Water plays an essential role in HTL. It is therefore critical to understand the fundamentals of water chemistry when subjected to high temperature conditions. Water is rather benign and will not likely react with organic molecules under standard environmental conditions (20°C and 101,325kPa). However, when the temperature increases, two properties of water mole­cules change substantially. First, the relative permittivity (dielectric constant), er, of water decreases quickly when the temperature increases. When the thermal energy increases, the shared electron by oxygen and hydrogen atoms tends to circulate more evenly and the electro­negativity of the oxygen molecule is reduced (less polar). For example, when temperature increases from 25°C to 300°C, the relative permittivity decreases from 78.85 to 19.66, result­ing in water molecules from very polar to fairly nonpolar, in a relative term. This polarity
change makes water more affinitive to the organic hydrocarbons, most of which are nonpolar molecules.

Second, the dissociation of water dramatically increases with the increase of temperature. Water, like any other aqueous solutions, split into H+ and OH+ ions in hydrolysis or dis­sociation. This process is reversible and the rate is sufficiently rapid so it can be considered to be in equilibrium at any instant. Based on Arrhenius reaction rate, the equilibrium con­stant (or the dissociation constant), Kw, affected by the temperature change, can be written as (Benjamin 2002):

where Kw1 and Kw2 are equilibrium constant at temperatures T1 and T2 , respectively; AEAr is the net change in heat content of the molecules in the overall reaction, also called the molar enthalpy of reaction; R is the universal gas constant, and T is the absolute temperature in Kelvin. AEAr is an empirical constant specific to particular reaction and in units of energy per mole.

The effect of temperature on water dissociation is illustrated in Figure 10.1, where the left side vertical axis is in pKw = — log10(Kw), and the right side vertical axis is the ratio of Kw to the Kw0. Kw0 is the water dissociation constant at a temperature of 25oC at atmospheric pressure, and is 10+14, From Figure 10.1, water molecules dissocia­tion constant at 300oC is about 500 times higher than that of 25oC at atmospheric pressure. The increase in the dissociation constant will increase the rate of both acid — and base-catalyzed reactions in water far beyond the natural acceleration due to increased temperature.

There is also a (usually small) dependence on pressure (ionization increases with increasing pressure). The dependence of the water ionization on temperature and pressure has been fully investigated (Figure 10.2) and a standard formulation exists (IAPWS 2004).

For the above two reasons, water at high temperatures becomes a good solvent for hydro­carbons that are typically nonpolar hydrophobic under standard environmental conditions. These changes in physical properties make the solvent properties of water at 300°C roughly equivalent to those of acetone at 25°C. Ionic reactions of organics should be favored by increased solubility in water. The enhancement of this solubility of hydrocarbons in water will further enhance the possibilities of contact of dissociated H+ with hydrocarbons, hence accelerate the activities of hydrolysis.

The dramatic changes in the physical and chemical properties of water as temperature increases suggest the possibility of organic chemical reactions to take place (Siskin and Katritzky 1991; Kruse and Gawlik 2003). In addition, water has the ability to carry out con­densation, cleavage, and hydrolysis reactions, and to affect selective ionic chemistry, which is not accessible thermally, largely due to changes in its chemical and physical properties, which become more compatible with the reactions of organics as the temperature is increased.

Some classes of organic molecules in biomass proved very susceptible to water’s influence. Hot water as a reactant and catalyst likely creates a second pathway for the cascade of molecular transformations that leads to oil. In this pathway, water causes organic material to disintegrate and reform (by adding H+ to open carbon bond) into fragments that then transform into hydrocarbons. This implies that hot water becomes a catalyst for a series of ionic reac­tions. The acidic and basic nature of hot water—rather than heat—drives this cascade. For example, water may function first as a base, nibbling away at certain linkages in the organic material. As new molecular fragments build up and modify the reaction environment, water can change its catalytic nature. It can then act as an acid, accelerating different reactions. The resulting products attack parts of the remaining molecules, further speeding the breakdown (Siskin and Katritzky 1991). The above analysis may also help to explain why HTL will more likely directly convert biomass into oil than pyrolysis in which water is not involved.