1. Understanding Coal Analysis
by Michael Hutagalung on 02/06/08 at 1:30 am
“How to understand a coal sample analysis? What is the difference between proximat
e and ultimate analysis? What is AR (as-received) basis? Is is the same with DAF
(dry, ash free) basis? How about AD (air-dried) basis? And what coal ash analys
is is all about?”
Well, it is indeed a long list of questions to answer but the explanation is act
ually not as twisted as it seems. The main purpose of coal sample analysis is to
determine the rank of the coal along with its intrinsic characteristics. Furthe
rmore, these data will be used as the fundamental consideration for future conce
rns, for instance: coal trading and its utilizations.
Coal Properties
Coal comes in four main types or ranks: lignite or brown coal, bituminous coal o
r black coal, anthracite and graphite. Each type of coal has a certain set of ph
ysical parameters which are mostly controlled by moisture, volatile content (in
terms of aliphatic or aromatic hydrocarbons) and carbon content.
1. Moisture
Moisture is an important property of coal, as all coals are mined wet. Groundwat
er and other extraneous moisture is known as adventitious moisture and is readil
y evaporated. Moisture held within the coal itself is known as inherent moisture
and is analyzed. Moisture may occur in four possible forms within coal:
o Surface moisture: water held on the surface of coal particles or maceral
s
o Hydroscopic moisture: water held by capillary action within the microfra
ctures of the coal
o Decomposition moisture: water held within the coal’s decomposed organic co
mpounds
o Mineral moisture: water which comprises part of the crystal structure of
hydrous silicates such as clays.
2. Volatile matter
Volatile matter in coal refers to the components of coal, except for moisture, w
hich are liberated at high temperature in the absence of air. This is usually a
mixture of short and long chain hydrocarbons, aromatic hydrocarbons and some sul
fur. The volatile matter of coal is determined under rigidly controlled standard
s. In Australian and British laboratories this involves heating the coal sample
to 900 ± 5 °C (1650 ±10 °F) for 7 minutes in a cylindrical silica crucible in a muffle f
urnace. American Standard procedures involve heating to 950 ± 25 °C (1740 ± 45 °F) in a
vertical platinum crucible.
3. Ash
Ash content of coal is the non-combustible residue left after coal is burnt. It
represents the bulk mineral matter after carbon, oxygen, sulfur and water (inclu
ding from clays) has been driven off during combustion. Analysis is fairly strai
ghtforward, with the coal thoroughly burnt and the ash material expressed as a p
ercentage of the original weight.
4. Fixed carbon
The fixed carbon content of the coal is the carbon found in the material which i
s left after volatile materials are driven off. This differs from the ultimate c
arbon content of the coal because some carbon is lost in hydrocarbons with the v
olatiles. Fixed carbon is used as an estimate of the amount of coke that will be
yielded from a sample of coal. Fixed carbon is determined by removing the mass
of volatiles determined by the volatility test, above, from the original mass of
the coal sample.
5.
Coal Proximate Analysis
The objective of coal ultimate analysis is to determine the amount of fixed carb
on (FC), volatile matters (VM), moisture, and ash within the coal sample. The va
riables are measured in weight percent (wt. %) and are calculated in several dif
ferent bases. AR (as-received) basis is the most widely used basis in industrial
applications. AR basis puts all variables into consideration and uses the total
weight as the basis of measurement. AD (air-dried) basis neglect the presence o
2. f moistures other than inherent moisture while DB (dry-basis) leaves out all moi
stures, including surface moisture, inherent moisture, and other moistures. DAF
(dry, ash free) basis neglect all moisture and ash constituent in coal while DMM
F (dry, mineral-matter-free) basis leaves out the presence of moisture and miner
al matters in coal, for example: quartz, pyrite, calcite, etc. Mineral matter is
not directly measured but may be obtained by one of a number of empirical formu
la based on the ultimate and proximate analysis.
Proximate Analysis unit (ar) (ad) (db) (daf)
Moisture (wt. %) 3.3 2.7
Ash (wt. %) 22.1 22.2 22.8
Volatile Matter (wt. %) 27.3 27.5 28.3 36.6
Fixed Carbon (wt. %) 47.3 47.6 48.9 63.4
Gross Calorific Value (MJ/kg) 24.73 24.88 25.57 33.13
(Kcal/kg) 24.73 24.88 25.57 33.13
A table is shown above containing an example of proximate analysis data of coal.
Conversion from one basis to another can be performed using mass balance equati
ons. The standard practice for proximate analysis of coal may be referred to AST
M D3172-07a or ISO 17246:2005.
Coal Ultimate Analysis
Similar to coal proximate analysis, the objective of coal ultimate analysis is t
o determine the constituent of coal, but rather in a form of its basic chemical
elements. The ultimate analysis determines the amount of carbon (C), hydrogen (H
), oxygen (O), sulfur (S), and other elements within the coal sample. These vari
ables are also measured in weight percent (wt. %) and are calculated in the base
s explained above.
Ultimate Analysis unit (ar) (ad) (db) (daf)
Carbon (C) (wt. %) 61.1 61.5 63.2 81.9
Hydrogen (H) (wt. %) 3.00 3.02 3.10 4.02
Nitrogen (N) (wt. %) 1.35 1.36 1.40 1.81
Total Sulfur (S) (wt. %) 0.4 0.39 0.39
Oxygen (O) (wt. %) 8.8 8.8 9.1
A table is shown above containing an example of coal ultimate analysis data and
showing significant elements only. Conversion from one basis to another can be p
erformed using mass balance equations. The standard practice for ultimate analys
is of coal may be referred to ASTM D3176-89(2002) or ISO 17247:2005.
Ash Analysis
Oxides wt.% of ash
(Calculated) Elements wt.% of ash
(Measured)
Na2O 0.35 Na 0.26
MgO 0.48 Mg 0.29
Al2O3 20.0 Al 10.6
SiO 74.1 Si 34.6
P2O5 0.05 P 0.05
K2O 1.1 K 0.92
CaO 0.68 Ca 0.49
TiO2 0.80 Ti 0.48
Mn3O4 0.06 Mn 0.05
Fe2O3 3.25 Fe 2.28
An analysis of coal ash may also be carried out to determine not only the compos
ition of coal ash, but also to determine the levels at which trace elements occu
r in ash. These data are useful for environmental impact modelling, and may be o
btained by spectroscopic methods such as ICP-OES or AAS. An example of coal ash
composition is shown on the right.
Beside composition of coal ash, ash fusion point is also one significant paramet
er in ash analysis. The optimum operating temperature of coal processing will de
pend on the gas temperature and also the ash fusion point. Melting of the ashes
3. may cause them to stick to the walls of the reactor and result in a build-up.
You might be interested to read an article of coal characterization equipments h
ere, illustrated with photos, including coal proximate analysis, ultimate analys
is, and ash fusion point analysis equipments.
Reference: CSIRO Energy Technology, ISO, ASTM, Wikipedia
