GB2043876A

GB2043876A - Determining Sulphur Content

Info

Publication number: GB2043876A
Application number: GB7850331A
Authority: GB
Original assignee: Coal Industry Patents Ltd
Current assignee: Coal Industry Patents Ltd
Priority date: 1978-12-29
Filing date: 1978-12-29
Publication date: 1980-10-08

Abstract

The sulphur content of a mixture of coal and ash is determined by irradiating the mixture with radiation, e.g. from plutonium 238 and detecting secondary and backscattered radiation from the mixture. Since the sulphur content is directly related to the iron content of the ash, by measuring the iron fluorescence (peak 17) sulphur can be determined therefrom. Background radiation 20, in the iron channel, is compensated for by measuring backscatter radiation in a higher energy range and combining this with the iron measurement according to a given formula, e.g. using a microprocessor. <IMAGE>

Description

SPECIFICATION Improvements in Methods of and Apparatus for Determining Sulphur Concentration of a Mixture This invention concerns improvements in methods of and apparatus for determining sulphur concentration of a mixture including coal and ash.

This invention finds particular application in connection with the monitoring and control of coal composition supplied for example to power stations. A known sampie of methods and apparatus for sulphur determination is described in our prior accepted British Patent Specification Serial No. 1 494 549. In that specification it is explained that a coal and ash mixture is subjected to radiation to cause iron atoms associated with sulphur atoms to emit a characteristic fluorescent radiation. Detection of the fluorescent radiation is one of the steps in determining the sulphur content.

An object of the present invention is to improve the detection of radiation arising within the mixture.

According to one aspect of the present invention, a method of determining sulphur concentration of a mixture including coal and ash comprises bombarding the mixture with primary radiation to cause radiative reactions in the mixture, detecting the secondary radiation generated by the radiative reactions over a spectrum of energies, determining an intensity of secondary radiation at a characteristic fluorescent energy of at least one of the elements in the mixture and correcting said at least one determined intensity by correlating it with other intensities of energies generated at other parts of the spectrum and processing the corrected value of intensity to determine the sulphur content.

Preferably, the intensity of secondary radiation at a characteristic fluorescent frequency of iron is determined.

According to another aspect of the present invention, apparatus for determining sulphur concentration of a mixture including coal and ash comprises a source of radiation arrangeable to bombard the mixture with radiation to generate secondary radiation therein over a spectrum of energies, detector means for detecting said secondary radiation and sensitive to different values of energy within said spectrum, the values including a characteristic energy of at least one element in the mixture, the detector means being able to derive electrical signals indicative of said values and electrical circuit means for processing the electrical signals to correlate at least the signals at the value of characteristic energy, thus to determine the sulphur content.

Preferably, the source of radiation comprises a nucleonic source such as plutonium 238.

The electrical circuit means comprises a microprocessor.

An embodiment of the present invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a block diagram of apparatus for investigating a mixture including coal, and Figure 2 is a graph showing radioactivity.

In order to determine sulphur content of a mixture of coal and ash, samples of the mixture are subjected to analysis by radiation techniques.

Referring now to Figure 1, there is shown apparatus for investigating a mixture including for example coal, ash and moisture. The ash contains elements such as iron, aluminium, silicon, chlorine, titanium, potassium and calcium and of course sulphur. The sulphur content is more or less related to the iron content as will be described below.

A source of radiation such as plutonium 238 is indicated at 1. The plutoriium 238 emits electromagnetic radiation in the X-ray spectrum and is arranged so that a beam 2 of the radiation bombards a layer of the above mentioned mixture 3. The beam of X-radiation 2 causes secondary reaction within the mixture 3 and X-rays in a spectrum of energies up to about 25 keV are emitted. The spectrum will be more fully described below with reference to Figure 2.

The secondary X-ray spectrum is detected by a detector 5 which derives a signal which is amplified by an amplifier 6 and the amplified signal is fed to a pulse amplitude selector 7 which selects only signals arising in a part of the secondary spectrum of interest. Operation of the pulse selector can be multiplexed so that more than one part of the secondary spectrum is selected or alternatively a second detector 5' can be arranged to detect the secondary reaction of a different part of the spectrum. Signals from at least two channels or parts of the spectrum are required as will be explained below.

The output signals from the detector 7 is fed to circuitry 10 for conversion to a form suitable for use in a microprocessor 1 2. The microprocessor 1 2 is connected to control devices 14 such as speed regulators for controlling transport and bunkering of the mixture of materials. A display 1 5 is provided for display of sulphur content or other signal arising in the circuitry.

Referring now to Figure 2, there is shown a typical secondary spectrum derived by interactions in the mixture of materials as measured by a proportional counter. Intensity of signals is plotted as ordinate and energy as abscissa. The resultant curve is indicated at 1 6.

The area of the graph is divided into two, i.e. the spectrum is divided into two channels. In the left channel there is shown a fluorescent intensity peak 1 7 which is derived from K-shell transitions in iron. In the right hand channel general background radiation is shown. A dotted curve 20 in the left hand channel shows a curve that would have been derived if no iron were present in the mixture being analysed and a dotted curve 22 in the left hand channel shows a curve that would have been derived if only iron was present.

It will therefore be appreciated that the iron signal is to some extent contaminated by the background signal and thus use of the value of the intensity of the iron peak could give a misleading indication of the quantity of iron in the mixture.

Fluorescence channel intensity is dependent upon the composition of a matrix which contains the fiuorescing element known in the art as a matrix absorption effect. In the case of iron fluorescence in a mixture including coal and ash, this effect is mainly a function of the ash forming elements and hence ash content. Consequently, the magnitude of the effect is related to the ash content and hence the backscatter signal. Investigations have shown that twenty per cent of the backscatter component can in fact be contained in the iron peak. Since for many types of mixture the iron is more or less directly related to the sulphur content of the mixture, it is important that a true value of the iron peak be obtained.

To correct the value of iron signal a fixed proportion of the value of the backscatter signal in the right hand channel is added to or subtracted from the iron peak in the left hand channel. The fixed proportion is determined empirically from calibration samples of known sulphur content S, according to the equation S=a+b (F+pB) where; a+b are constants of the equation, F the measured iron channel signal, B the measured backscatter channel signal and p the fixed proportion.

The proportion of the backscatter signal is programmed into the microprocessor 1 2 which does the correction on a continuous basis and which varies as the backscatter signal varies. In view of the foregoing, the above mentioned necessity for two channels will be appreciated, the channels being the iron and backscatter channels.

It will be appreciated that the backscatter signal can vary substantially dependent upon the nature of the mixture being analysed since different elements in the mixture will scatter the, incident radiation and iron fluorescent radiation differently. Moreover, even moisture content can alter the packing of the atoms of the elements in the mixture and thus affect the secondary radiation. Also of course, non-pyritic iron can be present in the mixture and this is not related to the sulphur content of the mixture. Similarly, sulphur can exist in the mixture otherwise than in association with iron.

Also factors other than the spectrum overlap of Figure 2 affect the intensity of iron counts, e.g. as ash content increases the intensity of incident radiation available at the sample for the generation of iron fluorescence decreases due to absorption. Similarly, iron fluorescence radiation generated within the coal at depths of more than about 1 mm is heavily attenuated by ash-forming minerals. This latter effect in particular is sensitive to changes in ash composition and correlates only roughly with ash content and hence backscatter signal. The two absorption effects together are numerically larger than the spectrum overlap. The net result is that where a backscatter correction can be applied it has to be determined empirically and does not necessarily correspond with the proportion indicated by curve 22 in magnitude or sign.

Investigations have shown that with low ash washed coals containing up to about ten per cent ash, iron channel intensity correlation with sulphur content is good and only minor improvements is obtained by correction of the iron peak with a proportion of the backscatter signal. However, when ash content varies widely, the backscatter component to the iron channel varies widely and improvements in accuracy are obtained by correction of the iron channel with a proportion of the backscatter signal. Naturally, accuracy is further enhanced if the quantity and disposition of other scattering elements in the mixture is substantially constant.

It is envisaged that other channels in the spectrum could be used for measuring energy peaks due to other elements, in which case the backscatter components would be added to or subtracted from these other channels. Knowledge of the quantity of the elements in the mixture would enable their scattering effect on the incident and secondary radiation to be quantified and then the accuracy of sulphur determination to be enhanced.

From the above description it can be seen that the present invention provides an improved method of and apparatus for sulphur determination.

Claims

1. A method of determining sulphur concentration of a mixture including coal and ash comprising bombarding the mixture with primary radiation to cause radiative reactions in the mixture, detecting the secondary radiation generated by the radiative reactions over a spectrum of energies, determining an intensity of a secondary radiation at a characteristic fluorescent energy of at least one of the elements in the mixture and correcting said at least one determined intensity by correlating it with other intensities of energies generated at other parts of the spectrum and processing the corrected value of intensity to determine the sulphur content.

2. A method as claimed in claim 1, wherein the intensity of secondary radiation at a characteristic fluorescent frequency of iron is determined.

3. Apparatus for determining sulphur concentration of a mixture including coal and ash comprising a source of radiation arrangeable to bombard the mixture with radiation to generate secondary radiation therein over a spectrum of energies, detector means for detecting said secondary radiation and sensitive to different values of energy within said spectrum, the values including a characteristic energy of at least one element in the mixture and able to derive electrical signals indicative of said values and electrical circuit means for processing the electrical signals to determine the sulphur content,

4. Apparatus as claimed in claim 3, wherein the source of radiation comprises a nucleonic source such as plutonium 238.

5. Apparatus as claimed in claim 3 and 4 wherein the electrical circuit means comprises a microprocessor.

6. A method of determining sulphur concentration of a mixture including coal and ash, substantially as hereinbefore described with reference to the accompanying drawings.

7. Apparatus for determining sulphur concentration of a mixture including coal and ash, substantially as hereinbefore described with reference to the accompanying drawings.