PAC Found to Collect Preferentially on ESP Discharge Electrode; Implications for Hg Capture by Injected Sorbents

Although full-scale sorbent injection tests have shown varying amounts of mercury removed within the ESP by powdered activated carbon (PAC), the actual behavior of PAC within an ESP is not well established. Preliminary indications from ATESR Lab testing by Vinit Prabhu have suggested that the electrical properties of PAC may cause its electrostatic precipitation to be significantly different from that of fly ash. In particular, experimental results indicate a potential for significant collection of PAC on ESP discharge electrodes (Figure 1), exceeding even that collected on the collection electrodes at high particle mass loading values.  Given the much larger (two orders of magnitude) surface area of the collection electrodes compared to the discharge electrode, the greater mass of  PAC collected on the discharge electrode suggests PAC collection in an ESP is strongly biased toward the discharge electrode on a per unit surface area basis.  PAC collection for mixtures of fly ash and PAC in the lab-scale ESP show similar trends.

Figure 1:  Mass of PAC collected in different locations of an ESP

For ESPs using wire discharge electrodes, this behavior could potentially result in a series of cylindrical, PAC-covered discharge electrodes that can adsorb mercury from the flue gas over extended periods of time beyond the typical rapping intervals of the ESP collection electrodes. Such a mechanism could explain the often-observed slow recovery of mercury concentrations measured at ESP outlets after cessation of sorbent injection.

In light of these experimental findings, a rudimentary mass transfer model has been developed to assess the impact on mercury concentrations at the outlet of an ESP in which PAC is partially captured on the discharge electrodes.  This model treats the carbon coated electrodes as a row of cylindrical mercury sinks, whose radii and ultimate capacity for adsorbing mercury vary in time. Results from simulations (Figure 2) show that the mercury concentration at the ESP outlet decreases abruptly at the start of PAC injection. Once PAC injection is stopped, PAC-covered discharge electrodes gradually become saturated with mercury and the measured mercury concentration at the ESP outlet begins to increase. This trend is in agreement with those observed in full-scale sorbent injection tests performed using PAC.

Figure 2:  Numerical mass transfer analysis results showing Hg concentrations over time downstream of the 5th, 10th, and 20th PAC-covered discharge electrodes in an ESP.