FUELS and COMBUSTION-1

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FUELS

Fuel

Coal

  • IT IS A MIXTURE OF ORGANIC CHEMICALS AND MINERAL MATERIALS PRODUCED BY A NATURAL PROCESS OF GROWTH AND DECAY, ACCUMULATION OF DEBRIS BOTH VEGETAL AND MINERAL WITH SOME SORTING AND STRATIFICATION, AND ACCOMPLISHED BY CHEMICAL, BIOLOGICAL,BACTERIOLOGICAL AND METAMORPHIC ACTION.
  • –CARBOHYDRATES + PRESSURE + TEMP. —>   HYDROCARBON
  • –THE ORGANIC CHEMICAL MATERIALS PRODUCE HEAT WHEN BURNED.
  • –THE MINERAL MATTER REMAINS AS RESIDUE CALLED ASH.
TYPE OF COAL FORMEDAGE OF COAL (MILLION YEARS)
ANTHRACITES250
BITUMINOUS150 – 210
SUBBITUMINOUS60 – 100
LIGNITE40 – 20
PEAT1

COAL  FORMATION

LIGNITE

•This type of coal contains a lot of moisture and breaks apart easily. Of the four types, lignite contains the least amount of carbon. Also called brown coal, lignite is used mainly at electricity-generating plants.

SUBBITUMINUS

•medium-soft. This type of coal has less moisture than lignite. Subbituminous coal is generally used to produce steam for electricity generation.

BITUMINUS

•medium-hard. This type of coal, which contains very little moisture, has high heat value. It is used to generate electricity and to produce coke, a coal residue used in the steel industry.

ANTHRACITE

•hard. This type of coal has the highest carbon content. Anthracite burns slowly and makes a good heating fuel.

The gradation of Indian coal based on its UHV is as follows:

GradeCalorific Value Range( in kCal/Kg)
AExceeding 6200
B5600 – 6200
C4940 – 5600
D4200 – 4940
E3360 – 4200
F2400 – 3360
G1300 – 2400

ANALYSIS OF COAL

  • PROXIMATE ANALYSIS.
  • ULTIMATE ANALYSIS.
  • Proximate analysis indicates the percentage by weight of the Fixed Carbon, Volatiles, Ash, and Moisture Content in coal.

TYPICAL PROXIMATE ANALYSIS OF VARIOUS COALS
(IN PERCENTAGE)

ParameterIndian Coal  Indonesian Coal  South African Coal
Moisture5.989.438.5
Ash38.6313.9917.0
Volatile matter 20.7029.7923.3
Fixed Carbon 34.6946.7951.2

Significance of Various Parameters in Proximate Analysis

a) Fixed carbon:

•Fixed carbon is the solid fuel left in the furnace after volatile matter is distilled off.

•It consists mostly of carbon but also contains some hydrogen, oxygen, sulphur and nitrogen not driven off with the gases.

•Fixed carbon gives a rough estimate of heating value of coal.

(b) Volatile Matter:

•Proportionately increases flame length, and helps in easier ignition of coal.

•Sets minimum limit on the furnace height and volume.

•Influences secondary air requirement and distribution aspects.

•Influences secondary oil support.

(c) Ash Content:

•Reduces handling and burning capacity.

•Increases handling costs.

•Affects combustion efficiency and boiler efficiency

•Causes clinkering and slagging.

(d) Moisture Content:

•Increases heat loss, due to evaporation and superheating of vapour.

ULTIMATE ANALYSIS

  • The ultimate analysis determines all coal component elements.
  • The ultimate analysis is determined in a properly equipped laboratory by a skilled chemist.
  • It is useful in determining the quantity of air required for combustion and the volume and composition of the combustion gases.
  • This information is required for the calculation of flame temperature and the flue duct design.
Parameter  Indian Coal, %Indonesian Coal, %
Moisture5.989.43
Mineral Matter38.6313.99
Carbon41.1158.96
Hydrogen2.764.16
Nitrogen1.221.02
Sulphur0.410.56
Oxygen9.8911.88

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