16 Data on the Distribution of Organic Sulfur Functional Groups in Coals AMIR ATTAR and FRANCOIS DUPUIS Downloaded by UNIV OF AUCKLAND on December 28, 2017 | http://pubs.acs.org Publication Date: June 1, 1981 | doi: 10.1021/ba-1981-0192.ch016
1
Department of Chemical Engineering, University of Houston, Houston, TX 77004
The distribution of the organic sulfur functional groups was determined in five coals and treated coals using thermokinetic analysis. The data suggest that 15-30% of the organic sulfur is sulfidic in all coals. About 30-40% of the organic sulfur in lignite is thiolic and the rest is thiophenic. In bituminous coals 40-60% of the organic sulfur is thiophenic. The data suggest that the organic sulfur is trapped in coal as thiolic sulfur, which subsequently condenses to sulfidic and eventually to thiophenic sulfur. Oxidation of coal reduces the accessibility of the organic sulfur to the reducing agent. Extraction of coal with hydrochloric acid dissolves the calcium and magnesium salts. A peak believed to belong to thiolates like [фSCa] is shifted. The sulfur is determined as фSH. An upper bound exists on the maximum portion of the organic sulfur that can be removed without the complete destruction of the coal matrix. Data are available that suggest that only thiolic and portions of the aliphatic sulfides can be removed by mild desulfurization. +
Coals contain inorganic sulfur compounds, like iron pyrite and gypsum and organic sulfur, which are bound to the organic matrix. Detailed reviews of sulfur functional groups in coal were published recently by Attar (J) and Attar and Corcoran (2). The chemistry, kinetics, and ther modynamics of the reactions of sulfur were described by Attar (3) and therefore will not be reviewed here in detail. This work had two objectives: (1) to study the structure of the organic sulfur groups in different coals, and (2) to examine the implications of the structure of the organic sulfur groups on potential desulfurization processes. Current address: Department of Chemical Engineering, North Carolina State University, Raleigh, N C 27650. 1
0065-2393/81/0192-0239$05.00/0 © 1981 American Chemical Society
Gorbaty and Ouchi; Coal Structure Advances in Chemistry; American Chemical Society: Washington, DC, 1981.
Downloaded by UNIV OF AUCKLAND on December 28, 2017 | http://pubs.acs.org Publication Date: June 1, 1981 | doi: 10.1021/ba-1981-0192.ch016
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COAL STRUCTURE
The main results of the test are as follows: 1. The majority of the organic sulfur in high-ranked coals (i.e., LVB) is thiophenic, while in low-ranked coals (i.e., lignites) most of the organic sulfur is thiolic or sulfidic. 2. The aliphatic sulfides constitute 18-25% of the organic sul fur in all coals. 3. Part of the organic thiols are present in the form of thiolates, probably of calcium. 4. Coals with a large content of organic sulfur can be divided into two groups: coals that can be partially desulfurized and coals that cannot be desulfurized easily. The portion of the organic sulfur that can be desulfurized is that present in the form of thiolic groups and aliphatic sulfides. Principle of the Method of Analysis A detailed description of the principle of the method of analysis was published by Attar and Dupuis (4). Therefore, only the main points will be described here. 1. A l l the organic sulfur functional groups can be reduced to H S if a sufficiently strong reducing agent is used. 2. Each sulfur group is reduced at a rate that can be charac terized by a unique activation energy and a frequency constant. 3. If a sample that contains many sulfur groups is reduced and the temperature is gradually increased, each sulfur group will release H S at a different temperature, given by 2
2
_
Ji_
where T is the temperature of the maximum rate of evolution of H S , E. and A are the activation energy and the frequency factor for the de composition, respectively, and a is the linear rate of temperature in crease. To a first-order approximation the frequency constant is inde pendent of the group reduced and depends on the reducing agent only. Therefore; 2
mi
{
where k' is Boltzmann's constant, h is Planck's constant, and Q* is the partition function of the activated complex of the reducing agent. The rate of evolution of H S from the reduction of the tth group can be des cribed by 2
Gorbaty and Ouchi; Coal Structure Advances in Chemistry; American Chemical Society: Washington, DC, 1981.
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t dtH S3 2
dt T
Downloaded by UNIV OF AUCKLAND on December 28, 2017 | http://pubs.acs.org Publication Date: June 1, 1981 | doi: 10.1021/ba-1981-0192.ch016
Figure 1. A typical kinetogram
where [ H S ] is the total amount of H S that reduction of this tth group would release. The detailed derivation of the previous equations was done by Juntgen (5) and by Jiintgen and Van Heek (6). 2
to
2
4. The value of T is a characteristic unique to the sulfur group reduced, and the area of each peak is proportional to the quantity of sulfur present in the form of the group reduced. mi
The implication of these discussions is that the area of the peak having a maximum at T is proportional to the concentration of sulfur present in the sample in the form of the tth group. Therefore, quanti tative determinations of the tth sulfur group can be accomplished by determining the area of each peak. Figure 1 shows a typical output graph. mi
Experimental A detailed description of the experimental system will not be given here since it was recently published by Attar and Dupuis (4). A step-by-step procedure is presented in Ref. 7. The data described in this paper were derived by using an improved version of the same experimental system. The following modifications were made: 1. Stronger reducing conditions were used in order to obtain more complete reductions of the organic sulfur. 2. The sensitivity of the detector was improved. 3. The cell design was changed; it is now possible to obtain detailed analysis on a routine basis. Results and Discussion The distributions of sulfur functional groups (DSFG) in four types of solids are described. The four types of solids are sulfur-containing poly mers with distinct sulfur functional groups; raw coals; treated coals; and iron pyrite.
Gorbaty and Ouchi; Coal Structure Advances in Chemistry; American Chemical Society: Washington, DC, 1981.
Downloaded by UNIV OF AUCKLAND on December 28, 2017 | http://pubs.acs.org Publication Date: June 1, 1981 | doi: 10.1021/ba-1981-0192.ch016
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COAL STRUCTURE
The analysis of the D S F G consists of two parts: (1) the qualitative assignment of a peak of a kinetogram to a given thermal structure; and (2) determination of the quantity of each sulfur group in the coal sample. Qualitative Identification of the Sulfur Groups. Tests of polymers with a known structure were used to identify the temperature at which each sulfur group releases its sulfur. Four polymers were tested: 1. Polyphenylene sulfide (8) as a representative of aromatic sulfides 2. Polythiophene as a representative of thiophenic sulfur 3. A copolymer produced from cyclohexene and 1, 2-ethylene dithiol (8) as a representative of aliphatic and alycyclic sulfides 4. Rubber as a representative of aliphatic sulfides and disulfides A l l the polymers contained some thiolic sulfur. The results of the tests are summarized in Table I, which shows the temperature at which the apex corresponding to each functional group appears. Sulfur-containing polymers can be used to identify the location where each sulfur group is reduced only if two conditions are fulfilled: (1) the rate of the chemical reaction controls the rate of release of H S , both when coal samples and when polymer samples are examined; and (2) the rate of the reduction of each sulfur functional group depends only on the hydrocarbon structures in its immediate vicinity. Table I shows the results of tests of the various polymers and the maximum temperature for each group. Quantitative Analysis of the Concentrations of Sulfur G r o u p s . 2
RECOVERY OF ORGANIC S U L F U R .
Quantitative analysis can be accomplished
provided that all the sulfur present in the form of each group is reduced to H S. It is also assumed that the distribution does not change during the analysis and that all the H S released is detected and determined. Each mole of sulfur, when reduced, produces 1 mol of H S . There fore, the number of moles of H S formed during the reduction of each group is proportional to the number of moles of sulfur present in the sample in that form. Table II shows the results of the quantitation of the kinetogram of six samples of a L V B coal with different particle sizes. The most important conclusions from these tests are the following: 2
2
2
2
1. The total recovery of organic sulfur was independent of the coal particle size used. 2. The consistency of the data from tests on the small coal particles is much better than that on the total six samples. The dimensionless standard deviation i n the case of the —325-mesh particles is consistently smaller than that of the six samples.
Gorbaty and Ouchi; Coal Structure Advances in Chemistry; American Chemical Society: Washington, DC, 1981.
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Organic Sulfur Groups in Coal
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