Dev. Chem Eng. Mineral Process., 9(3/4), pp.301-312.2001.

A Study of the Reactivity and Formation of

the Unburnt Carbon in CFB Fly Ashes

Yong Li*, Jiang-Sheng Zhang, Qing Liu, Ji-Ling Lu,

Guang-Xi Yue, Adel F. Sarofim’, Janos M. Be&#,

Yam Y. Leew and Baldur Eliasson=

Department of Thermal Engineering, Tsinghua University,

Beijing 100084, P R. China

Massachusetts Institute of Technology, Cambridge, R

Massachusetts, USA

Energy & Global Change Dept., ABB Corporate Research Ltd, ##

Bulduq CH-5401 Baden, Switzerland

A comprehensive study was conducted on the reactivify and the turbostratic structure

of the unburnt carbon in the CFB flu ashes. The observed deactivation of the

unburnt carbon and the pyrolysis chars prepared under various conditions was

studied and found to be due to the combination of the loss of catalytic effect of the

minerals and the cvstalline growth of carbon. Various forms of morphology of the

residual carbon particles were examined under a microscope. Both inertinite and

vitrinite seem to be possible sources of the unburnt carbon. The formation of the

unburnt carbon in the CFB fly ashes was likely caused by the deactivation of char

particles and the insufficient residence time of the corresponding f i e l particles.

* Author for correspondence.

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Yong Li, Jiang-Sheng Bang, Qing Liu, et al.

Introduction

In the literature, various possible causes or explanations have been suggested

concerning the formation of the unburnt carbon in fly ash. Yan and Ni (1994)

studied the influences of ash inhibition. Lu and Zhang (1989) investigated the

hgmentation of fuel particles at the top of a furnace and elutriation of fine particles

from the bed. Explanations also come from a coal petrography view. Non-reactive

(infusible) macerals are believed to be responsible for the unburnt carbon in fly ash

(Thomas and Gosnell, 1993). The observance of low reactivity of unburned carbon

extracted from commercial samples and evidence of deactivation caused by thermal

annealing under combustion relevant conditions indicated another attractive

explanation for the carbon loss (Charpenay and Serio, 1992; Senneca and Russo, 1997;

Hurt and Sun, 1998). Besides factors related to coal properties, characteristics and

adjustments of the burners and furnace are also critical to carbon loss.

The studies mentioned above were mainly under pulverized coal (PC) combustion

conditions. However, the combustion conditions of CFB are quite different fiom

those PC boilers, namely lower temperature in furnace (about 850-9OO0C), much

coarser feed coal particles (0-1 0 mm), long residence time of large particles, etc. In

this paper we present the reactivity and ordering of carbon structure of unburnt carbon

from CFB, and deactivation of carbon caused by thermal annealing under CFB

conditions.

Experimental Details

In our work, a bench scale CFB combustor (BS-CFB) has been constructed. The

302

Reactivity and Formation of the Unburnt Carbon in CFB Fly Ashes

bench scale CFB combustor used in our research basically consists of a 150 mm ID,

4.5 m high riser, a water-cooled cyclone separator and a J-valve. Further details of

the CFB combustor have been published by Jin and Lu (1999). Two bituminous

coals were burned in this combustor. Fly ashes fiom the bench scale CFB combustor

and two industrial CFB boilers were obtained. Using an isothermal thermogravimetric

analysis (TGA) method generally accepted by other researchers (Cai and Guell, 1996),

the reactivity of the residual carbon was determined. The crystaIline structure of the

residual carbon samples was examined by the powder X-ray d ihc t ion (XRD)

technique. In our study, the method presented by Short and Walker (1963) has been

used to interpret the X-ray diffraction data to obtain the crystalline parameters.

Table 1.

Coal Moisture Ash Volatile Fixed carbon Net calorific

Proximate analysis of YXand JJcoal.

(% air dried) (% dry) matter (YO daf) value (% daf) (MJkg)

Yx 9.47 21.84 37.61 62.39 19.70

JJ 1.35 44.94 41.07 58.93 17.27

Table 2.

Coal Vitrinite Inertinite Exinite Inorganic matter

The petrographic constituents of YXand JJ bituminous coal (~01%).

(%VN) (YoVN) (%VN) (%VN)

Yx 40.3 32.6 5 .O 22.1

JJ 44.4 23.8 0.4 31.4

To study the correlation between the deactivation of char and the crystalline

growth of its turbostratic carbon structure, XRD and TGA measurements have also

been performed on some laboratory chars produced under various pyrolysis

conditions in a tube oven. Morphology of residua1 carbon and pyrolysis chars has

been examined and photomicro,gaphs have been taken.

303

The proximate analysis of the coals is shown in Table 1, and Table 2 presents the

petrographic constituents determined by the standard procedure.

Results and Discussion

The deactivation as a function of residence time at various temperatures is shown in

Figure 1. Generally there was a constant decrease in the reactivity of char while at

high temperature. However, when the chars were pyrolyzed beyond a certain time at

a particular temperature, the reactivity approached an asymptotic value. Beyond that

time hardly any change was observed. Fiawe 1 also shows that the asymptotic

reactivity at a higher HTT is lower.

The values of the interlayer spacing ( 4 0 ~ ) decreased with the increase of the HTT

(highest heat treatment temperature). Further heat treatment at temperatures of

1200-14OO0C shows little effect on the interlayer spacing of turbostratic carbon,

indicating that the YX vitrinite was difficult to graphitize. ' The JJ chars are easier to

graphitize, because there is a trend of the further decrease of ~ c , o ~ to the value of the

graphite (3.3756 A) if pyrolyzed at 1400°C, as shown in Figure 2.

The properties of the residual carbon samples, including the reactivity, crystalline

structure parameters, porosity and total surface area are listed in Table 3. The results

of some pyrolysis chars are also included for comparison. The porosity and the total

surface area are measured by an Autoscan 33 mercury porosity instrument.

304

Reactivity and Fonnation of the Unburnt Carbon in CFB Fly Ashes

4.5

4 -

- 3.5 ", 2 3 - N

2.5

2 :

so

-

ZA FI -

-

I

A 0

4 30

E S .-

10

0

-U-YX 1073 K -M-YX 1173 K -0- JJ I173 K -*- JJ 1673 K

- 1r

- 12 - P

0 . o n

- 8 g . 3

5' - 6 Y * 4 4

A

- 2

I - l ~ l ~ l ~ l ~ l ~ l ~ l

20 A 0 60 80 100 120 la0 * 0-•

Residence time (min)

Figure 1. Deactivation as afunction of the residence time.

There are three unburnt carbon samples in Table 3, namely, JJASH, TUASH,

BS850. JJASH is collected from a 75 Uh boiler burning JJ coal, TUASH fkom a 20

tlh CFB boiler burning YX, and BS850 is obtained fiom the BSCFB burning YX coal.

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Yong Li, Jiang-Sheng Zhang. Qing Liu, et al.

The carbon content of JJASH, TUASH and BS850 is 8.1%, 10.3% and 30.2%

respectively. In all the CFB ashes, the medium size fiaction (60-100 pn)

contributes nearly 80% of the total unburnt carbon. The lower height of the riser of

BSCFB, giving less residence time, caused the much higher carbon content of BS850.

Table 3. Properties of the residual carbon samples.

Sample TN* TSA * * Rsoo Do02 (cc/@ (m2/@ (o/o/min) M) description

Pyrolyzed, 900°C, Yx

Samples

yx9-10 120min. 0.1295 4.1433 7.48 3.46

Y x 0.1537 3.0836 47.1 3.66 Devolatilized, 900"C, 7 min.

0.1458 4.4840 9.49 3.46 60 min. Residual carbon

WASH &om the 20 t/h Yx 1.3609 13.2035 17.3 3.52 CFB fly ash

Yx 0.5800 4.1264 15.4 3.51 BS850 Residual carbon fiom the BSCFB

0.5081 1.9294 2.69 3.48 120 min. JJ 0.0893 2.3757 3.21 3.52 Devolatilized,

9OO"C, 7 min JJ 1

Residual carbon JJASH from the 75 t/h JJ 0.4064 8.5609 1.36 3.51

YXl

yx9- Pyrolyzed, 9OO"C, yx

JJg-10 Pyrolyzed, 900°C, JJ

*Total intruded volume; **Total surface area.

Both the total surface area and the pore volume of the unburned carbon WASH

are the highest. Variation of residence time did not make much difference to pore

volume of the pyrolysis chars prepared fiom the YX bituminous coal. Generally, the

pore volume of the residual carbon samples is 4 to 9 times that of chars produced

from coal just after devolatilization (YXl and JJ1). This indicates that the particles

of the unburned carbon particles have experienced combustion in the furnace.

During the combustion period the total specific surface area did not decrease but

appeared to have increased.

306

Reactiviiy and Fonnation of the Lrnburnt Carbon in CFi3 Fly Ashes

The reactivity (Rsoo) of TUASH is nearly 3 times less than YX1, but larger than

the asymptotic value of YX9-10, 7.48 %/min. Reactivity of JJASH is even smaller

than that of JJ9-10, which was pyrolyzed at 900°C for 120 minutes. If normalized

by the total surface area, the reactivity of the unburned carbon samples wilI be even

smaller than the younger chars, namely, YXI and JJ1, respectively.

The observance of the deactivation chars pyrolyzed at temperatures below 900°C

for a long residence time, as well as the evidence of the low reactivity of CFB

unburned carbon, showed that the deactivation of unburned carbon also occurred in

the CFB furnace. The furnace temperature of the CFB boilers is around 850"C, but

the actual temperature of burning particles may well exceed the environmental

temperature. So the actual deactivation of carbon in the CFB h a c e will be more

severe than the pyrolysis chars made below 9OO"C, and this was partially confirmed

by the very low reactivity of the unburned carbon JJASH. For JJ coal, if the

temperature is as high as 1400"C, the reactivity Rsoo will decrease significantly to

almost zero due to the crystalline growth (see Fi,we 2).

For all the residual carbon samples, the interlayer spacing (b2) is larger than that

of the pyrolysis chars after devolatilization, i.e. JJ1 or YXl (See Table 3). This is

difficult to explain. It is possibly due to the short residence time of the unburned

fuel particles in a high temperature zone. But the direct comparison of the X-ray

diffraction patterns will allow us to check the ordering of carbon structure of the

particles during combustion and pyrolysis.

Figures 3 and 4 present the X-ray diffraction patterns of selected pyrolysis chars

307

Yong fi, Jiang-Sheng B a n g , Qing Liu, et al.

and residual carbons. But the two-dimensional dieaction peak, peak (lo), is hardly

observed in the difiaction patterns of the residual samples, i.e. BS850 and TUASH.

The lack of the appearance of peak (10) may indicate a short residence time of the

carbon particles. The emergence of the peak (10) in the intensity profile of the

JJASH is very possibly a result of the JJ bituminous coal is being easier to graphitize

than YX coal, as indicated by results of the XRD analysis of the pyrolysis chars.

5 15 25 35 45 55 65 75 85 s 15 2s 55 4 5 ' 55 65 75 85 Bragg angle (2Theta)

Figure 3. The difiaction patterns of Figure 4. The diflaction patterns of the residual carbon and the pyrolysis the residual carbons and the pyrolysis char. (7he parent coal is JJ) char. (The parent coal is YX)

Bragg angle (2The ta)

In summary, most of the particles containing the residual carbon have probably

only stayed in the furnace for a short residence time, but have experienced substantial

combustion. They may not have joined the circulating solids flow, otherwise

distinguishable peaks (10) should appear in the XRD patterns due to a much larger

residence time.

308

Reactivity and Formation of the Unburnt Carbon in CFB Fly Ashes

The XRD results show that below 8OO0C, the crystalline gowth, or the increase of

the order of the turbostratic carbon is negligible. This indicates that the deactivation

of the chars during pyrolysis at a temperature less than SOOOC, can not be correlated to

the increase of the order of the turbostratic carbon structure. Instead, it is very

possibly caused by the loss of catalytic effect, especially for YX coal.

Table 4.

residual carbon samples (Rso0, %/mrn) The reactrvrty of raw coal, demrnerallzed coal, pyrolysis chars and

Coal A B C D E Yx 43 6 12 0 47 I 7 38 17 3

JJ I0 0 7 7 3 21 2 69 136 A raw coal, €3 coal alter demineralization, C char formed after devolatilzzation of coal (7 minutes at

9OO"C), D char pyrolyzed beyond the asymptotic time at 900°C E unburned carbon in CFB ashes

Table 4 compares the deactivation caused by acid washing (a method according to

Radovic and Walker, 1983) and pyrolysis. These results, combined with those of the

reactivity, porosity and carbon srructure measurements, gave some important clues to

the different mechanisms of deactivation of the two coals, YX and JJ, which are about

the same coal rank. Under 900°C, RsW, of the YX chars from 43.6 to 7.48%/min,

and JJ from 10.0 to 3.3 %/min (see Figure 1 ) . But after demineralization, there was

a sudden drop in the reactivity of the YX coal, from 43.6 to 12.0%/min (see Table 4).

The reactivity of JJ coal however, only changed from 10.0 to 7.7%/min after

demineralization. So for JJ chars, the deactivation caused by acid washing is much

less than that by heat treatment. As the total surface area did not change much

among the pyrolysis chars, the deactivation during pyrolysis is possibly caused by the

observed increase of the order of the turbostratic carbon structure, rather than the

decrease of surface area.

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Yong Li, Jiang-Sheng Zhang, Qing Liu, et al.

Altogether it seems that the deactivation of pyrolysis chars and the residual carbon

from CFB fly ashes whose parent coal is YX, mainly resulted fiom the loss of the

catalytic effect of minerals upon demineralization of the coal. While for the

pyrolysis chars and residual carbon whose parent coal was JJ, the crystalline growth

with increase of temperature is significant and the deactivation was mainly caused by

the crystalline growth during pyrolysis or combustion.

Attempts were made to determine the origins of the organic remains. Figures 5a

and 5b shaow a comparison between the morphology of the pyrolysis chars and

residual carbon. Vesicles and rifts formed in organic solids (see Figure 5a) when

heat-treated. With fiuther heat treatment, the morphology did not change significantly.

Figure 5a. Pyrobsis chars. Figure 56. Residual carbon.

We found that in all the residual carbon samples, the morphology most frequently

seen is cenosphere (Figure 5b on right side). Others are the fiagments and

thick-walled bulky organic particles with few vesicles (Figure 5b, left side). The

many small fragments of the organic matter in JJASH and TUASH make it very

difficult to distinguish their maceral origins. The thick-walled ‘bulky’ char particles,

which constitute the slower burning char particles, can result from the pyrolysis of

macerals fiom both the vitrinite and inertinite.

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Reactivity and Formation of the Unburnt Carbon in CFB Fly Ashes

The inhomogeneity of coal is well known. It is said that fusible macerals

(vitrinite) bums faster than infusible macerals (inertinite). Attempts had been made

to relate the reactive and unreactive particles to the macerals or lithotypes of the coal.

However, for combustion there is ample evidence that listing only the three maceral

groups namely, vitrinite, liptinite and inertinite, can be quite misleading. Thomas

and Gosnell (1993) argued that the present classification system for black coals is

quite unsuitable for their use by combustion petrologists.

Conclusions and Suggestions

From the above discussion, several conclusions and suggestions may be made as

follows:

1 . The formation of most unburnt carbon in CFB fly ashes may be due to the

insufficient residence time of the fuel particles in the furnace.

2. Both inertinite and vitrinite were possible sources of unburnt carbon.

3. The deactivation of residual carbon and pyrolysis chars for YX coal occurred

under CFB combustion conditions and was mainly caused by the loss of the

catalytic effect of the minerals. The catalytic effect on the reactivity of the JJ

chars, though still observed, was much less significant than that of the YX coal.

The deactivation of JJ chars seemed to be mainly caused by the ordering of the

turbostratic carbon structure.

More studies on the deactivation mechanism are necessary, especially on the loss

of the catalytic effect of minerals during the combustion process.

4.

Yong Li, Jiang-Sheng ulang, Qing Liu, et al.

Acknowledgements

This work was supported by the Energy & Global Change Depamnent of ABB

headed by Dr. Baldur Eliasson.

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