Flow Characteristics Of Ceramic Balls And Pulverized Biomass Chars In A Vertical Tube

Down flow tube pyrolysis liquefaction apparatus is a reactor for fast pyrolysis of biomass with our own independent intellectual property rights.In the reactor,biomass powder is heated by the ceramic balls during flowing down along the tube and then pyrolyzed.It is essential to understand the flowing characteristics of particles in the down flow tube for studying the process of pyrolysis.In this study,the flowing characteristics and velocity distributions of ceramic balls(1.5~2mm in diameter), biomass-chars(60~80 mesh)and the mixture of ceramic balls and biomass chars were studied.According the requirements for using PIV system,equipment related to the experiments were designed and fabricated.The experimental results showed that the tundish type feeding device could feed the ceramics balls and biomass chars accurately.The velocities of the particles in the vertical duct under suction at the bottom of the tube(under suction)and with the bottom end open to the atmosphere(open end) were determined.Meanwhile,the velocity distributions of biomass chars and ceramics balls were measured and compared at different suction pump flow rates(10m~3/h, 15m~3/h,20m~3/h,25m~3/h,30m~3/h)and different heights below inlet.Finally the velocity distribution curves were obtained.With open end condition,the velocity distribution curves of biomass chars and ceramics balls showed that velocity in the central area is bigger than that of the area near the duct-wall.Whereas,under suction condition,the velocity distribution curve of biomass-chars in central area is a flat line,and at the same height,the velocity is bigger than that of with open end condition.The velocites of biomass chars and the mixture increase as the increase of the pump flow rate,but the pump flow rate has little effect on the ceramics balls.The velocity distributions of ceramics balls are more even when a sieve type distributer is used.The final settling velocity of biomass char calculated through the Stokes formula has been compared with final velocity of biomass-chars measured by PIV in the vertical duct.When the falling distance h=1200mm,the critical value of pump flow rate was 11m~3/h for biomass chars.When the pump flow rate is bigger than the critical value,the velocity of biomass char is bigger than that of open end.When the pump flow rate is smaller than the critical value,the velocity of biomass char is smaller than that of open end.Comparing the actual velocity of biomass-char with theoretical airflow velocity,the results indicated that under suction condition,the velocity of biomass-char was determined by the airflow velocity in the vertical duct.The velocity of biomass char is finally stabilized at about 0.8m/s with open end condition and 1.2m/s under suction at the bottom of the tube with the suction pump flow rate of 15m~3/h.Experiments for the mixed flow of biomass-chars and ceramic balls show that the velocity of the mixture is bigger than that of biomass-char alone under the same suction condition;the velocity of the mixture under suction condition is bigger than that with open end condition;the velocity of biomass-char is bigger than that of the mixture under suction condition.