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Product and Process Effects of Alternative Solid Fuels Usage at a Portland Cement Manufacturing Site

Steve R. Duke1, Srikanth Akkapeddi2, Dustin Swart2, Anton Schindler2, and Don Stafford3. (1) Chemical Engineering, Auburn University, 212 Ross Hall, Auburn University, AL 36849-5127, (2) Civil Engineering, Auburn University, 221 Harbert Engineering Center, Auburn University, AL 36849-5127, (3) US East Business Unit, Lafarge North America, 12735 Morris Road, Extension, Suite 300, Alpharetta, GA 30004

Portland cement manufacturing involves the combustion of solid fuels along with raw materials to produce clinker. Temperatures of 1,500 C are needed to carry out reactions for cement chemistry. Fuel costs and environmental impact concerns encourage the cement industry to explore alternatives to the use of fossil fuels. Potential alternative fuels must provide adequate and economical energy but they must also result in products of combustion that have negligible impact on cement quality.

Results will be presented from a multiyear study (supported by the Department of Energy) in which portland cement was produced at a full-scale cement plant with several 3-day trial burns of various alternative fuels along with coal. The fuels combinations investigated were: 1) coal only, 2) coal and tires, 3) coal, tires, and waste plastics, 4) coal, tires, and broiler litter, 5) coal, tires and switch grass, and 6) coal, tires, and wood chips. Samples of raw materials, fuels, cement kiln dust, clinker, cement, and emissions were collected systematically during each trial burn. Chemical compositions, physical characteristics, and mechanical properties were obtained for the samples. Chemical analyses showed that the primary chemical compounds of the cements exhibited no significant changes during the trial burn periods. Cement and concrete properties for each trial burn showed no significant changes in drying shrinkage development, splitting tensile strength, and permeability. The compressive strength of concrete from waste plastics and broiler litter burns showed an increase relative to the other burns, though it is not possible to attribute this result exclusively to the use of these fuels. The NOx emissions were the lowest for the coal only and broiler litter trial burns. The VOC emissions were the lowest for the coal only and waste plastics burns. The CO emissions of the cement plant were lower for all burns that included alternative fuels.

The trial burns also revealed a number of issues and technologies which are now being addressed to enable the large scale use of alternative solid fuels for cement manufacturing. Among these are alternative fuels handling facilities and best practices and process operation and control limitations that cap the substitution rates.

The burn trials showed that all of the alternative fuels had high potential as a coal substitute because they exhibited minimal impact on product quality and in some cases had positive impact on emissions.