The use of scanning electron microscopy for classification of coal chars during combustion

The use of scanning electron microscopy classification of coal chars during combustion Rosa Menbdez,

John

M. Vleeskens*

and Harry

for

Marsh?

lnstituto National del Carbbn, Oviedo, Spain *European Centre for Coal Specimens SBN, Petten, The Netherlands tuniversity of Newcastle-uponTyne, UK (Received 77 September 1991; revised 3 February 1992)

Scanning electron microscopy is demonstrated to be a useful tool for monitoring the performance of industrial boilers. For industrial applications the main advantage of scanning electron microscopy over optical microscopy is the short feedback time between sampling and reporting to the combustion engineer. The final stages of coal combustion are related to size, morphology and structure of the chars formed by pyrolysis of coal particles. Current classifications, based on optical microscopy, identify char particles as seen in cross-section and the parameters include pore width and size of inter-pore walls. Scanning electron microscopy, which has found less application, observes the outer shape of whole char particles. Internal features such as pore width and surface structure are relevant to combustion regimes I-II, but for the high-temperature II-III regimes, information on the outermost layer of the particles is needed. The analysis of chars sampled from a near-burner zone in a 400 MW power plant showed that the proportion of massive + 50pm material in half-burned coal could be taken as an index of burner performance. (Keywords:

combustion;

coal chars;

scanning

electron

microscopy)

During combustion of coal as a pulverized fuel in power generation, the coal is pyrolysed and carbonized to form a char particle which is then combusted. In reality, within the boiler, these separate stages may not be totally distinct such that oxidation, carbonization and combustion may occur simultaneously within the same particle. It has long been recognized that the morphology of the char particle, as developed from the coal particle, has an important effect on the combustibility. The morphologies of char particles are initially a function of the properties of the parent coal and also of the conditions of combustion. Three regimes of combustion can be distinguished as a function of temperature and particle size. For a given particle size the mechanism of combustion changes from internal combustion, controlled by chemical kinetics, to external combustion, limited by gas-to-surface diffusion. In reality, there are broad zones of overlap between the regimes. With increasing combustion temperature, the most important char characteristics are, successively:

(a) surface area and surface structure, i.e. the total surface which may be accessible to reacting gases, (b) the porosity which influences the rate of access of reacting gas to the surface, and (c) the size of the particle. Considerable attention has already been given to descriptions of the many morphologies of char particles as developed during combustion processes. Morphologies are dependent on coal rank, maceral composition and 0016-2361.!93.,05/061147 ( 1993 Butterworth-Heinemann

Ltd.

hardness, but cannot be realistically predicted from coal property analyses. As set out below, several nomenclatures of char morphologies exist and all are based on optical microscopy. Particles are mounted in resin and polished surfaces are prepared. Some form of image analysis, automatic or manual, is needed to classify the particle morphologies. The laboratory approach is to gain a broad understanding of pyrolysis/combustion processes. However, industrial assessments of combustion processes need to be both more rapid and simpler. This paper discusses the possibility of making a more efficient use of scanning electron microscopy (SEM), already a well-established technique, to assess external morphologies, using a simplified nomenclature. A major advantage of SEM is its greater depth of focus compared with a single, two-dimensional cross-sectional area of optical microscopy. External shape and part of the internal structure can be rapidly assessed by SEM. It is shown in this paper that SEM pictures of char particles can be prepared and analysed within 40min from the moment of sampling from a burner. The type of information presented may contribute to the knowledge of complex reactions occurring within a boiler which is basically a well-stired reactor. Better understanding may contribute to improved system design and operation. Char recognition studies date back to 19241930 (Sinnatt et al.‘) but was focused on coke properties. The first researchers to perform an extensive study on chars

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coal chars:

R. Menbndez

et al.

from pulverized coal were Littlejohn’ and Street et ~1.~ in the 1960s. The latter authors worked with selected lithotypes and observed the formation of ‘balloons’ from vitrinite-rich coals, and they related ‘solid’ chars to fusain. It was also observed that chars formed in nitrogen (650°C) were more open than those formed in air. However, the maximum temperature in their experiments was 950°C. Although the results have provided a valuable basis for later char classification work, they must be regarded with some caution when applied to present pulverized fuel combustion processes which operate at temperatures over 1500°C. Studies of char morphology were not resumed until the 1980s. This revival of char petrography was prompted by several developments. Firstly, there was an economic motive because an improved knowledge of what actually happens in the furnace was of increasing importance, with the coal trade extending outside its traditional borders and with supplies growing in diversity. Another incentive was the development of new processes, such as staged combustion, which came into being for environmental reasons. In low NO, processes the char conversion starts under fuel-rich conditions, different from those in the original pulverized fuel combustion process. An even stronger incentive is the concern with possible greenhouse effects, and a consequent demand for reduced coal use per unit of electric power by application of more efficient combustion processes, for example high-temperature conversion. Char petrography work between 1980 and 1990 has been reported by several research groups4-16. At that time no standardized char classification system was available to categorize results. Various systems were set up by the different research groups which resulted in disagreement in selection criteria, degree of subdivision and class names. A complicating factor in this work is the fact that chars are formed from particles that not only vary in size but also in maceral composition. Recently the International Committee on Coal Petrology (ICCP) formed a combustion working group to undertake the task of char classification. The work

Table 1

Parent

coal data

Sample SBN no. Country Rank Ash (wt% db) Volatile matter (wt% daf) Carbon (wt% daf) Hydrogen (wt% daf) Nitrogen (wt% daf) Oxygen (wt% daf) Sulphur. total (wt% daf) Vitrinite reflectance (%) Vitrinite (~01%) Exinite (~01%) lnertinite (~01%) Minerals (~01%)

United

Shell Mining

252 us USA hvb A/B 13.3 35.2 82.3 5.5 1.6 n.d. 1.0% 0.77 58.6 14.8 19.0 7.6

253 US USA hvb A/B 12.0 34.0 82.1 5.4 1.6 n.d. 0.97 0.79 71.8 10.6 13.6 4.0

of this group resulted in proposals”~” which have been applied in a modified form to pulverized coal combustion5. The ICCP method discriminates between char types by applying both morphologic (porosityrelated) and generic criteria. However, a shortcoming of the method is the exclusion of char size and the large number of morphologic types. On the other hand another system’ defines three size classes and four char types. A main criterion is the ‘plastic’ character of the char, a term used to indicate that the coal passed through a softening-resolidification stage during the process. Present systems of classification study particles in crosssection, using optical microscopy. Scanning electron microscopy has found little application so far, but it is a promising method, particularly when used in association with optical microscopy. The subject of this paper is to review briefly the relative advantages of both methods and their applicability to combustion systems. EXPERIMENTAL C/w

satnpling

Shell Mining and United coals were burnt in the Electric Power Facility, Borssele, The Netherlands. This is a 400 MW tangential boiler with five levels of burners (see Figure 1). Chars were sampled just below one of the burners at the lowest level. Fly ash samples were taken at the same time. Coal data are summarized in Table 1. Scunnitzg electron

COAL AIR =

Sampling

Volume

Figure 1 Schematic diagram of burner section with sampling in the Borssele 4OOMW tangential boiler

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Char particles were examined as an untreated powder, O-200lm diameter. About 20mg of sample (-5000 particles) were mounted on adhesive tape. The 1 cm diameter samples were coated with a 1Onm gold layer by argon sputtering and viewed in the secondary electron mode with Jeol JXA-840. Instant pictures were taken to analyse for type and size, using a point-count method. The surface percentages thus measured were recalculated to volume percentages using multiplication factors of 1, 3 and 6 for the size classes 0~50,5&100 and 10&200~m, respectively. A simplified classification system proposed for industrial applications is discussed below.

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RESULTS volume

AND DISCUSSION

Current systems of classification using optical microscopy discriminate between the following basic types of char:

Use of SEM

System A. Criterion: morphology (wall thickness) Char type Al Slightly porous coals A2 Cenospheres A3 Networks A4 Solids

in classifying

coal chars:

R. MenCndez

et al.

The number of classes in system A is often far greater than shown here, because subdivisions are made between thin- and thick-walled particles and between isotropic and anisotropic constituents. However, a trend is observed to simplify system A. System B has fewer classes and the parameters on which it is based are closely related to the criteria used in SEM analyses. The type of information derived from optical microscopy is based on recognition of particles in cross-section, whereas SEM visualizes the outer surface of unprepared chars. The latter method is not only quicker, because polishing is avoided, but also gives different information, complementary to optical microscopy. Whereas the inner structures of char determines the combustion properties at low temperatures, at higher temperatures the combustion zone is restricted to a relatively thin layer near the outer surface. Information from SEM thus becomes increasingly relevant with the development of hightemperature processes. An overall SEM image of a sample is shown in Figure 2. Although the average burn-off of this sample is about 50 wt%, the sample is seen to consist of a mixture of almost unaffected and completely pyrolysed (and partly burnt) coal particles. Optical microscopy (system

A) gives the following sample composition (mmf): 55 ~01% coal, 17 ~01% cenospheres, 15 ~01% networks and 13 ~01% solid char and other components. This sample is different from laboratory-prepared chars from drop tube furnaces which are considerably more homogeneous and show high percentages of spheres and networks. Particles with an essentially non-porous appearance and sharp edges are observed in association with others that are swollen, showing plastic features with large holes in the outer surface. This inhomogeneity, observed in the pyrolysis products of even a single coal, reflects the history of the coal particles in the furnace, where the feed particles are injected as a dense and turbulent flow at a speed of 27ms-‘. The SEM image suggests that at the sampling point, i.e. after less than 50ms in the furnace, the coal particles have followed different temperature paths. Possible reasons could be the turbulence of the flow and shielding by other coal particles near the coal feed point. This type of information by SEM can be obtained very quickly and can be used to monitor furnace operation. In a separate analysis step, optical microscopy could give additional information, if necessary. Optical microscopy can reveal whether the edges of the particles in the SEM image relate to coal or to coke particles. This is done by a visual estimation of the reflectance which is higher for coke than for coal. The balloons (cenospheres) observed with SEM show their internal structure when cut in polished blocks. Features observed by optical microscopy are the number and width of the pores and the wall thickness. A tentative SEM classification for individual char particles, as illustrated in Figure 2, can be based on the following characteristics: plastic-poroussopen, as distinct from non-plasticcmassiveeclosed. In this termin-

Figul

taken from the near-burner

System B. Criterion: fluidity Char type Bl Plastic B2 Non-plastic, B3 Non-plastic,

Overview (secondary

(porosity) porous non-porous

electron

image) of Shell Mining

char sample

zone of the Borssele power fat :ility

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ology ‘porous’ is used to indicate a strong, homogeneous perforation of the outer surface. ‘Open’ refers to the presence of large holes (e.g. >>10pm) in the shell. Comparisons of SEM and optical microscopy (OM) are given in Figures 3-5 using schematic drawings of salient features of the particles (OM, system A). Figure 3a (plastic-open) shows a highly plastic cenosphere with wall rupture. Particles such as this are usually thin walled. They are observed by OM mostly as thin-walled annuli, although thicker rings may also be observed depending on the sectioning plane. Figure 3h (porous-open) shows sectioning through a cenosphere with thick, strongly vesiculated walls. The local thickness of the wall could be assessed with OM. Figure 4a (porous-open) is similar to Figure 36, but with non-vesiculated walls. OM would show two types of particles: (1) a thick-walled cenosphere for a horizontal section: (2) a fragment for a section through the bridge between two large holes. Figure 4b (porous-closed) shows a particle that has been termed closed because the holes which are visible are rather small (< 1 pm). It is possible that the internal structure corresponds to this external surface and that a thick-walled network structure would be observed by OM. Figure 5a (upper part, massive-open; lower part, massive-closed). A section of the upper part is seen as a network and a section of the lower part as inertoid by OM. In vertical section the particle is classified as ‘mixed’ by OM. Figure 5b (massive-closed) has a fusinitic appearance observed by both SEM and OM. The particle is classified as inertoid in a polished section. The structures shown by SEM in Figures 3 and 4 are classified as ‘plastic’ by OM, system B. Non-plastic structures are shown in Figure 5. Whereas the spheres in

Figures 3 and 4, as seen by SEM, can be identified

under more than one category in system A by OM, the particles in Figure 5 are identical in size and shape in both OM and SEM. The size of the solid particles represents the thickness of char still to be burnt. For ‘plastic’ char particles (Figures 3 and 4) the outer wall thickness is an indication of burn-off time. In principle, burn-off time may be assessed by measuring wall thickness of char by optical image analysis with the limitation that different sections of one sphere may be classified as thin or thick and thus the optical method will give a thickness value which is generally high. Summarizing, OM gives information on such features of char particles as wall thickness and porosity. Internal pore size is a significant parameter for combustion under zone I-11 conditions. At higher temperature levels, where combustion is determined by flow restriction into the outer zone of the char, the description of particles by SEM is more appropriate to the combustion process. In industrial practice, SEM has the advantage that sample preparation and a consequent time loss of about 12 h are avoided. INDUSTRIAL

APPLICATION

Classification of half-burnt coal particles produced in a full-size power plant can provide the basis for a monitoring procedure. Combustion engineers would profit from a better knowledge of burner performance and be able to perform corrective actions, if necessary. Changes in coal feed may change size and composition of the char particles in the near-burner zone and this may necessitate a readjustment of the system conditions. Important considerations in the monitoring procedure suggested below are: choice of sampling location; time

b

Figure 3 (a) Left, SEM plastic-open; right, OM sections shown, right, OM section of cenosphere, with thick, vesiculated wall

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thin-walled

cenosphere

and thick-walled

cenosphere.

(b) Left, SEM porous-open;

Use of SEA.4 in classifying

coal chars: R. Menkndez

et al.

a

b

right, OM sections Figure 4 (a) Left, SEM, porous-open; Left, SEM, porous4osed; right, OM, thick-walled network

showing

thick- or thin-walled

cenosphere

and fragment

(section

connecting

holes),

tb)

a

b

(top) and massive-closed Figure 5 (a) Left SEM, massive-open (b) Left, SEM, massivexlosed; right, OM, inertoid

(bottom);

right,

OM sections

showing

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Use of SEM in classifying Table 2

Char characteristics

coal chars: R. Menkndez

used in proposed

classification

system

Type

Name

Features

Example

I II 111

Cenosphere Network Solid

Thin-walled, plastic Perforated, rounded Massive, with edges

Fipwe 3a

Volume

et al

Fic~urr.v 3h. 40, h Figuw.v So. h

pert.

4o:1

giving 22 ~01% as a tentative index for burn-off resistance. The partly reacted networks of the same size contribute 40% by volume. Sllell Mining These samples cover a wider range of burn-off value. Data are presented referring to 2,55 and 91 wt% burn-off. The middle data are comparable to those for United, a similar coal (Figure 7b). However, the proportion of + 50pm massive char is smaller (12 ~01%) than the 22 ~01% in United of the same burn-off. The corresponding values of unburnt coal in fly ash are 3.6 and 6.3 wt%. These results suggest a relationship between char class and boiler performance on which corrective actions in case of a change of feed coal could be based.

Volume

pert.

40

O-50 /Lcm

a

>lOOjrm

50-l 00 /ltm

30 0

Cenospheres

m

Networks

&%

Solids

Figure 6 Size-type distribution of United char sampled 400 MW boiler. Burn-off 56 wt%

from Borssele

necessary for feedback of the analysis results; and presentation of the SEM data. A critical point in existing boilers is that the design often does not permit sampling in the near-burner zone. It is possible in the Borssele power plant although the manual exercise requires considerable skill. Standardization of the procedure with the object to obtain burner/boiler representative samples is an important factor. The estimated feedback time between sampling and furnace operation is 40 min. In practice, this is no longer than the feedback time for reporting unburnt carbon in fly ash and it is considerably shorter than the time needed for optical microscopy. For reporting SEM results to the combustion engineer the use of the following simplified system is suggested. From Figures 3 to 5 three main types of char can be identified, with a decreasing open/porous structure and thus an increasing negative effect on oxidation and carbon-NO reactions. The proposed classification system uses a combination of char type and size, as shown in Table 2. A division is made into three size classes: 0_50pm, 5&100pm and 100~200~m. For sample analysis an SEM picture (Polaroid) is taken and a point-count is made of 500 particles to report size and type. Point-counting takes 40-50 min. Type histograms for each size class are given in Figures 6 and 7 for samples of different burn-off. It should be noted that the data for + 100pm particles are least accurate because in this size class one single particle represents 1% by volume. Unitetl Figure 6 presents the size-type composition for a sample of United with 56 wt% burn-off. Taking particles of all types together, the volume contribution is 38, 32 and 30 ~01% of sample over the classes of increasing size. The type distribution within all size classes is similar. with networks > solids > cenospheres. For an interpretation of the data in terms of burn-off, the solid percentages for + 50 pm particles are combined,

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20

10

1 O-50

Volume

50-l

pm

00

pm

>lOOpm

pert.

40

b I

30

20

10

_

0 1 O-50

pm

50-l

00

>lOOpm

firn

C 30

20

”

1 O-50 0

flrn Cenospheres

50-l

m

00

>lOOflm

pm

Networks

&%

Solids

Figure 7 Size-type distribution of Shell Mining chars sampled from Borssele 4OOMW boiler. (a) Almost no cenospheres; networks and solids (coal) in equal proportions (coal burn-off 2wt%). (b) Cenospheres appear; solids disappear from + 100pm fraction (coal burn-off 55wt%). (c) More cenospheres; shift from solids to networks (coal burn-off 91 wt%)

Use of SEM

The low burn-off sample shows a high contribution of solids which in this particular case are coal particles having hardly seen the process temperature. In the high burn-off sample (91 wt%) the proportion of networks is higher than in the other samples. The formation of networks is favoured at this stage because solid particles transform into networks and also because minerals are increasingly contributing to the particle shape. This effect of minerals was also observed in drop tube furnace experiments”. The information as obtained from Figures 6 and 7 can be tentatively interpreted as follows: a high proportion of type III, sized over 50pm, indicates poor burn-off; type II has intermediate combustion and also maintains fuel-rich conditions necessary for NO-reduction; type I may contribute little to unburnt carbon. Summarizing, it is proposed to use the concentration of larger solids (type III) in half-burnt coal as a burn-off resistance index. This approach is, as yet, only approximate but this is due to a lack of quantitative data on combustion properties of different char types. Back-up experiments in laboratory-size entrained flow reactors (drop tube furnaces) operating at 1500°C can provide such data. Chars produced in laboratory facilities are often more homogeneous than the samples taken from the Borssele power station and they can be used with advantage to correlate structural properties with burn-off and NO-reducing ability.

CONCLUSIONS 1. Scanning electron microscopy (SEM) is a useful tool to predict char conversion properties at high temperatures (1 SOOC). 2. For the case of pulverized fuel combustion (zone III), the outer structure as seen by SEM is more relevant to the process than the inner structure as seen by optical microscopy. 3. Because of the short feedback time (< 1 h), SEM shows promise for monitoring the performance of industrial burners. 4. Operational conditions can be adapted when the proportion of + 50 pm massive char at a representative sampling point passes a pre-set value. This may occur in the case of change-over to another coal. 5. Application in industrial practice requires back-up research to correlate structural characteristics with ‘reactivity’ at 1500°C. ACKNOWLEDGEMENTS This work is supported by the EEC under contract number JOUF-0050-C (TT). Chars were sampled by the

in classifying

et al.

coal chars: R. Mentkdez

Central Laboratory for the Dutch Power Facilities, KEMA, Arnhem, The Netherlands. Char classification was performed by Gerrit Hamburg and Marijke Roos, ECN, Petten, The Netherlands. Harry Marsh acknowledges the support of the Direction General de Investigacion Cientifica y Ttcnica (DGICYT), Spain, to study in the Instituto National de1 Carbon, Oviedo, Spain.

REFERENCES

7 8

9

10

11

12 13 14 15

16

17

18

19

Sinnatt, F. S., Newall, H. E. and McCullough, A. J. J. Sot. Chem. Ind. London 1927, 46, 331 Littlejohn, R. F. J. Inst. Fuel 1967, 40, 128 Street, P. J., Weight, R. P. and Lightman, P. Fuel 1969,48,343 Diessel, C. F. K. and Wolff-Fischer, E. M. Gluecka~f’Forsckungshefte 1986, 47, 203 Bailey, J. G., Tate, A., Diessel, C. F. K. and Wall, T. F. Fuel 1990, 69, 225 Shibaoka, M. and Thomas, C. G. in Proceedings of Australian Coal Science Conference, 3-5 December 1984, Churchill, Victoria, Australia, Gippsland Institute of Advanced Education, 1984, p. 167 Shibaoka, M. Fuel 1985, 64, 263 Shibaoka, M., Thomas, C. G., Gawronski, E. and Young, B. C. in Proceedings of International Coal Science Conference, 23-27 October 1989, Tokyo, Japan, NEDO, 1989, p. 1123 Thomas, C. G., Holcombe, D., Shibaoka, M., Young, B. C., Brunckhorst, L. F. and Gawronski, E. in Proceedings of International Coal Science Conference, 23-27 October 1989, Tokyo, Japan, NEDO, 1989, p. 257 Phong-anant, D., Selehi, M., Thomas, C. G., Baker, J. and Conroy, A. in Proceedings of International Coal Science Conference, 23-27 October 1989, Tokyo, Japan, NEDO, 1989, p. 253 Oka, N., Murayama, T., Matsuoka, H., Yamada, S., Yamada, T.. Shinozaki, S., Shibaoka, M. and Thomas, C. G. Fuel Process. Technol. 1987, 15, 213 Jones, R. B., McCourt, C. B., Morley, C. and King, K. Fuel 1985, 69, 1460 White, A., Davis, M. R. and Jones, S. D. Fuel 1989, 68, 2 Patrick, J. W., Green, P. D., Thomas, K. M. and Walker, A. Fuel 1989,68, 149 Bend, S., Edwards, I. A. S. and Marsh, H. in Proceedings of Coal Science Conference, 23-27 October 1989, Tokyo, Japan, NEDO, 1989, p.437 Skorupska, N. M., Sanyat, A., Hesselman, G. J., Edwards, I. A. S. and Marsh, H. M. in Proceedings of International Coal Science Conference, 26-30 October 1987, Maastricht, The Netherlands, Elsevier, 1987, p, 827 Bengtsson, M. ‘Proposed Classificaiion for Coal and Char Structures for Drop Tube Furnace’, Communication to the International Committee on Coal Petrology, June 1986 Diessel, C. F. K. and Wolff-Fischer, E. M. ‘Proposal for a Classification of Combustion Residues’, Communication to the International Committee on Coal Petrology. December 1987 Alvarez, D.. Menendez. R. M.. Fuertes. A. B.. Roos. C. M.. Vleeskens, J. M., Eenkhoorn, S. and Phong-anant, D. in Proceedings of International Coal Science Conference, September 1991,Newcastle, UK, Butterworth-Heinemann, 1991, pp. 3766379

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