Article pubs.acs.org/est
Sulfur Emission from Victorian Brown Coal Under Pyrolysis, Oxy-Fuel Combustion and Gasification Conditions Luguang Chen and Sankar Bhattacharya* Department of Chemical Engineering, Monash University, Melbourne, Australia ABSTRACT: Sulfur emission from a Victorian brown coal was quantitatively determined through controlled experiments in a continuously fed drop-tube furnace under three different atmospheres: pyrolysis, oxy-fuel combustion, and carbon dioxide gasification conditions. The species measured were H2S, SO2, COS, CS2, and more importantly SO3. The temperature (873−1273 K) and gas environment effects on the sulfur species emission were investigated. The effect of residence time on the emission of those species was also assessed under oxy-fuel condition. The emission of the sulfur species depended on the reaction environment. H2S, SO2, and CS2 are the major species during pyrolysis, oxy-fuel, and gasification. Up to 10% of coal sulfur was found to be converted to SO3 under oxy-fuel combustion, whereas SO3 was undetectable during pyrolysis and gasification. The trend of the experimental results was qualitatively matched by thermodynamic predictions. The residence time had little effect on the release of those species. The release of sulfur oxides, in particular both SO2 and SO3, is considerably high during oxy-fuel combustion even though the sulfur content in Morwell coal is only 0.80%. Therefore, for Morwell coal utilization during oxy-fuel combustion, additional sulfur removal, or polishing systems will be required in order to avoid corrosion in the boiler and in the CO2 separation units of the CO2 capture systems. TGA-MS, and a fixed-bed reactor by Zhang et al.4 SO2 and H2S were detected by MS and a mechanism of sulfur transformation during pyrolysis of lignite was proposed. Chu et al.7 studied sulfur release from direct liquefaction residue of Shenhua coal under pyrolysis and CO2 gasification condition. In their study, they focused only on H2S and COS detection by gas chromatography. Investigations on SO3 emission from coal combustion and gasification are rare, although there are a number of studies on sulfur emission from coal in general but they only concentrated on other gaseous sulfur compounds, such as H2S, SO2, and COS.3,4,8−11 There has been no reported study on emission of sulfur species during oxy-fuel combustion and gasification of Victorian brown coal. In this study we report information on release of sulfur species, including H2S, SO2, SO3, CS2, and COS during pyrolysis, oxy-fuel combustion, and CO2 gasification of a Victorian brown coal, Morwell. This comprehensive study attempts to fill that void and generate fundamental information that is also pertinent to design of the advanced power systems for use with CCS.
1. INTRODUCTION Victoria has large reserves of brown coal, about 430 years worth at current consumption rates. These brown coals are characterized by high moisture content, often up to 65%,1 that results in low overall efficiency and high CO2 emission during conventional pulverized coal-fired power generation. Consequently, new technologies that are amenable to carbon dioxide capture and storage (CCS), such as oxy-fuel combustion and gasification of coal, are currently being assessed for use with these brown coals. For operation with oxy-fuel and gasification based CCS systems, one important information is the CO2 concentration and presence of the trace gases therein, including the sulfur species. Sulfur contents in coal can vary from 0.2% up to 11%, present as organic, inorganic and elemental sulfur.2−4 Pyrite and sulfate are the common forms of inorganic sulfur. Organic sulfur is an integral part of the coal matrix, such as thiolic (R− H−S), sulphidic (R−S−R’), disulphidic (R−S−S−R), sulphoxide (R−S−O−R), and thiophenic (heterocyclic) structure; a minor amount of elemental sulfur also occurs in coal. Sulfur in coal participates in various reactions and ultimately ends up in solid and gaseous phases during combustion and gasification. The gaseous sulfur species, such as SO2, SO3, H2S, CS2, and COS, are of concern as environmental pollutants, which also impact on the CO2 purity for transport and storage5,6 as previously mentioned. Sulfur release under variable atmospheres has been studied by several researchers. Sulfur transformation of a Western Australian lignite during prolysis was investigated using TGA, © 2013 American Chemical Society
Received: Revised: Accepted: Published: 1729
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2. EXPERIMENTAL SECTION Morwell coal, which is currently used for base load power generation in Victoria, was used in the experiments. It was airdried, ground and sieved to 106∼150 μm size before being used. Proximate analysis was carried out by TGA and ultimate analysis was done by CHNS 2400 analyzer. The forms of sulfur were determined according to AS1038.11. Total sulfur content was determined according to AS1038.6.3.3. The coal contains 0.80% total sulfur on a dry basis, of which 0.50% is inorganic sulfur and the rest are organically bonded. Of the inorganic sulfur, the pyritic form sulfur is 0.27%. The composition of Morwell coal is listed in Table 1.
Sulfur release from Morwell coal under oxy-fuel condition with a longer residence time, from 2.5 s at 1073 K to 3.5 s at 1273 K, was also investigated in a larger drop-tube furnace with an inner reactor diameter of 50 mm. The coal feeding rate was 200 mg min−1 and the gas was fed at 10 L min−1 in order to control the coal/gas ratio as same as the experimental ratio in the small drop-tube furnace. The sulfur distribution among solid and liquid phases was measured according to AS1038.6.3.3, using a Leco Truspec analyzer. The outlet of the reactor gas was analyzed by a micro gas chromatograph (GC), which consists of two independent columns: column 1 was molecular sieve 5 Å and column 2 was PoraPLOT Q. The individual columns are designed to achieve optimum separation of a given set of gas components. Each of these columns has a different carrier gas: for the molecular sieve, ultra high purity argon is used and for PoraPLOT Q, ultra high purity helium is applied as a carrier gas. Coupled with the use of small-scale thermal conductivity detectors, elution time of gas species was much more rapid than in our conventional GC. During each experiment, a sequence with 30 runs was set and each run consisted of 210 s measurement time and 40 s equilibration time. The final results were based on the average of these runs. The molecular sieve column was used to detect H2, CO, and CH4; and the PoraPLOT Q column was primarily used to identify sulfur species and CO2. Individual calibration gases for H2, H2S, COS, CO, and CO2 were used. Gaseous CS2 was also measured by draeger tubes with detection limit 3 ppm. By using the combined DTF/micro-GC/draeger tubes, the system is able to identify quantitatively the individual sulfur species directly in the gaseous phase with the exception of SO3, which was measured separately as described in the following section. The schematic of the entire experimental setup is shown in Figure 1. 2.2. Determination of SO3. Several methods for SO2 detection and analysis have been developed in the literature.6,11−13 However, established methods for SO3 detection and analysis are very few. In even very small concentrations, SO3 has negative effects on plant operation mainly through the formation of sulphuric acid, which can lead to corrosion of
Table 1. Morwell Coal Analysis, wt% Air-Dried moisture 15.30 ash 4.60 voatile matter 29.90 fixed carbon 50.20 Ultimate Analysis, wt%, Dry Ash Free Basis carbon 74.85 hydrogen 4.96 oxygen 18.28 nitrogen 1.11 Sulfur wt% total 0.80 sulfate 0.23 pyritic 0.27 organic 0.30
2.1. Experimental Conditions, Char and Gas Composition Measurement. Two drop-tube furnaces (DTF) were used in the experiments. The first DTF has an inner diameter 22 mm, placed inside an electrically heated furnace with three separately controllable heating zones. The coal was fed from the top of the reactor at a controlled rate of approximately 20 mg min−1 by a screwfeeder. The residence time of the particles inside the reactor was around 0.4 s, calculated based on free-fall velocity of the particles. The reactant and/or carrier gas was fed at 1 L min−1 into the external reactor chamber, where it was heated to furnace temperature before entering the internal reaction chamber along with the coal at the top of the furnace. The second DTF is a larger one of 2m length and 50 mm diameter, and capable of having longer residence time up to 6 s. The reactor was heated up to the desired temperature in the range of 873−1273 K at least 1 h before coal feeding. To minimize the problem of sulfur adsorption inside the reactor and associated piping, a mixture of sulfur compounds in N2 was fed for an hour after the desired temperature was achieved. The reactor was then flushed with N2 before commencement of each experiment to ensure there was no desorption of sulfur species. The different reaction atmosphere conditions used in this study are listed in Table 2. Table 2. Reaction Environment (% Volume) reactant/carrier gas
pyrolysis
N2 O2 CO2
100
oxy-fuel
gasification
21 79
100
Figure 1. Schematic of the experimental setup. 1730
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surfaces in the heat recovery zones13,14 of a boiler. It also affects the CO2 quality required for CCS.5 Therefore, it is important to accurately measure SO3 when formed. The following SO3 determination method was used in this study.15 At temperature between 573 and 648 K, Calcium oxalate reacts with SO3(g), but not with SO2, following Scheme 1. Therefore, SO3 can be measured indirectly by measuring CO by the micro-GC in the molecular sieve column. SO3 + CaC2O4 → CaSO4 + CO2 + CO
(1)
CaC2O4·H2O was obtained from Sigma Aldrich and mixed with glass beads in a ratio of 3−10. The mix was placed in a three-neck round-bottom flask to form a 10 cm thick bed. The flask was connected to the reactor and heated for 1 h at 598 K with N2 purging through it to dehydrate the reagent and the flask interior. Following this, the flask was connected to the micro-GC and the entire system purged again with N2. During the entire experiment period, the temperature of the SO3 trap was carefully controlled between 598 and 623 K in order to ensure that only SO3 reacted with CaC2O4 and no reaction took place between SO2 and CaC2O4. 2.3. Thermodynamic Equilibrium Analysis. Factsage 5.3 used for the equilibrium calculations is capable of determining the distribution of chemical species in vapor as well in ash based on thermodynamic equilibrium at temperatures 1073 K to 1273 K. The input file includes all components of coal expressed in grams together with the combustion conditions including gas composition, temperature, and pressure.
Figure 2. Gas phase conversion of sulfur as COS, SO2, CS2, and H2S during pyrolysis as a function of temperature, H2S figures shown in the inset.
increase of the temperature to 1173 K before the emission stabilized. The sulfur in the pyrite is known to be released at relatively low temperature, ≥ 823 K. However, some of the H2S from the coal converts back to form a stable organic sulfur, with a stable thiophenic structure which is difficult to remove.2 This may be the reason that higher concentration H2S was detected at low temperature, but at higher temperature, the concentration was lower. The conversion of sulfur to COS, SO2, and CS2, presented in Figure 2, exhibit different trends. COS was observed to decrease in contribution to the gas phase conversion of sulfur from around 1.2% to around 0.4% from 873 to 1173 K, followed by subsequent increase to 0.5% when pyrolyzed at 1273 K. There were 0.24% sulfur emission as SO2 at 873 K, which then fell to undetectable level when the pyrolysis temperature was increased to 1073 K. However, when the coal was pyrolyzed at 1273 K, a trace amount of SO2 was measured. The evolution of SO2 at the lower temperature is due to the decomposition of the sulfates and the residual SO2 should be the result of the pyrolysis of organic sulfones.16 No amount of SO3 was detected under pyrolysis conditions. Carbonyl sulphide is formed by reaction of CO and elemental sulfur from pyrite decomposition and also by the decomposition of organic sulfur structures.16 CS2 was detected from the temperature of 1073 K onward. The release of CS2 peaked at 1173 K, with a conversion of 0.3%. It dropped with increasing pyrolysis temperature. We can make a comparison of our results from limited published results on other lignites. Compared with the sulfur distribution results of two lignites by Garcia-Labiano et al.,17 the release of total sulfur species in gaseous phase from pyrolysis of Morwell brown was in the range of between 14.1% and 14.4% in the temperature range of 873−1273 K, which was close to the value detected from the Spanish lignite A around 14.0%. We have made a thermodynamic equilibrium prediction of Sulfur species emission from Morwell coal during pyrolysis by using commercial software Factsage in the temperature range used in our study. Similar to our experimental results, H2S was the dominant sulfur species predicted by the thermodynamic simulation. The predicted trends of COS and CS2 emission also supported the experimentally observed trends. However, there
3. RESULTS AND DISCUSSION The results on sulfur species emission from Morwell coal under pyrolysis, oxy-fuel and CO2 gasification conditions will be discussed in the following sections. 3.1. Sulfur Release during Pyrolysis Experiments. The release of gaseous sulfur species of Morwell brown coal during pyrolysis in the drop-tube reactor was measured by the microGC at four temperatures: 873 K, 1073 K, 1173 K, and 1273 K. The sulfur distribution between the solid and liquid phases was measured according to AS1038.6.3.3, using a Leco Truspec analyzer, and the results are presented in Table 3. It can be seen Table 3. Sulphur Distribution by Phases (%) solid liquid gas (by diff.)
873 K
1073 K
1173 K
1273 K
18.9 53.6 27.5
32.2 53.4 14.4
32.9 53.0 14.1
33.0 52.6 14.4
that 18.9% sulfur remained in the solid phase when coal was pyrolyzed at 873 K and the rest were released in either gaseous phase or as tar. Sulfur retention in char beyond 873 K stabilized at around 33% of its original amount due to the cross-linking and reattachment mechanisms such that the released sulfur did not easily escape from the char intraparticle surface.2,3 Between 1073 and 1273 K, only small variations were observed in the sulfur distribution between the solid, liquid,and gas phases; this small variation most likely reflects the heterogeneity of the composition of coal and uncertainty of the instrument. The hydrogen sulphide release during pyrolysis as a function of temperature is presented in Figure 2. H2S was the dominant gaseous sulfur compound during the pyrolysis. About 27% of the coal sulfur released as H2S at 873 K temperature. The conversion to H2S dropped to approximately 14% with the 1731
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between H2S and COS. As it can be seen in Figure 3, the conversion of sulfur to SO2 between 1073 and 1273 K was over 47%, which was far more than the trace amounts detected during the pyrolysis experiments. The conversion of sulfur to SO2 was found to slightly increase with the increase of combustion temperature. This is expected as high temperature favors SO2 formation.19 Significant amount SO3 was detected in the gas phase during the oxy-fuel combustion of Morwell brown coal. As previously indicated, the emission of SO3 was determined by measuring the difference of CO with and without the CaC2O4 trap. The result is shown in Figure 3. The conversion of sulfur to SO3 was found to decrease with the combustion temperature. The maximum conversion detected was around 10.8% of coal sulfur at 1073 K before reducing to just over 3% at 1273 K. The decrease of conversion is likely to be caused by thermally stable sulphates formed, and remaining in the solid phase. Zheng et al.19 applied a chemical thermodynamics method to predict the SOx emission in the flue gas under oxy-fuel condition. Our SOx emission trend was similar to their conclusion, which was also supported by the study from Duan et al.20 The emission of SO2 increased while SO3 decreased with increasing combustion temperature under oxyfuel condition. Our experimental results support the trends in their experimental results. The experimental results of sulfur species release under oxyfuel combustion were also compared with the thermodynamic equilibrium predictions using Factsage. Sulfur oxides are the predicted predominant gaseous sulfur species during combusiton. The thermodynamic predictions confirmed that the sulfur trioxide emission would reduce with increasing combustion temperature. As shown in Figure 3, the dashed line represents the equilibrium prediction of SO3, which is slightly higher than the trend of SO3 measured in the experiments, shown by the solid line. However, the prediction is in good agreement with the experimental results at the higher combustion temperature range. The higher combustion temperature enhanced the emission of COS, which was supported by both experimental and simulation results. A trace amount CS2 was predicted by the model, but this was not detected during the experiments. The sulfur emission with 3.5 s residence time was also investigated in the larger drop-tube furnace. The feeding rate and the gas flow rate were adjusted to keep the same coal/gas ratio as in the small drop-tube furnace. The emission of sulfur species in the flue gas are displayed in Figure 4 No CS2 was detected under the 21% v/v oxygen oxy-fuel experimental conditions. The percentage of sulfur in the coal converted to SO2 ranged between 40% and 50%, which is similar to that measured in the small rig but with a clear increasing trend. The second dominant sulfur species was H2S under the current experimental condition, which equated to around 15% conversion of the coal sulfur. Approximately 10% of coal sulfur converted to SO3, and about 1% sulfur converted to COS at 1073 K. However, the coal sulfur conversion to SO3 decreased with the increase of the temperature. The conversion to COS changed significantly when the temperature reached 1173 K. Only around 3% of coal sulfur converted to SO3 but almost 15% coal sulfur converted to COS at 1273 K. Further observations of sulfur emission under oxy-fuel condition can be made. The release of sulfur oxides increased during combustion in an oxy-fuel environment. There was no detectable CS2 observed in the outlet gas. Over 10% of coal sulfur was released as SO3 at 1073 K, which then reduced to
was no SO2 formation predicted in the gaseous phase during pyrolysis by the thermodynamic simulations. A conclusion can be drawn from both experiment and modeling prediction. Emission of sulfur species reduced with the increase of pyrolysis temperature. H2S is the major sulfur species released during pyrolysis. Only a trace (less than 0.1 ppm) amount sulfur dioxide was detected during pyrolysis except at 873 K when 2.3% sulfur was released as SO2. The emission of sulfur species was found to follow the order of H2S > COS > CS2 > SO2 when the pyrolysis temperature was in the range of 1073−1273K. 3.2. Sulfur Release during Oxy-Fuel Combustion Experiments. The emission of sulfur species from Morwell brown coal during oxy-fuel combustion in the temperature range from 1073 to 1273 K was investigated. As shown in Table 2, the reacting gas composition in our experiments consisted of 21% v/v O2 balanced by 79% CO2. No CO and CH4 emissions were detected in the flue gas, which indicates that the combustion was complete in the gaseous environment used in our experiments. However interestingly, CS2 was not detected in the flue gas. This supports the proposed path for CS2 formation by reaction of H2S with carbon and/or the methane suggested by TorresOrdonez et al.18 Emission of the other sulfur species, especially sulfur oxides, in the gas phase will be discussed in the following section. The evolved Sulfur species, except SO3, were monitored by micro-GC during oxy-fuel combustion of Morwell coal at three different temperatures; the results are presented in Figure 3.
Figure 3. Gas phase conversion of sulfur as COS, SO2, H2S, and SO3 with a residence time of 0.4 s during oxy-fuel as a function of temperature, experimental data, and equilibrium prediction for SO3.
Hydrogen sulphide was found to be present, even though, as expected, its concentration during combustion was lower compared with pyrolysis at the same temperature; however, emission trends are similar. The maximum sulfur conversion as H2S was detected at the temperature of 1073 K with a value of 16% of the sulfur in coal, which was about 40% of the maximum value measured at 873 K during the pyrolysis. The rich oxygen environment resulted in further oxidation of H2S. The conversion of sulfur to COS was found to increase with the increase of combustion temperature. About 0.5% of coal sulfur was found to convert to COS when combusted at 1073 K. Thehe conversion percentage increased to 10.8% at 1273K. These values are also higher than the conversion during pyrolysis. The likely reason is the gas phase equilibrium 1732
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CO was one of the main product gases and this is most likely to be the cause for increase of COS production during gasification. FeS2 + CO → COS + FeS
(2)
Another dominant sulfur gas was CS2 and the result is shown in Figure 5. The sulfur conversion to CS2 during gasification illustrated an increasing trend with temperature. The maximum CS2 conversion percentage was measured at 1273 K of 29.5%, which was almost 24%- point higher than the value detected at 1073 K. We also made thermodynamic predictions to compare with our experimental results. As expected, H2S was the dominant species than COS when gasifying at the same temperature. The trend of released sulfur species predicted by Factsage was similar to that determined in the experiments in the temperature range of 1073−1273 K. Our experimental result of higher H2S emission than COS, although supported by the thermodynamic prediction, is different from the observations by Chu et al.7 from their investigation involving Shenhua char. In their study, higher level of COS was detected by gas chromatography during gasification at 1273 K. The major observations for the sulfur emission under gasification condition are as follows; when Morwell coal was gasified in CO2 at 1073 K, H2S was the predominant gaseous sulfur species. However, CS2 became to the major sulfur gas product when the gasification temperature reached at 1173 K and above, followed by H2S, COS, and SO2.
Figure 4. Gas phase conversion of sulfur as COS, SO2, H2S, and SO3 with a residence time of 3.5 s during oxy-fuel as a function of temperature.
3.4% by 1273 K. SO2 was major gaseous sulfur pollution in oxyfuel condition, followed by H2S, SO3 when combustion temperature is lower than 1173 K, or COS at the temperature of 1273 K. Moreover, there is a clear evidence from the results between 0.4 and 3.5 s residence time that the emission of sulfur species was not affected by residence time used in this study. 3.3. Sulfur Release during Gasification Experiments. The emission of sulfur species during CO2 gasification of Morwell coal will be discussed in this section. A trace amount (less than 0.1 ppm) of SO3 was detected during CO2 gasification in the experimental temperature range. The conversion of H2S, COS, and SO2 is presented in Figure 5.
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CONCLUSIONS The study of sulfur emission from Morwell brown coal under different reactive atmospheres has provided both fundamental information, and information that has practical implication for the utilization of Morwell coal, in particular its use under oxyfuel condition. The release of sulfur oxides, including both SO2 and SO3, is considerably high during oxy-fuel combustion even though the sulfur content in Morwell is only 0.80%. Therefore, for Morwell coal utilization during oxy-fuel combustion, additional sulfur removal or polishing systems should be considered in order to avoid corrosion in the boiler and in the CO2 separation units in CCS systems.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
Figure 5. Gas phase conversion of sulfur as COS, SO2, H2S, and CS2 during CO2 gasification as a function of temperature.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge the support from Brown Coal Innovation Australia (BCIA) and the Department of Resource, Energy and Tourism for their financial support. We also thank Ms.Bithi Roy for help with the thermodynamic simulation work.
The conversion of sulfur to H2S increased from 17.1% to a 25.7%. Compared with the conversion during combustion, the percentage of coal sulfur converted to H2S increased from about 1% at 1073 K to over 11% at 1273 K, which is also higher than the values measured during pyrolysis. The sulfur conversion to SO2 was the lowest at 1173 K temperature, with a conversion rate of 5.86%. A significant level of COS was detected during gasification. The conversion of sulfur to COS increased from 13.9% to 15.1% from the temperature of 1073− 1273 K. According to the study carried by Shao et al.,21 the following reaction, as shown in Scheme 2, is the controlling reaction for COS release in general. During CO2 gasification,
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REFERENCES
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