STEAM POWER PLANT 171
Efficient Combustion of Coal. The factors which affect the efficient combustion of coal are as
follows :
- Type of coal. The important factors which are considered for the selection of coal are as
follows :
(i) Sizing
(ii) Caking
(iii) Swelling properties
(iv) Ash fusion temperature.
The characteristics which control the selection of coal for a particular combustion equipment
are as follows :
(i) Size of coal
(ii) Ultimate and proximate analysis
(iii) Resistance of degradation
(iv) Grindability
(v) Caking characteristics
(vi) Slagging characteristics
(vii) Deterioration during storage
(viii) Corrosive characteristics
(ix) Ash Content.
The average ash content in Indian coal is about 20%. It is therefore desirable to design the
furnace in such a way as to burn the coal of high ash content. The high ash content in coal has the
following:
(i) It reduces thermal efficiency of the boiler as loss of heat through unburnt carbon, ex-
cessive clinker formation and heat in ashes is considerably high.
(ii) There is difficulty of hot ash disposal.
(iii) It increases size of plant.
(iv) It increases transportation cost of fuel per unit of heat produced.
(v) It makes the control difficult due to irregular combustion. High as content fuels can be
used more economically in pulverised form. Pulverised fuel burning increases the ther-
mal efficiency as high as 90% and controls can be simplified by just adjusting the
position of burners in pulverised fuel boilers. The recent steam power plants in India
are generally designed to use the pulverised coal.- Type of Combustion equipment. It includes the following:
(i) Type of furnace
(ii) Method of coal firing such as :
(a) Hand firing
(b) Stoker firing
(c) Pulverised fuel firing.
(iii) Method of air supply to the furnace. It is necessary to provide adequate quantity of
secondary air with sufficient turbulence.