A thermodynamic framework for the thermochemical valorisation of biomass

A general thermodynamic framework is developed that provides a unified perspective to analyse thermochemical biomass valorisation processes, such as pyrolysis, torrefaction, conventional, hydrothermal, and supercritical gasification, co-medium gasification, hydrothermal carbonisation. This approach is based on the element chemical potentials and the observation that the final equilibrium state of a thermochemical process consists mainly only of H2, CO, CO2, CH4, C() (char), and water, and depends only on its temperature, pressure, and overall elemental composition. It is completely independent of the particular compounds in the starting biomass feedstock. This allows the comparison of processes and operating conditions (e.g., selection of temperature and pressure, use of co-agents, etc.) to optimise efficiency and the product yield based on a particular feedstock. These results can be compactly summarised in terms of ternary diagrams, which can display information such as when char will form and the relative syngas yield. This framework is used to explore the use of water and carbon dioxide as a co-media for the gasification of different feed stocks to avoid char formation at high temperatures and to improve product yields and alternatively as co-carbonisation agents to increase char yields at low temperatures. It is then used to optimise the product yield and energy usage for the gasification of acetic acid and wheat straw, where it is found that maximum efficiency is achieved at ambient pressure and high temperatures, with sufficient added water or carbon dioxide to move the system out of the char-forming regime.