C.I tyuiA Sv*xa*j^j STC7DV OF SULFUR BEHAVIOR AND REMOVAL DURING THERMAL DESULFURIZATION OF ILLINOIS COALS Keith C. Hackley • Robert R. Frost Chao-Li Liu • Steven J. Hawk • Dennis D. Coleman CIRCULAR 545 1990 Department of Energy and Natural Resources ILLINOIS STATE GEOLOGICAL SURVEY ILLINOIS GEOLOG'CAIE SURVEY LIBR MAR 5 m LIBRARY. Cover photo: ISGS scientist working with pyrolysis system connected to quadrupole gas analyzer. ILLINOIS STATEGEOLOGICM |jj| 3 3051 00002 8518 Printed by authority of the State of Illinois/ 1990/1200. STUDY OF SULFUR BEHAVIOR AND REMOVAL DURING THERMAL DESULFURIZATION OF ILLINOIS COALS Keith C. Hackley • Robert R. Frost Chao-Li Liu • Steven J. Hawk • Dennis D. Coleman ILLINOIS GEOLOGICAL SURVEY LICRAXY HAR 5 m CIRCULAR 545 1990 ILLINOIS STATE GEOLOGICAL SURVEY Morris W. Leighton, Chief 615 East Peabody Drive Champaign, Illinois 61820 Digitized by the Internet Archive in 2012 with funding from University of Illinois Urbana-Champaign http://archive.org/details/studyofsulfurbeh545hack CONTENTS ABSTRACT v INTRODUCTION 1 EXPERIMENTAL PROCEDURES 1 Apparatus for Pyrolysis and Post-Pyrolysis Treatment 1 Monitoring Techniques 1 Coal Samples 3 RESULTS AND DISCUSSION 3 Pyrolysis under N 2 : Behavior of Organic and Pyritic Sulfur 3 Effect of Various Parameters on Sulfur Removal 5 Pyrolysis with a Trace of Oxygen 9 Post-Pyrolysis Desulfurization 13 Partial oxidation 13 Hydrodesulfurization 14 Combined Treatments for Thermal Desulfurization 17 Analytical and mineralogical results 17 Relative rates of sulfur removal 21 CONCLUSIONS AND RECOMMENDATIONS 22 ACKNOWLEDGMENTS 24 REFERENCES 25 APPENDIX 26 Apparatus 26 Experimental Methods and Techniques 28 FIGURES 1 Organic and pyritic sulfur removal as a function of pyrolysis temperature for three coals 4 2 Effect of heating rate on sulfur evolution during pyrolysis of - 20 + 35 mesh particles of IBC-101 9 3 Rate of sulfur evolution during charring experiments PH34, PH41, and PH35 9 4 Magnetic susceptibility measurements of chars prepared at various tem- peratures under pure N 2 and a 0.1 % 2 /N 2 mixture for coal sample RK-B-5 11 5 Proportions of original organic and pyritic sulfur remaining after charring and partial oxidation 13 6 Quadrupole gas analyzer data collected during pyrolysis of IBC-101 14 7 QG A data comparing the evolution of S0 2 and C0 2 during char oxidation at 455°C of IBC-101, experiment QMS8 14 8 Proportions of original organic and "pyritic" sulfur remaining in treated chars of RK-B-3 coal samples 15 9 Sulfur removal by charring at 750°C for 5 minutes and hydrotreatment at 800°C for 15 and 60 minutes 16 10 Removal of organic and "pyritic" sulfur from char by hydrodesulfurization at 800°C for different lengths of time 16 11 800°C hydrodesulfurization rate data for experiments QMS6, QMS7, and QMS9 prepared for IBC-101 17 12 Various combinations of the thermal desulfurization treatments investigated 18 13 Sulfur and carbon evolution versus pyrolysis temperature for four different coals 20 14 Sulfur evolution versus time for hydrodesulfurization at 800°C of three CR-B-1 chars 21 15 Sulfur evolution versus time for hydrodesulfurization at 800°C of three RK-B-4 chars 22 16 Sulfur evolution versus time for hydrodesulfurization at 800°C of three RK-B-5 chars 22 17 Sulfur evolution versus time for hydrodesulfurization at 800°C of three IBC-101 chars 23 18 QGAdata comparing the evolution of S0 2 and C0 2 during oxidation at 455°C of H 2 + 0.44% H 2 S-treated chars 23 A1 First coal pyrolysis apparatus 26 A2 Second coal pyrolysis-chardesulfurization apparatus 27 A3 pH monitoring system 28 A4 Quadrupole gas analyzer monitoring system 29 A5 Flash pyrolysis apparatus 29 A6 Isotopic mixing relationship of pyritic and organic sulfur in coal sample R-B-3 31 TABLES 1 Chemical analysis, sulfur isotopic composition, and mass balance cal- culations of Herrin (No. 6) Coal samples used in pyrolysis experiments 2 2 Amount and origin of sulfur removed by pyrolysis of three Illinois coal samples 3 3 Amount and origin of sulfur in the volatile gases of the stepwise pyrolysis of RK-B-3 4 4 Chemical analysis and isotopic composition of float and sink coal fractions of RK-B-3 samples 4 5 Results of 650°C pyrolyses of samples of RK-B-3 containing significantly different pyrite concentrations 5 6 Chemical analyses of two particle sizes of Illinois Basin Coal Sample Program IBC-101 and IBC-103 5 7 Sulfur removal by pyrolysis of IBC-101 6 8 Sulfur removal by pyrolysis of IBC-103 7 9 Effect of particle size, maximum pyrolysis temperature, and soak time on total sulfur removal and sulfide mineral content for IBC-101 8 10 Effect of heating rate and particle size on char pore structure (IBC-101 and IBC-103) 8 11 Sulfide and iron oxide mineral content of chars produced from coals pyro- lyzed 18 minutes at 550°C with various amounts of trace oxygen 10 12 Sulfide and iron oxide mineralogy of RK-B-5 chars produced by heating to various temperatures under pure nitrogen 11 13 Sulfide and iron oxide mineralogy of RK-B-5 chars treated with 0.1 percent oxygen after pure nitrogen treatment and preoxidation treatment 12 14 Pyrolysis and post-pyrolysis partial oxidation experiments on sink and float coal fractions 12 15 Post-pyrolysis oxidation results for IBC-101 13 16 Post-pyrolysis oxidation results for IBC-103 14 17 Distribution of sulfur forms in hydrogen-treated chars after acid leaching and partial oxidation 15 18 Pyrolysis and hydrodesulfurization results of coal sample RK-B-3 16 19 Separate pyrolysis and post-pyrolysis desulfurization experiments for IBC-101 17 20 Chemical analyses of the four coals used in the combined gas-phase thermal treatment tests 18 21 Sulfur concentration in chars and total char yields after combined thermal desulfurization treatments 19 22 Sulfide and associated iron minerals in chars after thermal desulfurization treatments 19 ABSTRACT In this 3-year program, we investigated the thermal desulfurization of Illinois high-sulfur coal for the ultimate purpose of producing clean low-sulfur solid fuel. The two principal objectives were to optimize the conditions for sulfur removal and to analyze the behavior of sulfur during gas-phase desulfurization. We developed several unique methods to monitor the sulfur mobility during thermal desulfurization: stable sulfur isotope tracing (used to follow the types of sulfur during desulfurization), pH monitoring, and quadrupole mass spectrometer gas analysis (used to measure the relative rates of sulfur removal). The thermal desulfurization processes studied were pyrolysis and post-pyrolysis treatments — most pyrolyses were carried out at 350° to 750°C under an atmosphere of nitrogen. From studying the behavior of the predominant forms of sulfur during pyrolysis, we determined that below 500°C the sulfur removed is almost entirely organic. Pyritic sulfur is not removed in any significant quantities until approximately 550°C and above. Other experiments show that pyrolyses carried out at or above 550°C produce the lowest sulfur content chars in the shortest time. During these studies a process was developed that is capable of converting nonmagnetic and weakly magnetic iron sulfides — the sulfides normally produced during pyrolysis — into a strongly magnetic iron sulfide that can potentially be removed by magnetic separation. The post-pyrolysis desulfurization processes studied include partial oxidation and hydrodesulfurization. Post-pyrolysis oxidation experiments showed that partial oxidation must be carried out below 550°C to produce maximum sulfur removal with minimum carbon loss. Various hydrodesulfurization treatments were also studied to determine their effects on the rate and amount of sulfur removal from char. From coals that originally had 4 to 6 percent sulfur, hydrodesulfurization experiments using pure hy- drogen produced low levels of sulfur — low enough to qualify as a compliance fuel. The effect of a small amount of hydrogen sulfide (H 2 S) in the hydrogen flow was also investigated to simulate the effect of excess H 2 S that occurs in large-scale systems during hydrodesulfurization in the absence of an H 2 S scavenger. Post-hydrodesulfuriza- tion oxidation helped to overcome the problem of back reactions of H 2 S with iron in the chars that occurred during hydrogen treatment. The final phase of this research combined pyrolysis and post-pyrolysis desulfurization techniques to show the potential of gas-phase thermal desulfurization. Results indicate that pyrolysis with a trace of oxygen followed by magnetic separation of iron sulfides may be a process capable of producing relatively low-sulfur chars from coals with low organic sulfur and relatively high pyritic sulfur. For most Illinois coals with moderate to high organic sulfur, some type of post-pyrolysis treatment such as hydrodesulfurization will probably be necessary because organic sulfur is so difficult to remove. A promising desulfurization treatment is hydrodesulfurization plus post-hydrodesulfurization oxida- tion with 5 percent oxygen or with a trace of oxygen followed by magnetic separation. INTRODUCTION Much of the coal in the Illinois Basin is high-sulfur coal, greater than 3 percent total sulfur, which limits its use as a fuel source. A substantial portion of the coal desul- furization research at the Illinois State Geological Sur- vey has been aimed at producing a clean low-sulfur solid fuel from Illinois high-sulfur coal. In this project our research efforts concentrated on pyrolysis and post- pyrolysis desulfurization processes. Our principal objec- tives were to optimize conditions for maximum sulfur removal and to better understand the behavior of sulfur forms during pyrolysis and post-pyrolysis desulfuriza- tion. To achieve these objectives, we developed several unique techniques to monitor the mobility of sulfur dur- ing the thermal desulfurization processes studied: sta- ble sulfur isotope tracing (to follow the types of sulfur during desulfurization), pH monitoring, and quadrupole mass spectrometer gas analysis (to measure the rela- tive rates of sulfur removal). Pyrolysis in an inert atmosphere removes a portion of organic and pyritic sulfur with the volatile gases (Kruse and Shimp, 1981). Elemental sulfur resulting from the decomposition of pyrite as well as hydrogen sulfide from any source can react with the organic matrix of the coal and remain in the char as organically bound sulfur (Given and Jones, 1966; Cleyle et al., 1984). Since the initial forms of sulfur undergo various reac- tions and can be converted to other forms during ther- mal treatment, the standard ASTM procedures for deter- mining the forms of sulfur in coal may not be reliable when applied to chars. In this project, we used naturally occurring differences in stable sulfur isotope compositions of pyritic and or- ganic sulfur in coals to monitor the mobility of these two major forms of sulfur during thermal desulfurization treatment. The isotopic composition of either the sulfur in the volatiles or that remaining in the char will give the proportion of organic and pyritic sulfur removed or remaining, no matter what new chemical form each type of sulfur has taken. We also looked into the effects that different parameters — particle size, heating rate, soak time, and maximum pyrolysis temperature — have on the rate and amount of sulfur removed from coal. After pyrolysis a significant amount of sulfur usually remains in the char, and many researchers have found that high-sulfur coals yield high-sulfur chars — thus further post-pyrolysis desulfurization treatments are usually considered necessary. We studied the removal of sulfur from char by partial oxidation and hydrodesul- furization because these gas-phase desulfurization treatments do not require subsequent washing, filtering, or dewatering steps. A review of recent literature on oxidation and hydrodesulfurization is given by Stephen- son et al. (1985). Generally, oxidation and hydrodesulfurization can sig- nificantly reduce the sulfur content of coals or chars. However, oxidation sometimes results in excessive car- bon loss with only minor sulfur removal, depending on the oxygen concentration and temperature used. Hy- drogen seems to be the most effective gas for desulfuri- zation. Fleming, Smith, and Aquiro (1977) showed, on the laboratory scale, that hydrodesulfurization of char can produce a solid fuel product with a sulfur content lower than the EPA direct combustion standard of 1.2 lb S0 2 /MMBtu. However, hydrogen sulfide can react back with the iron formed from the reduction of iron sulfides in the char if the H 2 S partial pressure becomes too high (Kor, 1977), resulting in little reduction in the inorganic sulfide sulfur content of the final char. We attempted to combine pyrolysis and post-pyrolysis pro- cesses in complementary ways to help minimize some major problems that can occur and to show the potential of gas-phase thermal desulfurization. EXPERIMENTAL PROCEDURES Detailed descriptions of the experimental apparatus and procedures are given in the appendix. Apparatus for Pyrolysis and Post-Pyrolysis Treatment Pyrolyses were carried out on 0.5- to 1-g size samples spread thinly in a quartz or ceramic boat. The experi- mental setup for the thin-bed pyrolysis system is very similar to that described in Frost, Auteri, and Ruch (1984) and Ruch, Chaven, and Kruse (1985). The pyrolysis apparatus consists of a bench-scale quartz tube reactor with two consecutive chambers. The first chamber is used for pyrolysis of coal samples at various temperatures under a nitrogen atmosphere, the second for combusting the volatile products to C0 2 and S0 2 with oxygen at 900°C. The sulfur dioxide formed by oxidation of the sulfur or sulfur compounds released during pyrolysis is trapped by hydrogen peroxide solu- tion and then quantitatively measured as BaS0 4 . The same pyrolysis apparatus was used for partial oxidation experiments. The N 2 flow was diluted with 4 to 5 percent 2 by volume for post-pyrolysis oxidations. A similar bench-scale system was set up for hydro- treating the chars at 800°C. The sulfur released as H 2 S during hydrodesulfurization was trapped and precipi- tated as CdS and converted to Ag 2 S using a dilute Ag 2 N0 3 solution. The Ag 2 S was then weighed to calcu- late the quantity of sulfur removed. The maximum heating rate that could be achieved with the tube-type pyrolysis/oxidation system in the 400° to 550°C temperature range, at which 90+ percent of coal devolatilization occurs, was about 60°C/min. There- fore, a special pyrolysis/oxidation system was con- structed to carry out pyrolyses with high heating rates. With the new system, heating rates of 100°C + /min in the 400° to 550°C temperature range could be obtained. Monitoring Techniques Stable sulfur isotope analysis was used to monitor the behavior of organic sulfur and pyritic sulfur individually during thermal treatment of coal. Coals with a natural difference between the 34 S/ 32 S ratios of the pyritic and organic sulfur were used for this work. If the isotopic composition of the pyritic and organic sulfur in a coal SULFUR BEHAVIOR DURING THERMAL DESULFURIZATION 1 is known, the relative proportions of pyritic and organic sulfur removed during a particular desulfurization proce- dure can be calculated by measuring the isotopic com- position of either the sulfur removed or the sulfur remain- ing in desulfurized coal or char. The use of stable sulfur isotopes as a tracer in desulfurization studies is also described in Liu, Hackley, and Coleman (1987). For the initial isotopic characterization of the predom- inant sulfur forms, the pyritic and organic sulfur were chemically separated from a coal sample. Pyritic sulfur was extracted by the reductive lithium aluminum hydride (LAH) method (Price and Shieh, 1979; Westgate and Anderson, 1982). The organic sulfur was collected by combusting the LAH-extracted coal in pure oxygen at 1350°C (modified ASTM D1377-82 procedures, Frost, Auteri, and Ruch, 1984). Both forms of sulfur were converted to S0 2 and analyzed on an isotope ratio mass spectrometer. The appendix provides details of the procedures, including the description of a quick screening method developed for locating isotopically appropriate coals and an explanation of stable sulfur isotope notation. For stable isotope monitoring during thermal desul- furization experiments, the sulfur liberated and the sul- fur remaining in the treated chars were quantitatively collected, converted to S0 2 , and analyzed isotopically on a Nuclide RMS 6-60 isotope ratio mass spectrome- ter. The relative proportions of organic and pyritic sulfur in the desulfurized products were then calculated. The excellent chemical and isotopic mass balances achieved on three different Illinois Herrin (No. 6) Coal samples prove the reliability of this novel stable isotope tracing method (table 1). Methods were also developed for continuous monitor- ing of the sulfur removed during pyrolysis and post- pyrolysis oxidation of coals. One method continuously measures the pH of a hydrogen peroxide solution used to trap the S0 2 produced in the bench-scale pyrolysis/ oxidation system. Because the solution pH is a direct function of the total amount of sulfur collected, a plot can be made showing the amount of sulfur evolved from the coal as a function of time. Differentiation of this curve provides information on sulfur removal rates. For more accurate rate data, a quadrupole gas analyzer (QGA) was used during the later experiments to monitor the evolution of both sulfur and carbon during pyrolysis and post-pyrolysis desulfurization experi- ments. Also, other gaseous species present during the experiments can be monitored with the QGA. The QGA and temperature controller for the pyrolysis/desulfuriza- tion tube furnace are interfaced with an IBM PC compu- ter, which collects gas composition and temperature data every 6 seconds during an experiment. Rate data can be calculated from the QGA data. Standard X-ray diffraction methods were used to de- termine the changes in iron-sulfur mineral content and structure that occurred when a coal was pyrolyzed or treated by post-pyrolysis desulfurization. Pyrrhotite crystal structure and stoichiometry were determined by comparing the sample X-ray diffraction patterns with X-ray patterns prepared from standard pyrrhotite sam- ples. The position of the major pyrrhotite X-ray diffrac- tion peaks, located between 43.2° and 44° 26, was used to monitor the changes in pyrrhotite composition; the lo- cation of this major X-ray peak depends on the pyrrho- tite crystal structure and stoichiometry (Smith et al., 1984). Estimates of the relative order of abundance of min- erals present after charring were made using the relative X-ray peak intensities. Although this method is not quan- titative, it generally gives a fairly good estimate of the relative abundance of minerals present (Arnold, 1966; Brindley, 1980; Hughes, personal communication). No attempt was made to quantify the percentage of each sulfide or iron-related mineral present because of the many variables that control these peak intensities, such as sample purity, structural factors, particle size, and crystallinity X-ray data were integrated with the other data collected in this study so that the reactions involved in coal desulfurization would be better understood. Table 1 Chemical analysis, sulfur isotopic composition, and mass balance calculations of Herrin (No. 6) Coal samples used in pyrolysis experiments Coal sample RK-B-3 RK-A-4 CR-B-4 Chemical analysis (%) Moisture 9.0 9.6 11.5 Volatile material 41.5 37.5 46.1 Fixed carbon 49.2 41.9 46.6 High-temp ash 9.3 20.6 7.2 Sulfate sulfur 0.01 0.01 0.01 Pyritic sulfur 0.73 0.74 0.33 Organic sulfur 2.33 2.02 3.03 Total sulfur 3.07 2.77 3.37 Isotopic composition of sulfur (' >/oo) Pyritic sulfur + 25.4 + 8.2 + 8.5 Organic sulfur -2.9 -5.1 -5.7 Total sulfur + 3.9 -1.4 -4.2 Isotopic mass balance (%o) Calculated values (total sulfur) + 3.8 1.5 4.3 *Mass balance equation: 8 St S - 8 S TS Xp^S, ) + Xor^So,) Xp Xor Xts Xts isotopic composition of total sulfur isotopic composition and percent pyritic sulfur isotopic composition and percent organic sulfur percent total sulfur ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 545 Coal Samples Bituminous Illinois Herrin (No. 6) Coal was used for this study. Many samples were collected by hand at freshly cut coal faces in underground mines. Chemical and isotopic analyses were performed as quickly as possi- ble. Three coal samples, RK-B-3, RK-A-4, and CR-B-4, were selected for further study because of the large difference between the stable sulfur isotopic composi- tion of the pyritic and organic sulfur. Another sample, RK-B-5, was used in some experiments because of its high pyrite content. In addition to the samples collected by hand, two Illinois Basin Coal Sample Program coals (IBC-101 and IBC-103) were also used in much of this project. RESULTS AND DISCUSSION Pyrolysis under N 2 : Behavior of Organic and Pyritic Sulfur The most abundant forms of sulfur in coal are organic and pyritic sulfur. Sulfur can also occur in coal to a lesser degree as sulfate, elemental sulfur, and other sulfide minerals. Some FeS 2 in Illinois coals exists as the polymorph marcasite, but for this paper all FeS 2 will be referred to as pyrite. The chemical and isotopic analyses of three Herrin Coal samples used in the initial pyrolysis studies on the behavior of pyritic and organic sulfur are shown in table 1. Pyrolyses were carried out at temperatures ranging from 350° to 750°C. The isotopic results shown in table 2 indicate that most of the sulfur removed with the volatiles is organic. Pyritic sulfur is not observed in the volatilized gases until 550°C and above. To study the release of organic and pyritic sulfur in more detail, a stepwise pyrolysis experiment was con- ducted on coal sample RK-B-3. The volatile sulfur was collected consecutively at three different temperature intervals (25°-350°C, 350°-500°C, and 500°-650°C); re- sults are shown in table 3. Note the change in the isotopic value of the sulfur volatilized between 500° and 650°C compared with the two intervals below 500°C. This finding indicates that the pyritic sulfur does not occur in the volatile gases (in any significant quantity) until above 500°C, supporting the initial pyrolysis data. Figure 1 displays the type of sulfur removed during pyrolysis from all three coals (RK-B-3, RK-A-4, and CR- B-4). Most of the removable organic sulfur is released at pyrolysis temperatures below approximately 550°C (fig. 1). Higher charring temperatures result primarily in re- moval of pyritic sulfur and additional volatile matter. These results suggest that relatively low-temperature charring should be used to remove organic sulfur, and less destructive procedures such as physical separation methods should be applied to remove pyritic sulfur. The greatest variation in the data occurs in the per- centage of pyritic sulfur released at the higher temper- atures. The differences in pyritic sulfur removed are probably a result of the different proportions of dissemi- nated (finely dispersed) and massive pyrite present in the coals. Sample RK-B-3 had a greater percentage of pyritic sulfur removed (28%) than did RK-A-4 and contained more massive pyrite. The third coal sample, CR-B-4, contained the least amount of pyrite (0.33%) and lost the lowest percentage of pyritic sulfur at 650°C. Furthermore, microscopic inspection showed that nearly all the pyrite in CR-B-4 is disseminated. Even though the pyrite contents of the samples were significantly different, similar percentages of organic sul- fur were removed from each of the three coals. The Table 2 Amount and origin of sulfur removed by pyrolysis of three Illinois coal samples Coal Charring temp (°C) Total sulfur removed (%) of volatile sulfur (%o) of char sulfur (%o) Origin of sulfur in volatiles (%) Total sulfur sample Organic Pyritic Ivlul JUIIUI recovered (%) RK-B-3 450 40 -2.9 — 100 — 550 53 -0.9 + 7.7 93 7 100.7 650 58.5 + 0.0 + 7.5 90 10 102.9 650 58 + 0.0 + 7.5 90 10 101.2 750 60 + 0.1 + 7.5 89 11 102.6 RK-A-4 350 13 -5.1 — 100 450 39 -5.1 + 1.5 100 97.4 550 53 -4.1 + 1.1 93 7 91.0 650 58 -3.6 + 0.8 89 11 94.9 CR-B-4 350 18 -5.7 — 100 101.8 450 46 -5.7 -3.5 100 98.8 550 59 -5.3 -3.1 97 3 101.2 650 63 -5.3 -2.9 97 3 100.1 SULFUR BEHAVIOR DURING THERMAL DESULFURIZATION Table 3 Amount and origin of sulfur in the volatile gases of the stepwise pyrolysis of RK-B-3 (& 34 S P y,mc= +25.4, S 34 S oiganle = -2.9) Temp interval Ramp time (min) Soak time (min) Sulfur removed (%o) S^Sof volatile sulfur (%o) Origin of sulfur involatiles% Sulfur col Organic ected % (°C) Organic Pyritic Pyritic 25-350 10 15 14.2 -2.7 >99 <1 18 <1 350-500 9 15 31.0 -2.5 98 2 39 2 500-650 10 15 10.0 + 10.2 54 46 7 22 implication here is that pyrite content does not affect the percentage of organic sulfur released during pyrolysis. Additional pyrolysis experiments were con- ducted to assess the effect of pyrite content. A sample of RK-B-3 was pulverized to less than 230 mesh (<63 u-m). A split of the pulverized sample was subjected to lithium aluminum hydride extraction to remove pyrite. Another split of RK-B-3 was subjected to a 1.4-specific gravity float-sink separation. Table 4 shows isotopic compositions of the pyritic and organic sulfur in the float and sink fractions, and table 5 shows the results of 200 400 600 Pyrolysis temperature (°C) 800 Figure 1 Organic and pyritic sulfur removal as a function of pyrolysis temperature for three coals 650°C pyrolyses of these samples. Note that the isotopic composition of the volatilized sulfur from the pyrite-free sample is identical to the isotopic composi- tion of the sulfur remaining in the char of that sample; both are indicative of the original total organic sulfur. The percentage of organic sulfur removed by pyrolyzing the pyrite-free sample is essentially identical to the per- centage removed when the pyrite was present. In addi- tion, the percentage of organic sulfur removed from the float and sink samples are virtually the same. These findings confirm that the presence of pyrite has no sig- nificant effect on the removal of organic sulfur under the conditions used in this study. Some authors suggest that sulfur forms in coal may be redistributed during pyrolysis (Cernic-Simic, 1962; Cleyle et al., 1984). Several experiments were con- ducted to determine if any redistribution of the sulfur forms could be detected by the inherent stable isotope tracing technique at 500°C and above. The coal with the largest isotopic difference between the pyritic and organic sulfur was pyrolyzed at 500°, 550°, and 650°C. The pyrolysis products were then pulverized to less than 230 mesh (63 p.m), and the inorganic sulfur was chemically removed and collected. The remaining sulfur was extracted from the chemically treated char by the ASTM high-temperature combustion method. The isotopic compositions of each fraction of sulfur removed were measured to determine the proportions of organic and "pyritic"* sulfur present. The isotopic results indicate that a portion of the pyritic sulfur does get trapped by the organic matrix of the coal during pyrolysis. Approxi- Table 4 Chemical analysis and isotopic composition of float and sink coal fractions of RK-B-3 samples (moisture-free basis) Chemical Float (%) Sink (%) 5S (%o) analysis Float Sink Volatile matter 41.3 31.3 Fixed carbon 50.6 44.2 High-temp ash 8.0 24.5 Sulfate sulfur 0.01 0.01 Pyritic sulfur 0.59 2.11 + 25.9 +21.6 Organic sulfur 2.55 1.76 -2.9 -2.9 Total sulfur 3.15 3.88 + 3.9 +10.4 "The term "pyritic" refers to the inorganic sulfide sulfur removed or remaining in a char even though pyrite begins to alter to pyrrhotite at approximately 500°C. "Pyritic" is used because much of the discussion is about the mobility of the two major forms of sulfur in coal (organic and pyritic) during thermal desulfurization. Distinctions are made between pyrite and pyrrhotite when appropriate. ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 545 Table 5 Results of 650°C pyrolyses of samples of RK-B-3 containing significantly different pyrite concentrations ("pyrite free" and "normal coal" are <230 mesh) Sample description Pyritic sulfur in Sulfur Organic sulfur volatile char Origin of sulfur in volatiles (%) coal (%) removed (%) removed (%) sulfur sulfur Organic Pyritic Pyrite free 67.4 67 -2.9 -2.9 100 Normal coal 0.7 57.7 67 + 0.4 + 8.4 88 12 Sink 2.1 47.6 68 + 5.7 + 13.0 65 35 Sink(dup) 2.1 48.6 69 + 5.7 + 13.3 65 35 Float 0.6 57.1 67 -1.7 + 6.3 96 4 mately 6, 9, and 12 percent of the originally pyritic sulfur was incorporated into the organic matrix at pyrolysis temperatures of 500°, 550°, and 650°C, respectively. (For these results, we assume that the chemical extrac- tion procedure used to dissolve the "pyritic" sulfur is approximately 95% efficient as determined by micro- scopic and Moessbauer analysis of extracted material.) Effect of Various Parameters on Sulfur Removal Considerable effort was directed toward determining the effect of process conditions — maximum tempera- ture, heating time (soak time), heating rate, particle size, and coal types — on sulfur removal during pyrolysis of coals. The rate and amount of sulfur evolved during pyrolysis were determined by the pH monitoring method, and the total sulfur content of the chars was determined by a modified ASTM D1377-82 method. About 90 pyrolysis tests were made using Illinois Basin Coal Sample Program coals (samples IBC-101 and IBC-103). Besides the very high ('Hash") heating rate (100°C + /min), heating rates of about 20°, 40°, and 60 c C/min were used. The sulfide mineral content was determined by X-ray diffraction for the flash pyrolyses experiments and for pyrolyses that were heated at ap- proximately 20°C/min. The porosity characteristics of selected nonf lash and flash chars were determined from surface area measurements. The proximate analysis and sulfur forms from IBC-101 and IBC-103 are given in table 6. The results of the various pyrolysis experiments are summarized in table 7 for IBC-101 and in table 8 for IBC-103; these data indicate that maximum pyrolysis temperature is the most important factor in determining the amount of sulfur evolved during pyrolysis. The impor- tance of soak time is dependent upon the maximum charring temperature. As indicated by the char yield data, at 500°C maximum pyrolysis temperature, de- volatilization of the coal is incomplete during heat up. However, at 600° and 700°C maximum pyrolysis tem- perature, devolatilization of the coal is virtually com- pleted during heat up so that the soak time has minimal effect upon char sulfur content. The effect that soak time has at a pyrolysis temperature of 600°C upon char sulfur content is a result of incomplete thermal decom- position of pyrite to pyrrhotite as shown by the X-ray Table 6 Chemical analyses of two particle sizes of Illinois Basin Coal Sample Program IBC- 101 and IBC- 103 (moisture- free basis) Chemical IBC-101 IBC-103 analysis (%) -20 + 35 -65 + 100 -20 + 35 -65 + 100 Volatile matter 43.3 43.7 37.4 39.0 Fixed carbon 46.9 46.7 53.7 53.1 High-temp ash 9.7 9.6 8.9 7.9 Sulfate sulfur 0.184 0.204 0.115 0.104 Pyritic sulfur 1.08 1.04 0.94 0.92 Organic sulfur 2.90 3.04 1.33 1.32 Total sulfur 4.17 4.29 2.39 2.35 diffraction data for pyrolyses conducted with IBC-101 (table 9). Heating rate does not have a significant effect upon char sulfur content (tables 7 and 8). But heating rate does have a significant effect on the rate of sulfur evolu- tion during pyrolysis (fig. 2). And as the heating rate increases, the temperature at which the maximum rate of sulfur evolution occurs also increases (fig. 2). Figure 3 shows a selected series of plots of sulfur evolution rates and the effects of different maximum temperatures. In figure 3, experiment PH35, the coal was heated to 500°C and held at that temperature; for experiments PH34 and PH41, the coal was heated to 600° and 700°C, respectively. Note that in all three cases the maximum sulfur removal rate occurs at about 500°C. This peak probably results from the removal of organic sulfur; isotope monitoring indicates that most of the removable organic sulfur is released by the time a temp- erature of 500°C is reached. The identification of this peak as organic sulfur is further substantiated by the X-ray data (table 9), which show that only a small frac- tion of the pyrite has been converted to pyrrhotite at this temperature. On the basis of stable isotope data discussed earlier and X-ray diffraction data of these SULFUR BEHAVIOR DURING THERMAL DESULFURIZATION Table 7 Sulfur removal by pyrolysis of IBC- 101 Heating Pyrolysis Char Char sulfur Coal sulfur rate Mesh temp(°C), yield content evolved Experiment (°C/min) size time(min) (%) (%) (%) PH81 20 -20 + 35 500,0 75.6 3.29 40.3 PH100 20 -65+100 500,0 72.7 3.24 45.1 PH97 20 -20 + 35 500,18 70.4 3.01 49.2 PH62 20 -65 + 100 500,18 67.6 3.15 50.4 PH70 20 -20 + 35 600,0 68.8 2.82 53.5 PH52 20 -65+100 600,0 66.5 2.99 53.7 PH69 20 -20 + 35 600,18 66.1 2.86 54.7 PH58 20 -65 + 100 600,18 65.0 2.92 55.8 PH46 20 -20 + 35 700,0 65.6 2.75 56.7 PH88 20 -65 + 100 700,0 64.8 2.86 56.8 PH66 20 -20 + 35 700,18 63.5 2.87 56.3 PH83 20 -65+100 700,18 63.4 2.94 56.6 PH130 40 20 + 35 550,18 67.5 2.80 54.7 PH82 40 -20 + 35 600,0 68.5 3.24 46.8 PH54 40 -65 + 100 600,0 66.4 2.95 54.3 PH61 40 -20 + 35 600,18 66.2 2.86 54.6 PH64 40 -65 + 100 600,18 64.2 3.06 54.2 PH80 60 -20 + 35 500,18 69.4 3.13 47.9 PH98 60 -65 + 100 500,18 68.4 3.18 49.3 PH57 60 -20 + 35 600,0 67.8 2.81 54.3 PH94 60 -65 + 100 600,0 67.1 3.12 51.2 PH79 60 -20 + 35 600,18 66.1 2.81 55.5 PH63 60 -65 + 100 600,18 64.0 2.87 57.2 PH96 60 -20 + 35 700,0 65.1 2.65 58.6 PH89 60 -65+100 700,0 64.1 2.88 57.0 PH67 60 -20 + 35 700,18 63.1 2.77 58.1 PH65 60 -65 + 100 700,18 62.1 2.82 59.2 1-5 200 + -20 + 35 500,0 78.1 3.46 32.5 1-6 200 + -65+100 500,0 80.0 3.64 32.2 1-12 200 + -20 + 35 500,30 69.3 3.02 49.8 1-13 200 + -65 + 100 500,30 69.8 3.21 49.7 1-2 200 + -20 + 35 600,0 67.6 3.02 51.1 1-4 200 + -65+100 600,0 67.4 3.25 48.9 1-14 200 + -20 + 35 600,10 64.9 2.98 53.6 1-15 200 + -65 + 100 600,10 64.0 2.95 56.0 1-7 200 + -20 + 35 600,30 638 2.78 57.5 1-8 200 + -65+100 600,30 63.3 2.94 56.6 1-1 200 + -20 + 35 700,0 63.6 2.77 57.8 1-3 200 + -65 + 100 700,0 63.0 2.89 57.6 1-9 200 + -20 + 35 700,30 61.0 2.83 58.6 1-10 200 + -65 + 100 700,30 60.8 2.93 58.4 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 545 Table 8 Sulfur removal by pyrolysis of IBC- 103 Heating Pyrolysis Char Char sulfur Coal sulfur rate Mesh tempfC), yield content evolved Experiment (°C/min) size time(min) (%) (%) (%) PH101 20 -20 + 35 500,0 78.2 1.83 40.1 PH73 20 -65 + 100 500,0 78.0 1.99 34.0 PH84 20 -20 + 35 500,18 72.6 1.78 45.9 PH91 20 -65+100 500,18 72.4 1.95 39.1 PH50 20 -20 + 35 600,0 70.6 1.82 46.2 PH51 20 -65+100 600,0 70.7 1.79 46.1 PH59 20 -20 + 35 600,18 69.2 1.67 51.6 PH72 20 -65+100 600,18 68.3 1.70 50.6 PH104 20 -20 + 35 700,0 68.2 1.57 55.2 PH53 20 -65+100 700,0 68.3 1.62 52.9 PH78 20 -20 + 35 700,18 67.0 1.59 55.4 PH74 20 -65 + 100 700,18 66.8 1.60 54.5 PH55 40 -20 + 35 600,0 71.8 1.86 44.1 PH71 40 -65+100 600,0 70.7 1.76 47.1 PH77 40 -20 + 35 600,18 69.5 1.78 48.2 PH60 40 -65 + 100 600,18 68.7 1.71 50.0 PH103 60 -20 + 35 500,18 73.3 1.86 43.0 PH99 60 -65 + 100 500,18 73.1 1.89 41.2 PH86 60 -20 + 35 600,0 71.8 1.91 42.6 PH95 60 -65+100 600,0 71.0 1.91 42.3 PH75 60 -20 + 35 600,18 69.1 1.64 52.6 PH87 60 -65+100 600,18 68.7 1.66 51.5 PH48 60 -20 + 35 700,0 68.8 1.67 51.9 PH93 60 -65 + 100 700,0 68.1 1.68 51.3 PH85 60 -20 + 35 700,18 66.7 1.68 53.1 PH90 60 -65 + 100 700,18 67.1 1.64 53.2 3-3 200 + -20 + 35 500,0 85.2 2.02 28.0 3-6 200 + -65+100 500,0 92.1 2.12 16.9 3-11 200 + -20 + 35 500,30 72.3 1.87 43.4 3-12 200 + -65 + 100 500,30 72.8 2.02 37.4 3-2 200 + -20 + 35 600,0 71.9 1.87 43.8 3-5 200 + -65+100 600,0 70.8 1.94 41.5 3-14 200 + -20 + 35 600,10 69.0 1.68 51.5 3-13 200 + -65 + 100 600,10 68.9 1.78 47.8 3-7 200 + -20 + 35 600,30 68.0 1.61 54.2 3-8 200 + -65 + 100 600,30 67.9 1.69 51.1 3-1 200 + -20 + 35 700,0 68.2 1.67 52.3 3-4 200 + -65+100 700,0 67.4 1.66 52.4 3-10 200 + -20 + 35 700,30 65.3 1.65 54.9 3-9 200 + -65+100 700,30 65.8 1.68 53.0 SULFUR BEHAVIOR DURING THERMAL DESULFURIZATION Table 9 Effect of particle size, maximum pyrolysis temperature, and soak time on total sulfur removal and sulfide mineral content for IBC-101 (heating rate was 18.5°C/min) Mesh size Charring temp(°C), time(min) Char yield (%) Char sulfur content (%) Sulfur evolved (%) X-ray diffraction dat; Pyrite- Pyrrhotite- peak area peak area (counts) (counts) i for chars Experiment Total Before max rate After max rate Pyrrhotite (mol% Fe) PH31A -20 + 35 500,5 69.1 3.27 41.1 18.2 22.9 6.3 4.0 47.6 PH33 -20 + 35 500,5 69.4 3.12 42.0 19.5 22.5 6.0 2.0 48.2 PH35 -65 + 100 500,5 68.3 3.34 41.2 18.7 22.5 6.0 3.0 47.6 PH43 -20+100 500, 18 68.4 3.20 40.7 18.0 22.7 6.0 5.5 48.2 PH36 -65 + 100 500, 30 66.5 3.22 43.0 18.4 24.6 5.0 6.0 47.4 PH40 -20+100 600,5 65.3 2.89 48.9 18.7 30.2 2.0 11.2 48.2 PH39 -20 + 35 600, 18 64.9 3.00 49.0 16.6 32.4 1.0 10.0 48.6 PH45 -20+100 600, 18 64.6 2.79 50.1 17.3 32.8 2.0 11.2 48.6 PH41 -65+100 600, 18 64.1 3.01 48.8 18.0 30.8 — 12.0 48.2 PH42 -20+100 600, 30 63.9 2.71 52.5 18.6 33.9 — 15.0 48.6 PH32 -20 + 35 700,5 62.8 2.82 54.1 18.0 36.1 — 12.8 49.4 PH34 -65+100 700,5 62.0 2.84 53.4 19.0 34.4 — 11.2 49.1 PH44 -20+100 700, 18 62.1 2.77 53.0 18.7 34.3 — 10.5 49.1 PH38 -20 + 35 700, 30 62.2 2.80 49.4 17.8 31.6 — 9.8 49.8 PH37 -65 + 100 700, 30 61.1 2.86 52.1 18.1 34.0 — 12.8 49.1 Table 10 Effect of heating rate and particle size on char pore structure (IBC- 101 and IBC- 103) Heating rate Mesh Char surface area Char yield Char sulfur content Coal sulfur N 2 C0 2 evolved Experiment (°C/min) size (m 2 /g) (m 2 /g) (%) (%) (%) IBC-101 PH69 20 -20 + 35 22.8 342.3 66.1 2.86 54.7 PH61 40 -20 + 35 27.7 338.3 66.2 2.86 54.6 PH79 60 -20 + 35 27.5 341.8 66.1 2.81 55.5 PH58 20 -65 + 100 13.3 354.5 65.0 2.92 55.8 PH64 40 -65 + 100 17.8 350.5 64.2 3.06 54.2 PH63 60 -65 + 100 19.9 361.1 64.0 2.87 57.2 IBC- 103 PH59 20 -20 + 35 7.1 317.8 69.2 1.67 51.6 PH77 40 -20 + 35 11.2 311.8 69.5 1.78 48.2 PH76 60 -20 + 35 10.8 300.4 69.1 1.83 47.1 PH72 20 -65 + 100 5.4 317.8 68.3 1.70 50.6 PH60 40 -65+100 8.2 306.8 68.7 1.71 50.0 PH87 60 -65+100 10.9 322.4 68.7 1.66 51.5 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 545 chars in figure 3, the second sulfur evolution peak for experiments PH34 and PH41 is probably a result of the thermal decomposition of pyrite. The X-ray diffraction data indicate that at 600° and 700°C, most or all of the pyrite in the sample has been converted to pyrrhotite (table 9). 900 T 300 340 380 420 460 500 Temperature (°C) 540 580 Figure 2 Effect of heating rate on sulfur evolution rate during pyrolysis of -20 + 35 mesh particles of IBC- 101 Tables 7 and 8 show the effect of two particle sizes on the quantity of sulfur removed during pyrolysis. Most chars produced from -20 + 35 mesh coal particles have slightly lower sulfur contents than those produced from -65+ 100 mesh coal particles. The effect of par- ticle size was investigated by more experiments using IBC-101 coal and a constant heating rate of 18.5°C/min. The time involved in heating the samples until the maximum sulfur liberation rate occurred (about 500°C) was constant for all runs. Therefore, by comparing the amount of sulfur liberated before the maximum desul- furization rate (R max ) was achieved, the effect of particle size on desulfurization rate could be evaluated. The fractions of sulfur evolved before and after R max are shown in table 9. From a comparison of the "before R max " percentages, particle size clearly had little or no effect on the amount of sulfur evolved. The results of surface area measurements on a select group of chars, produced at 600°C from IBC-101 and IBC-103 coals, are given in table 10. Very small pores were present in all the chars; the C0 2 surface areas are much higher than the N 2 surface areas. In most cases, the N 2 surface areas for chars produced from a given particle size fraction increase as the heating rate increases. In most cases, chars with higher N 2 surface area are produced from the larger particle size fraction, indicating that greater expansion occurs with larger par- ticles during pyrolysis. Significant differences are apparent between the physical and chemical characteristics of the two coal samples used for testing the effect of various parame- ters on the removal of sulfur during pyrolysis. For exam- ple, the organic sulfur content of IBC-101 is about 3 percent, whereas in IBC-103 it is about 1 percent. The 240- 180- 120- 60- — 240 | € 180 o o CO en E B re 120 - 60 - 240 180 - 120- 60- 496 C PH34 Max T = 700°C 511 °C PH41 Max T = 600°C 500°C PH35 Max T = 500°C 60 Time (min) Figure 3 Rate of sulfur evolution during charring experi- ments PH34, PH41, and PH35 (table 9) (samples contained 3.04% organic sulfur and 1.04% pyritic sulfur) volatile matter content of IBC-101 is about 4 percent higher than in IBC-103. In comparable pyrolysis exper- iments IBC-103 gives higher char yields and less evolu- tion of sulfur than does IBC-101. In addition all chars from IBC-103 exhibit a much higher degree of agglom- eration than do the chars produced under similar condi- tions from IBC-101. But even though the coals react differently under similar pyrolysis conditions, we found that for the parameters tested the relative effects on sulfur removal were similar for each coal. Pyrolysis with a Trace of Oxygen Normally when coal is heated above 500°C, pyrite (FeS 2 ) decomposes to nonmagnetic or weakly magnet- ic iron sulfides such as hexagonal pyrrhotites (Fe^S) and troilite (FeS). Adding a trace amount of oxygen into SULFUR BEHAVIOR DURING THERMAL DESULFURIZATION 9 Table 1 1 Sulfide and iron oxide mineral content of chars produced from coals pyrolyzed 18 minutes at 550°C with various amounts of trace oxygen Experiment % 2 in N 2 Char yield (%) Degree of magnetism* Char mineral contentf RK-83B-5 P-102 1.0 72.0 High Hem, mag P-123 0.5 71.1 High Mag, hem, mono-pyrr P-122 0.25 71.2 High Mono-pyrr, (mag) P-121 0.10 70.8 High Mono-pyrr, (mag) P-116 0.05 69.9 Moderate Hex-pyrr, (mono-pyrr), (mag) P-120 0.025 70.1 Slight Hex-pyrr, (mono-pyrr) P-117 0.0 69.5 Nonmagnetic Hex-pyrr IBC-103 P-135 0.1 71.9 Moderate Hex/mono-pyrr, (mag), (pyrite) * Degree of magnetism is an arbitrary estimate. t Pyrr, pyrrhotite; mono, monoclinic; hex, hexagonal; mag, magnetite; hem, hematite; ( ), indicates trace amounts. Minerals are listed in order of estimated abundance based on peak intensities from X-ray diffraction patterns. the nitrogen purging gas results in the formation of a significant amount of strongly magnetic, monoclinic pyr- rhotite during pyrolysis. A quick and rather crude mag- netic separation (by a hand magnet) made on a char produced at 550°C with a trace of oxygen showed that a significant amount of sulfur could be magnetically removed. The coal, RK-B-5, initially contained 3.8 per- cent total sulfur (see table 20 for proximate analysis), which was reduced to 2.5 percent sulfur when the coal (RK-B-5) was charred at 550°C with a trace of oxygen. After the crude magnetic separation, the sulfur content of the cleaned char was 1.2 percent. Although a signifi- cant amount of carbon material was also separated with the magnetic fraction of the char (about 48% total weight recovery), better magnetic separation tech- niques should significantly improve the separation yields. Because of the potential for reducing the "pyritic" sul- fur content of chars through conventional magnetic separation techniques, we further investigated the for- mation of magnetic pyrrhotite in chars. Efforts were directed toward studying parameters that affect the for- mation of magnetic pyrrhotite during pyrolysis: different amounts of trace oxygen used, pyrolysis temperature, and preoxidation. The mineralogical changes were mon- itored by X-ray diffraction. The mineralogical results of several pyrolyses at 550°C using various trace amounts of oxygen are shown in table 11. Adding 1.0 percent oxygen in the purging gas resulted in the formation primarily of magne- tite and hematite. A small amount of monoclinic pyrrho- tite was observed with 0.5 percent oxygen. The greatest formation of monoclinic pyrrhotite was observed when 0.25 and 0.1 percent oxygen were added to the purging gas. However, the char treated with 0.25 percent 2 contained noticeably more magnetite than did char treated with 0.1 percent oxygen. The chars from runs with oxygen contents lower than 0.10 percent contained primarily hexagonal pyrrhotite with only a trace of monoclinic pyrrhotite. As observed previously, the 550°C pyrolysis under pure nitrogen contained only hex- agonal pyrrhotite. To make certain that our results were not coal specific, a 550°C pyrolysis using 0.1 percent oxygen was performed on IBC-103; monoclinic pyrrho- tite was also found to be present in the IBC-103 char. A series of pyrolyses was run from 425° to 570°C to determine the sequence of sulfides that occur in the absence of oxygen. Table 12 shows the temperatures tested and the sulfide mineral contents of the resultant chars. Pyrite was the only sulfide mineral observed up to 500°C. This result differs from those of an initial exper- iment at 450°C in which a trace of monoclinic pyrrhotite was observed. However, Whiteway, Stuart, and Chan (1985) also observed some monoclinic pyrrhotite within the temperature range of 425°C to 500°C after pyro- lyzing some Nova Scotia coals. Table 12 shows that at 10 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 545 Table 12 Sulfide and iron oxide mineralogy of RK-B-5 chars produced by heating to various temperatures under pure nitrogen Experiment Pyrolysis temp(°C), time(min) Char yield (%) Degree of magnetism* Sulfide minerals in chart P-124 425, 18 82.5 Nonmagnetic Pyrite P-125 450, 18 76.0 Slight Pyrite, (mag) P-126 475, 18 74.1 Moderate Pyrite, (mag) P-127 500, 18 72.6 Moderate Pyrite, hex-pyrr, (mono-pyrr).(mag) P-128 525, 18 71.4 Moderate Hex-pyrr, pyrite, (mono-pyrr).(mag) P-130 550,9 70.2 Slight Hex-pyrr, (mono-pyrr) (pyrite) P-129 570,9 69.9 None to slight Hex-pyrr * Degree of magnetism is an arbitrary estimate. t Pyrr, pyrrhotite; mono, monoclinic, hex, hexagonal; mag, magnetite, hem, hematite, ( ), indicates trace amounts. Minerals are listed in order of estimated abundance based on peak intensities from X-ray diffraction patterns. 500° to 525°C a trace of monoclinic pyrrhotite was ob- served, but pyrite and hexagonal pyrrhotite were by far the most abundant sulfides. Perhaps a small amount of monoclinic pyrrhotite was formed due to the release of the inherent 2 in the coal. At 570°C, only hexagonal pyrrhotite was observed in the char. Another way to study the presence of magnetic min- erals in chars after pyrolysis is to measure the magnetic susceptibility of the chars with a magnetometer. Magnet- ic susceptibility tests were completed on two sets of 400 440 480 520 Temperature (°C) 560 600 Figure 4 Magnetic susceptibility measurements of chars prepared at various temperatures under pure N 2 and a 0.1% 2 /N 2 mixture for coal sample RK-B-5 chars from pyrolyses ranging from 425° to 600°C using RK-B-5 coal (fig. 4). The first set of pyrolyses was con- ducted under a pure nitrogen atmosphere, and the sec- ond with 0.1 percent oxygen (by volume) in the nitrogen flow. The magnetic susceptibility increases to a maxi- mum as the pyrolysis temperature increases to 475°C under pure nitrogen. This finding corresponds well with the X-ray diffraction data, which showed the presence of magnetite and monoclinic pyrrhotite in chars heated up to 475°C (table 12). By 550°C the magnetic suscep- tibility had fallen more than one order of magnitude in the pyrolyses conducted under pure nitrogen. The magnetic susceptibility of the chars heated under 0.1 percent oxygen also reached a maximum at about 475° to 500°C. However, the magnetic susceptibility did not drop much after 475°C but remained high for all the chars up to 600°C (fig. 4). X-ray diffraction data were collected on chars that were heated to 800°C and treated with H 2 /H 2 S gas. Some of these chars were cooled under pure nitrogen and others under 0.1 percent oxygen. The X-ray patterns show a shift in the sub- sequent iron sulfides from troilite to monoclinic pyrrho- tite because of the trace of oxygen. Thus by using a trace of oxygen we were able to produce a ferromag- netic monoclinic pyrrhotite in many different chars over a wide temperature range. From the sulfide studies of Taylor (1971) and Genkin (1971) and our own recent experiments on pyrrhotite in chars, we believe that the trace amount of oxygen added during pyrolysis drives the more iron-rich hex- agonal pyrrhotite to the more iron-poor, magnetic, mono clinic pyrrhotite. Two experiments were performed to verify this hypothesis. The coal was heated to 550° and 650°C under nitrogen and the temperature held for 18 minutes. Previous X-ray data have shown that these SULFUR BEHAVIOR DURING THERMAL DESULFURIZATION Table 13 Sulfide and iron oxide mineralogy of RK-B-5 chars treated with 0.1 percent oxygen after pure nitrogen treatment and preoxidation treatment Experiment Pyrolysis temp(°C), time(min) Treatment Char yield (%) Degree of magnetism* Char mineralogyt Pure nitrogen P-132 550,30 PureN 2 -18min 0.1%O 2 -12min 69.6 Moderate to high Hex-mono-pyrr, (mag) P-133 650, 30 PureN 2 -18min 0.1%O 2 -12min 66.5 Moderate to high Hex-pyrr, minor mono-pyrr Preoxidation P-131 550, 18 5%0 2 at295°C 0.1%O 2 at550°C 76.0 Moderate to high Hex-pyrr, minor mono-pyrr, (mag) * Description of magnetism is an arbitrary estimate. t Pyrr, pyrrhotite; mono, monoclinic; hex, hexagonal; mag, magnetite; hem, hematite; ( ), indicates trace amounts. Minerals are listed in order of estimated abundance based on peak intensitites from X-ray diffraction patterns. Table 14 Pyrolysis and post-pyrolysis partial oxidation experiments on sink and float coal fractions Charring condition Sulfur removed ( %) Volatile loss (wt%) Origin of sulfur in volatiles (%) Org* Pyr* Origin of remaining in Org* sulfur char (%) Run Total Org* Pyr* Pyr* Sink 1 650°C 47.6 68 31 27.5 65 35 35 65 2 650°C 48.6 69 31 27.8 65 35 34 66 3a 450°C 24.5 50 3 — 93 7 — — 3b Partoxid 50.1 18 77 33.3 16 84 68 32 Float 1 650°C 57.1 67 12 36.4 96 4 68 32 2a 450°C 37.4 46 — 100 — — 2b Partoxid 19.5 12 53 36.1 49 51 84 16 * Values calculated from isotopic compositions. conditions produce only hexagonal pyrrhotite. Mono- clinic pyrrhotite was successfully produced at both temperatures with the addition of 0.1 percent oxygen for 12 minutes after the above conditions had been established (table 13). To determine if preoxidation (used for deagglomera- tion purposes) would hinder the formation of monoclinic pyrrhotite, we ran a pyrolysis experiment in which we preoxidized the coal at 295°C with 5 percent oxygen and then used 0.1 percent oxygen at 550°C. Some magnetic pyrrhotite was successfully formed on the preoxidized char (table 13). All the pyrolyses up to this point had been carried out in our thin-bed bench-scale system using only 0.5 to 1.0 g of coal. Through the cooperation of the ISGS Minerals Engi- neering Section, we were able to run one test of our parameters on a larger scale in a fluidized-bed system. Approximately 100 g of IBC-101 was run in the fluidized- bed system. The sample was preoxidized at 250°C with 5 percent oxygen for 15 minutes, then heated to 550°C and held for 20 minutes with 0.1 percent oxygen added to the purging gas. The initial results were very en- couraging: X-ray diffraction showed that monoclinic pyr- rhotite was present in the fluidized-bed char, although only a relatively small amount of the magnetic pyrrhotite was detected. The conditions that lead to the formation must be optimized. Preliminary data collected on the magnetic separation of the magnetic iron sulfide from the char indicate a need to improve the magnetic sep- aration technique and equipment. The work reported above suggests that for some coals, especially those with low to moderate organic sulfur contents, magnetic separation of sulfide minerals alone is potentially useful for producing low-sulfur chars. 12 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 545 Post-Pyrolysis Desulfurization To produce clean low-sulfur solid fuel from most Illinois high-sulfur coals, post-pyrolysis desulfurization treat- ment is necessary. The two gas-phase post-pyrolysis treatments investigated were partial oxidation and hy- drodesulfurization. Partial oxidation To monitor the type of sulfur re- moved during post-pyrolysis partial oxidation, isotop- ically characterized coal samples were used in several partial oxidation experiments. In figure 5, an example of the results shows that pyritic sulfur is preferentially removed during partial oxidation. As already described, an Illinois Herrin Coal sample that had been previously well characterized isotopically was subjected to a float/ sink separation to provide a pyrite-rich and a pyrite-poor fraction (table 14). Table 14 compares the results of the pyrolysis and post-pyrolysis partial oxidation experi- ments on the sink and float fractions. As expected, the sink fraction showed a much greater sulfur loss during oxidation due to the preferential oxidation of the inor- ganic sulfide (pyrite and possibly a small amount of pyrrhotite). The much lower amount of pyritic sulfur re- moved during oxidation of the float fraction is probably a result of the pyrite in this fraction being primarily dis- seminated and thus not readily available for oxidation. 100 °^ 80- o> c 'c I 60 H 3 40 V) "co c O) 8 20H > >»X » » » > »' [ | Pyritic sulfur j Organic sulfur 450°C 450°C **&!***?* Coal Char Oxidized Figure 5 Proportions of original organic and pyritic sulfur remaining after charring and partial oxidation at 450°C The effects of the following process conditions on sulfur removal during post-pyrolysis oxidation were studied: oxidation temperature, oxidation time, oxygen concentration, and the presence of water vapor in the oxidizing gas stream. The results are summarized in table 15 for IBC-101 and in table 16 for IBC-103. With IBC-101, the lowest sulfur content chars were produced Table 15 Post-pyrolysis oxidation results for IBC-101 Oxidation conditions Char yield Char sulfur content Coal sulfur Temp Time 2 Concn evolved Experiment (°C) (min) (%) (%) (%) (%) 550°C/18 min PH130 — — — 67.5 2.80 54.7 PH132 450 15 1 66.7 2.70 56.8 PH131 450 5 5 67.1 2.46 60.4 PH129 450 15 5 61.7 2.30 56.0 600°C/18 min PH61 — — — 66.2 2.86 54.6 PH112 450 15 1 65.4 2.43 61.9 PH138 450 15 1* 65.4 2.39 62.5 PH114 450 5 5 65.8 2.33 63.2 PH110 450 15 5 60.5 2.25 67.4 PH136 450 15 5* 61.8 2.20 67.4 PH137 525 15 1* 64.3 2.47 61.9 PH109 525 10 3 62.6 2.44 63.4 PH115 525 10 5 60.3 2.37 65.7 PH135 525 15 5* 58.2 2.33 67.5 PH111 600 15 1 64.3 2.73 57.9 PH126 600 15 1* 61.9 2.54 62.3 PH113 600 5 5 63.1 2.62 60.4 PH133 600 15 5* 6.29 2.70 59.3 "In presence of water vapor. SULFUR BEHAVIOR DURING THERMAL DESULFURIZATION 13 Table 16 Post-pyrolysis oxidation results for IBC-103 (600°C, 18 min) Oxidation conditions Char yield Char sulfur content Coal sulfur Temp Time 2 concn evolved Experiment (°C) (min) (%) (%) (%) (%) PH77 — — — 69.5 1.78 48.2 PH121 450 15 1 69.1 1.43 58.7 PH123 450 15 1* 68.9 1.37 61.5 PH125 450 15 5 69.3 1.39 59.7 PH120 450 15 5 67.0 1.31 63.3 PH117 525 10 3 66.6 1.32 63.2 PH124 525 10 5 64.9 1.26 65.8 PH118 600 15 1 67.1 1.44 59.6 PH119 600 15 1* 66.3 1.45 59.8 PH122 600 5 5 66.6 1.42 60.4 *ln presence of water vapor. by oxidation for 15 minutes at 450°C with a 5 percent 2 gas stream. However, only a small reduction in char sulfur content is obtained at the expense of a significant reduction in char yield when the oxidation time is in- creased from 5 to 15 minutes. For pyrolysis of IBC-103, 525°C rather than 450°C appears to be the preferred oxidation temperature. However, chars from IBC-103 are highly agglomerated, suggesting that some carbon must be oxidized in order for some of the pyrite/pyrrho- tite to be subjected to oxidation. The presence of water vapor in the oxygen stream has little effect on the sulfur content of an oxidized char. Although the pH monitoring technique was only roughly quantitative in monitoring the sulfur evolution rates during the oxidation, results from it and the data in tables 15 and 16 do indicate that during the first few minutes of oxidation, the rate of sulfur oxidation is sig- nificantly higher than the rate of carbon oxidation. For reasons explained in the appendix a quadrupole gas analyzer (QGA) was used to gain more accurate rate data. The QGA data collected during a pyrolysis experiment and a post-pyrolysis oxidation experiment are shown in figures 6 and 7, respectively. Data col- lected by the pH monitoring technique showed that for a heating rate of approximately 20°C/min, the maximum sulfur evolution rate occurred at 460° to 500°C. How- ever, the QGA data (fig. 6) showed the maximum sulfur evolution occurred at about 435°C. We believe the latter temperature to be close to the true temperature for maximum sulfur evolution of Illinois coal during pyrolysis. The QGA data in figure 7 confirm pH monitor- ing results, which showed that during a 15-minute post- pyrolysis oxidation at about 450°C, sulfur was preferen- tially oxidized during the first few minutes, after which carbon was preferentially oxidized. Hydrodesulfurization Since hydrogen can be used in a gas-phase desulfurization process, the be- havior of the organic and pyritic sulfur was investigated 300 250- ~ 200- c £ § 150 c o _ 100 50 H 1 200 400 600 Temperature (°C) 800 Figure 6 Quadrupole gas analyzer (QGA) data collected during pyrolysis of IBC-101 (heating rate, approximately 20°C/ min) 100 80- a) 60 O o 40 20- SO, x 10 2 Time (min) Figure 7 QGA data comparing the evolution of S0 2 and C0 2 during char oxidation at 455°C of IBC- 101, experiment QMS8 (table 19) 14 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 545 using stable isotope analyses on untreated hydrodesul- furized chars, partially oxidized hydrodesulfurized chars, and acid-leached hydrodesulfurized chars. Since the hydrodesulfurization experiments were carried out at 800X, the pyrite would have been completely con- verted to pyrrhotite. However, as mentioned earlier for the pyrolysis experiments, when discussing the isotopic results we refer to the inorganic sulfur as "pyritic" sulfur. The isotopic data indicate that both organic and "pyri- tic" sulfur are removed during hydrogen treatment. How- ever, the proportions of "pyritic" and organic sulfur re- maining in the final products are quite different depend- ing on whether the char has been acid leached or par- tially oxidized before hydrogen treatment (table 17). Most of the sulfur remaining in the acid-leached hydro- desulfurized chars is "pyritic" sulfur. In fact, in two of the three coals tested (RK-A-4 and CR-B-4) no organic sulfur remained in the acid-leached hydrodesulfurized chars. The sulfur remaining in the two chars that were oxidized before hydrodesulfurization (RK-B-3 #26 and RK-A-4 #18) was completely organic. Figure 8 displays the proportions of organic and "pyritic" sulfur remaining in the charred products after each type of treatment for the RK-B-3 coal. We do not completely understand why most of the acid-leached hydrodesulfurized chars contained only "pyritic" sulfur and the partially oxidized hydrodesulfur- ized chars contained only organic sulfur (table 17). In the latter case, the fact that very little "pyritic" sulfur remains in the partially oxidized chars before hydrode- sulfurization probably explains why only organic sulfur remains in the final char. However, for the acid-leached hydrodesulfurized chars, there is no obvious explana- tion why only "pyritic" sulfur remains in most of the final chars. Table 17 Distribution of sulfur forms in hydrogen-treated chars after acid leaching and partial oxidation 100 Charring Sulfu in final char (%) Coal Total sample no. temp(°C) remaining Organic* Pyritic"* Acid-leached char RK-B-3 #13 450 7.5 41 59 RK-B-3 #16 650 3.3 35 65 RK-A-4 #17 450 4.1 100 RK-A-4 #19 550 4.0 100 RK-A-4 #20 650 2.6 100 CR-B-4 #21 450 4.1 100 CR-B-4 #23 550 3.2 100 CR-B-4 #24 650 2.0 100 Oxidized char RK-B-3 #26 450 7.1 100 RK-A-4 #18 450 10.0 100 gg 50 c c n E CD k_ _3 100 73 CO a c m bU o 450°C imnniitj Pyritic sulfur f-j-jmgjil Organic sulfur Oxidized 1 ::■:■:■:■:•:•:■:: J 450°C Leached 1111 11 Ii mmmW ^ Coal Char Treated Hydrotreated 'Values calculated from isotopic composition. Figure 8 Proportions of original organic and "pyritic" sulfur remaining in treated chars of RK-B-3 coal samples Four hydrodesulfurizations, lasting 15, 30, 45, and 60 minutes, were made after 750°C pyrolyses to deter- mine the type and amount of sulfur removed relative to the length of time for pure hydrogen treatment (table 18). The sulfur evolved during hydrogen treatment at 800°C is much more enriched in "pyritic" sulfur (44%) than the sulfur evolved during pyrolysis (14% "pyritic" sulfur). Figure 9 compares the desulfurization efficiency of 15- and 60-minute hydrodesulfurization. Pyrolysis plus hydrodesulfurization results in a significant removal of both organic and "pyritic" sulfur. The effect of hydro- desulfurization time can be seen more clearly in figure 10, which shows the amount and composition of sulfur removed by hydrodesulfurization and that remaining in the treated char. Of particular interest, sulfur removed by hydrodesulfurization was found to have a constant ratio of organic to "pyritic" sulfur with increasing hydro- desulfurization times. Much of the sulfur actually being removed by the hydrogen might have been primarily organically bound sulfur. Our earlier studies showed that during pyrolysis, some of the pyritic sulfur becomes bound into the organic structure and the amount of incorporation increases with temperature. The constant ratio of organic to "pyritic" sulfur observed during the different hydrodesulfurization times would be explained if the sulfur removed by the hydrogen is primarily organ- ically bound sulfur, which includes both original organic sulfur and original "pyritic" sulfur. Hydrodesulfurization rate data for a non-pretreated char, an acid-leached char, and a partially oxidized char are compared in figure 11. Obviously, acid leaching of the char (removal of iron) significantly increased its hydrodesulfurization rate. On the other hand, oxidation does not appear to have increased the initial rate of hydrodesulfurization, but because the rate did not di- minish as much with time, oxidation did appear to re- duce the time necessary for hydrodesulfurization to occur. This is addressed in more detail later when the relative rates of sulfur removal are discussed. SULFUR BEHAVIOR DURING THERMAL DESULFURIZATION 15 Table 18 Pyrolysis and hydrodesulfurization results of coal sample RK-B-3 Run no. Temp(°C), soak time (min) Sulfur evolved (%) Sulfur remaining in char (%) Total Org* Pyr* Total Org* "Pyr" Sulfur content in char (%) Char yield (%) Pyrolysis 197a 196a 195a 194a 750,5 750,5 750,5 750,5 Hydrodesulfurization 197b 800, 15 196b 800,30 195b 800,45 194b 800,60 60.2 58.9 58.6 59.8 16.8 24.4 26.4 27.1 86 86 86 86 56 56 56 57 14 14 14 14 44 45 44 43 39.8 41.1 41.4 40.2 23.7 16.8 14.9 12.9 63 63 63 62 69 74 76 77 37 1.88 64.4 37 1.94 64.9 37 1.96 64.3 38 1.90 64.3 31 1.21 60.6 26 0.87 58.8 24 0.76 60.1 23 0.68 59.6 "Values calculated from isotopic composition. 100- 80- cn £ 60 H c cb E CD ^ 40H 3 CO 20- i« .■.■ r . wwr . 30- Original coal Char, Hydrotreated, 750°C 15 min 100- 80- 9 60- c CO E CD ^ 40H 3 CO 20 -I Pyritic sulfur [;,,,,, : ;:;j Organic sulfur Original coal Char, Hydrotreated, 750°C 60 min Figure 9 Sulfur removal by charring at 750°C for 5 minutes and hydrodesulfurization at 800°C for 15 and 60 minutes 24- -o 18- i o > CD CO 6- OrgPyr=1.2 Org Pyr = 1.3 °^ll 1 - 3 Org Pyr =1.3 tt* « i m i n 15 min 30 min 45 min 60 min 30- 24- .E 18- c co E CD ^ 12- ,2 3 CO 6- OrgPyr = 2.2 Pyritic sulfur [__r^J Organic sulfur Org Pyr = 2.9 -m Org Pyr = 3.1 wnm ii .i Org Pyr = 3.4 15 min 30 min 45 min 60 min Figure 10 Removal of organic and "pyritic" sulfur from char by hydrodesulfurization at 800°C for different lengths of time (percentage of sulfur evolved and remaining refers to amount of sulfur originally present in untreated coal). See table 18 16 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 545 Table 19 Separate pyrolysis and post-pyrolysis desulfurization experiments for IBC- 101 (-20+ 100 mesh) Pyrolysis Oxidation Hydrodesulfurization Temp (°C), H 2 time concn (min) (%) Oxidation Char sulfur content (%) Experiment Temp (°C), time (min) Calc charS content (%) Temp (°C), time (min) 2 concn (%) Temp (°C), time (min) o 2 concn (%) Approx net char yield (%) QMS8 600, 18 2.79 455, 15 5 — — — — 2.12 61.1 QMS6 600, 18 2.84 — — 800, 60 100 — — 0.82 58.3 QMS7* 600, 18 2.80 — — 800,60 100 — — 0.48 51.9 QMS9 600, 18 2.82 455, 15 5 800, 60 100 — — 0.76 58.5 QMS10 600, 18 2.89 — — 800, 60 100 455,5 5 0.75 56.9 *Char was acid leached before post-pyrolysis desulfurization. The experimental conditions and sulfur contents of the desulfurized chars are summarized in table 19. Acid leaching had the greatest effect on reducing the char sulfur content, whereas post-pyrolysis oxidation had only a small effect on char sulfur content. In a char desulfurization process, oxidation should probably be used after rather than before hydrodesulfurization. This idea was tested in experiment QMS10, which showed a small reduction in char sulfur content compared with the untreated char. However, the actual amount of sulfur evolved during post-hydrodesulfurization oxidation was quite small, so the differences in char sulfur contents may be within the limits of experimental error. Combined Treatments for Thermal Desulfurization Results presented in previous sections were obtained mainly from experiments designed to determine the op- timum conditions for pyrolysis and various post- pyrolysis char desulfurization treatments. Results pre- sented in this section were obtained in a systematic study combining pyrolysis and post-pyrolysis treat- ments to show the potential of thermal desulfurization processes to produce low-sulfur chars from high-sulfur Illinois coals. Four coals that differ in their pyritic and organic sulfur contents were used in this part of the project. The chem- ical analyses of the coals are given in table 20. With the exception of IBC-101, the coals are not necessarily representative of process coals. Coals CR-B-1, RK-B-4, and RK-B-5 were chosen because of their relatively high pyritic sulfur contents. The various combinations of desulfurization treatments investigated are listed in figure 12. The list begins with untreated pyrolysis and shows the various combinations of post-pyrolysis treat- ments tested, including partial oxidation (with 5 and 0.1 % 2 ), acid leaching (with dilute HCI), hydrodesulfuri- zation, and magnetic separation of inorganic sulfides. Analytical and mineralogical results The analyt- ical results of the various treatments are summarized in table 21 (sulfur content and yield data) and the min- eralogical results are summarized in table 22 (X-ray 150 c E 03 o 120- 90 o o w g CD d 60- 30- 120- 90- 60 30- QMS7 Acid-leached 600°C char QMS9 Oxidized 600°C char QMS6 600°C char 40 Time (min) Figure 11 800°C hydrodesulfurization rate data for experi- ments QMS6, OMS7, and OMS9 prepared for IBC-101 (table 19) SULFUR BEHAVIOR DURING THERMAL DESULFURIZATION 17 diffraction analyses). The yield data given in table 21 were calculated relative to the original coal. Acid leach- ing followed by hydrodesulfurization (treatment 5) al- ways produces the lowest sulfur content chars, and thus is the treatment combination against which all other combinations should be compared. Data from the QGA concerning rates of sulfur removal are presented later. As can be seen in table 21, hydrodesulfurization with pure H 2 (treatment 3) reduces the sulfur content of chars significantly compared with pyrolysis alone (treatment 1) and post-pyrolysis partial oxidation (treatment 2). To obtain similar results in an industrial-scale reactor, an H 2 S scavenger would probably have to be mixed in with the char during hydrodesulfurization because of the ex- cess H 2 S present in the large-scale system. However, the sulfur-ladened scavenger would ultimately have to be removed from the char. The combination of post-pyrolysis partial oxidation of chars followed by hydrodesulfurization (treatment 4) Table 20 Chemical analyses of the four coals used in the combined gas-phase thermal treatment tests (moisture-free basis) Chemical Coal sample analysis (%) CR-B-1 RK-B-4 RK-B-5 IBC-101 Volatile matter 45.1 40.0 41.0 44.2 Fixed carbon 47.5 41.6 45.9 45.9 High-temp ash 7.4 18.4 13.1 9.8 CaO 0.28 0.43 1.86 0.50 Sulfate sulfur 0.067 0.022 0.044 0.158 Pyritic sulfur 1.90 3.77 2.04 1.08 Organic sulfur 3.01 2.38 1.74 2.77 Total sulfur 4.97 6.17 3.83 4.01 1. Coal pyrolysis (N 2 ) 750°C,5min char, pyrolysis (N 2 ) oxidation (5% 2 ) 2. Coal j> char, 750°C,5min 450°C,5min char, pyrolysis (N 2 ) hydrodesulfurization (H 2 ) 3. Coal r > char i — - p char 3 750°C, 5 min 800°C,60min 4. Coal 5. Coal pyrolysis (N 2 ) 750"C,5min pyrolysis (N 2 ) 750°C, 5 min char, char. oxidation (5% 2 ) 450°C, 5 min acid leach ambient temp hydrodesulfurization (H 2 ) char 2 £> char 4 800°C, 60 min char- hydrodesulfurization (H 2 ) 800°C, 60 min char.; 6. Coal pyrolysis (N 2 ) 750°C,5min char, hydrodesulfurization (H 2 + 0.44% H 2 S) 800°C,60min char,. oxidation (5% 2 ) 450°C,10min char fi 7. Coal pyrolysis (N 2 ) hydrodesulfurization (H 2 + 0.44% H 2 S) j>. char, j>. char 750°C, 5 min 800°C, 60 min oxidation (0.1 %0 2 ) ■ 1>- during cooling to ambient temp char. magnetic separation char 7 (nonmagnetic fraction) pyrolysis (N 2 + 0.1 %0 2 ) magnetic , . „ . , 8. Coal £>. char M1 j> char 8 (nonmagnetic fraction) 550°C,18min separation Figure 12 Various combinations of the thermal desulfurization treatments investigated 18 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 545 Table 21 Sulfur concentration in chars and total char yields (by weight) after combined thermal desulfurization treatments (outlined in fig. 12) 1 2 3 4 5 Oxidation Acid leach Coal Total S Pyrolysis (N 2 ) char. Oxidation (5%0 2 ) char 2 Hydrodesulf (Ha) char 3 (5%0 2 ) hydrodesulf char 4 (15%HCI) hydrodesulf char 5 sample S Yield St Yieldt S Yield S Yield S Yield CR-B-1 4.97 3.48 60.9 2.84 57.9 0.71 0.78* 57.9 0.64 53.1 0.41 49.4 RK-B-4 6.17 4.33 64.4 2.99 61.2 0.79 61.2 0.60§ 56.3 0.37 ±0.04 52.8 RK-B-5 3.83 2.86 63.4 2.38 60.2 1.58 ±0.07 60.2 1.24 55.8 0.30 ±0.02 52.0 IBC-101 4.01 2.65 ±0.09 63.1 ± 0.23 2.20 60.9 0.88 ±0.05 1.05* 60.9 0.70 55.1 0.42 52.1 6 7 7 8 8 Coal Total S Hydrodesulf (H 2 + 0.44%H 2 S) ox (5% 2 ) char 6 Hydrodesulf (H 2 + 0.44%H 2 S) ox(0.1%O) 2 char M2 Nonmagnetic fraction char 7 (N 2 + O.1%0 2 ) char M1 Nonmagnetic fraction char 8 sample S Yield S Yield S Yield S Yield S Yield CR-B-1 4.97 1.29 55.6 2.12 56.0 1.12 28.9 3.96 65.1 2.37 40.9 RK-B-4 6.17 1.36 59.2 3.93 60.1 1.27 16.4 5.29 69.7 2.03 44.6 RK-B-5 3.83 2.13 ±0.08 56.6 2.91 57.1 1.24 31.9 3.11 67.4 1.20 48.1 IBC-101 4.01 1.16 56.7 1.88 57.7 1.25 26.4 3.00 67.5 2.14 52.2 * Values are given as percentages. t 30-i nin hydrodesulfurization. t Estimated values. § 7-min oxidation. Table 22 Sulfide and associated iron minerals in chars after thermal desulfurization treatments (char subscripts, see fig. 12) ' Coal sample Coal Char, (750°C) pyrolysis Char 5 acid leached H 2 Char 4 2 ,H 2 Char, Char 6 H2 "+" H2O, 5% 2 Char HS H 2 + H 2 S Char M2 H2 + H2O, 0.1 %0 2 CR-B-1 RK-B-4 RK-B-5 IBC-101 FeS 2 t FeS, FeS, FeS, FeS FeS FeS Fe(N) Fe Fe Fe 2 3 FeS Fe 7 S e S(N) (CaS) (Fe 10 S„) Fe 3 4 (CaS) Fe,_ x S (Fe,_ x S) (CaS) FerS 8 (Fe 3 4 ) (CaS) Fe(fM) Fe Fe Fe 2 3 FeS Fe 7 S 8 S(N) (CaS) (CaS) Fe 3 4 (CaS) Fe^.S (Fe,_ x S) CaS (Fe 7 S 8 ) (Fe 3 4 ) (CaS) Fe(N) Fe CaS CaS CaS CaS, S(N) CaS Fe Fe 3 4 Fe 2 3 (Fe,_ x S) FeS Fe 7 S 8 Fe^.S) (Fe 3 4 ) Fe(N) Fe Fe — Fes Fe 7 S 8 S(N) (CaS) (Fe,_ x S) (CaS) (CaS) Fe^S (CaS) (Fe 3 4 ) " Mineral content as determined by X-ray diffraction listed in order of estimated relative abundance: (N), none; ( ), trace; Fe, iron; S, sulfides; FeS, troilite; CaS, oldhamite; Fe,_ x S, hexagonal pyrrhotite; and Fe 7 S 8 , monoclinic pyrrhotite. t Bold type represents the most abundant sulfides or iron minerals present. SULFUR BEHAVIOR DURING THERMAL DESULFURIZATION 19 slightly reduces the sulfur content compared with chars produced by hydrodesulfurization and pyrolysis alone (treatment 3). Although not apparent from table 21, post- pyrolysis oxidation also reduces the time required to reach a certain char sulfur content by hydrodesulfuriza- tion, especially for chars produced from high pyrite con- tent coals. Most of the hydrogen treatments were car- ried out for 60 minutes; however, two hydrogen treat- ments for CR-B-1 and IBC-101, run for only 30 minutes, removed almost as much sulfur as did the 60-minute hydrogen treatment times. Additional considerations of the time factor will be presented with the discussion of relative rates of sulfur removal. The combination of treatments 6 and 7, in which char hydrodesulfurization was carried out with the H 2 /H 2 S mixture, was included to simulate a large-scale system where excess H 2 S could cause back reactions with iron in the char (assuming no H 2 S scavenger is added to the system). We used 0.44 percent H 2 S, a concentration above the 0.2 percent level, at which the back reaction of H 2 S with iron becomes important (Stephenson et al., 1985). The chars treated with the H 2 + 0.44 percent H 2 S were then subjected to oxidation by either 5 or 0.1 percent 2 . For the 0.1 percent 2 , the conversion of iron sulfides present after hydrodesulfurization to a magnetic form of pyrrhotite was investigated using X-ray diffraction. The sulfur contents of chars treated with H 2 + 0.44 percent H 2 S mixture were not determined, but should be close to the sulfur contents of the chars treated with H 2 + 0.44 percent H 2 S mixture followed by the slight oxidation using the N 2 + 0.1 percent 2 mixture (table 21, treatment 7). According to QGA results, very little sulfur is actually lost during slight oxidation with 0.1 percent 2 . Obviously, the chars produced using the simulated excess H 2 S would have to undergo a post-hy- drodesulfurization treatment. Post-hydrodesulfurization oxidation helps to overcome the problem of H 2 S back reactions with iron in the chars. On the basis of the char sulfur contents, except for the RK-B-5 char, very little difference is apparent between post-hydrodesul- furization oxidation with 5 percent 2 and with 0.1 per- cent 2 plus a magnetic separation. The net process yield data included in table 21 would favor oxidation with 5 percent 2 as the post-hydrodesulfurization treat- ment. However, oxidation with 0.1 percent 2 plus magnetic separation is potentially useful if the yield of the nonmagnetic fraction can be significantly increased. A process including a magnetic separation step would be advantageous because it could produce chars with significantly lower ash contents. <£