Hydrothermal carbonization and pelletization of moistened wheat straw

Abstract

This study investigated the potential of hydrothermal carbonization (HTC) using the inherent moisture (50% wet basis) of wheat straw for improving the undesirable characteristics of wheat straw and producing high-quality pellets. The material was treated at temperatures of 100, 140, 180, and 220 °C for 15 and 30 min. When compared to the untreated sample, the pellets produced from samples treated at 220 °C for 30 min showed the lowest water adsorption of 10%, the highest bulk density of 147.6 kg/m3, and the highest HHV of 22.2 MJ/kg, though the ash content increased to 8.9%, and the durability was reduced to 87.0%. Pellets produced from samples treated at 140 °C for 30 min showed the highest durability of 97.7% and hardness of 5.1 N/mm2 with enhanced HHV of 18.3 MJ/kg, and unchanged ash content. The treatment time showed significant effect (p < 0.05) on pellet durability, hardness and water adsorption at temperature of 100 and 140 °C. However, there was no obvious effect when the temperature was 180 and 220 °C. The kinetic analysis indicates that the activation energy drops from 89.9 kJ/mol (raw wheat straw pellet) to 44.4 kJ/mol (220–30 pellet) and from 162.2 kJ/mol to 93.8 kJ/mol using the reaction order model and the 3D diffusion model, respectively. Pellets produced by HTC at 140 °C for 30 min make high-quality pellets with lower energy and water consumption as compared to the more severe HTC conditions previously reported.