Proximate Analysis of Coal

Proximate analysis reports moisture, volatile matter, ash and fixed carbon content of
a fuel by percentage weight. Moisture is the amount of water obtained from the fuel by
heating at a specific condition according to the standard method, without making any
chemical change to the fuel. Volatile matter is essentially a measure of the non-water
gases formed from a coal sample during heating, fixed carbon is the non-volatile fraction
of coal, and ash is the inorganic residue remaining after combustion. Proximate analysis
is the most often used analysis for characterization of coals.

Ash Content
Ash in coal, which is the remains when coal is burnt, is one of the materials of interest.
Ash is derived from the mineral matter content of coal. The inorganic materials which
were actually the part of the plant structures, constitute the ‘inherent’ mineral matter of
coal, whereas, the ‘extraneous’ mineral matter is that which was introduced probably as
air-borne dusts or waterborne silts at the later stage of coalification. Mineral matter of
coal predominantly consists of kaolinite, pyrite and calcite and upon combustion; results
in the oxides of silicons and metals, such as, aluminium, iron and calcium.
 Proximate Analysis of Coal (Cont.)
These oxides are the essential part of ash. When coal burns, shales and other hydrated
materials, which are also the constituents of mineral matter of coal, decompose and lose
their water of hydration and also emit carbon dioxide, sulfur dioxide gases. As there is a
loss in weight, so, amount of ash of coal is always less than its mineral matter content.

Moisture Content
Moisture of coal may also become inherent or extraneous. Inherent moisture is the
moisture associated with coal inherently, which cannot be removed by only air drying, it
can be removed when coal is heated above 1000 C. Extraneous moisture can be removed
by air drying of coal.

Volatile Matter
Volatile matter is one of the most common parameters measured in coal. It is part of a
standard proximate analysis. Volatile matter is essentially a measure of the non-water
gases formed from a coal sample during heating.
 Proximate Analysis of Coal (Cont.)
It is measured as the weight percent of gas (emissions) from a coal sample that is
released during heating to 950 °C in an oxygen-free environment , except for moisture
(which will evaporate as water vapor), at a standardized temperature. Volatile matter is
measured directly in the automated proximate analyzer. Results are presented in weight
percent.

Measurement of Moisture
• It is carried out by placing a weighted powder sample of raw power plant coal of size
less than 75 microns in an uncovered crucible and it is placed in the oven at 108 plus
minus 2-degree centigrade.
• The sample is cooled to room temperature and weighed again. The loss of weight
represents moisture.
 Proximate Analysis of Coal (Cont.)
Measurement of Volatile Matter
• Determination of volatile matter is carried out by taking a weighed fresh sample of coal
and is placed in the covered crucible and is heated in the furnace at 900 plus minus 15-
degree centigrade.
• The sample is cooled and weighed.
• The loss of weight represents Moisture+ Volatile matter
• Remainder is coke (Fix carbon + Ash)

Measurement of Carbon and Ash

• The cover from the crucible is to be removed and the crucible is to be heated over bushel
burner until all carbon is burnt.
• The residue is weighed which is Ash.
• 100 – ( Moisture + Volatile matter + Ash)= Fix carbon
Fix carbon is mostly consist of carbon but also contains some hydrogen, oxygen, sulfur,
nitrogen. Volatile matters are methane, hydrocarbons, carbon monoxide, hydrogen and
incombustible gases like CO2 and N2. Thus volatile matter is an index of gaseous fuels
present in coal.
 Proximate Analysis of Coal (Cont.)

Typical proximate coal analysis of a coal sample:

Constituents coal
Moisture 6
Ash 38
Volatile matter 21
Fix carbon 35

The significance of various parameters of proximate coal analysis:

Fix Carbon
• Fix carbon acts as a main heat generator during burning
• Fix carbon gives a rough estimate of heating value of coal
 Proximate Analysis of Coal (Cont.)

Volatile matter
• Volatile matter also contributes to the heating value of coal
• Increase in percentage of volatile matter in coal proportionately increases flame length
and helps in easier ignition of coal

Ash
• Ash is an impurity which will not burn
• Ash content is important in design of furnace grate, combustion volume, pollution
control equipment (ESP) and Ash handling plant
• Ash increases transportation, handling, storage cost
• Ash affects combustion efficiency and boiler efficiency
• Ash causes clinkering and slagging problems in boiler
 Proximate Analysis of Coal (Cont.)

Moisture
• Moisture increases transportation, handling, storage cost
• Moisture decreases the heat content per kg of power plant coal
• Moisture increases heat loss due to evaporation and superheating of vapor
 Mathematical Formulation of Proximate Analysis

Let,
𝑊𝑐 = Initial total weight of coal.
𝑊𝑚 = Weight of moisture content.
𝑊𝑣 = Weight of volatile matter.
𝑊 & 𝑊1 = Weight of coal after taken out from the crucible for the first and second time.

Moisture Content
𝑊𝑚 = 𝑊𝑐 − 𝑊

𝐿𝑜𝑠𝑠 𝑖𝑛 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑐𝑜𝑎𝑙 𝑠𝑎𝑚𝑝𝑙𝑒

%𝑀𝑜𝑖𝑠𝑡𝑢𝑟𝑒 = × 100
𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑐𝑜𝑎𝑙 𝑡𝑎𝑘𝑒𝑛

𝑊𝑚
= × 100
𝑊𝑐
 Mathematical Formulation of Proximate Analysis (Cont.)

Volatile Matter
𝑊𝑣 = 𝑊 − 𝑊1

𝐿𝑜𝑠𝑠 𝑖𝑛 𝑤𝑒𝑖𝑔ℎ𝑡 𝑎𝑡 𝑡𝑒𝑚𝑝 ± 15℃

%𝑉𝑜𝑙𝑎𝑡𝑖𝑙𝑒 𝑚𝑎𝑡𝑡𝑒𝑟 = × 100
𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑐𝑜𝑎𝑙 𝑡𝑎𝑘𝑒𝑛

𝑊𝑣
= × 100
𝑊𝑐
Ash Content

𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑟𝑒𝑠𝑖𝑑𝑢𝑒 𝑖𝑛 𝑡ℎ𝑒 𝑐𝑟𝑢𝑐𝑖𝑏𝑙𝑒

%𝐴𝑠ℎ = × 100
𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑐𝑜𝑎𝑙 𝑡𝑎𝑘𝑒𝑛
Fixed Carbon Content
%𝐹𝑖𝑥𝑒𝑑 𝑐𝑎𝑟𝑏𝑜𝑛 = 100 − % 𝑚𝑜𝑖𝑠𝑡𝑢𝑟𝑒 + 𝑣𝑜𝑙𝑎𝑡𝑖𝑙𝑒 𝑚𝑎𝑡𝑡𝑒𝑟 + 𝑎𝑠ℎ
 Mathematical Formulation of Proximate Analysis (Cont.)
Problem_1: Exactly 2.5g coal was weighted into a crucible. After heating for 1hour at
110˚C the residue weighted 2.415g, the crucible was then covered with a vented lid and
strongly heated for 7 minutes at 900±15˚C. The residue then weighted 1.528g. The
crucible was then heated without cover at 700˚C until a constant weight was obtained. The
last residue was found to weight 0.245g. Calculate the percentage result of above analysis.

Solution:
Given,
𝑊𝑐 = 2.5g 𝑊 = 2.415g 𝑊1 = 1.528g

Weight of residue = 0.245g

𝑊𝑚 = 𝑊𝑐 − 𝑊 = 2.5 − 2.415 = 0.085g

𝑊𝑚 0.085
%𝑀𝑜𝑖𝑠𝑡𝑢𝑟𝑒 = × 100 = × 100 = 3.4%
𝑊𝑐 2.5
 Mathematical Formulation of Proximate Analysis (Cont.)

𝑊𝑣 = 𝑊 − 𝑊1 = 2.415 − 1.528 = 0.887g

𝑊𝑣 0.887
%𝑉𝑜𝑙𝑎𝑡𝑖𝑙𝑒 𝑚𝑎𝑡𝑡𝑒𝑟 = × 100 = × 100 = 35.48%
𝑊𝑐 2.5

𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑟𝑒𝑠𝑖𝑑𝑢𝑒 𝑖𝑛 𝑡ℎ𝑒 𝑐𝑟𝑢𝑐𝑖𝑏𝑙𝑒 0.245

%𝐴𝑠ℎ = × 100 = × 100 = 9.8%
𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑐𝑜𝑎𝑙 𝑡𝑎𝑘𝑒𝑛 2.5

%𝐹𝑖𝑥𝑒𝑑 𝑐𝑎𝑟𝑏𝑜𝑛 = 100 − % 𝑚𝑜𝑖𝑠𝑡𝑢𝑟𝑒 + 𝑣𝑜𝑙𝑎𝑡𝑖𝑙𝑒 𝑚𝑎𝑡𝑡𝑒𝑟 + 𝑎𝑠ℎ

= 100 − 3.4 + 35.48 + 9.8 = 51.32%