Abstract
The urgent need to protect our forest, to mitigate health hazards faced by
the people from the use of firewood for cooking and to find an effective
means of managing agro wastes has prompted a research on improving the
properties of coal briquette using spear grass (Imperata cylindrica) and
elephant grass (Pennisetum purpureum). In the research, proximate
analysis and the elemental composition of the plant materials were carried
out alongside with a coal sample. Briquettes of different composition were
produced by blending the plant materials with the coal at various
concentrations: 0%, 10%, 20%, 30%, 40%, 50% and 100%. The physical,
mechanical and combustion properties of the briquettes were compared. It
was found that the ignition, burning rate and reduction in smoke emission
showed improvement with increase in biomass concentration. Compressive
strength and cooking efficiency – water boiling time and specific fuel
consumption showed initial improvement and rendered to
INTRODUCTION
1.1 Background of the Study
Nigeria, like other sub-Saharan countries, has faced forest degradation
problems due to combination of factors. Some of the factors are
clearing of land for agricultural and industrialization purposes, over
grazing, bush fires, drought, over exploitation, ever- increasing
deforestation along with the increased in the consumption of fuel wood
etc.
About 80% of Nigerians live in the rural or semi-urban areas and they
depend solely on fuel wood for their energy needs. Fuel wood accounts
for about 37% of the total energy demand of the country. Investigations
showed that out of the total wood demand from the forest, 90% goes to
fuel wood. Presently, Nigeria reportedly consumes about 43 x 109
kg of
fuel wood annually [1] and it will be far more than this by the end of
2010 if the present trend continues [2]. However, it is very obvious that
reduction in the use of fuel wood will drastically reduce the pressure
mounted on the forest in search of wood.
Meanwhile, it was reported that the total forest cover of Nigeria is still
less than 10% of the land area, which is far below the 25%
recommended by the United Nation Development Programme (UNDP)
[2]. Therefore, it is imperative that concerted efforts are needed to
address this situation.
2
Furthermore, in the recent years, global warming has become an
international concern. Global warming is caused by green house gasses
which carbon dioxide is among the major contributors. It was shown
that increased emission of CO2 in the atmosphere in the recent time has
exacerbated the global warming [3]. Part of the reasons for this can be
explained from the fact that the forest resources which act as major
absorbers of CO2 have been drastically reduced owing to the fact that
the rate of deforestation is higher than the afforestation effort in the
country.
Apart from environmental effects, the use of fuel wood for cooking has
health implications especially on women and children who are
disproportionately exposed to the smoke. Women in rural areas
frequently with young children carried on their backs or staying around
them, spend one to six hours each day cooking with fuel wood. In
some areas, the exposure is even higher especially when the cooking is
done in an unventilated place or where fuel wood is used for heating of
rooms. Generally, biomass smoke contains a large number of
pollutants which at varying concentrations pose substantial risk to
human health. Among hundreds of the pollutants and irritants are
particulate matters, carbon monoxide, formaldehyde and carcinogens
such as benzo[?]pyrene, 1,2–butadiene and benzene [4]. Studies
showed that indoor air pollution levels from combustion of bio fuels in
Africa are extremely high, and it is often many times above the
3
standard set by US Environmental Protection Agency (US-EPA) for
ambient level of these pollutants [5].
Also, consistent evidence revealed that exposure to biomass smoke
increases the risk of a range of common diseases both in children and
in adults. The smoke causes acute lower respiratory infection (ALRI)
particularly pneumonia in children [6, 7]. Among the women, it causes
chronic bronchitis and chronic obstructive pulmonary diseases (COPD)
(Progressive and incompletely reversible air ways obstruction) [8,9].
Eyes irritation (sore, red eyes, tears) from the smoke is also a common
experience in the use of fuel wood. A hospital based case – control
studies proved that a person exposed to smoke of biomass has high risk
of cataracts disease [10]. This evidence was further substantiated by an
experiment carried out on animals which showed that biomass smoke
is capable of damaging eye lens [11].
In the whole, it was summed up that the total deaths attributed to the
use of fuel wood in Nigeria are about 79,000. Also nearly 45% of the
national burden diseases are related to solid fuel use, according to a
WHO Survey [2]
. Again, combustion of raw coal has equally been
reported to have detrimental effects on both environments and the
health of the people. Among other effects, inhalation of coal smoke
increases the risk of lung cancer [12].
Frankly speaking, transition to electricity or gas would have been the
healthiest solution to these problems but the likelihood of a complete
4
transition in the poorer urban and rural communities in the near future
is minimal. Therefore it is pertinent that other intervention measures
especially ones recommended by WHO [4] should be adopted to
mitigate these health risks to the lowest possible level and equally to
relieve the forest resources from pressure mounted on it.
Fortunately, researches have shown that a cleaner, affordable fuel
source which is a substitute to fuel wood can be produced by blending
biomass (agricultural residues and wastes) with coal. Nigeria has large
coal deposit which has remained untapped since 1950’s, following the
discovery of petroleum in the country. Also, millions of tonnes of
agricultural wastes are generated in Nigeria annually. But it is
unfortunate that farmers still practise “slash–and-burn” agriculture.
These agricultural wastes they encounter during clearing of land for
farming or during processing of agricultural produce are usually burnt
off. By this practice, not only that the useful raw materials are wasted,
it further pollutes the environment and reduces soil fertility.
Fire affects soil below ground biodiversity, geomorphic process, and
volatilizes large amount of nutrients and carbon accumulated in the soil
organic matter [13]. Furthermore, during process of burning of
agricultural wastes on the field, if it is not properly controlled, it can
inadvertently lead to bush fire, destroying further the forest which has
suffered much from the hand of wood seekers.
5
Forest fire is one of the severe environmental problems in Nigeria and
every year various forest types are burnt as a result of fire set up
deliberately or inadvertently through careless or uncared acts. Forest
fire destroys the fresh saplings, seedlings and arrest regeneration of
native species [13].
However, these health hazard faced by people from the use of fuel
wood, along with the agricultural wastes management and reduction of
pressure mounted on the forest can be mitigated if Nigeria will switch
over to production and utilization of bio-coal briquette; a cleaner and
environmental friendly fuel wood substitute made from agricultural
wastes and coal. Moreover, this will offer a good potential for
utilization of a large coal reserve in Nigeria for economic
diversification and employment generation through bio-coal briquette
related SMEs.
1.2 Literature Review
1.2.1 Briquetting Process
Briquetting is a mechanical compaction process for increasing the
density of bulky materials. This process is used for forming fine
particles into a designed shape. It can be regarded as a waste control
measure in the case of production of briquettes from agricultural
wastes. However, depending on the material of interest, briquetting can
be used to provide fuel source as a preventive measure to many
ecological problems. Briquetting is a high- pressure process which can
6
be done at elevated temperature [14] or at ambient temperature [15, 16]
depending on the technology one wants to employ.
During this process, fine material is compacted into regular shape and
size which does not separate during transportation, storage or
combustion. In some briquetting techniques, the materials are simply
compressed without addition of adhesive (binderless briquettes) [17,
18] while in some, adhesive material is added to assist in holding the
particles of the material together [15, 16].
Generally, briquetting process has focused more on the production of
smokeless solid fuels from coal and agricultural wastes. There are
various techniques which have been used to produce smokeless solid
fuel from coal fine. The most common technique is the use of roller
press using only moderate pressure and binder. Note that the machines
employed for this process are also used to make other kind of non-fuel
briquettes from inorganic materials such as metal ores. However,
briquetting of organic materials (agricultural wastes) requires
significantly higher pressure as additional force is needed to overcome
the natural springness of these materials. Essentially, this involves the
destruction of the cell walls through some combination of pressure and
heat. High pressure involved in this process suggests that organic
briquetting is costlier than coal briquettes.
Various briquetting machines have been designed, ranging from very
simple types which are manually operated to more complex ones
7
mechanically or electrically powered. Generally, briquetting operations
have developed in two directions, mechanically compression
(hydraulic or pistons) and worm screw pressing types.
1.2.2 Historical Background of Briquetting Process [18- 21]
Although, compaction of loose combustible materials for fuel making
purposes is a technique which has been in existence thousands of years
ago but industrial method of briquetting seems to be dated back to
eighteenth century. In 1865, report was made on machines used for
making fuel briquettes from peats and are recognized as the
predecessors of the present briquetting machines. Since then, there has
been a wide spread use of briquettes made from brown coal and peat
etc.
The use of organic briquettes (biomass briquettes) started more
recently compared to coal briquette. It seams to have been common
during World War and during the 1930s depression. The modern
mechanical piston briquetting machine was developed in Switzerland
based upon German development in the 1930s. Briquetting of saw dust
are widespread in many countries in Europe and America during World
War II because of fuel shortages. However, after the World War,
briquettes were gradually phased out of the market because of
availability and cheapness of hydrocarbon fuels.
As time went on, the use of organic briquette was revitalized due to
high energy prices in the 1970s and early 1980s mainly for industrial
8
heating in USA, Canada and Scandinavia, etc. In Japan, the use of
briquette seems to be very common especially the use of “Ogalite” fuel
briquettes made from saw dust and rice husk. The Japan technology
has spread to Taiwan, and from there to other countries such as
Thailand, Asia, USA and some other European countries. This type of
briquette has been in use in Japan since 1950s as a substitute for
charcoal which was then a widespread fuel source.
Furthermore, in Great Britain, the first fuel briquette was manufactured
by the Powell Dul Fryn Company in 1938 by heating anthracite chips
bound with pitch to a temperature of around 750oC to produce a
briquette known as phurnacite and this production was taken over by
the National Coal Board in 1942. They were able to produce half a
million tonne of coal annually. It was also reported that the same
technique was tried on production of smokeless coal briquette from
low-rank coal containing as much as 30-40% volatile materials. But the
problem was how to reduce the volatile component of the coal to
prevent smoke formation and at the same time, retaining sufficient
active constituents to give an easily lighted bright fire with a high
radiation. It was found that moderate heating (carbonization) of the low
rank coal not only drives off a portion of the volatile matter but appears
to change the remaining volatile portion in such a way that it does not
smoke even with a volatile content as high as 23%. Phurnacite was
produced from low-rank coal by heating the coal bound with pitch to
9
200oC. Since then, other technologies for production of smokeless
briquettes were developed in Britain. These include the multi heat
briquette marketed by the National Coal Board. This brand of
smokeless briquette was made by curing pitch bound briquette in a bed
of sand which is fluidized and kept at a temperature of about 380oC.
Other types of briquettes developed were home fire and room heat.
Today, many other developing countries have adopted and developed
briquetting technology, owing to high cost and scarcity of fuel.
Common types of briquettes so far in use are coal briquettes, peat
briquettes, charcoal briquettes and biomass briquettes, etc. Most
recently, researches showed that blending of coal and biomass will give
rise to a briquette with better combustion properties and pollutants
emission reduction. This type of briquette is known as bio-coal
briquette. Some authors simply called it biobriquette [16].
1.3 Bio-Coal Briquettes
Bio-coal briquette is a type of solid fuel prepared by blending coal,
biomass, binder and sulphur fixation agent [23,24]. Other additives
may also be added. A research showed that bio-coal briquettes may be
prepared by blending the following [15]:
? Biomass (25% to 50%)
? Coal (75% to 50%)
? Sulphur fixation agent (up to 5%)
? Binder (up to 5%)
10
Also, according to clean coal technologies Japan, bio-coal briquettes
are prepared by blending:
? Biomass (10% to 25%)
? Coal (75% to 90%)
? Sulphur fixation agent (depending on the sulphur content of the
coal).
In this process, Ca(OH)2 acts as both sulphur fixation agent and the
binder [16]. Also, activators such as iron oxide, potassium manganate
and sodium chloride have been reported to have the ability of
improving the thermal efficiency of the briquette [26].
The high pressure involved in the process ensures that the coal particles
and the fibrous biomass material interlace and adhere to each other as a
result, do not separate from each other during combustion,
transportation and storage. During combustion, the low ignition
temperature of the biomass simultaneously combusts with the coal. The
combined combustion of both gives a favorable ignition and fire
properties; emits little dust and soot, generates sandy combustion ash,
leaving no clinkers [16, 24]. Also the desulfurizing agent such as
Ca(OH)2 in the briquette effectively reacts with the sulphur content of
the coal to fix about 60-80% of it into the ash [16]. It was showed that
lime (CaO) as a desulfurizing agent was able to capture up to 90-95%
of the total sulphur in the coal, leaving only 5-10% emitted as sulphur
oxides [25]. The equation of the reaction is as follows:
11
CaO(s)
+ SO2 (g) + ½ O2 (g) CaSO4(s).
Evidence also revealed that lime when used as desulfurizer also acts as
a binder. Also clay has been reported to be a good desulfurizing agent.
Clay contains CaO and MgO which acts as desulfurizing agents. Also
it contains Fe2O3 which has been shown to have a catalytic effect on
the sulfation reaction [25].
There are various biomass resources available for production of biocoal briquettes. Some of them are straw, sugar bagasse (fibrous residue
of processed sugar cane), corn stalk, groundnut – shell, wheat straw,
palm husk, rice husks, corn cob, forest wastes, and other agricultural
wastes. Several researches on bio–coal briquette have been carried out
using some of these biomass resources. There are records of researches
carried out on production of bio-coal briquettes using sawdust [27],
rice straw [28], olive stone [29, 30] and maize cob [24], etc.
Furthermore, it has been shown that any grades of coal can be used for
bio-coal production, even low grade coal containing high sulphur
contents [24, 26]. This implies that, by this technology, extra cost of
carbonizing low grade coal before briquetting is saved.
Binder is an adhesive material which helps to hold the particles of the
material together in the briquette. Apart from its function to hold the
particle from separation, it also protects the briquette against moisture
in case of long storage [13]. There are several binders that can be used.
Some of them are starch (from various starchy root such as cassava,
12
and cereals), molasses, clay and even tree gum, etc. Some chemical
substances have also been used as binding agent for production of
briquettes. Some of them are asphalt [31], potassium [32], magnesia
[33], ammonium nitrohumate [34] and commercial pitch [35]. Though
the use of starch as binder is satisfactory in every respect, it
disintegrates under moist or tropical condition. However, the use of
small additional hydrocarbon binder such as pitch or bitumen has been
reported to improve the water resisting property [36]. Moreover, the
nature of the binder has influence in the combustibility of the briquette
produced. For instance, briquette produced using clay takes longer time
to ignite than the one produced using starch [13]. The reason for this is
because of non-combustibility of clay compared to starch.
1.3.1 Characteristics of Bio-Coal Briquettes
(1) Bio-coal briquette decreases the generation of dust and soot up to
one-tenth that of direct combustion of coal [16]. Combustion of
coal generates dust and soot because, during the combustion, the
volatile components of the coal are released at low temperature
(200-4000C) as incomplete combusted volatile matter. For bio-coal
briquette, since the biomass component of the briquette ignites at
low temperature compare to the coal, this ensures that the volatile
matter in the coal which would have otherwise been liberated as
smoke at low combustion temperature combusts completely. By so
doing, there is a significant reduction in the amount of dust and
13
soot generated. Note that smoke is a complex mixture of air-borne
solid and liquid particulates as well as gases evolved on pyrolysis
or combustion of material [45].
(2) Bio-coal briquette has a significant shorter ignition time when
compared with coal or conventional coal briquette [15]. This is
because of the biomass component of the briquette. Biomass has
low ignition time.
(3) Bio-coal briquette has superior combustion-sustaining properties.
Because of low expansibility and caking properties of bio-coal
briquette, sufficient air flow is maintained between the briquettes
during combustion in a fire-place. Hence it has very good
combustion-sustaining properties and does not die out in a fireplace
or other heater even when the air supply is decreased [16]. This
property offers the opportunity of adjusting the combustion rate of
the bio-coal briquette easily.
(4) Bio-coal briquette emits less SO2. It contains desulfurizing agent
and the high pressure involved in the process enables the coal
particles to adhere strongly to the desulfurizing agent. During
combustion, the desulfurizing agent effectively reacts with the
sulphur content of the coal to form a solid compound instead of
being released as oxides of sulphur to the atmosphere. However, it
is widely accepted that bio-coal briquette technology is one of the
14
most promising technologies for the reduction of SO2 emission
associated with burning of coal [24,37].
(5) Bio-coal briquette has high breaking strength for easy
transportation. The high pressure involved in the process coupled
with the binder, compressed the raw materials into a rigid mass
which does not break easily, hence can be stored and transported
safely [16].
(6) Bio-coal briquette generates sandy ash which can be utilized in
agriculture for soil improvement [38]. In the briquette, since the
fibrous biomass intertwined with the coal particles, the resulted ash
after combustion does not adhere or form clinch-lump, therefore,
the ash is always sandy.
(7) Bio-coal briquette burns nearly perfect; therefore the flame has
significant higher temperature than simple biomass burning [39] or
coal [26].
15
1.3.2 Table 1: Comparative Tests of Bio-coal Briquettes [40]
Briquettes
Calorific
value
(kcal/kg)
Mass(in kg)
of
Evaporated
water by
1kg of the
briquette
Time(in minute) to
cook a mixture of
1kg of mansuli
rice, 0.25kg of
rahar dal and
0.5kg of potatoes
Mass (in
kg) of fuel
consumed.
75% coal
(Jumlepani ghorahi
coal) and 25%
biomass (Lumibini
bagasse.
4222.5 1.5 64 1.535
80% coal (40%
Jumlepani Ghorahi
coal and 40%
lignite) and 20%
biomass (Lumbini
bagasse)
____ ____ 104 1.650
20% coal (Abidara
coal) and 80%
biomass (Chitiwan
rice husk)
3806 1.0 _____ _____
40% of coal
(Abidara coal) and
58% biomass
(Nawalparasi rice
husk) and 2%
Chovar lime.
____ _____ 83 2.643
Coal (Indian coal) 5900 2.46 ____ ____
Cowdung 3710 1.2 144 4.00
Rice husk. 3688 0.86 112 3.035
Fuelwood. 3600 0.68 85 2.992
16
1.3.3 Comparison of Efficiency of Bio-Coal Briquette with Fuel
Wood.
Comparative tests of different bio-coal briquettes with fuel wood
showed that [40]:
? Bio-coal briquettes can boil more water than fuel wood using an
appropriate stove, under similar condition.
? Bio-coal briquette takes less time to cook the same amount of
foodstuffs than fuel wood.
? Only half the weight of bio-coal briquette (75% of coal and 25% of
biomass) is required to achieve the same results compared to fuel
wood.
? Bio-coal briquettes are easier to ignite and last for a longer period of
time than fuel wood.
? The combustion temperature of bio-coal briquette is higher than that
of fuel wood [29].
1.3.4 Production Process of Bio-Coal Briquette
The production process of bio-coal briquette is very simple and cost
effective. The raw materials; coal and biomass are pulverized to a size
of approximately 3mm and then dried. Research showed that 0-5mm is
the optimum particle size of the raw materials for a briquette [41]. The
dried pulverized materials, a desulfurizing agent and binder are mixed
together in appropriate proportions and are compressed with briquette
machine into a designed shape. The type of briquette machine
17
determines the shape and size of the briquette. Some briquette
machines have small mould while some have relatively larger mould.
For a large mould, there is always a facility which creates holes in the
briquettes when formed. These holes are necessary for efficient
combustion of the briquette. It allows for proper flowing of air needed
to maintain the combustion [42].
In this production process, high temperature is not required. The
process is simple, safe and does not require skilled operating technique.
Hence the process can easily be adopted and sustained in Nigeria. The
basic process flow for bio-coal production is shown in Fig.1.
18
Fig.1: Basic process flow for bio-coal production
Crushing
Drying
Briquetting
Mixing
Biomass
Pressure: 1-3t/cm2
Temperature: room temp.
Drying
Storage
Desulfurizer, binder, water
Raw coal
Drying
Crushing
19
Production of Bio-coal Briquette by Co–pyrolysis of Coal and
Biomass
In this method, the raw materials; coal and biomass are first cocarbonized. The coal and the biomass are dried up to 15% moisture of
the materials. And the material is ground and sieved to get fractions
between 0-5mm. After that, the fine coal and biomass are mixed
together and co-carbonized to a temperature of 500-600oC for 20-30
minutes. Then, a binder and desulfurizing agent are added in the
appropriate amount. The mixture is blended very well and compressed
into briquettes and the mechanical strength and water resistance is
improved by curing the briquette at a temperature between 120-180oC
for 2-4 hours. However, this method of production of bio-coal briquette
seems to be more expensive than the previous method due to extra
energy needed for carbonization of the raw materials.
1.3.5 Bio-coal Briquette Ash
The ash of bio-coal briquette has been shown to be effective for soil
improvement [43,37]. Table 2 shows the chemical composition of the
ash of a bio-coal briquette sample {coal (72.5%), biomass (13% sawdust, and 1.5% straw), CaO (7%)} [26]. The ash contains calcium
compounds such as Ca(OH)2, CaO, CaSO4.2H2O, CaSO3, CaCO3,
etc
which make it to have an acid neutralizing ability and as well,
functions as plant nutrient. Also, because sulphuric by-products in biocoal ash is generated from sulphur in the coal, bio-coal briquette
20
produced with a coal having higher sulphur contents might produce
more effective soil improving ash [26].
Furthermore, there was a plan by China to employ the use of bio-coal
briquette and its ash as a CDM (Clean Development Mechanism) to
obtain carbon credit in order to implement the reduction of CO2. The
plan was to switch from the use of coal to bio-coal briquette and
utilization of the ash to improve the soil on desert and semi-desert
areas along with planting of trees in the improved soil of the areas [26].
This suggests that it is possible to reduce CO2 and at the same time
keep coal consumed.
Table 2: chemical analysis on bio-coal briquette ash
Compounds Percentage composition (%)
Ca(OH)2 1
CaO 9
CaSO3 1
CaSO4.2H2O 10
SiO2 27
CaCO3 5
Al2O3 19
21
1.4 Preparation of other types of Briquettes
As it has been mentioned earlier, briquette is a kind of solid smokeless
fuel produced by compressing pulverized raw material under high
pressure at ambient or elevated temperature. The raw materials are
generally coal and biomass of various forms. The name given to any
fuel briquette depends on the materials of which it was made. For
instance, common briquettes; peat briquettes, charcoal briquettes,
biomass briquettes and coal briquettes are prepared as follows:
Peat briquettes: Peat briquette consists of shredded peat, compressed
to form a solid fuel [44].
Charcoal briquettes: Charcoal briquette is a common type of briquette
made by compressing pulverized wood charcoal with a binder.
However, other activator such as sodium nitrate may be added as an
accelerant.
Biomass briquettes: Biomass briquette is made from agricultural
wastes. It is a renewable source of energy. Lignin and cellulose are the
two major compounds of biomass. The lignin distributed among
cellulose determines the structural strength of biomass [39]. Lignin is a
non-crystallized aromatic polymer with no fixed melting point. When
heated to 200-300oC, lignin melts and liquefies. When pressure is
applied in this case, the melted lignin glues the cellulose together;
hence the biomass is briquetted when cooled. This method of
production of biomass briquette is based on lignin plasticization
22
mechanism [39]. However, biomass briquette can also be produced at
room temperature by the application of another briquetting technique;
in that case binder is used.
Fig.2: Basic process flow for production of biomass briquette by
plasticization mechanism
Coal briquette: Coal briquettes are made by compressing finely
divided coal particles. The coal is dried, crushed into appropriate
particle sizes. Binder and desulfurizing agents are added, then the
material is compressed into briquette [25]. Also, coal briquette can
Biomass
Crushing
Drying
Briquetting
Cooling
Storage
Diameter < 10mm
Moisture 6-14%
Pressure: 4-60Mpa
Temperature: 160-280oC
23
be produced by first carbonizing the coal before it is used [16].
During the carbonization, some of the volatile components of the
coal are driven off.
Fig.3: basic process flow for production of coal briquette
Crushing
Drying
Briquetting
Carbonization
Mixing
Diameter: 5-50mm
Moisture: 10% or lower
Pressure of 300-500kg/cm2
(roll-molding briquette machine)
and room temperature.
Raw coal
Drying
Storage
450oC
Desulfurizer, binder, water
24
1.5 Combustion Process
Burning process is a self-sustaining exothermic chemical reaction.
Burning is first initiated by initial supply of external heat. When this
initial heat is being supplied to a material, the temperature of the
material is raised until at a certain temperature, the material begins to
degrade into various gases (combustible and non-combustible gases) as
well as carbonaceous char. This process is known as pyrolysis. Then;
ignition occurs between the combustible gases produced and oxygen at
ignition temperature to produce flame [45]. If there is insufficient
supply of oxygen, there will be incomplete combustion resulting into
formation of carbonaceous products (such as char), smoke, unburnt
flammable volatile and non-flammable gases.
Smouldering:
Smouldering simply means burning of substance without flame. It is a
heterogeneous oxidation of a solid surface by a gaseous oxidant
(oxygen) [45]. Charcoal is an example of substance that smoulders.
Factors which Control Burning of Material
Some factors which control the burning of substances are listed below;
? Chemical composition of the material
? Geometry of the material (bulk, packing, orientation and surface
contour, etc)
? Size of the material
25
? Condition of burning such as initial temperature, relative humidity
and draught.
However, the chemical composition and physical properties of a
briquette such as thermal conductivity, porosity, calorific value and the
moisture contents, etc, will have a great influence on the combustion
performance of the briquette. Again, the rate of adsorption of oxygen
and desorption of the combustible products also have rolls to play in
the general combustion rate. Therefore, the spatial configuration of the
briquettes on the combustion boat is important. Loosely packed
briquettes in the combustion boat will have tendency to promote rapid
diffusion of the oxidants for a better combustion.
1.5.1 Pyrolysis and Combustion of Cellulosic Materials (Grasses)
As it has been mentioned earlier, burning of substance proceeds in two
stages; first, pyrolysis of the material and then the combustion
(oxidation) of the combustible pyrolysis products with release of
energy. In practice, pyrolysis and combustion of the resulted volatiles
may occur at almost the same time. The gas or gases evolved from
pyrolysing substances depend upon the heating rate and largely on the
nature of the substance. Common gaseous pyrolysates (pyrolysis
products) arising from pyrolysis of cellulosic materials include: carbon
dioxide (CO2), carbon monoxide (CO), ammonia (NH3), methane
(CH4), hydrogen (H2), hydrogen cyanide (HCN) and water vapor
(H2O), etc [45].
26
The degradation of cellulose may arise from the cleavage of the
glucosidic bond or by dehydration or breakdown of the anhydroglucose
units. Below 300oC, dehydration, elimination and breakdown occur.
The result of this process is gradual charring and depolymerization of
the macromolecules. At higher temperature, there is rapid cleavage of
the glucosidic bond accompanied by evaporation of the products.
Cellulosic pyrolysis takes either of these routes [45].
Laevoglucosan CO + other combustible
Volatiles eg. Alkanes,
1 alkenes, alkanols,
aldehydes and ketones.
(C6H10O5)n
2
H2O + C (Char).
The first route involves depolymerization of dehydrocellulose chain
forming laevoglucosan which further decomposes to form flammable
volatiles; carbon monoxide, alkanes, alkenes, alkanols, aldehydes etc.
The volatiles can subsequently form secondary pyrolysis products and
or combust in the presence of oxygen with release of heat.
27
The second route involves a complete dehydration of the material to
form water and char. The char is then oxidized by the oxygen to CO2
and CO with release of heat.
1.5.2 Pyrolysis and Combustion of Coal
Pyrolysis of coal (bituminous coal) converts about half of the coal
mass into gases including many fuel compounds. The subsequent
secondary pyrolysis and the combustion of these volatile compounds
accounts for large portions of the heat release, pollutant formation and
soot evolution during coal combustion. The pyrolysis of bituminous
coal at temperature between 2670C to 3620C revealed the presence of
the following compounds, in the volatile as determined by gas
chromatography and non-dispersive infrared analysis [46]. The
compounds are H2, CO, CH4, C2H4, C2H6, C2H2, C3H6, CO2, H2O and
light oil. H2, CH4 and CO was found to be more abundant in the
volatile composition while the remaining hydrocarbons contribute
about the half of the heat released during the combustion of the
volatiles. In fact, the light oil alone contributes the quarter of the heat
released. This is because of their high molecular weights, very small
mole fractions of these higher hydrocarbons contribute significant
amount of energy released.
However, it is expected that lower rank coal, on pyrolysis will produce
more amount of these volatiles than the ones released by bituminous
coal.
28
It is worthy to note that in the study of the pyrolysis products of
material, the volatiles isolated may consist of the primary and
secondary pyrolysis products. The secondary pyrolysis products result
from either the thermolysis of the primaries or the interaction of these
in either the gas or the condensed phase. This poses a complication in
the identifications of the pyrolysis products of a material.
1.6.1 Coal
Coal was formed by the remains of vegetable that were buried under
ground million of years ago under great pressure and temperature in the
absence of air. Coal is a complex mixture of compounds composed
mainly of carbon, hydrogen and oxygen with small amounts of sulphur,
nitrogen, and phosphorus as impurities. Dry anthracite was found to
have the following compositions [47].
Carbon – 90%
Hydrogen – 3%
Oxygen – 2%
Nitrogen – 1%
Sulphur – 1%
Ash – 3%.
Lower rank coal is expected to have lower percentage of carbon with
increased percentages of other component elements. Examination of
coal revealed that its structure is composed of aromatic and cyclic
structures [49]. Fig.4 shows an example of chemical structure of coal.
29
1.6.2 Types of Coal
Coals are classified according to their fuel properties. The higher the
carbon contents of coal, the better the fuel properties. Therefore, coal
classification is based on the degree to which the original plant
materials have been transformed into carbon. The older the coal, the
higher the carbon content and the better the fuel properties. This also
implies that the rank of coal is an indication of how old the coal is.
Types of coal are as follows [47,48]:
CH
OH OH
OOCH3
OH
OH H
HO OH
HO H
OH
OH
OH
Fig.4: Chemical structure of coal
30
? Peat: Peat is considered to be precursor of coal. It is used as fuel in
some countries. In a dehydrated form, peat is an effective absorbent for
oil spill on land and water.
? Lignite: Lignite coal is also called brown coal. It is the lowest rank
of coal, brownish black and has high moisture content (up to 45%),
calorific value of less than 5kw/kg and high sulfur content. It is
generally used as fuel for generation of electricity.
? Sub-bituminous coal: The properties of sub-bituminous coal ranges
from those of lignite to those of bituminous coal. It is black in colour.
It contains 20-30% moisture, calorific value of between 5-6.8kw/kg.
Sub-bituminous coal is primarily used as fuel for steam-electric power
generation.
? Bituminous coal: It is black and sometimes dark brown in colour. It
is most common coal, has moisture content of less than 20% and
calorific value ranging from 6.8-9kw/kg. It is mainly used for
generation of electricity.
? Anthracite: Anthracite is the highest rank of coal and is referred to
as hard coal. It is hard and lustrous. Anthracite is high in carbon
content, low in sulphur content and moisture content. The calorific
value is about 9kw/kg or above. It is mainly used for residential and
space heating.
31
1.6.3 Gasification of Coal
Coal gasification can be used to produce syngas. Syngas is a mixture of
carbon monoxide and hydrogen. The syngas can then be converted into
transportation fuel like gasoline and diesel through the Fischer-Tropsch
process [49]. This process has recently been used by the Sasol
Chemical Company of South Africa to make gasoline from coal and
natural gas [49]. Furthermore, the hydrogen obtained from the
gasification of coal can be used for other purposes such as powering of
hydrogen economy, making ammonia or upgrading fossil fuels.
During the process of gasification of coal, the coal is mixed with
oxygen and steam, heated and pressurized. The oxygen and the water
molecule oxidize the coal into carbon monoxide (CO) and hydrogen
(H2) is equally formed.
(Coal) + O2+ H2O H2 + CO ………………… (1)
Syngas