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Quality monitoring of coal

Quality Monitoring of Coal
Kalyan Sen, B.Mining.E (C.U), DrScTech (AGH, Poland)
Director, Central Fuel Research Institute, Dhanbad 828108 (India)

Introduction 
Quality of coal plays an important role in all its utilisation activities. Depending on the ultimate objective, be it steel making or power generation, the feed coal must satisfy some desired quality parameters. Coal being a highly heterogeneous substance in terms of inorganic and organic constituents, exhibits wide variability with respect to size and chemical compositions of the particles. Growing quality consciousness on the part of both suppliers and customers has made quality monitoring an integral part of the industrial activity. Proper quality monitoring of coal involves implementation of standardised procedures for sampling, sample preparation and analysis to provide a reliable estimate of the desired quality parameters at minimum variance.
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. Detailed documented procedures are laid down in National and International Standards for executing the job of representative sampling pertaining to different methods of sampling.  However, due to time, technical and other economic constraints strict implementation of the procedures are difficult to practice for routine monitoring job. Modified methodologies are implemented after mutual consent of 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 come from errors during its collection only. The important issue that is not always considered is the effect of modified procedures on the precision of the final result. This often leads to different controversies between the seller and the customer.
Before initiating discussions on the existing practice of sampling some basics of sampling need to be touched upon.
General principles of Sampling
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.

Variability of coal
Number of samples from a lot
Number of increments comprising each sample
Mass of sample relative to the nominal top size
While drawing increments great care should be taken to avoid the occurrence of bias in the results. The ideal method of sampling is the stopped belt method, which is considered free of bias. As implementation of such method will affect the continuity of plant operations, it is not always practicable for routine sampling.  However, any mechanical sampling device needs to be checked for bias by comparing with the results from stopped belt reference method.
General procedure for establishing a

sampling scheme

 

 

1	Decide for what purpose the samples are taken e.g. plant performance evaluation, process control, commercial transactions etc.
2	Identify the quality parameters to be determined, i.e. general analysis, total moisture, size analysis, washability, etc.
3	Define the lot
4	Define the precision required
5	Decide whether continuous or intermittent sampling is required
6	Determine the number of sub-lots and the number of increments per sub-lot to achieve the required precision.
7	Determine or estimate the nominal top size of the coal
8	Determine the minimum mass per increment and the minimum mass of the total sample
9	Decide on the method of combining the different increments to produce the gross sample
10	Decide on drawing common or separate samples, for general analysis and moisture

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 prevalent practice
For commercial transaction, the existing practice for quality monitoring of coal consignment is done either by the method of joint sampling or by third party sampling.  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.
Case study on sampling of washed coking coal
High ash caking coals are washed before despatching cleans 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 averaged on the railway rake basis. The collected samples are reduced by offline mechanical and/or manual means to produce the final test samples. In spite of the carrying sampling through mechanical means there were wide discrepancies in the results at the loading and unloading ends. To resolve the issue CFRI was involved as an independent third party to monitor the quality of washed coals. The detailed methodologies developed and implemented for a period of two and a half years (Dec. 1996 to March 1999) practically eliminated any controversy between BCCL and SAIL and made a significant impact in the economy of the both the seller and customer.
The sampling scheme designed by CFRI, if followed with all seriousness by the representatives of the two parties can significantly reduce the discrepancies in the results at both ends. If results are 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 the latter will definitely be affected by human discretions.
Sampling of Power coals.
Most of the non-coking coals are consumed by the power sector. State owned utilities and NTPC are the major consumers and 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 between NTPC and CIL, 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 most cases, sampling of thermal coals 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 of 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 size raw 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 washed or blended  coals (say, below 50mm), normally  by auto-mechanical auger systems.
CFRI has been involved in developing the standard methodologies for sampling in some of the power stations under NTPC. Studies carried on wagon top sampling through designed experiments reveal some interesting features. The presence of bias in wagon top sampling is indicated when compared with the ‘stopped belt’ sample. The difference in the ash value can be as high as 5 ash units. Further the variance between the increments is rather large and the precision value is rather low and lies in the range of 2-2.4 units.
In view of the MOEF restriction on supply of power coals to distant power plants at 34% ash level on yearly average basis, the existing practice of wagon top sampling can never be the right choice. The huge difference in the estimated value from samples collected from wagon top for a daily consignment with the ‘true value’ disqualifies the concept of average ash. Averaging of the values for a regular interval can only be done if the daily variation is within a defined tolerance limit. Inappropriate sampling will also affect the blending proposition, which is being considered as an immediate option to meet the MOEF directive.
Another relevant point that needs to be stressed is the non-availability of the complete set of mechanical size reduction units at some of the loading points. Manual practice of size reduction of a large quantity of samples, on a regular basis, is time consuming and involves human errors.

Recommendations for sampling of power coal.

• Sampling at both the loading and unloading point should be done through AMS. The AMS should be tested for bias before its use for routine monitoring.
• Proper sampling scheme to be designed to achieve the desired level of precision and the number of increments, sub-lots are to be identified.
• 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.
• Introduction of third party sampling by reputed organization
• During installation of new Coal Handling Plants or modernisation of existing ones, the nominal top size of coal for despatch should be below 50mm. This will reduce most of the problems of ‘non-availability’ and difficulties in operation of AMSs.
• In the absence of AMS facility, as an interim measure wagon top sampling can continue for a predefined period after designing a proper sampling scheme.  Sample preparation should preferably be carried through complete mechanical means.

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 auto-mechanical sampling on crushed coal of 20/25 mm size needs to be enforced.
Conclusion
Quality assessment of the primary fuel source assumes greater significance in this changed ambience of competitiveness and increased demand of improved process efficiency of the power sectors. 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, Auto-mechanical Sampling System has no substitute. However, AMS for 200mm size coals has some inherent disadvantages. These are i) high probability of spillage during collection of primary increment, ii) impact of large lumps on the primary cutter, iii) rapid wear & tear, iv) tedious and frequent maintenance v) non-availability in the Indian market, vi) high cost, etc.
 Considering the above factors utilisation of AMS for sampling at the loading or unloading point on crushed coals (size 20/25 mm) is the best option. This is normally the desired size for feed to the grinding circuits of the power plants. Further, sampling on crushed coal provides the additional advantage of switching to wagon sampling in case of failure of AMS, involving less manpower, more flexibility and handling of less amount of coal.
Continuance of the practice of wagon top sampling of 200mm size coal by manual method, known to be intrinsically biased becomes difficult to justify in this changed environment, when scope exists for implementing cost effective established alternatives.
Notwithstanding the fact that wagon top sampling is inherently biased, this practice is likely to continue for some more time and hence 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 estimated from a separate study can be considered by the customer and seller for inclusion in the coal linkage agreements.