the AIR firing scenarios. In addition, the radiative characteristics across all the scenarios were dominated by the gases and were not sensitive to the fly-ash PSD. Therefore, the higher particle concentrations in the OXY70 conditions did not negatively impact the heat extraction. Full article

Sewage sludge to energy conversion is a sustainable waste management technique and a means of militating against the environmental concerns associated with its disposal. Amongst the various conversion technologies, anaerobic digestion and gasification have been identified as the two most promising. Therefore, this study is focused on a detailed evaluation of the anaerobic digestion and gasification of sewage sludge for energy production. Moreover, the key challenges hindering both technologies are discussed, as well as the practical measures for addressing them. The applicable pretreatment measures for efficient transformation into valuable energy vectors were further evaluated. Specifically, the study evaluated various properties of sewage sludge in relation to gasification and anaerobic digestion. The findings showed that a high ash content in sewage sludge results in sintering and agglomeration, while a high moisture content promotes tar formation, which has been identified as one of the key limitations of sewage sludge gasification. More importantly, the application of pretreatment has been shown to have some beneficial features in promoting organic matter decomposition/degradation, thereby enhancing biogas as well as syngas production. However, this has additional energy requirements and operational costs, particularly for thermal and mechanical methods. Full article

The Biochemical Methane Potential (BMP) assay is a vital tool for quantifying the amount of methane that specific biodegradable materials can generate in landfills and similar anaerobic environments. Applications of the protocol are extensive and while simple in design, the BMP assay can use anaerobic seed from many different types of sources to determine the methane potential from most biodegradable substrates. Many researchers use differing protocols for this assay, both including and excluding the use of synthetic growth medias, intended to provide vital nutrients and trace elements that facilitate methanogenesis and leave the substrate being tested as the only limiting factor in methane generation potential. The variety of previous approaches inspired this effort to determine the efficacy of adding synthetic growth media to BMP assays. The presented findings suggest the use of M-1 synthetic growth media, defined in this study, at a volumetric ratio of 10% active sludge: 90% M-1 media yielded optimal results in terms of gas yield and reduced variability. Full article

Electrocatalytic CO₂ reduction to valued products is a promising way to mitigate the greenhouse effect, as this reaction makes use of the excess CO₂ in the atmosphere and at the same time forms valued fuels to partially fulfill the energy demand for human beings. Among these valued products, methane is considered a high-value product with a high energy density. This review systematically summarizes the recently studied reaction mechanisms for CO₂ electroreduction to CH₄. It guides us in designing effective electrocatalysts with an improved electrocatalytic performance. In addition, we briefly summarize the recent progress on CO₂ electroreduction into CH₄ from the instructive catalyst design, including catalyst structure engineering and catalyst component engineering, and then briefly discuss the electrolyte effect. Furthermore, we also provide a simplified techno-economic analysis of this technology. These summaries are helpful for beginners to rapidly master the contents related to the electroreduction of carbon dioxide to methane and also help to promote the further development of this field. Full article

Pd_xNi_y/TiO₂ bimetallic electrocatalysts were used in fuel cell polymeric electrolyte reactors (PER-FC) to convert methane into methanol through the partial oxidation of methane promoted by the activation of water at room temperature. X-ray diffraction measurements showed the presence of Pd and Ni phases and TiO₂ anatase phase. TEM images revealed mean particle sizes larger than those reported for PdNi materials supported, indicating that TiO₂ promotes particle aggregation on its surface. Information on the surface structure of electrocatalysts obtained by Raman spectra indicated the presence or formation of NiO. The PER-FC tests showed the highest power density for the electrocatalyst with the lowest amount of nickel Pd₈₀Ni₂₀/TiO₂ (0.58 mW cm⁻²). The quantification of methanol through the eluents collected from the reactor showed higher concentrations of methanol produced, revealing that the use of TiO₂ as a support also increased the reaction rate. Full article

Isotopic fractionation of methane between gas and solid hydrate phases provides data regarding hydrate-forming environments, but the effect of pressure on isotopic fractionation is not well understood. In this study, methane hydrates were synthesized in a pressure cell, and the hydrogen isotope compositions of the residual and hydrate-bound gases were determined. The δ²H of hydrate-bound methane formed below the freezing point of water was 5.7–10.3‰ lower than that of residual methane, indicating that methane hydrate generally encapsulates lighter molecules (CH₄) instead of CH₃²H. The fractionation factors α_H-V of the gas and hydrate phases were in the range 0.9881–0.9932 at a temperature and pressure of 223.3–268.2 K and 1.7–19.5 MPa, respectively. Furthermore, α_H-V increased with increasing formation pressure, suggesting that the difference in the hydrogen isotopes of the hydrate-bound methane and surrounding methane yields data regarding the formation pressure. Although the differences in the hydrogen isotopes observed in this study are insignificant, precise analyses of the isotopes of natural hydrates in the same area enable the determination of the pressure during hydrate formation. Full article

Bioenergy recovery from biomass by-products is a promising approach for the circular bioeconomy transition. However, the management of agri-food by-products in stand-alone treatment facilities is a challenge for the low-capacity food processing industry. In this study, the techno-economic assessment of a small-scale anaerobic digestion process was evaluated for the management of jabuticaba by-product and the production of biomethane, electricity, heat, and fertilizer. The process was simulated for a treatment capacity of 782.2 m³ y⁻¹ jabuticaba peel, considering the experimental methane production of 42.31 L CH₄ kg⁻¹ TVS. The results of the scaled-up simulated process demonstrated the production of biomethane (13,960.17 m³ y⁻¹), electricity (61.76 MWh y⁻¹), heat (197.62 GJ y⁻¹), and fertilizer (211.47 t y⁻¹). Economic analysis revealed that the process for biomethane recovery from biogas is not profitable, with a net margin of −19.58% and an internal rate of return of −1.77%. However, biogas application in a heat and power unit can improve project feasibility, with a net margin of 33.03%, an internal rate of return of 13.14%, and a payback of 5.03 years. In conclusion, the application of small-scale anaerobic digestion can prevent the wrongful open-air disposal of jabuticaba by-products, with the generation of renewable energy and biofertilizer supporting the green economy toward the transition to a circular economy. Full article

Since WO₃ is a relatively abundant metal oxide and features the ability to absorb in the visible spectrum, this non-toxic semiconductor is a promising photocatalyst among sustainable materials. These properties have delivered intriguing catalytic results in the conversion of methane to methanol; however, initial investigations indicate low photocatalytic efficiency resulting from fast recombination of photogenerated charges. To explore this aspect of inefficiency, five different morphologies of WO₃ consisting of micron, nanopowder, rods, wires, and flowers were obtained and characterized. In addition, several electron capture agents/oxidizers were investigated as a means of improving the separation of photogenerated charges. The photocatalytic activity of different morphologies was assessed via CH₃OH formation rates. Based on our results, WO₃ flowers produced the highest methanol productivity (38.17 ± 3.24 µmol/g-h) when 2 mM H₂O₂ was present, which is approximately four times higher in the absence of H₂O₂. This higher methanol production has been attributed to the unique structure-related properties of the flower-like structure. Photoluminescence emission spectra and diffuse reflectance data reveal that flower structures are highly catalytic due to their reduced electron/hole recombination and multiple light reflections via petal-like hollow chambers. Full article

CO₂ methanation was studied on Ni-based yttria-stabilised zirconia (Ni/YSZ) catalysts. The catalysts were prepared by the wet impregnation method, where the amount of Ni content was varied from 5% to 75%. Thereafter, the prepared catalysts were analysed by BET, XRD, SEM and H₂-TPR. BET results showed an initial increase in the surface area with an increase in Ni loading, then a decrease after 30% Ni loading. The XRD results revealed that the Ni crystallite size increased as the Ni loading increased, while the H₂-TPR showed a shift in reduction peak temperature to a higher temperature, indicating that the reducibility of the catalysts decreased as the Ni loading increased. The activity of the synthesised catalysts for CO₂ methanation was studied by passing a mixture of H₂, CO₂ and N₂ with a total flow of 135 mL min⁻¹ and GHSV of 40,500 mL h⁻¹ g⁻¹ through a continuous flow quartz tube fixed-bed reactor (I.D. = 5.5 mm, wall thickness = 2 mm) containing 200 mg of the catalyst at a temperature range of 473 to 703 K under atmospheric pressure and a H₂:CO₂ ratio of 4. The tested Ni/YSZ catalysts showed an improvement in activity as the reaction temperature increased from 473 K to around 613 to 653 K, depending on the Ni loading. Beyond the optimum temperature, the catalyst’s activity started to decline, irrespective of the Ni loading. In particular, the 40% Ni/YSZ catalyst displayed the best performance, followed by the 30% Ni/YSZ catalyst. The improved activity at high Ni loading (40% Ni) was attributed to the increase in hydrogen coverage and improved site for both H₂ and CO₂ adsorption and activation. Full article

This paper presents the implementation of natural gas vehicles (NGVs) in Tanzania’s road transportation sector. The peculiarity of this analysis is the evaluation of the technical and economic performance of the converted gasoline and diesel engines to use compressed natural gas (CNG) as the cleanest-burning hydrocarbon. The technical performance involved vehicle mileage (MiCNG), fuel consumption (Fcons), speed drop, engine fuel enhancement (Fenh), and fuel saving, while the economic performance involved conversion cost (Cc), fuel cost saving (FCsaving), and payback (PB). Considering the conversion of gasoline vehicles, the MiCNG could reach an average of 100 to 500 km per filling, depending on the CNG cylinder size. The Fenh and fuel saving were ranging between 1.9 and 3.9 and 71 and 78%. With a proportion of 30:70 diesel-CNG fuel, the heavy-duty truck with 180 kg of CNG could reach 1300 km, saving about 440 L, which is 78.6% per roundtrip, while the medium passenger car with 15 kg of CNG could reach 350 km, presenting a fuel saving of about 75%. From an economic point of view, gasoline retrofitted NGVs cost about 50 to 200 TZS/km, yielding a fuel cost saving of up to 79% and starting to pay off between 2 and 7 months or 10,000 and 40,000 km, depending on the engine capacity. Considering dual fuel, the heavy-duty truck consumes about 496 TZS/km, saving about 62.3% of diesel fuel and starting to pay off after 2.5 months or 29,304 km. To conclude, NGV technologies have been successfully implemented in Tanzania’s road transportation sector, presenting significant fuel savings and reducing reliance on imported oil. While taking measures, this study paves a way for Tanzania and other sub-Saharan countries to promote NGV growth. Full article

The reforming of methane with CO₂ was carried out efficiently in a fluidized bed reactor at 973 K under atmospheric pressure, taking advantage of the nickel catalyst efficiency achieved with a bed of particulate fines. The fluidization operation was characterized by determining a minimum velocity of 3.11 × 10⁻³ ms⁻¹ and higher velocities. The reactor worked with surface speeds of up to 1.84 × 10⁻² ms⁻¹, providing conversions from 45% to 51% and a syngas yield of 97%. The control base of the operation focused on the use of CO₂ was established through the reaction steps assumed for the process, including methane cracking, reverse Boudouard reaction, and RWGS (reverse reaction of water gas-shift). The reactor designed to operate in two zones was able to simultaneously process surface reactions and catalyst regeneration using feed with 50% excess CO₂ in relation to methane. Predictions indicating the production of syngas of different compositions quantified with the H₂/CO ratio from 2.30 to 0.91 decreasing with space-time were validated with the results available for process design. Full article

Here, Pd nanoparticles supported on ZnO were prepared by the alcohol-reduction and the borohydride-reduction methods, and their efficiency towards the photocatalytic conversion of methane under mild conditions were evaluated. The resulting Pd/ZnO photocatalysts were characterized by X-ray fluorescence, X-ray diffraction, X-ray photoelectron spectroscopy, UV–Vis, and transmission electron microscopy. The reactions were performed with the photocatalysts dispersed in water in a bubbling stream of methane under UV-light illumination. The products formed were identified and quantified by gas chromatography (GC-FID/TCD/MSD). The principal products formed were C₂H₆ and CO₂ with minor quantities of C₂H₄ and CO. No H₂ production was observed. The preparation methods influenced the size and dispersion of Pd nanoparticles on the ZnO, affecting the performance of the photocatalysts. The best performance was observed for the photocatalyst prepared by borohydride reduction with 0.5 wt% of Pd, reaching a C₂H₆ production rate of 686 µmol·h⁻¹·g⁻¹ and a C₂H₆ selectivity of 46%. Full article

The interest in Gas-to-Liquid technology (GTL) is growing worldwide because it involves a two-step indirect conversion of natural gas to higher hydrocarbons ranging from Liquefied Petroleum Gas (LPG) to paraffin wax. GTL makes it possible to obtain clean diesel, naphtha, lubes, olefins, and other industrially important organics from natural gas. This article is a brief review discussing the state-of-the-art of GTL, including the basics of syngas manufacturing as a source for Fischer-Tropsch synthesis (FTS), hydrocarbons synthesis (Fischer-Tropsch process), and product upgrading. Each one is analyzed, and the main characteristics of traditional and catalysts technologies are presented. For syngas generation, steam methane reforming, partial oxidation, two-step reforming, and autothermal reforming of methane are discussed. For Fischer–Tropsch, we highlight the role of catalysis and selectivity to high molecular weight hydrocarbons. Also, new reactors technologies, such as microreactors, are presented. The GTL technology still faces several challenges; the biggest is obtaining the right H₂:CO ratio when using a low steam-to-carbon ratio. Despite the great understanding of the carbon formation mechanism, little has been made in developing newer catalysts. Since 60–70% of a GTL plant cost is for syngas production, it needs more attention, particularly for developing the catalytic partial oxidation process (CPO), given that modern CPO processes using a ceramic membrane reactor reduce the plant’s capital cost. Improving the membrane’s mechanical, thermal, and chemical stability can commercialize the process. Catalytic challenges accompanying the FTS need attention to enhance the selectivity to produce high-octane gasoline, lower the production cost, develop new reactor systems, and enhance the selectivity to produce high molecular weight hydrocarbons. Catalytically, more attention should be given to the generation of a convenient catalyst layer and the coating process for a given configuration. Full article

Methane (CH₄) hydrate dissociation and CH₄ release are potential geohazards currently investigated using X-ray computed tomography (XCT). Image segmentation is an important data processing step for this type of research. However, it is often time consuming, computing resource-intensive, operator-dependent, and tailored for each XCT dataset due to differences in greyscale contrast. In this paper, an investigation is carried out using U-Nets, a class of Convolutional Neural Network, to segment synchrotron XCT images of CH₄-bearing sand during hydrate formation, and extract porosity and CH₄ gas saturation. Three U-Net deployments previously untried for this task are assessed: (1) a bespoke 3D hierarchical method, (2) a 2D multi-label, multi-axis method and (3) RootPainter, a 2D U-Net application with interactive corrections. U-Nets are trained using small, targeted hand-annotated datasets to reduce operator time. It was found that the segmentation accuracy of all three methods surpass mainstream watershed and thresholding techniques. Accuracy slightly reduces in low-contrast data, which affects volume fraction measurements, but errors are small compared with gravimetric methods. Moreover, U-Net models trained on low-contrast images can be used to segment higher-contrast datasets, without further training. This demonstrates model portability, which can expedite the segmentation of large datasets over short timespans. Full article

The consumption of methane and the production of biodegradable polymers using alphaproteobacterial methanotrophs offers a promising strategy to mitigate greenhouse gas emissions and reduce non-biodegradable plastic pollution. This study identified an ideal amount of added methane and N:C ratio in 100 mL batch cultures of the alphaproteobacterial methanotroph Methylocystis sp. Rockwell growing in 1-L sealed bottles using Response Surface Methodology (RSM) to achieve both high biomass and high polyhydroxybutyrate (PHB) production. RSM analysis showed achievement of optimal biomass at 474.7 ± 10.1 mg/L in nitrate mineral salts (NMS) medium and 480.0 ± 65.5 mg/L biomass in ammonium mineral salts (AMS) medium with 8 mmol of methane and an N:C ratio of 0.022. However, optimal PHB concentration was achieved with 6 mmol methane at N:C ratios of 0.012 in NMS medium (149.7 ± 16.1 mg/L) and 0.022 in AMS medium (200.3 ± 5.1 mg/L). A multi-objective RSM analysis projected maxima in PHB production and %PHB cell content (based on dry weight) when using 4.88 mmol methane and N:C ratio of 0.016 in NMS cultures, and 6.28 mmol methane and the 0.016 N:C ratio in AMS cultures. Cultures grown under these projected conditions produced 173.7 mg PHB/L with 46.8% PHB cell content in NMS and 196.9 mg/L with 53.1% PHB cell content in AMS. Taken together, these analyses predicted the optimal conditions for growth and PHB production in batch cultures of Methylocystis sp. Rockwell and confirmed a preference for ammonium as the N-source for PHB production. This information is valuable for media formulation in industrial scale-up of Methylocystis sp. Rockwell in PHB production. Full article

A significant portion of global greenhouse gas emissions is attributed to methane (CH₄₎, the primary greenhouse gas released by dairy animals. Thus, livestock farming has a new challenge in reducing enteric CH₄ for sustainability. In anaerobic microbial ecosystems such as the rumen, carbohydrates are converted into short-chain, volatile fatty acids that animals use for energy and protein synthesis. It is, therefore, essential to understand rumen physiology, population dynamics, and diversity to target methanogens. Thus far, numerous CH₄ mitigation strategies have been studied, including feeding management, nutrition, rumen modification, genetics, and other approaches for increasing animal production. As new molecular techniques are developed, scientists have more opportunities to select animals with higher genetic merit through next-generation sequencing. The amount of CH₄ produced per unit of milk or meat can be permanently and cumulatively reduced through genetic selection. Developing eco-friendly and practical nutrigenomic approaches to mitigating CH₄ and increasing ruminant productivity is possible using next-generation sequencing techniques. Therefore, this review summarizes current genetic and nutrigenomic approaches to reducing enteric CH₄ production without posing any danger to animals or the environment. Full article

Climate change and the urgent need to reduce greenhouse gas (GHG) emission from agriculture has resulted in significant pressure on the livestock industry for advanced practices that are environmentally more sustainable. Livestock is responsible for more than 15% of anthropogenic methane (CH₄) emission via enteric fermentation and improved strategies for mitigating enteric CH₄ production therefore represents a promising target to reduce the overall GHG contribution from agriculture. Ruminal CH₄ is produced by methanogenic archaea, combining CO₂ and hydrogen (H₂). Removal of H₂ is essential, as its accumulation inhibits many biological functions that are essential for maintaining a healthy rumen ecosystem. Although several other pathways occur in the rumen, including reductive acetogenesis, propionogenesis, nitrate, and sulfate reduction, methanogenesis seems to be the dominant pathway for H₂ removal. Global warming is not the only problem associated with the release of CH₄ from ruminants, but the released GHG also represent valuable metabolic energy that is lost to the animal and that needs to be replenished via its food. Therefore, reduction of enteric CH₄ emissions will benefit not only the environment but also be an important step toward the efficient production of high-quality animal-based protein. In recent decades, several approaches, relying on a diverse set of biological and chemical compounds, have been tested for their ability to inhibit rumen methanogenesis reliably and without negative effects for the ruminant animal. Although many of these strategies initially appeared to be promising, they turned out to be less sustainable on the industrial scale and when implemented over an extended period. The development of a long-term solution most likely has been hindered by our still incomplete understanding of microbial processes that are responsible for maintaining and dictating rumen function. Since manipulation of the overall structure of the rumen microbiome is still a significant challenge targeting key intermediates of rumen methanogenesis, such as H₂, and population that are responsible for maintaining the H₂ equilibrium in the rumen could be a more immediate approach. Addition of microorganisms capable of non-methanogenic H₂ sequestration or of reducing equivalents are potential avenues to divert molecular H₂ from methanogenesis and therefore for abate enteric CH₄. However, in order to achieve the best outcome, a detailed understanding of rumen microbiology is needed. Here we discuss some of the problems and benefits associated with alternate pathways, such as reductive acetogenesis, propionogenesis, and sulfate and nitrate reduction, which would allow us to bypass H₂ production and accumulation in the rumen. Full article

Biogas upgrading by a catalytic process has been studied in order to obtain syngas using renewable source of methane. This work evaluates the influence of metal dopant (Gd, Sm, and Zr) on the CeO₂ structure for the dry reforming of methane over Ni nanoparticle embedded catalysts. The doping with Zr improved the thermal stability of the catalyst, leading to the formation of small Ni nanoparticles, while Ni metal sintering was observed for Ni@CeO₂, Ni@CeGdO₂, and Ni@SmO₂, according to in situ XRD under reduction conditions. The ceria reducibility was affected by the dopant nature, for which the addition of Zr caused distortions in the ceria lattice, promoting the diffusion of oxygen bulk to surface. The doping with Gd and Sm created oxygen vacancies by charge compensation, and the saturation of oxygen vacancies in the fresh samples decreased the degree of Ce reduction, according to TPR results. The larger Ni particles and poor redox behavior for Ni@CeGdO₂ and Ni@CeSmO₂ were responsible for the high carbon formation on these catalysts during the DRM reaction. The Ni@CeZrO₂ catalyst did not present coke formation because of smaller Ni crystallite size and higher ceria reducibility. Therefore, the control of Ni particle size and the high oxygen mobility in the Ni@CeZrO₂ catalyst inhibits carbon deposition and enhances the mechanism of carbon removal, promoting the catalyst stability. Full article