Agglomeration collection efficiency

Ammonia can be injected into the flue gas prior to the ESP to modify the catalyst resistivity and agglomerate the line particles and improve the units collecting efficiency. Ammonia is most effective at operating temperatures lower than 550°F. Injection rates are typically 10-50 ppm of which > 50% of the ammonia is adsorbed on the catalyst lines and the remainder of the injected portion shows up as ammonia slip in the flue gas. Ammonia is good for ESP performance when used in moderation. Excessive injection can cause a degradation in performance due to collecting plate build-up. 

In gas suspensions, where very fine particles have to be removed, US action involves agglomeration of particles in order to increase their size and, consequently, to improve the collection efficiency of conventional filters (e.g. electrostatic precipitators, cyclone separators). These filters, while effective for large particle separation, are inefficient for retaining particles smaller than 2.5 pm. Therefore, acoustic agglomeration provides a means for separating fine particles released from industrial, domestic or vehicle sources, which, analytically, constitutes an excellent method for sampling in environmental analysis. 

For a long time, natural agglomeration has helped to improve the collection efficiency of, for example, filters and cyclones. Knees, bends, restrictions (e.g., valve seats), and even build-up in the pipes and channels of the cleaning device caused turbulence, particle collisions, and the formation of aggregates that, as a whole, have the combined mass of all the particles adhering to each other and a correspondingly larger cross section. Both effects allow the removal of particles that, individually, would be too small (Fig. 8.3). 

The overall collection efficiency is affected by the selective separation of the dust according to particle size in accordance with a separation curve. For constant gas flow rate, the overall collection efficiency increases if the following influencing quantities increase settling velocity of the dust particles, agglomerating tendency of the dust, dust content of the gas to be dedusted (within limits). 

The efficiency of the cyclone separator is greater for large than for small particles, and it increases with the throughput until the point is reached where excessive turbulence is created. Figure 1.55 shows the efficiency of collection plotted against particle size for an experimental separator for which the theoretical cut occurs at about 10 ttm. It may be noted that an appreciable quantity of fine material is collected, largely as a result of agglomeration, and that some of the coarse material is lost with the result that a sharp cut is not obtained. 

Amberlite XAD-2. XAD-2 polymeric sorbent (25) is composed of single resin beads consisting of an agglomeration of numerous minute microspheres (Figure 2). A clue to the efficiency of collection while minimizing reactivity may be found in the structure of the resin. 

The collection of the pyrolysis oils is difficult due to their tendency to form aerosols and also due to the volatile nature of many of the oil constituents. As the aerosols agglomerate into larger droplets, they can be removed by cyclonic separators. However, the submicron aerosols cannot be efficiently collected by cyclonic or inertial techniques, and collection by impact of the aerosols due to their Brownian or random motion must be utilized. A coalescing filter is relatively porous, but it contains a large surface area for the aerosol particles to impact by Brownian motion as they are swept through by the pyrolysis gases. Once the aerosol droplets impact the filter fibers, they are captured and coalesce into large drops that can flow down the fibers and be collected. 

A new field of application for agglomeration with increasing importance has developed in environmental control. The laws for abatement of water and air pollution require that fine waste particles must be enlarged to improve efficiency of collection and to avoid renewed contamination. 

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