Synthetic aggregates from coal-fired fluidized-bed combustion residues

Synthetic aggregates from coal-fired fluidized-bed combustion residues

Coal Science J.A. Pajares and J.M.D. Tasc6n (Editors) 9 1995 Elsevier Science B.V. All rights reserved. 1987 S y n t h e t i c a g g r e g a t e s ...

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Coal Science

J.A. Pajares and J.M.D. Tasc6n (Editors) 9 1995 Elsevier Science B.V. All rights reserved.


S y n t h e t i c a g g r e g a t e s from coal-fired fluidized-bed c o m b u s t i o n residues R. A. Winschel, M. M. Wu, and F. P. Burke CONSOL Inc., 4000 Brownsville Rd., Library, PA 15129, USA

1. INTRODUCTION The combustion of coal in a fluidized bed of limestone or dolomite is one method for controlling the emissions of sulfur dioxide (SO2) that may result from the production of power or steam from coal. In a fluidized-bed combustor (FBC), the coal sulfur is captured as CaS04, and a solid by-product consisting of CaSO4, coal ash, and unreacted sorbent (CaO) is produced. This FBC ash must be utilized, or disposed of at considerable expense. The cementitious property of FBC ash makes it possible to use it in the construction industry and the high volumes of materials used in construction make this an attractive potential market for ash. This paper will describe the development, preparation, and properties of synthetic aggregates produced from FBC ash for highway construction. The aggregates are produced in a three-step process consisting of hydration, disc pelletization, and curing. The durability of the synthetic aggregates is of particular concern; developments leading to increased durability of the aggregates will be described. 2. SUMMARY Synthetic aggregates were produced from the ash of a commercial coal-fired FBC. Even though the aggregates meet engineering specifications as Class A aggregate, they had poor durability in freeze/thaw tests. A cause of the poor durability was found to be the slow conversion of anhydrite to gypsum. Diluting the FBC ash with pulverized coal (p.c.) fly ash greatly improves the synthetic aggregate durability. The synthetic aggregates produced are strong, abrasion resistant and durable, and they meet engineering specifications for use as aggregates in portland cement and asphalt concretes. Preliminary economics suggest that it should be feasible to produce and market the aggregates, if avoided waste disposal costs are considered. 3. EXPERIMENTAL

The FBC ash was produced at a coal-fired circulating FBC unit (182,000 kg/h steam) at a cogeneration plant buming Pittsburgh seam coal. The material used (Table 1) was taken from the silo that contains both the fly ash and the bottom ash of the FBC. The pulverized coal (p.c.) fly ash was produced at an electric generating station that bums Pittsburgh seam coal. The analyses of these materials appear in Table 1.

1988 Table 1 Analysis of FBC by-product and p.c. fly ash Date Sampled: Moisture, wt % as rec'd Ultimate Analysis, wt % dry Carbon Hydrogen Nitrogen Sulfur, total Ash @ 399 ~ Maior Element, wt % dry SiO2 Al203 TiO2 Fe203 CaO MgO Na20 K20 p205

SOa Ca/S Mol. Ratio Lime Index (a), wt %

9/2/93 0.08

FBC Ash 3/14/94 0.08



p.c. Fly Ash 0.22

3.48 0.07 0.05 6.00 94.18

3.45 0.09 0.02 5.58 90.2

3.33 0.00 0.06 6.05 94.52

1.73 <0.01 < 0.01 0.41 97.23

12.03 5.30 0.22 4.36 47.81 7.19 0.24 0.69

13.85 5.75 0.22 4.48 45.08 8.85 0.25

14.10 6.52 0.27 5.19 41.02 9.09 0.38

47.43 21.38 1.02 21.42 2.85 0.89 0.40


0.88 0.15

1.18 0.15

2.03 0.37

15.41 4.43 28.0

14.48 4.45 25.7

17.14 3.87 21.3

1.06 -

(a) Based on CaO

Synthetic aggregates were produced semi-continuously in 110 kg batches from the FBC ash (or blends of FBC ash and p.c. fly ash) by a three-step process. The dry ash was first hydrated by mixing it with the appropriate amount of water in a Littleford Brothers LM-130 batch mixer. The hydrated ash was pelletized with an additional amount of water on a 91 cm ID rotary disc pelletizer (total water added during hydration and pelletization is about 18 wt % of the dry ash). The pellets were cured for 24 h at - 8 2 ~ and 90% relative humidity in a 208 L curing vessel. The size distribution of the pellets can be selected by varying the pelletizing conditions; most of the pellets were between 1.5 cm and 4 mesh for this work. The pellets are generally spherical in shape but they can be crushed to produce angular shapes for use in asphalt concrete. The pellets were evaluated as synthetic aggregates for use in portland cement and asphalt concrete by a variety of determinations and tests. Physical determinations included grain size distribution and crush strength (mean of 30 reported, typical standard deviation is 10 kg). American Society for Testing and Materials (ASTM) tests included bulk specific gravity (ASTM C127), LA abrasion index (ASTM C131), soundness index (ASTM C88-83), unit weight (ASTM C29), and water absorption (ASTM C127). Freeze/thaw tests were conducted in which the aggregates are sized into 1.3 x 0.95 cm and 0.95 c m x 4 mesh fractions, immersed in water for one day, temperature-cycled 50 or more times from -18 to 10 ~ over the course of seven days, then sized again to determine particle size degradation. The results of these tests were compared with American Society of State Highway Transportation Officers (AASHTO) specification M-283 for Class A aggregates.


Three samples of FBC by-product were taken over the course of 13 months to evaluate the variability of the material. During this time, the plant burned coals with sulfur contents from 1.4% to about 2.5%. As shown in Table 1, the properties of the by-product were fairly constant; the major changes are small variations in the Ca and S coments. Synthetic aggregates of essentially equivalent quality were readily produced from all three samples of by-product. The small variations in the properties of the three samples required small changes in the water addition rate. The physical properties of various batches of synthetic aggregates produced from FBC by-product are shown in Table 2. Shown for comparison are the properties of two commonly used natural Class A aggregates and AASHTO specifications for Class A aggregates. The aggregates made from 100% FBC by-product meet Class A specifications; however, testing over the course of one year showed that their soundness index degraded with storage time. The aggregates produced with blended fly ash did not suffer from this problem; therefore, later work was limited to FBC by-product/fly ash blends. Synthetic aggregates were produced from five blends of FBC ash and p.c. fly ash that ranged from 90/10 to 50/50 FBC ash/fly ash. All batches easily pass the LA abrasion index specification. The 90/10, 75/25 and 57.5/42.5 blends have crush strengths greater than 90 kg and all meet the soundness index specification; the 50/50 blend has poorer strength and soundness index. All batches that were tested meet the unit weight specification. Unit weight is a function of specific gravity and particle size distribution. The specific gravities of the aggregates from all batches are similar and, since the size distribution can be controlled by pelletizer operating conditions, any of the blends could meet the unit weight specification. However, it is apparent that both the specific gravity and unit weight of the synthetic aggregates are substantially lower than those of the natural aggregates. Freeze/thaw tests showed that, even though the 90/10 aggregates had a low soundness index, their durability was poor. Thermogravimetric analysis and X-ray diffraction analysis indicated that the slow conversion of anhydrite (CaSO4) to gypsum (CaSO 4 92H20) was one cause of the poor durability of the synthetic aggregates. The addition of fly ash improves the durability of the aggregates, presumably by diluting the gypsum precursor and by providing aluminum species that can react with anydrite to produce ettringite (Ca6A12(SO4)3(OH)I 2. 26H20), thus reducing the formation of gypsum. The freeze/thaw tests show that the 65/35 blend has the best durability. On the basis of durability and the other properties, the 65/35 blend was selected for further development. A longer curing time (48 h) reduced the water absorption of the 65/35 aggregate but was ineffective at further improving the durability. When used in bituminous concrete, aggregates are coated with asphalt. The aggregates produced from the 65/35 blend were coated with 5% AC-20 asphalt and subjected to the freeze/thaw test. The coating substantially reduces the freeze/thaw degradation of the aggregates (Table 2). Thus, when used in bituminous concrete, the 65/35 aggregates are strong, abrasion resistant, and durable and they meet all specifications for use as Class A aggregate. Class A aggregate prices are dependent on location. However, a typical price in the mid-Atlantic region of the U.S. is $5.50 to $7.50/tonne. Pelletization costs are

1990 estimated to be $8 to $11/tonne. Therefore, it appears that production of synthetic aggregates from an FBC/fly ash blend is economically feasible if waste disposal costs minus shipping costs are greater than $5.50/tonne. In the U.S., combustion by-product disposal costs for a new facility range from $9 to $18/tonne. z If the entire FBC byproduct generated at the plant in question (39,000 tonne/y) were blended with 35% fly ash and the appropriate amount of water, the yield of aggregates would be 51,000 tonnes/y. The market for aggregates is so large (over 1 billion tonnes/y for crushed stone alone in the U.S.2), that this additional production would have little impact on the market price of aggregates. Table 2 Properties and specifications of aggregates FBC Ash/Fly Ash

I Description





LA abrasion, % wear Soundneas,% loss (a) Unit wt, kg/m 3 Crush strength, kg Water absorption, % Specific gravity

27 7 1153 86 3.9 1.98

23 0 1137 95 4.1 2.01

25 6 1233 95 3.8 2.04

19 12 91 3.7 2.00

86 71

54 29

23 18

Freeze/thaw degrad., % 1.3 x 0.95 cm fraction 0.95 cm x 4 mesh fractk~

i 65135 (b)

65/35 (c)



AASHTO M-283 specs


28 8 1554 141 1.7 2.52

21 8 1650 92 0.5 2.67

40, max 12, max 1120, rain

74 3.6 2.02

47 100 1.8 2.19

37 17

40 25

8 4

2 2

1 2

57.5/ 42.5


18 39

22 28

94 3.3 2.06

43 20

(a) after 5 cycles with sodium sulfate (b) 48 hr cure

(c) 5% a s p ~


5. ACKNOWLEDGMF2~ This work was partially supported by the Ohio Coal Development Office under Grant Agreement No. CDO/D-902-9. REFERENCES 1. U.S. Department of Energy, Office of Fossil Energy, Morgantown Technology Center, "Report to Congress, Barriers to the Increased Utilization of Coal Combustion/ Desulfurization By Products by Governmental and Commercial Sectors", July 1994. 2. Terpodei, V. V. "Construction Aggregates", Mining Engineering June 1994, p. 530.