Insights into the swelling behaviours of biomass and biomass/thermoplastic briquettes under water penetration and moisture adsorption

This study investigated the swelling behaviours of briquettes made of forest residues (sawdust and bark) and their mixtures with linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE) and polypropylene (PP) under water penetration and 90% relative humidity. The swelling of briquettes under water penetration was observed by a time-lapse imaging methodology. The swelling behaviour under 90% RH was investigated by statistically analysing the changes in moisture content, thickness and width, as well as their adsorption/swelling rate constants. The time-lapse imaging study suggested that adding thermoplastics could improve the hydrophobicity of sawdust briquettes, especially, the briquettes with 6% of PP achieved a contacting time of 6 s, interaction time of >45 s and no swelling was observed during 51 s. This was superior to the use of 16% HDPE or LLDPE. Adding plastics showed no improvement to the hydrophobicity of bark briquettes under water penetration. Humidity tests suggested that adding thermoplastics to both sawdust and bark suppressed the adsorption of moisture and the swelling of briquettes. Adding 16% of PP achieved the best performance with increases in moisture content, thickness and width of similar to 12, similar to 14, and similar to 2% respectively for sawdust/PP, and similar to 10, similar to 8 and similar to 3% respectively for bark/PP. Analysis of the swelling kinetics suggested that the addition of thermoplastics to sawdust showed more significant suppression of the moisture adsorption and swelling of briquettes than adding thermoplastics to bark. The swelling of densified biomass degrades the characteristics as solid biofuels and needs to be avoided based on understandings of their swelling behaviours.