Multi-objective optimization of biomass pelletization from corn stalks and aqueous phase obtained from hydrothermal carbonization of tobacco stalks

This study focuses on optimizing the utilization of the hydrothermal carbonization aqueous phase (HTC-AP) by blending its extract, namely hydrothermal carbonization aqueous phase desiccated solid (HTC-APDS), with corn stalks in the synergistic torrefaction and pelleting process. The aim was to explore the impact of different process sequences on the performance of biomass fuel pellets and to identify new avenues for the resource utilization of HTC-AP. Using a combination of response surface methodology (RSM) and an artificial neural network integrated with a multi-objective genetic algorithm (ANN-MOGA), the effects of torrefaction temperature, moisture content, and HTC-AP addition on pellet performance were systematically analyzed. The results showed significant differences in pellet performance between two torrefaction process sequences: torrefaction followed by pelleting (TP) and pelleting followed by torrefaction (PT). In the TP mode, the optimal conditions were 10 % moisture, 10 % HTC-AP addition, and a torrefaction temperature of 251–255 °C. At these settings, the predicted calorific value, compressive strength, and density were 16.24 MJ/kg, 17.42 MPa, and 1173.27 kg/m3, respectively. PT mode yielded optimal results at 10 % moisture, 10 % HTC-AP addition, and a torrefaction temperature of 244–252 °C, with predicted calorific value, compressive strength, and density of 18.68 MJ/kg, 1.66 MPa, and 643.2 kg/m3, respectively. These experimental results are consistent with the predicted values under the optimized conditions. Microscopic characterization using Fourier transform infrared (FTIR) and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) highlighted the potential role of HTC-AP in the torrefaction and pelleting processes. Torrefaction promoted the diffusion of material components, and the presence of potassium chloride likely enhanced calorific value and reduced energy consumption. However, moisture evaporation and partial biomass decomposition may disrupt the formation of solid and liquid bridges, leading to reduced density and significantly weakened pellet strength. These effects may limit the effective utilization of HTC-AP. The TP mode achieved higher compressive strength and density, rendering it suitable for products that demand enhanced storage and transportation performance. In contrast, the PT mode exhibited a higher calorific value, a characteristic that is particularly advantageous for applications with elevated calorific value demands. Overall, this study provides a new pathway for HTC-AP utilization and offers a theoretical basis and process design guidelines for the improvement of biomass fuels and the efficient utilization of agricultural waste.