Calciner firing of refuse derived fuels
Alternative or refuse derived fuels (RDF) are mainly fired in kiln burners, preheater or calciner kilns. Of the greatest importance in this regard is homogenous fuel quality consisting of two-dimensional parts. Alternative fuel for the kiln burner has to be free of disruptive fractions (e.g. metals, stones, glass), which might cause problems in the feeding equipment or the burner itself.
Alternative fuel for the main burner has to possess the following dimensions:
Kiln burner fuel characteristics
- Grain size: Calorific value: >4,500 kcal/kg
- Density: >60 kg/m3 Moistrure content: <15%
The calorific value must amount to at least 3,500 kcal/kg and preferably more than 4,500 kcal/kg.
Whatever the case, fuel combustion is largely dependent upon moisture, physical properties, particle sizes, chemical composition and homogeneity, which in secondary fuels fluctuate considerably.
Calciner fuel characteristics
- Grain size: <100mm
- Density: >100 kg/m3 Moistrure content: <20%
- Calorific value: >3,500 kcal/kg
The homogenous composition of chemical and physical parameters, especially particle sizes and distribution, is also imperative for a calciner. However, in the calciner, the calorific value of the fuels can be lower with the result that even CV fractions with a minimum of 2,500 kcal/kg can be used. Most significant is a homogenous grain size.
From a technical standpoint, the design principles used by suppliers of calciners and other equipment are similar or comparable. What are important, as they also apply to other feed points in the rotary kiln, are the product characteristics of the RDF, temperature and residence times. The residence times of the alternative fuels and the size of the feed can both increase in line with the dimensions of the calciner. However, the generalisations often made when discussing the utilisation of alternative in a calciner, such as feed sizes of 50-80mm, are only rarely correct. Basically, the particle size of the alternative fuels must be selected in such a way that a consistent burnout within the calciner is possible.
The fuel particles may neither be fed into the kiln inlet with the meal stream, nor conveyed with the gas stream into one of the upstream cyclones. Otherwise the cold stream of meal-rich suspension and the hot combustion flue gas stream run parallel to each other and owing to their differing densities, require a long residence time for mixing. In order to avoid problems such as accumulations and blockages, process deterioration can be prevented by either the careful design of the feed location and the injection technology, or increased pre-treatment of the alternative or RDF to produce a finely ground, homogenous grain size. Using such homogenous feeding, depending upon the chemical parameters, RDF substitution rates of above 65% have become standard. Moreover, substitution rates of above 90% have been reported for large kilns with a highly voluminous calciner design and long retention times.
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