Coal Sampling in Power Plants
Coal sampling Importance
The purpose of collecting and preparing a sample of coal is to provide a test sample which, when analyzed, 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:
- ensure that the sample is representative of the bulk material, which means all parts of the material being sampled must have an equal probability of being collected and becoming part of the final sample for analysis, and
- ensure that the sample does not undergo any chemical or physical changes after completion of the sampling procedure and during the storage prior to analysis.
Coal is highly heterogeneous in nature consisting of particles of varied shapes and sizes each having different physical characteristics, chemical properties and residual ash content. It also has a tendency to segregate by size or mass. Sampling is further complicated by the sampling equipment available, the quantity to be represented by the sample mass, and the degree of precision required. In addition, the coal to be sampled may be a blend of different coal types. How the coal is blended, for instance whether it is intimately mixed or not, has a profound effect on the way a representative sample is obtained. As a result, coal is one of the most difficult materials to sample and the condition that a coal sample be completely representative of the whole as regards all aspects, is never fulfilled. National and international standards have been developed to provide guidelines to the coal sampling procedures under different conditions of sampling for the purpose of obtaining unbiased analysis samples.
The sampling of coal can take place from either stationary lots or from moving streams. Coal samples can be taken from a broad range of locations such as feed and product streams, conveyor belts, trucks, railway wagons, and stockpiles. Sampling can be carried out either manually or by mechanical sampling systems. Most coal samples require some kind of physical preparation prior to chemical analysis. The sample preparation process may include air drying, crushing, division, and mixing of a gross sample to produce the small individual portions required for laboratory analysis. Biased results can be introduced by the sampling procedure, during sample preparation and analysis. It is, therefore, important that all sampling systems are checked for bias. This chapter reviews the general principles of sampling, the methods and procedures of coal sampling, preparation and bias tests.
Coal Sampling
How samples are collected is dependent on the aspect of mining and the purpose for which the coal is being tested. Samples may be required for technical evaluation, process control, quality control, and/or for commercial transactions. It is important, before collecting a sample, to decide what the purpose of the sample will be, and then plan the sampling procedure accordingly so that it will be unbiased and sufficiently representative for the purpose.
General principles of sampling
The fundamental requirements of sampling include:
- 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;
- the dimension of the sampling device used should be sufficient to allow the largest particle to pass freely into it;
- the first stage of sampling known as primary increments is the collection of an adequate number of coal portions from positions distributed over the entire lot to reflect the variability of the coal. The primary increments are then combined into a sample, as taken or after reducing the mass of the sample to a manageable size. From this gross sample, the required number and types of test samples are prepared by a series of processes jointly known as sample preparation;
- 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.
Sampling Terminology
1. Coal Large – Coal with nominal size 15 to 5 cm, the upper limit not exceeding 23 cm.
2. Coal, Run-of-Mine – Unscreened coal containing all sizes, mainly 23, to 0 cm.
3. Coal, Small – Coal with nominal size 5 to 0 cm.
4. Composite Sample (for the Lot) -The quantity of coal obtained by mixing together equal quantities of coal from each of the laboratory samples representing the sub-lots into which the lot has been divided.
5. Gross Sample -Sample as collected from a sub-lot, that is, the quantity of coal obtained by aggregating together all the increments drawn from the same sub-lot.
6. Increment – The quantity of coal taken by a single operation of the sampling implement.
7. Laboratory Sample – The quantity of coal obtained by reducing a gross sample by following a specified procedure for laboratory testing.
8. Lot – The quantity of coal offered for disposal at one time,
9. Moisture Sample – A sample ‘to be used exclusively for the purpose of determining total moisture.
10. Sample Division-The -process whereby a part of the sample is retained and the remainder rejected.
11. Sample Reduction -The process of crushing or grinding .the sample to reduce the particle size and of mixing and sample-dividing in successive stages.
12. Sub-lot – The quantity of coal in each of the groups into which, a lot is divided for the purpose of sampling; a lot may consist of two or more sub-lots
Number of sub lot:
| Sr. No | Weight of the lot (MT) | No of sub lots |
| 1. | Up to 500 | 2 |
| 2. | 500-1000 | 3 |
| 3. | 1000-2000 | 4 |
| 4. | 2000-3000 | 5 |
| 5. | Over 3000 | 6 |
Type of sampling
There are two types of sampling
- Time based sampling – this sampling is done as per particular time interval. eg. Feeder sampling and conveyor belt sampling
- Mass Based sampling – This sampling is done as per the total lot in the particular rake and truck, ships, etc
Methods of samplings
There are different types of method of sampling
- Sampling from Rake
- Sampling from truck
- Sampling from ships
- Sampling from conveyors
- Sampling from stock piles
Sampling from Rake:
Rake sampling is done by manual method as well as mechanical method but we don’t have mechanical method so we took rake sampling by manual method as per Random chart given in IS 436 P1. There is 59 wagon in rail and we are collected 18 wagon as per random chart, number given by shift incharge from CCR. From each wagon we are collected from 5 location after discarding upper layer of coal and then taken increment. From each wagon we are collected 25 kg of sample means 5 increment from 5 location.
Sampling from Truck:
Truck sampling is done by manual method as well as mechanical method but we don’t have mechanical method so we took truck sampling by manual method as per Random chart given in IS 436 P1. Truck sampling done as per lot, every 10 truck is considered as a one lot and from one lot we are collected 3 truck sample as per random chart, number given by shift incharge from CCR. First truck is unloading then we are collected from 5 location after discarding upper layer of coal and then taken increment as sample. From each truck we are collected 25 kg of sample means 5 increment from 5 location.
Sampling From ships:
Ships is consider as one lot and weight near about 75000 MT coal loaded in a ship, first we will decided sub lots as mutual understanding between the receiver and sender and third party. Sample taken when ship unloading and sampling done from conveyor location or after the unloading in the coal yard.
Sampling from conveyor:
Conveyor belt sampling is the time based sampling where we can collect the sample from running belt at particular time interval. When belt is running we use scoop the sample from a moving conveyor belt using a cross belt cutter.
Sampling from stock piles:
This is a mass based sampling, here we have to find out the lot and then divided in a sub lots in particular equal quantity. Then we have to take sample form each sub lots from different location, should be 5 location for one sub lots and min 20-25 kg of sample withdraw from each sub lots.
Random chart:
Reference : IS 436 Part-1