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Effect of temperature on Lu’an bituminous char structure evolution in pyrolysis and combustion
Yandi ZHANG, Yinhe LIU, Xiaoli DUAN, Yao ZHOU, Xiaoqian LIU, Shijin XU
Frontiers in Energy. 2021, 15 (1): 14-25.
https://doi.org/10.1007/s11708-020-0805-x
In the process of pyrolysis and combustion of coal particles, coal structure evolution will be affected by the ash behavior, which will further affect the char reactivity, especially in the ash melting temperature zone. Lu’an bituminous char and ash samples were prepared at the N2 and air atmospheres respectively across ash melting temperature. A scanning electron microscope (SEM) was used to observe the morphology of char and ash. The specific surface area (SSA) analyzer and thermogravimetric analyzer were respectively adopted to obtain the pore structure characteristics of the coal chars and combustion parameters. Besides, an X-ray diffractometer (XRD) was applied to investigate the graphitization degree of coal chars prepared at different pyrolysis temperatures. The SEM results indicated that the number density and physical dimension of ash spheres exuded from the char particles both gradually increased with the increasing temperature, thus the coalescence of ash spheres could be observed obviously above 1100°C. Some flocculent materials appeared on the surface of the char particles at 1300°C, and it could be speculated that β-Si3N4 was generated in the pyrolysis process under N2. The SSA of the chars decreased with the increasing pyrolysis temperature. Inside the char particles, the micropore area and its proportion in the SSA also declined as the pyrolysis temperature increased. Furthermore, the constantly increasing pyrolysis temperature also caused the reactivity of char decrease, which is consistent with the results obtained by XRD. The higher combustion temperature resulted in the lower porosity and more fragments of the ash.
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循环流化床燃烧准东煤时掺烧油页岩半焦对烟道内沉积灰矿物组成和微观形貌的影响
刘卓, 李建波, Zhu Mingming, 卢啸风, Zhang Zhezi, Zhang Dongke
Frontiers in Energy. 2021, 15 (1): 26-37.
https://doi.org/10.1007/s11708-020-0668-1
本文利用实验室级别的循环流化床(CFB)系统,研究了CFB纯燃准东煤时烟道内受热面上的沉积灰矿物组成和微观形貌特征,并探究了掺烧10wt%和20wt%油页岩半焦对其的影响。为模拟烟道受热面上的灰沉积过程,该系统的烟道内安装了1个垂直(P1)放置和1个水平(P2)放置的取样探针。利用扫描电镜耦合能谱仪(SEM-EDX),X射线衍射仪(XRD),电感耦合等离子体发射光谱仪(ICP-OES)和粒度分析仪对取样探针上的迎风面(P1W, P2W)和背风面(P1L, P2W)沉积灰进行了详细表征。研究发现,当CFB纯燃准东煤时,P1W沉积灰富含CaSO4和Na2SiO3且灰颗粒已经通过烧结形成聚团(<30 μm)。然而,P1L和P2W沉积灰由矿物组成相似但粒径更粗的颗粒或团聚体组成。另外,P2L沉积灰主要由细灰组成,且Na2SiO3和Na2SO4矿物较少。掺烧油页岩半焦后,P1W和P1L上沉积灰中的团聚体显著降低。此时,SiO2和Ca/Na铝硅酸盐成为沉积灰的主要矿物相,而Na2SiO3和Na2SO4消失,沉积灰的粘性因此降低。同样,P2W沉积灰较少的覆盖在取样枪上,表明灰颗粒的沉积倾向降低。这是因为P2W沉积灰中的钠基矿物转变成了高熔点(Na,K)(Si3Al)O8和(Ca,Na)(Si,Al)4O8。ICP-OES分析结果表明,沉积灰中的Na含量,随着油页岩半焦的添加,从32 mg/g降至15 mg/g以下。因此,添加油页岩半焦有助于减轻准东煤CFB燃烧过程中飞灰在烟道内的受热面上的沉积倾向。
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A critical review of ash slagging mechanisms and viscosity measurement for low-rank coal and bio-slags
Md Tanvir ALAM, Baiqian DAI, Xiaojiang WU, Andrew HOADLEY, Lian ZHANG
Frontiers in Energy. 2021, 15 (1): 46-67.
https://doi.org/10.1007/s11708-020-0807-8
Gasification or combustion of coal and biomass is the most important form of power generation today. However, the use of coal/biomass at high temperatures has an inherent problem related to the ash generated. The formation of ash leads to a problematic phenomenon called slagging. Slagging is the accumulation of molten ash on the walls of the furnace, gasifier, or boiler and is detrimental as it reduces the heat transfer rate, and the combustion/gasification rate of unburnt carbon, causes mechanical failure, high-temperature corrosion and on occasions, superheater explosions. To improve the gasifier/combustor facility, it is very important to understand the key ash properties, slag characteristics, viscosity and critical viscosity temperature. This paper reviews the content, compositions, and melting characteristics of ashes in differently ranked coal and biomass, and discusses the formation mechanism, characteristics, and structure of slag. In particular, this paper focuses on low-rank coal and biomass that have been receiving increased attention recently. Besides, it reviews the available methodologies and formulae for slag viscosity measurement/prediction and summarizes the current limitations and potential applications. Moreover, it discusses the slagging behavior of different ranks of coal and biomass by examining the applicability of the current viscosity measurement methods to these fuels, and the viscosity prediction models and factors that affect the slag viscosity. This review shows that the existing viscosity models and slagging indices can only satisfactorily predict the viscosity and slagging propensity of high-rank coals but cannot predict the slagging propensity and slag viscosity of low-rank coal, and especially biomass ashes, even if they are limited to a particular composition only. Thus, there is a critical need for the development of an index, or a model or even a measurement method, which can predict/measure the slagging propensity and slag viscosity correctly for all low-rank coal and biomass ashes.
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Effects of pyrolyzed semi-char blend ratio on coal combustion and pollution emission in a 0.35 MW pulverized coal-fired furnace
Yonghong YAN, Liutao SUN, Zhengkang PENG, Hongliang QI, Li LIU, Rui SUN
Frontiers in Energy. 2021, 15 (1): 78-90.
https://doi.org/10.1007/s11708-020-0678-z
The effects of blend ratio on combustion and pollution emission characteristics for co-combustion of Shenmu pyrolyzed semi-char (SC), i.e., residuals of the coal pyrolysis chemical processing, and Shenhua bituminous coal (SB) were investigated in a 0.35 MW pilot-scale pulverized coal-fired furnace. The gas temperature and concentrations of gaseous species (O2, CO, CO2, NOx and HCN) were measured in the primary combustion zone at different blend ratios. It is found that the standoff distance of ignition changes monotonically from 132 to 384 mm with the increase in pyrolyzed semi-char blend ratio. The effects on the combustion characteristics may be neglected when the blend ratio is less than 30%. Above the 30% blend ratio, the increase in blend ratio postpones ignition in the primary stage and lowers the burnout rate. With the blend ratio increasing, NOx emission at the furnace exit is smallest for the 30% blend ratio and highest for the 100% SC. The NOx concentration was 425 mg/m3 at 6% O2 and char burnout was 76.23% for the 45% blend ratio. The above results indicate that the change of standoff distance and NOx emission were not obvious when the blend ratio of semi-char is less than 45%, and carbon burnout changed a little at all blend ratios. The goal of this study is to achieve blending combustion with a large proportion of semi-char without great changes in combustion characteristics. So, an SC blend ratio of no more than 45% can be suitable for the burning of semi-char.
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Modeling of single coal particle combustion in O2/N2 and O2/CO2 atmospheres under fluidized bed condition
Xiehe YANG, Yang ZHANG, Daoyin LIU, Jiansheng ZHANG, Hai ZHANG, Junfu LYU, Guangxi YUE
Frontiers in Energy. 2021, 15 (1): 99-111.
https://doi.org/10.1007/s11708-020-0685-0
A one-dimensional transient single coal particle combustion model was proposed to investigate the characteristics of single coal particle combustion in both O2/N2 and O2/CO2 atmospheres under the fluidized bed combustion condition. The model accounted for the fuel devolatilization, moisture evaporation, heterogeneous reaction as well as homogeneous reactions integrated with the heat and mass transfer from the fluidized bed environment to the coal particle. This model was validated by comparing the model prediction with the experimental results in the literature, and a satisfactory agreement between modeling and experiments proved the reliability of the model. The modeling results demonstrated that the carbon conversion rate of a single coal particle (diameter 6 to 8 mm) under fluidized bed conditions (bed temperature 1088 K) in an O2/CO2 (30:70) atmosphere was promoted by the gasification reaction, which was considerably greater than that in the O2/N2 (30:70) atmosphere. In addition, the surface and center temperatures of the particle evolved similarly, no matter it is under the O2/N2 condition or the O2/CO2 condition. A further analysis indicated that similar trends of the temperature evolution under different atmospheres were caused by the fact that the strong heat transfer under the fluidized bed condition overwhelmingly dominated the temperature evolution rather than the heat release of the chemical reaction.
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Large eddy simulation of a 660 MW utility boiler under variable load conditions
Haoshu SHEN, Yuxin WU, Minmin ZHOU, Hai ZHANG, Guangxi YUE, Junfu LYU
Frontiers in Energy. 2021, 15 (1): 124-131.
https://doi.org/10.1007/s11708-020-0659-2
Large eddy simulation (LES) has become a promising tool for pulverized coal combustion with the development of computational fluid dynamics (CFD) technologies in recent years. LES can better capture the unsteady features and turbulent structures of coal jet flame than Reynolds averaged Navier Stokes (RANS). The coal-fired power plants in China are now required to be operated in a wide load range and quickly respond to the electric grid. The boiler performance of variable loads should be evaluated in terms of flow, heat transfer, and combustion processes. In this paper, LES was applied to simulate a 660 MW ultra-supercritical boiler under BMCR (boiler maximum continue rate), 75%THA-100, and 50%THA-100 conditions. The predicted gas velocities agree well with the thermal calculation and the temperature error is less than 130 K. The simulation results show that the operation load has significant effects on the boiler performance. It is also proved that LES can provide guidance for the design and operation of advanced coal-fired boilers.
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Industrial-scale investigations on effects of tertiary-air declination angle on combustion and steam temperature characteristics in a 350-MW supercritical down-fired boiler
Xiaoguang LI, Lingyan ZENG, Hongye LIU, Yao LI, Yifu LI, Yunlong ZHAO, Bo JIAO, Minhang SONG, Shaofeng ZHANG, Zhichao CHEN, Zhengqi LI
Frontiers in Energy. 2021, 15 (1): 132-142.
https://doi.org/10.1007/s11708-020-0803-z
Industrial-scale experiments were conducted to study the effects of tertiary air declination angle (TDA) on the coal combustion and steam temperature characteristics in the first 350-MW supercritical down-fired boiler in China with the multiple-injection and multiple-staging combustion (MIMSC) technology at medium and high loads. The experimental results indicated that as the TDA increased from 0° to 15°, the overall gas temperature in the lower furnace rose and the symmetry of temperature field was enhanced. The ignition distance of the fuel-rich coal/air flow decreased. In near-burner region, the concentration of O2 decreased while the concentrations of CO and NO increased. The concentration of NO decreased in near-tertiary-air region. The carbon in fly ash decreased significantly from 8.40% to 6.45% at a load of 260 MW. At a TDA of 15°, the ignition distances were the shortest (2.07 m and 1.73 m) at a load of 210 MW and 260 MW, respectively. The main and reheat steam temperatures were the highest (557.2°C and 559.4°C at a load of 210 MW, 558.4°C and 560.3°C at a load of 260 MW). The carbon in fly ash was the lowest (4.83%) at a load of 210 MW. On changing the TDA from 15° to 25°, the flame kernel was found to move downward and the main and reheat steam temperatures dropped obviously. The change of TDA has little effect on NOx emissions(660–681 mg/m3 at 6% O2). In comprehensive consideration of the pulverized coal combustion characteristics and the unit economic performance, an optimal TDA of 15° is recommended.
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MPC-based interval number optimization for electric water heater scheduling in uncertain environments
Jidong WANG, Chenghao LI, Peng LI, Yanbo CHE, Yue ZHOU, Yinqi LI
Frontiers in Energy. 2021, 15 (1): 186-200.
https://doi.org/10.1007/s11708-019-0644-9
In this paper, interval number optimization and model predictive control are proposed to handle the uncertain-but-bounded parameters in electric water heater load scheduling. First of all, interval numbers are used to describe uncertain parameters including hot water demand, ambient temperature, and real-time price of electricity. Moreover, the traditional thermal dynamic model of electric water heater is transformed into an interval number model, based on which, the day-ahead load scheduling problem with uncertain parameters is formulated, and solved by interval number optimization. Different tolerance degrees for constraint violation and temperature preferences are also discussed for giving consumers more choices. Furthermore, the model predictive control which incorporates both forecasts and newly updated information is utilized to make and execute electric water heater load schedules on a rolling basis throughout the day. Simulation results demonstrate that interval number optimization either in day-ahead optimization or model predictive control format is robust to the uncertain hot water demand, ambient temperature, and real-time price of electricity, enabling customers to flexibly adjust electric water heater control strategy.
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Numerical study of thermal characteristics of double skin facade system with middle shade
Shaoning LIU, Xiangfei KONG, Hua YANG, Minchao FAN, Xin ZHAN
Frontiers in Energy. 2021, 15 (1): 222-234.
https://doi.org/10.1007/s11708-017-0480-8
Architectural shade is an effective method for improving building energy efficiency. A new shade combined with the double skin façade (DSF) system, called middle shade (MS), was introduced and developed for buildings. In this paper, a 3D dynamic simulation was conducted to analyze the influence of MS combined with DSF on the indoor thermal characteristics. The research on MS for DSF involves the temperature, the ventilation rate, the velocity distribution of the air flow duct, and the indoor temperature. The results show that the angle and position of the shade in the three seasons are different, and different conditions effectively enhance the indoor thermal characteristics. In summer, the appearance of MS in DSF makes the indoor temperature significantly lower. The indoor temperature is obviously lower than that of the air flow duct, and the temperature of the air flow duct is less affected by MS. The influence of the position of blinds on indoor temperature and ventilation rate is greater than the influence of the angle of blinds. According to the climate characteristics of winter and transition season, in winter, early spring, and late autumn, the indoor temperature decreases with the increase of the position of blinds at daytime, but the opposite is true at night. The results found in this paper can provide reference for the design and use of MS combined with DSF in hot summer and cold winter zone.
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Impact of “ultra low emission” technology of coal-fired power on PM2.5 pollution in the Jing-Jin-Ji Region
Xiao LIU, Zhilin LIU, Weidong JIAO, Xuan LI, Jintai LIN, Anthony KU
Frontiers in Energy. 2021, 15 (1): 235-239.
https://doi.org/10.1007/s11708-017-0518-y
In response to severe haze pollution, the Chinese government has announced a series of policies focusing on controlling emissions from coal consumption. “Ultra-low emission” (ULE) technologies have the potential to dramatically reduce emissions from coal-fired power plants, and have been deployed at some facilities in recent years. This paper estimated the potential environmental benefits of the widespread adoption of ULE in the Jing-Jin-Ji Region. Atmospheric modeling scenarios were analyzed for three cases: a “standard” scenario assuming no ULE deployment, a “best case” scenario assuming complete adoption of ULE across all power plants in the region, and a “natural gas” scenario, assuming emissions factors consistent with natural gas-fired power generation. The simulations show that the widespread adoption of ULE technologies can be an effective and economically competitive option for reducing the impacts of coal-fired power generation on air quality.
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Core designing of a new type of TVS-2M FAs: neutronics and thermal-hydraulics design basis limits
Saeed GHAEMI, Farshad FAGHIHI
Frontiers in Energy. 2021, 15 (1): 256-278.
https://doi.org/10.1007/s11708-018-0583-x
One of the most important aims of this study is to improve the core of the current VVER reactors to achieve more burn-up (or more cycle length) and more intrinsic safety. It is an independent study on the Russian new proposed FAs, called TVS-2M, which would be applied for the future advanced VVERs. Some important aspects of neutronics as well as thermal hydraulics investigations (and analysis) of the new type of Fas are conducted, and results are compared with the standards PWR CDBL. The TVS-2M FA contains gadolinium-oxide which is mixed with UO2 (for different Gd densities and U-235 enrichments which are given herein), but the core does not contain BARs. The new type TVS-2M Fas are modeled by the SARCS software package to find the PMAXS format for three states of CZP and HZP as well as HFP, and then the whole core is simulated by the PARCS code to investigate transient conditions. In addition, the WIMS-D5 code is suggested for steady core modeling including TVS-2M FAs and/or TVS FAs. Many neutronics aspects such as the first cycle length (first cycle burn up in terms of MWthd/kgU), the critical concentration of boric acid at the BOC as well as the cycle length, the axial, and radial power peaking factors, differential and integral worthy of the most reactive CPS-CRs, reactivity coefficients of the fuel, moderator, boric acid, and the under-moderation estimation of the core are conducted and benchmarked with the PWR CDBL. Specifically, the burn-up calculations indicate that the 45.6 d increase of the first cycle length (which corresponds to 1.18 MWthd/kgU increase of burn-up) is the best improving aim of the new FA type called TVS-2M. Moreover, thermal-hydraulics core design criteria such as MDNBR (based on W3 correlation) and the maximum of fuel and clad temperatures (radially and axially), are investigated, and discussed based on the CDBL.
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