Updated: July 2002

1. Purpose

The sewage sludge is supplied to the furnace after having been dried by the steam drier.k
The combustion exhaust gas generated within the furnace is supplied to the waste heat boiler arranged at the upper part of the furnace, and is recovered as steam. It is intended to save the energy by utilizing the recovered steam for a drying heat source.

2. Principle

The step grate type stoker furnace is made up of step grate type hearth as shown in Fig. 1, in which the movable stage and fixed stage are arranged alternately.

The sludge dried up to the moisture content of approx. 40% is supplied in the furnace from the hopper, and further supplied to the fixed stage downward by the operation of the movable stage. And it is moved downward sequentially within the furnace. The combustion air is supplied from the lower side of the fire bed, by which the air makes efficient contact with the sludge, to dry, ignite and burn the sludge.
The sludge is subjected to self-sustained burning at the sludge combustion unit at the temperature of 1,000-1,200ºC, and is discharged as half-melting slag-state incineration ash. (photos-1 & 2)
The steam recovered by the waste heat boiler is utilized for a drying heat source.
When the sludge contains limited calorific heat, the steam is compensated by burning oil with in the boiler, however, almost all sludges will not require any aux., fuel (oil).

3. Features

The waste heat during combustion is recovered by the boiler and is use for a drying heat source, etc. Therefore, it can be an energy-saving system. The stoker furnace of this type can be compatible with the high-calory sludge simply by changing the waste heat boiler construction. Since the incineration ash is in 0.5-50 mm semi-melting slag state, it is accompanied by little elusion of heavy metals, and can be disposed of without pollution, or utilized effectively.
The operation of the incineration furnace is fully automated within the range from sludge laying and ignition start-up to stopping of banked fire.

4. Performances

- Self-retained combustion limit: Calorific heat of dehydrated cake 1,300 kJ/kg or over
- Furnace outlet gas temperature: 900-1,100ºC
- Boiler outlet gas temperature: 250-300ºC
- Exhaust gas property (outlet of furnace):
Dust (0.5-3.0 g/m³N)
NOx 100 ppm or less
- Power consumption: 50-70 kW h/tcake
- Combustion capacity: 70-500 t/d (based on dehydrated cake)

5. Application Examples (based on dehydrated cake)

- Sapporo city:
80 t/d×2 furnaces (with 2 t/h boiler)
100 t/d×3 furnaces (with 3 t/h boiler)
100 t/d×4 furnaces (with 5 t/h boiler)
140 t/d×1 furnace (with 7 t/h boiler)
150 t/d×1 furnace (with 9 t/h boiler)
- Otaru city:
80 t/d×1 furnace (with 3 t/h boiler)
- Kiryu city:
100 t/d×1 furnace (with 4 t/h boiler)
- Shizuoka city:
80 t/d×1 furnace (with 3 t/h boiler)
- Kyoto city:
150 t/d×1 furnace (with 8 t/h boiler)
- Nara prefecture:
80 t/d×1 furnace (with 5 t/h boiler)
90 t/d×2 furnaces (with 5 t/h boiler)