Simulation of effect of internals on particulate mixing and heat transfer in downer reactor using discrete element method

Abstract

To investigate the effect of internals in downer reactor on the particulate mixing and heat transfer, a model of heat transfer based on discrete element method (DEM) has been developed and validated by experiments. The agreement between the simulated results and the experiment phenomena is able to prove the reasonability and correctness of the model mechanism of the particulate heat transfer using DEM. The effect of internals in downer reactor on the temperature increasing rate of particles has been predicted by the particulate heat transfer model based on DEM, which is important for the fast pyrolysis process. Two types of internals, the tube group internals and the baffle internals, have been designed to improve the mixing and heat transfer between fuel particles and solid heat carriers in downer reactor. The internals not only increase the particle residence time in downer reactor, but also enhance the mixing degree of binary particulate materials. Most importantly, the internals affect the temperature increasing rate of fuel particles controlled by the mixing degree of fuel particles and heat carriers. The terminal temperature fuel particle at the exit of downer reactor is determined by the average temperature increasing rate and the mean residence time of the fuel particle. As shown in the simulated results, the mixing degree of binary particulate materials in the baffle type downer reactor is remarkably better than that in the tube type downer reactor. At the same time, the temperature increasing rate of fuel particles in the baffle type downer reactor is also larger than that in the tube type downer reactor. It can be concluded that the baffle internals not only restrict the increase of particle residence time, but also mix the fuel particles and solid heat carriers sufficiently, which make the fuel particles heated rapidly. A shorter resistance time, a perfect mixing degree of two types of granular materials and a higher temperature increasing rate can confirm that the internal of baffle group is an excellent choice for the fast pyrolysis process in downer reactor.