Mechanical properties and characteristics of wheat straw and pellets

Abstract

Wheat straw pellets can be easily handled, transported and stored with reduced costs as compared with the raw material. The effect of pelletization process and densification parameters on the properties of the mixture of wheat straw powder and 40% epoxy 1092 as a binder was investigated. The samples were compressed into pellets using the lab-scale hydraulic press under various compacting pressures of 10, 12 and 15 bars and different die shapes and sizes (two cylindrical dies with diameters 10 and 18 mm (S1, S2), respectively, and a new hexagonal die of side length (s) = 6 mm (S3)). It was found that the pelleting process increased the fixed carbon content from 7.14% to 17.36%, the heating value from 15,600 kJ kg−1 to 27,800 kJ kg−1 and the bulk density 10 times when compared to a raw wheat straw powder. It was also found that type S2 at pressure 15 bar is the densest pellet and it had the maximum compression stress that reached 2798.54 kg m−3 and 70.02 MPa, respectively. The cylindrical pellet (type S1) of D = 18 mm at a pressure of 15 bar had the lowest water permeability of 2.35%. Pelletizing process had improved combustion characteristic parameters compared to raw biomass where combustion temperature ranges became higher, maximum weight loss rates and residues became lower that led to a higher combustion efficiency. Thermogravimetric analysis of wheat straw before and after pelleting process was analyzed to evaluate the combustion properties. Scanning electron microscopy images showed that the particle bonding was formed mainly from solid bridges, areas of cohesive failure due to lignin flow, and inter-diffusion between neighboring biomass particles. Images were investigated to explore the importance of compaction process. The images also showed that as the pressure decreases, the gap between the particles increases and produces the less durable pellet. It was found that the slagging index equals 0.38, which indicates that the pellet has a medium slagging inclination and for the fouling inclination (1.79), the wheat straw pellet has a relatively high fouling inclination.