Numericai Simulation Of The Thermal Performance Of Quad-sectional Air Preheater And Research On The System’s Dynamic Characteristics

In recent years, with the increasing pressure of energy saving and environmental protection, the increasing engineering scale of circulating fluidized bed boiler, and the development for large-capacity and high-parameter pulverized boiler, given the advantages of low air leakage rate, large heat capacity and high reliability, quad-sectional air preheater had been developed rapidly. Meanwhile, with the development of exhaust waste heat utilization technology, the 2nd generation technology that was high-low energy level coupled heat recovery system was about to enter the engineering applications. Air preheater was not only the tail of heat transfer equipment for boiler system, it was the core of the 2nd generation heat recovery system. Compared to the bisector and tri-sectional air preheater, the structure and heat transfer mechanisms of quad-sectional air preheater was more complex. The domestic research and information on the quad-sectional air preheater was very limited, which did not match with the background of wide range of engineering applications.A new thermal calculation model for quad-sectional air preheater was investigated based on the analysis of operating mode and heat transfer characteristics of quad-sectional air preheaters. The model included two calculating steps:the average outlet flue gas temperature and the weighted average outlet air temperature were obtained through the first step calculation; and based on the first step calculation results, the average outlet air temperatures were obtained in right-secondary airside, primary air side and left-secondary airside respectively by the second step calculation. The results showed that the maximum and minimum relative deviations between calculated values and design values were 2.27% and 0.21% respectively. Based on the visual basic programming language, a thermal calculation program which had a friendly interactive interface for quad-sectional air preheater was developed. For a 300 MW circulating fluidized bed boiler unit with quad-sectional air preheater, the maximum and average relative deviations between calculated values and actual operation data were 4% and 1.8% respectively, the accuracy and reliability of this thermal calculation model were verified.Base on CFD software, a 3D heat transfer model for quad-sectional air preheater was set up by defining the rotor heating surface temperature as user-defined scalar and by solving the scalar equations. The 3D temperature and density distribution of working fluid, the 3D temperature distribution of heating surface, the 3D temperature difference distribution between working fluid and heating surface, and the 3D heat transfer per unit volume distribution were obtained. Results showed that:(1)The different materials and type of heating panel was the main reason for the difference of heating transfer performance between hot-section and cold-section. (2)the temperature of working fluid was different between hot-section and cold-section, resulting in different volume flow, so that the fluid velocity of flushing hot-section and cold-section was different, which was the structural reason for the difference in heat transfer performance between hot-section and cold-section. (3)As the heating surface in cold-section adopted corrosion-resistant materials, low-temperature corrosion was easy to happen in the heating surface at hot-section inlet which was leaving right secondary air side and was about to enter gas side. Where the metal temperature should be kept above acid dew point so as to reduce or prevent the low-temperature corrosion.Finally, a thermal performance calculation procedure for high-low energy level coupled heat recovery system was developed, in which the quad-sectional air preheater was the core. The dynamic characteristics of quad-sectional air preheater in the system were discussed. Based on determination of the key parameters, a self-adaptive adjusting mechanism for the system was proposed. When running the system under varying conditions, the mechanism could meet the thermal performance needs for quad-sectional air preheater and low-temperature economizer.