|Portland cement manufacturing involves the combustion of fuels with various raw materials at approximately 2,700 °F (1,500 °C) to produce clinker. Fuel costs and environmental concerns have encouraged the cement industry to explore alternatives to the use of exclusive conventional fossil fuels. The key objective of using alternative fuels is to continue to produce high-quality cement while decreasing the use of conventional fuels and minimizing the impact on the environment.
In this study, portland cement was produced at a full-scale cement plant during 3-day trial burns of various alternative fuels along with coal. The fuel combinations investigated were: 1) coal only, 2) coal and scrap tires, 3) coal, scrap tires, and waste plastics, 4) coal, scrap tires, and broiler litter, 5) coal, scrap tires, and woodchips, and 6) coal, scrap tires, and switchgrass.
During these trial burns, the cement plant was able to maintain its target production rates while utilizing substantial replacement of conventional fuel with alternative fuels. Samples of raw materials, fuels, cement kiln dust, clinker, cement, and emissions were systematically collected. Chemical compositions, physical characteristics, and mechanical properties were obtained for all the samples collected.
Scrap tires and waste plastics were found to have higher heat values than coal. Although broiler litter, woodchips and switchgrass have heat values lower than coal, they burned well with no feed problems, and are available in abundance. Chemical analyses showed that the primary chemical compounds of the clinker, cement kiln dust and cement (i.e. Al2O3, CaO, Fe2O3, and SiO2) exhibited no changes of practical significance.
Various cement and concrete properties were determined for each trial burn. Tests of drying shrinkage, splitting tensile strength, and permeability of concrete showed no significant changes. The compressive strength of concrete from burns using alternative fuels showed an increase relative to the burn involving coal as the only fuel, though it is not possible to attribute this result exclusively to the use of these fuels. All the emission (NOx, CO, SO2 and Volatile organic compounds) levels were within the allowable limits set by Alabama Department of Environmental Management.
Overall, the cement plant was able to use alternative fuels to produce good quality cement with little impact on emissions levels. Therefore, it is concluded from the study described herein that scrap tires, waste plastics, broiler litter, woodchips and switchgrass are good potential alternative fuels for use during cement production. The final decision on the use of a specific alternative fuel will depend on the availability of the fuel, its cost, and its compatibility with the particular cement plant.