Collection Efficiency

Collection efficiencies for wet scrubbers are highly variable. Most conventional scrubbers can achieve high collection efficiencies for particles greater than 1.0 :m in diameter, however they are generally ineffective collection devices for submicrometer (<1 μm) particles. Some unconventional scrubbers, such as condensation and charged, are capable of high collection efficiencies, even for submicrometer particles. Collection efficiencies for conventional scrubbers depend on operating factors such as particle size distribution, inlet dust loading, and energy input. Figure 1 provides scrubber efficiency curves for coal and oil combustion, wood combustion, and coke production. Table 1 presents the PM-10 and PM-2.5 collection efficiencies.

Conventional scrubbers rely almost exclusively on inertial impaction for PM collection. As discussed above, scrubber efficiency that relies on inertial impaction collection mechanisms will increase as particle size increases. Therefore, collection efficiency for small particles (<1 μm) are expected to be low for these scrubbers. The efficiency of scrubbers that rely on inertial impaction can be improved, however, by increasing the relative velocity between the PM and the liquid droplets. Increasing velocity will result in more momentum for all PM, enabling smaller particles to be collected by impaction. This can be accomplished in most scrubbers by increasing the gas stream velocity. Unfortunately, increasing the gas velocity will also increase the pressure drop, energy demand, and operating costs for the scrubber.

Another factor which contributes to low scrubber efficiency for small particles is short residence times. Typically, a particle is in the contact zone of a scrubber for only a few seconds. This is sufficient time to collect large particles that are affected by impaction mechanisms. However, since submicrometer particles are most effectively collected by diffusion mechanisms that depend on the random motion of the particles, sufficient time in the contact zone is needed for this mechanism to be effective. Consequently, increasing the gas residence time should also increase the particle/liquid contact time and the collection efficiency for small particles.

An important relationship between inlet dust concentration (loading) and collection efficiency for fine PM in scrubbers has been recently found. Collection efficiency for scrubbers has been found to be directly proportional to the inlet dust concentration. That is, efficiency will increase with increasing dust loading. This suggests that scrubber removal efficiency is not constant for a given scrubber design unless it is referenced to a specific inlet dust loading. In contrast, it has been shown that scrubber outlet dust concentration is a constant, independent of inlet concentration.

Cumulative collection efficiency for PM wet scrubber at coal, oil, wood, and bark combustion source and coke production operation

 Table 1. PM-10 and PM-2.5 Cumulative Collection Efficiencies for Wet Scrubbers at Coal, Oil, Wood, and Bark Combustors; and Coke Production Units