Текст 7. Basics of combustion

Combustion can be defined as the complete, rapid exothermic oxidation of a fuel with sufficient amount of oxygen or air with the objective of producing heat, steam and/or electricity. The process of combustion occurs with a high speed and at a high temperature. Essentially, it is a controlled explosion. Combustion occurs when the elements in a fuel combine with oxygen and produce heat. All fuels, whether they are solid, liquid or in gaseous form, consist primarily of compounds of carbon and hydrogen called hydrocarbons (natural gas, coal fuel oil, wood, etc.), which are converted in the combustion process to carbon dioxide (CO2) and steam. Sulphur, nitrogen, and various other components are also present in these fuels.

Products of combustion

When the hydrogen and oxygen combine, intense heat and water vapor is formed. When carbon and oxygen combine, intense heat and the compounds of carbon monoxide or carbon dioxide are mixed. These chemical reactions take place in a furnace during the burning of fuel, provided there is sufficient air (oxygen) to completely burn the fuel. Very little of the released carbon is actually “consumed” in the combustion reaction because flame temperature seldom reaches the vapori- zation point of carbon. Most of it combines with oxygen to form CO2 and passes

out the vent. The final gaseous product of combustion is called a flue gas. As men- tioned in the introduction to this segment. Combustion can never be 100% effi- cient. All fuels contain moisture. Other fuel components may form by-products, such as ash, and gaseous pollutants that need emission control equipment.

Types of combustion

There are three types of combustion:

Perfect combustion is achieved when all the fuel is burned using only the theoretical amount of air, but as stated earlier, perfect combustion cannot be achieved in a boiler.

Complete combustion is achieved when all the fuel is burned using the min- imal amount of air above the theoretical amount of air needed to burn the fuel. Sol- id fuels, such as coal, peat or biomass are typically fired at air factors 1.1 – 1.5, i.e. 110 – 150% of the oxygen needed for perfect combustion.

Incomplete Combustion occurs when part of the fuel is not burned, which re- sults in the formation of soot and smoke.

Combustion of solid fuels

Solid fuels can be divided into high grade: coal and low grade: peat and bark. The most typical firing methods are grate firing, cyclone firing, pulverized firing and fluidized bed firing. Pulverized firing has been used in industrial and utility boilers from 60 MWt to 6000 MWt. Grate firing has been used to fire bio- fuels from 5 MWt to 600 MWt and cyclone firing has been used in small scale 3–6 MWt.

Combustion of coal

Oil and gas are always combusted with a burner, but there are three different ways to combust coal:

1. Fixed bed combustion (grate boilers)

2. Fluidized bed combustion

3. Entrained bed combustion (pulverized coal combustion)

In fixed bed combustion, larger-sized coal is combusted in the bottom part of the combustor with low-velocity air. Stoker boilers also employ this type of combustion. Large-capacity pulverized coal fired boilers for power plants usually employ entrained bed combustion. In fluidized bed combustion, fuel is introduced into the fluidized bed and combusted.