Air Pollution Control Technology in JAPAN
Exhaust Gas Treatment Equipment

Updated: November 1996



Millions of tons of waste are produced worldwide every day; most of it is dumped in landfills or discharged to rivers and sea without prior treatment. However, while the demand for waste disposal capacities is increasing, the availability of suitable sites is on the decline. Waste has become a critical problem for industrial society, particularly in big cities and densely areas. Therefore, the need to avoid or reduce the generation of waste and to recycle the waste produced will become an ever important political and economic issue. However, disposal capacities will continue to decrease in spite of the reduction in the volumes of waste produced. For this reason it is necessary to coordinate and optimize waste management.

The future development of incineration as a waste disposal method will of course depend mainly on whether optimum reductions in the total emissions of solid, liquid and gaseous products from waste incineration plants can be ensured without leaving environmental and economic aspects unconsidered. The prevailing trend can be clearly seen from the changes in the European and Japanese emission limits for flue gases from waste incineration plants.

HITACHI ZOSEN has been making every effort to research, develop and improve flue gas cleaning systems to meet clean air standards which are getting progressively more and more strict and severe in every countries in the world.
This document briefly introduces DRY TYPE FLUE GAS TREATMENT SYSTEM which is one of typical types of flue gas cleaning system.

This system consists of the following equipment as shown on fig.1;

- Evaporative Type Cooler
- Bag Filter
- Storage and injection equipment for Lime and Supplemental agent

(Waste Heat Boiler Type Cooler)

Fig.1 is simplified flow chart of this DRY TYPE FLUE GAS TREATMENT SYSTEM (Bag filter system).
Flue gas from a refuse incinerator is fed to Evaporative Type Cooler, in which the flue gas temperature is regulated by means of evaporation of injected water. The flue gas temperature at the inlet of Bag Filter is controlled to be appropriate for the reaction, which makes the removal of gaseous pollutants efficient and prevents the filter cloth from clogging caused by melting of reaction products (salt) such as CaCl2 and suffering heat damage.
After being temperature-conditioned, the flue gas is mixed with fine-grained Ca(OH)2 and supplemental agent which are supplied into the flue gas duct, where powdered Ca(OH)2 reacts with gaseous pollutants such as SO2, SO3, HCl and HF (if any). In simple terms, the following reactions take place;

     2SO2 + 2Ca(OH)2 -> 2CaSO3.(1/2)H2O + H2O --- (1)
      SO3  +  Ca(OH)2 -> CaSO4.2H2O      --- (2)
     2HCl +  Ca(OH)2 -> CaCl2 + 2H2O --- (3)
If HF is present in the flue gas,
      2HF  +  Ca(OH)2-> CaF2 + 2H2O       --- (4)
The powdered Ca(OH)2 reacts with the gaseous pollutants on its surface and core of particle remains unused. The gas/solid mixture is fed into the Bag Filter, in which solids(reaction product, unused Ca(OH)2 and flue gas dust) are effectively separated from the flue gas through the filter cloth. The gaseous pollutants will come in contact with unused Ca(OH)2 for further reaction.

The filtrate cake is periodically removed from the filter cloth by means of pulse jet of compressed air injected into clean side of the bag house with a lance(on-line cleaning).
Supplemental agent is added to the powdered Ca(OH)2 to facilitate the removal of the cake and the above mentioned reaction, furthermore in case of special supplemental agent supplying, it effects removal of another pollutants which are Dioxins and Hg, due to carbon contained in the supplemental agent.

The relation between the flue gas temperature controlled, Equivalent of Ca(OH)2 to HCl/SOx and the gaseous pollutants removal efficiency is shown on fig.2.

Fig.2 HCl and SO2 removal efficiency on a bag filter system

Nowadays this system has high performance of noxious gases removal at the same as with the Wet Type Process on constant research and much experience on this system (We at HITACHI ZOSEN has 44 plants of actual experience as of April, 1998).
This DRY TYPE FLUE GAS TREATMENT SYSTEM has main features as follows;

- High removal efficiency for noxious acid gases
- Simple and easy maintenance
- Stable and easy operation
- Low capital costs

We are very glad if this DRY TYPE FLUE GAS TREATMENT SYSTEM would be of service for worldwide clean air protection.


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