Combustion and Flame: One of the milestones in the history of civilization was the invention of fire. Fire is a source of heat energy. Cooking fuels include wood, cow dung cakes, crop wastes, kerosene oil, coke, or LPG gas. We utilise petrol or diesel oil to run automobiles. Wood, coke, LPG, petrol, diesel, and natural gas all have one thing in common: when they burn, they produce heat, which may then be used for a variety of purposes.

In addition, some materials emit flame when burned, whereas others do not. When a candle or coal gas is lit, it produces a flame, but the coal or charcoal does not. In this article, let’s look at the chemical process of burning and the different types of flames that result from it.

Combustion

The process in which a substance combines chemically with oxygen to give off heat and light is called combustion. As heat/energy is evolved during the process, combustion is an exothermic reaction.

Combustion

The substance which burns readily is called a combustible substance. For example, paper, wood, coal, CNG, LPG, kerosene, etc.

combustible substance

The substances which do not burn in the presence of air or oxygen are called non-combustible substances. For example, stone, iron, wall, glass, water, etc.

non-combustible substances

The substances which catch fire at a very low temperature and burn with flames are called inflammable substances. For example, LPG, petrol, spirit, kerosene, etc.

Conditions necessary for Combustion

There are three conditions necessary for combustion, which are as follows-

i. Availability of fuel: It is the substance that undergoes combustion. For example, wood, charcoal, kerosene, etc.

Conditions necessary for Combustion

ii. Supply of Air/oxygen: Oxygen is the supporter of combustion. Air with \({\rm{21 \% }}\) oxygen helps in the combustion.
iii. Ignition temperature: It is the minimum temperature to which a combustible substance must be heated before it catches fire. Combustion will not take place if the combustible substance does not reach or above its ignition temperature.

How to Control Fire?

Fire can be controlled by removing one or more of the conditions that are necessary for combustion. It can be done by-

• i. Getting rid of the gasoline
• ii. severing the air supply (or oxygen supply)
• iii. Turning off the heat or decreasing the fuel’s temperature.

Methods to Control Fire

The various methods to control fires are explained below:

By Water

Water is commonly used to put out fires. Water lowers the combustible substance’s temperature below the ignition point. The flammable material is surrounded by water vapour, which helps to block off the flow of oxygen. As a result, the fire was extinguished.

Fig: By Water

Water cannot be used to put out a fire caused by the burning of oil or petrol because water is heavier than oil and settles down the oil, causing the oil to continue to burn.

Water cannot be used when electrical equipment is on fire since water can transmit electricity and cause a shock to those who are putting out the fire.

Fires created by burning liquid fuels, such as kerosene oil, can be put out by dumping sand or soil on top of them.

Fig: sand or soil

By Fire Extinguisher

Extinguishers are devices that are used to extinguish fires. They either cut off the fire’s air supply, reduce the fuel temperature (below the ignition temperature), or both.

Fig: By Fire Extinguisher

There are many different types of fire extinguishers that can be used to put out a fire. One of them is a soda-acid fire extinguisher which is discussed below-

Working of soda-acid fire extinguisher- Carbon dioxide is liberated by the reaction of sulphuric acid with sodium bicarbonate solution and is released as a stream of carbon dioxide under high pressure. Water extinguishes the fire by reducing the temperature of the combustible substance below its ignition temperature, while carbon dioxide cuts off the combustible substance’s supply of oxygen.

Fig: soda-acid fire extinguisher

Types of Combustion

Combustion can be divided into three types- rapid, spontaneous and explosion.

Rapid Combustion

It is a type of combustion in which the gas burns quickly and produces heat and light.

When a lit matchstick is placed near a gas burner, and the gas tap is turned on, the gas instantly begins to burn, producing heat and light.

For example- Petrol, LPG, bourbon, and other alcoholic beverages.

Spontaneous Combustion

Combustion in which a material spontaneously bursts into flames without the application of heat.
For example-

• i. Phosphorus burns at room temperature.
• ii. Accidental fires in coal mines are frequently caused by the spontaneous combustion of coal dust.
• iii. Forest fires can also be sparked by the sun’s heat or by lightning.

Explosion

An explosion occurs when a material abruptly bursts to generate heat, light, and sound as a result of the application of heat or pressure.

For example- Crackers and pyrotechnics both emit a tremendous amount of gas.

Flame

When anything is burned, hot, luminous gas is released from it. This gas is called flame. Flames are caused by chemicals that evaporate as they burn. Some materials emit a flame, while others do not. Below given is the table that lists materials that form flame and materials that do not-

Fig: Flame

Structure of Flame

Fig: Structure of Flame

A flame has three zones:

• i. Outermost zone- The hottest portion of the flame is the outermost zone, which is blue in colour. This is also the point at which total combustion occurs.
• ii. Middle zone- The colour of the middle zone is yellow, and it is moderately heated. Partial combustion occurs in this zone.
• iii. Innermost zone- The least hottest zone is the innermost zone, which is black in colour. The unburned wax vapours of a candle can be found here.

Fuel

Fuel is any readily available substance that burns in air at a reasonable rate, creating a considerable amount of heat energy while leaving no unpleasantness behind.

For example, wood, charcoal, gasoline, kerosene, etc.

There is almost certainly no such thing as a perfect fuel. We should look for a fuel that meets the majority of the requirements for a particular application.

Characteristics of a Good Fuel

• i. It should be inexpensive and easily accessible.
• ii. It must be simple to store.
• iii. Its combustion should be controlled, and it should bum slowly.
• iv. It should have a low temperature of ignition.
• v. It should only produce a minor amount of ash as a byproduct.
• vi. It should have a high calorie content.
• vii. It should not emit polluting gases into the atmosphere.
• viii. During transportation, it should not pose any risks.

Classification of Fuels

Fuels are divided into three categories based on their physical state.

1. Solid fuels: These are the fuels that are found in nature. Solid fuels are those that exist in a solid-state at normal temperature. Wood, agricultural leftovers, charcoal, coal, coke, and other materials are examples.
2. Liquid fuels: Liquid fuels are fuels that exist in a liquid condition at normal temperature. Liquefied hydrogen, petrol, oil, kerosene, and diesel are among the examples.
3. Gaseous fuels: The fuels that exist in a gaseous state at normal temperature. Water-gas, producer gas, coal gas, compressed natural gas (CNG), and gobar gas are a few examples.

Fuel Efficiency

The calorific value of a fuel is the quantity of heat energy produced when one kilogram of fuel (in oxygen) is entirely burned. The higher the calorific value of a fuel, the more efficient it is. Kilojoules per kilogram \(({\rm{kJ}}/{\rm{kg}})\) or kilojoules per gram \(({\rm{kJ}}/{\rm{g}})\) are used to measure the calorific value of fuels.

Below given is a list of calorific value of some fuels-

Uses of Fuels

Common fuels are utilised in a variety of ways, including:

• i. Cars, scooters, and other vehicles that we use on a daily basis all run on diesel or petrol.
• ii. Natural gas is used in our houses for heating, water heaters, dryers, and stovetops.
• iii. Oil and natural gas are utilised to make a variety of items that we use on a daily basis. For example, hydrocarbons are used in making plastics, medications, and a variety of other goods we use daily.
• iv. Coal is the major fossil fuel used to generate electricity in many thermal power plants.

Harmful effects of burning fuels

1. Carbon-based fuels such as wood, coal, and petroleum emit ash and fine unburned carbon particles into the air, which can cause respiratory conditions such as asthma. These fine particles are Suspended Particulate Matter (SPM).

2. Incomplete combustion of fuels (such as coal, gasoline, and other fossil fuels) produces carbon monoxide, a very toxic gas that can kill persons sleeping in a room where coal is burning. \({\rm{CO}}\) reacts with haemoglobin in our blood, forming carboxyhemoglobin, which is unable to transport oxygen. Unburned hydrocarbons, many of which are carcinogenic (cause cancer), and other health hazards.

3. Most fuels release carbon dioxide during combustion, which causes a climate imbalance. As a result of deforestation, there are fewer trees available to absorb \({\rm{C}}{{\rm{O}}_2}\). Global warming is a result of this.

4. Sulphur dioxide gas is produced when coal and diesel are burned, which is caustic and causes discomfort in the nose, throat, and airways. Shortness of breath, wheezing, and a tightness in the chest are also symptoms. Nitrogen oxides are released by gasoline engines. Acid rain is caused when sulphur and nitrogen oxides dissolve in rainwater and produce acids.

Four Important Questions of Combustion and Flame

1. Kerosene oil on burning gives flames on the other hand charcoal does not produce flames? Give a reason.
Ans: The substance which evaporates during burning gives flames. Kerosene oil rises through the wick and is vaporized during burning and forms flames. Charcoal, on the other hand, does not evaporate and so does not produce flame.

2. Why is it not advised to use water if an electrical equipment is on fire?
Ans: If electrical equipment is on fire, water may conduct electricity and harm the people by electrocution who are trying to extinguish the fire. Hence, it is not advised to use water to put off the fire of electrical equipment.

3. Why isn’t hydrogen gas used as a domestic or industrial fuel, although it has a very high calorific value?
Ans: Despite its high calorific value, hydrogen gas is not employed as a home or industrial fuel for the following reasons:
i. It’s not cheap.
ii. It explodes as it burns.
iii. It is dangerous to store and transport hydrogen since it is extremely inflammable.

4. Can the process of cellular respiration be called combustion? Why?
Ans: Yes, Cellular respiration is a set of metabolic activities and processes that releases heat to break big molecules into smaller ones. As a result, this is a slow-burning reaction.

Summary

In this article, we studied the chapter combustion and flames- we studied the meaning of combustion, the conditions that are necessary for combustion and various types of combustion. We also studied the three zones of a flame and we got to know that not all materials that burn produce flame like that of a coal. We now know about the fuels and calorific value as well as characteristics of a good fuel.

FAQs on Combustion and Flame

Q.1. What is the difference between combustion and flame?
Ans: The main difference is that combustion produces heat but no flames, whereas burning converts most of the energy into light energy, resulting in less heat energy than combustion.

Q.2. How is the flame produced during combustion?
Ans: Heat is released during combustion. This heat is absorbed by other molecules of fuel and moves apart, forming vapour that produces flames.
Flames are formed at a specific point in the combustion reaction termed the ignition point.

Q.3. What is flame?
Ans: When anything is burned, a hot, luminous gas is released from it. This gas is called flame. Flames are caused by chemicals that evaporate as they burn. Kerosene oil, wax, and other similar substances are examples.

Q.4. What are the types of combustion?
Ans: Combustion can be divided into three types which are as follows-
1. Rapid combustion,
2. Spontaneous combustion, and
3. Explosion

Q.5. What are examples of combustion?
Ans: The examples of combustion are as follows-
1. Burning of Wood or Coal
2. Burning of Petrol or Diesel to run your Car.
3. Combustion of Natural Gas or LPG to cook on the stovetop.
4. For the production of energy in thermal power plants.
5. Fireworks.

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