Combustion and Flame

• Burning something is a chemical process in which substance reacts rapidly with oxygen and give off heat.
• Oxygen gas supports combustion and it is present in atmosphere in sufficient amount that is approx of total air.

1.0Introduction

People generally use wood, cow dung cakes, crop residues, kerosene oil, coke or LPG gas to cook food. For running vehicles, we use petrol or diesel oil. In factories, we use coal or fuel oil or natural gas. The materials, such as wood, coke, LPG (Liquefied petroleum gas), petrol, diesel, natural gas have one property in common i.e., they produce heat on burning which is then put into different uses. Furthermore, some materials on burning produce flame and some do not. For example, a candle or coal gas on burning produces a flame but not the coal or charcoal. So, let us study the chemical process of burning and the types of flame produced during this process.

2.0Combustion

Some substances, like magnesium catches fire and burns with a dazzling white flame when heated in air. Similarly, when a piece of paper is brought near a candle flame, it burns, producing heat and light. The charcoal and coal burns in air producing carbon dioxide, heat and light. Air or oxygen which helps in burning is called supporter of combustion and the chemical reaction which takes place with the release of heat and light energy is called combustion. A chemical process in which a substance reacts with oxygen to give off heat is called combustion. The substances which undergo combustion are known as combustible substances. It may also be called a fuel. Sometimes, heat is accompanied by light, either as a flame or as a glow, during combustion.

Q. Why is the person caught in fire, covered with a blanket?

• Explanation If the clothes of a person catch fire, he is immediately wrapped in a thick blanket. The blanket cuts off the supply of air i.e. the supporter of combustion and hence the fire is put off.
  
  Blankets during fire, cuts off the supply of oxygen

Different substances burn at different temperatures. Some minimum temperature is essential for burning a combustible substance, before it catches fire. This minimum temperature is called ignition temperature.

Ignition temperature: It is the minimum temperature to which a combustible substance must be heated, before it catches fire. Therefore, we see that a combustible substance cannot catch fire as long as its temperature is lower than its ignition temperature e.g. Ignition temperature of phosphorus is . So, unless phosphorus is heated to , it will not catch fire. However, if temperature of air is or more, phosphorus will catch fire without heating. Similarly, Kerosene oil and wood do not catch fire on their own at room temperature. But if both are heated a little, then kerosene oil will catch fire and not wood. This shows that ignition temperature of kerosene oil is lower than that of wood.

• Three conditions necessary for producing combustion.
  

Q. Why does the fire go off when water is poured over burning wood?

• Explanation It happens because water absorbs a large amount of heat energy, which results in fall in the temperature of wood below its ignition temperature and the fire goes off.

3.0How do we control fire?

We have seen homes, shops and factories caught in fire. How can the fire be controlled from spreading? Generally, water is used to control fire. Water brings down the temperature of the combustible substance below its ignition temperature. The water vapour surrounds the combustible material, thus helping in cutting off the supply of air. So, that the fire is extinguished.

Fire produced by the burning of oil or petrol cannot be controlled by throwing water on it because water being heavier than oil, settles down oil and oil continues to burn. In the case of fires caused by burning liquid fuels, such as kerosene oil can be controlled by throwing sand or soil over it. We know that there are three conditions necessary for producing and sustaining combustion. (1) Presence of a combustible substance. (2) Presence of a supporter of combustion. (3) Attainment of ignition or kindling temperature.

Thus, fire can be controlled by removing one or more of these requirements of fire control. The fire extinguisher also try to cut off the supply of air or bring down the temperature of the fuel or both, to control the fire. The combustible substance as such cannot be eliminated in most of the cases from the place of fire.

There are various types of fire extinguishers used for controlling a fire. The job of a fire extinguisher is to cut off the supply of air or bring down the temperature of the fuel.

Fire hazard symbol The symbol of the flame indicated a potential fire hazard and includes, flammable gases, liquids aerosols and solids.

• Soda - acid fire extinguisher: Carbon dioxide is liberated by the reaction of sulphuric acid with sodium bicarbonate solution which comes out with a stream of liquid water at high pressure. Water puts off the fire by lowering the temperature of the combustible material below its ignition temperature and carbon dioxide cut off the supply of air to the combustible substance.

To operate an extinguisher:

4.0Types of combustion

Combustion is mainly of three types

Rapid combustion:

The combustion in which the gas burns rapidly and produces heat and light energy is called rapid combustion. For example, when a burning matchstick is brought near a gas burner and the gas tap is opened, the gas immediately starts burning with the production of heat and light energy. For example, Petrol, LPG, spirit, etc.

Spontaneous combustion:

The combustion in which no external heat is given is known as spontaneous combustion. For example, Burning of white phosphorus in air.

Explosion:

The combustion in which large amount of gases are evolved with the production of a tremendous amount of heat, light and sound is called explosion. For example, when a cracker is ignited, a sudden reaction, oxidation process takes place, and in which at high speed large amount of gas is evolved with the production of tremendous amount of heat, light and sound.

Difference between rapid and spontaneous combustion

Q. Forest fires are a result of which type of combustion and why?

• Explanation Forest fires are the result of spontaneous combustion because of the following factors (i) presence of a combustible substance (ii) presence of right ignition temperature (iii) presence of supporter of combustion in air

5.0Flame and Its Structure

Look at the combustible substances around you and try to find whether on burning some of these materials (given below) forms a flame or not.

Flame: A region of burning gases is called flame.

Candle flame

A substance will burn with a flame, only if some gaseous substance is there to burn.

The fuel that evaporates on heating burns with flame. Example: candle, LPG, etc.

Q. Why does a matchstick produce a flame on burning?

• Explanation When the matchstick catches fire or is burnt, the heat released due to the burning of chemicals on the match head, partly decomposes the wood to form wood gas. The wood gas then catches fire and produces a flame.

Structure of a flame In order to understand the structure of a flame, light a wax candle and watch its flame. Carefully note the different coloured zones in the flame. Starting from the base of the flame, a flame has three zones. (i) Inner most zone: It is cooler than outer zones and it is dark. (ii) Middle zone: It is the largest zone of candle flame. This zone gives soot and smoke. (iii) Outermost zone: This zone of the flame is thin and blue in colour. This is the hottest zone of the flame. The temperature of this zone is maximum around .

Q. Why do goldsmiths use the outermost zone of the flame for melting gold and silver?

• Explanation Goldsmiths, use the outermost zone or non-luminous part of the flame for melting or shaping gold and silver ornaments. They direct the non-luminous part of the flame of candle on the gold with the help of a blow pipe. As the temperature of this part of the flame is around , it is sufficient to melt gold at specific point and thus helps goldsmiths to give proper shape to the gold ornaments.
  
  Goldsmith blowing flame through a metallic pipe

6.0What is fuel?

Any substance which is easily available and burns in air at a moderate rate, producing a large amount of heat energy, without leaving behind any undesirable residue is called fuel. For example, wood, charcoal, petrol, kerosene, etc.

Q. Why is Sulphur not used as a fuel even though it can burn in air to produce heat?

• Explanation Sulphur is easily available in nature and can burn in air to produce heat. However, it is not a fuel because on burning, it forms a poisonous gas, , which can cause serious respiratory problems and can even be fatal.
  
  CNG Bus

Ideal Fuel There is probably no fuel that could be considered as an ideal fuel. We should look for a fuel which fulfils most of the requirements for a particular use.

Characteristics of a good fuel

(1) It should be cheap and readily available. (2) It should be easy to store. (3) It should burn at a slow rate and its combustion should be controllable. (4) It should have moderate ignition temperature. (5) It should produce very small amount of residues such as ash. (6) It should have large calorific value. (7) It should not produce gases which pollute the air. (8) It should not produce any hazards during transportation.

• Natural gas and liquified petroleum gas (LPG) are examples of ideal fuel, as LPG and CNG satisfy most of the properties of the ideal fuel.

Classification of fuels

On the basis of physical state, fuels are classified into three parts.

1. Solid fuels: The fuels which occur in a solid state at room temperature are called solid fuels. For example, Wood, agricultural residues, charcoal, coal, coke, etc.
2. Liquids fuels: The fuels which occur in a liquid state at room temperature are called liquid fuels. For example, Liquefied hydrogen, petrol, oil, kerosene, diesel, etc.
3. Gaseous fuels: The fuels which occur in a gaseous state at room temperature are called gaseous fuels. For example, Water gas, producer gas, coal gas, compressed natural gas (CNG) and gobar gas, etc.

7.0Fuel Efficiency

The amount of heat energy produced on completely burning of one kilogram of fuel (in ) is called the calorific value of a fuel. The more is the calorific value of a fuel, more is the efficiency of the fuel. The calorific value of the fuels is expressed in kilojoules per kilogram ( ) or kilojoules per gram ( ). The calorific value of common solids, liquids and gaseous

Calorific value of different fuels Calorific value of solid fuels

Calorific value of liquid fuels

Calorific value of gaseous fuels

8.0Harmful effects of burning fuels

The increasing fuel consumption has harmful effects on the environment. The main products formed during the fuel combustion which produce harmful effect are

1. Carbon fuels like wood, coal, petroleum release unburnt carbon particles. These fine particles are dangerous pollutants causing respiratory disease, such as asthma.
2. Incomplete combustion forms carbon monoxide gas. It is very poisonous gas. It is dangerous to burn coal in a closed room. The carbon monoxide gas produced can kill persons sleeping in that room.
3. Combustion of most fuels releases carbon dioxide in the environment. Increased percentage of carbon dioxide in the air causes global warming. Global warming is the rise in temperature of the earth. This result in melting of polar glaciers. This leads to rise in sea level and floods in the seacoast.
4. Burning of coal and diesel release sulphur dioxide gas. It is an extremely suffocating and corrosive gas. Sulphur dioxide and nitrogen oxide dissolve in rainwater to form acid. Such rain is called acid rain. It is very harmful for crops, buildings and soil.
5. Wood is also used as a fuel. Burning of wood gives a lot of smoke which causes air pollution and is also very harmful for humans. It may lead to many respiratory problems. Cutting of trees for obtaining wood leads to deforestation which is quite harmful to environment.
6. Carbon particles of smoke or the ash get suspended in the air. Excessive amount of them in the air causes breathing problems.

9.0Basic terminology

1. Combustion: A chemical process in which a substance react with oxygen to give off heat.
2. Combustible: Those substances which catche fire easily.
3. Ignition temperature: It is a minimum temperature that is essential for burning a combustible substance.
4. Inflammable substances: The substance which have very low ignition temperature and can easily catch fire with a flame.
5. Spontaneous combustion: Combustion in which no external heat is required for combustion.
6. Explosion: The combustion is which large amount of gases are evolved with tremendous amount of heat, light and sound.
7. Flame: A region of burning gases is called flame.
8. Fuel efficiency: The amount of heat energy produced on completely burning of 1 Kilogram of fuel. It is also called calorific value.
9. Complete combustion: When combustion take place in excess of air, complete combustion takes place with production of gas.
10. Incomplete combustion: When combustion takes place in limited amount of air, incomplete combustion takes place with production of carbon monoxide and soot.