During the process of natural gas hydrate exploitation, the cementation state of the stratum changes due to the decomposition of skeleton hydrate. Fine sand in the reservoir will migrate to the wellbore with the fluid, which is the so-called sand production phenomenon. Sand production will bring great harm to hydrate exploitation. In this paper, a set of macroscopic visualized sand production vessel is designed, which can observe the migration of sand in the sediment layer under different working conditions, and the factors influencing the sand production characteristics are obtained by measuring the amount and the size of sand production. It is found that fine sand will invade and block the gravel layer driven by water and it is accompanied by the collapse and subsidence of the sand layer. Fluid velocity and initial hydrate saturation are important factors affecting the characteristics of sand production. In addition, it is of certain guiding significance to select the appropriate median diameter ratio of sand to gravel for sand control in the actual exploitation process.
Natural gas hydrate formation induced by the leakage of methane was observed in field investigation. The appearance of cold seep might be an indication of natural gas hydrate reservoir. In this study, CH4 hydrate formation in porous media with continuous gas flow was firstly investigated using magnetic resonance imaging. Three flow rates (0.2, 0.4, 0.6 mL/min) were experimentally conducted and the results indicated that the gas flow rate plays a crucial role in the hydrate formation. A high flow rate could reduce the CH4 displacement and hydrate formation time, however, more water was brought out from the sediment during displacement process, finally leading to a low hydrate saturation. It is suggested that for cold seep where gas bubble gushes violently, the hydrate reservoir may not be abundant. Our work can provide some insights of the mechanism of hydrate formation induced by cold seep, making contributions to future cold seep field investigation.
So far, the day-ahead scheduling research of photovoltaic (PV)-based battery swapping stations (BSSs) has not fully considered the uncertainties of swapping demand and PV output. To address this issue, a day-ahead economic scheduling method based on chance-constrained programming and probabilistic sequence operation is proposed in this paper. First of all, a BSS day-ahead scheduling model with chance constraints as swapping demand satisfaction and the confidence level of the minimum cost is established. The confidence level of chance constraints is set by BSS operators. Then, probabilistic sequences of stochastic variables are constructed, and the quantitative index to measure the day-ahead scheduling risk of BSS is proposed based on sequence operation. Thereafter, the feasible solution space is determined based on the battery controllable load margin, and then the fast optimization method for the BSS day-ahead scheduling model is developed by combining the feasible solution space and genetic algorithm (GA). Finally, the validity and applicability of the proposed method is verified in the case study.
Pyrolysis as the key stage of biomass thermochemical conversion processes draws increasing attention in past decades. Developing a kinetic model of biomass decomposition or bio-production is crucial for biomass pyrolysis process optimization and reactor design. In this paper, a novel kinetic model based on Weibull distribution is developed with the assumption of parallel reaction scheme during pyrolysis. The model is solved by an improved differential evolution algorithm and validated by the thermogravimetric analysis data of alpha cellulose, cardboard, and whitepaper. All the predicted results show good agreements with the corresponding experimental data. The present work provides a new model framework of biomass pyrolysis using Weibull distribution.
SNG (solidified natural gas) technology is a promising and potential technology for long-term and large-scale storage of methane in hydrates due to the plethora of benefits it offers in comparison to conventional methods of storage. In this study, we aim to investigate the kinetics of mixed hydrate formation using a ternary C1(93%)-C2 (5%)-C3 (2%) gas mixture representing the natural gas. The main objective is to study the effect of presence of higher hydrocarbons (ethane and propane) in influencing the mixed hydrate formation kinetics with the additional presence of thermodynamic promoter, tetrahydrofuran.
Experiments to study the effect of temperature and pressure on the mixed hydrate formation kinetics in an unstirred reactor configuration were performed. Results from this study will provide salient information in order to advance the SNG technology towards commercial implementation.
In this study, the water distribution in different flow channels of proton exchange membrane (PEM) fuel cell is investigated experimentally using microfocus X-ray tomography. The high-frequency resistance (HFR) and electrochemical impedance spectrum (EIS) are introduced to determine the electrochemical characteristics of a cell with different flow channels. It is found that although the serpentine-serpentine (s-s) flow channel configuration has the best drainage effect, its pumping loss and membrane resistance at low humidity are large. Conversely, the drainage effect of the parallel-parallel (p-p) flow channel is the worst, but its membrane resistance and pumping loss are the lowest. It is indicated that the serpentine-parallel (s-p) flow channel has better drainage capacity and performance than the p-p flow channel as well as lower pumping loss and better membrane hydration than the s-s channel.
In recent years, the subway system has experienced rapid development all over the world. However, the air quality in the station has attracted more public attention. This paper concentrates on the variation of the PM2.5 and PM10 concentrations on platformsin the entire operation periods of the train. The train frequency and piston wind are considered to investigate the influence of passing trains number, the train arrival and departure on the PM2.5 and PM10 concentrations on the platforms. The results show that the diurnal average concentrations of PM2.5 and PM10 on platforms are 81 and 147 μg/m3 , respectively. The maximum concentrations of PM2.5 and PM10 (104 and 173 μg/m3 , respectively) occur in the morning rush hours, when the number of passing trains is the largest. The piston wind can push the polluted air in the tunnel into the platform and increase the PM concentrations on platforms. Moreover, passengers walking can also cause the suddenly sharp increase of PM2.5 and PM10 concentrations when the train stops at the station. It can be deduced thatreasonable ventilation strategy and efficient air filtration system are necessary to be studied in the further research for the improvement of air quality and energy saving of the ventilation and air-conditioning system in subway stations.
Geothermal resources for global new energy development because of its wide distribution, huge reserves and environment-friendly. The enhanced geothermal system (EGS) is one of the key technologies for the extraction and utilization of geothermal energy in high-temperature rock masses in deep formations. The use of supercritical carbon dioxide (S-CO2) as a working medium in EGS has many advantages for heat exploitation. But the change of thermal properties of supercritical carbon dioxide is very rapidly during EGS production, which affects the transport of fluids and rock-fluid heat exchange. To investigate the effect of S-CO2 in EGS, in the present work, we develop the heat and mass transfer models for S-CO2 in EGS based on the Embedded Discrete Fracture Model (EDFM). The simulation results show that the complex fracture network has significant effects on the heat and mass transfer of supercritical carbon dioxide in the reservoir. In addition, S-CO2 not only has higher heating efficiency than water, but also captures and stores carbon dioxide.
The operational flexibility of coal-fired power plants during peak shaving processes becomes an urgent problem with the penetration of intermittent renewable power in the power grid. However, the original control systems for the power units have some deficiency during rapid peak shaving processes. In this paper, a revised control system by regulating high-pressure extraction steam was added to improve the operational flexibility during load change processes. Based on the transient models of a 660 MW supercritical coal-fired power unit, comparison on the original and revised control systems for the main parameters were proposed. It turns out that when compared with the original control system, the revised control system exhibits a better control performance during peak shaving processes, especially for the loading down process. The differences of the output power, live steam pressure, live steam pressure and reheat steam temperature between the original and revised control systems during load down process are −1006.41 MW·s, 21.26 MPa·s, 256.17 °C·s, and 680.17 °C·s, respectively. The study can provide a guidance for the control optimization and flexibility improvement of coal-fired power plants during peak shaving processes.
Pyrolysis is an effective way to convert aquatic plants into high-value products. The pyrolysis oil has high energy density but is difficult to utilize. In order to clean and economically convert the acids in pyrolysis oil into high grade esters, improving the quality of pyrolysis oil, this study chose seawater and freshwater reeds as raw materials, upgrading reed pyrolysis oil through catalytic esterification by reed biochar-based solid acid catalyst and comparing the catalytic effect of reed biochar-based solid acid catalysts with commercial 732 and NKC-9 solid acid catalysts. After analyzing the pyrolysis products of two kinds of reeds, seawater reed pyrolysis oil generated from 600°C (has the highest acid content) was chosen as raw oil to upgrade its quality, and biochar generated from 600°C and 700°C was chosen to produce catalysts. The acid content in upgraded oil was significantly reduced, and ester became the main component of upgraded oil. Among the six catalysts, 732-catalyzed upgraded oil had the highest ester content (21.97%). The 700°C freshwater reed biochar-based catalyst has comparable catalytic effect to that of NKC-9, the ester contents of their upgraded oil were 18.71% and 19.98%, respectively. Catalytic effects were proportional to surface sulfonic acid contents of the catalysts. Compared with raw pyrolysis oil, heat value of upgraded oil increased by 15.16%-76.54%, and viscosity decreased from 11.47Pa·s to about 3Pa·s. The commercial catalysts decreased the pH of upgraded oil to a lower value, but the reed biochar-based solid acid catalyst increased the pH value of upgraded oil to about 5, this was attributed to the fact that reed biochar-based solid acid catalyst has a superior thermal stability than that of the commercial catalysts, so that sulfonic acid group was not easily peeled off after being heated. The obtained results could provide a guidance for the relationship between biomass feedstocks, pyrolysis conditions, catalytic activity of biochar-based solid acid catalyst and upgrading effect of pyrolysis oil.