Fast and Precise Elemental Screening in Petroleum Coke and Coal

Product Highlight: Petra MAX

Rapid sulfur measurement plus 12 other critical elements
Petra MAX delivers advanced elemental analysis powered by EDXRF, utilizing XOS's patented doubly curved crystal optics coupled with a high-performance silicon drift detector and an intense monochromatic excitation beam. This industry-leading technology reduces background noise and increases signal-to-noise output, enabling low detection limits and high precision. Petra MAX offers low-level detection of iron, saving the marine industry money on engine replacement and lube oil costs, while also using a next-generation interface that saves users time.

Figure 1

For best results when using Petra MAX, it is recommended that the cup be almost completely filled prior to applying the film. This will result in the cup being at least 50% full in the vertical position after tapping to remove the air gaps so that the compacted powdered sample is within the analyzer focal point (which is about ⅓ of the way up from the bottom edge of the cup, as indicated by the hole in the sample basket in Figure 1). Lastly, if using Petra MAX Autosampler, it is recommended to run only one petcoke or coal sample at a time using a vent clip so that the ground, packed sample remains in the correct orientation and packed tightly throughout the analysis. 

Figure 2: Ground Samples Used in Experiment

Results

The measurement results in Table 1 demonstrate precise results as evidenced by a low relative standard deviation (%RSD) of <10% for each of the measured elements. The repeatability of Petra MAX can be compared with ASTM D6376 repeatability. Although Petra MAX does not comply with this method, as D6376 is a wavelength dispersive XRF method (WDXRF), Petra MAX does demonstrate repeatability that is within the method precision. This can be observed by comparing the ractual which is the difference between the maximum and minimum measurement values for each element, with the calculated D6376 precision (rD6376), both of which are listed in Table 1. The data in Table 2 shows a very similar high level for precision of sulfur in coal with an RSD of less than 1%. 

Petra MAX was calibrated with a combination of
LECO and NIST sulfur and coal standards prior to analysis. 

Conclusion

The results from both studies demonstrate that Petra MAX delivers precise results for sulfur in coal, as well as sulfur and metals in petcoke. This solution does not require the complex sample preparation, specialized skills, and additional chemicals and reagents as other methods and techniques. In addition, you can achieve results in 5 minutes or less—an ideal screening solution for these applications.

Sample Preparation

For similar powder applications, we typically recommend three repeat determinations at 100s, and then each be averaged and reported as a single result. While this is true when analyzing samples with percent level components, it is less applicable when the desired measurement components are present at ppm level concentrations. This is because a 300s measurement time is recommended for lower concentrations to achieve accurate measurements. If measurement time is not of concern, the average of three 300s determinations can be used to obtain a more precise and accurate measurement. If the sample is only being measured for percent level sulfur, a 100s measurement time is sufficient. In this case, use the average of three 100s determinations as a single test result. See below for procedural details.

XRF analyzers function best when analyzing homogenous samples, as the measurement results come from a relatively small focal point on the sample. For finished liquid hydrocarbon fuels, such as diesel or gasoline, this is generally not an issue as they tend to be homogeneous. Unfortunately, petcoke and coal tend to be inherently non-homogenous, which is why it is necessary to grind the sample to a fine powder and mix as well as possible before analysis. However, even grinding the powder to a very fine mesh size and mixing thoroughly may not be enough to ensure consistent results, especially if the sulfur or metals are still not evenly distributed in the sample. For this reason, in addition to grinding, mixing and tapping to remove air, the following procedure can be used to improve overall data quality for powder matrices:

• Prepare the sample using the recommended practices described above. Tap the sample on its side to compress the powder and eliminate air gaps, then introduce into Petra MAX in the correct orientation (using a vent clip for autosampler analysis). Measure for 300s for both trace metals and sulfur, or 100s if only measuring for sulfur.

• Prepare a second sample following the same process as above, or, using the first sample, shake to mix the powder, then re-tap the sample as before. Insert into Petra MAX and measure for 300s or 100s.

• Prepare a third sample or reanalyze the first sample again using the procedure above.

• Report the average of the three determinations as the measurement result. This will ensure that the user gets a more accurate value, that is, a value that is more consistent with the true value of the entire sample.