Technical Market Support » Thermal Coal
The major objective of the work is the establishment of an effective testing facility to measure the combustion reactivity of pulverised coals. The facility would use pyrometry techniques with measurements on individual particles being obtained relating to the following coal reactivity characteristics; Coal volatiles and ignition, Coal char and its reactivity (or burning rate).
Main findings and conclusions
Pyrometry techniques for determining of coal reactivity
Pyrometry techniques can be used for the determination of the selective amount of volatile matter released by pulverised coals, and the distribution of the release during heating. A single wavelength pyrometer can be used for such estimations.
The char reactivity can be determined using a dual wavelength pyrometer. Such values determined in this project agree with literature values reported for the same coals apart from those for a low reactivity coal for which the low burning temperatures give high noise to signal levels in the pyrometer output. The technique also provides estimates of the distribution of reactivity between particles, a factor which is not provided by measurements relating the average effect of multiple particles.
Coal devolatilisation measurements
The results of the experiments suggest that the time and the extent of devolatilisation during pf combustion depends on the existence of oxygen and the resultant flame sheet of burning volatiles which surround the particles. Devolatilisation measurements must therefore be undertaken with oxygen in the gas rather than in an inert gas as had previously been done. The time taken for volatile evolution and combustion was short in relation to char burning times and was not affected significantly by the proximate volatile content of the coal. Particle size was the most important controlling factor for the time required for volatile evolution and combustion.
Particle ignition characteristics
A parameter which is related to flame ignition can be estimated from the procedure. This is the ignition delay time, the time required for volatiles evolution and ignition to occur. This work showed the ignition delay time to increase with decreasing gas temperature and increasing particle size with little effect of changes in oxygen concentration in the range studied (15%v/v). In accordance with established theory, the ignition temperature was shown to decrease with increasing particle size for the coals studied. A general trend of increasing ignition temperature with increasing coal rank was also observed.
Char reactivity measurements
Char burnout rates were determined as a function of reaction time from measurement of burning temperatures of individual char particles. Combustion rates determined using the two colour pyrometry techniques developed in this project are found to be in general agreement with data obtained previously for similar coals using conventional combustion rate measurement techniques. The advantage of the new procedure is the relatively low cost associated with the equipment and the prospect of automation of the system to enable high throughput.
Work program description
3rd quarter, 1993: The first phase involved the rebuilding of the two colour pyrometer which was commenced in this quarter at CSIRO. At Newcastle University, work was initiated on coal particle combustion. The feasibility of relating the amount of volatile matter evolved to the intensity of the `volatile peak' from single colour pyrometry measurements on single particles was investigated. Comparisons of theoretical volatile release time and mass of volatile matter released were undertaken.
4th quarter, 1993: Following a substantial rebuild the two colour pyrometer was calibrated and transferred to Newcastle and interfaced to the particle combustion furnace.
1st quarter, 1994: Difficulties with the data .collection system. (which was purchased to replace the original system provided with the two colour pyrometer) were found and the system was substituted by a further system developed at Newcastle. Work progressed with coal particle combustion particularly with respect to volatile matter combustion data from the single colour pyrometer. Experiments using a number of size fractions (63-75, 75-90, 90, 106125 and 125-150µm) of coal have been undertaken in various furnace conditions (gas temperature (Tg ); 1200-1750K and 1.5% 02). The energy intensity measurements from the devolatilisation of these single particles was manipulated so that a direct comparison could be made with the predicted mass of volatile matter released.
2nd quarter, 1994: The development of a revised furnace with reduced axial temperature gradients progressed, with the transparent (glass) furnace being replaced by an insulated (Kaowool) furnace. Calculation of the char reactivity from particle temperatures assuming thermal equilibrium over-estimated the reactivities by only 2.4% in this furnace. This error, it is suggested, is reasonable given the other uncertainties in the analysis and will allow a simplified analysis of measurements. Investigations using the two colour pyrometer did not commence due to difficulties in the data collection system.
3rd quarter, 1994: The data collection system was perfected and initial results obtained for the two colour pyrometer. One channel of the pyrometer was found to be influenced by water vapour, with unacceptable errors with data at high temperatures. The maximum temperature at which experiments could be conducted was found to be 1300K. To overcome this problem the more appropriate filter in the wavelengths range for high temperature experiments was purchased. Low temperature experiments were conducted to prevent further delays.
4th quarter, 1994: The new filter was incorporated into the two colour pyrometer which was recalibrated and again interfaced with the furnace system. Detailed analysis was conducted on the data from the single channel pyrometer experiments which relate to volatile matter. It was found that a measure of the volatile matter released from a particle during devolatilisation could be related to the intensity of the energy emitted from a burning coal particle during the volatile matter combustion stage of the coal combustion process. Analysis was also conducted on the ignition temperature and delay times from these experiments.
1st quarter, 1995: The two colour pyrometer, with the new filter, was found to give reasonable outputs. Subsequent measurements on burning char particles of several coals yielded reactivities which agreed with previous measurements on the same coals.
2nd quarter, 1995: The detailed report was compiled, with a review of the related literature, and analysis of the single wavelength data for evaluation of volatile matter release and the two colour pyrometer data for char reactivity. The related physical analysis of collected char was also completed.
Potential for industrial applications
The techniques developed and detailed in this report have the potential to be additional, and commercial, characterisation procedures to those presently available.
The technique established for the determination of the relative volatiles appears to be the most appropriate for immediate application. The project has revealed that these measurements will be different in an atmosphere containing oxygen than in the usual pyrolysis (i.e. inert atmosphere) experiment