The thermal history of char samples taken from a gasifier

The thermal history from a gasifier Catherine

A. Johnson

and K. Mark

British Gas plc, London Research (Received 10 June 1986)

Station,

of char

samples

taken

Thomas Michael

Road,

London

SW6

2A0,

UK

The thermal history of coal chars obtained from a fixed bed gasifier is of importance in understanding gasifier performance and behaviour. Several methods of assessing thermal history have been considered. The results show that Raman spectroscopy is a good method for estimating the heat treatment temperature in a thermally homogeneous coal char sample, whereas reflectance measurements provide a rapid means of characterizing thermal heterogeneity of the char samples in the temperature range 40@1000 C. The advantages of the techniques over other char characterization methods are discussed. (Keywords: char; Raman microprobe

spectroscopy: reflectance; gasifier)

a fixed bed gasification process, for example, the British Gas/Lurgi Slagging Gasifier, there is a counter current flow of gases and solids’. The coal introduced into the top of the fuel bed is carbonized by the sensible heat of the gases passing upwards from the gasification zone, which is situated at the bottom of the gasifier. The coal undergoes devolatilization, it may cake and swell and char is formed’. A stirrer is located at the top of the fuel bed to ensure the smooth flow of gases and solids. The flow of gases and solids is also affected by the conditions in the raceway, for example, size and blast velocity, where the char produced in situ in the upper part of the gasifier undergoes reaction with steam and oxygen. The two processes of gasification and devolatilization occur in different regions of the gasifier and may be considered separately although interactions between them need to be taken into consideration. The efficiency of the slagging gasifier is high and there is little excess heat above that required for heating up the coal sufficiently for it to be carbonized in the upper part of the gasifier prior to gasification. The gas outlet temperatures are modest and are typically 3tXMOO”C. Good heat exchange is required for this and it is optimum when the flows of solids and gases are matched at every point. This situation is approached when the fuel bed descends uniformly and the gases flow evenly through the bed. Hence the flow pattern in the gasifier is an important feature of fixed bed gasifiers and more data are needed in this area to increase the understanding of gasilier behaviour and hence the ease of operation. This can be achieved by three methods: In

(1) various types of probes, for example thercan be used. These have the mocouples, disadvantage in that they tend to get damaged by the flow of coal and they may also perturb the local flow of char and gas (2) tracer studies, which are dynamic and provide information on the behaviour of a working gasifier 0016~2361/87/010017-OTS3.00 ~2 1987 Butterworth & Co. (Publishers)

Ltd

but the interpretation of the results is not always unambiguous (3) the characterization of gasifier char samples from a bed digout which is a post-mortem examination. This technique provides information on gasilier behaviour at shutdown. Its disadvantages are that there are cool down and bed collapse effects. The characterization of gasifier bed digout samples is seen as being complementary to probe and tracer studies. A knowledge of the thermal history of the coal/char as it passes down the gasifier is important in understanding the char properties and it is also indicative of the rate of descent of the particles and the gas flow distribution. The thermal history of the gasifier char bed digout samples can be inferred from systematic changes in either the structures of the minerals present in the original coal or the carbonaceous material produced in the gasifier. The former gives rise to temperature bands corresponding to the stability of various mineral phases which are usually identified by either X-ray diffraction or infrared spectroscopy. The latter is based on systematic changes in carbon structural characteristics with temperature. Since carbon is the major component of a char there are obvious advantages in studying changes in its structure. An important feature of char samples taken from a gasifier is that even if they are taken from a small area, they can be heterogeneous and this aspect needs to be quantified. Therefore, techniques which measure the average properties of a sample may give misleading results and techniques which are capable of characterizing the heterogeneity in thermal history are necessary. This paper illustrates the use of reflectance measurements and Raman microprobe spectroscopy for estimation of heat treatment temperatures (HTTs) and thermal heterogeneity in char samples. These techniques allow a microscopic examination of the char samples and therefore have the potential of quantifying some aspects of thermal heterogeneity.

FUEL,

1987,

Vol 66, January

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Thermal

history

240

-

200

-

160

-

120

-

80

-

40

-

of char from

a gasifier:

C. A. Johnson

curve fitting programme described previously3. This was used to estimate the heat treatment temperature (HTT) of the char. Repeat measurements made on a laboratory char heated to 1500°C showed that the Raman D bandwidth measurement was accurate to _+ 10 cm- ‘, which corresponds to an accuracy of 5 100°C for the estimated HTT. RESULTS

01

1

1500

1000 Temperature

I

I

I

500

0

2000

(‘C)

Figure 1 The variation of Raman D bandwidth with HTT for various laboratory coal chars and petroleum cokes: 0, reference sample, North Sea petroleum coke; A, Ashland A240 petroleum pitch coke; l , coal char A (used in gasifier); 0, coal char B (502): - - - - -, calibration curve calibration curve for coal char A for petroleum coke; -,

EXPERIMENTAL Samples used

The samples used were from a fixed bed gasilier. The coal used in the investigation had a rank of 702 in the NCB classification scheme. The sampling of a gasifier bed involves digging it out after a controlled shutdown and subsequent cooling of the residual char. Considerable care is needed in the sampling to ensure that contamination between various levels does not occur. Several kilograms of sample were taken at each sampling point and each sample was subdivided according to the guide lines outlined in BS1017. A few grams of each char (- 1 mm in particle size) were embedded in resin and ground and polished to a standard suitable for accurate reflectance measurements. Raman and reflectance measurements could then be taken from the same block. ReJ2ectance measurements

FUEL,

1987,

The techniques used in this study for the estimation of thermal history of gasilier chars have to be calibrated by comparison with standard chars made in the laboratory. The variation of Raman D bandwidth with HTT (5OG 2100°C) for chars/cokes prepared from various precursor materials (two coals4 and petroleum pitches) is shown in Figure 1. This result is comparable with those obtained3,4 for chars derived from six coals covering a range of rank from 203 to 501 in the NCB classification scheme. This shows that the Raman D bandwidth is not affected markedly by the nature of the precursor. Previous results also showed3 that the experimental conditions other than temperature, for example, heating rate, pressure and soak time, had very little effect. In contrast, char reflectance measurements are known to change markedly with the rank of the coal precursor but they are much less sensitive to changes in carbonization conditions’ 3. The variation of R,,, with HTT for the coal used in the gasilier is shown in Figure 2 (the vertical bars indicate _the standard deviation). The slight decrease4*’ in R,,, above a HTT lOOO-1200°C does not produce ambiguities because the narrow reflectance distributions with degree Rmax in the range S-97; indicate a reasonable of thermal homogeneity (HTT> 1OOOC); these samples are accordingly examined by Raman spectroscopy. Also the temperatures observed in the gasilier with the exception of the char/oxygen reaction zone are < 1500°C. The reflectance technique is useful in studying the

and Raman spectroscopy

The reflectance measurements were obtained using a Leitz Orthoplan microscope and MPV Compact photometer. The experimental methods for obtaining the measurements and details of measurements on laboratory chars including chars derived from the coal used in the gasilier run have been reported3g4 elsewhere. At least 100 measurements of maximum reflectance (R,,,,,) were made on each gasifier sample and the average value (R,,,) and distributions derived. In the case of samples with a broad reflectance distribution and heterogeneous thermal history up to 200measurements were made. Only samples which had a fairly homogeneous thermal history were chosen for examination by Raman spectroscopy, because only a weak Raman spectrum is observed for a coal char and consequently the data collection is time consuming. The time required to make a representative number of measurements on a heterogeneous char is prohibitive on a routine basis with the instrument available. The Raman spectra were, obtained using the Jobin Yvon MOLE. The experimental techniques used have been described previously4. A minimum of three spectra were taken from different particles in each sample. Further spectra were recorded if the agreement between the measurements was not within acceptable limits. The average bandwidth of the D band was measured using a

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and K. M. Thomas

Vol 66, January

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3 2 1

0

400

800 HTT

Figure 2 The variation used in the gasifier run

of reflectance

1200

1600

2000

(‘Cl (R,,,)

with HTT for the coal A

Thermal

history

of char from

a gasifier:

C. A. Johnson

and K. M. Thomas

shown in Figure 5. The D bandwidth (220cm-‘) is indicative of an HTT of 600°C (see calibration curve in Figure

” 0

2

4

6

8

R,,,

10

12

14

16

(%I

Figure 3 Reflectance (R,,,) distributions for gasifier chars A and B: A. j&=5.96%; B, VM=6.3%, R,,,=7.64% VM=7.9%,

35 30 25 20 z E

15

E 2

‘0 5

? ?i &

cl

2 ;

I).

Reflectance distributions of samples taken diametrically at the same level in the stirrer region of the gasifier are shown in Figure 6. Both of the distributions show the presence of material with a wide range of HTTs, but with two main components at Rmax values corresponding to 50&600_and 800-900°C (see calibration curve Figure 2, peak 1, R,,, = - 3 “/;,and peak 2, R,,, = - 7 ‘:C).In sample ST1 the latter predominates (see Figure 60) whereas in sample ST2 the former does (see Figure 6b). These results illustrate the effect of the stirrer mixing the char as it passes down the gasifier. Figure 7 shows a typical reflectance distribution (R,,,,) of a sample Bl taken near the bottom of the gasifier bed. The distribution indicates that the sample is reasonably thermally homogeneous and has a minimum HTT of 1000°C. However, reflectance distributions alone are insufficient for predicting the HTTs from these samples because of the shape of the reflectance-temperature calibration curve (Figure 2). Raman spectroscopy can

1

I

40 30 20

100 c.p.s

10 0 1

3

5

7

9

11

13

15

R max (%) Figure 4 Reflectance (R,,,) the top of the gasifier bed: (a) VM=11.3:,. R,,,=3.9”;;

distributions for chars taken from TI , VM = 28.4”/,, R,,, = 1.88 “/,: (b) T2,

heterogeneity in thermal history in the temperature range 400-l 000°C. Volatile matter determination (VM) is generally a useful method for characterizing chars with a uniform HTT< 1OOO’C but the limited applicability of this measurement to potentially heterogeneous gasifier chars is illustrated by the comparison of two samples taken from the same gasifier bed. The reflectance distributions of two chars, A (VM= 7.9%) and B (VM=6.3 “i,), are given in Figure 3. Although the volatile matters are similar, their compositions are quite different. Sample A contains very little coal, but the reflectance distribution indicates that the material has experienced a wide range of HTTs. On the other hand, sample B contains two distinct fractions; 15 % coal and the remainder which has been subjected to an HTT of at least 1000°C. Typical samples from three regions of the gasifier (top, stirrer region and bottom) are used to illustrate the application of the methods. Figure 4 shows the reflectance distributions of char samples taken from the top of the gasifier in two different runs. Sample Tl consists mainly of virtually unchanged coal (Rmax< 1.5 %) with some low temperature semicoke (Figure4a), whereas sample T2 is predominantly low temperature semicoke (Figure 4b). A typical Raman spectrum obtained from sample T2 is

J 1400

1200

1000

1600

’

Raman shift, cm-

Figure 5 A width=220cm-’

typical

Raman

spectrum

1800

of

char

T?.:

D-band

40 :

30

g

20

$

10

E 40 z I& 30 $

20

z’

10 0

0

2

4

6

8 Rmax

10

12

14

16

(%)

Figure 6 Reflectance (R,,,) distributions of chars taken from the stirrer region of the gasilier: (a) STI, VM=6.7”,,, &,,=5.87”,: (b) ST2, VM= 11.4”,,, i?,,,=4.06”,,

FUEL, 1987, Vol 66, January

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Thermal history of char from a gasifier: C. A. Johnson

and K. M. Thomas

c-3

E i $

25

E

20

%

15

b

10

2

5

:

0 3

5

7

9

11

13

15

17

R max 1%)

1000

Figure 7 A reflectance distribution for char Bl taken bottom of the gasilier: VM= 1.lS”,,; R,,,=9.01 “A

Table 1 Raman spectroscopy levels in the gasifier Raman

Level 1 Level 2 Level 3

data on char samples

D bandwidths (cm-‘) _____ Side 2

Side

120 120 107

97 115 120

125 107 90

1200 1200 1350

1

Figure 8 A typical Raman of the gasifier

( C)

Centre

Side 2

1470 1260 1200

1140 1350 1550

Raman D bandwidth measurements are accurate to k lOcm_i and the HTTs derived from these to f 1OO’C. I and 2 are diametrically opposite

provide the additional information. A typical Raman spectrum obtained for the sample is shown in Figures. The Raman bandwidth of 120 cm-’ is consistent with a HTT of 1200°C. This method can be used to obtain temperature profiles in the gasifier and this is illustrated in Table 1. This gives a comparison of Raman data from char samples taken diametrically across the gasifier at three levels. These data shown an even temperature distribution down and across the diameter of the gasifier.

DISCUSSION Exercises involving the shutting down and dissection of blast furnaces after water quench under normal operating conditions have been carried out previously6. These studies provide a comprehensive understanding of the change in coke properties during its descent to the tuyeres. In a fixed bed gasifier, the coal/char gradually experiences higher temperatures and devolatilizes as it passes down the gasilier. Therefore, sampling and char characterization exercises aimed at providing an understanding ofgasifier behaviour must follow either the devolatilization process, or structural parameters which change systematically with temperature and hence provide information on the thermal history of the samples. A logical starting point for these studies is to use standard volatile matter measurements on the samples. However, some of the samples taken from the gasifier are heterogeneous because of mixing and flow characteristics. In this circumstance, the average value of a parameter such as volatile matter can be misleading (see Figure 3). Also the volatile matter changes only between 350 and 1000°C’. Examination of the mineral phases in a thermally heterogeneous char would only provide temperature bands corresponding to the stability of minerals present. It will probably give a wide temperature

20

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1300

1400

1500

1000

1700

1800

SHIFT (cm-l)

spectrum

of char B 1 taken from the bottom

taken from three

Temperatures

Centre

1200

RAMAN

from near the

_

Side 1

1100

range and kinetic effects complicate the may interpretation of the results. Therefore, additional techniques are needed which are capable of providing further information on the following: (a) thermal profiles up to 2000°C in the gasifier; (b) heterogeneity in thermal history which provides information on mixing, heat transfer effects and flow patterns in the gasifier. The results clearly show that (a) can be achieved by Raman D bandwidth measurements and some information on (b) can be obtained from reflectance measurements. The temperature profiles shown in Table 1 indicate that the fixed bed gasifier has a fairly uniform diametrical temperature disfribution, together with a temperature plateau down the gasifier. The former is important for smooth operation. These are typical temperature profiles obtained from a fixed bed gasifier. Temperature distributions in the fuel bed are controlled by the rapid exothermic reaction of the char with oxygen and the slower endothermic reaction with steam. The maximum temperature occurs in the char/oxygen reaction zone and the temperature decreases as reaction with steam proceeds up the bed until the steam concentration is low and further changes are slow giving a temperature plateau. Heterogeneity in thermal history occurs mainly in the stirrer region of the gasifier. Figures 4 and 6 show that this can be followed by reflectance measurements providing useful information on the flow of coal down the gasifier. In principle, Raman microprobe spectroscopy can also examine thermal heterogeneity, but it is time consuming. However, the advent of multichannel detection techniques will improve the speed with which teh Raman data can be collected and the combination of Raman and reflectance measurements is, therefore, a powerful method for studying the thermal history of gasifier samples, capable of providing vital information for understanding the operation of a fixed bed gasifier.

ACKNOWLEDGEMENTS The authors would permission to publish

like to thank this paper.

British

Gas

for

REFERENCES 1

Evans, R., Thompson, B. H., Hiller, H. and Vierrath, H. E., Proceedings of 5th International Conference and Exhibition on Coal Utilization and Trade, 1985, 3, 659

Thermal history of char from a gasifier: 2 3 4

Dryden, 1. G. C. and Sparham, G. A. BCURA Monthly Bulktin 1963, 27, I Johnson. C. A.. Patrick, J. W. and Thomas, K. M. Furl 1986,65, 1284 Green. P. D., Johnson, C. A. and Thomas, K. M. Fuel 1983,62,1013

5 6 7

C. A. Johnson

and K. M. Thomas

Goodarzi, F. Fuel 1954,63, 820 Kojima, K., Nishi, T., Yaunaguchi, T., Nakama. H. and Ida, S.. Transactions qfthr Iron und Steel Institute qf’Jupan. 1977. 17. 401 BCRA Carbonization Research Report No. 80

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