Advances in Ma terials Physics and Che mist ry, 2012, 2, 158-161
doi:10.4236/ampc.2012.24B041 Published Online December 2012 (htt p://www.SciRP.org/journal/ampc)
Research on Supercritical Methanol Treatment of Lignite
Haiyan Luan1,2, Aiguo Wang2,3, Qian Zhang2,3, Fuming Chen2,3
1Dept. of Chemical Engineering, Tsinghua Universi ty, Beijing, China
2Research Institute of Tsinghua University in Shenzhen, S he n z hen, Ch ina
3Shenzhen Key Lab of Separation Technology, Shenzhen, China
Received 2012
ABSTRACT
China has rich lignite reserves which are the proper resources to be liquefied. As its low coalification degree, much hydrogen is
wasted. Solvent extraction can save hydrogen and improve its liquefaction performance. The paper studies supercritical methanol
treat ment of li gnit e with a devi ce at high temperatu re an d pressu re. Experi ment s mainl y focus o n th e effects of te mperatu re, p ressu re,
catalysts an d pretreatment ways on the extraction r ate. Results indicat e that the extracti on rate increases with raisi ng of temperatur e
and pressure below 330℃, 10 MP a. Wh en temperatu re ex ceeds 33 0℃, extracti on rat e decr eases sli gh tl y. After s wellin g pr etreatment
in methanol for 8 h, the lignite is treated for 60 min at 330℃, 8.2 MPa with NaOH as catal yst(1 %wt). The weight r atio o f methanol/
Xilinhaote lignite is 5/1. Under these conditions, the extraction rate can r each 22.88 %.
Keywords: Lignite; Supercritical Methanol Treatment ; Extraction Rate
1. Introduction
Lignite reser ves co v er 1 3% o f all th e co al reserves i n C hi na. As
its rich reserves and fin e liquefaction behavior, lignite becomes
high-quality resource to be liquefied. But expensive hydrogen
is wasted because water is formed during liquefaction
process[1,2]. To treat lignite before liquefaction can help save
hydrogen, improve the reactivity of lignite and increase oil
yield during liquefaction[3]. Therefore, the pretreatment is of
an important significance for its comprehensive utilization.
Solvent extraction of coal is a hot topic because it can study
coal structure and get small molecule compounds. Li et al[4]
did researches on the relationship of the extraction rate of ash-
free coal and extraction temperature in NMP. Hu et al[5] ex-
tracted coal with water under its supercritical and subcritical
state. The y found th at temperature and pressure were important
factors whi ch in fluence the extr action results. High temperature
and pressure improve solvent diffusion speed and dissolving
power as well exacerbate the resolvability of lignite. Yunus et
al[6] studied the extraction performance of about 20 kinds of
solvents with Soxhlet extraction. The extraction rate has a close
relation with solvent polarity. It shows a higher extraction rate
in polar solvents than in nonpolar solvents.
Coal st ructure and operation co ndition s are the key to extrac-
tion results. Coal is made up of condensation aromatic rings as
basic framework and side chains. Basic units are connected
with ether bonds and methylene bonds. Side chains include
alkyls and other functional groups. There is strong acting force
between coal molecules such as interionic force, hydrogen
bond s and Van d er Waals force[ 7] . Pr etreat ment sh ou ld weaken
the acting force between coal molecules and dissolve the ex-
tracts[8]. Besides coal structure and solvent properties, factors
which influence the speed during mass transfer process include
permeation and diffusion [9]. Treatment include two parts.
Solvent molecules permeate into coal micro pore structure and
then soluble substance spreads outside.
Fluids under sup ercritical cond itions are easier to en ter coal
molecules and can solve soluble substance better. So the paper
adopts supercritical methanol to pretreat lignite. Carbon emis-
sion reduces because less CO and CO2 is produced. Experi-
ments aim at the extraction rate and study the effects of tem-
perature, pressure, catalyst and pretreatment ways on the ex-
traction rate. Optimized technologies lay the foundation for
coal li quefaction .
2. Experiments
2.1. Instruments and Reagen ts
Ma in i n st r uments: Sartorius BS2109 electronic scale; RE2000E
Rotary Evaporator; FYXD2-20/40 0 aut ocl ave (Tmax=450, Pmax=
20MPa, V=2L); ZDXS3-5-1200 muffle.
Reagents: methanol, tetrahydrofuran, NaOH, H2SO4. All the
reagents are analytically pure.
2.2. Lignite Sample
The coal sample is Xilinhaote lignite from Inner Mongolia. The
sample has been grinded and sifted(200 mesh). Proximate and
ultimate analysis of the sample is shown in Table 1.
Table 1. Proximate and ultimate analysis of Xilinhaote lignite sam-
ple(wt%).
Proximate analysis Ultimate analysis, daf
Mad Aad Vdaf C H O* N S
9.95 10.21 47.49 65.87 5.13 27.37 1.07 0.56
*by difference.
*High-tech Zones Development Guidance Special of Guangdong Province-
Key
Problems T ackling and I ndustr ialization Type g(2010A011300038).

H. Y. LU A N ET AL.
159
2.3. Experiment M e thods
Mix 200 g coal sample, certain methanol and 2 g catalyst into
slurry. After swelling for 8 hours, put it into the autoclave. Heat
the mixture at rate o f 5 ℃/min and stir it at rate of 200 r/min.
Pressure is controlled by the intrant volume of methanol. Treat
at a constant temperature for a certain time. Turn on the tap to
cold down the system. When the temperature is below
70 ℃,take out all the material in the autoclave. Separate the
solid and liquid after treatment using vacuu m suction filtration.
Filter residue is washed three times by methanol and tetrahy-
drofuran. When it is dried, test its ash content and calculate the
extraction rate.
2.4. Analysis Methods
Define the mass of dry solid before and after treatment as M1,
M2, ash content as A1,A2, extraction rate as E. Suppose the
weight of ash will not change during the treatment process, so:
112 2
MA MA⋅= ⋅
(1)
( )()
( )
112 2
11
11
1
M AMA
EMA
−− −
=−
(2)
According to (1)(2):
( )
21
21
100%
1
AA
EAA
−
= ×
−
(3)
The ash content of the extracts is below 0.1% by the test.
That is all the ash is still in the solid. It is feasible to calculate
the extract ion r ate using the above ash balance method.
3. Results and Analysis
3.1. Effects of Different Treatment Conditions on
Extraction Performance
1) Effect o f temperatur e on extraction p er fo r mance
Take H2SO4 and NaOH as catalyst separately. Treat the lig-
nite for 60 min at9.0±0.5 MPa. The weight ratio of methanol:
Xilinhaote lignite is 5:1.Research the effect of temperature on
extraction performance(T=260-320 ℃). The variation of E
with T is shown in Figure 1.
250 260 270 280 290 300 310 320 330
5
10
15
20
25
E xtr ac tio n R a te / %
Temp erature / ℃
■, H2SO4 as catalyst; ▲, Na OH as catalyst
Figure 1. The effect of temperature on E.
As is shown in fig1,with the increase of tempera-
ture(260-320 ℃) at certain pressure, E increases obviously no
matter th e catalyst is s ou r or basic. At lo wer temperatu re, th at is
near or above the critical temperature of methanol, H2SO4 is
better than NaOH. When temperature surpasses 270℃, NaOH
is better than H2SO4.
With the rise of temperature, solvent viscosity decreases.
Solven t molecules are easi er to enter macro molecule st ructure,
leading dissociation of ether bonds. The dissociation speed
increase with the increasing temperature. Alcohols provide
active hydrogen, therefore free radicals and micro molecules
can be stable[13,14]. The solubility of compounds in methanol
increases. Hence the extraction rate increases with temper ature.
2) Effect o f pressure on extract ion performance
Take H2SO4 and NaOH as catalyst separately. Treat the lig-
nite for 60 min at 260℃. The weight ratio of methanol/Xilin-
haote lignite is 5/1. Research the effect of pressure on extrac-
tion performance(T=260-320℃). The variation of E with T is
shown in Figure 2.
As is shown in Figure 2, with the increase of pressure at
certain temperature, E increases obviously no matter the cata-
lyst is sour or basic. When the pressure surpasses 8.1 MPa,
NaOH is better th an H2SO4.
For supercritical fluids, the increase of pressure means in-
crease of solubility. During supercritical treatment, fluids of
high solubility makes free radicals move away from coal sub-
jects. Secondary reactions are avoided. At the same time, high
pressure can make fresh solvent permeate into coal molecules.
The mass transfer speed is raised because of higher turbulivity.
Hence th e ext raction rate increases with pressure.
3) Effect o f catalysts o n extracti on p erformance
Acid and base can help damage some strong chemical bonds.
Through 2.1.1 and 2.1.2, we can make the conclusion that
NaOH is bet ter than H 2SO4 when pressure surpasses methanol
critical pressure and temperature above 270 ℃. Oxygen exists
in coal in the form of carboxyl, hydroxyl and other functional
groups. Carboxyl and hydroxyl are acid groups[16-18]. Base
can also enforce hydrolyzation of oxygen bonds and increase
the content of hydroxyl[19]. Hence to choose base as catalyst is
better fo r raising extraction rate.
4) Effect o f pretreatment ways on ext r action performance
All the experiment samples above have been swelled in me-
thanol for 8 h. Pretreatment will influence coal molecule struc-
ture. The table below shows the effect of different pretreatment
ways on t he extraction rate at similar temperature an d pressure.
2 4 6 810
0
5
10
15
20
E xtr ac tio n R a te / %
Pressure / MPa
■, H2SO4 as catalyst; ▲, NaOH as catalyst
Figure 2. The effect of p ressure on E.
5) Swelling

H. Y. LU A N ET AL.
160
Compared 1# and 3# in Table 2, after swelling, the extraction
rate incr eases 4 .60% at similar t reatmen t co ndi tion s. It is p roper
to make the conclusion that swelling can help increase the ex-
traction rate.
Swelling can weaken the association between coal macro
molecules[ 20]. New stru cture makes it e asier for solven t mole-
cules to touch coal. What’s more, micro molecules enter super-
critical fluids. Secon dary reacti ons and reverse react ion s can be
avoided[21] .
6) Effect of moisture in coal
Compared 1# and 2# in Table 2，the extraction rate of 1# is
1.66% higher than 2#, in which the lignite sample is dried.
Treatment temperature and catalyst are th e same, but t reatment
pressure of the former is 1MPa higher than the latter. Accord-
ing to the analysis about the effect of pressure o n the extr action
rate, suppose treatment pressure was the same extraction rate
should be similar to each other. It is difficult for H2O as an
inorganic solvent to solve long-chain compounds, benzene
rings and condensed rings in coal. The supercritical condition
of H2O is Tr = 374.3℃, Pr=22.12MPa. Under the conditions
in this paper, H2O cannot damage the coal molecules. Com-
pared with large scale of methanol solvent, the effect of mois-
ture in coal can be ignored.
Through the analysis about effects of different experiment
conditions on the extractio n rate, some conclu si on s can be drew.
To get high extraction rate, we should swell the coal sample,
use base as catalyst and make the temperature and pressure
above methanol critical point.
3.2. Further Discussion on Treatment Temperature
Effects of temperature and pressure on the extraction rate are
preliminary investigated through above extraction experiments
under different conditions. In fact, pressure of the autoclave is
controlled by adjusting the volume of material. Considering
that the increase of pressure will bring higher requirement of
the equipments and subsequent liquefaction technologies, fur-
ther studies focus on the extraction rate at higher temperature
and pressure of 8.2 MPa. If temperature continues to increase,
the weight ratio of solvent/coal will reduce on the basis of the
6:1 . Treatment cost can be lower.
The experiment scheme is determined after comprehensive
analysis above. Firstly, swell the sample in methanol. With
NaOH as catalyst and pressure controlled at 8.2 MPa, increase
the temperature gradually from 240℃,which is methanol criti-
cal temperat ure. Wh en the tempe rature reaches the test te mper-
ature, stabilize for 60 min.
The result can be s een in Figure 3. During the procedure of
temperature varies from 260℃ to 320℃, the extraction rate
increases obviously, which is consistent with the results of the
Table 2. The effect of different pretreatment ways on the extraction
rate.
Pret r eatment way s T/
℃
P/MPa Catalyst E/%
Original sample(1#) 310 10.5 1%NaOH 17.87
Only drying(2#) 310 9 .5 1%NaOH 16.21
Swelling (3#) 310 9 .5 1%NaOH 22.47
260 280 300 320 340 360 380
15
20
25
E x trac tio n R a te /%
Temperat ure/ ℃
Figure 3. Variation of E with T.
foregoing. When the temperature rises to 330℃, the extraction
rate reaches the maximum. Continuing to rise the temperature
to 370℃, the extraction rate decrease slightly.
The rise of te mperatu re make s th e coal pyrol ysis ac celerating
and free rad icals generate in a very sho rt period . H-donor abili-
ty of methanol f is limited and free radicals cannot be stable
right away. As a part of free radicals poly-condense together,
the extraction rate is decreased[13,14]. Therefore, to obtain a
higher extraction rate, the reaction temperature should be
maintai ned at about 330℃.
4. Conclus ion s and Prospect
The extraction rate increases with the raising of temperature
and pressure below 330℃,10 MPa regardless of NaOH or
H2SO4 as catalyst and reaches its maximu m at 3 30℃. However,
there is a downward trend when continuing to raise temperature.
Experiment results show that taking NaOH as catalyst is more
condu cive to improve the extr action rate than H 2SO4.
After swelling pretreatment in methanol for 8 h, the lignite is
treated for 60 min at 330℃，8.2 MPa with NaOH as cata-
lyst(1%wt). The weight ratio of methanol/Xilinhaote lignite is
5/1. Under these conditions, the extraction rate can reach
22.88%.
In this paper, the influence factors such as temperature,
pressure, catalyst and pretreatment on supercritical methanol
processing lignite have been studied. But the analysis and se-
paration for th e ext r acts need fu r ther explorati on.
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