Enhancement of Low-Severity Coal Liquefaction by Mild Acidic

Three coals, Wyodak and Black Thunder mine subbituminous and North Dakota Beulah-Zap lignite, were pretreated with mildly acidic solutions of hydrochl...
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209

Enhancement of Low-Severity Coal Liquefaction by Mild Acidic Pretreatment and Cyclic Olefins Thomas M. Beasley and Christine W. Curtis* Chemical Engineering Department, Auburn University, Auburn, Alabama 36849-5127

James N. Hool Industrial Engineering Department, Auburn University, Auburn, Alabama 36849-5127 Received May 15, 1995X

Three coals, Wyodak and Black Thunder mine subbituminous and North Dakota Beulah-Zap lignite, were pretreated with mildly acidic solutions of hydrochloric or sulfurous acid in methanol, water, or hexane. These pretreated coals were reacted in the presence of one of three hydrogen donors: 1,4,5,8-tetrahydronaphthalene, a cyclic olefin commonly known as isotetralin; 1,2,3,4tetrahydronaphthalene, isotetralin’s hydroaromatic analog commonly known as tetralin; or 1,4,5,8,9,10-hexahydroanthracene, a three-ring cyclic olefin. Coal conversions obtained from combinations of selected pretreatments and hydrogen donors were examined in a factorial experiment. Supplemental reactions were performed evaluating the effect of other parameters. Reaction conditions were 350 °C for 30 min under 3.45 MPa of hydrogen introduced at ambient temperature. Both isotetralin and hexahydroanthracene yielded higher coal conversions than tetralin regardless of the pretreatment condition. Each coal responded differently to the pretreatments. The highest conversions for the three coals were achieved with isotetralin and different acid pretreatments. Black Thunder mine coal yielded 41.2% coal conversion after reacting with isotetralin following pretreatment with hydrochloric acid in methanol compared to 27.7% for the same system without HCl. Wyodak yielded 39.0% and Beulah-Zap yielded 35.0% coal conversion after reacting with isotetralin following pretreatment with hydrochloric acid in water compared to 24.9 and 23.2%, respectively, without HCl. Hexahydroanthracene was less effective than isotetralin with hydrochloric acid pretreatments but more effective than isotetralin with the sulfurous acid pretreatments. Conversions with hexane as a pretreatment solvent were similar but slightly lower than those with water. Combinations of some factors exhibited interactions.

Introduction High-severity coal liquefaction converts a hydrocarbonaceous solid to a liquid under high temperature (400-440 °C) and high hydrogen pressure conditions (18-20 MPa).1 Through liquefaction, the hydrogen content of the converted coal materials is increased while the heteroatom and mineral matter content are decreased. High-severity liquefaction reactions occur well above the activation energies of the major reactions involved and are dominated by unordered retrogressive condensation and cracking reactions as well as an array of transalkylation reactions.2 Upgrading catalysts used in high-severity liquefaction tend to deactivate because of the deposition of carbonaceous material from high concentrations of high molecular weight preasphaltenes and metals present in the liquefied materials.2 Liquefaction at low-severity conditions (350 °C or less) and lower hydrogen pressures (7-8 MPa) offers an alternative to traditional high-severity conditions. However, coal conversion is inherently low at these temper* Author to whom correspondence should be sent. X Abstract published in Advance ACS Abstracts, November 15, 1995. (1) Hessley, R. K.; Reasoner, J. W.; Riley, J. T. Coal Science: An Introduction to Chemistry, Technology and Utilization; John Wiley & Sons, Inc.: New York, 1986. (2) Moroni, E. C. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1986, 31 (4), 1-4.

0887-0624/96/2510-0209$12.00/0

atures and requires physical and chemical methods such as catalysts, hydrogen donation, and coal pretreatments to increase coal conversion. Moroni and Miller et al.2 stated that some advantages of low-severity liquefaction are reduction in hydrocarbon gas production resulting in reduced feed gas consumption, enhanced hydrogen utilization efficiency, and less severe materials handling and construction requirements. Retrogression of primary coal dissolution products is also reduced, which increases residuum product quality. In addition, coal products with low heteroatom content are produced, giving a liquefaction product more amenable to upgrading in a conventional catalytic cracker.2,3 By using solvent swelling and mild alkylation, Baldwin and co-workers4,5 improved conversion of low-rank coals by minimizing organic oxygen coupling sites which initiate retrogressive reactions. Other researchers6,7 have used ion exchange to improve conversion of low(3) Miller, R. L.; Shams, K.; Baldwin, R. M. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1991, 36 (1), 1-6. (4) Baldwin, R. M.; Kennar, D. R.; Nguanprasert, O.; Miller, R. L. Fuel 1991, 70 (3), 429-433. (5) Baldwin, R. M.; Nguanprasert, O.; Kennar, D. R; Miller, R. L. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1990, 35 (1), 70-76. (6) Serio, M. A.; Kroo E.; Charpenay, S.; Solomon, P. R. Prepr. Pap.sAm. Chem. Soc. Div. Fuel Chem. 1993, 38 (3), 1021-1030. (7) Ollendorf, R. L.; Cronauer, D. C.; Swanson, A. J.; Sajkowski, D. J. DOE Final Technical Report, 1992, Contract No. DEAC22-88PC88819.

© 1996 American Chemical Society

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Energy & Fuels, Vol. 10, No. 1, 1996

rank coals since this process removes Ca, Mg, K, and Na ions that inhibit hydrogen transfer. Similarly, lowrank coal conversions have been improved by pretreating coal with acidic solutions that also remove the alkali and alkaline earth metals and reduce organic oxygen coupling.8-11 Miller and his co-workers3,12-14 have performed numerous experiments involving pretreatment of low-rank coals with dilute solutions of hydrochloric acid (HCl) in methanol (CH3OH). Miller and Shams13 found that 2% HCl was an optimal amount for removing cations from low-rank coals. Shams and Miller14 hypothesized that Ca, specifically, can directly catalyze retrogressive reactions and, therefore, removal of Ca improves conversion, particularly at low-severity conditions. Serio and co-workers15,16 used the method of Hengel and Walker to pretreat coal with ammonium acetate (NH4Ac) for ion exchange15 and found that this procedure also improved coal conversion. Bedell and Curtis17 discovered that cyclic olefins, hydroaromatic species that do not contain aromatic rings, release hydrogen more readily than their hydroaromatic counterparts. The cyclic olefins transfer substantially more hydrogen to a model acceptor18 or coal19-21 than other types of hydrogen donors under both thermal and catalytic conditions. Huang22 increased coal conversion in low-severity liquefaction of Illinois No. 6 and Wyodak coals by using the cyclic olefin hexahydroanthracene as a hydrogen donor in catalyzed reactions. Preliminary reactions performed by Huang22 indicated that mild acidic pretreatment enhanced catalytic low-severity liquefaction of both low- and high-rank coals. The objective of this study was to improve lowseverity liquefaction of low-rank coals by combining mild acidic pretreatment to remove inhibiting ions prior to reaction with a highly reactive hydrogen donor. The importance of the types of acid and solvent in the pretreatment and of the type of hydrogen donor in the liquefaction reaction was assessed through a factorial experiment. These results were augmented by performing additional liquefaction reactions which evaluated other parameters such as the effect of reaction time and type of cyclic olefin on coal conversion. The amount of coal conversion achieved in the liquefaction reaction was the measure for determining the efficacy of the pretreatment method and hydrogen donor. Individual and interactive effects were examined. (8) Lafferty, C.; Hobday, M. Fuel 1990, 69, 78-83. (9) Lafferty, C.; Hobday, M. Fuel 1990, 69, 84-87. (10) Joseph, J. T.; Forrai, T. R. Fuel 1990, 69, 75-80. (11) Brannan, C. J.; Curtis, C. W.; Cronauer, D. C. Fuel Process. Technol., in press. (12) Kelkar, S.; Shams, K.; Miller, R. L.; Baldwin, R. M. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1993, 38 (1), 554-560. (13) Shams, K.; Miller, R. L.; Baldwin, R. M. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1992, 37 (1), 1017-1024. (14) Shams, K.; Miller, R. L,; Baldwin, R. M. Fuel 1992, 71 (9), 1015-1023. (15) Serio, M. A.; Solomon, P. R.; Kroo, E.; Bassilakis, R.; Malhotra, R.; McMillen, D. Fuel 1990, 69, 61-69. (16) Hengel, T. D.; Walker, P. L. Fuel 1986, 63, 1214-1220. (17) Bedell, M. W.; Curtis C. W. Energy Fuels 1991, 5, 469-476. (18) Wang, S. L.; Curtis, C. W. Energy Fuels 1994, 8, 446-454. (19) Bedell, M. W.; Curtis, C. W.; Hool, J. N. Energy Fuels 1993, 7, 200-207. (20) Bedell, M. W.; Curtis, C. W.; Hool, J. N. Fuel Process. Technol. 1994, 37, 1-18. (21) Kwon, K. C.; Curtis, C. W.; Guin, J. A. Fuel 1984, 63, 14041409. (22) Huang, A. P. Master’s Thesis, Auburn University, 1994.

Beasley et al. Table 1. Proximate and Ultimate Analyses of the Coals Studied Wyodaka

coal moisture ash volatile matter fixed carbon total carbon hydrogen nitrogen organic sulfur oxygen (by difference) total

North Dakotaa Beulah-Zap

Black Thunderb

Proximate (%) 28.1 32.2 6.3 6.6 32.2 30.5 32.9 30.2 100.00 100.0

11.2 5.4 44.5 38.8 100.0

Ultimate (%)c 75.0 5.4 1.1 0.5 18.0

72.9 4.8 1.2 0.7 20.3

74.5 4.9 1.0 0.4 19.2

100.0

99.9d

100.0

a

Voorhees, K. Users Handbook for the Argonne Premium Coal Sample Bank, 1993. b Commercial Testing and Engineering Co., Brimingham, AL. c Elemental content based on a moisture- and ash-free basis. d North Dakota Beulah-Zap lignite contains a small amount of chlorine.

Experimental Section Materials. Liquefaction reactions were performed with hexadecane (99% purity) containing one of three hydrogen donors, 1,2,3,4-tetrahydronaphthalene (tetralin, 99% purity) and 1,4,5,8,9,10-hexahydroanthracene (HHA, 97% purity) from Aldrich, and 1,4,5,8-tetrahydronaphthalene (isotetralin, 97% purity) prepared by K. W. Olsen23 and recrystallized using ethanol. Two coals from the Argonne Premium Coal Sample Bank, Wyodak subbituminous and North Dakota Beulah-Zap lignite, and one coal from Amoco Oil, Black Thunder mine subbituminous, were pretreated and subjected to low-severity liquefaction. Proximate and ultimate analyses for these coals are presented in Table 1. The feed coals were pretreated with distilled water, methanol (CH3OH, HPLC grade), or hexane (C6H14, HPLC grade). These solvents were used alone or in conjunction with hydrochloric acid (HCl, ACS reagent grade) or sulfurous acid (H2SO3, prepared by the method of Schein et al.24 ). Ammonium acetate (NH4Ac, HPLC grade) was also used to pretreat the coals and lignite. Tetrahydrofuran (THF, HPLC grade) was used to extract liquefaction products from the undissolved solids. All solvents and reagents were supplied by Fisher Scientific. Pretreatment Procedure. Pretreatment of low-rank coals with mild acidic solution was performed in this study following the method of Shams and Miller.13,14 The goal of the mild acid pretreatment was to remove alkali and alkaline earth metals. The procedure involved stirring a slurry of 5.0 g of coal with a solution of 0.4 mL of acid in 40 mL of solvent for 3 h in a sealed 125 mL Erlenmeyer flask. The coal slurry was then filtered using a Whatman No. 5 filter paper that had been wetted with solvent and was supported by a 114 mm Coors porcelain funnel with a fixed perforated plate. The coal was washed quickly with 150 mL of solvent (CH3OH, hexane and distilled water) and then with 150 mL of distilled water to remove any acid or solvent. The coal was then dried in a vacuum desiccator at room temperature for at least 12 h. The moisture and solvent content of the coal was determined after drying and was used to calculate the amount of coal used in the reaction on a moisture-, solvent-, and ash-free basis. In this study, alkali and alkaline earth cations were also removed using NH4Ac pretreatment; the procedure used followed the method described by Hengel and Walker.16 Longer contact times between the 1.5 M NH4Ac solution and (23) Olsen, K. W.; Bedell, M. W.; Curtis, C. W. Fuel Sci. Technol. Int. 1993, 11, 155, 1-75. (24) Schein, D. B.; Lee, T. L.; Hokama, J.; Zeitlin, H. Rev. Sci. Instrum. 1984, 55 (6), 989-990.

Enhancement of Low-Severity Coal Liquefaction Table 2. Calcium and Magnesium Analysis of Pretreated Coals in the Factorial Experiment pretreatment conditions

% calcium

% magnesium

from the grams of IOM recovered on a moisture and ash-free (maf) basis and is given by the following equation.

[ (

% conversion ) 1 -

Black Thunder 0.86 CH3OH only H2O only (no pretreatment) 0.84 H2SO3 in CH3OH 0.85 H2SO3 in H2O 0.82 HCl in CH3OH 0.086 HCl in H2O 0.16 NH4Ac 0.099

0.15 0.16 0.13 0.12 0.0075 0.028 0.0090

Wyodak CH3OH only 1.04 H2O only (no pretreatment) 1.02 H2SO3 in CH3OH NAa H2SO3 in H2O 0.97 HCl in CH3OH 0.059 HCl in H2O 0.12 NH4Ac 0.11

0.21 0.21 NA 0.18 0.0091 0.012 0.0098

a

Energy & Fuels, Vol. 10, No. 1, 1996 211

NA ) not available.

the coal were needed to achieve the same level of cation removal as 3 h of contact used in acid pretreatment. The entire method used by Hengel and Walker lasted about 40 h, but after the initial 6 h 91% of the Ca cations were removed.16 In the current study, pretreatment times of 6 and 40 h for NH4Ac were used. A factorial experiment was designed to analyze the effect of pretreatments on low-severity reactions promoted by cyclic olefins. The subbituminous coals were used in the factorial experiment; each reaction set was triplicated. Other auxiliary experiments were performed including the reactions with lignite; these reactions were duplicated. The pretreated coals used in the factorial design were analyzed for Ca and Mg content by Huffman Laboratories as shown in Table 2. The analysis data provided by the pretreatment with CH3OH or H2O served as baseline data for comparing the efficacy of different pretreatment since neither solvent should remove Ca or Mg from the coal. Liquefaction reactions were performed to compare the reactivity of dried and wet coals. The results showed that wet coals only produced ∼12% conversion with tetralin and ∼22% conversion with isotetralin compared to coal conversions of ∼20% for tetralin and ∼25% for isotetralin after the coals had been dried in a vacuum desiccator at room temperature; thereafter, all coals that had been pretreated were dried in a vacuum desiccator at room temperature for at least 12 h prior to reaction. Reaction and Extraction Procedure. Liquefaction reactions were performed in well-agitated ∼20 cm3 stainless steel tubular microreactors. These reactors were charged with 1.00 g of coal introduced on a moisture and solvent-free basis and 2.00 g of solvent. For most reactions, the solvent hexadecane was introduced at ∼1.3 g with ∼0.6 g of hydrogen donor which provided 1% donable hydrogen. The reactions were performed at 350 ˚C for 30 min with 3.45 MPa of hydrogen introduced at ambient temperature. The effect of reaction time on liquefaction behavior was evaluated by performing reactions for 15, 60, 90, and 120 min. All reactions were at least duplicated while the reactions in the factorial experiment were triplicated. To ensure each experiment was a true replicate, reactions having the same reaction conditions were performed separately. All reactions were performed in a random order. Coal liquefaction liquid products were extracted from the solid residue by adding approximately 200 mL of THF in 15 mL aliquots to the reactors and sonicating the solution for 3 min to dissolve the coal liquids. The THF solution containing coal liquids was filtered with Whatman No. 5 qualitative filter paper to separate solubles from insolubles. THF solubles were recovered by removing the THF with a Bu¨chi rotavapor while the THF insolubles were dried in a vaccum desiccator overnight at ambient temperature. Conversion was determined

g IOMmaf g feed coalmaf

)]

× 100%

(1)

Recovery was defined as the total grams of THF-soluble and THF-insoluble material removed from the reaction divided by the total grams charged and is given by the following equation.

% recovery ) g of THF solubles + g of THF insolubles × 100% (2) g of charged Recovery from the liquefaction reaction averaged 85%, although some recovery values were lower because of the volatility of tetralin and isotetralin. Some loss of these hydrogen donors occurred while THF was being evaporated from the extracted reaction products. To reduce this loss, the temperature of the solution being evaporated was lowered to and maintained at 50 ˚C. In addition, the evaporating system was kept near atmospheric pressure with only enough vacuum drawn to remove the nitrogen gas being blown across the surface of the THF solution.

Results and Discussion Factorial Experiment. A factorial experiment was designed to determine the interactions among the various factors involved in low-severity liquefaction reactions of pretreated low-rank coals that were promoted by cyclic olefins. The first of the four factors (factor A) varied was the acid used in the pretreatment of the feed coal. This factor contained three levels: no acid, HCl, and H2SO3. The second factor (factor B), consisting of two levels, varied the type of solvent used with the acid in coal pretreatment: H2O and CH3OH. The third factor (factor C) involved a two-level variation in the hydrogen donor type present in the liquefaction reaction: the hydroaromatic tetralin and the cyclic olefin isotetralin. The final factor (factor D) was a twolevel variation in the feed coal; the two coals used were Black Thunder mine and Wyodak subbituminous coals. Reactions were performed with all possible combinations of each of these factors. Ammonium acetate (NH4Ac) pretreatments were also performed to provide comparison to the mild acidic pretreatment. Reactions using 1,4,5,8,9,10-hexahydroanthracene as a cyclic olefin donor, hexane as a pretreatment solvent, and Beulah-Zap lignite as a lower rank feed coal were performed auxiliary to the factorial experiment. The effect of the individual factors and their interactions (AB, AC, etc.) were evaluated in the factorial experiment. Coal conversion was used as the measure for effectiveness of a particular parameter. The average coal conversions obtained in the factorial experiment are presented in Table 3. Each coal conversion is the average of three replicates; the standard deviation obtained at each condition is also given in Table 3. The solvent used in the acid pretreatment affected the coal conversions of both Black Thunder mine and Wyodak coals. The pretreatments of Black Thunder with CH3OH as the pretreatment solvent in conjunction with isotetralin as the reaction solvent were more effective than those using H2O. By contrast, H2O was the more effective pretreatment solvent for Wyodak when Wyodak was reacted with isotetralin. The type of coal used in these liquefaction reactions also affected coal reactivity and conversion at low-

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Table 3. Coal Conversions for Low-Severity Liquefaction Reactions in the Factorial Experiment Matrix pretreatment solvent

acid

coal conversion, % with tetralin

CH3OH H2O CH3OH H2O CH3OH H2O NH4AC

Black Thunder Coal none 21.7 ( 2.1 none 20.2 ( 1.9 H2SO3 20.3 ( 1.4 H2SO3 20.6 ( 3.2 HCl 16.5 ( 0.8 HCl 27.6 ( 3.9 none 21.5 ( 0.2

CH3OH H2O CH3OH H2O CH3OH H2O NH4AC

none none H2SO3 H2SO3 HCl HCl none

Wyodak 17.7 ( 1.9 17.4 ( 2.1 19.0 ( 0.7 21.4 ( 1.2 21.6 ( 1.2 22.3 ( 2.3 20.4 ( 3.0

with isotetralin 27.7 ( 2.2 24.7 ( 0.9 27.7 ( 0.6 28.2 ( 4.5 41.2 ( 4.0 35.1 ( 4.6 30.1 ( 3.4 22.7 ( 2.6 24.9 ( 0.6 24.1 ( 5.1 27.6 ( 3.0 30.5 ( 1.2 39.0 ( 2.1 27.1 ( 1.0

severity conditions. The conversion of Black Thunder mine coal was greater than that of Wyodak for most equivalent reaction conditions (Table 3). There were four exceptions of which only two reactions showed that Wyodak coal was more reactive than Black Thunder mine coal; these reactions were with isotetralin after pretreatment with HCl in H2O, which yielded the highest Wyodak conversion of 39.0%, and with tetralin following pretreatment with HCl in CH3OH, which yielded the lowest Black Thunder conversion of 16.5%. This low result is anomalous because the highest conversion for Black Thunder, 41.2%, occurred in a reaction with isotetralin after the same pretreatment. These results indicated that the choice of hydrogen donor used with Black Thunder pretreated with HCl in CH3OH had a larger impact than with other pretreatments of Black Thunder mine coal or of Wyodak coal. The type of acid or base used for pretreating the lowrank coals affected the amount of coal conversion obtained. Neither NH4Ac nor H2SO3 pretreatment of either Black Thunder or Wyodak coals increased coal conversion as much as HCl pretreatment. The NH4Ac pretreatment, though, was somewhat more effective than pretreatments using H2SO3. The reactivity differences between Black Thunder mine and Wyodak coals were also apparent with NH4Ac pretreatment since the conversion for NH4Ac pretreated Black Thunder reacted with isotetralin was 30.1% and for Wyodak was 27.1%. The reactions of H2SO3 with Black Thunder and isotetralin yielded 27.7% conversion with CH3OH as pretreatment solvent and 28.2% with H2O; similarly, when Wyodak was reacted with isotetralin after pretreatment with H2SO3, 24.1% conversion was obtained with CH3OH as pretreatment solvent and 27.6% with H2O. The removal of inhibitive cations from low-rank coals6,7,14 has been shown to result in increased coal reactivity. In this study, this relationship between coal conversion and removal of inhibitive cations was also observed. The amount of cation removal is given in Table 2. An inductively coupled plasma (ICP) analysis of the pretreated low-rank coals showed that the HCl and NH4Ac pretreatments of Black Thunder mine and Wyodak coals were the most effective at removing Ca and Mg. Since these pretreatments promoted higher conversions, this reduction in cations was related to the increases in reactivity and conversion of these coals at

low-severity conditions. Although the H2SO3 coal pretreatment was performed carefully, some oxidation of H2SO3 to H2SO4 may have occurred which would reduce the H2SO3 effectiveness for Ca removal. Comparing liquefaction reactions with isotetralin with and without mild acidic or NH4Ac pretreatment clearly showed that the pretreatments resulted in higher coal conversions. Statistical Analysis of the Factorial Experiment. The effect of each factor and their interaction was assessed through an analysis of variance (ANOVA) described by Miller and Freund.25 The factorial design approach, with corresponding ANOVA, was selected because (1) it allows simultaneous investigation of multiple factors (A, B, C, D); (2) it is more efficient than alternative experimental methods, such as, particularly, the one-factor-at-a-time method (i.e., it provides the opportunity to investigate more experimental effects while requiring fewer experimental observations), (3) it allows investigation of interactions (AB, AC, etc.); and (4) it allows independent statistical testing of experimental effects because of the inherent orthogonality of a factorial design. Degrees of freedom (df), sum of the squares (SS), and mean square (MS) for each source of variation (SV), for the total and for the experimental error were calculated as indicated by eqs 3, 4, ..., 11.25 The mathematical operations shown in eqs 3, 4, ... , 11 are a result of the factorial design, and their execution provides the opportunity to test independently the experimental effects. For each SV

dfsv ) nsv - 1

(3)

where nsv is the number of levels or level combinations for SV (where SV ) A, B, C, D, AB, AC, etc.). Equations 4-11 are examples for the total SS; one-, two-, three-, and four-factor interaction SS; and the SSerror calculations:

SST )

∑y2ijklm - CT y2i... - CT 24

(5)

y2ij... - CT - SSA - SSB 12

(6)

SSA )

SSAB )

SSABC )

(4)

∑

∑

y2ijk.. - CT - SSA - SSB - SSC - SSAB 6 SSAC - SSBC (7)

∑

SSABCD )

y2ijkl. - CT - SSA - SSB... - SSAB... 3 SSABC... - SSBCD (8)

∑

SSE ) SST - SSA - SSB... - SSAB - SSAC... SSABC - SSABD... - SSABCD (9) CT ) Y.....2/72

(10)

MSSV ) SSSV/dfSV

(11)

where The value FO obtained from eq 12 is used in a test to evaluate significance of a factor or an interaction. The subscript on F is conventional statistical notation for

Enhancement of Low-Severity Coal Liquefaction

Energy & Fuels, Vol. 10, No. 1, 1996 213

Table 4. Analysis of Variance (ANOVA) for the Factorial Experiment SV

df

SS

MS

F0

>F0.10

>F0.05

>F0.01

Aa Bb Cc Dd ABe AC AD BC BD CD ABC ABD ACD BCD ABCD error total

2 1 1 1 2 2 2 1 1 1 2 2 2 1 2 48 71

673.81 41.41 1434.69 66.89 53.92 276.37 9.42 6.36 31.73 8.96 27.80 0.83 14.19 112.00 170.20 428.81 3357.41

336.91 41.41 1434.69 66.89 26.96 138.19 4.71 6.36 31.73 8.96 13.90 0.42 7.10 112.00 85.10 8.93

37.71 4.64 160.60 7.49 3.02 15.47 0.53 0.71 3.55 1.00 1.56 0.05 0.79 12.54 9.53

X X X X X X

X X X X X X

X

X

X

X X

X X

X X

X X

a A ) variation of acid in pretreatment. b B ) variation of solvent in pretreatment. c C ) variation of hydrogen donor in liquefaction reaction. d D ) variation in feed coals. eCombinations of letters are interactions in the variations.

computed statistical test measures.

FO ) MSSV/MSError

(12)

The results of these calculations are summarized in Table 4. Each value of FO was then compared with tabular F values, FT (T for tabular) to identify if the FO value exceeded FT, hence indicating significance of the tested experimental effect at a particular level of significance, R, for a test. Levels of significance considered were 0.10, 0.05, and 0.01, and the smaller the level of significance for which FO exceeded the corresponding FT the more significant the effect is. The following FT values were used in the tests: FT for R ) 0.10

for dfSV ) 2 and dfERROR ) 48 is 2.42

FT for R ) 0.10

for dfSV ) 1 and dfERROR ) 48 is 2.82

FT for R ) 0.05

for dfSV ) 2 and dfERROR ) 48 is 3.20

FT for R ) 0.05

for dfSV ) 1 and dfERROR ) 48 is 4.05

FT for R ) 0.01

for dfSV ) 2 and dfERROR ) 48 is 5.10

FT for R ) 0.01

for dfSV ) 1 and dfERROR ) 48 is 7.22

Those FO values that exceeded the FT values are marked with X’s in the corresponding columns in Table 4. Factor Effects. Each of the four factors, acid and solvent used in pretreatment and hydrogen donor and feed coal used in reaction, had a significant effect as individual factors on coal conversion at low-severity conditions. The type of hydrogen donor had the greatest effect overall while the solvent in pretreatment had the least effect. Some factor combinations, such as the combination of acid used in pretreatment and hydrogen donor used in reaction, exhibited a significant interaction. This interaction was clearly observed when a greater difference in conversion occurred between reactions containing isotetralin and tetralin with HCl pretreated coals than with H2SO3 pretreated coals or (25) Miller, I.; Freund, J. E. Probability and Statistics for Engineers, 2nd ed.; Prentice Hall, Inc.: Englewood Cliffs, NJ, 1977; pp 331-418.

untreated coals. However, the reactions of isotetralin and tetralin with HCl and H2SO3 pretreated Black Thunder coals yielded a similar difference of ∼7.5% for both acid pretreatments. Pretreatment of Black Thunder mine and Wyodak coals increased conversion regardless of the type of pretreatment when compared to the conversions of 15.8 and 15.7% obtained for untreated Black Thunder and Wyodak coals, respectively. Interactive effects were observed from the combination of the solvent used in pretreatment and feed coal and from the solvent and acid used in pretreatment. Pretreatments that used H2O as the pretreatment solvent enhanced Wyodak conversion, while CH3OH as pretreatment solvent usually promoted Black Thunder conversion. The combined effect of pretreatment acid and solvent was manifested by the increased conversions of coals that had been pretreated with acid and CH3OH or H2O and reacted with isotetralin compared to those that had been pretreated with solvent alone and then reacted with isotetralin. One tertiary effect that involved the solvent used in pretreatment and the type of hydrogen donor and feed coal used in reaction proved to be significant. Wyodak reacted with isotetralin after H2O pretreatments yielded higher conversion than when reacted with isotetralin after CH3OH pretreatments. By contrast, Black Thunder yielded higher conversion when reacted with isotetralin after CH3OH pretreatment except for H2SO3 pretreated coal. The conversion trends for both Black Thunder mine and Wyodak coals when reacted with tetralin after solvent pretreatments were similar. The reactions performed in tetralin after solvent pretreatment gave higher conversions with CH3OH than H2O, while for reactions of coals pretreated with acids in solvents, H2O gave higher conversion than CH3OH. The difference between the conversions of Black Thunder reacted with isotetralin after CH3OH pretreatments and the conversions of Black Thunder with tetralin after CH3OH pretreatments, was larger (an average of 12.7% absolute) than the difference between the conversions of those donors using Black Thunder and H2O pretreatments, 6.5% absolute. For Wyodak, the difference between the conversions with the isotetralin and tetralin after CH3OH pretreatments, 6.4% absolute, was less than that of the two donors after H2O pretreatments, 10.1% absolute. Auxiliary Reactions. Additional reactions using a different pretreatment solvent, hydrogen donor, and feed coal were performed to evaluate further the effect of these parameters on the liquefaction behavior. Hexane (C6H14) was chosen as the pretreatment solvent based on the work of Miller and Shams.14 A three-ring cyclic olefin, 1,4,5,8,9,10-hexahydroanthracene (HHA), was selected since HHA had been used effectively as a hydrogen donor for low-severity liquefaction by Huang.22 Beulah-Zap lignite was selected and subjected to lowseverity liquefaction reactions to determine the efficacy of mild acidic pretreatment on lignite which typically contains a substantial amount of mineral matter. Since these auxiliary reactions were duplicated rather than triplicated, these reactions were not included in the ANOVA statistical analysis. The average standard deviation for these reactions was 1.7 with the highest being 5.2 and the lowest being 0.0.

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Table 5. Coal Conversions from Low-Severity Reactions Using Hexane as a Pretreatment Solvent and Using Hexahydroanthracene as a Hydrogen Donor coal conversion, % pretreatment solvent acid

with tetralin

with isotetralin

with HHA

C6H14 C6H14 CH3OH H2O CH3OH H2O

HCl H2SO3 HCl HCl H2SO3 H2SO3

Black Thunder Coal 21.6 ( 1.9 34.3 ( 1.1 18.6 ( 0.4 28.0 ( 0.7 16.5 ( 0.8 41.2 ( 4.0 27.6 ( 3.9 35.1 ( 4.6 20.3 ( 1.4 27.7 ( 0.6 20.6 ( 2.2 28.2 ( 4.5

NPa NP 34.3 ( 0.7 31.7 ( 0.7 28.4 ( 2.1 33.4 ( 3.4

C6H14 C6H14 CH3OH H2O CH3OH H2O

HCl H2SO3 HCl HCl H2SO3 H2SO3

Wyodak Coal 22.5 ( 0.3 34.2 ( 3.9 18.2 ( 3.2 32.7 ( 0.0 21.6 ( 1.2 30.5 ( 1.2 22.3 ( 2.3 39.0 ( 2.1 19.0 ( 0.7 24.1 ( 5.1 21.4 ( 1.2 27.6 ( 3.0

NP NP 30.5 ( 0.4 31.2 ( 1.0 25.5 ( 2.1 28.3 ( 1.9

a

Figure 1. Effect of time on conversion in Black Thunderisotetralin-HCl/CH3OH reactions.

NP ) not performed.

Table 6. Coal Conversions for Pretreated Beulah Zap Lignite Reacted with Tetralin and Isotetralin pretreatment

coal conversion, %

solvent

acid

with tetralin

with isotetralin

CH3OH H2O CH3OH H2O CH3OH H2O

none none H2SO3 H2SO3 HCl HCl

19.5 ( 1.0 16.8 ( 3.9 21.2 ( 0.2 20.4 ( 4.1 20.8 ( 0.3 18.5 ( 2.3

26.3 ( 2.0 23.2 ( 2.3 25.6 ( 1.5 25.8 ( 0.3 29.7 ( 1.2 35.0 ( 5.2

The solvent used with the acid in pretreatment affected the amount of coal conversion obtained. Coal conversions of Black Thunder mine and Wyodak coals with C6H14 as the pretreatment solvent and either isotetralin or tetralin as hydrogen donor are given in Table 5. In nearly half of the pretreatment combinations presented in Table 5, C6H14 provided a more favorable pretreatment solvent than either CH3OH or H2O; however, no consistent trends were observed. For both feed coals, C6H14 was most effective when used with HCl pretreatment in conjunction with hydrogen donation from isotetralin. These combinations yielded coal conversions of 34.3% for Black Thunder and 34.2% for Wyodak. The combination of C6H14 with H2SO3 gave a slightly less conversion of 32.7%. Hydrogen donation by the cyclic olefins in the liquefaction reactions consistently yielded higher coal conversions, regardless of the pretreatment used. In accordance with the literature,18-21,23 both cyclic olefins, isotetralin and HHA, yielded higher conversions than the hydroaromatic, tetralin, for any given feed coal. The highest conversion of 27.6% in any reaction with tetralin occurred using Black Thunder mine coal that had been pretreated with HCl in H2O. By contrast, the average coal conversion of Black Thunder that had been pretreated with solvent alone was 26.2%. Therefore, without acidic pretreatment, reaction with isotetralin achieved a coal conversion only slightly less than did tetralin with acid pretreatment. Mild acidic pretreatment of these low-rank coals increased their ability to accept hydrogen from the highly reactive cyclic olefins. When HHA was used as a hydrogen donor, the coal conversions produced in liquefaction reactions for both Wyodak and Black Thunder mine coals were higher than those achieved with tetralin (Table 5). Reactions using H2SO3 and H2O pretreated coals yielded higher

Figure 2. Effect of time on conversion in Wyodak-isotetralin-HCl/H2O reactions.

coal conversion than HCl pretreated coal when HHA was the hydrogen donor used. Coals pretreated with H2SO3 in CH3OH converted less than HCl pretreated coals when the same donor was used. Reactions using HCl and either H2O or CH3OH pretreated low-rank coals with isotetralin consistently produced higher coal conversion than those with H2SO3 pretreated coals. Low-severity liquefaction reactions with Beulah Zap lignite usually yielded lower conversions than either Black Thunder mine or Wyodak subbituminous coal for any corresponding set of pretreatment and reaction conditions, although Beulah Zap’s reactivity was more comparable to Wyodak. Both Wyodak and Beulah Zap lignite yielded their highest conversion after pretreatment with HCl in H2O and liquefaction with isotetralin as shown in Table 6. Wyodak’s conversion under these conditions was 39.0%, and Beulah Zap’s conversion was 35.0%. Since the liquefaction of pretreated coal with cyclic olefins promoted conversion, the effect of increased reaction time where the coal would be in the hydrogen donor rich environment for longer times was evaluated. Reactions were performed at four different reaction times to evaluate the effect of reaction time on lowseverity coal conversion. The reaction conditions chosen for the reaction time study were the conditions used in the two reactions from the factorial experiment that achieved the highest conversion for each coal after 30 min of reaction. These conditions were Black Thunder mine coal pretreated with HCl in CH3OH and reacted with isotetralin and Wyodak coal pretreated with HCl in H2O and also reacted with isotetralin. Coal conversions measured after 15, 30, 60, 90, and 120 min and are presented in Figures 1 and 2. The error bars given in the figures represent the standard deviations obtained in coal conversion. The reaction involving Black Thunder mine coal reached its maximum conversion at 30 min. Wyodak coal conversion continued to increase slightly after 30 min but at a much lower rate than in

Enhancement of Low-Severity Coal Liquefaction

the first 30 min. Because only two of each reaction were performed, a few of the error bars presented in Figures 1 and 2 are larger than desirable; however, the fact that conversion leveled off after 30 min is evident. Conclusions Cyclic olefins are effective hydrogen donors at lowseverity liquefaction conditions. Reactions with the cyclic olefins, isotetralin and hexahydroanthracene, consistently produced higher coal conversion from lowrank coals than reactions with the conventional hydroaromatic tetralin. The benefit of removing cations with mild acidic pretreatment to increase coal conversion at low-severity conditions when using cyclic olefins as hydrogen donors was also clearly demonstrated. Mild acidic pretreatment removed Ca and Mg and improved coal conversion of all three coals, Black Thunder mine, Wyodak, and Beulah-Zap. Combinations of mild acidic pretreatment and hydrogen donation from cyclic olefins increased coal conversion more than either of these two methods individually. The type of coal and pretreatment method used affected the amount of coal conversion achieved in lowseverity liquefaction reactions. The advantage of pretreating Black Thunder mine coal with mild acidic pretreatment was greater than that for Wyodak and Beulah Zap. Pretreating with hydrochloric acid (HCl) improved overall conversion more than sulfurous acid (H2SO3) or ammonium acetate (NH4Ac) pretreatments. Significant interactions between the factors were also evident. Feed coal and pretreatment solvent, pretreatment solvent and acid, and cyclic olefin and pretreatment acid showed combined effects. These interactive effects were greater than the individual effects. Discovery of these combined effects accomplished one of the

Energy & Fuels, Vol. 10, No. 1, 1996 215

primary goals of this research. Each physical or chemical method to improve low-severity liquefaction may increase conversion, but to achieve the goal of utilizing low-severity coal liquefaction the finding of mutually enhancing effects to promote increased conversion is vital. Acknowledgment. We greatly appreciate the support of the Department of Energy under Contract No. DE-FG22-91PC-91281. The technical and editorial assistance provided by Ying Tang is greatly appreciated. The technical assistance of Joe Aderholt, Frank Bowers, Henry Cobb, and Michael Hornsby is gratefully acknowledged. The word processing performed by Melanie Butcher is also sincerely appreciated. Nomenclature ACS ) American Chemical Society ANOVA ) analysis of variance CT ) sum of the squares correction term df ) degrees of freedom FO ) base value for sources of variation for F test FT ) tabular values used to compare with FO for F test H2SO3 ) sulfurous acid HCl ) hydrochloric acid HEX ) hexadecane HHA ) 1,4,5,8,9,10-hexahydroanthracene HPLC ) high-pressure liquid chromatography IOM ) insoluble organic material MS ) mean square NH4Ac ) ammonium acetate SS ) sum of the squares SV ) sources of variation THF ) tetrahydrofuran EF950093J