What Is Combustion and What Are the Types of Flames and Their Zones
What is Combustion?
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Combustion is a redox chemical reaction that takes place between fuel and an oxidant to give a mixture of gases and heat and light. Combustion does not always give rise to fire. But when it does, the flames are an indicator of the reaction. Incomplete combustion, the reactants get oxidized or burn in oxygen to give a limited number of byproducts. For Example- when 1 molecule of hydrocarbon burns in oxygen, it gives carbon dioxide and water. Thus, it can be deduced that end product from the burning of an element in oxygen gives the oxides of the same element. Which means, carbon yields carbon dioxide, sulfur yields sulfur dioxide, and so on.
When not, enough oxygen is available for the continuation of chemical reaction or burning of elements in oxygen, it produces more by products. For example- in incomplete combustion of Hydrocarbon, carbon, hydroxide and carbon monoxide are produced.
Combustible and Non-Combustible Substances
Substances that easily catch fire are combustible substances. Example- paper, coal, wood, etc.
Substances that do not catch fire easily are non-combustible substances. Example- water, glass, sand, etc.
Fuel
Fuel is a substance that produces a usable form of energy on combustion. Like fossil fuel, biogas, etc.
The physical state of the fuel may vary. That is, it may be solid, liquid or gas.
Fuels are of two types- Natural and Artificial depending upon the source.
Fire
When chemical combustion takes place between oxygen and fuel, it produces a visible heat and light source- fire. The fire keeps burning until there is enough oxygen and fuel to continue the combustion. The temperature at which a combustible substance catches fire when heated in the presence of oxygen is called ignition temperature. Thus, it’s a necessary parameter for generating fire. Some substances like diesel, petrol have very low ignition temperatures. Which means that they easily catch fire with a flame. These substances are known as inflammable substances.
A Fire Can Only Be Generated If
Fuel or combustible material is present.
A flame or heat source is present to bring the temperature to the ignition temperature of the fuel.
Oxygen is present to sustain combustion.
When any of these factors are removed or controlled, the fire is controlled.
Types of Combustion
Combustion is of Two Types
Rapid Combustion - In rapid combustion, heat and light is released rapidly in a short span of time.
Spontaneous Combustion - It is spontaneous and occurs without the application of heat. In such a type of combustion, the substance catches fire on its own. Forest fires are one of the examples of this type of combustion.
What is Flame?
A flame can be defined as a region where gaseous elements burn, generating heat and light. All combustible materials, whether liquid or gaseous, emit flames as they burn. Both the combustible substance and the combustion's supporter must be gases in order for combustion to result in a flame. For instance, when kerosene is burnt on a stove, it rises with the wick and evaporates before burning with flames. Similar circumstances apply in the case of a candle flame. However, charcoal does not vapourize. They shine rather than burn with a flame.
Structure of Candle Flame
Candle Flame
Three zones can be found in a candle flame:
The Outer Zone:
The outer zone of the Flame is referred to as the zone of full combustion or the non-luminous zone. Complete combustion happens because the wax vapours are able to get enough oxygen from the air. Due to total combustion, this area is completely blue, and anything that comes into touch with it leaves with no residue. It is hardly possible to see the hotter zone. Goldsmiths can benefit greatly from this region of the Flame.
The Middle Zone:
The bright zone, also known as the zone of incomplete combustion, is the pale-yellow zone. The Flame's middle region is just moderately hot. Here, there isn't enough oxygen for the wax vapours to totally burn. Carbon monoxide and some carbon particles are produced during partial combustion. Additionally, some carbon atoms are not completely burnt. Unburned carbon atoms heat up to a white-hot state and cause the flame to become yellow. On things that come into touch with this zone, black soot or carbon particles are left behind.
The Innermost Zone:
The region around the wick is known as the dark zone or the zone of no combustion. This region is black in colour, thus the name. Because it is the innermost zone of the Flame, there is no oxygen available for combustion in this zone, hence there is no combustion. Vaporised wax makes up the majority of it.
Classification of Flame
Flames can be categorised into two groups. Namely,
Diffusion flames, also known as non-premixed flames: candle flames.
Premixed Flames: Laminar premixed flames and turbulent premixed flames are the two forms of premixed flames. Example: a Bunsen burner, an LPG stove, etc.
Candle Flame
An illumination (light) source is a candle. It is made up of a solid fuel block with a built-in wick. Beeswax, soy wax, and other plant-based waxes can also be used to make candles.
Today's candles are often constructed of paraffin. Paraffin and plastic are combined to make gel candles. When you light a candle, a little quantity of wax is melted and vaporised by the match's heat.
It combines after being vaporised. Once it has evaporated, it combines with atmospheric oxygen to produce a flame. This Flame generates sufficient heat to maintain the candle's flame through a self-sustaining sequence of actions:
The solid fuel's top end melts due to the heat of the flame.
Through the wick, the liquid fuel then rises.
The evaporated fuel is then burned inside the flame of the candle.
Premixed Flame and its Structure
Structure of the Premixed Flame
Due to atmospheric oxygen diffusion, a diffusion flame arises in addition to the flame zone when a premixed flame burns in the open air with an excess of fuel.
For instance, the flow of a Bunsen flame generated by a burner with a controlled air intake can be changed from an extremely hot flux in which the majority of the fuel gases are oxidised to carbon dioxide and water to a low-temperature flux in which the majority of the fuel gases are only partially oxidised.
These flames consist of two zones, known as the reducing and oxidising zones, that are separated by an inner cone and an outer cone. The extra oxygen in the outer cone is what causes it to oxidise.
Important Questions
1. What are the different zones present in a candle flame?
Ans: There are three distinct zones present in a candle flame, namely the outer zone, the middle zone, and the innermost zone.
2. What is the definition of flame?
Ans: A flame is a region where gaseous components burn, releasing heat and light in the process.
Summary
A flame is a region where gaseous elements burn, creating heat and light in the process. Every combustible material, including liquids and gases, burns with a flame. A source of light is a candle. It consists of a block of solid fuel with an inserted wick. There are three zones in the candle flame. Specifically, the hotter zones are the outside zone, also known as the non-luminous zone, the middle zone, also known as the luminous zone, and the innermost zone, also known as the dark zone, which does not experience combustion.
FAQs on Combustion and Flames Complete Guide to Reactions and Flame Structure
1. What is combustion in chemistry?
Combustion is a chemical reaction in which a substance reacts rapidly with oxygen to release heat and usually light. It is an example of an exothermic reaction.
- Reactants: Fuel + O2
- Products: Oxides of elements (e.g., CO2, H2O)
- Energy is released as heat and often as flame
2. What are the necessary conditions for combustion?
The necessary conditions for combustion are the presence of fuel, oxygen, and ignition temperature, known as the fire triangle.
- Fuel: A combustible substance (e.g., wood, methane, petrol)
- Oxygen: Usually from air (about 21% O2)
- Ignition temperature: Minimum temperature required to start burning
3. What is ignition temperature?
Ignition temperature is the lowest temperature at which a substance catches fire and starts burning in air. Below this temperature, combustion cannot occur.
- Different substances have different ignition temperatures.
- Petrol has a lower ignition temperature than wood.
- Fire extinguishers work by lowering the temperature below ignition point.
4. What are the types of combustion?
The main types of combustion are rapid combustion, spontaneous combustion, explosion, and slow combustion.
- Rapid combustion: Burns quickly with heat and light (e.g., LPG).
- Spontaneous combustion: Starts without external ignition (e.g., white phosphorus).
- Explosion: Sudden reaction producing heat, light, sound, and gases.
- Slow combustion: Occurs slowly without flame (e.g., respiration).
5. What is the difference between complete and incomplete combustion?
Complete combustion occurs in excess oxygen producing CO2 and H2O, while incomplete combustion occurs in limited oxygen producing CO or C (soot).
- Complete combustion: Blue flame, more energy, no soot.
Example: CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) - Incomplete combustion: Yellow flame, less energy, forms carbon monoxide.
Example: 2CH4(g) + 3O2(g) → 2CO(g) + 4H2O(g)
6. Why does a candle flame have different zones?
A candle flame has different zones because combustion occurs with varying oxygen supply in different regions of the flame.
- Inner dark zone: Unburnt wax vapours, least hot.
- Luminous yellow zone: Partial combustion; glowing carbon particles produce light.
- Outer blue zone: Complete combustion; hottest part.
7. What is a flame in chemistry?
A flame is the visible, gaseous part of a burning substance where combustion occurs and light is emitted. It forms when:
- Fuel vaporizes
- Vapour reacts with oxygen
- Excited atoms emit light
8. Why is the flame of a Bunsen burner blue?
The Bunsen burner flame is blue because it undergoes complete combustion with sufficient oxygen supply.
- Air holes allow oxygen to mix with gas before burning.
- No unburnt carbon particles are formed.
- The blue color is due to excited CH radicals emitting blue light.
9. How do you write and balance a combustion reaction?
To write and balance a combustion reaction, react the fuel with O2 and balance C, H, and then O atoms step by step.
- Step 1: Write fuel + O2 → CO2 + H2O
- Step 2: Balance carbon atoms.
- Step 3: Balance hydrogen atoms.
- Step 4: Balance oxygen atoms.
2C2H6(g) + 7O2(g) → 4CO2(g) + 6H2O(g).
10. What are some common examples of combustion reactions?
Common examples of combustion reactions include burning of hydrocarbons, hydrogen, and carbon in oxygen.
- Hydrogen combustion: 2H2(g) + O2(g) → 2H2O(l)
- Carbon combustion: C(s) + O2(g) → CO2(g)
- Propane combustion: C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(g)
