lignite liquifaction

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Study on the liquefaction of Shengli lignite with NaOH/methanol

Zhiping Lei a, Muxin Liu a, Hengfu Shui a,⁎, Zhicai Wang a, Xianyong Wei b

a School of Chemistry and Chemical Engineering, Anhui Key Laboratory of Coal Clean Conversion and Utilization, Anhui University of Technology, Ma'anshan 243002, PR Chinab School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou, 221116, PR China

a b s t r a c ta r t i c l e i n f o

Article history:

Received 9 August 2009

Received in revised form 1 February 2010

Accepted 18 February 2010

Keywords:

Lignite

Liquefaction

NaOH

The behaviorof liquefaction of Shengli (SL)lignite withNaOH–methanolwas studied.Based on highcontent of

water in lignite and the economy of the process (amounts of NaOH used), the effects of NaOH concentration,

methanol content and water content on the liquefaction behavior of SL lignite werepreliminarilyinvestigated.

Theresultsshowthat SLlignitehas a good reaction activity,and itsconversionand product yieldreach98% and

99% at 300 °C for 1 h respectively, when the ratio of SL lignite, NaOH and methanol is for 1 g:1 g:10 ml. NaOH

participates in the reaction. The increase of the amount of NaOH significantly increases the amount of

tetrahydrofuran soluble (THFS) fraction.Methanolplays a promotionrole in the liquefaction, whichmakes the

product yield increase for about 16–23%. Water content has little effect on the SL lignite conversion, product

yield and the product distribution. Solvent-extraction components of liquefaction products of SL lignite with

NaOH–methanolare mainly THFS, toluene soluble (TS),hexane soluble (HS)and water soluble fractions(WS).

The FTIR analyses of solvent-extraction components show that all of the fractions contain OH group, aromatic

structure, carbonyl group and aromatic ether oxygen group.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

There are abundant lignite resourcesin China (more than 130 billion

tons), which is approximately 13% of thetotal coalreserve. Lignite on an

as-received basis has high moisture (18–24%, average), high ash (16–

26%, average), relatively low net calorific value (3281–3854 kcal/kg,

average), and low total sulfur contents (1.00–1.22%, average). Accord-

ingly,lignite is treatedas a low-gradefuel to befired to supplyelectricity

in many countries. More than 30% moisture content in lignite might

limit its use in direct coal liquefaction because of the large amount of

energy consumption for drying. Water in lignite might bond with

organic oxygen by hydrogen bonding, which will lead to a large amount

of energy consumption to remove water by evaporation drying, and the

dried lignite is dif ficult to store. In addition, the high content of oxygen

in lignite consumes a large amount of H2 for hydro-liquefaction of

lignite. Therefore, in order to better use lignite, the lignite should have

firstly effective dehydration and the high oxygen content of lignite

should be utilized. From this point of view, thegoal of the liquefaction of

lignite should be to produce value added products, such as oxygen-

contained chemical stocks.

In order to elucidate thechemicalstructureof coals, Ochi et al. [1–11]

used non-destructive reaction—methanolysis of coal with NaOH. They

found that the main reaction was hydrolysis and simultaneous

hydrogenationin whichetherlinkages were split andaromaticproducts

were hydrogenated. This made the reacted coal nearly all soluble in

pyridine except those from the high rank coals. All of those researchesindicated that methanolysis of lignite with NaOH can be used to break

lignite structure into oxygen-contained aromatic chemicals. But in our

preliminary study [12], it was found that when the ratio of NaOH/SL

lignite was at 1:1 on weight basis, the highest SL lignite conversion and

product yield were obtained. From the economic point of view, the

amounts of NaOH and methanol used should be significantly reduced

and the effect of water content on SL lignite conversion and product

yield should be studied. Little information about these is available in

literature based on our knowledge.

In thiswork, the liquefactionbehaviors of one of the Chineselignites—

Shengli (SL) lignite under different NaOH and methanol contents were

determined. In order to avoid drying of lignite before liquefaction and to

further decrease thedrying cost, theeffect of water on theliquefaction of

SL lignite with NaOH–methanol was investigated. Therefore, the raw

lignite can be directly used for reaction without drying. The objective of

this work is to investigate the effects of NaOH, methanol and water

contents on the liquefaction of SL lignite, which can facilitate the

development of ef ficient lignite utilization.

2. Experimental

2.1. Lignite and reagents

SL lignite was used in this study. The SL lignite as received was

ground to 200 mesh, stored under nitrogen atmosphere, and dried

under vacuum at 80 °C overnight before use. The ultimate and

Fuel Processing Technology 91 (2010) 783–788

⁎ Corresponding author. Tel.: +86 5552311552; fax: +86 5552311822.

E-mail address: [email protected] (H. Shui).

0378-3820/$ – see front matter © 2010 Elsevier B.V. All rights reserved.

doi:10.1016/j.fuproc.2010.02.014

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proximate analyses of the SL lignite are shown in Table 1. All solvents

used were commercially pure chemical reagents and used as received

without further purification.

2.2. Liquefaction

The liquefaction experiments were carried out in a 30 ml tubing

reactor shaken vertically. 1.0 g of the dried coal loaded with 1.0 g

NaOH was charged into the reactor together with 10 ml of methanol.Before the liquefaction experiment, the reactor was sealed and

flushed 3 times with nitrogen followed by tuning the system to the

desired initial pressure of 0.1 MPa with nitrogen. The reactor, agitated

vertically at 120 rpm, was submerged into a eutectic salt bath as

described in detail elsewhere [13], which had been heated to the

desired temperature and maintained for 60 min. The pressure of the

liquefaction at 300 °C was about 13 MPa. Then, the reactor was

quenched to ambient temperature in a water bath before the

overhead pressure in the reactor was released slowly. The reaction

mixture was removed by washing with methanol and separated by

solvent extraction.

2.3. Fractionation of liquefaction products

The liquefaction products were obtained by removing methanol

through rotary evaporation. Then the reaction products were acidified

with hydrochloric acid, washed with water, and filtrated until pH of

the filtrate was at 7. The filtrate was extracted by ether and the

product extracted by ether was dried by MgSO4. Then the water

soluble fraction (WS) was obtained by removing ether through rotary

evaporation followed by drying under vacuum at 50 °C for 12 h. Solid

products obtained by filtration were separated by Soxlet solvent

extraction with n-hexane, toluene, and tetrahydrofuran (THF) in turn.

The n-hexane soluble, n-hexane insoluble/toluene soluble, toluene

insoluble/THF soluble and THF insoluble fractions obtained were

defined as hexane soluble fraction (HS), toluene soluble fraction (TS),

THF soluble fraction (THFS) and THFI, respectively. The extraction

procedure on the liquefaction products is shown in Fig. 1.

The conversion of lignite ( X ) and product yield (Y ) were calculatedas:

X wt :%; daf ð Þ =mLignite 1− Adð Þ−mTHFI 1− ATHFIð Þ

mLignite 1− Adð Þ× 100

Y wt :%; daf ð Þ =mWS + mHS + mTS + mTHFSð Þ

mLignite 1− Adð Þ× 100

where mLignite, mWS, mHS, mTS, mTHFS and mTHFI are the weight of SL

lignite (g, daf), WS (g), HS (g), TS (g), THFS (g) and THFI (g),

respectively. Ad and ATHFI are the ash contents of SL lignite and THFI

(wt.%, d), respectively. All the reactions were duplicated to ensure

accuracy and the average errors were about ±5%.

2.4. Products analyses

The reaction products were characterized by IR spectra using a PE-

Spectrum One IR spectrometer at ambient temperature. In the IR

measurements, the sample was mixed with KBr (sample/KBr: 1/100)

and themixture waspressed into a pellet. Theliquid product (HS) was

determined by film coating method on KBr crystal plate. The

elemental analysis was carried out in Elementar Vario EL III.

3. Results and discussion

3.1. Effect of the ratio of NaOH/SL lignite

In order to reduce the amount of NaOH used and improve theeconomy of the liquefaction of SL lignite with NaOH–methanol, effects

of the NaOH/SL lignite ratio on the conversion of SL lignite and

liquefaction product yield were investigated and the results are

shown in Figs. 2 and 3. The liquefaction was carried out at 300 °C for

1 h with a 10 ml methanol addition and a varied amount of NaOH. It

can be seen that the conversion of SL lignite was only 8% without

NaOH addition. With the increase of NaOH/SL lignite ratio the

conversion of SL lignite increased significantly. From Fig. 2 it can be

seen that the conversion of SL lignite increased almost linearly with

the NaOH/SL lignite ratio up to 1. When theNaOH/SL lignite ratio got 1

(1 g NaOH addition), the conversion of SL lignite reached to 98%. This

may suggest that NaOH participates in the reaction system, which

agrees with Masataka's finding [1].

Fig. 3 shows the effect of the NaOH/SL lignite ratio on the

liquefaction product yield and distribution. From Fig. 3, it can be seen

that the liquefaction product yield markedly increased with the

increase of the NaOH/SL lignite ratio. The products yield was always

higher than the conversion of SL lignite as shown in Fig. 2, indicating

the effect of the combination of methanol with SL lignite. The solvent-

extraction components of the liquefaction product of SL lignite with

NaOH are mainly THFS and TS. The amounts of HS and WS are small.

With the increasing of the NaOH/SL lignite ratio, the amount of THFS

significantly increased from 5% to 79%, and the amount of WS+HS+

TS increased firstly, then stabilized at about 17%. This suggests that

the amount of the small molecular weight product in SL lignite

structure is small.

3.2. Effect of methanol content

It is clear so far that NaOH plays significant role in the liquefaction

of SL lignite with NaOH–methanol. Masataka Makabe [1] suggested

that NaOH reacted with methanol to produce H2, which could be used

for coal hydrogenation. Accordingly the amount of methanol is

important for increasing lignite conversion and product yield in the

liquefaction of SL lignite with NaOH–methanol. Then the effects of the

amount of methanol on SL lignite conversion andproduct yield should

be investigated and the results are shown in Figs. 4 and 5. In all

experiments the amounts of SL lignite and NaOH used were 1 g,

respectively.

From Fig. 4 it can be seen that the amount of methanol

insignificantly affects the conversion of SL lignite. Without methanol

addition, the conversion of SL lignite reached about 83%. It suggests

that NaOH plays about 83% of the role on the liquefaction of SL lignitewith NaOH–methanol. The main reaction of SL lignite with NaOH–

methanol is hydrolysis and SL lignite probably has more ether

linkages. With the increasing of the amount of methanol from 0 to

10 ml, the conversion of SL lignite slightly increased from 83% to 98%.

From the results above it can be concluded that methanol plays a

small part (up to 15%) of the role during the liquefaction of SL lignite

with NaOH–methanol. Then the role of the production of H2 from the

reaction of NaOH with methanol for lignite conversion should be

negligible.

Fig. 5 shows the effect of the amount of methanol on the

liquefaction product yield and distribution. Without methanol

addition, product yield was about 74% and main products were

THFS, WS and TS. The difference between yield and conversion is

probably due to the evolution of gases during reaction [9] and also the

Table 1

Proximate and ultimate analyses of the SL lignite sample.

Proximate analysis wt.% Ultimate analysis wt.%, daf

Ad V daf Mad C H O⁎ N S

18.3 49.5 20.6 67.95 4.50 24.78 0.98 1.29

*By difference.

784 Z. Lei et al. / Fuel Processing Technology 91 (2010) 783–788



material losses during product extraction may affect the product yield.

Fig. 5 shows that product yield was somewhat irregular with

methanol addition and the amounts of THFS, TS and HS significantly

increased with the addition of methanol, but the amount of WS

slightly decreased compared to that without methanol. These results

suggest that the main reaction is SL lignite hydrolysis by NaOH and

higher molecular weight products (such as THFS) may be split andtransformed to lower molecular structure(such as TS,HS), which may

be caused by the hydrogenation of H2 produced from the NaOH

reaction with methanol [1]. It can be concluded that the main

reactions of SL lignite with NaOH–methanol are the splitting of the SL

lignite structure by NaOH and the hydrogenation of liquefaction

products, which play about 85% and 15% role, respectively.

3.3. Effect of water content

SL lignite has a good activity with NaOH–methanol and is one of

the feasible liquefaction coals with NaOH as mentioned above. Inorder to probe the possibility of direct liquefaction of lignite without

dehydration and drying pre-treatment, the liquefaction experiments

of SL lignite with water were carried out. The results are shown in

Figs. 6 and 7.

Fig. 1. Extraction procedure for the liquefaction product of SL lignite with NaOH.

Fig. 2. Effect of the NaOH/SL lignite ratio on SL lignite conversion; 300 °C, 1 g SL lignite,

10 ml methanol, 1 h.

Fig. 3. Effect of the NaOH/SL lignite ratio on the liquefaction product yield of SL lignite;

300 °C, 1 g SL lignite, 10 ml methanol, 1 h.




Fig. 6 shows that the conversion of SL lignite slightly increased

with addition of water in the absence of methanol and reached to a

steady value of 86% with increasing the amount of water from 0.1 ml

to 0.5 ml, which is about equal to the SL lignite moisture contents

between 10% and50%. Under theexistence of methanol (10 ml)the SL

lignite conversion significantly increased for about 15%, then slightly

decreased and reached to a steady value of 95% with increasing the

amount of water from 0.1 ml to 0.5 ml.These results clearly show that

NaOH and methanol play mayor and promoting roles respectively in

the lignite liquefaction with NaOH–methanol.

It can be seen from Fig. 7 that the addition of water slightly

changes the product yield and distribution in the presence or absence

of methanol. The addition of water slightly increased the percentage

of TS+HS+WS, which suggested that the addition of water could

slightly promote the transformation of THFS to lower molecularweight products such as TS, HS and WS. It clearly demonstrates that

the addition of water does not affect the SL lignite conversion and

product yield in the presence or absence of methanol. This may

suggest that for the liquefaction of SL lignite with NaOH, there is no

need to dry the lignite before use, which can significantly improve the

economy of this process. Further work has been carried out in our

laboratory.

3.4. Characteristics of SL lignite liquefaction products

Table 2 shows the ultimate analysis of products of SL lignite which

reacted with NaOH–methanol. The products were obtained at 1 g SL

lignite reactionwith 1 g NaOH and 10 ml methanol.It can be seen that

carbon and hydrogen contents and the H/C ratio of all the liquefaction

products significantly increased compared with those of SL lignite

itself (as shown in Table 1), showing hydrogen addition during

reaction. It suggests that hydrogenation reaction takes place in theliquefaction of SL lignite with NaOH. It is important to note that the

sulfur content in HS was four times than that of SL lignite itself,

suggesting that sulfur in SL lignite could be transformed into a lower

molecular weight product—HS.

Fig. 8 shows the FTIR spectra of HS, TS and THFS fractions,

compared with that of SL lignite. Theband near 3400 cm−1 is assigned

Fig. 4. Effect of methanol content on SL lignite conversion; 300 °C, 1 g SL lignite and 1 g

NaOH, 1 h.

Fig. 5. Effect of methanol content on product yield; 300 °C, 1 g SL lignite and 1 g NaOH,

1 h.

Fig. 6. Effect of water content on SL lignite conversion; 300 °C, 1 g SL lignite, 1 g NaOH,

1 h.

Fig. 7. Effect of water content on reactionproductyield; 300 °C, 1 g SL lignite, 1 g NaOH,

1 h.




to theOH stretching mode [14,15]. It is obvious that theband intensity

of the OH stretching mode for THFS was much stronger than that for

TS and HS, suggesting the presence of more phenol group in THFS,

which is consistent with the higheroxygen content in THFS compared

to that in HS and TS (see Table 2). The bands between 3000 and

2800 cm−1 are assigned to the aliphatic C–H stretching vibration

mode and used to measure the aliphatic hydrogen content [14–19].

Fig. 8 shows that the intensities of aliphatic C–H stretching modes of

HS were higher than those of THFS and TS. The band near 1600 cm−1,

1500 cm−1 and 1450 cm−1 was assigned to aromatic ring stretching

vibration modes [20]. Fig. 8 shows that the order of intensities of

aromatic ring stretching vibration modes was THFSNTSNHS, which

agreed with the elemental analysis data in Table 2. The band near

1610 cm−1, which was assigned to aromatic ring streching vibration

modes, moved from 1610 cm−1 in HS to 1640 cm−1 in TS and THFS,

and indicated more poly-aromatic or heterocyclic compounds in TS

and THFS [21,22]. This is supported by the appearance of the carbonyl

vibration band at 1710 cm−1 [23,24] for TS and THFS. The bands near

1300 cm−1, 1260 cm−1 and 1220 cm−1 for THFS, which were

assigned to C–O (phenol), Car–O–Car, C–O (alcohol) and Car–O–Cal

structures respectively, were much stronger than those for TS and HS,

suggesting the presence of more phenol and/or ether groups in THFS,

corresponding to the higheroxygen content in THFS compared to that

in HS and TS (see Table 2). All the results above indicate that the

liquefaction products have a large number of polar functional groups,

such as OH group, aromatic structure, carbonyl group and aromaticether oxygengroup, which cannot be obtained from petroleumand its

derivatives. Liquefaction products partially reserve the oxygen

functional groups of SL lignite. The further separation and utilization

of the liquefaction products are now under investigation in our

laboratory.

4. Conclusions

SL lignite has a good reaction activity with NaOH–methanol. The

conversion of SL lignite and product yield reached to 98% and 99% at

300 °C for 1 h respectively, when the ratio of SL lignite, NaOH and

methanol was at 1 g:1 g:10 ml. The main products were THFS (about

70%) and TS (about 15%).

During the liquefaction, NaOH participated in the reaction and

played the main role (up to 85%). The conversion of SL ligniteincreased almost linearly with the NaOH/SL lignite ratio up to 1. The

conversion of SL lignite and product yield reached about 83% and 74%

in the SL lignite reaction with NaOH, respectively. Methanol played a

promotion role in the liquefaction of SL lignite with NaOH. Methanol

addition leaded to the conversion of SL lignite and product yield

increased for about 15% and 25%, respectively. Water content

(between 10% and 50%) insignificantly affected the conversion of SL

lignite and product yield.

Liquefaction products (THFS, TS and HS) contained OH group,

aromatic structure, carbonyl group and aromatic ether oxygen, and

the contents of polar function groups followed the order of

THFSNTSNHS.

Acknowledgements

The authors express their grateful appreciation for the financial

support from the National High Technology Research and Develop-

ment Program of China (863 Program 2007AA06Z113), the Natural

Scientific Foundation of China (20876001, 20776001), and the State

Key Laboratory of Coal Conversion (Grant No. 09-904). Authors are

also appreciative for the financial support from the Provincial

Innovative Group for Processing & Clean Utilization of Coal Resource.

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Table 2

Ultimate analysis of the products of SL lignite reacted with NaOH–methanol.

Ultimate analysis (wt.%, daf)

C H O⁎ N S H/C

HS 77.89 9.15 7.11 0.66 5.18 1.41

TS 76.83 7.91 13.09 1.39 0.78 1.24

THFS 71.50 6.42 19.28 1.60 1.20 1.08

*By difference.

Fig. 8. FTIR of SL lignite and extracted components of liquefaction products (HS, TS and

THFS).




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