ARTICLE pubs.acs.org/est
Carbon Dioxide Sorption Capacities of Coal Gasification Residues Thomas Kempka,*,† Tomas Fernandez-Steeger,‡ Dong-Yong Li,§,# Marc Schulten,^ Ralph Schl€uter,|| and Bernhard M. Krooss§ †
German Research Centre for Geosciences (GFZ), Helmholtz Centre Potsdam, Telegrafenberg, 14473 Potsdam, Germany Department of Engineering Geology and Hydrogeology, RWTH Aachen University, Lochnerstrasse 4-20, 52064 Aachen, Germany § Institute of Geology and Geochemistry of Petroleum and Coal, RWTH Aachen University, Lochnerstrasse 4-20, 52064 Aachen, Germany ^ Unit of Technology of Fuels, RWTH Aachen University, W€ullnerstrasse 2, 52056 Aachen, Germany DMT GmbH & Co. KG, Am Technologiepark 1, 45307 Essen, Germany
)
‡
ABSTRACT: Underground coal gasification is currently being considered as an economically and environmentally sustainable option for development and utilization of coal deposits not mineable by conventional methods. This emerging technology in combination with carbon capture and sorptive CO2 storage on the residual coke as well as free-gas CO2 storage in the cavities generated in the coal seams after gasification could provide a relevant contribution to the development of Clean Coal Technologies. Three hard coals of different rank from German mining districts were gasified in a laboratory-scale reactor (200 g of coal at 800 C subjected to 10 L/min air for 200 min). High-pressure CO2 excess sorption isotherms determined before and after gasification revealed an increase of sorption capacity by up to 42%. Thus, physical sorption represents a feasible option for CO2 storage in underground gasification cavities.
’ INTRODUCTION Deep coal deposits that cannot be economically developed using conventional methods are being currently considered as carbon dioxide sinks in combination with enhanced coal bed methane recovery.1-3 Carbon dioxide storage capacities of these deep deposits mainly depend on their accessibility to carbon dioxide with respect to pore space for free gas storage and surface area for storage by sorption. However, unworked deep coal seams are relatively inaccessible to pore gases, as porosity and permeability of coals tend to decrease with increasing overburden pressure.4,5 Underground coal gasification (UCG) is discussed as an option to circumvent the limitations resulting from low accessibilities by generation of energy in combination with carbon capture and its geological storage (CCS). The general UCG-CCS concept is discussed by many authors, though studies generally have not considered experimental or field data taking into account carbon dioxide sorption capacities.5-12 However, there is an agreement on the suitability of gasified coal seams for carbon dioxide storage (reactor zone carbon storage). Even though this concept has been widely discussed,5,7,13 qualified data on carbon storage capacities in the former reactor zone are not yet available. Sorption capacities of the coal gasification residues and the reactor faces are required for quantitative assessment of total carbon storage potential in coal gasification cavities. The aim of the present study was the determination of carbon dioxide storage capacities of coal gasification residues. For that purpose, representative hard coals were retrieved from deep German hard coal deposits and gasified using a laboratory gasification reactor. Coal properties and carbon dioxide sorption capacities were determined before and after the gasification process to assess the impacts of gasification. As illustrated in Figure 1, the UCG r 2011 American Chemical Society
reactor is represented by different zones: the void space (also involving accumulated ash and rubble) and the surrounding coal (char and tars) thermally influenced by the UCG process. By the methodology presented within this study, we investigated CO2 sorption capacities of the coal present close to the reactor wall and the virgin coal (raw coal) present outside the area influenced by the UCG process. These data can be applied for the assessment of CO2 storage potentials in coal seams gasified by the UCG method.
’ EXPERIMENTAL SECTION Samples. Samples were taken from three German mines producing Upper Carboniferous coals (mainly of Westphalian age) in the paralic “foredeep” of the Variscan foldbelt. This sequence contains approximately 200 seams within a total sediment thickness of more than 3000 m of clastic rocks (sand-, silt-, and mudstone). The highest percentage of coal within this stratigraphical sequence is reached in the Lower Westphalian B with almost 5%.14 Due to the thickness of the stratigraphic column, the lateral extension, and the complex burial and tectonic history, the coal shows a wide range of ranks from high-volatile bituminous coal to anthracite (vitrinite reflectance from 0.95 to 3.22%). The Westphalian B strata of the Prosper-Haniel mine deliver high-volatile bituminous coal used for power generation (33-38% volatile matter). Here, seams are mined in workings down to 1200 m, while the samples were retrieved at a depth of 915 m. In the Lippe mine;abandoned in 2008; Westphalian C and D with up to 40% volatile matter (high-volatile Received: August 18, 2010 Accepted: December 17, 2010 Revised: November 10, 2010 Published: January 6, 2011 1719
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Figure 1. Schematic vertical and horizontal cross sections of three UCG reactors developed by the CRIP (Controlled Retracting Injection Point) technology showing the areas influenced by the gasification process (not to scale).
bituminous) coals were mined. Here, samples were retrieved at a depth of about 1275 m. The Ibbenb€uren deposit, even though part of the same basin and of the same age, has undergone a different geological history than the Ruhr/Lower Rhine-Westphalia area. It represents an isolated uplifted block, which was exposed to an intense secondary coalification by the Bramsche Pluton.14 The actually mined seams are, therefore, of anthracitic rank (4-6% volatile matter). Mining operations are carried out down to depth of 1500 m where the sampling also took place. Coal Gasification. The process of underground coal gasification was simulated in a laboratory gasification reactor consisting of a furnace, a retort, and two thermocouples installed for temperature monitoring. Figure 2 shows the experimental process flow applied for the coal gasification experiments at in situ temperature conditions of 800 C. Samples with an initial mass of about 200 g were mounted in a retort with inner dimensions of 250 140 110 mm3. Air was applied as a gasification medium and injected into the reactor at a pressure of 0.2 MPa using a tube guide. Two tar separators were applied to extract higher hydrocarbons from the synthesis gas prior to its combustion in a gas burner. Experimental parameters were identical for all three coal samples. The gasification reactor was heated from 20 to 800 C within 3 h with a constant injection rate of 10 L air/min. Temperature and flow rate were kept constant for 20 min after attaining the final temperature of 800 C and shut down subsequently. The samples were removed from the reactor after a cool-down phase to 20 C lasting for about 1 h. Air was chosen as gasification medium to represent a base case for a typical UCG oxidant, since former and current UCG operations use oxygenenriched air mixtures involving water foam or vapor. CO2 Sorption Experiments. The volumetric method was used to determine the excess sorption isotherms of the raw and gasification residues with a grain size of 1-2 mm (as received) at
Figure 2. Scheme of the experimental setup applied for coal gasification (where Q and T represent the monitoring of the volumetric oxidant flow and temperature, respectively).
45 C (approximately 318 K). The experimental method has been described by Krooss et al.15 and Simons and Busch.16 The equation of state for CO2 reported by Span and Wagner17 was applied to compute the amount of gas in the reference and sample cells from the gas pressure, temperature, and the volumes of the cells. Interlaboratory comparisons among four European laboratory groups (TU Delft, Mons Polytechnique, INERIS, and RWTH Aachen University) showed that high-pressure CO2 sorption isotherms on activated carbon could be measured with a reproducibility of 1% within each of the participating laboratories.18 The deviations of sorption isotherms measured on the same sample in different laboratories were less than 5%. For natural coals19 the intralaboratory reproducibility was similar while deviations among different laboratories were somewhat larger. 1720
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Table 1. Properties of the Raw Coal Samples and the Gasification Residues Lippe parameter
raw
Prosper-Haniel gasified
raw
gasified
Ibbenb€uren raw
gasified
moisture (%)
0.0
0.0
0.4
0.0
0.6
0.0
ash (%)
3.10
3.75
2.70
12.65
5.20
N/A
volatile matter (%)
33.0
8.26
37.5
10.01
6.0
3.82
vitrinite reflectance (%)
0.99
6.59
0.95
6.17
3.22
5.73
vitrinite (%)
97.6
-
94.6
-
99.0
-
internite (%)
2.0
-
0.2
-
1.00
-
liptinite (%)
0.4
-
5.2
-
0.00
-
upper heating value (kj/kg) lower heating value (kj/kg)
31,310 30,237
-
33,300 32,116
-
33,310 32,587
-
fixed carbon (%)
84.34
65.30
84.83
60.27
89.80
73.19
hydrogen (%)
4.83
2.15
5.56
1.52
3.27
1.00
nitrogen (%)
1.46
3.15
1.63
1.71
1.09
1.33
sulfur (%)
1.15
0.92
1.04
4.87
0.50
0.70
density (g/cm3)
1.50
1.81
1.43
1.94
1.59
1.62
sample mass in sorption measurement cell (g)
6.01
3.24
5.54
4.09
6.14
6.47
maximum excess sorption (sm3 CO2/t coal)
22.62
39.43
20.78
30.42
28.65
41.56
Figure 3. Excess sorption isotherms for CO2 on the raw coal sample from the Lippe mine and the gasification residue at 45 C.
Figure 4. Excess sorption isotherms for CO2 on the raw coal sample from the Prosper-Haniel mine and the gasification residue at 45 C.
’ RESULTS
(Lippe). Volatile matter was reduced by 2.18 (Ibbenb€uren) to 27.49% (Prosper-Haniel), indicating that at least half of the coking coals were generated during the gasification process, while moisture content was reduced to zero for all samples. Vitrinite reflectance was increased by 5.22 (Prosper-Haniel) to 5.6% (Lippe) for the high-volatile bituminous coal gasification residues and by 2.51% (Ibbenb€uren) for the anthracite indicating its higher coalification. Sulfur content changed slightly by -0.23 (Lippe) to þ0.20% (Ibbenb€uren) for two samples, while an increase by 3.83% was observed for the Prosper-Haniel sample. CO2 Excess Sorption. CO2 excess sorption isotherms were measured for the raw coal at 45 C. Figures 3, 4, and 5 show the results of these experiments on three different raw coals and their gasification residues with the properties listed in Table 1. The examined raw coals exhibit maximum excess sorption capacities between 20.8 (Prosper-Haniel) and 28.7 sm3 CO2/t coal (Ibbenb€uren) while those of the gasification residues show variations from 30.4 (Prosper-Haniel) to 41.6 sm3 CO2/t coal (Ibbenb€uren) equivalent to an increase of sorption capacities
Coal Gasification. Table 1 shows the properties of the raw coals and their gasification residues. Ash and volatile matter content were determined according to the German Industry Standards DIN 51719 and DIN 51720. Carbon, hydrogen, and nitrogen contents were measured by combustion at a temperature of 1000 C in a pure oxygen atmosphere. An infrared sensor was applied for detection of carbon as CO2 and hydrogen as H2O, while nitrogen was determined in form of N2 using a conductivity sensor. After a gasification time of 200 min (180 min heating phase and 20 min constant temperature) the carbon content of the samples was reduced by 16.6 (Ibbenb€uren) to 24.6% (ProsperHaniel) and the ash content increased by 0.6 (Lippe) to 10% (Prosper-Haniel) accompanied by a density increase of 1.5 (Ibbenb€uren) to 26% (Prosper-Haniel). Hydrogen content was decreased by 2.3 (Lippe) to 4% (Prosper-Haniel) and the content of nitrogen increased by 0.1 (Prosper-Haniel) to 1.7%
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’ ACKNOWLEDGMENT The present study was conducted within the scope of the CO2 SINUS project within the framework of the GEOTECHNOLOGIEN R&D program funded by the German Ministry of Education and Research (publication GEOTECH-1328, BMBF grant 03G0691A/B). We express our gratitude for the financial support and also acknowledge the admission to the different longwall workings granted by the Deutsche Steinkohle AG (DSK AG) as well as Nadja Golz (DMT GmbH & Co. KG) for sample acquisition. Special thanks are dedicated to Susan Giffin and Philipp Weniger (both from RWTH Aachen University, Institute of Geology and Geochemistry of Petroleum and Coal) for petrographical analysis and to the anonymous reviewers for improving the quality of this manuscript. ’ REFERENCES Figure 5. Excess sorption isotherms for CO2 on the raw coal sample from the Ibbenb€uren mine and the gasification residue at 45 C.
subsequent to gasification by 31.1-42.6%. Considering the shapes of the isotherms the maximum excess sorption is located at 4.9-7.1 MPa for the raw coal and at 5.4- 6.6 MPa for the gasification residues.
’ DISCUSSION The aim of the present study was to provide quantitative data on carbon dioxide excess sorption capacities of coal gasification residues. For that purpose, representative hard coals were retrieved from deep German hard coal deposits and gasified using a laboratory gasification reactor. Coal properties and carbon dioxide sorption capacities were determined prior and subsequent to the gasification process to determine the impacts of gasification on the coal thermally influenced and surrounding the void volume of the UCG reactor. The experimental tests exhibit a general increase of carbon dioxide excess sorption capacities for the investigated coal gasification residues by 31-42% (equivalent to 30-41 sm3/t coal) accompanied by a decrease in carbon content by 16-24%, an increase in vitrinite reflectance by 2.51-5.6%, and an increase in density by 1.5-26% after gasification. These effects are induced by the coal gasification process which accounts for an increase in porosity and, therewith, an increase in accessibility and availability of surface area required for the physical sorption of carbon dioxide molecules. Based on the results of the present study an assessment of CO2 storage potentials related to the UCG-CCS technology, which is currently considered an economically and environmentally sustainable option for energy supply by several authors, is feasible in Central and Northern Europe. Further work should concentrate on the examination of in situ coal gasification residues to account for all combustion zones present in an underground coal gasification reactor. Furthermore, the influence of different oxidant choices on the thermally influenced coal surrounding the void space of the reactor should be determined in additional experimental studies.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Present Addresses #
Shell Projects and Technology, Kessler Park 1, 2288 GS Rijswijk, The Netherlands.
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