A Study Into the Cultivation of Algae - 2008

A STUDY INTO THE CULTIVATION OF ALGAE

FOR CARBON DIOXIDE SEQUESTRATION FROM A

POWER PLANT AND IT’S USE AS A BIO FUEL.

Adam A Marsh

0404304

15th December 2008

Cardiff University

A STUDY INTO THE CULTIVATION OF ALGAE

FOR CARBON DIOXIDE SEQUESTRATION FROM A POWER PLANT

AND IT’S USE AS A BIO FUEL.

ADAM A MARSH

0404304

15T H DECEMBER 2008

MSC SUSTAINABLE ENERGY AND ENVIRONMENT

CARDIFF SCHOOL OF ENGINEERING

CARDIFF UNIVERSITY

ABSTRACT

This review explores the possibilities of using algae to remove carbon dioxide

from power plant flue gas by photosynthesis through review of literature.

Different species of algae, algal products and factors affecting the photosynthetic

and carbon dioxide fixation rates were explored. The principle was applied to a

1500MWe coal fired power station whilst addressing the possibilities for using the

produced biomass as a fuel.

It is shown that the algal species Chlorella sorokiniana can be grown from the

flue gas emissions of a coal fired power plant in the South Western regions of

the United Kingdom. The entire carbon dioxide emissions from a 1500MWe coal

fired power plant can be absorbed in a 2x2km field of Chlorella sorokiniana with

a starting algal concentration of 5x107 cells.ml-1, producing biomass at an

average rate of 14.2 kg.s-1. Directly burning this biomass when dry will provide

5% of the fuel demand whilst anaerobic digestion will provide 7.3%. It has been

determined that the most useful product from such a product is to be biodiesel

from hydrocarbon producing algal species.

DECLARATION

1

CONTENTS

LIST OF FIGURES 3

LIST OF TABLES 6

1. INTRODUCTION 7

2. PHOTO-BIOREACTORS 10

3. ALGAE 13

3.1 Chlorella vulgaris 14

3.2 Chlorella sorokiniana 15

3.3 Botryococcus braunii 15

3.4 Chlamydomonas reinhardtii 16

4. PHOTOSYNTHESIS 20

4.1 Light 21

4.1.1 Light Intensity 23

4.1.2 Photoperiod 26

4.1.3 Photo-Limitation 27

4.1.4 Local Conditions 35

4.2 Carbon Dioxide 38

4.2.1 Concentration 38

4.2.2 Fixation Rate 40

4.3 Temperature 43

4.4 Nutrients 44

5. CARBON DIOXIDE SEQUESTRATION 46

2

6. ALGAE BIOFUEL 49

6.1 Biomass 49

6.2 Anaerobic Digestion 50

6.3 Biodiesel 52

7. CARBON STORAGE 55

8. HARVESTING 56

9. CONCLUSIONS 57

REFERENCES 59

APPENDIX

3

LIST OF FIGURES

Figure Description Page

1.1 2006 UK Electricity Generation Mix

2.1 (A) Arial view of a raceway pond

(B) A tubular photobioreactor with horizontal tubes

3.1 The changes in dissolved O 2, pH and volume of the H2

produced during the cultivation of sulphur deprived

algae under photoautotrophic conditions.

4.1 Chlorophyll light absorption spectra

4.2 The growth rates of four species of algae at 25oC

under different intensities of continuous light.

4.3 The impact of light intensity on Botryococcus Braunii

algal growth at 23oC, 12h light, 12h dark.

(A) Low light range (W.m-2); (B) High light range (W.m-2)

4.4 Relative total lipid content (rTLC) of Botryococcus

braunii under various light intensities.

4.5 The mean relative lipid content of three Botryococcus

braunii strains at different irradiance, 25oC

4.6 Impact of different photoperiods on algal growth of

Botryococcus braunii at 25oC and 30Wm-2

4.7 Light intensity change with depth at different algal

concentrations.

4.8 Effect of cell concentration on illuminated volume

fraction of a cubical photo-bioreactor.

4

A= Single side illumination at 325µmol.m -2.s-1

B= Two side illumination at 162.5µmol.m-2.s-1

4.9 Schematic representation of a cuboidal reactor,

illuminated exteriorly from above.

4.10 Changes in light distribution coefficient with increasing

depth of a cuboidal photobioreactor.

4.11 Schematic representation of a cylindrical reactor,

illuminated exteriorly.


diameter of an externally illuminated photobioreactor.

4.13 Schematic representation of a cylindrical reactor,

illuminated interiorly.


diameter of an internally illuminated photobioreactor.

4.15 Yearly total global horizontal irradiation

(a) Europe, (b) UK and Ireland

4.16 Monthly variation of Solar Irradiance in Cardiff

4.17 Effects of different concentrations of CO2 aeration

of the growth of Chlorella sp.

4.18 Effect of CO2 concentration on the growth of Chlorella

sorokiniana H-84 at 40oC

4.19 Effect of gas flow rate on CO2 fixation and O2 evolution,

4.20 Effect of luminous intensity on CO2 fixation and O2

evolution by Chlorella vulgaris,

4.21 Comparisons of the total amount and efficiency of CO2

` reduction in the single and the six-parallel

photobioreactor of semicontinuous Chlorella sp. Under

2%, 5%, 10% and 15% CO2 aeration.

4.22 Temperature effects on photosynthetic activity

4.23 Impact of temperature effects on growth over time,

5

(a) Botryococcus braunii

(b) Chlorella sorokiniana

5.1 Input and output characteristics of a coal fired power

station

5.2 Photobioreactor design dimensions

6.1 Relationship of optical density to algae cell number and

dry weight biomass.

6.2 Process flow schematic for bio-diesel production

6

LIST OF TABLES

Table Description Page

2.1 Comparison of the properties of different large-scale

algal culture systems

3.1 Chemical composition of C. sorokiniana H-84 grown in

air containing 10% CO2 at 35oC.

4.1 Optimum temperatures for four species of algae

4.2 Typical Nutrient Supplementation

6.1 Chemical composition of biomass

6.2 Range of biochemical methane potential data from

biomass or waste feedstock.

6.3 Comparison of some sources of Bio-diesel

6.4 Comparison between Extract and Cass-C Heavy Oil

8.1 Biomass recovery options

7

1. INTRODUCTION

Recent technologies have made it possible to take accurate measurements of

many of the processes at work in the atmosphere. Phenomenon such as global

warming, increased carbon dioxide levels and the correlations between the two

have been strongly researched and heavily debated in recent years. Human

industrial activities and energy production through the burning of fossil fuels are

currently regarded as the major causes of this increase in carbon dioxide and

average global temperature. Immediate drastic reductions in carbon dioxide

emissions are needed to quench the related environmental effects.

In the UK, approximately 90% of energy requirements are provided by oil, gas

and coal.1 Figure 1.1 shows the UK electricity generation mix for the year 2006,

where 76% is supplied by fossil fuels. Fossil fuel reserves remain plentiful,

especially coal, and are predicted to remain the predominant sources of energy

in 2020. The British Government do however, aim to reduce carbon dioxide

emissions from 1990 levels by 60% by 2050 lead by benchmark reductions of

26-32% by 2020 and 12% by 2012, the latter of which is a legally bound

agreement through the Kyoto Protocol. Outside of the UK, coal remains a rapidly

growing fuel. In 2006, China built 105GW of new coal-fired power stations whilst

the rapidly growing country of India will remain heavily reliant on coal for

decades to come. Both China and India are also signed to the Kyoto Protocol.

8

Figure 1.1; 2006 UK Electricity Generation Mix

Carbon sequestration or carbon capture and storage is the process of removing

carbon dioxide from the flue gasses produced by combustion, preventing them

from reaching the atmosphere. As coal is a very carbon intensive fuel, this

technology is required to reduce the environmental impacts of burning coal.

Carbon capture and storage is however in its infancy without suitable

commercial-scale projects in use.2 The addition of capture systems to a power

plant increases the cost of generated electricity and dramatically reduces the

efficiency of a power station. Having removed the carbon dioxide, the issue of

storage, never allowing it to reach the atmosphere remains. Current ideas lead to

burial in un-minable coal beds, depleted oil reserves and North Sea saline

reservoirs. The deep ocean is also a possibility, however far more research into

ecological effects must be addressed before this is a possibility.

The use of plants to absorb man made carbon dioxide is not a new idea. Trees

and forests are consistently planted for carbon offsetting, however the success of

such schemes for their intended use is doubtful due to the slow rates of carbon

capture and international location.3 Recently however, the use of fast growing

plants such as algae has been noted for their efficiencies of fixating carbon

9

dioxide from combustion systems, and the products that can be obtained from

them. It is actually the latter that has lead to the development of the former,

through research into the production of biofuels from algae. Algae have featured

in much press as the new saviour of the world’s energy and pollution crisis;

“Algae can grow from ambient (0.04%) to 100% carbon dioxide

concentrations”,4 “1kg of algae will ‘eat’ 3 kg of carbon dioxide”,5 they “can

produce copious amounts of hydrogen”6 and “algal oils could be made into a

kerosene-like fuel”7.

This study investigates these claims and aims to determine the practical

possibilities for the use of algae in power plant carbon capture, storage and

energy production.

10

2. PHOTO-BIOREACTORS

A photo-bioreactor is the term given to a construct used to grow biological

material with the medium of light. There exists a large range of commercial sized

photo-bioreactor style and table 2.1 compares a wide selection. They can

primarily be split into two groups, open or closed system. An open system has

contact with the surrounding environment and is therefore subject to similar

conditions such as temperature. A physical barrier separates a closed system

from the local environment allowing independent control of the conditions within,

but increasing cost. Open systems are generally ponds although there are a

number of variations of the simple system. Figure 2.1A shows the plan view of a

raceway pond. A pond will typically be very shallow, around 0.3m deep and

require a pump to cause a flow of culture around the pond. New culture will be

fed after the pump, whilst harvest of the grown culture will occur just before.8

Culture passing through a pump may well cause damage that must be avoided.

Because ponds are open to the atmosphere, they are easily contaminated by

bacteria and do not perform to a high carbon dioxide efficiency as much which is

added, is lost. Their main uses are to grow algae for food, utilising ambient

carbon dioxide from the surrounding atmosphere.

Figure 2.1; (A) Arial view of a raceway pond. (B) A tubular photobioreactor with horizontal tubes.

11

Table 2.1; Comparison of the properties of different large-scale algal culture systems

A tubular reactor, figure 2.1B, is typically constructed from glass or plastic, with

diameters of around 0.1m. The tubes can be arranged horizontally, orientated

North to South, or stacked vertically, needing less ground area, but reducing the

light levels incident upon the lower tubes. A pump is again required and

turbulence should be encouraged to allow mixing of gas, algae and supplying a

constant rotation of algae to the edges of the reactor. Tubular reactors are

subject to heavy shading of the interior areas in high algal concentrations,

resulting in small diameters being required.

Tubular photo-bioreactors are generally more effective at producing high yields

of algal biomass in a shorter period of time than ponds and have are more

efficient use of carbon dioxide. They are however more expensive, using more

materials and care must be taken to prevent clogging of the tubes by biomass,

an occurrence more likely in grouping, glutinous species. For carbon dioxide

fixation and biofuel production, closed system reactors are the only viable option

12

as any contamination from, and leaking of carbon dioxide to, the surrounding

environment cannot be tolerated.9

In experiment, small-scale tubular bioreactors are most commonly used, typically

under 4L in volume. They are illuminated from all directions by artificial lighting

or internally, using fibre optics. The results from such experiments therefore are

not true representations of natural conditions and some of the designs used are

simply not suitable for large-scale commercial applications.

13

3. ALGAE

Algae are the most basic form of plant life. There are tens of thousands of

different species ranging from single celled organisms to the most complex

marine seaweeds. Forming about 1.5 billion years ago, algae are a key cause in

the earth’s evolution of a hospitable atmosphere, reducing carbon dioxide and

increasing oxygen levels.

The more developed an organism, the more complex and less efficient the

energy harvesting process is, mainly due to the difficulty of distributing a carbon

dioxide supply around large organisms with many cell walls. Algae therefore,

being very basic in structure and often single cellular, have an extremely high

efficiency of solar energy use per unit of growth and are swift to adapt to new

environments. The high algal photosynthetic efficiencies lead to the incredibly

rapid growth rates causing algal blooms. Historically this rapid growth has

dominated life forms in rivers, lakes and shallow coasts when artificial

eutrophication by fertilisers has taken place. This has classed algae as a pollutant

in some cases, with vast numbers of papers written on the subject and how to

reduce the growth of rapid growth algae.10

A true alga is a eukaryotic organism with a nucleus and organelles such as

chloroplasts. Having evolved from prokaryotic organisms (organisms lacking

membrane bound organelles, such as a nucleus), cyanobacteria, or blue-green

algae is no longer classed as algae, but bacteria. Therefore, outside the algae

family and plant kingdom, many more organisms exist that obtain energy

through photosynthesis and therefore may also be eligible for use as is

suggested in this study.

14

Through literature review, four key species of algae emerge for they have much

written about them on products of growth or carbon dioxide sequestration. They

are Chlamydomonas reinhardtii, Botryococcus braunii and two from the genus

Chlorella, Chlorella vulgaris and Chlorella sorokiniana. These species are

discussed for their products and suitability for carbon dioxide sequestration

below.

3.1 CHLORELLA VULGARIS

Chlorella species have much literature as their nutritional value was noted early

in the 1940’s and hence batches for experimentation have long existed. It is

unicellular and easily cultured resulting in many photosynthesis related studies

having been carried out with this species before any other. This is the situation

we find ourselves in when it comes to determining rates of carbon dioxide

sequestration. The majority of algal behaviour discussed later in this study is

based on studies upon Chlorella species, so only brief descriptions are given

here.

Chlorella vulgaris can be found in lakes and ponds all over the world. It is

relatively fast growing with doubling times of under a day.11 Current studies

using Chlorella vulgaris for biological fixation of carbon dioxide have shown it to

be easy to utilise in engineering systems keeping high reproduction rates and

photosynthetic capacity.12

15

3.2 CHLORELLA SOROKINIANIA

Chlorella sorokiniania is very successful in high temperatures and high

concentrations of carbon dioxide,13 being found in hot springs. Under optimum

conditions, doubling times of 2.5 hours have been achieved, producing large

amounts of biomass.

Studies into the structure of Chlorella sorokiniana revealed its chemical

composition, shown in table 3.1.

Table 3.1; chemical composition of C. sorokiniana H-84 grown in air containing 10% CO2 at 35oC.

3.3 BOTRYOCOCCUS BRAUNII

Botryococcus braunii naturally produces hydrocarbons in the form of lipids.14,15

Although normally a green algae, during algal blooms the cells produce a huge

amount of carotenoids turning them yellow or orange. In this state, the lipid

composition can reach 80% of the cell’s dry weight. These lipids are formed

outside of the cell and embed single cells into a gelatinous mass.

16

The majority of hydrocarbons produced range from C27H52 to C34H58.15,16 Using

the Milne formula for calculating the calorific value of a fuel, given as equation

3.1, individually these hydrocarbons have high heating values of around 33.8

MJ.kg-1, a higher value than coal. In experiment, hydrocarbon contents of 50%

are regularly achieved. Assuming the remaining relative composition of the algal

cell is similar to that of Chlorella sorokiniana given previously, the dry high

heating value remains higher than 33 MJ.kg-1. If the hydrocarbon content

increases to 80%, then the calorific value increases to over 42 MJ.kg-1. If the

hydrocarbons are separated from the algal mass, they can be cracked into more

conventional fuels such as gasoline.17

HHVmilne = 0.341 C + 1.322 H - 0.12 O – 012 N + 0.0686 S – 0.0153 Ash (3.1)

Botryococcus is found all over the world and has the advantage of being able to

grow in brackish (slightly saline) waters. It does however have a rather slow

growth rate of a few days. Due to the grouping nature of Botryococcus, biofilms

may form in narrow spaces, blocking fluid flow. These prove a major problem in

the medical field with needle thin tubes, but may not prove to be more than a

cleaning inconvenience in larger diameter photo-bioreactors.

3.4 CHLAMYDOMONAS REINHARDTII

Chlamydomonas reinhardtii, when deprived of sulphur, can sustain the

production of hydrogen.18,19 The sulphur deprivation partially deactivates the

photosynthetic oxygen evolution complex, causing a transition to anaerobic

conditions,20 activating a hydrogenase enzyme that carries out the reversible

reduction of H+ ions with an electrons, shown by equation 3.2. Activation of the

17

hydrogenase enzyme occurs after several hours of anaerobic induction in the

dark. Under continuous light, this increased to a few days.

(3.2)

Most laboratory experiments on this process have required the addition of

acetate, a salt or ester of ethanoic acid, into the culture solution. This is

consumed during aerobic stages of growth, but not during hydrogen production.

Excluding the addition of acetate would dramatically reduce the cost of growing

Chlamydomonas reinhardtii on large scale operations. Under strict light pH

control and CO 2 supplementation during sulphur deprivation, hydrogen

production can be sustained with the addition of carbon dioxide gas instead of

acetate under photoautotrophic (synthesising food using light as the energy

source) conditions. If using flue gas from an electrical power plant, any sulphates

must be removed before entering the algal culture. The presence of oxygen will

prevent the anaerobic process and hence the formation of hydrogen as aerobic

processes can resume.21

The culture conditions must be strictly regimented to produce hydrogen and pre

stages of growth are required, as can be seen in figure 3.1. The first stage is

regular, aerobic photosynthesis forming oxygen, starch and inducing cell growth.

The carbon dioxide supply is then removed, starting the second phase and

forcing energy to be converted from stored starch whilst the previously created

oxygen is consumed. When there is no longer oxygen available in the growth

medium, anaerobic conditions form, initiating a delayed hydrogen production,

and stunting growth. Sustaining hydrogen production for a prolonged duration of

time will eventually cause the algae cell to die as the starch is consumed, so

aerobic photosynthesis must be reintroduced before this point if it is desired to

continue using the same cells.

18

Figure 3.1; The changes in dissolved O2, pH and volume of the H2 produced during the cultivation of

sulphur deprived algae under photoautotrophic conditions.

It is claimed that a maximum H2 production rate of 56.4 mL.L-1 can be achieved

under photoautotrophic conditions if the culture is supplied with carbon dioxide

for the first 24 hours of sulphur deprivation, exposed to high light intensities

(110-120 µ.E.m -2.s-1) during the oxygen producing stage and finally exposed to

low light intensities (20-25 µ.E.m-2.s-1) during the oxygen consumption and

hydrogen production stages.

19

Under aerobic growth conditions, a specific growth rate of 0.27h-1 can be

achieved.22 This is an equivalent doubling time of 3.7 hours under linear growth

conditions.

Papers were not found on the rates of carbon dioxide sequestration for the

hydrogen producing species, Chlamydomonas reinhardtii, however the hydrogen

production rate of 56.4 mL per L of culture for an average cell density of 15x10-3

kg.m -3 can be utilised to determine what mass of cells are needed to produce

required quantities of hydrogen. If hydrogen can be produced cleanly and

reliably by this species of algae, the possibilities for fuel can expand into a new

era. If bio-hydrogen were to be used in a power plant, the algae would require a

new source of carbon dioxide, as it is no longer a by-product of combustion.

There are also many channels for hydrogen use in the transport sector, car

manufacturers have invested heavily in hydrogen fuel cell powered cars.23

The rates of carbon dioxide sequestration by Chlamydomonas reinhardtii need to

be determined before this alga can be considered appropriate for removing flue

gas carbon dioxide. A less regimented process of light levels for the production is

also required if natural sunlight is to be harnessed.

20

4. PHOTOSYNTHESIS

Organisms of the plant kingdom and some bacteria create their own energy

supply by utilizing that of the sun. This process is known as photosynthesis, the

generic form of which is given as equation 4.1. Conditions allowing

photosynthesis are referred to as autotrophic. Carbon dioxide and water are

required in the process whilst oxygen is released. The carbon is effectively stored

as organic substances with the only external energy source being the sun, which

by human time scales, is inexhaustible.

(4.1)

If the carbon dioxide from flue gasses can be supplied effectively to a culture of

photosynthetic organisms, then these emissions after passing will be reduced

and organic structures produced, in other words, growth. This forms the basis of

this study.

This section discusses the factors that influence the rate of photosynthesis and

hence growth, explaining how they can be optimised for an algal culture.

21

4.1 LIGHT

Light drives photosynthesis by supplying the energy for two key systems; the

transfer of electrons from water to NADPH (nicotinamaide adenine dinucleotide

phosphate) and the generation of ATP (andenosine triphospate). The range of

electromagnetic radiation enabling photosynthesis is referred to as the

photosynthetically active radiation range, wavelengths between 400nm and

800nm.24 Chlorophyll typically absorbs light at around 450nm and 650nm,

perceived by the human eye as blue and red respectively, shown in figure 4.1. It

is therefore these wavelengths that need to be present for photosynthesis to

take place. Naturally, this light is sourced from the day’s sunlight but provided

the correct wavelengths are present, photosynthesis will take place under

artificial light also, with the added advantage of control. It should be noted that

almost all laboratory experiments are carried out under artificial light.

Figure 4.1; Chlorophyll light absorption spectra

There are three further light variables that will influence the process of

photosynthesis, and hence algal growth. The first is light intensity; how much

light energy enters the culture. Enough suitably energised photons must be

captured by chlorophyll pigments to initiate the chain of reactions that is

22

photosynthesis, but if too much energy is presented cell damage will occur,

inhibiting growth. Excess light stunting growth is referred to as photo-inhibition.

The second variable is photoperiod; the ratio of light and dark, typically

measured over a 24 hour period. If NADPH and ATP compounds are available,

the remaining processes of photosynthesis can take place without the necessity

of light. These reactions are referred to as ‘dark reactions’, 25 although naturally

they take place in both light and dark conditions. It may be hypothesised that

the fasted growth will take place under continuous light, however, algae has

evolved in systems with light and dark periods, therefore excess light may not be

utilised. This is an important factor when considering algae for large scale

production; will the energy used illuminating a culture overnight be recovered by

the extra levels of growth?

The third factor to be considered in large scale algal farms is photo-limitation or

light shading. By the nature of photosynthesis, photons captured by chlorophyll

cannot penetrate further into the culture. This results in an optimum depth of

algal culture where any deeper cells would not receive photons of suitable

wavelength, similar to the effect of the tree canopy in a forest.

For algae to be a renewable fuel source natural light must be utilised so local

solar irradiance, weather patterns and seasonal changes for suitability of algal

growth are also discussed.

23

4.1.1 LIGHT INTENSITY

Sorokin shows that the optimum light intensity for algal growth lies between 400

and 3000 foot candles 26 (6.30 and 47.28 W.m-2),27 figure 4.2. Each light intensity

curve starts with a light dependant region where growth rate increases with light

intensity, followed by a light independent plateau, finalising with a second light

dependant region where growth declines with higher light intensities. It can also

be seen that different species have distinguishable characteristics, although the

same tri-region curve fits all considered, with a similar plateau light intensity

range.

Figure 4.2; the growth rates of four species of algae at 25oC

under different intensities of continuous light.

A similar experiment by Qin on the algal species Botryococcus braunii showed

similar results,28 figure 4.3. The highest growth rates were produced under light

intensities of 30 and 60 Wm-2, with doubling periods of 2.61 and 2.51 days

respectively. The trend line gradients of intensities either side of 30 and 60W.m-2

24

indicate a very similar effect to that seen above with limited growth at intensities

above 100 W.m-2 and below 20 W.m -2.

Figure 4.3; The impact of light intensity on Botryococcus Braunii algal growth at 23oC,

12h light, 12h dark. (A) Low light range (W.m-2); (B) High light range (W.m-2)27

Some algal behavioural observations were also noted in this second experiment.

Under 20 W.m-2, the cells agglutinated together after stirring rather than

spreading to an even distribution as noticed between 30 and 60 W.m -2. A similar

effect was observed over 150 W.m-2 and at 300 W.m-2, most algal cells died.

Although the optimum light intensities are shown here to be over 30 W.m -2,

photosynthesis will occur at lower light intensities. The lowest value of

illumination to allow for photosynthesis is called the critical illumination, denoted

Ic. When looking at individual reactions, experiments have determined the

energy needed for each molecule of oxygen to be produced. Pirt describes the

minimum quantum demand to be in the order of 5.3 to 8.6 hv.O 2-1,29 where h =

plank’s constant (6.62x10-34 J.s) and v is the frequency (Hz, s-1). The minimum

quantum yield is therefore the equivalent of 5.3 to 8.6 joules. This energy can be

summated by the capture of lesser energy photons as the potential produced by

a single interaction can be stored for a brief amount of time in chlorophyll

antenna, reducing the lowest light intensities for photosynthesis further.

25

The optimum light intensity for growth was proven to also be the optimum for

hydrocarbon production for Botryococcus braunii, 27 as shown on figure 4.4. The

highest constituency was over 80%, but lowered dramatically under light

intensities of over 60 Wm-2.

Figure 4.4; Relative total lipid content (rTLC) of Botryococcus braunii under various light

intensities.

Further study comparing three different Botryococcus strains showed that a UK

strain (UK 807-2) contained the highest relative lipid content at 60 Wm -2 with a

comparable 75%,30 figure 4.5. It also indicates a drop off in productivity over 60

Wm-2.

Figure 4.5; the mean relative lipid content of three Botryococcus braunii strains at different

irradiance, 25oC.

26

4.1.2 PHOTOPERIOD

Qin also shows that higher growth rates take place under longer photoperiods,27

shown in figure 4.6. Algal growths are severely inhibited with only 4 hours of

sunlight a day and slow with 8 hours of light a day. It can also be seen that

there is a limited difference between 12 hours and 24 hours of light over a 24-

hour period, although continuous light does show a slight advantage at greater

optical densities.

Figure 4.6; Impact of different photoperiods on algal growth of

Botryococcus braunii at 25oC and 30Wm-2

This indicates that there will be no recoverable advantage illuminating an algal

culture over an entire 24 hours, but mechanical light may well prove a worthy

investment to ensure 12 hours of illumination during seasonal short days. This

will unfortunately increase costs, reducing profits the further away from the

equator the photobioreactor is based, unless the extra light received during

summer months can be utilised.

27

This artificial light does not have to be continuous however. The maximum

growth rate of a Chlorella species was shown by Pirt to be maintained if each cell

is exposed to 20 W.m-2 for 0.5s followed by a 9s dark period.31 If this finding can

realistically be put into practice, an hour’s growth can be produced with the

equivalent of 3.2 minutes of illumination.

4.1.3 PHOTO-LIMITATION

Light entering a dense algal culture is rapidly dispersed, forming many various

irradiances and light gradients amongst the culture whilst self-shading of cells

will occur.32 If the light is dispersed or captured too rapidly, only the outer edges

of a tubular reactor or the top surface of an algal pond will have high enough

illumination to support photosynthesis. These effects can be overcome by

lowering the cell density, increasing the dilution rate, increasing the irradiance

(remaining below a level to cause photo-inhibition), reducing the diameter of a

photobioreactor, reducing the depth of an algal pond or creating currents to

ensure individual cells pass through all irradiance levels for suitable periods of

time. A precise understanding of each of these variables is required to

understand how many algal cells will be under optimal growth conditions at a

particular time. Photo-bioreactors need to be designed to reduce the effects of

photo-limitation to allow the highest algal densities to cultivate within their

boundaries. There are a number of methods used in literature to determine the

light distribution in a photo-bioreactor.

Anderson et al.33 state that equation 4.2 can be used to determine the light

intensity at a required culture depth at a specific concentration;

dCkII o ××=

ln (4.2)

28

Where;

I = Light intensity at depth of penetration d

Io = Incident light intensity

C = Algae concentration (kg.m-3 or g.L-1)

d = Depth (m)

k = Light extinction coefficient (m2.kg-1)

The light extinction coefficient can be split up into the factors of each constituent

of the culture. In an algal culture, the constituents of consideration are non-

photosynthetic elements (water and minerals) and chlorophyll a, for which the

concentration (c) must also be considered.34 This leads to the Bannister linear

equation;

ckkk cw += (4.3)

A light extinction coefficient value of 200m2.kg-1 was determined by Ogbonna

and Tanaka for Chlorella sp.35 Using a concentration of 10g.L-1, the light levels at

a depth of 1mm will be 13% of those at the surface and at 2mm, only 2% of the

incident light will have penetrated, indicating how dramatic this phenomenon is,

shown in figure 4.7.

29

Figure 4.7; Light intensity change with depth at different algal concentrations.

This indicates that a photo-bioreactor requires a very high surface area to

volume ratio to absorb enough light for high algal densities, or very low algal

concentration values must be used. Low concentrations will require greater

volume and hence larger land areas. Ogbonna further discusses the need for a

new method of rating a photo-bioreactor, based upon its light distribution

characteristics.36

Using the same nomenclature as before, where in this case d=depth of reactor,

the average light intensity can be given by equation 4.4.

( )kCd

eII

kCdo

av

)(1 −−= (4.4)

The average light intensity will be under 50% of the incident light intensity in

any pond reactor over 2mm deep with a 10g.L-1 concentration. As explained

previously, there is a very rapid illumination gradient in an algal culture, so the

average light intensity favours shallow reactors, whatever the size or shape,

30

making scaling difficult. Instead it was proposed that the reactor could be divided

into illuminated and non-illuminated volume fractions. The ratio of the volume

receiving sufficient light for photosynthesis against the volume that does not

gives the illuminated volume fraction, denoted VF. VF may be close to unity at

low algal concentrations and will tend towards zero at high concentrations. This

is shown in figure 4.8. It is further proposed that an index of the light

distribution of a reactor be defined as the algal concentration that will reduce the

illuminated volume fraction to 50%. This index is named the light distribution

coefficient and is denoted Kiv.

Figure 4.8; Effect of cell concentration on illuminated volume fraction of a cubical photo-bioreactor.

A= Single side illumination at 325µmol.m-2.s-1, B= Two side illumination at 162.5µmol.m-2.s -1

If assuming that the critical light intensity for photosynthesis (Ic) is 7.65µmol.m-

2.s-1 (1.66W.m -2, from 1µmol=0.2176J), as stated in the paper, and that k

remained 200m2.kg-1, it was found empirically from figure 11 that the Kiv values

were 1.9kg.m-3 and 3.1kg.m-3 for condition A and B respectively. This clearly

informs that design B is a more efficient method, capable of housing a higher

concentration. Calculation of the light distribution coefficient differs for various

reactor shapes and illumination methods. The equations for an externally

illuminated cuboidal reactor, externally illuminated cylindrical reactor and

internally illuminated cylindrical reactor are given as equation 4.5, 4.6 and 4.8

31

respectively with figures 4.9, 4.11 and 4.13 respectively displaying schematics of

each reactor. The variation of the light distribution coefficient with

photobioreactor size for each model are shown as figures 4.10, 4.12 and 4.14

respectively. The following assumptions are applied to each model; Io = 60W.m-

2; Ic = 2W/m-2; k = 200m2.kg-1

It should be noted that these are greatly simplified models to what will exist in

commercial applications. The true behaviour will be a combination of the effects

existing mainly in pond and exteriorly illuminated conditions.

Figure 4.9; Schematic representation of a cuboidal reactor,

illuminated exteriorly from above.

DkII

K c

o

iv ××

=5.0

ln (4.5)

Figure 4.10; Changes in light distribution coefficient with increasing depth of a

cuboidal photobioreactor.

32

The depth values for a pond are very small with the largest depth being 10cm

providing a light distribution coefficient of under 4kg/m3. If ponds are to be used,

they will have to be very shallow, and hence occupying a large surface area

making for control of the culture very difficult.

Figure 4.11; Schematic representation of a cylindrical reactor, illuminated exteriorly.

L

Lc

o

iv LkLDI

DI

K×

−

=)2(

ln (4.6)

LL can be determined by calculation by equation 4.7, derived from the area

equation of a circle, provided the Kiv value is for 50% photosynthetically active

volume, ignoring effects at either end of the reactor.

DD

DLL 1464.022

1=

−= (4.7)

33

Figure 4.12; Changes in light distribution coefficient with increasing diameter of an externally

illuminated photobioreactor.

These are particularly low values, and in experiment, far better algal

concentrations have been grown. At smaller tube diameters, the pond effects will

play a larger role, especially as there will be a dominant solar irradiance direction

from above.

Figure 4.13; Schematic representation of a cylindrical reactor, illuminated interiorly.

L

Lc

o

iv LkdLI

dI

K×

+

=)2(

ln (4.8)

LL can be determined by calculation by equation 4.9, provided the Kiv value is for

50% photosynthetically active volume, ignoring effects at either end of the

reactor.

34

2

222

222

−

++−

=

dDdd

LL (4.9)

Figure 4.14; Changes in light distribution coefficient with increasing diameter of an internally

illuminated photobioreactor.

Assuming ; d = 0.05m

It appears that an internally illuminated reactor can support relatively high algal

concentration cultures. This suggests that any artificial illumination should

perhaps be utilised in this way.

35

4.1.4 LOCAL CONDITIONS

As much as possible of the energy needed from light to drive enough

photosynthesis to reduce the carbon dioxide emissions from flue gas should be

harnessed from the natural source of the sun. This will not only reduce costs

when compared to an artificially illuminated system, but will prevent extra

energy use, and hence carbon dioxide emissions directly from the power plant.

The natural solar illumination available will depend on the location from the

equator, local weather patterns and the time of year. The UK and Ireland’s total

annual horizontal solar irradiance is shown as figure 4.15, whilst the monthly

average conditions for Cardiff are shown in figure 4.16.

Figure 4.15; Yearly total global horizontal irradiation, (a) Europe, (b) UK and Ireland

36

Figure 4.16; Monthly variation of Solar Irradiance in Cardiff

It was shown previously that optimum growing conditions included 12 hours of

illumination at over 30W.m -2 a day. From figure 4.16 it can be seen that Cardiff

actually receives these conditions naturally between the months of April and

September, with substantially longer days in the summer months. The remaining

six months will require artificial lighting in the mornings and evenings. December

will prove to be the most draining of resources requiring six hours of artificial

light a day.

It has not been made clear from previous work if the photoperiod will have the

same effect if the illuminated period is split into different sections. Artificial

illumination over the winter months will coincide with the morning and evening

peak electricity demand periods. Not only will illuminating the culture risk the

possibility of causing too much electrical load during these times, but also the

electricity sold at these hours is at peak rate. If the culture can be illuminated at

night, during times of minimal loads, then the cost of artificial illumination will be

reduced. Issues surrounding light pollution may arise. Such a system should

however be designed for minimum light escaping the cultures, resulting in

maximum efficiency. An internally illuminated photobioreactor may prove to be

the best option under such conditions.

37

South of Cardiff, to the line of the equator, overall solar irradiation levels will

increase with less variation in lengths of day, resulting in steadier, easier to

manage conditions. North of Cardiff the overall illumination will reduce, seen in

figure 18, and the seasonal length of day will increase making for more difficult

conditions to utilise the algal potential. This puts Cardiff, with median conditions,

an excellent place to determine the validity of this study. Required reactor sizes

for an equivalent carbon producer will probably reduce in size in southerly

locations and increase in more northerly locations due to algal density

possibilities.

38

4.2 CARBON DIOXIDE

Carbon dioxide is received by an organism from the medium in which it lives, for

algae, this medium is water.37 In water, the majority of stored carbon dioxide is

in solution. About 1% is stored as carbonic acid, H2CO3, reducing the pH of the

solution. Increased acidity drives the reaction in the opposite direction, releasing

CO2. Gasses are generally less soluble in warmer solvents and this holds true of

carbon dioxide. The presence of salts and proteins in a solvent also reduces the

solubility of gasses.

The delivery of carbon dioxide to algae must therefore consist of two stages; the

carbon dioxide uptake into water solution and the removal of this carbon dioxide

from solution by algae. Optimum conditions of one may not be the optimum for

the other, but the overall effects can simultaneously be observed by bubbling

carbon dioxide through an algal culture and measuring the rates of carbon

dioxide removal. This is after all, the process that needs to be determined for

removing carbon dioxide from flue gas streams.

This section discusses the variations in concentration and fixation rate in

comparison to growth rates.

4.2.1 CONCENTRATION

Current atmospheric concentrations of carbon dioxide are around 0.038%. This is

a far lower concentration than what can be expected in flue gas, where the

concentrations will be closer to 13%. Figure 4.17 shows that the optimum level

of carbon dioxide for growth of Chlorella vulgaris is at 2%. The growth rate is

actually doubled with this higher concentration in comparison to air but is

severely inhibited at higher concentrations. After increasing the cell density, the

39

algae grew much better at 5% carbon dioxide, suggesting that higher density

cultures can be used with higher carbon dioxide concentrations.

Figure 4.17; Effects of different concentrations of CO2 aeration of the growth of Chlorella sp.

Inoculated at 300 µmol.m-2s -1, flow rate of 0.25 vvm, 26oC, starting density of

(a) 8x105 cells.mL-1, (b) 8x106 cells.mL-1

The huge range of results that exist by using different species of algae can be

seen clearly for Chlorella sorokiniana, shown in figure 4.18. This species appears

to grow very well, and far more rapidly than Chlorella vulgaris in much higher

carbon dioxide concentrations. The growth rate does not reduce until levels of

40% are reached, higher than those produced in power production plant flue gas

composition. For this reason, Chlorella sorokiniana may prove to be the most

suitable species for carbon dioxide sequestration.

Figure 4.18; Effect of CO2 concentration on the growth of

Chlorella sorokiniana H-84 at 40oC

40

4.2.2 FIXATION RATE

The true sizing of an algal carbon dioxide sequestration plant is dependant upon

the rate of carbon dioxide fixation. A faster rate per algal cell will reduce the

number of cells required, reducing the overall volume.

Published experimentation in this field has taken place using very small test

volumes. The rector used to produce the results in figure 4.19 had dimensions of

600mm height and 50mm diameter, providing an effective volume of 1.2 L. It

can be seen that higher gas flow rates reduce the values of carbon dioxide

fixation. This is because of the dramatically reduced retention time. The gas is

simply passing through the culture too fast to dissolve or be utilised by contact

with algae. In small reactor models, the gas flow rate must be reduced to

compensate for this.

Figure 4.19; Effect of gas flow rate on CO2 fixation and O2 evolution,

(T = 25oC, cell number = 5x107 cells.mL-1, luminous intensity = 5400 lx)

In larger reactors however, it is the author’s belief that this problem will largely

be overcome, as the retention time will be increased due to the extra lengths of

reactor. It is doubtful also that a commercial reactor will be orientated vertically,

as in experiment, further reducing the speed of gas through the culture and

41

increasing retention time. These factors may allow far higher gas flow rates to be

used. A higher flow rate will aid many other processes of the system. Induced

turbulence will improve fluid mixing and carbon dioxide solubility along with

movement of the algae in eddy currents, allowing each cell time in the higher

light intense regions of the photobioreactor. The true flow rate in a commercial

system is governed by the power output of the power plant at a particular time.

All the gasses that are emitted at the production rate must be accepted into the

culture, determining the flow rate. Requiring low flow rates for each reactor will

increase the overall size of the scrubbing system.

Figure 4.20; Effect of luminous intensity on CO2 fixation and O2 evolution by Chlorella vulgaris,

(T=25oC, cell number=5x107 cells.mL-1, gas flow rate=1.25L.min-1)

Figure 4.20 above has a very similar shape to the photo-dependant regions of

growth curves suggesting that carbon dioxide fixation rate is directly proportional

to growth rate. The optimum growth rate for Chlorella vulgaris was previously

stated to be in carbon dioxide concentrations of 2%. Figure 4.21 below shows

that the optimum efficiency of carbon dioxide reduction is greatest at 2% carbon

42

dioxide concentrations also, with an effective rate of 58%. This indicates the

assumption that the most effective carbon dioxide fixation rate occurs at the

maximum growth rate.

Figure 4.21; Comparisons of the total amount and efficiency of CO2 reduction in the single and the

six-parallel photobi oreactor of semicontinuous Chlorella sp. under 2%, 5%, 10% and 15% CO2

aeration.

43

4.3 TEMPERATURE

Temperature affects the rate of photosynthesis by changing the rates of enzyme

reactions involved in systems of the photosynthetic complex. The optimum

growth rate will occur at the optimum temperature, with decreasing rates either

side of the peak, figure 4.22.

Figure 4.22; Temperature effects on photosynthetic activity

The temperature response of different species varies considerably and the

optimum may well lie outside of the regular natural growth conditions. Typical

values fall between 25oC and 30oC. Table 4.1 indicates the optimum temperature

for a number of species, including Chlorella vulgaris and figure 4.23 shows

growth rates of Botryococcus braunii and Chlorella sorokiniana at different

temperatures.

Table 4.1; Optimum temperatures for four species of algae

44

Figure 4.23; impact of temperature effects on growth over time, (a) Botryococcus

braunii (b) Chlorella sorokiniana

The optimum temperatures for Chlorella vulgaris, Botryococcus braunii and

Chlorella sorokiniana are 30oC, 23oC and 40oC respectively.

45

4.4 NUTRIENTS

Nutrition is required by all plants to photosynthesise effectively. The range of

nutrients required varies on the plant and which processes are active will

determine the required supply rates.38 Nitrogen is heavily required for production

of proteins, especially light harvesting complexes that constitute almost 50% of

thylakoid membranes. Deficiency of nitrogen will have a wide effect on many

components whilst lesser -used nutrients will have specific effects, such as

electron transport in the case of iron. Phosphate is involved in many functions of

photosynthesis and deficiencies may change the activity of enzymes. Select

supply of nutrients can have favourable effects also, such as sulphur deprivation

in Chlamydomonas reinhardtii producing hydrogen. The highest hydrocarbon

content in Botryococcus braunii required very low levels of nitrates, phosphates

and potassium.39 A table of typical nutrients added to algal cultures in

experimentation is given as table 4.2.

Table 4.2; Typical Nutrient Supplementation

46

5. CARBON DIOXIDE SEQUESTRATION

Many of the factors that influence the rate of algal growth, and hence carbon

dioxide fixation have been discussed for a selection of different species. It is now

possible to apply these findings to a physical system. As coal is particularly dirty,

has many protesters but is in plentiful supply with respect to other fossil fuels, it

may prove to be the most useful application of algal carbon capture. Here, it is

applied to a medium sized 1500MWe coal burning power plant, figure 5.1.

Figure 5.1; Input and output characteristics of a coal fired power station

A 1500MWe coal-burning power plant with an electrical generation efficiency of

35% will require a thermal input of 4300MW. Using an ultimate analysis of 80%

carbon, 13% oxygen, 6% hydrogen and 1% sulphur, the calorific value of coal is

approximately 30MJ.kg-1. This equates to a fuel demand of 143kg.s-1. The carbon

dioxide producing reaction of burning coal is given as equation 5.1. Carbon has

an atomic mass of 12 whilst oxygen has an atomic mass of 16. Carbon dioxide

therefore has a molecular mass of 44. For each kilogram of carbon burnt as coal,

3.7kg of carbon dioxide is emitted. Running at full capacity, this station is

burning 114.6kg.s-1 of carbon, producing 420kg.s-1 of carbon dioxide.

47

(5.1)

Typical emissions from a coal plant are 13% carbon dioxide, 82% nitrogen, a

little oxygen and a trace of sulphur dioxide. 420kg of carbon dioxide will occupy

234m3 at 25oC and 1bar. The total volume of flue gas produced is therefore

1800m3.s-1 at 13% CO2. The temperature will be higher than 25oC, so these are

conservative volumetric estimates.

It was previously shown that Chlorella sorokiniana has the most rapid growth

rate, thrives in temperatures of 40oC and carbon dioxide concentrations of up to

20%. Chlorella sorokiniana is therefore the most suitable algae for carbon

dioxide sequestration mentioned in this study. Lihai et al suggest that the

optimum gas flow rate for carbon fixation of 1% CO2 through a Chlorella vulgaris

culture was 1.25 L.min-1.12 This was carried out at a lower than optimum light

intensity of 8 W.m-2, and 2% CO2 was earlier shown to be the optimum

concentration levels for vulgaris growth. None the less, with a starting algal

concentration of 5x107 cells.ml-1, a carbon dioxide fixation rate of 0.14g.L-1hr-1

was determined. Being from the same genus, we can make the assumption that

at its respective optimum growth conditions, the fixation rate of C. sorokiniana

will be similar to that of C. vulgaris.

Changing each of the figures to S.I. units, we have a carbon dioxide fixation rate

of 0.0023kg.m-3.s-1. Our power station is producing 420kg of carbon dioxide a

second. Assuming a constant fixation rate even though the concentration is

reducing down the reactor, a total reactor volume of 183x103m3 is required. The

experiment was carried out in a tube of diameter 130mm, resulting in a single

tube length of 14,100km. Using the arrangements shown in figure 5.2, allowing

light between adjacent tubes, four tubes can be housed in each metre width.

48

Having done this, a ground surface area of 3.5x106m2 is required. This can be

housed in a 1.87km square space. Adding a little safety factor by rounding the

numbers up, the carbon dioxide emissions from a 1500Mwe coal fired power

plant can be reduced to zero in a 2km x 2km square algal field.

Figure 5.2; Photobioreactor design using dimensions from lihai et al.

49

6. ALGAE BIOFUEL

6.1 BIOMASS

The flue gas carbon dioxide has been converted into Chlorella sorokiniana algal

biomass with a doubling time of 2.5 hours. If optimum growth conditions were to

be maintained, this equates to 9.6 doublings over a 24hour period. The original

cell density was 5x107 cells.ml-1, equal to a biomass dry weight of 0.7kg.m-3 by

use of the graph in figure 6.1.40 Our original mass was therefore 128x103 kg. If

this doubles 9.6 times, a mass of 1.23x106 kg will be produced daily, with an

average rate 14.2kg.s-1.

Figure 6.1; Relationship of optical density to algae cell number and dry weight biomass.

The next process to be determined is how to make use of this algal biomass. The

composition of Chlorella sorokiniana has been determined previously and table

6.1 below displays the chemical composition of each constituent of the algae.41

From this we can determine the suitability for this algal biomass for a fuel using

the Milne formula.

50

Table 6.1; Chemical composition of biomass

The dry high heating value of Chlorella sorokiniana proves to be 15.6 MJ.kg-1.

This value is typically regarded as too low to be used as a fuel. It is however,

higher than all four biomasses quoted in table 5. If the algal biomass was to be

dried and placed directly in the burner with coal, then 221MWth or 5%, of the

stations requirements will be supplied by algal biomass. This is the equivalent of

saving 7.4 kg.s-1 of coal. The carbon dioxide released by burning the algal

biomass will be filtered through the algae farm after combustion, re-fixing the

carbon dioxide. This cycle would continue during the plants operation, essentially

storing and recycling the carbon from the original coal combustions.

6.2 ANAEROBIC DIGESTION

The anaerobic digestion of the algal biomass can obtain greater amounts of

energy than its direct use as a biofuel. Anaerobic digestion is the degradation of

carbonaceous matter by micro organisms with the absence of oxygen, producing

carbon dioxide and methane with a reduction in biomass.42 A typical reactor will

be maintained at a temperature of 35oC, ‘fed’ once or twice a day at a rate of 1.7

kg(VS).m 3.d-1. The volatile substrate (VS) will be retained for 20 to 30 days and

will reduce by about 60%. The typical gas composition produced by anaerobic

digestion of algae is in the order of 70% methane (CH4), 25% carbon dioxide

with the remaining volumes consisting of water vapour, nitrogen and hydrogen

sulphide.43 Table 6.2 shows how much methane can be produced from various

51

biomass samples. Kelp, the closest mentioned biomass to algae, can produce

methane at a rate of 0.4 m3.kg-1 of added volatile substrate. Theoretically, the

yield could achieve 0.51 m3.kg-1 assuming the empirical formula for kelp to be

C2.32H3.73O1.48.44 Algae may be expected to perform better due to the simpler

individual cell structure.

Table 6.2; Ra nge of biochemical methane potential data from biomass or waste

feedstock.

Methane has a high calorific value of 55.5 MJ.kg-1, a far more efficient value for a

fuel. Using the methane production rate of 0.4m3.kg-1 for kelp and recalling our

dry algal production rate of 14.2 kg.s-1, methane can be produced at a rate of

5.68 m3.s-1. Directing this methane directly into the power plant furnace,

315MWth (7.3%) can be supplied by this process. This has actually released

more energy from the same yield of algae burnt as biomass. The remaining solid

matter is typically used as crop fertiliser or animal feed. The process of anaerobic

digestion has however created carbon dioxide once again. If 5.68m3 methane is

70% of the gasses produced and carbon dioxide occupies 25%, then carbon

dioxide is produced at a rate of 2.03m3.s-1. If this was to be released to the

atmosphere, we have still reduced the carbon emissions of the coal power plant

52

by 99% but there is no reason why this small amount of carbon dioxide cannot

be directed back through the photobioreactor.

The problem arises however at the long retention time due to the slow rates of

digestion. If the reactors can be ‘fed’ at a rate of 1.7kg(VS).m-3.d-1, then the total

volume of anaerobic digesters would lie around 723.5x103m3. If each reactor

were 10m in diameter and 10m tall, 116 reactors would be required, requiring a

land area of 11.6km2, the equivalent of a 108m square.

6.3 BIODIESEL

Biodiesel is a proven fuel in the transport industry and a technology that has

been around for over 50 years.8 Typically derived from plant and animal oils,

namely soybeans and sugar cane, it has been involved mainly with road vehicles.

The vastly more rapid growth of algae, along with its higher oil content will

greatly reduce the amount of land required to produce biodiesel, as can be seen

in table 6.3. It also has the advantage of not requiring agricultural land and

displacing food crops, a major issue surrounding energy crops. Use of algae for

biodiesel is yet to be put into commercial practice, but has interest from the air

transport industries looking for a ‘green’ fuel.

53

Table 6.3; Comparison of some sources of Bio-diesel

Table 6.4 compares oil extracted from Botryococcus braunii by ethyl acetate with

class-C heavy oil and shows that the oil has a similar calorific value whilst

containing less nitrogen and sulphur. When burnt, bio-diesel is relatively

environmentally friendly as it is significantly less polluting than conventional

petro-diesel.45

Table 6.4; Comparison between Extract and Cass-C Heavy Oil

Manufacturing bio-diesel from algal oils and lipids involves two stages, removal

of oil and chemical alteration into diesel, figure 6.2. Oil is extracted from the cell

culture by liquefaction or use of a solvent. 46 Super critical carbon dioxide can be

54

used, is non-toxic and cheap.47,48 Alternatively, acetone, methanol and n-hexane

can be used as solvents whilst breaking the cell wall will increase the extraction

yield of oils.17,49 After evaporating the solvent, biodiesel can be obtained from

the extracted oils by transesterification, the process of converting the relative

components into mono-methyl-esters.50,43 Biodiesel production for an algal farm

depends upon the species used, its relative lipid content and the growth rates

that can be achieved. The remaining solid algal residue can be used as animal

feed or be anaerobically digested.

Figure 6.2; Process flow schematic for bio-diesel production

It must be appreciated that the burning of biodiesel produced from a carbon

dioxide sequestration plant is simply releasing the removed carbon dioxide over a

wider area. The emissions have had two uses before their release, but unless

petro-diesel combustion reduces at the rate of biodiesel production, there will be

no net carbon dioxide reductions in the atmosphere.

55

7. CARBON STORAGE

Aside from biomass and oils, other compounds naturally produced by algae are

also of valuable use. β-carotene, omega-3 and eicosapentaenoic acid (EPA) are

of use in the pharmaceutical industry for heart and blood disorders,51 whilst the

oils extracted from algae can be used in building materials and bio plastics.52

Mixing Chlorella vulgaris with polyvinyl chloride (PVC) between 10-30% can

provide stable moulds, sufficiently strong for use for typical PVC building

materials. The use of a by-product of photosynthesis, polysaccharides, can be

added to concrete to pre-harden, removing the need of concrete compaction.

This is a rather undeveloped area, but the production of biodegradable, bio-

plastics would receive much interest.

56

8. HARVESTING

The harvest of algae currently proves to be a major challenge for industrial algal

cultivators.53 There are a number of methods available, compared in table 9. The

style of algae determines which harvesting methods are most suitable. The

easiest forms to harvest are those that flocculate together and sink in still water,

forming a dense sedimentary mass. A larger flocculation will sink more rapidly,

allowing greater rates of harvest. Flocculation can be induced by the addition of

chemicals, but this will increase costs and may contaminate by-products. After

harvesting, there remains much water which must be removed by heated

evaporation before the algae can be used for its products.

Table 8.1; Biomass recovery options

57

9. CONCLUSIONS

This review looked at the possibilities of using algae to remove carbon dioxide

from power plant flue gas and was applied to a 1500MWe coal fired station

whilst addressing the possibilities for using the produced biomass as a fuel.

It has been shown that the algal species Chlorella sorokiniana can be grown from

the flue gas emissions of a coal fired power plant in the South Western regions

of the United Kingdom. The entire carbon dioxide emissions from a 1500MWe

coal fired power plant can be absorbed in a 2x2km field of Chlorella sorokiniana

with a starting algal concentration of 5x107 cells.ml-1. This has been predicted to

produce algae at an average rate of 14.2kg.s-1. The algae produced cannot be

directly burnt as biomass in the furnace as all the carbon dioxide would be re-

released, requiring further sequestration whilst only providing 5% of the fuel

demand. Anaerobic digestion will provide 7.3% of the fuel demand, but will

require vast volumes of anaerobic digesters.

If algae were to be used to remove carbon dioxide from power plant emissions,

the produced biomass must be refined into suitable materials. The most viable

product is biodiesel; however it’s burning will re-release the stored carbon

dioxide. Some species can achieve relative lipid contents of 50-70%wt. These

species typically have far lower growth rates than that of Chlorella sorokiniana

although some Chlorella species have been found to produce hydrocarbons. Oils

extracted can be used to manufacture biodegradable bio-plastics, however much

research is required to make this a viable product.

Further research is required in order to determine if the assumptions made in

this study are accurate, specifically regarding the Chlorella sorokiniana carbon

dioxide fixation rate and optimisation of this process. Determining carbon dioxide

fixation rates for hydrocarbon producing algae will prove most valuable as

58

realistic production values of biodiesel can be obtained, providing an economic

enticement in support of the environmental incentive. Research involving

hydrogen producing species may reveal further fuel possibilities and genetic

engineering may open yet further doors in improving algal photosynthetic rates

in flue gas specific environments.

59

REFERENCE

1 HM Government, Department of Trade and Industry, 2007. Meeting the Energy Challenge. A

White Paper on Energy.

2 IEA Greenhouse Gas R&D Programme; CO2 Capture and Storage.

http://www.co2captureandstorage.info/what_is_co2.php

3 Friends of the Earth, 2007. Briefing Note: Carbon Offsetting.

http://www.foe.co.uk/resource/briefing_notes/carbon_offsetting.pdf

4 Eiichi Ono, Joel L. Cuello, 2004. Selection of optimal microalgae species for CO2 sequestration.

University of Arizona.

http://www.netl.doe.gov/publications/proceedings/03/carbon-seq/PDFs/158.pdf

5 Paul Henley, 2008. In bloom: growing algae for biofuel. BBC News, 9th October.

http://news.bbc.co.uk/1/hi/sci/tech/7661975.stm

6 Independent, 2000. Lowest form of life could save planet. Independent newspaper 22nd

February

http://www.independent.co.uk/environment/lowest-form-of-life-could-save-planet-724509.html

7 John Ashworth, 2006. Jet Fuel from Microalgal Lipids, National Renewable Energy Laboratory,

U.S. Department of Energy.

http://www.nrel.gov/docs/fy06osti/40352.pdf

8 Yusuf Chisti, 2007. Biodiesel from microalgae. Biotechnology Advances, Volume 25, pg. 294-306

9 Michael A Borowitzka, 1999. Commercial production of microalgae: ponds, tanks, tubes and

fermenters. Journal of Biotechnology; Volume 17, pg. 313-321

10 Morten Foldager Pederson, Jens Borum, 1996. Nutrient control of algal growth in estuarine

waters. Nutrient limitation and the importance of nitrogen requirements and nitrogen storage

60

among phytoplankton and species of macro algae, Marine Ecology Progress Series; Volume142,

pg. 261-272

11 Alain Dauta, 1990. Growth rate of four freshwater alga in relation to light and temperature,

Hydrobioligia; Volume 207, pg. 221-226

12 Lihai Fan et al., 2007. Optimization of Carbon Dioxide Fixation by Chlorella vulgaris Cultivated

in a Membrane-Photobioreactor, Chem. Eng. Technol.; Volume 30, Issue 8, pg. 1094-1099

13 R. Matsukawa et al., 2000. Antioxidants from carbon dioxide fixing Chlorella sorokiniana;

Journal of Applied Phycology; Volume 12, pg 263-267

14 J. Qin, 2005. Bio-Hydrocarbons from Algae: Impacts of temperature, light and salinity on algae

growth; A report for the Rural Industries Research and Development Corporation, Australia;

Publication No. 05/025; Project No. SQC-1A; pg.vii

15 Reinhard Bachofen, 1982. The production of hydrocarbons by Botryococcus braunii. Cellular

and Molecular Life Sciences; Volume 38, Issue 1, pg. 47-49

16 Shigeru Okada, 1997. Characterization of Hydrocarbons from the Yayoi Strain of the Green

Microalga Botryococcus braunii. Phytochemical Analysis; Volume 8, pg 198-203

17 C. Dayananda, 2006. Autotrophic cultivation of Botryococcus braunii for the production of

hydrocarbons and exopolysaccharides in various media, Biomass and Bioenergy, Volume 31, pg

87-93

18 Sergey Kosourov et al., 2001. Sustained Hydrogen Photoproduction by Chlamydomonas

reinhardtii: Effects of Culture Parameters.

19 Laurent Cournac et al., 2002. Limiting steps of hydrogen production in Chlamydomonas

reinhardtii and Synechocystis PCC 6803 as analysed by light-induced gas exchange transients,

International Journal of Hydrogen; Volume 27, pg. 1229-1237

61

20 Anatoly A. Tsygankov, 2006. Hydrogen production by sulfur-deprived Chlamydomonas

reinhardtii under photoautotrophic conditions, International Journal of Hydrogen Energy; Volume

31, pg. 1574-1584

21 Elias Greenbaum, James W. Lee, 1998. Photosynthetic hydrogen and oxygen production by

green algae – an overview. Biohydrogen; pg. 235-242

22 Marcel Janssen et al., 1999. Specific growth rate of Chlamydomonas reinhardtii and Chlorella

sorokiniana under medium duration light/dark cycles. Journal of Biotechnology; Volume 70, pg.

323-333

23 Ben Hankamer et al., 2007. Photosynthetic biomass and H2 production by green algae: from

bioengineering to bioreactor scale up. Physiologia Plantarum; Volume 131, pg. 10-21

24 D.W. Lawlor, 1993. Photosynthesis; Molecular, Physiological and Environmental Processes,

Second edition, Longman Scientific & Technical, pg.16



26 C. Sorokin and R.W. Krauss, 1958. The Effects of Light Intensity on the Growth Rates of Green

Algae, Plant Physiology; Volume 33, Issue 2, pg.109-113

27 Unit conversion tables; www.unitconversion.org

28 J. Qin, 2005. Bio-Hydrocarbons from Algae: Impacts of temperature, light and salinity on algae

growth; A report for the Rural Ind ustries Research and Development Corporation, Australia;

Publication No. 05/025; Project No. SQC-1A; pg.9

29 Pirt, S. J., 1983. Maximum photosynthetic efficiency: A problem to be resolved (abstract).

Biotechnology and Bioengineering; Volume 25, Issue 6, pg. 1915-22

30 Yan Li & Jian G. Qin, 2005. Comparison of growth and lipid content in three Botryococcus

braunii strains; Journal of Applied Phycology; Volume 17, pg. 551-556

62

31 S. John Pirt, 1985. The thermodynamic efficiency (quantum demand) and dynamics of

photosynthetic growth, New Phytologist; Volume 102, Issue 1, pg. 3-37

32 E. Molina Grima et al., 1995. A study on simultaneous photolimitation and photoinhibition in

dense microalgal cultures taking into account incident and averaged irradiances, Journal of

Biotechnology; Volume 45, pg. 59-69

33 Gary A. Anderson et al., 2002. Photobioreactor design. The Society for engineering in

agricultural, food and biological systems. Paper No: MBSK02-216

34 Daniel M. Jamu et al., 1998. Relationship between Secchi disk visibility and chlorophyll a in

aquaculture ponds, Aquaculture; Volume 170, pg. 205-214

35 James C. Ogbonna, Hirokazu Yada & H. Tanaka, 1995. Kinetic Study on Light-Limited batch

Cultivation of Photosynthetic Cells, Journal of Fermentation and Bioengineering; Volume 80,

Issue 3, pg. 259-264

36 James C. Ogbonna, Hirokazu Yada & H. Tanaka, 1995. Light Supply Coefficient: A New

Enginerring Parameter for Photobioreactor Design, Journal of Fermentation and Bioengineering;

Volume 80, Issue 4, pg. 369-376





39 C. Dayananda et al., 2005. Effect of media and culture conditions on growth and hydrocarbon

production by Botryococcus braunii; Process Biochemistry, Issue 40, pg. 3125-3131

40 Sheng-Yi Chiu et al., 2008. Reduction of CO2 by a high density culture of Chlorella sp. in a

semicontinuous photobioreactor, Bioresource Technology; Volume 99, pg. 3389-3396

63

41 Notton, D, 2005. Theoretical and Experimental Determination of Key Operating Parameters For

Composting Systems. Ph.D Thesis; pp.3-2

42 David P. Chynoweth et al., 2001. Renewable methane from anaerobic digestion of biomass;

Renewable Energy; Volume 22, pg. 1-8

43 E.P. Sánchez Hernández, L. Travieso Córdoba, 1993. Anaerobic digestion of Chlorella vulgaris

for energy production. Resources, Conservation and Recycling; Volume 9, pg. 127-132

44 V. Nallathambi Gunaseelan, 1997. Anaerobic digestion of biomass for methane production: A

review; Biomass and Bioenergy, Volume 13, Issue 1-2, pg. 83-114

45 Maria Gavrilescu, Yusuf Chisti, 2005. Biotechnology – a sustainable alternative for chemical

industry. BiotechnologyAfvances; Issue 23, pg. 471-499

46 Michele Aresta et al., 2005. Production of biodiesel from macroalgae by supercritical CO2

extraction and thermochemical liquefaction. Environ. Chem. Letters; Issue 3, pg.136-139

47 Rui L. Mendes et al., 1995. Supercritical CO2 extraction of carotenoids and other lipids from

Chlorella vulgaris. Food Chemistry; Issue 53, pg. 99-103

48 Yan Li et al., 2006. Utilization of carbon dioxide from coal-fired power plant for the production

of value-added products.

49 M. Zhu, 2002. Extraction of lipids from Mortierella alpina and enrichment of arachidonic acid

from the fungal lipids. Bioresource Technology; Issue 84, pg. 93-95

50 Han Xu et al,. High quality biodiesel production from a microalga Chlorella protothecoides by

heterotrophic growth in fermenters. Journal of Biotechnology; Issue 126, pg. 499-507

51 Yan Li et al., 2006. Utilization of carbon dioxide from coal-fired power plant for the production

of value-added products. pg. 28

64

52 Naoto Usui, Masahiro Ikenouchi, 1997. The biological CO2 fixation and utilization project by

RITE – Highly-effective photobioreactor system. Energy Consvers. Mgmt.; Volume 38, pg. S487-

S492.

53 Toon van Harmelen, Hans Oonk, 2006. Microalgae Biofixation Process: application and

potential contributions to greenhouse gas mitigation options. Prepared for the International

network on Biofixation of CO2 and Greenhouse Gas Abatement with Microalgae operated under

the international Energy Agency Greenhouse Gas R&D Programme.

APPENDIX I

CALCULATIONS

Coal power plant emissions:

Assumptions;

Efficiency = 0.35

Coal ultimate analysis = 80% C; 13% O; 6% H; 1% S.

Coal HHV = 30MJ.kg-1

Flue gas composition by vol. = 13% CO2; 0.005% SO2; 3.8% O 2; 82.9% N2.

1500/0.35 = 4285.7 ̃ 4300MWth

4300/30 = 143.3 kg.s-1 (coal)

143.3*0.8 = 114.6 kg.s-1 (C)

44/12 = 3.67

3.67*114.6 = 420.2 kg.s-1 (CO2)

420.2/44 = 9.55 kmol.s-1

1 mol gas occupies 24.5E-3 m3

9.55E3*24.5E-3 = 234 m3.s-1 CO2

234/0.13 = 1800 m3.s-1 Total

Carbon dioxide fixation volume/length/area:

Assumptions;

Optimum Chlorella gas flow = 1.25 L.min-1

Constant carbon dioxide fixation rate = 0.14 g.L-1.hr-1 (0.0023 kg.m-3.s-1)

Reactor diameter = 130 mm

Gap between each tube = 120 mm

420.2/0.0023 = 182696 ˜ 183E3 m3

0.132*? /4 = 0.013 m2

183E3/0.013 = 14,076,923m ̃ 14E6 m

(4*0.13)+(4*0.12) = 1.0m

1*14E6/4 = 3.5E6 m2

v (3.5E6) = 1870.8 ˜ 1870 m

Algal biomass production rate:

Assumtions;

Linear growth stage

Constant flue gas output

Doubling time = 2.5 hours

Original cell density = 5E7 cells.ml-1= 0.7 kg.m-3

24/2.5 = 9.6 day-1

0.7*183E3 = 128.1E3 kg

9.6*128.1E3 = 1229760 ˜ 1.23E6 kg.day-1

1.23E6/(24*602) = 14.2 kg.s-1

HHV Chlorella sorokiniana:

Assumptions;

Milne formula

Composition as given in table 2

Generic formulae and HHV of composition as given in table 5

HHV = 0.695*16.16 + 0.119*9.39 + 0.1*28.25 – 0.0153*0.069 = 15.6MJ.kg-1

Coal equivalent

Assumptions;

HHV Chlorella sorokiniana = 15.6 MJ.kg-1

15.6*14.2 = 221.52 MJ.s-1

221.52/30 = 7.4 kg coal

7.4/143 = 0.052 ˜ 5%

Anaerobic digestion:

Assumptions;

CH4 production rate of 0.4 m3.kg-1

HHV CH4 = 55.5 MJ.kg-1

70% CH4, 25% CO2 , 5% other.

0.4*14.2 = 5.68 m 3.s-1

55.5*5.68 = 315.2MJ.s-1

315.2/30 = 10.5 kg coal

10.5/143 = 0.073 ̃ 7.3%

(5.68/0.7)*0.25 = 2.03 m3.s-1 CO2

(2.03/234)*100 = 0.87 %

100-0.87 = 99.23% reduction in CO2

A Study Into the Cultivation of Algae - 2008

Documents

Transcript of A Study Into the Cultivation of Algae - 2008