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Water for Energy and Fuel Production
In the HTL process, other sets of complex reactions such as cracking and
reduction of polymers such as lignin and lipids, hydrolysis of cellulose and hemicellulose to glucose and other simple sugars, hydrogenolysis in the presence of
hydrogen, reduction of amino acids, dehydration, decarboxylation, C–O and
C–C bond ruptures, and hydrogenation of various functional groups result in the
production of liquids from biomass rather than solids as they occur in the HTC
process [41–52] (Elliott, 2012, pers. comm.). More fundamental work in this area
is still needed.
5.3.2 eFFeCTS oF oPerATing CondiTionS on hTl ProCeSS
Unlike the HTC process, the main objective of the HTL process is to generate biooil of high quality. The process is designed to minimize the production of solids
and gas. Numerous process parameters affect the performance of the HTL process
and these are well examined in the literature [41–48,53–66] (Elliott, 2012, pers.
comm.). The major process operating parameters are (1) temperature, (2) residence
time, (3) solids concentration, (4) pressure, (5) biomass heating rate, (6) biomass
particle size, (7) presence of hydrogen donor solvent and reducing gas environment,
and (8) pH of slurry. Since the most important variable is the nature of feedstock, its
effect on process performance is discussed in Section 5.3.3. The literature information on the effects of various operating parameters on the product distribution was
well summarized in an excellent review by Akhtar and Amin [41]. Sections 5.3.2.1
through 5.3.2.3 briefly summarize their assessments.
5.3.2.1 Pressure, temperature, and residence time
In any HTL process, pressure must be at least equal or above the saturation pressure
to maintain the process in a single-phase operation. High pressure allows a better
manipulation of hydrolysis reaction and the reaction pathways are thermodynamically favorable to produce liquids and gases. High pressure also increases the solvent
density, resulting in better extraction capability of the solvent. For a catalytic operation, however, high solvent density can block the active catalyst sites and therefore
reduce C–C bond breakage and the resulting degradation rate.
Generally, high temperature increases both the concentration of free radicals
and the probability of repolymerization of fragmented species. The hydrolysis
and fragmentation of free radicals dominate in the early stages of the reactions,
whereas repolymerization occurs in the later stages of the reaction, which in turn
forms char. Generally, at very high-temperature bio-oil production is reduced due
to (1) the secondary decompositions and Boudouard gas reactions that become
active at high temperature leading to high gas formation or (2) the recombination
of free radicals to form char. The overall process conditions and the presence of a
catalyst generally dictate the dominant reaction mechanism. For most feedstock,
however, the maximum bio-oil is obtained at temperatures around 300°C–350°C
[41]. Also, the literature results show that the largest shift in the optimum temperature for bio-oil occurs for algae [67–79]. Both softwood and grass are generally
more difficult to liquefy because of their higher lignin content and less reactive
cellulose content.
Water for Energy and Fuel Production
In the HTL process, other sets of complex reactions such as cracking and
reduction of polymers such as lignin and lipids, hydrolysis of cellulose and hemicellulose to glucose and other simple sugars, hydrogenolysis in the presence of
hydrogen, reduction of amino acids, dehydration, decarboxylation, C–O and
C–C bond ruptures, and hydrogenation of various functional groups result in the
production of liquids from biomass rather than solids as they occur in the HTC
process [41–52] (Elliott, 2012, pers. comm.). More fundamental work in this area
is still needed.
5.3.2 eFFeCTS oF oPerATing CondiTionS on hTl ProCeSS
Unlike the HTC process, the main objective of the HTL process is to generate biooil of high quality. The process is designed to minimize the production of solids
and gas. Numerous process parameters affect the performance of the HTL process
and these are well examined in the literature [41–48,53–66] (Elliott, 2012, pers.
comm.). The major process operating parameters are (1) temperature, (2) residence
time, (3) solids concentration, (4) pressure, (5) biomass heating rate, (6) biomass
particle size, (7) presence of hydrogen donor solvent and reducing gas environment,
and (8) pH of slurry. Since the most important variable is the nature of feedstock, its
effect on process performance is discussed in Section 5.3.3. The literature information on the effects of various operating parameters on the product distribution was
well summarized in an excellent review by Akhtar and Amin [41]. Sections 5.3.2.1
through 5.3.2.3 briefly summarize their assessments.
5.3.2.1 Pressure, temperature, and residence time
In any HTL process, pressure must be at least equal or above the saturation pressure
to maintain the process in a single-phase operation. High pressure allows a better
manipulation of hydrolysis reaction and the reaction pathways are thermodynamically favorable to produce liquids and gases. High pressure also increases the solvent
density, resulting in better extraction capability of the solvent. For a catalytic operation, however, high solvent density can block the active catalyst sites and therefore
reduce C–C bond breakage and the resulting degradation rate.
Generally, high temperature increases both the concentration of free radicals
and the probability of repolymerization of fragmented species. The hydrolysis
and fragmentation of free radicals dominate in the early stages of the reactions,
whereas repolymerization occurs in the later stages of the reaction, which in turn
forms char. Generally, at very high-temperature bio-oil production is reduced due
to (1) the secondary decompositions and Boudouard gas reactions that become
active at high temperature leading to high gas formation or (2) the recombination
of free radicals to form char. The overall process conditions and the presence of a
catalyst generally dictate the dominant reaction mechanism. For most feedstock,
however, the maximum bio-oil is obtained at temperatures around 300°C–350°C
[41]. Also, the literature results show that the largest shift in the optimum temperature for bio-oil occurs for algae [67–79]. Both softwood and grass are generally
more difficult to liquefy because of their higher lignin content and less reactive
cellulose content.