Alternative Fuels and Their Effects on Portland Cement
Abstract
The production of portland cement is a complex process involving several materials, complex systems, and temperatures on the order of 1500 °C. Non-renewable fossil fuels such as coal, petroleum coke, and natural gas have traditionally been used to generate these temperatures, but recent economic and environmental concerns have encouraged the cement industry to investigate alternative waste fuels as a viable replacement option. Forest trimmings, railway crossties, and liquid glycerin were the alternative fuels evaluated in this study. These industrial by-products were individually employed at a full-scale, cement production facility as a partial replacement of conventional fuels. Though the alternative fuels had lower heating value as compared to coal and petroleum coke, they supplied as much as 30 percent of the energy required for cement production when combined with waste tires and plastics. Cement was successfully produced during three- to four-day burn periods while maintaining target production rates. All primary emission (NOx, SO2, VOC, and CO) levels remained within the allowable limits set forth by the Alabama Department of Environmental Management. Carbon monoxide emissions were reduced by 40 percent during the forest trimmings trial burn relative to the baseline condition. VOC emissions were increased for all burns utilizing alternative fuels. Chemical analyses showed high variability in several process materials, which led to variations in cement compositions from each burn. However, many changes were of little practical significance and were primarily the result of variable plant conditions. Cement produced from the forest trimmings burn experienced low early-age but high long-term strength development. Cement produced from the glycerin burn experienced delayed setting times and low strengths at all ages, which may have been partially due to the large percentage of P2O5 in the glycerin fuel. Several additional physical properties of the cement produced from the alternative fuels also showed a significant difference relative to the baseline condition. However, plant conditions were highly variable throughout the study, which made it difficult to relate these effects to the addition of alternative fuels. Overall, the cement plant was able to utilize alternative fuels to produce high- quality, relatively consistent cement with little impact on emission levels and cement performance. Therefore, it is concluded from the study that forest trimmings, railway crossties, and liquid glycerin are all viable alternative fuel options for the production of portland cement pending consideration of local availability, associated costs, and compatibility with the local facility’s production operations.