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

The control of Suspended Particulate Matter (SPM) in the exit flue gas of boiler is very important from the pollution point of view. The most common method for bringing down the SPM level within the limit is through Electrostatic Precipitator. These limits are specified by respective State Regulating Authorities like Pollution Control Board.

In the Heavy Water Plant, Manuguru, Andhra Pradesh,(HWPM) there are three coal-fired boilers in the Captive Power Plant, which generate steam for producing electricity and also to meet the steam requirement of the main plant (process). The flue gas from each steam generator (boiler) is split into two passes and each pass is provided with an independent ESP, consisting of five electrical fields in series. The system is designed by M/s. ACC Babcock Limited.

The ESP is designed to maintain SPM concentration of 150 mg/Nm³ which was the stipulation from Andhra Pradesh Pollution Control Board (APPCB) in 1984, during the setting up of this project. Recently, APPCB has revised the SPM concentration to 115 mg/Nm³.

Our past experience of commissioning of ESP indicated the difficulty in maintaining SPM concentration in the range of 150 mg/Nm³ and a SPM variation of 300-700 mg/Nm³ was observed. The design review carried out with the manufacturer indicated that it is not possible to obtain the design ESP efficiency of the order of 99.6%.

The main cause for the poor performance of ESP was attributed to the resistivity of the ash particles. It was decided to condition the flue gas and alter its resistivity in order to improve the efficiency of the ESP. An exhaustive literature survey was undertaken to understand the resistivity problem. Research paper by Chittum et.al. as indicated by Schmidt led to a theory of conditioning the flue gas by principally altering the moisture-adsorption properties of dust surfaces. Essentially, the theory developed so far was for flue gas humidification as it was proposed that the moisture would reduce the electrical resistivity of most of the dusts. Ashworth et. al. in their paper submitted at the Joint ASME-IFEE Power Generation Conference in 1992 have also brought out that by flue gas humidification, the ESP efficiencies could be obtained greater than 99.8%.

A few experiments based on the Ashworth’s report were carried out at HWPM, by injecting steam and changing the moisture content of the flue gas. Though it showed improvement in the dust burden at ESP outlet, we could not achieve the value of SPM of 150 mg/Nm³. The maximum SPM level came down from existing 650 mg/Nm³ to about 360 mg/Nm³. The system as proposed by Ashworth et.al. for flue gas humidification using water instead of steam requires a dual fluid spray lances and specially designed nozzles which produced very small droplet of water with air atomisation and controls, but this system is highly capital intensive for our existing system. Therefore, an alternate system of Ammonia dosing was conceived at HWPM.

The choice of ammonia as a flue gas conditioner at HWPM is based on the ash characterisation including the ash leachate water analysis (Report by Sonde & Sadasivam,1994). The study indicated that ammonia would be an ideal choice for maintaining the right resistivity. The optimum resistivity range is generally considered to be in the range of 10⁸-10¹⁰ ohm.cm. at the operating conditions. This choice of flue gas conditioner should be such that the improvement in resistivity is substantial and this is decided by ash characterisation. The characterisation of ash indicated that the resistivity is of the order of 10¹⁰ ohm.cm. ammonia conditioning would reduce this resistivity into the narrow optimal range of 10⁸–10¹⁰ohm.cm. Experiments were carried out with injection of Ammonia through Ammonia cylinders and the results indicated change in dust burden value from 610 mg/Nm³ to about 130-190 mg/Nm³ (without altering the humidity). Encouraged by this result an ammonia injection system is designed. The schematic is shown in figure 1.

Figure 1. - Schematic of Ammonia Injection System