In supplying modern power plants it is no longer enough for producers to know merely the ash and moisture content of their coals. As power plants have become more technologically sophisticated so have the tolerance limits for varying ash content become more confined.
Online ash analysis became an important radiometric measuring tool which depended on nuclear methods such as dual energy gamma ray transmission techniques. Moisture content was determined using microwave techniques. But these methods have limitations, specifically related to high iron content in certain coals.
Major German mining manufacturer DBT, best known in Australia as a supplier of longwall systems, has developed in co-operation with another German company an “iron-compensation” technology as an extension to the dual-energy gamma ray transmission technique.
According to Thomas Sauer, measurement technology product and sales manager, DBT Automation, the x-ray technology is different from conventional radiometric online analysers in important ways. While higher atomic number elements such as iron influences the accuracy of the classic online radiometric systems, with the additional x-ray instrument these effects are reduced by "the energy-dependent absorption of the x-rays".
Standardised test procedures to assess coal quality were typically expensive and time-consuming, the company said. Data was often only available after several hours delay, which means intervention in the production and blending parameters was delayed. The x-ray instrument can test coal as an on-stream instrument at a conveyor belt or can be integrated into a laboratory for testing bulk samples. Sauer said this application had increased measurement accuracy by more than 30% while the standard deviation was 1%.
During the last two years an online working x-ray fluorescence coal analyser XRA 75 was developed. Capable of simultaneously determining individual elements in the coal, the technology was recently introduced into the Australian market.
The x-ray fluorescence technique incorporates sensor technology developed for NASA's Mars Pathfinder Mission. With special data acquisition, fitting and evaluation algorithms developed in the US by Process Control the contents of the main ash bearing elements are detectable especially the sulphur content.
This has become an important issue with the greater use of coals of different origins, which have increased the variation in the ultimate composition of the coals. This in turn has impacted negatively on the burning behaviour in modern boilers.
Technological developments in power plant technology have lead to the definition of new limit values for the sulphur, iron, calcium, potassium and sodium content of coals.
"The power industry has a great interest in coal constituents and coal composition," Sauer said. "The iron and sulphur content are some of main properties they want to know because of the impact on the boiling process. Increasingly coal producers have to meet these specifications because of international market pressure to meet limits and restrictions, particularly with sulphur dioxide emissions."
In conjunction with an online moisture meter, calorific value can also be determined. A technical limitation exists only for elements with ordinary number less than 12.
An important advantage the new developed x-ray fluorescence analyser XRA 100 has over other XRF-instruments for online analysers is that expensive liquid nitrogen cooling systems are no longer necessary because the detector is thermoelectrically cooled.
Already successfully introduced into Germany, South Africa and America, Sauer said Australia was the next logical entry point for the technology where DBT had a significant presence as a longwall equipment supplier.
The most likely applications for the technology would be at central coal loading terminals or at the preparation or blending facilities of power plants. According to Sauer the technology also has applicability in minerals processing including iron, manganese, chromium, nickel, copper and titanium.
