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Energetic hammering on the bed

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High temperature degradation by erosion-corrosion in bubbling fluidized bed combustors. Heat-exchanger tubes in fluidized bed combustors FBCs often suffer material loss due to combined corrosion and erosion.

Most severe damage is believed to be caused by the impact of dense packets of bed material on the lower parts of the tubes. In order to understand this phenomenon, a unique laboratory test rig at Berkeley was designed to simulate the particle hammering Energetic hammering on the bed between in-bed particles and tubes in bubbling fluidized bed combustors. In this design, a rod shaped specimen is actuated a short distance within a partially fluidized bed.

The downward specimen motion is controlled to produce similar frequencies, velocities and impact forces as those experienced by the impacting particle aggregates in practical systems. Room temperature studies have shown that the degradation mechanism is a three-body abrasion process. This paper describes the characteristics of this test rig, reviews results at elevated Energetic hammering on the bed and compares them to field experience. At higher temperatures, deposits of the bed material on tube surfaces can act as a protective layer.

The deposition depended strongly on the type of bed material, the degree of tube surface oxidation and the tube and bed temperatures.

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With HCl present in the bed, wastage was increased due to enhanced oxidation and reduced oxide scale adherence. Fluidized bed combustors, oxidation, erosion, HCl, carbon steel.

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The fluidized bed combustion of coal or other fuels as an energy production technology has received widespread usage. The excellent solids mixing in these reactors gives a number of advantages over conventional combustion techniques.

The results are more efficient combustion at a lower temperature so as to reduce NO x formation and the ability to reduce sulfur emissions by adding into the bed a sorbent material, such as limestone CaCO 3.

One of the approaches to these systems is a bubbling fluidized bed, where the gas velocity in the bed is only a little greater than the minimum velocity required to fluidize it; the excess gas flow appears as voids resembling bubbles in a bed which is relatively dense.

Despite the widespread usage of fluidized bed combustors FBCsthere is a problem in that both the walls of the combustor and the in-bed heat exchanger tubes in bubbling beds may suffer loss of section. The term 'wastage' is commonly used for this phenomenon, which is generally accepted to be a result of erosion or abrasive wear that may be accelerated by oxidation or high-temperature corrosion The wastage of heat-exchanger tubes in Energetic hammering on the bed FBCs has been an area of interest for many years.

The practical problem is that wear rates are very variable from unit to unit for reasons that are not clear. Furthermore, small operational changes often can cause large changes in wear behavior, again for reasons that have proved elusive.

The use of Cl-containing coals, for example, is one key area with conflicting plant and laboratory results To resolve these issues, it is desirable to have some knowledge of the mechanisms of tube wastage, and to identify the relative importance of various factors in a FBC environment that may contribute to the wastage process.

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To achieve this, a special test rig was constructed at Berkeley to study the erosion and corrosion phenomena in a simulated FBC environment where the conditions are known and controllable. The Berkeley test rig was built based on the belief Energetic hammering on the bed the energetic process within the bed capable of generating the kind of damage observed in practice is the impact of dense packets of bed material on the lower parts of the tubes These packets may be the wakes of rising bubbles within the bed.

They may also result from the cooperative motion of large volumes of the bed generating a process resembling cavitation beneath the tubes.

The wakes strike the bottoms of the tubes with a considerable force, which is easily detected and measured by appropriate sensorsat least for beds operating at atmospheric pressures.

Cold model studies have shown that upon bubble collapse, a block of essentially unfluidized bed particles is thrown against the tube bottom, and then drops away or slides upward across its surfaces To achieve such conditions in a laboratory, a horizontally held cylindrical specimen was allowed to actuate a short distance vertically within a minimally fluidized bedThe downward specimen motion is controlled to produce similar frequencies 0.

Typically, the specimen is in motion for between 20 and 40 ms. Between these periods of high particle loading, a relatively long period of inaction follows. This simulates the intermittent nature of the aggregate impacts found in practical situations. The first rig was built to operate at room temperature. Results from these earlier Energetic hammering on the bed verified the similarity in wastage behavior obtained from the actuation process of the rig and from BFBC cold tests The mechanism of wastage was identified to be that of low cycle fatigue resulting from three body abrasion The ability to internally cool the specimen to achieve a temperature difference between the bed and the specimen surface was later added to the high temperature rig, by passing water, air or a mixture of the two through the inside of the tube specimen Details of the coolant supply and temperature measurements of the tube outer and inner surface can be found in reference These fixtures were later taken down and the apparatus was further modified to add HCl gas to the fluidizing air The purpose of this paper is to summarize the results that have Energetic hammering on the bed obtained from the high temperature rig simulations.

The main focus is placed on tests performed at different temperatures with and without the presence of HCl in the bed. Emphases are placed on the effect of temperature, bed condition and bed material deposits on wastage.

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A schematic drawing of the high temperature rig that is presently in operation is shown in Fig. A single specimen is held horizontally in a stirrup that is immersed in a partially fluidized bed of sand particles.