Energy Fuels 2010, 24, 1745–1752 Published on Web 12/23/2009
: DOI:10.1021/ef901178d
Controlling the H2/CO Ratio of the Synthesis Gas in a Single Step by Catalytically Gasifying Coal in a Steam and Carbon Dioxide Mixed Environment at Low Temperatures Atul Sharma* and Toshimasa Takanohashi Advanced Fuel Group, Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology, 16-1, Onogawa, Tsukuba, Ibaraki 305-8569, Japan Received October 16, 2009. Revised Manuscript Received December 7, 2009
Production of synthesis gas suitable for the production of liquid fuels, chemicals, and methane by the Fischer-Tropsch (FT) synthesis process from catalytic gasification of coal has been investigated. Coal and HyperCoal were gasified in a steam and CO2 mixed environment as a gasifying agent at 700, 650, and 600 °C with K2CO3 as a catalyst. The H2/CO ratio of the synthesis gas was controlled by changing the steam/CO2 ratio of the gasifying agent. Gasification rates decreased with a decreasing temperature and an increasing CO2 fraction in the gas mixture but were high enough for practical application. The H2/CO ratio was primarily affected by the CO2 fraction in the mixed gas. Gasification rates in H2O þ CO2 mixed gas were not the sum of individual C-H2O and C-CO2 pure-gas rates. The role of the water-gas shift reaction in controlling gas composition in H2O þ CO2 mixed gas was investigated. The results showed that synthesis gas suitable as feedstock for dimethyl ether (DME), methanol, methane, and other chemicals production by the FT synthesis process can be produced in a single step at 700-600 °C by catalytic gasification of coal in steam/CO2 mixed gas as a gasifying agent. A single-step process to produce and control composition of synthesis gas from coal was proposed.
catalysts are alkali-metal salts, especially K2CO3.1-16 Exxon Mobil1 developed a K2CO3-catalyzed steam coal gasification process at high pressure to produce methane. Nearly all catalytic gasification processes developed were to produce either H2 or CH4. Not many attempts were made to produce synthesis gas by catalytic gasification.12 This was partly because alkali catalysts catalyzed both the gasification and water-gas shift reaction under atmospheric pressure conditions, leading to the formation of H2 and CO2 as main products, while higher pressure favors CH4 formation. The major drawback in the catalytic gasification of coals is the interaction of the catalyst with the mineral matters (ash) present in the coals, leading to the formation of compounds from which recovery of the catalyst is difficult.1-8,12 To overcome the problem of the loss of the catalyst, our research group has developed a process to remove mineral matter from coal by solvent extraction.13-17 The solvent-extracted coal, from hereon called HyperCoal (HPC), has less than 500 ppm of ash. Because of its ashless characteristics, a catalytic gasification process for coal may be developed using HPC as a feed material, leading to a low gasification temperature, easy recovery, and recycling of the catalyst. In our first study,13 we reported high gasification rates at temperatures as low as 775 °C, no catalyst deactivation, feasibility of
Introduction Gasification is a primary conversion route to produce synthesis gas (H2 and CO) from coal.1-12 Gasification is an endothermic reaction and requires temperatures above 1000 °C to achieve acceptable rates for commercial application.1-6 The product gas obtained at such high temperatures usually has low H2/CO (0.5-0.7), and it is also difficult to control the product gas composition because of equilibrium constraints at high temperature. Introduction of steam at some stage of the gasifier can increase H2/CO of the product gas but only marginally (∼0.9). A more common process approach is to upgrade the synthesis gas from the gasifier to the desired H2/CO (∼1-2) by the water-gas shift reaction at a lower temperature (300-400 °C) in a second stage often called as sweet shift process. Catalytic gasification of coal has been widely considered as an effective means to decrease the gasification temperature.8-12 The most favored *To whom correspondence should be addressed. E-mail: atul-sharma@ aist.go.jp. (1) Nahas, N. C. Fuel 1983, 62, 239–241. (2) Veraa, M. J.; Bell, A. T. Fuel 1978, 57, 194–200. (3) McKee, D. W.; Chatterji, D. Carbon 1975, 13, 381–390. (4) Kayembe, A.; Pulsifer, A. H. Fuel 1976, 55, 211–216. (5) Wigmans, T.; Elfring, R.; Moulijn, J. A. Carbon 1983, 21 (1), 1–12. (6) Formella, K.; Leonhardt, P.; Sulimma, A.; van Heek, K. H.; Juntgen, H. Fuel 1986, 65, 1470–1472. (7) Miura, K.; Aimi, M.; Naito, T.; Hashimoto, K. Fuel 1986, 65, 407– 411. (8) Kwon, T. W.; Kim, J. R.; Kim, S. D.; Park, W. H. Fuel 1989, 68, 416–421. (9) Lee, J. W.; Kim, S. D. Fuel 1995, 74, 1387–1383. (10) Schumacher, W.; Muhlen, H. J.; van Heek, K. H.; Juntgen, H. Fuel 1986, 65, 1360–1363. (11) Takarada, T.; Tamai, Y.; Tomita, A. Fuel 1985, 64, 1438–1442. (12) Meijer, R.; Kapteijn, F.; Moulijn, J. A. Fuel 1994, 73 (5), 723–730. r 2009 American Chemical Society
(13) Sharma, A.; Takanohashi, T.; Morishita, K.; Takarada, T.; Saito, I. Fuel 2008, 87, 491–497. (14) Sharma, A.; Takanohashi, T.; Saito, I. Fuel 2008, 87, 2866–2690. (15) Sharma, A.; Saito, I.; Takanohashi, T. Energy Fuels 2008, 22 (6), 3561–3565. (16) Sharma, A.; Kawashima, H.; Saito, I.; Takanohashi, T. Energy Fuels 2009, 23 (4), 1888–1895. (17) Sharma, A.; Saito, I.; Takanohashi, T. Energy Fuels 2009, 23 (10), 4887–4892.
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: DOI:10.1021/ef901178d
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Table 1. Properties of Coal and HPC elemental analysis (wt %, daf) coal
ash (wt %, db)
C
H
N
S
O
Pasir coal Pasir HPC
4.2 0.01
68.2 79.5
5.4 5.9
1.1 1.1
0.16 0.29
25.14 13.21
catalyst recovery and recycling, and H2 selectivity from catalytic gasification of Oaky Creek HPC. In a second study,14 the effect of catalyst addition on gasification reactivity of HPC and parent coal was compared at 700 and 775 °C and it was found that HPC and coal have nearly the same rates at 700 and 775 °C but at different catalyst loadings. In the following study,15 gasification rates of HPCs prepared from three different ranks of parent coals in the 600-775 °C range were compared. A subsequent study16 investigated the effect of the catalyst mixing procedure and catalyst loading ratio on the gasification rate. The production of synthesis gas (H2/CO) at such low temperatures is also an attractive proposition for wider application of HPC for gasification technology. However, results of our previous studies13-16 showed that, irrespective of the gasification temperature and coal type, the product gas from the K2CO3-catalyzed steam gasification of HPC contained H2 and CO2 as the major product gases with little CO and was mainly suitable for H2 production. To explore the application of HPC gasification technology for synthesis gas production, in the most recent study,17 we investigated the effect of steam partial pressure on gas composition from catalytic gasification of HPC at 700 °C. In this study, we report the production and control of the H2/CO ratio of synthesis gas by gasifying coal and HPC in a steam and carbon dioxide mixed environment. Coal and HPC were gasified in a steam and CO2 mixed environment as a gasifying agent at 700, 650, and 600 °C with K2CO3 as a catalyst. The effect of the temperature and ratio of steam/CO2 on the reaction rate and composition of the gas produced was investigated. A single-step process to produce and control the composition of synthesis gas from coal at 700-600 °C has been proposed.
Figure 1. Schematic of the experimental setup for catalytic steam gasification of HPC.
K2CO3 was added on the top of a measured sample already loaded into a test crucible as solid particles and stirred with a small spatula until white K2CO3 disappears by capturing moisture from the air. A common procedure is to mix K2CO3 as an aqueous solution for homogeneous dispersion7-11 but could not be applied because HPC does not mix with water. The particle size of the coal and HPC sample was under 75 μm. The gasification experiments were carried out with and without K2CO3 as a catalyst at 700, 650, and 600 °C with different steam/ carbon dioxide (H2O/CO2) ratios as the gasifying agent. Experiments were carried out in a thermogravimetric (TG-DTA 2020S, MAC) apparatus with about 20 mL/min argon (Ar) as the TG carrier gas flowing from the bottom (Figure 1). At the start of the experiment, 100 mL/min Ar was mixed with 20 mL/min oxygen (O2) and flowed from the top into the TG-DTA. Pre-oxidation was required for HPC samples because of their extremely high swelling propensity. To keep the same experimental conditions, coal samples were also subjected to pre-oxidation. In the actual process, pre-oxidation may not be necessary depending upon the type of feeding system. A desired amount of water was pumped by a high-performance liquid chromatography (HPLC) pump to a steam generator held at 250 °C. CO2 was flowed to the steam generator as a steam carrier gas. When the amount of water pumped by the HPLC pump to the steam generator was changed and the flow rate of CO2 was the carrier gas, different steam/CO2 ratios were achieved. For pure steam gasification, argon gas instead of CO2 was used as the carrier gas. In the case of steam and CO2 only conditions, partial pressure of steam and CO2 were kept at 0.5 atm by mixing argon gas. A four-way valve at the inlet of the TG-DTA was used to change Ar flow to CO2 þ steam flow. The flow lines were kept at 250 °C using ribbon heaters. First, a desired amount of sample was heated in Ar þ O2 flow up to 200 °C and held for 5 min to remove moisture and reduce the swelling propensity of the HPC by mild pre-oxidation. After a 5 min hold at 200 °C, the gas was switched to pure Ar flow for 60 min to remove the O2 from the reaction zone. After a 60 min hold, the sample was heated to the desired temperature at 20 °C/ min in pure Ar. When the desired temperature was reached without any hold time, the pure Ar gas was switched to the preset steam/CO2 gas mixture. The steam þ CO2 mixture flowing from the top comes into contact with the sample in the crucible. The evolved gases flow out together with the purge gas from the side and into an ice-cooled tar trap to remove tar before injecting into the micro gas chromatograph (Agilent 3000A). The total gas flow rate at the outlet was measured every 3 min by a film flow meter.
Experimental Procedures A sub-bituminous coal, Pasir (PAS) from Indonesia, was selected for the investigation. The HPC production method18 has been described in detail elsewhere. Briefly, HPC was produced by the solvent extraction of the coal with 1-methylnaphthalene at 360 °C and subsequently separating the extract (HPC) from the solvent. The extraction yield was 51% for Pasir coal. The properties of the Pasir coal and HPC produced from Pasir coal are shown in Table 1. HPC has nearly no mineral matter. Because of almost no mineral matter, all of the inorganically associated sulfur will be removed. The only sulfur in HPC will be the organically associated sulfur. A detailed characterization of the catalyst, HPC, original coal, and chars using X-ray diffraction (XRD), nuclear magnetic resonance (NMR), and scanning electron microscopy-energy-dispersive X-ray (SEMEDX) mapping techniques had been carried out and reported elsewhere.13,15,16 The experimental setup is shown in Figure 1. Samples for catalytic gasification experiments were prepared with 50% catalyst loading. Catalyst loading was on a dry and ash-free weight percent basis of coal and HPC. Both dry and wet mixing methods were investigated.16 The catalyst mixing method has been described in detail elsewhere.16 Briefly, a desired amount of
Results and Discussion Figure 2 shows a typical weight loss curve for a HPC þ 50% K2CO3 sample pyrolyzed in Ar up to 700 °C, followed by gasification with 50% steam þ 50% carbon dioxide (v/v) as the gasifying agent (from hereon in this paper, the steam/carbon
(18) Okuyama, N.; Komatsu, N.; Shigehisa, T.; Kaneko, T.; Tsuruya, S. Fuel Process. Technol. 2004, 85, 947–967.
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: DOI:10.1021/ef901178d
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Figure 2. Weight loss profile of HPC pyrolyzed in Ar up to 700 °C, followed by steam þ CO2 gasification.
dioxide ratio will be addressed as H2O/CO2). In a previous study,16 the gasification rate and gas composition were investigated at 10, 20, 40, and 50% catalyst mixing ratios. It was reported that the gasification rate was affected by the catalyst amount up to 50% loading, and above this, catalyst loading rates were almost independent of the catalyst amount. In addition, unlike the gasification rate, gas composition was not found to be affected by the catalyst amount at these catalyst loadings. Therefore, in the present study, only one catalyst mixing ratio, 50% catalyst loading, has been selected for investigation. In the actual process, 40-50% catalyst loadings would be very high because it may cause problems with the access of the reactant gas to the reaction surface at medium to high conversion levels and lower loadings, such as 20-30%, would be more appropriate. All experiments were carried out at atmospheric pressure. The weight loss curve can be roughly divided into three stages: the moisture removal, or drying stage, devolatilization stage, and fixed-carbon gasification stage. The initial weight loss (up to 200 °C) during heating from room temperature to 200 °C in an O2 þ Ar mixture is mainly due to moisture captured from air. Preoxidation was performed to reduce the extremely high swelling propensity of HPC. After the pre-oxidation stage, the sample was switched to 100% Ar for 60 min. The coal/HPC conversion on a dry ash and catalyst volatile free basis (dacvf) (from hereon called char conversion) was calculated during the fixedcarbon gasification stage by the following equation: X ðchar conversion, % dacvfÞ ¼
W0 -W 100 W0 ð1 -Wash -Wcat Þ
Figure 3. Effect of the H2O/CO2 ratio of the gasifying agent on (a) gasification profiles, (b) gas yields, and (c) H2/CO ratio of noncatalyzed coal and HPC at 700 °C.
carbon is very slow at low temperatures. Figure 3b shows that the gas yield of CO and H2 gases evolved from coal and HPC without K2CO3. Produced gas also contained CO2; however, because of the presence of CO2 as a gasifying agent, the amount of CO2 produced from gasification of coal cannot be obtained. At H2O/CO2 = 100:0, gas produced contained mainly H2 and very little CO. As H2O/CO2 changed to 50:50, H2 decreased and CO increased. At H2O/CO2 =0:100, produced gas contained mainly CO and very little H2. The formation of CH4 during char gasification was very small and can be assumed to be negligible for the present discussion. It should be noted that the total gas yield (H2 þ CO) was nearly the same at all three H2O/CO2 ratios for both coal and HPC. Figure 3c shows the change in the H2/CO ratio of the produced gas with the change in H2O/CO2 ratio of the gasifying agent. With an increasing CO2 fraction in the mixed gas, the H2/CO ratio decreased. These results suggest that controlling the H2/CO ratio of the synthesis gas may be possible at such low temperatures as 700 °C using the steam/CO2 gas mix as the gasifying agent. However, the production rates are very slow for any practical application.12 It is well-known that gasification rates can be
ð1Þ
where W0 is the weight when the gasification begins (db, mg) (weight at t=45, 41, and 39 min for T=700, 650, and 600 °C, respectively), W is the weight at any gasification time (db, mg, >39 min), Wash is the weight fraction of ash content in coal or HPC, and Wcat is weight fraction of the catalyst content. Results of the gasification rate and gas composition only in the char gasification stage have been discussed. Figure 3a shows the gasification profiles of coal and HPC at 700 °C without the catalyst in pure steam (100:0), 50% steam þ 50% CO2 (50:50), and pure CO2 (0:100). It can be seen that rates are very slow for both coal and HPC and not suitable for commercial application. The above results are in confirmation with previous results that steam and CO2 gasification of 1747
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Figure 4. Effect of the H2O/CO2 ratio of the gasifying agent on (a and b) gasification profiles, (c and d) gas yields, and (e and f) H2/CO ratios of K2CO3 catalyzed coal and HPC at 700 °C.
increased by either increasing the gasification temperature or adding a catalyst.1-12 In this study, the effect of catalyst addition to increase the gasification rate and on gas composition has been investigated. For coals, deactivation of the catalyst is a major problem for developing a commercial process, but with HPC, the catalyst can be used for long-time operations without deactivation. As reported in our previous reports,13 the catalyst was recycled 4 times without observing any significant decrease in the rate. The addition of the catalyst to coals with very low mineral content, such as Victorian brown coal (ash content of about 0.5%), may also be economically attractive, where the make up catalyst amount would not be very large. Panels a and d of Figure 4 show the gasification profiles of coal and HPC with 50 wt % catalyst in a steam and carbon dioxide (H2O/CO2) mixed environment at 700 °C. The H2O/ CO2 ratios selected were H2O/CO2 = 100:0, 70:30, 60:40, 50:50, 30:70, and 0:100. H2O/CO2 =70:30 means 70% steam and 30% carbon dioxide mixed gas on a volume basis. The gasification rate increased dramatically upon addition of the catalyst. Both coal and HPC with the catalyst show gasification rates high enough for commercial application. However, catalyst recycling was only possible for HPC, as reported in our previous report.13 Panels b and e of Figure 4 show H2
and CO amounts in the produced gas at different H2O/CO2 for coal and HPC at 700 °C. At H2O/CO2 =100:0, produced gas contained mainly H2 and very little CO. As H2O/CO2 changed to 70:30, 60:40, 50:50, and 30:70, H2 decreased and CO increased. At H2O/CO2 =0:100, produced gas contained mainly CO and very little H2. Panels c and f of Figure 4 show the change in the H2/CO ratio with the H2O/CO2 ratio of the gasifying gas. The H2/CO (mol/mol) ratio was 29, 4.4, 2.8, 2.1, 1.0, and 0.1 for the H2O/CO2 (v/v) ratio of 100:0, 70:30, 60:40, 50:50, 30:70, and 0:100 for coal. For HPC, the H2/CO (mol/ mol) ratio was 27, 5.0, 3.2, 2.5, 1.3, and 0.1 for the H2O/CO2 (v/v) ratio of 100:0, 70:30, 60:40, 50:50, 30:70, and 0:100. These results show that synthesis gas with the H2/CO ratio of 1-5 can be produced by the gasification of coal in a single step by changing the H2O/CO2 ratio of the gasifying agent. Synthesis gas with H2/CO = 1, 2, and 3 can be used as a feedstock for the Fischer-Tropsch (FT) synthesis process to produce dimethyl ether (DME), methanol, methane, and other chemicals. However, it should be noted that the above results were obtained when volatiles were not present and, therefore, do not take into account the effect of volatiles that will be formed during pyrolysis in a real gasification situation and can affect the gas composition. Volatiles and tar are expected to be gasified and decomposed by the catalyst just 1748
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Figure 5. Effect of the H2O/CO2 ratio of the gasifying agent on (a) gasification profiles and (b) gas yield of K2CO3-catalyzed HPC at different temperatures.
like char, and the effect may be more numerical in nature rather than affecting the mechanism.1 Figure 5 shows the gasification profiles and gas composition of produced gas from K2CO3-catalyzed HPC at 700, 650, and 600 °C as a function of the H2O/CO2 ratio. In general, at any given temperature within the temperature range investigated, the rate decreased with an increasing CO2 fraction in the gas mixture. Similarly, at any given temperature, H2 decreased and CO increased with an increasing CO2 fraction in the gas mixture. The effect of the CO2 fraction in the gas mixture on the gasification rate and H2/CO ratio with temperature can be more appropriately discussed by plotting the rate constant and H2/CO ratio against the H2O/CO2 ratio. For a simple understanding and comparison of gasification reactivity, time needed to reach 50% conversion (at 25% conversion where 50% conversion could not achieved) was obtained from the gasification profiles from panels a, c, and e of Figure 5. It has been reported1-5,7-17 that the K2CO3-catalyzed char reaction is a zero-order reaction, and therefore, the true rate constant obtained by integrating the rate over the gasification time will be close to the rate constant obtained by inversing the time needed for 50% conversion (except for 25% conversion, where the rate constant was obtained by [(1/(1 - x))(dx/dt)]). The rate constants
thus obtained were plotted against the CO2 fraction in the H2O þ CO2 mixed gas. Similarly, H2/CO (mol/mol) was obtained from panels b, d, and f of Figure 5 and plotted against the CO2 fraction in the H2O þ CO2 mixed gas. Panels a and b of Figure 6 show the effect of the CO2 fraction in the H2O þ CO2 mixed gas on the gasification rate and H2/CO ratio of the synthesis gas at 700, 650, and 600 °C. C þ H2 O f CO þ H2
ð2Þ
C þ CO2 h 2CO
ð3Þ
CO þ H2 O h H2 þ CO2
ð4Þ
The gasification rate was affected by both the temperature and CO2 fraction in the H2O þ CO2 mixed gas. At a given temperature, the gasification rate decreased with an increasing CO2 fraction. At a given CO2 fraction, the rate decreased with a decreasing temperature. The extent of decrease in the rate was large at a lower CO2 fraction and becomes smaller as the CO2 fraction increases, as shown in Figure 6a. Under a H2O/ CO2 mixed gas environment, three reactions, as shown above, C-H2O reaction 2, C-CO2 reaction 3, and water-gas shift (WGS) reaction 4, take place. If the overall gasification rate is 1749
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Figure 6. Effect of the CO2 fraction in the H2O þ CO2 mixed gas on the (a) gasification rate and (b) H2/CO ratio of the produced gas at different temperatures.
assumed to be the sum of individual contributions of carbon loss by reactions 2 and 3, a decrease in the overall gasification rate with an increasing CO2 fraction in the mixed gas can be expected (C-CO2 reaction 3 is slower than C-H2O reaction 22-5). Figure 6a compares the observed rates with the calculated rates obtained by simply adding the contribution of pure C-H2O and C-CO2 reactions. The results show that observed rates are far lower than those calculated and are definitely not the sum of two pure gas rates. These results suggest that a decrease in the overall rate on CO2 addition is not only due to the increased contribution of the C-CO2 reaction. Roberts et al.19 also reported similar results for a non-catalyzed char reaction in a mixture of CO2 and H2O and attributed this to the CO2 inhibition of C-H2O reaction sites. For K-catalyzed char gasification under a H2O and CO2 mixture, Meijer et al.12 attributed it to the chemisorptions of most of the active sites in the active alkali cluster from gasphase CO2 making fewer sites available for the fast oxygen transfer reaction, thus lowering the overall reaction. The observed gasification trends in Figure 6a are in accordance with these explanations. Unlike the gasification rate, the H2/CO ratio was primarily affected by the CO2 fraction in H2O þ CO2 mixed gas only and little or a negligible effect was observed with the temperature, as shown in Figure 6b. While the rate of the carbon loss or gasification rate in a H2O and CO2 mixture can be predicted using a complex combination of reactions 2 and 3 as reported by Robert et al.,19 for proper prediction of gas composition, inclusion of the WGS reaction 4 is essential. This is because the WGS reaction is one of the oxygen exchange reactions that are known to be catalyzed by the alkali-carbon system and, therefore, plays a major role in determining the gas
Figure 7. (a) Calculated gas yield from WGS equilibrium data for H2O/CO2 = 100:0, (b) comparison of the calculated and experimental H2 and CO fraction at 700 °C for H2O/CO2 = 100:0, and (c) comparison of the H2 and CO fractions to the calculated values by shifting experimental H2 and CO fractions at 700 °C for H2O/ CO2 = 70:30, 50:50, and 30:70.
composition.12 However, in the case of H2O þ CO2 mixed gas, all three reactions take place and gas composition may be determined by these competing reactions.12 Therefore, it is essential to investigate if the WGS reaction is still a primary gas composition controlling reaction in a H2O þ CO2 mixed gas environment or if the other two heterogeneous reactions 2 and 3 became dominant. To investigate this, the equilibrium gas composition of H2, CO, CO2, and H2O in C-H2O and C-(H2O þ CO2) systems was calculated from the WGS equilibrium data, and a typical result is shown in Figure 7a for H2O/CO2 = 100:0. Similar data were obtained at other H2O/CO2 ratios. For comparison with the experimental results and because H2 and CO were the only gases measured experimentally (CO2 was not measured in experiments because of CO2 being a reactant), the fractions of H2 and CO in H2 þ CO from the calculation were obtained from results in Figure 7a. Similarly, experimental fractions of H2
(19) Roberts, D. G.; Harris, D. J. Fuel 2007, 86, 2672–2678.
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Figure 8. Correlation between the H2/CO ratio, gasification time, and H2O/CO2 ratio of the gasifying agent as a function of the temperature.
Figure 9. Schematic process flow of a new low-temperature singlestep coal to synthesis gas production process.
H2/CO ratio of synthesis gas, gasification time, and H2O/ CO2 ratio of the gasifying agent at 700, 650, and 600 °C. Takarada et al.11 suggested that the gasification rate of about 0.3 h-1 (18 min) at low temperatures, such as 700 °C and below, would be necessary for commercial application. From Figure 8, it can be seen that synthesis gas with H2/CO=1-3 can be produced in a single step from catalytic coal gasification at 700 °C with gasification time
