Proximate analysis: In this analysis, the percentage of carbon is indirectly determined.
It is a
quantitative analysis of the following parameters.
1. Moisture content 2. Volatile matter 3. Ash 4. Fixed carbon
Moisture Content: About 1 gram of finely powdered air-dried coal sample is weighed in a
crucible. The crucible is placed inside an electric hot air-oven, maintained at 105 to 110 0C for one
hour. The crucible is allowed to remain in oven for 1 hour and then taken out, coo
cooled in desiccators
and weighed. The process of heating, cooling and weighing of crucible is repeated until we get a
constant weight. Loss in weight is reported as moisture.
Volatile Matter: The volatile matter present in the coal may be combustible gases (CH 4, CO, H2
etc.) or non-combustible gases (CO2, N2).
The dried sample (moisture free coal) taken in a crucible in and then covered with a lid and placed
in an electric furnace or muffle furnace, maintained at 925 ± 20 oC. The crucible is taken out of
the oven after 7 minutes of heating. The crucible is cooled first in air, then inside desiccator
desiccators and
weighed again. The process of heating, cooling and weighing of crucible is repeated until we get
a constant weight. Loss in weight is reported as volatile matter on percentage-basis.
Ash: Ash is the non combustible, useless matter which is left behind after the combustion of coal.
Ash content reduces the calorific value of coal.
The weighed amount of coal sample is taken in a crucible and then heated without lid in a muffle
furnace at 700 ± 50 oC for ½ hour. The crucible is then taken out, cooled first in air, then in
desiccators and weighed. Hearing, cooling and weighing are repeated, till a constant weight is
obtained. The residue is reported as ash on percentage-basis.
2
Fixed carbon:
Percentage of fixed carbon = 100 - % of (Moisture + Volatile matter + Ash)
Significance of proximate analysis: Proximate analysis provides following valuable
Moisture: Moisture is coal evaporates during the burning of coal and it takes some of the liberated
heat in the form of latent heat of evaporation. Therefore, moisture lowers the effective calorific
value of coal. Moreover over, it quenches the fire in the furnace, hence, lesser, the moisture
content, better the quality of coal as a fuel. However, presence of moisture, up to 10%, produces a
more uniform fuel- -
Volatile matter: A high volatile matter content means that a high proportion of fuel will distil
over as gas or vapour, a large proportion of which escapes un-burnt, So, higher volatile content in
coal s undesirable. A high volatile matter containing coal burns with a long flame, high smoke and
has low calorific value. Hence, lesser the volatile matter, better the rank of the coal.
Ash: Ash is a useless, non-combustible matter, which reduces the calorific value of coal.
Moreover, ash causes the hindrance to the flow of air and heat, thereby lowering the temperature.
Also, it often causes trouble during firing by forming clinkers, which block the interspaces of the
grate, on which coal is being burnt. This in-turn causes obstruction to air supply; thereby the
burning of coal becomes irregular. Hence, lower the ash content, better the quality of coal. The
presence of ash also increases transporting, handling and storage costs. It also involves additional
cost in ash disposal. The presence of ash also causes early wear of furnace walls, burning of
apparatus and feeding mechanism.
Fixed carbon: calorific and betters the quality
coal. Greater the percentage of fixed carbon, smaller is the percentage of volatile matter. This also
represents the quantity of carbon that can be burnt by a primary current of air drawn through the
hot bed of a fuel. Hence, high percentage of fixed carbon is desirable. The percentage of fixed
carbon helps in designing the furnace and the shape of the fire-box, because it is the fixed carbon
that burns in the solid state.
