Elemental Analyzers for Quality Control of Coal

Coal is one of the most essential fossil fuels. In 2012, the worldwide stone coal output was about 6.6 billion metric tons [1]. A large amount of the global traded stone coal is mined in China, Russia, USA, and India. Compared to the huge amount of mined coal, the required sample volume for the characterization of coal, varying from a few mg up to 1 g, seems incredibly small. The characterization of coal is crucial for its quality assessment and additional use. Based on the product quality, coal is appropriate for steel production, coking, or electrical power generation. The following article analyzes the chemical background of proximate and ultimate coal analysis and how these parameters are measured with ELTRA’s combustion and thermogravimetric analyzers.

The most common types of coal (bituminous, lignite, and anthracite) can be differentiated by their diverse physical and chemical properties. The elements carbon (C), nitrogen (N), sulfur (S), hydrogen (H), and oxygen (O) are the most regularly measured elements. Also, the mineral content of the ash (especially silica, ferric oxide, alumina, etc.) is determined. Coal is characterized by more or less physically established parameters such as volatiles, moisture, and ash content, and also calorific value and ash fusion temperature.

Proximate Coal Analysis (Physical Testing)

Given the diversity of parameters which impact the quality of coal, it appears rather ambitious to name one parameter which ideally describes the coal quality. Due to the fact that coal is typically used as fuel the calorific value is ideal to give a first impression of the product quality. For a first (“proximate”) analysis of coal, the calorific value, ash, moisture, and volatile content are measured. Based on these data the so-called fixed carbon content is calculated. Table 1 illustrates the key types of coal with their calorific value and the content of volatiles.

Table 1. Main types of coal [1]

Type of coal Calorific value Kj/Kg Volatile Content
Lignite 6,700- 25,000 45 – 60%
Bituminous 25,000 – 35,000 14- 45
Anthracite 35,000 and more < 14

The calorific value can be established with standard bomb calorimeters available in the market.

Calorimeters for coal analysis can be split into adiabatic and isoperibolic. In both types of calorimeter, a formerly dried coal sample is added in a calorimetric bomb, oxygen is introduced and the coal is combusted. The combustion heat is measured and provides the gross calorific value. Very crucial for documentation is the “base” on which all further values are measured. The ISO Standard 17247:2013 describes the listed reporting basis (refer Table 2).

Table 2. Reporting units [2]

Base Acronym Description
As received ar, m Including hydrogen and oxygen from moisture
As received ar Excluding hydrogen and oxygen from moisture
Air dried ad Dried at air; and excluding hydrogen and oxygen from moisture
Dry d Dried until steady mass and analyzed
As Analyzed -- Including hydrogen and oxygen from moisture

AR (as received) is the most extensively used basis for coal analysis. For a proper proximate analysis, it is vital to know the moisture content of the coal as this value impacts all other parameters. Moisture can be divided into surface, decomposition, hydroscopic, and mineral moisture and can be determined with a thermogravimetric analyzer (TGA) or by the use of a furnace and an external balance. The standard ASTM D7582-10 defines the proximate coal analysis by using macro thermogravimetric analyzers. These analyzers, such as ELTRA’s TGA Thermostep (Figure 1), integrate the heating and the weighing process for a convenient analysis of volatiles, moisture, and ash content in one analysis cycle. Micro thermogravimetric analyzers are not ideal for coal analysis because of the limited sample weight of less than 10 mg. Macro TGA analyzers accept a sample weight of up to 1 g which is weighed into ceramic crucibles. These crucibles are positioned on a carousel offering 19 positions for different samples. The carousel is situated in a heating chamber which can be purged with an inert gas (nitrogen) or oxidizing atmosphere (oxygen or air). This chamber can be heated from room temperature to 1,000 °C and is connected to an in-built weighing cell by a ceramic pedestal.

ELTRA’s Thermostep

Figure 1. ELTRA’s Thermostep

The moisture content of the coal is nowadays established by filling 1 g sample into the ceramic crucible and heating the TGA analyzer up to 107 °C. The temperature must be held until a constant mass is detected. In contrast to the moisture content of coal, the volatile content does not consist of one species (water) but of a blend of aromatic and aliphatic hydrocarbons. For a precise determination of the volatiles, a TGA analyzer has to raise the temperature within 30 minutes to 950 °C, holding this temperature for seven minutes. During this heating process, the crucibles are covered with lids and the heating chamber is purged with nitrogen. For the ensuing determination of the ash content the TGA analyzer cools down from 900 °C to 600 °C, changes to an oxygen atmosphere, and heats up to 750 °C, holding this temperature for one hour.

The entire analysis cycle is performed automatically. Table 3 illustrates typical results of a stone coal sample tested with ELTRA’s Thermostep.

Table 3. Analysis of the coal standard AR-1721

Value Content in (%)
Moisture 1.5 +- 0.1
Volatiles 42. 96 +-0.3 (dry base)
Ash 7.33 +- 0.02 (dry base)

The fixed carbon content is used as an estimate of the quantity of coke that will be produced from a coal sample. It can be calculated by subtracting the measured quantity of the volatile content from the sample mass which was added into the ceramic crucible. The calculated fixed carbon is lower than the total carbon content as certain volatile hydrocarbons are eliminated during the analysis process.

Ultimate Coal Analysis (Chemical Analysis)

Besides proximate coal analysis, ultimate coal analysis also requires the determination of the carbon (C), nitrogen (N), sulfur (S), hydrogen (H), ash and oxygen (O) content by difference. According to the ISO standard 17247, a coal sample’s oxygen content is not measured directly. This content is calculated by incorporating all other values. The remaining difference to 100% is defined to be the quantity of oxygen.

For determination of the elements C, N, H, and S, elemental analyzers are available. A common elemental analyzer combusts the coal sample and measures the combustion gas with a thermal conductivity cell, infrared cells, or a combination of both. Available analyzers vary with regards to combustion temperature, required sample weight, and measured elements.

Micro elemental analyzers offer the chance to determine C, N, H, and S in a single analysis cycle, but only accept a very small sample weight of maximum 10 mg. This makes the sample preparation for these analyzers susceptible to error. The requirements for the determination of the elements C, H, and N are stipulated in the standard ISO 29541. Typical macro elemental analyzers, which fulfill these requirements, use a steel or quartz combustion tube and examine sample weights of normally 60 - 80 mg. When using these types of combustion tubes, the applied temperature is restricted to approximately 1,000 °C.

The precise determination of sulfur requires higher temperatures, as the sulfur, which is bonded as sulfate, will not be discharged in the gas phase at lower temperatures. As a consequence, common sulfur analyzers (which are defined in the standard ISO 19579) use ceramic combustion tubes which can withstand higher temperatures. Alternatively, the applied temperature can be raised by adding tin to the coal sample. Owing to the extra combustion energy of the tin, the local temperature of the sample is higher than the temperature in the furnace. Tin is not required when using ELTRA’s CHS 580 analyzer (Figure 2). This analyzer also defines the carbon and hydrogen content and uses a furnace with ceramic combustion tube which can apply temperatures up to 1500 °C.

To determine the sulfur content approximately 150 mg of sample is weighed into a sample carrier (for example - a ceramic boat). The sample carrier is introduced into the hot furnace and the released combustion gases (CO2, H2O, SO2) are measured with infrared cells.

In the CHS-580 analyzer, the sample is weighed into ceramic boats for subsequent combustion

Figure 2. In the CHS-580 analyzer, the sample is weighed into ceramic boats for subsequent combustion

Characterization of Ash

The combustion residue of the coal (ash) can also be of interest. Like for coal, the ash parameters can be divided into chemical or physical properties.

For steam power generation, for instance, the physical ash behavior at various temperatures (ash fusion test) is very crucial. When coal is combusted in a furnace of an electrical power plant, a glassy slag (clinker) or a powder-shaped residue, which has to be removed as molten liquid, are unwelcome by-products. Not every electrical power plant is able to manage clinker-forming coal because of the cost-intensive furnace cleaning that is necessary.

When using an ash fusion analyzer (such as CAF Digital from Carbolite Gero) the ash forms in the shape of a pyramid, cone, or a cube and is added into a furnace with a special window. Through this window, a camera can view the sample’s behavior while heating. Normally, temperatures up to 1,600 °C are applied. During the heating process parameters such as softening, hemisphere, deformation, and flow temperature are recorded. The flow temperature is essential for deciding on the additional use of the coal, for instance in steam power generation.

A spectrometer can be used for chemical analysis of ash. The element concentration of sodium, aluminum, magnesium, etc. is important for the assessment of the environmental impact. A non-destructive analysis technique is X-ray fluorescence. Dissolution of the ash is required when other methods like ICP-OES or AAS are selected.

Conclusion

Coal and coal ash analysis requires a range of different instrumentation. Elemental and thermogravimetric analyzers are vital tools for quality control which offer quick and precise results and are easy to run. ELTRA offers a wide range of combustion analyzers for the determination of C, N, O, H, and S in solids as well as thermogravimetric analyzers.

References and Further Reading

  1. www.kohlenstatistik.de
  2. ISO 17247: 2013

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

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

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