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Water for Energy and Fuel Production
fuels. HTL involves a direct liquefaction of biomass in the presence of water (and may
be a catalyst) to liquid fuels in subcritical conditions. The process conditions for HTL
thus differ from the HTC process described earlier in that generally HTL process
requires the temperature range of about 250°C–400°C, a range that is higher than that
used in the HTC process and that does not use the conditions of supercritical water.
It is analogous to intermediate-temperature dry pyrolysis in that the objective of both
processes is to produce liquid with minimum amounts of solids and gases.
The HTL process is not a selective catalytic process such as aqueous-phase reforming (APR) and its derivative technologies described in Chapter 6, in which hydrogen, alkanes, and monofunctional groups are produced at low temperature (range of
215°C–265°C) from a selective group of oxygenated biomass by a set of selective catalysts or biofine process described in Chapter 7. When high-temperature HTL reaction
conditions are used with a selective group of catalysts, HTG process evolves. This
process largely generates methane and carbon dioxide with some hydrogen. The HTG
process is described in Section 5.4. Numerous excellent reviews on HTL are available
in the literature [41–47] (Elliott, 2012, pers. comm.).
5.3.1 reACTion meChAniSmS
HTL is a chemical transformation process of biomass in a heated and pressurized
water environment where long-chain organic compounds break into short-chain
hydrocarbons. All fossil fuels found underground, petroleum, natural gas and coal,
and so on based on biogenic hypothesis, are formed through the HTL process from
biomass buried beneath the ground and subjected to high pressure and temperature.
In the recent years, it has been found that kerogens (which are a large part of oil
shale) break down much easily in the presence of water than without it [41–48]
(Elliott, 2012, pers. comm.). Gas hydrates and several carbon isotope studies have
shown the involvement of water (and hydrogen from it) in the creation of natural
gas. HTL process reaction paths depend on the temperature, the pressure, the
reaction time, the water pH, the solids particle size, and the nature of the catalysts
(if present).
While the exact reaction pathway for the HTL process is as yet not known, the
study of Appell et al. [49–52] at Pittsburgh Energy Technology Center made some
important points for the process. They studied liquefaction of wood particles with
hydrogen and carbon monoxide at 370°C and 27 MPa pressure in the presence of
sodium carbonate catalyst. Alkali salts such as sodium carbonate and potassium
carbonate can initiate the hydrolysis of cellulose and hemicellulose into smaller fragments. The degradation of biomass into smaller products mainly proceeds by depolymerization and deoxygenation reactions. The amount of solids residue remained
depends on the lignin content. Lignin contains alkyl phenols and free phenoxyl radicals formed by its thermal decomposition above 250°C, and it is likely to recombine
and form the solids residue through condensation or repolymerization reaction.
Appell et al. suggested that during the conversion of carbohydrates to oil, sodium
carbonate reacts with carbon monoxide and water to form sodium formate as
(5.3)
Na CO
CO H O
HCO Na CO
2
2
3
2
2
2
2
+
+
→
+