Updated: July 2002

1. Outline of system

To decrease the volume of sewage sludge and convert it to the resources, an epoch-marking sewage sludge melting system has been made fit for practical use. That is the Mitsubishi vertical whirl sewage sludge melting system based on our integral technologies, results and experiences including those for enriching oxygen and reducing NOx when coal gas is burnt in incinerators, boilers and furnaces, and those for vertical whirl sewage sludge 2-step slag tap incinerators.
Figure 1 shows the flow diagram of this system, where the melting furnace and secondary combustion chamber are of boiler structure to cool down the furnace and recover the heat, and to recover the electric power using a steam turbine. The exhaust gas is treated collectively through an exhaust gas treatment tower without using dust removing bag filter due to the limited amount of the dust contained in the exhaust gas. As the oxygen generator, a pressure swing type adsorption tower [Mitsubishi pressure swing adsorption (PSA)] is adopted.

2. Characteristics

(1) No more melting agent or auxiliary fuel:
Oxygen-enriched air generated by a compact oxygen generator can incinerate and melt the sludge at a high temperature without melting agent or auxiliary fuel. It is possible to decrease the amount of total volume because the amount of slag is little.
(2) Stabilized operation:
The vertical swirling furnace exhausts gas above and extracts the slag at the center of the bottom, enabling even distribution of the slag layers flowing down on the walls, and stabilized operation.
(3) Highly controlled load variations:
Oxygen-enriched incineration facilitates the control of furnace temperature using the volume OXYGEN only, realizing stabilized slag outflow.
(4) Structure ensuring a long life:
To endure the strict conditions applicable to the walls of the melting furnace including the temperature exceeding 1,500ºC, they are designed in a water-cooled refractory structure. Even adhesion of the sludge on the surface of the refractories (self-coating) realized a long life.
(5) Controlled amount of NOx emission:
Sludge is incinerated by enriched oxygen or reduced in the melting furnace. Combustion gas is denitrated by self-denitration.
Consequently, gas with low NOx is exhausted. No cost is necessary for denitration and other treatment.
(6) Less exhaust gas: Oxygen-enriched incineration decreases the amount of the oxygen contained in the supply air and the amount of gas exhausted by incineration, compacting the exhaust gas treatment facilities.

3. Performance

Table 1 shows the results of the operation using various types of sewage sludge.

Table 1. Data of a Melting Furnace in Operation (Capacity of the furnace used for demonstration: 15 tons per day)

4. Applicability and objectives

(1) Applicability: waste melting furnaces (sewage sludge)
(2) Objectives: sewage sludge

Figure 1 Flow diagram of a sludge melting system