Rapid Thermogravimetric Analysis of Coal

Introduction 

The quantitative determination of moisture, volatile compounds, chemically bound carbon, and ash content has long been used to determine the quality and economic value of different types of coal. High ash content is undesirable for the operation of thermal power stations because inert material increases transport and waste disposal costs, and also means that the heat exchangers have to be cleaned more frequently. To make sure that assays can be properly compared, the analysis procedures have been standardized and described in many standard methods [1-10]. Quite early on, measurement routines were developed for thermogravimetric instruments that enabled faster and more automated analyses to be performed. These techniques have been compared with the standard manual methods [11-14].

TGA (thermogravimetric analysis) is, however, also very useful for coal research, e.g. to compare combustion profiles or to determine the nature of volatile components with TGA-MS. Even the lime deposits (fur) formed in hot water systems have been investigated with TGA [15].

The determination of moisture, volatile content, soot, ash or fillers is also required for other applications. Analogous to the analysis of coal, a standardized thermogravimetric procedure is nowadays used to determine the content of elastomers, thermoplastics and thermosets, as well as lubricants [16, 17]. Conversely, procedures developed for the determination of carbon black in rubber [17] are used for the analysis of brown coal, lignite and or other renewable fossil fuels [e.g. 18].

Speeding up the TGA Procedure

As already indicated, the standard methods for coal analysis are laborious and often time-consuming. To determine the moisture content exactly, the measurement is usually started at room temperature, or slightly above.

This leads to long cooling times before the next sample can be measured, which in turn limits the throughput of samples. The METTLER TOLEDO STARe system allows the analysis time to be reduced by half compared to the ASTM E1131 standard method. In this case, the measurement is started directly at 110 °C (or even higher), which means the otherwise long cooling time down to 30 °C is avoided. The moisture is, however, still accurately measured - the weight loss up to the start is automatically measured by determining the starting weight on reaching 110 °C. This is done by measuring the weight of the sample and the crucible during the sample preparation, which is in fact normal in the manual procedure [1 -10]. Buoyancy effects are compensated in the TGA measurement by subtraction of a blank curve. Besides this, oxygen is used instead of air, resulting in much shorter combustion times. One must of course make sure that coal particles are not blown out of the crucible by the rapid generation of gas. This can manifest itself for example in widely differing measurement values for the ash content. As with all analytical methods, it is essential that the coal samples are homogeneous and representative if one, for example, wants to characterize one hundred tons of coal. Vaporization, degassing and combustion proceed more rapidly when shallow, open crucibles are used and small sample weights are analyzed.

Experimental Details

A METTLER TOLEDO TGA/SDTA851e with the small furnace (up to 1100 °C) was used for the measurements. The system was automated with a TSO800RO sample robot and a TSO800GC1 gas controller for gas switching. The balance was purged with 40 ml/min of nitrogen as protective gas. During the measurement, first 80 ml/min of nitrogen, and afterward 80 ml/min of oxygen were used as reactive gas.

Conclusions 

The assay of moisture, volatile content, carbon and ash in coal and coke using TGA is a standardized routine method. The time required for the method can be shortened by up to 50% without affecting the accuracy by using the METTLER TOLEDO STARe system and adapting the temperature program. In automated operation, this allows at least 35 analyses to be performed per day. The TGA curves show whether degassing and combustion are complete. This is necessary in order to optimize the method and for the control of routine measurements. Thermogravimetry, in addition, allows drying and combustion behavior to be investigated and different coal types to be characterized. Inhomogeneous samples must be well ground to achieve a high degree of reproducibility. If necessary, larger amounts of sample can be measured thermogravimetrically in 900 µl crucibles.

_{Rapid Thermogravimetric Analysis of Coal | Thermal Analysis Application No. UC 147 | Application published in METTLER TOLEDO Thermal Analysis UserCom 14}