What is hydrothermal upgrading? Water above 100 °C changes from a liquid phase to a gas phase. One can change the temperature boundary by changing the pressure – a low pressure lowers the boiling point, similarly increasing the pressure maintains a liquid phase at higher temperatures than the boiling point – precisely why pressure cookers work so efficiently and why there is no water left on Mars as it is boiled away in the atmosphere.
Water, among some other substances like CO2 can also exist as a supercritical fluid. For water this boundary is right around a temperature of 374 °C and a pressure of 218 atm. At this point the boundary between liquid and gaseous phase water disappears and we get what is known in the technical jargon as a supercritical fluid. Academicians and researchers across the globe have found a plethora of catalyst-like uses for supercritical fluids. In the biofuels context, supercritical water in combination with a chemical catalyst deconstructs the organic polymeric backbone of the biomass into very desirable energy products and produces clean oil that is entirely fungible with the petroleum counterparts.
The grand challenge in the biomass to liquids (BtL) technology is to remove water from the material. It is the > 50% water content or as some like to define “the inherent wetness” of biomass limits the energy density and thus the energy value of most biomass feed. Removing “oxygen” content from the highly functionalized biomass feed while maintaining or retaining maximum “carbon” is a major challenge in the field.
Using supercritical water to deconstruct biomass can be looked upon as a complementary technology towards the more traditional pyrolysis or gasification processes. While the impact of these processes are similar, one obtains a lot more oil as a product and there is usually no methane and no biochar, in addition one may use biomass straight from the fields circumventing the energy intensive pre-drying step. What makes this process different is that there are no polyaromatics, and the product can be processed in a conventional refinery. The resulting water stream generated from the HTU process would consist of valuable chemicals such as low-molecular weight aliphatics, lignin monomer molecules, oxidised lignin monomers, aromatic diacids, aromatic polyols, quinones, aromatics, O-heterocyclic compounds, and phenolics. A few of the commercially important aromatics can be seen in Table 1.
The HTU process offers a unique pathway to a product that can substitute a major portion of the products obtained from conventional crude oil spectrum. The process is well defined for forestry, agricultural waste, wood, paper/pulp waste, and algae and is competitive with $50 oil based on $100 per ton biomass – economically right in the sweet spot with enough margin to make the risk-return attractive. Figure showing HTU. With the highly experienced consultants at Lee Enterprises Consulting Inc., we are able to offer insight into this niche technology and further upgrading technologies for commercial partners.
Table 1. A handful commercially relevant aromatic compoundsa
|Benzoic acid||Rubbers, preservatives, dyes|
|Catechol||Printing, photochemicals, pharma, corrosion inhibitors|
|Hydroquinone||Antiseptics, photography, inhibitors, skin bleaching agents|
|1,4-dimethoxybenzene||Solvents, food, antioxidant|
|1,2-dimethoxybenzene||Synthetic precursor, food|
|2-methoxyphenol||Synthetic precursor to eugenol, vanillin, food|
|p-cresol||Synthetic precursor to antioxidants, solvents|
aProduced at levels greater than 1,000 tonnes per annum and are priced greater than ≥1800 USD/t
What is HTU?
Feedstocks: forestry, agricultural waste, wood and paper/pulp waste, algae
Conditions: Temperature: 300 – 350 °C; Pressure: 120 – 180 bar, Time: 5 – 20 minutes,
Solvent: liquid water present, Catalyst, Thermal Efficiency: 70-90%
Chemistry: Removing “Oxygen” from biomass as “Carbon-dioxide” and “Water”
- 45% biocrude (%w on feedstock)
- 25% gas (>90% CO2 remainder CO, lower alkanes e.g., methane, ethane etc.)
- 20% H2O
- 10% dissolved organics (e.g., acetic acid, ethanol, low-molecular weight aliphatics,
- oxidised monomers, aromatic diacids, aromatic polyols, quinones, aromatics, O-heterocyclic compounds, and phenolics)
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