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  • 2016 Volume 36 Issue 10
    Published: 15 October 2016
      
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  • NOx Emission Optimization of a 1 000 MW Tower Boiler During Medium and Low Load Operation
    WANG Huajian, ZHAO Sinan, FANG Qingyan, ZHANG Cheng, WEI Tongsheng, ZHOU Hongguang, CHEN Gang
    2016, 36(10): 765-772.
    Abstract ( ) Download PDF ( )   Knowledge map   Save
    To solve the problem of high NOx emission commonly occurring at furnace outlet of large-scale coal-fired boilers during medium and low load operation, numerical studies were conducted on the flow, heat transfer, combustion and pollutant emission in a 1 000 MW ultra supercritical tower boiler, following which the problem was tackled by changing the damper openings of circumferential air and reducing the content of running oxygen. Results show that the simulation data agree well with actual measurements. During medium and low load operation, when the damper opening of circumferential air is reduced, the stoichiometric ratio is to be reduced in the initial period of combustion, while the NOx emission concentration would be reduced accordingly by 31.2% and 17.0% respectively at 70% and 50% unit load. The NOx emission concentration can also be reduced by appropriately lowering the volumetric fraction of running oxygen, when the burnout rate of pulverized coal would basically keep constant.
  • Study on Structure Optimization and Heat-transfer Characteristics of High-temperature Convective Heating Surface in Pressurized Oxy-fuel Boilers
    GAO Zhengyang, ZHAO Hang, FAN Junhui, YIN Libao, LIAO Yongjin
    2016, 36(10): 773-780.
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    Taking a 300 MW coal-fired boiler as an object of study, the properties of flue gas formed in pressurized oxy-fuel combustion were studied at 1 MPa with CO2 to O2 volumetric ratio of 70:30, while the heat-transfer characteristics of high-temperature reheater and superheater were numerically simulated using Fluent software combined with radiation model DO at different flue gas velocities, to which heat-transfer optimization was carried out by iterative method, and subsequently an analysis was conducted on parameter variation of above heat exchangers. Results show that with the rise of flue gas flow rate, the convective heat-transfer coefficients of structurally-optimized high-temperature superheater and reheater gradually increase, but their radiative heat-transfer coefficients generally reduce, and the heat-transfer area required for high-temperature superheater and reheater reduces accordingly under pressurized oxy-fuel conditions, with lowering variation amplitude. The dimensional parameters of optimized heat-exchangers decrease, such as the flue height, flue width, horizontal pitch and tube size, etc., meaning less material consumption, whereas the pressure drop of flue gas rises significantly, resulting in increased auxiliary power consumption.
  • Modelling of Turbine Heat Rate Based on Krill Herd Algorithm and Its Application
    NIU Peifeng, CHEN Ke, MA Yunpeng, ZHAO Qingchong, LI Guoqiang
    2016, 36(10): 781-787.
    Abstract ( ) Download PDF ( )   Knowledge map   Save
    To accurately predict the heat rate of steam turbine, a model was established with the sample data of a 600 MW supercritical steam turbine unit in a thermal power plant using opposition adaptive krill herd algorithm (OAKH) and fast learning network (FLN), of which the prediction results were compared with that of basic FLN model and those FLN models whose parameters were optimized by particle swarm optimization, biogeography-based optimization and krill herd algorithm. Results show that compared with other algorithms and models, the model of turbine heat rate based on OAKH algorithm has a higher accuracy in prediction and stronger capability in parameter optimization and generation, which may help to accurately and effectively predict the heat rate of steam turbines.
  • Dynamic Analysis of a Rotor with Coupling Faults of Crack and Rub-Impact Under Nonlinear Oil-film Force
    XIANG Ling, GAO Xueyuan, ZHANG Lijia, DI Weiwei
    2016, 36(10): 788-794.
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    A nonlinear dynamic model was established for the rotor with coupling faults of crack and rub-impact, considering the time-varying rigidity of crack, rub-impact force and nonlinear oil-film force, which was solved by numerical integration method. Meanwhile, the bifurcation diagrams, poincaré maps, axis orbit and the largest Lyapunov exponent (LLE) were used to analyze the effects of non-dimensional crack depth and rotating speed on the system response, system stability and the rub-impact force in both qualitative and quantitative ways. Results indicate that, the system has undergone diverse nonlinear motions, such as 2T-periodic motion, 4T-periodic motion, 8T-periodic motion, quasi-periodic motion and chaos. As the non-dimensional crack depth increases, the speed of the first bifurcation rises, and the time reaching the critical speeds of quasi-periodic motion and chaos advances. Besides, with the increase of non-dimensional crack depth, the quasi-periodic motion evolves into alteration of chaos and multi-periodic motion in the high-speed area. Moreover, the influence of crack depth on rub-impact force varies in different speed areas, which becomes particularly noticeable in the high-speed area.
  • Study on Segmented Model for Fouling Monitoring of Tri-sectional Rotary Air Heaters
    YUE Junfeng, DING Jianliang, XU Ligang, ZOU Lei, ZHANG Enxian, GUAN Shipian, HUANG Yaji, CHEN Bo
    2016, 36(10): 795-800.
    Abstract ( ) Download PDF ( )   Knowledge map   Save
    To realize the segmented monitoring of ash fouling respectively in hot and cold section of a tri-sectional rotary air heater, a segmented model was set up based on the heat transfer model of air heater and by defining the utilization coefficient as the cleanliness factor, of which the calculation steps were determined according to the heat balance principle and the real-time operating data. Taking the tri-sectional rotary air heater in a 1 000 MW ultra supercritical once-through boiler as an example, variation trends of cleanliness factor in both the hot and cold section were analyzed during soot formation and blowing period. Results show that the variation trends of segmented cleanliness factor calculated by the model can reflect actual fouling situation in hot and cold section during stable load operation of the air heater; however, in variable load operation, the data may have some deviations, which would hardly bring problems, since soot-blowing is rarely performed under varying load conditions. The segmented model may therefore serve as a reference for ash fouling monitoring in hot and cold section of tri-sectional rotary air heaters.
  • Influence Mechanism of Axial Spacing on Stall Inception of a Two-stage Axial-flow Fan
    ZHANG Lei, YUAN Wei, WANG Songling, ZHANG Qian
    2016, 36(10): 801-809.
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    Based on throttle valve function and SST k-ω turbulence model, the unsteady flow process in stall condition of an axial-flow fan with adjustable vanes in a certain power plant was numerically studied, so as to analyze the influence mechanism of three different axial spacings on the initiation position, form and development of the stall. Results show that the stable operation range of axial flow fan is widened with decreasing axial spacing; all the stall inceptions are in the same form of spike type under different axial spacings, which firstly appears in the tip region of second rotor; with the rise of axial spacing, the time taken by stall inception from the second rotor to the first rotor increases; when the axial spacing is relatively small, the formation of separation vortex is resulted from the interaction between main flow and trailing edge back flow, whereas when the axial spacing is relatively large, the formation of separation vortex is related to the leakage flow from tip clearance.
  • CFD-based Numerical Simulations on Optimization of the Ammonia Spraying Strategy in a 1 000 MW SCR Denitrification System
    SUN Hong, HUA Wei, HUANG Zhijun, SUN Shuanzhu, YU Zhijian, DUAN Lunbo
    2016, 36(10): 810-815.
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    Based on actual velocity field and NOx concentration distribution on the inlet cross section of selective catalytic reduction (SCR) system in a 1 000 MW coal-fired boiler, CFD-based numerical simulations were carried out to determine the outlet parameters by integrating the turbulence flow, multiple species transport and chemical reactions in the SCR system. Subsequently, an optimal ammonia injection strategy was acquired through repeated calculations according to the consistent distribution theory of ammonia-nitrogen ratio, which was validated through on-site experiments. Results show that spraying ammonia uniformly under the uneven inlet conditions would bring out prominent deviation of ammonia-nitrogen ratio at the inlet and of ammonia distribution at the outlet, causing serious NH3 escape. On-site validation experiments prove the strategy to be effective in optimizing ammonia spraying, which helps to reduce the blindness of ammonia injection adjustment and improve the on-site efficiency of the system.
  • Study on the Technology of Low-temperature Flue Gas Denitrification by Carbon-based Catalyst
    LI Bo, HAI Yunlong, XING Deshan, CHENG Wenyu, YAN Weiping
    2016, 36(10): 816-821.
    Abstract ( ) Download PDF ( )   Knowledge map   Save
    Denitrification performance tests and intrinsic kinetics research of activated coke were conducted in experimental environment, during which, the effects of following factors on the denitration efficiency of activated coke were analyzed, such as the oxygen concentration, water content and the ratio of nitrogen to ammonia, etc., while the mechanism of activated coke denitration technology was explored. Results show that the denitration efficiency increases with the rise of oxygen concentration; when the oxygen concentration gets up to 8%, the denitration efficiency achieves a higher value, which would basically keep unchanged as the oxygen concentration continues to increase. The presence of water has an inhibiting effect on the activity of activated coke. Higher content of ammonia does not help to improve the denitration efficiency, and may even lead to secondary pollution. The optimum ratio of ammonia to nitrogen oxides is between 1.1 and 1.2.
  • Modeling and Operation Optimization of Wet Flue Gas Desulfurization Systems
    HAO Runlong, ZHAO Yi, GUO Tianxiang
    2016, 36(10): 822-826.
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    To solve the problem of high energy consumption existing in wet limestone-gypsum desulfurization process, mathematical models were set up for the booster fan, slurry circulating pump and oxidation fan by studying the working conditions of the gas-gas heater (GGH), demister, booster fan, slurry circulating pump and oxidation fan, etc., and based on the theory of fluid mechanics. With these models, characteristic curves between the differential pressure of GGH/demister and the system blocking rate were got for the desulfurization system in a certain 600 MW coal-fired power plant, while relations between the total resistance coefficient and the blocking rate of GGH/demister were obtained. Results show that when the blocking rate is over 0.3, the GGH and demister should be swept, and the total resistance coefficient should be kept below 0.003 during operation. Through optimal combination of slurry circulating pumps, energy saving of the wet flue gas desulfurization (WFGD) system and operation optimization of the booster fan could be realized; the operation optimization of WFGD system could also be achieved by optimizing the oxidation fan according to the variation of SO2 mass concentration and flue gas flow.
  • Study on Key Parameters of a Supercritical Fossil-fired Power System with CO2 Recompression and Reheat Cycles
    ZHANG Yifan, WANG Shengpeng, LIU Wenjuan, CHEN Yunan, WANG Yueming, LI Hongzhi
    2016, 36(10): 827-833.
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    A mathematical model was established for the supercritical fossil-fired power system with CO2 recompression and reheat Brayton cycles, based on which the effects of following key parameters on the cycle efficiency were analyzed through detailed calculation with programs developed on the Fortran platform, such as the split ratio of flow, the inlet and outlet pressure of compressor, inlet temperature of turbine etc. Results show that the cycle efficiency increases linearly with the temperature rise of primary and secondary working medium. Different from traditional Rankine cycles, above parameters in Brayton cycles show non-monotonic relationship with the cycle efficiency due to the features of spercritical CO2 physical properties and the constraints of minimum temperature difference for heat exchange. There exists an optimum combination of compressor inlet pressure, compressor outlet pressure and split ratio for supercritical CO2 Brayton cycles, in which case, the cycle efficiency reaches the maximum.
  • Numerical Study of New Air-cooling Units
    CHENG Youliang, REN Zemin, ZHANG Ning
    2016, 36(10): 834-841.
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    To solve the problems existing in the "Λ" shaped air cooling unit and enhance the cooling effectiveness of air-cooled condensers, numerical simulations were conducted on the hyperboloid and frustum cone-shaped two new air-cooling units using Fluent software by taking the 600 MW power generation set in Longshan Power Plant as the comparison object, so as to obtain its temperature and velocity field, and analyze the influence of porosity of top baffle on the heat-transfer effectiveness of the air-cooling unit. To achieve good heat-transfer effectiveness, the power generation set was structurally optimized. Results show that both the new air-cooling units are better than the "Λ" shaped one, which are able to reduce the back pressure of the steam turbine and lower the coal consumption of the generation set, and when the porosity of top baffle is 0, optimum effects can be obtained; the frustum cone-shaped air-cooling unit is better than the hyperboloid one not only in heat-transfer effectiveness but also in safety and economy.
  • Effects of Swirl Vane on Flow Field and Dehumidification Characteristics of a Chimney
    DAI Liping, ZHOU Qiang, YAO Shigang, ZHANG Hui, WANG Xiaodong
    2016, 36(10): 842-848.
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    Simulation and analysis were conducted on the flow field and droplet trajectory in a boiler chimney with and without swirl vane using CFD software combined with RANS equations, RNG k-ε turbulence model and discrete phase model, based on which, the effects of following factors on the flow field and dehumidification efficiency of the chimney were analyzed, such as the vane elevation, installation height and stage number, etc. Results show that by comprehensively considering the pressure loss and dehumidification effect, the optimal swirl vane would be of the single-stage kind, with a vane elevation of 45° and an installation height of 30 m.
  • Microstructure and Mechanical Properties of 617B Welded Joint for Test Platform of 700℃ Critical Components
    WANG Shuo, DIAO Wangzhan, YANG Quan, TAN Shuping, OU Haiyan
    2016, 36(10): 849-852.
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    Welding procedure tests of alloy 617B were conducted respectively by hot wire and manual TIG process, and the welded joints were subsequently researched by tensile, bending and hardness test, while the microstructure of welded joints was analyzed by means of optical microscope. Results show that the welded joint of alloy 617B has good ductility, and its mechanical properties including hardness values are within allowable range. The microstructure in base metal and heat affected zone is of the single austenite type, and good thermal stability is found in the heat affected zone; the grain size is found not to grow near the fusion line and the width of heat affected zone is narrow; the microstructure of the weld metal has the typical cellular dendrite morphology. The 617B alloy can be successfully applied to the test platform for 700℃ critical components by appropriately controlling the interpass temperature and welding parameters without post weld heat treatment.