stability of liquids derived from reaction of coal and lignin

Fuel Processing Technology, 33 (1993) 175-190 175 Elsevier Science Publishers B.V., Amsterdam

Stability of liquids derived from reaction of coal and lignin

J o n g w o n Kim b, Bilal A. Akash a, Charles B. MuchmoreL J o h n A. Koropchak ~ and Shashi B. La lvani a'*

aMechanical Engineering and Energy Processes, Southern Illinois University at Carbondale, Carbondale, Illinois 62901 (USA) bKorea Institute of Energy Research, Taejon, 305 343 (Korea)

(Received November 9th, 1992; accepted in revised form January 22nd, 1993)

A b s t r a c t

Addition of lignin to coal is synergetic in that it significantly improves the quality and yield of the liquid products produced from coal under relatively mild condition of pressure and temperature, This paper describes the stability of coal- and coal-lignin-derived liquid, because the usefulness of coal-derived liquids is dependent to a certain extent on their stability during storage. The stability of liquid products was characterized by determining their solubility in pentane and benzene, and by analyzing the change of boiling point and molecular weight distributions with aging condition. Percent of conversion, and individual atomic mass balances on various experiments conducted with coal, lignin and coal-lignin mixtures are also reported.

INTRODUCTION

It is an t ic ipa ted tha t due to unce r t a in supplies of crude oil, more markets will develop for low-cost energy subst i tutes tha t take advan tage of the wide d is t r ibut ion and renewabi l i ty of l ignocel lulosics and other biomass resources. It has been shown by Coughl in and Davoudzadeh [1] tha t addi t ion of l ignin to coal in the presence of a hydrogen donor solvent and high pressure (13.3 MPa and above) resul ts in an enhancem en t in coal l iquefaction. Also, Coughlin, and Davoudzadeh in ano the r s tudy [2] have charac ter ized the products of col iquefact ion of l ignin and b i tuminous coal at 400 °C and 13.3-16.8 MPa. Sato et al. [3] have repor ted tha t the influence of l ignin on coal l iquefact ion yields is t empera tu re dependent . Recent work in our labora tor ies [4-7] has shown an e nhancemen t in coal l iquefact ion yields and rates due to l ignin addi t ion at lower pressures (1.1 4.6 MPa) and tempera tures in the range 325-375 °C. It was found tha t the liquid products obta ined from coa l - l i gn in mixtures conta in apprec iably lower amounts of benzene-insoluble compounds and h igher quan- tities of the desirable lower molecular weight asphal tenes and pentane-soluble

*To whom correspondence should be addressed.

0378-3820/93/$06.00 © 1993 Elsevier Science Publishers B.V.

176 J. Kim et al./ Fuel Processing Technol. 33 (1993) 175-190

fractions over the corresponding fraction of liquids that are obtained by processing either coal or lignin alone.

The stability of coal derived or coal-l ignin derived liquids under storage is of great importance if coal liquefaction products are to replace petroleum products. The aging characteristics of coal derived liquids has been studied by several investigators in an attempt to identify the reactive components respon- sible for the degradation. They monitored the liquid samples using measurements of viscosity [8, 9], solubility [9], molecular weight [8], refractive index [9], percent oxygen [9], carbon dioxide and water [10].

In this work, the stability of liquid products was characterized by determining their solubility in pentane and benzene, and by analyzing the change of boiling range and molecular weight distribution with aging condition.

EXPERIMENTAL

Coal

The coal sample was a bituminous coal (Illinois Basin Coal # 105 obtained from the Argonne Premium Coal Bank) ground to -200 mesh. The elemental and proximate analyses are given in Table 1.

Lign in

The Indulin AT lignin, a by-product of the paper industry was used, which was obtained from Westvaco. The analyses are shown in Table 1.

Liquefaction procedure

Experiments were conducted using 4.0 g total solids (lignin, coal, and coal-l ignin mixture with a lignin-to-coal ratio of 3:2) in 120 ml tetralin at 375 °C for I h under initial hydrogen gas pressure of 140 psig (1.07 MPa).

TABLE 1

Elemental and proximate analysis of raw material (solids)

Analysis Coal Indulin AT lignin

Elemental (wt%) C 63.56 63.83 H 4.48 5.58 N 1.30 < 0.50 S 4.54 1.52 Proximate (wt%) Volatile matter 35.97 60.47 Carbon fraction 45.34 17.85 Ash 18.69 1.94

J. Kim et al./ Fuel Processing Technol. 33 (1993) 175 190 177

A number of experiments were conducted for the following three reasons: (i) to generate enough quantit ies of liquid products so that their stability in various environments could be ascertained, (ii) to closely characterize the react ion product, so that individual atomic mass balances could be performed, and (iii) to determine the reproducibili ty of the experiments conducted.

A glass-lined autoclave was used as a reactor. It was charged with solid sample and tetralin, and pressurized to 140 psig (1.07 MPa). A constant temperature of 375 "C was maintained for the durat ion of 1 hour after which the heat supply to the reactor was shut off. After cooling to room temperature, final temperature and pressure were recorded and produced gas was collected by a gas sampler for analysis. Gas collected was analyzed by a gas chromatograph, which was equipped with a 15 ft x 2.1 mm i.d., 60/80 Carboxen 1000 packed column. Liquid products were separated from solids by filtration. The solids were dried to a constant weight in a vacuum oven at 115 C.

Overall conversion is defined as the mass change per total mass of solids charged to the reactor. From the final temperature and pressure of the reactor and its gaseous composition as determined by the GC analysis, the total amount of gases produced was calculated using the ideal gas law. The free space volume of the reactor was 130 cm 3. Conversion to liquid was determined by the difference between the solid charged and the sum of gas produced and solid residue.

Hydrogen consumption from the gas phase is the difference between the initial amount of hydrogen gas charged and the final amount of hydrogen present in the gas phase after reaction. Gas composition measurements and the ideal gas law were used to calculate the difference between the amounts of hydrogen present in the gas phase before and after the liquefaction reaction. Hydrogen consumption from tetral in was determined from the ratio of naphta- lene to tetral in obtained by GC analysis.

S t a b i l i t y test p r o c e d u r e

The solubility of liquid samples in pentane and benzene was determined to characterize the stability of the products. Stability tests for coal-liquid and coal-lignin-liquid samples were conducted in both air and nitrogen environments. Tetral in in the liquid residue was removed using a rotary vacuum evaporator. The concentrated liquid sample was charged to several 100 ml glass containers, after which the containers containing the liquid sample were filled with air or ni trogen and submerged in two stirred, constant temperature oil baths maintained at 40 °C and 80 ~C, respectively. The liquid under storage was sampled for analysis periodically. The liquid sample was subsequently separated into benzene-soluble and benzene-insoluble fractions; the benzene soluble fraction was then extracted with pentane to generate a pentane-soluble fraction [11]. Size-exclusion chromatography was performed to determine average molecular weight, using the procedure described by Yau et al. [12]. The procedure is also given in another publication involving coal and lignin- derived liquids [5].


The liquid samples after 65 days of aging period were analyzed by the ASTM D-2887 procedure, which is used to determine the boiling range distribution by gas chromatography. This method is applicable to coal liquefaction products as well as petroleum products. The applicability of the procedure toward analysis of coal liquids has been reported by Pannell and Sood [13]. They compared the results of simulated distillation on nine coal liquid samples obtained from separate liquefaction experiments, and found excellent correla- tion with the results of the true-boiling-point (TBP) values obtained according to the ASTM D2892 procedure. In this work, the boiling range distribution of the liquid residue was analyzed in a Sigma-3 gas chromatograph equipped with an FID detector and SE-30 packed column, that separated hydrocarbons in boiling point order. Some n-alkanes and aromatics (C6-C19) were used as liquid standard solutions in order to calibrate the gas chromatograph by correlating their corresponding boiling temperature and retention times. The aliphatic standard n-alkanes used were: n-hexane, n-octane, n-nonane, nodecane, n- undecane, n-dodecane, n-tridecane, n-tetradecane, n-pentadcane, n-hexadecane, n-heptadecane, n-octadecane, and n-nonadecane. The aromatic-aliphatic standard hydrocarbons were: benzene, toluene, ethylbezene, p-xylene, m-xylene, o-xylene, cumene, n-propylbenzene, mesitylene, p-cymene, n-butylbenzene, n- hexylbenzene, n-octylbenzene, and n-decylbenzene.

RESULTS AND DISCUSSION

M a s s ba lance

The solid conversion data are reported in Table 2, which is based upon the amount of solids reacted as a fraction of the total solid mass charged to the reactor (moisture ash-free basis).

Each set of data (i.e. coal, lignin or coal-lignin) was assumed to follow a normal distribution. For each set of data points, the standard deviation, o, was determined. The standard deviation (of coal, lignin and coal-lignin) from these experiments was estimated to be 6.37, 1.29, and 3.82, respectively. The data show a great consistency in the results obtained from lignin and coal-lignin experiments, compared to coal alone. Thus, a greater degree of reproducibility can result upon the addition of lignin to coal during liquefaction. Chauvenet's criterion for rejection of unsuitable data was used for all sets of data; this statistical approach provides an unbiased way of evaluating the data.

The average overall coal, lignin and coal-l ignin conversions (m.a.f) are 54.8%, 69.9% and 66.8%, respectively. Assuming that lignin conversion is unaffected by the presence of coal, enhancement in coal depolymerization (i.e. solid conversion to gas and liquid) due to lignin addition was determined to be 11.9%.

The gas composition for coal, lignin and coal-l ignin liquefaction experiments is shown in Table 3. Lignin reaction resulted in a greater gas production than that observed for coal and coal-l ignin mixtures. The material recovery data for a number of experiments using lignin show that the overall material

J. Kim et al./ Fuel Processing Technol. 33 (1993) 175-190 179

TABLE 2

Solid conversion ~ in tetralin b. 4 g coal, 4 g lignin (Indulin AT) and (1.6 g coal + 2.4 g lignin) were reacted in 120 ml tetralin at 375 °C for 1 h under initial hydrogen gas pressure of 140 psig

Coal Lignin Coal + lignin

Conversion a Conversion Conversion ~ Conversion Conversion ~ Conversion (% m.a.f.) (% m.a.f.) (% m.a.f.)

Average

38.75 47.31 72.25 c - - 62.25 68.39 57.50 63.44 71.50 d 65.25 70.90 47.50 54.83 68.50 71.51 66.00 71.53 45.25 52.90 65.85 69.11 75.00 d 33.25 d - - 67.75 70.83 57.50 64.41 50.00 56.99 67.50 70.60 61.50 67.76 49.00 56.13 66.00 69.24 58.75 65.46 50.25 57.20 66.25 69.47 59.00 65.67 46.50 53.97 52.00 d - - 58.00 64.83 49.25 56.34 65.50 68.79 55.00 62.74 41.00 49.24

47.50 54.83 66.76 69.94 60.42 66.85

~Calculated as: (massi,-massout)/(massl,). bDistilled tetralin was used to remove impurities. CData not included in estimating average conversion (as per statistical test). dData not included in estimating average conversion (due to possible experimental error).

TABLE 3

Analysis of the gas products. The free space volume (gas) is 130 ml

Experiment Gas composition (vol %)

P, (psi) a H2 CO CH4 CO2

Coal -0.24 97.85 0.56 0.78 0.81 Lignin 13.07 87.33 2.03 7.77 2.88 Coal + lignin 5.27 89.46 1.65 7.06 1.83

aDefined as the difference in pressure between the initial H2 pressure (140 psig) and final pressure after cooling at the end of the experiment.

r e c o v e r y is f a i r l y good, b e t w e e n 94-98~o of t h e o r i g i n a l mass c h a r g e d to t h e

r e a c t o r . H y d r o g e n c o n s u m p t i o n f r o m t h e gas p h a s e a n d f rom t h e t e t r a l i n was

a l so e s t i m a t e d by gas c h r o m a t o g r a p h y , as s h o w n in T a b l e 4. T h e s e d a t a

a l l o w e d us to e v a l u a t e t h e o v e r a l l m a s s b a l a n c e (Tab le 5). I t s h o u l d be n o t e d in

T a b l e 5 t h a t t he a m o u n t o f l i qu ids p r o d u c e d (L2) is d e t e r m i n e d by t h e d i f f e r ence

m e t h o d . C o a l c o n v e r s i o n to l i q u i d s due to t h e l i g n i n a d d i t i o n is e n h a n c e d by


TABLE 4

Hydrogen consumption from gas phase and tetralin. The amount of hydrogen consumption from tetralin was determined by gas chromatography

Experiment H2 consumption (mol × 103)

Gas phase Liquid phase

Coal 1.279 8.197 Lignin 2.941 10.112 Coal + lignin 4.165 9.105

TABLE 6

Elemental and proximate analyses of samples

Sample Elemental analysis (wt%) Proximate analysis (dry basis)

C H N S Volatile Fixed Ash matter carbon

Raw materials (solids) Coal 63.56 4.48 1.30 4.59 35.97 45.34 18.69 Indulin AT lignin 63.83 5 .58 <0.50 1.52 60.47 17.85 1.94

Liquid products Coal liquids 87.08 7 .83 <0.50 0,91 Lignin liquids 80.29 7 .39 <0.50 0,46 Coal+lignin liquids 83.25 7.74 0.57 0.50

Solid products Coal 56.57 3.84 0.84 4.37 31.18 38.77 30.05 Lignin 70.24 5.22 0.76 2.25 46.37 42.37 11.26 Coal+lignin 57.66 4.04 0.90 4.19 38.13 38.49 23.38

6.1% assuming tha t the l ignin convers ion is unaffected by coal. The elemental and proximate analyses of the solid and liquid samples are shown in Table 6. The data reported in Tables 1-6 are used to ca lcula te the elemental mass balance. The balance, Table 7, shows tha t there is more carbon in the products t han in the reactants . This is a t t r ibu ted to the inclusion of some residual amoun t s of te t ra l in in the products. Similar type of behavior was observed for the hydrogen balance. The oxygen balance shows tha t there is more oxygen in the reac tan t s than in the products. It is possible tha t some mois ture formed dur ing the reac t ion is adsorbed on the solid mater ia l and subsequent ly lost dur ing the drying of the solids. Ano the r possibili ty is tha t mois ture present in the liquids is also lost due to evapora t ion (during the dist i l lat ion step to remove te t ra l in from the liquid products obta ined after the reaction). The sulfur balance also shows tha t there is more sulfur in the reac tan t s than in the

TABLE 5

Overall mass balance

.%

Experiment In (g)

Solids (S1) H2, gas (G1)

Coal Lignin

H 2 consumed from tetralin (T1)

Out (g)

Solids (S2)

Gases (G2)

Liquids produced (L2) ~

Conversion (%)

Solids b

mass- m.a.f- basis basis

Conversion to liquids ~

Conversion to gas d

Coal 4.0 0.00 0.1112 0.0164 Lignin 0.0 4.00 0.1112 0.0202 Coal + lignin 1.6 2.40 0.1112 0.0182

2.1 1.3296 1.5832

0.1440 0.2909 0.2406

1.8835 2.5109 2.3056

47.50 66.76 60.42

54.83 69.94 66.85

47.09 62.77 57.64

0.76 4.35 3.03

aL2 = (S1 + G1 + T1) - ($2 + G2). b(S1 - $2)x 100/S1. ¢L2 × 100/S1. d(G2 -- G1 - ,H2 consumed from gas phase)x 100/S1.

-&


TABLE 7

Elemental mass balances of C, H, O and S

A. Carbon balance

Experiment In (g) Out (g)

Solid Solid Gas Liquid

I n - Out (g)a


-0.30 ( - 11.8) - 0.49 ( - 19.2) -0.35 (-13.7)

aPercent value is given in parentheses.

B. Hydrogen balance

Experiment In (g)

Solid Consumed in gas phase

Consumed from tetralin

Out (g)

Solid Liquid

I n - O u t (g)a

Gas

Coal 0.18 0.00 Lignin 0.22 0.01 Coal + lignin 0.21 0.01

0.02 0.02 0.02

0.08 0.07 0.06

0.15 0.19 0.18

0 -0.03 (-15.0) 0.02 -0.03 (-11.5) 0.02 -0.02 (-8.3)


C. Oxygen balance



I n - O u t (g)a


0.11 (36.7) 0.55 (51.4) 0.37 (48.7)


D. Sulfur balance



I n - O u t (g)"


0.07 (38.9) 0.02 (33.3) 0.03 (27.3)



products. Sulfur compounds were not detected in the gas phase (since we did not use the column for H2S and COS detection) and this could explain the data on the sulfur balance. It was difficult to obtain the nitrogen balance for experiments involving lignin as it contains little nitrogen. Since the lignin used contains a very small amount of ash (1.94%), a satisfactory ash balance was not obtained for the lignin liquefaction experiment.

Boil ing range distribution

Boiling range distribution was obtained for liquid products from coal and coal-l ignin mixtures after 65 days of aging time. Figs. 1 and 2 show compari- sons between boiling range distribution of coal liquid products and coal-l ignin liquid products. About 65% of the starting material of coal liquids stored 65 days at 40°C under nitrogen or oxygen was distillable at 250°C. However, as shown in Fig. 2, about 50% distillable products were obtained from the coal-l ignin mixture under comparable conditions. However, the percent of distillable product up to 380 °C was about the same in both cases. These figures show the trend of change of boiling range distribution according to aging conditions. Severe aging conditions (higher temperature and oxygen atmosphere) made the amount of distillable product at low temperature decrease.

Soxhlet extraction and molecular weight distribution

Table 8 shows the solvent analyses of coal-l ignin and coal-derived liquid aged under various conditions. The extent of increase in benzene insolubles

,.I,- JE El)

.¢_ t~ -1

E -1

( J

11o

lOO-

9o

8 0

70

6o

5o

4o

3o

2o

lO

o oo --25o 3oo 35o 4oo 45o 500

Boiling Temperofure, °C

Fig. 1. Boiling range distribution of coal-derived liquid, according to aging condition. Legend: (R) 0/40/65, (A) N/25/65, (*) 0/80/65, ([~) N/40/65 and (x) N/80/65


¢.,

O~

Q)

.. . .

E - f

110

1 0 0 -

9 0 -

8 0 -

7 0 -

60 -

50

40

30

2O

10

0 2OO --2 o 360 4oo

Boiling Temperature, °C 450 500

Fig. 2. Boiling range distribution of coal-lignin derived liquid, according to aging condition. (Legend as in Fig. 1.)

was calculated, based on the liquid sample at zero aging time. Figure 3, which is based on data given in Table 8, shows a comparison between the pentane- soluble, asphaltene, and benzene-insoluble fractions of the original coal-liquid and its degraded liquid samples. In this figure, the following nomenclature was adapted to label the samples: x / y / z where x represents the environment (O for air or N for nitrogen), y is the temperature in °C and z refers to the number of days of exposure. Benzene insolubles increased from 15°7o to 24% due to a four-day exposure in air at 80°C, and increased to 21% due to a 14-day exposure in air at 40 °C. On the other hand they showed only a slight increase to about 17% due to a 14-day exposure in nitrogen at 40°C. The asphaltene fractions decreased from 38°7o to 32°7o when the sample was exposed to air for 14 days at 40 °C. Figure 3's counterpart, the coal-lignin liquid sample, is shown in Fig. 4. The benzene-insoluble fraction showed a slight increase from about 10% of the original sample to 11.5°7o due to a 14oday exposure in air at 40°C. There was not any significant increase in benzene insoluble fraction due to exposure in nitrogen. The highest increase in benzene insolubles occurred when the sample was exposed to air at 80°C (increased to about 1707o). The pentane- soluble fraction decreased from 40°7o to about 30%. The original samples contained 50°70 asphaltene fraction. Asphaltenes slightly increased under the air environment, but were virtually unchanged, at 48%, in the nitrogen environment. Thus, coal-l ignin derived liquid was more stable than that obtained by coal liquefaction. The data show that liquids exposed to air at elevated temperature are less stable than the original liquid obtained from coal and coal-l ignin liquefaction.


TABLE 8

So lven t ana lys i s of coa l - l i gn in - and coal-derived l iquid af te r 0-65 days ag ing t ime at d i f ferent t empe ra tu r e s

Sample No. P e n t a n e Benzene Benzene Increase in a and solubles solubles insolubles benzene Aging t ime (%) (%) (%) insolubles (%) (af ter 0-65 days) CLDT CDT CLDT CDT CLDT CDT CLDT CDT

Unaged (ambient) 0 39.8 46.4 50.5 38.1 9.7 15.4 n.a. n.a.

N2-aged (40 "C) 14 43.9 48.3 48.0 35.0 8.0 16.7 17.5 8.4 21 52.0 38.5 39.4 36.1 8.7 25.4 - 10.3 64.9 65 41.0 36.2 48.2 38.0 10.8 25.8 11.3 67.5

02-aged (40 °C) 8 31.7 39.0 58.8 37.4 9.5 23.6 - 2.0 53.2

14 38.4 46.3 50.1 32.5 11.5 21.3 18.6 38.3 21 44.7 39.6 44.1 32.9 11.2 27.4 15.5 77.9 65 32.2 39.4 52.3 32.7 15.4 27.9 62.2 81.1

N2-aged (80 °C) 8 43.3 35.3 48.1 37.8 8.5 26.9 - 12.3 74.6

14 45.7 34.3 43.1 32.9 11.2 32.8 15.4 113.0 65 36.0 31.9 46.2 33.2 17.8 34.9 83.5 126.6

OE-aged (80 °C) 4 29.2 41.4 54.2 34.3 16.6 24.3 71.1 57.8

14 47.8 40.1 35.7 25.7 16.6 34.2 71.1 122.1 65 30.3 30.9 36.0 27.5 33.7 41.5 247.4 169.5

~'Calculated based on l iquid sample at 0 ag ing time. n.a. = not appl icable.

50

40

d 30

~o

10

CBT

o} o/4o/a o/4o/14 o/ao/4 N/40/14

Fig. 3. Pen tane-so lub le , a spha l t ene , and benzene- insoluble f rac t ions of o r ig ina l coal- l iquid (CDT) and i ts degraded l iquid samples. Per group from left-to-right: p e n t a n e soluble, asphal- t ene soluble, and benzene insoluble , respect ively.

186 J. K im et al./ Fuel Processing Technol. 33 (1993) 175-190

60-

4O

. o

~ 30

l

CLDT 0//40//8 0//40114 0//00//4 N~40/14 N/80/B

Fig. 4. Pentane-soluble, asphaltene, and benzene-insoluble fractions of original coal-lignin- liquid (CLDT) and its degraded liquid samples. (Legend as in Fig. 3.)

Referring to Table 8, in the case of coal-derived liquid aged at 40 °C under a nitrogen atmosphere for 65 days, increases in the amount of benzene-insoluble fractions corresponded to the decreases in pentane solubles and a relatively constant amount of asphaltenes. But under an air atmosphere for 65 days at the same temperature, a decrease in asphaltenes was observed, as well as an increase of benzene insolubles. It would appear that the pentane solubles and asphaltenes are polymerizing to form the larger benzene-insoluble components, either through chemical incorporation of oxygen or with oxygen func- t ionality as a catalyst. Coal-l ignin derived liquids aged for 65 days under nitrogen at 40°C show only a relatively small change (11.3%) in the distribution of these fractions, and under air for the same time and temperature exhibited much less of an increase in benzene insolubles compared to the liquids from coal held under the same conditions.

Molecular weight distributions were also compared. As expected, the average molecular weight of each fraction increases in the order, pentane solubles < asphaltenes < benzene insolubles. This trend corresponds to our pre- vious results [5, 6] and the report of an other investigator, who determined molecular weight by vapor phase osmometry measurements [9]. Figure 5 shows the weight average molecular weight (Mw) of the original coal-liquid and its degraded liquid samples for all three fractions, as well as of the overall Mw. For example, the Mw of the benzene-insoluble fraction increased with increasing severity of the storage conditions, although some increase was also noted for storage under nitrogen. The Mw of benzene-insolubles increased from 1830 to 2024 due to a 14-day exposure in air at 40 °C, and even increased further to 2566 due to a four-day exposure in air at 80°C. Figure 6 shows the weight average molecular weight (Mw) of the original coal-l ignin liquid and its degraded liquid samples, for all three fractions, as well as the overall Mw. The Mw of the benzene-insoluble fraction of the coal-lignin-liquid increased at a lower rate when compared to its counterpart, the Mw of the benzene insoluble coal-liquid fraction. It remained the same at 1760 during an eight-day exposure in air at

J, Kim et al./ Fuel Processing Technol. 33 (1993) 175~190 187

3000

2500

2000

1500

1000-

500-

0 i i , I

CDT 0//40//8 0/40//14 0/80//4 N/40/14

Fig. 5. Weight average molecular weight of pentane-soluble, asphaltene, and benzene- insoluble fractions and overall M~ of original coal-liquid (CDT) and its degraded liquid samples. Per group from left-to-right: pentane soluble, asphaltene soluble, benzene insoluble and overall M~, respectively.

~E

2500-

2000

I000

500

CLDT 0/40/B 0/40/14 01B0/4 N/40/14 N/B0/fl

Fig. 6. Weight average molecular weight of pentane-soluble, asphaltene, and benzene- insoluble fractions and overall M~ of original coal lignin-liquid (CLDT) and its degraded liquid samples. (Legend as in Fig. 5.)

40 °C, and inc reased to 2140 due to a 14-day exposure in a i r a t 40 °C. The Mw of the benzene insoluble f r ac t ion r eached a m a x i m u m va lue of abou t 2200 at the h ighes t sever i ty condi t ion (i.e., a four-day a i r exposure at 80°C). The overa l l Mw of the coal l ignin l iquid sample inc reased wi th inc reas ing sever i ty , f rom 788 to 994 when exposed to a i r for 4 days a t 80 °C. S to rage unde r n i t rogen for 14 days at 40 °C did not cause any s ignif icant inc rease in overa l l Mw; only a s l ight inc rease was no ted for 8 days a t 80°C. F igu re 7 shows the n u m b e r a v e r a g e m o l e c u l a r weigh t (Mn) of the or ig ina l coal- l iquid and its degraded liquid samples , for all th ree f ract ions: pentane-so lubles , a spha l tenes , and benzene- insolubles , as well as the overa l l Mn. The Mn of the benzene insoluble f r ac t ion


1000

1200

.~ Boo

400

CIff 0/40/8 0/40114 O/flO/4 N/40/14

Fig. 7. Number average molecular weight of pentane-soluble, asphaltene, and benzene- insoluble fractions and overall Mn of original coal-liquid (CDT) and its degraded liquid samples. (Legend as in Fig. 5.)

c

1200

800

400

CLOT o/4o/~ o/4o/14 o/oo/4 N/4o/14 N/oo/o

Fig. 8. Number average molecular weight of pentane soluble, asphaltene, and benzene insoluble fractions and overall M. of original coal-lignin-liquid (CLDT) and its degraded liquid samples. (Legend as in Fig. 5.)

inc reased wi th inc reas ing sever i ty of the s to rage condi t ions; some increase was also no ted for s to rage unde r n i t rogen. The n u m b e r a v e r a g e we igh t of benzene insolubles inc reased f rom 1231 to 1284 due to a 14-day exposure in a i r a t 40°C, r e a c h i n g a m a x i m u m va lue of 1430 due to a four-day exposure in a i r a t 80 °C. The overa l l M, inc reased f rom 680 to 774 upon exposure to a i r a t 80°C, for a four-day period. The inc rease was g radua l wi th sever i ty condi t ions , the i n t e rmed ia t e va lues were 693 and 726, those co r re spond ing to an e ight-day and a 14-day exposure in a i r a t 40°C, respect ive ly . The overa l l M. also showed an inc rease when s tored unde r n i t rogen for 14 days a t 40°C. F igure 8 shows the n u m b e r a v e r a g e m o l e c u l a r weigh t (M.) of the or ig ina l coa l - l ign in- l iqu id and i ts degraded l iquid samples of all th ree f rac t ions , and of the overa l l M,. The

J. Kim et al./Fuel Processing Technol. 33 (1993) 175-190 189

overall M, increased with increasing severity, from 634 to 648 during an eight-day exposure to air at 40 °C, and to about 710 at a four-day exposure in air at 80c'C. The overall M, remained essentially unchanged under ni trogen storage at 40°C for 14 days and 80°C for 8 days, in contrast to the behavior of the coal-l iquid sample. The increase in molecular weight could be directly related to peroxidation and the concomitant polymeriz- ation that would follow.

CONCLUSIONS

The data of average coal, lignin and coal- l ignin conversions (m.a.f.) showed that enhancement in coal depolymerization (i.e. solid conversion) due to lignin addition was about 11.9%, assuming that lignin conversion is unaffected by the presence of coal.

The stability of coal liquids, as measured by boiling point distribution and benzene insolubles, is affected negatively by oxygen and high temperature, and positively by lignin addition. Exposure to air at elevated temperature results in a decrease in the stability of liquid samples examined in this study (as characterized by a decrease in pentane-soluble and asphaltene fractions with a simul- taneous increase in the benzene-insoluble fraction and in molecular weight). However, the coal-l ignin-derived liquids are generally more stable than those obtained by coal liquefaction and subjected to identical environment under accelerated stability tests.

ACKNOWLEDGEMENTS

This work was prepared with the support, in part by grants made possible by the Illinois Department of Energy and Natural Resources through its Coal Development Board and Center for Research on Sulfur in Coal, and by the U.S. Department of Energy (Grant Number DE-FG22-91PC91334). However, any opinions, findings, conclusions, or recommendations expressed herein are those of the author(s) and do not necessarily reflect the views of IDENR, CRSC, and the DOE.

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