Average Production Rate Research Articles

Equilibrium analysis of dissolved oxygen in Lake Wuliangsuhai during ice-covered period

为探明中国北方浅水湖泊乌梁素海冰封期水体溶解氧平衡的内在机理，于2021年1—2月在湖心处布设了一台水质在线监测浮标，收集到包括溶解氧等在内的水质数据. 通过对溶解氧数据的小波降噪处理，结合气象资料，模拟分析了冰生长及稳定期内水体溶解氧的变化趋势，定性分析了水体溶解氧的平衡机理. 结果表明：湖泊的日均最高产氧速率为7.19 mg/(L ·d)，最低产氧速率为2.01 mg/(L ·d); 日均最高耗氧速率为7.13 mg/(L ·d)，最低耗氧速率为2.37 mg/(L ·d). 24 h的单位时间平均最高产氧速率为0.55 mg/(L ·h)，最低产氧速率为0 mg/(L ·h); 单位时间平均最高耗氧速率为0.36 mg/(L ·h)，最低耗氧速率为0.08 mg/(L ·h). 由此说明小时间尺度下溶解氧的补充消耗不均衡导致了大时间尺度下的溶解氧不平衡，进而产生了冬季湖泊的亏氧现象. 通过进一步溶解氧驱动因素与水环境因子响应关系的分析发现，浊度、水温与产氧速率呈显著负相关，叶绿素a与产氧速率和耗氧速率均呈显著正相关，表明了这些限制性水环境因子在一定程度上影响了冰下水体的溶解氧平衡.;In order to probe into the internal mechanism of dissolved oxygen (DO) equilibrium in the shallow Lake Wuliangsuhai in Northern China during the ice-covered period, a set of online water monitoring buoys were arranged in the center of lake for collection of water quality data from January to February, 2021. Upon the wavelet denoising processing of DO and based on the meteorological data, the DO changes during the growth and stability period of ice was simulated and analyzed, followed by a qualitative analysis of the mechanism of the DO equilibrium. Results show as follow: The lake's maximum daily average oxygen production rate was 7.19 mg/(L ·d); the minimum was 2.01 mg/(L ·d). The maximum daily oxygen consumption rate was 7.13 mg/(L ·d) and the minimum was 2.37 mg/(L ·d). The maximum 24-hour per-unit-time average oxygen production rate was 0.55 mg/(L ·h) and the minimum was 0 mg/(L ·d). The maximum per-unit-time average oxygen consumption rate was 0.36 mg/(L ·h) and the minimum was 0.08 mg/(L ·h). This study indicates that the disequilibrium between the DO production and consumption on small time scales results in DO disequilibrium on large time scales, causing oxygen deficiency in the lake in winter. Further analysis on the relationship between DO driving factors and water environment factors suggests a negative correlation between turbidity, water temperature and oxygen production rate and a positive correlation between chlorophyll-a and oxygen production and consumption rate, which indicates that these restrictive water environmental factors influence the DO equilibrium in water body under ice.

Variation characteristics of dissolved oxygen and metabolic rate in lakes during the ice-covered period

冰封期湖泊与大气的气体交换受冰盖阻碍，影响湖泊溶解氧含量，进而影响湖泊水质.为探究冰封期湖泊溶解氧和新陈代谢速率变化特征及影响因素，本研究通过监测典型季节性冰封湖泊不同深度的溶解氧（DO）、水温和光合有效辐射（PAR），结合水质检测结果，分析冰封期湖泊DO变化及代谢速率影响因素，对湖泊日新陈代谢速率计算方法进行改进并估算冰封期湖泊新陈代谢速率.结果表明：2021年1—3月间，岱海DO浓度平均值为15.49 mg/L，并出现昼夜变化和分层现象.DO在冰封期变化趋势呈现出先逐渐升高，然后保持稳定，进入消融期（2021年3月2—11日）分层现象逐渐消失的规律.岱海冰封期净初级生产力和生态系统呼吸速率平均值分别为0.11和0.10 mg/（L·d），在水温未出现分层时净初级生产力呈现较高水平；之后水温沿水深方向出现分层，净初级生产力明显下降；当冰层融化后，PAR显著上升，净初级生产力又逐渐恢复至0.10 mg/（L·d）.统计分析表明，岱海冰封期DO与水温、PAR、总氮等变化具有一定相关关系，且由于湖泊流域生态环境条件及冰封期物候特征不同，岱海与内蒙古其他湖泊相比，冰封期DO变化趋势存在一定差异.;In ice-covered periods, the gas exchange between lakes and the atmosphere is blocked by ice sheets. The dissolved oxygen concentration and consequently the water quality of the lakes are thus affected. To explore the metabolic rates and variation characteristics of the dissolved oxygen, and its influencing factors in typical seasonal ice-covered lakes, dissolved oxygen concentration, water temperature and photo-synthetically active radiation (PAR) were monitored at different depths of the lakes. In combination with the water quality monitoring results, the variations of the dissolved oxygen and the influencing factors of the metabolic rates were analyzed to optimize the calculation methods of the everyday metabolic rates, thus to estimate the metabolic rates of the ice-covered lakes. The results showed that the average dissolved oxygen concentration in Lake Daihai was 15.49 mg/L, and diurnal variation and stratification had appeared. The dissolved oxygen concentration initially increased gradually and then stabilized from Jan. to Mar. 2021, the ice-covered period. The stratification phenomena disappeared from Mar. 2nd to 11th, 2021, the thawing period. The average net primary productivity and ecosystem respiration rate of Lake Daihai were 0.11 mg/(L·d) and 0.10 mg/(L·d) respectively in the ice-covered period. The net primary productivity appeared to be higher when the water temperature was not stratified. Along with the rise of the water temperature, the water temperature began to stratify along the depth of water, and the net primary productivity decreased significantly. After the ice had melted, the PAR increased significantly, and the net primary productivity gradually increased back to 0.10 mg/(L·d). Statistical analysis demonstrated that water temperature, PAR and total nitrogen had exerted significant effects on the dissolved oxygen concentration during the ice-covered period in Lake Daihai. Due to the different ecological environment conditions and phenological characteristics of the ice-covered period in the lake basin, compared with other lakes in Inner Mongolia, Lake Daihai has certain differences in the trend of DO changes during the ice-covered period.

Accelerating gas production of the depressurization-induced natural gas hydrate by electrical heating

Natural gas hydrate (NGH) will be one of the major future energy sources due to its properties of clean energy and large reserves. Depressurization is proposed as an effective method to extract natural gas from hydrate, however, the gas production from hydrate dissociation may be interrupted by ice generation and hydrate re-formation due to insufficient heat supply in the single depressurization process. To solve this issue, this work conducted simulation on accelerating gas production from the depressurization-induced methane hydrate by electrical heating. The continuous heating and intermittent heating modes were employed and then the electrical heating scheme was optimized for the comprehensive effect of high energy efficiency and high gas production rate. The results show that electrical heating is conducive to gas production from hydrate dissociation at a rapid rate. In the continuous heating, a high initial hydration saturation, low initial water saturation, low specific heat capacity, and high thermal conductivity result in the high gas generation rate and efficient electrical energy utilization (a large energy efficiency ratio). The intermittent heating has a higher efficient utilization of electrical energy than continuous heating. The optimal scheme is determined as the first-half heating type with the optimized electrical heating power of 25.6 W and the heating time of 12.5 min by the gradient descent method of AdaGrad. Compared to the baseline continuous heating case, the energy efficiency ratio (10.70) of the optimal scheme is enhanced by 24.7% with the average gas production rate (2.55 SmL/s) enhanced by 18.2%. It's hoped that the findings of this work can provide some insights into extracting natural gas from gas hydrate deposits. Electrical heating is employed to provide heat for the endothermic dissociation of methane hydrate induced by depressurization for accelerating natural gas production in this work. The electrical heating scheme is optimized to achieve high-efficient utilization of electrical energy for the fast extraction of natural gas from hydrate dissociation. • Electrical heating accelerates the depressurization induced dissociation of hydrate. • The effects of phase components and porous media properties are analyzed. • Intermittent heating has higher efficiency of energy utilization than continuous heating. • The electrical heating power is optimized by the gradient descent method of AdaGrad. • The optimal scheme is first-half heating with the optimized electrical heating power.

Radiogenic heat production from natural radionuclides in sediments of the Tigris river in Mosul City, Iraq

In this paper, the concentration of the activity of natural radionuclides is represented by 238U, 232Th and 40K. This concentration was measured by gamma spectroscopy, which was used to determine and calculate radioactive heat production for all radionuclides and their distribution pattern in the sediment samples from the Tigris River in Iraq. The results showed that the activity concentration of these radionuclides was within the allowed limits by the UNSCEAR. Field observations show that the sediments occurred due to the weathering of pre-existing sedimentary rocks that make up the area's geology. The estimated average radiogenic heat production rate was 0.3634 µW/m³. It ranged from 0.2430 to 0.6453 µW/m³, and within the internationally permissible limits, the radiogenic heat production rate in Tigris River sediments was low. It is recommended to conduct a comprehensive radiometric survey to cover all the areas through which the Tigris River passes in Iraq. It will help characterise the radiogenic heat produced for each geological area.

On Local Optima Distribution in Buffer Allocation Problem for Production Line with Unreliable Machines

In this paper, we consider a buffer allocation problem in manufacturing flow lines with series-parallel network structure where nodes correspond to buffers of finite capacity, and arcs correspond to the machines. The machines are supposed to be unreliable, their time to failure and repair time are assumed to be exponentially distributed. Different machines may have different production rates and the production rates of all machines are assumed to be deterministic. The buffer allocation problem is to determine the capacities of all buffers with respect to a given optimality criterion, which is a function of the average production rate of the line, the buffer acquisition and installation cost and the inventory cost. In search for the optimum, the tentative solutions are evaluated by means of an approximate method based on the Markov models aggregation. We carry out computational experiments with the local search and genetic algorithms. It turns out that the “massif central” or “big valley” structure of the fitness landscape is present but only partially: The fitness of the local optima is negatively correlated with the distance to the best found solution, yet the set of local optima can not be encompassed by a ball of relatively small radius. Moreover, we show that in many problem instances, several clusters of local optima can be identified. The symmetries of the fitness function are discussed and suggested as the possible cause of the local optima clustering. Finally the performance of genetic algorithms is bfiefly discussed with respect to solutions clustering.

Techno-Economic Performance of Closed-Loop Geothermal Systems for Heat Production and Electricity Generation

• Closed-loop geothermal systems evaluated for heat and electricity production • Slow heat transfer in rock and limited heat transfer area pose challenges • Targeting high temperatures, existing wells, and multi-laterals beneficial • Relatively high levelized costs for greenfields, unless drilling at low cost Closed-loop geothermal systems, recently referred to as advanced geothermal systems (AGS), have received renewed interest for geothermal heat and power production. These systems consist of a co-axial, U-loop, or other configuration in which the heat transfer or working fluid does not permeate the reservoir but remains within a closed-loop subsurface heat exchanger. Advocates indicate its potential for developing geothermal energy anywhere, independent of site-specific geologic uncertainties, and with limited risk of induced seismicity. Here, we present a technical and economic analysis of closed-loop geothermal systems using a Slender-Body Theory (SBT) model, COMSOL Multiphysics simulator, and the GEOPHIRES analysis tool. We consider a number of different scenarios and evaluate the influence of variations in reservoir temperature (100 to 500℃), well termination depth (2 to 4 km), mass flow rate (10 to 40 kg/s), injection temperature (10 to 40℃), fluid type (liquid water vs. supercritical carbon dioxide), design configuration (co-axial vs. U-loop), and degree of reservoir convection (natural, forced or conduction-only). The resulting average heat production rates range from about 2 to 15 GWh per year for cases considering a co-axial design and from 9 to 67 GWh per year for cases with a U-loop design. Assuming generous economic and operating conditions, estimates of levelized cost of heat range from ∼$20 – $110 per MWh (∼$6 – 32/MMBtu) and ∼$10 – $70 per MWh (∼$3 – $20/MMBtu) for greenfield co-axial and U-loop cases, respectively. In the scenarios in which electricity generation is considered, annual electricity production ranged between 0.12 and 7.5 GWh per year at a levelized cost of electricity from roughly $83 to $2,200 per MWh. In all scenarios, the results exhibit a large rapid drop in production temperature after initiation of operations that levels off to a steady value significantly below the initial reservoir temperature. Operating at lower flow rate increases the production temperature but also lowers the total heat production. The consistently low production temperatures hinder efficient electricity generation in most cases considered. Natural or forced convection can increase thermal output but requires sufficiently high reservoir permeability or formation fluid flow. As expected, overall system costs are heavily dependent on drilling costs; hence, repurposing existing wells could significantly lower capital and levelized costs. In comparison with other types of geothermal systems, our results for closed-loop geothermal systems predict long-term production temperatures considerably below the initial reservoir temperature, and relatively high levelized costs for greenfield closed-loop geothermal systems, particularly for electricity production, unless significant reductions in drilling costs are obtained.

Social and Political Drivers Affecting Wheat Production in Nigeria

Social and political factors are believed to be affecting agricultural operations in a way that stagnates their growth or leads to the total collapse of the system. A study on social and political factors affecting wheat crop production in Nigeria was conducted with the aim of exploring those factors for proper solutions. Primary data was collected through national stakeholders’ focus group discussion while time series data of the country's wheat production, harvested area, and imports was collected. MAXQDA statistical software was used to analyze the focus group discussion report, while compound growth rate analysis was used to compare the growth rate of the variables under study. The results indicated the extent to which political factors affect wheat production in Nigeria more than others. The degree to which external social factors affect the sector was less when compared with political factors and more than internal social factors. Non-adoption of recommended agronomic practices and the knowledge level of the farmers were the major internal social factors. While the major external social factors were consumer food habits, consumer demand for convenience, low produce prices, and high input costs. Political factors identified were social security issues, inconsistent government policies, and intricacies in implementation, the role of the publication media, role playing by the milling industries, international trade interests, and lack of political will. The result also indicated that the total average growth rate of production was negative, and positive growth was recorded in the harvested area, with a high percentage recorded in imports. For Nigeria to achieve the desired outcome, focused commitments and the adoption of a multi-dimensional approach are required.

Observation and simulation of HOx radicals in an urban area in Shanghai, China

A continuous wintertime observation of ambient OH and HO2 radicals was first carried out in Shanghai, in 2019. This effort coincided with the second China International Import Expo (CIIE), during which strict emission controls were implemented in Shanghai, resulting in an average PM2.5 concentration of less than 35 μg/m3. The self-developed instrument based on the laser-induced fluorescence (LIF) technique reported that the average OH radical concentration at noontime (11:00–13:00) was 2.7 × 106 cm−3, while the HO2 concentration was 0.8 × 108 cm−3. A chemical box model utilizing the Regional Atmospheric Chemical Mechanism 2 (RACM2), which is used to simulate pollutant reactions and other processes in the troposphere and which incorporates the Leuven isoprene mechanism (LIM1), reproduced the OH concentrations on most days. The HO2 concentration was underestimated, and the observed-to-modelled ratio demonstrated poor performance by the model, especially during the elevated photochemistry period. Missing primary peroxy radical sources or unknown behaviors of RO2 for high-NOx regimes are possible reasons for the discrepancy. The daytime ROx production was controlled by various sources. HONO photolysis accounted for more than one half (0.83 ppb/h), and the contribution from formaldehyde, OVOCs and ozone photolysis was relatively similar. Active oxidation paths accelerated the rapid ozone increase in winter. The average ozone production rate was 15.1 ppb/h, which is comparable to that of a Beijing suburb (10 ppb/h for the ‘BEST-ONE’) but much lower than that of Beijing's center (39 ppb/h in ‘PKU’ and 71 ppb/h in ‘APHH’) in wintertime. Cumulative local ozone based on observed peroxy radicals was five times higher than the value simulated by the current model due to the underprediction of HO2 and RO2 under the high-NOx regime. This analysis provides crucial information for subsequent pollution control policies in Shanghai.

Skewness and kurtosis in stochastic thermodynamics

The thermodynamic uncertainty relation is a prominent result in stochastic thermodynamics that provides a bound on the fluctuations of any thermodynamic flux, also known as current, in terms of the average rate of entropy production. Such fluctuations are quantified by the second moment of the probability distribution of the current. The role of higher order standardized moments such as skewness and kurtosis remains largely unexplored. We analyze the skewness and kurtosis associated with the first passage time of thermodynamic currents within the framework of stochastic thermodynamics. We develop a method to evaluate higher order standardized moments associated with the first passage time of any current. For systems with a unicyclic network of states, we conjecture upper and lower bounds on skewness and kurtosis associated with entropy production. These bounds depend on the number of states and the thermodynamic force that drives the system out of equilibrium. We show that these bounds for skewness and kurtosis do not hold for multicyclic networks. We discuss the application of our results to infer an underlying network of states.

Rapid dissociation and on-site saturation evaluation of methane hydrate sediment samples for natural gas hydrate exploitation

Natural gas hydrate is a kind of clean energy with huge reserves, and the saturation (volume percentage of hydrate in pore space of sediments) is the key parameter for determining whether the reservoir is worthy of exploitation. In this work, rapid hydrate dissociation by the combination of heat injection and NaCl inhibitor addition was studied, and an on-site evaluation method for hydrate saturation in sediment samples was proposed by using a core sampler to transfer hydrate samples under pressure. The results showed that the average gas production rate per unit volume was increased significantly to reach 7.22 L/Lr·min-1 by the injection of NaCl aqueous solution with 50.9 °C, which was attributed to the increase of the chemical potential to further accelerate the rate of hydrate dissociation in the presence of NaCl. Furthermore, for the measurement of methane hydrate samples saturation with a volume of 673 cm 3 (which contained 1.4 mol hydrates with the saturation of 58%), hydrate saturation could be accurately achieved within 30 min with a relative error lower than 11.7% This work may provide new thoughts for on-site saturation evaluation and rapid dissociation of hydrate samples during natural gas hydrate exploitation.

Investigation of Energy Recovery from Municipal Solid Waste: A Case Study of Al-Karak City / Jordan

Abstract Municipal solid waste (MSW) from Al-Karak city is monitored and characterized. The MSW is sorted into 14 different waste categories. In addition, the MSW is separated into several size fractions, followed by sorting and characterization. The average daily production rate of waste is 61.5 ton/day. The MSW generated per capita is estimated to be 0.55 kg/capita/day. The main input MSW comprised a modest proportion of organic content 38.8 %. Other combustible fractions are present in significant proportions, allowing the waste to be utilized for energy recovery. Refuse-derived fuels (RDFs) may be made from waste fractions > 100 mm and 50 – 100 mm.

Comparing production and life-history traits of a key amphipod species within and between estuaries under different levels of anthropogenic pressure

Inter-population variability may arise as a response to adverse natural and anthropogenic stressors. The dynamics of a key amphipod species (Echinogammarus marinus) was followed during ≈ 1-year at its southerly range (NW Portugal), in three estuaries with different levels of anthropogenic pressure (High - Ave estuary; Moderate - Mondego estuary; Low - Minho estuary). We hypothesised that E. marinus populations would present lower production and fitness, and higher intersexuality incidence with increasing anthropogenic pressure. According to a GAM model explaining ≈70% of the observed variability, E. marinus biomass depends on temperature, organic matter, Fucus area and time of the year. Significant differences were found between the gammarid biomass in Minho and Mondego estuaries and within sites from the same estuary. As expected, Ave estuary exhibited the lowest average annual production, abundance and fecundity rates. However, the highest average production was found in Mondego and not in Minho estuary, although the turnover ratio (P/B) of both estuaries was very similar.Besides the system's global ecological status, E. marinus also seems to respond to microhabitat conditions, which may explain the spatial heterogeneity observed in the amphipod production within the same estuary. Intersexuality prevalence was negligible in the three populations, which does not support the idea of a link between anthropogenic pressure and intersexuality in E. marinus. We argue that the dependence of E. marinus on Fucus sp. should be further investigated to fully understand the role of both fucoids and the gammarid in coastal foodwebs.

Enhancing the photocatalytic activity of Ruddlesden-Popper Sr2TiO4 for hydrogen evolution through synergistic silver doping and moderate reducing pretreatment

The photocatalytic hydrogen (H2) generation relies heavily on the properties of photocatalysts, including their light absorption, charge transport, and oxygen vacancy of photocatalysts. In this work, we have successfully synthesized Ruddlesden-Popper perovskite Sr2TiO4 as photocatalysts for H2 evolution reaction. Importantly, we adopted a facile and effective strategy of Ag doping and reducing pretreatment to tailor the properties of Sr2TiO4 photocatalysts. The synergistic effect of Ag doping and moderate reducing pretreatment confers Sr2TiO4 photocatalyst with improved morphological characteristics, increased specific surface area, optimized light absorption capability, and bandgap, suppressed charge carrier recombination, as well as an increased amount of surface oxygen vacancies. Benefiting from these merits, r-Ag0.05Sr1.95TiO4 with a suitable amount of Ag doping and moderate reducing pretreatment displays an excellent photocatalytic performance for H2 generation, which presents average H2 production rates of 1695 μmol h−1 g−1 and 541 μmol h−1 g−1 under full-range light irradiation (λ ≥ 250 nm) and visible light irradiation (λ ≥ 400 nm), respectively.

Towards high light conversion efficiency from photo-fermentative hydrogen production of Arundo donax L. By light–dark duration alternation strategy

Suitable illumination project would help in achieving high light conversion efficiency (LCE) for photo-fermentation. This study proposed an improvement strategy for LCE of photo-fermentative hydrogen production (PFHP) with a photosynthetic consortium by adopting light–dark duration alternation. For this purpose, 6 projects (continues light, 24 h light + 24 h dark, 24 h dark + 24 h light, 48 h light + 48 h light, 48 h dark + 48 h light, and continues dark) light disturbances were carried out to estimate the strategy. The fluctuation of cell growth (OD660) was corresponded to the light–dark alternation. 24 h dark + 24 h light alternation achieved the maximum hydrogen yield (HY) of 390.9 mL/g TS cell (6.7 % higher than continuous light) and maximum improvement of LCE of 114.7%. Moreover, heat map analysis revealed that the light period after inoculation had the closest relation (Pearson’s r = 1) with the average hydrogen production rate (HPR) of photo-fermentation. Besides, decreased dark period after inoculation would increase the hydrogen yield of photo-fermentation.

Enhanced Morphological Preservation and Redox Activity in Al-Incorporated NiFe2O4 for Chemical Looping Hydrogen Production

Although chemical looping technology has enabled energy-efficient H2 production by combining separation and reaction through the lattice oxygen in oxygen storage materials (OSMs), metal oxides undergoing successive reduction–oxidation cycles generally suffer from a decline in long-term performance. This work investigates an Al-incorporated Ni-ferrite (NiFe2O4) particle to demonstrate a detailed deactivation mechanism induced by redox stress and suggests a strategy to prolong the lifespan of the particle. It is revealed that the grain coalescence and surface densification hamper the redox performance by decreased electrical conductivity and retarded gas transfer, eventually leading to deactivation over the cycles. The formation of a spinel solid solution with aluminum (Al) incorporation was found to be an effective strategy to prevent densification and maintain the long-term performance. Al incorporation at 3.3 wt % is determined as the best particle that can maintain the highest H2 yield (8.2 mmol H2·g–1) and average production rate (0.41 mmol H2·g–1·min–1) for 11 cycles.

High-rate biohydrogen production from xylose using a dynamic membrane bioreactor

This study aimed a high-rate dark fermentative H2 production from xylose using a dynamic membrane module bioreactor (DMBR) with a 444-μm pore polyester mesh. 20 g xylose/L was fed continuously to the DMBR at different hydraulic retention times (HRTs) from 12 to 3 h at 37 °C. The maximum average H2 yield (HY) and H2 production rate (HPR) at 3 h HRT were found to be 1.40 ± 0.07 mol H2/mol xyloseconsumed and 30.26 ± 1.19 L H2/L-d, respectively. The short HRT resulted in the maximum suspended biomass concentration (8.92 ± 0.40 g VSS/L) along with significant attached biomass retention (7.88 ± 0.22 g VSS/L). H2 was produced by both butyric and acetic acid pathways. Low HY was concurrent with lactic acid production. The bacterial population shifted from non-H2 producers, such as Lactobacillus and Sporolactobacillus spp., to Clostridium sp., when HY increased. Thus, xylose from lignocellulose is a feasible substrate for dark fermentative H2 production using DMBR.

Geomechanical Response Induced by Multiphase (Gas/Water) Flow in the Mallik Hydrate Reservoir of Canada

Summary Natural gas hydrate (NGH) is regarded as an important alternative future energy resource. In recent years, a few short-term production tests have been successfully conducted with both permafrost and marine sediments. However, long-term hydrate production performance and the potential geomechanical problems are not very clear. According to the available geological data at the Mallik site, a more realistic hydrate reservoir model that considers the heterogeneity of porosity, permeability, and hydrate saturation was developed and validated by reproducing the field depressurization test. The coupled multiphase and heat flow and geomechanical response induced by depressurization were fully investigated for long-term gas production from the validated hydrate reservoir model. The results indicate that long-term gas production through depressurization from a vertically heterogeneous hydrate reservoir is technically feasible, but the production efficiency is generally modest, with the low average gas production rate of 4.93 × 103 ST m3/d (ST represents the standard conditions) over a 1-year period. The hydrate dissociation region is significantly affected by the reservoir heterogeneity and reveals a heterogeneous dissociation front in the reservoir. The depressurization production results in significant increase of shear stress and vertical compaction in the hydrate reservoir. The response of shear stress indicates that the potential region of sand migration is mainly in the sand-dominant layer during gas production from the hydraulically heterogeneous hydrate reservoir (e.g., sand layers interbedded with clay layers). The maximum subsidence is approximately 78 mm and occurred at the 72nd day, whereas the final subsidence is slowly dropped to 63 mm after 1-year of depressurization production. The vertical subsidence is greatly dependent on the elastic properties and the permeability anisotropy. In particular, the maximum subsidence increased by approximately 81% when the ratio of permeability anisotropy was set at 5:1. Furthermore, the potential shear failure in the hydrate reservoir is strongly correlated to the in-situ stress state. For the normal fault stress regime, the greater the initial horizontal stress is, the less likely the hydrate reservoir is to undergo shear failure during depressurization production.

Research on the Enhanced Oil Recovery Technique of Horizontal Well Volume Fracturing and CO2 Huff-n-Puff in Tight Oil Reservoirs.

The low oil productivity of tight oil reservoirs is mainly caused by poor reservoir physical properties such as low porosity and permeability, a small pore and throat ratio, and bad well connectivity, which lead to bad water injection capability and rapid pressure decline for the reservoir. In this paper, an enhanced oil recovery technique for tight oil reservoirs is proposed by combination of horizontal well volume fracturing and CO2 huff-n-puff to improve the reservoir physical properties and increase the flow capability of crude oil. In the initial reservoir development stage, the precise horizontal well trajectory and reservoir volume fracturing scale are designed by analyzing the distribution of sand body thickness and the oily sedimentary facies. The micro-seismic tracking technique is also used to monitor the fracture elongation. When the reservoir energy cannot satisfy the economic limit of oil productivity, the CO2 huff-n-puff technique is applied to increase the reservoir energy quickly. After the precise fracturing technique is used in tight oil Block X, the average oil production rate of six fractured horizontal wells increases by 5 ton (1 ton = 7.33 bbl) at the initial production stage, and the effective oil production increase life lasts for 32 months. When the reservoir energy is supplemented using the CO2 huff-n-puff technique, the oil production rate of pilot experiment well SP-1 increases from 1.9 to 12.8 ton with a cumulative oil increase of 1333.8 ton.

(Invited) Bioelectrochemical Systems for Resource Recovery and H2 Production from Wastewater

Wastewaters contain abundant nutrients and energy, which can be recovered by using proper bioelectrochemical systems. We developed a three-chamber resource recovery microbial fuel cell (RRMFC) to treat synthetic urine-containing wastewater with various organic pollutants and simultaneously recover nutrients (e.g., N, P, and S). Over one cycle (∼3 days), 99% of urea, 97% of COD, 99% of histidine, 91% of creatinine, 99% of sodium acetate, 98% of SO4 2−, and 99% of PO4 3− were removed from the wastewater, and simultaneously 42% of total nitrogen, 37% of PO4 3−, 59% of SO4 2−, and 33% of total salts were recovered from the wastewater. We also developed a solar microbial electrolysis cell (solar-MEC) with a Fe2O3 photoanode, a bioanode, and a black-silicon (b-Si) photocathode for H2 production from wastewater without external bias. The solar-MEC had a maximum current density of 0.8 mA/cm2 and an average H2 production rate of 5.1 µmol/h/cm2 under one sun irradiation.

Vacancy-modified g-C3N4 nanosheets via one-step thermal polymerization of thiosemicarbazide precursor for visible-light-driven photocatalytic activity

The graphitic carbon nitride (g-C3N4) with nanosheets structure or carbon vacancies is considered as the promising photocatalysts duo to their fascinating properties. Herein the carbon vacancy modified g-C3N4 nanosheet (TCN-600) was synthesized via one-step thermal polymerization of thiosemicarbazide precursor without auxiliary agent and extra gas protection. The higher calcination temperature and self-generating gas atmosphere (N2H4, CS2 and H2S) during the precursor polymerization impel introduction of carbon vacancies and simultaneous formation of nanosheet structures. The as-prepared TCN-600 exhibits excellent photocatalytic activity for tetracycline (TC) degradation and hydrogen production. TCN-600 shows average hydrogen production rate of 1.86 mmol g−1 h−1, which is 11.6 times higher than that of bulk g-C3N4. Meanwhile, TCN-600 displays high TC degradation efficiency of 83.3% within 90 min. The photodegradation rate of tetracycline by TCN-600 is 5.6 times higher than that by bulk g-C3N4. The significant improvement in photocatalytic activity is mainly attributed to its huge specific surface areas (144.5 m2/g), extended visible-light absorption, negatively shifted conduction band position, longer charge carries lifetime and faster migration of the photo-induced electron-holes. This study provides a new strategy to synthesize g-C3N4 with carbon vacancies and simultaneous nanosheet structure.