Conversion of recycled sawdust into high HHV and low NOx emission bio-char pellets using lignin and calcium hydroxide blended binders

Spruce wood sawdust (S), as biomass waste, could be utilized as a renewable fuel, but it suffers from its bulky, low energy density, high volatiles content and NOx emission. This study investigated the possibility of conversion S into bio-char pellets (SC-Ps) as renewable and CO2-neutral bio-fuel. Sawdust derived bio-char (SC) was produced through pyrolysis, and subsequently compressed into SC-Ps bonded by lignin (L) and hardened by Ca(OH)2, NaOH, CaCl2, CaO. The combustion characteristics of S and SC, the physical properties of SC-Ps including abrasive resistances (ARs), impact resistance index (IRI) and compress strengthens (CS) were evaluated. Results showed that the high heat value (HHV) of SC increased by 95% and its NOx emission decreased due to the release of N-containing volatiles. Among these hardeners, addition of 5% Ca(OH)2–10% L reduced the disruptive force created by uptake moisture and played an effect of hydration on hardening the bonds. In addition, the catalysis of hydroxide promoted the polymer chain growing into three-dimensional cross-linking that strengthened the bonds. Thus, the mechanical strengths of the SC-Ps bonded by Ca(OH)2/L were sufficient for directly transportation and being charged into the blast furnace.