Moisture, Volatiles and Ash Determination in Coal & Coke

Coal remains one of the most significant fossil fuels in the global energy and industrial sectors. For instance, in 2018, worldwide production of hard coal reached approximately 7 billion metric tons.¹ A substantial portion of this output comes from major producers such as China, the United States, Russia, and India.

Accurate characterization of coal is essential for evaluating its quality and determining its suitability for various applications. Depending on its composition, coal can be used for coking, steel manufacturing, electricity generation, and more.

This article focuses on the "proximate" analysis of coal, as outlined in ASTM D3172 (2021). This type of analysis involves determining three key parameters: moisture content, ash content, and volatile matter. These indicators play a critical role in assessing coal’s value and performance for its intended use.

“Automated” Proximate Coal Analysis with TGA Thermostep

The key analytical procedures for coal and coke characterization are governed by international standards issued by organizations such as ASTM, DIN, and ISO. This is also true for proximate analysis, which assesses coal and coke based on moisture, ash, and volatile matter content.

Standards like ASTM D3172 and ISO 3172 provide the general framework for proximate analysis, but they reference more specific standards, such as ISO 562, ASTM D3175, and ASTM D7582, that detail the procedures for determining individual parameters or for conducting automated analyses.

Proximate analysis can be carried out in two main ways. One approach follows ISO 562, using conventional furnaces and balances for manual determination. The other, more streamlined method—outlined in ASTM D7582—employs automated thermogravimetric analyzers like ELTRA’s TGA Thermostep (see Fig. 1).

This system offers significant advantages in efficiency and ease of use, allowing simultaneous measurement of up to 19 samples and automated determination of all three key parameters: moisture, volatile matter, and ash content. Compared to manual methods, the TGA Thermostep provides a much more convenient and time-saving solution.

Accurate, standard-compliant determination of moisture, ash, and volatile content in coal and coke requires careful control of various parameters, such as temperature, atmosphere, measurement duration, and weight stability.

When using individual furnaces and balances, applying these different settings can be quite time-consuming. After each step, samples need to cool down in a desiccator before a final weight can be recorded, adding significant delays to the overall analysis process.

Fig 1. TGA Thermostep. Image Credit: ELTRA GmbH

Automated thermogravimetric analyzers like the ELTRA TGA Thermostep series consist of a programmable furnace with integrated balance, a carousel with 19 sample positions and can measure all requested proximate coal parameters in a single run.

To begin the analysis, users simply select the appropriate application in the software, add approximately one gram of sample into the ceramic crucibles, and start the measurement.

During the process, the system’s rotating carousel automatically positions each crucible onto a weighing pedestal, where the current weight is recorded at each analysis stage.

Fig 2. Filling a sample into a ceramic TGA crucible: Automated measurement of up to 19 samples. Image Credit: ELTRA GmbH

During the whole measuring process, different temperatures and atmospheres are automatically applied in the TGA Thermostep depending on the analysis step. To compensate for the thermal buoyancy, an additional empty crucible at position 20 of the carousel is measured to reliably determine the thermogravimetric parameters.

The utilization of the TGA Thermostep significantly reduces analysis time for proximate coal analysis, as samples no longer need to cool down in a desiccator to ensure accurate measurement. The settings for automated determination of moisture, volatiles, and ash are defined in the ASTM D7582 standard (Table 1).

Table 1. Proximate coal analysis parameter according to ASTM D 7582. Source: ELTRA GmbH

One specific feature of a thermogravimetric analyzer is the automated placement of lids on crucibles to ensure accurate measurement of volatile content. The ELTRA TGA Thermostep is equipped with an additional lid carousel that automatically places the lids during the analysis process.

A full proximate coal analysis cycle using the ELTRA TGA Thermostep takes approximately four hours. Typical results from a simultaneous measurement of 19 samples are presented in Table 2 and Figure 5.

Fig 3/4. TGA Thermostep with additional lid carousel in open and closed positions. Image Credit: ELTRA GmbH

Table 2. Typical results of the TGA Thermostep series analyzer: Dry base means that the given value takes the released amount of water into account. In comparison to a sample which is measured as received, the measured ash and volatile content are respectively higher (ISO 17247). Source: ELTRA GmbH

Figure 5. Typical measuring graph of ELTRA TGA Thermostep series analyzer. Image Credit: ELTRA GmbH

• Y-axis (left): Relative weight of the sample
• Y-axis (right): Temperature
• Red curve: Temperature
• Graph 1-19: Weight loss of applied coal samples

The first three samples listed in Table 2 were analyzed using a cool-down temperature of 750 °C. This setting allows the TGA Thermostep to complete the proximate coal analysis in a relatively short time of around four hours. However, for full compliance with standard requirements, the software also offers the option to apply a lower cool-down temperature of 600 °C.

While this setting slightly improves the repeatability of the ash measurement, it extends the overall measurement time by approximately one hour. Moisture and volatile matter measurements remain unaffected by this change.

In proximate analysis, the fixed carbon content is a calculated value that provides an estimate of the amount of coke that can be produced from the coal sample. It is determined by subtracting the measured volatile content from the initial sample mass introduced into the ceramic crucible. Because volatile hydrocarbons are released during the analysis, the calculated fixed carbon value is typically lower than the sample’s total carbon content.

Summary

Proximate analysis of coal and coke is a globally important application with significant economic relevance. While it can be performed manually using separate furnaces and balances, the process is far more efficient, user-friendly, and faster when carried out with automated TGA analyzers such as the TGA Thermostep.

Reference

1. Home - STATISTIK DER KOHLENWIRTSCHAFT e. V. (2025). https://www.kohlenstatistik.de/.

This information has been sourced, reviewed and adapted from materials provided by ELTRA GmbH.

For more information on this source, please visit ELTRA GmbH.