Effects of length-to-diameter ratio, pinewood sawdust, and sodium lignosulfonate on quality of rice straw pellets produced via a flat die pellet mill

Poor pelleting ability of rice straw (RS) discourages its production and utilization as a biofuel. Woody biomass and lignin are believed to improve the quality of herbaceous and agro-residual pellets. This study investigated the influence of length-to-diameter ratio (L/φ = 6 and 10), pine sawdust (PS) (0, 20, and 40 wt%) and sodium lignosulfonate (SL) (0, 10 and 20 wt%) on the quality of RS pellets produced via a mini-pilot scale flat-die pellet mill. L/φ positively affected quality of RS pellets and specific energy consumption (SEC) of pelleting process. PS improved significantly the energy content and reduced the ash content of the RS pellets simultaneously, however, it decreased the true, bulk densities and tensile strength of RS pellets. SL positively influenced pellet physical properties for SL up to 10%. Average unit, bulk, tapped densities and tensile strength of RS pellets were 1272–1330 kg m−3, 596–687 kg m−3, 661–760 kg m−3, and 4.4–5.6 MPa in L/φ = 6 and L/φ = 10, respectively. PS remarkably affected combustion behavior of RS pellets, however the effect of SL was not critical. The SEC of pelleting process ranged between 1.9 and 2.7 GJ t−1 in L/φ = 6 and 2.4–3.6 GJ t−1 in L/φ = 10. The corresponding energy ratios were 3.1–4.4 and 2.5–3.6, respectively. The optimal heating value and energy density were 15.79 MJ kg−1 and 9102.81 MJ m−3 in L/φ = 6, and 15.27 MJ kg−1 and 10818.08 MJ m−3 in L/φ = 10.