Mist removal performance of a novel electrostatic precipitation type mist eliminator with a narrow gap at high velocity for coal-fired power plant

To remove fine particles from a wet FGD (Flue gas desulfurization) in coal-fired thermal power plants, we developed a flat-plate type wet electrostatic mist eliminator (EME) with narrow gap and high gas velocity, compared to conventional electrostatic precipitation (ESP) design for the thermal power plants. The particle collection and pressure drop performance tests of the lab scale EME having single channel geometry with a collection area of 0.5 m² were conducted at various gas velocities and applied voltages, and also compared to those with the Chevron type mist eliminator. The pilot scale EME designed from the lab scale with collection area (4.6 m²) was tested at a fixed velocity of 4 m/s with and without the Chevron type ME, and its long term performance was evaluated for approximate 400 minutes. Mists smaller than 10 μm including CaCO₃ were generated and an optical particle counter and EPA test method 5 were used to measure particle collection efficiency and emission concentration. Experimental results showed that collection efficiency of the lab scale EME at 4 m/s was approximately 3 times higher than that with the Chevron type, while pressure drop was only 8% of it. Particle emission from the pilot scale EME with multiple channels was 2 mg/m³ and decreased to 1.4 mg/m³ when it combined with the Chevron. Even though the initial particle collection efficiency of the pilot scale EME, 71.2%, was decreased to 60.5%, the efficiency was recovered almost to the initial state after 10 min water cleaning with 43.5 L/min.

I. Introduction

Adverse health effects seem to be linked with smaller particles, probably largely because of the increase in the number of particles and particle surface area per unit mass with decreasing particle size, among various factors [1]. Ambient particulate matter (PM) exposure was listed among the top ten health risk factors, based on the Global Burden of Disease study conducted in 2010 [2]. In addition, many time series studies in Europe, USA and Asia etc. have provided evidence of associations between daily mortality and changes in air pollution level [3]–[6]. Moreover, most of Asian countries such as China and India etc. are suffering from high level of particle mater concentrations [4], [7]. In particular, many researches insisted that concentration and the duration of periods with elevated small particles such as PM2.5 and PM10 were associated with increased risk for mortality in East Asia [8]. Fossil fuel combustion for power generation is one of the major sources of the small particle generation for major Asian countries [9], [10]. In general, a gas cleaning system for the fossil fuel combustion in power plants consisted of a selective catalytic reduction (SCR), an electrostatic precipitator (ESP), and flue gas desulfurization (FGD) [11] so the pressure to reduce emissions from the power plants to very low levels has created renewed interest in technologies for the gas cleaning systems. In order to meet the stringent emission standard, the installation of a wet ESP is considered to be one of the most effective way to remove small particles such as PM10, PM2.5. [12]–[17]. The wet ESP is similar to a dry ESP except that water is added to the top of the collecting electrode assembly to bathe the entire collecting surface with flowing water [18]. However, adding the general wet ESP using the gas velocity of 1–2 m/s and the wide gap of 200–300 mm needs a large scale of installation space and economical cost so low temperature dry ESP system (~50 mg/m³) and a wet FGD system including a mist eliminator as final particle removal device (~5 mg/m³) have been applied for thermal power plants recent years because they can grantee final PM emission nearly 5 mg/m³ based on mass concentration [16], [19], [20]. In order to achieve super low PM emission less than 2 mg/m³, it is necessary to upgrade mist eliminators for FGDs because particle removal mechanism of the mist eliminators is inertial impaction of fine mists at the high velocity over 4 m/s. However, particles generated from wet FGDs were known to be very fine less than PM10 [21] which is difficult to remove by inertial impaction so it is difficult for a general mist eliminator in the FGD as the final PM removal device to remove such small particles. In this study, we used a novel low temperature ESP with significantly narrow gap between electrodes of 30 mm and with high gas velocity over 4 m/s because the ESP with the high velocity can be installed as a final mist eliminator just on top of the wet FGD to remove fine particles such as PM2.5 and PM10. We investigated the comparison between a well-known vane type mist eliminator, specifically the Chevron type, used for FGDs for thermal power plants [22], [23], and the novel ESP in terms of particle removal and pressure drop by changing gas velocity. Particle collection performance was also compared with theoretical result obtained from Deutsch's and Cochet's models. Finally, continuous particle loading with mists of CaCO₃ solution was performed to assess the long-term performance of the ESP. Fig. 1.

Schematics of the experimental apparatus for lab scale: a) chevron and b) electrostatic mist eliminators.

Fig. 2.

The experimental set up to compare the particle collection and pressure drop performance of the a) inertial and b) electrostatic type mist eliminators.

Fig. 3.

Experimental set ups for the pilot scale model of the EME and chevron mes.

Table 1. Experimental conditions for the performance tests of lab and pilot scale mes and emes.

	Type		Volume [m3]	Collection area [m2]	Distance between discharge plate and collection plate [mm]	Voltage [−kV]	Current [mA]	Face Velocity [m/s]
Lab scale	ME		0.008	‐	‐	‐	‐	2
								3
								4
								5
	EME		0.015	0.5	30	10	0.04-0.82 [30 mm] 0.01-0.17 [40 mm] 0.01 ~ 0.03 [50 mm]	2
					40	12		3
					50	14		4
								5
Pilot scale	ME		0.0658	‐	‐	‐	‐	2
								3
								4
								5
	EME		0.1404	4.68	30	10	0.7~5.7	2
						12		3
						14		4
								5