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Ind. Eng. Chem. Res. 2009, 48, 5024–5034
Effects of Coal Interaction with Supercritical CO2: Physical Structure Benson B. Gathitu, Wei-Yin Chen,* and Michael McClure Department of Chemical Engineering, UniVersity of Mississippi, 134 Anderson Hall, UniVersity, Mississippi 38677
It is known that polar solvents swell coal, break hydrogen-bonds in the macromolecular structure, and enhance coal liquefaction efficiencies. The effects of drying, interaction with supercritical CO2 and degassing on the physical structure of coal have been studied using gas sorption technique and a scanning electron microscope (SEM). Both drying and interaction with supercritical CO2 drastically change the micropore and mesopore surface area, absolute volume, and volume distribution in both bituminous coal and lignite. Degassing removes debris in the pore space which allows for better analysis of the changes in the morphology that were induced by drying and exposure to supercritical CO2. SEM reveals that interaction of bituminous coal with supercritical CO2 results in an abundance of carbon structures similar to the maceral collinite. 1. Introduction The study of the interactions between CO2 and coal has been of great importance lately due to the necessity to mitigate climate change. CO2 is a greenhouse gas which is produced in vast quantities by utility boilers and means to reduce its presence in the environment are sought. One of the most viable means of CO2 disposal is capture of enriched flue gas and then sequestration deep underground. Along with other rock formations, coal beds are a viable option because coal has a known capacity to adsorb CO2. At temperatures as low as 298 K, CO2 is known to induce significant irreversible swelling in coal volume by up to 4.18%, pore volume enlargement up to 20 to 50%, and depression in the glass-transition temperature from 120 to 82 °C.1-3 These effects increase with increasing temperature up to 200 °C and pressure up to 30 atm (subcritical). Separately, it was demonstrated that coal swells in pyridine4 and amines.5 Prior contact of coal with subcritical steam at 50 atm and 300 to 370 °C enhances coal dissolution in pyridine.6-8 Moreover, exposure of coals to N2 at 150 to 200 °C for 1 h induces increases in volatile yield in the subsequent pyrolysis.9 The objective of this study is to enhance knowledge of solvent effects on coal pore volume and structure. Coal is exposed to supercritical CO2, a known powerful solvent,10,11 to induce changes in the coals morphology. The change after interaction is thoroughly examined using the gas sorption technique to analyze the surface area and pore structure of the coal. A scanning electron microscope (SEM) is also used to visually examine the changes in the coal after its interaction with supercritical CO2. Adsorption of N2 at 77 K has been used extensively to measure the surface areas of porous materials such as coal. However, it yields lower surface area than expected because it cannot access the entire micropore structure due to an activated diffusion process and shrinkage of pores. On the other hand, it has been stated that at room temperature, using CO2 as the adsorbate, it is possible to access the entire porosity in coal.12 Consequently, using N2 at 77 K will enable us to access the larger pore structure, while using CO2 at 273 and 298 K will enable us to access the entire pore structure. In this study, we classify pore sizes according to the standards of the International Union of Pure and Applied Chemistry * To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: (662) 915-5651. Fax: (662) 915-7023.
(IUPAC)12 where pores greater than 500 Å in diameter are macropores, pores with diameters in the range of 20-500 Å are mesopores, pores with diameters in the range of 8-20 Å are micropores and pores less than 8 Å are submicropores. 2. Experimental Details 2.1. Apparatus and Procedure for Supercritical CO2 Interaction with Coal. The reactor (Figure 1) consisted of a 2.54 cm o.d., 1.75 cm i.d., and 50 cm long galvanized steel pipe purchased from a local hardware store. The pipe was threaded and capped at both ends. These end-caps were fitted with stainless steel fittings to form inlet and outlet ports. The
Figure 1. Experimental setup.
10.1021/ie9000162 CCC: $40.75 2009 American Chemical Society Published on Web 04/09/2009
Ind. Eng. Chem. Res., Vol. 48, No. 10, 2009
inlet and outlet ports were connected to high-temperature highpressure trunnion ball valves supplied by Swagelok via 0.635 cm o.d. tubing with a wall thickness of 0.089 cm. A vacuum and a high pressure gauge were connected to the reactor assembly with each gauge separated from the main reactor body by the trunnion ball valves. This allowed for the high pressure gauge to be isolated from the system during the vacuum cycle and the vacuum gauge to be isolated from the system during the high pressure cycle. An analysis of the pressure ratings of the parts used to make the reactor indicated that the weakest component was the 0.635 cm o.d. stainless steel tubing with a wall thickness of 0.089 cm. According to the Swagelok Tubing Data Catalog,13 at an elevated temperature of 204 °C, these tubes can handle pressures up to 4896 psig. During our experiments, we carefully operated below this temperature and pressure. Future plans include the addition of an appropriate pressure relief valve. The caps at both ends of the steel tube were sealed using Teflon tape and pressure leak tested several times using helium at a pressure of 1400 psig by means of a pressure gauge and Snoop liquid leak detector to ensure that the seals could hold. The trunnion ball valves have been tested by the manufacturer using N2 at 1000 psi to ensure that they leak less than 0.1 standard cm3/min. In-house, we observed a small leak using a sensitive gauge connected via a trunnion ball valve to the hot CO2-filled pressurized reactor, this leak was quantified and found to be much less than the manufacturer’s specification. We also determined that over a 48 h period, which is the typical exposure duration, we lose less than 0.1% of the CO2 present in the filled reactor through valve leakages and therefore the trunnion ball valves are adequate for this work. Owing to this valve leakage, the vacuum gauge which has a range of between 30 in Hg vacuum to 30 psig was detachable via a Swagelok quick connect fitting to avoid damaging it. The coal was placed in pouches fabricated from stainless steel wire mesh cloth (width of opening ) 10 µm) and placed appropriately within the length of the steel tube so that after assembly of the reactor and placement within the furnace, the pouches would be within the 30 cm long heated section of the Lindberg model 55035 electrically heated furnace. A short stainless steel tube was placed in the lower section of the reactor to ensure that the pouches did not drop beneath this heated section. Those sections of the reactor assembly not enclosed within the furnace (transfer lines) were wrapped with heating tape and heated up to the furnace temperature. The temperature of the transfer lines was monitored using a thermocouple. To avoid both damaging the high-pressure gauge and errors in pressure measurement due to heating of the high-pressure gauge during the process, a flexible armored capillary was used to connect the gauge to the hot reactor thereby isolating it from the heat. After loading the two coal-filled pouches containing a total of 30 g of coal, the tube was sealed and the reactor assembly vacuumed to 30 in. Hg while the furnace and the transfer lines were heated up to 26 °C. The reactor was then filled with instrument grade (99.99% purity) CO2 delivered from a siphontube fitted CO2 gas cylinder. The cylinder was wrapped with a domestic electrically heated blanket to raise its temperature above room temperature up to 34 °C. This increased the cylinder pressure and allowed for the reactor to be filled with a fluid of greater density than would have been the case if the tank were at room temperature. This greater density allowed for higher pressures to be achieved at a given process temperature than
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would have been achieved if the reactor was filled using a cylinder at room temperature. Before isolating the reactor from the CO2 gas cylinder, adequate time was allowed for the pressure to stabilize within the reactor (about 10 min) and then the furnace was turned on to heat up the reactor to the desired temperature. With the reactor loaded with 30 g of coal and saturated liquid CO2 at 26 °C, the approximate solvent to coal ratio (by mass) was 2.81 for asreceived bituminous coal and 2.92 for dried bituminous coal; for lignite it was 2.59 and 2.26, respectively. The differences in the solvent to coal ratios above are due to the different mass densities of the coals as measured using the water displacement method. An exposure duration of 48 h was chosen based on a rough extrapolation of the time it took to achieve steady state in a study conducted by Reucroft and Sethuraman.2 At the end of the process, the CO2 was released over an approximate duration of 10 min. This rate of release was adequate to allow for the gas to be taken up by the fume hood and also to avoid movement of the reactor body which could break the delicate lining of the heating-element on the furnace. The reactor was cooled down to ambient before removal of the coal from the reactor. For the sample contacted with supercritical CO2 at pressures beyond the operating limit of the reactor fabricated in-house, this was done on a Supercritical Fluid Technologies model FCF 150. When 43 g of as-received coal was placed in the reactor’s 80 mL sample cell and the reactor was filled with saturated liquid CO2 at 26 °C, the solvent to coal ratio (by mass) was 1.3. 2.2. Description of Samples. This study was conducted using a suite of as-received and dried coals in their raw form and after contact with supercritical CO2. They are described below: 1. Illinois #6 bituminous coal, ground and sieved to between 75 and 106 µm. The drying process was done in a vacuumed oven heated to between 100 and 105 °C overnight. 2. Beulah lignite, ground and sieved to between 75 and 106 µm. The drying process was done in a vacuumed oven heated to between 100 and 105 °C overnight. 3. D80B48@2000 psig, dried, raw bituminous coal was contacted with supercritical CO2 at 80 °C and 2000 psig for 48 h. 4. A80B48@8100 psig, as-received bituminous coal was contacted with supercritical CO2 at 80 °C and 8100 psig for 48 h. 5. A130B48@3075 psig, as-received bituminous coal was contacted with supercritical CO2 at 130 °C and 3075 psig for 48 h. 6. D80L48@2600 psig, dried, raw lignite was contacted with supercritical CO2 at 80 °C and 2600 psig for 48 h. 7. A130L48@3300 psig, as-received lignite was contacted with supercritical CO2 at 130 °C and 3300 psig for 48 h. The analytical results of these samples are tabulated in Tables 1 and 2. These came from the suppliers of the raw coals (see Acknowledgments section) while Huffman Laboratories analyzed and reported those of coals contacted with supercritical CO2. 2.3. Collection of Isotherms. The isotherms were collected using a Quantachrome Instruments NOVA 1200 gas sorption analyzer running Firmware Version 3.70. During analysis, the samples were held in a long 9 mm cell with a large bulb and a filler rod. After consulting with an application specialist at Quantachrome,13 we were advised to use the following settings for the adsorbate dosing routine parameters.
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Table 1. Ultimate Analysis of the Samples (wt %)
Illinois #6 bituminous coal (as-received) Illinois #6 bituminous coal (dried) D80B48@2000 psig (dried)a D80B48@8100 psig (dried) A130B48@3075 psig (as-received) Beulah lignite (as-received) Beulah lignite (dried) D80L48@2600 psig (dried) A130L48@3300 psig (as-received)b
loss on drying
C
H
O
N
S
mineral matter
atomic H/C ratio
atomic O/C ratio
% mass change after interactionc
2.30 2.30 1.44 1.23 1.85 11 11 7.09 10.36
71.26 72.9 70.4 69.57 70.13 58.57 65.01 59.28 57.61
4.79 4.9 4.59 4.7 4.72 3.88 4.31 3.99 4.73
6.45 6.6 12.24 11.02 12.52 16.38 18.18 29.64 31.36
1.47 1.5 0.97 1.44 1.43 0.88 0.98 0.8 0.84
2.74 2.8 2.67 2.63 2.49 0.74 0.82 0.95 0.97
10.56 10.80 9.73 9.40 9.71 6.01 10.78 9.44 8.18
0.81 0.81 0.78 0.81 0.81 0.79 0.80 0.81 0.99
0.06 0.06 0.12 0.11 0.12 0.16 0.18 0.29 0.30
na na 7 -4 -3 na na 10 -10
a D80B48@2000 psig: D stands for dried, 80 for process temperature in °C, B for bituminous coal, 48 for process duration in hours and 2000 for the process pressure in psig. b A130L48@3300 psig: A stands for as-received, 130 for process temperature in °C, L for lignite, 48 for process duration in hours and 3300 for the process pressure in psig. c na ) not applicable.
Table 2. Mineral Analysis of Coal (% of Mineral) Sample
Na2O
MgO
Al2O3
SiO2
P2O5
SO3
K2O
CaO
TiO2
MnO
Fe2O3
Illinois No. 6 bituminous coal A130B48@3075 psig Beulah lignite A130L48@3300 psig
0.51 0.51 4.60 3.59
1.02 0.95 7.00 7.50
20.49 19.91 9.97 10.20
51.65 51.05 22.62 17.80
0.07
