Effect of process parameters on pelletisation of herbaceous crops

Straw, sawdust and other herbaceous crops benefit from pelletisation, as the process increases the materials bulk density therefore reducing transportation and storage costs; providing better material feeding with less dust formation. This study investigated a pressure pelletisation method of switchgrass for five types of material preparation: raw cut switchgrass, raw shredded switchgrass, torrefied switchgrass, switchgrass combined with heavy pyrolysis oil and wheat straw. The effects of pelletisation pressure and temperature on the quality of pellets were evaluated in terms of density, mechanical strength and durability. Temperature had a greater effect than pressure on pellet quality, where at elevated temperatures, the lignin present in the biomass softened and acted as a binding agent. The cut switchgrass produced more desirable pellets over shredded switchgrass due to an additional binding effect of intertwined fibres. The torrefaction of grass was not an attractive pre-process as the pellets were very brittle and possessed little mechanical strength and reduced bulk density. At elevated temperatures, with a grass to tar ratio of 2:1, the pellets were twice as strong as pellets made by cut switchgrass. The increased durability was a result of lignin present in the biomass and the heavy oil dispersing inside the pelletiser, which filled in the gaps between the switchgrass fibres upon heating. Finally, the pellets were burnt in a fixed bed combustor and the ignition rate and average burning rates were evaluated. The results have provided an indication to how the pellets would perform compared to other fuels. Future work should focus on the standardisation of herbaceous crop pelletisation and in particular, upper and lower bound limits for the moisture content should be investigated, as moisture was found to have a significant impact on pellet quality.