LIGAND ASSISTED DESULFURIZATION OF LIGNITE USING THE PERCHLOROETHYLENE COAL REFINING PROCESS

ABSTRACT

The perchloroethylene coal refining process was used on North Dakota lignite coal. Effects of extraction time, perchloroethylenexoal ratio and artificial weathering were studied to determine their effects on the removal of organic sulfur. The artificial weathering conditions used were found to be too harsh and caused a decrease in the amount of organic sulfur removed, unlike normal weathering conditions. Additional studies were done involving various ligands, and showed that the addition of certain ligands during the process aided in the extraction of organic sulfur from the coal.     

EFFECT OF PYRITIC SULFUR AND MINERAL HATTER ON ORGANIC SULFUR REMOVAL FROM COAL

Abstract

The perchloroethylene coal cleaning process uses perchloroethylene as the solvent to remove both organic and inorganic forms of sulfur without any significant loss to its calorific value. The process removes these forms of sulfur in two sequential unit steps. The objective of this investigation was to determine the exact sequence of operations in the Process. Hence, organosulfur was removed before and after depyriting and demineralizing the coal. The extent of total sulfur as well as organic sulfur removal were compared in both cases. It was found that the desulfurization is more efficient when organosulfur is extracted before pyritic sulfur and not vice versa, in the sequential removal of organic and inorganic forms of sulfur. The data presented in this paper reestablishes a fact that the mineral matter content in coal is quintessential to its organosulfur extractability.     

A Review of: “Environmental Consequences of,and Control Processes for Energy Technologies” Pollution Technology Review No. 181 Argonne National Laboratory,Argonne, Illinois 1990, xxii + 453 pp.

ABSTRACT

Sulfur in coal causes problems in coal processing and utilization, therefore the chemical beneficiation of coal has gained widespread attention. Using three coal samples with high sulfur content the effect of desulfurization has been studied. For Nantong coal containing 4.25% total sulfur under the condition sulfur reduction increased from 38.8% upto 62.4% the released sulfur was generally sulfate and pyrite, but the organic sulfur almost remained constant. When alkali treatment was performed at 370^°C, organic sulfur removal of Donglin and Yanzhou coal was to 50.6% and 57.3% respectively.     

IMPREGNATION AND RECOVERABILITY OF ELEMENTAL SULFUR IN COAL USING THE PERCHLOROETHYLENE DESULFURIZATION PROCESS

ABSTRACT

During the perchloroethylene extraction process, C-S bond cleavage reactions occur, which liberate labile sulfur from the organic matrix of coal into the solvent medium i.e., perchloroethylene, where it is dissolved and extracted. In this paper, the effect of impregnation of elemental sulfur in raw coal on its forms of sulfur analyses has been investigated. The effect of the same sulfur-impregnated coal on its organosulfur extractability has also been explored. Studies were conducted to observe whether the impregnated elemental sulfur was fully recoverable by the perchloroethylene extraction process. It was observed that sulfur can be very easily impregnated into the microstructure of coal. On the basis of tests on raw and impregnated coals, based on ASTM D-2492 standard, the impregnated sulfur reflects mostly in the form of organic sulfur. The impregnated sulfur which appears in the form of organic sulfur is fully recoverable via the perchloroethylene extraction     

SELECTIVE REMOVAL OF ORGANIC SULFUR FROM COAL BY PERCHLOROETHYLENE EXTRACTION

ABSTRACT

Desulfurization of coal involves the removal of both the inorganic and organic forms of sulfur. Several physical methods are available for the removal of inorganic sulfur which is normally represented by pyritic and sulfatic sulfur. Removal of organic sulfur requires the use of chemical cleaning methods. This paper presents the results of an organic sulfur removal technique which employs an organic solvent. This desulfurization technique is selective enough to reject organic sulfur without significantly reducing the calorific value of the treated coal. The sulfur containing organic species in the liquid product from the desulfurization procedure have been completely characterized using GC/MS techniques. These results provide further insights into the nature of the sulfur forms in the parent coal as well as the metamorphism of sulfur species in the coal, and the selective nature of the desulfurization process.     

MICROBIAL DESULFURIZATION OF COAL

ABSTRACT

The sulfur found in coal is generally divided into three forms: pyritic, sulfate, and organic sulfur. Originating from irt-situ oxidation of metal sulfides, sulfates axe present in coal at low concentrations. Being soluble in water, they are relatively easy to be leached from the coal. Similarly, microbial depyritization promotes the oxidative conversion of inorganic sulfur compounds to water-soluble products. The pyrite removal results from the combined effects of direct bacterial attack and indirect chemical solubilization. In the former, pyrite (FeS₂) is oxidized by bacteria into Fe₂(SO4) ₃; in the latter, ferric iron is the actual oxidizing agent and microorganisms serve to regenerate the ferric iron from ferrous iron.

Organic sulfur is chemically bound to the carbon skeleton of coal. It is believed to be present mainly in the forms of organic sulfides, disulfides, thiols and thiophenes. Two pathways have been proposed for microbial desulfurization of heterocycles. One emphasized the oxidation of carbon ring structures to polar teachable derivatives. The other stressed the sulfur-specific metabolism without degradation of the carbon skeleton. Both pathways have been demonstrated with model organosulfur compounds, especially dibenzothiophene (DBT). Reports on organic sulfur removal from coal and water-soluble coal-derived products by microorganisms are also available. Unfortunately, due to the difficulty in accurate analysis.of organic sulfur content, the technical feasibility of microbial organic sulfur removal from coal is still in dispute.

The costs of independent microbial depyritization and organic:sulfur removal were estimated to be $10–14 and $14 per ton of coal, respectively. A cost analysis for a tow-step process, which was designed to achieve complete removal of inorganic sulfur and a 40% reduction of organic sulfur, yieded $11 per ton of coal. These are lower than the costs of other techniques, e.g. $20–40 per ton of coal for flue-gas desulfurization.     

AN EXAMINATION OF ASTM PROCEDURES FOR DETERMINING SULFUR FORMS IN COAL

ABSTRACT

Several raw coals of various ranks were used to examine the effects of some of the variations in ASTM procedures on analytical results obtained for total sulfur and sulfur forms. ASTM procedures were also used to determine pyritic sulfur in two coals cleaned by selective agglomeration processes.In addition, organic sulfur values obtained by the indirect ASTM procedure were compared to those obtained directly by leaching coal with hot HNO3 followed by total sulfur determinations on the coal residue.

Differences in the ASTM leaching procedure with HNO3 did not significantly affect the pyritic sulfur results for any of the coals. However, small differences between the referee (leaching coal with HNO3) versus alternate (leaching coal ash with HCl) ASTM procedures for determining pyritic sulfur were observed.Neithei: procedure was consistently higher than the other.     

STUDY ON CHEMICAL STABILITY OF ORGANIC SULFUR IN COAL AND ITS STRUCTURE*

In this paper, two high-sulfur coals were selected from Yazhou and Donglin mines of China. Inorganic sulfur was completely removed first, then the tested samples were experimented under various conditions: hydrolysis, oxidation, reduction, and supercritical extraction with methanol. The change in organic sulfur was investigated. It was found that approximately 40-60% organic sulfur in coal was released at the temperature of 350°C. The removed organic sulfur was thioalcohol and thioether, while the remaining part in coal was thiophenic sulfur.     

TEXAS LIGNITE DESULFURIZATION

ABSTRACT

Texas lignites are a low-Btu, high-ash and moderately high-sulfur fuel that can be readily upgraded before combustion using physical and microbial desulfurization technology. Although physical and chemical processes and some microbial processes are effective in removing pyritic sulfur, they are ineffective for removing organic sulfur. Microbial processing of Texas lignite with sulfur content of up to 15 wt.% was investigated using laboratory shake-flask experiments. Organisms of the genus Thiobucillus ferrooxiduns, Leptospirillum Jerrooxidans and Pseudomonas putidawere investigated. It was found that up to 77% of the sulfur compounds present in the Texas lignite with more than 90% of the pyritic sulfur could be removed by the use of these microbes. Texas lignite samples were pulverized to pass through 149-μim sieve. Thiobucillus ferrooxiduns reduced up to 54% of the total sulfur and 78% of the pyritic sulfur in 14 days and Leptospirillum ferrooxidans removed up to 41% and 60%, respectively. A mixed culture of these two bacteria was also studied for sulfur removal The lignite was retreated with P. putidu which further removed 50% of the total sulfur in 7 days. P. puiidu could successfully remove up to 37% of the organic sulfur in 7 days.     

SELECTIVE REMOVAL OF ORGANIC SULFUR FROM COAL BY TRICHLOROETHANE EXTRACTION

ABSTRACT

Desulfurization of coal involves removal of both organic and inorganic forms of sulfur. In this paper, the process feasibility of organic desulfurization of coal using 1,1,1-trichloroethane (TCA) as the solvent, is established. Sulfur solubility curve in tnchloroethane was obtained. The process conditions of this novel process were critically assessed. From the solubility curve it was concluded that the process should be carried out at or near the normal boiling point of the solvent. Comparisons between the TCA extraction and the perchloroeihylene (PCE) process have been presented. Since the boiling point of tricbloroethane is quite low, the process conditions are very mild in comparision to the PCE process. It was found that the TCA process is capable of removing organosulfur from coal more selectively than the PCE process. The sulfur containing organic species in the extract obtained from     

WASHABILITY CURVES FOR PYRITE REMOVAL IN COAL USING PERCHLOROETHYLENE AS HEAVY MEDIUM

Abstract

Pyritic sulfur is removed from raw, high sulfur coal by gravitational separation using a suitable solvent, or heavy medium. This is possible due to the inherent difference in the specific gravity of clean coal and the mineral matter in it. The effectiveness of perchloroethylene (PCE) as a heavy medium was experimentally evaluated. The most important factors governing the efficiency of this process are the quantity of clean coal yield and depyriting efficiency. It was found that the pyritic sulfur removal efficiency as well as the clean coal yield depended strongly on its particle size distribution and mineral matter content. This paper presents valuable data on the effect of particle size of coal on clean coal yield as well as pyritic sulfur removal efficiency. A “master” curve is obtained to determine a workable size range which gives the most optimal yield of clean and depyrited coal.     

EFFECT OF FILTRATION TEMPERATURE ON ORGANIC SULFUR REMOVAL FROM COAL BY PERCHLOROETHYLENE COAL CLEANING PROCESS

Abstract

The perchloroethylene extraction desulfurization process removes the organic sulfur in coal via a hybrid mechanism of solvent extraction and chemical reaction. The nature and extent of the reaction is controlled by the extraction time and temperature of operation. Although the extraction temperature is kept identical for all types of coals (120°C), the organosulfur extraction time still depends upon the type of coal. If the reaction mixture is left too long in the extraction environment, the intermediate labile sulfur released by the reaction forms cross-links with the organic matter in the macromolecule of coal. This is detrimental to the process efficiency. Constant temperature has to be maintained throughout the extraction, till coal is separated from the solvent. If not, the extracted labile sulfur re-enters the coal macromolecule to form inter-penetrating polymer networks with the organic matter in coal. In this paper, it has been established that the time required for separation and isothermality of the process are crucial to maintain the reaction progressing toward sulfur and organic sulfur liberation from the macromolecule. The data presented in this paper are important from the viewpoint of process development, because the process mandates the separation of coal and solvent at the operating temperature.     

PERCHLOROETHYLENE EXTRACTION DESULFURIZATION OF LOW SULFATE COALS

ABSTRACT

The perchloroethylene coal refining process utilizes perchloroethylene (PCE) as its solvent in all phases of the precombustion desulfurization process, including wet grinding, organic sulfur removal, gravitational separation of pyrites and mineral matter, and recovery of elemental sulfur (S₈). The Process is capable of producing compliance coal which emits less than 1.2?lb SO_X/MBTU when burnt, starting from 5 mass percent sulfur Midwestern and Eastern U.S. coals. However, the process efficiency was found to be very strongly dependent upon the degree of weathering or the level of coal oxidation. In this paper, perchloroethylene extraction data of fresh, low-sulfate coals are summarized and critically assessed. The extraction efficiency of the organic sulfur removal ranged from 5 to 30 percent for fresh coals, while that for weathered coals ranged from 30 to 60 percent. This study provides a valuable insight into the chemical reaction mechanism of perchloroethylene desulfurization process.     

温和条件下煤中有机硫的脱除

在超声波辐射或热处理条件下,考察碘甲烷对煤中有机硫的脱除效果,对碘甲烷和甲醇在煤中有机硫的脱除过程中的协同作用也进行了考察。随着碘甲烷用量的增加和加热时间的延长,有机硫的脱除率也随之增加。用GC／MS对脱除下来的有机硫化合物进行了检测。     

PERCHL0R0ETHYLENE EXTRACTION DESULFURIZATION OF WEATHERED COALS

Abstract

The perchloroethylene extraction process has proven to be an effective pre- combustion coal desulfurization process which offers a complete process package including wet grinding, organic sulfur removal, pyrite and mineral matter separation, solvent recovery, and byproducts and sulfur recovery. In this paper, coal weatherability was investigated for various Midwestern and Eastern U.S. coals, and its effect on organosulfur extractability by the perchloroethylene process was identified. Both “natural” and “artificial” weathering of these coals were experimentally investigated. A statistically significant difference in the extraction efficiency between fresh and weathered coals vas observed. A strong relation between the extractability and degree of weathering of the coal was established. The results provide a valuable insight into the process engineering of this process.     

LIGAND ASSISTED DESULFURIZATION OF LIGNITE USING THE PERCHLOROETHYLENE COAL REFINING PROCESS

The perchloroethylene coal refining process was used on North Dakota lignite coal. Effects of extraction time, perchloroethylenexoal ratio and artificial weathering were studied to determine their effects on the removal of organic sulfur. The artificial weathering conditions used were found to be too harsh and caused a decrease in the amount of organic sulfur removed, unlike normal weathering conditions. Additional studies were done involving various ligands, and showed that the addition of certain ligands during the process aided in the extraction of organic sulfur from the coal.     

KINETICS OF PERCHLOROETHYLENE EXTRACTION DESULFURIZATION OF COALS

ABSTRACT

The perchloroethylene coal cleaning process selectively removes the organic sulfur from coal via a hybrid mechanism of chemical reaction and physical solvation It was found that the chemical reaction was catalyzed by the inorganic species present in the coal. In this paper, a kinetic study was experimentally carried out to determine rate constants of the reaction. It was confirmed that the extent of organosulfur extraction depended strongly on the type of coal, and also that there is a critical extraction time which is required as the minimum time for each type of coal. Isothermal batch kinetic studies were done for various types of coal. A relation was established between the type of coal and its kinetics and hence the minimumtime for extraction.     

SUPERCRITICAL EXTRACTION OF ORGANOSULFUR FROM COAL USING ACETONE-VATER MIXTURES

Abstract

Binary mixtures of acetone and water at their supercritical conditions have been investigated for their ability to remove organic sulfur from high sulfur Midwestern coals. The reduction of organic sulfur from Ohio 5/6 and Indiana 5 coals has been as high as 61% (based on a BTU basis) The experiments have been carried out following a statistical experimental design and the optimal process conditions and discerning characteristics of the process have been identified. The solvent composition and the extraction conditions can be tailored in such a way as to selectively remove sulfur and further increase the calorific value of the treated coal.     

EFFECT OF MOISTURE IN COAL ON ITS ORGANIC SULFUR EXTRACTABILITY

Abstract

The extent of organic sulfur removed by the perchloroethylene desulfurization process depends upon several factors including the type of coal, the amount of catalyst present in it, and the temperature of organosulfiir extraction. Moisture in coal also plays a very important role in this extraction process. In this paper, the role played by moisture and its subsequent effect on the process efficiency has been investigated. It has been found that the moisture in coal affects the extraction process in two ways. Firstly, in presence of water, the temperature of the operation is reduced. This affects the organosulfiir extraction efficiency adversely. Secondly, the naturally available catalytic ingredients in coal, essential for the organosulfiir extraction, are soluble in water. Therefore, in presence of water, the catalytic potency of these catalytic species is lost, and thus reducing the organosulfiir extractability. The data presented in this paper are also important from the point of view of process development, because it has been experimentally established that the moisture content in coal has to be sufficiently reduced in order to improve the overall process efficiency.     

ENVIRONMENTAL AND ECONOMIC ISSUES IN THE USE OF PERCHLOROETHYLENE AS THE SOLVENT IN PERCHLOROETHYLENE COAL CLEANING PROCESS

ABSTRACT

The Perchloroethylene coal cleaning process effectively removes both organic and inorganic forms of sulfur in coal. Complete recyclability of the solvent is the key to both economic and environmental issues concerning the use of perchloroethylene in this process. Recyclability of the solvent has been fully established by repeated batch operation as well as FTIR structural investigations of solvent molecules. In the current investigation, the solvent has been subjected to various analyses after each stage in the operation i.e., before extraction, after extraction and after distillation. The organic desulfurization is based on an extraction-reaction mechanism and is catalyzed by the mineral matter inherently and naturally present in coal. This paper also aims at studying the role played by the solvent (perchloroethylene) in the extraction process as well as in the catalytic reaction occurring in the system. This paper also presents data on the effect of re-using ‘sulfur-rich mother liquor’, rich in extracted sulfur, on the organosulfur extraction efficiency. These data are very important from the point of view of process engineering and economics.