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Design and experiment on biochar second-stage cooling system with spiral-flow.

TitleDesign and experiment on biochar second-stage cooling system with spiral-flow.
Publication TypeJournal Article
Year of Publication2015
AuthorsZongLu, Yao, Min Li, LiXin Zhao, HaiBo Meng, Hongbin Cong, and Shulin Hou
JournalTransactions of the Chinese Society of Agricultural Engineering

Biomass pyrolysis is an energy converting technology, but during the pyrolysis process, the outlet temperature of biochar is high due to high final pyrolysis temperature. As such biochar continue burning when exposed in air. So only after cooling, biochar can be transported and used. Currently, common practice is that biochar are natural cooled in a sealed container or by sprinkling water directly to biochar after pyrolysis. Both of the methods need a long time, besides, a great deal of residual heat of biochar is wasted during cooling. This leads to the long production cycle, lowly utilization rate of biochar heat in pyrolysis process. So a sealed cooling device is needed to solve the problem. Through this device, the outlet temperature of biochar should be <40-50°C. Because biomass heat transfer coefficient is low, the cooling process needs more time than other materials, and the new device should be long enough to allow sufficient cooling. As such, a two-stage heat exchange device is needed, using the principle of solid-liquid indirect heat exchange. Such design can ensure that the cooled biochar can be used directly for package, transportation or experiment. In this study, we first examined the details about device structure and size design. Based on analysis and comparison of different structure and biochar sizes, we selected water cooled screw type as the main structure. Biochar was delivered by screw blade in the inner tube with circulating water in out surface of the tube. For such device, the most important part was baffles, it was added between the inner tube and external tube, making circulating water flow in the form of spiral-flow around the inner tube, thus increasing heat transfer time between circulating water and wall surface of inner tube. Based on this, a second-stage biochar cooling system was designed with temperature acquisition terminal and control device, the temperature acquisition terminal was used to monitor the temperature of circulating water and biochar, including two temperature sensors and temperature patrol instrument. The control device regulated water flow and rotation speed of screw axis. We then conducted different cold water flow test,different rotation speeds of screw axis test and different biochar test by use of the system. The application results showed that, the average outlet temperature of biochar was 30°C, it reached the design requirement. Besides, the application results showed that the temperature difference increased with the rise of cold water flow. The temperature difference of peanut biochar was 272, 242, 222°C when rotation speeds of screw axis was 5 r/min, cold water flow was 8, 6.5, 4 m³/h. Meanwhile the outlet temperature of peanut biochar was 40°C, while rotation speed of screw axis was 25 r/min, and cold water flow was 6.5 me³/h. In contrast, when rotation speed of screw axis was 5 r/min, the outlet temperature of peanut biochar was 25°C. In all heat transfer coefficient was different with different biochar. The experiments showed that corn stalk biochar, peanut biochar, corncob biochar overall heat transfer coefficient was 100.6, 85.3, 57.6 W/(m²∙K), respectively. The cooling process completed smoothly with the cooperation of cooling system and pyrolysis equipment, this system could meet the need of continuous cooling process for various kinds of biochar.