Characteristics Of Pressure Fluctuations And Cluster In Dense Transport Bed

As a new gasification technology, Dense Transport Bed (DTB) gasification is suitable for low rank coal and various gasification modes. Pressure fluctuations and cluster characteristics in DTB risers are key research topics of gas-solid hydrodynamics. In this thesis, we have studied pressure fluctuations and cluster characteristics in a cold dense transport bed (17m-H×0.102m-ID) with silica sand belonging to Geldart group B. The main results are as follows:1. The sources of pressure fluctuations in DTB could be divided into two types. One is global pressure fluctuation, which is characterized by frequency less than2Hz and intensity of1Hz pressure fluctuation increasing with the increase of solid circulating rate (Gs), and is caused by fluctuations in gas velocity and macro dense solid flow. The other is local pressure fluctuation, which corresponds to frequency greater than2Hz and is mainly originated from cluster behavior, gas-solid interaction, etc.2. The wavelet analysis was employed to extract global pressure fluctuations from the original pressure signals. It was found that the global pressure fluctuations propagated along the riser from bottom to top. The dynamic behaviors of gas-solids flow in there different flow patterns (e.g. pneumatic conveying, fast fluidization and dense suspension upflow) were studied. It was found that the pressure fluctuation behavior of dense suspension upflow was different from pneumatic conveying and fast fluidization. For dense suspension upflow, the dominant frequencies of pressure fluctuation are mainly below0.6Hz, and there is only one zone of power spectral density decaying by power function; while for pneumatic conveying and fast fluidization, the dominant fluctuation frequencies are below1Hz, and there are two zones of power spectral density decaying by power function.3. The spatial distribution of cluster frequency and duration in the riser is uneven. Cluster frequency decreases from center (>240Hz) to wall (<100Hz); cluster existence time increases from center (<0.003s) to wall (up to0.04s) of the riser. In the bottom zone of the riser, cluster frequency is lower and cluster existence time is longer, indicating that the large scale clusters are easier to form in near wall and bottom zones.4. Cluster frequency increases with the increasing superficial gas velocity (Ug), while cluster existence time decreases with the increasing U%. In the bottom zone of the riser, cluster frequency decreases with the increasing Gs, while cluster existence time increases with the increasing Gs.5. Compared with circulating fluidized bed (CFB), dense transport bed has a higher cluster frequency and a shorter cluster existence time. In DTB riser, cluster frequency is in the range of40to350Hz, and cluster existence time is between0.0018and0.04s.