Article pubs.acs.org/EF
Solvent−Coal−Mineral Interaction during Solvent Extraction of Coal Mariangel Rivolta Hernández, Carolina Figueroa Murcia, Rajender Gupta, and Arno de Klerk* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, T6G 2 V4, Canada ABSTRACT: Solvent extraction of coal is a coal-to-liquids conversion process whereby the organic matter in coal is dissolved in a solvent to produce a substantially mineral matter free product. The solvent extraction of Poplar lignite coal was studied with three model solvents (tetralin, quinoline, and 1-naphtol) and one industrial coal liquid derived solvent. Of interest was the fate of hydrogen, nitrogen, sulfur, and mineral constituents during physical dissolution at 200 °C and during reactive dissolution at 400 °C. The hydrogen redistribution in all solvents at 400 °C was within a narrow range, even though hydrogen-donor solvents resulted in higher extraction yield and in a higher H/C ratio of the residue. Quinoline strongly interacted with the coal, which masked its high physical extraction yield at 200 °C and enabled more mineral matter to be stabilized in solution. This behavior is typical of nitrogen bases in the coal, but was amplified by quinoline. Nitrogen redistribution took place with all solvents to nitrogen enrich the residue. Little or no sulfur redistribution took place during solvent extraction at 200 °C with any of the solvents, but at 400 °C, sulfur was redistributed. Evidence that sulfur is removed from the mineral matter during solvent extraction was provided by an observed decrease in S/Fe ratio, decrease in sulfur content of the ash, and increase in S/C ratio of extraction residues.
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radical reactions.14 There is also evidence that the clay minerals are responsible for char-forming reactions.15 At the temperatures typical of industrial solvent extraction and catalytic liquefaction processes, 400−450 °C, mineral catalysis has a minor contribution compared to that of thermal reactions. It can therefore be concluded that the detrimental impact of mineral matter in coal outweighs the potential beneficial effect mineral matter may have in coal conversion processes. Conventional coal cleaning methods are not able to completely remove the mineral matter from coal, due to the inherent limitations of physical solid−solid separation in segregating strongly associated organic and mineral fractions. Even though conventional physical cleaning methods cannot remove all the mineral matter, it is a useful pretreatment step before chemical cleaning, which is capable of removing the remaining mineral matter from the coal. Solvent extraction of coal is successful in producing ash-free coal.16 Solvent extraction of coal is a coal-to-liquids process whereby organic coal molecules are dissolved in an organic solvent (Figure 1). Extraction is usually performed in an autoclave with inert or hydrogen atmosphere at sufficient pressure to maintain the solvent in the liquid phase at reaction temperature. Physical dissolution dominates at lower temperature, around 200 °C and lower temperatures for lignites; the role of the solvent is to relax the coal matrix and drag soluble molecules from the coal into the bulk solvent phase.17,18 At higher temperatures, thermal decomposition of the coal takes place. All of the processes that were developed for the solvent extraction of coal operate at temperatures in the thermal decomposition range, typically, 400−450 °C and around 10 MPa.1,11,19 The free radical species produced by thermal decomposition can be stabilized with hydrogen to form lighter more soluble products.
INTRODUCTION Currently there is a renewed interest in alternative carbon sources for the production of fuels and chemicals. Coal is a prime candidate. Thanks to its wide distribution and large reserves, coal is a feasible local substitute feed material for conventional crude oil in many countries. Apart from the strategic benefit provided by coal-to-liquids conversion, there is also an economic incentive due the price differential between crude oil and coal, which is presently around US$ 15/GJ. Several processes have been studied to convert coal into more valuable components, among those, solvent extraction of coal was widely investigated.1,2 One of the main purposes of the solvent extraction of coal process is the removal of the mineral matter from the organic matter in coal to produce ash-free coal (AFC). Mineral matter in coal degrades the thermal efficiency of high temperature conversion processes, and mineral matter restricts coal utilization to processes that can handle solids. Mineral matter forms ash during coal combustion, and its disposal and handling is sometimes troublesome. The high pyritic content of some coals is a source of sulfur dioxide and when released into the atmosphere have an impact on the environment.3,4 In liquefaction processes, mineral matter may cause problems in the filtration steps, abrasion of equipment, and solid build up in the liquefaction reactor. Trace elements in coal also represent a potential hazard for the environment and health.5,6 The impact of trace elements extends to the coal conversion process, too. Some specific metals (Ni, V, As, Cu, Mn) are known to be poisonous for the catalysts used in upgrading the coal-derived liquids. Conversely, it is reported that mineral matter and pyrites in particular can have a catalytic effect in solvent extraction of coal.7−10 Whether the effect is really catalytic is a matter of debate. It was reported that hydrogen sulfide,10−12 and dilution effects,13 could offer an alternative explanation of the observed rate enhancement in the presence of mineral matter. Catalysts are rapidly deactivated during coal dissolution, and conversion is dominated by free © 2012 American Chemical Society
Received: June 30, 2012 Revised: September 30, 2012 Published: October 26, 2012 6834
dx.doi.org/10.1021/ef3011004 | Energy Fuels 2012, 26, 6834−6842
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Article
Table 1. Proximate and Ultimate Analyses for Poplar Lignite Coala ultimate analysis (wt % dry ashfree)
proximate analysis (wt %)
a b
description
x
s
element
x
s
moisture ash volatile matter fixed carbonb
23.4 29.8 30.4 16.4
0.6 2.8 1.4
carbon hydrogen nitrogen sulfur oxygenb
63.1 2.8 0.9 0.7 32.5
1.2 0.2 0.1 0.2
Average values (x) and sample standard deviations (s) are indicated. Calculated by difference.
Figure 1. Flow diagram of a generic solvent extraction of coal process. After the extraction, the microreactor was washed using fresh tetrahydrofuran (THF) ReagentPlus 99% as provided by SigmaAldrich. Equipment and Procedure. The solvent extraction of Poplar lignite was carried out in a stainless steel microreactor of 15 mL capacity. The reactor was charged with 2.7 g of coal and solvent in a 1:3 mass ratio. The reactor was purged three times with N2 to remove air and was then pressurized to 4 MPa with N2. At the reaction conditions studied, the N2 is an inert gas and it does not affect the organic nitrogen content studied. The reaction mixture was heated from room temperature up to the extraction temperature in a constant temperature fluidized sand bath. The heat-up time to reach an internal temperature of 400 °C was 8 min. The duration of each experiment was 1 h; this included the heat-up time. Experiments were performed at 200 and 400 °C. At the end of the experiment, the reactors were taken from the sand bath and cleaned to remove sand particles. Reactors were cooled with air before being depressurized. The coal− solvent mixture was vacuum filtered. The reactor and solid residue were washed with THF at room temperature. The solid material obtained (filter cake) was vacuum-dried at 80 °C overnight. The ash content of the filter cake was afterward determined following the ASTM D3174-11 standard test procedure.26 Analyses and Calculations. Coal residues obtained after extraction were analyzed to determine CHNS composition using a Vario MICRO Cube. The analysis was performed in triplicate with sample sizes between 2 and 4 mg. Energy dispersive X-ray fluorescence (XRF) was employed to determine and quantify the elemental composition of the raw coal, residues, and ashes from raw coal and residues. The XRF analyses were performed with a Bruker S2 Ranger with a silicon drift detector. Analyses were performed at 35 kV tube voltage and 30 μA tube current. The X-ray source used a Pd target, and no filters or secondary targets were employed. The results from XRF analyses were used only for relative comparisons and an external calibration was not performed for all elements. The results are thus semiquantitative and not absolute concentration values. Fourier transformed infrared (FTIR) analyses of raw coal and residues were performed with an ABB MB3000 FTIR instrument. All samples were analyzed using the MIRacle attenuated total reflectance (ATR) diamond accessory, which allows the direct collection of spectra from the solid samples. Spectra were collected with a resolution of 2 cm−1 as the average of 200 scans. The extraction yield was determined gravimetrically on the basis of solvent-free dried residue and calculated on a dry ash-free (daf) basis, rather than on a specific solvent solubility fraction (eq 1):27
However, many side-reactions are possible, including some that lead to the formation of heavier and more carbonaceous products. An efficient process for the production of ash-free coal liquid products should have the following attributes: (a) A clear separation between organic and mineral matter must be obtained. (b) Hydrogen must not be wasted on the conversion of mineral matter. (c) Nitrogen and sulfur must preferentially remain with the mineral matter. (d) The mineral matter must not be converted into a soluble form. The solvent plays a role in facilitating the coal dissolution, but it may also affect the way in which the mineral matter interacts with the system. A number of studies observed transfer of sulfur between mineral matter and coal.20−23 However, the relationship between the nature of the solvent and transfer of material to and from the mineral received little attention. The present work attempts to answer to the following questions with respect to the dissolution of lignite coal in different solvents: How does the solvent−mineral interaction affect the hydrogen to carbon ratio (H/C ratio) change, and how the hydrogen is redistributed? How is sulfur and nitrogen redistributed, and is selective extraction possible with respect to nitrogen and sulfur in coal through appropriate solvent selection? Is mineral matter chemically modified during extraction in different solvents, and is mineral matter quantitatively rejected with the residue?
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EXPERIMENTAL SECTION
Materials. Poplar, a Canadian lignite coal from Saskatchewan, was used for the solvent extraction experiments. The proximate and ultimate analyses are given in Table 1. These analyses were performed on as received coal. The lignite was not dried, since removing the water may increase the cross-link strength and thereby reduce the extraction yield.24 The value for the oxygen content was calculated by difference and is of little real value.25 The particle size of the coal used for all the experiments was in the range 355−1000 μm. The solvents used were as follows: 1,2,3,4-tetrahydronaphthalene, (tetralin) as provided by Sigma Aldrich ReagentPlus 99%; quinoline, as provided by Sigma-Aldrich, reagent grade 98%; 1-naphthol as provided by Sigma-Aldrich, ReagentPlus 99%; and a hydrotreated industrial coal derived solvent (HT-1006). The HT-1006 solvent is the hydrotreated product from a coal liquefaction process. The most abundant species in the HT-1006 solvent are 1−3 ring aromatics and naphthenes. The rational for investigating these solvents in particular are explained later on.
extraction yield =
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(feed coal (daf) − residue (daf)) feed coal (daf) × 100 [wt%daf]
(1)
RESULTS Coal Extraction Yield. Two parameters were varied during the investigation, namely, extraction temperature and solvent
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type. The extraction temperatures considered, 200 and 400 °C, were representative of solvent extraction that is dominated by physical dissolution and chemical decomposition, respectively. Four solvents were selected to evaluate the relative importance of different solvent properties during physical and chemical extraction of coal. Tetralin is a good hydrogen-donor solvent, with proven advantages during chemical extraction of coal and is often included in extraction studies to facilitate comparison with other studies. Quinoline is a basic solvent (Kb = 3 × 10−10), representative of nitrogen bases in coal liquids. 1Naphtol is an acidic solvent (Ka = 6 × 10−10), representative of tar acids in coal liquids. The HT-1006 solvent was selected for its industrial significance. HT-1006 is also a solvent with good hydrogen-donor properties, such as tetralin, but it is a complex mixture and not a pure compound solvent. The extraction yields determined after solvent extraction with different solvents are listed in Table 2. Physical dissolution
the solvents resulted in a residue with lower H/C ratio, higher S/C ratio, and higher N/C ratio. The N/C ratio of the residue obtained by quinoline extraction was very high and indicated some interaction of the residue with the solvent. Additional extraction experiments were performed with quinoline at 320 and 350 °C, resulting in residue N/C mass ratios of 0.034 ± 0.001 and 0.040 ± 0.001, respectively. Inorganic Composition. To produce ash-free coal, it is important that the mineral matter remains in the residue. Apart from the elements typically associated with organic material (mainly C, H, N, S, and O), the elemental composition of the residue should not change. It is also important that the solvent extraction does not dissolve inorganic sulfur. A semiquantitative analysis of the most abundant elements, Na and heavier, present in the raw coal was performed with XRF (Table 4). Other elements that were present at low Table 4. Concentration of the Most Abundant Elements Heavier than Na in Poplar Coala
Table 2. Extraction Yield Obtained with Poplar Lignite Coal and Different Solvents for 1 h at 200 and 400 °C under Nitrogen Atmosphere
raw coal (relative wt %)b
extraction yield (wt % dry ash-free) solvent
200 °C
400 °C
tetralin quinoline 1-naphtol HT-1006
18 3 −1 9
65 46 52 69
of lignite is overall quite low, with only the less polar solvents (tetralin and HT-1006) resulting in a meaningful extraction yield. Low or negative extraction yields found for quinoline and 1-naphtol are due to chemical interaction with the coal, as described in the literature.28,29 Under the influence of thermal decomposition, the extraction yields were higher for all solvents, with the hydrogen-donor solvents (tetralin and HT1006) performing better than the rest. CHNS Composition of Organic Residue. The CHNS composition of the organic residue after solvent extraction indicated whether there was selective extraction of hydrogen, sulfur, and nitrogen relative to the raw coal. This is a measure of product quality. The extract will have a higher quality if the residue has a lower H/C ratio but higher S/C and N/C ratios than the raw coal. The CHNS analyses of the residues obtained after solvent extraction are listed in Table 3. Little change is seen in the elemental ratios at low temperature, which is partly due to the low extraction yield at 200 °C (Table 2). Directionally, all of
element
x
s
Al Ca Cl Fe Mg P S Si Ti
1.07 0.85 0.08 0.17 0.10 0.02 0.13 1.43 0.07
0.02 0.02
