INCREASED MICROALGAE PRODUCTION METHODS TO MEET
BIODIESEL DEMAND
Jose Cruz Rios, Jr.
B.S., California State University, Sacramento, 2008
PROJECT
Submitted in partial satisfaction of
the requirements for the degree of
MASTER OF SCIENCE
in
MECHANICAL ENGINEERING
at
CALIFORNIA STATE UNIVERSITY, SACRAMENTO
SUMMER
2012
© 2012
Jose Cruz Rios, Jr.
ALL RIGHTS RESERVED
ii
INCREASED MICROALGAE PRODUCTION METHODS TO MEET
BIODIESEL DEMAND
A Project
by
Jose Cruz Rios, Jr.
Approved by:
__________________________________, Committee Chair
Akihiko Kumagai, Ph.D
____________________________
Date
iii
Student: Jose Cruz Rios, Jr.
I certify that this student has met the requirements for format contained in the University
format manual, and that this project is suitable for shelving in the Library and credit is to
be awarded for the project.
__________________________, Department Chair ___________________
Susan Holl, Ph.D
Date
Department of Mechanical Engineering
iv
Abstract
of
INCREASED MICROALGAE PRODUCTION METHODS TO MEET
BIODIESEL DEMAND
by
Jose Cruz Rios, Jr.
There are many advantages to microalgae when used in biofuel production. Compared to
current food or energy crops, micro-algal growth for biodiesel production does not need
to compete for arable land. While algae may theoretically be capable of producing 10 to
100 times more oil per acre, such capacities have not been validated at the commercial
scale level. The algal biofuel industry is in need of a sustainable solution to overcome
low productivity of algal cultures. A critical review of current production processes are
identified, including algal growth facilities with a focus on higher oil yields and algal
culture population. A system is proposed and serves as one solution to the abovementioned problem. A hybrid phototrophic energy manufacturing system integrates
methods such as nitrogen starvation and cell attachment to improve algal oil production
and harvesting. A conveyor-belt type system floating on a water surface has been
proposed in the past utilizing flat surfaces as the attachment medium for the algae. A
v
similar system implementing higher productivity methods is identified. Although
significant literature exists on micro-algal growth and biochemistry, considerably more
work is needed with regards to harvesting methods and hybrid closed/open production
systems.
, Committee Chair
Akihiko Kumagai, Ph.D
______________________
Date
vi
ACKNOWLEDGMENTS
The author wishes to express sincere appreciation to the Department of Mechanical
Engineering for their extended long-term support and especially to Professors, Akihiko
Kumagai, Sue Holl, and Timothy Marbach for their patience and knowledge. This project
would never have been completed without the encouragement and devotion of my
beautiful wife, family and friends.
vii
TABLE OF CONTENTS
Page
Acknowledgments............................................................................................. vii
List of Tables ..................................................................................................... ix
List of Figures ...................................................................................................... x
Chapter
1. INTRODUCTION ....................................................................................... 1
1.1 Alternative Energies: A Necessity .....................................................1
1.2 Why Biofuel as an Alternative Energy? ...........................................4
1.3 Purpose of Study ................................................................................4
2. BIOFUEL BASICS .........................................................................................6
2.1 History of Biofuel ..............................................................................6
2.2 Biomass Sources ................................................................................9
2.3 Conversion Process of Feedstock to Biofuel ...................................10
2.4 Positive and Negative Impacts of Biofuels ......................................13
3. MICROALGAE ............................................................................................15
3.1 Algae Basics.....................................................................................16
3.2 Algal Biodiesel Production Pathways ..............................................17
3.3 Improving Oil Yields .......................................................................17
3.4 Current Production Methods ............................................................24
4. AN IMPROVED ENERGY MANUFACTURING SYSTEM .....................29
4.1 Algal Growth and Production Process .............................................29
4.2 Algal Harvesting ..............................................................................30
4.3 Economic and Environmental Sustainability ...................................31
4.4 Discussions and Conclusions ...........................................................32
Bibliography .......................................................................................................33
viii
LIST OF TABLES
Page
1. Table 3.3 Lipid Content of Commonly Researched Microalgae Species……18
ix
LIST OF FIGURES
Page
1. Figure 1.1 Annual Energy Consumption in Quadrillion Btu…………..……….3
2. Figure 2.1 Pathways for Biofuel Production from Different Biomass
Feedstocks………………………………………………………......8
3. Figure 2.4 Percent Reduction in Pollutants for Biodiesel(B20)
As Compared to Petroleum Based Diesel(B100) ….……………..13
4. Figure 3.1 Products of an Algae Cell Through Photosynthesis.…..………….16
5. Figure 3.3.4 Effect of Light Intensity on Specific Growth Rate of
Microalgae………………………………………………………….21
6. Figure 3.4.1.1 Model of an Open Raceway Pond………..……..……………..25
7. Figure 3.4.1.2 Model of Closed Photobioreactor………………..…………....27
8. Figure 4.4 Concept of a Sustainable Mechanical Biological
Manufacturing System ...................................................................31
x
1
Chapter 1
INTRODUCTION
With the U.S. being the largest consumer of energy on the planet, it is imperative
that the U.S. take initiative in discovering more sustainable sources of energy and
decreasing its dependence of foreign oil, coal, and natural gas. The goal set by the U.S.
government is to replace 20% of fossil-based transportation fuels with biofuels by the
year 2030. If biodiesel were the sole biofuel used to meet this goal, 5.1 x 1010 gasolineequivalent gallons of biodiesel would be needed each year at the current rate of
consumption (Biomass Research and Development Initiative, 2006). According to the
United States Energy Information Administration, the average price of gasoline in the
U.S. as of June 4, 2012 has reached $3.612. This influx in energy prices has brought
increased attention to biofuels as a renewable energy option.
1.1 Alterative Energies: A Necessity
Political, economic, and environmental pressures have challenged us to look
toward other means of energy. Specifically, energies that are economically and
environmentally sustainable. Alternative fuel sources were heavily investigated during
the energy crisis of the 1970’s. At that time, most industrialized economies were highly
dependent on crude oil. The Organization Petroleum Exporting Countries (OPEC)
controlled the majority of the oil supply and price. In 1973, the US government decided
to back the Israeli military during the Yom Kippur War. This decision did not bode well
with the Arab nations, thus the Organization of Arab Petroleum Exporting Countries
2
(OAPEC) voted on an oil embargo against the US. This resulted in escalating gas prices
for the United States. Since this event, the US has invested heavily in alternative energies
in order to be less dependent on foreign oils and more politically autonomous (Horton,
2010).
Global economic issues can affect the oil supply through war, terrorist attacks,
and natural disasters, which can drastically affect oil prices in the US. In August 2005,
there was an interruption in oil supply due to Hurricane Katrina. Major ports in the
regions affected were not able to receive foreign oil, which resulted in the temporary
shutdown of many US refineries and pipelines. It is apparent that a disruption in the
foreign oil supply of this magnitude may have been averted by having domestically
harvested oil available to the regions in need (Horton, 2010). The availability of
additional fuel sources will also help decrease the effects of depleting oil reserves and an
increasing demand. Emerging economies such as China and India are now competing for
oil supply. Therefore, it is imperative that the US investigates other avenues of energy so
that we can become less dependent on foreign oil (Figure 1.1).
3
Annual Energy Consumption
1992-2006
(Quadrillion (10 15) Btu)
120
100
United States
80
Russia
60
China
40
Europe
20
India
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
0
Figure 1.1: Annual Energy Consumption in Quadrillion Btu
Source: (U.S. Energy Information Administration)
Environmental factors such as air quality and global climate change are just two
reasons why biofuels need continued investigation. Automotive emissions such as carbon
dioxide and unburned hydrocarbons can lead to ozone depletion. The result is respiratory
and cardiovascular issues among humans. An increase in average temperature around the
world will also cause higher energy consumption and thus, further deplete world oil
reserves.
4
1.2 Why Biofuel as an Alternative Energy?
Renewable energy is something the world desperately needs to reduce greenhouse
gas emissions. Fossil fuels are not sustainable and are in limited quantity. The security of
our energy supply lies rooted in biofuels for our growing transportation sector. In the
United States, the production of biofuel is in limited quantities. The majority of which are
ethanol and biodiesel. Today, ethanol displaces 2% of all gasoline. Advancements in
technology may allow us to produce ethanol out of cellulosic material, ultimately
reducing our dependence on gasoline (Wenqiao, Cui, & Pei).
For now, corn is still the main feedstock of ethanol production today. Using food
crops as fuel may seem like a convenient short-term solution but the effect is that it
increases the demand of food crops. Assuming supply remains constant, if demand goes
up, then prices will go up as well. As a result, the price of corn has shot up with the
increased demand for ethanol. From a moral standpoint, food should not have to compete
with energy when hunger is such a major issue around the world. The right thing to do is
to make alternative fuels from non-food crops and wastes.
1.3 Purpose of Study
It takes roughly one gallon of oil to produce one gallon of ethanol using current
technologies, whereas, it takes one gallon of oil to produce between two to three gallons
of biodiesel. Biodiesel produced from agricultural waste and non-food crops is beneficial
to the environment, reduces carbon emissions and does not compete with food crops.
Therefore, this author has chosen to focus his research on algae as a feedstock for
biodiesel production, based on initial findings and interest in the subject matter. An
5
investigation of the production pathways for current biofuels will be conducted, followed
by an overview of the production pathway of microalgae for biodiesel production. The
author will also evaluate methods of increased algal production and will conclude with a
proposal of an energy manufacturing system with the potential for increased yield of
microalgae. Closing thoughts and future research are identified.
6
Chapter 2
BIOFUEL BASICS
There are many renewable energy sources; biomass is one of the few biofuels that
converts directly into a liquid fuel. In order to gain a better understanding of how biofuels
are manufactured, their historical use, biomass sources, and conversion processes must be
investigated. An exploration of traditional pathways of biofuels is also necessary in order
to understand how biomass is converted into biofuels.
2.1 History of Biofuel
Biofuel is defined as fuel derived from biological materials, including materials
from organisms that died relatively recently and from the metabolic by-products of living
organisms (Demirbas, 2009). Biofuel can come in the form of solid or liquid fuels, as
well as various biogases. Some examples include low nutrient input with high per acre
yield crops, agricultural or forestry waste, and other sustainable biomass feedstock like
algae.
Early applications of biofuels in the solid form have been used since man
discovered fire. Wood was the first form of biofuel that was used even by the ancient
people for cooking and heating. Liquid biofuels such as olive oil soon followed. Other
oils were derived from plants and animals and used for lamp oil (Sussman, 1983). Whale
oil was also commonly used until the modern methods of refining kerosene were
developed in 1846 by Abraham Gesner (Russell, 2003).
7
By the 19th century, gasoline and petrol-fueled engines were being invented.
Rudolf Diesel was a German inventor who invented the diesel engine. He designed his
diesel engine to run on peanut oil (Knothe, 2001). The Model T car was invented by
Henry Ford in 1903. His car was completely designed to use hemp derived biofuel as a
fuel source(New York Times, 1925). World War II saw an increase in demand for
biofuels because participating countries found it more cost effective over importing fuel.
Germany had developed the use of a gasoline mixture that included alcohol that was
derived from potatoes. Great Britain soon followed and discovered a way to mix grain
alcohol with petroleum (Nag, 2007). After WWII, countries in the Gulf and Middle East
supplied western countries with cheap oil, which had a negative effect on the further
development of biofuels.
It was not until the 1970’s, after the oil crisis, that man renewed his interest in
biofuels. Growing realizations of the world’s environmental problems and critical
instabilities in the Middle East have brought biofuels back on the table and have become
the center of attention of world governments.
Interest in biofuel began to reemerge in 2004 with policies in the US to increase
biofuel consumption in its economy. The two main types of liquid biofuels in use are
ethanol and biodiesel. Ethanol is used in gasoline engines and is derived from grains and
sugarcane crops while biodiesel is used in diesel engines and is derived from oil
producing crops, such as rapeseed and oil palm. Figure 2.1 illustrates the most current
biofuel pathways for various biomass feedstocks. It is also worthy to note that there is a
differences between “first generation” biofuels and “advanced” biofuels. First generation
8
biofuels originate from agricultural crops and processes. Production processes for these
biofuels are established. For example, producing ethanol requires fermentation or
distillation additionally biodiesel requires a process known as transesterification.
Advanced biofuels come from non-food crops or residues, such as trees, grasses,
agricultural or forestry residues, and algae. The next few subchapters will attempt to
explain each step in the biofuel pathways flowchart.
Figure 2.1: Pathways for Biofuel Production from Different Biomass Feedstocks
Source: (Pena and Sheehan, 2007, in USAID, 2009)
9
2.2 Biomass Sources
Biofuels begin as biomass, biological material consisting of living or recently
living things. There are two key types of biomass sources, energy crops and biomass
waste. Energy crops not used for human consumption are grown for immediate use as
fuel or transformed into biofuels through a conversion process discussed later. The main
crops that fall into this category include oilseed, grains, and sugar crops. These first
generation biofuels are obtained using conventional techniques of production. Each
feedstock consists of oils, starches and sugars. Some of the most prevalent types of first
generation biofuels include biodiesel, vegetable oil, biogas, bioalcohols, and syngas.
(First Generation Biofuels, 2010).
Second generation biofuels are derived from biomass waste. According to the US
Energy Information Administration, biomass waste is an organic non-fossil material of
biological origin that is a byproduct or a discarded product. Biomass waste includes
municipal solid waste from biogenic sources, landfill gas, sludge waste, agricultural crop
byproducts, straw, and other biomass solids, liquids, and gases; but excludes wood and
wood-derived fuels (including black liquor), biofuels feedstock, biodiesel, and fuel
ethanol (U.S. Energy Information Administration).
Crops grown outside of tropical regions around the globe produce wastes
exceeding one billion tons per year. These wastes primarily come from wheat and corn
residues, which remain largely unused. Biological wastes such as manure, sewage sludge,
and municipal solid wastes are treated and converted into biofuels. Landfill gases
generated by garbage consisting of biomass, are also a source of bioenergy (Boyle, 2004).
10
2.3 Conversion Process of Feedstock to Biofuel
A number of different conversion processes exist for the conversion of cellulosic
biomass to biofuels. The predominant differentiation between the conversion options is
the primary catalysis system. Biomass can be converted into biofuel through three
different conversion processes, chemical, biological, and thermochemical.
2.3.1 Chemical Conversion
A chemical conversion process usually consists of a sequence of steps, each of
which involves making some sort of change in either chemical makeup, concentration,
phase state, energy level, or a combination of these, in the materials passing through the
particular step. Transesterification and hydrotreating are among the more common
chemical conversion processes which makes biodiesel production possible.
2.3.1.1 Transesterification
Biodiesel is produced through transesterification. It is an environmentally friendly
diesel substitute that is made up of fatty acid methyl esters (FAME). Renewable
biological sources such as vegetable oil and animal fats are mixed with alcohol, in the
presence of a homogeneous and heterogeneous catalyst. The product consists of two
components, biodiesel and glycerol (Meher, Vidyasagar, & Naik, 2006)
11
2.3.1.2 Hydrotreating
Hydrotreating is a chemical process applied to natural gas and refined petroleum.
The process is also known as hydrodesulfurization. The goal of the process is to decrease
the amount of sulfur in the petroleum by increasing the amount of hydrogen in the
product. The end result is a fuel that has less environmental impact after combustion
(Newth, 2003).
2.3.2 Biological Conversion
Biological or Biochemical conversion routes rely on biocatalysts, such as
enzymes and microbial cells, in addition to heat and chemicals to convert biomass first to
an intermediate mixed sugar stream and then to ethanol or other fermentation produced
biofuel.
2.3.2.1 Alcohol Fermentation
The term fermentation can generally be defined as the metabolic process in which
an organic substrate goes under chemical changes due to activities of enzymes secreted
by micro-organisms.
2.3.2.2 Anaerobic Digestion
Anaerobic digestion is a series of processes where microorganisms break down
biomass in the absence of oxygen. The product is a gas such as methane which can be
used to generate power. It is a common process in waste management plants and is also
an effective way to release energy (Kaltschmitt, 2007).
12
2.3.2.3 Enzymatic Hydrolysis and Fermentation
Enzymatic hydrolysis is a process that occurs when bacteria release enzymes that
break down cellulose into glucose. This process occurs naturally in cows when they
consume straw or other cellulosic matter. Glucose is a sugar molecule that can be
converted into ethanol for use as an automotive fuel (Lynd, 1996).
2.3.3 Thermochemical Conversion
Thermochemical conversion technologies rely on heat and physical catalysts to
convert biomass to an intermediate gas or liquid, followed by a conversion step to
convert that intermediate to a biofuel. Thermochemical conversion technologies tend to
be grouped in two distinct categories for fuel production: gasification and pyrolysis
(Energy Production From Biomass, 2002).
2.3.3.1 Gasification
Gasification is a complete depolymerization of biomass with limited oxygen at
high temperatures, typically greater than 850˚C, to a gaseous intermediate fuel known as
syngas, which consists of H2 and CO. Syngas or synthetic gas can be used for heat and
power generation (Boyle, 2004).
2.3.3.2 Pyrolysis
Pyrolysis is the milder depolymerization of biomass producing a liquid
intermediate known as bio-oil in the absence of oxygen at lower temperatures, typically
in the range of 400-650 ˚C (Bridgwater, 2000). Bio-oil typically has about half the energy
value of crude. Further processing is also required in order to remove contaminants. Biooils can be utilized for heat and power generation as well as fuel for transportation.
13
2.4 Positive and Negative Impacts of Biofuels
Biofuels have several benefits and impacts. The most important being its low
environmental impact. Photosynthesis of carbon dioxide with water creates biomass. This
process extracts carbon dioxide from the atmosphere. However, as biomass burns during
combustion, CO2 is released back into the atmosphere. Unlike biofuels, fossil fuels are
the result of man tapping into resources sequestered in the ground for millions of years.
As fossil fuels burn during combustion, previously sequestered carbon dioxide is released
into the atmosphere (Deng, Li, & Fei, 2009). Emission studies of the combustion of
biodiesel as compared to petroleum diesel show significant reduction in pollutants
(Figure 2.4).
Figure 2.4: Percent Reduction in Pollutants for Biodiesel(B20) as Compared to
Petroleum Based Diesel(B100)
Source: (Drapcho, Nhuan, & Walker, 2008)
14
The data indicates that 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 (Drapcho, Nhuan, &
Walker, 2008). Biofuels are renewable as new crops are grown and waste materials
collected. Political and economic relief are other benefits of biofuels. Biofuels help
reduce our reliance on foreign oil and insures the country’s political autonomy. More jobs
are also created from the manufacturing of biofuels and acts as an economic stimulus.
Some drawbacks to biofuels include competition for land use among energy
crops. Since large amounts of crops are necessary to create significant quantities of
biofuels, land is required to meet growing demands. Land previously allocated towards
growing food crops are replaced for energy producing crops. This will result in a shortage
of food and will lead to a starving population. Thus, land use between food crops and
energy crops need to be optimized in order to keep demand under control and prices
down. High yield crops are ideal for biofuels therefore; new fuels based on algae need to
be researched and developed in order to minimize land competition.
15
Chapter 3
MICROALGAE
Global climate change, population growth and limited oil supplies have motivated
countries to search for sustainable energy supplies. Renewable energies like wind, wave,
tidal and geothermal have the potential to supplement the high-energy needs of the US.
Unfortunately, these types of energies are not easily converted into liquid fuels for use in
the transportation sector. Although biofuels have the potential to supply transportation
fuels, first generation energy crops are restricted to agriculturally productive areas and
most likely depend on freshwater sources for irrigation. Studies have shown that further
expansion of these types of energy crops will be unsustainable in the future. Therefore,
additional strategies are necessary if biofuel production is to be sustainable at a
commercial level.
Microalgae can be advantageous over higher plant forms. For example, algae are
able to grow at very high rates and are able to make use of a large fraction of energy from
the sun. Some algal strains can convert about 10% of the total solar energy into biomass
and grow in conditions unsuitable for land energy crops (Carlsson, Beilenvan, Moller, &
Clayton, 2007). It is also possible to cultivate marine microalgae in a commercial sized
operation where salt water or brackish water can be utilized. Although microalgae has
many benefits, it currently struggles to be economically feasible with respect to
cultivation, harvesting and extraction processes (Brennan & Owende, 2010).
Additionally, as with agriculture, a nutrient supply in the form of fertilizers or organic
waste would still be required for algal culture. This chapter will investigate the
16
production pathway of algae, primarily focusing on cultivation and lipid content
optimization for biodiesel production and identify cultivation methods that can increase
oil yields.
3.1 Algae Basics
Algae are a large and diverse collection of simple, usually autotrophic organisms,
varying from single-cellular to multi-cellular forms. Seaweeds are among the largest and
more intricate marine forms. Like plants, algae can consume water and carbon dioxide
along with sunlight to produce organic compounds such as sugar. The chemical reaction
is illustrated below:
6CO2 + 6H2O + light energy → C6H12O6 + 6O2
This reaction shows how biological material or biomass is able to gain water,
carbon dioxide and sunlight then convert it to glucose and oxygen. When these products
ignite, combustion occurs and energy is releases in the form of heat (Boyle, 2004).Most
algae are somewhat flat, which
maximizes the surface area for
absorbing water, minerals, and
sunlight. Through this process,
most species of algae are able to
produce cellulose, starches, and
oils as shown in Fig. 3.1
Figure 3.1: Products of an Algae Cell Through Photosynthesis
17
3.2 Algal Biodiesel Production Pathways
To determine the most favorable pathway for algal biodiesel production based on
current technologies, a life cycle assessment (LCA) analyzing the advantages and
disadvantages of different production methods must be implemented. Factors such as
energy input, energy output, and environmental impact should be considered. One study
identified a pathway for biodiesel production with a net energy savings 85% greater than
the average biodiesel pathway used today (Stephenson, Kazamia, Dennis, Howe, Scott, &
Smith, 2010). The production of algal biodiesel can be modeled in five stages. First,
cultivation of an ideal strain of algae that has high areal productivity and good oil content
must be identified. Once the algae are cultivated, a harvesting method is implemented.
After the algae has been harvested, lipid extraction is necessary. Afterwards, the bio oil
produced is then converted into biodiesel through the transesterification process. Lastly,
options to reuse or disposal of the leftover biomass should be considered to minimize
environmental impact.
3.3 Improving Oil Yields
Numerous algae strains can produce substantial levels of bio oil or lipids.
Botryococcus braunii can achieve lipid contents as high as 75%, but the total lipid yield
depends mainly on both, the area of productivity and lipid content (Mata, Martins, &
Caetano, 2010). Although some strains may produce more lipids, they may not be ideal
candidates as the main population culture because of their low growth rates and/or
inability to produce dense cultures. Most microalgae strains usually used in research
laboratories are model strains, for their ease of growth and availability of previous
18
research, not for their lipid production, or their likeliness for biofuels production. Table
3.3 lists commonly researched microalgae species and their respective lipid content.
Table 3.3: Lipid Content of Commonly Researched Microalgae Species
Source: (Deng, Li, & Fei, 2009)
A key restriction in the production of algal biomass for the conversion to
biodiesel is that algal growth and photosynthesis are greatly reduced when the production
of lipids in algae are exploited. One solution to this problem could be through the genetic
engineering of microalgae in order to increase lipid production without sacrificing overall
productivity. Steps in this direction have been made using Chlamydomonas and Chlorella
mutants that block starch synthesis leading to increased oil (Ramazanov & Ramazanov,
2006).
19
3.3.1 Species Selection
Currently, there is an extensive amount of continuing work aimed at enhancing
the understanding of how lipid synthesis in algae may be genetically engineered to
increase lipid productivity. It is thought that lipid biochemical pathways in algal cells are
similar to those of terrestrial plants, but evidence is still somewhat limited in this respect
(Hu, et al., 2008). This type of approach would complement efforts to increase fatty acid
synthesis in algae.
Nonetheless, a selection of species needs to be identified that are capable of
accumulating the largest amount of lipids and optimized culture conditions before genetic
alterations begin. When growing algae to achieve high yields, one needs to allow them
access to basic nutrients: light, carbon dioxide, water and inorganic nutrients. The
biochemical composition of algae can be modified by manipulating their environment,
this includes nutrient availability. Several studies indicated that lipid concentrations in
algae can vary depending on changes in growth conditions or nutrient concentrations
(Converti, Casazza, Ortiz, Perego, & Del Borghi, 2009). One study has identified a
marine strain known as Nannochloropsis salina that can yield a lipid content of 69%
under a two-stage growth condition. CO2 and Nitrogen are provided in the first phase to
promote growth of the algae followed by a phase of nitrogen starvation, which induces an
increase in lipid content within the algae cells (Sforza, Bertucco, Morosinotto, &
Giacometti, 2010).
20
3.3.2 Nitrogen Starvation
Reports indicate that photoautotrophic microalgae under nitrogen starvation have
shown increased lipid accumulation. Nitrogen starvation is one of the more reliable
methods for increasing lipid content (Hsieh & Wu, 2009). However, implementation of
nitrogen starvation commonly causes a decrease in algal growth rates, resulting in lower
lipid production.
3.3.3 Temperature
Water temperature can affect algal growth depending on algal species. The
optimal temperature for phytoplankton cultures is generally between 20˚C and 30˚C
(Converti, Casazza, Ortiz, Perego, & Del Borghi, 2009). Temperatures below 16˚C can
dramatically decrease growth while temperatures higher than 35˚C can be lethal for most
species.
3.3.4 Light Saturation
Light is necessary for healthy growth and production. Light must not be too
strong or too weak. In most algal-cultivation systems light only penetrates the top 3 to 4
inches of water. This is due to the growth and multiplication of the algae. They become
so dense that they obstruct light from reaching the deeper parts of a pond or tank. Algae
requires about 1/10th the amount of light they receive from direct sunlight, which can
often be too strong (Chisti, 2007).
Light saturation is characterized by a light saturation constant, which is the
intensity of light at which the specific biomass growth rate is half its maximum value,
µmax. Light saturation constants for microalgae tend to be much lower than the maximum
21
sunlight level that occurs at midday. The biomass growth rate is much lower near the
equator because light saturation is much more intense. Above a certain value of light
intensity will result in a reduction in algal growth (Fig. 3.2.2). This occurrence is known
as photoinhibition. Algae become photo inhibited when light intesities reach beyond the
light level at which the specific growth rate peaks (Chisti, 2007). Minimizing the effects
of photoinhibition can greatly increase the average daily growth rate of algae.
Figure 3.3.4: Effect of Light Intensity On Specific Growth Rate of Microalgae
Source: (Chisti, 2007)
22
3.3.5 Carbon Dioxide Fixation
Microalgae need carbon dioxide fixation during the photosynthesis process in
order to produce the sugars necessary to produce bio-oil. Therefore, a steady supply of
CO2 is required to grow algae for biodiesel production. Methods to harness CO2 waste
from gas burning power generation stations are currently being researched. The benefit
being that the previously harmful greenhouse gases can now be redirected into an algal
oil production facility as nutrients for algal growth.
The amount of CO2 fixation necessary to promote algal growth can be calculated
based on the carbon fraction of the specific species of micro algal biomass using an
elemental analyzer. The cumulative CO2 fixation amount (FACO2) can be determined
using the equations below (de Morais & Costa, 2007).
23
3.3.6 Suspended Growth
Suspended algae production using open or closed systems produce low algae
concentrations in the medium. A usual concentration for these methods is one gram of
dry algae per liter of water. This equates to a mass ratio of about one to 1000(algae to
water). Minimal algal concentrations require large volumes of water, which may not meet
the goals set by the US Department of Energy. One of the key factors in designing an
open or closed system is that large water volume means large system sizes. A large
system size will unequivocally result in high overhead costs and more acreage demand
(Hoffman, 1998).
3.3.7 Immobilized Growth On Textured Surfaces
All existing algal production systems harvest suspended algae that is allowed to
float freely in a medium whether in open pond systems or closed photobioreactor
systems. The process of removing the algae from the medium can be costly and energy
demanding. This is due in part to low concentrations of algae in the medium. Flotation,
sedimentation, filtration, centrifugation, and flocculation are methods that will need
further investigation to make algae harvesting more feasible for large-scale algal
production. Results have found no simple or cost-effective solution suitable to large-scale
algae production (Wenqiao, Cui, & Pei). Research now indicates that micro scale
textured stainless steel sheets submerged in an algal medium can enhance the attachment
of algae cells, resulting in a potential increase of algae concentrations.
24
3.4 Current Production Methods
There are three different types of technologies currently capable of algal
production. These technologies include photoautotrophic, heterotrophic, and mixotrophic
production. This subchapter will further explore these technologies and assess their
viability.
3.4.1 Photoautotrophic Production
Photoautotrophic production is currently, the only production method that is
economically and technically feasible for large-scale biomass production. The author will
investigate three key 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 other factors such as climatic, economic,
land, and water considerations.
3.4.1.1 Open Pond Production Systems
Algae can be grown under natural or artificial conditions. Raceway ponds are the
most common of open pond production systems. They are oval shaped, closed loop
recirculation channels typically operating at water depths of 0.2-0.3 meters. Areal
dimensions range from 1 hectare for circular ponds to 200 hectares for large ponds used
in Australia for D. salina produciton. Water management procedures include direct CO2
fixation under automated pH-stat control in shallow raceways (Del Campo, GarciaGonzalez, & Guerrero, 2007). Mixing and circulation of the algae occurs with the help of
25
a paddle wheel continuously driving the medium around the channel to prevent biomass
sedimentation and stagnation (Brennan & Owende, 2010).
Figure 3.4.1.1: Model of an Open Raceway Pond
Advantages of open pond production systems include cost effectiveness and ease
of operation. Open pond production systems require less capital equipment than other
production methods. Unfortunately, open pond production systems need a steady inflow
of clean water due to evaporative losses. Contamination is also a large issue, open ponds
are susceptible to unwanted algal species, bacterial growth, grazers and sudden
environment changes such as harsh weather conditions and large temperature swings.
These variables are more difficult to control in open pond production systems (Brennan
& Owende, 2010).
26
3.4.1.2 Closed Photobioreactor Systems
Closed photobioreactor production systems offer many theoretical advantages to
the inherent problems associated with open pond production systems. Closed
photobioreactors can more readily avoid contamination and yields higher culture
densities while providing closer control over physical and chemical conditions. They also
do not suffer from evaporative losses nor do they take up extensive areas of land. There
are three types of photobioreactors; they include tubular reactors, plate reactors, and
bubble column reactors.
Tubular reactors are generally designed to feature shorter optical paths under
external illumination constructed of an array of glass or plastic tubes (Figure 3.3.1.2).
Flat plate reactors are typically thin rectangular chambers oriented vertically or inclined
towards the sun. These designs are intended to minimize light attenuation between the
wall and the center of the culture vessel, with typical tube/plate thicknesses of 0.05m
(Greenwell, Laurens, Shields, Lovitt, & Flynn, 2010). This type of production system
uses typically uses a mechanical pump or an airlift system to re-circulate the microalgae
medium. Efficient gas transfer is critical to closed photo bioreactors because the system
needs to both sufficiently provide CO2 as the source of inorganic carbon for algal growth
and remove synthetically generated O2, which can inhibit photosynthetic efficiency or be
directly toxic to algae at high concentrations (Carvalho, Meireles, & Malcata, 2006).
27
Figure 3.4.1.2: Model of Closed Photobioreactor
Large-scale commercial closed photobioreactors are currently not economically feasible.
The problem lies with high initial construction and operating costs. Several kilometers of
tubes are necessary to produce significant amount of fuel. Designing an economical
photobioreactor system is limited to the length of the costly tubes with other potential
problems arising such as, potential oxygen accumulation, carbon dioxide depletion, and
pH disparity in the system (Deng, Li, & Fei, 2009). Further research and development is
required to make this system economically feasible.
3.4.1.3 Hybrid Production System
A hybrid production system consists of a combination of the open pond
production system and closed photobioreactor system. It is a cost effective solution to
cultivating high yielding strains of microalgae for production of biodiesel. The hybrid
production system has two components; the first component is to cultivate a microalgae
strain with a high lipid content in a closed photobioreactor system for biomass
28
production. The second component is to deliver the concentrated microalgae medium into
an open pond production system, which applies nutrient restrictions and other stressors to
promote biosynthesis of oil (Brennan & Owende, 2010).
3.4.2 Heterotrophic Production
Heterotrophic production systems grow microalgae with carbon substrates inside
stirred tank bioreactors or fermenters. During which 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 systems have much lower set-up costs than photoautotrophic
production systems. However, heterotrophic production system uses more energy than
photoautotrophic production systems. Several studies have concluded that heterotrophic
production systems have higher technical viability for large-scale biodiesel production
than photoautotrophic production systems (Brennan & Owende, 2010).
3.4.3 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 (Brennan & Owende, 2010).
29
Chapter 4
AN IMPROVED ENERGY MANUFACTURING SYSTEM
The
projected
mechanical-biological
energy
manufacturing
system
is
conceptualized for theoretical purposes in Fig. 4.4. The proposed system is intended to be
a guideline to produce a sustainable algal production facility that intensifies algal lipid
content and population concentrations in a growth medium with current available
technologies. The system contains three components, which integrate methodologies for
increasing micro algal production as discussed in Chapter 3. They include an ideal algal
growth production process, better harvesting practices, and methods to make this system
more economically and environmentally sustainable.
4.1 Algal Growth and Production Process
Algae growth will begin with a carefully screened algal strain, capable of high lipid
production. A marine species should be selected to minimize freshwater use and
eliminate unnecessary demand of the world’s limited source. Selection of a freshwater
strain could lead to price increases in drinking water. The production system will be a
hybrid phototrophic production system where two stages will ensure high algal
production and lipid content.
The first stage includes a closed photobioreactor system where algae can grow
without unwanted contaminants. It will be fixed with ideal amounts of CO2 based on the
algae strain selected along with proper O2 removal. Light intensity and nutrients will also
be supplemented based on the algae strain selected.
30
The second stage will include the transferring of the concentrated algal medium to
an open pond system where nitrogen will be restricted, inducing algal cells to accumulate
high lipid contents. The open pond will feature a stainless steel conveyor belt like system
where algae can attach and continue to grow. The entire production system will be
contained in an indoor facility to minimize contamination and increase water temperature
control. Increased facility costs will be supplemented with photovoltaic and smart
lighting systems along with roof windows, which can allow for natural lighting.
4.2 Algal Harvesting
The proposed system will use corrosion resistant cylindrical steel surfaces,
submerged under salt or brackish water to the ideal depth determined by the algal species.
The steel surfaces will promote further growth in the medium during the nitrogen
starvation phase. These cylindrical sheets will feature micro dimples to promote strong
cell attachment of the algae. The cylinders will rotate about their center axis
simultaneously being driven forward much like a conveyor belt. The rotation rate and
light exposure of the cylinder surface will be optimized for the specific algal strain
selected. Once a layer of algae has grown on the cylinder surface, a mechanical arm with
a rubber end similar to a squeegee will gently scrape off the excess biomass for biodiesel
production. While the cylinder rotates, small amounts of algae in the micro dimples will
be left behind acting as inoculum, which will jumpstart new growth. Centrifugation of the
low concentration algal medium may be necessary to prevent biomass sedimentation.
This system has the potential to be designed at a large enough scale where continuous
algae growth and harvesting is possible.
31
4.3 Economical and Environmental Sustainability
A renewable, sustainable power source will be used to operate this energy
manufacturing system. Possible energy sources that meet these criteria are hydroelectric,
hydrothermal vents, photovoltaic systems, or wind turbines. Since a marine algae strain
Figure 4.4: Concept of a Sustainable Mechanical Biological Manufacturing System
would be selected as the ideal algal population, it may be reasonable to position this
facility near a salty body of water. A gas burning power generation station located near
the facility can act as a cogeneration plant supplying its waste CO2 gases. This will
minimize greenhouse gases and lower the cost of algal carbon fixation. Nitrogen rich
waters such as the effluent medium that comes from a regional water treatment facility
can be redirected to the proposed system and act as nutrients for the growing algae
through anaerobic digestion. Biomass wastes and biproducts from this system can also be
recycled as nutrients for algae growth. This can not only save production costs on
expensive manufactured nutrients, it can also reduce greenhouse gases. Integrating
environmentally friendly ideas such as the ones proposed above can lead to a fully
sustainable system in the near future
32
4.4 Discussions and Conclusions
A system such as the one proposed above allows for the integration of multiple
disciplines of study including mechanical engineering, chemical engineering, biology,
and mass/continuous manufacturing concepts. Further research and development is
required to identify other renewable energy sources and processes that can potentially
increase algal production. Basic biology of microalgae, species selection, genetic
manipulation, molecular characterization of control for carbon sequestering and storage
still need further investigation.
Economic feasibility of this energy manufacturing system will depend on
lowering costs and increasing the efficiency of the following:
1. Production systems, including land, water, nutrient and CO2 requirements.
2. Modification of the photosynthetic capability and productivity of microalgae.
3. Algal cell harvest methods
4. Lipid extraction methods
5. Transesterification methods
6. Recycling byproducts from algal biomass
This study identifies effective methods to increase algal production and lipid
content. Future research includes adjustment of both environment and nutritional
conditions of optimal algal strains for biodiesel production. Continued research in algal
cell production and other alternative energies will help decrease our need for foreign oil
and help us meet the goal of replacing 20% of fossil-based transportation fuels with
biofuels by the year 2030.
33
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