The effect of using thermally dried sewage sludge as an alternative fuel on Portland cement clinker production

This study considers the potential of a sewage sludge previously dried using a novel drying process as an alternative fuel in the cement industry. A comprehensive characterisation study of the sewage sludge is presented. The calorific value of the sludge was around 8300 J/g, easily meeting cement industry requirements of 6250 J/g. To account for inorganic ashes that would remain from sewage sludge after burning, it was found that up to 14% of the raw materials used in cement manufacture could be replaced with sewage sludge, without changing important raw meal moduli. It was estimated that the sludge raw meal would reduce fossil fuel consumption in a modern cement kiln by around 70%. Clinkers made from sewage sludge containing raw meal had consistently lower free lime contents. However, a slight increase in belite formation was also noted due to the presence of P from the sewage sludge component.

► Dried sewage sludge is shown to be a potential hazardous waste.
► Organic component of sludge can reduce kiln fuel consumption up to 66%.
►  Inorganic component of sludge can reduce raw material consumption up to 14%.
► Sludge ashes decrease free lime content of clinker, an advantage.
► P from sludge favours extra belite formation at expense of alite, a disadvantage.