Coal is a highly heterogeneous substance in terms of the inorganic and organic constituents and exhibits wide variability with respect to size and chemical composition of the particles. An estimation of the true value of the desired parameters of a bulk material, to a certain of degree confidence, through analysis on a few grams of test sample is definitely a daunting problem. The basic purpose of collecting and preparing a sample of coal is to provide a test sample which when analysed will provide the test results representative of the lot sampled. In order that the sample represents the coal from which it is taken, it is collected by taking a definite number of increments distributed throughout the whole volume of coal.

The procedure for sampling will, however, differ with the purpose and method of sampling. Samples may be required for technical evaluation, process control, quality control or for commercial transactions. For quality assessment of coals from new sources, samples are to be drawn from in-situ coal seams, either as rectangular blocks or pillars cut from full seam height, or from seam channels or from borehole cores. To check the quality of coal consignments, it is desirable to sample from conveyor belts. The reference method of ‘stopped belt’ sampling is often implemented to standardise any other mechanical automatic sampling systems. Detailed documented procedures are laid down in National and International standards for executing the job of representative sampling pertaining to different methods of execution.

The major consumers of indigenous coals belong to the core sectors such as steel, power, cement, chemicals and domestic sectors. Quality monitoring of coal is an important activity for any commercial transactions between the consumers and the producers. As already mentioned the method of sampling for quality monitoring differs and is governed by many factors. The sampling procedure will depend mainly on the nature of sample collection i.e. by mechanical or manual means, from moving belt or from stationary lots like wagons, stockpiles, etc. Normally any sampling scheme is supposed to conform to relevant national or international standards. However, due to technical, cost and time constraints, very often some modifications are made in the method of sampling jointly by the seller and the purchaser. It is a known fact that about 80% of the total variances involved at the different stages of sample collection, preparation and analysis comes from errors during its collection only.

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Seminar entitled " New Challenges for Indian Coals"

Before initiating discussions on the existing practice of sampling some basics of sampling need to be touched upon.

The fundamental requirements of sampling are,

• 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 take care of 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.
• To ensure that the result obtained has the required precision the following issues are to be considered.

Sampling scheme has to be designed based on the purpose of sampling and after ascertaining at what stage of coal handling operation the sample is required.

• Division of lots: A lot may be sampled as a whole or a series of sublots. Each sublot will constitute one sample.
• Basis of sampling : It can be either time basis or mass basis. In time basis the sampling interval is defined in minutes/seconds and mass is proportional to the flow rate, whereas in mass basis the interval is defined in tonne and the mass of increments is uniform.
• Precision : In all methods of sampling, sample preparation and analysis, errors may be introduced at every stage and the measured value may differ from the true value of the parameter. As the true value is not exactly known it is difficult to assess the accuracy of the results, but an estimate of precision of the results can always be made.

The required precision for a lot for each parameter has to be decided and then the number of sub-lots, number and mass of increment are to be estimated.

Normally, joint sampling is carried out at the loading end by the representatives of the producer and the customer, following a methodology mutually agreed upon by both parties. Depending on the nature of the agreement, the loading point results can be taken exclusively for commercial transactions. In some cases the mean value of the results of joint sampling at both the loading and unloading ends is considered. The tolerance values in the quality parameters are often defined, beyond which several bonus/penalty clauses are imposed. What needs to be stressed is whether the tolerance value identified is compatible with the sampling scheme. More clearly, whether the tolerance value lies within the precision limit that can be achieved through the implementation of a particular sampling scheme. This requires periodic testing, which unfortunately is rarely practiced in India.

It is a common experience that in spite of joint sampling, there often exist wide discrepancies in the results at the two different ends. There may be multiple reasons for this :-

• Identical procedures for sampling and sample preparation are not followed at the two ends.
• In case of manual sampling human discretion becomes a significant factor
• Deviation from the procedures identified in the agreement
• The level of precision remained undefined while designing the sampling scheme.

High ash coking coals are washed before despatching to steel plants. Most of the coking coal washeries in the eastern region have automatic mechanical samplers. Samples are drawn from the automatic samplers and results are given on the railway rake basis. The collected samples are reduced by offline mechanical and/or manual means to produce the final test samples. The automatic samplers are different from Automechanical Sampling Systems (AMS) which have integrated size reduction units. For day to day commercial transactions the quality parameters are ash and total moisture percentages.

The methodology that needs to be followed can be derived from the following tests.

• Carry out tests to estimate the increment variance and preparation & testing variation.
• Decide on the level of precision of the ash value
• With known values of the two variances, calculate the number of sub-lots and the number of increments per sub-lot to arrive at the desired precision level
• Carry duplicate sampling with the existing procedure and estimate the precision achieved. Modify the number of increments and number of sub-lots accordingly to arrive at the required level of precision.
• The minimum mass per sublot can be obtained from standard table, which considers the nominal top size of the coal and the precision level. The minimum mass per increment can then be derived accordingly.

Increasing the mass of the increments above the minimum requirement does not improve the precision. Depending on the variability of the fuel the precision of results can be improved by increasing the number of increments and number of sub-lots. It is desirable to conduct the sampling on sub-lot basis and the mean result of the sub-lots can be considered as the final result of daily rake despatched.

The tests enumerated above can be simultaneously carried without disrupting the routine sampling. The tests are essential as they provide the required basis for the agreement between the purchaser and the consumer.

Furthermore, the off line sample preparation and size reduction until the pulverisation stage, should be done by mechanical means. Manual grinding should be strictly avoided.

The sampling scheme designed on the basis of the above tests, if followed with all seriousness by the representatives of the two parties can significantly reduce the discrepancies in the results at both ends. If result is to be given as an average of those at the two ends, then identical means of sampling and preparation are to be adopted. Results obtained from automatic samplers at one end cannot be compared with those of manual sampling at the other end, as it will definitely be affected by human discretions.

Most of the non-coking coals, which are mined today, are consumed by the power sector. Thermal Power Stations under NTPC are the major consumers of power coals. NTPC stations receive coals of widely varying characteristics from different coalfields under Coal India Limited/ Subsidiaries. Very often power stations receive coals from multiple sources and for commercial purposes, source wise results are required. As per the agreement, sampling for general analysis and total moisture has to done at the loading point. If Auto-mechanical Sampling System (AMS) exists at the loading point sampling should be done through AMS. In case of nonexistence of AMS or malfunctioning of AMS, sampling for general analysis is to be done at the unloading end, provided AMS exists. If AMS is not installed at either ends, sampling is carried out at the loading point from the wagons by manual means.

In some cases, sampling at the loading point is done manually from wagons, as the AMS even if installed, are not functioning. The coal that is being despatched is generally upto 200mm size. Depending on the existing facility, the coals are loaded into the wagons by rapid loading system, pay loaders etc.

Manual method of wagon top sampling of large sized coals is not only difficult but also violates some of the fundamental principles of sampling. As per requirement samples are to be drawn from the full depth of the wagons which is impossible to be collected manually. Furthermore, due to size segregation the samples collected from the wagon top does not satisfy the criteria of representativeness of the whole samples. Since the ash distribution in the different size fractions is not homogeneous, results from the samples which do not reflect the true size distribution of the lot are likely to be biased. More importantly, sample collection by a shovel from the top is a function of human discretion and not governed by the probability rule. Wagon sampling when practiced in other parts of the globe is done on smaller and uniform sized coals, generally washed and blended, and preferably by automechanical auger systems.

Other important points that need to be stressed are the steps that are taken for sample preparation. Most of the loading point sites are not equipped with mechanical size reduction units. Manual practice of size reduction of a large quantity of samples on a regular basis is time consuming and involves human errors.

To get a correct assessment of the quality parameters, it is recommended that sampling should be done through auto mechanical sampling systems. Immediate steps need to be taken to bring the existing AMS in working conditions, followed by testing of bias. The system should be studied for a prolonged period to identify its limitations and constraints.

The best alternative for monitoring power coals at the loading point is to use AMS on coals crushed to size below 50 mm. Since at the power plant end the coals are further crushed to 20/25 mm size as a feed to grinding mills, the loading point coal size can be rationally fixed to 20/25 mm size. Automatic sampling on crushed coal of 20/25 mm size has several advantages. The AMS for 25 mm size is less costly, available in Indian market, requires less maintenance and generally has less wear & tear compared to AMS for 200 mm size. It also ensures true representative sampling and less handling of material. The additional advantage of crushed coal is that in case of nonfunctioning of AMS, wagon top sampling can be resorted to, which will provide more representative sample than can be obtained from large size coals.

In the absence of any immediate option to switch over till the AMSs are installed or the existing systems are not brought into working conditions, wagon top sampling on the large sized coals has to continue. However, the procedure needs to be established considering the respective variances of sampling and sample preparation & analysis. It is always desirable to perform the sampling on the basis of different sub-lots. A sub-lot can constitute a fixed number of rakes, depending on the total daily despatch.

Concerted efforts have to be made by both the seller and purchaser to operate the existing AMS at the loading points and test its effectiveness for a prolonged period. For futuristic planning of quality monitoring of power coals, the automechanical sampling on crushed coal of 20/25 mm size needs to be enforced.

It is a well known fact that sampling from wagon top introduces bias. However, as this practice is likely to continue for some more time there is a need to evolve a procedure that will give results with lesser variance. It needs to be stressed that meticulous supervision of sampling and preparation procedure, increasing the number of increments and expressing the daily result as a mean of different sub-lots may increase the precision but in no way decrease the bias that is inherently present in the system. The precision value can be used by the customer and seller to match the results from the loading and unloading ends to settle the disputes on the results, if any.

In this changed ambience of competitiveness and efficiency enhancement imperatives of the core sectors, quality assessment of the primary fuel assumes greater significance. The matter is not simply confined to commercial transactions only, but has direct relevance to the efficiency of end use. If the quality parameters are to be determined at a reasonable precision level and nearer to the true value, automechanical sampling system has no substitute. Considering the technical constraints like spillage, impact of large lumps, rapid wear & tear etc. associated with AMS for large sized coals and other factors involving ease of handling, maintenance, availability in the Indian market, utilisation of AMS for sampling at the loading or unloading point on crushed coals (size 20/25 mm) is the best option. Sampling on crushed coal provides the advantage of switching to wagon sampling in case of failure of AMS, involving less manpower, more flexibility and handling of less amount of coal. This will require initial cost involvement but considering the benefits in terms of accuracy of results, lesser maintenance and manpower costs, this will pay in the long run. Adhering to a system which is intrinsically biased when alternatives are known and established may be difficult to justify in the perspective environment.