Control Problems Waste Feeds in Fluid Beds (PDF - File size 71K)

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High concentrations of alkali salts, iron, and phosphorus in sludge can cause operating problems in fluid-bed incinerators. Understand how clinker and scale form, and how to prevent difficulties.

A major advantage of a high-temperature fluid bed (HTFB) is its ability to accept unsteady feed. Because a large quantity of heat is stored in the granular bed material, fluctuations in feed rate, composition, or water content are better tolerated in the HTFB than in other incinerators.

However, high concentrations of alkali salts, iron, and phosphorus in the sludge can cause problems. Because sodium and potassium chlorides have low melting points, large quantities of these materials in the sludge can lead to glassification of the bed media. The bed materials, usually sand, can become sticky, and agglomerates (or clinkers) can form, which results in segregation of the bed materials and eventually defluidization of the bed.

Furthermore, if the sludge contains iron and phosphorus, deposition can occur. If there are high concentrations of these elements, scale can accumulate on the freeboard wall, the roof of the reactor, the exhaust gas duct, and the heat exchanger.

The scale can obstruct the exhaust gas duct, which can result in excessive backpressure and lead to operating difficulties and shutdown of the plant. A potential solution to the agglomeration and scaling problems is to operate the fluid bed at temperatures below the melting points of the compounds or eutectics. Although this has worked in certain cases in the past, low-temperature operation will often produce hydrocarbon or carbon monoxide emissions above emission limits, and it will limit potential heat recovery. Further, such operation must be carefully monitored to prevent temperature excursions.

The purpose of this article is to shed some light on the problem of clinker and scale formation and to outline solutions, which are illustrated by examples from two operating plants.

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