Challenges and Opportunities of Hydrothermal Carbonization of Biomass
Hydrothermal Carbonization (HTC) is one of the most promising methods for biomass treatment. Research on the HTC in the lab-scale has progressed recently. However, due to complex reaction mechanisms and operating conditions (e.g., high temperature and pressure), more improvements are required in order to account it as a commercial technology. This thesis is an effort to find solutions for some of the impediments and gaps towards commercialization of HTC including research on the effects of recycling water and biomass particle size on the HTC, finding a generic model to predict the main products from HTC, developing a reliable heat and mass transfer model that considers the complexities such as the heat of the reactions and the porous nature of the biomass, development of a continuous reactor and characterization of its products, and the integration of HTC with other processes. The results indicate that in order to move towards an industrial set-up, the recovery of water and heat to the process not only decreases the energy and water consumption but also improves the quality of the products. In addition, a proper kinetic and heat and mass transfer model is a necessary step for design of an industrial HTC reactor. By knowing the time and the amount of energy released by the exothermic reactions during the HTC process, there are opportunities for consuming less energy. Moreover, the results of performing HTC in a novel continuous reactor designed and developed in this research revealed that continuous HTC at temperatures above 240 oC, reaches higher degrees of carbonization and delivers products with higher porosities and thermal stabilities. The research also succeeded in finding generic equations for prediction of HTC with regards to the biomass composition (cellulose, hemicellulose, and lignin), and process intensity (temperature and time). Finally, by applying the knowledge obtained from the previous parts, an integrated HTC and anaerobic digestion (AD) system was proposed and it was shown that using HTC for conversion of biomass with high moisture content to energy is promising.