CA1316800C - Method and apparatus for analyzing different sulphur forms - Google Patents

Abstract

Abstract A method and apparatus are described for quantitatively determining the different forms of sulphur present in a solid matrix, e.g. coal. The novel method comprises intimately mixing finely divided coal with a finely divided solid diluent material, e.g. silica, which is inert to sulphur under reaction conditions. This mixed sample is then burned within a confined combustion zone at a predetermined ele-vated temperature and the combustion gases from the combus-tion chamber are continuously removed. These gases are passed through an infrared analyzer which continuously moni-tors the intensity of the infrared spectra for sulphur oxy-compounds in the combustion gases. The infrared intensity is measured as a function of evolution time of sulphur oxy-compound from the sample to obtain peaks in an infrared intensity-time pattern indicative of different forms of sulphur.

Description

-~316~0 Method and apparatus for analyzing different sulphur forms Background of the Invention .
This invention relates to a method and apparatus for quantitatively determining the different forms of sulphur S present in a matrix, such as coal.
One of the more serious environmental problems through-out the world is air pollution due to the emission o~
sulphur oxides when sulphur-containing fuels are burned.
It is now widely recognized that sulphur oxides are particularly harmful pollutants, producing what is now known as acid rain.
Coal remains one of the world's most important fuel sources and large quantities are burned in thermo-generating plants for conversion into electrical energy.
Many coals contain substantial amounts of sulphur which generate unacceptable amounts of sulphur oxides on burning.
Coal combustion is by far the largest single source of sulphur dioxide pollution in the United States.
The sulphur content of coal, nearly all of which is emitted as sulphur dioxides during combustion, is present in essentially three forms: pyritic sulphur, organic sulphur and sulphate sulphur. Distribution between the different forms of sulphur varies widely among various coals and can even vary quite substantially within a single coal deposit.
It is, of course, highly desirable to be able to remove cubstantial portions of the sulphur present in coal before ' -:
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the coal is burned. Since the different forms of sulphur must be removed by different techniques, how a given supply of coal will be processed will be largely dependent on the relative proportions of the different forms of sulphur present in the coal. The present ASTM methods of analyzing for the different forms of sulphur present in coal are exceedingly time consuming and re~uire highly trained personnel. For instance, the current practice utilizes wet analysis o~ pyritic and sulphatic sulphur to get the content of organic sulphur by difference from the total sulphur contents.
There are many different instruments available on the market that can quickly analyze the total sulphur content of coal. For instance, one commercial analyzer oxidizes the coal sample in a resistance furnace, where the sulphur in the coal is combusted to a gas of sulphur oxycompounds (S0x) which is detected by an infrared detector. These sulphur oxycompounds are primary sulphur dioxide together with minor amounts of other sulphur oxycompounds. However, this analyzer is capable only of giving the total infrared intensity, time integrated as the total sulphur content.
There is a need for a method and apparatus which can quantitatively determine the different forms of sulphur present in a matrix, such as coal, as simply as total sulphurs can now be determined. With that object in mind, the present inventors developed a method for quantitativel~
determining the different forms of sulphur present in a sulphur containing material, such as coal, in which a finely divided sulphur-containing sample was burned within a confined combustion chamber. This combustion chamber was at a predetermined elevated temperature, and the com-bustion gases from the combustion chamber were continuously removed. These removed combustion gases were passed through an infrared analyzer which continuously monitored the intensity of the infrared spectra for Sx in the combustion gases. The infrared intensity was measured as :
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a function of evolution time of Sx from the coal sample to obtain peaks in an infrared intensity-time pattern indicative of different forms of sulphur. Based upon the shape of these pattern peaks and the area under the peaks, the quantity of each form of sulphur in the sample was de~ermined.
The above study showed that the different forms of sulphur within coal or other matrix have sufficiently different oxidation or dissociation rates that these can be detected and measured on the basis of Sx emissions during oxidation. ~he different forms of sulphur can be shown as separate and distinct peaks on an infrared spectro-chronogram. Thus, the area under the total curve of such spectro-chronogram represents the total sulphur content of the sample and when the different peaks in the curve are resolved into individual cur~es, the areas under the individuaL curves can be identified with the amounts of the different forms of sulphur in the total sample.
The multi-peak curve can be resolved into individual curves by known techniques utilizing microprocessor technology.
However, there was a major difficulty with the above procedure in that replicate specto-chronograms on a common test sample varied widely in characteristics. This poor reproducibility of spectro-chronographic results made the deconvolution of different sulphur peaks virtually imposs-ible.
Summary of the Invention _ _ _ _ _____________ According to the present invention, the above problem was solved by intimately mixing the finely divided solid sulphur containing sample, e.g. coal, with a finely divided solid diluent material inert to sulphur under reaction conditions. This mixed sample was then burned within a confined combustion zone at a predetermined elevated temperature and the combustion gases from the combustion chamber were continuously removed. These removed combus-tion gases were passed through an infrared analyzer which ~ 31 6~o continuously monitored the intensity of the infrared spectra for Sx in the combustion gases. The infrared intensity was measured as a function of evolution time of Sx from the sample to obtain peaks in an infrared inten-sity-time pattern indicative of different forms of sulphur.
With the mixed sample of the present invention, reproduci-bility of spectro-chronograms improved dramatically.
It is believed that the reason for this improvement in results is that when powdered coal is tested without a solid inert diluent, erratic high local temperatures are created which interfere with the orderly burning of the different sulphur forms within the coal sample. It is believed that the presence of the small particles of inert diluent uniformly mixed among the coal particles protects the coal from the erratic high local temperatures.
The finely divided solid diluent material is preferably used in an excess relative to the coal with a ratio of finely divided solid inert diluent material to coal of at least 2:1 being preferred. Particularly good resolution hetween different sulphur peaks is obtained at a finely divided solid inert diluent to coal ratio of 5 to 10:1.
Increasing the content of inert diluent beyond this level generally results in the broadening of all peaks of the spectro-chronograms, allowing the use of higher tempera-tures in the reaction chamber. A variety of different materials may be used as the solid inert diluent and silica has been found to be particularly desirable because it pro-vides excellent results while being readily available and inexpensive. Other non-catalytic inert materials may also be used.
For best results, both the coal sample and ~he inert diluent should be very finely divided, e.g. in the form of powders. Thus, they generally should have particle sizes of less than 60 ~m, with particle sizes of less than 10 ~m being particularly preferred. The coal and the inert diluent must be thoroughly blended together so that the - :, , ~: , ' ' ' .. .

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diluent particles are well distributed throughout the coal particles. This finely divided mixture is then thinly spread in a uniform layer in a sample container, e.g. a sample boat.
The temperature in the combustion chamber can change the peak positions of the spectro-chronograms, as well as the characteristics or shape of the curves. Thus, the peaks become broader and lower at lower chamber tempera~
tures, and with increasing temperatures, the peaks become more sharply defined and the oxidation and/or dissociation kinetics become faster. Preferably the temperature or the combustion chamber is maintained within the range of about 500C to 2000C. Within this general range, an optimum temperature is selected to provide the best definitions of the different components.
The invention also relates to an apparatus for carrying out the above method and comprising (a) a combustion fur-nace including a combustion chamber for receiving a finely divided sample of sulphur-containing material and for com-busting said sample to form combustion gases containingS0x, means for controlling the temperature of the combus-tion chamber and means for feeding oxygen to the combustion chamber at a controlled rate, (b) conduit means connected to an outlet of said combustion chamber for continuously removing combustion gases from the combustion chamber, (c) detector means for detecting Sx concentration of gases passing through said conduit and for generating output signals indicative of said concentration at short time intervals during substantially the total period of combus-tion gas production from said sample, and (d) processor means for receiving the output signals from (i) the Sx concentration detector and (ii) time of combustion gas production and determining therefrom the quantity of each form of sulphur in the sample.
A series of experiments were conducted on a modified ~ 31 ~

"LECO SC-32 Sulphur Detector". In the conventional opera-tion of this analyzer, the output of the device (inverse of infrared adsorption intensity for a fixed wavelength of S02) is collected in the form of digital data to arrive at the total sulphur content of a coal sample. For the present studies, the above analyzer was modified so that the infrared signal output was continuously recorded as a function of oxidizing time to give spectro-chronograms.
DescriE~on of the Preferred Embodlments In the drawings which illustrate the invention:
Figure 1 is a block diagram of components forming the apparatus of the invention;
Figure 2 is two replicate IR spectro-chronograms for Sx from oxidizing a coal sample without inert diluent;
Figure 3 is six replicate IR spectro-chronograms for Sx from oxidizing a coal sample with silica diluent;
Figure 4 is ten replicate IR spectro-chronograms for Sx from oxiding a coal sample with silica diluent; and Figure 5 is duplicate IR spectro-chronograms for Sx from oxidizing five different coal samples with silica and from silica only;
Figure 6 is IR spectro-chronograms for ~x from oxidizing coal with silica diluent at different temperatures.
Referring to Figure 1, the system includes a combustion furnace 10 with a combustion chamber 11 positioned within the furnace. The furnace 10 is a resistance-type furnace having resistance heating elements 12. The heating elements and combustion chamber are housed within a refrac-tory block including side walls 13, a rear wall 14, and a front wall 15 having an access opening 16 through an instrument front panel 17. The refractory linings of the furnace 10 include a floor and a top. Thus, the combustion chamber 11 is totally enclosed within the refractory lining of the furnace. The system includes a pressurized source 19 of oxygen gas coupled to a pair of flow controllers 20 t~

~ 3 ~ 0 and 21 which supply the oxygen gas to the combustion chamber 11 via supply conduits 22 and 23 respectively.
Thus, the specimen material is combusted by furnace 10 in the presence of oxygen to convert the sulphur contained in the sample to Sx for subsequent analysis.
Gases from the specimens being combusted within boat 18 in the combustion chamber 11 are withdrawn by a dis-charge tube 24 which extends into the combustion chamber 11 and communicates with a filter 25, a pump 26 for drawing the gas through the filter and a flow controller 27 for adjusting the flow rate of gas to an IR cell 28~
The gas from IR cell 28 is vented to the atmosphere and this IR cell includes a detector 29 which is electrically coupled to a microprocessor 30 for receiving signals from the detector 29.
The microprocessor 30 also is connected to a timer and to the controls of the furnace so that the microprocessor receives signals determining the concentration of Sx in the gas and signals indicating the time of combustion.
The microprocessor is capable of determining from these signals the quantity of each form of sulphur in the sample.
It may also be connected to a printer 31 and/or CRT display 32 which can illustrate the results in the form of a spectro-chronogram.
The following examples are provided to more specific-ally illustrate the invention described herein.

Exa_~le For the purpose of comparison, a first test was con-ducted on the above modified analyzer using a sample of coal by itself. The coals used were standard samples from various sources. They were ground to a very fine powder size (-60 llm). Two 100 mg samples of one coal sample were analyzed by being placed in sample boats and analyzed with-in the modified analyzer at a combustion chamber tempera-ture of 525C. The results obtained are shown in Figure , 131 ~8~

2 and it will be seen that there is very poor reproduction of the spectro-chronograms.

Example 2 ____ ___ For this test, one coal sample was obtained and was finely ground to ~60 ~Im particle size. Six 100 mg samples of this coal were prepared and six further samples were prepared consisting of 100 mg o~ the individual coal powders thorougly mixed with 900 mg of silica powder (-60 ~m). The twelve different samples thus prepared were then tested in the above modified analyzer at a combustion chamber temperature of 525C. The result obtained are shown in Table 1 below.
Table 1 ________ ______ __________________________ _________ Coal _ % Total SulE~ur _ Relativ Sample _ _ Differenc Number Coal(100 mg) only Coal(100 mg)+
Silica (900 mg) _____ _____________________________ __ ___________ 1 0.66 i 0.02 0.67 + 0.01 +1.5 2 1.01 _ 0.01 1.01 + 0.01 +0.0 3 2.12 i 0.02 2.14 + 0.02 +0.9 4 2.44 i 0.05 2.43 + 0.02 -0.4 3.58 i 0.01 3.61 ~ 0.05 +0.8 6 _ _ 4.54 + 0.01 4.55 + 0.01 +0.2 ______ _______= ___________________ _ _ _ _ ___________ From the above results, it will be seen that the total sulphur content was the same, regardless the addition of silica to coal. This clearly shows that silica is inert and does not interfere with the sulphur analysis.

ExamE~

This test was carried out using a single sample of coal which was ground to a powder of -60 ~m particle size. Six samples were prepared each consisting an intimate mixture of 100 mg of the above coal powder and 900 mg of silica powder (-60 ~m). The six samples were then each analyzed within the ;. , :
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modified analyzer at a combustion chamber temperature of 525C. The results are shown in Figure 3 and it will be seen that all six tracings are substantially identical, clearly showing that there is a high level of reproducibility.

Exam~e 4 The same general procedure as described in Example 3 was repeated using samples consisting of 100 mg of the coal powder and 900 mg of the silica powder, but for these tests the combustion chamber temperature was lowered to 500C. The results for ten replicate tests are shown in Figure 4 and it will be seen that an excellent reproducibility of sulphur content was obtained as well as excellent definition and separation of the peak positions and heights for the primary and secondary peaks.

_xample 5 For this example, five different coal samples were used which were known to vary widely in their amounts of different sulphur forms. The same general procedure as described in Example 3 was used with samples consisting of carefully blended mixtures of 100 mg of coal powder and 900 mg of silica powder. A sample of silica powder only was also tested. The tests were all conducted at a combustion chamber temperature of S50C and the results of six duplicate tests are shown in Figure 5.
Excellent reproducibility was obtained and the spectro-chronogram tracings obtained showed the results in Table 2 . ~

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below:
Table_2 __________ ___________ ____________ ___________________ . .
_ _ __ _ ~ Sulphur Forms Trace No. Sam~le No . Or~an Pyritic SulE~atic Tota]
1 & 2 Silica only 0.00 0.00 0.00 0.00 3 & 4 Coal S-l Oa 42 0.03 0.06 0.51 5 & 6 Coal S-2 0.71 0.38 0.02 1 11 7 & 8 Coal S-3 0.73 1.42 0.10 2 2 9 & 10 Coal S-4 1.18 1.00 0.73 2.91 11 &_12 Coal_S-5 ~_ 1.36 2.09 0.66 ~.11 The above results clearly show that the reproducible peaks are associated with different forms of sulphur.

Exam~le 6 _ _ _ ___ To study the effect of reaction temperature on the shape of the spectro-chronograms, a 100 mg sample of coal (-60 l~m) was mixed with 900 mg of silica powder (-60 ~m) and analyzed at 500, 525, 550, 600 and 700C~ The results obtained are shown in Figure 6 and it will be seen that as the reaction temperature increased from 500 to 700C, the intensity of the primary and secondary peaks grew from about 43.5 to 141.0 mm and from about 17.0 to about 40.0 mm respectively. On the other hand, the peak position shifted from 264 to 84 seconds for the primary peak and from 114 to about 48 seconds for the secondary peak. The changes in peak, height and position were more pronounced for the primary peak than those for the secondary peak.
As the reaction temperature increased, the resolution between the primary and secondary peaks became progressively poorer. The same coal sample was used for this complete temperature study and the area under the spectro-chronograms remained unchanged, indicating that the tot al salphur GOnt en t was i de n t i ca l for al 1 cas es .

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Having thus described the present invention, it should be noted that various other alternatives, adaptations and modifications may be used within the scope of the present invention. For instance, while the description relates primarily to the detection of different forms of sulphur present in coal, the sulphur being detected may be present in many materials other than coal.

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Claims (11)

1. A method for quantitatively determining the different forms of sulphur present in a solid matrix, which comprises burning a finely divided sulphur-containing sample within a confined combustion chamber, said combustion chamber being at a predetermined elevated temperature, continuously removing the combustion gases from the chamber, continuously generating a signal-time pattern indicative of the amount of sulphur dioxide in the collected combustion gas as a function of combustion time of the sulphur-containing sample to obtain peaks in the generated signal-time pattern indicative of different forms of sulphur and determining from the pattern peaks the quantity of each form of sulphur in the sample;
characterized in that the finely divided sulphur-containing sample is mixed prior to burning with a finely divided solid diluent inert to sulphur under the burning conditions.

2. A method according to claim 1 wherein the sulphur-containing sample is coal.

3. A method according to claim 2 wherein the inert diluent is silica.

4. A method according to claim 3 wherein the coal and silica are ground powders and are thoroughly mixed.

5. A method according to claim 4 wherein the silica and coal are in the ratio of at least 2:1.

6. A method according to claim 5 wherein the ratio of silica to coal is in the range of about 5 to 10:1.

7. A method according to claim 3 wherein the combustion chamber is at a temperature in the range of 500° to 2000°C.

8. A method according to claim 1 wherein the signal generated is obtained by continuously monitoring the intensity of the infrared spectra for sulphur oxycompounds.

9. A method according to claim 8 wherein the signals from the infrared intensity monitoring are fed to a micro-processor which detects the infrared intensity-time pattern and determines therefrom the quantity of each form of sulphur in the sample.

10. A method according to claim 1,2,3,4,5,6,7,8 or 9 wherein the sulphur-containing sample and the diluent have particle sizes of less than 60 µm.

11. An apparatus for quantitatively determining the different forms of sulphur present in a solid matrix according to the method of claim 1, which comprises:
(a) a combustion furnace including a combustion chamber for receiving a finely divided sample of sulphur-containing material mixed with a solid diliuent inert to sulphur under combustion chamber conditions and for combusting said finely divided sample to form combustion gases containing sulphur oxycompounds, means for controlling the temperature of the combustion chamber and means for feeding oxygen to the combustion chamber at a controlled rate, (b) conduit means connected to an outlet of said combustion chamber for continuously removing combustion gases from the combustion chamber, (c) detector means for detecting sulphur oxycompounds concentration of gases passing through said conduit and for generating output signals indicative of said concentration at short time intervals during substantially the total period of combustion gas production from said sample, and (d) processor means for receiving the output signals from (i) the sulphur dioxide concentration detector and (ii) time of combustion gas production and determining therefrom the quantity of each form of sulphur in the sample.