Production and characterization of densified briquettes from nanocomposite biochar-cellulose nanocrystal (Biochar-CNC) reinforced with polyvinyl alcohol (PVA)

Abstract

Biomass briquettes are still important to communities in developing nations because they are cheap, sustainable, and generated from solid waste that can be utilized to produce energy. However, the low quality of the briquettes when compared to traditional cooking fuels hampers their widespread adoption; yet, there is an opportunity for expanding the briquette market due to the rise in charcoal prices, increasing scarcity of forest resources, and more environmental awareness among consumers. The main objective of this study was to develop a bio-based briquette with improved combustion characteristics through the use of an innovative binder. A novel nanocomposite briquette (biochar/cellulose nanocrystals (CNC)/polyvinyl alcohol (PVA) was produced using the solution casting method, with CNC/PVA nanocomposite as a binder. A total of five (5) nanocomposite briquettes having biochar-to-binder ratios of; 90:10, 80:20, 70:30, 60:40, and 50:50 and designated as BCP (9/1), BCP (8/2), BCP (7/3), BCP (6/4) and BCP (5/5), respectively were developed. The nanocomposite briquette samples were characterized for thermal stability, mechanical properties, elemental composition, surface morphology, proximate composition, and combustion characteristics using established methods. The produced briquettes had a very low ash content of less than 2% and a low average moisture content of 8%. The surface morphology of the briquettes revealed a rough and porous structure that can enhance combustion. The BCP (9/1) briquette had the highest calorific value of 27 MJ/kg, followed by BCP (8/2) and BCP (7/3) which had a calorific value of 26 MJ/kg. The BCP (7/3) nanocomposite briquette was the most thermally stable, with the lowest onset degradation temperature (220 °C), highest peak temperature (514 °C), least char residue, and the most compressive strength of 11 MPa. The BCP (9/1), BCP (8/2), and BCP (7/3) nanocomposite briquettes also satisfied the combustion indices criteria, demonstrating their potential to replace coal in industrial applications. The thermal degradation and kinetics of the nanocomposites were studied using TGA-DTG techniques at three different heating rates; 5 °C/min, 10 °C/min, 20 °C/min in an oxygen environment. The kinetic parameters, that is, the activation energy and pre-exponential factor were calculated using the Coats-Redfern method. The combustion of the briquettes happened in three distinct phases with a higher activation energy required at higher heating rates to initiate the first stage of combustion. Also, up to 40% of the binder can be added without affecting the ignition, combustion, and burn-out properties of the briquette. This study, therefore, demonstrates that Biochar-PVA-CNC nanocomposite briquettes are a potential biofuel for industrial and household applications.