ABSTRACT
The effect of torrefaction temperature and residence time on the fuel properties of lignite, biomass
(coconut shells and cassava peels) and their blends was investigated. The samples were subjected to
three torrecfaction temperatures (200, 260 and 300oC) and at two residence times (10 and 20
minutes) using programmable muffle furnace. Blends of torrefied lignite and biomass were prepared
in two different ratios (80:20 and 70:30). The energy content, proximate and ultimate analyses of the
samples were determined using ASTM methods. Scanning electron microscope (SEM) was used to
evaluate the pore size, fiber content, topography and the morphology of the samples. The potential
emissions of SO2, CO2 and NOx from the torrefied samples were evaluated using emission estimation
model for fossil fuel electric power generation. The proximate analysis showed that the ash (8.0 %)
and moisture (30.0 %) contents of lignite were higher than that of the biomass. The coconut shells
and cassava peels had higher volatile matter of 72.9 % and 68.1 % respectively and much lower
fixed carbon. The data showed that release of volatile matter decreased at severe torrefaction
condition. The content of fixed carbon and energy increased with the severity of the torrefaction
condition except for cassava peels which decreased at 300 oC. One–way analysis of variance on the
results of the proximate analysis showed that there was significant difference (P<0.05) between the
volatile matter, fixed carbon, energy and ash content of lignite, coconut shells and cassava peels, but
no significant difference between the moisture and solid yield. For the blends, volatile matter was
found to be higher than that of lignite alone. Increase of biomass ratio in the blends decreased the
carbon, nitrogen, oxygen and sulfur content of the
samples. Lignite/coconut shells (70:30) had better fuel properties compared to (80:20) and lignite/ca
ssava peels (at both ratios). Results of the ultimate analysis showed that after torrefaction there were
large reduction in oxygen and hydrogen content. However, 15 % carbon, 26 % nitrogen and
72 % sulfur was reduced from cassava peels while lignite recorded an increase of 40 % carbon,5
6 % nitrogen and 48 % sulfur after torrefaction. The SEM image showed that torrefied lignite had a
uniform and denser structure compared to the raw. The torrefied coconut shells showed a
destroyed and less fibrous structure than the raw while the torrefied cassava peels showed a smootsu
rface. The fiber length of lignite, coconut shells and cassava peels decreased after torrefaction.
Results of the emission potential showed that emissions of SO2,CO2 and NOX from lignite and
coconut shells increased after torrefaction, while cassava peels decreased. It was also found that b
lending biomass and lignite reduced emissions of SO2, CO2 and NOX from lignite. Torrefaction
improved the fuel properties of lignite and biomass such as heating value grindability, hydrophobicit
y, and uniformity. Blending the two fuels (lignite/biomass) provided a way to compensate the
negative effects of each other. Therefore, producers of power and heat should explore the use
of torrefied lignite, coconut shells, cassava peels and their blends as suitable fuels.
TABLE OF CONTENTS
TITLE PAGE – – – – – – – – – – -i
CERTIFICATION- – – – – – – – – – -ii
DEDICATION- – – – – – – – – – -iii
ACKNOWLEDGMENTS- – – – – – – – – – iv
TABLE OF CONTENTS- – – – – – – – – -v
LIST OF TABLES- – – – – – – – – – -xiii
LIST OF FIGURES- – – – – – – – – – – x
ABSTRACT- – – – – – – – – – – -xi
CHAPTER ONE: INTRODUCTION
1.1 Coal- – – – – – – – – – – -1
1.2 Coal Formation- – – – – – – – – – -1
1.3 Coal Combustion- – – – – – – – – -3
1.4 Environmental Impacts of Coal- – – – – – – -3
1.5 Biomass- – – – – – – – – – – -4
1.6 Justification of the Study- – – – – – – – -5
1.7 Objectives of the Study- – – – – – – – – -6
CHAPTER TWO: LITERATURE REVIEW
7
2.1 Coal as a Source of Energy- – – – – – – – -7
2.2 Coal Classification- – – – – – – – – -7
2.3 Physical and Chemical Properties of Coal – – – – – -9
2.4 Analysis of Coal- – – – – – – – – -9
2.4.1 Scanning Electron Microscope– – – – – – – -10
2.5 Reliance on Coal- – – – – – – – – – -10
2.6 Lignite as a Source of Energy- – – – – – – – -11
2.6.1 Emissions from Lignite- – – – – – – – -13
2.6.2 Lignite Treatment- – – – – – – – – -14
2.7 Biomass as a Source of Energy- – – – – – – – -15
2.7.1 Air Pollution from Biomass- – – – – – – – -17
2.7.2 Biomass Energy density- – – – – – – – -18
2.8 Biomass Torrefaction- – – – – – – – – -18
2.8.1 Basic Principle of Torrefaction- – – – – – – -23
2.8.2 Advantages of Torrefaction- – – – – – – – -24
2.8.3 Applications of Torrefied Biomass- – – – – – – -25
2.9 Biomass Co-Firing- – – – – – – – – -26
2.9.1 Biomass Co-firing and gaseous emissions – – – – – – -27
CHAPTER THREE: MATERIALS AND METHODS
3.1 Sampling /Sample Collection- – – – – – – – -29
8
3.2 Sample Preparation- – – – – – – – – -30
3.3 Proximate Analysis- – – – – – – – – -31
3.3.1 Moisture Content- – – – – – – – – -31
3.3.2 Volatile Matter- – – – – – – – – -31
3.3.3 Ash Content- – – – – – – – – – -32
3.3.4 Fixed Carbon- – – – – – – – – – -32
3.4. Calorific Value- – – – – – – – – -32
3.5 Ultimate Analysis- – – – – – – – – -33
3.6 Torrefaction- – – – – – – – – – -33
3.7 Scanning Electron Microscopic Test (SEM) – – – – – – -33
3.8 Data analysis- – – – – – – – – – -34
3.9 Potential Emission of Green House Gases from the Torrefied samples- – -34
CHAPTER FOUR: RESULTS AND DISCUSION
4.1 RESULTS- – – – – – – – – – -36
4.1.1 Proximate Analysis Results- – – – – – – – -36
4.1.2 Ultimate Analysis Results- – – – – – – – -39
4.1.3 Statistical Analysis Results- – – – – – – – -44
4.1.4 Comparisons of Significance- – – – – – – – -45
4.1.5 Results of the Potential Emissions of the Raw and Torrefied Samples- – -48
4.1.6 Scanning Electron Microscope (SEM) Results of samples- – – – -50
4.2 DISCUSSION- – – – – – – – – – -58
4.3 CONCLUSION- – – – – – – – – – -66
9
REFERENCES- – – – – – – – – – -68
APPENDIX- – – – – – – – – – -79
CHAPTER ONE
INTRODUCTION
1.2 Coal
One of the most important field of study in the realm of science and technology is that of
fuel because the whole of our world’s civilization is based upon an unceasing availability of
power. Coal (from the Old English term col, which meant “mineral of fossilized carbon” since the
13th century) [1] is a combustible black or brownish-black sedimentary rock usually occurring in
rock strata in layers or veins called coal beds or coal seams. It is one of the most important of the
primary fossil fuels and is composed primarily of carbon along with variable quantities of other
elements, chiefly hydrogen, sulfur, oxygen, and nitrogen [2].
Although fossil fuels have their origin in ancient biomass, they are not considered biomass
by the generally accepted definition because they contain carbon that has been “out” of the carbon
cycle for a very long time. Their combustion therefore disturbs the carbon dioxide content in the
atmosphere. Their structure varies based on their age and also the amount of pressure applied over
time.
Coal is the most abundant fuel in the fossil family [2]. United States has more coal reserves
than any other country in the world. In fact, one-fourth of all known coal in the world is in the
United States, with large deposits located in 38 states [2]. Like all fossil fuels, coal can be burned to
release energy.
1.2 Coal Formation
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Coal forms from the accumulation of plant debris usually in a swamp environment. When
plant dies and falls into the swamp, the standing water of the swamp protects it from decay.
Swamp waters are usually deficient in oxygen, which would react with the plant debris and cause it
to decay. This lack of oxygen allows the plant debris to persist. In addition, insects and other
organisms that might consume the plant debris on land do not survive well under water in an
oxygen deficient environment. To form the thick layer of plant debris required to produce a coal
seam, the rate of plant debris accumulation must be greater than the rate of decay. Once a thick
layer of plant debris is formed, it must be buried by sediments such as mud or sand. The weight of
these materials compacts the plant debris and aids in its transformation into coal. About ten feet of
plant debris will compact into just one foot of coal. Plant debris accumulates very
slowly. Therefore, accumulating ten feet of plant debris will take a long time. The fifty feet of
plant debris needed to make a five-foot thick coal seam would require thousands of years to
accumulate.
Due to the variety of materials buried over time in the creation of fossil fuels and the
length of time the coal was forming, several types were created. Depending upon its composition,
each type of coal burns differently and releases different types of emissions. The four types (or
“ranks”) of coal mined today are lignite, sub-bituminous, bituminous, and anthracite. Coal forms
when dead plant matter is converted into peat, which in turn is converted into lignite, then subbituminous
coal, bituminous coal and lastly anthracite. This involves biochemical and geological
processes (diagenesis and catagenesis respectively). The major methods of mining coals are
surface (opencast or open cut) mining, underground (deep) mining and underground gasification.
Lignite is a soft brownish-black coal; it forms the lowest rank of the coal family. It has
higher moisture and less carbon content than the higher rank coal. Nigeria has the largest deposit
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of lignite in Africa e.g. Garinmaiganga and Tai mines in Gombe state [3]. Sub-bituminous is a dull
black coal. It gives off a little more energy (heat) than lignite when burned. In Nigeria, subbituminous
coal is mined in Onekama and Okpara mine in Enugu state and Okaba mine in Benue
state. [4] Bituminous coal has more energy than sub-bituminous but Anthracite is the hardest coal
and gives off the greatest amount of heat upon combustion. Unfortunately, in Nigeria, as elsewhere
in the world, there is little anthracite coal to be mined.
1.3 Coal Combustion
Coal combustion is the burning of coal in the presence of oxygen. This aims at heat
(energy) generation. When the combustible materials such as carbon, hydrogen or compounds
containing these are ignited in the presence of air (oxygen), combustion takes place. The
combination of carbon, hydrogen and sulfur with oxygen may be expressed by the following
equations.
C + O2 CO2 ……………..… (1)
2H2 + O2 2H2O ………….….. (2)
S + O2 SO2…………….… (3)
Sulfur in coal burns off as gaseous sulfur which combines with oxygen to form SO2 and probably
SO3 on further oxidation. The combustion of coal releases several environmental pollutant such as
SO2, NO2, CO, CO2, and CH4
[5].
1.4 Environmental Impacts of Coal
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The environmental impact of coal includes issues such as land use, waste management,
water, and air pollution. Starting from coal mining, blasting, processing, transportation and use of
its products, SO2, NO2, CO, CO2, and CH4 are formed [6, 7]. These gases are hazardous to
health. They affect the vegetation and aquatics when diffused to streams, rivers and air [8-10]. In
addition to atmospheric pollution, coal combustion produces hundreds of millions of tonnes of
solid waste products annually, including fly ash, bottom ash, and flue-gas desulfurization
sludge [11-13].
1.5 Biomass
Biomass is a renewable energy source not only because its energy comes from the sun but
also because biomass can re-grow over a relatively short period of time. Through the process of
photosynthesis, chlorophyll in plants captures the sun’s energy by converting carbon dioxide from
the air and water from the ground into carbohydrates—complex compounds composed of carbon,
hydrogen, and oxygen [14]. When these carbohydrates are burnt, they turn back into carbon dioxide
and water and release the energy they captured from the sun [15-18]. In this way, biomass functions
as a sort of natural battery for storing solar energy. Biomass is a biological material derived from
living or recently living organisms. It often refers to plants or plant-based materials which are
specifically called lignocellulose biomass [19-24]. It refers to organic matter that has stored energy
through the process of photosynthesis [25, 26]. It exists in one form as plants and may be transferred
through the food chain to animal’s bodies and their wastes. All of which can be converted for
everyday human use through processes such as combustion, which releases the carbon dioxide
17
stored in the plant material [27, 28]. Many of the biomass fuels used today come in the form of wood
products, dried vegetation, crop residues and aquatic plants.
1.6 Justification of the Study
The importance of energy for a nation’s development cannot be overemphasized. This is
because energy is the cornerstone of economic and social development. In Nigeria, the energy
demand is high and is increasing geometrically while the supply remains inadequate. The energy
supply mix must thus be diversified through promoting and developing the abundant energy
resources present in the country to enhance the security of supply.
Coal which generates 40% of the world’s electricity has however been neglected for a long
time in Nigeria because the existed coal power production facilities degraded the environment
through pollution. Alternatively, co-firing biomass along with coal offers advantages but mostly
boilers are specifically designed for coals of certain ranks such as bituminous and subbituminous
coal. This is because bituminous and sub bituminous coal has higher carbon and lower
moisture contents compared with lignite. There are less similar ranking for lignite and biomass.
And since their physical properties are highly diverse, so are the costs for getting these fuels from
the field or into the boiler. Biomass and lignite have a relatively low-energy density and high
moisture content and as such tend to rot during storage. Biomass has the tendency to have a fibrous
nature that can make it difficult to grind into small particles. In order to successfully co-fire
biomass with lignite, both fuels need pretreatment to increase the heating value, hydrophobicity,
bulk density, stability during storage and grindability.
18
Torrefaction has been proposed as a method to improve biomass`s properties for
gasification and combustion. The purpose of this research is to evaluate the impact of such a
treatment on the fuel properties of lignite, cassava peels and coconut shell.
1.7 Objectives of the Study
The objectives of this study are as follows:
1. To produce suitable torrefied biomass that can improve the fuel properties of lignite.
2. To investigate the effect of torrefaction temperature and residence time on the physical and
chemical properties of lignite, coconut shells and cassava peels.
3. To investigate the effect of torrefaction on the pore size, fiber content, topography and
the morphology structure of lignite, coconut shells and cassava peels.
4. To determine the influence of blend ratio on the fuel properties of the torrefied lignite, coconut
shells and cassava peels.
5. To investigate the effect of torrefaction on the possible reduction of pollutant elements from the
lignite, coconut shells and cassava peels.
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