Envlron. Sci. Technol. 1984, 18, 592-600
Trace Sulfur-Containing Species in the Offgas from Two Oil Shale Retorting Processes Deborah S. Sklarew," Deborah J. Hayes,+ Michael R. Petersen,$ and Khrls B. Olsen
Pacific Northwest Laboratories, Richland, Washington 99352 C. David Pearson Phillips Petroleum Co., Bartlesville, Oklahoma 74004
Six non-HzS sulfur species were identified and quantitated from two different retorting processes, a modified in situ process and an aboveground process. Carbonyl sulfide (COS), methyl mercaptan (CH,SH), carbon disulfide (CS,), thiophene, methylthiophene, and sulfur dioxide (SO,) were determined by gas chromatography combined with a flame photometric detector. One of the in situ retorts produced the highest concentrations of sulfur species while the aboveground retort produced the least of the three retorts studied. However, other modified in situ retorts have been reported to produce relatively low concentrations of sulfur species. Variability in the quantitative data was attributed to variability both in sulfur content of the raw oil shales and in retort conditions. Possible reaction mechanisms were considered. The non-HzS sulfur species comprise 0.4-6% of the low molecular weight sulfur species measured in the retort offgases. Abatement strategies must consider the presence of these trace species if the air quality standards proposed by Colorado are to be maintained.
Introduction If alternative energy sources such as oil shale begin to play an expanded role in the nation's energy budget, it will become necessary both to minimize the environmental impacts of these new industries and to make the most efficient use of the resources. In the oil shale industry, the gaseous byproduct from the retort process may either be used as a fuel or be incinerated as waste. In either case, significant quantities of nonhydrocarbon species, especially those containing sulfur and nitrogen (I),must be controlled to prevent technical, health, and/or environmental problems. Such potential problems include corrosion, fugitive emissions, and SOJNO, stack emissions that might exceed current EPA limits if not effectively controlled. In this study the offgases from two different retorting technologies were analyzed. The main objectives were (1) to identify and quantitate the non-H2Strace sulfur species present in the gases, (2) to ascertain the effect of different retort phases and parameters on the concentration of the sulfur species, and (3) to consider the potential environmental significance of these species. The retorts from which gases were sampled included Rio Blanco's vertical modified in situ Retorts 0 and 1 and the Paraho directheated aboveground retort. All three retorts used Green River oil shale from various locations in Colorado. Since the retorts were in the pilot plant stage, effluents are not necessarily representative of those which will be obtained in future commercial operations, but the results are thought to be indicative of problems the commercial industry will need to consider. Until recently, very little information was available about the gaseous non-HzS sulfur species produced from 'Present address: Battelle-Houston, Houston, TX 77027. *Presentaddress: 335 Spring Creek Road, Providence, UT 84332. 502
Environ. Scl. Technol., Vol. 18, No. 8, 1984
Green River oil shale retorts. For example at a Paraho retort, COS and CSz were reported as not detected although 16 ppm of SOzwas determined in the same product gas (2). Thiophene was detected in Paraho retort gas at levels of 8-22 ppm (3). Trace sulfur species have only very recently been determined at the Geokinetics in situ retorts ( 4 , 5 ) . At the in situ Occidental retort 6, measurements were made for COS and SOz. COS ranged from 1 to 40 ppm during a 3-month period, generally decreasing with time, and SOz was generally below detection limits (6). The offgas from the LETC retort of the relatively high sulfur Michigan Antrim shale [ -3.5 w t % sulfur in the raw fuel vs. an average of -0.7 w t % sulfur in the Green River oil shale, (7, B ) ] has been shown to contain several non-HzS sulfur species, including COS, CH,SH, C2H5SH, CSz, and thiophene (9). We report here the identification and quantitation of six sulfur species from two different retorting processes.
Experimental Section Sampling Trips and Methods. The two Rio Blanco modified in situ retorts, Retorts 0 and 1, were operated for periods of -3 and 6 months, respectively. Retort 0 was sampled -1 (lo/&)), -3 (11/80), and -5-6 weeks (11/80) after the start of the burn. Retort 1 was sampled -1 (7/81) and -4 months (10/81) after ignition. For both Retorts 0 and 1,the last sampling trip was scheduled close to the predicted termination of the retort. However, operation of the retorts did continue for several weeks beyond our final sampling trips. The Paraho aboveground retort was operated for -1 week (9/80) and sampled several times during that period. Since startup and shutdown are relatively rapid compared to an in situ retort, no attempt was made to sample during these periods. Analytical methods described here were first developed during sampling at the Paraho retort and applied to the Rio Blanco retorts. Sampling locations (Figures 1and 2) were determined by availability of ports, tubing, and pumps. In all cases, the gas was sampled prior to any treatment such as incineration/scrubbing and release via the stack. A t Rio Blanco Retort 0, during trips 1and 2,the offgas was sampled after the knockout drum which contributed to the removal of water and possibly other condensable species. The gas was transferred to the analysis trailer via 100 yards of unheated in. stainless steel tubing. Because the line was unheated, values obtained for condensable gases may be considered minimum numbers. During the third trip to Retort 0, which occurred in late November, condensation in the line caused frequent clogging, and the sampling point for this'and subsequent Retort 1trips was changed to a port prior to the knockout drum and closer to the sampling trailer. This port was connected to the analytical equipment by -30 f t of unheated in. stainless steel tubing. At Paraho, the recycle gas was sampled after the electrostatic precipitator where oil was condensed
0013-936X/84/0918-0592$01.50/0
-
0 1984 American Chemlcal Society
FLUE GAS
w1
0 BOILER FEED WATER
BOILER
-
PURIFIED FLUE GAS
x
GAS
I I AIR
I
- u
t
Y
INCINERATOR
WATER
WATER WATER
STORAGE TANK
tw
SCRUBBER
EVAPORAl'ION PONDS
U STORAGE TANK
WATER PUMP
Flgure 1. Sampling locatlons at the Rio Blanco retort, points A and E.
RAW
OIL MIST
AND SHALE PREHEATING ZONE
COMBUSTION ZONE
ELECTROSTATIC
BLOWER CONTROLLER RETORTED SHALE
Flgure 2. Sampling location at the Paraho retort, point E. Taken from ref 3.
and separated. The temperature of the Paraho plant gas lines was controlled by steam tracing at -160 O F . About 6 ft of tubing was needed to connect the sampling point to the analytical instrumentation. Positive pressure was necessary for sampling and was provided by a positive displacement pump when sampling points were at a negative pressure. Teflon or stainless steel tubing and fittings were used to minimize adsorption of the sulfur species. Continual gas flow was maintained to maximize equilibration of the gases with the sampling system; therefore, losses were thought to be minimal. Oil and water condensing in the line were trapped by a glass impinger containing quartz wool placed ,- 1ft from the gas chromatograph. Grab samples were taken in Teflon-coated stainless steel bombs, at approximately ambient pressure ( 580-605 mmHg). N
Analytical Methods. Most analysis was completed on site due to the instability of a number of the retort gas constituents. The grab samples were returned to the laboratory to confirm field identifications of stable species. Sulfur and hydrocarbon species were analyzed by gas chromatography (HP5840A) using a flame photometric detector (FPD) and flame ionization detector (FID), respectively. Gas flows described in the HP5840 GC manuals were determined to be optimal. Because of problems with the FPD supplied with the 5840 GC, an alternate FPD, the Meloy 285 SO2 analyzer, was used with an HP3370B integrator during the first trip to Retort 1and connected to the 5840 GC electronics during the second trip to Retort 1. A Teflon-lined gas sampling valve with a 0.72-mL Teflon sample loop allowed quantitative introduction of the gas samples. The loop was flushed with sample, and the contents were brought to atmospheric pressure prior to injection (except when reduced pressure injection was desired). Glass, 6 ft X 2 mm i.d., columns were connected to the sample loop by a short (-1 ft) piece of l/&. Teflon or stainless steel tubing which was packed with the same material as the glass column to minimize dead volume. Teflon tubing presented problems with leaks at the connections; therefore, stainless steel tubing was substituted during smipling at Retort 1. No differences were observed (IO). Porapak QS was found to be a satisfactory packing material for many of the sulfur species present. Chromosorb 107 was also used for sulfur analysis. Hydrocarbons were evaluated on both columns because of the necessity to optimize the separation of the sulfur species from the predominant hydrocarbons and thereby minimize the pronounced quenching effect of hydrocarbons on the flame photometric signal (11,121. Temperature programs described in the literature (9, 11,13) had to be modified Envlron. Sci. Technol., Voi. 18, No. 8, 1984
593
Table I. Permeation Tubes and Gas Standards
concn, volume ppm
Paraho
Rio Blanco 0
Permeation Tubes" 47 40 32 45 35 27
cos cos cos cos
CHBSH CH3SH
so2 SO2
so2
Gas Cylindersc 23 X 96 X 221 1028 51 X 205 11 X 95 54
Rio Blanco 1 Xb X X (XI X X
X X
X
X X
I
I
I
i
6q'C
X X
I
ISOTHERMAL
120
60.
I
0
10
20
40
30
50
MINUTES
X X
I
(XI (XI
Figure 3. Gas chromatogram of sulfur species in undiluted offgas. Porapak QS column. Flame photometric detector. HzS doublet is due to detector saturatlon.
X X
H2S
RIO BLANC0 RETORT 1 10 14181
'Os
a Permeation tubes were supplied and certified by Metronics, Santa Clara, CA. Concentrations listed are based on the permeation rate at the temperature at which certified, atmospheric pressure, and 175 mL/min. b X = standard used for quantitation. (X) = standard available but not used for quantitation. Gas cylinders were supplied and certified by Matheson. Sulfur and nitrogen species were diluted in NZ Concentrations in volume ppm.
to accommodate the complexity of these retort gas samples. The one found most useful for both Porapak QS and Chromosorb 107 columns started at 60 OC initial temperature, was held isothermal for 6 min, then was programmed at 6 OC/min to 180 OC, and was held isothermal for -40 min. A slower temperature program was needed for SO, separation: the column was held isothermal at 40 OC for 20 min and then increased to 60 "C at 2 OC/min, followed by an increase to 84 "C at 6 OC/min and then a rapid ramping to 180 "C. Three types of standards, depending on availability, were used to identify and quantitate the samples: certified permeation tubes, certified gas standards, and liquid solutions. Table I lists the permeation tubes and gas standards available during sampling at the three retorts. All liquid sulfur standards were accurately diluted in either pentane, hexane, or heptane to avoid overlapping retention times and consequent hydrocarbon quenching of the sulfur signal. Dilutions of the sample and/or standards were necessary to bring the sample gas within the dynamic range of the detector (11,12). The N2 dilution apparatus used at all three retorts consisted of two calibrated rotameter (601 and 602 tubes) and a 1-L Pyrex mixing chamber. Adequate time for mixing was allowed. At Retort 1, an alternate dilution method made use of a vacuum pump to evacuate the gas sampling loop. The gas sample was introduced into the gas sampling loop until the required pressure was reached. The loop contents, at reduced pressure, were then injected into the GC. The relationship that is used for calculating concentrations is C, = C , ( A , / A , ) l / E
(1)
where C, = concentration of compound in sample (volume or mol ppm), C, = concentration of compound in calibration standard, A , = area response of compound in sample, A , = area response of compound in calibration standard, and E = slope of calibration log plot. Because of FPD nonlinearity, the best quantitative data were considered to be those in which the area of a sample peak approximately matched that of the calibration standard; then E could be assumed to be 2. The latter was especially 594
Envlron. Sci. Technol., Vol. 18, No. 8, 1984
4OC
40T 0
10
20
60° 84
30
180) 40
ISOTHERMAL
50
I
60
MINUTES
Figure 4. Gas chromatogram of sulfur species in undiluted offgas. Chromosorb 107 column. Flame photometric detector. Extended temperature program to separate SOz from the hydrocarbons.
necessary during the first trip to Retort 1when the Meloy 285 FPD electronics was used. The Meloy FPD linearizes the output based on the not necessarily correct assumption of a square law relationship between sulfur mass flow and FPD response (14). Bomb samples were returned to the lab for GC-MS analysis (HP5982) to confirm field identifications where stability and detection limits allowed. A Chromosorb 107 column was used. Because of field problems with the FPD during the second and third trips to Retort 0, GC-MS was also used to quantitate several sulfur species in grab samples obtained during these periods. It should be noted that instability of some sulfur species (notably H2Sand CH,SH) and lack of a gas sampling loop contribute to decreased accuracy for these values.
Results and Discussion Identification. Six sulfur species, in addition to hydrogen sulfide (H,S), have been identified in the retort gases at Rio Blanco and Paraho by GC and/or GC-MS: carbonyl sulfide (COS), carbon disulfide (CS,), thiophene (C4H4S),methylthiophene (C5H6S),methyl mercaptan (CH,SH), and sulfur dioxide (SO2) (Table 11; sample chromatograms, Figures 3 and 4). The first four compounds were matched with retention times of their respective standards on both Porapak QS and Chromosorb 107 GC columns. CH3SH was identified on the Porapak QS column, and SO, was identified on the Chromosorb 107 column. GC-MS analysis of a grab sample on a Chromosorb 107 column confirmed the presence of COS, CS,, SO,, thiophene, and methylthiophene. The other species either were too unstable to survive in the bomb or were
Table 11. Sulfur Species in Retort Gases
compounds analyzed hydrogen sulfide carbonyl sulfide methyl mercaptan carbon disulfide thiophene methylthiophene sulfur dioxide ethyl mercaptan isopropyl mercaptan dimethyl eulfide dimethyl disulfide
method of standard identification GC-FPD GC-MS Porapak Chromosorb Chromosorb QS 107 107
observed in Rio Blanco Retorts 0 Paraho and 1 gasesa retort gas
X X X X X X
X X X X X X
x
X X
X X
X
x
b X
X
X
b
X
X
X X X X
X
X X
X X
ODetection limit -1-10 pprn. *Stan-ird does not chromatograph on this cc--mn. Table 111. Quantitation of Sulfur Species in Retort Gas at Rio Blanco Retort 0'
cos
CH3SH
cs2
12:17
486*
dc
d
d
d
14:16 15:23 23:38 01:36 03:20 04:48
509 516 n.q.' n.q. n.q. n.q.
d d 80* 80* 90* 86*
d d 28* 27* 25* 25*
d d 55 37 40
d d 61 34 41 38
bomb heated bomb
419 350
n.p.8 n.p.
17 31
61 288
n.q. n.q.
0 (3.4%) 0
bomb
85
n.p.
15
238
n.q.
0 (3.7%)
394 f 164
84 f 5
24 f 6
date trip 0-1 10120180
10121180 trip 0-2f 11f 7 f 80 trip 0-38 11f 20f 80 average
HZS
thiophene methylthiophene
n.q. (2.0% ) d
44 f 12
'Volume ppm in the total gas (retort gas plus dilution gas) unless otherwise ihdicated. bAsterisk = best values. c d = too dilute to measure (due to high dilution factor). "All data in parentheses are from Rio Blanco Oil Shale Co. en.q. = not quantitated (detector saturated or standard not available). fGC-MS data. 8n.p. = no peak; Le., standard does not chromatograph well on GC column used.
masked by other components. For example, H2S, which was present at percent levels at the time of sampling at Retort 0, was no longer detectable 2-3 weeks after collection when the sample was analyzed in the laboratory. Additional sulfur species which were searched for by GC and/or GC-MS but were below our detection limits (1-10 ppm) include ethyl mercaptan, isopropyl mercaptan, dimethyl sulfide, and dimethyl disulfide. Prior to quantitation, efforts were made to optimize the separation of the six sulfur species from the predominant hydrocarbons. Hydrocarbon species observed in the Rio Blanco retort gases by GC-MS included C1-Clo alkanes and alkenes, a number of branched and mono- and diunsaturated hydrocarbons, benzene, and Cl-C,-alkylated benzenes. The temperature program previously described (see Experimental Section) resulted in separation of the hydrocarbons from COS, CH,SH, and CS, (Figure 5). The thiophene and methylthiophene peaks still overlapped with the C6 and C, hydrocarbons, respectively, so that these data are somewhat less reliable and the actual value would be slightly higher than that given in the tables. It should be noted that quenching could not be completely eliminated or reliably assessed for any of the components due to the slow, continual elution of retained hydrocarbons from previous analyses. This factor may account for some of the variability seen in the data. Quantitation. Quantitative data for the sulfur species at Rio Blanco Retorts 0 and 1 and the Paraho retort are
_____
60°C
60°
1206
laon
k F - - L - l - - l 20
FID
ISOTHERMAL
30 1
I
40
I 50
MINUTES
Flgure 5. Overlap of sulfur and hydrocarbonspecies in a retort offgas
on a Porapak OS column.
shown in Tables 111-V, respectively. Since retort gases are diluted with varying amounts of steam, air, or inert gas, the tabulated data represent the concentration of sulfur species in the total gas flow rather than that in the actual product gas. Since dilution flows are generally different at aboveground and in situ retorts, it is not possible to make a direct comparison between them. The H2Svalues supplied by the analytical personnel at each retort were highest at Rio Blanco Retort 0 (as high as 3.7 %) and lowest at Paraho (-0.3%); the variation at Retort 1 was considerable (0.4% to 2.5%). No attempt was made to Environ. Sci. Technol., Vol. 18, No. 8, 1984
595
Table IV. Quantitation of Sulfur Species in Retort Gas at Rio Blanco Retort la
date 1-1 7/14/81
7/15/81
7/16/81
7/17/81
1-2 1017181 10/8/81
10/9/81
10/10/81
10/11/81
10/14/81
CHSSH
CS2
thiophene
methylthiophene
1836 19:48 21:27 23:42 0053 03:55 0727 1041 12:55 1400 1500 0900 1245 14:OO 1920 0837 1500 1620 1935 21:35
49 n.q. n.q. n.q. 49 n.q. 48* n.q. 34* 44* 51* 34* 34* 27 33* 46* n.q. n.q. 57* 54
8 9 9 9 8 8 4 9 4 4 6* 9* 9 8
30* 29* 39* 47 35 42 30* n.q. 38* 34* 27* 27* 9 n.q.
56* 44* 66 66 n.q. 70 n.q. n.q. 63* 103 85 71 n.q. n.q. 24* 30* n.q. n.q. n.q. 19*
12:02 1420 2044 09:06 10:ll 11:17 12:30 16:23 21:28 0017 13:14 17:Ol 18:12 2307 00:17 02:20 07:49 1040 1309 1519 1659 22:14 2320 0018 01:23 02:36 12:02 14:47 1623 13:39 16:51 22:25
175 160 110 74 82 99 125 130 102 112* 75 54* 73 85 113 86 91 75 79 83 77 109 84 106 88 72 n.q. n.q. n.q. n.q. n.q. n.q.
9*
11
13 5 4 6 8*
10 n.q. n.q. 16 20
24 21 13 8 8*
49 53 33 27 19 28 28 51 35 33 35* 23* 39 23* 16 22* 38 26 24* 24 20 31 n.q. 37 25 28 37* 30 35
11
13 17 12 12 9* 7 9* 10* 10 10 14 9 13 11 12
14 12 20 12 15 n.q. n.q. n.q. n.q. n.q. n.q.
n.q. n.q. n.q.
35* 38 n.q. 19 8 20 21 34 30 14 17 16 24 21* 19 19 20* 24 19 18 n.q. 33 28 37 23 27 29* 31 37 n.q. n.q. n.q.
a Volume ppm in the total gas (retort gas plus dilution gas) unless otherwise indicated. n.q. = not quantitated but present (poor integration or detector saturated or too dilute to measure due to high dilution factor). CAsterisk= best values. dAll data in parentheses are from Rio Blanco Oil Shale Co.
quantitate H2Sby GC-FPD because of the extremely large dilutions necessary to approach the dynamic range of the FPD; indeed, H2S levels were high enough to saturate the detector and produce a doublet in the chromatogram (Figures 3 and 5). However, H2S was measured at retort 0 by using an Interscan H2S analyzer; these data agreed well with process H2S measurements made by Rio Blanco Oil Shale Co. personnel. The COS concentration was highest at the beginning of Retort 0 (-500 ppm) and generally decreased during this retort. These data are consistent with those obtained by Lawrence Livermore National Laboratory personnel using an on-line mass spectrometer (15),suggesting that these COS values were valid and that COS was reasonably stable in the bomb samples analyzed by laboratory GC-MS. COS 598
Environ. Sci. Technol., Vol. 18, No. 8, 1984
values at Retort 1were generally considerably lower (
