Approximately 63 million tons of fly ash are generated annually in the U.S. Of this amount, about 66% is discarded. Half of the utilized fly ash is used in concrete. Market forces generally limit the carbon content of fly ash for use in concrete to a maximum of 2%.

With the implementation of the Clean Air Act to reduce NOx emissions from coal-fired power plants, it is anticipated that fly ashes will contain significantly higher amounts of unburned carbon. As a result, it is anticipated that the increased carbon content of the average fly ash may result in less fly ash being used in concrete, and more fly ash will be discarded.

Table 2:
Target Composition of the Raw Feed

Analyte	Wt (%)
SiO2	13.74
Al2O3	3.91
Fe2O3	1.76
CaO	41.79
MgO	1.64
SO3	0.57
Na2O	0.32
K2O	0.75
Loss on Ignition (950°C)	35.08
Calculated Compounds*	Wt (%)
C3S	57
C2S	18
C3A	11
C4AF	8
* Cement notation: C = CaO; S = SiO2; A = Al2O3; and F = Fe2O3

Cement manufacture, by nature, is an energy-intensive process that consumes raw materials rich in silica, alumina, iron, and calcium. Fly ash is rich in all or some of these contents and is often used as a raw material. High-carbon fly ash has the additional benefits of being a source of fuel.

Recently, Construction Technology Laboratories (CTL) sought to utilize high-carbon fly ash as both a source of fuel and raw material. Using many different fly ashes and cement plant raw materials, CTL has successfully shown on the laboratory- and pilot-scales that some high-carbon fly ashes can be used simultaneously as a raw material and fuel in the manufacture of portland cement. Incorporation of the high-carbon fly ash also was observed to reduce the clinkering temperature and decrease the alkali content of the resulting cement.

Recently, this concept was demonstrated on a commercial scale. This article summarizes the preliminary analyses, demonstration, and results.

Cement plant

The cement plant was located in the Midwest and produced a typical Type I/II cement. The plant has a multi-stage preheater.

Fly ash

The high-carbon fly ash was a locally available dry fly ash (not ponded) with an average particle size of less than 45 microns. Representative fly ash samples were collected and analyzed to determine composition and fuel value. Analyses were used to optimize the fly ash content in the raw feed. Table 1 provides the oxide analyses of the fly ash.

Table 1:
Oxide Composition of Fly Ash

Analyte	Wt (%)
SiO2	42.95
Al2O3	15.46
Fe2O3	7.10
CaO	4.47
MgO	1.30
SO3	0.49
Na2O	1.88
K2O	2.50
Misc. minor elements	3.02
Carbon content	20.83

Differential scanning calorimetry (DSC) was performed to determine the heat content of fly ash. DSC results also were used to predict if the fly ash would increase the emission rate of volatile organic compounds (VOCs). Results indicated that the total heat content (calorific value) of the ash was in excess of 300 Btu/lb, and that no increase of VOCs were anticipated, as all organic compounds were of low volatility.

Blending and formulation

Prior to the demonstration, approximately 50 tons of the high-carbon fly ash was blended with an appropriate amount of raw materials (crushed limestone and a small amount of shale), ground into raw feed, and placed in a blending silo. The use of fly ash was maximized, essentially by replacing a majority of the shale. However, the chemistry of the limestone and remaining shale limited the fly ash content to 6% by weight of the total raw mix. Based on this utilization rate, the anticipated energy contribution from the fly ash was 57 kBtu per ton of clinker.

The composition of the demonstration raw feed (i.e., containing fly ash) was targeted to be identical to that of the normally produced raw feed. Table 2 presents the target composition of the raw feed. The lime saturation factor was 94.98, the silica ratio was 2.42, and the iron modulus was 2.22.

Kiln operation observations

Key processing, operational, and environmental parameters were observed before, during, and after the demonstration. Select observations are described below.

Preheater Temperatures and Pressures

No abnormal or adverse variation in the temperature profiles of the preheater were observed during the demonstration run. Likewise, there was no abnormal pressure variation within the preheaters as a sign of material build up leading to blockage during the demonstration. Typically, pressure increases could be an indication of buildups in the preheaters.

Table 3:
ASTM C 150 data for cements produced before and during demonstration

Cement	Compressive Strength (psi)			Air Content	Time of Set (minutes)		Early Stiffening Potential	Autoclave Expansion
	3-day	7-day	28-day	%	Initial	Final	Penetration (mm)	%
Before	3,750	4,490	5,880	7.8	85	180	10	0.12
During	3,650	4,190	6,080	8.1	105	210	50	0.07
After	3,560	4,390	5,670	6.4	90	195	8	0.05

Burning Zone Temperature

During the demonstration, the temperature of the burning zone increased significantly, because the kiln feed was better thermally prepared in the preheater primarily due to the combustion of residual carbon in the kiln feed. If the burning zone temperature had been reduced commensurately, addition fuel savings could have been realized.

Kiln Feed Rate

The feed rate into the preheater increased nearly 10% during the demonstration. After the demonstration, the rate returned to a level similar to that of before the demonstration. The increased production rate is directly attributed to the use of the high-carbon fly ash, which enhanced calcination of raw feed in the preheater prior to entry into the kiln.

One drawback of the increase in temperature was that the load in the clinker cooler increased. The increased burning zone temperature also resulted in an increase of the clinker exit temperature. After exiting the clinker cooler, clinker was still overly hot for the conveyer belts to tolerate, hence the clinker had to be stockpiled on the ground rather than placed directly into a storage silo. Modifications in the operating parameters are potential long-term solutions to this problem.

Fuel Rate

Fuel supply was reduced during the demonstration to accommodate the additional energy provided by the high-carbon in the fly ash. Energy savings due to fuel reduction alone averaged about 4% during the demonstration. This represents an approximate fuel savings of 91 kBtu per ton of clinker. A comparison of this figure with the anticipated benefit of 57 kBtu per ton attests to the efficiency of the heat recovery in actual operation.

General

During the demonstration, the kiln ran in an extremely smooth manner without any problems of material flow (blending and feeding), operations (feed flow through the preheater, blocking, plugging), temperature profiles, or kiln temperatures. No snowman formation in the cooler occurred; the product was satisfactory with respect to clinker formation and grinding characteristics; and no environmental problems occurred (i.e., CO, stack opacity, detached plumes). The opacity was essentially unchanged during the demonstration. From an operational standpoint, the demonstration was successful, in that production increased and fuel economy exceeded expectations.

Clinker and cement evaluation

Samples of clinker and cement produced before, during, and after the demonstration were analyzed to compare their composition and properties.

Clinker Characterization

Analytical and microscopical examinations of the clinkers revealed that clinker produced during the demonstration had a lower free lime, lower sulfate, and lower alkali levels than the clinkers produced before and after. These parameters reflect product improvements resulting from the use of the high-carbon fly ash. Clinker XRD patterns shown in Figure 1 (page 33) indicate the desired peaks of C₃S, C₂S, C₃A, C₄AF phases associated with portland cement clinker. The shape of these peaks indicated that the clinker phases had the desired morphology.

The microscopical examination confirms all of the major phases found in the XRD analyses and showed a normal formation and distribution of alite, belite, interstitial, and free lime and/or periclase. In general, minor differences were observed between phases formed in the demonstration and control clinkers.

The kiln feed was rather uniform, so any changes are attributable to corresponding changes in the kiln operation. Figure 2 presents photomicrographs of clinker produced before, during, and after the demonstration. Alite crystals in the clinker produced during the demonstration were small but well defined, as compared to the clinkers made before and after. This morphology most likely resulted from the fast heating rate followed by rapid cooling of the clinker. Such clinkers exhibit better strength as shown in the present case.

Cement Evaluation

Cements produced at the cement plant from the before, during, and after clinkers were tested in accordance with ASTM C 150, ”Standard Specification for Portland Cement.”

Analytical testing showed a large reduction in the in total alkalis of the cement produced from the demonstration clinker. The incorporation of fly ash into the raw feed reduced the alkali content of the cement by approximately 20%, without any equipment modifications. Such cement is favored in concrete making for reduced in alkali-silica reactivity and better durability characteristics.

Selected test results from ASTM C 150 physical requirements for all cements are presented in Table 3 (page 33).

Of interest are the comparative setting time results. The demonstration cement had slightly longer initial and final setting times. This can lead to reduced water demand in concrete, permitting the ready-mix operator to reduce the water content while producing concrete of the same consistency. This could increase the compressive strength of the resulting concrete.

All cements meet the standard physical requirements and the demonstration cement is comparable to the control cements. It should be mentioned that cements produced before and after the demonstration have the potential for moderate to severe flash set in part because of their high alkalis. However, cement produced during the demonstration has only a mild false set tendency. This most likely can reduce water demand and consequently improve the strength at equivalent consistency as discussed above.

Conclusions

The commercial demonstration has shown that high-carbon fly ash can effectively be used in making clinker and cement that are comparable or superior in characteristics to those prepared from the normal kiln feeds. Additionally, the use of high-carbon fly ash imparts many manufacturing benefits such as increased production and reduced fuel consumption.

Acknowledgment

The authors gratefully acknowledge the financial support for the demonstration by the Illinois Department of Commerce and Community Affairs through the Office of Coal Development and Marketing, and the Illinois Clean Coal Institute.

Javed I. Bhatty, John Gajda, and F. M. Miller are senior research scientist, senior engineer, and senior principal process scientist, respectively, at Construction Technology Laboratories, Inc., 5420 Old Orchard Road, Skokie, IL 60077; (+1) 847-965-7500.