Development and utilization of a Raman characterization method for Hydrothermal char

Avery B. Brown, Worcester Polytechnic Institute


Hydrothermal carbonization is a process by which biomass in water is thermochemically converted to a brown activated carbon known as hydrothermal char. As a biomass upgrading process hydrothermal carbonization has several advantages, the process is conducted in liquid water meaning pretreatment in the form of drying is not required. The process is normally conducted in batch reactors at temperatures ranging from 180 to 350°C and reactions of 0.5 to 24 hours. These mild conditions are not energy intensive and allow for the technique to deployed cheaply. The process involves no dangerous solvents or chemicals, and the liquid product has potential applications as a liquid fuel. Hydrothermal carbonization is a cost-effective technology that provides a solution to the growing waste and biomass production of the United States. However, there is a growing interest in the processes potential to produce a material that can be utilized for advanced materials. Hydrothermal chars have been shown to be highly susceptible to post-treatment by acid, temperature and mechanical methods. With a wide variety of potential feedstocks, treatment and post-treatment conditions a deeper understanding of the structure of material and how this structure is impacted by these conditions is needed to allow for its optimal use. Vibrational spectroscopy is a powerful tool for elucidating the structure of materials. Specifically, Raman spectroscopy is largely untapped method for the characterization of Hydrothermal chars. Raman spectroscopy is a scattering method that like IR spectroscopy utilizes an incident laser to produce vibrational responses in the molecule being studied. Raman spectroscopy can potentially be used to observe the carbon structure of a molecule. Typically, observations of this structure are conducted using Nuclear Magnetic Resonance, which is a relatively slow an expensive technique. Previous use of Raman spectroscopy in the field of hydrothermal carbonization has either underutilized the technique making observations of the presence of carbon or has been mischaracterized incorrectly stating the structure. In this work we have used Density Functional Theory to elucidate the Raman patterns of polycyclic aromatic hydrocarbons such as the ones that are the building blocks of hydrothermal chars. As a result of this work we have established a fitting method that explains the origins of Raman bands that are consistent with the structural motifs observed by more expensive techniques. The theoretical method is next deployed to the Raman spectrum of hydrothermal char derived from the treatment of glucose, a common model biomass. It is shown that common Raman spectroscopy methods when applied to hydrothermal char severely change the surface of material. How Raman spectroscopy impacts the surface of hydrothermal chars was studied and methods of mitigating these changes were developed. With a full compliment of theoretical and practical tools at our disposal we then apply these methods in attempt to understand the changes that occur to the structure of glucose based hydrothermal chars as a function of temperature and time.