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

1

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.

2

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

3

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

4

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

5

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.

6

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

8

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

9

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

15

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

17

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.

21

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

24

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