![]() ![]() In the field of hydrothermal carbonization, some publications have attempted to model this complex system of chemical reactions 14, 29- 38. Therefore, it is not surprising that several publications reported increasing hydrochar yield with increasing feedstock concentration 14, 27- 29.ĭue to these circumstances, the optimization within a kinetic model depends not only from the temperature and time, but also from the feedstock concentration. The formation mechanism, which is similar to a polymerization, is very likely a reaction with higher order then one. This mechanism results in an oligomer that further precipitates from the solution, thus, forming a new condensed phase that agglomerates to spherical particles 25, 26. ![]() It is postulated that the formation mechanism consists of an aldol condensation of HMF with its consecutive product 2,5-dioxo-6-hydroxy-hexanal, followed by further addition of HMF 24. Most knowledge in this field is certainly derived by a reaction network starting from cellulose, followed by hydrolysis to glucose, isomerization to fructose, dehydration to hydroxymethylfurfural (HMF) and finally aldol condensation that forms carbon spheres/secondary char. It is, therefore, likely that the spheres are liquid droplets, behaving like an emulsion under hydrothermal conditions. It can be demonstrated that these spheres further grow after conversion of the relevant intermediates/precursor, hence, it can be said that the growth mechanisms follows a coagulation mechanism 14. The scientific literature covers a lot of examples, dealing with the formation of these carbon spheres from various feedstock materials, such as lignin hydrolysates 11, glucose 7, 9, 12, 13, fructose 14- 16, sucrose 17, agarose 18, phenolic compounds 19, 20, xylose 20, cellulose 21, cyclodextrin 22 and starch 23. The size of the particles varies from several hundred nanometers to maximum several tens of micrometers. The latter can be regarded as some sort of precipitate from the solution, these particles appear like solidified droplets with spherical shape and are, therefore, often referred as carbon spheres. Consequently, the final hydrochar from lignocellulosic biomass is a mixture of primary char that is covered by secondary char. The unhydrolyzed fraction, mainly lignin and some parts of the cellulose undergo solid-to-solid conversion yielding so called primary char. Those mostly dissolved intermediates can either degrade to a large variety of organic compounds, mainly organic acids 9, or polymerize to form so-called secondary char 10. Low amounts of phenolic compounds are formed, e.g., by partial lignin hydrolysis. The carbohydrates hydrolyze under hydrothermal conditions, yielding intermediates such as xylose or glucose. The three major components of lignocellulose are hemicellulose, cellulose and lignin. In HTC, the reaction network consists of several consecutive and parallel reactions. The main product is a solid lignite-like char, often referred to as hydrochar, which can be used as fuel or further processed to advanced carbon materials 8. ![]() Hydrothermal carbonization (HTC) is a conversion technology for wet biomasses, such as digestate, sewage sludge, municipal organic waste as well as lignocellulosic biomass in general 1- 7. ![]()
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