Updated: July 2002

1. Abstract

When municipal waste is incinerated, its exhaust gas includes dust and toxic gas such as dust, hydrogen chloride, sulfur oxide, NOx etc. Currently, dioxin and heavy metals included in dust from waste incineration have become significant social concern. The Waste Treatment Law has become much more strongly enforced than previously.
Hitachi Zosen has developed a comprehensive exhaust gas / ash treatment system that removes toxic gas and dust efficiently. We are currently applying these systems to waste incineration plant.

2. Features

1. Exhaust gas treatment equipment
The equipment consists mainly of a gas cooler, a bag filter, a gas re-heater and a catalytic reduction reactor.
(1) Gas cooler
Combustion gas from the incinerator is rapidly cooled by a water spraying process to a temperature suitable for exhaust gas treatment. As exhaust gas is rapidly cooled, the generation of dioxin is limited. This equipment is designed to prevent corrosion and dust obstruction by adopting a perfect evaporation cooling system.
(2) Bag filter
Cooled exhaust gas is introduced to the bag filter together with injected special agent and slaked lime. HCl and SOx react efficiently with the slaked lime. Dioxin and dust absorbed by the special agent are also removed efficiently.
Filter cloth material and filtering speed appropriate to the composition of exhaust gas are selected to provide high efficiency and compact filtering system.
(3) Catalytic reduction reactor
Exhaust gas treated in the bag filter is sent to the catalytic reduction reactor with ammonia gas after it has been heated to a temperature suitable for the denitrification reaction using the gas re-heating device. The reactor, using a titanium catalyst, removes very small amounts of dioxin that were not removed by the bag filter.
(4) Activated carbon adsorption tower
To eliminate dioxin more efficiently, an activated carbon adsorption tower is applied.

2. Ash processing equipment
The residues (hereafter fly ash) generated in the above flue gas treatment system contain toxic and harmful substances such as dioxins and water-soluble heavy metal compounds. The following systems are designed and installed so that any problem does not occur after their final disposal:
(1) Dioxins thermal decomposition
The fly ash is heated up to a temperature of 350-450ºC for 1hr in condition of very low oxygen concentration, and then cooled down to approx. 60ºC promptly. Dioxins' decomposition and elimination takes place in this thermal treatment. This system operates automatically without any full-time operator, is compacted and requires low operating cost.
(2) Heavy metals stabilization
The fly ash is mixed and kneaded with moistening water and chemical agent in a twin-axle type of kneading machine which is specially designed for this use. Water soluble heavy metals react with the agent and alters into insoluble compounds.

System Flow