Optimization and statistical analysis of the effect of main operation conditions on the physical characteristics of solid and hollow cylindrical pellets

Nowadays, thermal energy production from solid fuels as biomass materials focused on the densified biofuels to overcome the disadvantages of raw biomass as irregular shape, low bulk density, and high moisture content. The quality of biomass pellets is sensitive to the pelletization conditions. Consequently, in this work the impact of the operating parameters as applied pressure, die temperature, and moisture content on the physical properties of solid and hollow pellets as pellet length ( L p ), normal volume expansion ( V exp, n ), initial and relaxed pellet densities ( ρ p, i and ρ p, rel ), moisture content of pellet ( M p ), water absorption rate ( k ), and absorption volume expansion ( V exp, ab ) of rice straw (RS) pellets was studied. Furthermore, the synergic effect of these parameters on the pellet properties was evaluated at the optimum conditions. The operating parameters were optimized using a multi-objective optimization approach of response surface method (RSM) based on the predicted significant models that describe the physical properties of the pellets. The optimization results showed that high-quality pellets could be produced at a pressure of 72.76 MPa, temperature of 110°C, and moisture of 7.23% for solid pellets and 81.5 MPa, 109°C, and 7.72% for hollow pellets. Significant individual and interaction effects of all independent parameters on the pellets properties were investigated using statistical analysis results, main plot curves, and 2D interaction contour plots. New four significant empirical dimensionless relationships that correlate the properties of pellets were proposed for calculating ρ p, rel , M p , k , and V exp, ab directly based on other physical and mechanical properties and indirectly based on the operating parameters through the predicted models for different properties. These relationships can be generalized to any type of pellets that are produced under various operating conditions. Hollow pellets are recommended being used in the industrial sector due to their enhanced heat transfer which resulted from the high surface to volume ratio besides their ease of production at normal operating condition similar to that required for solid pellets. Graphical abstract