3. CONTENTS
Introduction
Fuels and their classification
Combustion reaction of common fuels
Air-fuel ratio
Enthalpy and Internal energy of combustion
Application of First law of Thermodynamics to
chemical reaction (combustion)
Analysis of products of combustion
Orsat apparatus.
4. INTRODUCTION
Fuel is a combustible substance, containing carbon as a
main constituent, which on proper burning gives large
amount of heat, which can be used economically for
domestic and industrial purpose. Example : Wood,
charcoal, coal, kerosene, petrol, diesel, producer gas, oil
gas etc. During the process of combustion, carbon,
hydrogen, etc., combine with oxygen with a liberation of
heat.
The combustion reaction can be explained as
C + O2 CO2 + 94 kcals
2H2 + O2 2H2O + 68.5 kcals
The calorific value of a fuel depends mainly on the
amount of Carbon and Hydrogen.
5. Requirements of a Good Fuel
A good fuel should have the following characteristics:
High calorific value.
Moderate ignition temperature.
Low contents of non-combustible matters.
Low moisture content.
Free from objectionable and harmful gases like CO, SOx , H2 S.
Moderate velocity of combustion.
Combustion should be controllable.
Easy to transport and readily available at low cost
6.
7. SOLID FUELS AND THEIR CHARACTERISTICS
Solid fuels are mainly classified into two categories, i.e. natural fuels, such as wood, coal, etc.
and manufactured fuels, such as charcoal, coke, briquettes, etc.
Advantages
(a) They are easy to transport.
(b) They are convenient to store without any risk of spontaneous explosion.
(c) (c) Their cost of production is low.
(d) They posses moderate ignition temperature.
Disadvantages
(a) Their ash content is high.
(b) Their large proportion of heat is wasted.
(c) They burn with clinker formation.
(d) Their combustion operation cannot be controlled easily.
(e) Their cost of handling is high.
8. LIQUID FUELS AND THEIR CHARACTERISTICS
The liquid fuels can be classified as follows : (a) Natural or crude oil, and (b) Artificial or manufactured oils
Advantages
(a) They posses higher calorific value per unit mass than solid fuels.
(b) They burn without dust, ash, clinkers, etc.
(c) Their firing is easier and also fire can be extinguished easily by stopping liquid fuel supply.
(d) They are easy to transport through pipes.
(e) They can be stored indefinitely without any loss.
(f) They are clean in use and economic to handle.
(g) They require less excess air for complete combustion.
Disadvantages
(a) The cost of liquid fuel is relatively much higher as compared to solid fuel.
(b) Costly special storage tanks are required for storing liquid fuels.
(c) There is a greater risk of five hazards, particularly, in case of highly inflammable and volatile liquid fuels.
(d) For efficient burning of liquid fuels, specially constructed burners and spraying apparatus are required.
9. GASEOUS FUELS AND THEIR CHARACTERISTICS
Gaseous fuels occur in nature, besides being manufactured from solid and liquid fuels. The advantages and
disadvantages of gaseous fuels are given below : Advantages Gaseous fuels due to erase and flexibility of their
applications.
Advantages
(a) They can be conveyed easily through pipelines to the actual place of need, thereby eliminating manual labor
in transportation.
(b) They can be lighted at ease.
(c) They have high heat contents and hence help us in having higher temperatures. (d) They can be pre-heated by
the heat of hot waste gases, thereby affecting economy in heat.
(e) Their combustion can readily by controlled for change in demand like oxidizing or reducing atmosphere,
length flame, temperature, etc.
(f) They are clean in use.
(g) They do not require any special burner.
(h) They burn without any shoot, or smoke and ashes.
Disadvantages
(a) Very large storage tanks are needed.
(b) They are highly inflammable, so chances of fire hazards in their use is high.
10. COMBUSTION REACTIONS
A combustion reaction is a major class of chemical reactions, commonly referred to as
"burning." In the most general sense, combustion involves a reaction between any
combustible material and an oxidizer to form an oxidized product. It usually occurs
when a hydrocarbon reacts with oxygen to produce carbon dioxide and water. Good
signs that you're dealing with a combustion reaction include the presence of oxygen
as a reactant and carbon dioxide, water, and heat as products. Inorganic combustion
reactions might not form all of those products but remain recognizable by the
reaction of oxygen.
Combustion Doesn't Necessarily Mean Fire
Combustion is an exothermic reaction, meaning it releases heat, but sometimes the
reaction proceeds so slowly that the change in temperature is not noticeable.
Combustion doesn't always result in fire, but when it does, a flame is a characteristic
indicator of the reaction. While the activation energy must be overcome to initiate
combustion (i.e., using a lit match to light a fire), the heat from a flame may provide
enough energy to make the reaction self-sustaining.
General Form of a Combustion Reaction
hydrocarbon + oxygen → carbon dioxide + water
11. COMBUSTION REACTIONS
Examples of Combustion Reactions
It's important to remember that combustion reactions are easy to recognize because the
products always contain carbon dioxide and water. Here are several examples of balanced
equations for combustion reactions. Note that while oxygen gas is always present as a reactant,
in the trickier examples, the oxygen comes from another reactant.
Combustion of methane
CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(g)
Burning of naphthalene
C10H8 + 12 O2 → 10 CO2 + 4 H2O
Combustion of ethane
2 C2H6 + 7 O2 → 4 CO2 + 6 H2O
Combustion of butane (commonly found in lighters)
2C4H10(g) +13O2(g) → 8CO2(g) +10H2O(g)
Combustion of methanol (also known as wood alcohol)
2CH3OH(g) + 3O2(g) → 2CO2(g) + 4H2O(g)
Combustion of propane (used in gas grills, fireplaces, and some cook stoves)
2C3H8(g) + 7O2(g) → 6CO2(g) + 8H2O(g)
Combustion of hydrogen
2H2 + O2 → 2 H2O
12. Complete Versus Incomplete Combustion
Combustion, like all chemical reactions, does not always proceed with 100% efficiency. It's
prone to limiting reactants the same as other processes. As a result, there are two types of
combustion you're likely to encounter:
Complete Combustion: Also called "clean combustion," complete combustion is the
oxidation of a hydrocarbon that produces only carbon dioxide and water. An example of
clean combustion would be burning a wax candle: The heat from the flaming wick
vaporizes the wax (a hydrocarbon), which in turn, reacts with oxygen in the air to release
carbon dioxide and water. Ideally, all the wax burns so nothing remains once the candle is
consumed, while the water vapor and carbon dioxide dissipate into the air.
Incomplete Combustion: Also called "dirty combustion," incomplete combustion is
hydrocarbon oxidation that produces carbon monoxide and/or carbon (soot) in addition
to carbon dioxide. An example of incomplete combustion would be burning coal (a fossil
fuel), during which quantities of soot and carbon monoxide are released. In fact, many
fossil fuels—including coal—burn incompletely, releasing waste products into the
environment.
13. Stoichiometric air fuel ratio
The stoichiometric air fuel ratio is the ratio that gives the amount of air required
for the complete combustion of the unit amount of fuel.
This ratio is calculated on the molecular level. The stoichiometric ratio gives the
exact amount of air required for the complete combustion of any fuel.
If the mixture has a lower air-fuel ratio than a stoichiometric ratio then it is
known as a lean mixture.
And if the air-fuel ratio of the mixture is higher than the stoichiometric ratio then
this mixture is considered a rich mixture
14. Stoichiometric air fuel ratio
Stoichiometric air fuel ratio for different fuels:
Here are the stoichiometric ratios for the different fuels:-
Fuels Stoichiometric air-fuel ratio
Gasoline 14.7:1
Octane 15:1
Hydrogen 34.5
Methane 17
Methanol 6.5:1
Diesel 14.5:1
Ethanol 9:1
15. Stoichiometric air fuel ratio
How to calculate stoichiometric air fuel ratio?
Here are the steps to find the stoichiometric ratio of the fuel.
Step 1] Find the chemical equation of the oxidation of the fuel.
Step 2] Balance the equation.
Step 3] Find the molecular weight of fuel and the weight of the oxygen.
Step 4] Find the mass of oxygen: mass of fuel ratio
Step 5] Divide this value by 0.232 as there is only 23.2 percent of oxygen is present in
atmospheric air. This is the value of stoichiometric air-fuel for the particular fuel.
16. Stoichiometric air fuel ratio
Stoichiometric air fuel ratio example:
Stoichiometric air-fuel ratio for methane:-
The balanced chemical equation for the oxidation of methane is,
CH4+2O2→CO2+2H2OCH4+2O2→CO2+2H2O
Atomic weight of C = 12.01
Atomic weight of H = 1.00
Atomic weight of O = 16
∴ Molecular weight of CH4 = 12.01 + 4(1) = 16.01
Molecular weight of O2O2 = 2(16) = 32
The oxygen: fuel ratio is,
=Weight of oxygen Weight of methane Weight of oxygen Weight of methane
=Molecular Weight of 2O2Molecular Weight of methane CH4Molecular Weight of 2O2Molecular Weight of methane CH4
=2×3216.01=2×3216.01
= 3.997 %
The stoichiometric air fuel ratio is given by,
=3.9970.232=3.9970.232
= 17.2
Hence the stoichiometric air-fuel ratio for methane is 17.2:1.
17. Enthalpy and Internal energy of combustion
Enthalpy of combustion of a substance
It is the change in enthalpy produced when one mole of the substance is completely
burnt in air or oxygen at a given temperature.
In combustion reactions, some substances will release more energy than others.
Enthalpies of combustion can be used to compare which fuels or substances release the
most energy when they are burned. They can be calculated using a bomb calorimeter.
18. Enthalpy and Internal energy of combustion
Enthalpy of combustion of a substance
Fuel is burned and the temperature increase measured. The mass of fuel corresponding to
the temperature increase can be used to calculate the enthalpy change of the reaction,
which in turn can be used to calculate the enthalpy of combustion of that fuel.
The enthalpy of combustion of a substance is defined as the heat energy given out when
one mole of a substance burns completely in oxygen.
Combustion reactions are exothermic so the value for the enthalpy change (ΔH) is always
negative.
19. Enthalpy and Internal energy of combustion
Internal energy
It is defined as the energy associated with the random, disordered motion of molecules.
It is separated in scale from the macroscopic ordered energy associated with moving
objects;
it refers to the invisible microscopic energy on the atomic and molecular scale.
For example, a room temperature glass of water sitting on a table has no apparent energy,
either potential or kinetic. But on the microscopic scale it is a seething mass of high speed
molecules traveling at hundreds of meters per second. If the water were tossed across the
room, this microscopic energy would not necessarily be changed when we superimpose an
ordered large scale motion on the water as a whole.
20. Analysis of products of combustion
Combustion analysis is a method used in both organic chemistry and analytical
chemistry to determine the elemental composition (more precisely empirical formula) of
pure organic compound by combusting the sample under conditions where the resulting
combustion products can be quantitatively analyzed. Once the number of moles of each
combustion product has been determined the empirical formula or a partial empirical
formula of the original compound can be calculated.
Applications for combustion analysis involve only the elements of carbon (C), hydrogen
(H), nitrogen (N), and sulfur (S) as combustion of materials containing them convert
elements to their oxidized form (CO2, H2O, NO or NO2, and SO2) under high
high oxygen conditions. Notable interests for these elements involve measuring total
nitrogen in food or feed to determine protein percentage, measuring sulfur in petroleum
products, or measuring total organic carbon (TOC) in water.
21. Analysis of products of combustion
Combustion analysis, the products, carbon dioxide and water vapor, are trapped by
absorption onto reactive solids located in tubes above the reaction vessel. These tubes can
then be weighed to determine the absorbed masses of carbon dioxide and water.
The mass of carbon in the starting material is determined by a 1:1 ratio with the mass of
carbon dioxide produced (as in the combustion reaction for methane already displayed).
The initial hydrogen mass is determined by a 2:1 ratio with the amount of water produced.
The data and the ratios can then be used to calculate the empirical formula of the unknown
sample. Combustion analysis can also be performed using a CHN analyzer, which uses gas
chromatography to analyze the combustion products.
22. Analysis of products of combustion
Why Perform Combustion Analysis?
Improve Fuel Efficiency
Reduce
Improve Safety
What’s Measured?
Combustion analysis involves the measurement of gas concentrations, temperatures and pressure for boiler
tune-ups, emissions checks and safety improvements.
Parameters that are commonly examined include:
• Oxygen (O2)
• Carbon Monoxide (CO)
• Carbon Dioxide (CO2)
• Exhaust gas temperature
• Supplied combustion air temperature
• Draft
• Nitric Oxide (NO)
• Nitrogen Dioxide (NO2) • Sulphur Dioxide (SO2)
23. Orsat apparatus
The Orsat Apparatus is used to Analysis the products combustion, this is done by the process explained
below.
It consists of burette to measure and there are three flasks.
There is a chemical in the flasks to absorb different types of gasses, like CO2, O2, CO.
This flask has Potassium hydroxide (KOH), Pyrogallic acid which is an alkali, Cuprous chloride. this is to
observe the gases.
By the lowering, the leveling bottle a small number of combustion products gets into the measuring
burette.
This small amount of sample is passed through every flask which has reagents that are used to absorbs
the gases.
This sample is returned to the vessel where the volume is measured.
At the absorption process, there is a volume decrease this used to represent the partial volume of each
constituent
Which has been absorbed.
In this, the CO2 is taken as N2 After the absorption which is remainder of flue gas.
There is a sequence in absorption CO2, O2, and CO.
In this, if the Absorbent is used to absorbing O2 which can absorb a small amount of CO2, by this amount
of CO2 is less which is given by the Or sat apparatus if there is the first CO2 is observed and next CO2.
25. Orsat apparatus
This small amount of sample is passed through every flask which has reagents that are used
to absorbs the gases.
This sample is returned to the vessel where the volume is measured.
At the absorption process, there is a volume decrease this used to represent the partial
volume of each constituent
Which has been absorbed.
In this, the CO2 is taken as N2 After the absorption which is remainder of flue gas.
There is a sequence in absorption CO2, O2, and CO.
In this, if the Absorbent is used to absorbing O2 which can absorb a small amount of CO2, by
this amount of CO2 is less which is given by the Or sat apparatus if there is the first CO2 is
observed and next CO2.
The analysis of this apparatus gives on the due basis, this is given by the temperature which
is lower than the dew point temperature of combustion temperature.
The analysis of this apparatus gives on the due basis, this is given by the temperature which
is lower than the dew point temperature of combustion temperature.
