Research On Gas-solid Fluidization Characteristics Of Fluidized Beds Based On Pressure Fluctuations In Wind Caps

The circulating fluidized bed combustion is a mature clean-coal combustion technology that has been widely used in power plant boilers. Because of the complex gas-solid two-phase flow inside the furnace during the operation, a circulating fluidized bed boiler usually has a higher failure rate than an ordinary pulverized coal fired boiler. To solve this problem, the in-bed and wall pressure fluctuations are generally measured to be used for the analysis of gas-solid fluidization characteristics in fluidized beds under different operation conditions. Based on the analysis of gas-solid fluidization characteristics in fluidized beds, the operating state of fluidized beds can be monitored, and the corresponding faults that happen can be early diagnosed. However, these in-bed and wall pressure measuring points are exposed to the gas-solid flow, so that they are easily to be blocked and wore. Once the pressure measuring points have been destroyed, the monitoring parameters will lapse. For this reason, the analysis of pressure fluctuation signals measured from wind caps which was a new method was proposed to realize the analysis of gas-solid fluidization characteristics in fluidized beds. With the help of a cold bubbling fluidized bed and a cold circulating fluidized bed, the state of gas-solid flow under different operating conditions and the typical faults were simulated. To research the relationships between the characteristics of pressure fluctuations in wind caps and the gas-solid fluidization characteristics inside the fluidized bed, the pressure fluctuation signals measured from the inlets of wind caps were analyzed with the methods both in time and frequency domain, such as statistical functions, power spectrum density estimation, wavelet analysis, energy weighted average frequency, homogeneous index and so on. The purposes of the research also included attesting to the practicability of the proposed method in this paper and providing a theoretical basis for the failure diagnosis of circulating fluidized beds based on pressure fluctuations in wind caps.Firstly, the state of gas-solid two phases flow in a bubbling fluidized bed was simulated at different superficial gas velocities, bed material particle sizes, static bed heights and the disturbance of feeding and discharging materials. The pressure fluctuation signals were measured from the central and side-wall wind caps at simultaneity. The signals were decomposed into sub-signals of different frequency bands with wavelet analysis, and then the energy of sub-signals and the ratios of energy at different levels to total energy were calculated. Besides, the power spectrum densities of signals were got with Welch spectrum estimation method in order to analyze the characteristics of signals in frequency domain. Secondly, the state of gas-solid flow inside a circulating fluidized bed was simulated at different primary air velocities, particle size distributions, solids inventories and secondary air flow rates. And the state of gas-solid flow inside a loop seal was also simulated at different recycling gas velocities and solid circulation rates. After measuring the pressure fluctuation signals from the wind caps located at the different cross-section positions of the circulating fluidized bed and the loop seal, the signals were analyzed with statistical average and standard deviation in order to get the general trends and fluctuation amplitudes of the pressures in wind caps. The signals were also decomposed into sub-signals of different frequency bands based on wavelet analysis, and the energy of sub-signals and the ratios of energy at different levels to total energy were calculated. Besides, the power spectrum densities of signals were got with Pre-whitening and Post-color spectrum estimation method for the analysis of signals in frequency domain and the homogeneous indexes of signals were calculated for the analysis of fluidization homogeneity and regimes.After the simulations of normal operating conditions of bubbling fluidized bed and circulating fluidized bed, the partial blockages of wind caps near the side walls, both the materials-returning side and the materials-feeding side, were simulated. By feeding bed materials of large particle size with different qualities and simultaneously removing some original bed materials of the same qualities, the agglomeration of different levels in a circulating fluidized bed was simulated. Under the failure operation conditions, the pressure fluctuation signals of the wind caps located at the corresponding positions of the distributor were measured to analyze the effect of wind cap partial blockage and agglomeration to the pressure fluctuations characteristics and gas-solid fluidization characteristics based on the methods of statistical functions, wavelet analysis and homogeneous index.The experimental results show that the pressure fluctuations characteristics of wind caps in a bubbling fluidized bed can reflect the gas-solid fluidization characteristics in the bubbling fluidized bed at different superficial gas velocities, bed material particle sizes, static bed heights, cross-section positions and the disturbance of feeding and discharging materials; the pressure fluctuations characteristics of wind caps in a circulating fluidized bed can reflect the gas-solid fluidization characteristics in the circulating fluidized bed at different primary air velocities, particle size distributions, solids inventories, secondary air flow rates and cross-section positions; the pressure fluctuations characteristics of wind caps in a loop seal can reflect the gas-solid fluidization characteristics in the loop seal at different recycling gas velocities and solid circulation rates; the pressure fluctuations characteristics of wind caps in a fault fluidized bed with wind cap partial blockage or agglomeration can reflect the corresponding fault that happens in the fluidized bed.