PRECIP 
NEWSLETTER 

December 2009
No. 407

In Japan, the latest gas treatment system for a coal-fired power station consists of a moving electrode type ESP, a DeSO_x system and a wet ESP. The gas temperature of the moving electrode type ESP is a cool 80 to 90°C. This system can reduce outlet dust concentration to less than 1 mg/m³N and outlet sulfuric acid mist to less than 0.1 ppm. At ICESP XI, Toshiaki Misaka, with others from Hitachi Plant Technologies, Co, Ltd., Tokyo, reviewed the operating costs for this type of ESP.

In 1979, Hitachi Plant Technologies developed the moving electrode system for ESPs. It prevents back corona by removing the collected dust using rotating brushes and movable collecting plates. The moving electrode system is effective for collecting high resistivity dust with high efficiency. It is compact compared with a conventional fixed electrode system and can also reduce electrical power consumption.

Figure 1 shows an illustration of a moving electrode type ESP. The first and second sections at the gas inlet side are fixed electrodes and the outlet section is the moving electrode system. The collecting plates of the moving electrode system are divided into strips, coupled with chains and are moved slowly by driving wheels. The discharge electrodes are installed between collecting plates at the collection zone. Dust is collected on the collecting plates by electrostatic forces. Dust attached to the collecting plate is transferred to the hopper before the dust layer becomes thick enough to cause back corona.

Figure 1. Overall structure of Moving Electrode Type Electrostatic Precipitator (MEEP)

The moving electrode system is equipped with brushes that sweep off dust from the movable collecting plate. Rotating brushes for dust removal are installed in the hopper. Since this area is a non-collection zone, it is free from gas flow or electricity. The collected dust is completely removed by the rotating brushes. Thus the collecting plates are kept clean at all times. In addition rapping reentrainment does not occur by brushing. This system maintains stable collection efficiency even when collecting high resistivity dust.

The moving electrode type ESP has various advantages compared to the fixed electrode type. It can decrease power consumption to 67 percent, reduce the installation area to 74 percent and it requires one less field. In the case of a retrofit of an existing ESP, a moving electrode type ESP is smaller than a fixed electrode type ESP. Therefore the requirement of higher performance can easily be met by adding the moving electrode type ESP in the area where the existing fixed electrode type ESP had been installed.

Figure 2 shows the latest installation list of the moving electrode type ESP, with a total of 57 units supplied as of 2008. Four new units are now under construction and the applications cover a wide variety of fields. There is an increasing demand for the use of the moving electrode type ESP for coal-fired boilers as outlet dust concentrations become more strictly regulated. Another increasing application is for sintering machine exhaust gas in iron works because the conventional fixed electrode type ESP cannot maintain stable collection efficiency due to high resistivity dust problems.

Figure 2 –Supply Record of Moving Electrode Type ESP

Until recently, moving electrode type ESPs have mainly been supplied in Japan. There are now various inquiries from foreign countries. In China, a moving electrode type ESP was supplied for the No. 5 coal-fired boiler of Changzhou Guangyuan Cogeneration Co., Ltd. in 2006. The ESP consists of two fixed electrode sections designed by Enelco Environment Technology Co. in China and one moving electrode section designed by Hitachi Plant Technologies Ltd. The measured performance data of this unit showed a collection efficiency of 98.89 percent. The second unit of a moving electrode type ESP for the No. 6 boiler has now started operation at the same site.

Cost comparisons of a moving electrode type ESP and a conventional fixed electrode type ESP were studied using actual operation results. In the case of the conventional fixed electrode type ESP, the total running cost over a span of 15 years is almost the same as that of the initial facility cost. Approximately 90 percent of the running cost is accounted for by the utility cost, which is used for the electric power required by the high voltage power supplied to the ESP. The moving electrode type ESP operates with less than 70 percent power consumption of the fixed electrode type ESP.

Maintenance and repair account for 7 to 12 percent of the running cost, and this is almost the same for both types of ESPs. However, the running cost of the moving electrode type ESP is only 68 percent that of the fixed electrode type ESP.

In general, the facility cost of a fixed electrode type ESP is initially cheaper by 10 percent compared to the moving electrode type ESP. The total cost including facility and running cost will, however, become the same as that of the moving electrode type ESP in 5 years. In 15 years, the total cost for the fixed electrode type ESP increases by 20 percent or more than that of the moving electrode type ESP. Thus, the moving electrode type ESP has an economical advantage over the fixed electrode type ESP.

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