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STUDIES ON ELECTROSTATIC PRECIPITATION
AT TEMPERATURES AROUND ACID DEWPOINT

Electrostatic Precipitators (ESP) for coal-fired boilers are generally operated at a temperature above acid dewpoint. Recently, an alternative technique has been developed from the view point of high efficiency, in which the ESP is operated below acid dewpoint temperature. Because of the formation of sulfuric acid mist, the collecting performance of the ESP is improved. This formation is mainly due to condensation of sulfuric acid vapor, created by the reaction between H2O and sulfur trioxide (SO3) vapor, in the stack gases.

In this paper we describe an experimental study of the collecting performance of an ESP at temperatures around acid dewpoint, and the behavior of condensed SO3. The results may be summarized as follows:

(1) The collecting performance of the ESP is improved by the effect of condensed SO3. (2) Condensed SO3 exists as sulfuric acid mist on the surface of dust particles. (3) At a temperature below acid dewpoint, it is possible to remove SO3 and dust particles from the stack gas with high efficiency.

The combustion gases from a coal-fired boiler consist of inert gas, reactive gases such as sulfur dioxide(SO2), fly ash particles, and condensable vapors (sulfuric acid and water vapor). The ESP is the most widely used system for solid particulate removal from these gases. In the smoke plume, the opacity may increase where hot stack gas is mixed with the cooler atmosphere. This increased opacity is due to aerosol formation resulting from the condensation of vapors, such as sulfuric acid, present in the stack gases. Sulfuric acid is formed by the reaction between S03 and water vapor. The plume opacity of stack gas has been calculated by various authors^1),2). The ESP is generally operated above acid dewpoint. In cold-side ESPs the injection of S03 into the gas stream has been attempted in order to improve the collecting performance³⁾. On the other hand, the ESP can remove sulfuric acid mist and dust particles simultaneously from stack gas at temperatures below the acid dewpoint. In addition the plume opacity can be decreased by the removal of sulfuric acid mist. Fig. 1 shows the SO3 condensation on the surface of dust particles in the stack gas. It is considered that gaseous SO3, SO3 mist and dust particles bearing condensed SO3 mist influence the collecting performance of the ESP. We therefore investigated the behavior of SO3 at temperatures around that of acid dewpoint.

Electrostatic Precipitator
A schematic of a typical ESP (Max flow rate : 1500m³N/h) is shown in Fig. 2. To ensure similarity to actual stack gases, water vapor, dust particles (fly ash) and gaseous SO3 were introduced into the heated air stream. A dust monitor and an SO3 sampler were used to measure the dust and SO3 concentrations in the stack gases. Experimental conditions are shown in Table. 1. Acid dewpoint, calculated for these experimental conditions, is 130^OC (SO3 10ppm, H2O 8 Vol%)⁴⁾.

                                                                Gas Temperature                         9 0,   I 1 0,   1 3 5,   1 5 O ^OC

                                                                Average Electric Field               1 9 0 ~ 4 6 0 kV/m
                                                                Force
                                                                _____________________________________________

Experimental SO3 Sampling Apparatus

An SO3 sampler is shown in Fig.3. Using this sampler, the amount of gaseous SO3(1), SO3-mist(2) and formed sulfate(3) in the stack gas can be individually determine . Dust particles in the stack gas are separated by a dust collector heated to 250^OC (packed with quartz wool). SO3-Mist(2) in the stack gas is vaporized by this collector. Vaporized SO3-mist(2) and gaseous SO3(1) are condensed in a spiral glass tube(SO3 Collector) at 70^OC. The amount of formed sulfate(3) is determined by analysis of the separated dust particles in the heated dust collector. The total amounts SO3-mist(2) and gaseous SO3
(1) are determined by analysis of the condensed SO3 in the spiral glass tube. In addition the dust particles and the SO3-mist are separated in a cylindrical tube at the temperature of the stack gases. Only gaseous SO3 is condensed in the spiral glass tube. The amount of gaseous SO3 is determined by analyses of the contents of the spiral glass tube. SO3 and sulfate are determined from absorptiometry of 70% isopropyl alcohol using barium chloranilate^5),6).

                                                                                                                                                Fig.4 The Effect of S03 on ESP
                                                                                                                                                            Collection Efficiency

Condensation of SO3 onto dust particles
At an SO3 concentration of 10 ppm and gas temperatures of 90^OC and 135^OC, dust particles were sampled from the plate electrodes inside the ESP. In order to measure pH changes, the dust particles were suspended in distilled water. The pH changes of suspended dust are shown in Fig.5. By way of comparison with this suspension, original dust (prefed dust) was similarly suspended. From Fig.5, it can be seen that the pH of suspended dust changes sharply to acidic in the case of SO3 injected dust. Following this, the pH changes slowly to alkaline. On the other hand the pH of original dust changes directly to alkaline. From this result, it is considered that condensed SO3 exists as sulfuric acid mist on the surface of the dust particles.

CONCLUSIONS

It became clear after the investigation on electrostatic precipitation at a temperature around acid dewpoint that the lower the gas temperature falls, the more the collecting performance of the ESP is improved. At a temperature below that of the acid dewpoint, say 90^OC, the ESP can simultaneously remove both SO3 and dust particles from stack gases with high efficiency.

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