MICROALGAE BIODIESEL AS A SUBSTITUTE FOR JET FUEL
Chandan Sohi
B.S., University of California, Davis, 2005
THESIS
Submitted in partial satisfaction of
the requirements for the degree of
MASTER OF SCIENCE
in
MECHANICAL ENGINEERING
at
CALIFORNIA STATE UNIVERSITY, SACRAMENTO
SPRING
2010
© 2010
Chandan Sohi
ALL RIGHTS RESERVED
ii
MICROALGAE BIODIESEL AS A SUBSTITUTE FOR JET FUEL
A Thesis
by
Chandan Sohi
Approved by:
__________________________________, Committee Chair
Timothy Marbach, Ph.D
__________________________________, Second Reader
Ilhan Tuzcu, Ph.D
____________________________
Date
iii
Student: Chandan Sohi
I certify that this student has met the requirements for format contained in the University format
manual, and that this thesis is suitable for shelving in the Library and credit is to be awarded for
the thesis.
__________________________, Department Chair ___________________
Susan Holl, Ph.D
Date
Department of Mechanical Engineering
iv
Abstract
of
MICROALGAE BIODIESEL AS A SUBSTITUTE FOR JET FUEL
by
Chandan Sohi
With dwindling petroleum resources, the need for alternate fuel resources has become
immense. Any new fuel source needs to be home grown, economically feasible, and
environmentally friendly. Although many such fuels are available for ground
transportation, such as ethanol, there are not many options for alternate aviation fuels.
One possible replacement fuel for jet fuel is biodiesel. Biodiesel has many similar
properties to jet fuel, such as energy density and specific energy. However, production
issues, low temperature properties, oxidative degradation provide significant challenges
for implementation of biodiesel as an aviation fuel. This author studied these challenges
by examining biodiesel produced from microalgae feedstock. The high production rates
of microalgae make it an ideal feedstock. Furthermore, the growth of microalgae does
not require arable land for growth. Hence, it does not figure into the land concerns of the
“fuel vs. food” debate. The author examined methods of improving low temperature
properties such as winterization and additives. For fighting oxidative degradation, this
author examined research evaluating the procedure of adding antioxidants to lengthen
oxidative stability. The study concluded that although pure microalgae biodiesel fuel
would meet the criteria of being home grown, economically feasible, and
environmentally friendly, the implementation of the fuel is still several years away.
However, blends of petroleum diesel and microalgae biodiesel containing up to 30-vol%
biodiesel can be implemented due to the better fuel properties of petroleum diesel.
_______________________, Committee Chair
Timothy Marbach, Ph.D
_______________________
Date
v
TABLE OF CONTENTS
Page
List of Tables ......................................................................................................................... vii
List of Figures ....................................................................................................................... viii
Chapter
1. INTRODUCTION ……………...……………………………………………………….. 1
1.1 Need for Alternative Fuels .................................................................................... 2
1.2 Choosing a Fuel ................................................................................................... 5
2. BIOFUELS… ..................................................................................................................... 7
2.1 Brief History ......................................................................................................... 7
2.2 Biomass……………………………………………………………………….… 9
2.3 Biofuel Sources…………………………………………………………..…..…. 9
2.4 Production of Biofuels……………………………………………………..….... 11
2.5 Benefits and Impacts………………………………………………..…………... 17
3. WHY MICROALGAE BIODIESEL?................................................................................ 20
3.1 Production............................................................................................................. 20
3.2 Fuel Properties...................................................................................................... 23
4. PRODUCTION................................................................................................................... 31
4.1 Strain Selection.......................................................................................................... 31
4.2 Production Technologies...................................................................................... 33
4.3 Harvesting............................................................................................................. 38
4.4 Conversion Technologies...................................................................................... 38
4.5 Microalgae Biodiesel Pathways............................................................................ 43
5.
AVIATION CHALLENGES............................................................................................ 48
5.1 Low Temperature Properties................................................................................. 48
5.2 Oxidative Degradation.......................................................................................... 58
6. PRODUCTION................................................................................................................... 60
Bibliography……........................................................................................…….………….… 63
vi
LIST OF TABLES
Page
3.1
Oil Yields of Feedstock Crops……………………….………………………... 22
3.2
Characteristics of Different Fuel Types..………….……….………………...... 24
3.3
Comparison of Biodiesel vs. Conventional Jet Fuel……………………….….. 26
4.1
Lipid Content of Many Microalgae Species…………………………………... 32
4.2
Expected Yield for Pyrolysis Conversion Process…………………………….. 42
5.1
Effects of Winterization on Fatty Acid Composition of Long Chain Methyl
Ester……………………………………………………………………………. 52
5.2
Effects of Additives on Cloud Point and Pour Point Properties of Biodiesel
Based Fuels…………….………………………………………………………. 55
5.3
Effects of Additives on Kinematic Viscosity of Biodiesel Based Fuels….......... 57
vii
LIST OF FIGURES
Page
1.1
Annual Energy Consumption Values for Selected Countries.…………………. 4
2.1
Current Biofuel Pathways.………………….……………………………..……. 8
2.2
Two-Stage Gasification…………………….…………………….……….……. 12
2.3
Alcohol Fermentation……………………….…………………………….……. 15
2.4
Anaerobic Digestion……………………………………………………….…… 16
2.5
Percent Reduction in Pollutants for Biodiesel as Compared to Petroleum Based
Diesel…………………………………………………………..………….……. 18
3.1
Mass of Fuel vs. Volume of Fuel per Unit Energy………………………........... 25
4.1
Tubular Bioreactors…………………………………………………….………. 35
4.2
Algal Biomass Conversion Pathways…………………………………..….….... 39
4.3
Transesterification Process……………………………………………….…….. 45
4.4
Current and Emerging Pathways for Biofuels………………………………….. 46
4.5
Microalgal Biofuel Production Cycle…………………………………………... 47
5.1
Airplane Fuel System………….………………………………………………... 49
viii
1
Chapter 1
INTRODUCTION
Purpose of the Study
One of the most pressing political/economic needs faced today is the need to
reduce our dependence on foreign oil.
There are several new options available as
alternatives to gasoline for ground transportation. Over time, as new technology and
processes are developed, our dependence on gasoline will become negligible. Some of
this will come through better fuel cell technology for hybrid or electric vehicles while
other improvements will come through continuous development of biofuels. Today we
have biofuels such as ethanol and biodiesel available in the marketplace. Currently
ethanol displaces 2% of all gasoline. Further advances in technology will allow us to
produce the ethanol out of cellulosic material, which will further decrease our
dependence on gasoline [1].
However, we still do not have a suitable replacement for jet fuel. The prevalent
alternative fuel options for ground transportation are not suitable for the aviation
industry. For airplanes, the specific energy, energy density, and the low temperature fuel
properties for any alternative option are quite important. Ethanol does not have the
specific energy or energy density that is suitable for aircrafts. Biodiesel has suitable
energy density (about 80% that of jet fuel) however it has a propensity to freeze at the
low temperatures that airplanes are likely to encounter at high altitude cruising. Another
limitation that biofuels face is the production capacity of these fuels. The amount of
2
biofuel presently produced is very limited. In order to increase production of biofuels,
more land and resources will need redirection. These resources would need shifting from
use for crops whose primary function is to provide food to crops whose primary function
is of feedstock to produce biofuels. Redirection of only a limited amount of these
resources is possible, before the redirection starts hampering food production and causing
shortages in food supply. Hence, biofuel feedstock crops that do not require arable land
need further investigation. Technology for production of these alternative fuels need to
be developed and improved.
1.1 Need for Alternative Fuels
Political, economic, and environmental issues that have risen over the past several
decades have brought fourth the need for alternative jet fuels that are home grown,
economically viable, and environmentally clean.
The need for alternative fuel sources first came to the forefront during the energy
crisis of the 1970’s. During this period, most of the industrialized economies were
heavily dependent on crude oil. The oil supply along with the oil prices were at the time
controlled heavily by the Organization Petroleum Exporting Countries (OPEC). In 1973,
the US government decided to re-supply the Israeli military during the Yom Kippur War.
A move that did not go down well with the Arab nations, so as a response the
Organization of Arab Petroleum Exporting Countries (OAPEC) a large portion of OPEC
decided to place an oil embargo on the United States thus limiting oil supplies and
sending oil prices skywards. The oil embargo highlighted the United States’ dependence
3
on foreign oil and the need for alternate fuel sources to guarantee its political autonomy
[2].
Politics is just one of the reasons for the need for alternate fuels. There are other
considerations as well. Supply chain issues due to war, terrorism, or weather can also
bring about a severe hike in oil prices. In August of 2005, there was an interruption in oil
supply due to Hurricane Katrina. Ports in the region were not able to receive foreign oil
shipments and refineries and pipelines had to be shutdown.
This disruption in oil
highlights the need for more fuel sources to lessen the impact of disruption in supply of
fuel from one particular region. With other fuel options available, the disruption in oil
supplies could have not been as drastic and the economic affect not been as severe [2].
The availability of additional fuel sources will also help dampen the affects of depleting
oil reserves and an increasing demand. With emerging economies such as China and
India having more demand for fuel sources, the rate of depletion for the fossil fuel
sources should go up (Figure 1.1). With a crowded market place for fuel sources, the
competition for fuel should be extreme with the prices for crude oil going up drastically
[3] .
Additional motivations for the need for alternate energy sources include
environmental factors. There are air quality and global climate concerns that underline
the need for more environmentally friendly fuel source. Aviation emissions such as NO x,
CO, and unburned hydrocarbons can lead to ozone and smog problems. This degradation
4
Figure 1.1: Annual Energy Consumption Values for Selected Countries [3]
5
in air quality can cause health issues relating to the respiratory and cardiovascular
systems. Global climate considerations are also important when discussing alternate
fuels. There is a need to decrease the life-cycle CO2 emissions. CO2 is one of the leading
sources of global warming. Since 1900, average temperatures have risen 1.5 °F and sea
levels have risen over seven inches. If CO2 emissions are not drastically controlled, there
can be drastic planetary changes such as frequent severe weather, higher temperatures,
higher sea levels, glacier retreats, and habitat and eco-system losses [4].
1.2 Choosing a Fuel
There is certain criterion that is important in the search for alternative fuel sources
for the aviation industry. This criterion includes “drop in” factor of the fuel, production
capability, efficiency gains and losses, physical and chemical properties of the fuel, CO2
life cycle analysis, infrastructure requirements, and aircraft design issues and
maneuverability. The “drop-in” fuel term refers to fuels capable of blending with or
directly replacing jet fuel without any major changes to aircraft design or present
infrastructure and without a sacrifice in airplane maneuverability. Hence, the “drop-in”
factor of the fuel is very important in picking a suitable alternative to jet fuel.
There are many alternative fuel options available. Some of these are long term,
some short term, and some considered as “drop-in” solutions. The options available
include Synthetic Fuels (Jet A and Synthetic Paraffinic Kerosene), Biofuels (Ethanol,
Butanol, Biodiesel, Biokerosene, Biojet), and Cryogenic Fuels (Liquid Hydrogen and
Liquid Methane). These fuels must be studied thoroughly and their advantages and
6
disadvantages as a “drop-in” fuel correctly deciphered. Cryogenic fuels are a long-term
solution for aviation fuels. Design changes along with technological advances to the
airplanes will be necessary to utilize these fuels. Such fuel technology is at the least a
couple decades away from fruition [5]. Biofuels are a better option as replacement for jet
fuel then synthetic fuels due to the high levels of CO2 emissions during the production of
synthetic fuels. Hence, this author decided to focus this research on the biofuels, and
more specifically microalgae based biodiesel in particular.
This author will look at the biodiesel with an emphasis towards aviation use. This
thesis will compare the properties of biodiesel with those of conventional jet fuel.
Discussion performed on the advantages and disadvantages of microalgae as a feedstock
and biodiesel as a fuel. Examination of the factors preventing the fuel from becoming a
“drop-in” fuel conducted. For biodiesel, these factors include production capacity and
low temperature properties of biodiesel.
Production pathways for microalgae based
biofuels need examination, areas of improvement and concerns identified.
7
Chapter 2
BIOFUELS
A better understanding of biofuels is required before performing any discussion
on the topic. It is important to understand their history, sources, and the technology
necessary to manufacture them. This section will give a brief overview on these topics. It
will examine different traditional biofuel pathways (Figure 2.1) that use varying
conversion processes to convert biomass sources into biofuels.
2.1 Brief History
Man has been using biofuels for thousands of years. It is not until as recent as
two hundred years, that fossil fuels became available to the world and have since become
prevalent.
The contributing factor behind such a change was the need of high
temperatures for iron smelting. Up until the industrial revolution, the smelting process
primarily used charcoal. Coal had remained largely unsuitable for this function due to its
impurities and variable nature. However, with the introduction of coal-charcoal, a fuel,
in the early 1700 has changed all that. Coal-charcoal, now called coke, started replacing
charcoal as the primary fuel source. By the end of the 19th century, coal was in wide
demand across the industrialized countries. The 20th century would see this demand for
coal go up five folds even with the introduction of other fuel sources such as oil and
natural gas.
It was not until the late 1970’s, after the oil crisis, that man has had a renewed
interest in biofuels. The shortage of oil during the time showed us our dependency on
8
Figure 2.1: Current Biofuel Pathways [6]
9
fossil fuels. Hence, extensive research funding has gone towards finding alternate fuel
sources that are environmentally clean, sustainable, and reliable. Scientists and engineers
have also been looking to develop the technology to go with these fuels. Currently
several different biofuels are available in the marketplace. Among them are ethanol,
butanol, propanol, and biodiesel (fatty acid methyl ester) [7].
2.2 Biomass
Biofuels are fuels formed from biomass, biological material made up of living or
recently living things. Through photosynthesis, the biological material is able to procure
water and carbon dioxide and use the energy produced by the sun to convert them into
organic compounds such as sugars. The chemical reaction below illustrates this process:
6CO2 + 6H2O + light energy → C6H12O6 + 6O2
The reaction shows how a plant (biological material) is able to gain water, carbon
dioxide, and energy through the sun and convert it into glucose and oxygen. During
combustion, the oxygen is used and energy release as heat [7].
2.3 Biofuel Sources
There are two main sources for biofuels; these are energy crops and wastes. This
section will explore these sources and their many types.
2.3.1
Energy Crops
Energy crops are those crops that are cultivated for use as fuels or for conversion
into biofuels. These crops include wood for direct use as fuels, and corn, soy seeds, and
sugarcanes for conversion to biofuels. They include plants who have oil rich seeds and
10
not grown primarily as a source for food. The relatively clean nature of the biofuels
produced from these crops coupled with the desire to find domestic sources of oil have
primarily led to the popularization of energy crops. There are two primary types of
energy crops; these are woody crops and agricultural crops.
The two most widely grown energy crops are sugarcane and corn. These crops
are ideal due to their high yield. Other examples of energy crops are those crops that are
cultivated mainly for their oily seeds. These crops include sunflowers and soybeans [7].
2.3.2
Wastes
There are many types of wastes that can be used as bioenergy sources. These
wastes include wood residues, temperate crop wastes, tropical crop wastes, animal
wastes, municipal solid wastes, landfill gasses, and commercial and industrial wastes.
Wood residue accumulates largely through trimming of plants and trees.
These
trimmings are often times not utilized.
Temperate crop wastes include residues from wheat and corn. The residue from
these plants measures to more than one billion tones per year. In large yield areas of
these crops, the residue remains largely unused. Contrastingly, tropical crop wastes come
mainly from residues of tropical food crops such as rice (rice husks) and sugar canes
(Bagasse).
Animal wastes, such as manure, sewage sludge, and poultry litter, and
municipal solid wastes are treated and converted into biofuels. Landfill gases generated
by municipal solid wastes, are also a source of bioenergy [7].
11
2.4 Production of Biofuels
It is possible to transform biomass directly into solid, liquid, and gaseous fuels
through different processes.
These processes can be divided into thermo-chemical,
physical-chemical, and bio-chemical conversion processes.
2.4.1
Thermo-chemical Process
One of the main ways to convert solid biofuels into solid, liquid, and gaseous
energy carriers is thermo-chemical conversion. There are three main types of thermochemical processes. They are gasification, carbonization, and pyrolysis. Often times
these conversion processes work in unison.
An example of this is the biomass
gasification process (Figure 2.2). This two-stage process initially requires pyrolysis
before actual gasification takes place
2.4.1.1 Gasification
Gasification is the process of transforming solid biofuels into gaseous energy
carriers. During this process, solid biofuel is reacted at high temperatures with oxygencontaining substance such as air. The resultant of this process is syngas, or synthetic gas.
The produced syngas has many uses such as heat and power generation. Alternate energy
carriers are also created using syngas. Fischer-Tropsch method transforms the synthetic
gas into fuels such as methanol and hydrogen [7].
2.4.1.2 Pyrolysis
Pyrolysis is the process of converting solid biofuels into liquid products. The process
involves decomposition of the biomaterial through the addition of heat without the
12
Figure 2.2: Two-Stage Gasification [3]
13
presence of oxygen [8]. The process can result in gaseous, liquid, and solid byproducts.
It is imperative that the biomaterial does not burn so that gasification is minimal and the
resulting byproduct is a liquid fuel. The liquid product of pyrolysis, bio-oil, generally has
about half the energy value of crude. It also requires further processing to remove acid
contaminants. Bio-oils can be used for can be used for heat and power generation as well
as fuel for transportation [7].
2.4.1.3 Carbonization
The third thermo-chemical process is carbonization.
During Carbonization,
organic substance (biomaterial) decomposes and transform into carbon or carbon creating
residue. The process insures the maximum output of solid reaction products such as
charcoal. The process is quite similar to gasification and pyrolysis; however, this process
results in the largest byproduct of the conversion being solid. Heat and power generation
activities often utilize the resulting solid biomass that is generally charcoal [8].
2.4.2
Physical-chemical Conversion
Physical-chemical conversion requires biomass (oil seeds) containing vegetable
oil or fat. The initial step of the process separates the liquid part of the oil from the solid
part through mechanical pressing. However, the same result is plausible by extraction
using a solvent. Generally, physical-chemical conversion uses both processes. First, the
oil seeds go through the mechanical press and later the extracted with the use of a
solvent. The resulting oil is a viable byproduct; however, chemical conversion is also
available to turn the resulting into Fatty Acid Methyl Ester [8].
14
2.4.3
Bio-chemical Conversion
Bio-chemical conversion uses living organisms or their product to convert
biomaterials into biofuels. There are three types of bio-chemical conversion processes;
alcohol fermentation, anaerobic digestion, and aerobic fermentation.
2.4.3.1 Alcohol Fermentation
Alcohol fermentation takes sugar, starch, or cellulose containing biomass and
converts it into an alcohol and carbon dioxide (Figure 2.3). The process involves using
micro-organisms, generally yeast, to break down the sugars. The next step involves
separating ethanol from the rest of the byproducts. Engines and combined heat and
power plants (cogeneration) are two uses for this pure ethanol fuel.
Alternatively,
blending ethanol with gasoline provides a substitute fuel for gasoline. Currently this fuel
is available containing up to 85% ethanol. However, blends of a larger percentage of
ethanol should be available in the future with more research and development of
technology that is able to utilize the fuel [8].
2.4.3.2 Anaerobic Digestion
Anaerobic digestion creates vapor-saturated gas mixture that is roughly sixty percent
methane and forty percent carbon dioxide (Figure 2.4). During the process, biomaterials
are broken-down by bacteria into biogas. Waste and water treatment plants use anaerobic
digestion; however, it also occurs naturally at landfills and bottom of lakes. The resulting
biogas useful as an alternative for natural gas, transportation and power generation are
possible [8].
15
Figure 2.3: Alcohol Fermentation [3]
16
Figure 2.4: Anaerobic Digestion [3]
17
2.4.3.3 Aerobic Fermentation
During aerobic fermentation, biological processes, in the presence of air, decompose
biomaterial. These conditions are necessary for yeast to grow rapidly to a size conducive
of alcohol production. The primary product of this process is carbon dioxide (compost)
and heat [8].
2.5 Benefits and Impacts
There are several benefits and impacts of biofuels. The most important of which is its
environmentally friendly nature. Photosynthesis of carbon dioxide with water creates
biomass. This process ends up extracting carbon dioxide from atmosphere. However, as
biomass burns during combustion, CO2 releases back into the atmosphere. Biofuels have
shown to be significantly more environmentally friendly. Fossil fuels on the other hand
are a result of atmospheric carbon dioxide sequestered in the ground for millions of years.
As fossil fuels burn during combustion, previously sequestered carbon dioxide releases
into the atmosphere [9]. Emission studies of combustion of biodiesel as compared to
petroleum diesel show significant reduction in pollutants (Figure 2.5). There is up to
100% reduction in sulfur dioxide, 80% reduction in carbon monoxide, 67% reduction in
unburned hydrocarbons, 47% reduction in particulate matter, and up to 90% reduction in
mutagenicity [3]. Biofuels are renewable as new crops are grown and waste material
collected. Political and economic reliefs are other benefits of biofuels. Biofuels help
loosen our reliance on foreign oil and insuring the country’s political autonomy. Large
number of jobs created to manufacture biofuel acts as an economic stimulus.
18
Figure 2.5: Percent Reduction in Pollutants for Biodiesel as Compared to Petroleum
Based Diesel [3]
19
Some of the drawbacks of biofuels are due to the land use for growing energy
crops. Since large amounts of crops are necessary to create significant quantities of
biofuels, the land needed to grow these crops has to come from somewhere. Land
previously allocated towards growing crops for food now grows crops for fuel. Doing so
will hamper the food supply. Hence, too much land cannot be set aside to grow crops for
fuel. This limitation on land use really dictates the type of energy crops used for fuel
production. High yield crops are ideal for biofuels. New fuels based on algae and
halophytes are being researched and developed that will help eliminate this limitation.
20
Chapter 3
WHY MICROALGAE BIODIESEL?
Biofuels derived from corn, cotton, soybean, mustard seed, sunflower, rapeseed,
jatoropha, oil palm, and algae are available in the market. Several factors set aside
microalgae based biodiesel from other fuel options. These factors include production
capacity, energy content, performance, availability, and price. In this chapter, microalgae
biodiesel is compared to other options based on the above given criteria. This author will
attempt to quantify the critical factors and present the results and conclusion.
3.1 Production
Production is important because any viable alternative to jet fuel needs to be readily
available to the consumers. Any disruption in production or supply chains is likely to
send fuel prices skyrocketing.
Hence, it is extremely important to evaluate the
characteristics of production that are likely to increase or enhance the production capacity
of a biofuel. Microalgae, as a feedstock for production of biofuels, provide several
benefits over other feedstock; mainly oil yield and land use. This section will discuss in
details these benefits and provide a description of challenges faced in large-scale
production of microalgae biofuels.
3.1.1
Oil Yield
Oil Yield is a very important characteristic when deciding which feedstock to use
to produce biofuels. Generally, you need a feedstock with high oil yields because oil
yield has a direct correlation with the production capacity of oil. Plant oils or lipids are
21
the starting point to manufacture biodiesel or jet fuel. In Table 3.1, a list of oil yields of
varying biofuel feedstock crops is available. Common feedstock crops such as corn and
soybean have oil yields of eighteen and forty eight gallons per acre respectively.
Rapeseed has an oil yield of 127 gallons per acre. Compared to these numbers, the oil
yield for algae is ten to fifty times higher than other terrestrial plants. Algae can yield
between twelve hundred with technology currently available and ten thousand gallons of
oil per acre with technology available in the future [10].
3.1.2
Land Use
Land use is a very important factor in production capacity. For every acre of
arable land used to grow feedstock, there is one less acre of land to grow food. Hence, an
increase in production of fuels based on feedstock grown on arable land decreases food
supply and in turn increases food prices for vegetable oils. An estimate of one quarter of
one third of all price increase in vegetables oils results from an increase in production of
energy from land. Since feedstock cost account for a large percentage of biofuel price,
any increase in vegetable oil prices sends biofuel prices higher. At that point, biodiesel is
no longer competitive with fossil fuels [11].
Microalgae provide a distinct advantage in this aspect.
It can grow on
marginal/non-arable land. Algae grown from twenty to thirty million acres of non-arable
land can replace the entire oil supply imported by the United States. There is no need to
use any arable land for growing feedstock for biofuels.
Furthermore, any fluctuation in
vegetable oil prices does not have an impact on biofuels derived from microalgae.
22
Oil Yield
Crop
Gallons/Acre
Corn
18
Cotton
35
Soybean
48
Mustard Seed
61
Sunflower
102
Rapeseed
127
Jatropha
202
Oil Palm
635
Algae
1,200 – 10,000
Table 3.1: Oil Yields for Feedstock Crops [10]
23
Furthermore, algae can grow with saline or brackish water. Thus, this is no need to
allocate any fresh water for growth of microalgae. This can be very beneficial during
draughts, as there will be little to no interruption in production of microalgae based
biomass [10].
3.1.3
Production Issues for Cultivating Microalgae
One of the main production concerns for producing microalgae based biofuels is
technology. Currently microalgae production is performed at a much smaller scale than
that would be required to replace jet fuel.
New technology and processes need
development and testing to increase the production capacity of the feedstock. The current
technology is also very expensive. Production of any alternative fuel for replacement of
jet fuel needs to be economically feasible.
Future technology needs to make
economically acceptable so that it does not price the fuel generated out of market due to
cost. This author will discuss the entire production cycle for producing microalgae
biodiesel in further detail in Chapter 4.
3.2 Fuel Properties
A way to distinguish between possible alternative jet fuels is through fuel
properties of the fuel. Determination of how a fuel will affect the airplane’s design,
maneuverability and operations need evaluation. Since the main purpose of the fuel is to
provide energy, specific energy and energy density are important characteristics of a fuel
that need evaluation before contemplation in any changes in type of fuel. Table 3.2 and
Fig. 3.1 provide values for these criteria for varying fuel types. In addition to providing
24
Table 3.2: Characteristic of Different Fuel Types [12]
25
Figure 3.1: Mass of Fuel vs. Volume of Fuel per Unit Energy [12]
26
Table 3.3: Comparison of Biodiesel vs. Conventional Jet Fuel [12]
27
energy, fuel is also use as a lubricant in engine control systems and pumps, to absorb
excess heat, and as hydraulic fluid in engine control systems.
3.2.1 Specific Energy
Specific energy, the energy per unit mass, is dependent on the energy content of the
fuel. Specific energy is used to asses the lower heating value of the fuel. In general, jet
fuel provides aircrafts with the necessary energy to propel the aircraft. The turbines
convert the chemical energy present within the fuel into mechanical energy; thus
providing necessary thrust to operate the airplane. The reaction between the fuel and
oxygen at high temperatures release the chemical energy stored within the fuel. This
released energy is the heat of combustion of the fuel. If the water formed during the
combustion process is in gaseous form, the heat of combustion is termed as lower heating
value, LHV. However if the water formed during the combustion process is in condensed
liquid state, the heat of combustion is termed as the higher heating value, HHV. Since
aircraft engines generally release water in a vapor phase, we use the LHV as the
determining factor [12].
Several potential alternative jet fuels tend to contain oxygen. These fuels, such as
ethanol and butanol, tend to have lower LHV then those without oxygen since the oxygen
present in the fuel molecule does not contribute any energy to the combustion process.
Due to the lower LHV, the specific energy of the fuel also tends to be lower.
The specific energy of a fuel is an important characteristic in determining the range
of an airplane. Aircrafts receive ratings at their maximum take-off weight. This take-off
28
weight includes cargo, passengers, and the weight of the fuel. The lower the specific
energy of the fuel, the larger the quantity of fuel (in weight) needed to fly similar
distance. Hence, reducing the amount of cargo and passengers the airplane can carry. A
fuel with larger specific energy is able to fly more cargo and passengers and for longer
distances [12].
3.2.2
Energy Density
Similar to specific energy, energy density is also an important characteristic while
distinguishing between fuels. Energy density is the measure of energy per unit volume of
a fuel. The design of an airplane limits how much fuel it can carry. This limiting factor
in the design is the volume capacity of the fuel tanks of the airplane. Fuels with higher
energy density will enable the airplane to have a greater range [12].
3.2.3
Fuel Comparison
This section will provide the advantages and disadvantages of using biodiesel as a
replacement fuel for conventional jet fuel. This author will evaluate the fuel properties of
biodiesel against those of jet fuel.
Areas of technological improvement and future
research identified.
3.2.3.1 Advantages
Table 3.2 provides values for the specific energy and energy density of the fuels.
Generally, a good fuel needs to have high specific energy and high energy density.
Currently acceptable jet fuel, Jet A or Jet A-1, has a specific energy of 43.2 MJ/kg and an
energy density of 34.9 MJ/l. Setting this as a standard, we look for fuels with similar or
29
better properties.
Liquid hydrogen has excellent specific energy of 120 MJ/kg.
However, it has very poor energy density of 8.4 MJ/l. Adapting such a fuel would
require design changes in an airplane. The airplane would require larger tanks to store
the required fuel.
Ethanol, a prevalent ground transportation fuel, lacks in both
categories. It has a specific energy of 27.2 MJ/kg and an energy density of 21.6 MJ/l.
These values fall shorter of the required criteria for fuels. Biodiesel on the other hand has
specific energy and energy density properties similar to currently used jet fuel. It has a
specific energy of 38.9 MJ/kg and energy density of 33.9 MJ/l. This correlation presents
itself by plotting mass per unit energy versus volume per unit energy of the fuels (Figure
3.1) [12].
Flash point deals with the fuel’s ability to absorb heat. Conventional jet fuel has a
flash point between 40 to 45 °C. Meaning that the lowest temperature at which the fuel
can vaporize to form an ignitable mixture in the air is between 40 and 45 °C. Biodiesel
generally has much higher flash point of 100 °C. In result, biodiesel is able to absorb
much more heat than conventional jet fuel.
Although the viscosity of biodiesel, 4.7 cSt, is a little higher than that of jet fuel at
1.2 cSt, it is in an acceptable range and is not a concern. An advantage presented by
biodiesel over conventional jet fuel is the amount of sulfur contained in the fuel.
Conventional biofuels generally have between 0.05% and 0.15% sulfur included in them.
Biodiesel on the other hand has less than 0.05% present within it [12].
30
3.2.3.2 Disadvantages
Along with all the benefits provided by biodiesel, over other renewable fuels, as
replacement for jet fuel there are several disadvantages or challenges that need research.
One such disadvantage is that biodiesel is biodegradable. This will result in increased
biological growth during storage. During this stage, the fuel will be broken down into
simpler components by bacteria, fungi, or other simple organisms. This could lead to
formation of emulsions and have an affect on water separation.
Biodiesel also has a very high freezing point when compared to conventional jet
fuel. Biodiesel has a freezing point of approximately 0 °C as compared to the - 40 °C for
conventional jet fuel. The standard operations of an airplane expose it to high altitudes
where the temperature is likely below the 0 °C limit of the biodiesel. This author will
discuss the current research and technology on this issue in Chapter 5.
31
Chapter 4
PRODUCTION
The production capacity of the feedstock of any biofuel is very important in
determining the fuels feasibility as a short-term replacement fuel source for jet fuel. Key
features of the production cycle need evaluation. This chapter will look at the production
pathways of microalgae based biofuels, biodiesel in particular. This author will isolate
the processes and methods employed during production and isolate avenues of
improvement within the various production pathways.
4.1 Strain Selection
There are several different types of microalgae strains. The two most important
factors to consider while selecting specific strains are oil content and growth rate.
Microalgae species possess more oil per hectare than other traditional biofuel feedstocks
such as soybeans, sugarcanes, and corn among others. Oil extracted from microalgae,
depending on specific strain, grown on 20-30 million acres of marginal (difficult to
cultivate) land can replace the entire US supply of imported oil [11]. Hence, it is
important that the strain of microalgae selected meet the needs of production (oil content
and growth rate). Strains with lower oil content generally grow a lot faster than those
strains with higher oil content. A list of several microalgae species and their oil content
is available in Table 4.1.
32
Table 4.1: Lipid Content of Many Microalgae Species [11]
33
Another important factor for selecting which strain to grow becomes a factor due to
performance criteria of the required fuel. The composition of fatty acids contained within
the microalgae play an important role on the properties of the biodiesel produced from
the microalgae. For instance, reducing the saturated fatty acid content of oil retrieved
from a plant can help alter the cold weather properties of the biodiesel fuel derived from
it.
4.2 Production Technologies
Several different production technologies are available for microalgal biomass
production.
These technologies fall into three main categories and several sub-
categories. These three main categories of production technologies are Photoautotrophic,
Heterotrophic, and Mixotrophic production.
This section will further explore these
technologies and assess their strengths and weaknesses.
4.2.1
Photoautotrophic Production
Photoautotrophic production is the only current production method that is currently
economically and technically feasible for large-scale biomass production. There are three
main types of photoautotrophic production of microalgae.
They are Open Pond
Production Systems, Closed Photobioreactor Systems, and Hybrid Production systems.
The performance and viability of these systems depend on the type of microalgae strain
selected for production as well as climatic, economic, land, and water considerations.
34
4.2.1.1 Open Pond Production Systems
Algae can be grown in natural or man-made ponds. Raceway ponds are the most
common of open pond production systems. Raceway ponds are oval shaped closed loop
recirculation channels generally operating at water depths of 0.2 meters to 0.5 meters. In
this model, baffles located in the channel, direct the flow around turns. Mixing and
circulation of the algae occurs with the help of a paddle wheel. The paddle wheels
continuously drive water around the channel to prevent biomass sedimentation [11] [13].
The single main advantage of open pond production systems is economic. Open
pond production systems require less capital equipment than other production methods.
This production system is also very easy to operate. This type of production system also
has several disadvantages. Open pond production systems tend to loose water through
evaporation, are susceptible to pollution by unwanted species, and culture environment
such as light and temperature are much harder to control in open pond production
systems [13].
4.2.1.2 Closed Photobioreactor System
The main objective of closed photobioreactor production systems is to overcome
major problems associated with open pond production systems. Closed photobioreactors
can keep out atmospheric contaminants and save water, energy, and chemical as
compared to open pond production systems. Further advantages of this system include
the propensity of closed photobioreactors to erected over any open space and operate at
high biomass concentration. There are three types of photobioreactors; they are tubular
35
Figure 4.1: Tubular Bioreactors [14]
36
reactors, plate reactors, and bubble column reactors. Out of these three types, tubular
reactors (Figure 4.1) are the most prevalent. Tubular photobioreactors consists of an array
of glass or plastic tubes. This array of solar collectors captures sunlight and aligns in
vertical, horizontal, inclined, or a helix pattern. The diameter of these tubes needs to be
less than 100 cm since light does not penetrate the dense culture broth [13]. In this
production system, mechanical pump or an airlift system continuously re-circulates the
microalgae broth. Every so often, the broth returns to a degassing area where the
accumulated oxygen (generated during photosynthesis) is striped.
Photobioreactors also have several drawbacks. The equipment is very expensive
and has design limitation dependent on the tube lengths. Several kilometers of tubes are
necessary to produce significant amount of fuel, this requires extensive capital, as the
tubes are very expensive. The design of photobioreactor production system is dependent
on the length of the tubes due to potential oxygen accumulation, carbon dioxide
depletion, and pH disparity in the system [11]. These challenges have been receiving
major research attention. Breakthroughs are required to make this production system
economically feasible.
4.2.1.3 Hybrid Production System
Hybrid production systems consist of a combination of Open Pond Production
System and Closed Photobioreactor System. It is a cost effective way of cultivating high
yielding strains of microalgae for production of biodiesel.
The Hybrid Production
System consists of two parts, in the first part high oil yielding microalgae strains grow in
37
closed photobioreactors to produce biomass. In the second part, microalgae enter an
Open Pond Production System consisting of nutrient restrictions and other stressors to
promote biosynthesis of oil [13].
4.2.2 Heterotrophic Production
Heterotrophic production system grows microalgae on carbon substrates inside
stirred tank bioreactors or fermenters. During this process, the growth of the microalgae
is independent of light. Therefore, this system provides more control over the growth of
the microalgae and lowers the cost of harvesting due to higher cell density of the
microalgae. Heterotrophic production system has much lower set-up costs then the
Photoautotrophic production system. However, heterotrophic production system uses
more energy then photoautotrophic production systems. Several studies have concluded
that heterotrophic production systems have higher technical viability for large-scale
biodiesel production than photoautotrophic production systems [13].
4.2.3
Mixotrophic Production
Some microalgae strains can be grown using either photoautotrophic or
heterotrophic production systems. In this process the algae is able to photosynthesize as
well as ingest organic matter. For this process, light is not a constricting factor since
mixotrophs are not completely dependent on photosynthesis; organic substrates can also
support the growth of the microalgae. The photoautotrophic process uses light to grow
the microalgae while the heterotrophic part of the process such as aerobic respiration uses
carbon substrates to grow the plant [13].
38
4.3 Harvesting
Recovery of microalgae biomass is an important part of the microalgae based
biodiesel production cycle. Harvesting can result into upwards off twenty percent to
thirty percent of total production costs. Hence, selection of harvesting technology is
crucial to the overall economic feasibility of the production of microalgae biomass.
Generally, harvesting consists of two steps. The first step being bulk harvesting, during
which biomass is separated from its bulk suspension. The second step of the process
involves thickening; the purpose of this step is to concentrate the slurry [13].
Although harvesting is an important and essential part of the production cycle, this
author has decided not to go into much more detail on the topic. This author has decided
to spend his efforts on other parts of the production cycle that are of more interest and
within the scope of the paper.
4.4 Conversion Technologies
There are many different pathways of converting microalgae biomass into usable
fuels. The conversion technologies associated with converting the biomass into fuels
falls into two primary categories. They are biochemical and thermochemical conversion.
This section will explore the conversion technologies that fall within these categories and
the type of fuels associated with them. Finally, this author will identify the most suitable
pathway for producing algae based alternate jet fuel.
39
Figure 4.2: Algal Biomass Conversion Pathways [13]
40
4.4.1
Biochemical Conversion
To biochemically convert microalgae biomass to fuel three main biochemical
conversion technologies are applied. These three technologies are anaerobic digestion,
alcoholic fermentation, and photobiological hydrogen production. Anaerobic digestion
converts the biomass into a biogas consisting primarily of methane, carbon dioxide,
hydrogen, and sulphide. The process requires the breakdown of the biomass to produce a
gas that has a lower heating value that is twenty to forty percent lower than that of
feedstock. Anaerobic digestion requires biomass with high moisture content, generally
with eighty to ninety percent moisture. The process requires three stages. In the first
stage, compounds are broken down into soluble sugars. Then fermentative bacteria
converts these sugars into alcohols, volatile fatty acids, acetic acid, gas containing
hydrogen and carbon dioxide, which converts to methane and carbon dioxide by use of
methanogens.
The second biochemical conversion process, alcoholic fermentation, converts
biomass containing sugars, starch, or cellulose into ethanol. This process converts starch
into sugars by first grounding down the biomass. The next step requires mixing sugar
with water and yeast. The resulting mixture is transferred into fermenting tanks are used
to store this mixture, where it is kept warm. The yeast in the mixture breaks down the
sugars and converts it into ethanol. The ethanol goes through a purification process to
remove water and other impurities. The concentrated ethanol is useful as an alternate for
41
petroleum and the remaining solid waste from the process treated as feed or used for
gasification.
Photobiological hydrogen production is the third and final biochemical conversion
process involved with converting microalgae into fuel. This process produces hydrogen,
under anaerobic conditions, from eukaryotic microalgae. Under particular conditions, the
pigments in specific types of microalgae absorb solar energy. An enzyme in the cell acts
as a mechanism to split water molecules. Some bacteria produce hydrogen after they
grow on a substrate [13].
4.4.2
Theromochemical Conversion
Thermochemical conversion process of creating fuel requires the thermal
decomposition of biomass.
There are four different processes to do so. They are
gasification, liquefaction, pyrolysis, and direct combustion. Gasification is the process of
oxidizing microalgae biomass into a combustion gas mixture (refer to section 2.4.1.1).
The resulting fuel from this process is syngas.
The thermochemical conversion process of liquefaction converts wet microalgae
biomass into liquid fuel. This is a low temperature process which in the presence of
hydrogen and aided by a catalyst yields bio-oil. This process uses high water activity to
decompose biomass materials into smaller molecular materials with higher energy
density. Direct combustion is a thermochemical conversion process that involves burning
biomass in the presence of air and storing the chemical energy of the biomass into gases.
42
Mode
Conditions
Bio-oil (%)
Char (%)
Syngas (%)
75
2
13
50
20
30
30
35
35
Moderate temperature (500 °C),
Flash Pyrolysis
short hot vapor residence time
(about 1s)
Moderate temperature (500 °C),
Fast Pyrolysis
moderate hot vapor residence time
(about 10 - 20s)
Low temperature (400 °C), very
Slow Pyrolysis
long solids residence time
Table 4.2: Expected Yield for Pyrolysis Conversion Process [13]
43
The heat produced during this process need using immediately since storing of these
gases is not possible [13].
Pyrolysis is the process of converting biomass to bio-oil, syngas, and charcoal. There are
three types of pyrolysis; they are flash pyrolysis, fast pyrolysis, and slow pyrolysis.
During slow pyrolysis, the temperature is at 400 °C and the results in 30% bio-oil, 35%
charcoal, and 35% syngas. The temperature is at 500 °C for fast pyrolysis and the
resultant fuels are 50% bio-oil, 20% charcoal, and 30% syngas. For flash pyrolysis, the
temperature is also at 500 °C and the resultant fuels are 75% bio-oil, 2% charcoal, and
13% syngas (Table 4.2). The large biomass to bio-oil conversion rates obtained through
pyrolysis; make it ideal for producing microalgae based biodiesel. Flash pyrolysis, which
produces the largest fraction of bio-oil, is the ideal conversion technology to replace
fossil fuels with those derived from biomass. However, there are several challenges that
need research. Oils retrieved through pyrolysis tend to be unstable, viscous, acidic, and
contain solids and chemically dissolved water. The viscous nature of the fuels is very
important for aviation fuels. Hence, this issue needs further research and development.
4.5 Microalgae Biodiesel Pathways
The first step in any pathway, for producing microalgae based fuel, is to select the
specific strain of microalgae to grow. The selection of the strain is dependent upon oil
content, growth rates, and specific requirements of the desired fuel. After strain selection,
a production system needs choosing. This selection takes into account several factors
44
including production capacity, cost, and feasibility. Once microalgae have grown, it
needs harvesting. After harvesting, the microalgae is ready to be processed into fuel.
The next part of the production pathway, requires conversion of microalgae biomass into
bio-oil. The best way to do so, as previously discussed is through thermochemical
conversion process of flash pyrolysis. The resulting bio-oil, from the pyrolysis process,
needs converting to bio-diesel. This done so through transesterification (Fig. 4.3), a
process that converts fatty acids retrieved from the bio-oil into biodiesel. It is a chemical
reaction in which the triglycerides and alcohol react together to produce fatty acid methyl
esters (FAME) and glycerol. FAME has chemical and physical properties that are very
similar to conventional diesel fuel. Figure 4.4 presents the different biofuel pathways
with the inclusion of algae based biofuels and Fig. 4.5 presents the production cycle
discussed in this chapter.
45
Figure 4.3: Transesterification Process [12]
46
Figure 4.4: Current and Emerging Pathways for Biofuels [6]
47
Fuel Requirements
Strain Selection
Production System
Photoautotrophic
Heterotrophic
Open Pond
Closed
Photobioreactor
Hybrid
Mixotrophic
Harvesting
Conversion
Technologies
Thermochemical
Biochemical
Gasification
Anaerobic Digestion
Thermochemical
Liquefaction
Alcoholic
Fermentation
Pyrolysis
Photobiological
Hydrogen Production
Direct Combustion
Fuels
Syngas
Charcoal
Electricity
Bio-Oil
Transesterification
Biodiesel
Methane
Hydrogen
Ethanol
Figure 4.5: Microalgae Biofuel Production Cycle
48
Chapter 5
AVIATION CHALLENGES
Being able to use microalgae based biodiesel, as an alternate for jet fuel would be
greatly beneficial. It will help ease the environmental, political, and economic concerns
that come along with the use of conventional jet fuel. Currently, several factors are
preventing biodiesel from replacing jet fuel. Chief among these are the low temperature
properties of biodiesel and oxidative degradation of the biodiesel over time.
5.1 Low Temperature Properties
Due to standard flight procedures in low temperature conditions, such as those
experienced during high altitude cruising, aviation fuel requires certain low temperature
properties. These properties include cloud point temperature, pour point temperature, and
viscosity.
5.1.1
Cloud Point Temperature
Cloud point temperature refers to the temperature below which wax in biodiesel
forms a cloudy appearance. This causes the wax to thicken the oil, which in turn clogs
the fuel filters (Fig. 5.1) and injectors within the engine [15].
5.1.2
Pour Point Temperature
Pour point temperature of biodiesel refers to the lowest temperature at which
pumping it is possible. It is of significant importance for aviation fuel due to the low
temperatures at the normal cruising altitude. At low temperatures where biodiesel starts
49
Figure 5.1: Airplane Fuel System [17]
50
to solidify, biodiesel can plug pipelines thus causing damage to pumps by overstressing.
A decrease in feed rate of the fuel for lubrication purposes is also possible [16].
5.1.3
Viscosity
Viscosity describes the fluids resistance to flow. Flights at high altitude, approximately
30,000 ft, experience much higher viscosity than airplanes flying at lower altitude. Thus,
it is much tougher for the fuel to flow through the pipes for highflying airplanes.
5.1.4
Current Research
Although not enough research is available on improving the low temperature
properties of microalgae, there has been extensive research done on improving the low
temperature properties of biodiesel produced from other feedstock. Researchers have
used several different approaches to fight the challenges provided by the low temperature
properties of biodiesel. Primary among these approaches are winterization and addition
of cold-flow additives.
Implementation of similar approaches on microalgae based
biodiesel to improve the low temperature properties of the fuel needs investigation.
5.1.4.1 Winterization
Winterization is not a new process or technique.
It refers to a process of
separating the part of oil that has a solidification temperature that is below a specific cutoff value. The food industry employs this process for easier handling and pouring of
vegetable oils and their byproducts.
Winterization of biodiesel refers to balancing an inactive mixture of methyl esters
at a temperature that is between its cloud point and pour point. During this process,
51
saturated methyl esters tend to “precipitate and form a suspension of small, wax- like
crystals in a liquid phase.” Filtering out these formed solid particles results in a liquid
biodiesel with improved methyl esters.
This liquid should have better cold flow
properties.
A similar study performed on soybean-based biodiesel has provided encouraging
results. The study winterized biodiesel until it withstood a minimum of three hours at a
bath temperature of -10 °C with no clouding visible. During the many trial of this
process, the cloud point and pour point temperatures never measured above -16 °C. The
results achieved were not quite at the level required however the significant
improvements made in lowering the cloud point temperature and pour point temperature
from roughly 0 °C to -16 °C is significant none the less.
The process however is not without its flaws. The multiple iterations of the
process that were required to lower the cloud point temperature and the pour point
temperature ended up decreasing the total amount of methyl esters. The results from the
experimentation showed an average product yield, from one trial to the other, of about
25% or 0.32 Kg. A mass balance from the trailing trial revealed a total loss of starting
material to be 20%. Information detailing the losses in methyl esters is available in Table
5.1. During the study, methyl hexadecanoate experienced the largest decrease of over
three times its initial value. On the other hand, methyl octadecatrienoate had the largest
increase in the methyl ester content, almost increasing by 50%.
52
Fatty Acid
Untreated (wt %)
Winterized (wt %)
Hexadecanoate (C16:0)
12.9
4.3
Octadecanoate (C18:0)
5.2
1.3
Octadecenoate (C18:1)
23.8
30.3
Octadecadieonate (C18:2)
46.6
49.6
Octadecatrieonate (C18:3)
7.8
11.9
Other
3.7
2.6
Table 5.1: Effects of Winterization on Fatty Acid Composition of Long Chain Methyl
Esters [18]
53
The study found that a small amount of saturated long chained methyl esters
within the winterized methyl esters had drastic effect on the cold temperature properties
on the transesterified and winterized soybean methyl esters. A blend containing 68.3%
linoleate, 26% methyl oleate, and 5.7% methyl linolenate produced a cloud point of -23
°C and pour point of -48 °C; about 7-32 °C below those for winterized soy bean methyl
esters containing only 5.6 wt% of saturated methyl esters. Removal of such a large
fraction of material is not ideal for large-scale production of biodiesel [18].
An alternate approach would be to alter the biodiesel feedstock to change
genetically the fatty acid portion of oilseeds. The feedstock would need modification to
produce oil with elevated oleic acid and reduced polyunsaturated and saturated fatty
acids. This altered oil would produce biodiesel that has increased oxidative stability with
improved cold temperature properties [18]. A similar approach can be utilized for largescale production of microalgae based biodiesel with improved low temperature
properties.
However, much research and development is required before the
implementation of such techniques.
5.1.4.2 Additives
Currently low temperature property targets, cloud point temperature of -10 °C, are
reachable by adding petroleum diesel to biodiesel. However, achieving this requires
using low proportions of biodiesel, to about 30%.
Using similar approach low
temperature property targets for biodiesel based aviation fuel is also possible. This would
require further reduction in the proportion of biodiesel to as little as 5 % [18].
54
Generally, additives can be categorized within three types; they are pour point
depressants, wax crystalline modifiers, and cloud point depressants.
Pour point
depressants inhibit growth of crystalline and wax crystalline modifiers disrupt the
crystalline process by growing a large quantity of small and compact wax crystals. The
filters capture most of these crystals; however, “the cake layers formed on the filter
surface is considerably more permeable to fuel flow.” As the fuel re-cycles back to the
fuel tank, warming of the fuel will melt the formed cake layer and the engine will operate
as intended. Research conducted on cloud point depressants has produced encouraging
results. The process involves increasing the solubility of long chained paraffins and
reducing the temperature of the nucleation (a physical reaction that occurs when
components in the solution start to precipitate out) [18]. Similar to winterization, much
research is not available in adding additives to microalgae based biodiesel. However,
researchers have investigated the affects of additives to biodiesel from other feedstock.
A study performed by Dunn et al. investigated the effects of additives on low temperature
properties. The first part of the study examined the effects of additives on the pour point
temperature.
The study required mixing additives to 3 different fuel types; 100%
soybean biodiesel, a 30% soybean biodiesel blended with petroleum diesel, and 20%
soybean biodiesel blended with petroleum diesel. The results of the experimentation are
available in Table 5.2. For the 20% and 30% blends, the pour point temperatures were
quite low. For the 20% blend, the pour point temperatures ranged from -18 °C to -29 °C
for the varying additives. For the 30% blend, the pour point temperature ranged from
55
Table 5.2: Effects of Additives on Cloud Point and Pour Point Properties of Biodiesel
Based Fuels [18]
56
-25 °C to -49°C. Compared to these values the addition of soybean biodiesel and
additives provided pour point temperatures ranging between -2 °C and -8 °C.
The cloud point temperature values were also quite low but not as much as those
for the pour point were. For the 20% blend, the cloud point temperature ranged from -12
°C to -15 °C for the varying additives. For the 30% blend, the cloud point temperatures
ranged from -13 °C to -21 °C. Similarly, to how the pour point temperatures were higher
for the pure biodiesel, the cloud point temperatures were also higher for the pure
biodiesel than the two blends. For the pure biodiesel, the cloud point temperatures
ranged from 6 °C to -2 °C.
Dunn et al also examined the effects of additives on kinematic viscosity. The
experiment required adding varying additives at different doses to the 20% blend fuel at
two separate temperatures. The experiment measured kinematic viscosity at 40 °C and -3
°C for 0, 500, 1000, 1500, and 2000 ppm of additives. As expected, the results showed
viscosities to be much higher for -3 °C to be much higher than those at 40 °C. The
amount of additives added reduced the viscosity (Table 5.3) [18].
The results from the study were a bit inconclusive. They showed that addition of
additives helped lower the pour point temperature but had little effect on the cloud point
temperature. Increasing the amount of additives to the fuel had no effect on the cloud
point temperature. The data gathered also showed that the addition of additives helped
lower the kinematic viscosity. However, the effects of additives used in the study were
minimal. To make the method a viable option for improving the low-temperature
57
Table 5.3: Effects of Additives on Kinematic Viscosity of Biodiesel Based Fuels [18]
58
properties of biodiesel produced from microalgae feedstock it requires further research
and development.
New additives need exploration and processes to lower the low
temperature properties need refinement.
5.2 Oxidative Degradation
One of the main issues with using biodiesel as a replacement fuel for jet fuel is
oxidation. Oxidation occurs due to any of several factors, chief among them are exposure
to oxygen through air, presence of light, high temperatures, and peroxides. Since the biooils, used to produce biodiesel through trasesterification, carry a large fraction of fatty
acids with double bonds, oxidative stability becomes an issue when storing the fuel for
extended period.
aerobically.
Once that happens, the fuel starts to degrade, breaking down
Hence, storing biodiesel for extended period is an issue that needs
investigation.
Presently there is very little research on improving the oxidative stability of
microalgae based biodiesel. However, researches conducted on other forms of biodiesel
provide the framework for tackling the issue with microalgae base biodiesel. A possible
solution is through the addition of oxidation inhibitors or antioxidants, which delay the
start of oxidation. Treatment of fuel with antioxidants is beneficial since doing so allows
keeping current storage tanks and systems for fuel handling. No new technological
advancements in technology are required. The design of the aircraft can stay constant
[19].
59
There are two types of antioxidants. Those that occur naturally within the bio-oils,
such as vitamin E, and the ones synthetically added.
There are several synthetic
antioxidants; some examples include butylated hydroxytoluene, butylated hydroxyanisol,
and propyl gallate. Several factors determine the success of an antioxidant. These
factors include fatty acid profile of the oil, amount of naturally occurring antioxidants,
and storage conditions.
It requires much research and development to identify those antioxidants that are
most suitable for microalgae based biodiesel. Other methods to improve the oxidative
stability of microalgae based biodiesel need investigation before implementing it as a
replacement for jet fuel.
60
Chapter 6
CONCLUSION
Biodiesel produced from microalgae feedstock is an excellent short-term
replacement for jet fuel. Production of the microalgae biodiesel is possible within the
United States, it is environmental friendly, and overtime can become an economically
feasible. As shown previously, completion of the entire production cycle of microalgae
biodiesel within the United State is possible. Many of the challenges associated with
large-scale production of other biofuels do not affect microalgae biodiesel. Growth of
microalgae requires a lot less land than biofuels produced from other feedstock such as
corn and soybean would.
Furthermore, the land required can be non-arable.
The
production also has very little issues with water supply as all the water requirements can
be met with brackish water.
Still much research and development is required within the production cycle to
make the development of microalgae biodiesel feasible technologically and
economically. Current production capacity for growing microalgae does not match the
amount of feedstock that would be required to meet the demand of replacing jet fuel
completely. In addition, the cost of producing such large capacity of microalgae with
current technology is quite high. Thus technological advancement in production systems
for growing microalgae need to made. Mixotrophic production systems, a combination
of both photoautotrophic and heterotrophic, need further investigation.
61
Ways to increase the bio-oil output needs pursuing. These desired results are
reachable through one of two ways, biological or thermochemically. Algae strains can be
bio-engineered to carry more oil or processes that convert biomass into bio-oil need
improvement. Special attention toward processes that produce bio-oils is necessary.
Flash Pyrolisis produces up 75% of its resulting fuel as bio-oil; ways to increase this
percentage need research and development.
Some aviation factors that also cause hindrance in replacing jet fuel with
microalgae based biodiesel need further research and development. Ways to improve the
low temperature properties (cloud point temperature, pour point temperature, and
kinematic viscosity) need further study. Improvements to current approaches such as
winterization and addition of additives should be made as well as new approaches
investigated.
Studies of the effects of winterization and addition of additives on
microalgae biodiesel need accomplishing.
Without improvement in these low
temperature properties replacement of jet fuel with microalgae biodiesel is impossible.
Another aspect microalgae biodiesel that needs advancement is the fuels storage
life. Due to oxidative degradation, microalgae biodiesel have very limited shelf life.
Ways of improving this shelf life is necessary. Current research is using antioxidants to
bring oxidative stability and hence extending the shelf life of the fuel seems to be
working for blends of petroleum diesel and biodiesel. However, the results are very
limited for 100% biodiesel fuels. It requires much research to prolong the shelf life for
microalgae biodiesel.
62
Due to the above listed issues, replacement of jet fuel by 100% microalgae
biodiesel is not possible with the current technology. Improvements in all aspects of
production and implementation are necessary. However, blends of biodiesel have shown
to be suitable for replacement of jet fuel. These blends are suitable drop-in fuels for
aviation until the necessary advancements with 100% microalgae biodiesel are
completed. The use of these blend of biodiesel, to a certain extent, help ease our reliance
on foreign oil. The biodiesel blends are also likely to provide the environment a break by
reducing the amount of petroleum burned.
All in all microalgae based biodiesel is an excellent candidate to replace jet fuel.
There are still issues that are being resolved, that need technological advancement.
However, we should continue to investigate other fuel sources as well.
63
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