Chemical Engineering Session A10
6197
Disclaimer—This paper partially fulfills a writing requirement for first year (freshman) engineering students at the University
of Pittsburgh Swanson School of Engineering. This paper is a student, not a professional, paper. This paper is based on publicly
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HYDROTHERMAL LIQUEFACTION OF ALGAE TO PRODUCE BIOFUEL
FOR THE TRANSPORTATION INDUSTRY
Abigale Gray, aeg51@pitt.edu, Vidic 2:00, Nicole Iden, nmi9@pitt.edu, Mena 4:00
based products are being heavily researched. Although solar
Abstract- As the search for an alternative to petroleum and wind are being introduced to replace the electricity
progresses, biofuels emerge as a viable contender. traditionally produced from burning coal, there is still no
Hydrothermal liquefaction (HTL) is a promising biofuel economically and environmentally viable alternative to the
production process that converts organic wastes into a “combustible fuels” used in transportation “that can fit into
workable biofuel. This paper focuses on the hydrothermal the existing hydrocarbon fuel infrastructure”[2]. Biofuels
liquefaction of algae to produce biofuels by describing the have been proposed as a viable alternative, however there are
technical process, the benefits algal biofuel provides, and various negative effects that can be associated with biofuels
how it can contribute to a solution for the world’s energy such as food scarcity, lack of arable land, and lowered
crisis. Hydrothermal liquefaction decomposes biological efficiencies [2]. To remedy some of these issues,
material into monomers by applying high heat and pressure. hydrothermal liquefaction technology is being investigated
Although the exact pathway that transforms the biomass is and refined in order to provide a more sustainable biofuel
still unknown, it is likely that it has two basic steps: hydrolysis production process and lessen the deteriorating effect of fossil
and repolymerization. Compared to other techniques of fuel combustion on the environment.
biofuel production HTL’s efficiency is quite high.
Additionally, hydrothermal liquefaction produces a biocrude
WHAT ARE BIOFUELS?
similar to crude oil used today. Thus, hydrothermal
liquefaction is one of the most favorable technologies
Organic matter can be manipulated in various ways: the
currently being researched as a fossil fuel replacement, one burning of woody biomass, combustion of plant by-products,
of the most pressing challenges society faces.
gasification, and many other processes. Each method provides
energy, but the energy is suited to a particular application. For
Key Words— Algae, Biofuels, Biomass, Green energy, example, if the energy is going to be used to heat a house, the
Hydrothermal liquefaction, Organic waste
most logical source would be burning wood as it provides a
large amount of heat. Similarly, the transportation industry
THE NEED FOR RENEWABLE ENERGY relies almost exclusively on liquid phase energy sources.
Therefore, a sustainable alternative would need to be
Energy demands are growing as the world population comparable to the liquid fuels that are currently in use. This
continues to increase rapidly. Alternative methods of energy creates the need for biofuels, an alternative energy source
generation are being explored to meet these demands, but, created through the decomposition of biomass transformed
when considering a source of energy, properties such as into a liquid bio crude with many of the same properties of
availability, energy content, and effectiveness must be taken petroleum based fuels. Elena Kazamia from the University of
into account. The search for a renewable fuel examines these Cambridge’s Department of Plant Sciences summarizes the
factors and aims to find a replacement for petroleum fuel biofuel production process as following a particular set of
products. Fossil fuels have been linked to the deterioration of steps: “cultivation of chosen crop, harvesting, processing, and
the ozone layer and global warming. The depletion of ozone extracting fuel substrate, converting into biofuel” and the
occurs as harmful chemicals such as CO2 and NOx are likely use of the biofuel in a combustion engine [3]. Biofuels
released from the burning of fossil fuels [1]. These chemicals have been researched since the late twentieth century, but they
decompose the ozone (O3), the atmospheric layer responsible have become much more prevalent recently as the world’s
for deflecting the majority of ultraviolet rays [1]. UV rays are energy crisis and environmental concerns worsen. Biofuel’s
a leading cause of diseases such as skin cancer and also sustainability stems from the fact that it absorbs CO2 as the
change the Earth’s climate. In an effort to lessen these feedstocks are cultivated along with lowered emissions during
negative environmental impacts, a substitute for fossil fuel
combustion. These factors lower biofuel’s global warming
potential and make it a sustainable alternative [3].
University of Pittsburgh Swanson School of Engineering
2016/03/04
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Abigale Gray
Nicole Iden
Third generation biofuel, as seen in Figure 1, is produced
from algae and microbes. These feedstocks are considered
promising as algae is not edible and requires little space in
which to grow, solving many of the problems encountered
with the first and second generations [5]. Third generation
biofuels have shown encouraging initial results, producing a
diesel that is closer in properties to petroleum crude oil [6].
However, less is known about the processes that can be
employed to create third generation biofuels. Promising
mechanisms include biochemical conversion, chemical
reaction, direct combustion, and thermochemical conversion
[5]. One such thermochemical process, hydrothermal
liquefaction, has recently garnered attention due to its high
energy output.
TYPES OF LIQUID BIOFUELS
Biofuels can be made from a variety of sources, such as
manure, corn, or woodchips, among others. The viability of
biofuels depends heavily on the feedstock source used, as
certain types have advantages over the others. As biofuel
research is advanced biofuels become more sophisticated and
more likely to provide a sustainable source of fuel.
First and second generation biofuels can be grouped
together due to their similarities in feedstock sources and
product similarities. Figure 1 provides an outline of these
basic ideas and compares the generations to one another. One
product from first and second generation biofuels, bioethanol,
has been recorded to have 66 % of the energy that gasoline
has and can be mixed with gasoline to further increase energy
efficiency
[4].
HYDROTHERMAL LIQUEFACTION
Hydrothermal liquefaction is the thermochemical
“conversion of biomass into liquid fuels and chemicals by
processing in a hot, pressurized water environment for
sufficient time to break down the solid biopolymeric structure
to mainly liquid components” [7]. HTL research focuses on
wet biomass such as algae due to its suitability at processing
these types of feedstocks [7]. Since HTL is still is a relatively
new process, there are uncertainties in the chemical pathways.
Overall, research has shown that “two reactions occur in
hydrothermal processing of microalgae: hydrolysis and
polymerization” [8]. The basic goals of these reactions are to
transform the macromolecules in microalgae (proteins, lipids,
and carbohydrates) into long chains of hydrocarbons that can
store energy and to remove oxygen [2].
Hydrolysis
Hydrolysis, which is just the chemical breakdown of
compounds in the presence of water, is a chemical process
that is used in many well-known, simplistic reactions such as
acid-base neutralization or photosynthesis [7]. In HTL
hydrolysis breaks down the proteins, lipids or carbohydrates
into smaller components that can then be recombined to form
the hydrocarbons required for fuel. Each group is broken
down differently [8]. Figure 2 shows the basic steps that occur
for each type of macromolecule.
Chao Gai and colleagues from the Chinese Academy of
Sciences provide an in depth analysis of the hydrolysis
process in HTL of biofuels. They explain how hydrolysis
starts with lipids. The figure below provides an overview of
all the steps, beginning with the degradation of
triacylglycerols (TAGs) into “initial fractions in terms of
glycerol and fatty acids” [8]. This step can be completed
without the need for catalysts [8]. Although these fatty acids
are stable in the reaction medium, glycerol can then be
decomposed further to produce alcohols or aldehydes such as
“methanol, ethanol, acrolein, allyl alcohol, formaldehyde,
propionaldehyde, acetaldehyde, and gas products” [8]. This
decomposition takes place through the process of
FIGURE 1: [5] This diagram shows the different
types of biofuels and how they are categorized.
Primary biofuels are generally unprocessed and have
uses other than that of the transportation sector [5].
Secondary biofuels are processed biomass intended for
transportation use [5].
This bioethanol can be used in transportation and has been
researched extensively to investigate its capabilities. The
results show that although the emissions are recorded to be
lower in carbon monoxide, CO, content, there is little to no
improvement in NOx emissions [4]. These feedstocks also
raise the question of whether crops should be used as fuel or
food. According to the science journalist Peter Fairley, “the
rapid scale-up of biofuels, fermented or refined from
foodstuffs such as corn (maize), sugarcane and soybeans, has
contributed to higher food prices and deforestation” [2]. This,
combined with the rising population, creates a scenario in
which first and second generation biofuels cannot be used as
a sustainable alternative.
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decarboxylation which, like its name suggests, involves the
removal of a carboxyl group. These compounds are all
composed of long chains of carbons, an ideal form for fuel.
Furthermore, decarboxylation removes oxygen from the
compounds by creating CO2 [8]. This is important to the
overall process as the goal is to remove as much oxygen as
possible to create a product similar in properties to that of
petroleum. Specifically, the goal is to lower bio diesel’s
oxygen content from 20% to the 1% traditionally found in
petroleum [2].
important to the overall production of hydrocarbon chains that
make up biocrude.
Polymerization
While hydrolysis has been extensively researched, both
for hydrothermal liquefaction and in general, polymerization
has not been at the forefront of scientific work. For this
reason, very little is known about the exact reactions that
occur during polymerization. Only basic components of
polymerization are fully understood and specifics concerning
biofuels and algae are still being investigated. Polymerization
is essentially the opposite reaction of hydrolysis. While
hydrolysis is responsible for decomposing polymers into
smaller compounds, polymerization aims to recreate larger
molecules [8]. In hydrothermal liquefaction, polymerization
involves taking the triglycerides and methanol from the
hydrolysis of lipids and protein and transforming them into
glycerine and methyl esters, the main component of
biodiesels, by the mechanism of the reaction shown below in
Figure 4. This process is actually very similar to how other
production methods create biofuels. The glycerine formed is
one of the many by-products of hydrothermal liquefaction
that can be recycled back into this or other processes [10].
EQUATION 1 [10]: The basic equation that biodiesel
reactions follow
PRODUCTS
FIGURE 2 [8]: A general outline of how each
macromolecule in algae is broken down in hydrolysis
HTL yields four phases of product: biocrude oil, aqueous
nutrients, gaseous particles, and solid residues [6]. The
biocrude oil phase is a dark viscous layer that has many of the
same qualities that crude oil in nature possesses. It is the
desired product from the HTL process, and therefore is made
from a summation of all the steps. The aqueous phase is
comprised of the ions, PO3−4, NH+4, CH3COO−, and metallic
cations such as K+, Na+ or Mg2+ [6]. These ions have the
potential to be recycled back into the algae growth medium as
nutrients, which improves the capability of a continuous
reaction process. Approximately 20% of the total product
yield of HTL is in the gaseous phase. This 20% is mostly
comprised of CO2, but a large amount of H2 is also present
[6]. The CO2 produced can also be recycled back into the
production process, as the growth of algae requires CO2 for
photosynthesis. Solid residues have an approximate yield of
10% for the whole HTL process, and are comprised in varying
degrees of carbon, nitrogen, hydrogen and sulfur. Diego
Barreiro from the Department of Biosystems Engineering of
Ghent University describes the ash as “attractive as a soil
amendment. Moreover, [the solid residue] may be used as
Proteins undergo a hydrolysis process similar to lipids. There
are two pathways that protein hydrolysis can follow:
decarboxylation and deamination [8]. These reactions can
occur simultaneously and neither is more prevalent than the
other [8]. Figure 3 shows the basic reaction, including the
initial hydrolysis of the protein into amino acids and later the
different paths that the amino acid can take. Deamination
ultimately leads to the production of the same types of alkanes
as produced in lipid hydrolysis while decarboxylation
produces amines and carbon dioxide [8]. The production of
carbon dioxide removes the oxygen originally present in the
proteins, leaving behind mostly hydrocarbons [8].
Carbohydrates, the final components of algae, are
hydrolyzed to produce simpler sugars [9]. This process is
quite simple; it involves breaking a few bonds in the larger
carbohydrate chains to form smaller molecules [8]. These
simple sugars produced from carbohydrate hydrolysis are
reused in other hydrolysis reactions, eventually creating the
final product. However basic this step may be, it is very
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feedstock for subsequent thermochemical processes, such as
pyrolysis or gasification” creating another product that can be
recycled back into the process itself [6]. No studies were
presented to support this proposition, but it is an example of
how products can be reused to improve the sustainability and
net energy quotient of HTL with algae as a feedstock.
The HTL process is performed under high pressures and
temperatures to break and reform new bonds. The temperature
at which HTL is performed greatly affects the reaction
process and produces different amounts of each type of
product. Barreiro’s review also observes that, “increasing the
temperature leads to a decrease in the oxygen content and a
consequent higher HHV [higher heating value]” closer to that
of petroleum [6]. A peak in biocrude production appears to
occur during the 375 degrees Celsius mark (see graphs
below), yet other concerns arise at these temperatures [6].
Nitrogen content also increases when temperatures exceed
375 ◦, “likely due to the promotion of protein degradation at
high temperatures” and creates more of the harmful chemicals
mentioned previously [8].
VARIABLES AFFECTING HTL
Products from the HTL process vary widely. Conditions
such as type of feedstock, temperature, or pressure can all
change the quality and quantity of biocrude produced. These
factors all contribute to the overall sustainability and net
energy yield produced from the biofuel.
Type of feedstock
Feedstocks introduce the greatest amount of variability in
HTL due to differences in cell composition. All cells
incorporate lipids, proteins, and carbohydrates, all of which
differ in elemental analysis. Traditional biocrude production
methods, such as pyrolysis and gasification, only use lipids,
while HTL uses all three macromolecules. This is both
supplemental and detrimental to the biocrude’s overall
sustainability. In order to optimize HTL to produce the
highest yield with the lowest environmental impact, the
effects of macromolecule content must be understood.
High lipid content feedstocks with lower levels of
carbohydrates and proteins are favored for sustainability.
According to Diego Barreiro an equation summarizing this
trend is:
GRAPH 1 [8] a. Chlorella pyrenoidosa (CP) and b)
Spirulina platensis (SP) percentage of each phase
This claim is supported with Graph 2 showing different algae
species biocrude yield over increasing temperature. Graph 1
shows a slightly different view, displaying the change in
percentage of each phase of product produced at various
temperatures. For these two species of algae, a peak in graph
biocrude yield occurs between 280 and 300° Celsius. The
combination of these figures gives the temperature trend that
biocrude yield increases with temperature up to a certain
temperature, after which detrimental effects begin to occur.
EQUATION 2 [6] The equation that explains how
macromolecule content relates to biocrude oil yield
Through experimentation, it was found that lipids are
consumed entirely during the reaction. The lipids were then
augmented through the partial degradation of carbohydrates
and proteins.
This addition of carbohydrates and proteins only
decreases the overall sustainability of the biocrude, as the
elemental composition of each contributes more than just
carbon and hydrogen. Proteins contain nitrogen and sulfur,
which, when combusted in the presence of oxygen, will
produce the harmful chemicals NOx and SOx [6]. These
chemicals only further the issues of ozone depletion and
subtract from the overall sustainability of HTL. Therefore, the
optimal source for the HTL process will be high in lipid
content, and low in both carbohydrate and protein content.
GRAPH 2 [6]: Shows the percent yield of different algae
species vs temperature of the HTL reaction.
These effects include harmful emissions released upon
combustion, increased amount of solid residue, and decreased
amount of recyclable materials in the aqueous phase. Further
Pressure/Temperature
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conversion processes, HTL is “accomplished at the lower end
of the process temperature range” [7]. This lessens the initial
energy input required to maintain high temperatures. For
example, hydrothermal liquefaction pathways occur at
between 200-370℃ compared to the 500℃ needed for
gasification [7]. By lowering the processing energy, the
overall efficiency is greatly improved, allowing hydrothermal
liquefaction to emerge as an energy efficient leader biofuel
production.
research is needed to ascertain the optimal temperature for
biocrude produced during HTL. This will improve
marketability of the HTL process, as more biofuel will be
made from the same amount of stock under such optimal
conditions.
HYDROTHERMAL LIQUEFACTION’S
ADVANTAGES OVER OTHER BIOFUEL
PRODUCTION METHODS
Water as an Ideal Medium
HTL has the potential to alleviate many of the
complications that are obstacles to the implementation of
biofuels in the transportation sector. Although there are
multiple reactions that could transform biomass into biofuel,
HTL is the most promising technology currently being
researched because it “offers a number of potential
advantages over other biofuel production methods” [2]. These
advantages include the ability to use alternative feedstocks
like algae, efficiency, application of water, and products that
are very similar to today’s fuels.
As stated in the book “Biofuels from Algae” edited by A.
Panday, “water is an ecologically safe substance that is
widespread through nature” [9]. This availability coupled
with water’s ecological properties contributes to
hydrothermal liquefaction’s suitability as a fossil fuel
replacement. HTL utilizes water heated to high temperatures
and under immense pressure (250 ℃ and 22.1 MPa) to
facilitate the reactions necessary to convert biomass to usable
bio crude. Water at these temperatures would naturally
evaporate into the vapor phase, but the intense pressure
compresses the molecules so that they remain in a liquid form.
This point is known as the supercritical stage [9]. One
property of water that makes it work well as a reaction
medium is its dielectric constant, which is the permittivity of
electric fields through a substance [9]. A lower dielectric
means electrons have more freedom to move. When water is
heated to the temperatures used in HTL, its dielectric constant
drastically decreases. This in turn allows “the shared electron
between oxygen and hydrogen atoms to circulate more
evenly” and lowers the polarity of water, making the water
“more affinitive to the organic hydrocarbons, most of which
are nonpolar” [9]. Furthermore, water is in a constant state of
dissociation and recombination of H+ and OH-. When water is
heated to high temperatures, the ion product value increases,
enhancing ionic reactions without the need for an additional
catalyst [9]. These properties promote the reactions of
hydrothermal liquefaction and contribute to its sustainability.
Agricultural industry
Throughout the search for an alternative fuel, one of the
largest objections to using biofuels has been the harmful
effects on the agricultural industry. Traditional biofuel feed
sources, corn or grasses, have been criticized by opponents
arguing that “agricultural products are being produced as
feedstocks for biofuels rather than as food for humans or feed
for animals” [2]. Furthermore, the resources needed to grow
these crops (land, water, pesticides) almost completely use up
the resources they help save, resulting in low overall
efficiency [2]. However, using HTL enables the use of low
energy crops like algae. These crops “can ideally reduce
energy inputs and reduce other environmental impacts” [2].
Algae can be grown in limited space, does not require a lot of
maintenance, and is not a source of food for the world’s
population. Therefore, using HTL solves one of the largest
problems currently facing biofuel implementation.
Product similarities and quality of biofuel
Efficiency
Hydrothermal liquefaction produces biocrude that has
many similar characteristics to that of petroleum crude oil.
Along with the lower oxygen levels already mentioned, HTL
biocrude has an energy content of anywhere between, “70–
95% of that of petroleum fuel oil” [6]. For this reason,
implementation of biofuels created from hydrothermal
liquefaction to the transportation fuel sector will be easier
than other methods. Also, refinement methods used to
produce transportation fuels from biocrude will be similar to
those used for petroleum. This is promising, as it means HTL
could be implemented on a large scale without a complete
reinvention of the current plants and processes that are in use
today and less research will be needed.
Hydrothermal liquefaction is overall much more efficient
than traditional biofuel production methods. Its efficiency
stems, in part, from eliminating the need to pretreat the
biomass to remove excess water. HTL actually requires the
presence of water and so it avoids the “large negative impact
on the overall process efficiency” that is caused by drying the
biomass [7]. Also, the “efficient separations of product and
by-product streams” caused by the changes in temperature
and pressure that occur throughout the reactions allow for the
“portioning of products or by-products into separate phases”
and purification of products [2]. This removes the need for a
separate separation step, creating a more energy efficient
process. Furthermore, compared to other thermochemical
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pathways are only some of the possibilities” and that future
studies are required for further understanding [8].
Another important quality of biocrude produced from
HTL is the high Energy Return on Investment (EROI). EROI
relates the amount of energy expended to produce biocrude to
the amount of energy released upon consumption [11].
Factors that affect EROI are Higher Heating Values (HHV),
energy consumption during formation, and energy output
when combusted. Higher EROI’s have been achieved through
the HTL process than other methods through the use of high
lipid content algae reacted at lower temperatures [11]. HTL’s
biocrude also produces fewer harmful emissions when
combusted in engines due to a purer crude formed from the
natural self-separation of the various phases of products
formed during HTL [6]. HTL’s high EROI couple with lower
emissions contribute to the high level of sustainability.
Engineering Reactors
One of the largest fields in hydrothermal liquefaction
research is concerned with engineering the proper reactors
capable of holding the high temperature and pressure water
without significant amounts of degradation. Issues that arise
when considering a process plant for hydrothermal
liquefaction are presented in Andrew Peterson’s article which
first appeared in The Royal Society of Chemistry’s journal.
Peterson and colleagues describe the critical need for
engineering reactors that can handle heat integration,
“recovering the heat from the hot stream to heat the cold
stream” [2]. Basically, this draws attention to the need for a
reactor that can effectively recycle heat to keep the production
energy of hydrothermal liquefaction low.
Furthermore, there is a void in technologies that can be
employed to separate the products in full-scale plants.
Reactors currently being researched include batch reactors or
continuous reactors. To summarize the differences, batch
reactors are large reactors that take in “batches” of raw
material and return them some time later as products [3].
Alternatively, continuous reactors are usually smaller and
take in a continuous flow of reactants and return a continuous
flow of products. Both types of reactors are currently being
researched to investigate which will be the most effective for
the production of biofuels from HTL.
CURRENT RESEARCH INTO
HYDROTHERMAL LIQUEFACTION’S
OBSTACLES
Although the preliminary results from HTL studies are
promising, there is still much to be done before large-scale
biofuel production will be a reality. Scientists and engineers
are currently working on techniques to derive and better the
chemical pathways utilized in hydrothermal liquefaction.
Little is known about the polymerization process that is
responsible for recombining the small hydrocarbons, along
with a large amount of variability in results from hydrolysis
of differing species of algae. Furthermore, there are process
plants and systems that need to be engineered before this
technology can implemented commercially.
POLICIES AFFECTING HYDROTHERMAL
LIQUEFACTION RESEARCH
Unearthing Chemical Pathways
Policies impacting biofuel development generally consist
of two sides: a timeline of when a level of biofuel usage
should be attainable, and a list of qualities that define what
sustainability is and what feedstocks fall under this category
[12]. Leaders in policies for the biofuel division are the United
States, the European Union (EU), and Brazil. A main concern
for each governing body is the definition of sustainability.
Priorities are different for each organization, yet the general
ideas can be stated as such: “first, ecological targets like
climate protection and sustainable use of land, second,
societal issues such as employment and social welfare, third,
economical profitability in the current and future competitive
environment, fourth, technological development along
sustainability criteria and finally fifth, a supportive
governmental strategy providing an efficient legislative
framework” [13]. Altogether sustainability can be defined as
the ability for a fuel to be available for mass production
without altering the environment or economy to great
extents.
The policy directly related to biofuels in the EU is called
the Renewable Energy Directive (RED) [13]. This policy sets
goals for biofuel usage in the future, a standard for
One of the largest concerns currently facing HTL are the
ambiguities in the chemical pathways (hydrolysis and
polymerization). Each type of algae reacts differently and
produces different results, and so pinpointing an exact process
has been difficult [7]. In order for hydrothermal liquefaction
to be considered a viable technology for biofuel production,
more research must be conducted to clarify these pathways.
Through continued research of these pathways engineers will
have the ability to modify the process to fit specific needs or
to increase efficiency.
Current studies aim to shed light on these reactions. For
example, “An investigation of reaction pathways of
hydrothermal liquefaction using Chlorella pyrenoidosa and
Spirulina platensis” performed by Chao Gai at the University
of Illinois in 2015 provided an in depth analysis of the
hydrolysis portion of hydrothermal liquefaction processes [8].
He and his team discovered possible pathways that could exist
for the transformation of the particular algae they were
working with. They also tested factors affecting the rate of
reactions, such as temperature and the properties of the
feedstock in question [8]. These findings are a step towards
HTL implementation, but even Gai states that “the predicted
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Nicole Iden
sustainability, and sponsors monetarily those attempting to
market biofuels through tax breaks and grants. The US has a
similar policy in place called the Energy Independence and
Security Act (EISA), yet it is tailored to fit the US market
more adequately [12].
As of 2014, the US is the largest producer of biofuels at
13.3 billion gallons of bioethanol, followed by Brazil at 5.6
billion gallons, and lastly the EU with 1.3 billion gallons [12].
A majority of fuels is made from first generation feedstocks,
as those are the cheapest to produce at this time. Research is
being funded through the RED and EISA to search for more
effective ways to produce biofuels, as well as creating tax
breaks, which make biofuel more competitive in the
transportation sector. Without the support of these governing
bodies and their funding, biofuels will not flourish. Growth in
the biofuel sector will directly lie in the funding from these
organizations
in
the
future.
Sustainability is a complex idea involving not only
environmental but also market impacts. As sustainability
becomes a more prevalent issue for society, governments are
providing policies and guidelines to explicitly define
sustainability depending on their countries needs and
capabilities. The future of HTL is currently being determined
by studies intending to provide insight into the techniques that
will improve HTL. Continuing research is imperative for the
implementation of hydrothermal liquefaction and biofuels
into the global market. Above all, hydrothermal liquefaction
has the potential to provide green energy and help mollify
global energy concerns.
REFERENCES
[1] P. Fairley.(2011) .“Next generation biofuels.” Nature.
(Article).
[2] A. A. Peterson et al. “Thermochemical biofuel production
in hydrothermal media: A review of sub- and supercritical
water technologies.” The Royal Society of Chemistry. (Online
article).
https://www.eeducation.psu.edu/drupal6/files/be497b/pdf/Peterson_et_al._
2008_Hydrothermal_review.pdf
[3] E. Kazamia, A.G. Smith. (2014). “Assessing the
environmental sustainability of biofuels.” Trends in Plant
Science. (Article).
[4] D. Rickeard. (2009). “Liquid Biofuel for Transport
Prospects, risks and opportunities.” Green Facts: Facts on
Health
and
the
Environment.
(Digest).
http://www.greenfacts.org/en/biofuels/l-3/1definition.htm#1p0
[5] F. Alam, S. Mobin, H. Chowdhury. (2015). “Third
generation biofuel from Algae.” Elsevier Ltd.(Conference
Article).
[6] D. López Barreiroa, W. Prinsa, F. Ronssea, W. Brilmanb.
(2013). “Hydrothermal liquefaction (HTL) of microalgae for
biofuel production: State of the art review and future
prospects” Biomass and Bioenergy. (Journal article). J.
Becker. (2013). “Hydrothermal liquefaction -- the most
promising path to a sustainable bio-oil production.” Eureka
Alert!. http://www.eurekalert.org/pub_releases/2013-02/auhl020613.php
[7] D. C. Elliott. (2011). “Chapter 7: Hydrothermal
Processing.” Thermochemical Processing of Biomass:
Conversion into Fuels, Chemicals and Power, 1 st ed, C.
Brown, Robert. Hoboken, NJ: John Wiley & Sons, Ltd.
(Ebook). pp. 200-231.
[8]C. Gai et al. (2015). “An investigation of reaction
pathways of hydrothermal liquefaction using Chlorella
pyrenoidosa and Spirulina platensis.” Energy Conversion
and
Management.
(Online
article).
http://dx.doi.org/10.1016/j.enconman.2015.02.056
[9] R. Singh et al. (2013). “Chapter 11: Hydrothermal
Upgradation of Algae into Value-added Hydrocarbons.”
FIGURE 3 [3] An overview of the various areas that
affect sustainability.
HYDROTHERMAL LIQUEFACTION: A
SUMMARY
Hydrothermal liquefaction is one of the most promising
biofuel production processes being discussed in the field of
green energy. HTL seeks to mitigate environmental
ramifications caused by petroleum based products through the
use of third generation biofuels. Its combination of high
temperatures and pressures efficiently convert non-food
based crops such as algae into a usable biocrude.
Additionally, HTL employs water as an eco-friendly reaction
medium to achieve a higher level of efficiency. The further
optimization of HTL is fundamental for the integration of
biofuels to a global market. The investigation of the trends
mentioned above will facilitate research into the conditions
needed to achieve the highest level of sustainability.
7
Abigale Gray
Nicole Iden
Biofuels from algae, 1st ed. Pandey, Ashok. Amsterdam:
Elsvier Science and Technology. (Ebook). pp 235-255.
[10] A. Demirbas, M. Faith Demirbas. (2011). “Importance
of algae oil as a source of biodiesel.” Energy Conversion
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http://www.sciencedirect.com/science/article/pii/S01968904
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[11]G. Yoo, M. Park, J. Yang, M. Choi. (2015). “Lipid
content in microalgae determines the quality of biocrude and
Energy Return on Investment of hydrothermal liquefaction.”
Applied Energy. (Article).
[12]Krissana Treesilvattanakul , Farzad Taheripour , and
Wallace E. Tyner. (2014). “Application of US and EU
Sustainability Criteria to Analysis of Biofuels-Induced Land
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http://www.mdpi.com/1996-1073/7/8/5119/htm
[13]M. Kircher. (2015). “Sustainability of biofuels and
renewable chemicals production from biomass.” Current
Opinion in Chemical Biology. (Article).
SOURCES CONSULTED
S. Bracco. (2015). “Effectiveness of EU biofuels
sustainability criteria in the context of land acquisitions in
Africa.” Renewable and Sustainable Energy Reviews.
(Review).
G. Roberts. (2015). “Hydrothermal liquefaction of municipal
wastewater cultivated algae: increasing overall sustainability
and value streams of algal biofuels.” KU ScholarWorks.
(Article).
R. Singh et al. (2015). “Chapter 10: Hydrothermal
Liquefaction of Biomass”. Recent Advances in
Thermochemical Conversion of Biomass, 1st ed., Pandey,
Ashok. Amsterdam: Elsevier Science and Technology.
(Ebook). pp 270-279.
AKNOWLEDGEMENTS
We would like to thank Sarah Ireland, our co-chair, for
providing us with valuable advice throughout this entire
process. We would like to thank our families for supporting
us, our teachers for giving us the knowledge to actually
understand what our topic was, and our writing instructor for
reading through all of our work and providing help when we
needed it.
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