Particles, atmospheric removal

A device used to remove fly-ash from boiler exit-gases in order to reduce the atmospheric pollution load. It places an electric charge on the dust particle and removes the particle onto a collecting plate. 

Influence of the downstream plants. Up to now, we have regarded the coal gasification reactor with the waste heat recovery system as an isolated unit. In the event that the gas generated is intended to be used as fuel gas, for example in a combined power station, this approach is justified. If, however, the gas is to be used as synthesis gas, the effect of the downstream units must be taken into consideration. In such cases it is necessary to feed the gas to a CO shift conversion unit in order to obtain the C0/H2 ratio required for the synthesis process. Apart from gasification at atmospheric pressure, which requires an intermediate compression step, it has proved advisable to locate the CO shift conversion directly downstream of the gasification section. A stage in which dust particles are removed from the gas is situated between these two units. It is assumed that exergy losses do not occur in this unit. 

Aerosol particles are removed from the atmosphere by wet deposition through incorporation into precipitation either in or below clouds, and by dry deposition through sedimentation and impaction on or diffusion to soil, leaves, or the like. 

The principal processes that govern the concentration and properties of atmospheric aerosols are emission of aerosol particles and precursor gases, gas-to-particle conversion and other pertinent atmospheric chemical reactions, transport, and processes by which particles are removed from the atmosphere. There is a substantial hterature on the characterization of these processes from laboratory studies and field measurements (cf. Section 4.04.1), so only a brief overview is provided here. 

Debris injected into the troposphere circles the earth within one to two weeks, and particles are removed preferentially by rainfall but also by dry deposition onto the ground or water surface. The deposition occurs continuously and mostly within the latitude band of injection. Particles remain suspended in the troposphere for time periods of a few weeks. For high yield tests, with cloud formation extending into the stratosphere, debris may be suspended for time periods of one year or more. Atmospheric dispersion and interhemispheric exchange result in widespread global dispersal and deposition of the radionuclides produced in the test. 

Figure 4. Residence time of particles in seconds (left axis) and days (right axis) as a function of particle radius. The shaded areas represent estimates of the lifetimes made as follows 1, molecular or ionic clusters C, coagulation of particles P, removal by precipitation F, gravitational settling A, derived from spatial distribution of Aitken particles R, derived from the distribution of small radioactive particles. From Kreidenweis et al. (1999) in Atmospheric Chemistry and Global Change by Brassem et al. 1999 by Oxford University Press, Inc. Used by permission.

Wet deposition and dry deposition are the ultimate paths by which trace gases and particles are removed from the atmosphere. The relative importance of dry deposition, as compared with wet deposition, for removal of a particular species depends on the following factors ... 

Friedlander (11) has examined the effects of flocculation by Brownian diffusion and removal by sedimentation on the shape of the particle size distribution function as expressed by Equation 9. The examination is conceptual the predictions are consistent with some observations of atmospheric aerosols. For small particles, where flocculation by Brownian diffusion is predominant, p is predicted to be 2.5. For larger particles, where removal by settling occurs, p is predicted to be 4.75. Hunt (JO) has extended this analysis to include flocculation by fluid shear (velocity gradients) and by differential settling. For these processes, p is predicted to be 4 for flocculation by fluid shear and 4.5 when flocculation by differential settling predominates. These theoretical predictions are consistent with the range of values for p observed in aquatic systems. 

This ADS finally dissipates when the particles are removed from the atmosphere by dry and wet removal processes. Gravitational settling of large particles (>10 pm) occurs near the source within the first day of transport. Wet removal occurs sporadically throughout the 5-10 days lifetime of the remaining smaller size dust particles. 

As in a LOCA design basis accident (see Section 6.2.1.2.), the aerosol concentration in the containment atmosphere will also be reduced by the action of the containment spray system. Since larger aerosol particles are removed more readily by the spray droplets than are small particles, the removal efficiency to be expected in a severe accident is higher than in a design basis accident which has much lower aerosol concentrations in the containment atmosphere and, consequently, smaller dimensions of the aerosol particles (Pasedag et al., 1981). 

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