Search Results (308)

Early-stage estimation of total biogas yield is useful for preliminary screening of anaerobic digestion substrates when detailed kinetic parameters, long-term reactor data and substrate-specific biochemical methane potential tests are not yet available. This study develops a compact Gradient Boosting Regression model for estimating total biogas yield from basic substrate and operating descriptors commonly reported in anaerobic digestion studies. A literature-informed reconstructed benchmark dataset containing 120 records from twelve organic substrate groups was used. The response variable was total biogas yield, expressed as mL g⁻¹ volatile solids (VS) added; methane yield and methane content were not used as interchangeable target variables. Seven input variables were considered: volatile solids content, total solids content, carbon-to-nitrogen ratio, residence time descriptor, digestion temperature, pH and organic loading rate. The model was evaluated using the coefficient of determination (R²), mean absolute error (MAE), root mean square error (RMSE), mean absolute percentage error (MAPE), five-fold cross-validation and residual diagnostics. On the test subset, the model achieved R² = 0.9885, MAE = 13.40 mL g⁻¹ VS, RMSE = 16.16 mL g⁻¹ VS and MAPE = 3.73%. The mean five-fold cross-validation score was R² = 0.9720 ± 0.0180, indicating stable interpolation performance within the investigated data domain. Feature-importance analysis identified C/N ratio as the dominant predictor, followed by volatile solids and total solids. However, residual diagnostics showed heteroscedasticity, and the low apparent importance of pH and temperature was attributed to their narrow ranges in the compiled dataset, rather than to biological irrelevance. The proposed model is therefore intended as an early-stage screening tool for total biogas yield estimation, not as a universal methane yield or design-level prediction model. Full article

This study presents a comprehensive thermodynamic evaluation of several CO₂ conversion pathways, both non-catalytic and catalyst-assisted, using density functional theory (DFT) calculations in Gaussian 16 (B3LYP/6-311++G(d,p)). In the non-catalyzed section, three key routes are examined: hydrogenation (CO₂ + H₂ → CO + H₂O), dry methane reforming, and the reverse water–gas shift (RWGS). For the hydrogenation reaction, the Gibbs free energy change (ΔG) decreases from +0.018 to +0.005 Hartree as the temperature increases from 298.15 K to 1173.15 K, indicating a slight improvement in feasibility but still a high activation barrier of 0.326 Hartree, underscoring the need for catalysis. Dry methane reforming is both exothermic and spontaneous, with ΔG ≈ = −0.049 Hartree at 298.15 K and −0.030 Hartree at 593.15 K; however, operating under harsh conditions may accelerate degradation of reactor materials. In the catalyst-assisted section, copper surfaces and Cu₃M clusters (M = Sc, V, Ni, Cu, Co and Fe) are evaluated alongside two bimetallic catalysts, Fe₂ and Ni₂, under electrochemical CO₂ reduction (eCO₂RR) conditions. Scandium- and vanadium-doped clusters exhibit significant CO₂ adsorption, as evidenced by shifted vibrational frequencies between 800 and 1800 cm⁻¹ that signal C=O bond weakening. Under the evaluated thermobarometric conditions, Ni₂-containing systems displayed lower Gibbs energy values within their own optimized intermediate set and higher entropy values than the corresponding Fe₂-containing set, suggesting greater configurational flexibility and favorable stabilization trends. However, because Fe₂ and Ni₂ systems are chemically different, absolute total energies were not used as a standalone criterion for intrinsic catalytic superiority. Overall, while some non-catalytic routes become thermodynamically more favorable only at high temperature, the explicit inclusion of catalytic models, particularly doped Cu₃M clusters and Ni-containing systems, indicates enhanced CO₂ activation through stronger catalyst–adsorbate interactions, vibrational weakening of C=O bonds, and favorable electronic descriptors. These results suggest that catalytic systems may enable CO₂ conversion under milder conditions, although full kinetic confirmation requires comparative transition state calculations for each elementary catalytic step. Full article

This study, conducted in the Ecuadorian Andes, evaluated the anaerobic co-digestion of local crop residues (amaranth and quinoa) with llama, vicuña, and pig manure to analyze methane production kinetics. The raw materials were characterized by proximate, elemental, and structural analyses, and biogas volume and the CH₄ fraction were monitored daily. The Amaranth-vicuña and Amaranth-llama treatments reached 77.29 ± 5.63 and 64.62 ± 3.62 mL biogas/g VS and 36.20 ± 7.29 and 31.78 ± 3.62 mL CH₄/g VS, respectively; in contrast, Quinoa-vicuña and Quinoa-llama produced only 1.04 ± 0.25 and 0.24 ± 0.03 mL CH₄/g VS. Monod-model parameters were estimated using an apparent formulation based on the methane production rate, and the kinetic behavior was compared with first-order, modified Gompertz, and modified logistic models. In addition, artificial neural networks (ANNs) were evaluated to predict the methane production curve from substrate characterization. Network 44, with a 13-15-10-1 architecture, yielded an overall R² = 0.998, validation R² = 0.997, and validation MSE = 0.415. Ten-times repeated five-fold cross-validation of the same architecture yielded R² = 0.985 ± 0.007 and RMSE = 1.21 ± 0.28 mL CH₄/g VS, supporting its interpolation capability within the experimental domain, although this does not demonstrate extrapolation to new substrate combinations. Overall, the proposed approach combines interpretable kinetic parameters with ANN-based prediction, but external validation with independent datasets is still required. The reported yields correspond to the specific production achieved in a low-cost batch system operated at room temperature and should not be interpreted as standardized biochemical methane potential (BMP) values. Full article

Anaerobic digestion (AD) is a common waste management method for producing renewable energy from biogas. However, animal manures typically have low C/N ratios, which can limit biogas recovery. This study aims to optimize biogas production through the co-digestion of goat manure (GM) with high-C/N lignocellulosic rice husk (RH) and sludge as the inoculum. Characterization of the substrate and inoculum revealed a low GM C/N ratio (GM = 21.3), whereas RH has a high-C/N (RH = 107.3). The volatile solids-to-total solids (VS/TS) ratios were around 82–85% for GM and RH. Batch experiments were conducted at different organic loading rates, with an inoculum-to-substrate ratio of 2:1 (mL:g), at 36 ± 1 °C for 65 days. This study investigates optimizing biomethane recovery by using serum-bottle biomethane potential (BMP) and compares kinetic performance and yields across different GM-to-RH ratios with the characteristics of the influent and effluent. The highest BMP values (mL CH₄/g_VS) occurred at 100% GM (245.1), followed by 90% GM (233.6) and 30% GM (232.5), indicating a strong synergy between GM and RH at specific mixing ratios. Kinetic modeling using both the modified Gompertz and first-order models effectively described digestion dynamics, allowing for estimation of potential lag phases and maximum production rates. These models aligned well with experimental data across substrates, aiding process design. Overall, the results show that strategic co-digestion of GM with RH can maximize methane recovery, with defined substrate ratios and a clear understanding of the kinetics essential for scale-up and sustainable biogas production. Full article

Combined cooling, heating, and power (CCHP) tri-generation systems can greatly improve comprehensive energy utilization efficiency thanks to their energy-cascade utilization concept. However, traditional fossil-fuel-based CCHP systems still suffer from intensive carbon emissions, hindering their further development in the current low-carbon scenario. To solve this issue, a new low-carbon CCHP system (LC-CCHP) integrating a calcium-looping, sorption-enhanced steam methane reforming (CL-SE-SMR) unit, a lithium bromide absorption chiller, and a hydrogen gas turbine is proposed in this work, and the corresponding system model is built to evaluate its performance. The proposed system features an innovative architecture that integrates carbon capture directly into the reforming process, which simultaneously enables a high hydrogen yield and low carbon-capture penalty. Moreover, instead of the widely used thermodynamic equilibrium assumption, a detailed kinetic model is employed for the CL-SE-SMR unit, which provides more realistic predictions and greater reference value for practical engineering applications. Then, multi-objective optimization is conducted using a particle swarm optimization algorithm to identify the optimal operating conditions. It is found that the proposed system performs best at a steam-to-carbon molar ratio of 4.37, a calcium-to-carbon mass ratio of 6.23, an air-equivalency molar ratio of 1.39 for a hydrogen gas turbine and a reaction temperature of 600 °C for SE-SMR. Under these operating conditions, the system can achieve a carbon-capture rate of 89.2%, an exergy efficiency of 45.7%, an energy efficiency of 95.4%, and a levelized cost of exergy of 0.109 $/kWh. Full article

As exploration expands into deep and unconventional petroleum systems, light oil and condensate have become key targets for reserve growth and production enhancement. This study employs the gold tube pyrolysis of kerogens from Cretaceous Qingshankou shale to investigate the role of hydrocarbon (HC) expulsion in light oil and condensate generation during thermal maturation. The results show that HC expulsion significantly reduces overall HC yields and alters their chemical composition. Specifically, compared with immature kerogen, n-hexane-extracted mature kerogen (EasyR_o = 0.96%) exhibited reductions of 60%, 57%, and 50% in C₁₅₊ compounds, C_6–14 HCs, and C_1–5 gases, respectively. Moreover, the generation window of C_6–14 HCs (a proxy for light oil) is narrowed and shifted toward lower maturity. Kinetic parameters were further used to establish two separate evolutionary models for methane, wet gas, light oil, and heavy oil. Based on these models, the shale oil resource potential of the first member of the Qingshankou Formation, the Qijia–Gulong Sag, is estimated to be (6.95–8.80) × 10⁶ ton/km² for the no-HC-expulsion scenario and (3.63–3.85) × 10⁶ ton/km² for the significant-HC-expulsion scenario (HEE = 84.35%). These results provide a valuable reference for assessing the light oil and condensate potential of high-maturity Qingshankou shale in the Songliao Basin. Full article

Photocatalytic CO₂-to-CH₄ conversion is usually assessed through catalyst activity, selectivity, and methane formation rate, although the practical usefulness of the resulting gas stream also depends on methane recovery from CO₂-rich mixtures. Here, a phenomenological Langmuir–Hinshelwood reactor model was used to evaluate batch gas-phase CO₂ photoreduction under different reactor thicknesses, water-availability conditions, photocatalyst activities, and idealized methane-selective extraction configurations. The model considered competitive adsorption, transient gas-phase balances, finite or buffered water supply, and a lumped selective CH₄ extraction term. Under finite vapor inventory, water depletion limited reaction progress, particularly in thin reactors. Buffered-water operation increased CO₂ conversion and methane formation, while reactor thickness produced a trade-off: thin reactors favored apparent conversion and methane enrichment, whereas thicker reactors provided a larger CO₂ reservoir and higher cumulative methane formation. At the baseline kinetic condition, methane extraction strongly decreased the in-reactor CH₄ fraction but only modestly increased methane formation. When photocatalyst activity was increased, the non-membrane thin reactor reached a product-accumulation-limited regime, and methane-selective extraction became kinetically relevant. These results indicate that methane-selective extraction concepts should be evaluated not only as separation devices, but as conditional reactor-intensification tools whose relevance depends on water availability, reactor geometry, catalyst productivity, and product dilution. Full article

The aim of this study was to evaluate the effect of Sida hermaphrodita harvest timing on biomass composition, properties, and methane fermentation performance. In addition, an energy–economic assessment was performed for biomass obtained at different stages of the growing season. The economic assessment assumed CHP electrical and thermal efficiencies of 38% and 47%, electricity and heat prices of 0.18 and 0.05 EUR/kWh, respectively, month-specific agrotechnical costs, and OPEX equal to 30% of total energy revenue. The biomass exhibited clear seasonal changes, transitioning from a material with high bioavailability during the summer period to a structurally more recalcitrant substrate in the autumn and winter months, as indicated by increasing lignification and fibrous fraction contents. The highest CH₄ production yields, ranging from 300 to 320 mL/g VS, and maximum production rates of up to 33.5 mL/g VS·d were obtained between June and August. In December, the CH₄ yield decreased to 180 ± 9 mL/g VS, accompanied by a substantial deterioration in kinetic performance. Despite the relatively stable theoretical methane potential, which ranged from 405 to 430 mL/g VS, its conversion efficiency declined from 77.1% in the summer period to 41.9% in the winter period. Regression analysis confirmed the key influence of the C/N ratio and total solids content, with model fits reaching R² values of 0.74–0.80, while the structure of lignocellulosic complexes had a less pronounced but still relevant effect. The maximum CH₄ production per unit cultivation area, approaching 3380 m³/ha, was achieved in July–August, reflecting a balance between high specific methane yield and biomass productivity. At the same time, the results demonstrated that the maximum biomass yield did not translate into the highest energy and economic performance. The highest net economic return, 1789 ± 330 EUR/ha, was obtained in July, despite biomass yield being 13.6% higher in September. These findings indicate a seasonal decoupling between biomass yield and energy performance, highlight biomass quality as a critical determinant of anaerobic digestion efficiency, and support harvest-date optimization as a low-cost strategy for the practical use of S. hermaphrodita in agricultural biogas plants. Further long-term continuous and semi-continuous studies are required to validate process stability and performance under industrial operating conditions. Full article

A working-state model is proposed for the MnOx–Na₂WO₄/SiO₂ catalyst in oxidative coupling of methane (OCM), where a Na₂WO₄-rich surface environment forms an adaptive interphase that buffers the effective interfacial oxygen chemical potential and stabilizes cooperative MnO_x/Na–WO_x/Mn–O–W motifs. A thermodynamic-kinetic scheme is developed that relates (1) reaction-induced surface enrichment (structural stabilization), (2) oxygen spillover (damping of local oxygen gradients), and (3) Mn ↔ W redox exchange as an electron-oxygen buffer channel. Ex situ XPS/EDS/EPR data indicate a dynamically stratified near-surface region with chemically heterogeneous environments of Mn, W, and O. The W 4f region remains dominated by the W⁶⁺ contribution in the presence of a minor reduced component after OCM. In oxygen-deficient mixtures (CH₄/O₂ > 4), interfacial reconstruction becomes more pronounced: Mn-centered Mars–van Krevelen chemistry determines CH₄ activation and oxygen exchange, while the Na₂WO₄-rich phase ensures fast ion/oxygen transport. Observation of the EPR signal from W⁵⁺ ions in the tungstate matrix indicates the existence of reduced W intermediates at low oxygen potential. Optimization of C₂ selectivity and stability is suggested to require maintaining the catalyst within the selective window of effective interfacial μO by adjusting CH₂/O₂ and contact time, as well as controlling the architecture of the Na–W–O/MnO_x interfacial region. Full article

Radiant tube burners (RTBs) are widely used in industrial heat-treatment furnaces, yet the coupled effects of hydrogen co-firing and exhaust gas recirculation (EGR) on their thermal fields remain insufficiently understood. This study presents a three-dimensional CFD analysis of 28 operating conditions, spanning hydrogen fractions from 0 to 100% and EGR rates from 0 to 20% at a fixed excess air ratio of 10%. The model employs the eddy dissipation concept with a reduced two-step methane mechanism, detailed hydrogen kinetics, and a Discrete Ordinates radiation model with a weighted-sum-of-gray-gases approach. All cases exhibit splitting flames: hydrogen enrichment intrinsically raises the laminar flame speed above the flame morphological transition threshold, while in pure methane, radiative preheating increases the flame speed by 29%, eliminating the triangular flame mode. The volumetric temperature uniformity index peaks near 30% H₂, whereas EGR improves uniformity in hydrogen-rich cases but slightly degrades it in methane-rich conditions. Surface temperature uniformity is maximized at 20% EGR due to near-wall thermal blanketing. Thermal efficiency increases with hydrogen fraction, from 59.1% at 0% H₂ without EGR to 68.6% at 100% H₂ with 10% EGR, while higher EGR suppresses peak temperatures. These findings provide guidance for balancing energy efficiency and temperature uniformity in hydrogen-ready RTBs. Full article

This study presents an integrated optimization framework for enhancing biogas and methane production through anaerobic digestion, addressing the challenge of identifying optimal operating conditions across multiple interacting parameters. Biochemical methane potential tests were conducted to evaluate the individual effects of four critical operational parameters: temperature, mixing regime, inoculum-to-substrate (I-S) ratio, and chemical oxygen demand load (COD-L). Experimental findings confirmed that thermophilic conditions, mixing once a day, I-S ratio of 1:2, and moderate COD loading consistently delivered the most favorable biogas and methane yields. Kinetic modeling, including the Modified Gompertz and Logistic models, showed strong predictive agreement with experimental data (R² > 0.90), reliably capturing production dynamics across all tested conditions. Polynomial response surface methodology further identified COD-L as the dominant driver of methane yield, with optimal operating conditions falling within moderate temperature and COD-L ranges. This revealed significant nonlinear interactions between parameters. Building on these findings, the Crayfish Optimization algorithm successfully determined global optimal conditions, achieving a maximum biogas production of 0.371 Nm³/kg.VS. These results highlight how combining experimental investigation with predictive modeling and metaheuristic optimization creates a powerful decision-support framework for improving the efficiency and stability of anaerobic digestion systems. Full article

Anaerobic digestion (AD) of lignocellulosic biomass is often constrained by biomass recalcitrance, limiting methane recovery. This study investigated whether low-temperature dilute-acid hydrothermal pretreatment could enhance methane production from pine nut shells (PNSs), a lignin-rich and underutilized agro-industrial residue, and whether derivative-based kinetic landmarks could provide a more systematic characterization of batch AD performance. Methane production was significantly improved by dilute sulfuric acid and hydrothermal pretreatments. The highest methane yield (201.8 mL CH₄ g⁻¹ VS) was achieved under the combined 100 °C hydrothermal and 2.5% H₂SO₄ condition, representing approximately 1.8-fold and 3.3-fold increases compared with hydrothermal-only and untreated PNSs, respectively. Enhanced performance was attributed to hemicellulose solubilization, lignin disruption, and improved substrate accessibility. In contrast, excessive acid severity resulted in process instability, associated with total volatile fatty acid accumulation and pH reduction. The Modified Logistic Model (MLM) was further used to derive five kinetic landmarks (PAA, PAM, PI, PDM, and PDA) describing phase-specific features of cumulative methane production curves. While these landmarks provide a model-based framework for comparing batch AD kinetics, their nearly constant normalized yields primarily reflect the geometry of the fitted logistic function rather than independent biological invariants. Overall, the results identify 100 °C hydrothermal pretreatment with 2.5% H₂SO₄ as an effective moderate-severity strategy for enhancing methane recovery from PNSs and demonstrate the utility of MLM-derived landmarks as comparative descriptors of phase-resolved methane production. Full article

Anaerobic digestion of municipal wastewater sludge is often limited by slow hydrolysis rates. This study evaluated the effects of ozone pretreatment on methane production during mesophilic batch digestion. Ozone was applied at 0–10% for 30–90 s, with inoculum-to-substrate ratios of 1.0–2.0. Methane production was monitored using the AMPTS II system. The maximum methane yield (736 NmL CH₄ g⁻¹ VS; 1381 NmL total) was obtained at 10% ozone for 30 s and I/S = 1.5. Kinetic modelling showed enhanced methane production rates and reduced lag phases, with the Gompertz and Logistic models providing the best fit. Full article

The efficiency of anaerobic digestion (AD) of lignocellulosic biomass is strongly determined by biomass yield, chemical composition, and bioavailability, all of which undergo substantial seasonal variation. However, integrated analyses linking these factors with AD performance, process kinetics, and energy-economic efficiency remain limited. This study aimed to evaluate the effect of seasonal variability in the chemical composition of Helianthus annuus biomass on AD efficiency from a technological and economic perspective. The novelty of this study lies in integrating seasonal changes in biomass composition with AD kinetics, CH₄ productivity per hectare, and CHP techno-economic performance to identify the optimal harvest window for Helianthus annuus. The experiments were conducted using biomass harvested from June to December. The results showed significant (p < 0.05) variability in biomass properties, including a progressive increase in lignocellulosic fractions over the growing season, with neutral detergent fiber (NDF) increasing from 30.58 ± 1.8 to 66.58 ± 3.1% TS and acid detergent lignin (ADL) from 5.13 ± 0.5 to 10.35 ± 0.9% TS, accompanied by a decline in substrate bioavailability. The maximum CH₄ yield of 258 ± 13 mL/g VS was obtained in August, with a process rate of 29.0 ± 3.4 mL/g VS·d and the highest utilization of methane potential, reaching 62.5 ± 3.8% (BMP_CH4/TBMP). Correlation and regression analyses indicated that ADL and NDF were the strongest empirical predictors of AD performance within the analyzed dataset, showing a negative association with both CH₄ production yield and kinetics (R² up to 0.86), whereas reducing sugars had a stimulatory effect. Multiple regression models showed high predictive performance, with R² = 0.889 for BMP_CH4. The highest energy and economic efficiency was achieved in summer. In August, CH₄ production reached 3214 ± 596 m³/ha, corresponding to 11.2 ± 2.1 MWh/ha of electricity and a net result of 1559 ± 417 EUR/ha. Increased lignification in the later part of the season led to reduced process efficiency and a deterioration of the economic balance. From a practical perspective, these results demonstrate that harvest scheduling should be based on the trade-off between biomass quantity and biodegradability rather than on biomass yield alone. Full article

Tri-reforming of methane (TRM) has emerged as a promising pathway for low-carbon syngas production by integrating steam reforming, dry reforming, and partial oxidation within a single process. This coupling enables simultaneous CH₄ utilization and CO₂ valorization while enabling internal heat generation and flexible adjustment of the H₂/CO ratio for downstream synthesis. However, TRM performance cannot be adequately evaluated using conversion or energy efficiency alone, because the process involves complex interactions among competing reaction pathways, transport phenomena, catalyst stability, and thermodynamic irreversibility. This review provides a multiscale critical assessment of TRM from both first-law energy and second-law exergy perspectives, linking reaction-network fundamentals to reactor-level behavior and system-level performance. The literature evidence shows that although high temperatures and near-autothermal operation can enhance CH₄ conversion and reduce external heat demand, these conditions may simultaneously intensify deep oxidation, hotspot formation, carbon-forming tendencies, and exergy destruction. While equilibrium analyses help define feasible operating windows, they are insufficient without kinetic modeling and reactor-scale studies that capture spatial non-uniformities and pathway competition. Across reported TRM systems, exergy destruction is consistently concentrated within the reformer, identifying the reacting core as the dominant thermodynamic bottleneck. Accordingly, the key challenge in TRM is not simply to maximize conversion but to preserve chemical work potential while maintaining syngas quality and operational stability. Viewed from this perspective, TRM is better understood as an irreversibility-aware multiscale design problem in which optimal performance depends on the integrated optimization of catalyst functionality, reactor architecture, heat management, and system-level operation. Full article