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Biomass pelletization technology overcame the poor utilization and handling properties of loose materials. The quality of pellets is sensitive to the pelletization conditions. In this work the impact of the operating parameters as pressure, temperature, and moisture content on the mechanical characteristics as the compression strength ( $${\sigma }_{max}$$ ) and durability ( $$Du$$ ) and energy consumption for both production ( $${E}_{c,p}$$ ) and ejection ( $${E}_{c,ej}$$ ) of rice straw (RS) pellets was studied. Furthermore, the synergic effect of these parameters on the pellet characteristics 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 characteristics of the pellets. The optimization results showed that high quality pellets could be produced at a pressure of 68.4 MPa, temperature of 110.0 °C, and moisture of 8.2% for solid pellets and 63.6 MPa, 110.0 °C, and 8.7% for hollow pellets. Significant individual and interaction effects of all independent parameters on the pellets characteristics were investigated using statistical analysis results, main plot curves and 2D interaction contour plots. New significant empirical dimensionless relationships that correlate the characteristics of pellets were proposed. These relationships can be generalized for any type of pellets that produced under various operating conditions. Hollow pellets are recommended to be used in the industrial sector due to their enhanced heat transfer which resulted from the high surface to volume ratio besides their ease production at normal operating condition similar to that required for solid pellets.

Abstract Biomass raw materials are widely regarded as a significant source of renewable energy, which significantly reduces the dependence on traditional fossil fuels, especially in the case of countries that are able to obtain biomass from various sources. Recently, pelletization has been widely used for mass and energy densification of biomass to overcome the problems associated with raw material use. As the global pellet market has developed quickly, the use of wood residues became no longer sufficient to fulfill the market needs. The standards of pellets provide limits for both physical and mechanical characteristics of produced pellets. Characteristics of produced pellets depend mainly on the feedstock characteristics as particle size and moisture content and operating conditions as applied pressure and die temperature. Thus, this paper provides rich information on the factors affecting the physical and mechanical properties of granules included in pellets. The main goal of the paper is to review the latest and comprehensive research on the physical and mechanical properties of most types of single and mixed pellets from biomass. The analysis of the effect of properties, adhesives, humidity, pressure and temperature as well as the physical and mechanical properties of the pellets studied was carried out. In addition, the critical and optimal values of various factors for different materials in which the following is of importance: high quality of pellets and biomass from which they are produced were analyzed. The principle and effect of applying post-production conditions as steam explosion and torrefaction on the characteristics of the pellets were reviewed in details. Moreover, this study proposes some recommendations for further development of the pelletization analysis and characteristics.

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 (Lp), normal volume expansion (Vexp, n), initial and relaxed pellet densities (ρp, i and ρp, rel), moisture content of pellet (Mp), water absorption rate (k), and absorption volume expansion (Vexp, 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 ρp, rel, Mp, k, and Vexp, 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.