The Process of Combustion

Combustion is a chemical process. It is a reaction between the fossil fuels such as coal or natural gas and oxygen. The main products of the combustion process are carbon dioxide and water. The combustion process is always exothermic and it liberates heat. The actual reaction taking place due to combustion can be written down

Fuel + O2 -> Product + Heat

Due to the combustion of fuel and air, heat energy is generated where the internal energy of the fuel species is the chemical energy. This energy is associated with the chemical bonds and intermolecular attractions. The heat of reaction is the quantity of heat exchanged between a system and its surroundings during a chemical reaction at a specified temperature. The heat of combustion is defined as the quantity of heat released in combustion of a fuel with oxygen.

C + O2 ->CO2 + 8,084 kcal/kg of carbon

2C + O2 -> 2CO +2430 kcal/kg of carbon

S+ O2 -> SO2 + 2224 kcal/kg of sulphur

2H2 + O2 -> 2H2O + 28922 kcal/kg of hydrogen

The process of combustion can be completely carried out only in the presence of adequate amount of oxygen. Rapid fuel oxidation results in large amount of heat. Almost 79% of air which is not oxygen is nitrogen. Nitrogen is considered as temperature reducing diluter.

Nitrogen absorbs heat and reduces the combustion efficiency of the fuel. The amount of heat which is available for use is also reduced. At high temperatures, nitrogen combines with oxygen to produce oxides of nitrogen, which are toxic pollutants.

Under certain conditions, carbon combines with oxygen to form carbon monoxide. This incomplete combustion releases small amount of energy as compared when carbon combines with carbon dioxide.

The process of combustion in solid fuels consists of following phases:

1.Drying

2.Pyrolysis

3.Devolatilisation

4.Gasification

A separation of drying/ pryolysis/gasification is applicable in ‘in stage combustion’. In batch combustion, there will be distinct separation between volatile matter and char combustion phase.

For large particles there will be a certain degree of overlap between the phases, while in batch combustion there will be large degree of overlap.

Drying
Moisture will evaporate at low temperature i.e 50-1000C. Vaporisation uses the energy released from the combustion process; it lowers the temperature in the combustion chamber, which slows down the combustion process. For Example, wood requires that amount of energy to evaporate contained moisture, and subsequently to heat the water vapour, that temperatures are reduced below the minimum temperature required to sustain combustion. Moisture content is a very important fuel variable.

Pyrolysis
Pyrolysis can be defined as the thermal degradation (devolatilisation) in absence of externally supplied oxidising agent. The pyrolysis products are mostly tar and carbonaceous charcoal, and low molecular weight gases. CO and CO2 can also be formed. Fuel type, temperature, pressure, heating rate and reaction time are all variables that affect the amounts and properties of the products formed.

Devolatilisation
Devolatilisation in case of wood starts at about 2000C. Rate of devolatilisation increases as the temperature is raised. At 4000C, most of the volatiles are gone and the devolatisation rate decreases rapidly. Low devolatilisation rate can be observed in the temperature range of 400-5000C

Gasification
It can be defined as thermal degradation in presence of an externally supplied oxidising agent. Gasification can be carried out with air, oxygen, steam or CO2 as oxidising agent.

Combustion
Ideally can be defined as complete oxidation of fuel. Drying and pyrolysis will always be the first step in a solid-fuel combustion process.