Effects of Natural Additives on the Properties of Sawdust Fuel Pellets

Additives play a key role in the quality of fuel pellets. It can improve the physical, chemical, and thermal properties of the pellets. In this study, the effects of various natural additives on the quality of the sawdust fuel pellets have been investigated, and then the emissions resulting from gasification of the best pellet formulation were determined at various gasification conditions. The quality of pellets was evaluated based on density, mechanical strength (durability and hardness), porosity, and water resistance. For the pelletization process, spruce sawdust was used as a feedstock and lignin (L), lignosulfonate (LS), proline (P), corn starch (CS), and torrefied oat hull (TOH) were used as bioadditives. A lab-scale single-pelleting unit was used to compress sawdust pellets at 100 degrees C and 4000 N for 60 s. Results showed that lignin and proline produced the best pellets using sawdust feedstock with a preadjusted moisture content to 16%. Central composite design (CCD) was used to determine the impacts of proline and lignin contents on the quality of the pellets. Increasing proline and decreasing lignin had a positive impact on the density and mechanical strength of the pellets. Adding TOH to the pellet formulation increased heating value and slightly water resistance, but it decreased density and mechanical strength. Computed tomography (CT) analysis of the pellets showed that increasing the proline content in the pellet formulation decreased the porosity of the pellet, whereas increasing lignin or torrefied binder increased the porosity of the pellets. In the second stage, the best pellet formulation, which contained 5% lignin and 10% proline, was used to investigate the effects of gasification conditions, such as equivalence ratio (ER) and temperature, on the distribution of gaseous, liquid, and solid products as well as the composition of the produced syngas. Using a noncatalytic fixed bed downdraft gasifier, steam gasification of sawdust pellets showed that increasing ER and temperature increased total gas and syngas yields and H-2 concentration, and decreased CH4 and C2H4 concentrations as well as char and tar yields.