Antibiotics have been widely used to enhance disease resistance and growth rate in the livestock and poultry breeding industry. However, residual antibiotics can enter the water and soil environment via manure, easily leading to an increase in bacterial resistance to the environment. Thus, it is often required to remove residual antibiotics from the ecological environment. Conventional fecal treatments cannot fully meet the large-scale production in recent years, such as aerobic composting and anaerobic fermentation, due to the long degradation cycles and limited effects on antibiotics. In contrast, an efficient thermochemical conversion, hydrothermal treatment, can be expected to effectively promote the rapid and complete degradation of antibiotics. This study aims to simulate oxytetracycline hydrothermal degradation in pig manure using molecular dynamics. Oxytetracycline, which has the highest proportion of antibiotic residues in pig manure, was selected as the research object. The oxytetracycline-water model was established and then optimized using Materials Studio software. The degradation of oxytetracycline was conducted under hydrothermal conditions using molecular dynamics simulation with LAMMPS software and the ReaxxFF reaction force field. The hydrothermal degradation of oxytetracycline was explored in the hydrothermal system and the reaction path. A systematic analysis was implemented to explore the trends of water and hydroxyl radicals and their reaction mechanisms. Results indicate that hydrothermal temperature was the critical factor controlling oxytetracycline degradation. As the temperature increased, long-chain oxytetracycline molecules progressively fragmented into short-chain small-molecule fragments: The proportion of C_1-5 fragments rose from 45.45% at 1 800 K to 80.68% at 3 000 K, while the proportion of complex long-chain molecules with C>15 decreased from 51.52% to 6.82%. The hydrothermal degradation of oxytetracycline was characterized by rapid and slow increases in fragment formation, and then molecular decomposition intensified with rising simulation temperature. In the temperature range of 1 800 to 2 000 K, there was a low increase in the total number of molecular fragments produced by the degradation of oxytetracycline. In contrast, there was a significant increase in temperature over 2 200 K. The number of H₂O molecules was closely related to the dominant reaction types, lower than the initial value in the initial stage of the reaction at a lower hydrothermal temperature (1 800-2 200 K). Oxytetracycline also shared the hydrolysis reaction. The number of H₂O molecules gradually increased as the hydrothermal temperature rose, and a dehydration reaction also occurred. In addition, the number of hydroxyl radicals gradually increased and fluctuated within a higher range as the hydrothermal temperature increased. These hydroxyl radicals preferentially attacked high-electron-density active sites on the oxytetracycline molecule, thus promoting a series of complex reactions, such as demethylation, deamination, dehydration, hydroxylation, and ring-opening of oxytetracycline. Ultimately, there was the degradation of oxytetracycline into small-molecule substances. Molecular dynamics can be expected to explain the hydrothermal degradation of oxytetracycline at the molecular level. This finding can also provide theoretical guidance and perspective for the efficient removal of antibiotics from swine manure.

Biomass pelletizing treatment can enhance the energy density and decrease the transportation and storage cost of the resulted fuel. In the study, wood sawdust (WS) pretreated by hydrothermal and hydrothermal oxidation process at 170～260 ℃ was used to prepare biomass pellet. The effects of the hydrothermal and hydrothermal oxidation pretreatment on the mechanical property and combustion performance of the wood pellet were investigated using Van Soest method, X-ray diffraction (XRD), thermogravimetric analysis (DTG), industrial analysis and oxygen bomb calorimeter. The results showed that the hemicellulose and amorphous cellulose decomposed rapidly at 170～230 ℃, the crystalline cellulose began to decompose at 260 ℃ and the relative mass fraction of lignin gradually increased with an increase of temperature because of the decomposition of hemicellulose and cellulose after hydrothermal pretreatment. While the hemicellulose was completely decomposed before 230 ℃ and the temperature of crystal cellulose decomposition was advanced to 230 ℃ after hydrothermal oxidation pretreatment. Compared with hydrothermal pretreatment, the relative mass fraction of lignin increased more obviously. The pyrolysis characteristics of wood sawdust pretreated by hydrothermal and hydrothermal oxidation were analyzed. It was found that the samples after hydrothermal pretreatment, the height of hemicellulose pyrolysis peak gradually decreased and completely disappeared at 230 ℃. The pyrolysis peak of cellulose first increased and then decreased with the increase of pretreatment temperature, and the peak of lignin was positively correlated with the increase of pretreatment temperature. Compared with hydrothermal pretreatment, with the increase of hydrothermal oxidation temperature, the pyrolysis peak of hemicellulose disappeared obviously; the pyrolysis peak of cellulose showed a downward trend and the peak was significantly advanced while the pyrolysis peak of lignin continued to rise. The hydrothermal and hydrothermal oxidation pretreatments can greatly enhance the mechanical properties of the resulted biomass pellets compared with that of the unpretreated WS pellets. After hydrothermal pretreatment, the apparent density increased from 1 110.2 to 1 381.6 kg/m³ and the compressive strength increased from 16.2 to 38.9 MPa at 230 ℃, which increased by 24.4% and 140.1%, respectively. The apparent density increased to 1 429.2 kg/m³, and the compressive strength increased to 41.1 MPa at 200 ℃ after hydrothermal oxidation pretreatment, which increased by 28.7% and 153.7%, respectively. Compared with hydrothermal pretreatment, hydrothermal oxidation pretreatment can promote the degradation of the hemicellulose and cellulose contained in WS and enhance its mechanical property, thus, the pretreatment intensity can be alleviated. In the process of hydrothermal and hydrothermal oxidation pretreatment, the crystallinity of samples increased first and then decreased with the increase of pretreatment temperature. After hydrothermal pretreatment, the crystallinity of cellulose increased from 41.4% to 50.7% at 230 ℃. The crystallinity of cellulose reached the maximum value at 200 ℃ after hydrothermal oxidation pretreatment which was higher than the maximum value of hydrothermal pretreatment. When the temperature of hydrothermal oxidation pretreatment rose to 260 ℃, the crystallinity finally decreased to none. The mechanical property of the pellets is found to be positively correlated with the change trend of the cellulose crystallinity and the crystalline cellulose is the key component for the biomass pelletizing process. The hydrothermal and hydrothermal oxidation pretreatments can significantly enhance the heating value, the comprehensive combustion characteristic index and combustion stability index of the resulted WS pellets. Compared with hydrothermal pretreatment, the hydrothermal oxidation pretreatment can promote the carbonization degree of the resulted samples. In sum, the quality and combustion performance of the fuel pellet can be improved by the pretreatment. These results can supply some support for the production of good quality WS pellets.

With the gradual development of large-scale and intensive animal husbandry in China, a large number of concentrated discharge of livestock and poultry manure has posed a serious threat to the environment, and the rational disposal of the waste can effectively avoid environmental pollution. As the traditional disposal methods, aerobic composting and anaerobic fermentation have their own limitations. As a more reasonable technical route, hydrothermal treatment technology has attracted more and more attention because it is beneficial to the high-value and harmless utilization of livestock and poultry manure. Based on the above background, the swine manure with the largest yield was selected as raw material, and the hydrothermal residue was prepared by 180～260 ℃ hydrothermal treatment (the residence time is 0 min) with the solid-liquid ratio at 1:12 (in dry basis). The effects of the hydrothermal treatment on the moisture content, hydrothermal residue yield, fuel characteristics and combustion performance of the hydrothermal residue were studied by means of elemental analysis, industrial analysis and thermogravimetric experiment. Based on the experimental results, the hydrothermal treatment system was simulated by Aspen plus software and the influence of hydrothermal temperature on the steam supply and demand of the system is explored. The results show that the hydrothermal treatment can improve the dehydration performance of the hydrothermal residues. The moisture content of the wet hydrothermal residue decreases gradually with the increasing hydrothermal temperature. Hydrothermal treatment improves the quality of the residue as fuel. Hydrothermal treatment reduces the hydrogen-carbon ratio and oxygen-carbon ratio of the raw material, which deepens the carbonization degree of the raw material, and the fuel performance of the residue is similar to that of lignite. The hydrothermal treatment increases the ignition temperature of the residue and reduces the combustion temperature range, which is beneficial to the application of the hydrothermal residue as fuel. In the hydrothermal treatment system, part of the flash steam produced by the pressure relief of the swine manure after hydrothermal treatment was used for the drying of wet hydrothermal residue, and the rest was used for the raw swine manure preheating in the early stage to increase the temperature of the raw swine manure and reduce the moisture content of the wet hydrothermal residue into the furnace. The output steam of the swine manure hydrothermal treatment system at 230 ℃ is the largest. Therefore, the energy consumption of swine manure hydrothermal treatment system at 230 ℃ was analyzed. The hydrothermal treatment system for 12500 kg swine manure was designed under 230 ℃. The energy generated by the system can not only meet the consumption of the hydrothermal process, but also supply 833.49 MJ for external heating.