ABSTRACT
Biogas generation is accomplished by anaerobic digestion of
biodegradable material in a biogas digester. Anaerobic digestion as a process
requires dynamic model for predicting process performance. Mathematical
model for biogas production from anaerobic digesters described in this study
was used to predict biogas generation using experimental data. Multiple
regression analysis was used in analyzing two sets of data, the first sets of
data contains four different experimental results of biogas yield in which the
biogas production rate was determined weekly. The second sets of data
containing four different experimental results had biogas production rate
determined daily, while the first data sets were used as a curve fitting model,
the second sets were used for the verification of the model derived. The
predicted values of biogas yield were close to the measured values with a
maximum correlation coefficient, R = 0.91.
It was further showed that the regression constants were dependent on
temperature only. Hence, the predictive capability of the model can be
improved by making those regression constants dependent on temperature.
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TABLE OF CONTENTS
Certification ————————————————————————i
Dedication ————————————————————————–ii
Acknowledgement —————————————————————-iii
Abstract —————————————————————————–iv
Table of contents ——————————————————————v
CHAPTER ONE: INTRODUCTION ————————————— 1
1.1 Background of study —————————————————— 3
1.2 Research Problem ——————————————————— 3
1.3 Significant of study——————————————————– 4
1.4 Research Objectives ——————————————————4
CHAPTER TWO: LITERATURE REVIEW
2.1 The history of biogas technology——————————————-6
2.2 The need for a biogas system ———————————————–8
2.3 Biogas production using Biogradable substances ———————– 10
2.3.1 Pretreatment of water Hyacinth to accelerate its Biodigestivity into
biogas ———————————————————————— 10
2.3.2 Biogas production using water hyacinth ——————————– 11
2.3.3 Biogas production from blends of cassava (MANIHOT UTILISSIMA)
peels with some animal wastes —————————————–12
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2.3.4 Biogas production from waste using biofilm reactor:
Factor analysis in two stages system ———————————– 13
2.3.5 Biodegradation of distillery spent wash in anaerobic
Hybrid reactor ————————————————————-14
2.4 Anaerobic digestion ———————————————————- 15
2.4.1 Substrate qualities for anaerobic digestion —————————– 16
2.4.2 Digestion environment—————————————————– 17
2.4.3 Batch digesters ————————————————————– 18
2.4.4 Continuous digester ——————————————————– 18
2.4.5 Semi-batch digester ——————————————————–19
2.5 Biogas digester feed stocks————————————————–19
2.5.1 Feeding the digester ——————————————————–21
2.5.2 Types of biogas digester—————————————————23
2.5.2.1 Complete mix digester—————————————————23
2.5.2.2 Plug flow digester ——————————————————–24
2.5.2.3 Lagoon ——————————————————————— 24
2.6 The law of Biogas production———————————————–25
2.7 Factors that affect biogas production ————————————– 28
2.7.1 Digester operating parameters ——————————————–28
2.7.2 Seeding and start-up procedure——————————————-28
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2.7.3 Nutrient balance————————————————————- 29
2.7.4 Solid contents —————————————————————31
2.7.5 Organic loading ————————————————————- 31
2.7.6 Retention time—————————————————————33
2.7.7 Volatile acid concentration———————————————— 36
2.7.8 Stirring or Mixing of Digester content ———————————- 37
2.7.9 Inhibition and toxicity —————————————————– 38
2.7.10 Temperature—————————————————————- 39
2.7.11 pH and Alkalinity ——————————————————— 42
2.7.12 Solid-water ratio———————————————————–44
2.7.13 Quality and characteristics of waste material————————-44
2.7.14 Loading rate—————————————————————- 44
2.7.15 Carbon nitrogen ratio —————————————————–45
2.8 Kinetics of biogas production———————————————–45
2.8.1 Design models—————————————————————47
2.8.2 Stoichiometric models —————————————————– 51
2.8.3 Model trend—————————————————————— 57
2.9 Maximum likelihood ——————————————————— 60
2.10 Method of moments estimation ——————————————- 63
2.11 Regression ——————————————————————– 64
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2.11.1 Multiple linear regression———————————————— 64
CHAPTER THREE: METHODOLOGY
3.1 Source of data —————————————————————–68
3.2 Use of statistical tools for analysis —————————————–68
3.2.1 Multiple regression analysis ———————————————-68
3.2.2 Relationship between regression constants and design variables — 70
3.3 Importance to design ——————————————————— 71
3.4 Model performance ———————————————————–71
CHAPTER FOUR: RESULTS AND DISCUSSION
4.1 Analysis ————————————————————————73
4.2 Verification of the relationships ——————————————– 79
CHAPTER FIVE: CONCLUSION AND RECOMMENDATION
5.1 Conclusion ——————————————————————— 81
5.2 Recommendation————————————————————– 81
References ————————————————————————–82-89
Appendix————————————————————————-90-103
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CHAPTER ONE
INTRODUCTION
1.0 BACKGROUND
Nature does not produce any wastes. All by – products and final
products of natural processes are used in a continuous cycle of the
composition and mineralization of organic substances. The biosphere has a
high buffer potential giving it a wide tolerance range for all natural products
and processes.
Only with the growth of human population and its economical
activities that wastes become a serious danger to the steady – state of the
natural metabolic processes. With the continuous growth of the cities and the
concentration of an increasing part of the population in municipal areas, a
solution of the waste problem becomes more and more inevitable. While a
big part of the inorganic wastes – like glass, plastics, metals etc – meanwhile
are recycled, the biggest part of the organic waste fraction is still simply put
on waste disposal sites. The uncontrolled decomposition of these materials
adds another stress factor to our endangered environment. The partially
anaerobic conditions cause gaseous emissions of carbon dioxide, ammonia
and methane to the atmosphere, while the products of the mineralization
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processes contaminate the ground water with phosphates, nitrates and other
mineral salts, thus poisoning the basic resources of human life.
On the other hand, organic wastes – contain significant energy
potentials as well as valuable plant nutrients and the capability of improving
and conserving agricultural soils. For these reasons, efforts have been taken
during the last decades on the development of waste processing technologies
which are ecologically safe and, at the same time, make use of the valuable
components and characteristics of the material. Technologies which have
been developed for the large-scale processing of organic wastes are the
composting and the anaerobic fermentation. Both methods have their
specific advantages and disadvantages. The decision for one of these
technologies can only be taken with regard to infrastructural technical, and
environmental conditions of the particular area.
The method employed by the developing countries like Nigeria in
disposing or treatment of their waste have brought more harm than good in
their environment and the world at large. Apart from the global warming and
harmful toxins that are being released by man made machinery, the
dependence on wood as a source of energy is on the increase and is really
affecting the natural reserves of forest and desert encroachment.
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Biogas has many advantages as a replacement for petrol, diesel and
the generation of light. Biogas gives the smallest emissions of carbon
dioxide and particulate matter of all vehicle fuels in the market today. The
methane molecule is the simplest of hydrocarbons, which means that the
exhaust produced by combustion is very clean.
The biogas reactor constants can be estimated using the multiple
regression analysis. The estimated yield of gas can be calculated and
compared with the measured yield of gas.
1.1 BACKGROUND OF THE STUDY
Biogas process has been used for long in some part of the country, but
only in small quantity. Though the development technology is still in its
embryonic stage, there were few large scale biogas plant of 1800m3 capacity
in 1996 at the energy research centre of Nigeria (ECN)(Sambo, 1992).
However, its potential is promising, is the energy research centre
Sokoto in Nigeria putting more effort to create awareness to our local farmer
on the construction and use of Biogas.
1.2 RESEARCH PROBLEM
Around the world, pollution of the air, water and soil from municipal,
industrial and agricultural operations continues to grow. Government,
industries, organizations and individuals are constantly searching for
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technologies that will allow for more efficient and cost-effective waste
treatment and disposal.
One technology that can successfully treat the organic fraction of wastes
(biogenic wastes) is indeed anaerobic digestion through fermentation. The
fermentation provides pollution prevention and also allows for sustainable
energy, water, fertilizer and nutrient recovery. Thus, this technology can
convert a disposal problem into a profit centre, and provides a realistic
solution to environmental, social and economical problems for developed and
developing nations.
1.3 SIGNIFICANT OF THE STUDY
The importance of the study are:
· To provide the best method for estimation of yield;
· Several existing models do not predict measured values well, therefore
the need for better predictive modes; and
· No kinetic equations available for design will help to generate the
equations for design of this particular design.
1.4 OBJECTIVES OF THE STUDY
· Determine the equation for yield estimation using multiple regression
analysis;
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· Investigate relationships between regression constants and design
parameters;
· Compare the results using data in the literature;
· Apply the derived relationships in design; and
· Compare the predicted design parameters from the relationship with
measured data.
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