ELECTROSTATIC PRESIPITATOR (ESP) 1. General

Coal combustion generates more than hundred times of dust compared to oil burning thus requires a dust collecting facilities to meet stringent environmental restriction. Coal being consumed by Japanese power companies has relatively low sulphur content, say less than 1.0%, which gives high apparent electric resistibility of dust thus high efficient electrostatic precipitator (ESP) is required. In general high sulphur content of coal increases dust collecting efficiency thus requires relatively simple electrostatic precipitator. 2. Principle of ESP

Basic principle of ESP is that all dust particles in flue gas are electrically charged by corona discharge and attracted by electrodes and pile up on electrode plates. When the dust piled up on electrodes becomes thick, it would be removed by hammer shock and drop to the hoppers. Electrodes would normally being connected by link chain which rotates with relatively low speed (0.5 m/min) and piled up dust is removed by rotating brushes above the hoppers.

Electrode Discharge Pole

3. Electric resistibility of dust and dust collecting efficiency

Dust collecting efficiency is largely affected by electric resistibility of dust and electric resistibility changes according to followings as:

a. flue gas temperature b. moisture and SO3 contents in flue gas c. coal properties and ash composition

In general electric resistibility ranging 106~1011Ω-cm would normally give favorable collecting efficiency. 4. Type of ESP a. Low temperature ESP

Low temperature ESP is installed after air heater where gas temperature would normally be 120oC~150oC.

< 30 mg/Nm3

iler De- NOX AH ESP Bo

Electric resistibility of dust with the temperature range of 120oC~150oC is relatively high which would results in reducing dust collecting efficiency. Even with some improving technologies 30 mg/Nm3 at the outlet would be considered the limits.

b. High temperature ESP

High temperature ESP is installed after economizer where flue gas temperature would normally be 320oC~380oC.

< 30 mg/Nm3

Original aim of this system was to increase dust collecting efficiency in relatively high gas temperature where electric resistibility of dust would be relatively low. However it is observed that this system has turned out to be no significant advantages compared to low temperature ESP. 30 mg/Nm3 at the outlet of ESP would be considered the limit.

iler ESP De- NOX AH Bo

High Temperature ESP

Low Temperature ESP

High Alkaline Low Sulphur C l

High Alkaline High Sulphur Coal

High Efficiency ESP

100 200 300 400

Flue Gas Temperature (oC)

1012

1011

108

1010

Elec

tric

Res

istib

ility

(Ω-c

m)

109

100

106 1014108 10121010

Electric Resistibility of Dust (Ω-cm)

Dus

t Col

lect

ing

Effic

ienc

y

c. High efficiency ESP This system has been developed to cope with very stringent environmental restriction. ESP is installed after air heater and GGH where flue gas temperature would normally be 95oC~105oC.

Boiler

DeNOX

AH ESP DeSOX

G G H

GGH

< 10 mg/Nm3

Non-leak GGH

chimney

5. Ash distribution

Location of Hopper Ash (%) Type of Ash Clinker hopper 5 ~ 15 clinker Economizer hopper 2 ~ 5 Fly ash Air heater hopper 1 ~ 2 Fly ash ESP 92 ~ 78 Fly ash (dust) Pyrite hopper (pulverizer) 0.1 ~ 1.0 Pyrite

c