Interaction between capture mechanisms

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INTRODUCTION

This thesis reports the results of a study on the efficiency of a moving granular bed used for air pollution control. The study was prompted by the advantages which this type of gas cleaning apparatus offers in special applications where the conventional types show certain shortcomings. The objective is to find generally applicable and robust design methods for moving granular beds, including the effect of applied electrostatic fields on the efficiency of such beds. Equipment for the engineering control of particulate air pollution can be divided into four broad classes (Boegman 1979) Each of these main types has certain limitations.
Wet scrubbers for instance have a high energy consumption and reduce the gas temperature, thus limiting its atmospheric dispersion potential and creating water pollution and sludge disposal problems, while bag filters are limited in the handling of high temperature gases and « sticky » dusts (Cooper and Alley 1994). It is therefore difficult to apply equipment from the three last-mentioned classes to situations where dust has to be removed from gas at high temperature, such as would for example be required where gas from fluidized bed combustors is to be used in gas turbines (Andries 1993, Ahmadi and Smith 2002). Another problematic case is the removal of hygroscopic or water soluble salts, which tend to complicate cleaning of filter bags and cause corrosion problems in wet electrostatic precipitators or wet scrubbers. Such an application, for the filtration of particulate matter containing condensed fume particles of potassium and sodium salts from exhaust gas of a sinter plant in the South African iron and steel industrial sector (von Reiche et at. 1983, von Reiche et al. 1992) gave direct rise to the work described here. For such applications, the granular bed offers a possible solution. The granular bed utilises a granular medium, with the granules typically being two orders of magnitude or more larger than the particles which are to be removed. The gas stream that is to be cleaned moves through the granular bed in countercurrent, crosscurrent or cocurrent mode and the particles are removed from the gas by attachment to the granules. The bed acts as a depth filter rather than a surface filter (Rajagopalan and Tien 1979). For use on an industrial scale, the filter medium must be replaced or cleaned.
This can be done by replacement with new medium on an intermittent basis (Arras and Berz 1972) but more conveniently by removing the medium, cleaning it off-line and returning it to use in a continuous manner (Combustion Power Company 1979, Abatzoglu et at. 2002). The filtration of liquids and gases through a bed of granular medium developed as an industrial process during the 19th century. A patent for the clarification of molasses in sugar factories was for example granted in 1888 (Arras and Berz op cit.). Early applications to gas filtration are summarized by Squires and Pfeffer (1970) and Tien (1989). A static bed was mostly used, being replaced when the pressure drop became excessive.
Large-scale commercial applications followed the introduction of in situ cleaning methods, for example by mechanical shaking (Berz and Maus 1977) or by reverse flow together with rotating rakes (Hermann 1973). Although this arrangement required cyclical operation, it was used on large scale for the off-gas of cement clinker coolers, lime and gypsum kilns, zinc furnaces, anode coke production and for ammonium chloride fume (Hoffmann and Brachthauser 1988) The first continuously operated granular bed filters were of the horizontal type, operated in the fluidized bed mode. Cook et at. (1971) and Rush et at. (1973) describe filters with a granular alumina medium treating emissions from electrolytic aluminium cells. They were fed mechanically and overflowed directly into the cells.

CONTENT: :

• CHAPTER 1: Scope of the thesis
  • Granular bed description
  • Historical overview
  • 1.3.1 Static granular beds
  • 1.3.2 Continuous granular beds
  • 1.3.3 Electrostatic augmentation
  • 1.3.4 Particle charging. This study. References
• CHAPTER 2:
  • Introduction
  • Capture mechanisms
  • 2.2.1 Gravity
  • 2.2.2 Direct interception
  • 2.2.3 Inertial interception
  • 2.2.4 Diffusion
  • 2.2.5 London- or van der Waals forces
  • 2.2.6 Electrostatic capture
• CHAPTER 2:
  • Interaction between capture mechanisms
  • Particle retention
  • Bed loading
  • Particle charging
  • References
  • Introduction
  • Bed capture efficiency derived from single granule efficiency
  • Single granule capture efficiencies
  • 3.3.1 Direct interception
  • 3.3.2 Inertial interception or impaction
  • 3.3.3 Diffusion
  • 3.3.4 The effect of granule sphericity
  • 3.3.5 The effect of particle bounce on impaction efficiency
  • 3.3.6 Electrostatic mechanisms. Particle release mechanisms
  • 3.4.1 Drag forces due to gas flow
  • 3.4.2 Effect of bed movement
  • Particle charging
  • The effect of bed loading
  • The combined use of the equations
  • References
• CHAPTER 4:
  • 4.1.1 Apparatus
  • 4.1.1 Bed configurations
  • 4.1.2 Electrostatic augmentation
  • 4.1.3 Dust feed
  • 4.2 Test dusts
  • Measurement of particle concentration
  • Experimental procedure
  • 4.4.1 Static bed
  • 4.4.2 Moving bed
  • References
  • Static bed
• CHAPTER 5:
  • 5.1.1 Confirmation of basic theory
  • 5.1.2 The influence of bed loading. Moving bed
  • 5.2.1 The influence of bed movement
  • 5.2.2 The influence of the pre-charger
  • 5.2.3 The influence of bed movement with the pre-charger
  • References
  • Rationale for use of precharger
  • Conclusions on bed load parameters
  • Conclusions on re-entrainment of particles due to bed movement
  • Design procedure outline
  • Further research recommended

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