Combustion and Flame
What is combustion?
Combustion or burning is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidised, often gaseous mixture termed as smoke. In some reactions, water is also produced along with smoke and other chemicals.
Types of combustion
Combustion is categorised as the following :
1. Complete and Incomplete Combustion
Complete Combustion –
- In complete combustion, the reactant burns in oxygen, producing a limited number of products.
- When a hydrocarbon burns in oxygen, the reaction will yield carbon dioxide and water.
- When elements are burned, the products are primarily the most common oxides. Carbon will give carbon dioxide, sulphur will give sulphur dioxide.
- Nitrogen is not a combustible substance when oxygen is the oxidant, but small amounts of various nitrogen oxides form when air is the oxidant.
Incomplete Combustion –
- Incomplete combustion will occur when there is not enough oxygen to allow the fuel to react completely, to produce carbon dioxide and water.
- It also occurs if external devices or sources does not allow the combustion to take place completely. Carbon and carbon monoxide are the by products and not carbon dioxide.
- Certain substances like diesel, oil, plastic, tyres, coal or wood, pyrolysis occurs before combustion. Pyrolysis is the process where complex molecules or polymers are broken down into simpler molecules. Pyrolysis generally occurs without oxygen. It is used in waste management to alter the waste generated into a more usable material.
- Incomplete combustion adds harmful compounds to the environment, in the form of smog and other contaminants.
This type of combustion, though categorised by the presence of incandescence and smoke, produces no flame.
A relatively slow process, smouldering occurs between the oxygen in air and the surfaces of solid fuels such as coal, peat, wood, tobacco and synthetic foams. These solid fuels glow when smouldering, indicating temperatures in excess of one thousand degrees celcius. Sometimes it occurs for some time in a hot environment, despite lack of oxygen. Although under such conditions, it produces high amounts of carbon monoxide.
3. Diffusion Combustion
Diffusion combustion results from the transfer of fuel vapours and oxygen across a concentration gradient into a reaction area that is characterised by high temperatures and correct proportion of reactants. Vapours may come initially from a solid fuel such as candle wax, a liquid fuel like alcohol or kerosene or a gaseous fuel like methane, or even the ordinary LPG cylinders we use in our homes.
The flames produced from diffusion combustion begins as smooth, laminar flame, increasing in turbulence as it grows and consumes more fuel and oxygen.
4. Rapid Combustion
Rapid combustion releases massive amounts of energy in the form of heat and light as is the case with fire. In some cases, combustion occurs so fast that large amounts of gases are released, along with heat and light, causing a significant pressure shift in the surrounding atmosphere. This pressure shift, often accompanied by a very loud noise, is called an explosion.
Internal combustion engines convert the energy produced by rapid combustion into usable kinetic energy.
5. Spontaneous Heating and Combustion
Spontaneous heating and combustion differs from most other types of combustion in that no external ignition source is required for it to proceed. An extremely slow process, spontaneous can take upto a few weeks. It consists of a gradual oxidation of certain material. As heat builds up, the rate of reaction increases, eventually causing smoldering or flaming combustion when the temperature rises. It may occur with petrochemicals, hydrocarbons, hay, cotton, etc.
What is a flame?
A flame is the visible gaseous part of a fire. It is caused by a highly exothermic reaction taking place in a thin zone.
Very hot flames are hot enough to have ionised gases as components, which may be considered plasma.
Structure of a candle flame
A candle flame consists of three zones.
- The innermost zone of a flame is dark or black and is the coldest part of the flame and is made of unburnt vapours of combustible material.
- The middle zone of a flame is yellow, bright and luminous. The fuel vapours burn partially in the middle zone, because there is not enough air for burning in this zone. The partial burning of fuel in the middle zone produces carbon particles. These carbon particles then leave the flame as smoke and soot. It has moderate temperature.
- The outer zone of the flame is blue. It is a non luminous zone. In this zone, complete combustion takes place, as it has enough supply of oxygen.
What is fuel?
Fuel maybe defined as any material that can be made to react with other substances, so that it releases chemical energy as heat.
Classification of fuels
1. Solid Fuel: Coal, wood, charcoal, peat and agricultural waste
2. Liquid Fuel: Kerosene, gasoline
3. Gaseous Fuel: Liquified Petroleum Gas, natural gas
4. Biofuels: Biofuel is defined as derived from biomass
5. Fossil Fuel: Fossils fuels are hydrocarbons, coal, petroleum, natural gas, coal. Fossils fuels are formed from plants dead and fossilised millions of years ago. They are non-renewable sources of energy
What are the characteristics of a good fuel?
The characteristics of a good fuel are :
- High calorific value
- Moderate ignition temperature
- Low moisture content
- Low noncombustible matter
- Moderate velocity of combustion
- Products of combustion not harmful
- Low cost
- Easy to transport
- Combustion should be controllable
- No spontaneous combustion
- Low storage cost
- Should burn in air with efficiency
Uses of combustion chemistry
The study of combustion chemistry helps us to design and monitor better and more efficient machines and engines. It also helps us to avoid using fuels that irreversibly damage our environment.