5 Instrumental analytical techniques
5.1 X-‐ray spectroscopy
5.1.2 X-‐ray fluorescence
X-‐ray fluorescence (XRF) is the emission of characteristic ‘secondary’ (or fluorescent) x-‐rays from a material that has been excited by bombarding with high-‐energy ‘primary’ x-‐rays or gamma rays. This phenomenon has been widely used for elemental analysis and chemical analysis. XRF is a mature technology and has the advantage of being nondestructive, rapid, simple, and cost-‐effective, which is
probably unsurpassed by any other method when used for multi-‐elements determinations in the same prepared coal sample. This technique has been adopted by both ASTM and ISO as a standard test method for determining major and minor elements in coal ash (see Section 4.4.2). XRF has also been used extensively for determining sulphur in coal.
XRF spectrometry can be undertaken by two distinct methods, energy dispersive XRF (ED-‐XRF) and wavelength dispersive XRF (WD-‐XRF). The ED–XRF is more cost effective compared to WD–XRF. It also allows for smaller units with fewer components resulting in a cheaper and more reliable instrument. Such instruments can be easily tailored to the needs of different customers, integrated with industrial installations, and also miniaturised for the purpose of in-‐situ analysis. Detection precision and accuracy of an XRF instrument are driven by several factors including x-‐ray excitation source and strength, type of detector used, time exposure, sample surface conditions, physical and chemical matrix effects, as well as primary elements of interest and inherent x-‐ray spectral line interference from element overlap. Today, bench-‐top or portable XRF elemental analyser or XRF sulphur analyser for coal/coal ash are widely available from a number of manufacturers. Figure 16 shows an example of a bench-‐top ED-‐XRF elemental analyser by Applied Rigaku Technologies, Inc (USA). In the ED–XRF method, the coal sample is air dried and ground to required particles size. Approximately 7 or 8 gram of homogeneous coal powder is pressed into even, compact sample holder or a pellet. The secondary x-‐rays emitted by the sample are directed into a solid-‐state detector. Incoming photons ionise the atoms within the detector, producing electrical pulses which are proportional to the levels of energy being detected. These pulses are amplified and interpreted using a computer that calculates the elemental composition of the sample. The resulting information is then enhanced by referencing an onboard database and/or user defined information that provides additional data about the sample. The spectrum of the sample are adjusted for a number of other variables that might distort the results including (Niton UK, 2013):
• geometric effects caused by the sample’s shape, surface texture, thickness and density;
• spectral interference such as a variety of scattering effects originated within the sample;
• sample matrix effects such as absorption of the characteristic x-‐rays of one element by other
elements in the sample, and secondary and tertiary x-‐ray excitation of one or more elements by other elements in the sample.
Figure 16 – A bench-‐top ED-‐XRF elemental analyser (Rigagu, 2013)
In an inter-‐laboratory study carried out by the US Electric Power Research Institute (EPRI), the analytical methods for measuring Hg and Cl in coal were evaluated and compared. The study found that the Cl values obtained using an XRF analyser were in excellent agreement with the consensus values. A lower quantitative limit for Cl could be achieved with XRF compared to standard test methods. However, the determination of Cl using XRF suffered interferences from sulphur in coal. This interference became significant for coals with sulphur content greater than 1% (EPRI, 2000). Wang and others (2005) evaluated the determination of iodine in coal using XRF and found that under optimum conditions, coal samples with iodine concentrations higher than 5 ppm can be determined using this ED-‐XRF method.
Song and others (2006) used XRF to simultaneously determine As, P, S, Cl in coal. They found that the measured values of As, P, S, Cl in the coal samples agreed well with the results obtained from standard test methods, and the test limit of the XRF were 1.2 μg/g for As, 22 μg/g for S, 2.1 μg/g for P and 2.0 μg/g for Cl. However, x-‐rays are unable to penetrate the coal particles beyond 3.175 mm and therefore this method requires finely ground homogeneous samples, limiting its applications in online analysis as only the material surface can be analysed, prohibiting a sound representation of the entire product (Willett and Corbin, 2011).
XRF is a rapid, simple and accurate method of determining the concentration of major and minor elements in coal ash. The coal ashing procedure removes most of the combustible and volatile components. XRF analysis of whole coal is more challenging. One problem is that calibration standards for XRF analysis of whole coal must themselves be whole coals. Only a few coal standards exist, and these are certified for only a few elements. Non-‐metals like S, P, B and C are difficult or impossible to determine using XRF. This technique is also greatly affected by matrix effects and numerous standards are required in order to match the sample matrix (Davidson and Clarke, 1996; Huggin, 2002). Recent advances in excitation and detection have made it possible to determine light elements and non-‐metals such as S. This has been done by the use of a Rh end window tube as a universal tube and light elements can be excited effectively by Rh L radiation (Khuder and others, 2007).
Instrumentation
model Rigaku NEX QC x–ray tube 4 W Ag-anode detector semiconductor sample type coal (powder)
film Mylar
analysis time 300 seconds 240 seconds for S 60 seconds for Ca, Ti, Fe environment air
options autosampler manual sample press
The development of polarised ED-‐XRF offers additional improvement in the technique, especially for extending the XRF technique to trace elements or for carrying out the analysis on whole coal samples. The use of polarised incident radiation reduces background fluorescence radiation thereby increasing the signal/noise ratio resulting in significantly lowered detection limits for determining trace elements in coal. Recently, Moriyama and others (2010) investigated the use of an ED–XRF spectrometer with polarised optics and new quantification software for trace elements determination. They claimed that accurate analysis down to ppm level could be achieved even in complex sample composition with the quantification software which estimates non-‐measuring sample matrices using scattering intensities and full profile fitting method combined with the Fundamental Parameter (FP) method. The scattering FP method corrects for non-‐measuring components in samples such as coal fly ash, soils and biological samples by using Compton and Thomson scattering intensities from a Mo secondary target. The measured concentrations of the trace hazardous elements (As, Cd, Cr, Hg, Pb, and Se) and major elements in a coal fly ash sample using the ED-‐XRF spectrometer with secondary targets, polarised optics, and high speed detector with pile-‐up rejection demonstrated good agreements with the certified values.
Figure 17 – The integrated XRF and XRD spectrometer (Bonvin and others, 1998)
Bonvin and others (1998) proposed an integrated XRD–XRF system for online process control applications. The combined XRD and XRF instrument, as illustrated in Figure 17, has separate proportional counters to detect diffracted beams and fluorescence radiation. A standard XRD platform is used with fixed geometry goniometer and an energy dispersive x-‐ray detector for the XRF analysis. This
electronic module
water cooling
exchangerheat generator
x–ray tube fixed channels vacuum tank
sample in analysis position
sample changer
molecular pump
vacuum pump
approach has been applied in industrial processes such as iron and steel, and cement making but is yet to be tested and validated for coal analysis.