ABSTRACT
This work studied the effect of groundnut shell and maize cob on coal
briquette. The ratio of coal: biomass prepared were 90:10, 85:15, 80:20,
75:25, 70:30, 100:0. The mixture was treated with Ca(OH) which serves
as a desulphurizing agent, before briquetting. The chemical analysis
carried out on the raw materials (i.e. groundnut shell, maize cob and
coal) indicated the presence of Ca, Mg, Al, Na, Fe, Cu, K, Zn, Mn, Pb,
Ni, Cr, As, S. The proximate analysis of the raw materials was also
carried out. Burning and viability tests carried out revealed that maize
cob-coal briquettes ignite and burn faster, smoke less, produce flame
and small quantity of ash after burning, than the other briquettes.
Hardness compressive strength and density test of the briquettes
produced showed that coal briquette has better hardness, compressive
strength, and density results than the other briquettes. Also, the bio-coal
briquette with the highest percentage of biomass (i.e. 30%) gave the
best viability, burning, porosity, porosity index, ash content, calorific
value results than the other briquettes. However, maize cob-coal
briquettes gave the best results compared to the groundnut shell-coal
briquettes and the coal briquette which was used as the standard.
TABLE OF CONTENTS
Title page………………………………………………………………… i
Certification page………………………………………………………… ii
Dedication ………………………………………………………………… iii
Acknowledgements…………………………………………………… iv
Abstract…………………………………………………………………… v
Table of contents………………………………………………………. vi
List of tables………………………………………………………………. x
List of figures……………………………………………………………… xi
List of plates………………………………………………………………. xii
CHAPTER ONE
1.0 INTRODUCTION…………………………………………………… 1
1.1 Coal …………………………………………………………………. 3
1.1.1 Concept of coal…………………………………………………….. 3
1.1.2 Uses of coal…………………………………………………………. 8
1.1.3 Coal as an alternative energy resources ………………………… 11
1.1.4 Nigerians’ overdependence on oil and gas………………………. 12
1.1.5 A forecast of coal demand in Nigeria……………………………… 12
1.1.6 Environmental issues………………………………………………. 13
1.1.7 Coal Analysis………………………………………………………… 14
1.2 Briquetting technology………………………………………………… 15
1.2.1. Advantages of briquette production ……………………………… 16
1.2.2. Types of briquettes ………………………………………………… 16
1.2.2.1 Coal briquetting ………………………………………………….. 17
1.2.2.2 Biomass briquetting ……………………………………………… 22
1.2.2.3 Bio-coal briquetting………………………………………………… 23
1.2.2.3.1 Characteristics of bio-coal briquettes……………………….. 23
vii
1.2.2.3.2 Advantages of bio-coal briquettes…………………………… 25
1.2.2.3.3 Bio-coal briquetting technology……………………………….. 25
1.2.3. Biomass as a feedstock for the production of bio-coal briquette 29
1.2.4 Groundnut shell as an appropriate residue for the production
of bio-coal briquettes……………………………………………… 42
1.2.4.1 Analyses of groundnut shell……………………………………… 43
1.2.4.2. Uses of groundnut shell…………………………………………. 44
1.2.5 Maize/Corn cob as an appropriate residue for the production of
bio- coal
briquettes……………………………………………………….45
1.2.5.1. Analyses of corn cob………………………………………………46
1.2.5.2. Uses of corn cob……………………………………………………47
1.2.6. Binders used in the production of bio-coal briquettes……………47
1.2.6.1 Starches as a binder for the production of bio-coal briquette….48
1.2.6.1.1 Other applications of starch……………………………………..50
1.2.7 Burning process of bio-coal briquette …………………………….51
1.2.8 Characteristics of a good fuel (bio-coal briquette),……………….52
1.3 The aim of the research………………………………………………54
CHAPTER TWO
2.0 EXPERIMENTALS:……………………………………………………55
2.1 Materials and methods. ………………………………………………55
2.1.1 Materials and their sources…………………………………………..55
2.1.2 Apparatus used for the experiment. …………………………………55
2.1.3 Reagents used for the experiment…………………………………..56
2.1.4 Preparation of materials ……………………………………………..57
2.2 Characterization of the raw materials…………………………………58
2.2.1 Determination of the colour and texture of the raw materials…….58
2.2.2 Determination of chemical composition of the raw materials…….58
2.2.3 Proximate analysis of the raw materials…………………………..59
viii
2.2.3.1 Determination of the moisture content of the raw materials……59
2.2.3.2 Determination of the volatile matter in the raw materials……….60
2.2.3.3 Determination of the ash content of the raw materials………….60
2.2.3.4 Determination of the carbon content of the raw materials………61
2.2.3.5 Determination of calorific value of the raw materials ……………61
2.2.3.6 Determination of the fixed carbon content of the raw materials.63
2.3 Bio-coal briquette formulation …………………………………….63
2.4 Characterization of the bio-coal briquette samples…………………65
2.4.1 Determination of the calorific value of the briquette samples……65
2.4.2 Determination of the moisture content of the briquette samples..67
2.4.3 Determination of the ash content of the briquette samples………68
2.4.4 Determination of the porosity of the briquette samples…………..68
2.4.5 Determination of the porosity index of the briquette samples……69
2.4.6 Determination of the density of the briquette samples……………69
2.4.7 Determination of the hardness of the briquette samples…………69
2.4.8 Determination of the compressive strength of the briquette
samples………………………………………………………………..70
2.4.9 Water boiling tests of the briquette samples………………………72
2.4.10 Viability tests of the briquette samples …………………………..72
CHAPTER THREE
3.0 RESULTS AND DISCUSSION………………………………………73
3.1 Colour and texture of the materials………………………………….73
3.2 Chemical composition of the materials…………………………….73
3.3 Analysis of the materials…………………………………………….76
3.4 The effect of biomass on the production of briquettes …………..79
3.5 Characterization of the briquette samples…………………………80
3.5.1 Effect of biomass on the calorific value of the briquette samples.80
3.5.2 Effect of biomass on the moisture content of
the briquette samples………………………………………………82
ix
3.5.3 Effect of biomass on the ash content of the briquette samples….83
3.5.4 Effect of biomass on the porosity of the briquette samples…….85
3.5.5 Effect of biomass on the porosity index of the
briquette samples…………………………………………………..87
3.5.6 Effect of biomass on the density of the briquette samples……..89
3.5.7 Effect of biomass on the hardness of the briquette samples…..90
3.5.8 Effect of biomass on the compressive strength of the
briquette samples…………………………………………………92
3.5.9 Effect of biomass on the water boiling test of the
briquette samples……………………………………………………94
3.5.10 Effect of biomass on the viability tests of the briquette samples.95
3.6 Cost analysis……………………………………………………………99
3.7 Conclusion …………………………………………………………….100
3.8 Recommendation……………………………………………………..101
References…………………………………………………………………103
CHAPTER ONE
INTRODUCTION:
Energy is the ability to do work. Sources of energy include
electricity, petroleum, nuclear power, solar energy, tar sand, burning of
coal, wood and biomass, etc. Nigeria is blessed with abundant energy
resources: oil, gas, coal, wood, biomass, solar, wind, nuclear and
hydropower.
Energy availability in Nigeria and its supply has been a source of
constant friction between the people and the government. This however,
should not be so because, among the abundant energy resources
available in Nigeria, only oil and gas sector have so far been well
developed. The industrial and domestic sectors of the Nigerian economy
continue to suffer from perennial shortage of energy. This shortage has
led to accurate energy crisis at the household level. The bulk of the
energy used for cooking at the household level in Nigeria is mainly
derived from wood fuel and fossil fuel (kerosene).
The fossil fuels are produced and delivered at a cost most
Nigerians cannot afford. As a result, a greater percentage of the ever
growing population of the country have resorted to depend on the
country’s forest waste as a source of fuel for agricultural, domestic and
small-scale industrial activities in semi urban and rural areas. The use of
wood fuel encourages cutting down/felling of trees (deforestation). This
leads to desertification in the Northern part of Nigeria; and flooding, soil
erosion and loss of top soil fertility in the Southern part of Nigeria. In
some cases, it can lead to extinction of wild life.
Energy is the key factor in economic development in most
countries today. In Nigeria, there is overdependence on oil and gas for
energy for industrial and domestic purposes, since it is the only source of
2
energy that is well developed. Hence, there is need to develop the other
sources of energy so that energy supply will be enough and affordable
for industrial and domestic purposes, and our oil and gas be conserved
(and used for transportation). Most advanced countries today are
adapting the concept of preserving and also retaining their natural
resources. As the world adjusts itself to the new millennium and its
technology, the demand for fuel and energy increases, therefore, it
should be conserved.
Of all the available energy resources in Nigeria, coal and coal
derivatives such as smokeless coal briquettes, bio-coal briquettes, and
biomass briquettes have been shown to have the highest potential for
use as suitable alternative to coal/wood fuel in industrial boiler and brick
kiln for thermal application and domestic purposes, therefore, it will serve
as the most direct and effective method of combating deforestation in the
country. Coal and biomass are available, and cheap.
There is a worldwide acceptance of briquettes and growing
demand for the briquetting plants. In June 2009, a workshop on
“Investment Potentials of the Nigerian Coal Industry” was organized by
the Nigerian Coal Coporation. It was clear from the workshop that
substantial progress has been made in briquetting technology and
practice in recent years.
In countries like Japan, China and India, it was observed that
agricultural waste (agro residues) can also be briquetted and used as
substitute for wood fuel. Every year, millions of tons of agricultural waste
are generated. These are either not used or burnt inefficiently in their
loose form causing air pollution to the environment. The major residues
are rice husk, corn cob, coconut shell, jute stick, groundnut shell, cotton
stalk, etc. These wastes provide energy by converting into high-density
fuel briquettes. These briquettes are very cheap, even cheaper than coal
3
briquettes. Adoption of briquette technology will not only create a safe
and hygienic way of disposing the waste, but turn into a cash rich
venture by converting waste into energy and also contributing towards a
better environment.
Coal can be blended with a small quantity of these agricultural
waste (agro residues) to produce briquettes (bio-coal briquettes) which
ignites fast, burn efficiently, producing little or no smoke and are cheaper
than coal briquettes.
Briquetting technology is yet to get a strong foothold in developing
countries including Nigeria, because of the technical constrains involved
and lack of knowledge to adopt the technology to suit local conditions.
Overcoming the many operational problems associated with this
technology and ensuring the quantity of the raw material used are crucial
factors in determining its commercial success. In addition to this
commercial aspect, this technology encourages conservation of wood.
Hence, briquette production technology can prevent flooding and serve
as a global warming countermeasure through the conservation of forest
resources.
1.1 Coal:
1.1.1 Concept of coal:
Coal is a carbon containing, combustible solid, usually stratified which
is formed by debris from the decay of ferns, vines, trees and other plants
which flourished in swamps millions of years ago. Over time, the debris
became buried and the actions of bacteria, heat, and pressure
transformed the debris first into peat (a precursor of coal) and then into
the various types of coal .This process of transformation is referred to as
metamorphosis, coalition or lithification. Coal is composed chiefly of
carbon, hydrogen, oxygen, with a minor amount of nitrogen and sulphur,
and varying amounts of moisture and mineral impurities such as
4
phosphorus. Coal lumps are black or dark brown in colour, its colour,
luster, texture, etc vary with the type, rank and grade [1]
Classification of coal:
There are four main classifications of coal, arising from progressive
variation in their carbon content.
i. Peat: contains about 60% carbon.
ii. Lignite coal: contains about 65% carbon.
iii. Sub bituminous coal: contains about 70% carbon.
iv. Bituminous coal: contains about 85% carbon.
v. Anthracite coal: contains about 94% carbon [2].
Destructive distillation of coal: This involves heating coal to a very high temperature (600-12000C) in
the absence of air. During this process, the coal decomposes to give
coal gas, coal tar, coke and ammoniacal liquor.
Coal heat coke + coal tar + coal gas + ammoniacal liquor. i. Coal gas: This is a mixture of hydrogen, carbon(iv) oxide and small
amount of ethane, hydrogen sulphide, and sulphur (iv) oxide. The
main use of coal gas is as fuel.
ii. Coal tar: A thick brownish-black liquid is a mixture of many organic
chemicals including benzene, toluene, phenol, naphthalene and
anthracene. The component can be separated by fractional
distillation and are used for the manufacture of commercial products
including drugs, dyes, paints, insecticides , etc.
iii. Coke: This is non-volatile residue which contains about 90%
amorphous carbon and is chemically similar to hard coal. Coke is
used in the manufacture of carbide as fuel and as reducing agent in
the extraction of metals. Coke is used to make producer gas and
water gas.
5
iv. Ammoniacal liquor: This is an aqueous solution containing mainly
ammonia, and is used in the manufacture of ammonium
tetraoxosulphate (iv) [3].
Coal mining in Nigeria:
Coal was first discovered in Nigeria in 1909 near Udi by the mineral
survey of Southern Nigeria. Between 1909 and 1913, more coal
outcrops were located. Coal is found in the following Nigerian States:
Enugu, Imo, Kogi, Delta, Plateau, Abia , Benue ,Edo, Bauchi, Adamawa,
Gombe, Cross River States. In 1950, the Nigerian Coal Corporation
(NCC) was formed and given the responsibility for exploration,
development and mining of the coal resources [4].
Nigerian coal resources:
Nigerian coal resources has been found suitable for boiler fuel,
production of high calorific gas, domestic heating, briquettes, formed
coke, and the manufacture of a wide range of chemicals including
waxes, resins, adhesives and dyes. The characteristic properties of
Nigerian coal (low sulfur, and ash content and low thermoplastic
properties) make these sub-bituminous coals ideal for coal-fired electric
power plant [4].
Coal deposits of Nigeria:
Coal exploration in Nigeria started as far back as 1916. Available
data show that coal (mainly sub-bituminous steam coals except for the
Lafi-obi bituminous coking coal) occurrences in Nigeria have been
indicated in more than 22 coal field spread over 13 States of the
Federation. The proven coal reserves so far in Nigeria total about 639
million metric tones while the inferred reserves sum up to 2.75 billion
metric tones. In addition, an estimated 400 million tones of coal lie
untapped under the soil of Enugu. [5]
6
Presently, the Nigeria coal industry has four existing mines at Okpara
and Onyeama underground mines in Enugu State, Okaba surface mine
in Kogi State and Owukpa underground mine in Benue state. In addition,
there are more than 13 undeveloped coal fields. The undeveloped coal
fields in Nigeria are of two categories:
The virgin coal fields where further detailed exploration work and/or
access roadways are required and the developing coal fields where
reserved have been proven and mine access roadways developed. The
developed coal fields include Azagba Lignite field in Delta State,
Ogboyoga coal field in Kogi State, Ezimo coal field in Enugu State, Lafi
obi coal field in Nassarawa State and Inyi coal field in Enugu State while
others are located in Amansiodo in Enugu state, Ute in Ondo State,
Lamja area of Adamawa State, Gindi-Akunti in Plateau State, Afuze in
Edo State, Janata-Koji area of Kwara State and extension of Okpara
mine south in Enugu State [6].
Table1: EXISTING POTENTIAL COAL MINE SITES WITH RESERVES IN
NIGERIA [6]
S/N
Mine
location
State Type of
coal
Estimated
reserves
(million
tonnes)
Proven
reserves
(million
tonne)
Depth of
coal (m)
Mining
Method
1 Okpara mine
Enugu Sub
bituminous
100
24
180
Underground
2 Onyeama mine
Enugu Sub
bituminous
150
40
180
Underground
3 Ihioma Imo Lignite 40
NA
20-80
Open cast
4 Ogboyoga Kogi Sub bituminous
427
107
20-100
Open cast/
underground
5 Ogwashi Azagba
Delta Lignite
250
63
15-100
Open cast/
underground
7
6 Ezimo Enugu Sub bituminous
156
56
30-45
Open cast/
underground
7 Inyi Enugu Sub bituminous
50
20
25-78
Open cast/
underground
8 Lafia/obi Nassarawa Bituminous 156
21-42
80
Underground
9 Nnewi / Ota
Anambra Lignite 30
NA
18-38
Underground
10 Amasiodo Enugu Bituminous 1000 NA 563 Underground
11 Afikpo/ Okigwe
Ebonyi/ Imo Sub
bituminous
50
N.A
20-100
Underground
12 Okaba Kogi Sub bituminous
250
3
20-100
Underground
13 Owukpa Benue Sub bituminous
75
57
10-100
Opencast/
underground
14 Ogugu/ Agwu
Enugu Sub
bituminous
NA
NA
NA
Underground
15 Afuji Edo Sub bituminous
NA
NA
NA
Underground
16 Ute Ondo Sub bituminous
NA
NA
NA
Underground
17 Doho Bauchi Sub bituminous
NA
NA
NA
Underground
18 KurumuPindosa
Bauchi Sub
bituminous
NA
NA
NA
Underground
19 Garin Maigunga
Bauchi Sub
bituminous
NA
NA
NA
Underground
20 Lamja Adamawa Sub bituminous
NA
NA
NA
Underground
21 Janata koji Kwara Sub bituminous
NA
NA
NA
Underground
22 Gindi akwati
Plateau Sub
bituminous
NA
NA
NA
Underground
N/B: NA= Not available.
8
1.1.2 Uses of coal:
(a). Cement production: Coal is used for cement manufacture. In
Nigeria, Okaba, Ogboyoga and Owukpa coals are suitable for cement
manufacturing. Their physical properties qualify them for the purpose [6]
(b). Power generation: Coal is one of the two most principal sources of
fuel and energy, the other being petroleum [7]
Power plays a central and crucial role in national development.
Nigeria‘s power supply falls far short of demand. This inadequacy
represents a major constraint on industrial growth, and underscores the
need to make electricity more widely available, and more specifically in
the rural areas. This is in order to encourage the development of cottage
industries in the countryside, ameliorate the living conditions of the rural
dwellers and thus reduce the incidence of flight to the cities in search of
gainful employment, especially by the youth and trained man power. To
ensure a regular and dependable supply of the requisite amount of
power in the country, coal can be used for power generation [8]. In
Nigeria, Okaba, Ogboyoga, and Onyeama coals are suitable for power
generation. Their physical properties which include high calorific value,
low sulphur content (about 0.69%), low ash, low moisture, and high
volatility qualify them for this purpose [6].
(c). Metallurgical purposes: The most important non fuel use of coal is in
smelting of iron ores. The main process for iron production from its ore is
still the blast furnace. The blast furnace process of making iron and steel
employs coke (the solid product from coal carbonization) as a major raw
material [7]. Not all coal can yield the type of coke (metallurgical coke)
that can be utilized in a blast furnace. The Nigerian coals are generally
non–coking, and hence, the coals derived from there are not directly
utilizable in blast furnace [8]. Onyeama mine and Okpara West area
9
mine coals are suitable as the process require very high temperatures
[6].
(d). Coal for export: Onyeama mine and Okpara mine coals have been
mapped out mainly for export.
(e). Industrial fuel: Coke char will also find widespread use in a variety of
industrial enterprises such as cement factories, foundries, ceramics
plants, bakeries, laundries and brick manufacture. Because of the
unreliability of electric supply, coke char and solid briquettes could also
be effectively deployed as non-polluting prime energy resources by rural
cottage industries [8].
(f). Coal is also used in making chemicals: For instance, the solvent
extraction studies of Enugu coals using benzene /methanol
(C6H6/CH3OH) as the extracting solvent system, it was possible to
fractionate the extract into pre-asphaltenes (benzene insoluble, pyridine
soluble), asphaltene (n-hexane insoluble) and oils (n-hexane solubles)
and determine the n-paraffin content of the oils by urea adduction
technique. Also, montan wax has been obtained from brown coal by
solvent extraction. The waxes have immense industrial uses in candle
making, waxing paper, medicinal and cosmetics preparation among
others [9].
Coal Chemicals:-
For about 100 yrs, chemicals obtained as by-product in the primary
processing of coal to metallurgical coke have been the main source of
aromatic compounds used as intermediates in the synthesis of dyes,
drugs, antiseptics and solvent. Although some aromatic hydrocarbons
such as toluene and xylene are now obtained largely from petroleum
refineries, the main sources of others such as benzene, naphthalene,
anthracene, and phenanthrene is still the by-product of coke oven.
10
Heterocyclic nitrogen compounds such as pyridines and quinolines are
also obtained largely from coal tar.
Table 2: Coal tar chemicals:
Compound Use
Naphthalene Phthalic acid
Acenaphthenes Dye intermediates
Fluorene Organic synthesis
Phenanthrene Dyes, explosives
Anthracene Dye intermediates
Carbazole and other similar compounds Dye intermediates
Phenol Plastics
Cresols and xylenols Antiseptics
Pyridine, picolines, Intidines, quinolines, Drugs, dyes,antioxidants
acridine, and other tar bases.
Coal can also be converted to liquid fuels by:
a. Fischer Tropsch process:- Here, coal is heated in the presence of steam to a temperature of 12000C to give water gas.
C+ H2O 12000C CO + H2
b. Bergius process: Here, coal is heated in the presence of hydrogen to the temperature of 4500C and pressure of 200 atm to give
gasoline.
C + H2 4500C/ 200 atm gasoline
The use of a particular coal depends on its rank (i.e. peat, lignite,
bituminious, anthracite). The diagram below provides the estimated
percentage of the world’s coal reserves for each coal rank and also the
use of each coal rank.
11
% of world resources
Carbon and heating value high
High moisture content
Low rank coal (47%) Hard coal (53%)
Lignite (17%) sub bituminious (30%) anthracite (1%)
Bituminious (52%)
Largely power generation domestic industries
Power generation, cement manufacture, industrial uses .
thermal steam coal metallurgical coking coal
power generation, cement manufacture of iron steel manufacture, industrial uses.
Fig 1: Diagram of the typical uses and the estimated percentage of
the worlds’ coal reserves for each coal rank [10].
1.1.3 Coal as an alternative energy resource:
The great exploitation of fossil fuel began with the industrial
revolution, about two centuries ago. The newly built steam consumed
large quantity of fuel, but in England, where the revolution began, wood
was no longer readily available. Most of the forest has already been cut
down. Coal turned out to be an even better energy source than wood
because it yields more heat per gram. This difference in heat of
combustion is a consequence of differences in chemical composition.
When wood or coal burns, a major energy source is the conversion of
carbon to carbon (iv) oxide. Coal is a better fuel than wood because it
contains a high percentage of carbon and low percentage of oxygen and
water. Although coal is not a single compound, it can be approximated
by the chemical formula C135H96O19NS. This formula corresponds to a
carbon content of 85% by mass [11].
12
The exploitation of coal for energy (electricity) generation and the
production of bio-coal briquettes for domestic and industrial heating will
[12,13]:
i. Provide a more reliable energy (electricity supply),
ii. lower the cost of electrical supply,
iii. expand industrialization of the economy,
iv. increase employment and human recourses development,
v. increase capacity utilization of existing industry,
vi. increase national income through taxes,
vii. reduce deforestation and prevent desert encroachment in the
Northern part of the country.
1.1.4 Nigerians’ overdependence on oil and gas:
At the peak of its importance, coal was a major article of world trade
because it was the source of fuel for industrial and domestic purposes. It
was used in steam engine to generate power to drive ships, railway
locomotives and industrial machines.
Petroleum was discovered in commercial quantity at Oloibori in
Rivers state in the year 1956. Since the inception of petroleum in
Nigeria, the use of coal for electricity generation, cooking and for heating
up houses in the cold period to create warmth has long been neglected
inspite of its abundance in the country, because of the overdependence
on oil and gas. This results to constant failure in power supply, political
and economical instability due to insufficient and increase in price of
petroleum product [14].
1.1.5 A forecast of coal demand in Nigeria:
Presently in Nigeria, coal is not in demand. Infact, people depend
on oil and gas as source of fuel for domestic and commercial purposes.
13
With the introduction of briquette fuel for domestic and commercial
purposes, it is expected that the demand of coal in Nigeria will rise. In
countries like China, Nepal, Japan, India and United States where these
briquettes are already being used constantly and effectively [15], coal
demand has tremendously increased. For instance, in China, domestic
coal demand in 2002 reached 1370 metric tonne, accounting for 66% of
the total primary energy consumption [16], Japan total primary energy
supply, which was 459 million tonne oil equivalent (toe) in 1990 reached
466 million toe in 2001, indicating an increase of 1-6% for the period
[17]. In these countries, the demand for coal is expected to increase
from 1.051 billion tonnes in 2001 to 1.444 billion tonne in 2025 and coal
for electricity generation will constitute about 90% of total coal demand in
United States of America [18].
China and India together account for almost three quarters of the
increase in world coal demand. In all regions, the coal use becomes
increasingly concentrated in power generation which accounts for almost
90% of the increases in demand between 2000 and 2030 [19].
If Nigerian coal will be utilized in power generation and as domestic
fuel, its demand will increase, coal mining will be effective again, and our
oil and gas will be conserved for transportation purposes.
1.1.6 Environmental issues.
Coal contains carbon, hydrogen, sulphur, and other minerals. When
coal is burnt, carbon, hydrogen and sulphur react with oxygen in the
atmosphere to form carbon (iv) oxide, water and sulphur (iv) oxide. The
sulphurdioxide can react with more oxygen to form sulphur trioxide, SO3.
2S02(g) + O2(g) ————->2S03(g)
The SO3 dissolves readily in water droplets in the atmosphere to form
an aerosol of sulphuric acid which falls as rain.
14
H2O(l) + SO3(g)—————->H2SO4
When inhaled, the suphuric acid aerosol is small enough to be
trapped in the lung tissues, where they cause severe damage. Acid rain
destroys vegetation and forest as well as life in the sea, lake, ocean,
streams, etc. Also, CO2 is produced when coal is burnt. The total
quantity of CO2 released by the human activities of deforestation and
burning of fossil fuel is 6-7 billion metric tonnes per year. Carbon (iv)
oxide causes global warming and depletes the ozone layer.
Bio-coal briquette contains less percentage of coal than in coal
briquette (since there is partial substitution of coal with biomass). Hence,
there will be lesser emission of carbon, sulphur, dust, etc, into the
environment.
In order to reduce the emission of these gases into the environment,
lime based products such as Ca(OH)2 can be incorporated into the
mixture to fix the pollutants to the sandy ash, or the coal can be
carbonized.
Since the use of bio-coal briquettes will reduce cutting down of trees
for the purpose of using them as fire wood, briquette technology can
serve as global warming countermeasure by conserving forest resources
which absorbs CO2, through provision of bio-coal briquettes.
1.1.7 Coal analysis:
The composition of a coal is usually reported in terms of its proximate
analysis and its ultimate analysis: The proximate analysis consists of
four items: fixed carbon, volatile matter, moisture and ash, all on a
weight percent basis.
Volatile matter: The portion of a coal sample which, when heated in the
absence of air at prescribed conditions, is released as gases. It includes
15
carbon (iv) oxide, volatile organic and inorganic gases containing sulphur
and nitrogen.
Moisture: The water inherently contained within the coal and existing in
the coal in its natural state of deposition. It is measured as the amount of
water released when a coal sample is heated at prescribed conditions. It
does not include any free water on the surface of the coal. Such free
water is removed by air drying the coal sample being tested.
Ash: The inorganic residue remaining after the coal sample is
completely burned and is largely composed of compounds of silica,
aluminum, iron, calcium, magnesium and others. The ash may vary
considerably from the mineral matter present in the coal (such as clay,
quartz, pyrites, and gypsum) before being burned.
Fixed carbon: This is the remaining organic matter remaining after the
volatile matter and moisture have been released. It is typically calculated
by subtracting from 100 the percentages of volatile matter, moisture and
ash. It is composed primarily of carbon with lesser amounts of hydrogen,
nitrogen and sulphur. The ultimate analysis provides an element-by
element composition of the coal’s organic fraction, namely: carbon,
hydrogen, oxygen, and sulphur, all on a weight percent basis.
Coal can also be analysed in terms of mineral value and heating
value. Mineral matter consists of the various minerals contained in the
coal. Heating value is the energy released as heat when coal undergoes
complete combustion with oxygen [20].
1.2 Briquetting Technology:
Introduction:
Briquetting is the agglomeration of fine particles charred or
uncharred, by applying pressure to them and compacting them into
various shapes using binding agent. Pressure is applied to coal,
16
biomass, etc in a mould so that the particles can adhere to each other in
a stable manner for subsequent handling [21]. A briquette is a block of
compressed coal, biomass or charcoal dust that is used as fuel. It can
also be said to be a block of flammable matter which is used as fuel to
start and maintain fire [21].
1.2.1 Advantages of briquette production:
Briquette production will:
i. provide a cheap source of fuel for domestic purposes, which will be
affordable by all Nigerians.
ii. provide a good means of converting coal fines, low rank coal, waste
agro residue into a resourceful substance of economic value.
iii. Help to conserve some of our natural resources since it is a good
substitute for fire wood. Therefore, it will help to reduce the quantity of
firewood, oil and gas that is used in the production of energy for
domestic uses and generating plants.
iv. Help to develop the demand for coal. Coal is used in making bio-coal
and coal briquette. This will in turn promote coal mining which seems
dormant for sometime.
v. Create employment opportunities for people since people will be
needed to operate the briquette machine, get the raw materials (i.e. coal
and agro-residue, etc), sell the briquettes produced, etc [22].
1.2.2 Types of briquettes:-
i. Coal briquettes:- These are briquettes formed by agglomeration and
application of pressure to coal fines (i.e. coal particles) [16].
ii. Charcoal briquettes:- They are briquettes formed by agglomeration
of fine particles of charcoal and applying pressure to give shapes
17
[23]. Charcoal is a form of carbon consisting of black residue from
partially burnt wood.
iii. Biomass briquettes:- These are briquettes formed by agglomeration
of biomass (e.g. rice husk, corncob, cotton stalks, coconut shell,
groundnut shell, saw dust, etc) and applying pressure to them to
give them shapes. Biomass briquettes are a renewable source of
energy and it avoids adding fossil carbon to the atmosphere.
iv. Bio-coal briquettes: They are briquettes formed by blending coal
with vegetable matter (biomass), and then treating with
desulphurizing agent (Ca(OH2)), using an amount corresponding to
the sulphur content in the coal. When high pressure is applied in the
briquetting process, the coal particles and fibrous vegetable matter
in the bio-briquette strongly intertwined and adhere to each other,
and do not separate from each other during combustion [24].
v. Wood briquettes: are made of dry untreated wood chips (e.g. wood
shavings). They have lower ash and sulphur content compared to
the fossil fuels. The CO2 balance is even, because wood briquette
release just as much as CO2 to the atmosphere as the tree absorbs
through growth by photosynthesis [24, 25].
1.2.2.1 Coal briquetting:
This is the agglomeration of coal (fines) particles by applying
pressure to them and compacting them into various shapes with binding
agents.
History of coal briquetting:
The first patent for the briquetting of fines dates back to the mid
1800s. In 1848, a patent was granted to William Easby for a method of
converting fines into lumps. In his application, Easby made only one
claim “the formation of particles of any variety of coal into solid lumps by
18
pressure”. A list of work has been done on briquetting technology of
biomass, charcoal and coal briquetting as an alternative to fuel wood to
prevent deforestation that results in desertification in some countries like
Northern India, Kyrgyz Republic, Karaganda, Nepal, Kenya, Japan, etc.
A. Coal briquetting in the Kyrgyz Republic:
An assessment team, during its field work in October and
November 1994, identified and reported work done by researchers in
Kyrgyz Republic in recent years. Their findings were summarized as
follows: In November 1994, a briquetting laboratory existed in Osh, in
apparently new quarters for the institution of integrated use of natural
resources of the Academy of Science, Kyrgyz Republic. A brief visit to
this facility showed that it was operational with two piston and mould
heating capacity. In 1995, a modified brick press was used to produce
about 700 briquettes to test various binders and additives such as clay,
lime powders, and cotton processing residues, etc. Also, briquettes with
adequate thermal stability could be made using residues from cotton and
animal fat processing in the Fegana Valley [26].
B. Bentonite-clay binder coal briquetting technology in Kyrgyz
Republic.
The potential of absorbent clay, a binder for the manufacture of a
briquette from whole (uncarbonized) coal, which would be smokeless
during combustion appears to have been first recognized in researches
at Chulakongkorn University in Bangkok, Thailand during 1980’s. The
original clay used for the work was fuller’s earth, but latter work focused
on the use of bentonites because of superior result in suppressing the
emission of unburnt volatile materials from the combustion of the coal.
Hydrated lime was incorporated in the recipe for the purpose of
suppressing the emission of sulphur (iv) oxides. Kyrgyz coals do not
have as much sulphur as the coals used in the Pakistan briquetting
19
work, the amount of the lime that was incorporated in the recipe for a
Kyrgyz briquetting industry was significantly less. However, lime could
have a beneficial effect on the ultimate strength of the briquette to
withstand handling and shipping. Some potassium nitrate was also
incorporated in the recipe in order to promote ease of initial ignition.
When the briquettes produced were burnt, it was observed that they
were smokeless [24, 19, 20]
C. The beehive charcoal briquette and briquette stove in the Khubu
region, Nepal:
The Nepal biomass (such as firewood and agricultural by-products) is
used almost exclusively for cooking and space heating in the rural areas.
Indoor air pollution from open fire causes eye irritation and lung disease
dominantly for women and children [28]. This is especially so for high
altitude areas, where open fires are also used for space heating.
Processing forest waste and agro biomass (by product) first into
charcoal and then compacting the charcoal into briquettes allow this to
be used inside the house for cooking and heating purposes. To minimize
the smoke, the beehive stove was invented. The smoke emission from
the beehive briquettes is far less as compared to an open air.
The beehive charcoal briquettes are made from charcoal produced
from agricultural residue such as rice husk, wheat chaff and forest waste
vegetation (fallen pine needles, pinecones, grassy weeds, etc). This is
an efficient method of utilizing bio-waste. All woody biomass material
can form the raw material for charcoal briquettes. Emphasis was placed
on using agricultural residues and invasive biomass. The biomass
material is first sun dried until it has humidity below 15%. It is then
heated in a 220 litres metal charring drum. The resulting charcoal is
ground to dust and mixed with 30% dry clay-soil (in volume). Water is
added to make a paste. Using a 5kg hand weight, the paste is
20
compacted into a round mould which produces a charcoal briquette with
19 holes after moulding. These holes not only allow the briquette to dry
evenly but also enhance burning by allowing flames and gases to
escape evenly from the briquette [29,30].
It was also observed that in the North-Eastern region of India, there
were considerable reserves of oil and gas. With a view to conserve oil
and gas for more productive uses in industrial establishments and mass
transportation, the reserves of coal can be more usefully exploited for
domestic applications. The raw materials that are used are low grade
coal and coal particles, bentonite, molasses, plastic clay, lime and
sodium silicate as binders. The processes they used in manufacturing
the briquettes are as follows:
i. Crushing/ grinding of coal to below 2mm particle size.
ii. Preparation of binder in a semi-liquid form
iii. Mixing of coal with binders
iv. Briquette making using the briquetting machines
v. Drying
vi. Packaging [31]
Technology of coal briquetting:
Coal briquetting requires a binder to be mixed with coal fines, a press
to form the mixture into a cake or briquette, which is then passed
through a drying oven to cure or set it by drying out the water so that the
briquette will be strong enough to be used in a stove [32]. Coal
briquettes can be produced through the following technique:
Carbonization production process:
The coal briquette carbonization production process, consist of a
carbonization stage and a forming stage.
21
Carbonization stage:
The raw coal (5-50mm particle size) is preliminarily dried in a rotary
dryer. The dried raw coal is put in a furnace, and subjected to fluidization carbonization at temperature of about 450oC. The semi-coke is
discharged from the top of the furnace together with the carbonization
gas. The semi-coke is separated from the carbonization gas by the
cyclone, producing smokeless semi-coke containing approximately 20%
of volatile matter.
Forming stage:
The smokeless semi-coke and auxiliary raw materials, hydrated lime
(desulfurizer), clay (binder) and water (caking additive) are mixed by
kneading. The mixture is formed using a briquetting machine (i.e. a
moulding machine) at normal temperature and pressure of about 3-5
MPa. The briquettes formed are dried in the continuous drier, cooled and
packaged for sell, or it is ready for use [33].
Fig 2: Process flow of briquette production.
22
1.2.2.2 Biomass briquetting:-
This is the agglomeration of biomass (such as rice husk, groundnut
shell, coconut shell, corn cob, etc) by applying pressures to them to
compact them into various shapes with binding agents.
Technology of biomass briquetting:
Biomass, particularly agricultural residues seems to be one of the
most promising energy resources for developing countries. The idea of
utilizing the residues from agricultural sectors as primary or secondary
energy resources is considerably attractive. This kind of waste is
available as free, indigenous and environmentally friendly energy
sources. Moreover, decreasing availability of firewood has necessitated
that efforts be made towards efficient utilization of agricultural wastes.
They have acquired considerable importance as fuels for many purposes
viz: domestic cooking, industrial process heating, power generation etc.
Some of agricultural residues such as coconut shell, wood chip and
wood waste, are ready to be directly used as fuel. Nevertheless, the
majority of these bulky materials are not appropriate to be used directly
as fuel without a suitable process, because of the fact they have low
density, high moisture content, and low energy density. All of these
issues may cause problems in transportation, handling, storage,
entrained particulate emission control including direct combustion [34].
Biomass briquetting requires a binder to be mixed with pulverized
biomass, and a press to form the mixture into cake or briquette, which is
then passed through a drying oven so that any water contained in the
briquette will be driven off. The biomass to be used must be dried and
pulverized. It must also be combustible and should not constitute
environmental hazard [35].
23
1.2.2.3 Bio-coal briquetting:-
Bio-coal briquette or biomass-coal briquette may be defined as a type
of solid fuel prepared from coal and biomass. When pressure is applied
during the process, the coal particles and biomass material adhere and
interface to each other. Thus, these two materials do not separate from
each other during the storage, traveling and combustion. During
combustion, the biomass is simultaneously burnt with the coal at low
ignition temperature. Since the biomass part in the briquette has lower
ignition temperature as compared to the other part, it can assure that the
quality of combustion of the coal volatile matter at low temperature is
improved. Besides, it is widely accepted that biomass-coal briquette
technique is one of the most promising technologies for reducing SO2
emission. With regard to the sulphur content of the coal, a desulfurizing
agent can be added for sulphur capturing purpose. The agent effectively
reacts with suphur in the coal to fix the sulphur into the sandy ash. Thus,
several coal ranks, including low grade coal containing high sulphur and
ash contents can be used for producing biomass coal briquettes [34].
Many researches about biomass- coal briquettes have been carried out.
Olive stone [35-35], sawdust [37], rice straw [38] are examples of the
biomass material in the briquettes.
1.2.2.3.1 Characteristics of bio-coal briquettes:
Bio-coal briquettes has the following characteristics:
1. It is made from coal and biomass: Bio-coal briquettes can be
produced from either high quality coal or low quality coal.
2. Bio-coal briquette ash is also beneficial to improving soil in the desert
and semi-desert [39]. The table below shows the chemical components
of bio-coal briquettes ash. It contains higher silica and alumina, but lower
calcium compounds such as CaO, CaCO3, and CaSO4, in general
24
similar to gypsum. An experiment performed in Shenyang in 2001 using
gypsum, produced gypsum improved soil, and actually it helps corn
growth significantly [39,40]. The experiment also concludes that ash of
bio-coal briquettes have similar effect on soil improvement, although it is
not so successful as gypsum but effective, because it contains less
calcium compounds than gypsum.
Table 3: Chemical analysis on bio-coal briquette ash [40].
Gypsum from
desulphurization
Bio-coal briquette
ash
Ca(OH)2 2% 1%
CaO 31% 9%
CaSO3 2% 1%
CaCO3 29% 5%
CaSO4 .2H2O 32% 10%
SiO2 9% 27%
Al2O3 4% 19%
3. They can be in different shapes- rectangular, cylindrical, square
shapes, etc.
4. Since fibrous biomass is intertwined with the coal particles, there is no
fear of the fused ash in the coal adhering and forming clinker lumps
during combustion.
5. The bio-coal briquettes are formed under high compressive force.
Because of this, the desulphurizing agent and coal particles strongly
adhere to each other, and they effectively react during combustion. With
the addition of a desulphurizing agent at a ratio approximately 1.2:2 of
Ca/S, 60-80% of the sulphur in the coal is fixed in the ash [22].
25
1.2.2.3.2 Advantages of bio-coal briquettes:
1. Briquettes from biomass and coal are cheaper than briquette from
coal. This is so, since some of the biomass materials used are of less
economic importance and are always left to waste, except in cases
where they are to be used, which is rare. E.g. in Abakaliki rice mill, the
rice husk is left to waste.
2. High sulphur content of oil and coal when burnt pollutes the
environment. In bio-coal briquettes, part of the coal is substituted with
biomass, hence the sulphur content is reduced [22].
3. Bio-coal briquettes have a consistent quality high burning efficiency,
and are ideally sized for complete combustion.
4. Combustion of bio-coal briquettes produces ashes which can be
added to soil to improve soil fertility.
5. Bio–coal briquettes are usually produced near the consumption
centers and supplies do not depend on erratic transportation from long
distance.
Based on these facts, bio-coal can replace the following conventional
fuel that are used in mass quantities: diesel, kerosene furnace oil, fire
wood, coal, lignite, etc [41].
1.2.2.3.3 Bio-coal briquetting technology:-
Bio-coal briquettes are a very valuable source of energy. They are
made by thermal conversion of biomass and coal into bio-coal, which is
subsequently compacted. Practically, any type of solid residue or waste
from forestry, agriculture and the wood processing and agro-industries
can be used as raw materials, e.g. wood waste, corncob, rice husk,
bagasse, coffee husk, etc. The calorific value of bio-coal briquettes is
almost double that of biomass alone. They also have clean combustion
26
behaviour (little smoke, a low level of toxic emissions), and are easy to
store and transport [42]. Bio-coal briquettes can be produced using
i. manual briquetting machine e.g. briquetting press machine, and
ii. sophisticated briquetting machine such as briquetting plants.
The manual briquetting machine is cheaper than the briquetting plant,
which is more efficient.
The need for a smokeless bio-coal briquette:
Smoke is a collection of airborne solid and liquid particles and gases
emitted when a material undergoes combustion or pyrolysis, together
with the quantity of air that is entrained or mixed into the mass. It is
commonly an unwanted by product of fire [43]. All fires produce smoke,
the nature and density of which depend on the burning material. Smoke
contains particulate matter, liquids, as well as gases. The major
problems caused by smoke are eye irritation and reduced visibility,
coughing and sneezing (i.e. when the smoke is inhaled deep into the
lungs) [44]. Also, inhalation of smoke is the primary cause of death in
victims of indoor fires. Smoke kills by a combination of thermal damage,
poisoning, and pulmonary irritation caused by CO, HCN and other
combustion products [43]. Hence, it is very important that the briquettes
produced should be smokeless to avoid or prevent smoke when burning
the briquettes. This can be achieved by carbonizing the coal into coalite
(semi-coke), or by incorporating additives which can help to drive off the
volatile matters that cause smoke.
Production processes of bio-coal briquettes:
Method 1:
By this method, the coal is carbonized. This involves internal heating
at low temperature in a fluidized-bed carbonization furnace (approximately 4500C) to produce a smokeless semi-coke containing
approximately 20% volatile matter. The smokeless semi-coke, biomass
27
(pulverized) and auxiliary raw materials; hydrated lime and binder e.g.
starch, clay, etc, are thoroughly mixed at a predetermined mixing ratio.
After pulverizing, the mixture is blended with a caking additive while
water is added to adjust the water content of the mixture. The mixture is
kneaded to uniformly distribute the caking additive, and to increase the
viscosity in order to make the forming of the briquettes easy. The mixture
is then introduced into the molding machine to prepare the briquettes.
The briquettes are then dried [45]. A cross sectional view of
carbonization furnace and a basic process flow for bio-coal briquette
production are shown in Fig 3 and 4 [57].
Fig 3: Cross- sectional view of carbonization furnace.
Fig 4: Basic process flow for bio-coal briquette production [45].
28
Method 2:
The biomass is dried, pulverized and sieved with a sieve of known
mesh size. The biomass is pre-treated (if need be ) with 3% wt/wt sodium hydroxide solution at 900C for 1 hr. Coal fines is mixed with the
sodium hydroxide treated biomass at a predetermined ratio. The mixture
is then compacted at ambient temperature by using a hydraulic press,
producing bio-coal briquettes which is then dried [46].
The bio-coal briquettes can also be dried at high temperature (curing)
to remove volatile matters.
Method 3:-
This is the most widely used method. By this method, the coal and
biomass (dried) are pulverized, and mixed with sulphur and chlorine
fixation agents such as calcium carbonate, calcium hydroxide, etc (ie
lime based products). The fixing agent added is equal to the amount of
sulphur in the coal. These desulphurizng agents fix the sulphur into the
sandy ash during combustion, making the ash rich in nutrients (that can
be used by plants). Thus, several coal ranks, including low grade coal
containing high suphur and ash contents can be used for producing bio
coal briquettes [47].
An experiment performed on the desulphurizing efficiencies of
different coals using desulphurizing agents such as calcium carbonate,
calcium hydroxide, magnesium carbonate showed that calcium
hydroxide or calcium oxide is the best desulphurizing agent with
desulphurizing efficiency reaching over 80%. The desulphurization
efficiencies related calcium are above 80% MgCO3 was also used, but
do not show promising results with desulphurization efficiencies below
50% for most coals [47].
29
Calcium hydroxide is the best desulphurizing agent because of the following: Calcium hydroxide was decomposed at 3500C, and H2S is
released from volatile matter,
Ca(OH)2 ——->CaO + H2O
CaO + H2S ———> CaS + H2O
Ca(OH)2+ SO2 ———>CaSO3. ½ H2O + ½ H2O
The schematic manufacturing process of bio-coal briquette is shown in
Fig 5.
Coal Biomass
dried up sulphur and chlorine fixation agent dried up
pulverized pulverized
heating and mixing
briquetting ( using briquetting machine)
Biocoal briquettes.
Fig 5: Schematic manufacturing process of bio-coal briquette.
1.2.3. Biomass as a feedstock for the production of bio-coal
briquette.
Biomass is made through process of photosynthesis, which uses
carbon (iv) oxide from the atmosphere, while releasing oxygen. During
photosynthesis, the solar energy (light) is captured by pigments in plants
and is used to reduce atmospheric carbon dioxide gas into
30
carbohydrates through biological reaction with water. The solar energy is
stored in the form of carbohydrate chemicals such as cellulose,
hemicelluloses and lignin.
Cellulose and hemicelluloses are polysaccharides of glucose (i.e.,
they are polymers of glucose). Hemicelluloses have a less ordered
structure than cellulose, and can be more easily hydrolyzed to simple
sugars and other products. Lignin is an amorphous polymer, and plays
an important role in developing structure of the plants [48].
Biomass feedstock utilized in energy system.
Biomass feedstock used for energy purposes can be generally
divided into dedicated energy crops and residues (wastes or by-products
of various process and activities), and range from woody to grassy
materials, as shown in Table 4 below. Examples of common biomass
include crop residues (wheat straw, corn stalks, nut shells, orchard
pruning, vineyard stakes, sugar cane bagasse, etc), forest residence
(slash, forest thinning, urban wood waste (construction residues, grass
dipping and backyard pruning), and several energy crops [48]. Many
different types of biomass can be utilized for bio-briquette production and
also in co-firing systems. Co-firing experience includes wood, residues
from forestry and related industries, agricultural residues, as well as
various biomasses in refined form such as pellets, are popular in
Denmark and the Netherlands. Also, oil, sugar and starch energy crops
can be used for production of liquid fuels with high energy value (bio
diesel and bio-ethanol respectively) for use in the transport sector and
their utilization for power production is not economically justified at
current [49].
31
Table 4: Types of biomass feedstock used for energy purposes
[50,51]
SUPPLY SECTOR TYPE EXAMPLES
1 Agricultural residues Dry lignocellulosic
agricultural residues.
Straw (maize, cereal,
rice) sugar beet
leaves, residue flows
from bulb sector, etc.
Livestock waste Solid manure (chicken
manure) cattle, pig,
sheep dung, etc.
2 Dedicated energy
crops
Dry lignocellulosic
woody energy crops.
Willow,
Popular Eucalyptus,
etc.
Dry lignocellulosic
herbaceous energy
crops .
Miscanthus, switch
grass, common reed,
reed canary grass,
giant reed, cynara
cardunculus, Indian
shrub, etc.
Oil energy crops. Rapeseed, sunflower
seeds, soybean, olive-
kernel, calotropis
procera, groundnut
(nut, shell), etc
Sugar energy crops Sugar beet, cane beet,
sweet sorghum,
Jerusalem Artichole,
sugar millet, etc.
Starch energy crops Wheat, potatoes,
32
maize, barley, triticae,
amaranth ,corn cob.
Others Flax (linum), Hemp
(cannabis) Tobacco
stems, Aquatic plants,
(lipids from algae),
cotton stalks, kenaf,
etc.
3 Forestry Forestry by- products Bark, wood blocks,
wood chips from tops
and branches, wood
chips from thinning,
logs from thinnings.
4 Industry Wood industry
residues
Industrial waste wood
from sawmills and
industrial waste wood
from timber mills
(bark, sawdust, wood
chips, slabs, off- cuts)
Fibrous vegetable
waste from virgin pulp
production and from
production of paper
from pulp, including
black liquor.
Food industry
residues
Wet cellulosic
materials (beet root
tails) fat (used cooking
oils) tallow, yellow
33
grease, protein
(slaughter house
waste)
Industrial products Pellets from sawdust
and shavings.
Briquettes from saw
dust and shavings.
Bio-oil (pyrolysis oil)
Ethanol, Bio- diesel.
Parks and gardens Herbaceous Grass
Woody Pruning
Waste Contaminated waste Demolition wood
Biodegradable
municipal waste,
Sewage sludge, land
fill gas, sewage gas
Others Roadside hay Grass/hay
Husks/shells Almond, olive, walnut,
coconut, palm pit
(imported),cacao
(imported).
Note: Dry lignocellulosic feedstock is the category of feedstock, which
can be used for thermochemical conversion (gasification, combustion
and liquefaction). Wet lignocellulosic feedstock is a feed stock that can
be used for biological conversion (digestion).
Properties of biomass in relation to co-firing:
Generally, proximate analysis of biomass gives 80% volatile matter
and 20% fixed carbon (moisture free and ash free bases), whereas
34
bituminous coal (for instance), gives 70-80% fixed carbon and just 20
30% volatile matter [48, 52].
There are factors to consider before a biomass qualifies for use as
feed stock for briquetting. Apart from its availability in large quantities, it
should have the following properties:
i. Low moisture content: Biomass usually has high moisture content,
resulting in a relatively low calorific value of the fuel [52,53]. Fresh wood
typically contains 50% of water by weight, whereas the moisture content
for bituminous coal is approximately 5% [52]. Moisture content of
biomass affects its combustion properties. Higher moisture content will
reduce the maximum combustion temperature, and increase the
necessary residence time of feedstock in a combustion chamber, and
consequently could result in an incomplete combustion and increased
emissions related to it (volume of flue gas produced per energy unit)
[50]. Therefore moisture content should be as low as possible, generally
in the range of 10-15% or less. High moisture content will pose problems
in burning and excessive energy is required for drying [54].
ii. Ash content and composition: Typical biomass contains fewer
ashes than coal, and their composition is based on the chemical
components required for plant growth, whereas coal ashes reflect the
mineralogical composition [55]. In both coal and biomass, ash forming
matter can be present in four general forms: easily leachable salts,
inorganic elements associated with the organic matter of the biomass,
minerals included in the fuel structure, and inorganic materials, typically,
sand, salt or clay [52].
Alkaline metals that are usually responsible for fouling of heat transfer
surfaces are high in biomass ashes, and are released in the gas phase
during combustion. In biomass, these inorganic compounds are in the
form of salts or bound in the organic matter, but in peat, for example,
35
inorganic matter is bound mostly in silicates, which are more stable at
high temperature. The elemental composition of ash, (alkali metals (e.g
potash, phosphorus, chlorine, silicon and calcium), affects ash melting
behaviour. Even a small concentration of chlorine in the fuel can result in
deposition of harmful alkaline and chlorine compounds on boiler heat
transfer surfaces [52]. The ash content of some types of biomass are
given in the Table 5 below:
Table 5: Ash content of different biomass types [54].
Biomass Ash content (%) Biomass Ash content (%)
Corncob 1.2 Tannin waste 4.8
Jute stick 1.2 Almond shell 4.8
Sawdust (mixed 1.3 Areca nut shell 5.1
Pine needle 1.5 Castor stick 5.4
Soya bean stalk 1.5 Groundnut shell 6.0
Bagasse 1.8 Coir pith 6.0
Coffee spent 1.8 Bagasse pith 8.0
Coconut shell 1.9 Bean straw 10.2
Sunflower stalk 1.9 Barley straw 10.3
Jowar straw 3.1 Paddy straw 15.5
Olive pits 3.2 Tobacco dust 19.1
Arhar Stalk 3.4 Jute dust 19.1
Lantana
Camara
3.5 Rice husk 22.4
Subabul leaves 3.6 Tamarind husk 4.2
Teawaste 3.8 Deoiled Bran 28.2
iii Chemical properties: With regard to chemical properties of biomass,
it generally has less sulphur, fixed carbon, and fuel bond nitrogen, but
more oxygen than coal.
36
iv Also, biomass should have low bulk energy density, hydrophillic and
non- friable character.
Most of the challenges that co-firing poses to boiler operation
originate from fuel properties (the differences in characteristics of coal
and biomass) and can be summarized as follows [55]: Pyrolysis starts earlier for biomass than for coal. The volatile matter content of biomass is higher than in coal. The fractional heat contribution by volatile substances in
biomass is approximately 70% compared with 30-40% in coal. The specific heating value (kJ/kg) of volatiles is lower for
biomass compared with coal. Biomass char has more oxygen compared with coal and it is
more porous and reactive. Biomass ash is more alkaline in nature, which may aggravate
the fouling problems.
Characteristics of biomass feedstock and their effect on co-firing are
shown in Table 6.
Table 6: The physical and chemical characteristics of biomass
feedstock and their effects on co-firing [55].
Properties Effects
PHYSICAL Moisture content Storage durability,
dry- matter losses ,
self ignition.
Bulk density Fuel logistics (storage,
transport, handling) cost.
Ash content Dust, particle emissions,
ash utilization/disposal costs.
Particle dimension
and size distribution.
Determines fuel feeding system,
Determines combustion
37
technology ,
Drying properties,
Dust formation,
Operational safety during fuel
conveying.
CHEMICAL Carbon, C GCV (positive)
Chlorine, Cl Corrosion
Nitrogen, N NOx, N2o, HCN emissions.
Sulphur, S SOx emissions, corrosion
Fluorine, F Hf emissions, corrosion.
Potassium, K Corrosion (heat exchangers,)
Super heaters
Lowering of ash melting
temperatures.
Aerosol formation
Ash utilization (plant nutrient)
Sodium, Na Corrosion (heat exchangers,
super heaters)
Lowering of ash melting
temperatures,
Aerosol formation,
Ash utilization (plant nutrient).
Magnesium, Mg Increase of ash melting
temperature,
Ash utilization (Plant nutrient).
Calcium, Ca Increase of ash melting
temperature.
Ash utilization (plant nutrient)
Phosphorus, P Increase of ash melting point.
38
Ash utilization (plant nutrient).
Heavy metals Emissions of pollutants,
Ash utilization and disposal
issues, Aerosol formation.
Most of the challenges that co-firing poses to boiler operation
originate from biomass properties, and therefore improving the
properties e.g. by pre-treatment can be applied as one of the measures
to avoid or reduce these challenges.
Possibility for utilization of biomass ash:-
(a) Recycling of biomass ash: Biomass contains only those
inorganic elements which have been extracted from the environment
where it was grown thus returning those components in the form of ash
to a place where it originated, closes the mineral cycles and is the most
sustainable form of ash utilization [56]. The biomass ash produced is
either disposed or recycled on agricultural fields or forests, however
generally, recycling is not a controlled practice. Recycling of biomass
ash has been done in Austria, Denmark, Germany, etc.
Setting up a close ash recycling system for agriculture might be
difficult, because ashes have unpredictable composition and possibly
large amounts of heavy metals compared to nutrients [55].
(b) Utilization of biomass ash as fertilizer / fertilizer production:
Potentially, ashes could be used as soil improvers, especially when
originating from combustion or gasification where dolomite has been
used as bed material, because they are rich in magnesium and calcium
[57]. However, there are a number of factors constraining utilization of
biomass ash as fertilizers. Biomass ashes cannot be considered as
completed fertilizers due to lack of nitrogen and soluble phosphorus low
39
nutrient content in comparism with heavy metals and high inert content
[56,57]. Biomass ashes could be used as a raw material for fertilizer
production [55].
(c) Utilization of biomass ash in building application:
Direct utilization of biomass ash in building application might be
possible only, for some types of bottom ash from biomass combustion,
other ashes might be used as raw materials in the production of building
materials [56].
(i) Bottom ashes from fluidized bed combustion or gasification which
contains a lot of sand could be utilized as building material to replace
sand and granulates in road construction and landscaping.
(ii) Fly ash from combustion of biomass are not likely to find a direct
application as building material, because they have a powder like form,
so it cannot replace particles like sand or gravel. It could be utilized as
component in cement or concrete, but the alkali and chlorine content can
be problematic [56].
(d) Utilization of biomass ash as fuel:
Utilization as fuel for power and heat generation is possible for ashes
with high energy content, thus, basically only the fly ash from fluidized
bed gasification of biomass can qualify for this. These ashes have high
calorific value due to high amount of unburnt carbon in a form of a fine
powder (in PF boiler) or compacted by pelletisation or granulation [56].
Issues related to supply of biomass:
Biomass supply chain consists of the following factors: transportation,
handling, storage, sizing, pre-processing (drying and/or other
pretreatment) and feeding.
Transportation, receiving, handling, storage, pre-processing and
feeding of (especially raw) biomass can cause problems [49,58,59]. For
instance, receiving, handling and storage of fresh wood chips or other
40
wet fuels may cause odour or spore element emissions [49].
Preparation, storage and handling of raw biomass are difficult due to
often high moisture content, hydrophilic and non-friable character,
particle size variation, and high fibre content [52,58], as well as
susceptibility to rotting and heating.
A. Storage: Storage of biomass is usually necessary in order to
guarantee its continuous supply to the co-firing power plant during the
year. There are a number of issues associated with storage. During
storage, biomass can lose its moisture content, energy value and dry
matter content due to degradation processes (microbiological activities)
[42]. The storage conditions can have considerable influence on
biomass properties essential for its use. The temperature in a biomass
pile rises as the material starts to decay, leading, in extreme cases, to
self-ignition and potentially fire [55]. Decomposing of biomass material
also leads to material and energy losses. The change in temperature of
biomass pile is dependent on the moisture content of biomass- in
general, the higher the initial moisture content of the stored feedstock,
the higher the dry matter loses. The temperature changes of biomass
pile can be also influenced by the size of stored biomass. The biological
activity of microorganisms, which results in heat production, takes place
on the surface of the chip, thus, the smaller the chip size, the larger the
surface area per volume, and consequently, the higher the biological
activity resulting in higher temperature [42].
The dry matter losses can, in some cases, be compensated by the
lower moisture content due to the drying process occurring as a
consequence of heat production during decomposition process. Since
moisture content and size of biomass influence its energy content (dry
matter loses), the pretreated applied (e.g. pelletizing drying, chipping)
could help to stabilize biomass properties in relation to potential changes
41
in its energy content during storage. It should be noted that storage of
pre treatment biomass has certain demands with regard to storages
conditions (e.g. pellets should be stored in closed halls or rooms, silos
and bunkers, as they are sensitive to moisture, and after contact with
water, can loose quality and cause handling problems).
B. Transportation: Transport of biomass is expensive due to
generally low bulk densities of biomass fuels, and since the cost of
biomass fuel is a critical factor in the economics of co-firing, the costs of
transportation are very important issues. One way to reduce the
transport costs of biomass is to apply densification processes to biomass
and bring it in the form of briquettes, bales, pellets etc.
C. Preparation: Preparation of biomass feedstock is often applied,
most commonly by its drying and sizing, and there are number of
problematic issues associated with them. Sizing of biomass can be
difficult due to the non-friable and fibrous character of biomass material.
It is usually unnecessary to bring biomass feedstock to the same size as
coal particles, however large and spherical biomass particles can cause
problems with fuel conversion efficiency [58].
Drying of biomass is applied to improve the combustion or
gasification efficiency, reduce the biomass susceptibility to
decomposition and consequent dry matter losses, fire and health
hazards (fungi production due to biological degradation), and to avoid
the need for complex combustion technology and process control, which
is required by fuels with varying moisture content. Drying of biomass
requires energy, and can cause environmental problems.
D. Handling: This depends on the type of biomass, for example:
i. Solid residues from the palm oil and olive oil industries (granular
agricultural materials) handle well at normal delivered moisture contents.
42
ii. Handling of woody biomass, in the form of chips, chunks and sawdust
is usually more difficult, mainly due to the wide range of particle sizes
and moisture contents.
iii. Herbaceous (grassy) biomass is commonly handled, transported and
stored in baled form, and is more difficult and more expensive to handle
than other biomass types.
iv. Pelletized biomass is generally good for handling, but some of these
materials handle very poorly when wet. Pellets absorb moisture from the
surrounding air and can swell. They should be stored in dry conditions
[55].
E. Others are feeding, grindability, etc.
1.2.4 Groundnut shell as an appropriate residue for the
production of bio-coal briquette.
Grondnuts, Arachius hypogea, are legumes whose fruits are formed
underground, each fruit or nut usually contains two or three seeds,
enclosed by the shell. It is one of the most important annual cash crops
grown in West Africa. In Nigeria, the crop is grown mainly in Kano State,
but also in the Sokoto, Bornu and Kaduna States.
Groundnuts require rich, light, sandy loam soils, since such light soils
allow the ovary to push easily into the soil, making harvesting easier. It
requires annual rainfall of 80-120cm, abundant sunshine and fairly high
temperatures. These conditions are obtained in the savanna areas.
Groundnuts are propagated by seed. Planting is done with the early
rains in March-April in South, and May-June in the North. Groundnuts
reach maturity in 4-5 months. In wetter areas, groundnuts are harvested
in August, while in the dries savannah, harvesting is done in October-
November. Harvested pods are spread on concrete floors or plat forms
to dry. They are later beaten with sticks or pounded or using threshing
43
machine to remove the shells. This is called shelling or decortication.
The seeds are separated from the shells by winnowing or using a
shelling machine. The seeds are further dried and packed in jute bags,
while the shells are dried and kept [60].
Groundnuts are normally baked before eating. Groundnut oil is used
in cooking and also in the manufacture of margarine and soap. It is also
used in canning sardines. The solid portion which remains after the oil is
extracted is used in the manufacture of biscuits and for animal feed in
the form of groundnut cake. This cake is richer in protein than other
cakes such as palm kernel and coconut cakes. Groundnuts may be
crushed and used in the preparation of groundnut soup. The whole plant
may also be used as a fodder crop or ploughed into the soil as an
organic manure. It is a most useful rotational crop since it enriches the
soil with nitrogenous material [60]. Groundnut shell is obtained after the
groundnut seeds have been removed from the pod. Hence, it is an agro
residue.
Plate 1: Groundnut shell.
1.2.4.1 Analysis of groundnut shell.
The chemical composition of groundnut shell is shown in Table 7 below
Table 7: Chemical composition of groundnut shell [61].
Constituent Percentage
Cellulose 65.7
44
Carbohydrate 21.2
Protein 7.3
Mineral 4.5
Lipids 1.2
Ash analysis of groundnut shell.
An analysis of the ash obtained after groundnut shell was burnt was
given as follows:
Table 8: Ash analysis of groundnut shell [62].
Calcium oxide (CaO) 10.91%
Iron oxide (Fe2O3) 2.16%
Magnesium Oxide (MgO) 4.72%
Silicon Oxide (SiO2) 33.36%
Aluminum oxide (Al2O3) 1.75%
Potassium oxide (K2O) 16.18%
Sodium oxide (Na2O) 9.30%
Sulphur oxide (SO3) 6.40%
Carbonate (CO3) 6.02%
Hydrogen Carbonate (HCO3) 9.20%
1.2.4.2 Uses of groundnut shell.
Groundnut shell is used as fuel, for manufacturing coarse boards,
cork substitutes, etc. Groundnut shell can also be grounded and mixed
with feed, to be used in feeding livestock. A recent experiment carried
out, showed that groundnut shell can be used as partial replacement of
ordinary Portland cement. In the experiment, the ash analysis of the
groundnut shell was carried out, and it was observed that the
constituents in groundnut shell (which was given in the table above)
have cement properties that would be beneficial to the concrete [62].
45
Groundnut shell, when ground is an appropriate agro waste for the
production of bio-coal briquettes, since it burns smoothly and very fast
when it is dried.
1.2.5 Maize /Corn cob as an appropriate residue for the production
of bio-coal briquette.
Corn cob is a part of maize plant. Maize (Zea mays) is cultivated
throughout the tropics wherever rainfall is adequate. It is one of the most
important food crops of West Africa. Maize is propagated by seed. The
main planting seasons in Nigeria are between March and April in the
South, and May and June in the North. It can also be planted at any time
of the year provided there is sufficient moisture in the soil. Maize is
harvested either dry or green. The period from planting to harvesting
varies from 90-120 days, depending on the maturity rating of the variety
planted. In Nigeria, a large quantity of the maize is harvested green and
roasted or boiled for human consumption, while the corn cob remains. It
can also be harvested dry, the cereals pluck out of the cob and also
used for human consumption. The cereals can further be dried and
stored in concrete or aluminum silos.
Maize is an essential part of the diet of many West African people. It
may be boiled or roasted on the cob, or the grains removed from the cob
and used for other purposes. The maize plant can be fed to farm animals
like cattle, sheep, goats etc. It can also be cut fresh and prepared as
silage. Analysis of the grains shows that it is a rich source of
carbohydrates, protein and oils [60].
Corncob is the central core of maize that is left after the grains are
removed. It is also an agro residue. As the maize plant matures, the cob
becomes tougher. When harvesting corn, the corn cob is collected
together with the grains, leaving the corn stover in the field [43]. Corn
cob is obtained when the maize is removed.
46
1.2.5.1 Analyses of maize / corn cob:
Different analyses of corn cob are shown in Table 9-11. [63-64]
Table 9: Proximate analysis of corn cob obtained using ASTM
standards [63]
Table 10: Proximate analysis of corn cob.
Property Value
Higher heating value (kJg-1) 18.4
Volatile matter (wt%) 79.8
Moisture (wt%) 5.03
Ash (wt%) 2.36
Table 10: Elemental Analysis of corn cob obtained using Leco
CHN analyzer [63,64].
Element Percentage.
Carbon 44.00 – 47.79
Hydrogen 5.64 – 7.00
Nitrogen 0.44
Oxygen 47.0
Sulphur 0.001
Chlorine 0.21
47
Table 11: Ash Analysis of corn cob [64].
Sulphate SO3 8.74 wt%
Phosphate P2O5 6.87 wt%
Silicon oxide SiO2 40.30 wt%
Calcium oxide CaO 1.27 wt%
Magnesium oxide MgO 2.49 wt%
Sodium oxide Na2O 1.19 wt%
Potassium oxide K2O 2.04 wt%
1.2.5.2 Uses of corn cob:
Corn cobs are important source of furfural, an aromatic aldehyde
used in a wide variety of industrial processes. Cellulose from powdered
corn cobs is a rodenticide applied inside buildings to control rats and
mice. Corn cobs make a quick, hot fire and burns smoothly. It can also
be used as fibre in ruminant fodder. Over many years corncobs can also
be made into charcoal [43]. Corncobs can also be processed into grit,
flour, and beeswing products, for instance, the pith. chaff and beeswing
can be used for making absorbent, while the woody ring which is the
hardest part of corncob is useful for abrasive applications [64].
1.2.6 Binders used in the production of bio-coal briquettes:
Binders are substances, organic or inorganic, natural or synthetic,
that can hold (bind) two things or something together. Two types are
combustible and non-combustible binders.
Combustible binders are binders that support combustion and can
burn. Examples are starch, petroleum residues, molasses, cottonseed
oil, etc. Non combustible binders are binders that can not support
combustion examples are clay, cement, limestone, etc [24]. Starches
have proved very satisfactory as binders.
48
1.2.6.1 Starches as a binder for the production of bio-coal
briquettes:
Starch or amylum is a carbohydrate consisting of a large number of
glucose units joined together by glycosidic bonds. This polysaccharide is
produced by all green plants as an energy store. It is the most important
carbohydrate in human diet and is contained in such staple foods as
potatoes, wheat, maize (corn), rice, and cassava. Pure starch is a white,
tasteless and odourless powder that is insoluble in cold water or alcohol.
It consists of two types of molecules: the linear and helical amylose and
the branched amylopectin. Starch is processed to produce many of the
sugars in processed foods. When dissolved in warm water, it can be
used as a thickening stiffening or gluing agent, giving white paste [65].
Biosynthesis:
Plants produce starch by first converting glucose- 1 – phosphate to
ADP-glucose using the enzyme glucose-1-phosphate adenyl
transferase. This step requires energy in the form of ATP. The enzyme
starch synthase then adds the ADP- glucose via a 1,4- alpha glycosidic
bond to a growing chain of glucose residues, liberating ADP, and
creating amylase. Starch branching enzyme introduces 1, 6- alpha
glycosidic bonds between these chains, creating the branched
amylopectin. The starch debranching enzyme isoamylase removes
some of these enzymes exist, leading to a highly complex synthesis
process [66]. Hence, starch is synthesized from ADP- glucose.
Starch is a white powder with molecular formula (C6H10 O5)n, and density of about 1.5 glcm3. Starch becomes soluble in water when
heated. The granules swell and burst, the semi-crystalline structure is
lost and the smaller amylose molecules start leaching out of the granule,
forming a network that holds water and increasing the mixture’s
viscosity. This process is called starch gelatinization. During cooking, the
49
starch becomes a paste and increases further in viscosity. During
cooling or prolonged storage of the paste, the semi crystalline structure,
partially recover and the starch paste thicken, expelling water. This is
mainly caused by the retrogradation of the amylose. This process is
responsible for the hardening of bread or staling, and for the water layer
on top of a starch gel (syneresis).
If starch is subjected to dry heat, it breaks down to form pyrodextrins,
in the process known as dextrinization. Pyrodextrins are brown in colour.
This process is partially responsible for the browning of toasted bread.
Iodine solution is used to test for starch; a dark blue colour indicates
the presence of starch. The details of this reaction are not yet fully known, but it is thought that the iodine (I-3 and I-5 ions) fit inside the coils
of amylase, the charge transfers between the iodine and the starch, and
the energy level spacing in the resulting complex correspond to the
absorption spectrum in the visible light region. The strength of the
resulting blue colour depends on the amount of amylose present.
Starch indicator solution consisting of water, starch and iodine is
often used in redox titrations: in the presence of an oxidizing agent, the
solution turns blue, in the presence of reducing agent, the blue colour disappears because triiodide (1-3) ions break up into three iodide ions,
disassembling the starch iodine complex. A 0.3% w/w solution is the
standard concentration for a starch indicator. It is made by adding 3
grams of soluble starch to 1 litre of heated water, the solution is cooled
before use.
N/B Starch-iodine complex becomes unstable at temperatures above 35oC.
Starch as food:
Starch is the most important carbohydrate in the human diet and is
contained in many staple foods. The major sources of starch intake
50
world wide are rice, wheat, maize (corn), potatoes, cassava, etc [67].
1.2.6.1.1 Other applications of starch.
Starch can be extracted and refined from seeds, roots, and tubers, by
wet grinding, washing, sieving and drying; and used for other
applications. Today, the main commercial refined starches are
cornstarch, tapioca, and wheat and potato starch. Starch can also be
modified. A modified starch is starch that has been chemically modified
to allow the starch to function properly under conditions frequently
encountered during processing or storage, such as high heat, high
shear, low pH, freeze /thaw and cooling [67]. The industrial applications
of starch are as follows:
a. Starch adhesive: Its relatively high viscosity affords an appreciable
binding capacity. Starch becomes sticky when mixed with water or
certain chemicals. It stays sticky over a very long period of time. It can
also be used for making industrial glues.
Starch adhesives are used for the following:
i. hot- melt glues,
ii. stamps, book binding, envelopes,
iii. labels (regular and water proof),
iv. wood adhesives, laminations,
v. automotive, engineering,
vi. pressure sensitive adhesives,
vii. corrugation,
viii. paper sacks, etc.
b. Pharmacy and cosmetics: Native Starch is used as binder, fillers
and disintegrating agents for tablet production and cosmetics production.
Starch is used for the following:
i. dusting powder,
ii. make up,
51
iii. soap filler / extender
iv. face creams,
v. pill coating, dusting agent,
vi. tablet binder/dispersing agent, etc .
c. Textiles: Starch is perfect for textile application. This is why it is
widely used in the sizing of yarns and finishing of cotton and polyester
fabrics. It is also used in printing of fabrics.
d. Paper making: In paper making industry, starch is used for internal
sizing, filler retention, surface sizing, paper coating, carbonless paper,
stilt material, disposable diaper, feminine products, etc.
e. Mining: Starch is also used for the following in mining industry: ore
floatation, ore sedimentation, oil well drilling muds, etc
f. Miscellaneous: Starch is also used for the following purposes:
biodegradable plastics film, dry cell batteries, leather finishing, printed
circuit boards, etc [68].
Simple process for cassava starch production.
cassava roots peeling washing
grating/ rasping mixing with water filtering / screening
settling starch washing settling or dewatering
drying milling cassava starch [69].
1.2.7 Burning process of bio-coal briquette:
Combustion occurs when a combustible material is heated to its
ignition temperature in the presence of an oxidizing agent, which may be
air. Oxidization of a material takes place continuously as long as it is
exposed to an oxidizing agent. At ambient temperature, oxidation is
usually so slow that the process is not noticeable to human sense e.g.
rusting of iron. As temperature rises above the ambient temperature, the
rate of oxidation becomes more rapid and generates heat. When ignition
52
temperature is reached, ignition occurs and flame appears. Combustion
is the continuous burning that follows after ignition.
For the briquettes to burn, it must be combustible (hence, it needs to
be very dry), and there must be an oxidizing agent. When they are
heated to their ignition temperature, through a source (an ignition
source), pyrolysis takes place. Pyrolysis involves the decomposition of
the briquettes which slowly gives off gases. As the temperature
increases, gas evolution also increases and a flame appears, producing
heat. After some time, the flame goes off and the briquettes glow. (i.e.
becomes reddish in colour) producing more quantity of heat. When the
particles in the briquettes have been properly burnt, the glow begins to
go off and the heat it produces reduces until only ash remains.
It should be noted that combustion will continue until
i. the briquettes are remove or consumed,
ii. the oxidizing agent concentration is lowered below that
essential
iii. the combustible material is cooled below its ignition
temperature.
1.2.8 Characteristics of a good fuel (bio-coal briquette).
A good fuel should posses the following characteristics:
1. High calorific value, since the amount of heat liberated and
temperature attained depends on this.
2. Low moisture content: High moisture content quenches the fire in
the furnace, producing fly ash which can cause air pollution.
3. Moderate rate of combustion.
53
4. Low non-combustible matter content, (i.e. ash or clinker). This
reduces the heating value, adding cost of storage, handling,
disposal, etc.
5. Harmless combustion products, i.e., it should not pollute the
atmosphere by emitting CO, SO2, H2S, etc.
6. Low cost of production.
7. Easy transportation; hence it should have good hardness and
compressive strength.
8. Low storage cost.
9. Uniform size so that combustion is regular
10. A fuel should burn in air with efficiency, without much smoke.
11. Moderate ignition temperature and low ignition time
12. Controllable combustion so that combustion can be started or stopped when required
13. Low ash content: This is so because ash:
i. Reduces the calorific value of a fuel
ii. Causes hindrance of the flow of air and heat, thereby decreasing the efficiency.
iii. Increases transportation, handling and storage costs as
additional cost is involved in ash disposal
iv. Causes clinkers (ie fused ash lumps) which block the
interspaces of the grate on which the fuel is being burnt.
Therefore, the lower the ash content, the better quality of the
coal.
54
14. Moderate density: A solid fuel should have moderate density so
that it will not produce fly ash during combustion. Fly ash causes
air pollution.
15. High fixed carbon content: Fixed carbon represents the quantity
of carbon that can be burnt by a primary current of air drawn
through the hot bet of a fuel. The higher the fixed carbon content
of a fuel, the greater the calorific value the smaller the volatile
matter, the lower the ash and moisture content and the better
the quality of the fuel [70].
1.3 The aim of the research:
The aim of the research work is to investigate the effect of the use of
different biomass (groundnut shell and corn cob) on the properties of
coal briquettes.
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