Sampling of Coal:
Sampling
of coal is a crucial process used to determine the characteristics and quality
of coal. It involves collecting representative samples from different parts of
a coal deposit, shipment, or stockpile to analyze its properties. The collected
data helps in assessing the coal's suitability for specific applications,
determining its value, and ensuring compliance with various quality standards.
The purpose of collecting and
preparing a sample of Coal is to provide a test sample which, when analysed,
will provide test results representative of the lot sampled. This helps ensure
an accurate characterization of the coal from which the sample is taken. Proper
sampling and sample preparation are critical for accurate analysis.
Statistically, about 80% of the total variances involved at the different
stages of sample collection, preparation and analysis come from errors during
its collection. There are two criteria that must be followed when sampling to
ensure the overall precision and accuracy of the results:
General Principles of
Sampling:
The
fundamental requirements of sampling include the following:
1.
All particles of coal in the lot to be sampled are accessible to the sampling
equipment and each individual particle shall have an equal probability of being
selected and included in the sample.
2.
The dimension of the sampling device used should be sufficient to allow the
largest particle to pass freely into it.
3.
Collection of an adequate number of coal portions from positions distributed
over the entire lot to reflect the variability of the coal.
4.
The minimum mass of the gross sample should be sufficient to enable particles
to be present in the same proportions as in the lot of coal from which it is
taken.
BIS Formulations for the
Purpose of Coal Sampling and Analysis:Sl.
No
Code
|
Sl.
No |
Code |
Title |
Purpose |
|
1 |
IS:
436 (Part-I/Sec-1)- 1964 (Reaffirmed
2013) |
Methods
for sampling of Coal
and Coke |
Manual
Sampling |
|
2 |
IS:
436 (Part-I/Sec-2)- 1976 (Reaffirmed
2010) |
Methods
for sampling of Coal
and Coke |
Mechanical
Sampling |
|
3 |
IS:
1350 (Part-I)- 1984 (Reaffirmed
2002) |
Methods
for test of Coal and
Coke |
Proximate
Analysis of Coal (Ash, Moisture,
VM & FC) |
|
4 |
IS:
1350 (Part-II)- 1970 (Reaffirmed
2005) |
Methods
for test of Coal and
Coke |
Determination
of Calorific Value |
Preparation of Sample:
The
preparation consists of following steps:
1) Primary Crushing: The gross sample collected is fed to primary crusher and the coal size is reduced to 12.5mm size with help of mechanical crushing.
2) After Primary crushing of coal sample, one portion (one fourth of the gross sample) called Part-1 will be used for determination of total moisture and the other portion (three fourth of the gross sample) called Part-2 will be used for Testing and analysis.
3) Secondary Crushing: After primary crushing of coal, Part-2 of the coal sample is sent to secondary crusher, coning and quartering of coal sample is carried out at secondary crusher and the sample is further reduced to 3.35 mm of size.
4) Pulveriser: Coning and quartering of coal sample is done and pulveriser will reduce the coal sample to powdered form and the top size of 212 Micron is attained. Precaution will be taken so that further sieving and pulverising is not needed at the time of testing.
*In
case the pulveriser is not in working condition, the testing is done at sample
of 3.35-micron size.
The
final pulverized sample will be divided into four equal parts viz. Set – I, set
– II, Set-III
and
Set-IV.
·
Set
– I is handed over to Third Party for testing and analysis.
·
Set-II
of the sample shall be handed over by the Coal Company.
·
Set-III
of the sample shall:
- In case of rail mode, be handed
over to the Purchaser.
- In
case of road mode, be handed over in equal portions to all of the Purchasers on
that particular day; and
· Set-IV of the sample called referee sample is sealed jointly by Third Party and representatives of each of the Parties and is kept at the sample preparation room with lock and key arrangement under custody of Third Party. The referee sample is retained in double sealed condition (duly signed by the Third Party and the representatives of the Parties) for 30 (thirty) days from the date of sample collection.
The
testing and analysis of coal sample are carried out by Third Party at
independent coal
testing
labs as per BIS Standards. The determination of Ash, moisture, and GCV is done
on
equilibrated
moisture basis. The findings of the analysis report are communicated to both
Coal Company and
consumer.
The
part 1(one fourth of gross sample) of size 12.5 micron is used for total
moisture
test.
The total moisture test is carried out at coal company labs, witnessed by each
of the
representative
of the party.
Testing
and Analysis:
The
main purpose of coal sample analysis is to determine the quality or rank of the
coal along with its intrinsic characteristics. Testing data are used for coal
trading and its utilization.
General
coal analysis and testing include the following:
1. Proximate analysis (determination of moisture content, ash content, volatile matter, fixed carbon)
2.
Ultimate analysis (determination of carbon, hydrogen, oxygen, nitrogen,
sulphur)
3.
Ash analysis (major and minor elements in coal and coal ash)
4.
Calorific value (also known as heating value or specific energy).
Proximate analysis: The proximate analysis of coal determines the Moisture content, Volatile matter (VM), Ash and, by difference, Fixed Carbon within the coal sample. It is the simplest and most common form of coal evaluation, and constitutes the basis of many coal purchasing and performance prediction indices used by utility operators. Proximate analysis is reported by percent on as-received, moisture-free, and moist- and ash-free bases.
Ultimate analysis : is determined also using set procedures for ash, carbon, hydrogen, nitrogen, oxygen, and sulfur contents, which are reported as percent on as-received and moisture- and ash-free bases. Ultimate analysis is important for classification of coal by rank.
Both
analyses are important for utilization, commercial, and industrial purposes.
Few
Definitions:
o As-received (AR): Data are expressed
as percentages of the coal with moisture. This category is also sometimes
referred to as as-fired and is commonly used by the combustion engineer to
monitor operations and for performing calculations as it is the whole coal that
is being utilized.
o Dry basis (d)—Data are
expressed as percentages of the coal after the moisture has been removed.
o Dry, ash-free
(DAF)—Data
are expressed as percentages of the coal with the moisture and ash removed.
o Dry, mineral-matter-free (dmmf)—The coal is
assumed to be free of both moisture and mineral matter, and the data are a
measure of only the organic portion of the coal.
o Moist, ash-free (maf)—The coal is
assumed to be free of ash but still contains moisture.
o Moist, mineral-matter-free (mmmf)—The coal is
assumed to be free of mineral matter but still contains moisture.
Steps of Sampling:
Here
are the key steps involved in coal sampling:
i.
Selection of Sampling Points: Depending on
the type of coal deposit or stockpile, sampling points are strategically chosen
to cover the entire area and ensure a representative sample. These points
should be located at regular intervals and distributed across different levels
and positions within the coal.
ii.
Sample Collection: Once the sampling
points are identified, various techniques can be used to collect coal samples.
Common methods include using a mechanical sample collector, auger sampling, or
manual collection with a sampling scoop.
iii. Sample
Preparation: After collecting the samples, they need to be prepared to create a
homogenous mixture. Larger samples are often crushed and divided into smaller
portions. The goal is to ensure that the final sample represents the entire lot
or deposit accurately.
iv. Sample
Reduction: From the prepared sample, smaller subsamples are taken using
techniques like the riffle splitter or rotary divider. The idea is to further
reduce the size while maintaining a representative representation of the
original sample.
v.
Laboratory Analysis: The final subsamples
are then sent to a laboratory for analysis. Various tests are performed to
determine the coal's properties, such as moisture content, ash content,
volatile matter, sulphur content, calorific value, and various other
parameters. These tests help characterize the coal and predict its behaviour
during combustion or other industrial processes.
vi. Reporting
and Quality Control: The laboratory analysis results are compiled into a coal
quality report. This report is essential for trading and commercial purposes,
as it provides detailed information about the coal's quality and compliance
with industry standards.
Proper
coal sampling is vital to ensure accurate assessments of the coal's quality and
performance characteristics. It enables coal producers, consumers, and traders
to make informed decisions regarding its utilization and value in various
applications, such as power generation, steelmaking, and industrial processes.
Additionally, it is crucial for regulatory compliance and ensuring fair trade
practices in the coal industry.
Methods
of Sampling:
There
are several methods for sampling coal to ensure that the collected samples
accurately represent the overall quality and characteristics of the coal. Here
are some common coal sampling methods:
i. Mechanical
Sampling Systems: Mechanical sampling systems use automated equipment to
extract samples from a moving stream of coal. These systems are often used in
coal mines, ports, and other bulk handling facilities. They can provide a
continuous and representative sample of the coal.
ii. Auger
Sampling: Auger sampling involves using a hollow, screw-like device to collect
coal samples from a vertical or horizontal borehole. This method is useful for
sampling coal from stockpiles, truck beds, and railcars. It is relatively
simple and cost-effective.
iii. Cross-Belt
Sampling: In cross-belt sampling, a device is placed across a conveyor belt to
collect samples as the coal passes by. This method is commonly used in coal
transportation and handling systems.
iv. Manual
Grab Sampling: Manual grab sampling involves physically collecting coal samples
using tools like shovels or scoops. This method is simple and can be effective
if proper care is taken to ensure representative sampling. However, it may be
less accurate compared to automated methods.
v. Core
Sampling: Core sampling is similar to auger sampling but involves extracting a
cylindrical core of coal from a borehole using a specialized coring tool. This
method is often used in geological exploration to obtain detailed samples from
coal seams.
vi. Coning
and Quartering: This manual method involves pouring the collected sample onto a
flat surface and forming it into a cone shape. The cone is then divided into
quarters, and two opposite quarters are discarded. The remaining quarters are
combined and mixed, creating a representative sample.
vii. Riffle
Splitting: Riffle splitters are devices that divide a sample into two smaller
portions by allowing the material to flow over a series of inclined chutes with
V-shaped openings. This method is commonly used to reduce the size of large
samples for laboratory analysis.
viii. Rotary
Divider: A rotary divider is a mechanical device that evenly divides a sample
into smaller portions by rotating a circular plate with evenly spaced
compartments. It provides a precise and consistent method of sample reduction.
ix. Incremental
Sampling: Incremental sampling involves collecting small portions of coal at
regular intervals as it is being loaded or unloaded. The collected increments
are combined to create a representative composite sample.
x. Probal
Sampling: Probal sampling uses a probe to extract samples from a bulk material.
The probe is inserted into the coal mass, and a sample is collected by closing
a valve on the probe. This method is often used in situations where continuous
sampling is required.
It's
important to note that the choice of sampling method depends on various
factors, including the type of coal, the location of sampling, the intended use
of the coal, and the level of accuracy required. Whichever method is chosen,
proper care and attention to detail are essential to ensure that the collected
samples accurately represent the overall quality of the coal.
SAMPLING OF COAL SEAM (in
‘situ):
Sampling
of coal in situ gives a measure of the quality of coal to be mined. The
analysis of this sample, however, will seldom give the same results as those of
a run-of-mine sample.
The
section of seam to be sampled shall be exposed from the roof to the floor. The
exposed surface shall be as smooth as possible so that a rectangular channel
may be cut. It may often be necessary to penetrate into the seam for a few
decimetres to avoid taking weathered coal especially if the ‘ be ’ has been
standing exposed .for some time.’ The seam. sample shall, be taken in a channel
representing the entire cross-section of the seam having the dimensions of 30 x
10 cm, that is, 30 cm in width and 10 cm in depth. For this purpose, two
parallel lines, 30 cm apart end at right angles to the bedding planes of the
seam shall be marked by a chalked
string~on
the smooth, freshly exposed surface of the seam. Obvious dirt bands exceeding
10 cm in thickness shall be excluded. The channel between the marked chalk
lines in the seam shall be cut to a depth of 10 cm and the coal sample
collected on a clean strong cloth or tarpaulin placed immediately at’ the
bottcm so that the chances of pieces flying off during
excavation
of coal are minim&d. The total height of the channel shall be measured and
noted. The excluded dirt bands shall, if required, be separately collected and
analysed.
REDUCTION
OF GROSS SAMPLE:
10.1 Run-of Mine Coal: The gross sample shall be crushed to 5 cm, preferably by mechanical means, mixed thoroughly and quartered. Two opposite quarters shall. be retained and the rest rejected. The retained material shall be further mixed together, halved and one & half retained. Thus, ultimately one quarter of the original gross sample is retained and the rest rejected. The material so obtained shall be crushed to 12.5 mm by a jaw crusher and then to 3.35 mm by a palmac type of reduction mill ( see Fig. 3 ). The crushed material shall be reduced either by coning and quartering or by ri6ling ( see Appendix C ) till 2 kg of sample is obtained.
10.1.1
Laboratory Sample: The sample as reduced under 10.1 shall be finally ground to
pass through 212-micron IS Sieve (See also C-4 ). From the ground material, 1.5
kg shall be taken which shall constitute the laboratory sample.
10.1.1.1
Each laboratory sample shall be divided into 3 equal parts, one for the
purchaser, another for the supplier and the third for the referee. The samples
shall be kept in glass or polyethylene containers and shall be sealed and
marked properly.
REDUCTION OF GROSS SAMPLES:
The procedures is already defined in IS : 436 ( Part I/Sec I ) – 1964 (Reaffirmed in 2001) as follows:
A.
GENERAL
PRECAUTIONS:
1. The
place set apart for the treatment of gross samples shall preferably be
enclosed, roofed over, cool, and free from draughts. Where this is not
possible, precautions shall be taken against (a) loss of fine wind-borne
sample, (b) contamination with moisture, and (c) contamination with foreign
matter.
2. Select
a hard and clean surface free of cracks for sample mixing, quartering and other
operations. Do not let cinders, sand, chippings from the floor or any other
foreign matter get into the sample.
B.
CONING
AND QUARTERING:
1. The
material which has been crushed to 3.35 mm (see 10.1) shall be heaped into the
shape of a cone by pouring one scoopful of the material after another at the
apex of the cone till the entire sample has been coned. The material shall be
allowed to slide down the sides of the cone only under the influence of
gravity.
2. Flatten
the cone evenly so that it forms a low circular pile. Cut the pile into four
quarters along two diameters which intersect at right angles. Retain one pair
of opposite quarters and reject the other. Repeat till the size of the retained
sample is reduced to the required weight of 2 kg.
RIFFLING:
1. The
material which has been crushed to 3.35 mm shall be dropped uniformly in the
riffle. One half shall be retained and the other half rejected. This procedure
shall be repeated several times till 2 kg of material is obtained.
GRINDING
(FINE SIZE):
In
grinding the sample to pass 212-micron IS Sieve (see 10.1.)1, it has been found
that unnecessarily fine grinding is harmful. The ground coal should have the
following approximate particle size distribution:
a) a) Passing
212-micron IS Sieve and retained on 125-micron IS Sieve: 35 percent.
b) b) Passing
125-micron IS Sieve and retained on 63-micron IS Sieve : 30 to 35 percent.
c) c) Passing
63-micron IS Sieve : Remainder
CRITERIA
FOR REJECTION OF SUSPECT TEST RESULTS
GENERAL:
It may sometimes happen that a test. result is obtained which deviate considerably from the other test results and therefore arouses suspicion that it may have arisen from a mistake in sampling or sample preparation rather than in the course of normal variation. If the analyst knows that a mistake has occurred, the test result must be rejected irrespective of it’s magnitude. If, however, only a suspicion exists, it may be desirable to determine whether such a result may be rejected or whether it must be accepted as part of the normal variation expected.
CRITERIA
The
procedure as given in D-2.1.1 may be followed for determining the acceptability
or rejectability of the suspect test result.
C.
Collection of samples from Conveyor
Belt/Ropeways/Pipelines:
a) Sampling each day’s supply shall be
considered as one lot in case of Coal supplies by Conveyor
Belt/Ropeways/Pipelines.
b) In case of supply by conveyer belt
sample shall be collected in increments of full cross section and thickness of
the stream in one operation in a regular interval of time as mutually decided
by both Seller and Purchaser and lot shall consist of samples so collected
during a day i.e. 0:00 Hr to 0:00 Hr. of the following day.
c) Before collecting the increments, the
speed of the conveyer and quantum of material passing a certain point in a
given time shall be ascertained so that an appropriate spacing of time between increments
may be arranged over the whole of the lot.
d) If it is practicable to stop the belt
periodically, increment may be collected from the whole cross section of the
stream by sweeping the whole of the Coal lying between the sides of a suitable
frame placed across the belt. The frame should be inserted in the Coal until it
is in contact with the belt across its full width.
e) Minimum 150 kgs of samples to be
collected for daily Gross Sample.
f) Any stone/shale of size more than that
specified in FSA Schedule-II shall be removed/ discarded from the sample.
However, all stone/shale of size as mentioned in FSA Schedule shall form part
of the sample collected.
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