Introduction

Fly ash is a volumetrically dominant type of coal combustion product; we must understand its properties in order to better utilize it. Petrographic composition is one of these properties. Petrographic analysis provides information about the contribution and type of carbon as well as relating to other parameters, such as surface area, and the analysis is also relatively inexpensive. In recent years, two distinct systems for describing ash petrography have been used. The system devised by Bailey (Bailey and others, 1990), and modified by Lester (Lester and others, 1997) and Alvarez (Alvarez and others, 1997), emphasizes textural description of the char structure. This system is the base of char classification considered by the International Committee of Coal Petrology. The system used at the Center for Applied Energy Research, UK (Hower and others, 1995 and later), places more of an emphasis on the forms of carbon and inorganic matter in fly ash. New classification by Hower and Mastalerz (2001) combines the two classifications, including carbon forms, inorganic constituents, and the textural features of chars.

Texture-based classification

(modified from Bailey and others, 1990, Lester and others, 1997, and Alvarez and others, 1997)

1. Gives a textural description of char particles that may be related to the origin (T-boiler temperature, coal rank, etc.);
2. Does not provide information on the individual components in mixed categories;
3. Does not differentiate mineral matter in fly ash;
4. Determines porosity, and cenosphere wall thickness in a subjective manner.

Texture-based classification chart

(considered by International Committee for Coal and Organic Matter)

Click the image below to open a PDF file with a larger view (56k, requires Adobe® Reader®).

Photomicrographs of selected types of fly ash, texture-based classification

(from the collection of authors)

Reflected light, horizontal axis 240 µm. Click the images below to see a larger view.

Solid	Tenuisphere
Tenuisphere	Crassisphere/Crassinetwork
Mixed Porous	Mixed Porous

Carbon type-based classification (based on Hower and others, 1995).

1. Gives information on carbon types that is related to the surface properties of the fly ash;
2. Gives detailed information on the contribution of different types of mineral matter (main component of the fly ash);
3. Does not provide textural information of the organic matter in fly ash.

Photomicrographs of selected types of fly ash, carbon type-based classification

(from the collection of authors)

Reflected light, horizontal axis 240 µm.

- Isotropic carbon (coke) - Glass - Mullite - Spinel - Quartz - Sulfide - Oxides - Uncombusted coal - Petroleum coke - Anisotropic carbon (coke) - Inertinite

Combined classification by Hower and Mastalerz

1. In order to gain more information from petrographic analysis, we propose combining the two classifications (Hower and Mastalerz, 2001, Energy & Fuels, no.15, p. 1319-1321)
2. Such a combination would include carbon forms, inorganic constituents and the textural features of chars.
   This information directly translates into the amount of a certain component within a textural type.
   
3. The combined classification system provides information that can be related to the surface area of the fly ash (from the contribution of individual components) and also some genetic information inferred from a char type.
4. The two-digit numerical code represents the components, and texture; for example; code 24 indicates anisotropic carbon in crassinetwork.

Combined classification by Hower and Mastalerz

Click the image below to open a PDF file with a larger view (19k, requires Adobe® Reader®).

Values are given to show how the contribution of individual categories are calculated.

FIRST DIGIT - FLY ASH COMPONENT 1 isotropic carbon 2 anisotropic carbon 3 inertinite 4 glass 5 mullite 6 spinel 7 quartz 8 sulfide 9 oxides 1- uncombusted coal 2- petroleum coke 3- other organic 4- other inorganic

SECOND DIGIT - TEXTURAL CHARACTERISTIC 1 tenuisphere 2 crassisphere 3 tenuinetwork 4 crassinetwork 5 mixed porous 6 mixed dense 7 inertoid 8 fusinoid 9 solid 0 mineroid

Photomicrographs of selected types of fly ash, combined classification
by Hower and Mastalerz (from the collection of authors).

Reflected light, horizontal axis 240 µm.

37-inertinite in inertoid	60-spinel in mineroid
12-isotropic carbon in crassisphere	37-inertinite in inertoid
60-spinel in mineroid	39-inertinite in solid
11-isotropic carbon in tenuisphere	12-isotropic carbon in crassisphere
22-anisotropic carbon in crassisphere	14-isotropic carbon in crassinetrork
10-isotropic carbon in mineroid 40-glass in mineroid	25-anisotropic carbon in mixed porous 35-inertinite in mixed porous

Conclusions

1. Fly ash is a complex material and understanding its properties is important in its utilization.
2. Fly ash has a low carbon content for commercial purposes. Therefore, a classification is necessary that accounts for both the carbon and inorganic fraction of the fly ash.
3. The classification also must describe associations of components within individual particles in order to better predict the behavior of the fly ash.
4. The combined classification we are proposing attempts to address some of these aspects, although further modification may be needed depending upon individual purposes.

Acknowledgments

The authors would like to thank Indiana and Kentucky power plants for allowing us to collect samples.