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8.4 Biofuels 251

In continuous operation, slurry feeding is a challenge because of the water
evaporation at the end of the pipe. Poor conversions were reported because of the
loss of water by condensation. Serious erosion and/or corrosion occurred on several
spirals of the auger where the reaction was actually took place before the feedstock
enters the reactor. Char formation was found to occur downstream because of lack
of water brought about by back condensation. Some differences have been found in
the chemical composition of the oil from the batch process and the semi-continuous
process. A higher percentage of aromatic and/or unsaturated carbon was found in
the oil produced using the semi-continuous process.

8.4.4.2 Hydrothermal Liquefaction

Hydrothermal liquefaction employs reactive hydrogen or carbon monoxide carrier
gases to produce liquid fuel from organic matter at moderate temperatures. Tem-
peratures between 300 and 400 °C correspond to the maximum yield of oil products
[50]. Direct liquefaction involves rapid pyrolysis to produce liquids and/or organic
vapors, whereas indirect liquefaction employs catalysts to convert non-condensable,
gaseous products of pyrolysis, or gasification into liquid products.
Technically, any matter with high organic content can be converted into another
form by thermochemical conversion, but the conversion rate depends on the
feedstock and the process itself. A variety of feedstocks are proven technically
successful including high-carbon content materials such as coal, peat, and ligno-
cellulosic material. Low-quality feedstock includes municipal and industrial wastes
and agricultural residues. Studies on thermochemical conversion of biomass were
predominantly focused on materials that are highly cellulosic, until recently an
emerging interest in liquid waste such as livestock manure (e.g., [50]) and algae
[34].
Many factors affect the conversion rate of a HTC process. They include, but not
limited to, temperature, pressure, retention time, reactant gases, feedstock, catalysts,
if any, and the reactor design. These parameters were found to be most important as
they dictate the yield and quality of the products. Regardless of the process, the new
fuel is inevitably more expensive than its parent fuel due to the extra energy and
equipment required for the conversion.
Hydrothermal conversion is a process that is similar to cooking of food. The
major differences between this process and other processes are use of wet feedstock
as raw material because it saves energy from drying the feedstock. Also, the
presence of water is beneficial for the conversion process.
The primary reactions in the conversion of biomass to oil likely involve the
formation of low-molar weight, water-soluble compounds such as glucose. Water
also acts as solvent and alkaline catalysts. More importantly, water plays a critical
role in the water-gas shift reaction