Whichever sampling method is used, the sampling equipment used should meet the design criteria set by the relevant Standards and correct procedures must be followed in order for representative and correct sampling.
Advances in technologies have led to automated mechanical sampling systems ranging from single stage to multistage sampling systems incorporating crushing, discrete sub-‐sampling and reject handling.
Modern mechanical sampling systems are more accurate and precise, and are designed to accommodate the installation of an online coal analyser within its subsystem..
Sample preparation and bias testing
The sample preparation process may involve constitution of samples, reduction (crushing), division, mixing and drying, or all or a combination of these. Issues such as loss or gain of moisture, improper mixing of constituents, improper crushing and grinding, and oxidation of coal may arise during the sampling and sample preparation processes. To minimise moisture contamination, all standard methods include an air-‐drying stage in the preparation of the analysis sample to provide a stabilised sample that is not subject to further moisture change. The sample is then reduced and divided to provide an analysis sample. Sample division can be carried out mechanically or manually. Various types of mechanical divider are commercially available which meet the design criteria specified by relevant standards. The minimum mass of divided samples depends on top size and is specified in the relevant standards. In general, the coal preparation steps should be done rapidly, and in as few steps as possible. The sample should be protected from moisture change, oxidation and contamination.
Testing for bias in a coal sampling system is an essential part of coal analysis and is of significant importance. Bias testing on sampling systems should be carried out periodically. All bias testing is based on a matched pairs experimental design. A minimum number of 20 pairs of observations is recommended by international standards. Tests for bias can be carried out for ash, moisture or any other variable required but two parameters, moisture and ash, are thought to be sufficient for a bias test.
Standard laboratory analysis of coal
Routine coal analysis and testing generally include proximate analysis, ultimate analysis, ash analysis and heating value. In addition, special coal analyses such as determination of trace elements, coal hardness, ash fusion temperature may also be executed. The proximate analysis of coal is an assay of the moisture, ash, volatile matter, and fixed carbon as determined by series of prescribed or standard test methods.
Moisture, volatile matter, and ash are all determined by subjecting the coal to prescribed temperature levels for prescribed time intervals. The losses in weight are, by stipulation, due to loss of moisture and, at the higher temperature, loss of volatile matter. The residue remaining after ignition at the final temperature is called ash. Fixed carbon is the difference of these three values summed and subtracted from 100.
Ultimate analysis determines the percentage mass fraction of the major constituents of coal like carbon, hydrogen, sulphur, nitrogen, and, usually by difference, oxygen. Carbon and hydrogen are determined by
combustion of weighed sample of coal in dry oxygen in a closed system at specified temperatures to convert all the hydrogen to H2O and all the carbon to CO2. These products are absorbed by suitable reagents and determined gravimetrically.
The Kjeldahl method for determining nitrogen in coal has been used for many years. However, the international standards that describe the procedures of the Kjeldahl method have recently been withdrawn but some national standard organisations still recognise this method as a standard test method for nitrogen. International and various national standards recommend that the total carbon, hydrogen and nitrogen contents be determined by instrumental methods.
The three most widely used test methods for sulphur determination are the Eschka method, the bomb washing method, and the high-‐temperature combustion method, all based on combustion of the sulphur-‐
containing coal to produce sulphate, which can be measured either gravimetrically or volumetrically.
Total sulphur values alone are not adequate in assessing a cleaning process for reducing the sulphur content of coal. Methods for determination of the different forms of sulphur in coal are specified by various standards.
The calorific value is determined in a bomb calorimeter either by a static (isothermal) method or by an adiabatic method, with a correction made if net calorific value is of interest.
The ash is prepared by heating the coal under different prescribed conditions depending on the standard test methods selected for determination of major and minor and/or trace elements in coal ash. The elemental composition of coal can be determined by instrumental analysis methods such as AAS/AES, ICP-‐AES, ICP-‐MS, AFS, XRF or a combination of these techniques.
The analytical techniques used to measure mineral matter in coal are virtually the same as those used to measure trace elements in coal and coal ash.
Tests for determination of chlorine are performed by burning a coal in oxygen, and the chlorine formed is collected and analysed. There are two standard methods of determining chlorine in coal, oxygen bomb combustion/ion selective electrode method and combustion methods with or without Eschka mixture.
Several techniques can be used for the determination of Hg in coal and combustion residue. Various methods are selected by different standards organisations as standard methods.
The ash fusibility determination is an empirical test designed to simulate as closely as possible the behaviour of coal ash when it is heated in contact with either a reducing or an oxidising atmosphere. The free-‐swelling index is a measure of the volume increase of a coal when heated under specific conditions and is reported in numbers from 0 to 9, with higher values considered superior from a coking standpoint.
The swelling property of hard coal/bituminous coal can be measured using a dilatometer. The grindability of coal is determined using a Hardgrove machine.
Due to the heterogeneous nature of coal and the fact that many of the test methods applied to coal analysis are empirical in nature, strict adherence to the standard procedures is necessary to obtain repeatable and reproducible results.
Instrumental analytical techniques
A large number of instrumental analytical techniques have been shown to have wide applicability to coal analysis and are now widely applied for analysis of coal and coal products. These techniques are based on a wide range of technologies including x-‐ray spectroscopy, electron microscopy, atomic spectroscopy, mass spectrometry, neutron activation analysis, Fourier transform infrared spectroscopy, and laser-‐
induced breakdown spectroscopy. Many of these techniques are fast, sensitive, simple to operate, have low detection limits, and if properly calibrated, accurate. In particular, many of the instruments are capable of determining multi-‐elements simultaneously. Additional information such as chemical forms of the elements may be obtained. Some instruments require minimal sample preparation and can be used for in-‐situ measurements. Many of such instrumental methods have been well developed and adopted by national and international standard organisations as standard test methods for coal analysis. Other instrumental test methods have been applied to coal analysis by companies and in laboratories in may parts of the world although they have not obtained Standard status.
Online analysers provide an automatic, fast, relatively accurate, and instantaneous method of coal analysis for pricing, quality or process control, and SO2 emissions control. There are three types of online coal analysers: moisture meters, ash gauges and elemental analysers. Various technologies can be applied.
Most moisture meters employ microwave technology but magnetic resonance is also used for moisture determination. Microwave moisture meters are low cost, simple to use and reliable. MR measures all (both free and bound) moisture but it is large, sophisticated and expensive. Two common techniques employed by ash gauges are dual-‐energy γ-‐ray transmission (also called dual-‐gamma or low-‐energy transmission LED), and natural gamma detection. DUET is the most widely used due to its ability to measure the ash content of coal directly on a conveyor belt independently of the mass of coal on the belt and of the belt speed. However, it is incapable of detecting changes of iron oxide (Fe2O3) in ash and its results are affected by changes in the percentages of Fe or Ca in the ash. Natural gamma detection is ideal for coals with highly variable compositions of Fe and Ca and is cheaper. However, it is less accurate than some other technologies that requires significant shielding.
Technologies used in online coal elemental analyse include PFTNA, XRF and PGNAA and more recently LIBS. PGNAA is capable of measuring most of the major elements (except for O, and Na if its percentage in coal is low) in coal. PGNAA can be sensitive and accurate depending on the element analysed. Its calibration is independent of the coal rank. It is the most widely used technology for full elemental coal analysis. The biggest drawback is the requirement of maintaining a nuclear isotope source to provide the neutrons. PFTNA is capable of measuring the major and minor elements contained in coal. The main advantages of the PFTNA include self calibration independent of the coal rank, and better accuracy in the determination of elements such as C, O, and Na, which PGNAA cannot or has difficulty in detecting. XRF
cannot match the precision of PGNAA, and is unable to measure C, H, N, and O in coal. However, it has low costs so it can be economically viable for selected applications. LIBS is emerging as a potential competitive technology for online coal elemental analysers. LIBS can detect the major ash constituents in coal such as Si, Al, Fe, Ca, K and Ti, as well as Na, Mg which PGNAA cannot in most coals. The main disadvantage is that a LIBS analyser, like XRF analysers, analyses surface chemistry of the coal particles and therefore requires finely ground homogeneous samples.
An online analyser can be installed on the coal conveyor belt (cross-‐belt online analyser) or on the sample stream of the online sampling system (sample-‐stream online analyser). The cross-‐belt analysers are less accurate but they have advantages such as little or no sampling requirement, lower cost and quicker installation. Also, the analyser can be located where it is needed, not where there is a defined place.
However, they also have some limitations like the difficulty in obtaining physical samples to calibrate and diminished accuracy due to changes in belt loading and cross-‐sectional profile. Careful consideration of size and access, and application requirements is required when determining the optimum location for placing an analyser.
The choice of an analyser is mainly between an ash gauge and an elemental analyser, whether or not coupled with a moisture meter. Criteria of greatest importance in choosing a suitable online analyser include the purpose of the installation, coal complexity and coal quality variability, accuracy requirement and costs.
A disadvantage of most of the instrumental methods is the small sample size used which may not be representative of the quantity of coal being analysed. In addition, most instruments need careful calibration in order to perform precise, accurate, and bias free coal analysis. Most often, the accuracy of the results is highly dependent on the quality and suitability of the standard materials used to standardise the instruments.