Preparation, quality analysis and bonding mechanism of densified bio-briquettes from cotton stalk and walnut shell wastes

Abstract

To contribute to regional sustainable development, briquetting of cotton stalk (CS), walnut shell (WS) in different particle sizes, and their blend was investigated at different temperatures at a constant pressure of 34.92 MPa. A total of 133 tests were conducted. The proximate and ultimate analysis, lower heating values (LHV), densities, moisture contents, and surface morphologies were determined for quality assessment. In results, the LHVs of raw CS/WS feedstocks and CS briquettes of coarse particles densified at 110–120 °C were 16.15/18.44 and 16.07–16.69 MJkg⁻¹, respectively. The medium and fine WS particles compressed at 110–120 °C and 100–120 °C, had the maximum LHVs around 18.96–19.99 MJ kg⁻¹, respectively. Some representative briquettes were selected based on their high LHVs for further evaluation. Their densities were greater than 1.0 g cm⁻³, and hybrids of coarse CS particles with medium WS particles compacted in the ratio range of 5:5–7:3 at 100 °C had higher LHVs (≥19.0 MJ kg⁻¹) and densities, complying with class A1 non-woody briquettes in ISO 17225–7:2021 and class TW2 commercial and industrial biofuels in ISO 17225–8. The molecules of the particles were solidified by forming new hydrogen bonds, dipole-dipole forces, and inter-particle solid bridges. The softened lignin in CS played an important role in particle binding in the absence of sufficient water, smooth and non-porous briquette surfaces were observed for the representative briquettes. In conclusion, the briquetting of coarse CS and medium WS particles in a 7:3 ratio at 100 °C was recommended for residential and industrial use due to its suitability for fully utilizing local agricultural waste.