Proceedings of International Forum for Green Asia 2017
Special issue of Evergreen - Joint Journal of Novel Carbon Resource Sciences & Green Asia Strategy

P-06

Fraction estimation and O2 gasification of solid products from coal pyrolysis ina pressurized drop tube furnace
Cheolyong Choi,1 and Jun-ichiro Hayashi2
1Interdisciplinary Graduate School of Engineering, Kyushu University
Institute for Materials Chemistry and Engineering, Kyushu University
Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan

2Abstract
Coal gasification leaves more or less amounts of solid products, char and soot, the origins andreactivities of which are different from each other. The soot and char is inevitably recovered
as a mixture, and this makes it difficult to investigate their reactivities individually as well as
separate them from each other. The present study aimed to investigate the reactivity of thesoot that had been recovered from a drop-tube reactor (as an experimental simulator of a partof practical entrained-flow type gasifier). A soot/char mixture was immersed in ethanol, andsubjected to ultrasonic irradiation to break strong physical interaction between the char andsoot, and then emulsion/solid phase separation. This sequence successfully isolated the charand soot particles from each other. The soot/char mass fractions were in agreement roughlywith those judged from thermogravimetry for the O2 gasification of the mixture. The isolatedchar and soot were subjected individually to the gasification. The results, taken together withthat for the mixture, showed a kinetic interaction between the char and soot in the early stageof the gasification, and this was attributed to the presence of catalytic species with the soot.
More rapid gasification of the soot than commercially available carbon blacks was at least
partly due to such catalysis.
around 600°C (secondary pyrolysis). Althoughpractical gasifiers are operated at temperature
well above 1000°C, the char still remains as abyproduct due to its low reactivity with gasifying agent such as CO2. More problematically,soot is formed from the vapor of aromatics (orthose from aromatization of light hydrocarbons
such as acetylene), growth in size of aromaticring systems, nucleation, and then agglomeration. Reactivity of the soot is lower than that of

1. Introduction
Coal gasification is a promising technology that is flexibly applicable to power generation and production of synthesis gas. Coal undergoes the primary pyrolysis at temperature
over 200°C releasing inorganic gases, light hydrocarbon gases, and tar as fragments of macromolecules. The solid residue from the primary pyrolysis, termed char, experiences gasification in parallel with reforming of tar overFigure 1. Morphological characteristics of char and soot.

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Proceedings of International Forum for Green Asia 2017
Special issue of Evergreen - Joint Journal of Novel Carbon Resource Sciences & Green Asia StrategyP-06
char when prepared in inert gas.[1] Difference inthe mechanism between the char and soot gasification arises primarily from that in their carbonaceous structure (see Figure 1). The char isa porous particle that involves a number of micro- (sizes < 2 nm) and mesopores (< 50 nm).
Gasifying agents diffuse into pores and reactwith active sites as well as the external surface.
Whereas, the soot is a non-porous submicronparticle that easily agglomerate each other. Instead, the reactant gases can easily diffuse intogaps among soot particles, but the gasification
takes place only on the soot surface.
Char and soot, if survive in the gasifier,leave it forming mixed solids. Separation of
mixture of char and soot and their quantification have been reported. Miura et al.[1] quantified the char/soot fractions by temperature-programmed gasification. Umemoto etal.[2] proposed a laser-diffraction particle size
analysis taking notice of size differences between char and soot particles. They also compared the quantification methods including an
estimation of char/soot fractions from proximate and ultimate analyses of the parent coal interms of carbon-based yield of pyrolysis products, showing that all methods were in broad
agreement. Veranth et al.[3] prepared a char/sootsuspension in ethanol by ultrasonication, thenextracted the relatively stable emulsion of sootparticles after the settling, and recovered theprecipitated char. This method was then scaled
up by Apicella et al.[4]This ongoing study will examine fractionof char and soot, which are products of coalpyrolysis in a pressurized drop tube furnace.
Then, the reactivities of those solids will beinvestigated by isothermal O2 gasification in athermogravimetric analyzer.

MPa) in a pressurized drop tube furnace(PDTF). The particle residence time was estimated as around 3 s.
A 150 mg portion of a char/soot mixture
recovered from the PDTF was placed in a 50 mlof glass tube (Tube 1) filled with 40 ml of ethanol. The tube was agitated by ultrasonicationfor 3min, and was settled for 1–1.5 h. The suspension was then transferred to another tube(Tube 2) leaving the precipitated solid at thebottom of Tube 1. Tube 1 was refilled withethanol, and both tubes were agitated and settled again. In the same manner, the upper suspension was transferred into the next tube, anda new tube was added up to five. The extractionof the upper emulsion part in the last tube after
settling was repeated five times or more until
no particle was detected in the upper part of thesuspension in Tube 1. The total number of agitation and settling was 10–15. The char andsoot were thus recovered from the bottom precipitates and the upper emulsion phases. Ethanol was removed by filtration using a membrane filter of which pore size was 0.45 μm.
This separation method was modified fromthe previous studies so as to reduce time andethanol consumption since a major portion ofsoot was extracted within the first and second
cycles. Moreover, incomplete separation wasexperienced even after 30 repetitions. Although
the particle concentration was much higher thanthat recommended by Varanth et al.[3], the sootseparation from the char was hardly disturbed
by rapid aggregation of char/soot particles.
2.2. Thermogravimetric analysis
The gasification of the as-received
char/soot mixture, the isolated char and sootwas performed in a thermogravimetric analyzer(TGA). The sample was heated at a rate of
25 °C/min to 600°C in atmospheric N2 flowing
at a rate of 700 ml/min temperature increased.
The N2 flow was switched to that of O2/N2 with
O2 concentration of 2 vol.%. It was found in
preliminary runs for the char/soot mixture that

2. Experimental
2.1. Separation of solid products

Pulverized Tandinsky (TS) coal was pyrolyzed at 1200°C in flow of pressurized N2 (2Table 1. Result of the separation of char and soot.
Char [mg]Soot [mg]Char/soot mixture [mg](Fraction [%])
(Fraction [%])
301[a]
196 (66.0)
82.6 (27.8)[a] Two separation results were merged.

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Unseparated(Fraction [%])
18.6 (6.2)

Recovered [mg](Recovery [%])
297 (98.7)

Proceedings of International Forum for Green Asia 2017
Special issue of Evergreen - Joint Journal of Novel Carbon Resource Sciences & Green Asia Strategy

P-06
every mass release curve had an inflection point(see Figure 2), which was arisen probably fromthe difference in the rate of gasification between the char and soot. The mass fraction ofthe soot was thus estimated roughly from thegasification of the mixture gasification.
However, such a way of estimating thechar/soot fraction had no quantitative basis, andthe physical separation of them was therefore
necessary.

and soot, respectively, were in agreement withboth those from the physical isolation andthermogravimetry.
However, the char fraction determined bythat of the isolated char slightly smaller thanthose from the thermogravimetry and also
shown in Table 2. This suggested that a small
portion of the char was involved in the isolatedsoot. This was qualitatively confirmed by observation of the isolated soot by scanning electron microscopy.

3. Results and discussion
3.1. Estimation of soot fraction
Table 1 presents the result of the char/sootseparation. The total mass recovery of the isolated char and soot was as high as 94%, whileleaving only 6% of the residue. Assuming thatthe residue consisted solely of the char (i.e., nosoot had been left in the residue), the char andsoot fraction were estimated as 72.2% and27.8%, respectively. These fractions wereroughly in good agreement with those estimatedfrom the thermogravimetry of the mixture (seeFigure 1).
The validity of the char/soot fractions wasfurther examined through the following assumption and calculation[2]; (1) Oxygen wascompletely removed (released) from the solidby devolatilization of coal as CO, (2) the char
was comprised of fixed carbon and ash, whichwere determined by proximate analysis, (3) thesoot consisted of the other C, i.e., volatile carbon except for that as CO. In the estimation, thefractions of char and soot were considered on
the coal carbon basis. Considering that char andsoot were highly carbon-rich materials, the estimation was expected to give a reasonable andacceptable result. As seen in Table 2, the calculated fractions, 75.3% and 24.7% for the char

1-X, -1.0E+00

3.2. O2 gasification of char and soot
Derivative thermogravimetry curves of theisolated char and soot during the O2 gasification
are compared in Figure 3. X denotes the massconversion. dX/dt can be expressed by;
𝑑𝑋= 𝑘𝑠𝑝 (1 − 𝑋)𝑑𝑡where, ksp is the specific rate of gasification.
The synthesized curve has been drawn bysimply assuming that the separated char andsoot with a mass ratio of 3:1 were gasified simultaneously and independently according to theresults as mentioned above.
It is seen that the isolated char showsslightly higher reactivity than the char/sootmixture. This is in agreement with the fact thatthe soot produced in inert atmosphere had reactivity lower than that of the char. The reactivityof the mixture seems to be maximized in theearly stage of gasification. It is clear that thistrend is arisen from that for the char gasification. On the other hand, the dX/dt for the soot
Table 2. Composition of Tandinsky coal andestimation of carbon yields in solid products.

74% – Char

1.0E-01
1.0E-0226% – Soot
1.0E-03

050

100

150

time, minFigure 2. Change in residual mass fraction withtime during the gasification char/soot mixture.

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Ultimate anal.

wt% (d.a.f. basis)

C

80.70

H

5.16

O

11.37

N

2.39

S

0.38

Proximate anal.

wt% (dry basis)

FC

54.34

VM

35.95

Ash

9.72

Estimation

wt%-C (solid carbon)

Char

75.3

Soot

24.7

Proceedings of International Forum for Green Asia 2017
Special issue of Evergreen - Joint Journal of Novel Carbon Resource Sciences & Green Asia Strategy

P-06
slightly decreases in the early stage, increases
very gradually to X ≈ 0.8, and then decreases
later. The initial decrease in the dX/dt would be
due to contamination of the soot with a small
amount of char, as suggested previously. More
importantly, in overall, the dX/dt of the soot ismaintained over a wide range of conversion.
Such a trend implies the catalysis of metallic
species held by the soot.
The synthesized dX/dt slightly but steadily
lower than that for the mixture, indicatingchemical interaction between the char and soot.
Assuming the presence of catalytic species on
the soot, the positive synergistic effect of themixing can be interpreted by solid-to-solidtransfer of such species, or otherwise, by thatthe char and soot (in contact with each other)shared the species at their interface. It seems
that the positive synergistic effect diminishes
along the gasification and turns even negative.
This could be chemical incorporation of thecatalytic species (metallic species) with themineral matter of the char.
In order to confirm catalysis during thesoot gasification, a commercial carbon black,
Ketjen Black EC-300J, was gasified under thedX/dt, mg min-1

0.06

4. Conclusion
The char/soot mixture prepared by the pyrolysis of Tandinsky coal in PDTF was separated into the char and soot, and gasified with
O2 in TGA. The soot particles aggregated overthe char surface was successfully isolated fromthe char by repetitive ultrasonication and settling in ethanol. The kinetic analysis revealedchemical interaction between the char and sootin contact. This was, taken together with thekinetics of the soot gasification, explained by
the presence of catalytic species, which wereheld by the soot initially but transferred to thechar catalyzing its gasification.

Char/soot mixture
Char separated
Soot separated
Synthesized curve

0.04
0.02
0.00

0.00

0.25

0.50
X, -0.75

1.00

Figure 3. dX/dt profiles for the gasification ofchar/soot mixture, char, and soot.
0.03
dX/dt, mg min-1

same conditions. Figure 4 reveals much lower
reactivity of the carbon black than that of thesoot. The dX/dt for the carbon black changes
with X simply showing the surface area increase
and then decrease (pore development and collapsion). Thus, the clear difference in the dX/dt
profile between the carbon black and soot supports the catalysis in the gasification of the latter.
The higher reactivity of the separated sootmight also have resulted from the pyrolysis atmosphere. Senneca et al.[5] found that sootformed under N2 atmosphere was a “young soot”
based on relatively smaller aggregate size,
higher H content, lower structural order of aromatic ring systems, more C-O complexes, andhigher reactivity than mature soot formed in a
CO2 atmosphere.

TS soot separated
Ketjen Black EC-300J

0.02
0.01
0.00

0.00

0.25

0.50
X, -0.75

1.00

Figure 4. dX/dt profiles for the gasification of theisolated soot and carbon black.

Reference[1] K. Miura, H. Nakagawa, S. Nakai, S.
Kajitani, Chem. Eng. Sci. 2004, 59, 5261.
[2] S. Umemoto, S. Kajitani, S. Hara, M. Kawase,
Fuel 2016, 167, 280.
[3] J.M. Veranth, T.H. Fletcher, D.W. Pershing,A.F. Sarofim, Fuel 2000, 79, 1067.
[4] B. Apicella, O. Senneca, C. Russo, S. Heuer, L.
Cortese, F. Cerciello, V. Scherer, M. Schiemann, A. Ciajolo, Fuel 2017, 201, 118.
[5] O. Senneca, B. Apicella, S. Heuer, M. Schiemann, V. Scherer, F. Stanzione, A. Ciajolo, C.
Russo, Combust. Flame 2016, 172, 302.

Email: 3ES15024R@s.kyushu-u.ac.jp

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