Experimental Study of Gaseous Sulfur Species Formation during the

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Experimental Study of Gaseous Sulfur Species Formation during the Steam Hydrogasification of Coal Qian Luo, Chan S. Park,* Arun S. K. Raju, and Joseph M. Norbeck Bourns College of Engineering−Center for Environmental Research and Technology (CE−CERT), University of California, Riverside, California 92521-0425, United States ABSTRACT: Steam hydrogasification is the thermochemical conversion of carbonaceous materials into synthesis gas in a steamand hydrogen-rich environment. The formation of gas-phase sulfur species during the pyrolysis and gasification of sulfurcontaining feedstocks, such as coal, presents well-known challenges. Gas-phase sulfur component formation under typical steam hydrogasification conditions was studied experimentally, and the impact of process parameters has been evaluated. The experimental results show that sulfur in the feedstock is mainly converted to hydrogen sulfide (H2S) during the steam hydrogasification reaction (SHR) process. Carbonyl sulfide (COS) and carbon disulfide (CS2) were not observed in the gas phase. The H2- and steam-rich environment in the SHR process is favorable for the formation of H2S and suppresses COS and CS2 formation. The H2S concentration in the gas phase increased consistently with increasing temperatures (700−800 °C) and increasing water/coal mass ratios (0.5−3). These results are important in the development of a downstream sulfur cleanup system during SHR-based coal conversion.

1. INTRODUCTION Coal is one of the major energy sources with high availability and usage around the world and is expected to play a major role in the world energy consumption for the foreseeable future. However, concerns over the environmental impact of coal use is widespread, and efficient technologies that minimize both greenhouse gas and criteria pollutant emissions are sought by technology developers and regulatory agencies. One of the wellknown challenges associated with the use of coal is the presence of sulfur species. As a consequence, the chemical behavior of sulfur species during pyrolysis and gasification of coal has received considerable attention. The formation of specific sulfur compounds during coal pyrolysis is influenced by many factors, including the specific type of coal used (rank of coal, petrographic structure, mineral matter content, sulfur content, and its forms), the parameters of the process (temperature, pressure, reaction time, and size distribution of coal),1 and other factors, such as heating rate, residence time, etc.2 Volatile hydrogen sulfide (H2S) is the most abundant of all of the potential chemical species present in pyrolysis and gasification product gas.3 Carbonyl sulfide (COS), sulfur dioxide (SO2), sulfur trioxide (SO3), and carbon disulfide (CS2) have been observed in pyrolysis gases but in lower concentrations.4 The amount of sulfur found as H2S has been observed to decrease as the temperature and duration of pyrolysis increase.5 Ibarra et al.6 reported that coal samples with higher pyrite contents released very little sulfur as H2S at temperatures higher than 630 °C, whereas samples with lower pyrite contents and, hence, higher organic matter resulted in more H2S. Ibarra et al.7 investigated the behavior of sulfur structures in low-rank coal with high organic and pyritic sulfur contents during lowtemperature pyrolysis. They found that the evolution of H2S as a function of the temperature passes through two Fourier transform infrared spectroscopy (FTIR) peaks between 500 and 560 °C and between 630 and 700 °C, which correspond to the decomposition of organic and pyritic sulfur, respectively. © 2014 American Chemical Society

The formation of COS with pyrolysis temperature followed a trend similar to that for H2S. The gaseous atmosphere during coal pyrolysis plays an important effect on the evolution of sulfur-containing gases. Hydrogen gas (H2) improves the formation of hydrogen sulfide (H2S) and inhibits the formation of other sulfur-containing gases.8 Chen et al.9 investigated the chemical transformation of sulfur in high-sulfur coals from China using a fixed-bed reactor under a pressure of 3 MPa at a heating rate of 10 K/min. They concluded that sulfur was removed from coal more effectively in hydropyrolysis than in pyrolysis under an inert atmosphere. During steam gasification, only H2S was produced and COS was not detected.10 The Bourns College of Engineering−Center for Environmental Research and Technology (CE−CERT) at the University of California, Riverside (UCR) has developed a thermal conversion process referred to as steam hydrogasification that has been shown to convert carbonaceous feedstock into syngas (H2 and CO mixture) in a cost-effective manner.10 Comparatively large quantities of H2 and steam are present during the steam hydrogasification process, and the behavior of the chemical species, including sulfur, is often different from conventional thermochemical processes. The effect of process parameters on the formation of sulfur species in the gas phase in an environment of excessive steam and hydrogen has not been reported in the literature. The major objective of this study is to investigate the formation of sulfur species during steam hydrogasification. The effect of water/coal mass ratio, temperature, and hydrogen partial pressure on the gas-phase sulfur composition has been evaluated. These results will provide information necessary to Received: October 23, 2013 Revised: April 8, 2014 Published: April 9, 2014 3399

dx.doi.org/10.1021/ef4021087 | Energy Fuels 2014, 28, 3399−3402

Energy & Fuels

Article

rate of 40 K/min by immersing it into a tubular electrical heater. The experiment is concluded when the temperature and pressure of the system are stable and no changes are observed. The product gas containing the volatile sulfur components was collected in a Tedlar bag and analyzed using a Kitagawa H2S detector tube and gas chromatograph (GC) with a flame photometric detector (FPD) using a Supel-Q PLOT, 30 m × 0.53 mm column.

develop an appropriate sulfur cleanup system for protection of the downstream catalysts.

2. EXPERIMENTAL SECTION Bituminous coal from the Illinois region in the United States was used in this study. The coal particle size was less than 150 μm. Properties of coal are listed in Table 1. A batch reactor setup with a reactor volume

3. RESULTS AND DISCUSSION 3.1. Sulfur Formation in the Gas Phase. During coal pyrolysis and gasification, the sulfur in the fuel is primarily converted to H2S, while small amounts of gas-phase carbonyl sulfide (COS) and CS2 are also produced.11 The formation of sulfur-containing pollutants during the steam hydrogasification can be described in a simplified manner through the following set of reactions:12,13

Table 1. Proximate and Ultimate Analyses of the Coal Samplea ultimate analysis (dry basis)

a

proximate analysis (as received)

component

wt %

parameter

wt %

C H O N S

67.4 5.1 13.3 1.3 3.3

moisture (M) volatile carbon fixed carbon (FC) ash

3.2 36.2 50.9 9.8

coal S + heat → H 2S + COS... + char S

(R1)

gas-phase reactions

High heating value = 12 083 Btu/lb.

of 260 cm3 (15.87 in.3) was used for the experiments. The reactor, shown in Figure 1, was specifically designed to enable continuous

CO + H 2S ↔ H 2 + COS

(R2)

CO2 + H 2S ↔ COS + H 2O

(R3)

H 2S + COS ↔ CS2 + H 2O

(R4)

The steam hydrogasification experiments conducted during this study did not yield detectable amounts of COS and CS2. The gas-phase reactions R2−R4 indicate that both COS and CS2 can be converted to H2S in the presence of large amounts of H2 and steam in the reactor. Zhou et al.13 have suggested that the presence of hydrogen in the gas phase improves the hydrogen sulfide formation and inhibits the formation of other sulfurcontaining gases. Previous studies have reported that H2S was the only sulfur-containing species detected in the gas phase during coal hydropyrolysis.14 Similar results have been reported for steam gasification processes.15 Furthermore, Calkins16 noted that, only above 850 °C, some CS2 is formed at the expense of H2S. A review by Attar17 concluded that H2S is the dominant sulfur-containing product for reactions in a H2 environment. Because the temperature of the steam hydrogasification reaction (SHR) process is at or below 850 °C in a H2- and steam-rich environment, the formation of H2S is favorable and the formation of COS and CS2 is likely suppressed. This explains that H2S is the only gaseous sulfur species detected during the SHR process. The gas-phase sulfur concentration represents only a portion of sulfur in the feedstock. It is assumed that the remaining sulfur in the feedstock is present in the unreacted char from the steam hydrogasification process. 3.2. Effect of the Temperature. The experimental results agree with the accepted trend of increased sulfur component release into the gas phase along with increasing temperatures. The results are shown in Figures 2−5. H2S concentrations and mass of sulfur in the gas phase consistently increased with increasing temperatures during the steam hydrogasification process. For example, for the temperature range of 700−800 °C, at a water/coal mass ratio equal to 3 and the initial H2 pressure equal to 25 psi, H2S in the gas phase increased from 1200 ppmv (1.2 × 10−3) to 3000 ppmv (3 × 10−3) and the mass of sulfur in the gas phase increased from 2.6 to 8.0 mg, as shown in Figures 4 and 5. Similar results have been reported in earlier studies of rapid pyrolysis18 and hydropyrolysis of coal, where the sulfur yield in the gas increased over the temperature range between 700 and

Figure 1. Schematic diagram of the stirred batch reactor (1, magnetic agitator; 2, thermalcouple; 3, heater; 4, reactor; and 5, impeller). stirring under high pressures. The reactor is made of Inconel alloy and can be operated at pressures and temperatures as high as 3.45 × 106 Pa (500 psi) and 1070 K (800 °C), respectively. The reactor setup is composed of a magnetic agitator, thermocouple, heater impeller, and reactor vessel. Reaction parameters, including temperature, pressure, and heater duty, were measured continuously during the experiments. To minimize the problem of sulfur absorption, the reactor and all of the connecting lines were treated with Silcolloy 1000 from the Silcotek Company, which is a passivation layer inert to sulfur-containing gases. A coal sample weighing 0.5 g was loaded at the bottom of the batch reactor (Figure 1) with the desired amount of water according to the coal/water mass ratio. The reactor was then pressurized with H2 to the desired pressure of 25 or 50 psi. The reactor was heated at a heating 3400

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Figure 2. Effect of the water/coal mass ratio on the H2S concentration at different temperatures (1 ppmv = 10−6) (initial H2 pressure = 50 psi).

Figure 5. Effect of the water/coal mass ratio on the mass of sulfur in the gas phase at different temperatures (initial H2 pressure = 25 psi).

water/coal mass ratio from 0.5 to 3 at 50 psi H2 pressure resulted in an increase in the concentration from 400 ppmv (4 × 10−4) to 1500 ppmv (1.5 × 10−3) and an increase in the mass of sulfur in the gas phase from 0.5 to 4.8 mg. These results are in agreement with trends reported by Shao et al.,22 who propose that the increase in the concentration is the result of steam reacting with pyrite to promote the formation of H2S. Similar results were obtained by Czaplocki and Smolka,23 who found that the addition of steam to the pyrolysis process results in the increase of sulfur in the gas phase. 3.4. Effect of the H2 Partial Pressure. The concentration of H2S and mass of sulfur in the gas phase decreased when the initial H2 partial pressure increased from 25 to 50 psi at 800 °C. These results are shown in Figure 6. These results are not in

Figure 3. Effect of the water/coal mass ratio on the mass of sulfur in the gas phase at different temperatures (initial H2 pressure = 50 psi).

Figure 6. H2 partial pressure effect on the mass of sulfur in the gas phase at 800 °C. Figure 4. Effect of the water/coal mass ratio on the H2S concentration at different temperatures (1 ppmv = 10−6) (initial H2 pressure = 25 psi).

agreement with previous reports by Chen et al.9 that state that hydropyrolysis is a more effective method of coal desulfurization than pyrolysis because the presence of H2 enhances H2S formation during hydropyrolysis. However, Cleyle et al. and others have reported that, in a hydrogen-rich environment, H2S may become trapped in the carbon matrix as organic sulfur at higher temperatures.24 This will lead to pyritic sulfur being converted into organic sulfur; i.e., sulfur does not remain in the pyrolysis char only as FeS, but an additional percentage is incorporated into the matrix as organic sulfur. The observed reduction in the mass of gas-phase sulfur at higher H2 partial pressures is likely caused by the removal of H2S from the gas phase through this phenomena. It is possible that there are other mechanisms involved, and additional experimental work will be conducted to identify the precise mechanisms involved.

950 °C.19,20 Kuramochi et al.21 concluded that lower temperatures favor the formation of species, such as FeS, ZnS, and MnS, retained in the solid phase, resulting in a lower concentration of H2S in the gas phase. 3.3. Effect of the Steam (Water). The results presented in Figures 2−5 show that the H2S concentration and mass of sulfur in the gas phase increased with an increase in the water/ coal mass ratio. For example, at 800 °C with an initial H2 pressure of 25 psi, the H2S concentration increased from 760 ppmv (7.6 × 10−4) to 3000 ppmv (3 × 10−3) and the mass of sulfur in the gas phase increased from 0.6 to 8.0 mg as the water/coal mass ratio increased from 0.5 to 3. Increasing the 3401

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(6) Ibarra, J. V.; Palacios, J. M.; Moliner, R.; Bonet, A. J. Evidence of reciprocal organic matter-pyrite interactions affecting sulfur removal during coal pyrolysis. Fuel 1994, 73 (7), 1046−1050. (7) Ibarra, J. V.; Bonet, A. J.; Moliner, R. Release of volatile sulfur compounds during low temperature pyrolysis of coal. Fuel 1994, 73 (6), 933−939. (8) Zhou, Q.; Hu, H.; Liu, Q.; Zhu, S.; Zhao, R. The atmosphere of pyrolysis also have important effect on evolution of sulfur-containing gases during coal pyrolysis. Energy Fuels 2005, 19 (3), 892−897. (9) Chen, H.; Li, B.-q.; Yang, J.-l.; Zhang, B.-j. Transformation of sulfur during pyrolysis and hydropyrolysis of coal. Fuel 1998, 77 (6), 487−493. (10) Park, C. S.; Singh, S. P.; Norbeck, J. M. Steam hydrogasification of carbonaceous matter to liquid fuels. Proceedings of the 24th Annual International Pittsburgh Coal Conference; Johannesburg, South Africa, Sept 10−14, 2007. (11) Meng, X. M.; De Jong, W.; Verkooijen, A. H. M. Thermodynamic analysis and kinetics model of H2S sorption using different sorbents. Environ. Prog. Sustainable Energy 2009, 28, 360− 371. (12) Zevenhoven, R.; Kilpinen, P. Control of Pollutants in Flue Gases and Fuel Gases, 3rd ed.; Helsinki University of Technology: Espoo, Finland, 2004. (13) Zhou, Q.; Hu, H.; Liu, Q.; Zhu, S. Effect of atmospheres on evolution of sulfur containing gases during coal pyrolysis. Prepr. Pap. Am. Chem. Soc., Div. Fuel Chem. 2004, 49, 927−928. (14) Chu, X.; Li, W.; Li, B.; Chen, H. Sulfur transfers from pyrolysis and gasification of direct liquefaction residue of Shenhua coal. Fuel 2008, 87, 211−215. (15) Chen, H.; Li, B.; Zhang, B. Effects of mineral matter on products and sulfur distributions in hydropyrolysis. Fuel 1999, 78, 713−719. (16) Calkins, W. H. Investigation of organic sulfur-containing structures in coal by flash pyrolysis experiments. Energy Fuels 1987, 1, 59−64. (17) Attar, A. Chemistry, thermodynamics and kinetics of reactions of sulphur in coal−gas reactions: A review. Fuel 1978, 57, 201−212. (18) Garcis-Labiano, F.; Hampartsoumian, W.; Williams, A. Determination of sulfur release and its kinetics in rapid pyrolysis of coal. Fuel 1995, 74, 1072−1079. (19) Xu, W. C.; Kumagai, M. Sulfur transformation during rapid hydropyrolysis of coal under high pressure by using a continuous free fall pyrolyzer. Fuel 2003, 82, 245−254. (20) Dial, M.; Gulyurtlu, I. H2S and HCl formation during RDF and coal co-gasification: A comparison between the predictions and experimental results. Proceedings of the Biomass Gasification Technologies ̇ AK Marmara Research Center (MRC) Campus in Workshop; TÜ BIT Gebze, Kocaeli, Turkey, April 9−11, 2008. (21) Kuramochi, H.; Nakajima, D.; Goto, S.; Sugita, K.; Wu, W. HCl emission during co-pyrolysis of demolition wood with a small amount of PVC film and the effect of wood constituents on HCl emission reduction. Fuel 2008, 87, 3155−3157. (22) Shao, D.; Hutchinson, E. J.; Heidbrink, J.; Pan, W.; Chou, C. Behavior of sulfur during coal pyrolysis. J. Anal. Appl. Pyrolysis 1994, 30, 91−100. (23) Czaplicki, A.; Smolka, W. Sulfur distribution within coal pyrolysis products. Fuel Process. Technol. 1998, 55 (1), 1−11. (24) Cleyle, P. J.; Caley, W. F.; Stewart, I.; Whiteway, S. G. Decomposition of pyrite and trapping of sulphur in a coal matrix during pyrolysis of coal. Fuel 1984, 63, 1579−1582.

4. CONCLUSION Experimental work was conducted on the steam hydrogasification of coal, and the effect of process parameters on gas-phase sulfur formation has been studied. The results are summarized as follows: (1) H2S is the only sulfur-containing species detected in the gas phase during the SHR process under typical process conditions. COS and CS2 were not observed in the gas phase. These results are promising. H2S removal technologies are mature and widely practiced in commercial scales, and the sulfur cleanup system design will be simplified because of the absence of other sulfur species in the gas phase. (2) The H2S formation increased with increasing temperatures in the reactor, because of more sulfur leaving the solid coal matrix as reaction temperatures increase. H2S concentrations in the gas phase also increased as the water/coal mass ratio increased. This is likely due to the steam reacting with pyrite, thereby promoting the formation of H2S. (3) The H2S concentration decreased with an increasing H2 partial pressure. It is speculated that H2S produced may have been trapped in the carbon matrix as organic sulfur, partially because of increased active sites in coal at higher H2 partial pressures. It has been shown by this work that, under typical SHR process conditions, the formation of gaseous sulfur species is strongly influenced by the temperature and water/coal ratio. Our results generally agree with the trends observed in previous reports in the literature under similar relevant process conditions. The SHR conversion process operates at lower temperatures and higher water/coal ratios than conventional gasification processes. These experimental results indicate that commercially available H2S removal systems may be used during SHR-based conversion to protect downstream catalysts in catalytic processes, such as steam reforming and Fischer− Tropsch synthesis.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 1-951-781-5771. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The author thank Junior Castillo for assistance with the fabrication of the experiment setup and the California Energy Commission for financial support.

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REFERENCES

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dx.doi.org/10.1021/ef4021087 | Energy Fuels 2014, 28, 3399−3402