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Cite This: Energy Fuels XXXX, XXX, XXX−XXX
Behavior of Mercury in a Multicomponent Slurry Gasifier: Effects of Waste Cogasification and Water Redox Condition Yuying Du,† Qifei Huang,*,† Dahai Yan,† Li Li,† Xuebing Li,† Xihui Shen,‡ Jian Chen,‡ Zechun Huang,† Ning Wang,† and Meijia Liu† †
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State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing CN 100012, China ‡ Zhejiang Fengdeng Environmental Co., Ltd., Lanxi CN 321103, China ABSTRACT: The behavior of mercury in a multicomponent slurry gasifier (MCSG) is investigated in this work. An industrial scale MCSG for ammonia production operated at 1300−1400 °C and 1.0−1.6 MPa is used for the study. The effects of waste cogasification and redox condition of the process water are explored. The redox condition is found to be an important factor in determining Hg distribution in the MCSG output steams. Hg is mainly distributed to the clarified water (81.6−93.2%) in the tests with oxidizing water conditions. However, Hg primarily partitions into the raw syngas (82.5−99.2%) for the tests with reducing water conditions. Compared with gasification of coal only, Hg in the raw syngas from cogasification of waste with coal is slightly increased, even though the Hg level in the waste is generally high. An increase in the oxygen content in the process water is beneficial for Hg capture in the water and in the solid stream. The process water contributes 64.5−100.0% of the total Hg in MCSG when water recycling is employed. Attention should be paid to Hg removal in the process water.
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or fly ash but mainly partitioned to the gas phase. On the basis of the literature cited, the behavior of mercury depends on the specific gasification modes and their operating parameters. Multicomponent slurry gasifier (MCSG) is a type of coal− water slurry fed entrained-flow gasifier,16 which was developed in China and now widely applied around the world. This type of gasifier has been commercially used for waste treatment17,18 because it is designed to suit a large variety of feedstock. However, few studies have focused on the behavior of mercury in the MCSG gasification scheme, and the related available information is very limited. In this work, the behavior of mercury in the MCSG gasification scheme is investigated in an industrial scale chemical plant. The effects of waste cogasification and redox condition of process water are studied.
INTRODUCTION Coal gasification has been undertaken industrially for syngas production for over 200 years.1 According to the GSTC Database 2017 update,2 there are currently 2886 gasifiers in the world, with the cumulative gasification capacity of syngas expected to reach 400 MWth in 2020. The release of volatile trace elements, such as Hg,3 in the gasification process is always of environmental and Occupational Safety and Health4 concern. Many thermodynamic equilibrium and bench-scale experimental5−7 studies have reported that a significant fraction of mercury vaporizes during coal gasification. The reducing environment prevailing within the gasifier is not favorable for mercury oxidation via gas-phase reactions alone. Mercury remains in the vapor phase in the temperature range of 500− 600 °C, which is associated with the operating temperatures in most hot gas removal systems. Fly ash could promote surface oxidation of elemental mercury with typical Cl and S species in syngas, playing a remarkable role in mercury removal. Pilot and industrial investigations have been carried out on fixed-bed, fluidized-bed, and entrained-flow gasifiers, the three principal commercial gasification modes. In the fixed-bed gasifier,8−10 the mercury content decreased from the drying to combustion zones, with almost all of the mercury changed to the gaseous phase at the end of the pyrolysis zone. Ascribed to the carbon adsorption occurring within the lower half of the bed, 1% of mercury content is present in the discharged ash. In the fluidized-bed gasifier,11 mercury is detected in the gas rather than in the hot gas filter fine dust or bed char. In the entrained-flow gasifier,12−15 mercury is distributed among the slag, fly ash, and effluents from gas scrubbing but scarcely detected in the treated syngas or elemental sulfur from the sulfur recovery unit. Complete opposite phenomena, however, were also reported, that mercury was not found in either slag © XXXX American Chemical Society
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METHODS
MCSG Gasification System. Testing was carried out in an MCSG system at a 100 t/d ammonia plant. Schematic diagram of the MCSG is given in Figure 1. The gasifier operates under nominal conditions of 1300−1400 °C and 1.0−1.6 MPa to assure slagging operation and high carbon conversion. Coal, waste, and water are used to prepare the slurry feed; pure oxygen is used as the gasification agent. The raw syngas and the molten ash from the gasifier directly enter the water quench below for cooling. The ash is then removed in the form of slag by a lock hopper. Fine solids leave with the syngas stream. The raw syngas undergoes a series of wet gas-treatment processes: the separator for dewatering, the venturi scrubber for particulate removal, the shift unit for CO conversion, and the acid gas removal unit for desulfurization. The water from the quench, separator, and scrubber are sent for flashing, where the dissolved gases are driven from the water and discharged as exhaust gas. The Received: March 22, 2019 Revised: May 31, 2019
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DOI: 10.1021/acs.energyfuels.9b00897 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
of the shift unit were collected for analysis in tests 5−8. The samples were digested and analyzed for mercury concentration using inductively coupled plasma mass spectrometry, which is capable of detecting and quantifying a range of metals at concentrations in the parts per billion range and below.
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RESULTS AND DISCUSSION Slurry Feeds. The concentration of mercury in the slurry feeds are given in Table 2. The level of mercury in the coal− waste−water slurry (tests 3, 4, 7, and 8) was much higher than in the coal−water slurry (tests 1, 2, 5, and 6) and quite variable in the range of 0.008−0.142 ppm because of waste heterogeneity. The mercury in the slurry feeds was expected to be totally released into the raw syngas in gasification at 1300−1400 °C within the MCSG reactor. Raw Syngas. Mercury concentration in the raw syngas is shown in Figure 2. It can be seen that mercury was detected in the raw syngas in all measurements. This is attributed to the reducing condition within the gasifier reactor, which is not favorable for Hg0 oxidation. The concentration of mercury in the raw syngas in the tests with water-oxidizing condition (tests 1−4) is much lower than those for the tests with waterreducing condition (tests 5−8); this may be caused by the oxidization of dissolved gaseous Hg0 in oxygen-containing water via a multiphase reaction and the dissolving of the product Hg2+ in water. A high level of mercury is seen in the raw syngas in the tests with the coal−water slurry and waterreducing condition (tests 5 and 6), despite the absence in the slurry feeds, indicating the existence of other mercury input stream in the MCSG. Clarified Water. The concentrations of mercury in the clarified water are shown in Figure 3. The mercury in the clarified water is found in the range of 0.41−0.79 ppm in the tests with water-oxidizing condition (tests 1−4). The reason for the high mercury level in water has been described in the above section. This high level of mercury gives an indication that the process water could be another mercury input source in the MCSG because a large portion of the clarified water was recycled to the direct water quench as process water. Bottom Slag. Mercury concentrations in the bottom slags are given in Figure 4. Measurements below the analytical limit of detection are given as 0 for comparison purpose in this paper. It can be seen that mercury levels in the bottom slags were detected in the range of 0.33−1.48 ppm in the tests with water-oxidizing condition (tests 1−4), much higher than those observed for the tests with water-reducing condition (tests 5−
Figure 1. Schematic diagram of MCSG. wastewater from flash is sent to the clarifier for sediment. An average of 85% of the clarified water is typically recycled as process water, mostly to the direct water quench, while the other 15% is discharged after water treatment. Test Arrangement. The test conditions for investigations in this work are given in Table 1. Wastes used in these tests were organic hazardous wastes from leather processing and pharmaceutical industries. These incoming wastes had a high moisture content (ca. 80% on an as-received basis). Tests with and without waste were included to study its effect on mercury behavior. In the tests with wastes, the wastes were 56 wt % of the sum of coal and waste on an as-received basis. In all tests, the slurry solid−liquid mass ratio was controlled between 55:45 and 60:40. The raw syngas was mainly composed of H2, CO, and CO2. The ratio of (CO + H2) in the total raw syngas was in the range of 74−77%. The clarifier was modified from the advection settling basin (ASB) to the microeddy tank (MET). The former with water exposed to air caused a wateroxidizing condition; the latter without water exposed to air led to a water-reducing condition. Tests with the ASB and the MET were included to provide information on the effect of redox condition of process water on mercury behavior. Mercury Sampling and Analysis. The samples of the raw syngas were taken after the venturi scrubber in all tests, using modified EPA method 29; the sour syngas was taken downstream of the shift unit in tests 5−8. The slurry feeds and bottom slags were sampled in all tests. The former was taken upstream of the gasifier and the latter from the slag tank. Samples of the clarified water and clarified solids were taken in tests 1−4, whereas those of the flash wastewater and exhaust gas were taken in tests 5−8. The condensates from and liquids upstream
Table 1. MCSG Test Conditionsa gasifier
clarifier syngas composition %
test
time
1 2 3 4 5 6 7 8
Y1D1AM Y1D1PM Y1D2AM Y1D2PM Y2D1AM Y2D1PM Y2D2AM Y2D2PM
coal coal coal coal coal coal coal coal
slurry feed
H2
CO
CO2
type
water redox condition
+ + + + + + + +
27.5 27.5 28.6 28.6 26.3 26.3 27.8 27.8
47.3 47.3 47.2 47.2 50.3 50.3 46.2 46.2
20.4 20.4 19.5 19.5 16.9 16.9 19.4 19.4
ASB ASB ASB ASB MET MET MET MET
oxidizing oxidizing oxidizing oxidizing reducing reducing reducing reducing
water water waste waste water water waste waste
+ water + water
+ water + water
a Y1D1 is the first day of the first year; Y1D2 is the second day of the first year; Y2D1 is the first day of the second year; Y2D2 is the second day of the second year.
B
DOI: 10.1021/acs.energyfuels.9b00897 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 2. Mercury Analyses of the Slurry Feedsa mercury concentration, ppm, as received test
1
2
3
4
5
6
7
8
slurry
0.016
0.003
0.065
0.008
ND
ND
0.142
0.136
a ND, not detected; tests 1, 2, 5, and 6 are the tests with coal−water slurry as the feed; tests 3, 4, 7, and 8 are the tests with coal−waste−water slurry as the feed.
Figure 2. Raw syngas mercury measurements (tests 1−8).
Figure 3. Measurements of mercury in the clarified water for the tests with water under oxidizing condition (tests 1−4).
Exhaust Gas and Wastewater from Flash. Mercury concentrations in the flash wastewater and exhaust gas are given in Figure 6. Mercury level in the flash wastewater in the tests with reducing water (tests 5−8) was seen in the range of 1.2−26.4 ppb, 3 orders of magnitude lower than that in the clarified water under oxidizing condition (tests 1−4), indicating that the reducing water is less capable of mercury capture, in line with our previous observation. The level of mercury in the exhaust gas from flash was shown in the range of 7.3−32.0 μg/Nm3, comparable to that in the flash wastewater, suggesting that the gaseous dissolved Hg0 in the reducing water would be released into the gas phase rather than oxidized into Hg2+ that is soluble and more stable in water.
8). This is attributed to the oxidization of dissolved gaseous Hg0 in the presence of oxygen in water and thereby adsorption of the product Hg2+ on slag particle surfaces, consistent with our previous analyses. Clarifier Solid. The concentrations of mercury in the clarifier solids are shown in Figure 5. Clarifier solids originate from the fine solids generated in the gasifier reactor, carried downstream with syngas, captured by wet cleaning equipment, and eventually settled in the clarifier. Mercury levels in the clarifier solids for the tests with oxidizing water (tests 1−4) were observed in the range of 2.59−9.53 ppm, much higher than in the bottom slags, mainly due to the finer particle size, higher carbon content, and longer residence time in water. C
DOI: 10.1021/acs.energyfuels.9b00897 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
Figure 4. Bottom slag mercury measurements (tests 1−8).
Figure 5. Measurements of mercury in the clarifier solid for the tests with water under oxidizing condition (tests 1−4).
Shift Unit. The concentrations of mercury in the condensate from and liquid upstream of the shift are shown in Figure 7. Mercury level in the liquids condensed during syngas sampling upstream of the shift were detected in the range of 7.1−33.7 μg/L for the tests with reducing water (tests 5−8), whereas that in the condensates from the shift was in the range of 0−4.6 μg/L. The evident decrease in the mercury concentration in the liquid phase after shift conversion is a reflection of the reducing mercury level in the syngas, as confirmed by the mercury level in the raw syngas upstream of the shift (270−540 μg/Nm3) and that in the sour syngas downstream of the shift (close to 0 μg/Nm3). This indicates that the shift unit is capable of mercury capture, probably due to the oxidization and adsorption of mercury on the shift catalyst surface in the presence of sulfide. Lots of research have reported on the catalysts developed for mercury capture in reducing atmosphere, such as Fe2O3. However, little is reported on the catalysts that are typically applied in the shift unit. Input Distribution. Mercury distribution in the input streams from MCSG is given in Table 3. It can be seen that the process water contributed most of the Hg in all tests (64.5−
100.0%). The slurry feed also made a significant contribution to the input of Hg in the tests with waste addition and waterreducing condition (tests 7 and 8). Output Distribution. The sum of the quantifiable mass flow rates of mercury in MCSG output streams is assumed to be the total amount. Distribution data for mercury is given in Table 4. The mercury (81.6−93.2%) distributed toward the clarified water for the tests with water-oxidizing condition (tests 1−4). This is due to the effect of the ASBs in dissolving oxygen in water and thereby oxidizing the dissolved gaseous Hg0 from the raw syngas. The mercury (82.5−99.2%) partitioned to the raw syngas in the tests with water-reducing condition (tests 5−8), only 0.8−17.5% to the water. This is because of the reducing condition in water because of the MET and the release of dissolved gaseous Hg0 in water to the gas phase. Effect of Waste Cogasification. The output distribution data for mercury indicates that the major proportion of mercury was in the clarified water in the tests with wateroxidizing condition (tests 1−4) and in the raw syngas in the tests with water-reducing condition (tests 5−8), and the overall distribution to the output streams was only slightly D
DOI: 10.1021/acs.energyfuels.9b00897 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
Figure 6. Measurements of mercury in the flash wastewater and exhaust gas for the tests with water under reducing condition (tests 5−8).
Figure 7. Measurements of mercury in the condensate from and liquid upstream of the shift for the tests with water-reducing condition (tests 5−8).
affected by cogasification of wastes within the variety studied here. The distribution of mercury to the raw syngas appears to be slightly higher in the tests with wastes addition (tests 3, 4, 7, and 8) than in the tests without wastes (tests 1, 2, 5, and 6) when compared under the same water redox condition. This is probably due to the capture of Hg by the water becoming lower as the Hg concentration in the raw syngas leaving the gasifier increases (as a result of the higher Hg input in the tests with wastes addition). The mercury proportion to the water from tests 1, 2, 5, and 6 (without wastes) and tests 3, 4, 7, and 8 (with wastes) are compared under the same water redox condition; the difference between tests 1−2 (without wastes) and tests 3−4 (with wastes) and between tests 5−6 (without wastes) and tests 7−8 (with wastes) is less than 20%.
Table 3. Distribution of Mercury in MCSG Input Streams mercury distribution, wt % test
slurry feed
process water
1 2 3 4 5 6 7 8
0.5 0.1 3.3 0.4 0.0 0.0 35.4 35.5
99.5 99.9 96.7 99.6 100.0 100.0 64.6 64.5
E
DOI: 10.1021/acs.energyfuels.9b00897 Energy Fuels XXXX, XXX, XXX−XXX
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ACKNOWLEDGMENTS This work was supported by the China Postdoctoral Science Foundation (2018M641441) and the National Key R&D Program of China (2018YFC1900102).
Table 4. Mercury Distribution in the Output Streams of MCSG mercury distribution, wt % test
bottom slag
raw syngas
clarified water
clarifier solid
1 2 3 4 5 6 7 8
3.1 1.5 3.6 1.1 0.0a 0.0a 0.0a 0.0a
0.5 2.2 3.0 3.7 93.8 82.5 99.2 98.9
92.0 93.2 90.1 81.6 6.2b 17.5b 0.8b 1.1b
4.4 3.1 3.3 13.6
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REFERENCES
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a
Measurement below the detection limit is assumed to be 0 in calculation. bMercury distribution to the flash wastewater, which is equivalent to the sum of the clarified water and the clarifier solid.
Effect of Water Redox Condition. The level of Hg in the raw syngas is seen evidently higher in the tests with waterreducing condition (tests 5−8) than in the tests with wateroxidizing condition (tests 1−4), whereas that in the bottom slag is significantly lower. Therefore, the mercury output distribution is very different for the tests with water-oxidizing condition (tests 1−4) than in the tests with reducing condition (tests 5−8), and this has been shown to have a profound effect on the possibility of mercury recycling in the MCSG system.
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CONCLUSIONS When ASBs were used as the clarifier, Hg is mainly distributed to the clarified water (81.6−93.2%) because of the oxidizing condition in water. In this case, Hg only slightly partitions to the water and the solids. In contrast, when a MET was used, Hg primarily partitions to the raw syngas (82.5−99.2%) because of the reducing condition of water; Hg only slightly partitions to the water (0.8−17.5%) and scarcely partitions to the solid. The redox condition of the process water is an important factor in determining the Hg distribution in the MCSG output steams. Thus, an increase in the oxygen content of the process water is beneficial for capturing Hg in the water and solids. Compared with the gasification of coal only, Hg in the raw syngas from cogasification of waste with coal is slightly increased, even though the Hg level in the waste is generally high. In the meanwhile, the proportion of Hg that is distributed to the water from cogasification of waste with coal drops. The process water contributes 64.5−100.0% of the total Hg in MCSG when water recycling was adopted. Therefore, attention should be paid to water treatment for Hg removal in the process water.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Qifei Huang: 0000-0003-1364-6321 Li Li: 0000-0001-6649-0215 Notes
The authors declare no competing financial interest. F
DOI: 10.1021/acs.energyfuels.9b00897 Energy Fuels XXXX, XXX, XXX−XXX
