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

Abstract

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 °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 °C/min.

Downstream Torrefaction of Wood Pellets in a Rotary Kiln Reactor—Impact on Solid Biofuel Properties and Torr-Gas Quality

“…ers also concluded similar results regarding the darkened color of the torrefied pellets with increasing process temperature [5,19,21,31,38]. It was reported in the literature that the color change takes place between 150 and 300 °C due to chemical changes caused by several reactions such as decomposition, oxidation, cross-linking, condensation, and enzyme-mediated reactions within the biomass structure [29].…”

“…The higher net calorific values of the torrefied fuels produced at 250 and 270 • C are comparable to the net calorific value of coal varying between 25 and 35 MJ/kg (d.b. ), which is an important improvement in fuel properties considering further potential applications [19,29,31,40].…”

“…Utilization of torr-gas and other valuable chemicals such as furfural in further steps is also not well-established. In the literature, there are several publications on downstream torrefaction of biomass pellets, as given in Table 1, which mostly focus on the effect of torrefaction temperature, residence time, or torrefaction atmosphere on different aspects such as fuel properties (proximate, ultimate analysis, or calorific value) [15][16][17][18][19][20][21][22], torr-gas analysis [16,23], bulk or energy density [15][16][17]23], grindability [15,18,20], hydrophobicity or water uptake [16,17,20,21], and hardness or durability [17]. Both upstream and downstream torrefaction is still under research, and more commitments are required to ensure economically feasible market uptake.…”

Improving the Conversion of Biomass in Catalytic Pyrolysis via Intensification of Biomass—Catalyst Contact by Co-Pressing

“…Similar to the devolatilization temperature obtained from cellulose thermal decomposition, Yang et al [11] and Li et al [41] reported decomposition temperature ranges of 588-673 K and 573-673 K during the non-catalytic TGA heating of cellulose. On the other hand, Liu et al [48] and Wang et al [49] also reported an increase in the maximum degradation temperature of cellulose when heating rate was increased. The restricted catalytic reaction during the TGA pyrolysis of cellulose can be explained by the kinetic diameter of cellulose (8.6 Å) which is larger than the pore size of HY zeolite (7.4 × 7.4 Å) [50].…”