Sampling of Coal Dr kalyan sen Director, Central Fuel Research Institute, Dhanbad, 2003

6/1/20142 Quality Monitoring (QM) of Coal is an essential requirement for process control, plant performance or for any commercial transaction between Consumer and Producer QM requires proper implementation of standard sampling, preparation and test procedures

6/1/20143 Objective To collect a representative portion of fuel (coal) for determination of quality parameters.

6/1/20144 Purpose Commercial Transaction LocationNormally at Loading point ParametersAsh, TM, 60% RH moisture, GCV Quality Parameters for Power Generation LocationNormally at unloading point ParametersAsh, TM, GCV, VM S, N, Maceral composition

6/1/20145 Steps for quality assessment Sampling Sample preparation Analysis

6/1/20146 H Coal is heterogeneous material H Very difficult to achieve highest level of Sampling precision H In terms of variance D 80 % is from Sampling D 20 % is from preparation & analysis H Overall precision is influenced primarily due to Sampling H Utmost importance need to be given for Sampling

6/1/20147 Sampling methods depend on < mechanical or manual sampling < sampling from moving belt < sampling from stationary lot (wagon, stockpile, etc.)

6/1/20148 Sampling types Moving Stream v Auto-mechanical sampling system v Manual Stationary lot, Wagon, Stockpiles, etc. F Auto-mechanical Auger F Manual

6/1/20149 Sampling variance is a function of product variability i.e. different results can be obtained from u same increments for different coal u different increments for same coal

6/1/ The objective is to reduce the sampling variance as far as practicable

6/1/ Any Sampling scheme normally conforms with the national or international standards (BIS/ISO/ASTM, etc.) Constraint - technical, cost and time Thus modifications in sampling procedures are necessary with mutual agreement between parties

6/1/ Precision u measures the closeness of data in given condition u indicates the reproducibility of the results u measures the chance error as expressed by variance SMALLER THE RANDOM ERROR, PRECISE IS THE METHOD A commonly accepted index of precision is twice the population standard deviation

6/1/ Precision depends on u Variability of coal u number of samples from a lot u number of increments comprising each sample u mass of sample related to the nominal top size

6/1/ Precision P L = 2 * Sqrt [V I /m*n + V PT /m] n = no. of increments m = no. of sub lots V I = Primary increment variance V PT = Preparation & testing variance P L = Overall precision at 95% confidence level

6/1/ Bias Systematic error which leads to the average value of a series of results being persistently higher or lower than those which are obtained using a reference sampling method which is intrinsically unbiased

6/1/ Reference method of sampling is Stop Belt Method (free of Bias) (free of Bias)

6/1/ Design of Sampling Scheme Basic Principles Both for Mechanical & Manual systems

6/1/ General scheme for sampling... u Decide purpose of sampling e.g. plant performance, process control, commercial transaction u Identify the quality parameters, viz., general analysis, TM, size, washability, etc. u Define the lot u Define the precision required u Decide whether continuous or intermittent sampling is required

6/1/ u Determine the number of sub-lots, increments to achieve the required precision. u Determine the nominal top size of the coal u Determine the min. mass/ increment and the min. mass of the total sample u Decide on the method of combining the different increments for gross sample u Decide on drawing common or separate samples, for analysis General scheme for sampling.. contd..

6/1/ General principle of Sampling u Primary increments should account for the Variability u Equal probability to all particles to be selected and included in the sample u Largest particle of the lot should pass freely through the sample device u Sufficient mass of the sample to enable particles to be present in the same ratio as in the lot

6/1/ SAMPLING FOR COMMERCIAL TRANSACTION Joint Sampling Joint Sampling Washed coking coal Washed coking coal Power coal Power coal

6/1/ Joint sampling u at loading point - by customer and producer on mutually agreed methods u at both ends - mean value u bonus/penalty to producer for values beyond agreed tolerance limits u requires periodic testing... Unfortunately rarely practiced in India

6/1/ Reasons for discrepancies in results u level of precision not defined u Non-identical procedures for sampling at both ends u manual sampling results in large human error u deviation in procedures from agreed one

6/1/ Primary requirements for development of a methodology u testing for estimation of the variances, V i and V pt u decision on level of precision of the ash value u calculation for no. of sub-lot and increment /sublot at desired precision from known values of variances u estimation of precision for the existing procedure u estimation of min. mass/ sub-lot form the std. Table u estimation of min. mass/ increment

6/1/ SAMPLING SCHEME is designed based on the above test The procedure can significantly reduce the discrepancies in the results at both ends

6/1/ Sampling of washed coking coal Samples are drawn from the u Automatic mechanical Sampler (AMS) u Conveyor Belt For day to day quality monitoring, samples are reduced by offline and/or manual means to analyze ASH & TOTAL MOISTURE

6/1/ Sampling of Power Coal Best option: AMS at loading/ unloading point AMS for coal x200mm or above is not a proper choice to ascertain quality parameter Suggestion: sampling on crushed coal below 50mm or preferably at 20mm

6/1/ Where AMS is non-existing/ non-functioning, sampling may be done for the time being, at loading point from the wagon by manual means Wagon top sampling is difficult, because segregation occurs because of large size segregation occurs because of large size impractical to collect sample from the full depth impractical to collect sample from the full depth introduces bias due to manual operation introduces bias due to manual operation Suggestion: smaller size (< 50mm) of the sample

6/1/ Periodic tests Estimate of overall precision Estimation of increment variance Estimation of preparation & testing variance If the overall precision is different from the desired one then, number of increment & sub lot to be modified accordingly.

6/1/ Alternative Long Term Auto mechanical sampling system at Sampling points Auto mechanical Augers from the wagons

6/1/ Alternative Short Term Estimation of variance of sampling, sample preparation & testing Design a practical procedure for routine implementation Perform periodic test for checking

6/1/ Conclusion: Choice of Sampling methodology depending on the purpose Choice of Sampling methodology depending on the purpose Efforts to reduce the sampling variance to a min. possible limit Efforts to reduce the sampling variance to a min. possible limit Sampling on mechanically crushed coal below 50mm Sampling on mechanically crushed coal below 50mm Preferable size is 20mm ( feed to most power plants) Preferable size is 20mm ( feed to most power plants)

6/1/ Replacement of manual sampling method by AMSReplacement of manual sampling method by AMS In absence of AMS, manual wagon top sampling of this size would give better resultsIn absence of AMS, manual wagon top sampling of this size would give better results In absence of AMS, manual sampling from wagon top can be done as an temporary option, following the prescribed methodology In absence of AMS, manual sampling from wagon top can be done as an temporary option, following the prescribed methodology Conclusion…………contd.

6/1/ Ash Sample Preparation P Gross Sample P Air dry & Crush to 12.5 mm P Air dry (if required oven dry at 35 deg but not more than 2 h at a time) P Reduce the sample to 2 kg P Divide into 4 parts P 2 parts (Reserve); 1 part (212 micron) & 1 part (212 micron) P 1 part preserve as check sample P 1 part divide using laboratory divider (2 samples) P Sample A 1 Sample A 2

6/1/ Total Moisture Sample Preparation Gross Sample Air dryRecord wt. Crush to 12.5mm Reduce 2.5kg Crush to 2.8mm Divide into 4 parts Sample A Sample B RESERVE Size 12.5 mm above ? Record Initial wt. Stage 1 loss% Stage 2 loss% Oven dry 35deg not exceed 2h Record wt. Stage 3 Moist. 10g, 108deg, 2h Y N

6/1/ Common Sample Preparation Common Bottle Sample Air dry Record wt. Crush to 2.8mm Divide into 2 parts Sample A Sample B RESERVE Record Initial wt. Stage 1 loss% Stage 2 loss% Oven dry 35deg not exceed 2h Record wt. Stage 3 Moist. 10g, 108deg, 2h Divide Crush to 2.8mm Sample C Ash Divide into 2 parts

6/1/ THANKS