CBAP
Coal Blends Accreditation Program (CBAP)
The analysis of coal blends is a field in which there are no other techniques that can provide information similar to that from organic petrology. Petrographic analysis is the only way to unambiguously know the presence of coals of different characteristics in a blend, their relative proportions and their individual petrographic characteristics.
The ICCP Coal blends working group has developed over the last 13 years a set of analytical procedures to analyse coal blends in highly reproducible manner. The ISO standards applicable are those for sample preparation for petrographic analysis (ISO 7404-2; 2009), maceral group composition of coal (ISO 7404-3; 2009) and vitrinite reflectance of coal (ISO 7404-5; 2009) but specific instructions are required for the analysis of the various parameters relevant for coal blend petrographic analysis.
Unmounted coal blends will be distributed containing several component coals and will have to be prepared and polished at participants laboratory according to ISO 7404-2; 2009.
The parameters to be evaluated are:
- Number of coals in each sample (This field is mandatory to further proceed in the accreditation process)
- Overall mean vitrinite reflectance (based on ISO 7404-5; 2009)
- Vitrinite reflectance for each mode reported (based on ISO 7404-5; 2009)
- Overall maceral composition assessed in terms of vitrinite content (m.f. and unassigned free basis); (based on ISO 7404-3; 2009 and on the procedures developed by the Coal Blend Working Group, Commission III, ICCP)
- Maceral composition of each component coal assessed in terms of vitrinite content (m.f. and unassigned free basis. (based on ISO 7404-3; 2009 and on the procedures developed by the Coal Blend Working Group, Commission III, ICCP)
- Blend composition; how much of each rank component? (based on ISO 7404-5; 2009)
The analyses to be performed in order to assess the parameters mentioned above are:
1) Reflectance Analysis.
Two pellets should be prepared for analysis of each blend sample and 250 reflectance measurements have to be made on each of the two pellets to give a total of 500 points for the blend. The ISO standard 7404-5 ; 2009 is to be followed and readings should be taken in random reflectance (not maximum) mode. Particular care should be taken when making measurements on particles by using a regular grid pattern over the entire surface of the pellet using a graduate stage. When you have moved to a new grid point, take a measurement on whatever vitrinite is closest to the cross-hair if its surface is suitable for reading. Particles of vitrinite that, because of their size or features (i.e. weathered vitrinites), are not suitable for reflectance measurements should be rejected. Then move to the next grid point. Do not take more than one reflectance reading on any one vitrinite particle.
A histogram should be constructed at the end of the analysis with the results obtained for the two pellets from each sample. Each histogram should present two or more modes representing the vitrinites from the various coal components. You should decide from the form of the histogram on the probable reflectance thresholds or points of separation between the various component coals in the blend or after the direct assignment of vitrinite fields to their parent populations during the measurements of the vitrinite reflectances. Once the various vitrinite populations have been separated, calculate their mean reflectances and their standard deviations. Also the overall blend vitrinite reflectance and standard deviation should be provided.
Next, the number of vitrinite reflectance readings for each of the coals identified in the sample must be counted and the percentage calculated considering the total counts (500). The percentages represent the proportion in volume of each component coal in the blend free.
2)- Point-Count Analysis.
This procedure allows to determine not only the coal blend composition in terms of percentage of coals from different rank, but also the maceral composition of each of the component coals. It is recommended that this analysis be carried out following the criteria of maceral groups identification explained in the ISO 7404-3; 2009 standard. The analysis is to be conducted using non-polarised light and 500 points on each of the two pellets with spacing between particles of 0.5 mm and between traverses of 1.0 mm should be counted. The macerals from the various rank coals should be distinguished by the visual appearance of the vitrinite and liptinite anywhere in a particle falling below the cross-hair. If necessary, reflectance measurements on the vitrinite associated with a particle to be counted could be made for appropriate differentiation.
This method yields the maceral composition of the blend expressed on a mineral matter and unassigned particles free basis.
At the conclusion of the exercise, the results from all participating petrographers are pooled. If a participant fails in identifying the number of coals in the blend he/she will fail accreditation. If the number of coals is correct, the other parameters will be evaluated. A statistical evaluation identifies those petrographers whose results do not fall within acceptable limits of departure from the group mean or that report results which are consistently higher or lower than the group mean (see Statistical Evaluation in Detail).
Petrographers who are not accredited in all the parameters tested for coal blends petrographic analysis or are accredited only on some of them (that is reflectance of the component coals or proportion of the coals in the blend, or maceral composition of the component coals) will have to perform a complete set of analyses for either both parameters or the parameter in which they were not accredited. The new set of analyses will then form the petrographer’s new data file that will be statistically assessed and to which all subsequent analyses are added and evaluated.
For the second round, the AUMSD for the worst sample will be eliminated.
REFERENCES
International Committee for Coal and Organic Petrology, (ICCP), 1998. The new vitrinite classification (ICCP System 1994). Fuel 77, 349–358.
ISO 7404-2, 2009. Methods for the Petrographic Analysis of Coals — Part 2: Methods of Preparing Coal Samples. International Organization for Standardization, Geneva, Switzerland. 12 pp.
ISO 7404-3, 2009. Methods for the Petrographic Analysis of Coals—Part 3: Method of Determining Maceral Group Composition. International Organization for Standardization, Geneva, Switzerland. 7 pp.
ISO 7404-5, 2009. Methods for the Petrographic Analysis of Coal—Part 5: Methods of Determining Microscopically the Reflectance of Vitrinite. International Organization for Standardization, Geneva, Switzerland. 14 pp.