SCHOOL OF ENGINEERING
DEPARTMENT OF MECHANICAL AND PRODUCTION ENGINEERING
MPE 580: FINAL YEAR PROJECT REPORT
TITLE: DESIGN AND ANALYSIS OF FOOD WASTE BIOGAS DIGESTER AT MOI UNIVERSITY
JOEL MWANGI NJUGUNA MPE/15/13
ALFRED KIPKOECH LANG’AT MPE/08/13
Mr. S. KIMUTA1
DATE: 19TH JUNE, 2017
This documents content is our original work based on the research we made to the best of our abilities and it hasn’t been presented anywhere else for academic purpose.
JOEL MWANGI NJUGUNA MPE/15/13
Signed…………………… Date: ………………….…
ALFRED KIPKOECH LANG’AT MPE/08/13
Mr. S. KIMUTAISigned: …………………….. Date: ………………………
DEDICATIONWe dedicate the project to our parents for their continued love and support throughout our studies.
ACKNOWLEDGEMENTWe wish to sincerely acknowledge and appreciate the significant contribution made by all those in one way or the other assisted us during the process of preparing this report.We thank the Almighty GOD for giving us good health, strength and the opportunity to undertake this project.
Special thanks go to Mr. Kimutai our research lecturer for his insights, objective suggestions and guidance throughout the undertaking of this project.
ABSTRACTWaste management continues to present an eminent problem in the society today. Although our knowledge on the various ways to utilize the wastes for energy generation is quite widespread, little is been done to actually put it in practice. Moi University has several hostels and messes around, where daily a large amount of kitchen waste is obtained which can be utilized for better purposes. Biogas production requires anaerobic digestion. This Project aims to create small scale Organic Processing Facility to create biogas which will be more cost effective, eco-friendly, cut down on landfill waste, generate a high-quality renewable fuel, and reduce carbon dioxide ; methane emissions. Overall by creating a biogas reactors on campus in the backyard of our hostels will be beneficial. Kitchen (food waste) are collected from different hostels as feedstock for our reactor which works as anaerobic digester system to produce biogas energy. The anaerobic digestion of kitchen waste produces biogas, a valuable energy resource anaerobic digestion is a microbial process for production of biogas, which consist of primarily methane (CH4) ; carbon dioxide (CO2). Biogas can be used as energy source and also for numerous purposes. But, any possible applications requires knowledge ; information about the composition and quantity of constituents in the biogas produced.
The scope of this project targets local institution, in this case Moi University to address the issue of food wastes that is a common problem. A large amount of food waste remains over in the various residences as well as other eating joints within the institution.
TOC o “1-3” h z u DECLARATION PAGEREF _Toc485637907 h iDEDICATION PAGEREF _Toc485637908 h iiACKNOWLEDGEMENT PAGEREF _Toc485637909 h iiiABSTRACT PAGEREF _Toc485637910 h ivCHAPTER 1 PAGEREF _Toc485637911 h 1INTRODUCTION PAGEREF _Toc485637912 h 11.1 BACKGROUND INFORMATION PAGEREF _Toc485637913 h 11.2 STATEMENT OF PROBLEM PAGEREF _Toc485637914 h 21.3 OBJECTIVES PAGEREF _Toc485637915 h 31.4 JUSTIFICATION OF PROBLEM PAGEREF _Toc485637916 h 4CHAPTER TWO PAGEREF _Toc485637917 h 5LITERATURE REVIEW PAGEREF _Toc485637918 h 52.1 INTRODUCTION PAGEREF _Toc485637919 h 52.2 PRODUCTION PROCESS PAGEREF _Toc485637920 h 82.3 MATERIALS THAT ARE USED FOR BIOGAS PRODUCTION PAGEREF _Toc485637921 h 102.4 THINGS TO CONSIDER WHEN IT COMES TO BIOGAS PRODUCTION PAGEREF _Toc485637922 h 112.5 DIGESTER TECHNOLOGIES PAGEREF _Toc485637923 h 112.6 BIOGAS ENERGY UTILIZATION IN KENYA PAGEREF _Toc485637924 h 14CHAPTER 3 PAGEREF _Toc485637925 h 16METHODOLOGY PAGEREF _Toc485637926 h 163.1 INTRODUCTION PAGEREF _Toc485637927 h 163.2 PROCEDURE FOR FABRICATING THE BIOGAS PLANT MODEL PAGEREF _Toc485637928 h 163.3STEPS IN FABRICATION OF THE SMALL SCALE BIGAS PLANT PAGEREF _Toc485637929 h 173.4 FEEDING THE WASTE INTO THE DIGESTER PAGEREF _Toc485637930 h 213.5GAS FORMATION, COLLECTION AND MEASUREMENT PAGEREF _Toc485637931 h 233.6 STIRRING PAGEREF _Toc485637932 h 25CHAPTER 4 PAGEREF _Toc485637933 h 26RESULTS AND DISCUSSION PAGEREF _Toc485637934 h 264.1 RESULTS PAGEREF _Toc485637935 h 264.2 DISCUSSION PAGEREF _Toc485637936 h 27CHAPTER FIVE PAGEREF _Toc485637937 h 29ANALYSIS PAGEREF _Toc485637938 h 295.1 INTRODUCTION TO ANALYSIS PAGEREF _Toc485637939 h 295.2 ANALYSIS OF THE RESULTS OBTAINED EXPERIMENTALLY PAGEREF _Toc485637940 h 295.3 AN ANALYSIS TO COVER THE ENTIRE SCOPE OF OUR PROJECT PAGEREF _Toc485637941 h 305.4DESIGN OF A BIOGAS DIGESTER BASED ON THE DATA ANALYSED PAGEREF _Toc485637942 h 315.4.1 CHOICE OF BIOGAS DIGESTER PAGEREF _Toc485637943 h 315.4.2 DESIGN OF THE DOME DIGESTER PAGEREF _Toc485637944 h 325.4.3 THE DESIGN DRAWING PAGEREF _Toc485637945 h 375.5 ENERGY EQUIVALENCE THAT CAN BE DERIVED FROM THE BIOGAS DIGESTER DESIGNED PAGEREF _Toc485637946 h 38CHAPTER 6 PAGEREF _Toc485637947 h 40COST ANALYSIS PAGEREF _Toc485637948 h 40CHAPTER SEVEN PAGEREF _Toc485637949 h 41CONCLUSION AND RECCOMENDATION PAGEREF _Toc485637950 h 417.1 CONCLUSION PAGEREF _Toc485637951 h 417.2 RECOMMENDATION PAGEREF _Toc485637952 h 42REFERENCES PAGEREF _Toc485637953 h 43
CHAPTER 1INTRODUCTION1.1 BACKGROUND INFORMATIONFood waste has always been disposed of in a manner that endangers the environment or is simply unreliable. A common practice is draining into a designated pit that fills up after some time, and worse still, produces a stinking smell. This can be resolved by using the wastes to produce biogas that will be beneficial in cooking.
Food waste is composed mainly of organic material. If decomposed in a suitable digester mixed with a specified amount of dung, biogas can be produce and be used as an alternative method of cooking in the university. The project has never been tested in Moi before, but owing to the fact that there is sufficient food wastes shows that there is a big possibility to harness the food wastes to produce energy. All the hostels produce a lot of food waste due to the population of students in there. It is therefore important that this is looked into as an alternative source of cooking in the cafeterias instead of traditional firewood, charcoal or even the more expensive LPG gas. Due to scarcity of petroleum and coal it threatens supply of fuel throughout the world also problem of their combustion leads to research in different corners to get access the new sources of energy, like renewable energy resources. Solar energy, wind energy, different thermal and hydro sources of energy, biogas are all renewable energy resources. But, biogas is distinct from other renewable energies because of its characteristics of using, controlling and collecting organic wastes and at the same time producing fertilizer and water for use in agricultural irrigation. Biogas does not have any geographical limitations nor does it requires advanced technology for producing energy, also it is very simple to use and apply.
Cooking at Moi University mainly involves the use of electricity in the hostels, and in the mess charcoal or LPG gas are mainly used. Not only are both alternatives more expensive but also charcoal is one of the biggest factors for deforestation. Biogas on the other end is from food, with no impact on the environment.
1.2 STATEMENT OF PROBLEMAs the population at Moi University continues to grow by the day, the energy requirements continue to escalate. With the ‘traditional’ use of coils for cooking increasing each passing semester, the electricity bills continue to grow bigger for the institution. Despite the bills, power shortages and failure in the residential areas have become a norm presently due to overloading in the consumer units, sometimes leading to short-circuiting among other complications arising from the same. In the past, demonstrations have been witnessed by the students due to power shortages experienced in the residential areas. These have been without destruction of some property in the university.
A growing population of students also means increased volumes in the waste produced. This has put a lot of pressure on the waste management facilities that are already present and disposal mechanisms available. In Moi today, the waste management infrastructure is almost outdated, with declining levels of capital investment and maintenance on the same.
In light of the above challenges, it is necessary to come up with a system to mitigate these problems. The policy of reduce, reuse and recycle comes in handy. Most food wastes can be converted into useful energy by utilizing a biogas plant. The kitchen wastes present organic materials that have a very high calorific value and the energy generated here can be used for cooking purposes. This will go a long way in alleviating the problem of the high electricity bills in the institution currently. It will also offer a cleaner waste management system where all the biodegradable wastes are utilized for energy generation while the residue form very nutritious manure that can be used in the farms around. As the entire world move towards cleaner, greener energy solutions, we recommend for biogas utilization which is not only economical but also environmental friendly as a means for solving the energy problem in our institution.
1.3 OBJECTIVESThe general objective of this project is to come up with a food waste management policy for Moi University through biogas production from the food wastes around.
In order to meet this objective, we will put into consideration the following specific objectives:
To determine the best biological recovery of the food waste
To determine the amount of biogas that can be produced form the food wastes in Moi University.
To design and recommend a suitable biogas digester for Moi University
1.4 JUSTIFICATION OF PROBLEMFood waste management has become a demanding developmental problem in Moi University in recent times. It is therefore very important that waste management institutions, corporate bodies, non-governmental organizations and individuals alike find a lasting solution. Sadly, in Kenya, little has been done to harness the capability of food waste to be a reliable energy source. Its continuous supply is a good indicator to feasibility. According to the hostels department, over 100 kg of food waste are collected daily. The bins in hostels are divided into food waste bins and plastics section. The food waste bins are located mainly in the sinks.
Despite the seriousness of the problem, very little research has been conducted into food waste management in Moi today. Through this project, it will provide a clear understanding of the nature of the problem and the remedying strategy that can be adapted to solve the problem Apart from offering reference to agencies involved in food waste management, this project will go a long way in providing a solution to the same through the waste management methods discussed here and the design of a simple food waste to energy project that could be implemented as well. It will further encourage studies into this problem as well by others who intend to make a change in the world of waste management in this country.
This project is mainly to contribute to the ongoing efforts to ring about a sustainable environment. Since Earth has dwindling resources, it is important to put everything into good use- including food waste materials. A cheaper alternative of cooking would be a welcome idea for the university to ease its financial burdens.
CHAPTER TWOLITERATURE REVIEW2.1 INTRODUCTION2.1.1 WHAT IS BIOGAS?
BIOGAS is a renewable energy source that belong to the category of biofuels. It is produced by through the bio-degradation of organic material by bacteria under anaerobic conditions. The gases primarily produced are methane (CH4) and carbon dioxide (CO2) with other trace gases. It can be used both at a small scale (rural) as well as in commercial and industrial applications.
Component Concentration (by volume)
Methane (CH4) 50-75 %
Carbon dioxide (CO2) 25-45 %
Water (H2O) 2-7 %
Hydrogen sulphide (H2S) 0-1%
Ammonia (NH3) 0-1 %
Nitrogen (N2) 0-2 %
Oxygen (O2) 0-2 %
Hydrogen (H) 0-1 %
Table-1. Composition of biogas. (From the Biogas Handbook pg. 41)
2.1.2 CHARACTERSTICS OF BIOGAS
Composition of biogas depends upon feed material also. Biogas is about 20% lighter than air has an ignition temperature in range of 650 to 750 0C. Methane is an odorless ; colourless gas that burns with blue flame similar to LPG gas. Its caloric value is 20 Mega Joules (MJ) /m3 and it usually burns with 60 % efficiency in a conventional biogas stove. However, due to mixture of many gases in the digester tank, there is characteristic rotten egg smell from the same.
This gas is useful as fuel to substitute firewood, petrol, LPG, diesel, ; electricity, depending on the nature of the task, and local supply conditions and constraints.
Biogas digester systems provides a residue organic waste, after its anaerobic digestion(AD) that has superior nutrient qualities over normal organic fertilizer, as it is in the form of ammonia and can be used as manure. Anaerobic biogas digesters also function as waste disposal systems, and can, therefore, prevent potential sources of environmental contamination and the spread of pathogens and disease causing bacteria. Biogas technology is particularly valuable in residual treatment of animal excreta and kitchen refuse (residuals) in a domestic, institutional, municipal as well as industrial level.
Energy Content 6-6.5 kWh/m3
Fuel Equivalent 0.6-0.65 l oil/m3 biogas
Explosion Limits 6-12 % biogas in air
Ignition Temperature 650-750 0C
Critical Pressure 75-89 bar
Critical temperature -82.5 0C
Normal Density 1.2 kg/m3
Smell Rotten eggs
Table-2:- GENERAL FEATURES OF BIOGAS (from Biogas utilization and cleanup by extension)
2.1.3 PROPERTIES OF BIOGAS
Various variables affect the production of biogas as described below:
Change in volume as a function of temperature and pressure.
Change in calorific value as function of temperature, pressure and water vapour content.
Change in water vapour as a function of temperature and pressure.
2.1.4 FACTORS AFFECTING YIELD AND PRODUCTION OF BIOGAS
Factors affecting the fermentation process of organic substances under anaerobic condition include:
The quantity and nature of organic matter
Acidity and alkalinity (PH value) of substrate
The flow and dilution of material
The C:N ratio (carbon to nitrogen ratio)
2.1.5 BENEFITS OF BIOGAS TECHNOLOGY:
The benefits of biogas technology could be grouped majorly into four as shown below:
Production of energy through a renewable energy source.
Transformation of organic wastes to very high quality fertilizer.
Improvement on the environment as well as sanitary and hygienic conditions through consumption of the biodegradable wastes and reduction of pathogens.
It is a cheaper and much easier way to produce energy on a small scale when compared to other bio-fuels.
Environmental benefits on a global scale due to a reduction in the greenhouse emissions (especially methane) into the earth’s atmosphere
2.1.6 Disadvantages attached to this technology
Although Biogas is no more dangerous than other gases, here are some things to look out for
Safety precautions should be paramount as improper gas mixing ratios and uncontrolled pressures could lead to explosions on a large scale.
It is very difficult to enhance the efficiency of biogas plants
On a large industrial scale, the process is not very attractive economically (volume of biogas produced is limited).
2.2 PRODUCTION PROCESS2.2.1 COMPONENTS OF A BIOGAS COMPONENT
A typical biogas system consists of the following components:
Collection system of the biodegradable wastes
2.2.2 PRINCIPLES FOR PRODUCTION OF BIOGAS
Organic substances exist in wide variety from living beings to dead organisms. Organic matters are composed of Carbon (C), combined with elements such as Hydrogen (H), Oxygen (O), Nitrogen (N), Sulphur (S) to form variety of organic compounds such as carbohydrates, proteins & lipids. In nature microorganisms, through digestion process breaks the complex carbon into smaller substances.
There are 2 types of digestion process:
The digestion process occurring in presence of Oxygen is called Aerobic digestion and produces mixtures of gases having carbon dioxide (CO2), one of the main “green-house gases” responsible for global warming.
The digestion process occurring without (in the absence of) oxygen is called Anaerobic digestion and generates a mixtures of gases. The gas produced, mainly methane produces 5200-5800 KJ/m3 which when burned at normal room temperature and presents a viable environmentally friendly energy source to replace fossil fuels (non-renewable).
2.2.3 ANAEROBIC DIGESTION
It is also referred to as biomethanization, is a natural process that takes place in absence of air (oxygen). It involves biochemical decomposition of complex organic material by various biochemical processes with release of energy rich biogas and production of nutritious effluents.
220.127.116.11 BIOLOGICAL PROCESS (MICROBIOLOGY)
The biological process involves the following:
HYDROLYSIS: In the first step the organic matter is enzymolysed externally by extracellular enzymes, cellulose, amylase, protease & lipase. Bacteria decompose long chains of complex carbohydrates, proteins, & lipids into small chains. For example, Polysaccharides are converted into monosaccharide while proteins are split into peptides and amino acids.
ACIDIFICATION: Acid-producing bacteria involved in this step, convert the intermediates of fermenting bacteria into acetic acid, hydrogen and carbon dioxide. These bacteria are anaerobic and can grow under acidic conditions. To produce acetic acid, they need oxygen and carbon. For this, they use dissolved O2 or bounded-oxygen. Hereby, the acid-producing bacteria creates anaerobic condition which is essential for the methane producing microorganisms. Also, they reduce the compounds with low molecular weights into alcohols, organic acids, amino acids, carbon dioxide, hydrogen sulphide and traces of methane. From a chemical point, this process is partially endergonic (i.e. only possible with energy input), since bacteria alone are not capable of sustaining that type of reaction.
METHANOGENESIS: (Methane formation) Methane-producing bacteria, which were involved in the third step, decompose compounds having low molecular weight. They utilize hydrogen, carbon dioxide and acetic acid to form methane and carbon dioxide. Under natural conditions, CH4 producing microorganisms occur to the extent that anaerobic conditions are provided, e.g. under water (for example in marine sediments), and in marshes. They are basically anaerobic and very sensitive to environmental changes, if any occurs. In essence, Methanogens provide the last link in a chain of microorganisms which degrade organic material and returns product of decomposition to the environment.
18.104.22.168 SYMBIOSIS OF BACTERIA
Methane and acid-producing bacteria act in a symbiotically-mutual manner. Acid producing bacteria create an atmosphere with ideal parameters for methane producing bacteria (anaerobic conditions, compounds with a low molecular weight). On the other hand, methane-producing microorganisms use the intermediates of the acid producing bacteria. Without consuming them, toxic conditions for the acid-producing microorganisms would develop. In real time fermentation processes the metabolic actions of various bacteria acts in a design. No single bacteria is able to produce fermentation products alone as it requires others.
2.3 MATERIALS THAT ARE USED FOR BIOGAS PRODUCTIONAlmost all biodegradable wastes can be used for Biogas Production. Here are some of the digestible feedstock types with their characteristics.
Types of feedstock Organic content C:N ratio Dry matter (%) Volatile solids (%) Biogas yield
Pig slurry Carbohydrates, proteins, lipids 3-10 3-8 70-80 0.25-0.35
Cattle slurry Carbohydrates, proteins, lipids 3-10 5-12 80 0.2-0.3
Poultry slurry Carbohydrates, proteins, lipids 3-10 10-30 80 0.35-0.5
Stomach/intestine contents 3.5 15 80 0.4-0.55
Food remains 3.5 15-20 75 0.2-0.45
Garden waste 10-15 70-90 80-85 0.15-0.25
grass 12-25 20-25 90 0.2-0.4
Fruit wastes 3.5 15-20 75 0.2-04
Table-3: The characteristics of some digestible feedstock types (The Biology Handbook)
The values used here have been obtained theoretically and may actually vary in practical use. Very organic waste material you can use in the anaerobic digester. Make sure you chop them into small pieces as much as possible. However, these common domestic kitchen wastes should be avoided as they inhibit the process of methane production in the digester.
Dry skins of Onion and Garlic
Fibrous materials like coconut husk
Bones, raw or cooked
The materials fed in the biogas plant determine by far the C: N (carbon to nitrogen) ratio which is a key factor when it comes to the production of biogas. As such it is important to feed materials that will not alter this ratio to ensure optimal gas production parameters.
2.4 THINGS TO CONSIDER WHEN IT COMES TO BIOGAS PRODUCTIONVarious parameters play a big role when it comes to the process of biogas production. The bacteria found in the digester are very sensitive to any changes in these parameters:
C: N ratio
These factors determine the retention time required to produce gas. In order to maintain C: N ratio and the pH constant, the type and amount of feedstock fed should not be varied so much.
2.5 DIGESTER TECHNOLOGIESThere are a number of different designs of small scale biogas digesters and recording the type of digester is important for estimating its capacity, particularly in cases where the digester is partly underground. The most common digester technologies are:
2.5.1 Fixed dome plant
The digester in a fixed dome plant consists of an underground pit lined with concrete or brick, with an inlet pipe that is used to add feed to the digester.
Gas is produced under pressure and is stored under the dome at the top of the digester. Biogas is removed from the digester using a pipe attached to the top of the dome.
As biogas is produced, slurry is pushed out from the digester through the outlet pipe into a displacement tank. When the biogas is used, this slurry flows back into the digester. Some designs may also include an additional gas storage tank connected to the gas outlet pipe.
Figure 1. Fixed dome plant
2.5.2 Floating drum plant
The floating drum plant comprises of a brick lined pit that is often partly underground (the digester) and a drum above ground is used as the gas collector. The drum is typically made of steel although some newer designs use fiberglass reinforced plastic.
Water and feedstock are combined in a mixing pit which then flows into the underground digester through the inlet pipe. As gas is produced, it is collected in the drum, which moves up and down a central guide pipe depending on the amount of gas being stored. The gas is held under pressure from the weight of the drum, which can be increased with the addition of weights.
As more feedstock is added, slurry flows out through the outlet pipe. A gas outlet pipe is also attached to the drum to remove gas from the plant.
A variation of this design is the small-scale above ground floating drum plant can be developed in which the digestion of household food waste and is made from two plastic water tanks with their tops removed and the smaller tank placed upside down inside the larger one. Pipes are added to the outer tank to add feedstock and remove the slurry and a gas outlet pipe is added to the top of the inner tank.
Figure 2. Floating drum plant
2.5.3 Balloon/bag digester
This type of digester is usually made from a large, strong plastic bag connected to a piece of drainpipe at either end, with these pipes being used to add feedstock and remove slurry.
To avoid damage to the bag, the digester is usually placed in a trench and the trench is slightly deeper at the slurry outlet so that the slurry will settle there.
As gas is produced the top of the bag inflates and the gas can be removed through an outlet pipe in the top of the bag. Gas pressure can be increased by placing weights on top of the bag.
A common variation of this design is the bag digester, which is a circular concrete, brick or plastic lined container covered with a plastic bag or tent. As with a fixed dome plant, inlet and outlet pipes are used to add feedstock and remove slurry and the gas is collected and removed from the bag.
Figure 3. Balloon digester
2.6 BIOGAS ENERGY UTILIZATION IN KENYAEnergy utilization in terms of biogas consumption in Kenya remains very low. However, as the entire world continue to push for the use of greener energies, attempts to turn into Biogas use in Kenya are slowly starting to show up. Currently, there are several biogas plants in Kenya, the biggest one being Naivasha Biogas Plant.
Biogas plants in Kenya
Naivasha biogas plant Located at Naivasha, producing ?1.8MW
Nyogara Biogas plant Located at Dagorretti area of Nairobi, next to Nyogara Slaughter House
Takamoto Biogas Firm that operates to promote small scale biogas production by fabricating and installing digesters to farmers countrywide
James Finlay Biogas Located at Kericho,it has a digester size of 1700m3
TakaNguvu-from waste to energy Located in Mombasa
Kisumu Biogas plant Located in Kisumu
Table 4. Major biogas contributors in Kenya. (Source. Internet research)
Naivasha Biogas Plant
The project, commissioned in 2007 has a volume of 54 m3 with two expansion chambers. The underground structure is located about 0.5 m below surface. The required area for the toilet building and biogas plant is approximately 10 x 15 m. Design parameters for the biogas plant assume 1,000 visitors per day. The dimensions of the plant were based on a sufficient settlement of solids which is achieved with a hydraulic retention time (HRT) of 5 days. The solids settle and remain in the system for digestion and biogas production.
The digester is of fixed dome type. This type of fixed-dome biogas plant was selected due to its robust technology that works without moving parts. It is able to operate with inflow fluctuations caused either by lack or overuse of water. The production of biogas is continuous and the fixed dome technology provides for biogas storage.
Here is a technical drawing of the biogas plant project.
Figure 4: Technical drawing of the biogas plant (built completely underground). Diameter of dome-shaped digester is 5 m.
Besides this plant, there are many small scale biogas plants spread all over, especially in the rural areas. Takamoto Biogas, for example, supply flexible prefabricated digesters which can easily be installed by those willing to take up this energy solution. Nyongara Biogas Project has also been set up with partnership from KIRDI adjacent to the Nyogara Slaughter House to demonstrate that waste can be used for energy generation.
Biogas use continues to get acceptability among people as an effective alternative to replace the rather expensive non-renewable energy sources, which are unavailable in some rural area as well.
CHAPTER 3METHODOLOGY3.1 INTRODUCTIONThe project hereby will involve the fabrication of a small scale biogas plant with an aim of gathering data on the amount of biogas that can be produced from the various kitchen waste as well as the biodegradable wastes generated from the hostels within the institutions. As such, we will take samples from Soweto Mess as well as Ngeria hostels for one week to determine the amount of biogas that can produced from the two places. This will then be extrapolated for the entire institution. After analyzing the data obtained, we will carry out design calculations on the size of the digester, gas collection and storage capacity among others while coming up with a design of a biogas plant that perfectly suits the biodegradable wastes available at Moi University. The calculations will include a projected equivalent energy that can be harnessed from such a plant.
3.2 PROCEDURE FOR FABRICATING THE BIOGAS PLANT MODELCOMPONENTS
Inlet pipe-feeding the kitchen waste
Outlet pipe- for digested slurry
Gas collection and storage tank
Slurry collection can
. MATERIALS REQUIRED
Empty PVC can 20lts.
Inlet PVC pipe 63.5mm diameter, 600mm long
Outlet PVC pipe 38mm diameter, 450mm long
Rubber (hose) pipe 13mm diameter, 300mm long
Plastic bucket 15lts.
Funnel 88mm diameter
Plastic pipe elbow 38mm fitting
Stirrer 50mm long
Glue, tape thread and any other adhesive
Sharp knife (preferably hot)
A medium sized hammer
A marker pen
STEPS IN FABRICATION OF THE SMALL SCALE BIGAS PLANT3.3.1 DIGESTER PREPARATION
For this experiment, a plastic can of 20 liter capacity was used. This was arrived at based on the amount of food waste that could be easily collected within. Also, in the market, the only other digester capacity available was 50 liter plastic container that would not only be bulky but also complex for our experiment. Based on the size of this digester, the other physical parameter for the same are as tabulated below.
Total capacity 20 liters
Digester height 400 mm
Side water depth (liquid height) 290 mm
Free board line (empty volume) 110 mm
liquid slurry volume 13.5 liters
Table 5. Physical characteristics of the digester (from online biogas calculator)
In order to get conclusive data, method employed was based on the principle of continuous feeding into the digester. Thus the digester was designed in such a way so that the waste can be easily put inside it. Following sequential steps were taken for designing of the digester.
At the top of the digester, the following holes were marked and cut out:
A 64mm diameter hole on upper part towards one end (for the inlet pipe fitting)
A 51mm diameter hole on the upper part at the middle (for the stirrer to be fixed on)
A 13mm diameter hole at the upper part towards the other end (For gas outlet)
On the side, one hole was made:
A 38mm diameter hole on the side of the tank at the described position (for slurry outlet)
3.3.2 Inlet System:
A PVC pipe of 50 cm in length was fitted on the hole made on the top of the digester having a diameter of 2.0 in. At the top, a cap of the same size was fitted to ensure it remained closed when there was no feeding of wastes taking place. This pipe was suck to a depth of 33cm in the digester tank.
3.3.3 over flow line:
The over flow line was connected at the height of 27 cm of the digester. The PVC pipe of diameter 1.5 in. was bent by means of an elbow and immersed into water can at the pipe outlet. The function of the over flow line is to take the excess slurry out of the digester when feeding of new wastes into the digester tank.
3.3.4 Gas line:
A rubber pipe was attached on the top hole of the digester having a diameter of 0.25 in. to carry the gas from the digester. The gas was collected and measured by means of a water displacement method.
An outlet was provided at the bottom of the digester. Its function was to take the digested slurry out of the digester
At the top of the digester, a thin metallic rod was fitted in the middle to act as a stirrer. It was placed at a depth of 37cm inside the digester.
To ensure air-tightness of the unit, all the joints were reinforced using a necessary seal. Before feeding the kitchen waste into the unit for the first time, it was dipped in a bath of water to test for air-tightness. All the outlets were closed apart from the gas delivery pipe. Air was blown through it to see if any bubbles came out of the unit. Since no bubble emerged, we concluded that it was air tight and ready for the first feeding.
3.3.7 Set up drawing
Figure 5. Digester schematic
Key for the parts:
1 funnel 4 outlet pipe for slurry
2 gas outlet 6 inlet pipe for inoculum
3 stirrer 7 digester tank
Figure 6. A picture showing the complete set-up for the experiment
3.4 FEEDING THE WASTE INTO THE DIGESTER3.4.1 PREPARATION OF THE INOCULUM AND FEEDING THE DIGESTER FOR THE FIRST TIME
Inoculum used in preparation for the first feeding of the digester consisted mostly of cow dung and small quantities of kitchen waste. Cow dung is essential in order to start the whole process of digestion and fermentation of the wastes. Initially, we visited the school farm to get the dung. On the same day, we got our first supply of wastes from the Soweto Mess. After physical assessing the kitchen waste, we found it to contain cooked rice, beans and some vegetable peelings. Following the thumb rule for biogas production which states that 1kg of solid waste should be mixed in about 3 liters of water, we measured about 2.5kg of cow dung and 1kg of kitchen waste which was mixed in about ten liters of borehole water. Use of tap water is discouraged since it is usually treated using chlorine, which inhibits the growth of the bacteria responsible for digesting the various wastes to produce the biogas. The mixture was then stirred and homogenized nicely to make a fine slurry. The resulting slurry, about 13 liters in capacity, is fed into the digester through the inlet which is then closed by a lid and kept air-tight. Care must be taken to prevent any leakages.
3.4.2 THE FEEDING RATE
Besides feeding the biogas for the first time, it has to be fed with new kitchen waste regularly to ensure the process proceeds as required. However, the rate must be determined and kept as constant as possible. The materials to be fed into the digester also matters a lot since the chemical composition has to be kept as constant as possible. Slight changes in pH, the C: N (carbon-nitrogen) ratio and temperature are among the parameters that should be maintained at a constant rate
Since our digester capacity is 20 liters only, an optimal feeding rate would be around 50g of kitchen waste after every day. This is done after gas formation is noticed. We arrived at this rate based on the retardation time required to fully digest the various wastes in the digester. By adding wastes in these small quantities, it will allow the wastes to get fully digested and release optimum gas and get discharged through the slurry outlet pipe. Overfeeding the digester will drain out the partially digested wastes, which will increase acidity of the slurry in the digester as well as decreasing the colony of microorganisms working on the wastes thereby inhibiting the production of methane gas. Similarly, if the digester is underfed, the gas production will also be low. The picture below shows feeding done on the first day of gas formation.
Figure 7: feeding the biogas digester.
3.4.3 MATERIALS TO BE FED IN THE DIGESTER
For our project, we used kitchen waste from the Soweto Mess. In order to maintain the C: N ratio as constant as possible, we used only the left-over food which mainly included cooked rice, beans, and ugali among other common left-over foods. Any kitchen waste that composed more than these wastes had to be sorted and removed. Grinding of the waste food stuff was also done before introducing them to the digester in order to increase the surface area for chemical reaction by the bacteria in the digester
GAS FORMATION, COLLECTION AND MEASUREMENT
Gas formation started after about five days after feeding the digester for the first time. However, this gas contained a lot of impurities and would not burn. Moreover, there was air present in the gas tank when we placed it over the digester so that initially there was aerobic digestion leading to the production of CO2 gas.
Gas collection and measurement was done after every day by the water displacement method. In this method the gas line from the digester was connected to the inverted glass beaker of one liter capacity at bottom. The glass beaker was graduated and the divisions of the beaker were used to measure the gas in milliliters. The beaker was completely filled with the water and kept upside down with a cap at its mouth. A hole was made in the cap for the pipe to get fitted into it. When the gas was produced it could easily displace the water and go inside the beaker. The picture below shows gas level taken arbitrarily on a particular feeding day
Figure 8. Picture showing the gas level on one of the days.
3.6 STIRRINGAn agitation mechanism was achieved through the use of a stirrer. It was done twice per day by manually moving the stirrer in a circular manner. This ensured that the materials inside did not accumulate at a central point on the bottom of the digester which would greater reduce the surface area for reaction, thence lowering the efficiency of the unit.
CHAPTER 4RESULTS AND DISCUSSION4.1 RESULTSInitial gas formation was observed on the 23rd day. The table below shows the amount of food waste added daily and the amount of biogas that was produced.
DATE DAY ( after retention time) Addition food waste (g) Quantity of biogas produced in ml (incremental)
26th MAY 1 50 350
27th MAY 2 50 750
28th MAY 3 50 1150
29th MAY 4 50 1650
30th MAY 5 50 2060
Table 6. Amount of gas produced
After the fifth day, the amount of biogas produced began to decline. However, our experiment had produced enough amount of biogas that could help us draw important conclusions from the same.
4.2 DISCUSSIONVolume build up with time
After a retention time of 23 days, volume buildup in the biogas production was directly proportional with time. This indicates that the materials inside the digester decomposed to result in biogas production. However, this declined from the fifth day moving forward.
The Hydraulic retention time.
Although the hydraulic retention time for kitchen waste is way less than what we achieved, various factors played a big part in lengthening this duration. To start with, the temperatures were very low which made the process of digestion to proceed at much lower rates than that of a normal biogas production process. Placing the digester in the open would mean relatively high temperatures during the day and low temperature during the night. Such temperature variations do not auger well with the bacteria involved in the digestion process and could interfere with the gas production. Insulation was done on the digester to prevent major heat loss that would lead to temperature drops.
Quality of gas produced
An initial attempt to burn the gas did not produce any flame. This is because the gas produced was mostly CO2 emanating from aerobic digestion when the inoculum was fed for the first time. With time, the concentration of methane gas increased and it burned with a smooth pale blue flame.
Figure 9. Biogas burning
Reasons for Biogas failure
The process of production of biogas is sensitive to very many changes. The obvious reason for the decline in biogas production was due to inhibition of the bacteria responsible for the decomposition process. Although care was taken to ensure the amount of waste fed into the system was constant, a change in the type of food fed into the system interfered with the C: N ratio. Also, though we did not measure the pH, it is suspected that introduction of new food waste interfered with the same. Lastly, leaving the biogas outside during the day to boost the temperatures caused a big enough variation on the same that led to the failure of the same.
CHAPTER FIVEANALYSIS5.1 INTRODUCTION TO ANALYSISIn analysis, we have covered the following:
Analysis on the results from the experiment
Extrapolated the analysis from our results to give the broader scope of biogas production at Moi University
Based on the analysis carried out, design a biogas digester that best suits the available wastes at Moi University.
Finally, determine the energy equivalence of the biogas that can be produced from this design.
5.2 ANALYSIS OF THE RESULTS OBTAINED EXPERIMENTALLYAn analysis on the results is carried out to determine the amount of biogas that can be obtained from a known amount of kitchen waste.
From the experimental data, a volume of about 2060ml of biogas was produced within five days by adding 50 gram of waste each day. Using simple mathematical relationships, the amount of biogas that can be produced from 1kg of food waste can be determined based on the conditions in this area.
We have the following:
50 grams= 0.05 kg of food waste fed daily
Approximately, the amount of gas produced daily
= 2060÷5. This gives 412ml/day of biogas
If 50grams of waste we used daily, the approximate amount of gas that can be produced from 1kg of waste thus is
= 412÷0.05. This gives 8240ml/kg of biogas daily.
This is the same as 0.00824m3/kg of biogas produced each day.
Thus, we deduce that the amount of biogas that can be produced at the prevailing conditions around Kesses is approximately 0.00824ml/ kg of kitchen waste.
However, theoretically it has have shown that the value of biogas that can be produced from fully digester kitchen waste at optimal conditions is between 0.0105 to 0.015ml of biogas per kg of kitchen waste. However, in our case, we had to initiate the decomposition by use of cow dung which produces much lower amount of biogas when compared to kitchen waste. The temperatures here are also quite low. The efficiency of our digester can be estimated to be around:?=practical valuetheoretical value ×100?=0.008240.0105 ×100?=78.5%5.3 AN ANALYSIS TO COVER THE ENTIRE SCOPE OF OUR PROJECTThe results obtained experimentally are extrapolated to cover the entire scope of our project, which is Moi University. By determining the amount of food wastes that can be generated in the various hostels as well as the cafeteria at the institution, Gas Production Rate from all the kitchen wastes at the institution can be determined. Here is a tabulated data on the amount of wastes that can be collected from the various hostels and messes within the campus for utilization in biogas production.
Location Amount of digestible wastes generated in KG (daily)
Soweto Hostels 40
UPPER HILL 5
C HOUSES 10
D HOUSES 15
E HOUSES 10
F HOUSES 10
NGERIA MESS 15
SOWETO MESS 20
STUDENTS CENTRE MESS 30
Table 7. Estimated waste generation
With an approximated weight of about 200kg of kitchen waste been generated daily, a gas production rate (G) can be determined using the following equation.
=1.65m3 of gas produced daily
Based on this amount of gas produced daily, design of an appropriate biogas digester is given below that best suits the kitchen waste available.
DESIGN OF A BIOGAS DIGESTER BASED ON THE DATA ANALYSEDIn this section, choice of the best digester is given and calculations have been made as well to determine the dimensions that best suit a digester based on the analysis made in the preceding sections of this chapter.
5.4.1 CHOICE OF BIOGAS DIGESTERTo get the most suitable digester, it is important to take into consideration a number of factors.
•Utilization of Local Materials
•Type of Inputs
•Soil conditions and water table
•Gas consumption pattern of the average household
The size of the digester is also very critical in order to obtain maximum capacity utilization of the available biodegradable wastes.
For the kind of situation at hand, we resorted to recommend the fixed dome digester because of the following advantages:
Requires only locally and easily available materials for construction
It is relatively cheap to construct
Easy to construct
5.4.2 DESIGN OF THE DOME DIGESTERThe following equations have been used to determine the various dimensions of the dome digester. The equations used herein are standard equations that apply to biogas digester design and have been obtained from the Mathematical Modeling and Parameters Estimation of an Anaerobic Digestion in the International Journal of Energy and Environment (IJEE)
The various parameters to be determined are:
G (Gas production rate) Vs (Active slurry volume)
H (Height of cylindrical digester) D (Diameter of digester)
d ( slurry displacement) h (height of slurry displacement)
b (Breath of outlet rectangular tank) l (Length of outlet rectangular tank)
H (height of outlet rectangular tank) Dc (diameter of cylindrical inlet)
Hc (Height of cylindrical inlet) dh (Height of spherical dome)
r (Radius of spherical dome)
The gas production rate has already been determined previously as 1.65m3 of biogas.
To determine the other parameters:
Active slurry volume (Vs), the volume of digester which is filled by undigested feed (kitchen waste +water) is calculated by using following formula.
Vs=HRT×2W100050×2×1501000=15m2Calculation of Height & diameter of digester (H and D). In biogas digester the ratio of height and diameter are usually set as D =2H (Niangua, 2006) because it is directly proportional to the cost of the digester. As we know the volume of cylinder is found by following formula.
Vs=?4D2HThus, H=(Vs?)13=(15?)13=1.68H= 1.68m and the diameter D= 2H which is equal to 3.36m
Slurry displacement inside digester (d), the slurry displacement inside the digester depends upon the gas usage pattern during cooking. The pressure of the gas pushes the digested slurry down. Supposing three quarter of the gas produced is available for cooking which is done for three hours daily, we have the following:
First, get the variable gas storage volume (Vsd) as follows:
324G+Vsd=0.75GVsd=0.625GFrom this, d is calculated as:
?4D2d=Vsd=0.625Gd=0.625×1.65×4?×3.362=0.12m The slurry displacement height is given as:
Since the maximum pressure attained by the gas is equal to the pressure of the water (slurry) column above the lowest slurry level in the inlet/outlet tanks, this pressure is usually selected to be 0.85 m water gauge as a safe limit for brick.
I.e. h + d=0.85
Therefore, h= 0.85-0.12= 0.73m
Calculation of the dome height.
The gas collection space is known as Dome, so to find out the height of dome we have to determine the volume of dome Vd, which is a section of sphere given as:
Vd=?12dh2D22+dh2The total volume of the gas space, as mentioned earlier is taken as equal to G. As the slurry or gas volume Vsd is already fixed as 0.625G, the remaining gas space volume, which is the volume of the dome, will be equal to G-0.625G to give 0.375G
Thus:0.375G=?12dh23.3622+dh2 0.6188=?12dh5.645+dh22.3634=dh5.645+dh2dh=0.41mRadius of dome r: The radius is obtained by the equation:
r=D22+dh22×dh=3.3622+0.4122×0.41=3.65m The outlet tank. The outlet tank is rectangular with the following measurementsl×b×ho?VsdUsually, l is taken to be 1.5b. The equation becomes:
1.5b×b×ho?Vsdb=Vsd1.5h12=0.625×1.651.5×0.7312=0.97mThe length is given as 1.5b, which is 1.5×0.97 to give 1.455m
As indicated in the equation, the height of the outlet tank (ho) is taken to be 0.73m as well.
9. Inlet tank calculation: The inlet tank is set cylindrical in shape for this digester. This circular shape makes it easier to stir the slurry to ensure uniform mixing before going to the digester.
Volume of the intake tank is given as ?4Di2HiBoth a diameter and height of 0.75m are selected. This gives a volume that is bigger than the active slurry displacement.
All these parameters are as tabulated below
G (Gas production rate) 1.65cum/day
Vs (Active slurry volume) 15cum/day
H (Height of cylindrical digester) 1.68m
D (Diameter of digester) 3.36m
d ( slurry displacement) 0.12m
h (height of slurry displacement) 0.73m
b (Breath of outlet rectangular tank) 0.97m
l (Length of outlet rectangular tank) 1.455m
ho (height of the outlet rectangular tank) 0.73m
H (height of outlet rectangular tank) 0.5m
Dc (diameter of cylindrical inlet ) 0.75m
Hc (Height of cylindrical inlet) 0.75m
dh (Height of spherical dome) 0.41m
r (Radius of spherical dome) 3.65m
Table 8. Design parameters (as calculated above)
5.4.3 THE DESIGN DRAWINGHere is a technical drawing of the dome digester, representing both a sectional view as well as a plan for the same.
Figure 10. Design drawing of dome digester
5.5 ENERGY EQUIVALENCE THAT CAN BE DERIVED FROM THE BIOGAS DIGESTER DESIGNEDThe energy available in one month for a digester producing 1.65m3 of biogas daily is equivalent to:
The lower calorific value of the biogas is between 21300 to 23400KJ/m3, and is always a function of the percentage methane content.
Net energy in 1.65 m3 of biogas = 1.65 x 21.3
This gives an energy of 35.145MJ per day
Energy provided per month = 35.145 x 30
Energy provided per month = 1054.35 MJ
However, only three-quarter of the gas is available for cooking daily. (As indicated in the digester design calculations). This gives an effective energy consumption of 0.75 x 1054.35 to give energy equivalence of 790.76 MJ.
Energy available per month is thus 790.76 MJ.
Equivalence with LPG
Calorific value of LPG = 46.1 MJ/kg
One cylinder weighs 13 kg (the big size)
Therefore, net energy in 1 cylinder = 13× 46.1
The net energy in 1 cylinder = 593.3 MJ
With an energy of about 790.76 MJ per month using the biogas digester, it is equivalent to
=790.76593.3=1.33Therefore a biogas plant of 1.65 m3 capacity with an efficiency of 0.75 has an energy equivalence of 133% of an LPG cylinder per month.
Equivalence with Charcoal
Calorific value of charcoal is 29.6MJ/kg
In order to generate 790.76 MJ of energy per month, the amount of charcoal required is
=790.7629.6 =26.71 kg of charcoalUsing traditional kilns to convert wood into charcoal requires about 8-12kg of wood to produce 1kg of charcoal and have efficiencies as low as 12%. (From charcoal production-energypedia.info). The amount of firewood used is thus:
26.71×8=213.68kg of firewoodTherefore a biogas plant of 1.65 m3 capacity with an efficiency of 0.75 produces an energy equivalent to burning of 213.68 kg of firewood per month.
CHAPTER 6COST ANALYSISA: MATERIALS QUANTITY COST PER PIECE (KSH) TOTAL COST (KSH)
Cement 5 bags 900 4500
Bricks 1000 10 10000
Stone 0.5 tons 20,000 per ton 10000
Sand 1.5 tons 4000 per ton 6000
Aggregate 0.5 tons 15,000 per ton 7500
Binding wire 3kg 800 per kg 2400
Paint 3 liter 250 per liter 750
SUB TOTAL A 41,150
B: APPLIANCES Main gas valve 1 piece 1000 1000
Gas tap 2 pieces 800 1600
Water drain 1 piece 500 500
Rubber hose pipe 4 meters 100 per meter 400
Mixer 1 set 1000 1000
Pipes and fittings 5 sets 200 per set 1000
Pressure gauge 1 piece 1800 1800
Skilled labour (2) Days 1000 per day 2000
Unskilled labour (10) Days 350 per day 3500
SUB TOTAL B: 12,800
Table 9. Projected cost analysis of implementing the design suggested in chapter 5
CHAPTER SEVENCONCLUSION AND RECCOMENDATION7.1 CONCLUSIONThe concept of kitchen waste utilization in a fixed dome biogas plant for biogas Production offers effective Waste Management and Resource Development solutions with positive measures for the economy, improved air quality and sustained energy security. Moi University can therefore look into such a cost effective alternative to cooking using electricity and charcoal.
The results can help with designing new biogas systems that can be used in Moi University. However, the biogas volume should be increased by outsourcing food waste once it becomes operational. The technique is relatively unexplored and as such will need demonstration in other dense population areas to further the information on these systems and confirm the experiment’s conclusion. The overall outcome of this project if implemented is the optimization costs investment, operating and maintenance. This attainment allowed us to correct the calculation methods for future sites with more efficiency and effectiveness.
The biogas system will work normally if operated properly and well designed.
7.2 RECOMMENDATIONFollowing the compilation of this report, the following recommendations are given:
Adoption of biogas production and use by the institution should be embraced. Biogas produced can be used in the various cooking joints around the school, and more importantly within the school cafeteria. This will turn out to be economical at the long run when compared to the use of LPG as currently is. As the entire world moves towards cleaner energy, initiating the use of biogas at Moi University would signal the willingness to join others on saving our environment and adopting eco-friendly energy sources.
We also recommend the adoption of the digester design given in this report. We designed the dome digester based on the amount of kitchen waste that is produced at Moi University. Although the exact amount was impossible to determine, only slight changes are required on the dimensions made.
This report is also recommended on any particular individuals who have particular interest in the world of biogas production. It has covered exhaustively almost all areas regarding to biogas production. Nonetheless, further research can be carried out to determine the viability of biogas production in an institution such as Moi University among other areas regarding to biogas not covered herein.
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