5 Instrumental analytical techniques
5.7 Online analysis systems
5.7.3 Selecting an online analyser
The choice of the type 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 between ash gauges and elemental coal analysers include parameters of interest, coal complexity and coal quality variability, and accuracy requirement. There is a large gap in price and performance between elemental analysers and ash gauges. The economics and choice of technology will depend on why the system is being installed.
If the coal producer or utility is interested primarily in ash, moisture, and calorific value, and there is no need to measure sulphur online, the simple ash gauges coupled with moisture analysers, both mounted over the existing conveyor, may be adequate for the task and cost only around one third that of elemental analysers. In multiple seam applications and in cases where the iron fraction in the ash varies
significantly, a dual gamma ash gauge is unlikely to perform acceptably. As the accuracy requirements become more stringent or if the determination of sulphur is required, the choice may be a cross-‐belt elemental analyser or, for the best accuracy possible, a sample-‐stream elemental analyser (Moodley and Minnitt, 2009; Woodward and others, 2003).
When choosing between the sample-‐stream analyser and the cross-‐belt analyser, although it is related to the quality requirement, sample-‐stream analysers are more appropriate for load out situations where quality is paramount. Further upstream, where control decisions can be less exact, a cross belt analyser is often more appropriate. Proximity to, or existence of, a sampling system will also determine the appropriateness of the technology. The sample-‐stream analyser is an appropriate technology if a complementary sampling system is already available; where no proximal sample position is available, a cross-‐belt analyser becomes relatively more attractive.
In selecting appropriate technology, the main decision criteria usually include the objective of the installation, system performance and pricing. In choosing between the PGNAA and PFTNA technologies, if the major ash constituents Si, Al, Fe, Ca, Ti, K, and S as well as ash, volatile matter and CV are the main interests then a PGNAA instrument may be sufficient. If measurement of the elements Na, Cl, C, O are also required, then PFTNA is the more suitable option. Na, Cl and C can be measured by PGNAA only if their concentration in coal is sufficiently high.
In choosing between XRF and PGNAA, the advantages of XRF over a PGNAA system are: (1) the cost is lower, (2) the technique does not use neutron sources. The energy levels of the x-‐rays used are low, and they can be easily stopped by the steel enclosure; (3) easier to calibrate and to maintain. The main disadvantages of the technology are: (1) it is less precise than PGNAA; (2) it does not provide full elemental analysis; (3) it measures surface composition only. Table 15 and Table 16 compare the XRF and PGNAA instruments, and their capabilities, respectively.
Table 15 – Comparison of PGNAA and XRF (Hallee, 2010)
PGNAA XRF
Californium source:
• average 2 year lifetime x-‐ray tube:
• more than 6 year lifetime
High maintenance cost Low maintenance cost
Difficult licensing Easy licensing
Measures nearly all elements (no Oxygen) Measures elements with atomic number >10
Better precision Less accurate than PGNAA
Non-‐integral volume measurement Surface measurement
Dependent on the load below the saturation volume Independent of the load
Difficult to calibrate, large samples required Easy to calibrate, small samples required
Table 16 – Elemental measurement capabilities of PGNAA and XRF (Hallee, 2010)
Measurements PGNAA XRF
Proximate analysis
Moisture ✓ ✓
Ash ✓ ✓
Sulphur ✓ ✓
CV ✓ ✓
SO2 ✓ ✓
Elemental analysis
C ✓ -‐
H ✓ -‐
N ✓ -‐
S ✓ ✓
Cl ✓ ✓
Si ✓ ✓
Al ✓ ✓
Fe ✓ ✓
Ti ✓ ✓
Ca ✓ ✓
K ✓ ✓
When compared with PGNAA, LIBS has advantages which include: (1) it can measure elements such as O, Na, Mg which PGNAA cannot or has difficulty to measure; (2) it is fast; (3) has lower costs; and (4) it does not use a radioactive source. No heavy shielding is required so it is light, has a small footprint and is easy to install. The major disadvantage of LIBS is that it measures only the surface of the coal particles.
Because XRF and LIBS only analyse the surface of coal particles, for best accuracy, the preferred location for XRF and LIBS online elemental analysers is on the sample stream conveyor where the coal has been ground to a fine particle size and homogenised.
Comments
Certain coal properties (for instance, the elemental composition of coal) can be determined by several instrumental methods. Each method has its advantages and limitations. In general, no single method yields a complete analysis of coal and it is often necessary to employ a combination of methods. A disadvantage of some 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 and 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.