Behaviour of different char components in fluidized bed combustion: a char petrography study

Behaviour of different char components fluidized bed combustion: a char petrography study John M. Vleeskens, Jan Gerrits”

Ton W. M. 6. van Haasteren*t,

Marijke

in

Roos and

Netherlands Energy Research Foundation, ECN, PO Box 7, 1755 ZG Petten, The Netherlands * TNO, Netherlands Organization for Applied Scientific Research, PO Box 342, 7300 AH Apeldoorn, The Netherlands (Received 22 January 1987; revised 8 May 1987) Residues of fluidized combustion were shown to contain slightly more anisotropic char than is consistent with the vitrinite content of the parent coals. The composition of the cyclone residues gradually shifted to higher contents of anisotropic char as the recycle ratio increased, but remained constant over a wide range of air-tofuel ratios. Cyclone chars were richer in anisotropic matter than filter chars, in which the inertinite residues tended to concentrate. The surface structure, as determined by the size of mosaic units, was not influenced by either residence time or air-to-fuel ratio. The combined data suggest that greater abrasion resistance, rather than reduced chemical reactivity of vitrinite char, may explain its slower combustion. A practical implication is that fly ash recycling may be more effective with high inertinite coals than with coking coals. (Keywords: fly ash; floidized beds: char)

Combustion residues, like coals, differ in composition and structure. In a complicated way, they reflect not only the type of the parent coal, but also the conditions of combustion. The study of conversion products may provide detailed information on the combustion history of coal particles, which ultimately determines the combustion efficiency. A high degree of burnout is necessary to maximize heat generating capacity and fly ash quality. Burnout depends on combustion conditions, such as temperature, residence time and air-to-fuel ratio. It depends, moreover, on coal characteristics such as particle size, rank, composition and mineral content. Microscopic studies on pulverized coal residues have been reported previously’ -4. For the case of fluidized bed combustion (85o”C), low rank is known to have a beneficial effect on burnout, but the role of maceral composition deserves further investigation. This paper discusses the possible effect of vitrinite char formation on the relative burnout of macerals in the same coal. The effect has been studied under different conditions of recycling and excess air in a fluidized bed.

could be enhanced through recycling of the fly ash. A flow diagram is given in Figure I. A more detailed description of the plant has been documented previously5s6. Coal and limestone are pneumatically transported to the boiler and injected under-bed into the 2.25 m2 fluidized bed. The coal is burnt partially in the 1.05 m high bed section and partially in the 3.50 m high freeboard section. After the freeboard, the dust-laden flue-gas is cooled to about 240°C and the solids removed using cyclones and a fabric filter. An appropriate parameter to characterize the number of cycles of the recycled ash and char particles through the combustion zone is the recycle ratio (R) which is defined in this context as: R = MR/ M,

(1) CYCLONES

FREE BOARD

EXPERIMENTAL Fluidized bed combustion The facility used is a 4 MW (thermal) coal fired atmospheric fluidized bed boiler (AFBB), designed for research on the combustion of coal and other fuels such as lignite, petroleum coke, oil and gas. This facility has been used to study effects of increased residence time and airto-fuel ratio on combustion efficiency. The mean residence time of unburnt char particles in the combustion zones (bed section and freeboard section)

SAMPLING VALVE FLY ASH REMOVAL ,

t Present address: 6160 MD G&en.

DSM Research, The Netherlands

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0016-236l~X8iO30426~5$3.00 0 1988 Buttcrworth & Co. (Publishers)

426

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FABRIC FILTER [-,

Engineering,

PO Box 18,

Figure 1

system Ltd.

FUEL, 1988, Vol 67, March

RECYCLED

ASH

1

Flow sheet for atmospheric fluidized bed boiler with recycle

Behaviour

of different chars in fluidized

in which M, is the mass flow of recycled fly ash (kg/h) and MD is the mass flow of discharged fly ash (kg/h). If the mean residence time of a char particle in the fly_ ash during one cycle through the combustion zone is t seconds, the mean residence time of a char particle which is recycled R times is R x tseconds. Another series of experiments was performed in a 0.04 MW (thermal) atmospheric fluidized bed combustor with in-bed feeding, operating at 855”C, 1.4msK’ fluidizing velocity. The air-to-coal ratio was kept at 1.34. After reaching constant conditions of operation several 8 h runs were executed in succession. The coal was double sieved and its nominal size was + 1 to - 10mm. In the middle of each run three ash samples were taken from the cyclone at 1 h intervals to determine concentration and composition of the chars. This facility does not permit fly ash recycling. It has been used, primarily, to study the effect of coal rank on combustion efflciency7. Char classification

The study of residue samples was performed, basically, using the method of Goodarzi’, who investigated the petrographic composition of char samples which had not been recycled. The microscopic analysis was performed by point counting using reflected polarized light. The residues were divided into three groups (Figure 2), according to the criteria shown in Table 1.

50Fm

’

bed combustion..

J. M. Vleeskens

et al.

The classification system is based on structural rather than morphological criteria. It has been chosen to discriminate between chars derived from coking and noncoking components. The ‘cenosphere’ group, which is defined by shape only, did not seem very useful in this respect, and the classification method was therefore modified accordingly. All but a few char particles, including thick-walled (> 5 pm) cenospheres. were reported as either A-char or I-char according to their anisotropic or isotropic nature (Figure Al). Structureless thin-walled balloons never formed more than 5 “I by mass of the total residue. For practical purposes all combustion residues could be considered to consist of A-char and Ichar only. The name A-char is being used throughout this text to designate the sum of anisotropic materials, which may include minor amounts of converted semifusinite”.

Table 1 Group ‘Vitrinite*

Char classification

char’

‘Inertinitet char’ ‘Cenosphere’ ___~

* Derived t Derived

Criterion Anisotropic char (partly mosaic structure) Isotropic char Balloon shape; walls < 5 pm _... __~~~ ~~~

from vitrinite, mainly from non-reactive inertinite,

Designation CA CI

mainly

50 urn

Figure 2 Specimens of char structures. Leitz MPV-2, oil immersion objective (50 x ): (a) isotropic char, I-char; (b) anisotropic char, A-char, mosaic structure; (c) ‘cenosphere’ thin-walled (< 5 pm); (d) ‘cenosphere’, thick-walled (> 5 pm), showing anisotropy under crossed nicols

FUEL, 1988, Vol 67, March

showing

427

Behaviour of different chars in fluidized bed combustion: J. M. Vleeskens et al.

The following fly ashes were analysed for A-char to Ichar ratio, with, in some cases, an estimate of mosaic size also being made: 1.

2. 3.

Cyclone ashes from coals of different rank and type after combustion in the once-through, 0.04 MW fluidized bed combustor. ‘Mixed’ fly ash samples taken from the 4 MW TNOfacility at various air-to-coal ratios. Cyclone ash, filter ash and ‘mixed’ fly ash taken from the same facility at different recycle ratios.

RESULTS AND DISCUSSION No recycling Residues were produced in the 0.04MW facility from bituminous coals, varying from low-volatile (20% VM) to high-volatile B (40 % VM) (Table 2). All cyclone residues contained slightly more A-char than consistent with the coke-forming content* of the parent coal (Figure 3). After combustion a shift is observed from V/V + I % vitrinite (including natural coke) in the coal to CA/CA+CI % anisotropic char (Figure 4) in the residue. For all coals (with the exception of coal R) CA/CA + CI is about 10% in excess of V/V + I. This increase is independent of rank and small for most coals. It can be attributed, partly, to the formation of anisotropic char from ‘reactive’ inertinite”. The South African sample R, which contained only 24% vitrinite, produced a residue containing 66% Achar. This effect at first sight suggests a preferential burnout of inertinite, but coke formation from semifusinite of this coal must be taken into account. The total coke-forming content in this coal is not 24x, but 60x?. Again, the amount of anisotropic coke in the char is slightly higher, but comparable to the percentage of ‘reactive’ components in the coal. * Expressed as V/V + I t Pyrolysis (SSO’C, l&lOmin, 70% A-char

Table 2

80

70

v/v+1 (OhIFigure 3 Anisotropic char content (CA/CA+(X) in chars versus vitrinite content (V/V +I) of parent coals. Coals vary in rank between hvb-B and mvb. Bed temperature, 850°C; air-tocoal ratio, 1.24(mk 1.34 (0)

4

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I 0.8

I 1.2

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BE P 2 EA V C Gl P3 P4 G3 ES R

produced

samples

I

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Figure 4 Combustion efficiency versus air-to-coal ratio (coal P4). Bed temperature, 850°C; bed height, 1.05 m; recycle ratio, 8.5

Coal data

Moisture SBN no.”

(wt %)

Ash (dry) (wt %)

508 509 b

0.7 3.9 2.8 1.2 3.0 2.2 3.8 3.1 1.5 1.9 3.4 6.8

6.8 8.4 4.9 3.7 9.8 5.3 16.2 9.5 9.0 7.1 7.9 16.6

501

e

510 d

c c 1 513 P

a Dutch Centre for Coal Specimens, SBN b Commercial c Commercial used in recycling experiments d Westar Mining Ltd. ECNcombustor ‘Commercial. Coal type comparable to SBN no. 512 rdaf=dry, ash free @dmmf = dry, mineral matter free. Balance = exinite * In&ding 0.4 % natural coke ** Including 1.7 % natural coke ***Including 2.8% natural coke

428

I 1.4

AIR TO COAL RATIO 4

nitrogen)

Vitrinite, Sample code

90

FUEL, 1988, Vol 67, March

Volatile matter (wt %, daf)r

V-reflectance (R,, %)

20.1 22.8 30.2 34.4 31.2 34.5 29.3 35.3 34.5 32.5 44.0 30.1

1.51 1.30 1.04 1.Ol 0.98 0.96 0.92 0.87 0.87 0.89 0.76 0.73

V

Inertinite,

(vol%, dmmf)O 79.8 84.0 78.3 55.7 12.6* 19.2 65.9 69.1** 73.4*** 60.2 69.2 22.6

19.8 15.9 16.6 28.6 21.4 13.8 29.4 21.0 18.4 36.8 17.5 72.2

I

Behaviour

of different

From the above results, it can be concluded that the combustion rates of coking and non-coking components are similar at all ranks of coal. The anisotropic coke content of char reflects the percentage of either vitrinite or vitrinite plus ‘reactive’ inertinite in the coal before combustion. Higher-rank coals of the suite under discussion have been reported to produce larger mosaic structures which were similar in size to those observed in metallurgical coke’. These coarse surfaces may affect the intrinsic reactivity and contribute to the well-known reduction of burnout at increasing rank. Because the noncokeforming inertinite is burned at a rate which is comparable with that of A-char, it is apparent that inertinite also experiences some effect of rank. Effect of air-to-coal ratio

One of the factors which influences burnout of coal during fluidized combustion, is the air-to-coal ratio. Figure 4 gives the results of a series of experiments on coal P4 during which the air factor was changed while the other boiler conditions, including the recycle ratio, were kept constant. An increase in combustion efficiency from 90% to 980/l is shown as the air-to-coal ratio increases from 0.81 to 1.34. The composition of the chars did not change with excess air. A virtually constant CA to V ratio of 1.10+0.02 was determined on the five mixed fly ash residues. In addition, the structure of the anisotropic char was independent of degree of burnout. The percentage of coarse (> 5 pm) to tine mosaic structures was 5 f 1% for the whole range of air-to-coal ratios. These data show again that A-char values are only slightly higher than the V-content of the parent coal. They also imply that this small difference is not a result of a difference in chemical reactivity between A-char and Ichar, because the coal is burned off to values ranging from 10% down to 2% unburnt coal without a systematic change in char composition or structure. Recycling In Figure 5 some results are presented to illustrate the

effect of recycling ratio on combustion efficiency. An increase of combustion efficiency with increasing recycle ratio is observed, as expected. Clearly visible is the difference in combustion efficiency (2-3 %) between a Polish coal (P3) and a USA coal (Vl) at the same recycle ratio and identical boiler conditions. This difference

100 r

95

90 ;;;

0

10

20 RECYCLE

30 RATIO

-

Figure 5 Combustion efficiency versus recycle ratio (*, Coal P3; A, Coal Vl). Bed temperature, 850°C; bed height, 1.05 m; air-tocoal ratio, 1.24

chars in fluidized

bed combustion:

J. M. Vleeskens et al.

.I

0

10

30

20

RECYCLE RATIO

-

Figure 6 Percentage anisotropic char (CA/CA+CI) versus recycle ratio (Polish coal P3). Dashed line indicates vitrinite content (V/V + I) of parent coal. * Filter ash; A fly ash; 0 cyclone ash. Bed temperature, 850°C; bed height, 1.05 m; air-toxoal ratio, 1.24

‘r 1 100 90

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601

I

0

I

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1

I

20

30

RECYCLE RATIO

I

-

Figure 7 Percentage anisotropic char (CA/CA+Cl) versus recycle ratio (USA coal Vl). Dashed line indicates vitrinite content (V/V + I) of parent coal. * Filter ash; A fly ash; 0 cyclone ash. Bed temperature, 850°C; bed height, 1.05 m; air-toxoal ratio, 1.24

persists up to high recycle ratios. It may be described as an effect of rank, which has been mentioned before. Analysis ofcyclone ashes revealed an increase of A-char at the expense of I-char at increasing recycle ratios. The A-char concentration was observed to increase from 78 (84) ~01% in the non-recycled char to 85 (90) ~01% at the highest recycle ratios (Figures 6 and 7). Analysis of the filter ash, moreover, provided additional information. At the left-hand side of the curves in Figures 6 and 7, which represent the shortest residence times, a major disproportion is observed between coarser and finer fractions. The line filter ash (-8 pm) contains considerably more inertinite than the + 30 pm cyclone ash, in which Achar is concentrated*. At longer residence times the difference in composition between the residues decreases. These phenomena could reflect either a reduced chemical reactivity, or a better mechanical stability of vitrinitechar compared with inertinite char. In view of the results obtained at different air ratios, a chemical factor is not likely to be effective in this case. The extended residence times could have resulted, perhaps, in mosaic growth, with consequent smoothing of the A-char surface and diminished burnout. However, as the percentage of coarse mosaic structures was 6 f 2 vol Y0for all samples, which is virtually constant in view of the limited accuracy of the determination, there is no evidence of changes in surface structure. This is not in support of a mechanism *Natural coke present in the feed coal, by contrast, tended to concentrate in the fines (Ml.5 mm). The tine coal fraction contained a higher proportion of natural coke than the whole coal, but no difference between inertinite contents was observed (Tab/r 3)

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Behaviour of different chars in fluidized bed combustion: J. M. Vleeskens et al.

Table 3 Maceral composition of whole coals and < 0.5 mm fractions for samples VI, P3, P4, containing natural coke

Sample code

Coal size (mm)

Vitrinite vol %b

Nat. coke” vol %b

Vitrinite + Nat. coke” vol%*

Inertinite volOAb

Exinite vol %b

VI Vl

+o SO.5

72.2 69.7

0.4 3.9

72.6 73.6

21.4 21.2

5.9 5.2

P3 P3

+o CO.5

67.4 65.6

1.7 5.8

69.1 71.4

21.0 21.4

9.9 7.3

P4 P4

+o go.5

70.6 69.5

2.8 6.7

73.4 76.2

18.4 18.4

8.2 5.4

‘Coarse mosaics, > 5 pm *Mineral matter free

affecting the intrinsic reactivity of A-char with increasing residence time. A greater mechanical strength of anisotropic char compared to isotropic inertinite char, is more probable. Summarizing, the observed increase of A-char concentration in residues with extended residence time and the capture of coke-like A-chars in coarse cyclone fractions are consistent with a mechanism where isotropic inertinite char experiences an effect of in-reactor wear with simultaneous combustion. This is a specific case of the general ‘attrition’ phenomenon proposed by Massimila and colleagues’ I. A practical implication of these findings is that fly ash recycling may be more effective in promoting burnout of high inertinite Southern Hemisphere coals, than with coking coals. Strength properties of coal chars other than those examined here should be tested in the reactor to confirm the validity of this suggestion.

ACKNOWLEDGEMENTS This work was jointly performed by TN0 and ECN. Support was received from PEO, Netherlands Project Office for Energy Research, Contract no. 20.16-009.10 and 20.35-041.10.

REFERENCES 1 2 3

4 5

CONCLUSIONS The most probable explanation of the reported data is that reduced burnout of vitrinite char relative to inertinite char under recycling conditions is the result of a better resistance to wear rather than of a lower chemical reactivity. The observed preferential combustion of inertinite is consistent with a mechanism of gradual inreactor wear, i.e. specific attrition, with simultaneous combustion.

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7 8 9 10 11

Street, P. J., Weight, R. P. and Lightman, P. Fue[ 1969,&I, 343 Shibaoka, M. Fuel 1985,64, 263 Oka, N., Murayama, T., Matsuoka, H., Yamada, S., Yamada, T., Shinozaki, S., Shibaoka, M. and Thomas, C. G. Fuel Proc. Tech. 1987, 15, 213 Bengtsson, M. Fuel Proc. Tech. 1987, 15, 201 Meulink, J., van Haasteren, A. W. M. B. and Temmink, H. M. G. ‘Operating experience with a 4MWth AFBB research facility’, in Proc. 8th Int. Conf. on lluidized bed combustion, March 1985, Houston, Texas, USA van Haasteren, A. W. M. B. ‘Effects of fly ash retiring on the combustion efficiency and the stack emissions during fluidized combustion of coal. Experiments on the TN0 4 MWth AFBB’, TNO-report no. 85-013220, November 1985 Vleeskens, J. M. and Nandi, B. N. Fuel 1986,65, 797 Goodarzi, F. Private communication Grint, A. and Marsh, H. Fuel 1981, 66, 1115 Diessel, C. F. K. and Wolff-Fischer, E. M. GIiickauf Forschungshejie 1986, 47, 203 Arena, M., D’Amore, M. and Massimila, L. AIChE 1983,29,40