Kyushu University Global COE Program
Journal of Novel Carbon Resource Sciences, Vol. 5, pp. 1-4, Feb. 2012

Tar Reforming of Lignite at Low Temperatures Using SupportedPotassium Carbonate
Young-Kwang Kim*1, Jin Miyawaki*2, Isao Mochida*3, Seong-Ho Yoon*2*1Interdisciplinary Graduate School of Engineering Sciences, Kyushu University*2
Institute for Materials Chemistry and Engineering, Kyushu University*3
Research and Education Center of Carbon Resources, Kyushu University(Received November 16, 2011; accepted November 24, 2011)
Alumina–supported K 2CO3–LaMn0.8Cu0.2O3 was investigated for use in the catalytic conversion of tar, produced
from lignite, into syngas under inert and steam–reforming conditions. A double–bubble fluidized bed reactor
system, equipped with a micro gas chromatograph to analyze permanent gases and a collecting system to analyze
condensable species, was developed to screen the catalytic conversion of tar components at 700ºC. Samples ofAdaro coal were characterized based on elemental, proximate, and thermal analyses. The redox properties ofcatalysts, estimated by hydrogen temperature programmed reduction analyses, were correlated with their catalyticperformance in tar conversion. The synthesized catalyst effectively converted tars into hydrogen–rich syngas andalso improved tar reforming by inhibiting coke deposition.

1. Introduction

tar–conversion efficiencies higher than 99 % and very
effective reforming capabilities for light hydrocarbons
in gaseous products. However, the lifetime of Ni–based
catalysts is limited due to the deposition of large amountsof carbon on the catalyst surface10).
In the present study, catalysts consisting of an alumina - supported K 2CO3, perovskite oxide - supported
K 2CO3 were prepared, and their ability to catalyze theconversion of tar which could be produced through thepyrolysis of Adaro coal under steam-reforming conditions was investigated. A double-bubble fluidized bed reactor system equipped with a micro gas chromatographto analyze synthesized gases and a tar collecting system
to analyze condensable species, was designed and selfmanufactured to study the catalytic conversion of tar
components at 700ºC. To confirm the catalytic tar conversion into syngas without the formation of depositedcoke on the catalyst surface is another objective of thisstudy.

At low temperatures, the steam coal gasification
process produces hydrogen by reducing H2O. However,the low reactivity of coal leads to very slow conversion
rates and tar emission. In addition, at temperatures
lower than its dew point, tar formation severely limits
the stability of the system by blocking up and/or foulingsystem components such as fuel lines, filters, engines,and turbines. Therefore, the removal or conversion oftar compounds is required to transform coal (or otherbiomass) into a suitable fuel for internal combustion
engines, fuel cells, or feed stock for chemical synthesis.
Due to the advantages inherent to converting tar into
useful gases and adjusting the composition of the productgases, both mechanical and thermal cracking methods
have been proposed1). Mechanical methods do not allow
energy recovery from tars, while thermal cracking
requires temperatures higher than 1100ºC to convert thetars into lighter gases. Catalytic decomposition has been
proposed to overcome these drawbacks. In addition, theuse of a catalyst can yield hydrogen–rich product gas
compositions, which promotes reforming reactions.
Various catalysts, such as nickel–based catalysts, dolomites, olivine, and zeolites, have been investigated foruse in catalytic tar decomposition. However, these catalyst systems have mostly been employed in secondary
reactors, where volatile tars generated from the thermal
decomposition of coal or biomass2-5) and from coke ovengas (COG)6-9) in the primary gasifier are broken down
into product gases on the catalyst surface in the presenceof gasifying agents such as steam, oxygen, or air. Dolomites and nickel–based catalysts are the most conventional catalysts for tar decomposition in secondary reactors at 700–900ºC. Nevertheless, dolomites suffer fromlow mechanical strength and poor activity in the modification of gaseous hydrocarbons. Ni–based catalysts have

2. Experimental
2.1 Materials

Indonesian lignite, also known as Adaro coal, wasground into 45–125 μm particles and dried at 80ºC for
24 h in an oven. Table 1 shows selected properties of theprepared Adaro coal.
Table 1

Properties of Adaro coal.

Proximate analysis (wt.%)
CoalAdaro
Coal-1-Elemental analysis(wt.%, dry & ash free base)

VM

Water

Fixedcarbon

AshCHNSOdiff

43.3

3.7

48.6

4.4

70.9

5.1

1.0

0.1

22.9

Journal of Novel Carbon Resource Sciences, Vol.5, Feb. 2012

Perovskite–type oxides, LaMn1–xCuxO3 (LMC82, x =0.2), and La1–xK xMnO3 (LKM91, x = 0.1) were preparedaccording to a sol–gel method11,12). Quantitative amountsof lanthanum nitrate, manganese nitrate, and copper nitrate were stirred for 8 h in a solution of citric acid andwater. The water was evaporated from the mixed solution by a rotary evaporator at 45°C until a viscous gel
was obtained. For LMC82/γ–alumina, γ–alumina wasmixed in at this step. The ratio of LMC82/γ–alumina was20/80 wt.%. The gel was kept at 110°C overnight and theresultant mass was ground and then calcined at 750°Cfor 5 h. All of the prepared supports impregnated with a10 wt.% K 2CO3 by immersion in a mixture of K 2CO3 andethanol/H2O. The solvent was removed under vacuum at
110ºC in accordance with a previously published methodof wet impregnation13). The catalysts were double–sievedto select an appropriate size range after calcination andK 2CO3 impregnation. Table 2 reports the particle size
ranges and the surface area of the coal samples and catalysts.
Table 2first condenser, and steam and light tar were collectedat –10ºC by the second condenser. The first and second
condensers were filled with THF (tetrahydrofuran),
respectively. Lighter tars were collected through a quartz
filter at 25ºC by the final condenser. The lines running
between the steam generator and the reactor, and thosebetween the reactor and the first condenser of the tarcollector, were heated to 200ºC to prevent tar products
from condensing.

Particle size ranges, densities, and surface area.
Material

Adaro coal alone

Size range

SBET(m 2/g)45µm<d<125µm

6.6γ–alumina (A)149.9

10 wt.% K2CO3/γ–alumina(K10@A)

115.5

LMC82 (L)

12 µm<d<250µmFig. 1 Experimental apparatus. (1) Gas preheating
section, (2) fluidization column 1 for coal, (3)fluidization column 2 for catalyst, (4) gas distributor and separator, (5) electric furnace, (6)thermocouple, (7) condenser 1, (8) condenser 2,(9) quartz microfiber filter, (10) gas analyzer.

9.8

10 wt.% K2CO3/LMC82 (K10@L)

7.3

10 wt.% K2CO3/LMC82/γ–alumina(K10@L–A)

89.9

2.3 Procedures

To investigate tar reforming ability, Adaro coal (1 g)
was introduced into the bottom (first) reactor. Catalyst(1 g) was introduced into the upper (second) reactor
separated by nickel mesh, as illustrated in Figure 1. The
temperature of the reactor was raised at 200°C min–1 in
N2 (400 mL·min–1 flow rate, U = 0.035 m·s –1) or N2/H2Ovapor flows (320/80 mL·min–1) and kept at 700ºC for 30min. The resulting syngas (H2, CO, CO2, and CH4) wassent to the micro GC through the three condensers. The
collected tars were dissolved in THF. Then the steam
and THF were evaporated at 105ºC for 24 h in an oven,and the collected tars were weighed. At the end of eachexperiment, the amount of coke formed on the catalysts
was quantified by thermogravimetric analysis.

The specific surface areas of prepared catalysts werecalculated using the BET method applied to N2 adsorption isotherms at 77 K. Redox activities were measured
by TPR. Briefly, a sample (~50 mg) was heated at a constant rate (5 K/min) in a U–shaped quartz reactor fromroom temperature to 1000ºC in a flowing hydrogen/helium mixture (50 mL/min, 5 % H2 in He). Hydrogenconsumption was monitored using a quadruple mass detector.

2.2 Apparatus

A circular stainless steel steam bubbling fluidized bed
reactor (15.7 mm ID and 0.3 m high) was used for the catalytic steam gasification of Adaro coal, as shown in Figure 1. The fluidization gas distributor and separate fluidized bed were 1 mm thick and composed of 10 stacked
pieces of nickel mesh with holes 50 µm in diameter. Thefluidization columns (50 mm) and preheating section(0.1 m) were heated by a cylindrical 2.4 kW electric furnace. The temperature of the bed, measured by a K–typethermocouple placed 30 mm above the distributor, waskept constant by a PID controller. Temperature variationin the two continuous fluidization columns was always
within ± 1ºC of the set point.
The syngas–containing steam flowed to a three–stage
condenser. Heavier tars were collected at 25ºC by the

3. Results and discussion

3.1 Catalyst characterization

In Table 2, LMC82 exhibited a very low BET surface area of 9.8 m2/g. The surface areas of the supports,
LMC82, and γ–alumina decreased following K 2CO3 impregnation and LMC82 coating.
The redox properties of the catalysts were evaluatedby TPR, as shown in Figure 2. The redox properties ofeach catalyst were correlated with catalyst performance.
LMC82 yielded two main reduction peaks at 250ºC and700ºC. The peaks shifted to slightly higher temperatures
following K 2CO3 impregnation. Although γ–alumina had-2-Tar Reforming of Lignite at Low Temperatures Using Supported Potassium Carbonatea larger surface are than LMC82, almost no reduction
activity was observed. The reduction of γ–alumina
increased slightly after K 2CO3 impregnation and LMC82
coating. K10@L–A exhibited broad reduction peaks
below 700ºC.

The inevitable carbon loss (show Table 3) is generally
conjectured to come from the coke formation on thereactor wall and light aromatics, which are un-detectablein our system. However, at the present state, the exact
reason is hard to be clear. More precious GC-MS analysisof the aromatics should be established to close to thecomplete carbon balance in future work.
Table 3

Tar collected from Adaro coal withoutcatalysts.

Contents
Condensed tar

Amount (g for 1 g coal, at 700 oC)
Pyrolysis

20 vol.% steam in N2

0.0772 (10.9 %)0.0718 (10.1 %)Char(67.8 %)(66.2 %)

Syngas(8.5 %)(15.6 %)

Carbon loss(12.8 %)(8.1 %)( ): Carbon percent for whole carbon

3.3 Catalytic tar reforming and coke formation oncatalysts

Fig. 2 TPR analyses of catalysts.

The use of γ–alumina under pyrolytic conditions resulted in the removal of large amounts of tar, as shown inTable 4. However, about 20 % of the available carbon wasdeposited as coke on γ–alumina, as shown in Table 5.

3.2 Volatile products and tar formation at low
temperatures
Before the catalytic reforming tests, blank pyrolysis
experiments with Adaro coal were performed under inert
and steam reforming conditions. The release of volatiles
under inert conditions was mostly complete in 30 min,as shown in Figure 3. Under steam reforming conditions,slow char gasification progressed with a regular slope
following volatile release.

Table 4

Tar collected after reforming in the presenceof catalysts.

Catalyst

Pyrolysis

Steam reformingA

0.0246 (3.47 %)0.0302 (4.26 %)K10@A

0.0231 (3.26)

0.0218 (3.08)

L

0.0442 (6.24)

0.0403 (5.69)

K10@L

0.0274 (3.87)

0.0246 (3.47)

K10@L–A

0.0203 (2.87)

0.0167 (2.36)

Table 5

Coke formation on the catalyst surface.

Catalyst

Fig. 3 Comparison of carbon conversion for Adaro
coal under pyrolytic and steam reforming conditions at 700ºC.

Condensed tar amount (g for 1 g coal)

Coke formation (g on 1 g catalyst)
Pyrolysis

Steam reformingA

0.141 (19.90 %)0.124 (17.50 %)K10@A

0.105 (14.82)

0.072 (10.16)

L

0.047 (6.63)

0.005 (0.71)

K10@L

0.010 (1.41)< 0.001

K10@L–A

0.023 (3.25)< 0.001

This means that while tar compounds were capturedby the high surface area of γ–alumina, tar reforming
was not remarkable. The reactivity of γ–alumina wasimproved by K 2CO3 impregnation, but large amounts ofcoke, constituting 10.16 % of the available carbon, still
remained on the K10@A catalyst surface under steam
reforming conditions. LMC82 exhibited lower tar removal but also lower coke formation than γ–alumina.
This shows that the lower surface area of LMC82 did not

Tar formations were observed in inert and steam atmospheres at 700ºC. Table 3 shows the amounts of tar compounds collected by the three–stage condenser. Without
any catalyst, tar compounds represented 10.9 % of thetotal carbon in the coal sample under inert gas flow, andabout 10.1 % in the presence of 20 vol.% steam.

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Journal of Novel Carbon Resource Sciences, Vol.5, Feb. 2012

Fig. 4 Composition of syngas produced under (a) pyrolytic and (b) steam reforming conditions.
effectively adsorb tar even though the surface reactivity
was higher than that of γ–alumina. K 2CO3 impregnationof LMC82 improved its surface reactivity by effectively
inhibiting coke formation on the catalyst surface. However, the potential for tar removal was limited by its relatively low surface area. K 2CO3 impregnation resulted ina further decrease in surface area. Catalysts consisting of
γ–alumina (high surface area) hybridized with K 2CO3–
LMC82 (high surface reactivity) showed the highest
degree of tar reforming and relatively low amounts ofdeposited coke on the catalyst surface. Under steam reforming conditions, this hybrid catalyst showed remarkably high performance with regard to both tar reforming
and the inhibition of coke (below 0.1 % of the availablecarbon) formation.

way, adsorbed tar compounds can be converted into hydrogen–rich syngas. The remaining compounds formcoke on the surface of the catalyst. In the present study, acatalyst composed of γ–alumina (high surface area) hybridized with K 2CO3–LMC82 (high surface reactivity)
showed the highest degree of tar reforming with minimal coke formation. Consequently, catalysts for effective
tar reforming should be of high surface area and highsurface reactivity. The feasibility of various catalysts requires further study for use with brown coal and biomass
tar reforming with NOx and SOx emissions.
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1)

3.4 Changes in syngas composition by tar reforming

Figure 4 shows the results of a syngas analysis after tarreforming tests in inert and steam reforming conditions.
Γ–alumina barely affected syngas production under anyconditions. The addition of K 2CO3 increased the hydrogen production of γ–alumina. This enhancement was theresult of coke reforming on the catalyst surface because
the composition of condensed tars in the presence of
γ–alumina with or without K 2CO3 impregnation varied
only slightly (Table 5). Although LMC82 had relatively
little impact on syngas composition under inert conditions, similar to that of γ–alumina, hydrogen–rich syngas
was obtained under steam reforming conditions. The useof K10@LMC82 resulted in higher hydrogen production
than K10@A. K10@L–A was the highest–performing
catalyst with regard to hydrogen–rich syngas production with minimal tar and coke formation under steam
reforming conditions.

4. Conclusions

Tar reforming can be achieved in two steps: adsorption of tar compounds onto the catalyst surface after
the release of volatile materials, followed by catalytic
reforming of these tar compounds into syngas. In this-4-