Hydrothermal carbonization of fructose—effect of salts and reactor stirring on the growth and formation of carbon spheres

Abstract

Hydrothermal carbonization (HTC) has become a promising technology for the production of hydrochar and carbon spheres. Several studies indicate a strong dependency of the reaction conditions on the sphere diameter. The usage of additives, such as salts, is one possibility to increase the size of the spheres. However, the growth mechanism which leads to larger particles is not fully understood. In this work, kinetic studies of HTC with fructose were performed with different salts as additives. The growth of the particles (the increase in size) has been compared to the formation rates (increase in yield) of hydrochar by using the reaction rate constants from the kinetic model. The results indicate that the acceleration of the growth rate is independent of the formation rate. It is therefore assumed that coagulation, as a growth mechanism, took place. With longer reaction times, the particles reached a stable particle size, independently from the added salts; therefore, it was assumed that the particles underwent some sort of solidification. The state of matter can therefore be described as an intermediate state between liquid and solid, similar to mesophase pitch. Experiments with a stirrer resulted in squashed particles, which supports the model, that the particles exhibit emulsion-like behavior.

Low-Temperature Solvothermal Method for Coal Tar-Based Carbonaceous Mesophase Preparation and Its Excellent Performance

“…Figure presents the polarized light microscopy images and schematic illustration of the formation process of CMs. As shown in Figure d, the formation process of CMs aligns with the conventional mechanism. ,, …”

“…As shown in Figure 1d, the formation process of CMs aligns with the conventional mechanism. 18,22,25 Initially, the precursor undergoes dehydrogenation and aromatization of alkanes. Subsequently, aromatic compounds undergo condensation and carbonization, leading to the formation of carbonaceous mesophase pellets, as shown in Figure 1a.…”

Computational Modeling Approaches of Hydrothermal Carbonization: A Critical Review

“…Studies that investigate the influences of performing HTC with stirred compared to non-stirred reactors as well as the effect of the stirring speed are uncommon. A study by Jung et al [70] compared non-stirred and stirred experiments on the HTC of fructose for the formation of carbon spheres. They found that with the stirred HTC experiments, the hydrochar was composed of more agglomerates and experienced distorted morphologies.…”

Integrated Humin Formation and Separation Studied In Situ by Centrifugation

“…8,22−26 However, some recent research gives evidence that humin spheres form as highly viscous liquid droplets instead of solid particles because the growth of the spheres is recognized after feedstock depletion, indicating coagulation, and a strongly altered humin shape due to shearing by stirring of the reaction mixture is observed. 21,27 Clearly, the spherical morphology of some particles indicates the presence of a liquid state during formation, but it has been neither observed systematically nor formulated as a complete hypothesis of humin formation as highly viscous droplets up to the formation of solid layers of deposits. Based on the reported assumption that liquid humin droplets could be altered in morphology by shearing such as at stirred reaction conditions, we additionally hypothesize that liquid humin droplets could be forced into coalescence by application of a strong gravitational field.…”