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Biogas Production Using Anaerobic Digestion of Agricultural Vegetal
Residue

Using Sugar Cane Molasses as Substrate



BY


OLAWUYI, IRETIOLUWA

062135


BEING A PROJECT SUBMITTED TO:

DEPARTMENT OF MECHANICAL ENGINEERING

FACULTY OF ENGINEERING AND TECHNOLOGY

LADOKE AKINTOLA UNIVERSITY OF TECHNOLOGY, OGBOMOSO, NIGERIA.


IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE AWARD OF THE
DEGREE OF BA
CHELOR OF TECHNOLOGY (B. TECH) IN MECHANICAL
ENGINEERING




JULY, 2012

ii


CERTIFICATION

This is to certify that this project work titled: Biogas Production using Anaerobic Digestion of
Agricultural vegetal Residue was carried out by Olawuyi, Iretioluwa

with matriculation number
062135 of the department of Mechanical Engineeering, ladoke Akintola University of
Technology, Ogbomoso, Nigeria.




________________________





___________________


Dr. Oladeji J.T.







Date


Supervis
or






________________________





___________________


Dr.
Durowoju M.O.







Date


Head of Department



iii


DEDICATION

This
project work is dedicated to the Almighty God.




iv


ACKNOWLEDGEMENT
S

Grateful

acknowledgement is made to the following people whose interest and contribution has
made this work come to life. Their effort is sincerely appreciated.

First of all, I would like to
thank
Dr. J.T. Oladeji

for the opportunity, and express my pleasure
in be
ing able to work under her supervision.

I will also like to thank Engr. Adebayo for His
help and precious advice during this study. I will also use this medium to appreciate friend and
colleagues whose has contributed in one way or the other to make this w
ork possible.

I say a big thank you to everyone.




v


TABLE OF CONTENTS

CERTIFICATION

................................
................................
................................
.........................

ii

DEDICATION

................................
................................
................................
..............................

iii

ACKNOWLEDGEMENTS

................................
................................
................................
..........

iv

TABLE OF CONTENTS

................................
................................
................................
...............

v

LIST OF TABLES

................................
................................
................................
.......................

vii

LIST OF FIGURES

................................
................................
................................
....................

viii

CHAPTER ON
E

................................
................................
................................
............................

1

1.0

Introduction
................................
................................
................................
................................
..

1

1.1

Preamble


................................
................................
................................
................................
...

1

1.2

Statement of

the Problem

................................
................................
................................
...........

2

1.3

Aim and Objectives of the Study

................................
................................
...............................

4

1.4

Significance of the Study

................................
................................
................................
...........

4

1.5

Scope of the Study

................................
................................
................................
.....................

5

CHAPTER TWO

................................
................................
................................
................................
.....

6

2.0


Literature Review

................................
................................
................................
.......................

6

2.1

What is Biogas and how to generate it?

................................
................................
.....................

6

2.2

Biogas Impurities and Substrates

................................
................................
...............................

7

2.3 Overview of biogas
components
................................
................................
................................
..........

7

2.4

Substrate


................................
................................
................................
................................
.

10

2.4.1

Substrate Preparation

................................
................................
................................
.......

15

2.4.2

Temperature

................................
................................
................................
.....................

16

2.5

Process and Mechanism of Biogas

................................
................................
..........................

16

2.5.1

Main Bacteria which are Responsible for Biogas Process

................................
...............

18

2.6

Advantages of Biogas

................................
................................
................................
..............

20

2.6.1

A clean fuel

................................
................................
................................
......................

21

2.6.2

Biogas never runs out

................................
................................
................................
......

21

2.6.3

Biogas for Heating:

................................
................................
................................
..........

22

vi


2.6.4. Biogas for electricity generation:

................................
................................
...........................

22

2.6.5. Biogas as vehicle fuel:

................................
................................
................................
...........

22

2.6.6. Digester gas for cooking

................................
................................
................................
........

23

2.7

Disadvantages of Biogas

................................
................................
................................
..........

23

CHAPTER THREE

................................
................................
................................
...............................

25

3.0


MATERIALS AND METHOD

................................
................................
...............................

25

3.1

Collection of Materials

................................
................................
................................
............

25

3.2

Material Preparation

................................
................................
................................
................

25

3.3

Design of Biogas Digester

................................
................................
................................
.......

25

3.3.1

Materials Needed for the Floating Drum Digester:

................................
.........................

25

3.3.2

Set
-
Up

................................
................................
................................
..............................

26

REFERENCES

................................
................................
................................
................................
......

29





vii


LIST OF TABLES

Table 2.1: General features of biogas.

................................
................................
..........................

7

Table 2.2: Typical components and impurities in biogas.

................................
............................

9

Table 2.3: Biogas yields and methane contents from agricultural feedstocks.

...........................

12

Table 2.4: Biogas Compar
ison with other fuels
................................
................................
..........

23






viii


LIST OF FIGURES

Figure 2.1: Factors affecting the net energy yields
which could be obtained from biomass.

.....

12

Figure 2.2: Anaerobic conversion of biomass into methane.
................................
.....................

17

Figure 2.3: Competition between sulfate
-
reducing bacteria and methane
-
forming b
acteria for
acetate and hydrogen..
................................
................................
................................
.................

19

Figure 3.1: Set
-
Up of a Floating Drum Digester

................................
................................
........

26

Figure 3.2: Stage Two of Digester Construction

................................
................................
........

27

Figure 3.3: Final Set
-
Up of the Digester

................................
................................
.....................

28


1


CHAPTER ONE


1.0

Introduction

1.1

Preamble


Energy is one of the most important factors to global

prosperity. The dependence on
fossil fuels
as primary energy

source has led to global climate change, environmental

degradation, and
human health problems. In the year 2040, the

world predicted will have 9

10 billion people and
must be

provide
d with energy and materials

(Okkerse & Bekkum, 1999)
.

It is estimated that in terms of primary source

used, by 2030, the structure of energy production

will be based on: 75
-
85 % of conventional fuel

combustion, 10
-
20 % of nuclear fission, 3
-
5 %
of

waterpower, approx. 3
% of solar and wind energy

(Popescu & Mastorakis, 2010)
.

Some of the major
problems associated with the use of fossil fuel as a source of energy are

highlighted below;

i
.
Fossil
fuels

are non
-
renewable;
fossil fuels don't last
forever and they run out.

ii
.
When fossil fuels are used (i.e. burned) there is

air pollution.

Burning them has been
attributed to global warming or the greenhouse effect.

iii
. Some

delicate ecosystems

may be disturbed

or
destroyed in man's zeal to obtain
evermore

fossil fuel.

They take ages for more to develop.

iv
. Cost of fossil fuels is relentlessly going up, eating into common people's ability to pay for
other necessary items

(Jimschem, 2007)
.

The most common forms of
renewable energy

are considered the solar, geothermal energy,
water,

wind and finally the biomass related energy. Some

of the most important benefits of
using renewable

energy are based on the organic composition, lack

of fossil driven CO
2

2


emission, does u
se mainly

locally available resources and are solutions for all

needs
(Popescu
& Mastorakis, 2010)
, covering best and directly the local

community.

Through anaerobic digestion, solar energy stored in the algal biomass as a resu
lt of the
photosynthesis reaction could be released as biogas

(Yen & Brune, 2007)
.

There are two major streams of biodegradable

wastes: (i)
green waste
from parks, gardens etc.
and

(ii)
kitchen waste
. The former includes
usually 50
-
60% water and more wood
(lignocelluloses), the

latter

contains no wood, but up to 80
%, by mass,

water
(Ionel, 2010)
.

Bio
-
energy from renewable resources is already

a viable alternative to fossil fuels; however, to
meet

the increasing need for bio
-
energy several raw

materials have to be considered. Ligno
-
cellulose is

the most abundant organic material on earth, in

diverse quantities, qualities and
forms, and is

therefore a promising raw material for bio
-
energy

produc
tion
(Petersson
et al
,
2007)
.

Biogas is a flammable gas produced when organic materials are fermented under anaerobic
condition. It contains methane and carbon (IV) oxide with traces of hydrogen sulphide and
water vapour. It burns with pale blue flame and
has a calorific value of between 25.9
-
30J/m
3

depending on the percentage of methane in the gas. The gas is called by several other names,
such as: dung gas, marsh gas, gobar gas, sewage gas and swamp gas
(Dangoggo & Fernando,
1986)
.

1.2

Statement of the Problem

Nigeria as an agricultural country produced a large quantity of agricultural residues and wastes
annually and this can play a
significant role in meeting her energy demand

(Oladeji

et al
,
2009)
.


Human engagements both at

the domestic front

and in industrial operations are inevitably
accompanied by

waste generation. Even in compliance to the goal or

concept of cleaner
3


production which requires that a higher

percentage of raw materials are converted into
products,

solid was
te generation is unavoidable. But the recycling

option of cleaner production
can be considered appropriate

means of combating the menace of solid wastes. This

usually
involves the collection of the waste and reuse in

the same or a different part of product
ion (on
-
site recovery

and reuse) or collection and treating wastes so that they

can be sold to consumers
or other companies. In line with

this, biogas technology employs the use of anaerobic

digestion
of wastes to produce methane
-
rich gas known as

biogas.

This has been an emerging technology that has

become a major focus of interest even in waste

man
agement throughout the world
(Elango

et al
, 2007)
. It is an identified

veritable option in
the integrated waste management of

municipal solid waste involve in
waste
-
to
-
energy

transformation
(Igoni

et al
, 2008)
.

In Africa, water pollution and access to energy resources

present challenges to human health,
environmental

health, and economic development. In 21 sub
-
Saharan

African countries, less
than 10% of the pop
ulation have

access to electricity. The need for alternative renew
-
able

energy sources from
locally available resources can
not

be over emphasised. Appropriate and
economically

feasible technologies that combine solid waste and

wastewater treatment and
ener
gy production can simultaneously

protect the surrounding water resources and

enhance
energy availability. Biogas technology in which

biogas is derived through anaerobic digestion
of

biomass, such as agricultural wastes, municipal and

Industrial waste (wate
r), is one such
appropriate

technology Africa should adopt to easy its energy and

environmental problems
.
Anaerobic digestion consists of

several interdependent, complex sequential and parallel

biological reactions in the absence of oxygen, during

which
the products from one group of
microorganisms

serve as the substrates for the next, resulting in

transformation of organic
matter (biomass) mainly into a

mixture of methane and carbon dioxide
(Parawira, 2004)
.

4


Besides being a n
on
-
polluting,

environmentally feasible and cost effective process, biogas

generations have many applications such as for cooking,

electricity generation and hatching of
chickens
(Usman

et al
, 2012)
.

1.3

Aim and Objectives of the Study

The aim of this project work is to produce
biogas from (Agricultural residue). In order to
achieve this, the specific objectives are
:

i.

To prepare material to be converted into biogas.

ii.

To design and fabricate digester.

1.4

Significance of the Study

Biogas is

not a thin
g to throw away just like that,
because, having it will remove the fear that
the world will one day face shortage crisis of natural gas. And Government will spend less for
imported gas.

Secondly, biogas will encourage majority participation in t
he natural resources trade, that is,
there would no longer be a
cartel

comprising of few nations who control the prices of natural
gas, in a monopolistic manner.

Thirdly, biogas would remove this attitude of some nations in the world today, who engage in
the practice of using natural gas to sign agreements to assist the other in time of war, and get a
certain percentages of these resources as the benefit.

Fourthly, biogas will enable majority of communities in different countries, to participate
actively i
n the power sector, since these raw materials would be extracted from these
communities in various countries of the world. And thereby encourage rapid development from
the Rural Areas in various countries of the world.

Fifth, biogas will encourage large pr
oduction output, and less production cost, due to the fact
that we would no longer need to, for instance, go far to order for the resources needed for
5


production, and no
time wastage
, but limited time, due to the fact that everything would
become within re
ach. Furthermore,
inflation

would be completely reduced to the ground, since
we would now have enough resources to set a balance in prices of goods and production cost.

Sixth, biogas would bring about
jobs creation
, and encourages new inventions like chemi
cal
experiments, Agricultural development and methods of improving large scale farming. Then,
more industries would be created, and a new improved living would be experienced by low
income earners in the populace, and this would reduce urban congestion in
most countries.

(Egedesea, 2012)

The benefit of digester residue includes;



Contains micro and macronutrients, such as Nitrogen Phosphorus and Potassium and a fair
amount of Manganese, Zinc, Copper and Iron. These are simultaneou
sly added to the soil.
Besides, it increases the microbial activity.



Carbon as an energy source and other nutrients are provided to soil microbes, which result
in augmentation of their population. This helps in organic matter decomposition, biological
nitrogen fixation, solubilization of insoluble phosphates and availabilit
y of plant micro
nutrients.

1.5

Scope of the Study

This study covered design and construction of mini biogas plant suitable for household use

using sugarcane molasses as substrate
.






6


CHAPTER TWO

2.0






Literature Review

2.1

What is Biogas

and how to generate it
?

Biogas is a
colourless
, flammable gas produced via anaerobic digestion of

biogenic wastes to
give mainly methane (50
-
70%), CO2 (20
-
40%) and traces of other gases such

as H
2
S, NH
3
,
CO, N
2
, H
2
, O
2

and water vapour etc.

(Edelmann

et
al
, 1999)
. The composition of the gas
depends on

the source of feedstock and the management of the digestion process
(Anunputtikul
& Rodtong, 2004)
. The gas becomes

flammable when the methane content is at least 45%
(Anonymous, 2003)
.

Biogas is produced when organic material is broken down by microorganisms in an

oxygen
free environment, so called anaerobic digestion. This process occurs naturally

in many
environments with limited availability of
oxygen, such as in swamps, rice

fields and in the
stomach of ruminants. In a biogas plant the organic matter is pumped

into a digester, which is a
completely airtight container. The products formed are both

biogas and digestate which is a
nutrient
-
rich fer
tilizer.

The energy
-
rich part of the biogas consists of methane. Depending on the production

conditions biogas consists of 45
-
85 % methane and 15
-
45 % carbon dioxide. In

addition, it will
include hydrogen sulfide, ammonia and nitrogen in small amounts.

Bio
gas is usually saturated
with water vapour. The quantity or volume Biogas is

usually of the unit normal cubic meter
(Nm
3
), at 0º C and atmospheric pressure.

The biogas process involves a variety of microorganisms in a complex interplay

leading to the
intri
cate organic compounds, such as carbohydrates, fats and proteins

are broken down to the
final products of methane and carbon dioxide

(Deublein & Steinhauser, 2008)
.

7


2.2

Biogas Impurities and Substrates

Biogas mainly consists of

methane, carbon dioxide and several impurities. Composition,
energy content, density and molar mass of biogas are listed in Tab. 2.
1
.
(Deublein &
Steinhauser, 2008)
.

T
able 2.1:

General features of biogas.

Composition

55
-
70%
methane (CH
4
)

30
-
45% carbon dioxide (CO
2
)

Traces of other gases

Energy content

6.0
-
6.5 kWh m
-
3

Fuel equivalent

0.60
-
0.65L oil/m
3

biogas

Explosion limits

6
-
12% biogas in air

Ignition temperature

650
-
750
o
C

Critical pressure

75
-
89 bar

Critical temperature

-
82.5
o
C

Normal density

1.2 kg m
-
3


Smell

Bad eggs

Molar Mass

16.043 kgkmol
-
1


Source: (Deublein & Steinhauser, 2008).

2.3 Overview of biogas components

Typical components and impurities in biogas which are described below are listed in Tab. 2
(Deublein & Steinhauser, 2008)
.



Methane and carbon dioxide:

The composition of gas depends on the following factors:

8


The amount of
long
-
chain hydrocarbon compounds.

i.

A longer retention time increases the anaerobic degredation of biomass.

ii.

If the material in the bioreactor is well stirred and homogenous, fermentation takes place
faster.

iii.

The type of disintegration is important when the su
bstrate is well enclosed in lignin
structures.

iv.

Higher fluid content in the bioreactor results in lower level of CO2 in the gas phase.

v.

The higher temperature and the higher pressure causes higher dissolved level of CO2 in the
water.

vi.

The substrate should be
well prepared
(Deublein & Steinhauser, 2008)
.



Nitrogen and oxygen:

Normally, biogas contains a 4:1 ratio of nitrogen and oxygen. However, when the ventilation is
switched on in order to remove the sulfide and if the gas pipes a
re not fully tight, this ratio can
be changed
(Deublein & Steinhauser, 2008)
.



Carbon monoxide:

It is under the detection limit of 0.2% by vol
ume

(Deublein & Steinhauser, 2008)
.



Ammonia:

Normally t
he level of ammonia is very low. It may exceed 1.5 mg m
-
3

when a high amount of
Nitrogen rich substrates are used in the plants
(Deublein & Steinhauser, 2008)
.




9


Table 2.
2
: Typical components and impurities in biogas.

Component

Content

Effect

CO
2

25
-
50% by vol

-
Lowers the calorific value

-
Increases the methane number and the anti
-
knock properties of engines.

-
Causes corrosion (low concentrated carbon
acid). If the gas is wet.

-
Damages alkali fuel cells.

H
2
S

0
-
0.5% by vol

-
Corrosive effect on equipment and piping
systems (stress corrosion): many manufacturers
of engines therefore set an upper limit of 0.05%
by vol.

-
SO2 emmissions after burners or H2S
emmissions

with imperfect combustion


異灥r
汩浩琠〮ㄥ⁢y⁶潬⸠

-
p灯楬猠sa瑡ty獴s.

NH
3

0
-
0.5% by vol

-
NO
x

emmissions after burners damage fuel
cells.

-
Increases the anti
-
knock properties of engines.

Water
vapour

1
-
5% by vol

-
Causes corrosion of equipment and piping
system.

-
Condonsates damage instruments and plants.

-
Risk of freezing of piping systems and nozzles

Dust

>5μm

-
B汯捫猠湯lz汥猠l湤⁦ne氠ce汬献

N
2

0
-
5% by vol

-
Lowers the calorific value.

-

Increases the
anti
-
knock properties of engines.

Siloxanes

0
-
50 mg m
-
3

-
Act like an abrasive and damages engines.

S
ource:
(Deublein & Steinhauser, 2008)

10




Hydrogen sulfide:

The concentration of hydrogen sulfide mainly depends on the process
and type of waste. The
concentration of H
2
S may exceed 0.2% by volume without desulfurizing step. Due to the
harmful effects on plant components downstream, H
2
S should be kept at the lowest level
possible. Therefore biogas is desulfurized when it is still in the reactor.



Siloxanes:

In the digestion tower, high concenrations of siloxanes are carried over into the sewage gas. At
high temperatures, siloxanes and
oxygen form SiO
2

which remains on the surface of the
machine and cause a decline in the flow levels
(Deublein & Steinhauser, 2008)
.

While the biogas digestion process continues, the concentration of the substrate decreases, an
d
acids and acetates are formed as the rate of methane production increases. Kinetics of the
process depend on the rates at which biogas is produced from the fermentation materials or the
rates at which organic materials are decomposed. Consequently, there

are various proposals on
the rate
-
limiting steps in biogas generation (Zuru
et al
, 2004).

2.4

Substrate

Many types of organic materials are suitable as substrates for digestion, for example

sewage
sludge, food waste from households, restaurants and
shops, manure, various

plant materials and
grey water from the food industry. Co
-
digesting of different

materials often gives a higher
methane yield, i.e. the produced amount of methane per

inserted amount of organic material
increases as compared with any

material digested

separately

(Gabriela, 2010)
.

Sources

that generate biogas are numerous and

varied. These include landfill sites,

wastewater
treatment plants and anaerobic

digesters. Landfills and wastewater treatment

plants
emit biogas
from decaying waste. To

date, the waste industry has focused on

controlling these emissions to
our

environment and in some cases, tapping this

potential source of fuel to power gas turbines,

11


thus generating electricity. The primary

components o
f landfill gas are methane

(CH
4
), carbon
dioxide (CO
2
), and nitrogen

(N
2
). The average concentration of methane is

~45%, CO
2

is ~36%
and nitrogen is ~18%.

Other components in the gas are oxygen (O
2
),

water vapour and trace
amounts of a wide

range of non
-
me
thane organic compounds

(NMOCs)

(Abdeen, 2011)
.

Accordi
ng to
Sagagi
(
2009)
,
difference in the production of

biogas to a large extent depends on
the nature of the substrate.

Generally, as long as any type of biomass contains
carbohydrates, proteins, fats, cellulose as
main components, they can be used for biogas production.

When selecting the biomass as
substrate following information should be considered firstly:

i.

Substrates should be chosen depending on their contents,

ii.

Higher nutritional value gives higher biogas yield,

iii.

Chosen substrates should be without any pathogens,

iv.

For digestion success, harmful substances should be in smaller amounts,

v.

Biogas from the digestion should be appliable for further applications,

vi.

Digestion residue should be useful as fertilizer
(Deublein & Steinhauser, 2008)
.

Energy crops, manures and other types of wastes and their mixtures are appropriate substrate
for biogas production. In Table 2.
3

the general char
acteristics and biogas yields of some
agricultural
feedstock’s

are shown.

Land for crop production is limited. The surface area of the world is mainly covered by
oceans;

only 149.106 km
2

is terrestial

land which consists of 9.4% arable area. Therefore crops should
be chosen mainly depending on their biomass yields per hectare, climatic conditions,
availability of irrigation water and robustness against diseases. Secondly, when the selection of
substrat
es, economic aspects like energy needs of substrates, should be taken into account.
After that it may be said which substrate is more convenient for biogas production
(IEA, 2011)
.
12


Factors affecting the net energy yields which c
ould be obtained from biomass are shown in
Fig
ure
.
2.3
.


Table 2.3: Biogas yields and methane contents from agricultural feedstocks.

Feedstocks

Total Solids

(% dissolved
solids (DS))

Volatile
solids

(% DS)

Retention
time

(days)

Biogas
Yields

(m3/kg VS)

CH4

content

(%)

Pig slurry

3
-
81

70
-
80

20
-
40

0.25
-
0.50

70
-
80

Cow slurry

5
-
12

75
-
85

20
-
30

0.20
-
0.30

55
-
75

Chicken
slurry

10
-
30

70
-
80

>30

0.35
-
0.60

60
-
80

Whey

1
-
5

80
-
95

3
-
10

0.80
-
0.95

60
-
80

Leaves

80

90

8
-
20

0.10
-
0.30

NA

Straw

70

90

10
-
50

0.35
-
0.45

NA

Garden
wastes

60
-
70

90

8
-
30

0.20
-
0.50

NA

Grass silage

15
-
25

90

10

0.56

NA

Fruit
wastes

15
-
20

75

8
-
20

0.25
-
0.50

NA

Food
remains

10

80

10
-
20

0.50
-
0.60

70
-
80

Source:
(Khanal, 2008)
.



Figure
2
.1:

Factors affecting the net energy yields which could be obtained from biomass.




Liquid manure and co
-
substrates:

Manures from a variety of animals have large potential for utilization in an anaerobic digester.
Their
biodegradability

is high
(Mandal & Mandal, 1997)
. Liquid manure from all animals may
involve foreign matter. Some of these, such as litter and residue can be processed in the
digester, whilst the rest of the foreign matters
such as sand, sawdust, soil, skin and tail hair,
cords, wires,

plastics and stones are unwanted due to their harmful effects on the digestion.
Generally in liquid manure, organic acids, antibiotics,
chemotherapeutic

agents and
13


disinfectants that cause an i
ncreased complexity and even
disruption

of the biogas production,
might be found
(Deublein & Steinhauser, 2008)
.

Cultivation and management influence the methane yields of energy crops. Therefore, the
quality of the substrate for biogas production should be optimized
(Bruni, 2012)
. The most
important parameter for choosing crops is biomass

yield per hectare. Crops should be easily
cultivated, harvested and stored.

They should be able to tolerate diseases and pests, be able to grow in the soil with low nutrient
levels. Many energy crops are familiar to farmers and are easy to cultivate and
they produce
large amounts of biomass. They are often characterized by good digestibility. They are also
good substrates to use in biogas plant. Some crop residues, such as sugar beet tops and straw,
generated in large amounts in agriculture could also be
used as a substrate in biogas production.
Harvesting crop residues for energy utilization has the benefit that the direct production cost of
these materials are often cheap, and collecting them from the fields promotes nitrogen
recycling and reduces eutrop
hication due to nitrogen leaching
(Lehtomäki, 2006)
.



Algae:

Today, most of the plants such as crop plants, sugar cane, sugar beets, and
canola

are being
used for energy generation that causes competition with food. Therefore, the use of plant
biomass for energy generation is problematic
(Muss
gnug
et al
, 2010)
. Algae use sunlight as
energy and get CO
2

from the atmosphere and synthesize their carbon needs
(Deublein &
Steinhauser, 2008)
. Algae have many advantages in comparison with higher plants because of
faster growth rates and the possibility of cultivation on non
-
arable
land areas or in lakes or the
ocean
(Muss
gnug
et al
, 2010)
. Harvesting of algae has many advantages:



Restoration of conditions favourable for fauna and flora,



Decrease in bad smell,



Increase in removal of nitrogen and phosphorus from coasts,

14




Decrease in sink of nutrients in sediments
(EU Life, 2011)
.

Because of these advantages, quite a number of research projects have been carried out.
Recently, new harvesting techniques have been invented and valuable co
-
products that have
been produced by some algae strains have been discovered. These improvements c
ause a raise
in the interest to use these organisms for bioenergy generation
(Mussgnug

et al
, 2010)
.
However, their low C/N ratio might cause problems in the digester
(Yen & Brune, 2007)
.



Wood, straw:

Lignocellulosic

biomasses that contain lignocellulose, such as wood and straw, could be
degraded better with pretreatment such as thermal and chemical treatments. Unlike cellulose
and hemicellulose, lignin is a cross linked network by a hydrophobic polymer. Lignin is
res
istant to anaerobic degredation
(Fernandes

et al
, 2009)
. Degredation time takes at least 25
days
(Deublein & Steinhauser, 2008)
, and mostly it causes interruptions in hydrolysis step
(Fernandes

et al
, 2009)
.

Straw is a
lignocellulosic substrate.
Kaparaju

et al
, (2009)

say that lignocellulose consists of
cellulose (40


50%), hemicelluloses (25


35%) and lignin (15


20%) is extremely resistant to
enzymatic digestion. Enzymatic
degradation

of lignocelluloses is usually not so efficient due to
high stability of the materials to enzymatic or bacterial attacks
(Taherzadeh & Karimi, 2008)
.
Utilization of hemicellulose and pentose
-
sugar is still problem for bacteria i
n the digestion
system

(Kaparaju

et al
, 2009)
. Lignin is a very complex molecule composed of phenylpropane
units linked in a three dimensional structure which is hard to degrade. There are

chemical
bonds between lignin and hemicellulose and even cellulose. Lignin is one of the disadvantages
of using lignocellulosic materials in biogas production, as it makes lignocellulose resistant to
biological degradation
(Taherzade
h & Karimi, 2008)
.

Pretreatment accelerates the hydrolysis stages, improves the biogas production and decreases
the
hydraulic

retention time. In the future, fermenting these
kinds

of biomasses in the biogas
plant will provide substantial power. It could be considered economically unattractive due to
15


the high chemical prices in comparison with low operational costs
(Fernandes

et al
, 2009)
, but
it also helps to protect the environmen
t. After digestion, low amounts of harmful or unpleasant
materials would be released. Digestion of wood and straw with liquid manure is much more
preferable, since the digestion runs more stable
(Deublein & Steinhauser, 2008)
.

2.4.1

Substrate Preparation

In some cases, the substrate is pre
-
treated before being fed into the process. Dry

materials may
need to soak up water, while excessive water
-
rich substrates, such as

wastewater and sewage
sludge, must be dewatered in order not
to take too much

digester
-
volume. Before digestion, of
course the organic material needs to be

separated from the organic material, e.g. food separated
from packaging. Metals can

be removed by magnetic separation. If food waste is collected in
plastic bags
, these

have to be opened and sifted away

(Gabriela, 2010)
.

Many fruits and vegetables have been evaluated as anaerobic digestion substrates. They are

characterized by high moisture and volatile solids content, as well as high
biodegradability.

The
two main factors hindering the anaerobic digestion of fruits and vegetables are low

alkalinity
and high fibre content. Alkalinity should not be less than 1500 mg L
-
1 in order to

avoid process
failure
(Gunaseela
n, 1997)
. Regarding fibre content, some pre
-
treatments

have been proposed
to improve biodegradability.
Madhukara

et al,

(
1997)

used 15 days and 6

months of ensilaging
as pre
-
treatment, reducing fibre content and improving methane yields

during the anaerobic
digestion of green peas.
Bruni

et al
, (2010)

evaluated different pretreatments

such as size
reduction, CaO addition, enzymatic and partial aerobic microbial

conversion or stem treatment
with catalyst before the anaerobic digestion of b
iofibers

separated from digested manure. This
resulted in the chemical treatment (CaO addition) and

steam treatment with NaOH giving the
highest methane yield increases
.

The plant wastes
will be

allowed to dry up and degrade for about one month to reduce t
he
toxicity

of the waste due to acidity. They
will

then

be

cut into small sizes (about 2") for ease of
stirring

while inside the digester and better reaction due to size reduction. They
will be

16


subsequently

soaked in water at 50% level for two weeks to all
ow for partial decomposition of
the waste by

aerobic microbes which have been reported to facilitate cellulosic breakdown
(Fulford, 1998)
.

2.
4
.
2

Temperature

The temperature is an important factor to take into account during
anaerobic

digestion. The
temperatures that are usually used in biogas processes range from

about 37°C (mesophilic) to
about 55ºC (thermophilic). It is at these temperatures that

micro
-
organisms grow best in the
mesophilic and thermophilic area. Since the b
iogas

process, as opposed to an aerated compost,
does not heat itself, the heat must be

supplied. It is also important that the digester is
sufficiently thermally insulated

(Gabriela, 2010)

2.
5

Process and Mechanism of Biogas

Biogas digestion process is divided into four stages. Fig. 1 illustrates these stages. In all these
different stages, different microbial activities occur. These stages are hydrolysis, acidogenesis,
acetogenesis and methanogenesis. Process proceeds without

any problems, if degradation of all
of the stages occurs well. If one of them is inhibited, then the methane production decreases or
all the process may be shut down. Biogas digestion process consists of different groups of
bacteria that work in sequence
(Gerardi, 2003)
.

Biogas production is a three stage

complex biochemical process that takes place under
anaerobic conditions in the presence of

highly pH sensitive biocatalysts that are mainly
bacteria. The process involves solu
blization

(hydrolysis), acidification (Acidogenesis

/

Acetogenesis) and methane formation

(methanogenesis). The methanogens also operate within
three temperature ranges namely;

Psychrophilic temperature (< 25°C), mesophilic (25
-
40°C)
and thermophilic

(45
-
60°C)
(El
-

Mashad

et al
, 2004)
.

17








Figure 2
.
2
:


Anaerobic conversion of biomass into methane. Modified from

(Demirel & Scherer, 2008)
.



Stage 1
-

Hydrolysis: Hydrolysis is
the breaking (“lysis”) of a large compound into small
compounds by adding water (“hydro”). Insoluble components such as carbohydrates, fats
and proteins undergo hydrolysis in this stage. Complex components are degraded into small
soluble components by brea
king their chemical bonds. Hydrolytic or facultative anaerobes
or anaerobes are responsible for this stage
(Gerardi, 2003)
.

Complex carbohydrates


simple sugars

Complex lipids


Fatty acids

Complex proteins


Amino acids



Stage 2
-

Acidogenesis: In this stage, soluble components that were produced through
hydrolysis; are degraded by facultative anaerobes and anaerobes. During degradation,
carbon dioxide, hydrogen gas, alcohols, organic acids, some organic
-
nitrogen compounds
18


and some organic
-
sulfur compounds are produced. Some of the other compounds are used
to form new bacterial cells
(Gerardi, 2003)
.



Stage 3
-

Acetogenesis: Acetogenesis occurs in the acid
-
forming stage. Many of the acids
and alcoh
ols such as butyrate, propionate and ethanol may be degraded into acetate that will
be used as a substrate by methane
-
forming bacteria and also carbon dioxide and hydrogen
can form directly acetate by fermentative bacteria
(Gerardi,

2003)
.



Stage 4
-

Methanogenesis: In this step, methane is mainly produced from acetate and carbon
dioxide and hydrogen gas. Here all of the compounds must be converted into compounds
that can be used by methane forming bacteria. Acids, alcohols and ot
her organic
-
nitrogen
compounds cannot be used directly by methane
-
forming bacteria,

as a result these
components accumulate
in the digester supernatant
(Gerardi, 2003)
.

2.5.1

Main Bacteria which are Responsible for Biogas
Process



Acetate
-
forming bacteria:


Acetate
-
forming bacteria have a symbiotic relationship with methane
-
forming bacteria. Acetate
which is produced by acetate
-
forming
bacteria

is directly used as a substrate for producing
methane by methane
-
forming bacteria. When the acetate is produced, hydrogen is also
produced. This hydrogen creates pressure that affects acetate
-
forming bacteria adversely in the
system.

Acetate
-
forming bacte
ria are so sensitive to hydrogen.

They can only survive if their
metabolic waste (hydrogen) is
continuously

removed. However, methane forming bacteria use
this hydrogen to produce methane and significant hydrogen pressure is prevented. The growing
rate of
acetate
-
forming bacteria is very slow
(Gerardi, 2003)
.



Sulfate
-
reducing bacteria:

Sulfate
-
reducing bacteria are also found in the biogas

digestion system. When sulfate

is in the
system, they start to multiply themselves by using hydrogen and acetate. This situation causes a
competition between sulfate
-
reducing bacteria and methane
-
forming bacteria (Fig. 2). Under
19


low acetate concentrations,
substrate to sulfate ratios <

2
, sulfate forming bacteria obtain
hydrogen and acetate easier than methane
-
forming bacteria. When substrate

to

sulphate
ratios
are 2 and 3, competition is particularly intense. At substrate to sulfate ratios > 3, methane
forming bacteria obtain hydrogen
and acetate easily
(Gerardi, 2003)
.


Figure 2.3:

Competition between sulfate
-
reducing bacteria and methane
-
forming bacteria for acetate and
hydrogen. Data source:
(Gerardi, 2003)
.



Methane
-
forming bacteria:

Methane
-
forming bacteria are some of the oldest bacteria with many types of shapes, growth,
patterns and sizes. They are oxygen sensitive and their cells have unique chemical composition
that makes the bacteria sensitive to toxic
ity. All type of methane forming bacteria can produce
methane. However, they have different structures, enzymes, substrate utilizations and
temperate range of growth. In nature, methane
-
forming bacteria participate in the degradation
of many organic compou
nds. Methane forming bacteria can grow well in the strict anaerobic
environment. Their generatio
n times range from 3 days at 35
o
C to 50 days at 10
o
C. In order to
obtain a large population of methane
-
forming bacteria at least 12 days are needed. There are
t
hree types of methane
-
forming bacteria which are different from each

other by substrate
utilization
(Gerardi, 2003)
.

-
Group 1 Hydrogenotrophic methanogens: They use hydrogen to transform carbon dioxide into
methane. As a result

they help to reduce hydrogen pressure.

CO
2

+
4H
2


CH
4

+ 2H
2
O

20


-
Group 2 Acetotrophic methanogens: They convert acetate into methane and carbon
dioxide.This carbon dioxide can be used by hydrogentrophic methanogens.

This group of
methanogens are affected
more by hydrogen pressure.

4CH
3
COOH


4CO
2

+ 2H
2

-
Group 3 Methylotrophic methanogens: They produce methane from methyl groups such as
methanol (CH
3
COH) and methylamines [(CH
3
)
3
-
N].

3CH
3
COH + 6H


3CH
4

+ 3H
2
O
(Gerardi, 2003)
.

2.6

Advantages of Biogas

The most important environmental benefit of the anaerobic digestion process is the

production
of biogas, a renewable energy source, which can be used as fuel for the internal

combustion
engines, for direct heating and, under better

efficiency, in cogeneration, for

electricity
production as well
(Demirel & Scherer, 2008)
.

Biogas can be used for variable energy services; the question is which is the best use for

biogas. Traditionally biogas has been used
as fuel for boilers but generally the best usage
depends on several factors such as amounts of biogas produced, energy costs, energy demand
of the plant and incentives. Normally in the biogas plants, during the digestion process, more
gas is generated than

needed in

order to support the process. This excess biogas is a potential to
use for other functions. In Fig
ure

2.
4, the potential gas use scenarios are illustrated
(Khanal,
2008)
.

There are numerous environmental benefits. Bi
ogas is an effective, renewable, non
-
fossil fuel
with a high methane content that does not compound the greenhouse effect. Furthermore,
emissions of nitrogen oxides, carbon oxide, carbon monoxides and hydrocarbons are small in
comparison to petrol and dies
el.

21


2.6
.1

A clean fuel

Biogas is an energy rich fuel and can be used to produce heat and power and can also be used
as vehicle fuel. Compared to the use of diesel for vehicles, biogas emits 80 per cent less
hydrocarbons and 60 per cent less nitrogen
oxides. The concentration of particles and dust is
also negligible in when biogas is burned.

2.
6
.2

Biogas never runs out

Animal and food waste and sewage sludge undergo natural anaerobic digestion in an oxygen
-
free environment. This means that all animal a
nd human waste can be broken down by a
bacterial process. The rest products of the anaerobic digestion are returned to Nature, so biogas
is included in the category of renewable energy sources. In short, as long as humans and
animals exist on the planet, b
iogas will go on being produced. The supply is unlimited

(What
are the advantages of biogas?, 2012)
.

Other advantages relating

to energy,

agriculture and environment problems are

foreseeable
both regionally and globally and

can
be summarised as follows:

• Reduction of dependence on import of

energy and related products, and

reduction of
environmental impact of

energy production (greenhouse effect, air

pollution, waste
degradation).

• Substitution of food crops and reduction

of fo
od surpluses and of related

economic burdens.

• Utilisation of marginal lands and of set

aside lands and reduction of related

socio
-
economic
and environmental

problems (soil erosion, urbanisation,

landscape deterioration, etc.).

• Development of new know
-
h
ow and

product
ion of technological innovation
(Abdeen, 2011)
.

22


2.6.3

Biogas for Heating:

Utilization of heat produced from the biogas has a direct effect on the economy of a biogas
plant. The gas burns with a clean, clear flame. No soot and slag are present in boilers and other
equipment, and the plant lasts longer
(Ene
rgy map, 2011)
. The heat could be used for:



heating swimming pools, industrial plants and greenhouses,



warmth transformation in cold,



treatment of products,



cleaning and disinfection of the milking equipment,



heating stables as for the breeding of you
ng animals
(Deublein & Steinhauser, 2008)
.

2.
6
.
4
. Biogas for electricity generation:

Power generation is the most common use for biogas. This electricity can be used for operation
of plant, for sale or credit for the local
power utility. Electricity

generation with biogas can be
produced from engine generators, turbine generators, micro turbines and fuel cells
(Khanal,
2008)
.

2.
6
.
5
. Biogas as vehicle fuel:

Petroleum fuels will gradually become e
xtinct and these will have to be replaced by sustainable

fuels. Replacement of petroleum fuels with biofuels has been addressed by the European
Commission in the directive 2003/30/EG where the following targets were set:

• 2% biofuels by the end of 2005


5.
75% biofuels by the end of 2010
(Jönsson, 2004)

Biogas must be transformed to natural gas quality for use in vehicles. This process requires the
removal of particulate
s, carbon dioxide, hydrogen sulph
ide, moisture and other contaminants
(Khanal, 2008)
.

23


When biogas is used as vehicle fuel, it gives the lowest emissions of carbon dioxide and
particles of all the fuels currently on the market
(Energy ma
p, 2011)
. Biogas utilization as fuel
is in general less problematic and cheaper in comparison with the cost of feeding biogas into
the natural gas network.

2.
6
.
6
. Digester gas for cooking

The biogas produced from anaerobic digestion of waste streams can be used through
conventional low pressure gas burners for cooking
(Khanal, 2008)
.

Table 2.
4
: Biogas Comparison with other fuels
Source:

Abdeen
, 2011

2.7

Disadvantages of Biogas

The disadvantages of biogas are as highlighted below:

i.

Biogas would encourage deflation of goods prices, due to many producers, and surplus
goods availability to the populace.

ii.

Pollution

would be on the increase both s
ound, air, and w
ater pollution
due to many
industrial waste materials in the society.

iii.

There would be
food shortage
, in the countries in the world, especially those nations with
limited land for food production. And since
biogas

would involve using raw materials from
both cash crops and otherwise. And the fact that not all the nations in the world, have land
24


for farming activities
e.g.
,
the

nation’s

living on Islands, and others in the deserts.
These
sets

of nations would be deni
ed the necessary means of survival

(Egedesea, 2012)
.



25


CHAPTER THREE

3.0




MATERIALS AND METHOD

3.1

Collection of Materials

The material

which will be used
as substrates

is

sugarcane molasses

which
is an

agricultural
waste

material

and
will be collected
fresh
from the various market dumpsites present in
Ogbomosho

metropolis.


3.2

Material Preparation

Laboratory test will be carried out to

for
sampling, analysis, recording and report generation

functionalities which
will be
used to determine

the

most effective substrate pre
-
treatment
method or the need

for nutrient addition to accelerate the anaerobic digestion

process and
increase the biogas yield.

Laboratory equipment and services will be requested at
Clarke Energy,13b Oban
ta Road Apapa
Lagos Nigeria

The waste materials
will
be

sun

dried for
about
twenty days then oven dried at 110
o
C for

10hrs
before use.

3.3

Design of Biogas Digester

A portable
floating drum
digester will be fabricated for the purpose of this work.

3.3.1

Materials Needed for the Floating Drum Digester:



2 50
-
gallon drums (one with a tight fitting lid)



1 smaller drum (we use a 40 gallon plastic garbage can)



4' piece of PVC pipe 5" diameter



2 pieces of plastic tubing 8' long and 1" diameter



1 valve to moderat
e flow of methane

26


3.3.2

Set
-
Up

A
hole

will be cut

in the lid of one of the 50
-
gallon drums, near the outer edge. The hole

will be
made of

the same diameter as the PVC pipe.
Then a

two foot section of

half the PVC pipe

will
be cut away

as shown in the illustration
of Figure 3.1 below

(PVC can be cut with a saw).

T
he pipe
will be slided
into the hole and all the way down until it rests on the bottom of the

drum.
T
he fitting between the pipe and the lid
will be sealed
with waterproof epox
y sealant or

any other tight sealing method.






Source:
(Jason & Charlie, 2001)

Figure
3.1:
Set
-
Up of a Floating Drum Digester

27


Now

a smaller hole
will be cut

(sized to fit the tubing) near the opposite edge of
the lid and one
of the pieces of 1"diameter tubing

will be attached to it

with waterproof epoxy.
T
he tubing
will
then be made to run
into the bottom of the 40 gallon
drum

and
slide
s

into the drum so that it
runs the depth of

the drum.
A
ll fittings

carefull
y sealed

with epoxy or with an equivalent
sealing method.

Source:
(Jason & Charlie, 2001)

Figure 3.2: Stage Two of Digester Construction

Another

hole

will be cut

in the bottom of the 40
-
gallon drum, sized to fit the

valve from the
tubing. T
he valve
will be attached
to the hole.
T
he

second 50
-
gallon drum (without a lid)

will
be filled

with water.
T
he 40
-
gallon drum

will be inverted
,
with

the valve

opened
, the drum
will
be slided
into the 50
-
gallon

drum with water, and

then the valve

will be closed
. Now
the

digester is set up and ready to be filled with
substrate

slurry
.

28


Source:
(Jason & Charlie, 2001)

Figure
3.3:
Final Set
-
Up of the Digester





29


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