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In learning English, learners are faced with various challenges, such as listening, speaking, reading and writing. Among them, writing is the most important and difficult one, and grammatical errors are the most common type of errors in English writing. The research and implementation of grammatical error detection and correction in English writing is of great significance to both English learners and English teachers. In this paper, we propose a sequence annotation model based on recurrent neural network to solve the problem of the influence of grammar errors and other noises in English writing corpus on sequence information extraction. Aiming at the grammatical errors in English writing, this paper proposes two methods of English grammatical error detection and correction. The experimental results show that the accuracy of RNN is higher than that of CAMB after 10 to 20 iterations, ranging from 0.848 to 0.916.

As a new form of education, the language education software based on AI (Artificial Intelligence) technology effectively improves the timeliness and interest of language teaching, and enrich and improve the English education system to a certain degree. Second language acquisition (SLA) theory integrates linguistics, language education, sociology, psychology and other disciplines, focusing on the study of people’s acquisition of foreign languages and the mechanism of language ability. This article is based on teaching practice materials and students’ English learning data, and uses the Noticing Hypothesis theory to try to solve the problems emerged from artificial intelligence teaching courses.

The combination of oxy-fuel and blended-coal combustion may be one of these effective methods to both reduce CO2 emissions and improve energy utilization efficiency in coal-fired power stations. The aim of this study is to investigate oxy-fuel combustion interactions of blended coals under different conditions using a thermo-gravimetric analyzer. The results show that compared with those in an O2/N2 mixture, the promotive and inhibitive effect and the comprehensive interactions are considerably weaker in an O2/CO2 mixture. In the O2/CO2 mixture, both increasing the O2 concentration and decreasing the particle size result in decreasing the promotive effect but increasing the inhibitive effect and the comprehensive interactions, which increase the non-additive combustion characteristics. Enhancement of the heating rate increases the promotive effect but decreases the inhibitive effect and the comprehensive interactions, which weaken the non-additive combustion characteristics. Of these factors, the effects of the oxygen concentration and heating rate on comprehensive interactions are greater than that of particle size. This study provides useful information for the design and optimization of thermo-chemical conversion systems of coal blends in the O2/CO2 atmosphere.

Interactions, known as the ignition promotion and burnout inhibition, exist during the combustion of blended coals in the air. The oxy-fuel conditions influence these interactions due to the change of the gas physical properties and the gasification reaction. However, little research has focused on the oxy-fuel combustion interactions of blended coals. To investigate the combustion interactions of blended coals in an O2/CO2 atmosphere, experimental and numerical studies of the mixed combustion of less- and more-volatile coals are performed in a drop tube furnace. The results show that the more-volatile coal combustion brings the promotive effect on the devolatilization of the less-volatile coal and the inhibitive effect on the char combustion of the less-volatile coal. Compared with that in the air, the promotive effect is weaker, but the inhibitive effect is stronger in the 21%O2/79%CO2 environment due to the different physical properties of the gases. In the O2/CO2, increasing the oxygen concentration increases the promotive effect but weakens the inhibitive effect owing to the change of the physical properties of gases and the reactivity improvement of the less-volatile coal char. In addition, the promotive and inhibitive effects achieve a level similar to that in the air when the oxygen concentration is increased to approximately 30–35%. Furthermore, the char-CO2 gasification reaction in the O2/CO2 has little impact on the promotive effect but weakens the inhibitive effect. Increasing the reactor wall temperature weakens both the promotive effect and the inhibitive effect.

Coal blends are commonly used in thermal power plants and oxy-fuel combustion also attracts great concerns recently. However, little has been done on oxy-fuel combustion of coal blends. In this paper, combustion tests are performed in a drop tube furnace for various coal blends under O2/CO2 mixtures, which are also reproduced numerically. Strong combustion interactions between the parent coals in a blend are observed. The ignition of the low-volatile coal is promoted due to the rapid combustion of the volatiles from the high-volatile coal. However, the char burnout of the low-volatile coal is compromised due to the rapid O2 consumption by the large amount of volatiles. The interactions under oxy-fuel conditions are more sensitive to the excess O2, inlet oxidizer temperature and coal particle size. 1) The increase in the excess O2 tends to weaken the combustion interactions, in which both the ignition promotion and burnout inhibition effects on the low-volatile coal become less apparent. 2) The increase in the inlet oxidizer temperature further promotes the ignition of the low-volatile coal whereas weakens the burnout inhibition of the low-volatile coal. 3) The effects of reducing particle size on the combustion interactions are similar to those of increasing inlet oxidizer temperature. Since biomass often contains a high volatile content, the findings may shed light on the synergistic effects in oxy-fuel co-combustion of biomass and coal.

A novel idea is proposed to improve the SO2 tolerance of low-temperature selective catalytic reduction (SCR) catalysts by using a RHO zeolite membrane to separate NO from SO2 to reduce the contact between SO2 and the SCR catalysts. RHO zeolite membranes were prepared on a macroporous stainless steel tube. The results from XRD and SEM measurements indicated that the crystals on the surface were pure RHO zeolites and a uniform and continuous layer of RHO zeolite membrane covered the surface of the support. Permeation measurements using different feed gases indicated that the prepared RHO zeolite membrane could effectively separate NO from SO2. A maximum NO/SO2 selectivity of 36.14 was obtained when using NO/SO2/N2 mixtures. The SO2 permeation decreased a lot in the present of water. Surface diffusion and activated diffusion were believed to be the main transport mechanisms of the RHO zeolite membrane.

Gas temperature deviation is a common problem for tangentially fired boilers, affecting the boiler operation safety and the boiler generation efficiency. The gas temperature deviation in the tangentially fired π-type boilers has been well studied. However, the gas temperature deviation in the tangentially fired tower-type boilers has yet to be fully understood. This paper develops insights into the causes and controlling strategies of gas temperature deviation in a 660 MW tangentially fired tower-type boiler with obvious steam temperature deviation by experiment and simulation. Firstly, four upper furnace structures are established to fully reveal the causes of gas temperature deviation. The gas temperature deviation is mainly caused by the flue gas rotation, the heating tubes division, and the suction effect of the induced draft fan. More evident recirculation is observed near the right side than the left side in the platen heater region, resulting in that the flue gas deflects to the left side, and the left side region presents higher temperature. Furthermore, the controlling strategies of gas temperature deviation are studied comprehensively by adjusting SOFA (Separated Over-fire Air) horizontal angle and setting SOFA left–right-side velocity offset. Properly SOFA horizontal angle or enhancing the left-side SOFA velocity may effectively reduce the gas temperature deviation. 34–45° for the SOFA horizontal angle and 1.2 for the SOFA left–right-side velocity ratio are recommended. In the actual operation, offsetting the SOFA left–right-side velocity (i. e. SOFA air damper opening degree: left side—80% and right side—50%) is successfully applied to reduce the re-heated steam temperature deviation from 18 K to 1 K without bringing adverse effects under the low load. These deep insights may provide useful information to control the steam temperature deviation in the platen heater zone.

As a highly toxic element of coal, arsenic (As) is mainly converted into As2O3 vapor during coal combustion. Mineral oxides are effective and abundant sorbents to control As2O3 emissions. In this paper, the characteristics of As2O3 adsorption by mineral oxides were studied in a two-stage reactor, and the density of states, adsorption sites, and adsorption energies for the adsorption reaction were investigated using molecular simulation. The results demonstrate that CaO is an excellent As2O3 sorbent. The As2O3 adsorption effect of CaO, MgO, and Al2O3 was found to become better with increasing temperature within the range of 300–900 °C. The order of As2O3 adsorption at 300–700 °C follows CaO > Fe2O3 > Na2O > MgO > Al2O3 > SiO2, and SiO2 hardly adsorbs any As2O3. At 900 °C, the melting of Na2O and Fe2O3 greatly reduces their As2O3 adsorption capacity. The O atoms in Na2O/CaO/MgO/Al2O3/SiO2 and the Fe atoms in Fe2O3 are the adsorption active sites for As2O3. The order of the calculated adsorption energy follows Fe2O3 > Na2O > CaO > Al2O3 > MgO > SiO2, and the As2O3 adsorption involves chemical adsorption. The adsorption products are Ca3(AsO4)2, FeAsO4, AlAsO4, etc., for which As exists in the form of As3+ and As5+, with As3+ transformed into As5+ with increasing adsorption temperature.

The burner tilt angle has a significant influence on the combustion characteristics of a furnace, but few studies have focused on this topic. In this study, a numerical model was developed and validated to investigate the effect of the burner tilt angle on the combustion and NOX emission characteristics of a 700 MWe pulverized-coal-fired boiler with deep-air-staging technology. The turbulence, heat transfer and chemical reactions in the furnace were reproduced in detail by the model. Deep insight into the effects of burner tilt angle on combustion and NOX emission characteristics was acquired. The results show that the turbulence intensity in the burnout zone of furnace increases as the burner tilt angle increases, leading to a significant increase in the reaction rate of CO combustion. Hence, the level of CO emission is continuously reduced. The char burnout rate changes little with increasing burner tilt angle since the kinetic reaction rate and oxygen diffusion rate increase while the particle residence time decreases. The NOX emission increases as the burner is tilted farther upward because of an enhancement in fuel NO formation and a decrease in NO reburning. This study provides useful guidelines for improving the performance of deep-air-staged tangentially coal-fired boilers.

Down-fired boilers are commonly used to burn low-rank coals for power generation. However, they often have operational problems such as low char burnout and high NOx emissions. This paper comprehensively investigates how to mitigate such problems in a 300 MWe down-fired utility boiler burning a low-volatile coal by optimizing the staged-air windbox structure. First, experiments and simulations are conducted on a 1:1-scaled model of the staged-air windbox to study the cold air flow in the original and two new windbox structures for understanding the effect of different staged-air windbox structures. Then, combustion simulations are performed for the utility boiler to further study the influence of the newly designed staged-air windboxes on the boiler performance, based on which the optimization scheme of the staged-air windbox in the boiler is more reliably concluded. Finally, the utility boiler is retrofitted by using a newly designed staged-air windbox. Performance tests are made for the boiler before and after the retrofit. The new staged-air windbox is confirmed to greatly improve the performance of the utility boiler, e.g., NOx reduction by 20.7% and thermal efficiency increase by nearly 2 percentage points (from 88.76% to 90.48%) at full load. The idea of the new staged-air windbox design can be readily implemented into down-fired boilers of this kind for more efficient and environmentally friendly firing of low-rank coal for heat and power generation.

To assure stable ignition and combustion of low-volatile coals in conventionally configured boilers, storage systems are commonly used to separate the ground coal from the air or gas and evaporated moisture prior to the combustion process, and the stored ground coal is then transported by hot air to the combustion process. This study is to develop a novel combustion system, in order to facilitate the firing of different types of coals in existing conventional corner-fired boilers originally configured for low-volatile semianthracite-firing and to improve the boiler performance. Numerical simulation, boiler retrofit and full-scale performance test are conducted. After successful demonstration in a 330 MWe semianthracite corner-fired utility boiler, the novel combustion scheme is implemented in retrofit of two other similar boilers. The simulation and full-scale performance test on all the three boilers show that the use of the new combustion system achieves a significant reduction in NOx emissions and a remarkable increase in boiler efficiency whereas induces no side effect in operation. After retrofit, these boilers can also readily accommodate low-volatile semianthracite (the design coal) and high-volatile bituminous coal (the retrofit coal), largely improving the coal flexibility.

The Melut Basin is an important petroliferous rift basin in Africa, and a large number of structural hydrocarbon accumulations of the Paleogene Yabus and Samma Formations have been discovered in it. However, there are few published research results and exploration discoveries on stratigraphic hydrocarbon accumulations in the Melut Basin. Based on information from the evaluation of the lithology assemblage, oil-source, migration pathway, preservation condition and the detailed analysis of typical exploration cases, the favorable exploratory interval, formation conditions and accumulation characteristics of the stratigraphic hydrocarbon accumulations have been studied in the northern Melut sub-basin. The sedimentary facies study indicated that the Upper Yabus Formation developed widespread delta front deposits with the lithology assemblage of “sand wrapped by mud”, which was conducive to forming depositional stratigraphic traps dominated by facies change or depositional pinchout. The Adar Formation is a set of mudstone deposition with a thickness of 200–600 m above the Yabus Formation and well sealed the Yabus and Samma hydrocarbon accumulations as the regional top seal. The parameters of organic matter, and the thickness and distribution of source rocks showed that the Al Renk Formation has good source rock quality with an average TOC value of 2.08% wt. and type II kerogen prone to oil generation. The thermal modelling revealed that the Al Renk source rocks continuously generated and expelled hydrocarbon since the Late Cretaceous to the present and provided sufficient oil source for the Upper Yabus stratigraphic traps. The tectonic evolution research showed that the multi-stage rifting cycles made the Melut Basin develop numerous normal faults, which provided essential vertical migration pathways for the hydrocarbon charging of the Paleogene stratigraphic traps. Among these faults, the basinal border faults and the intra-basinal late-rifting faults are the primary hydrocarbon conduit faults. The details of exploration cases showed that the hydrocarbon conduit faults are the key controlling factors for the formation of the Upper Yabus stratigraphic hydrocarbon accumulations, and the stratigraphic traps jointly dominated by lithology change (depositional pinchout) and hydrocarbon conduit faults have low hydrocarbon filling risk. The sandstones of the Upper Yabus Formation are shallow (700–2500 m) and have high porosity with a value of greater than 20% and high oil production of a few hundred to more than 1000 barrels per day. Therefore, the Upper Yabus Formation develops favorable petroleum geology conditions for the formation of stratigraphic hydrocarbon accumulations, and exploring such stratigraphic traps is of great significance to the increasing exploration in the mature northern Melut sub-basin.

Mineral oxides are effective in-furnace sorbents used to control lead (Pb) emissions in flue gas at high temperatures. In this paper, the PbO/PbCl2 adsorption characteristics of sorbents were investigated via experimental and density functional theory (DFT) methods. The results show that Si/Al-based compounds can chemically adsorb Pb, and the adsorption is related to the Si-O and Al-O bonds in the sorbents. Exposed Si and O atoms on SiO2 surfaces and exposed Al and O atoms on Al2O3 surfaces are the active sites for Pb adsorption, and PbO is easier to remove than PbCl2. Pb adsorption is promoted in a mixture of SiO2 and Al2O3. Doping Si atoms into Al2O3(100) promotes PbO adsorption, and doped three-coordinate Si atoms have a more obvious promotion effect than doped two-coordinate Si atoms. Doping Al atoms into SiO2(001) has no obvious effect on PbO adsorption. The effect of temperature on Pb adsorption was studied by thermodynamic analysis. The Gibbs free energy difference for PbO adsorption on SiO2(001) increases from −373.04–32.42 kJ/mol as the temperature increases from 300 to 1800 K. High-temperature calcination changes the bond length and bond angle of the system, affecting the stabilities of atomic configurations and decreasing the Pb adsorption capacity.

Coal combustion is one of the major pathways by which arsenic enters the ecological environment. An effective method to control arsenic emissions in-furnace is to transform the arsenic from a vapour to fly ash particles using mineral sorbents. However, flue gas components have a significant effect on gas-phase arsenic adsorption, which limits the industrial application of mineral sorbents. In this paper, the effect mechanism of flue gas components (NaCl/HCl/SO2/CO2) on gas-phase arsenic adsorption over different mineral oxide sorbents was investigated. The results demonstrate that the order of arsenic adsorption is CaO > MgO/Fe2O3> NaCl > Al2O3 > SiO2. NaCl promotes the arsenic adsorption of CaO above 800 °C, and the arsenic removal efficiency of CaO with 5 % NaCl is 52.51 % at 900 °C. NaCl inhibits the arsenic adsorption of MgO and Fe2O3, and promotes arsenic adsorption by Al2O3 and SiO2. Acid gases inhibit arsenic adsorption by the sorbents and the order of the inhibition intensity of acid gases at 700 °C is HCl > SO2 > CO2. The active sites (CaO, FeO, or AlO bonds) in the sorbents are the main reaction sites for arsenic adsorption, and captured arsenic is in the form of AsO21- and AsO43-.

Coal combustion is one of the major pathways by which lead (Pb) enters the ecological environment. An effective method to control Pb emissions in a furnace is to transform the Pb from vapors to particles using Si-Al-based sorbents. However, the high price, poor thermal stability, and difficult molding of sorbents limits their application. Fly ash cenospheres (FACs) prepared from the waste of coal-fired power plants, which are mainly composed of SiO2 and Al2O3, are ideal materials for Pb sorbents because of their high strength, light weight and low price. In this study, the adsorption and reaction mechanisms of FACs and several Si-Al-based sorbents for Pb vapors were investigated in a two-stage fixed-bed reaction system, and the thermal stability and regeneration characteristics of FACs were tested. The results indicate that the adsorption capacity for Pb vapors is FAC > kaolin > SiO2 > γ-Al2O3. The acidic sites, such as Si-O and Al-O bonds in FACs, were the main reaction sites for Pb adsorption and captured Pb in the form of stable Pb compounds (Pb2+). As the temperature increased from 700 to 1200 °C, the Pb adsorbed by FACs increased, and 55.21% of the Pb in the simulated flue gas was removed at 1200 °C. Kaolin is an excellent sorbent for Pb vapors. The adsorption capacity of kaolin for Pb first increases and then decreases with increasing temperature, and the maximum removal rate occurs at 1000 °C, reaching 33.46%. In addition, FACs are thermally stable and reusable. Therefore, the high-temperature application of FACs to remove Pb vapors has good prospects.

A novel combustion system has been applied to a 600 MWe down-fired boiler to reduce NOx emissions without producing an obvious increase in the carbon content of fly ash. The system mainly includes moving fuel-lean nozzles from the arches to the front/rear walls, and re-arranging the staged air, as well as introducing separated-over-fire air (SOFA). This paper evaluates the effects of the SOFA locations (on the arches, on the throat, and on the upper furnace) on the combustion and NOx emissions characteristics using simulations. The numerical results are in good agreement with the measured results. Compared to the original combustion system, significant NOx reduction (approximately 50%) is found for all three SOFA location settings. Taking economic efficiency and NOx emissions into account, the SOFA on the upper furnace is adopted in the actual modification, and no negative effects are observed.

Co-combustion of sludge with coal is a promising sludge disposal approach. In this study, three different combustion models, including different turbulent gas-phase combustion and char surface combustion sub-models, were employed to simulate the combustion and emission characteristics of co-combusting sludge with coal in a 100 MW tangentially coal-fired utility boiler. By comparison of the numerical results from different combustion models to a set of full-scale coal–sludge co-combustion field experiments, a precise and suitable computational fluid dynamic (CFD) model was finally developed. The comparison result demonstrated that the finite-rate/eddy-dissipation model together with the multiple-surface-reaction model, which have carefully considered the effect of high moisture content on the fuel ignition and char combustion processes, were competent in modeling the characteristics of coal–sludge co-combustion. On the basis of the developed numerical model, the ignition, char burnout, and NOx emission cha...

This study attempts to better understand the mechanisms of wind farm blockage effect by analysing momentum balance in realistic atmospheric flow over an idealised large offshore wind farm. The analysis is performed following the two-scale momentum theory, which predicts the importance of three different terms in the farm-scale momentum balance, namely the streamwise pressure gradient, Coriolis force and acceleration/deceleration terms. A numerical weather prediction (NWP) model is used as a realistic farm-scale flow model in this study to investigate how these three terms tend to change in time. Initial results suggest that the streamwise pressure gradient may be enhanced substantially by the resistance caused by the wind farm, whereas its influence on the other two terms appears to be relatively minor. These results suggest the importance of modelling the farm-induced pressure gradient accurately for various weather conditions in future studies of wind farm blockage.

Sodium, calcium, sulfur and chlorine in the flue gas of coal-fired boilers are the main cause of ash deposition/fouling in heat exchangers. However, there are still many debates on the mechanism of ash deposition/fouling due to the lack of direct micro evidence. This paper aims to clarify the initial deposition/fouling characteristics of a high-alkali coal by computer-controlled scanning electron microscopy (CC-SEM). The results demonstrate that temperature has an influence on the deposition/fouling compounds. The crystalline compounds in the samples at 900 °C were mainly CaSO4 and Ca2Al2SiO7, while the compounds in the samples at 500 °C were mainly NaCl. Sulfur had a significant adsorption effect on sodium and calcium at 900 °C, while chlorine had a significant adsorption effect on sodium at 500 °C. A 3-layer microstructure of deposition/fouling was found after the samples were stripped by molecular force. Additionally, deposition/fouling could be divided into three stages: the formation of sodium salt precursors, the growth of loose structural aluminosilicates and the adhesion of particulates on the surface. Sulfates, especially Na2SO4 and CaSO4 formed a precursor for the initial deposition/fouling of high-alkali coal.

To reduce NOx emissions without introducing an obvious increase in the carbon content of fly ash, a novel combustion system was applied to a 600 MWe Foster Wheeler (FW) down-fired boiler. This approach mainly consisted of moving fuel-lean nozzles from the arches to the front/rear walls, rearranging staged air, and introducing separated overfire air (SOFA). The aim of this work was to evaluate the overall performance of the novel combustion system relative to different SOFA ratios (i.e., 15, 20, 25, and 30%) and different SOFA locations in the upper furnace (1.0, 2.0, and 3.0 m above the arches) using numerical simulations and experimental measurements. Both numerical and experimental results showed that, with increasing the SOFA ratio from 15 to 20%, NOx emissions were greatly reduced but the carbon content in the fly ash increased slightly. With a further increase from 20 to 30%, NOx emissions slightly decreased but the carbon content in the fly ash increased substantially. Considering both the environme...