​

UNIT – IV

Fuels and Combustion

​

Dr.A.GEETHA

ASSOCIATE PROFESSOR & HEAD

DEPARTMENT OF CHEMISTRY

KONGU ENGINEERING COLLEGE

PERUNDURAI

​

*

​

UNIT – IV

​

Fuels and Combustion:

Introduction – classification of fuels - characteristics of a good fuel - combustion - calorific values – gross and net calorific values - Dulong‘s formula (simple problems) - Flue gas analysis by Orsat‘s method - solid fuels - coal and its varieties – proximate analysis – significance – metallurgical coke - Otto-Hoffman byproduct method - liquid fuel - refining of petroleum – manufacture of synthetic petrol - hydrogenation of coal - Bergius process - knocking - octane number – cetane number - gaseous fuel - water gas

Fuels: Any substance used to produce heat or power by combustion. Any chemical process accompanied by the evolution of light or heat is called combustion.

​

Fuel is a combustible substance containing carbon as a main constituent whichon proper burning gives large quantity of heat, which can be used economically for domestic and industrial purpose

Fuel + Oxygen → Combustion products + Heat

Example of Fuels:

wood

Charcoal

Coal

Kerosene

Petrol

Diesel

Producer gas

Oil gas

​

CLASSIFICATION

​

FUEL ------------------- Primary or Natural

Secondary or Artificial

​

​

Primary----------Solid

Liquid

Gaseous

​

Secondary ----------Solid

Liquid

Gaseous

Solid fuel (e.g., wood, coal)

Characteristics of Good Fuels:

• High Calorific Values

• Moderate Ignition Temperature

• Low Moisture Content

• Low Ash Content

• Moderate Velocity of Combustion

• Should not produce harmful products

• Low Cost

• Easy Storage & Transportation

• Easily available

COMBUSTION

Combustion is defined as the process of burning of a substance in the presence of air or oxygen with the liberation of light and heat.

​

Mechanism:

C + O₂ CO₂ + 94 k cals

2H₂ + O₂ H₂O + 68.5 k cals

​

​

​

​

​

​

​

Calorific Value:

​

It is defined as “the total amount of heat liberated, when a unit mass of fuel is burnt completely”.

​

• It is used to identified “the Efficiency of a Fuel.

​

• The amount of heat can be measured by the following units :

Calorie

Kilocalorie

British Thermal Unit (B.T.U)

Centigrade Heat Unit (C.H.U)

​

Calorie: It is the amount of heat required to raise the temperature of 1g of water by 1˚C (i.e. 15 to 16 ˚C)

​

​

TYPES OF CALORIFIC VALUES

​

♦ Higher or Gross Calorific Value

​

♦ Lower or Net Calorific Value

​

Higher or Gross C.V.

​

It is defined as the total amount of heat liberated, when one unit of fuel is burnt completely and the combustion products are cooled to room temperature. (closed vessel)

​

Dulong formula can be expressed as

​

​

​

​

​

(• If both hydrogen and oxygen are present, it may be assumed that all the oxygen are already combined with 1/8 of its weight of hydrogen to form water.

• This fraction is then deducted from the hydrogen content of the fuel in the calculation)

​

Lower or net C.V.

​

It is defined as the amount of heat liberated, when one unit of the fuel is burnt completely and the combustion product are allowed to escape.

​

Net or Low C.V. = Gross C.V. – loss due to water formed

​

​

​

​

Or

​

LCV=Gross C.V – Mass of hydrogen ´ 9 ´ Latent heat of steam (587 cal/g)

​

• (Because 1 part by weight of hydrogen produces 9 parts (1 + 8) by mass of water)

​

Problem:

Calculate the grass and net calorific values of coal sample containing 84% carbon, 1.5% sulphur, 6% nitrogen, 5.5% hydrogen and 8.4% oxygen. The calorific values of carbon, hydrogen and sulphur are 8080 kcal/kg, 34500 kcal/kg and 2240 kcal/kg, respectively, and latent heat of steam is 587 cal/g.

Solution:

According to Dulong’s formula for calculating calorific value,

​

​

​

where C, O, H, S are % of carbon, oxygen, hydrogen and sulphur, respectively.

Given that C = 84%, O = 8.4%, H = 5.5% and S = 1.5%. Subtracting these in the above equation:

Gross calorific value : 8356.05 kcal/kg

​

Therefore,

​

Net calorific value = Gross calorific value – 0.09H * 597

= 8356.05 – 0.09 * 5.5* 587

= 8065.485 kcal/kg

​

​

​

​

​

​

​

​

​

​

FLUE GAS ANALYSIS

(ORSAT APPARATUS)

​

INTRODUCTION

​

• The mixture of gases like CO₂, O₂, CO etc., coming-out from the combustion chamber is called flue gases.
• The analysis of a flue gas will give an insight into the complete or incomplete combustion process and also the efficiency of the engine.
• The flue gases are estimated by using Orsat’s method.

​

​

​

​

​

​

​

​

​

​

​

Construction and working of Orsat’s Apparatus

• It consists of a horizontal tube having a 3-way stopcock at one end and water-jacketed measuring burette at the other end.
• The horizontal tube also connected to 3-different absorption bulbs for the absorption of CO₂, O₂ and CO respectively. The lower end of the burette is connected to water reservoir (rubber tube)
• The bulb I: Potassium hydroxide solution.
• The bulb II: Alkaline pyrogallol solution.
• The Bulb III: Ammonical cuprous chloride solution

ORSAT’S APPARATUS

​

Absorption of gases in reagent bottles:

• It is quite necessary to follow the order of absorbing the gases, CO₂ - first, O₂ - second and CO - last.
• This is because the absorbent used for O₂ (i.e: alkaline pyrogallol) can also absorb some amount of CO₂ and the % of CO₂ left would be less.

 (i) Absorption of CO₂: CO₂ present in the flue gas is adsorbed by KOH.

(ii) Absorption of O₂ : O₂ present in the flue gas is adsorbed by alkaline pyrogallol.

(iii) Absorption of CO: CO present in the flue gas is adsorbed by ammonical cuprouschloride.

Significance of Flue-gas Analysis

• This analysis gives an idea about the complete or incomplete combustion process.
• The presence of a high % of CO in the flue gases shows incomplete combustion of the fuel and also indicates the short supply of oxygen.
• If the flue gases contain considerable amount of oxygen it indicates an excess supply of oxygen and possibility of complete combustion.

​

• Solid fuels refers to various forms of solid material that can be burnt to release energy , providing heat and light through the process of combustion.

​

• Common examples of solid fuels include wood, charcoal, peat, coal etc.

​

• Solid fuels have been used throughout human history to create fire and solid fuel is still in widespread use throughout the world in the present day.

​

• Also called black gold.

​

• Found in sedimentary layers of soil.

​

• Contains carbon, volatile matter, moisture and ash.

​

• Mostly used for power generation and metallurgy.

​

• Coal reserves are six times greater than oil and petroleum reserves.

​

Classification of Coal

​

• Coal is classified on the basis of its rank.
• The rank of coal denotes its degree of maturity.
• Based on carbon, moisture, calorific value and ash content coal is classified as follows:

​

Coalification or metamorphism of coal

The process of transformation (coalification) of lignite to anthracite

Contd…..

• Peat-Peat is not actually coal, but rather the precursor of coal.
• Lignite- Also known as brown coal, is the lowest grade coal with the least concentration of carbon. Used for generating electricity. Principle reserves in Neyveli in Tamil Nadu.
• Sub-bituminous coal-Sub-bituminous coal is black in color and has a higher heating value than lignite, primarily as fuel for steam electric power generation.
• Bituminous coal- Bituminous coal is a middle rank coal between sub-bituminous and anthracite. Bituminous usually has a high heating value and is the most common type of coal used in electricity generation.
• Anthracite- The highest rank of coal. It is a hard, brittle, and black lustrous coal, often referred to as hard coal, containing a high percentage of fixed carbon and a low percentage of volatile matter.

​

​

• Proximate analysis

​

Estimation of moisture content

It is determined by heating about 1g of finely powdered coal at 100^◦ C to 105 ^◦C for an hour in a hot-air oven. The loss in weight is reported as due to moisture.

​

% of moisture in coal = [loss in weight of coal/weight of coal initially taken]×100

​

Significance

• Decreases calorific value of fuel
• Lengthens the time of heating

​

​

​

​

​

​

Contd…..

​

Estimation of volatile matter

For determining volatile matter, a known weight of moisture free coal is taken in a crucible with properly fitting lid. It is then heated at 950 ^◦C ±20 ^◦C for exactly seven minutes in previously heated muffle furnace. The loss in weight is due to volatile matter which is calculated as

​

% of volatile matter in coal = [loss in weight of moisture free coal/weight of coal sample]×100

​

Significance

• Decreases calorific value of fuel
• Forms smoke and pollutes air

​

​

​

​

​

​

Contd…..

​

Estimation of ash in coal

A known weight of coal is taken in a crucible and the coal is burnt completely at 700 ^◦C-750 ^◦C in muffle furnace (for half an hour) until a constant weight is obtained. The residue left in the crucible is ash content in coal which is calculated as

​

% of ash in coal = [weight of ash formed/weight of coal sample taken]×100

​

Significance

• Decreases calorific value as it is non combustible matter
• Ash disposal is a problem

​

​

​

​

​

​

Contd…..

​

Estimation of Fixed Carbon

% of Fixed Carbon = 100-[% of moisture+% of VM+% of ash]

​

​

​

​

CARBONISATION OF COAL

• It is the process of heating the coal in the absence of air to a high temperature to produce a residue coke, tar and coal gas is called as carbonisation of coal.

​

• Thus the process of preparing coke from coal in the absence of air is called as carbonisation of coal.
• Caking of coal:

When coal is heated strongly, the mass becomes soft and coherent, then it is called caking of coal.

• Coking of coal:

When coal is heated strongly, the mass produced is hard, porous and strong, then it is called coking of coal.

​

All the caking coals do not form strong, hard and coherent residue coke. Hence all the caking coals are not necessarily coking coal but all the coking coals have to be necessarily caking in nature.

Metallurgical coke

• When bituminous coal is heated strongly in the absence of air, the dense strong, porous mass obtained is called metallurgical coke.

​

• Characteristics of Metallurgical coke:-

i) Purity: -It should contain less percentage of moisture, ash, phosphorus and sulphur.

ii) Porosity: -It should be porous, so that combustion should be uniform and complete.

iii) Strength: -The mechanical strength of coke should be very high.

iv)Size:-Coke should be have medium size.

v) Calorific value:-Coke should possess a very calorific value.

vi) Cost:-Coke should be cheap and easily available.

vii)Combustibility:-Coke should burn easily.

viii) Reactivity:-The reactivity of coke should not be very high.

​

Metallurgical coke is superior to coal for the following reasons:

• Coke is stronger and more porous than coal.
• Coke contains lesser amount of sulphur than coal.
• Coke does not contain much volatile matter.

Manufacture of Metallurgical coke by Otto-Hoffmann’s method:

Significance of Otto-Hoffman’s method :

• To increases the thermal efficiency of the carbonization process.
•  To recover the valuable by products (like coal gas, ammonia, benzyl oil, etc).

Working :

• Coal is charged into the chamber having a size of about 10-12m long, 3-4m height and 0.4-0.45m wide.
• The coke ovens are heated to 1200°C by burning gaseous fuels and usually employ a regenerative principle to attain economical heating.
• The flue gases produced during combustion are passed to the regenerators that takes up the heat and gets heated to about 1000°C.
• The flow of heating gases is reversed so that the hot flue gases preheat the other chambers.
• Carbonization time is about 12-18 hours.
• Yield is 70%.

​

Liquid fuels – Petroleum�

​

• Petroleum is made from the remains of plants and animals buried millions of years ago.
• It is a non-renewable resource.
• It contains straight or cycloparaffins.
• Olefins
• Aromatics
• Other organic compounds containing N, O, S.

​

*

• Composition

​

• Petroleum is a dark, greenish brown, viscous liquid that is found underground. It comprises hydrocarbons such as:

​

1. Straight paraffins or cycloparaffins such as methane, ethane, propane, butane, isobutane, pentane, hexane.
2. Olefins such as ethylene, butene, isobutene and acetylene, butadienes.
3. Aromatics such as benzene, naphthalene, cyclohexane, methyl cyclopentane.
4. Some organic compounds containing nitrogen, oxygen and sulphur.

​

*

PETROLEUM�

• Petroleum or crude oil is naturally occuring liquid fuel. It is a dark brown or black coloured viscous oil found deep in earth’s crust.
• The oil is usually floating over a brine solution and above the oil, natural gas is present. Crude oil is a mixture of paraffinic, olefinic and aromatic hydrocarbons with small amounts of organic compounds like N, O and S

*

The average composition of crude oil is as follows

Classification of Petroleum

The average composition of crude oil is as follows

1. Paraffinic-Base type crude oil
2. Naphthenic (or) Asphaltic Base type crude oil
3. Mixed Base type crude oil

​

*

REFINING OF PETROLEUM CRUDE OIL

• The crude oil obtained from the earth is a mixture of oil, water and unwanted impurities. After the removal of water and other impurities, the crude oil is subjected to fractional distillation. During fractional distillation, the crude oil is separated into various fractions.

*

Removal of Impurities

• Step : 1

Separation of water (Cottrell’s process)

• Step : 2

Removal of harmful sulphur compound &salts

• Step : 3

Fractional distillation

​

*

Step : 1 Separation of water � (Cottrell’s Process)��

• The crude oil from oil well is an extremely stable emulsion of oil and salt water. The crude oil is allowed to flow between two highly charged electrodes, where colloidal water droplets combine to form large drops, which is then separated out from the oil

*

Step 2: Removal of harmful sulphur compounds & Salts�

• Sulphur compounds are removed by treating the crude oil with copper oxide. The copper sulphide formed is separated out by filtration.
• Electrical desalting are used to remove NaCl and MgCl₂

​

*

Step 3: Fractional distillation

• The purified crude oil is then heated to about 400°C in an iron retort, where the oil gets vapourised. The hot vapours are then passed into the bottom of a “fractionating column” (Fig).
• The fractionating column is a tall cylindrical tower containing a number of horizontal stainless steel trays at short distances. Each tray is provided with small chimney covered with a loose cap.

​

*

*

• When the vapours of the oil go up in the fractionating column, they become cooler and get condensed at different trays.

​

• The fractions having higher boiling points condense at lower trays whereas the fractions having lower boiling points condense at higher trays.

​

• The gasoline obtained by this fractional distillation is called straight-run gasoline. Various fractions obtained at different trays are given in table

*

Various fractions, compositions and their uses

*

Petrol or Gasoline

• It is a low boiling fraction of petroleum obtained between

40-120⁰C

• It is a mixture of hydrocarbons pentane to nonane (C₅ –C₉)
• Calorific value is about 11,250 kcal/kg
• Used as fuel in ICE of automobiles and aeroplanes.
• Antiknock value can be improved by the addition of TEL

​

To Avoid Knocking in engine Gasoline is prepared by

• Cracking Method
• Manufacture of Synthetic petrol

*

SYNTHETIC PETROL

• The gasoline, obtained from the fractional distillation of crude petroleum oil, is called straight run petrol.
• As the use of gasoline is increased, the amount of straight run gasoline is not enough to meet the requirement of the present community.
• Hence, we are in need of finding out a method of synthesizing petrol.

Hydrogenation of coal (or) Manufacture of synthetic petrol

• Coal contains about 4.5% hydrogen compared to about 18% in petroleum.
• Coal is a hydrogen deficient compound. If coal is heated with hydrogen to high temperature under high pressure, it is converted to gasoline.
• The preparation of liquid fuels from solid coal is called hydrogenation of coal (or) synthetic petrol.

There are two methods available for the hydrogenation of coal

• Bergius process (or direct method).

​

• Fischer-Tropsch process (or indirect method).

In this process, (In Fig) the finely powdered coal is made into a paste with heavy oil and a catalyst powder (tin or nickel oleate) is mixed with it.

The paste is pumped along with hydrogen gas into the converter, where the paste is heated to 400 − 450°C under a pressure of 200 − 250 atm.

1.Bergius process (direct method)

Synthetic Petrol

Synthetic petrol is a mixture of alkanes with composition resembling that of petrol, obtained artificially from coal.

Bergius Process

During this process hydrogen combines with coal to form saturated higher hydrocarbons, which undergo further decomposition at higher temperature to yield mixture of lower hydrocarbons.

The mixture is led to a condenser, where the crude oil is obtained.

The crude oil is then fractionated to yield

1. Gasoline
2. Middle oil
3. Heavy oil.

The middle oil is further hydrogenated in vapour phase to yield more gasoline. The heavy oil is recycled for making paste with fresh coal dust. The yield of gasoline is about 60% of the coal used.

Definition

Knocking is a kind of explosion due to rapid pressure rise occurring in an IC engine.

​

Causes of knocking in S.I (Spark Ignition) Engine [Petrol engines]

In a petrol engine, a mixture of gasoline vapour and air at 1:17 ratio is used as fuel.

KNOCKING

KNOCKING

Knocking - Detonation- Pinking

An unwanted Sound

​

What are Engine fuels

Petrol and Diesel are used as engine fuels.

​

PISTON ENGINE

• Piston Engine can be divided into
• Spark Ignition (SI) Engine
• Compression Ignition Engine (CI) Engine

​

• SI Engine consumes Petrol Octane Rating
• CI Engine consumes Diesel Cetane Rating

​

​

​

Historical View

• There was a demand for more powerful engines. So they added more and larger pistons to the engines. Some were even built with 16 cylinders that means we will increase the size of the engines to increase their power and this is not a practical method.

​

Practical Method

• The obvious method to get more power from an engine of a given size was to increase its compression ratio (a measure of the extent to which the gasoline–air mixture is compressed in the cylinder of an engine) The more the mixture is compressed before ignition, the more power the engine can deliver.

• The auto ignition of fuels causes a “knocking”.

​

• It ignites twice; once due to the high pressure and again when the spark ignites the gasoline. This causes the car engine to be less efficient and it can also damage the engine.

​

• Hence quality of an automobile gasoline is determined by its resistance to knock ”detonation during service”.

​

• The antiknock quality of the fuel limits the power and economy that an engine using that fuel can produce.
• The higher the antiknock quality of the fuel, the more the power and efficiency of the engine.

SI Engine

In a four stroke engine,

Petrol + air gets mixed in the carburettor.

• Mixture sucked into the cylinder during suction stroke.
• Mixture gets compressed during compression stroke.
• The compressed mixture gets ignited by an electric spark

during power or ignition stroke.

• The product of combustion increases pressure and pushes

the piston out and expels the exhaust gases from the

cylinder.

​

​

Knocking in SI Engine

In Petrol Engine,

​

Sometimes, in some cases,

​

• The fuel – air mixture may get heated to a temperature greater than its ignition temperature and spontaneous combustion occurs even before sparking. This is called pre-ignition.

​

• Further, the spark also occurs and again combustion take place with the rest of the mixture faster and causes explosive.
• This is called knocking.

​

• Knocking lowers the efficiency of engine which results in loss of energy.

​

Chemical Structure and Knocking

The knocking tendency decreases as follows

​

n-alkanes isoparaffins olefins naphthenes aromatics

​

(aromatics have highest antiknock value whereas n-alkanes have lowest antiknock value)

​

So, the presence of maximum quantity of aromatics and minimum quantity of n-alkanes is desirable in petrol.

Octane Number

• Octane number is used for gasoline(petrol).

​

Definition: �It is the % of iso-octane in a mixture of iso-octane and n-heptane.�Octane Number is rated from 0 to 100.

​

• Comparisons are made with blends of two pure hydrocarbons,

n-heptane and iso-octane (2,2,4-trimethylpentane).

Octane Number

• Higher octane number fuel will have lesser chance of knocking. Hence such fuel can be compressed to higher extent without observing any knocking.

​

• Lesser octane number fuel will have higher chance of knocking. Hence such fuel can't be compressed to higher extent.

​

• Octane number actually measures the resistance to auto-ignition. Higher the Octane Number, Higher will be the resistance to auto-ignition.

​

For example,

Gasoline with a knocking ability that matches that of a blend of

90% isooctane and 10% n-heptane

has an octane number of 90 •

​

How octane number of a fuel can be increased ?

1. By putting special additives into the fuel which discourage auto ignition.
2. By blending high-octane fuels in with the ordinary petrol.

​

Anti-knocking additives

• Anti-knocking additives are substances which reduce the tendency of a fuel to auto-ignite, and so increase the octane number.

• Small amounts of lead compounds have been used as economical and effective anti-knock additives. tetraethyl lead

• But it damage the environment.

​

• Other Additives
• Toluene , Xylene, tertiary-butyl-ether (MTBE) , Methanol or Ethanol Alcohol and Tertiary Butyl Alcohol

CI Engine

In a CI engine,

• Air alone gets compressed.
• Diesel is sprayed which must ignite spontaneously
• The product of combustion increases pressure and pushes

the piston out and expels the exhaust gases from the

cylinder.

​

​

Knocking in CI Engine

In Diesel Engine,

​

Sometimes, in some cases,

​

• Even after the compression stroke is over and even after the diesel oil is sprayed, burning may not start.

​

• So more and more fuel is injected automatically and sudden ignition may occur . This is called delayed-ignition.

​

• This delayed ignition causes explosive.

​

• This is called knocking.

​

​

Chemical Structure and Knocking

The knocking tendency increases as follows

​

n-alkanes isoparaffins olefins naphthenes aromatics

​

(n-alkanes have highest antiknock value whereas aromatics have lowest antiknock value)

​

So, the presence of maximum quantity of n-alkanes and minimum quantity of aromatics is desirable in diesel.

Cetane Number

• Cetane number is used for diesel.

​

Definition: �It is the % of n-hexadecane (n-cetane) in a mixture of n-hexadecane and 1-methyl naphthalene.

�Cetane Number is rated from 0 to 100.

​

Comparisons are made with blends of n-hexadecane and 1-methyl naphthalene.

​

​

Additives (Improvement of cetane number)

• Ethyl nitrate and iso amyl nitrate

​

CONCLUSION

OCTANE NUMBER AND CETANE NUMBER

​

• Octane Number and Cetane Number are the standards to measure the tendency of fuel to ignite spontaneously.

​

• Octane number measures the performance of gasoline while Cetane number measure the performance of the diesel.

​

• The fuel having high octane number has the low cetane number and high cetane number fuel has low octane number.

​

• This is one reason why we can’t use petrol in a diesel engine and diesel in petrol engine.

Comparison of gasoline oil and diesel oil

Gaseous Fuel-

WATER GAS

• It is a mixture of CO and H₂ with small amount of N₂.
• The average composition of water gas is as follows

• Its calorific value is about 2800 kcal/m³

Manufacture

• The water gas producer consists of a tall steel vessel, lined inside with refractory bricks.
• It is provided with cup and cone feeder at the top and a side opening for water gas exit.
• At the bottom it is provided with two inlet pipes for passing air and steam (Fig 4.8)

C + H₂O−−−> CO + H₂ - 28 kcal

When steam and little air is passed alternatively over a red hot coke maintained at about 900 − 1000°C in a reactor, water gas is produced.

Various Reactions

The reactions of water gas production involves the following two steps.

I - Step

In the first stage, steam is passed through the red hot coke, where CO & H₂ gases are produced. The reaction is endothermic. Hence, the temperature of the coke bed falls.

​

C + 2H₂O−−−> CO₂ + 2H₂ - 19 kcal

In the second stage, in order to raise the temperature of the coke bed to 1000°C, the steam supply is temporarily cut off and air is blown in. The reaction is exothermic.

C + O₂ −−−> CO₂ +97 kcal

2C + O₂ −−−> 2CO +59 kcal

​

II - Step

Thus the steam-run and air-blow are repeated alternatively to maintain proper temperature.

Since both CO2 and H2 burn with a non-luminous or blue flame, water gas some times known as “blue gas”

​

Properties:

• High Calorific values
• Burns with nonluminous flame
• Flame is short but very hot

*

Uses

• It is used for the production of H₂ and in the synthesis of ammonia.
• It is used to synthesis gasoline in Fischer Tropsch process.
• It is used as an illuminating gas and a fuel.
• It is also used in the manufacture of power alcohol and carburetted water gas (water gas + oil gas).

THANK YOU