A new approach to the use of alternative fuels for firing of rotary kilns in cement plants consists in the use of ground pellets made from agglomerated fluff. Alternative fuels may originate from private or municipal treatment plants, but also from DSD sorting plants, DSD being a German waste management system. The individual components or material mixtures are not burned in bulk form as usual, but pelleted and then ground and blown into the main burner of the kiln together with coal dust. The advantage of this process lies in a much better proportioning of the fuels into the dust firing system. There is considerably less variation in the lower calorific value of pelleted alternative fuels as they have a lower moisture content and a more homogeneous composition than bulk alternative fuels. Hard pellets allow a specific grinding in hammer or roller mills, and thus the production of the required material fineness; the pulverized pellets exhibit a better ignition and combustion behaviour. This prevents a drop in flame temperatures in the rotary kiln, which ultimately leads to a constant clinker quality. Alternative fuels derived from the recycling of waste and recyclable fractions are among the materials which are most difficult to process. Depending on the season and the regions, fractions of municipal waste differ in terms of composition, particle sizes and the initial moisture content. Unsorted plastics are, for example, plastic waste from domestic waste (DSD), plastic waste mixed with paper and aluminium (e.g. beverage cartons) or carpet waste with a mixture of plastics in combination with other substances. Industrial and packaging waste is also mixed waste. Compared to municipal waste, however, it is generally dry and much more homogeneous. In most cases, recycling of these high-volume material mixtures is problematic in terms of storage, transport and bulk density. Pelleting offers the ideal solution for more economical storage and transport as well as a defined bulk density. In the future, it is also conceivable that decentralised, modular processing plants might be used which combine the necessary processing steps of pre-crushing, screening, drying/conditioning, pelleting and fine grinding directly in the cement plants. © 2020 Verlag Bau und Technik GmbH. All rights reserved.
