Inverstigation On Hydrodynamic Properties Of The Fluidized Bed With Coarse Granules For Firing Cement Clinker

For the purpose of developing a new kind of clinker firing device used for manufacture of ecological cement, an experiment project has been carried out. The device planned is a certain type of reactor in which coarse granules are fluidized in a bubbling fluidization state. This thesis presents main results and cognitions achieved in the program, concentrating on the dynamic characteristics of coarse granules fluidized bed and its operation stability.The major results are as follows: 1. Experimental findings on hydrodynamic properties of the fluidized bedIn a two-dimensional fluidized bed with coarse granules, creation, growing, and movements of bubbles were observed by means of high-speed photograph. From typical analysis of those photographs of the bed, the four mechanisms of bubble growth were first recognized: a) bubble expanding by pushing aside of particles, b) particles draining downward from the bubble top, c) bubble coalescence and d) gas penetrating into the bubble.Bubbles grow rapidly to large size while rising along the bed height above distributor and the bed is prone to run in slugging regime;Upward-moving particles are distantly thrown into the free-space with high splashing velocities as the bubble bursts at the bed top level.In a three-dimensional fluidized bed with coarse granules, it was discovered for the first time that minimum fluidizing velocities are lower than that determined in the two-dimensional bed and the tendency is more distinct for larger particles system. Slugging phenomenon in the fluidized bed can be restrained by decreasing particles size or widening size distribution of particles in order to get better fluidizing quality.From the above findings, granules of mean diameter of 3.4 mm with relatively wide range of particle size distribution are recommended for cement firing in industrialscale of fluidization process.Pressure fluctuation signals were analyzed by means of time/frequency decomposition. It was shown that variance in pressure fluctuation increased remarkably with increasing fluidization numbers. With detailed analysis of variance in pressure fluctuation, information of upward movements of bubbles may be acquired in the three-dimensional fluidized bed. Original bubble formation region was first discovered to locate at 100 mm above from bed bottom based on power spectrum analysis of pressure fluctuation signals. Dominant frequency of the signals is enhanced with increasing fluidization number, which implies the increase in bubble rising velocities.Under the condition of the ratio of static height of bed to the diameter of bed controlled about unity and fluidization number being below 2.06, the overexpansion and slug flow of the fluidized bed can be restricted, and the bed is found to be more stable. 2. Numerical simulation on the fluidized bed with coarse granulesA set of computer programs was written out to simulate fluidizing behaviors of both gas and solid phases with FORTRAN computing code. Eulerian approach was used to describe the gas flow and Lagrangian approach used to describe the movement of particles in the bed. A new developed method, the Discrete Element Method (DEM) is used to treat the displacement of particles and velocity in a given instant. Soft sphere model was used to treat collision force among granules. SIMPLE (Semi-Implicit Method for Pressure Linked Equations) was adopted in the numerical calculation of the gas equations. As both the particles counterforce to gas and the voidage in the gas calculating cell significantly affect the gas behavior, two-way coupling technology between gas and particles was used and the voidage was estimated carefully. These resulted in more accuracy of numerical simulating results. Reliability of the simulating program developed to coarse granules bed was verified with test data.With different temperatures (room and high temperature) and three different fluidization numbers, a lot of numerical simulations were performed in a two-dimensional fluidized bed with 11,000 spherical particles of 3 mm in diameter. The findings were summarized as follows:1) Mixing of particles is more rapid with an increasing in fluidization number;2) The permissible and desirable operating range of fluidization number is rather narrow;3) As bubbles form in the bed, the gas flows pattern changes apparently. In the totally rising low, a gas stream occurs starting from the emulsion phase region of relatively high pressure entering into the bubble space where the pressure being relatively low. As bubbles forming and rising , the cross-section-average velocity of gaspassing the bed remains essentially unchanged. In a bubble, there exists pressure gradient in a certain degree. As gas flow rate through the bed increases, the volume fraction of bubble phase and in-bubble gas velocity increase apparently, however, gas flow rate in the emulsion phase decreases correspondingly. Even some regions of zero gas velocity were resulted from the numerical computation when the input of fluidization number being 1.67. This may causes gases short circuit through bubbles and decrease in contact efficiency between gases and solids.4) Granules circulating movement rapidly decreases with an increase of fluidization number. At the top surface of the bed, particles spring with rather high velocities resulted from momentum transfer between gas and solid in y-direction. It is the chief reason why fluidized bed of coarse granules runs more rigorous than other type of fluidized bed in the same fluidization number.5) Appreciable particles showering happen in bubbles in fluidized beds. The particles falling from bubble top to bottom in a bubble can intensify heat exchange between particles and hot gas.6) Numerical simulation was implemented for the first time at different temperatures, especially at high temperature in the fluidized bed of coarse granules. It was shown that there is a complex relationship between the minimum fluidizing velocity and temperature, whereas minimum fluidizing Reynolds number decreases with temperature increasing until it descends more smoothly over the temperature of 800K. Minimum fluidizing velocity increases to 1.3m/s at 1700K compared as 1.25m/s at 300K. A correlation between minimum fluidizing velocity and temperature is found in the fluidizing bed with particles of 3 mm in diameter.7) Numerical simulating results indicated that the fluidized bed can run more steadily in high temperature than in room temperature, because mixing degree of granules, bouncing height of granules and expansion ratio of bed are decreased.8) Forces exerted to coarse granules were simulated at 1700K.The forces had the following properties: it is the gas drag force exerted to particles which make the bed fluidizing. The higher the fluidization number is, the more drastically the particles collide each other. It was found that at the fluidization number exceeded 1.33 granules movement in certain regions were mainly controlled by collisions between particles, since effects of contact force predominated over gravity and gas drag . In the emulsion region between bubble and bed wall, granule movement is controlled by collision force at fluidization number of 1.67. If cement raw granules are fired at the fluidization number, the crack occurrence of raw granules may become severe.9) In the coarse granules fluidized bed at the temperature of 1700K, majority ofgranules in emulsion phase retains a quasi-packing state, as seen in packing bed. Moreover, enlargement of the transition region between the bubble-and emulsion-phase is responsible for promotion of average voidage in emulsion phase.10) A nephogram of Nusselt number distribution was presented. Excellent property of convection heat transfer was verified in the fluidization. Nusselt number ranges from 4 to 6 as fluidization number changes from 1.33 to 1.67.In a word, the range of operating fluidization number can be extended at high temperature compared at room temperature. However, big size bubbles were still discovered at the fluidization number of 1.67 in the bed and high collision forces were also exerted to granules, so fluidization number should be still controlled at low value. Range of fluidization number from 1.33 to 1.67 is recommended by numerical simulation in the high-temperature bed. 3. Hot model experiment with fluidized bedA small scale hot fluidized bed of coarse granules was established and successfully operated for firing cement clinker, based on the investigating results of hydrodynamic behaviors of the fluidized bed. Hot model experimental results showed that the design of the new firing process with fluidized bed of coarse granules is reasonable and the system can steadily run. The suitable starting work temperature of the bed is about 600°C and the optimal static bed height is 300mm.At 1400"C, operating fluidization number was controlled in the rang from 1.5 to 2.0 to avoid 'dead bed' due to the fluctuation of gas flow. The range of the fluidization number controlled was a bit wider than recommended range. Pressure drop across the fluidized bed in the whole system ranges from 3500 to 5000Pa. Point of zero pressure was restricted in the range from bed bottom to bed height of 300mm. The quality of the fire clinker fired with this new technology is superior to clinker fired by the ordinary shaft kiln and national standard of 52.5s cement is satisfied. In the meantime, the clinker made with this new technology has advantage of small clinker size and more pores in it and so on. So, it is possible that the energy consumption of the clinker grinding could be obviously decreased.