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  • 2014 Volume 34 Issue 6
    Published: 15 June 2014
      
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  • Effects of Separator Performance on Dynamic Balance of Bed Material During Startup of CFB Boilers
    MA Suxia,GUO Jun,WU Weihong
    2014, 34(6): 417.
    Abstract ( )   Knowledge map   Save
    A dynamic model was set up for bed material balance in CFB boilers during startup process, so as to study the influence of separator efficiency, critical size d99 and cut size d50 on the bed material size distribution during and after dynamic balancing period, as well as on the startup time. Results show that the smaller the d50 and d99, the higher the separation efficiency for particles of different sizes, the finer the bed material, the higher the quality of bed material; the dynamic balancing time increases with the reduction of d50. The effect of d99 on dynamic balance of bed material is reflected by the mass fraction of particles in initial bed materials with the size ranging from d99 to the upper limit of circulating ash particles; the larger the mass fraction, the shorter the dynamic balancing time.
  • Effects of Oxygen Volume Fraction on Induction of Coal Spontaneous Combustion
    ZHOU Yonggang,LI Pei,NI Shian,QIAN Haiping,ZHAO Hong
    2014, 34(6): 421.
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    To analyze the internal mechanism of coal spontaneous combustion influenced by oxygen volume fraction from the point of view of reaction energy barrier, experimental tests were carried out to study the effects of oxygen volume fraction on the lowtemperature oxidation of Menghun coal at different reaction stages, and to study the transition characteristics of active energy in spontaneous combustion process according to chemical dynamic theory by slow pyrolysis using thermobalance. Results show that the oxygen volume fraction affects little in dewatering and weight loss process. Both the oxidation process and combustion process change little for the oxygen volume fraction varying from 5% to 15%, and the reactivity enhances rapidly when the oxygen volume fraction increasing from 15% to 20%. The maximum reaction rate and heat output under the oxygen volume fraction of 5% are respectively 11% and 60% of that under the oxygen volume fraction of 20%. In initial oxidation stage, the active energy under oxygen volume fractions of 5%, 10% and 15% is respectively 2.40, 2.11 and 1.56 times of that under the oxygen volume fraction of 20%. The spontaneous combustion has selfacceleration quality when the oxygen volume fraction is in 15%20%, with the continuous reduction of active energy in oxidation process. When the oxygen volume fraction varies from 5% to 10%, the coal oxidation will be inhibited due to higher active energy in final oxidation stage than in middle oxidation stage.
  • Experimental Study on Combustion Characteristics of Wucaiwan Coal
    LI Hong,WANG Guifang
    2014, 34(6): 427.
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    Physical and chemical properties of Wucaiwan coal, Wudong coal and their mixtures with different blending ratios were respectively analyzed, including the ignition temperature test for pulverized coal, the combustion and slagging tests in a onedimensional furnace, and the test for contamination properties of coal ash with a 200 kW contamination test facility, while the ignition, burnout and slagging properties of above coal mixtures were evaluated. Final evaluation was performed to main properties of the coal samples using expert system, and the coal ash contamination property was verified with two contamination criteria, based on which suggested design and checking coal were proposed. Results show that the coal mixtures have strong slagging tendency; Wucaiwan coal has strong contamination tendency, while Wudong coal has low contamination tendency. The contamination tendency of Wucaiwan coal can be reduced by blending Wudong coal. The suggested design and checking coal are both mixtures of Wucaiwan and Wudong coal respectively with blending ratios of 7∶3 and ∶5.
  • Study on Morphology Changes of Element Fe During Coal Ash Fusion Based on XAFS Method
    YANG Junbo,FAN Haojie,LIU Junjie,YANG Rundong,ZHANG Zhongxiao
    2014, 34(6): 432.
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    The fusion process of ash samples respectively with high and low ash fusion point was observed under reducing atmosphere with the Xray absorption fine structure (XAFS) spectrum of Shanghai Synchrotron Radiation Facility (SSRF). Results show that during fusion process, the valence state of Fe changes from Fe3+ to Fe2+; for ash samples with high fusion point, the conversion generally happens at the temperature ranging from 900 ℃ to 1 300 ℃, with a Fe transition from tetrahedral to octahedral coordination; whereas, for those samples with low fusion point, tetrahedral Fe forms firstly, which transforms to octahedral Fe later on; these morphology changes happen before fusion process; for ash samples with low fusion point, significant changes are observed in FeO structure of the firstlayer coordination at the moment of complete fusion, while those with high fusion point, no evident change is observed in the coordination structure.
  • Transition Behavior of Main Elements in Fly Ash During High Alkali Coal Combustion
    DAI Baiqian,WU Xiaojiang,ZHANG Zhongxiao
    2014, 34(6): 438.
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    To study the fouling and slagging properties of fly ash from combustion of Xinjiang high alkali coal, XRF and SEMEDX analysis were carried out to ash samples taken from various heating surfaces in a 300 MW subcritical boiler burning high alkali coal. Results show that in the gas temperature range of 8001 100 ℃, mass fractions of CaO, MgO and SO3 in ash samples are relatively high; whereas for in temperature range of 600800 ℃, mass fractions of Na2O, Fe2O3 and SO3 are relatively high, in which case serious fouling on hightemperature convection heating surfaces is caused by Na2SO4, CaSO4 and NaFeSO4 during combustion of high alkali coal. For locations where the gas temperature is in the range of 669915 ℃, both the initial melting temperature of ash samples and corresponding liquid slag ratio are higher than those in other places. Na2SO4 particles with sizes of 24 μm rich in Na and S are found to stick on surface of ash sample C, which at high temperatures have high sticky properties and low melting temperatures, and therefore may lead to serious fouling problems.
  • Study on Coupled Vibration Between Longblade Bending and Shafting Torsion of Steam Turbine
    L Fangming,WEI Qi,LU Luyi,L Kai,WANG Kun,SUN Qing,LI Wangfan,WANG Xiujin
    2014, 34(6): 443.
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    To investigate the coupled vibration between longblade bending and shafting torsion of turbogenerator sets, three finite element models were developed according to the geometry and materials of a certain steam turbine, which were respectively the bladedisc model, reduced shaft model and the coupling cascadeshafting model, of which the vibration characteristics were analyzed under static and service conditions. Results show that three kinds of vibration modes exist in the coupling cascadeshafting model, namely the blade mode, shafting mode and the coupling mode, reflecting the effects of blade bending vibration on the shafting torsion. The vibration characteristics of blades in the coupling cascadeshaft model are slightly different from that in the bladedisc model, due to the effect of shafting torsion. For the purpose of blade safety, the static and dynamic frequency of blades should be considered simultaneously during product design, taking the vibration characteristics as the basis, with focus on the coupled vibration of shafting torsion and long blade bending, so as to avoid the risk of resonance.
  • Numerical Simulation on Turbulent Flow in a Dualelbow Channel Behind Butterfly Valve
    HUI Wei,LIU Yingzheng,WANG Shaofei
    2014, 34(6): 450.
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    The complex turbulent flow in a dualelbow channel behind butterfly valve was systematically studied using computational fluid dynamics, during which the influence of inlet Reynolds number and valve disc inclination angle on the flow pattern was analyzed. By comparing the torque values between calculated results and experimental data, the SST turbulence model was adopted for the subsequent simulations. Results show that the inlet Reynolods number has quantitative influence on the flow field; with the rise of Reynolds number, the torque value increases, the front stagnation point of valve disc does not change, and the size of the two separation regions in the downstream reduces. The inclination angle of valve disc has qualitative influence on the flow field, which can be explained by the flow separation, the secondary flow and the flow shear & collision theories; among all the four inclination angles selected, the larger the inclination angle, the stronger the blocking effect of valve disc on the flow, when the front stagnation point moves gradually toward the upstream face center, and two flow separation regions occur; when the inclination angle varies from 30° to 45°, corresponding flow parameters change rapidly, and accordingly two obvious flow separation regions appear.
  • PIV Flow Field Measurement for Rotor Blade Cascade of a Variable Geometry Turbine
    MA Chao,ZANG Shusheng,HUANG Minghai
    2014, 34(6): 458.
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    IV experimental studies were carried out to a variable geometry turbine at different guide vane angles, during which detail data concerning the flow field and vorticity field in the passage of rotor blade cascade and downstream areas were obtained and analyzed. Results show that in the process of guide vane angle changing from 6° to -6°, the maximum flow velocity at outlet section and in flow passage of the rotor blade are increased respectively by 11% and 40.6%, the impellor output power is increased by 42%, and the flow angle changes monotonically; however, with the reduction of guide vane angles, flow loss in the passage downstream the rotor blade rises accordingly.
  • Twodegreeoffreedom IMCbased Water Level Control for Nuclear Steam Generators
    FANG Fang,XIONG Ying,CHEN Kai,CHENG Ran
    2014, 34(6): 463.
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    An internal model control (IMC) strategy with twodegreeoffreedom structure was proposed to ensure the control performance over full operating range for water level in unclear steam generators (SG). Considering the impact of steam flow fluctuation on the water level, a feedforward controller was added in the control structure to improve the disturbance rejection property, and to achieve good global control performance for all power levels, the gainscheduling method was utilized to combine the feedback and feedforward controller together under five typical conditions, and subsequently parameter tuning rules were presented based on theoretical analysis of the control system. Simulation results show that the proposed method has good control performance for all power levels, which is better than adaptive neural network control (ACNFC) and selftuning PID control.
  • Thermal System Fault Diagnosis in Loadvarying Dynamic Process Based on PSO Algorithm and Nearest Neighbor Classifier
    WANG Xiaoxia,MA Liangyu,QI Zaishan
    2014, 34(6): 469.
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    A novel approach based on particle swarm optimization (PSO) algorithm and nearest neighbor classifier is proposed for fault diagnosis of power plant thermal systems in loadvarying dynamic process, which applies improved particle swarm optimization (IPSO) algorithm to generate ideal set of typical fault prototypes, and uses evolutionary Elman neural network to predict target values of fault feature variables, and subsequently calculates realtime fault symptoms with predicted and real values of the feature variables. Fault diagnosis can then be implemented by calculating the similarity between realtime fault symptoms and generated fault prototypes with the nearest neighbor classifier. Taking the feedwater heater system of a 600 MW power unit as an example, fault diagnosis tests were carried out with its fullscope simulator. Results show that via the proposed method, good diagnosis results may be achieved for thermal system of power plants during loadvarying dynamic process.
  • Operation Status of SCR Denitrification System for Domestic Power Boilers
    LI Debo,LIAO Yongjin,XU Qisheng
    2014, 34(6): 477.
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    Field tests were carried out to main performance parameters of SCR denitrification system in typical domestic power plants, including the flue gas temperature field, velocity field, flue gas volume, denitrification efficiency and ammonia escape rate, etc. Results show that the distribution of both the outlet NO concentration and the inlet flow velocity is uneven; obvious positive correlation exists between the deviation of inlet velocity and that of outlet NO concentration; the nonuniformity of flow field distribution and the ageing of catalysts are the main factors affecting the performance of SCR denitrification system in domestic power plants.
  • Study on Simultaneous Desulfurization and Demercurization with CaBr2 in a 300 MW Coalfired Power Plant
    SHI Xiaohong,WEN Wubin,XUE Zhigang,LIU Xin,CHAI Fahe,ZHI Guorui,REN Yanjun,MA Jinghua,GAO Wei
    2014, 34(6): 482.
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    To reduce the mercury emission from coalfired power plants, CaBr2 solution was added to the coal to improve the conversion proportion of elemental mercury into oxidized mercury, and therefore to promote the efficiency of simultaneous desulfurization and demercurization. Relevant mercury control tests were performed to a 300 MW coalfired power plant. Results show that the mass fraction of oxidized mercury is to be raised significantly from 35% to 90% in the total gaseous mercury when the brominecoal ratio is 20 mg/kg; under current flue gas desulfurization conditions, higher demercurization efficiencies can be obtained in the case of the brominecoal ratio being within 50100 mg/kg, and the highest demercurization efficiency is to be reached when the brominecoal ratio is 100 mg/kg.
  • Effects of NO and HCl Components in Flue Gas on the Mercury Adsorption by Activated Carbon Fiber
    REN Jianli,HU Yanjun,SHEN Caiqin,PING Chuanjuan,YAN Mi
    2014, 34(6): 487.
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    The activated carbon fiber (ACF) was modeled as a single layer grapheme with four member rings, based on which the adsorption mechanism of mercury on ACF surface influenced by NO and HCl components in flue gas was studied using density functional theory B3LYP of quantum chemistry and lan12dz basis set. Results show that once the NO and HCl are adsorbed, they are strongly bound to ACF cluster surface in the form of ACF—NO and ACF—Cl. The binding energies of ACF—NO and ACF—Cl are found to be 119.48 kJ/mol and 348.68 kJ/mol respectively, indicating strong chemisorption between them. NO and HCl are directly chemisorbed by active sites and homogeneously distributed on ACF surface, where stable bonds are founded, active sites on ACF surface are increased, and therefore the chemisorption of elemental mercury is promoted.
  • Prediction Model for Penetration Depth of Laser Keyhole Welding
    WAN Haibo，LI Qiang，LENG Xiaochun
    2014, 34(6): 495.
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    A prediction model for penetration depth of laser keyhole welding was developed by dynamically analyzing the evolution of front keyhole wall and considering the factors, such as the power density distribution of laser beam and the plasma absorption, etc., with which the penetration depth of an austenite stainless steel weld joined by Yb fiber laser was predicted for different welding velocities, and subsequently a comparison was made between the predicted values and experimental measurements. The model was further used to study the relationship between the penetration depth and the defocusing distance. Results show that the penetration depth predicted by the model agrees well with that of actual measurement, which increases with rising negative defocusing distance and reducing positive defocusing distance.