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5.3.3 role oF FeedSToCk
The nature of feedstock is the most important variable affecting the quantity and
quality of bio-oil by the HTL process. HTL produces high-density liquid fuels and
operates in the presence of water eliminating the need for feedstock drying that
is important in dry pyrolysis process. The HTL process uses agricultural biomass
and biowaste including crop residues and wood, food processing waste, animal and
human manure, and algae. The HTL process can also be used as a pretreatment
process for the subsequent fermentation of the feedstock that are difficult to convert.
While animal and food processing waste contain lipids, proteins, and small
amounts of lignocellulose, crop residues and wood primarily contain lignocellulose.
The primary basic compounds in these feedstock are various isomers of glucose
(such as d-glucose and l-glucose), hemicellulose, cellulose, lignin, amino acids,
proteins, lipids including fatty acids such as stearic acid and palimitic acid, and so
on. Over the years, more and more efforts have been made to examine the effectiveness of the HTL process for a variety of lignocellulosic wastes containing different amounts of lignins and crystalline cellulose. Some of the materials examined
are swine manure, garbage, Indonesian biomass residue, birch wood, sawdust, rice
husk, phytomass, and chlorella [41–47] (Elliott, 2012, pers. comm.). These feedstock have been tested in the temperature range of 280°C–375°C, the pressure range
of 5–50 MPa, and the residence time of 5–180 min depending on the feedstock.
In some cases, sodium carbonate was used as a catalyst. The final yield varied
from 21% (in case of garbage) to as high as 61% (in case of swine manure) [41–47]
(Elliott, 2012, pers. comm.). More recently, HTL has been applied to algae for making biofuel. HTL can thus be applied to a variety of biomass with a varying degree
of success. In the following discussion, feedstock is broken into three categories:
biowastes, lignocellulosic wastes, and algae. These three categories cover the range
of feedstock property variation.
5.3.3.1 Biowastes
The best raw materials for the HTL process are perhaps biowastes such as various
types of manures and wastewaters because they are mostly cellulosic (with very little
lignin) and can be easily converted into bio-oil under hydrothermal conditions. In
general, presence of high cellulose and hemicellulose content in biomass yields more
bio-oil. Appell et al. [49–52] were the first to examine various waste streams such
as urban refuse, cellulosic wastes, and sewage sludge, and found that at a temperature around 380°C, a pressure of 1500 psig, and a residence time of 20 min, even
in the absence of a catalyst, an oil yield of about 24.5% was obtained. At low temperatures, oil was largely paraffinic and cycloparaffinic in the presence of carboxyl
and carbonyl groups, whereas at high temperatures some aromatics were present.
Following this pioneering study, a significant number of additional studies were published [41–48,53–68] (Elliott, 2012, pers. comm.), indicating that the applicability
of the HTL process to cellulosic wastes is easy. These studies are also extensively
described in a number of recent reviews [41–48] (Elliott, 2012, pers. comm.). The
studies included swine manure, bovine manure, cellulose, activated sludge, sewage
sludge, artificial garbage, protein-containing biomass (such as food wastes), various