GB2054140A

GB2054140A - Improvements to X- and gamma -ray Techniques for Determination of the Ash Content of Coal

Info

Publication number: GB2054140A
Application number: GB8020526A
Authority: GB
Original assignee: Australian Atomic Energy Commission
Current assignee: Australian Atomic Energy Commission
Priority date: 1979-06-22
Filing date: 1980-06-23
Publication date: 1981-02-11
Also published as: AU528536B2; DE3023453A1; AU5945080A; GB2054140B

Abstract

An X- or gamma -ray scatter assembly for use in quantitative determination of the concentration of ash in coke or coal 5, comprises X- or gamma -ray source 1, energy sensitive detector 3, 4 to detect X- or gamma -rays 8, resulting from X- or gamma -ray scatter in the coal or coke. Shielding means 2 is adapted to reduce the intensity of direct X-rays or gamma -rays and to collimate partly the beam of X-rays and/or gamma -rays to ensure matched volumes and deep penetration in the coal or coke. The assembly has an associated electronic analyser adapted to select all or one or more parts of the detected X-ray or gamma - ray spectrum, thereby modifying the output of the assembly. Two or more of these assemblies are employed in an apparatus and method for analysing coal or coke. <IMAGE>

Description

SPECIFICATION Improvements to X and Y-Ray Techniques for Determination of the Ash Content of Coal The present invention relates to improvements in the determination of the ash content of coal or coke employing measurements of X-rays and/or y-rays. The application of X-ray and/or y-ray techniques to determine ash content of coal or coke is described by J. S. Watt and V. L. Gravitis in 'Analysis of Coal', Australian Patent No. 501 427.

Accurate analysis of coal is made difficult by its heterogeneous nature. For example, coal on conveyor beits is known to segregate on different parts of the belt depending on its particle size and density. As a result the composition of small samples may differ in ash content, particle size distribution and density from that of the average of large samples. It is therefore desirable to average the ash content determination over as large a volume of coal as possibie so that an analysis representative of the whole coal sample or coal stream to be analysed is obtained.

In practice, using X-ray or y-ray techniques, this would be achieved by detecting the X-rays or y- rays which have penetrated deeply into the coal.

If ash content is determined by combining measurements from two or more X-ray and/or y-ray assemblies, accurate ash determination would require that essentially the same volume of coal is 'seen' by each separate measurement, i.e., the detected X-ray intensity, for each measurement has resulted from interactions of X-rays or y-rays over the same volume of coal. Since the mass absorption coefficient of X-rays or y-rays in coal changes considerably with energy, sample volumes 'seen' by each X-ray measurement normally overlap only to a limited extent.

The present invention relates to the determination of ash content of coal using X-ray and/or y-ray techniques in which the volume of coal 'seen' by each separate measurement is approximately the same i.e. matched, and in which X-rays and/or y-rays penetrate deeply within the coal. The basis of matching the volumes, and obtaining deep penetration, is the use of partial collimation of X-ray and/or y-ray beams. The separate measurements with matching volumes can be made with two or more assemblies.

The present invention therefore provides an X- or y-ray scatter assembly for use in quantitative determination of the concentration of ash in coke or coal, said assembly comprising an X- or y-ray source, an energy sensitive detector to detect X- or y-rays resulting from scatter in said coal or coke of X- or y-rays from said source, and shielding means adapted to reduce the intensity of direct X-rays or y-rays and to collimate partly the beam of X-rays and/or y-rays to ensure matched volumes and deep penetration in the coal, said assembly having associated therewith an electronic analyser adapted to select all or one or more parts of the detected X-ray or y-ray spectrum, thereby modifying the output of said assembly.

The invention also provides a measuring apparatus for quantitative determination of the concentration of ash in coal or coke, which apparatus comprises at least two X-ray or y-ray scatter assemblies, and means to compute said ash concentration from the outputs of said assemblies, the geometries of said assemblies each being arranged so that the detected X-ray or y-ray intensity is a measure over substantially the same volume of coal or coke sample, said assemblies each comprising an X-ray or y-ray source, an energy-sensitive detector to detect said scattered X-rays or y-rays resulting from scattering in the coal or coke of X-rays or y-rays from said source, an electronic analyser associated therewith to select all or one or more parts of the detected X-ray or y-ray spectrum as output from said assemblies, and shielding means adapted to reduce the intensity of direct X-rays or y- rays and to coliimate partly the beam of X-rays and/or y-rays to ensure matched volumes and deep penetration in the coal,- and wherein the energy of the X-rays or y-rays emitted by said assemblies differ from each other.

Also within the scope of the invention is a method of analysing coal or coke comprising determining the concentration of ash or mineral matter in coal or coke by the steps comprising measuring the transmission or scatter of X-rays or y-rays of a first energy chosen such that there is a significant difference in absorption of radiation per unit weight in coal matter and mineral matter excluding iron, in combination with either or both of (a) effecting at least one further measurement of transmission of scatter of X-rays or y-rays at a different energy so chosen that there is a significant difference in absorption of radiation per unit weight of coal matter and mineral matter and that the relative absorption per unit weights by said coal matter and said mineral matter at any one energy is significantly different from the relative absorption at each other energy including said first energy, (b) effecting a measurement of the bulk density or mass per unit area of the coal or coke by measuring the transmission or scatter of X-rays or y-rays at an energy chosen such that there is no significant difference in absorption of radiation per unit weight in coal matter and mineral matter, and wherein the detected X-ray or y-ray intensity is a measure over substantially the same volume of coal or coke sample in each case.

Figure 1 is an illustration of back-scatter assembly suitable for use in the invention.

Figure 2 shows a normalised count rate plotted against depth of water and depth of coal.

Figure 3 is a diagrammatic illustration of a preferred embodiment of the invention.

Figure 4 is an illustration of scatter-transmission assemblies suitable for use in the invention.

Figure 1 shows a geometrical arrangement of an X-ray or y-ray back-scatter assembly showing radioisotope source 1, shielded container 2, scintillation detector 4, with sodium iodide crystal 3, and coal or coke sample 5.

Figure 2 shows a normalised count rate plotted against depth of water and depth of coal determined using Am-241, Gd-1 53, and Ba-1 33 radioisotope sources. The effect of depth of sample material on normalised count rate is approximately the same for each source.

The matching of volumes has been obtained by apparatus shown diagrammatically in Figure 1.

The radioisotope source 1, was located within a shielded container 2, and the sodium iodide crystal 3, of a scintillation detector 4, was located against the outside of the shielded container. The coal sample 5, in an aluminium container 6, is placed above the shielded assembly. The path of a y-ray or X-ray 7, from the radioisotope source scattering in the coal sample, and scattered y-ray 8, to detector, is also shown. Multiple scatter of y-rays in the coal sample also occurs, and results in detection of the multi scattered y-ray.

The measurements described below were all made using the assembly shown in Figure 1. A sodium iodide crystal of diameter 38 mm, height 25 mm was used, and all X-rays or y-rays detected were countered by the scintillation detector.

Results are in Figure 2. The continuous curves were obtained using water as the sample, and 'X' and '0' represent measurements on coal containing 10 wt. % ash. The detected intensity was determined as a function of depth of water or coal in the container for each of the three sources, Am241 (60 keV y-rays), Gd-i 53 (100 keV) and Ba-133 (mainly 356 keV). The normalised count rate (vertical/axis, Figure 2) was obtained by taking the ratio of measured count rate at the water depth to the count rate obtained for great depths corresponding to saturation of intensity of back-scattered Xrays. Figure 2 shows that good matching with depth of water or coal sample has been obtained for the three sources used. For Am-241, the effective penetration has been increased by a factor of three compared with that for an uncollimated beam.

The results above demonstrate that good matching has been obtained with each source in exactly the same position in the shielded container. An even closer coincidence of curves in Figure 2 for Am-241, Gd-1 53, and Ba-133 could be obtained altering the Am-241 source position relative to the position of the other two sources or the distance of the assembly from the coal sample. Matching in lateral directions has not been considered above, but can also be achieved by the use of selective collimation of the radioisotope sources.

The use of matched volumes is also of advantage when samples of coal are of thickness less than that corresponding to saturation of backscattered intensity of X-rays or y-rays. The use of 'matched volumes makes it possible by a correction to be applied to compensate for changes in coal depth and hence determine ash content more accurately.

The following Examples illustrate preferred embodiments of the invention. In each case the coal being analysed can be a static sample, or in a continuously flowing sample by-line, or the main stream of coal moving on a conveyor or through a hopper.

Example 1 A preferred embodiment of the invention is described in the following example with reference to Figure 3. Coal 10 on a moving conveyor belt 11 is viewed by beams of X-rays or y-rays from backscatter assemblies 12, 13 and 14, each substantially similar to that in Figure 1. The radioisotope sources in each assembly could be Am-241 (59.5 keV), Gd-1 53 (-100 keV) and Ba-1 33 (356 keV and others).However, there are many other combinations of sources that would be suitable, e.g., Sm-I 45 or a secondary excitation source, Am-241, and Ba-1 33 or Cs-i 37. The electronics used with scintillation detectors (4 in Figure 1) are known art and comprise high voltage units 1 5 to polarise the scintillation detectors, amplifiers 16, single channel analysers 1 7 or discriminator 1 8 to select electrical pulse heights corresponding to the appropriate y-rays or X-rays detected, and interface units 1 9 to link outputs from the units 1 7, 18 with the digital computer 20 which scales the electrical pulses and calculates the ash content.

The preferred embodiment has been used to determine the ash content of coal samples supplied by Utah Development Co. (ash range 5-27 wt.%, iron in ash 3-19 wit.%, 24 samples). The three radioisotope sources used in the three matched backscatter assemblies were Am-241, Gd-1 53 and Ba-1 33. The measured intensities lAm, 13d and gBa were combined to give the ash content Cash by the equation.

a2 a3 a5 CaSh=a 1 + + + Am (lGd+a4) 1Ba The rms error in ash content determination for these samples was 1.6 wt.% ash. The subscripted values of a in this and the next equation are constants derived by a least squares calculation to give the minimum error in Cash.

This error of 1.6 wt.% ash compares with an error of 3.0 wt.% ash for the same samples when using only the measured intensities lAm and IBa from the matched backscatter assemblies using Am241 and Ba-133 radioisotope sources.

When selected parts of the measured spectra from the Am-241 and Gd-1 53 assemblies are used instead of the whole spectra as above, it would be expected that the rms error of 1.6 wt.% ash in ash determination would be decreased.

A simpler form of the preferred embodiment has been used to determine the ash content of coal samples having a more limited range of variation of ash content and iron in the ash. (Samples supplied by Utah Development Co., ash range 5-1 8 wt.%, iron in ash 6-14 wt.%,13 samples). As the range of iron concentrations in these samples was not very great, the measurement for ash only had to be corrected for density variations. Only two matched backscatter assemblies were therefore required.

The two radioisotopes used in the two matched backscatter assemblies were Am-241 and Ba-i 33.

The measured intensities lAm and IBa were combined to give the ash content C ash by the equation.

Cash=a,+a2 iog (1m)+a3 log (1boa).

The rms error in ash content determination for these samples was 1.0 wt.% ash.

Example 2 A second preferred embodiment is based on scatter-transmission of y-rays or X-rays (Figure 4).

The geometrical arrangement of scatter-transmission assemblies about the coal 30 on conveyor 31 comprise radioisotope sources 32, in shielded containers 33, with further shields 34 between sources 32 and scintillation detectors 35, with further shields 36 about the sodium iodide crystals 37 of the scintillation detectors 35. The shields 34 stop y-rays or X-rays from the radioisotope sources from reaching the detectors directly, i.e. all detected y-rays or X-rays must have resulted from a scattering interaction in the coal. The shields 33 and 36, by limiting the directions of y-rays or X-rays incident on the coal and emergent detected y-rays or X-rays ensure that most of the detected y-rays or X-rays have resulted from interactions in the same 'sensitive' volume of coal. Hence the sensitive volume is matched for various energy y-rays.

The electronic equipment is similar to that described in Example 1.

The sources of X- and y-rays described hereinabove are illustrative only of suitable sources. Other known sources such as those described in Australian Patent 501 427, the disclosure of which is incorporated herein by reference, will be apparent to those skilled in the art.

Claims

1. An X- or y-ray scatter assembly for use in quantitive determination of the concentration of ash in coke or coal, said assembly comprising an X- or y-ray source, an energy sensitive detector to detect X- or y-rays resulting from scatter in said coal or coke of X- or y-rays from said source, and shielding means adapted to reduce the intensity of direct X-rays or y-rays and to collimate partly the beam of Xrays and/or y-rays to ensure matched volumes and deep penetration in the coal, said assembly having associated therewith an electronic analyser adapted to select all or one or more parts of the detected X-ray or y-ray spectrum, thereby modifying the output of said assembly.

2. An assembly as defined in claim 1 wherein the source of X- or y-rays is Am-241, Gd-i 53, Eu155, Ba-i 33, Cd-109 and/or Co-57.

3. A measuring apparatus for quantitative determination of the concentration of ash in coal or coke, which apparatus comprises at least two X-ray or y-ray scatter assemblies, and means to compute said ash concentration from the outputs of said assemblies, the geometries of said assemblies each being arranged so that the detected X-ray or y-ray intensity is a measure over substantially the same volume of coal or coke sample, said assemblies each comprising an X-ray or y- ray source, an energy-sensitive detector to detect said scattered X-rays or y-rays resulting from scattering in the coal or coke of X-rays or y-rays from said source, an electronic analyser associated therewith to select all or one or more parts of the detected X-ray or y-ray spectrum as output from said assemblies, and shielding means adapted to reduce the intensity of direct X-rays or y-rays and to collimate partly the beam of X-rays and/or y-rays to ensure matched volumes and deep penetration in the coal, and wherein the energy of the X-rays or y-rays emitted by said assemblies differ from each other.

4. A measuring apparatus as defined in claim 3 wherein said scatter assembly has its source and detector on the same side of said coal or coke sample.

5. A measuring apparatus as defined in claim 3 wherein said scatter assembly has its source and detector on opposite sides of said coal or coke sample.

6. A measuring apparatus as defined in claim 3 further comprising one or more measuring means which measure moisture content, hydrogen content, or iron content.

7. A method of analysing coal or coke comprising determining the concentration of ash or mineral matter in coal or coke by the steps comprising measuring the transmission or scatter of X-rays or y- rays of a first energy chosen such that there is a significant difference in absorption of radiation per unit weight in coal matter and mineral matter excluding iron, in combination with either or both of (a) effecting at least one further measurement of transmission of scatter of X-rays or y-rays at a different energy so chosen that there is a significant difference in absorption of radiation per unit weight of coal matter and mineral matter and that the relative absorption per unit weights by said coal matter and said mineral matter at any one energy is significantly different from the relative absorption at each other energy including said first energy, or (b) effecting a measurement of the bulk density or mass per unit area of the coal or coke by measuring the transmission or scatter of X-rays or y-rays at an energy chosen such that there is no significant difference in absorption of radiation per unit weight in coal matter and mineral matter, and wherein the detected X-ray or y-ray intensity is a measure over substantially the same volume of coal or coke sample in each case.

8. A method as defined in claim 7 wherein the step (a) is constituted by transmission or scatter of X-rays or y-rays at one further energy chosen that there is a significant difference in absorption of radiation per unit weight in coal matter and mineral matter and that the relative absorption per unit weights by said coal matter and said mineral matter at said first energy is significantly different from the relative absorption at said second energy.

9. The method wherein the concentration of ash or mineral matter in coal or coke is determined as defined in claim 8 coupled with a further measurement of iron concentration by neutron capture of y-ray techniques.

10. The method wherein the concentration of ash or mineral matter in coal or coke is determined as defined in claim 7 coupled with a measurement of moisture or hydrogen content.

ii. The method as defined in claim 10 wherein the moisture or hydrogen content is measured by neutron scatter or transmission, or capture y-rays from neutron absorption by hydrogen.

12. The method as defined in claim 7 wherein the X- or y-rays are obtained from Am-241, Gd- 153, Cd-i 09, Eu-i 55, Ba-i 33, Cs-i 37 and/or Co-57.

13. A method of analysing coal or coke substantially as hereinbefore described with reference to the accompanying drawings.

14. Measuring apparatus for quantitative determination of the concentration of ash in coal or coke, or a scatter assembly for use in such measuring apparatus, substantially as hereinbefore described with reference to the accompanying drawings.