The HTL process can be described by the following conceptual equation:

Biomass ^ Gas + water dissolved organics + solvent soluble [18 1]

hydrophobic organics + solvent insoluble hydrophobic organics + H2O

The gas consists primarily out of CO2 (and some CO and CH4), while the aqueous phase contains relatively small oxygenated hydrocarbons. The organics in the hydrophobic phase contain considerably less oxygen than the feedstock, in the order of 10-30 wt.% compared to more than 45% in the original oil. Without separation, the solvent-soluble (e. g. acetone) organics and solvent-insoluble organics define one product phase, sometimes referred to as bio-crude. This product has a thick paste-like appearance. The separated solvent-soluble organics form an oily substance: HLO. Table 18.1 lists selected properties of HLO. Solvent-insoluble organics are solid and char-like at room temperature.

Reported yields are in the following ranges: gas: 5-15 wt.%; water-dissolved organics: 10-25 wt.%; solvent-soluble hydrophobic organics: 30-60 wt.%; solvent — insoluble hydrophobic organics: 0-30 wt.%; and water: 10-30 wt.%. The residence time is a dominant process parameter with respect to the product distribution. After ca. ten minutes reaction time, the gas, water and water-dissolved organics yields become constant. After that time, a significant part of the oil (solvent-soluble organics) is transferred to solvent-insoluble organics (char).2 At low residence times (less than five minutes), it is possible to produce only very limited amounts of char-like product, but deoxygenation is then obviously limited. The effect of residence time on the product distribution is visualized in Fig. 18.2.

The chemistry of HTL is complex. Nevertheless, all HTL reactions can be classified according to their mechanism: ionic and free-radical reactions.14-16 Hydrolysis reactions, a class of decomposition reactions of organics involving breakdown by water, are typical ionic reactions catalyzed with bases or acids. Cellulose and hemicelluloses may be completely hydrolyzed under HTL conditions, while only partial hydrolysis of lignin is possible without a catalyst.17 It should be realized here that in the biomass structure, hemicellulose is bound partially to lignin as well, complicating the hydrolysis reactions and possibly promoting char formation as well. Complete dissolution (hydrolysis) of woody biomass was, however, recently demonstrated with Na2CO3.18 Ionic reactions are accompanied/followed by free radical decomposition reactions. These thermal reactions are favoured over ionic reactions at lower pressures, lower densities (gases) and higher temperatures.19,20 Susceptibility of biomass constituents towards thermal degradation decreases in the order: hemicelluloses, cellulose, lignin.21,22 In hot compressed water, thermal reactions tend to produce primary char, similar to the char-forming reactions in pyrolysis. During HTL, two types of char have been identified, viz. primary char and secondary char. Primary char is the residue that remains after conversion of solid biomass particles. Secondary char is the char produced via polymerization reactions of liquid decay products. Both phenol (lignin) and sugar derivatives have been identified as being susceptible
towards polymerization, but it may be reasonable to assume that primary char is caused primarily by the lignin and secondary char by the sugar derivatives.1,18,23 Anyway, these polymerization and polycondensation reactions lead to the increase of the average molecular weight of the oil and eventually lead to the formation of solvent-insoluble components (char). Due to the higher order in the reactants (~2) of these polymerization reactions, polymerization can be reduced by dilution. Na2CO3, as a catalyst, is known to prevent polymerization reactions and thus char formation, but its effect is more complex than just dilution.24-26 Appell et al.21 proposed the following biomass liquefaction mechanism with Na2CO3 and CO

Although the char yield (primary and secondary) is reduced by using alkalis, their use in HTC also has drawbacks. Alkalis react in the process, and the recovery of sodium or potassium from liquid and solid products could be a complicated and

costly procedure.26 In contrast, heterogeneous catalysts do not react away and are easy to recover. For that reason, Knezevic et al.2 studied the influence of a Ru/TiO2 catalyst on the HTL process. Compared to non-catalytic experiments, they found that the tested catalyst (1) increased the gas yield, (2) decreased the char yield and

(3) had a small effect on the oil yield. The lower char yields in catalytic tests suggest that the catalyst was able to convert the solvent-insoluble product (char) into gas. This was confirmed in an independent experiment using the solvent — insoluble product of glucose as a feedstock. In this test of 60 minutes at 350°C, ca. 30 wt.% of gas was produced that consists of CO2 and CH4. Without catalyst, under otherwise identical conditions, gas production was only 2-3 wt.%.1

Although the exact reaction pathway of HTL is not yet unravelled, at least four reactions have to be incorporated in a lumped (engineering) reaction path model: (1) depolymerization reactions, (2) decay reactions of the monomers (e. g. dehydration: glucose ^ HMF), (3) reactions causing the formation of gas (CO2 and/or CO by decarboxylation/decarbonylation), and (4) polymerization/ polycondensation reactions.

Knezevic et al.28 visualized the liquefaction of wood in closed quartz capillaries (see Fig. 18.3a). The formation of secondary char from glucose (Fig. 18.3) was also visualized in those capillaries.1

For a detailed overview of the chemical reactions in hot compressed water, the reader can refer to several reviews.14-16,29-31