The primary distinction lies in the fact that water exhibits greater polarity than CO2, thereby enabling it to extract more polar molecules, such as sugars and proteins, compared to CO2. In contrast to CO2, where escalating pressure results in increased polarity, the opposite effect is observed in water. As we raise the temperature, the density and dielectric constant of water diminishes, thereby reducing its polarity. Lastly, in extraction processes, we typically operate within the subcritical region of water, as the supercritical state can potentially damage various compounds. In contrast, with CO2, we have the flexibility to operate in both its subcritical and supercritical regions.

While both CO2 and water systems manage pressure, flow, and temperature, their construction varies significantly. A primary distinction arises due to the possibility of encountering chlorides when using water with biomass. To prevent related issues, high-nickel alloys, such as Hastelloy or Inconel 625, are employed in the construction of these systems. Additionally, when we extract with CO2, we operate at lower temperatures compared to water, which is above 100 degrees Celsius. Therefore, the resultant water stream must be cooled to safe temperatures prior to collection.

We can operate under a broad spectrum of pressures and temperatures within the subcritical region of water. As long as our operations are conducted below the supercritical threshold of water, which is 373 degrees Celsius and 220 bar, and above 100 degrees Celsius, we are indeed operating within the subcritical region of water.
Water phase diagram – Redraw (liquid phase = subcritical phase).

Subcritical water can facilitate several reactive chemistries, leading to the breakdown or decomposition of diverse biomass sources. Depending on the pressure and temperature, two primary reactions - hydrothermal carbonization or hydrothermal liquefaction - can take place. Hydrothermal carbonization typically occurs at temperatures below 245 degrees Celsius, while hydrothermal liquefaction generally happens within the temperature range of 245 to 373 degrees Celsius.

Hydrothermal carbonization (HTC) is a thermochemical process that converts organic material, such as biomass, into a carbon-rich solid known as hydrochar. The mechanism of HTC involves several reactions including hydrolysis, dehydration, decarboxylation, and aromatization, leading to the formation of the hydrochar. The overall process reduces the oxygen and hydrogen content of the biomass while increasing the relative carbon content, thereby increasing its energy density.

Hydrochar generated through hydrothermal carbonization (HTC) exhibits coal-like properties and can serve as a solid fuel. Additionally, it possesses the potential for further refinement into valuable materials such as activated carbon. One significant advantage of hydrothermal carbonization is that the feedstock does not require prior drying before undergoing the process.

Hydrothermal liquefaction (HTL) is a thermochemical conversion process that involves the conversion of biomass or organic feedstock into a liquid product under high temperature and pressure conditions in the presence of water. During hydrothermal liquefaction, the biomass or organic material is typically mixed with water and subjected to temperatures ranging from 245 to 373 degrees Celsius and pressures ranging from 100 and 250 bar. Under these conditions, the biomass undergoes a series of chemical reactions, including hydrolysis, dehydration, decarboxylation, and hydrogenation.

The result of hydrothermal liquefaction is a liquid product known as bio-oil or biocrude, which has similarities to fossil crude oil. This bio-oil can be further refined and upgraded to produce transportation fuels, such as gasoline, diesel, and jet fuel. Additionally, other valuable co-products like solid biochar and aqueous phase can be obtained during the process.

Hydrothermal liquefaction offers several advantages, including the ability to process a wide range of feedstocks, including wet biomass and waste materials.