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  • 2017 Volume 37 Issue 6
    Published: 15 June 2017
      
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  • Numerical Analysis on Low NOx Combustion Near-wall Air Distribution System of an Opposed Firing Boiler for 530 MW Supercritical Units
    DU Zhihua, MENG Yi, SUN Jun
    2017, 37(6): 425-431.
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    To reduce NOx emission of an opposed firing boiler and to prevent high-temperature corrosion of the water wall, a low-NOx combustion retrofit was carried out by adding an air distribution system in the main combustion zone to supply a non-symmetrical rectangular high-speed direct-flow air curtain over the wall. Results show that after retrofit, the measured near-wall atmosphere has been improved significantly with a remarkable increase in volume fraction of near-wall oxygen from 0.3% to 3.0%, while the near-wall gas temperature around front and back wall has been reduced by about 100 K, lowering the risk of high-temperature tube burst of the water wall.
  • NO Emission Characteristics of Typical Coals Under O2/CO2/H2O Atmosphere at Intermediate and High Temperatures
    LEI Ming, HUANG Xingzhi, WANG Chunbo
    2017, 37(6): 432-439.
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    NO emission characteristics of Datong bituminous coal (DT) and Yangquan anthracite (YQ) under O2/H2O/CO2 atmosphere at intermediate and high temperatures were investigated in an isothermal thermal analysis system, and the experimental results were subsequently compared with those under O2/N2 and O2/CO2 atmospheres. Results indicate that there is an obvious peak in the NO release curve of DT under O2/N2 and O2/CO2 atmospheres at 5% oxygen concentration, but the only NO peak turns into two separated ones under O2/CO2/H2O atmosphere. However, when the oxygen concentration gets up to 21%, the two separated peaks turn back into one peak under O2/CO2/H2O atmosphere. The NO release profiles of YQ are similar to those of DT. The NO emission of DT in O2/CO2 is always lower than that in O2/N2. At high temperatures, the NO emission of DT in O2/CO2/H2O is higher than that in O2/CO2 due to H2O gasification at low oxygen concentrations. But as the oxygen concentration increases, the NO emission of DT in O2/CO2/H2O becomes lower compared to that in O2/CO2. The NO emission of YQ is higher than that of DT, but the changing trend is similar to that of DT under different atmospheres.
  • Effects of CO2 Dilution on Flameless Combustion and NO Generation of Different Fuels
    TIAN Hong, LIU Zhengwei, WANG Feifei, HU Zhangmao
    2017, 37(6): 440-446.
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    To study the impact of CO2 dilution on flameless combustion mechanism of different fuels, the effects of CO2 dilution rate on the flameless combustion, NO generation and diffusion flame temperature of following fuels were investigated experimentally and numerically, such as the fuels CH4, C3H8 and H2, etc. Results show that, as the CO2 dilution rate increases, the peak temperature and NO emission decrease, while the distance between the peak temperature point and the burner nozzle increases gradually, and the temperature distribution in the furnace becomes more homogeneous, which makes it more conducive to reach flameless combustion state. For the same dilution rate, the effect of CO2 dilution on the decreasing of peak temperature and NO emission of different fuels from strong to weak is as follows: H2>CH4>C3H8. The combustion in furnace would be in a flameless state as long as the dilution rate of CO2 is high enough.
  • Dynamic Mode Decomposition of the Unsteady Flow in a Vaneless Diffuser
    DING Jie, HU Chenxing, LIU Pengyin, ZHU Xiaocheng, DU Zhaohui
    2017, 37(6): 447-453.
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    Taking the simulation results under design condition 1.8 kg/s and unsteady condition 1.4 kg/s as the basic data, an analysis was conducted on the complex flow field in the vaneless diffuser of a centrifugal compressor using dynamic mode decomposition (DMD), so as to obtain the contours of tangential and radial velocity modes at 10% span plane as well as the characteristic frequencies, and to evaluate the applicability of DMD method in analyzing the unsteady flow within the vaneless diffuser. In addition, flow field reconstruction was conducted at different characteristic frequencies to figure out the transformation of unsteady flow in the diffuser. Results show that at small mass flow rates, the predominant effect of blade passing frequency is suppressed; the eigen mode corresponding to stall does not rotate along the circumferential direction, where three stall cells can be observed, and the frequency of stall is about 193 Hz.
  • Simulation on Dynamic Characteristics of a Cracked Rotor Considering Nonlinear Oil Film Force
    XIANG Ling, GAO Xueyuan
    2017, 37(6): 454-460.
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    Considering the time varying stiffness and nonlinear oil film force, a dynamic model was established for the double disc rotor-bearing system with crack faults, and subsequently numerical integration method was used to solve the model, so as to study the effects of rotating speed, crack depth and unbalance force on the response, instability and bifurcation of the system by using bifurcation diagram, axis orbits, Poincaré maps and cascade spectrum. Results show that when the rotating speed changes, rich nonlinear dynamic behaviors would occur to the system, such as multiple periodic, quasi-periodic and chaotic motions. Under deep crack conditions, double period bifurcation and multi period motion would appear at a lower speed, when the system nonlinearity and instability tend to be enhanced. The increase of unbalance force simplifies the dynamic behavior of the system, makes the appearance of system instability lag behind, but does not affect the oil-whip.
  • Numerical Analysis of a Rotary Recuperative Type Catalytic Combustion Reactor Burning Ultra-low Calorific Value Gas
    SANG Zhenkun, BO Zemin, ZHANG Qianqian, WENG Yiwu
    2017, 37(6): 461-466.
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    Aiming at the problem that the ultra-low calorific value gas is hard to be ignited and burned, which is almost entirely discharged into the atmosphere, resulting in issues of environmental pollution and energy waste, a novel reactor was developed to burn the ultra-low calorific value gas, namely rotary recuperative type catalytic combustion reactor. According to the characteristics of this reactor, such as periodic rotation, thermal storage, heat release and catalytic combustion, with the help of computational fluid dynamics software, the performance of the reactor was simulated and analyzed. Results show that the reactor can efficiently oxidize the ultra-low calorific value gas (with CH4 volumetric fraction of 2% and inlet velocity of 20 m/s), and can continuously generate high-temperature gas in the range of 1 035 K to 1 200 K, proving the reactor to be feasible in burning ultra-low calorific value gas. In the state I (or II) of rotary period, the temperature of outlet fuel gas and flue gas increases in approximately linear law, which can be used to evaluate the thermal performance of the reactor. During periodic rotation of the reactor, the top wall temperature varies in 1 200-1 600 K, that is beneficial to the generation of catalytic combustion, preventing heat accumulation in the reactor and avoiding high-temperature deactivation of the catalysts.
  • Mechanism Modeling and Simulation on Pulverizing System of a Double-inlet Double-outlet Coal Mill
    GU Junjie, CHEN Jianyong, ZHANG Yan
    2017, 37(6): 467-474.
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    To improve the safety and economy of a pulverizing system for double-inlet double-outlet coal mills, a mathematical prediction model was established to study the relationship between the ball wear and the grinding condition by analyzing the wear mechanism of the steel ball. Meanwhile, a three-input three-output dynamic model was set up for the double-inlet double-outlet mill based on the mass and energy conservation considering the heat absorption in raw coal moisture evaporation and the mechanical heat produced in coal grinding, with which, calculations and simulations were conducted for a MGS-4060-A double-inlet double-outlet coal mill matching 600 MW power units. Results show that under the step disturbance of hot air flow, cold air flow and coal feed rate at rated operating conditions, the simulated outlet temperature, material level and mill output agree well with actual operation data of the coal mill.
  • Study on Catalytic Activity of MnOx/ZSM-5 for NO Oxidation
    SUN Baomin, YANG Xiaochu, XIAO Haiping, WAN Zhentian, WANG Tao
    2017, 37(6): 475-482.
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    To achieve fast SCR reaction conditions, low-cost MnOx/ZSM-5 catalysts with different Mn loadings were prepared for NO oxidation using incipient impregnation method, which were subsequently characterized by ICP, XRD, SEM and N2 adsorption/desorption technology, so as to study their activities on NO oxidation. Results show that the catalytic activity is significantly affected by the Mn loading. High Mn loading provides more reaction sites, but excessively high Mn loading would cause aggregation of active components, thus reducing the specific surface area and lowering the surface dispersion of catalysts. The sample with 5.2% Mn loading has the best catalytic activity, in which case, the NO conversion rate can reach the maximum value of 50.6% at 400 ℃. However, the catalytic activity for NO oxidation is unsatisfactory at lower temperatures, therefore, to further improve the reaction performance of MnOx/ZSM-5 catalysts, the preparation conditions need to be optimized.
  • Numerical Analysis of Mercury Emission Speciation in Simulated Flue Gas
    ZENG Fang, LIU Feng, JIN Fei, ZHANG Xiaoguang, LÜ Jianyi, WANG Yunyang
    2017, 37(6): 483-488.
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    Based on analysis of the transformation process of mercury speciation and related influencing factors, numerical simulations were conducted to study the control technology of coal-fired mercury emission with chemical kinetics calculation software Chemkin by setting the initial concentrations of various components (such as HCl, O2, NO, H2O, CO2 and SO2) in simulated flue gas using control variable method. Results show that the existence of chlorine atoms can promote the transformation of elemental mercury (Hg0) into oxidized mercury; increasing the amount of HCl within a certain range is beneficial to the oxidation of Hg0. O2 can promote the oxidation of mercury, but not as obvious as HCl. Depending on the flue gas atmosphere, NO can not only promote but also inhibit the oxidation of Hg0; in the oxide atmosphere, NO has a positive effect on the oxidation of Hg0. H2O is beneficial to the oxidation of mercury, while presence of CO2 would inhibt the oxidation of mercury due to the strong reducing atmosphere caused by CO2. Under different working conditions, SO2 can promote or inhibit the oxidation of Hg0.
  • Arrangement of Outer Steam Coolers for a 1 000 MW Double Reheat Ultra-supercritical Unit
    FU Wenfeng, SHI Yu, LI Jiahua, YANG Yongping
    2017, 37(6): 489-494.
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    Taking the 1 000 MW double reheat ultra-supercritical unit as an example, based on analysis of the specific fuel consumption, the effects of following arrangement modes of outer steam coolers on the energy consumption of unit were studied, such as the single series connection, double series connection and double parallel connection, etc., and subsequently the optimum arrangement mode of outer steam coolers was obtained, in which case the variation law of specific fuel consumption was analyzed for each part of the thermal system and for the whole unit. Results show that by adopting the outer steam coolers, the feedwater temperature is raised, the irreversible loss of boiler is reduced, thus lowering the specific energy consumption of unit. In the single connection mode, the specific fuel consumption can be reduced by 0.632 g/(kW·h) at most when the outer steam cooler is arranged at No.2 high-pressure heater; whereas in the double connection mode, the specific fuel consumption can be reduced by 1.122 g/(kW·h) at most when the coolers are arranged in series at No.2 and No.4 high-pressure heater. With the reduction of unit load, the energy-saving effect will have slight decrease if double outer steam coolers are arranged in series connection.
  • Thermodynamic Analysis on the Superheating Degree Utilization Modes of 1 000 MW Ultra-supercritical Units
    ZHOU Luyao, XU Gang, BAI Pu, XU Cheng, YANG Yongping
    2017, 37(6): 495-500.
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    To solve the problem of too high superheating degree of extracted steam in 1 000 MW ultra-supercritical (USC) units, the energy saving effect of two systems respectively with one or two steam coolers was analyzed and compared with that of the small turbine regenerative system in the case of 85% and 90% internal efficiency of the small turbine. Meanwhile, the energy saving effect was analyzed for various utilization modes of the superheating degree under different loading conditions. Results show that the energy saving effect of small turbine regenerative system is always better than that of outer cooler system under THA load condition. The energy saving effect of small turbine regenerative system increases with the rise of internal efficiency, while that of outer cooler system improves with the rise of cooler stages. When the load decreases, the energy saving effect of small turbine regenerative system declines, while that of outer cooler system rises. When the load reduces to 50% THA state, the energy saving effect of both the systems is equivalent.
  • Microstructure Stability of Haynes282 Alloy During Long-term Aging and Creep-rupture Tests
    FU Rui, ZHAO Shuangqun
    2017, 37(6): 501-507.
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    For the Haynes282 alloy, aging tests were conducted at 700 ℃, 760 ℃ and 800 ℃ for 10 000 h, respectively, while creep-rupture tests were carried out at 700-850 ℃ and at 80-520 MPa, so as to study the microstructure stability during above tests by the way of scanning electron microscopy, energy dispersive spectrometer and transmission electron microscopy. Results show that the γ' phase in Haynes282 alloy grows slowly at the aging temperature of 760 ℃, which grows rapidly at 800 ℃. After being aged at 700 ℃, 760℃ and 800 ℃ for more than 1 000 h, the impact energy lowers quickly. During long-term aging at 800 ℃ and creep-rupture test at 850 ℃, molybdenum-rich μ phase and degenerated MC carbide precipitate at grain boundaries and within grains of Hayness282 alloy.
  • Design Improvement and Testing of a Weir Distribution Box for Passive Containments
    WANG Yanzhi, LU Yanghui, WANG Yan
    2017, 37(6): 508-512.
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    Taking the cooling water flow in a weir distribution box as an object of study, a water distribution model was set up for two kinds of weir distribution box, namely riser box and float box, of which the key parameters were determined through necessary calculations, and subsequently verification experiments were conducted on a water distribution test bench. Results show that when the Reynolds number is around 220, the coverage rate of water film would be increased by 4.9% and 3.8% respectively via riser box and float box, and when the Reynolds number is around 130, the coverage rate would be increased by 3.1% and 5.3% accordingly. Both the design schemes can reduce the mean square error of the collection levels and improve the coverage rate of the containment under small flow rates.