CEE Jan-Mar 2012

Useful heat Drying Pyrolysis Ignition Bum out Radiant energy Figure 2. Simplified operational sequence regarding the implementation of solid fuels with subsequert coke burnout (Baier 2009). concepts. In order to implement such concepts, naturally also cement plants come into focus. Frequently, however, this makes us forget that cement is a standardized mass product of a highly energy-efficient production process. Generally, "substitute fuel" still implies cheap "disposal" to the extent that even legislation talks about "co-combustion". But actually, these are highly complex physical-chemical conversion processes, which have not yet been entirely clarified even with regard to the combustion of coal. Simplifying the diffusion controlled combustion process (Figure 2), it can be described with drying, pyrolysis, ignition and burnout as well as the oxygen concentration at the fuel particle. With an increasing thermal substitution rate, this sequence dominates the entire cement-clinker process. Usually, Commercial wastes and particle fractions of high calorific value are separately or jointly pre-reduced and classified, removing metals, harmful constituents or PVC in the process. Solid substitute fuels with an average lump size of dmax. 250 or 80 mm to be fed via the kiln inlet or a calciner of a rotary kiln, therefore, are subject to a different sample preparation procedure and quality management than those fuels, which must be fed via the main burner (Figure 3). For a better burnout, these must be comminuted again into finer pieces (dmax. 25 or 3 mm) which are still several powers of ten compared to coal dust. If the processing should be finer yet, the processing reaches increasingly technical and economic limits. Normally, in the cement plant, ready to be burnt fuels are transported pneumatically to the respective firing point. Especially, if they are fed via the main burner, the inhomogeneous substitute fuel mixtures are differentiated again into individual particle fractions burning at different levels: While thin, large-area particles (e.g. packaging film 2 - 5001-Jm thick) are consumed by the flame floating, three-dimensional particles (e.g. hard plastic, rubber, wood , etc .) form the tip of the flame or even fly through it ending in the kiln charge material and leading to a reduced clinker burning or the formation of sulphur cycles. For these reasons, FueiTrack takes the approach of going over processed and quality-monitored fuel mixtures < 80 mm mechanically, while very large-sized substitute fuels > 80 to 250 mm with a known quality can be thermally processed . Mechanical processing for the use in a clinkering zone burner Due to the insulating effect, the geometry and other surface effects, the diffusions in fuel particles and the combustion speed proceed at different speeds. Mechanically, the Figure 3. Waste treatment and objectives of use must be co-ordinated with regard to process engineering and quality. 41