Hydrothermal Carbonization of Fructose: Growth Mechanism and Kinetic Model

Abstract

Hydrothermal carbonization experiments were performed with fructose solutions as feedstock. A kinetic model is proposed that models hydrochar formation based on molar concentrations. On the basis of SEM pictures, the mean particles size of the hydrochar was determined, and it could be demonstrated that the growth of the particles is not directly related to the formation rate of hydrochar or the decreasing rate of the intermediates concentration in the aqueous solution. Four additives (salts) have been tested, and the results point out that the ionic strength is an important factor to control the particle size. Together, these observations lead to the conclusion that coalescence is an important driver for the growth of the particles and that a LaMer-like nucleation followed by diffusion-controlled growth is unlikely to be the formation and growth mechanism of hydrochar.

Influence of the pH Value on the Hydrothermal Degradation of Fructose

“…While for real biomass it could be demonstrated that the lignin content significantly affects yield and properties of the hydrochar, humin obtained from the hydrothermal treatment of pure sugars is considered polycondensated HMF. This is evident from the elemental composition of humin, which complies well with HMF minus one water molecule . What kind of condensation reaction takes place, is still matters of discussion.…”

“…When performing hydrothermal treatment of sugars or biomass, a solid, insoluble by‐product is usually observed, which is labelled as humin or hydrochar. Hydrochar is characterised by a high carbon content of around 67 % w/w . Hence, the hydrothermal carbonisation especially of agricultural residues features a field of growing importance in order to produce absorbents, fertilizers etc.…”

“…propose that intermediate 2,5‐dioxo‐6‐hexanal (DHH) is generated from HMF, which undergoes Aldol condensation with additional HMF and DHH molecules. Jung et al . propose that HMF might enolise and enolised HMF undergoes Aldol condensation with HMF.…”

“…Hydrochar is characterised by a high carbon content of around 67 % w/w. [28,29] Hence, the hydrothermal carbonisation especially of agricultural residues features a field of growing importance in order to produce absorbents, fertilizers etc. from renewable resources.…”

“…This is evident from the elemental composition of humin, which complies well with HMF minus one water molecule. [29] What kind of condensation reaction takes place, is still matters of discussion. Patil et al [30] propose that intermediate 2,5-dioxo-6-hexanal (DHH) is generated from HMF, which undergoes Aldol condensation with additional HMF and DHH molecules.…”