Big Interest in Online Technologies at Coal Quality Conference
Focus on Productivity Drives Technology Push
A recurring theme expressed by metallurgists and coal processing specialists at this years Australian Coal Preparation Society Conference (ACPS) was the challenge to produce more with less whilst maintaining, if not improving, quality.
In this regard, allot of discussion focused on technology, particularly the costs and benefits of adoption under the current economic climate facing the Australian coal industry.
One topic discussed at length was using performance data to make better processing decisions. A typical coal plant is “awash” with data. However, often this is not used in a meaningful way.
A major limitation of data is that it is collected and analysed “after the event”. By then, hundreds of tonnes of coal has been processed, but less than optimal. On-line coal quality analysers, also referred to as cross belt analyzers, can overcome this limitation. Such instruments are fitted to a conveyor belt and continuously generate key quality data in real time.
The main cross-belt analysers used in the coal industry measure moisture, ash and elemental composition. Three technologies dominate. These are microwave to measure moisture, dual energy gamma ray transmission (DET) to measure ash, and prompt gamma neutron activation analysis (PGNAA) to measure elemental composition.
Moisture analysis using microwave transmission
When a microwave signal is passed through coal on a conveyor belt water in the coal will absorb more energy than the coal and mineral components of the material. The loss in energy of the transmitted microwaves can be quantified in terms of an attenuation of microwaves and a phase shift of the transmitted beam with respect to the incident beam.
Ash Analysis using dual energy gamma ray transmission
In this system, ash value is determined by combining measurements of the absorption intensities of narrow beams of low and high energy gamma rays, transmitted through the coal from two radioactive sources. The two sources most commonly used are Am-241 (for the low energy beam) and Cs-137 (for the high energy beam). The transmission of the high energy beam depends primarily on the mass per unit area of coal, while the low energy beam depends both on the mass per unit area of coal, and also the mineral content of the coal. The intensities of the two energy beams, after passing through the coal, are determined separately be scintillation detectors and then combined to give a measure of the ash value which is independent of both the thickness and the mass of the coal.
Elemental analysis using prompt gamma neutron activation analysis
PGNAA incorporates the bombardment of elemental nuclei present in coal with neutrons and observing the signature gamma-ray emissions that result. When coal is exposed to a neutron emitting radioactive isotope such as Cf-252, interactions of the neutrons with the nuclei present form excited nuclei. The resultant gamma ray emissions from the excited nuclei are classed as either prompt (occurring within 10-12 seconds) or delayed (arising from the decay of the induced radioactivity). The sensing of the characteristic prompt gamma ray emissions from the excited nuclei by a scintillation device forms the base for determination of the elemental composition of the coal, hence the term prompt gamma neutron activation analysis (PGNAA).
With the presence of elements constituting the ash components of coal determined (such as Si, Al, Fe, Ca, Na, K and Ti etc..) an overall ash value of the coal can be inferred and sulphur can be measured. In addition important trace elements may also be measured.
Cost-benefit of online analysis
Utilization of online analysis technology can significantly improve coal quality and saleable yield. To maximise the benefit of such technologies due attention needs to be given to assessing the application to determine what type of analyser is most suitable. RTII specialize in online instrumentation and the commissioning of instrumentation to deliver the best results. For further information on our product range please visit our website.