Torrefaction of Sorghum Straw Pellets in a Stationary Reactor with a Feeding Screw

The effects of temperature and rotating speed of a feeding screw on the key performance parameters (mass yield, energy yield, and energy densification index) of torrefaction in a stationary reactor with a feeding screw were studied. A comparative experiment of sorghum straw pellet torrefaction was performed on a fixed tube furnace. The results obtained from torrefaction in two reactors indicated that the increase in temperature resulted in a decrease in mass yield and energy yield. As the temperature increased, the volatile content in the solid products decreased and the fixed carbon content increased, indicating that the increase in temperature leads to a deeper degree of biomass thermal degradation. The thermal degradation characteristics of sorghum straw were supplemented by thermogravimetric experimental results measured at 2.5, 5, 10, 20, 40, and 60 degrees C/min. Compared with the fixed tube furnace, the pellets torrefied in the stationary reactor with a feeding screw were heated more uniformly due to the mechanical rotation of the feeding screw. The effect of the feeding screw on the heat transfer mechanism in the stationary reactor was analyzed based on the results of the torrefaction tests in the two reactors. The temperature distribution in the stationary reactor with a feeding screw was measured by five thermocouple measuring rods, which were evenly installed at 0.05 m to the inner pipe wall along the screw axis. It can be seen from the measured temperature profiles that the heating rate of the pellets heated in the stationary reactor with the feeding screw was variable. For the stationary reactor with the feeding screw loaded with biomass pellets, the optimal rotating speed of the feeding screw for torrefaction in the stationary reactor can be estimated by temperature and the theoretical average heating rate (HRestimation). Comparing the results obtained from the torrefaction tests and thermogravimetric experiments, it can be concluded that the optimal theoretical average heating rate is 4-4.5 degrees C/min.