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This study investigated the enhancement of corn stover (CS) anaerobic digestion (AD) performance through low-temperature hydrothermal modification (HM) with Fe/C catalysts and developed two predictive models for biomethane yield (BY). CS was modified with Fe/C at 50 °C and then anaerobically digested. The results indicated that Fe/C significantly improved CS hydrolysis efficiency, indicated by increasing concentrations of glucose, mannose, xylose, and volatile fatty acids (VFAs), which were 1.9, 1.7, 3.0, and 1.8 times higher than those of HM alone, respectively. The enhanced hydrolysis of CS effectively improved AD performance, leading to a BY increase of 25.5% as compared to the control group. The time to reach 90% of the maximum BY (T90) was also reduced by 7 days. Furthermore, the developed GM(1,N) gray system model effectively simulated multi-parameter coupling effects in AD processes under small-sample conditions (n < 20), demonstrating high accuracy (average percentage deviation [APD] = 4.50%) and enabling correlation analysis between modification parameters and BY. The ANN-GA model exhibited superior accuracy in BY prediction. This study demonstrated the effectiveness of low-temperature HM-Fe/C in enhancing BY and the accuracy of two models in predicting BY.

This study investigated the enhancement of anaerobic digestion (AD) performance of corn stover (CS) through Fe/Ni catalytic hydrothermal depolymerization with ethanol. The CS depolymerization process was conducted using Fe/C, Ni/C, Fe/CNT and Ni/CNT catalysts in combination with ethanol or water/ethanol solvents. The results revealed that the depolymerization with catalyst-ethanol (DC-E) effectively disrupted the physical encapsulation of cellulose by lignin. It also showed that the Ni/CNT catalyst in ethanol significantly promoted β-O-4 bond cleavage in lignin, achieving a lignin conversion rate of 48.5% and 2.7 g/L total phenol concentration (TPC). The water/ethanol (9:1) system effectively degraded hemicellulose (53.6% conversion) while retaining over 90% cellulose for AD. Structural analysis revealed that DC disrupted cellulose hydrogen bonds, reducing crystallinity index (CrI decreased from 38.4% to 32.6%) and increasing cellulose accessibility to 909 mg/g (2.6 times higher than untreated CS). The efficient depolymerization of CS obviously improved the biodegradability of cellulose and hemicellulose, contributing to the increase of biomethane production. Biomethane yield (BY) of E-Ni/CNT was 18.1% and 27.6% higher than that of E-HP and the control group, respectively. These findings indicated that ethanol-promoted catalytic depolymerization of CS can enhance the performance of AD.

This research investigated the effects of hydrothermal depolymerization with Fe/Ni loaded C catalysts on the anaerobic digestion (AD) performance of corn stover (CS). CS was depolymerized at 140 °C for 20 min with Fe/C or Ni/C catalysts, and then anaerobically digested. The results showed that the biomethane yield with Fe/C-600 increased by 36.6% compared to that of the control. This increase could be attributed to effective CS depolymerization with catalysts (DC), indicated by modified structures of solid fraction and enriched available components of liquid fraction. SEM analyses showed that CS microphysical structure after DC was obviously disrupted, resulting in more accessibility of cellulose and hemicellulose. The crystallinity index (CrI) of depolymerized CS was significantly reduced from 32.5% to 23.5%, allowing for a more easily biodegradable non-crystalline area to be available for enzymes. Meanwhile, the DC group produced 4.7 times more reducing sugar (RS), and a 3.4 times increase in total volatile fatty acids (VFAs) as compared to the control. Furthermore, these enhancements in DC led to an increased relative abundance of cellulolytic bacteria (Hydrogenispora and Fermentimonas) and anaerobic methanogenic archaea (Methanosarcina) in following the AD process. This could explain the reason for the biomethane yield increase with DC from microbial perspectives. This study demonstrated that hydrothermal depolymerization with Fe/C or Ni/C could provide an effective approach for obtaining more biomethane from CS via AD.

for stationary hypersonic-limit Euler flows passing a solid body in three-dimensional space, the shock-front coincides with the upwind surface of the body, hence there is an infinite-thin layer of concentrated mass, in which all particles hitting the body move along its upwind surface. By proposing a concept of Radon measure solutions of boundary value problems of the multi-dimensional compressible Euler equations, which incorporates the large-scale of three-dimensional distributions of upcoming hypersonic flows and the small-scale of particles moving on two-dimensional surfaces, the authors derive the compressible Euler equations for flows in concentration layers, which is a stationary pressureless compressible Euler system with source terms and independent variables on curved surface. As a by-product, they obtain a formula for pressure distribution on surfaces of general obstacles in hypersonic flows, which is a generalization of the classical Newton-Busemann law for drag/lift in hypersonic aerodynamics.

This study aims to investigate the effect of different applied voltages on the biomethanation performance and microbial community characteristics of corn stover (CS) in a microbial electrolysis cell (MEC)-assisted anaerobic digestion (AD) system (MEC-AD). The results showed that the MEC-AD system operating at 0.8 V achieved the highest methane yield of 192.40 mL CH4/g VS (volatile solids), an increase of 14.98% compared to the conventional AD. The system obtained methane yields of 187.74 to 191.18 mL CH4/g VS at lower voltages (0.4 V and 0.6 V), and 156.11–182.75 mL CH4/g VS at higher voltages (1.0 V and 1.2 V), respectively, suggesting that lower or higher voltages would have adversely impacted the methane yield. Correspondingly, the MEC-AD system operating at 0.4–0.8 V achieved over 71.47% conversion rates of total solids (TS), VS, and cellulose. The microbial community analysis revealed that 0.8 V optimally enriched fermentative acidogenic bacteria (FABs, 24.55%) and electroactive bacteria (13.50%), enhancing both hydrolysis acidification efficiency and direct interspecies electron transfer (DIET). Both Methanosarcina and Methanoculleus demonstrated significant positive correlations with FABs, SOBs, and electroactive bacteria. This study reveals that 0.8 V represents the optimal operating voltage for biomethane production in MEC-AD systems, providing critical insights for agricultural waste valorization.

This study developed a system (MEC-AD) by integrating a single-chamber microbial electrolysis cell (MEC) with anaerobic digestion (AD), aiming to enhance the conversion efficiency of kitchen waste (KW) into biomethane and optimize metabolic pathways. The performance and microbial metabolic mechanisms of MEC-AD were investigated and compared with those of conventional AD, through inoculation with original inoculum (UAD) and electrically domesticated inoculum (EAD), respectively. The results show that the MEC-AD system achieved a CH4 yield of 223.12 mL/g VS, which was 31.27% and 25.24% higher than that of conventional UAD and EAD, respectively. The system also obtained total solid (TS) and volatile solid (VS) conversion rates of 82.32% and 83.39%, respectively. Furthermore, the MEC-AD system enhanced the degradation of soluble chemical oxygen demand (SCOD) and mitigated biogas production stagnation by reducing the accumulation of volatile fatty acids (VFAs) as intermediate products. Microbial metagenomics analysis revealed that the MEC-AD system enhanced microbial diversity and enriched functional genera abundance, facilitating substrate degradation and syntrophic relationships. At the molecular level, the system upregulated the expression of key enzyme-encoding genes, thereby simultaneously strengthening both direct interspecies electron transfer (DIET) and mediated interspecies electron transfer (MIET) pathways for methanogenesis. These findings demonstrate that MEC-AD significantly improves methane production through multi-pathway synergies, representing an innovative solution for efficient KW-to-biomethane conversion.

We consider the singular Riemann problem for the rectilinear isentropic compressible Euler equations with discontinuous flux, more specifically, for pressureless flow on the left and polytropic flow on the right separated by a discontinuity x = x ( t ). We prove that this problem admits global Radon measure solutions for all kinds of initial data. The over-compressing condition on the discontinuity x = x ( t ) is not enough to ensure the uniqueness of the solution. However, there is a unique piecewise smooth solution if one proposes a slip condition on the right-side of the curve x = x ( t ) + 0, in addition to the full adhesion condition on its left-side. As an application, we study a free piston problem with the piston in a tube surrounded initially by uniform pressureless flow and a polytropic gas. In particular, we obtain the existence of a piecewise smooth solution for the motion of the piston between a vacuum and a polytropic gas. This indicates that the singular Riemann problem looks like a control problem in the sense that one could adjust the condition on the discontinuity of the flux to obtain the desired flow field.

In this paper we study the transonic shock in steady compressible flow passing a duct. The flow is a given supersonic one at the entrance of the duct and becomes subsonic across a shock front, which passes through a given point on the wall of the duct. The flow is governed by the three-dimensional steady full Euler system, which is purely hyperbolic ahead of the shock and is of elliptic–hyperbolic composed type behind the shock. The upstream flow is a uniform supersonic one with the addition of a three-dimensional perturbation, while the pressure of the downstream flow at the exit of the duct is assigned apart from a constant difference. The problem of determining the transonic shock and the flow behind the shock is reduced to a free-boundary value problem. In order to solve the free-boundary problem of the elliptic–hyperbolic system one crucial point is to decompose the whole system to a canonical form, in which the elliptic part and the hyperbolic part are separated at the level of the principal part. Due to the complexity of the characteristic varieties for the three-dimensional Euler system the calculus of symbols is employed to complete the decomposition. The new ingredient of our analysis also contains the process of determining the shock front governed by a pair of partial differential equations, which are coupled with the three-dimensional Euler system.

As a lignocellulose-based substrate for anaerobic digestion, rice straw is characterized by low density, high water absorbability, and poor fluidity. Its mixing performances in digestion are completely different from traditional substrates such as animal manures. Computational fluid dynamics (CFD) simulation was employed to investigate mixing performances and determine suitable stirring parameters for efficient biogas production from rice straw. The results from CFD simulation were applied in the anaerobic digestion tests to further investigate their reliability. The results indicated that the mixing performances could be improved by triple impellers with pitched blade, and complete mixing was easily achieved at the stirring rate of 80 rpm, as compared to 20–60 rpm. However, mixing could not be significantly improved when the stirring rate was further increased from 80 to 160 rpm. The simulation results agreed well with the experimental results. The determined mixing parameters could achieve the highest biogas yield of 370 mL (g TS) −1 (729 mL (g TS digested ) −1 ) and 431 mL (g TS) −1 (632 mL (g TS digested ) −1 ) with the shortest technical digestion time ( T 80 ) of 46 days. The results obtained in this work could provide useful guides for the design and operation of biogas plants using rice straw as substrates.

To overcome major limiting factors of microbial processes in anaerobic digestion (AD), metal and metal ions have been extensively studied. However, there is confusion about the effects of metals and metal ions on biomethane productivity in previous research. In this study, Zn and Zn2+ were selected as representatives of metals and metal ions, respectively, to investigate the effects on biomethane productivity. After the metals and metal ions at different concentrations were added to the batch AD experiments under the same mesophilic conditions, a Zn dose of 1 g/L and a Zn2+ dose of 4 mg/L were found to cause the highest biomethane production, respectively. The results indicate that metal (Zn) and metal ion (Zn2+) have different mechanisms to improve AD performance. There may be two possible explanations. To act as conductive materials in interspecies electron transfer (IET), relatively high doses of metals (e.g., 1 g/L of Zn, 10 g/L of Fe) are needed to bridge the electron transfer from syntrophic bacteria to methanogenic archaea in the AD process. As essential mineral nutrients, the AD system requires relatively low doses of metal ions (e.g., 4 mg/L of Zn2+, 5 mg/L of Fe2+) to supplement the component of various enzymes that catalyze anaerobic reactions and transformations. This research will provide clear insight for selecting appropriate amounts of metals or metal ions to enhance biomethane productivity for industrial AD processes.