Sample preparation and analysis
As secondary fuels are often soft and elastic materials, primary size reduction has to be effected with cutting and shearing forces such as are applied in cutting mills. Here, the same grinding procedure as for hard and brittle material has to be followed: Pre-cutting followed by sample division and fine grinding. Since the secondary fuels usually are very heterogeneous, the sampling process becomes evident for correct analytical results. RETSCH’s heavy-duty cutting mill SM 300 easily handles inhomogeneous sample materials with different breaking properties and is especially safe to operate. It is available with heavy-metal-free grinding tools for neutral-to-analysis sample preparation which makes it ideal for processing secondary fuels. With the three exchangeable cutting rotors and its high powered drive, textile and leather parts, plastic caps and ductile metal cans are cut up just as effectively as abrasive electronic scrap and non-metallic car-shredding parts. As there are limits to this cutting mechanism, i.e. steel and iron parts should be separated by a magnetic separator before the cutting process. The final fineness is mainly determined by replaceable bottom sieves with defined aperture sizes. Depending on the sample material, final particle sizes < 2 mm can be achieved. A cyclone increases the material throughput and helps to dissipate the grinding heat. For the fine grinding down to 200 μm, the Ultra-Centrifugal Mill ZM 200 from RETSCH is the ideal tool. It grinds extremely fast and thanks to the patented cassette system it is very easy to clean which allows for high sample throughput. The sample size again is determined by an exchangeable sieve. Experience shows that aperture size of 0.5 or 1 mm is fully adequate for obtaining analytical fineness. Very elastic material like rubber has to be embrittled with liquid nitrogen before pre cutting and final grinding to avoid melting of the sample.
Due to their different combustion behaviour (compared to inorganic material), secondary fuels cannot be processed in an induction furnace. The most suitable choice is a measurement with a resistance furnace at a temperature of 1,300°C. As cement factories use different types of fuels with different calorific values, it is essential to measure the carbon content as a perfect indicator for the calorific value of a material. Sulphur, on the other hand, is an important parameter for processing the combustion gas desulphurisation. Whereas coal has a rather high sulphur content of up to 5%, it is only around 0.02% for secondary fuels such as wood or biomass. With ELTRA elemental analyzers samples of greatly varying sulphur content can be examined precisely and reliably.
