Different types of plants show different flexibility in terms of accepting sludge as an input material. in addition, the thermal balance of an existing plant mayfurther restrict what may be acceptable in addition to the existing load. The recognized advantages of energy recovery from sludge include: t the high calorifıc value (similar to lignite) of dewatered sludge t the use of dewatered sludge as a carbon dioxide (C02) neutral substitute for primary fuels such as oil, gas and coal t the use of dewatered sludge is a 'sink' for pollutants such as heavy metals, toxic organic compounds and pharmaceutical residues, thus offering a potential disposal route for these substances provided the combustion plant has adequate flue gas cleaning t the potential, under certain circumstances, to utilize the inorganic residue from sludge incineration (incinerator ash), such as in cement or gravel. As a result, this can offer a complete solution to the problem of disposing of the WWTP sludge. But there are also concerns. lncineration ofmechanically dewatered sludge cake will produce zero or only a small amount of excess energy. lndeed, additional energy is often needed to maintain combustion of such sludge. While thermally dried sludge will have a calorific value comparable to that of lignite or light coal, the drying process also requires energy; thus the overall energy balance is not always clear and positive. Much depends on the heat source and the drying process applied. Use ofwaste heatfrom the incineration piant to dry the sludge is, however, benefıcial. Where does this leave us? it is evident that the drying of sludge plays an important role in both its utilization as a fertilizer and for the recovery ofenergy. Fully dried sludge offers a flexible product that can be channelled into either route. lts application depends to some extenton its dried solids (DS) content. The demands placed on the drying system are therefore critical and include: t high process stability t high mechanical reliability t high safety standards under all operating conditions t compliance with environmental legislation, such as emission limits t a product with properties suitable for a wide range of uses. Such demands are not easy to meet. The Andritz fluidized bed approach The sludge drying system developed by Andritz Fliess-bett Systeme GmbH has been successfully applied for more than 1 5 years in a growing number of installations. The system includes a fluidized bed dryer, the direct feeding system, the gas recycling system, cooling loop and product handling. The main elements are outlined below. The dryer Shows the operation of the fluidized bed dryer. it is essential that the fully dried, granular sludge particles are kept in suspension against gravity by a constant gas flow. Blowing the fluidizing gas uniformly across the entire area of the dryer generates a fluidized layer of dry, granular sludge particles that look much like a boiling liquid. The direct (sludge) feed system Unlike most other sludge drying systems, the Andritz system does not need to recycle dry granules in order to generate wet granules in a mixer, which would then subsequently be fed to a dryer. The direct feed system disintegrates the wet dewatered sludge cake pumped into the dryer into small particles. This method ofgranulation (or disintegration) makes it independent of the nature of the incoming sludge and/or cake moisture content. it is applied, forexample, in regional sludge drying centres where sludge is accepted from up to 40 differentWWTPs without blending or any other kind of homogenization. Gas recycling system The fluidized bed dryer operates in a closed inert gas loop . The fluidizing gas leaving the dryer carries fines and evaporated water from the fluidized bed dryer. The fines are separated in a cyclone and the evaporated water is condensed from thegas stream in a scrubber-condenser using a direct water spray. The gas is recycled to the dryer using a blower. The system generates small quantities of inert gas which are enough to keep the entire dryer loop permanently under a low-oxygen atmosphere. Oxygen concentrations of 1 %-3% measured in the dried sample gas are typical. The excess inertgasfrom the dryer 1 ENERJi DÜNYASI ŞUBAT 2007 ♦ _ 66--.J... =..:...:.=..:...:.:..:....::_:_:.:.:�=..::.::..::..:..:..:.:..============- "Enerjide Sürdürülebilirlik ve Küreselleşme: Verimlilik, Emisyonlar, Yeni PiyasaOluşumlan"
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