Effect of compacted dose of pure straw and blends of straw with calcium carbonate or cassava starch on pelletising process and pellet quality

Abstract

In this study, changes in the nature of particle size distribution (PSD) of wheat straw (WS) were analysed after the addition of calcium carbonate (CC) additives or cassava starch (CS) with 2 and 6% wt/wt ratios, respectively. Pelletising was conducted for materials with moisture contents of 20% w.b. at a temperature of 78 °C, in a die with an open compaction chamber with a height of 80 mm, using single doses of 50, 100, and 150 mg. The straw blend with 6% wt/wt CC exhibited the most even PSD and the lowest particle size geometric mean of 0.71 mm. These features positively influenced the mechanism of bonds among the particles, compaction effect, and quality of the pellets. Based on mathematical models, it was observed that under optimal conditions, agglomeration of the blend with 4% wt/wt CC using the 100 mg dose aided in achieving the maximum single pellet density of 850 kg m−3. This required a specific compaction work of 36 kJ kg−1 followed by a pellet shift of 0.63 mm in the die, by using a piston realising a specific work of 10.92 kJ kg−1. The produced pellets were characterised by a superior elasticity modulus of 6.15 MPa and high compression strength; for their deformation until cracking, a specific compaction work of 5.52 mJ mm−2 was required. Furthermore, the water absorption capacity of the crushed pellets was 4.00 g H2O g–1 DM (dry matter), which was higher than those of pure WS, the blend with CS, and uncrushed pellets.