PERFORMANCE ENHANCEMENT OF ELECTROSTATIC PRECIPITATOR (ESP) BY AMMONIA INJECTION IN THE FLUE GAS

The ammonia is dosed at the inlet of each pass of the ESP through a fine control provided through a needle valve. Air is mixed through air blower system to maintain the concentration of ammonia in the inlet at a value of about 0.5% to 1.5% depending on the level of ammonia required for the system. This also helps in proper mixing of ammonia in the flue gas. The ammonia concentration in the flue gas can be varied from 20 ppm to 70 ppm maximum.

The ammonia injection system includes ammonia storage tank, ammonia vaporiser, knock out drum, steam system, instrumentation and controls. Ammonia is evaporated through a steam coil vaporizer and the ammonia vapor is maintained at a constant pressure through a pressure regulating and a pressure control system provided on the LP stream supply. A knock-out drum is provided to de-entrain any ammonia liquid droplets entering the system. Down stream of the ammonia knock-out drum, an auto regulating, pressure reducing valve with a pilot valve facility is provided in order to maintain a constant down stream pressure of 2.8 kg/cm²(g). The entire ammonia storage system is also provided with safety relieving devices, pressure and temperature gauges, and a vent header at an elevation of 22 meters in order to safely disperse ammonia in an unlikely emergency situation.

ESP performance parameters before and after ammonia injection

(without ESP fine tuning)

In the second experiment, fine tuning of the ESP settings were made. The designer of ESP had specified certain settings of the DC voltage and the DC current to be maintained on the discharge electrode. Overriding all these settings were the spark rate setting for each field which was maintained at value specified by the designer (less than 3 sparks/min.). Before ammonia injection, the set values were away from designed values since the spark rates were increasing even before the design values could be reached. Therefore while the secondary voltage specified was in the region of 40KV to 50 KV and the secondary current from 300mA to 600 mA, the system was operating with a value of secondary voltage between 20KV to 40 KV and the secondary current anywhere in the 30mA to 300 mA. It was not possible to apply voltage higher than these values due to back corona discharge. On injection of ammonia into the flue gas, the spark rate got reduced and there was an apparent increase in the secondary voltage level. In the second set of readings after injection of ammonia the settings for the current could be changed without any increase in the spark rates or flash over. In fact, after dosing it was possible to maintain voltage and current levels close to the design values. These adjustments resulted in bringing down the SPM dramatically to a value less than 140 mg/Nm3 and the SPM value came down to as low as 20 and 70 mg/Nm3.