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TRANSCRIPT
CHAPTER ONE
1.0 INTRODUCTION
‘’\1.1 BACKGROUND STUDY
The continuous inflation in the price of source of energy for cooking,
lighting, mobility, and so on has become alarming and unaffordable
compels demand for innovation of other less alternatives means of power,
such as generation of biogas from animal waste.
The most commonly used fuel is firewood. Even though sizeable
proportions of urban and semi-urban dwellers are fuel wood, the majority
of users of this fuel are the rural dwellers that constitute between 75-90%
of the nations population. The problems emerging from sole dependence
on this source of energy are many. For the fact that the effect increases
desert encroachment, soil erosion and loss of soil fertility, the source of
this energy undergo along period to regenerate. Consequently,
dependence on fuel wood as a only energy results to environmental
degradation which requires a large input and great expense to
rehabilitate.
To protect the environment from further deterioration and also
supplement the energy needs of the rural dwellers, a technically can be
effectively utilized (i.e. Biogas Technology). Using suitable organic
materials such as agricultural waste, industries wastes and municipal solid
wastes in digester have several advantages. The process produces energy
in the form of a combustible gas known as biogas which has no
undesirable effects on the environment. The end product of this process is
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a residual needed for healthy plant growth known as biofertilizer which,
when applied to the soil, enriches it with no harmful effects on the
environment.
The development and construction of biogas digesters which started in
the 1920’s has reached a significant level today (Primary and Perimental,
1979; Sambo, 1992) such that biogas technology has supplemented a
large proportion of energy requirements of the rural majority in several
developing countries like Nigeria. The availability of raw materials,
coupled with the ever increasing price of fossil fuel, have made this
technology attractive (Maishanu et al, 1990). The strategy can be utilized
to provide energy for households, rural communities, farms and
industries.
In addition, developed and developing countries and several international
organizations have shown interest in biogas systems with respect to
various benefits: a renewable source of energy, biofertilizer, waste
recycling, rural development, Public health and hygiene, pollution control,
environmental management, appropriate technology, and technical co-
operation. Within the context of UNEP/UNESCO/ICRO microbiology
Programme, which is sponsored jointly by the United Nations
Environmental Programme, UNESCO, and the international cell Research
Organisation, several workshops have been held in an attempt to catalyze
the applications of this acknowledge low-cost, non waste –producing
technology that is increasingly being deployed to manage the
environment and to ameliorate the search for substitute sources of fuel,
food, and fertilizer.
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The utilization of microbial activity to treat agricultural, industrial, and
domestic wastes had been common practice for a half century. Treatment
includes the aerobic, activated slude process and the anaerobic or
methane fermentation method; the latter is simple, does not require
imported know-how or components, is suited to small family or village-
scale digestion, and is the only process utilizing waste as a valuable
resource.
Most importantly the use of methane has been restricted available and
cheaper energy sources to the developing countries.
1.2 OBJECTIVES
1.2.1GENERAL OBJECTIVE
The general objective of this investigation is generation of biogas from
animal wastes i.e. (cattle dungs).
1.2.2SPECIFIC OBJECTIVES
The specific objectives are as follows.
1. To show that biogas is generated through anaerobic digester.
2. To indicate the importance of animal waste as a source of energy.
1.3 JUSTIFICATION
Really countries like China, Philippines, India and many Asian countries
are where biogas technology is being practiced. Though, African countries
with much available raw materials for biogas production, much has been
written but little has been done in the practical aspect. It is very
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paramount for development in this regard as it also helps in prompting a
healthier climate, reduces deforestation and generates revenue.
Obviously, in the rural areas where energy is being short, it is a necessity
for finding alternative source of energy.
CHAPTER TWO
2.0 LITERATURE REVIEW
Hence, analysis has been carried out on the use of different animal
waste in the production of biogas (wastes from beef, cattle, dairy
cattle, poultry layers, etc.). The heat content (British thermal units) of
the biogas from the different animal waste gives the highest quantity
of biogas production (in cubic feet per day).
The application of biogas in electricity generation, powering of
machineries, domestic use cooking, lighting and heating), cogeneration
and the limitation involved in biogas application has been given a
detailed study in this project. The cost-benefit analysis involve in
setting-up a biogas plant is also provided so as to know amount of net
income that can be realized from the plant (Owumi, 2002).
Lastly, this project is focused principally on generation of biogas as a
subject which is treated in order to provide a full-scale definition of
“Biogas” the raw materials needed to generate biogas; the chemical
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reaction involved; and the environmental and operational
considerations that is, the factors militating biogas generation. A full
description of the biogas plant the basic elements contain in this
thesis.
2.1 DEFINITION OF BIOGAS
Biogas is a flammable gas produced by the anaerobic fermentation of
organic waste materials such as cattle dung, agricultural wastes, water
hyacinth, human excreta, solid organic wastes, etc. It is a mixture of
methane (55 - 65%), carbon dioxide (30 - 40%) and traces of other
gases such as Nitrogen, Hydrogen, Carbon monoxide, water vapour,
ammonia and Hydrogen sulphide.
Biogas consists mainly of methane which is colourless, odourless,
inflammable gas, it is referred to as sewage gas, klar gas, march gas,
refuse-derived fuel (RDF), or sludge gas.
A thousand cubic feet of processed biogas is equivalent to 600cubic
feet of natural gas, 6.4 gallons of diesel oil. For cooking and lighting a
family of four would consume 150cubic feet of biogas per day, an
amount that is generated from the family’s night soil and the dung of
three cows.
2.2 CHEMISTRY OF BIOGAS PRODUCTION
In the first place, the biogas production process involves the biological
fermentation of organic materials such as agricultural waste, manure
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and industrial effluents in anaerobic (oxygen deficient) environment to
produce methane, carbon dioxide and traces of hydrogen sulphide.
Anaerobic digestion may be described as a three stage process. The
first consists of micro-organisms attacking the organic matter, that is,
complex organic compounds such as glucose and fructose. Polymers
are transformed into soluble monomers through enzymatic hydrolysis.
n(C6H10O5) + nH2O hydrolysis n(C6H12O6) (1)
The monomers become the substrate for the micro-organism in the
second stage where soluble organic compounds are converted into
organic acids by a group of bacteria collectively called “acid formers”.
n(C6H12O6) acid forming bacteria 3nCH3COOH (2)
soluble organic acids consisting primarily of acetic acid, form the
substrate for the third stage.
3nCH3COOH methane forming bacteria CH4 + CO2 (3)
Finally, methanogenic bacteria, which are strictly anaerobic in nature,
can generate methane by two different routes.
One is by fermenting acetic acid to methane and carbon dioxide, the
other consists of reducing carbon dioxide through hydrogen gas
generated by other bacteria species:
CO2 + 4H2 Reduction CH4 +2H2O (4)
Carbon dioxide can also be hydrolysed to carbon acid as follows.
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CO2 + H2O hydrolysis H2CO3 (5)
The carbonic acid generated is reduced to methane and water by
hydrogen as follows:
H2CO3 +4H2reduction CH4 +3H2O
The organic matter used in the methane fermentation generally
contains volatile and ash. The volatile solids are made up of
carbohydrates, fats, proteins, tannins etc.
SAMIRS. S. & OSKAR R. C (Biomass conversion processes for energy
and fuels)
2.3 ILLUSTRATION OF THE BIOGAS PLANT
It consists of two components: a digester and a gasholder. The digester
is a cylindrical water proof container with an inlet into which the
fermentable mixture is introduced in the form of liquid slurry. The
gasholder cuts off air to the digester (anaerobiosis) and collects the gas
generated.
The construction, operation and maintenance determine the success of
a biogas. Furthermore, several types of designs of biogas plants are in
existence but the fixed done and the floating gasholder types are more
popular. For biogas plant construction, important criteria are: (a) the
amount of gas required for a specific use or uses, and (b) the amount
of waste available for processing. Fry (8), singh (15), and others (10)
have documented several guidelines for consideration in the designing
of batch (periodic feeding) and continuous (daily feeding)
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compartmentalized and non – compartmentalized biogas plants that
are of either the vertical or horizontal type. In addition, researchers
have recently dealt with the scientific principles, process engineering,
and shapes of digestion reactors, and with the economics of the
technology.
Digester reactors are constructed from brick, cement, concrete, and
steel. In Indonesia, where rural skills in bricklaying, plastering, and
bamboo craft are well established, clay bricks have successfully
replaced cement blocks and concrete.
CHAPTER THREE
3.0 THE DIGESTER
It is an integrated part of biogas digester where digestion of raw material
takes place. Knowingly, it is the heart of biogas plant. Different classes of
digesters with different mode of operations are analysed as stated:
3.1.0MULTISTAGE DIGESTER
Two groups of micro organisms viz: the comparatively fast growing acid
forming (non-methanogenic) bacteria and much slower sensitive
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methanogenic bacteria these are two classes of biomethanations.
Multistage digesters are designed to isolate the groups of bacteria into
separate vessel and to optimize each environment for maximum reaction
rate. This kind of digester differs in their physiology, sensitivity to
environmental stress, growth capabilities and nutritional requirements.
For the fact that production of a gaseous fuel and residual solids with
fertilizer valve, anaerobic digesters have a bad reputation because they
are prone to operational problems i.e. hydraulic organic and toxic over
loading. Initially, the dilution rate exceeds the growth rate of digester
microbes, which are then washed out of the unit. High organic substance
concentrations, on the other hand, cause increase in volatile acids
formation. Methanogenic bacteria are inhibited, and the digester “Sours”
as PH falls and failures ensures.
Obviously, incase substances toxic to the methane bacteria enter the
digester in adequate amounts, washout of this population causes failure of
the overall process.
3.1.1BATCH DIGESTER
This is a procedure where organic matter is placed in a close tank and
allowed to be digested anaerobically over a period of two to six months
depending upon the feed materials and other parameters like
temperature, pressure etc. it is usual to heat and maintain the digester at
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the desired temperature. Batch digester is very simple to run and very
little attention need be paid to it between starting up and empting out.
Maximum efficiency of digestion can be obtained if the digester is loaded
carefully to avoid wastage of space and pockets of air trapped in the
sludge, because these inhibit the onset of methanogenesis. This type of
digester is generally used in the laboratory to asses the digestibility of
particular waste. Batch digesters posses some advantages, in the sense
that they can be used when the waste is only available at irregular
intervals and even if it has a very high solid content (25%). If the waste is
fibrous or difficult to digest, batch digestion may be more suitable than
continuous flow types, because the digestion time can be increased
easily.
The Demerits of this type of digester are as follows:
1. Removing some of the contents and replacing with fresh waste is
time consuming, messy and inefficient operation.
2. Quantity of usable gas is relatively small.
3. Initial gas yields could be high in carbon dioxide, the first volume
of gas should be vented to atmosphere since it usually contains
air which forms an explosive mixture with methane.
3.1.2HIGH RATE DIGESTERS
The designation of this integrates stirring or shaking of the contents to
achieve good mixing and also has a means of heating to ensure a
stable favourable temperature inside the digester. This type of digester
is necessary for maximum efficiency couple with short retention time
(RT). One application of the high rate concept is in sewage treatment.
High rate digestion speeds up the process and digestion are usually
completed well with a month. Temperature usually associated with
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high-rate anaerobic digestion sewage takes place in the mesophilic
range of about 200c to 400c. Operating the digester in the Thermophic
range of about 450c to 600c can reduce the retention time.
3.1.3ANAEROBIC FILTER RECTORS
It was innovated in 1950’s to use relatively dilute and soluble waste
waters with low levels of suspended solids. It primarily consists of a
column or chamber filled with a packing medium and are not carried
out of the digester with the effluence. With the light of the above,
these digesters are also known as fixed film or retained film digesters.
The liquid enters at the bottom and flows up through the packing
medium as the organisms in the liquid pass over the bacteria film, they
are converted to biogas. One to the high concentration of bacteria, the
gas production rates in these digesters is much higher than in
conventional digesters. Gas rates of up to 5M3/day have been reported.
The systems usually have loading rates which range from 8-16M3/day
and retention times ranging from 5hours to 12days.
L. P White & L. G. Plaskett (Biomass as fuel)
3.1.4CONTINUALLY FED DIGESTER
It is a form of digester that involves the feed as influent to be
deposited into the vessel at regular intervals, probably once a day. The
feed rate, in theory should be continuous for maximum efficiency, but
in practice it could be intermittent. Considering equilibrium, the digest
must also be emptied by a similar amount. On simple designs, this is
automatically catered for, but in sophisticated types, influent and
effluents rates are determined by pumps and associated equipment.
3.1.5 ANAEROBIC CONTACT REACTORS
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The process is downward flow developed in the 1950’s use a
completely mixed reactor. It feeds at close head and is drawn off at the
bottom. The liquid flow to a setting tank where the sludge containing
methane forming micro-organisms settles out and is then returned to
the digester. Anaerobic contact digesters are used in at lest nine agro-
industrial plants in Europe. There have been some problems with these
digesters related to the unpredictable and slow settling of the micro-
organisms from the digester liquid and the need for high sludge
recycling rates. The retention time is about 20 to 30days.
3.2.0ENVIRONMENTAL AND OPERATIONAL CONSIDERATION
3.2.1RAW MATERIALS
There are various sources for obtaining raw materials –poultry waste and
livestock, crop residues, night soil, food processing and paper water
hyacinth, filamentous algae and seaweed.
Different problems are encountered with each of these wastes with regard
to collection transportation processing, storage, residue utilization and
ultimate use.
Agriculture residues like spent straw, hay, cane trash, corn and plant
stubble and bagasse need to be shredded in order to facilitative their flow
into the digester reactor as well as to increase the efficiency of bacteria
action succulent plant material yields more gas then dried matter does,
and hence materials like brush and weeds need semi-drying. The storage
of raw materials in a damp, confined space for oven ten days, initiates
anaerobic bacterial action. That, though causing some gas loss, reduces
the time for the digester to become operational.
3.2.2 INFLUENT SOLIDS CONTENT
Importantly, if fermentation materials are too dilute or too concentrated
the biogas produced would be inactive resulting in low biogas production
and insufficient fermentation activity, respectively. Practically the ratio of
raw materials (domestic and poultry wastes and manure) to water should
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be 1.1 i.e. 100kg of excretes to 100kg of water in the slurry, this
corresponds to a total solids concentration of 8% to 11%by weight.
3.2.3 LOADING
Definitely, the determinant for the scale of the digester is loading, which is
determining by the influent solid content, retention time and the digester
temperature.
Optimum loading rates vary with different digesters and their sites or
location. Higher loading rates have been used when the ambient
temperature is high. In general, the literature is filled with a variety of
conflicting loading rates in practice, the loading rate should be an
expression of either (a) the weight of total volatile solids (TVS) added per
unit volume of the digester, or (b) the weight of TVS added per unit weight
of TVS in the digester. The latter principle is normally used for smooth
operation of the digester.
3.2.4 SEEDING
Seeding with an adequate population of both the acid-forming and
methanogenic bacteria is commonly practiced. Actively digestion sludge
from a sewage plant constitutes ideal “seed” material. As a general
guideline, the seed material should be twice the volume of the fresh
manure slurry during the start-up phase, with a gradual decrease in the
amount added over a three week period. If the digester accumulates
volatile acids as a result of overloading, the situation can be remedied by
reseeding, or by the addition of time or other alkali.
3.2.5 PH
If the PH is low it means the growth of the methanogenic bacteria and gas
generation and is sometimes the result of overloading. The range of 6.0-
8.0 is known as successful PH for anaerobic digestion while efficient
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digestion occurs at a PH near neutrality. A slightly alkaline state is an
indication that PH fluctuations are not too drastic. Through the addition of
lime or dilution low PH may be remedied.
3.2.6 TEMPERATURE
The choice of the temperature to be used is influenced by climate
considerations. With a mesophilic flora digestion proceeds best at 30 –
400c with thermophilic, the optimum range is 50 – 600c. In general there
is no rule of thumb, but for optimum process stability, the temperature
should be carefully controlled within a narrow range of the operating
temperature. In warm climates, with no freezing temperatures, digesters
may be operated without added heat. As a safety measure, it is common
practice either to bury the digesters in the ground on account of the
advantageous green house covering. Heating requirements and,
consequently, costs, can be minimized through the use of natural
materials such as leaves, sawdust, straw, etc, which are composted in
batches in a separated compartment around the digester.
3.2.7 NUTRIENTS
In the digester, the maintenance of favourable microbiological activity is
decisive to gas emission and consequently is related to nutrient
availability.
Carbon and Nitrogen are two most important nutrients and a critical
factor for raw materials choice is the overall C\N ratio. Notably, animal
poultry wastes and domestic sewage are example of Nitrogen rich
materials that provide nutrients for the growth and multiplication of the
anaerobic organisms.
In contrast, N- poor materials like green, corn stubble etc are rich in
carbohydrates substances that are essential for gas production. Excess
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available of Nitrogen leads to the formation of NH3, the concentration of
which inhibits further growth. Ammonia toxicity can be remedied by low
loading or by dilution. In practice, it is imperative to maintain, by weight a
C\N ratio close to 30.1 for achieving an optimum rate of digestion.
Addition of materials low in Carbon with those that are high in Nitrogen,
and vice versa can wisely control the C\N ratio.
3.2.8 TOXIC MATERIALS
A variety of pollutants that could inhibit digestion usually accompanied
wastes and biogradable residue. Potential toxicity due to ammonia can be
corrected by remedying the C\N ratio of manure through the addition of
shredded biogases or straw, or by dilution. The soluble salts of Copper,
Zinc, Nickel, Mercury, and Chromium are common toxic substances.
Contrarily, salts of Sodium, Potassium, Calcium and Magnesium may be
stimulatory or toxic in action, both manifestations begin associated with
the caution rather than the anionic portion of the salt. Synthetic and
pesticide detergents may also be troublesome to the process.
3.2.9 STIRRING
Gas generation may be hitched by the formation of a scum that is
compressed of these low-density solids that are enmeshed in a
filamentous matrix, when solid materials not well shredded are present in
the digester. In time the scum hardens, disruption the digestion process
and causing stratification. Mechanically through the use of a plunger or by
means of rotational spraying of fresh influent internal agitation can be
done successfully. In batch digester agitation is normally required, which
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is ensured in exposing of new surfaces to bacteria action, presents viscid
stratification and slow down of bacteria activity and promotes uniform
dispersion of the influent materials throughout the fermentation liquor,
thereby speed up digestion.
3.3.0 RETENTION TIME
Loading rate, dilute, temperature e.t.c. these other factors influence
retention time. Bio-digestion occurs faster at high temperature, reducing
the time requirement. Normal period for the digestion of dung would be
two to our weeks.
3.3.1 DEVELOPMENTS AND PROCESSES FOR RURAL AREAS
A survey was adopted by the Economic and social council of the United
Nations two years ago, presented in 1978 to the Committee on Science
and Technology for development, listing the on-going research and
development in unconventional source of energy. From this point of view
of the developing countries, it is good to note that the use of farm wastes
to produce methane, has also been identified in the United Nation World
plan of Action for the Application of Science and Technology, to
Development.
The Economic and Social Council for Asia and the Pacific, moreover,
adopted the Colombo Declaration at its thirtieth session which determined
that the most urgent priorities for action are in the fields of food, energy,
raw materials, and fertilizers, and that these priorities would be best met
by the Integrated Biogas System (IBS).
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An integration system aims at the generation of fertilizer and acquisition
of energy, production operation through the growth of algae and fish in
oxidation ponds, hygienic disposal of sewage and other refuse and
tangible efforts to counteract-environmental pollution. The core of the
system is the biogas process. It has the ability to “seed” self –reliance in
relatively crude economics.
The development of rural industry, the provision of local job opportunities,
and the progressive eradication of hunger and poverty are allied benefits.
The combination of a photosynthetic step with digestion provides for the
transformation of the minerals left by digestion directory into algae that
can be used as fodder, as feed for fish, as fertilizer or for increased energy
production by returning them to the digester process.
Putting back into soil what has been taken from it and increasing the
amount of nutrients by fixing CO2 and N2 from the atmosphere into the soil
and water through photosynthesis by algae is the aim of IBS. Embracing
low cash investments on a decentralized basis, the implementation of IBS
provides employment to the whole work force without disrepute of the
rural structure.
Moreover, it is an example of soft technology that does not pollute or
destroy the physical environment.
Preliminary work on a small scale has started at the college of Agriculture
of the University of the Philippines. An eco-house has been built by
Graham Caine on the Thamas Polytechnic playing field at Eltham, South
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East of London in England. However, the results on the project are not yet
available.
3.4 COST – BENEFIT ANALYSIS
No general answer to the economic feasibility of biogas production.
National economic consideration plays a prominent role. For instance, in
South Korea where wood is in short supply and domestic fuel substitutes
like rice and barley straw, and coal and oil could be conserved. Wood
could be a foreign exchanger earner in the field of hand – crafts.
Transportation costs of coal and oil to the rural areas is high in India and
an extra budded on an already poor farmer.
The consumption of commercial and non commercial energy for the whole
of India, as determined for the period 1960-1971 by the fuel policy
committee report is provided in Table 1 below.
TABLE 1 Consumption of Commercial and Non-Commercial Energy in India.
Year Coal Oil
[Million
Tons]
Electricity
[Million
Tons]
Firewood
[Billion
Kwh]
Cowding
[Million
Tons]
Vegetable
[Million
Tons]
1960-1961 47.1 6.75 16.9 101.04 55.38
1965-1966 64.2 9.94 30.9 111.82 61.28
1970-1971 71.1 14.95 48. 7 122.75 67.28
Source: Ghosh, S.N. 1974, Report of the fuel policy committee.
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The rural share in the energy consumption of electricity and coal is not
considerable because, as the report of the panel of the National
Committee on Science and Technology on fuel and power indicates, the
large towns and cities with population of 500,000 and more accommodate
only 6percent of India’s total population but consume about 50percent of
the total commercial energy produced in the country. However, in the
villages where kerosene is used for lighting purpose, but it is clear that
with increasing population biogas generation seems to offer solutions in
the area of fuel availability, electricity fertilizer for cash crops and would
provide other socio-economic benefits.
On the other hand, cost benefit analysis of methane generation varies
widely, depending upon the uses and actual benefits of biogas production.
A perfect example is the fact that a village –model gas plant which cost Rs
500 some years ago, cost Rs 1,500 in 1974 and Rs 2,000 in 1977.
Moreover, a significant problem is whether rural people who cannot spend
Rs 2,000 can cope with increasing inflationary and digester construction
materials costs.
TABLE 2 COST – BENEFIT ANALYSIS OF BIOGAS PLANT (IN
NIGERIA) - (VILLAGE – MODEL GAS PLANT)
a. Capital cost
Gas holder and frame 18,700
Piping and stove 6,940
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Civil engineering construction
(tank inlet and outlet etc) 42,020
Total 67,660
b. Annual expenditure
The interest on investment at 9% 6,080
Depreciation on gas holder and frame at 10% 1,860
Depreciation on piping and stove at 5% 400
Depreciation on structure at 3% 1,260
Cost of painting, once a year 1,340
Total 10,940
c. Annual Income
Gas 3m3/day at N300 per 29m3 (1,000cu Ft)
10,060
Manure (7tons, composted) with refuse 16tons
at N800 per ton 12,800
Total 22,860
d. Net Income (B- C) 11,920
Source: Based on the feasibility evaluation/analysis Escap document
NR/EGNBD/4, June 1978.
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CHAPTER FOUR
4.0 DESIGN ANALYSIS AND CALCULATION
The act whereby engineers formulate plans for the physical realization of
machine devices and system in respect of decision making process is
known as design. It practically aimed at solving human problems.
However, the following design consideration are made.
i. There should be simplicity in the design and construction.
ii. Easy replacement of the unit part on account of damage.
iii. The machine power condition should be minimal.
iv. The construction of the biogas unit should be at minimum cost
compatible with its efficiency.
4.1.1SIZING THE DIGESTER AND GAS HOLDER
The determined factors for the size of the digester and gas holder are the
volume of the biogas to be generated per day. The mass of waste and
slurry needed, the height and diameter, mass of water and waste ratio.
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4.1.2VOLUME OF DIGESTER
In this design, a module experiment is carried out to confirm. An empty
graduated cylinder was weighted, then 1kg of cow dung was put into it
and corresponding water weighting 1kg was added and the volume was
recorded. The underlisted result was obtained.
1kg of dung + 1kg of water = 2kg of slurry = 2×10-3m
0.05m3 of biogas
(B.O.R.D.A. 1991)
Based on the above result, I am designing for 0.1m3 of slurry. Applying
the method of proportionality – the mss of waste slurry and volume of
biogas are calculated.
1kg of dung = 2kg of slurry = 2×10-3m3of slurry.
= 0.01m3 of biogas
Y1 Y2 0.1M3 Y3
From proportionality
Y2 = 2 × 0.01
2 × 10-3 = 10kg
Y2 = 10kg which is the weight of the slurry needed to produce 0.01m3
volume of the slurry.
Hence Y1 = 10/2 = 5kg
Y1 = 5kg which is the weight of the poultry waste to produce
0.01m3 of the slurry.
Therefore
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Y3 = 0.01 ×0.05
2 × 10-3 = 0.25m3
Y3 = 0.25m3 which is the total volume of biogas generated per day by
0.01m3 of the slurry.
4.1.3DIGESTER DIAMETER AND HEIGHT
For the fact that manure is usually retained in the digestion chamber for
the period of about 7weeks, then the digester volume is 15× volume of
the slurry (15 × 0.01m3) which equals 0.15m3.
The height should not be too high, diameter should not wide to give room
for proper mixing .
The dimension is analyzed as follows:
Vd
Hd
d1
Fig 1 Reactor vessel
Let Vd be the volume of the reactor in ms.
Hd be the height of the reactor in m di be the internal diameter of the
reactor in m.
Volume = area × height
By considering a square cross- section
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Vd = Hd3
0.30 = Hd3
Hd = 0.655 - 0.66
But Vd = ∏di 2 × Hd
4
0.30 = 3.142 × di2 × 0.66m
di = 0.30 × 4
2.07372
di = 0.76m
Reactor diameter is 0.76m while Reactor height is 0.66 but for ease of
construction purpose (Hd) height is taken as 0.892m and Diameter (di) is
taken as 0.6m
4.1.4 GAS HOLDER SIZE
The Gas holding capacity represents the cone section of the reactor
100
2200
Fig (2)
Let Hc be the height of the cone in on
Vc be the radius of the cone in m
By considering the volume of the cone
Volume + 1/3∏ r2h
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V = 1/3 ×3.142 × (0.2) × 0.1
V =0.02513m3 0.025m3
4.1.5DIGESTER THICKNESS
tD
d1
Fig 3 Digester Thickness
The diameter of the digester thickness is to ensure that the system works
safely i.e. reducing the built in presence with reactor, so the principle of
thin walled pressure vessels is used to analysed for the thickness
(Banmested 8th edition).
Let Pmax be maximum pressure within the reactor (N/m2)
di is the internal diameter of the reactor (m)
tD is the digester thickness (m)
T is the tensile stress of the digester materials in (N/m2)
Tangential stress acting uniformly over the stressed area is given as
δT = Pmax ×d1 (1)
2tD
Maximum tangential stress (Baumester), 8th edition) is
δtmax = Pmax (d1 +tp) (2)
2tp
Since it is a sealed vessel, longitudinal stress is also considered
25
δtmax = Pmax d1 (3)
4tp
From equation (2)
tD = Pmax * d1 (4)
(δ Tmax – Pmax)
From the gas equation
Pv +MRT (5)
Where: P is the pressure of the biogas in (N/m2)
V is the volume of the biogas in (m3)
M is the mass of the biogas in (kg), k is the gas constant of the biogas in
(Nm/kgk).
T is the temperature at which the biogas is generated or stored in the
reactor in (kg).
But Pmax = Allowable pressure × factor of safety
= P × n
When n is the factor of safety and P is the allowable pressure substituted
for Pmax in equation (4)
TD = nP × d1 (6)
(2 δ1 – np)
Mass of the gas = nPg × Vb where Pg is the density of the biogas which is
1.693kg/m3 (C. M. S 1996)
Vb is the volume of the biogas generated per day which is 0.25m3 now
m=1.693 × 0.25 = 0.292kg.
The gas constant R of the biogas is 312.45Nm/kgk (C.M.S. 1996).
26
Temperature at which the gas is generated is 350c which is 308k
substituting at the above in equation (5)
PV = MRT
P = MRT
V
P = 0.292 ×317.5 × 308
0.25
P = 114,218.72N/m2
P = 114.218KN/m2
Let the assumed factor of safety be 1.5, therefore
7 ×P = 1.5 ×114.218 × 103
= 171.327KN/m2
From the table, tensile stress δT for mild steel is
150 × 106/m2 (Baymestur)
d1 = 0.76m
From equation (4)
tD = 171.327 × 0.76
(2 × 150 ×106 – 171.327)
= 130.20852 = 4.91 × 10-4m
3.0 × 108
= 4.91 × 10-1mm ≈ 0.49/mm
For longitudinal stress from equation
Tp= 171.327 × 0.76
4 × 150 ×106
1 30.20852
27
6.0 108
tp = 2.170 * 10-4m
= 0.2170mm
Since 0.491mm is greater than 0.2170mm, 0.491 is chosen as the
thickness of reactor but for ease of construction purpose 1mm is chosen.
4.1.6 BASE PLATE THICKNESS OF THE DIGESTER
The base plate thickness is affected by the weight of the slurry, which can
cause buckling or bending of the plate. For a uniformly distributed load,
the bending moment Mb (Baumester, 8th Edition) is
WI 2 = Wd 2 (10)
8 8
The bending stress δb is
Mb c (11)
I
Where I is the polar moment of the inertia
C is the distance from the neutral axis
I for rectangular object is bh 3 = ditB 3
2 12
C = tB
2
Substituting I, Mb and C in equation (ii)
δb = 3Wdi
4tB2
28
tB = ----------------------------- (12)
4δb
Where δb = is the bending stress of the plate material in (N/m2).
W is the weight of the slurry in (N/m)
Di is the digester internal diameter in (m)
W = mg = 10 ×9.81 = 98.1N/m
Di = 0.76m
δb= 280 × 106N/m2 for mild steel (Agriya, 2000) substitute the above
valves into equation (12)
tB =3 ×98.1 × 0.76 = 14.95553409
4 × 280 × 106 33466.40106 =4.48 × 10-4m
Multiplying by factor of safety, N = 1.5
tB = 6.70 ×10-4m
= 0.67mm
4.1.7SHAFT DESIGN
In designing the shaft, the blade or impeller is calculated along side the
diameter of the shaft
IMPELLER BLADE h - x
h x
h 2x
29
Hd
h - x
h – 2x
fig. 4 Blade arrangement
Now, Hd = h –xth - = 2x + h – x + h – 2x
= 4h – 6x
h = Hd + 6x ------------------------------- (7)
4
Let 5% Hd
H = 0.3225 Hd -------------------------------------(8)
Where h is the Blade height in (m) Hd is the height of the digested in (m)
X is the distance from the edge of the blade at the second side.
Substituting the earlier calculated valve of Hd into equation (8).
H = 0.76 × 0.3225
= 0.2451 ≈0.25m which is the height of the blade.
The blade is positioned at angle 1200 to each other as shown below.
1200 Fig. 5 Blade Orientation
30
4.1.8SHAFT DIAMETER
Considering the design, torsional effect is certain from the viscous drag
force by the slurry on the shaft through the blade.
From fig. 4, since the radius of the impeller (v) is related to the reactor.
i.e. r = 2di ---------------------------------- (9)
6
(Ayrinya 2000)
Area of the blade Ab = h × r substituting the above equation into equation
(9)
Ab = 2hdi -----------------------------------(10)
6
Substituting the earlier calculated valves for di and h into equation (ii)
Ab = 2 ×0.2 × 0.76 = 0.506m2
6
The drag force on the blade Fd is T × Ab ------------(11)
Where T is the viscous shear stress on the blade but,
T = ηdv = ηdu --------------------------------(12)
dx dv
where η is the dynamic viscosity of the slurry
dv is the velocity gradient
dr
31
The shaft velocity V = ωr
Where r is the radius of the stirred in (rad/s)
ω is the angular velocity of he stirred blade in (m)
Now, V = 2∏N × r
60
Where N is the shaft speed in (rev/min) which in 4rpm (Agrinya 2000)
From equation (9) r = 2 × 0.76 = 0.253 = 0.25
6
Which is the radius of the blade substituting the valve of r and N into v
V = 12.1 × 10-2m/s
The dynamic viscosity of the slurry is 8.1413 × 10-4Ns/m2 (Agrinya 2000).
Substituting the valve of v, r and 7 into equation (12)
= 12.1 × 10 -2 ×8.1413 × 10 -4
0.25
= 9.850973 = 3.9403
0.25
= 3.940 × 10-4N/m2
Substituting the earlier calculated valves for Ab and T into equation (ii)
Fd = 3.9 × 10-4 × 0.0506
= 1.973 × 10-5 which is the drag force exerted on the blade by the
slurry.
32
The torque on the shaft now will be T = fd × r -------------------- (13)
R is the radius of the shaft substitute the valve of fd and r into the
equation (13)
T = 1.973 × 10-5 × 0.25
= 4.9325 × 10-6Nm
Since the blade is three, total torque Tt - 3
3× 4.93 × 10-6
T = 1.479 × 10-5Nm
From the relation (Baumester, 8th edition)
I = T -------------------------------------------- (14)
j r
Where T is the torque on the shaft in (Nm) j is the polar moment of inertia
for a solid shaft.
= ∏d 4
32
T is the shear stress in N/m2
r is the radius of the shaft
Substitute j in equation below
r = 2Tt -------------------------------------- (15)
33
∏T
From the table 1 for mild steel is 150 × 106N/m2 (Ibogbe 1999)
Substitute the earlier calculated valves Tt and T into equation (15)
r = 2× 1.695 ×10 -5
∏ × 150 × 106
4.1.9 GAS FLOW ANALYSIS
Essence of the flow-analysis is for adequate dimensioning of the outlets
opening and
the valves V1
Vo = 0 A1 P1P1
P0 P0 Fig. Gas flow
Analysis
From the above diagram
eo = density at upstream
Po = Pressure at upstream
Vo = velocity at upstream
e1 = density at downstream
P1 = Pressure at downstream
34
V1 = velocity at downstream
Taking flow analysis as adabatic
From Bernoulli equation
(Eastop 1999)
[ ] { [P0] – [P1]} + ½ [v12 – v0
2 ] = 0 ---------------- (22)
[r – 1] { [ Pe] e1]}
Since the tank is large, assume vo = 0 and v1 = 1 then where is the ratio
of specific heat of liquid for isotropic flow
e = e0 (P ) 1/
P0
P = P . P0 [P0]1/ ------------------------------------------------------- (23)
e e0 P0 [P ]
P = P0 (P) -1/ - ---------------------------------------------------------- (24)
e e0 (P0)
Substitute equation 24 in 23
][ (P0 ) (P)] 1/ V2/2 --------------------------- (25)
[ -1] [ (e0) (P0)]
V = 2( ) (P0) (P) -1/
( -1) (e0) (P0) ---------------------------------- (26)
35
The velocity down stream of the orifice mass flow, Mr is given by
Mr = Qf × e
Qr = volume flow rate (m3/s)
C = density of the biogas (kg/m3)
Qr =A1V
Mr = A1Ve0
Substituting for V
Mr = A1e0 2 [P0] – [P] -1
[ -1] [e0] [P0] --------------------------------- (27)
Actual mass flow, Mact = cdmr
Cd = Coefficient
The specific heat ratio for
CH4 = 1.313
CO2 = 1.304
eg = density of biogas = 1.1693kg/m3
g is the heat ratio for methane and CO2 generated in the ratio 60:40 and
is given by
36
g = (0.6 × 1.313) + (0.4 ×1.304)
= 1.304
Therefore,
Mass flow rate Mr = Q/eg
= Vbd/eg
= 0.26/1.1693kg/day
= 2.475 × 10-6kg/s
4.2.0 PRESSURE RATIO
The pressure ratio is obtained such that P1 ‹ Pv where Pv is the vapour
pressure of the gas stream.
In order that the ratio does not graduate above the vapour pressure which
could lead to cavitations.
For the pressure ratio we first determine the vapour pressure of the gas.
By regression analysis data from compressed gas handbook (Agrinya,
2000).
Pv = aTb
Where Pv is the vapour pressure.
T is the temperature
A and b are the constants from the table.
At T = 400c
a = 1337.62
b = 1.45
37
Corr = 0.9993
Pv = 1337.62 × 401.45
= 281397.18N/m2
Choosing diameter of 0.01m
A = ∏d 2 = ∏ × (0.01) 2
4 4
= 7.85 × 10-5m2
= 1.3094, e0 = 1.1693kg/m3, Mr = 2.475 × 10-6kg/s substituting the
valve above in equation (27)
r = 1 – [ (2.475 × 10 -6 ) 2 (7.85 × 10 -5 ) (2 × 1.715 × 10 5 × 1.3094 )]
(0.3094) (1.1693) -1
= 1 - 2.0076 ×10-63
= 1
Since P0/P1 = 1
P1 = P0 (1) = 1.715 × 105 (1)
= 1.715 × 105N/m2
P1 is allowed because P is less than Pv
4.2 1 HEAT TRANSFER ANALYSIS
The generated heat by the digester should be able to:
i. Compensate for heat loss by evaporation of water into gas.
ii. Raise the temperature of the slurry.
38
iii. Compensate for heat loss from the tank by conduction
Heat transfer equation
Q = MCs T
Qs = MsCs T
Qs = MsCs (Ttank – Tslurry) -------------------------------------- (16)
Where Qs is the quantity of heat needed to heat the slurry (J).
Cs is the specific heat capacity of the slurry (kJ/kgk)
Tslurry is the slurry inlet temperature (k).
Ttank is the tank temperature (k).
Ms is the mass flow rate of the slurry (kg/m).
Ms = X3 × Vd × tr
Where X3 is the theoretical loading rate of total digestible matter (kg/m3/day)× × Vd is the volume of the slurry = 0.01m3
Tr is the retention time 45days Ms = 2 × 0.01 × 45
= 0.9kg
Cs = 4.2 × 103J/kgk (Agrinya, 2000)
Tslurry = 15 + 273 = 288k
Ttank = 35 + 273 = 308k
Substitute the valves into equation ------------------------------ (16)
Qs = 0.9 × 4.2 × 103 × (308 – 288) × 103
= 75600J
The heat transfer resistance R = x/KA where K is the thermal conductivity of the
39
base plate = 113.11W/mk (Baumester).
X is the material thickness = 0.491 × 10-3 m
A is the surface area = ∏di 2 4
= ∏ × (0.76) 2 4
= 0.455m2
The thermal resistance
R = 0.491 × 10 -3 113.11 × 0.581
= 7.4714 × 10-6k/W
This thermal resistance is negligible. The heat transfer will be through
direct
conduction since there is no heat lost and the convection will be a natural
connection.
However, because the part of the biogas generated will be used to supply
the energy required should be calculated, checked for its effectiveness
and compared with the volume generated per day.
Energy required Qs = Cv × volume where Cv is the calorific valve of the
gas.
Cv of methane is 33.934 × 106J/m3Qs/Cv
= 75600 33.934 × 106
= 0.002228m3
This is very small in quantity compared to 0.25m3 of the biogas to be
generated per day.
40
4.2.2BASE STAND STRENGTH
This unit is the strength of the base support to be able to withstand the
load.
W
0.38
R2 R1
Considering a beam having a concentrated load on it as shown above.
Taking moment about R1
W × 0.38× = R2 × 0.76
W is the weight of the slurry and the vessel weight.
W = 10kg +450kg +34 = 494kg
W = 4846.14N
R2 = 4846.14 × 0.38
0.76
R2 = 1841.5332
0.76
R2 = 2423.07N
Equating the upward and downward forces.
W = R1 + R2
4846.14 = R1 + 2423.07
41
R1 = 4846.14 – 2423.07
R1 = 2423.07N
4.2.3 FACTOR OF SAFETY
The ideology that ultimate load is considerably larger than the load the
components will be allowed to carry under normal working condition was
applied to design machine component.
A fraction of the ultimate load carrying capacity is utilized when the
allowable load is applied for this design, a factor of safety ranging
between 1 and 2 is used.
42
CHAPTER FIVE
5.0 MATERIAL SELECTION, CONSTRUCTION AND COSTING
Material selection means proper thought is exercised in choosing durable
and suitable materials which property would uphold the environmental
effect.
The construction of the project also must be done by priorities accuracy of
measurement of materials along side with perfection of welding.
Costing of the plant must be bearable to encourage masses.
5.1 MATERIAL SELECTION
Several factors have to be considered so as to make the process efficient
and economical. Important factors known to be applicable in the design of
biogas plants;
i. Availability of materials
ii. Available manufacturing technique
iii. Safety under operation
iv. Cost
v. Workability and machine ability materials for this
project are therefore chosen using the following
criteria.
Categorically the product was analysed to actualize minimum acceptable
valves for all the relevant materials properties.
43
An evaluation of the materials which gave the best overall combination of
properties and the least were selected.
The degree of relative importance of the various required properties from
essential to desirable and for each property, the potential materials were
placed in a ranking order.
Materials that do not posses the least require criteria were eliminated at
initial stage of selection.
5.1.1 IMPELLER MATERIAL
Based on it purpose, galvanized sheet was chosen which is readily
available and relatively cheap. Its high strength suitable under the given
working condition and also weld able.
5.1.2 VESSEL MATERIAL
It consists of mild steel which is available and comparatively cheap. Its
strength accounts for suitability under working condition and weld-able
within the locality.
5.1.3 SHAFT MATERIALS
Considerably, mild steel was chosen for this component just for its
availability, cheapness strength and machineability.
5.1.4 BEARING
The bearings are of the self lubrication type because the air tightness of
the unit which requires perfect sealing.
44
Properties of Materials
COMPONENTS MATERIALS PROPERTIES
Vessel and base plate Mild steel Tmax = 150mpa
dy = 280mpa
e = 7859kg/m3
E = 207gpa
Shaft Mild steel
1mm thick
Tmax = 150mpa
dy = 280mpa
e = 7850kg/m3
E = 207gpa
Impeller Galvanized sheet
1mm thick
d1 = 415mpa
e = 7850kg/m3
5.1.5 CONSTRUCTION
It involves fabrication and assembling of various parts to build the system
as a whole.
i. VESSEL
The working drawing serves the purpose of construction for the
construction of vessel with respect to the required diameter, height and
water liter capacity drum. Likewise, gas holder was welded and the slurry
inlet and outlet was drilled in the workshop, with filler drilling machines to
the required diameter for fitting.
45
ii SHAFT IMPELLER
Machining of shaft was done on lathe machine on the ground of working
drawing. Turning operation was carried out to provide a place for the
bearing to be forced fitted. Fabrication and welding also done according to
the precision of impeller.
iii. BASE SUPPORT
Stability of the digester is very paramount in that wise carefulness was
adopted in cutting the standing precisely. The welding also done base on
the indication on the drawing.
iv. ASSEMBLERS
This action succeeded construction process, fittings were done and base
support was screwed to the base.
5.1.6 COSTING
It implies price of various materials and other expenses involved during
the construction of the project.
46
CHAPTER SIX
6.0 CONCLUSION AND RECOMMENDATIONS
6.1 CONCLUSION
This project features various types of biogas digesters that could be set up
to generate the methane gas purposely for lighting, cooking and so on. It
also analyses the most viable animal waste suitable for the production of
biogas, which is, the cattle dung. The application of biogas as stated in
this research work covers the various areas which biogas can be
efficiently, effectively and
economically utilized. e.g domestic use; cooking, heating and lighting.
The generation of biogas has indicated the importance of the residue
(slurry) for agricultural use i.e fertilizer.
6.2 RECOMMENDATIONS
(a) Government in conjunction with other research and development
centres should encourage the establishment of a database on biogas in
order to enable existing and promising designs, application and analysis
to be assessed more accurately and should also identify with greater
precision the places where biogas plants could become acceptable and
economically viable.
47
(b) Cattle dung and poultry waste are potential source of methane.
Investigation indicates that more of methane was produced at mesophilic
temperature than at room temperature.
© Negligence of government to the importance of biogas production
and application in Nigeria hinders its fame.
48