Accurate Instrumental Determination of Carbon in Coal

Traditionally, thermal coal has been traded with its value being primarily determined by calorific value as well as properties relevant to particular customers (such as grindability and slagging/fouling behaviour). With the need to estimate greenhouse gas emissions and the possible introduction of carbon taxes, it is likely that carbon content may also be an important parameter. Thus accuracy and precision in its determination are required.

The objective of this research is to provide data to support the adoption of rigorous calibration procedures to be used with modern instrumental analysers now almost exclusively used to determined carbon (as well as hydrogen and nitrogen) in coal. The ultimate aim is to ensure the accuracy in the measurement of carbon. These analysers are comparators that require calibration with a standard reference material.

Part of the study included a survey of use of CHN analysers, worldwide. It is concluded that the diversity of calibrating standards and the manner in which they are used are of concern. Also of concern is the measurement or estimation of moisture in the calibration standards. Although the number of laboratories that responded was small, the answers to the questions certainly confirmed that there is a need to "standardise" the approach taken to calibrate instrumental analysers used to determine carbon (as well as hydrogen and nitrogen) in coals.

The data obtained in the second part of this study (interlaboratory tests) are limited to those supplied by the Australian laboratories. The results of the interlaboratory tests demonstrate that it is difficult to prepare coal reference materials with the required stability to exclude the effect of oxidation on the recommended value of carbon concentration. As a consequence, there is an increase in the uncertainty of the carbon concentration with the increase of storage time of the coal. It is also demonstrated that organic compounds of known stoichiometry can be (indeed should be) used to calibrate instrumental analysers used to determine carbon in coal. Consideration of the data obtained also leads to the conclusion that both national and international Standard Methods for the instrumental procedures for the determination of carbon in coal should not allow calibration procedures that require the use of standard reference coals, unless it can be demonstrated that a certified (i.e. accurate and stable) value can be specified for the concentration of carbon.

An e-newsletter has also been published for this project, highlighting its significance for the industry.