Use of High-Carbon Illinois Coal Fly Ash in Portland Cement Manufacturing

FINAL TECHNICAL REPORT

November 1, 1998, through October 31, 1999

Project Title: USE OF HIGH-CARBON ILLINOIS COAL FLY ASH IN PORTLAND CEMENT MANUFACTURING

ICCI Project Number: 98-1/3.1C-2

Principal Investigator: Javed I. Bhatty, Construction Technology Laboratories, Inc.

Other Investigators: F.M. Miller and John Gajda, Construction Technology Laboratories, Inc.

ICCI Project Manager: Ronald H. Carty, ICCI

ABSTRACT

A two-phase project on the use of high-carbon Illinois coal fly ash as a raw feed ingredient for producing portland cement was undertaken. Such fly ashes can replace shale and clay in the raw feed, and the carbon will provide supplementary fuel value.

During Phase I, several Illinois fly ash samples were collected and characterized. Three fly ashes, owing to their chemical compositions and high carbon contents, were selected for testing as a raw feed component in cement manufacture. Fly ashes were procured from the City Water Light and Power, Vermilion, and Grand Tower power plants. Raw materials and kiln feeds from the Lone Star, Illinois Cement, Dixon-Marquette, and Lafarge cement plants were obtained for parallel testing and comparison. These four cement plants represent all of the cement manufacturing capacity in Illinois.

Based on chemical analyses and relative proximity, fly ashes were paired with cement plants. Seven mix formulations incorporating fly ash with the cement plant raw materials were prepared and burned to form clinker. Clinkers were subsequently analyzed by chemical and physical methods. Tests confirmed that clinkers produced from the fly ash-containing mix formulations were comparable in characteristics to those produced directly from the respective kiln feeds. Results suggest that the fly ash can potentially improve the burnability of the kiln feed.

During Phase II, one optimum fly ash raw mix formulation for each cement plant, along with the companion kiln feed, was burned in CTL's pilot scale rotary kiln to form clinker. Clinkers were subsequently analyzed by chemical and physical methods. Tests confirmed that the clinkers produced from the fly ash-containing raw mixes were comparable in characteristics to those produced directly from the respective kiln feeds. Cements were produced from clinkers for evaluation in accordance with selected ASTM C 150 specifications. Results indicated that cements made from fly ash-containing clinkers meet the requirements of an ASTM C 150 Type I Portland cement, and had improved compressive strength development in comparison to control clinkers. Estimated energy savings from incorporation of Illinois high carbon fly ash in the kiln feed ranged from 122,100 to 298,940 Btu per ton of clinker, a 3.6 to 8.8% savings.

EXECUTIVE SUMMARY

This project consisted of a two-phase study to evaluate the potential for using Illinois high carbon fly ash as a partial raw feed and fuel substitute. During Phase I, samples of high-carbon fly ashes from Illinois power plants, including those in close proximity to the Illinois cement plants were collected. Several fly ashes were received and tested for their chemistry and compound compositions (including the unburned carbon content based upon the loss-on-ignition and thermal analyses).

Likewise, typical raw materials (limestone, shale, clay, sand, etc.) from the four Illinois cement plants- Lone Star at Oglesby, Dixon-Marquette at Dixon, Illinois at LaSalle, and Lafarge at Joppa, were also collected. Regular production kiln feeds from these plants were also obtained as the control or target materials.

The fly ash samples and the raw materials including the target kiln feeds were analyzed for their contents of SiO2, Al2O3, Fe2O3, CaO, MgO, alkalies, etc., and loss-on-ignition (LOI). The fly ashes were examined for their carbon content to determine the potential fuel value. Three fly ashes were selected based on their chemical composition, carbon contents, and lack of volatile release at low temperature (to avoid emission problems). These were the ashes obtained from the CWLP, Vermilion, and Grand Tower power plants.

Seven cement raw mixes were formulated using the raw materials with the fly ashes. The use of fly ash was maximized as the principle source of alumina, silica, and iron. Insofar as possible within compositional constraints, the mixes were to consist of fly ash and limestone only. Minor amounts of corrective materials were added to the raw mix, only if necessary to adjust the lime saturation factor, silica modulus, and other burnability factors. Fly ashes replaced between 4.7 and 9.0 percent of the current raw mix of the cement plant kiln feeds.

Each raw mixture was press-pelletized and subjected to firing at temperatures sufficient to complete clinkering. The clinkering temperature was typically 1450°C with a residence time of approximately one-hour. For burnability tests, two additional temperatures of 1350°C and 1400°C were included. The clinkers were tested for the appropriateness of the major phases by X-ray diffraction (XRD), X-ray fluorescence (XRF), and optical microscopy. The Franke free-lime method was used to determine free lime in the clinkers. The data were compared and contrasted with those of the target clinkers made directly from the kiln feed.

The tests confirmed that the clinkers produced from the fly ash containing mix formulations were comparable in characteristics to those produced directly from the respective kiln feeds. The tests have also indicated significant energy benefits of the carbon contents in fly ashes. For example, a 5% replacement by the CWLP fly ash would contribute nearly 50,000 Btu/ton of clinker; a 9% Vermilion fly ash replacement would supplement over 115,000 Btu/ton clinker, and a 9% replacement by Grand Tower fly ash would give nearly 275,000 Btu/ton clinker of produced. Furthermore, there was no evidence of any adverse effects of carbon in fly ash on the release of volatile matter when tested for thermal behavior by differential scanning calorimetery (DSC). Therefore, no emission problems are expected from the incorporation of these high-carbon fly ashes.

During Phase II, one fly ash raw mix formulation was selected for each of the Illinois cement plants. Due to a material shortage, additional CWLP fly ash was procured. All raw materials were reanalyzed and the four fly ash-containing raw mixes were formulated. The fly ash-containing raw mixes along with companion kiln feeds from the cement plants (controls) were pelletized and burned in CTL's pilot scale rotary kiln. The burning temperature was in excess of 1400°C, with an estimated residence time of 40 minutes. Free lime content was checked periodically through XRD analyses. Due to the limitations of the kiln, it was not possible to measure the fuel savings due to the fly ash.

Resulting clinkers were analyzed by XRF, XRD, and optical microscopy. Fly ash and control clinkers had nearly identical chemical constituents. Optical microscopy showed comparable clinker microstructure, evidencing comparable quality. The grindability of the clinkers was nearly identical.

All cements made from the fly ash-containing and control clinkers complied with the requirements for a Type I Portland cement, as specified in ASTM C 150, "Standard Specification for Portland Cement". Additionally, cement produced from fly ash containing clinkers showed improved compressive strength development over those of the companion control clinkers.

The estimated energy savings due to the incorporation of Illinois high-carbon fly ash in the kiln feeds, as burned in the pilot scale production of clinker ranged from 122,100 to 298,940 Btu per ton of clinker. Given that the average preheater/precalciner cement plant consumes approximately 3.4 MBtu of fuel energy per ton of clinker produced, the estimated energy savings range from 3.6 to 8.8 percent. This represents a significant energy savings.