Mixing Of Particles And Coal Pyrolysis Behaviors In Screw Reactors

In the coal topping technology, hot particles are applied as heating media to heat the coal particles to desired temperature. The temperature distribution in reactor is strongly dependent on the mixing and flow pattern of particles. A high degree of mixing is required not only for good heat and mass transfer, but also for a high yield of products with desired composition. Screw reactors may be a suitable option for fast mixing and pyrolysis for the coal topping technology, due to their versatility for handling the mixing, metering, transportation and reaction simultaneously. The understanding of mixing between the coal particles and the hot particles is of essential important, which will affect the pyrolysis process.Experimental and theoretical studies are carried out on the axial backmixing, transverse mixing of the particles, and pyrolysis of coal in a screw reactor. The objective of the project is to understand the mechanisms of particle mixing, heat and mass transfer between particles in screw reactor, which may provide information for optimized designs and handling of screw reactors in coal topping process.The axial backmixing characteristics of particles in a screw reactor are studied by impulse injection of tracer particles. The tracer particles are prepared by coating a highly water soluble red dye (Acid red 18) on the sand particles. The preliminary results show that the particles are evenly coated with good reproducibility. The effects of screw speed, screw feeder inclination and particle size on the residence time distribution (RTD) of particles in a screw reactor are investigated. Results show that the backflow of the particles is significant with the increased screw inclination, resulting in an enhanced backmixing. An increase in the screw speed and the particle size results in a decreased backmixing. The oversized particles may be crashed to small particles by the screw during mixing and transporting, leading to an increased backmixing.The two-dimensional Markov chain is applied to describe the flow pattern and RTDs in the screw reactor. Each pitch of the screw is divided into an active zone and a stagnant zone. The model can well describe the phenomena observed in the experiments for the horizontal and upward inclination cases. The size of stagnant zone will decrease with a decrease of the screw inclination and eventually disappear.A digitized image analysis method coupled with a solidification technique is developed to study the transverse mixing of particles in a screw. An adaptive threshold technique based on the R-B color contrast of materials is proposed to recognize the particles with different colors. The technique can be used to track the particles at any defined cross section along the screw, and thus to reveal the transverse mixing degree of particles in the location. The effects of screw rotation speed, particle filling level and particle size on the transverse mixing index and mixing rate constant in a steady state can be investigated by this technique. The results show that the mixing degree depends only on the particle properties. Segregation will occur for the large difference in the size of the two groups of particles, which results in a bad transverse mixing. Operational conditions, such as screw speed, particle filling level and inclination will influence the mixing rate, but not the mixing degree. The low screw speed and filling level, as well as the upward inclination of screw enhance the transverse mixing rate.Furthermore, a pilot scale screw reactor is designed and constructed for coal pyrolysis. Continuous operation of the apparatus is achieved. The influence of the solid heat carrier temperature, screw speed and the mass ratio of solid heat carrier and coal on the pyrolysis gas yield and the composition are studied. Results show that gas yield is enhanced by increasing the solid heat carrier temperature and mass ratio of solid heat carrier to coal. The high screw speed can not only enhance the mixing rate, but also reduce the residence time of coal, and an optimum screw speed could be found. It is also found that temperature of casing drops sharply along the conveying direction. Thus, the design of the screw pyrolyzer needs to consider the appropriate screw length for balancing the mixing degree and heat loss along the screw reactor.Finally, a model based on Markov chain theory is developed to predict the temperature distribution and volatile yield in a coal screw pyrolyzer, by taking the heat transfer and reactions into account. The results show that nearly 80-85% of heat transferred from solid heat carrier to coal is from radiation and conduction. The heating rate of the coal particles depends on the particle size. The heating rate can be as high as 240 K/s for the coal particles smaller than 0.5 mm. In a non-uniform particle size distribution system, the fine particles have a higher pyrolysis temperature and volatile yield. The total volatile yield increases by widening the particle size distribution. However, the time for reaching the steady state is longer for this case. Furthermore, a cluster-dispersing model is proposed to describe the effect of the mixing on coal pyrolysis. An increase of the cluster size caused by the low degree of mixing will result in state has large cluster size and low effective heat conductivity coefficient and eventually the low heating rate of coal particles.