3.4. Starch and lignin as the binder
Since both lignin and starch have been widely used as the natural binders for making biochar and charcoal pellets/briquettes in commercial operations, it is important to compare the performance of biomass sawdust binder with lignin and starch in our current lab-scale single die test unit. Table 9 shows the properties of biochar pellets made from a biochar sample prepared at 330C (39.4 wt.% biochar yield) with starch, lignin and untreated pine sawdust as the binder. The density of single biochar pellets made from the biochar sample mixed with binders ranged from 820 to 870 kg/m3, which is comparable to 810 kg/m3 for SPF pellets made from pure biochar sample at a 230C die temperature [15]. The higher heating value of biochar pellets decreased with the increase of binder fraction because the heating values of those binders were lower than the biochar sample. The Meyer hardness of biochar pellets was close to the control wood pellets, which was much higher than the SPF biochar pellets (4 N/mm2) made from the pure biochar sample [15]. Verhoeff et al. and Mallory also reported that the useof starchasthe binder couldresultinstrongbiocharpellets [13,19]. The saturated moisture content of biochar pellets was around 9–12wt.%, which was much lower than control pellets but slightly higher than biochar pellets without binders. The volumetric energy density of biochar pellets was around 13 GJ/m3, close to that of control wood pellets, which was also similar to the values found for torrefied pellets with raw sawdust as the binder, shown in the previous section. Similar to biomass sawdust, starch and lignin as the binder can improve the durability of biochar pellets because starch increases the adhesion forces between biochar particles and lignin builds the solid bridges between biochar particles. Starch and lignin binders, however, cannot enhance the energy density of biochar pellets because, like raw sawdust, they cannot fill the new pores created in the biomass torrefaction process at a die temperature below their melting temperature.