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
1.0 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 evergrowing 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, Lafiobi 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.
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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.
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% 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-byelement 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].