Updated: July 2002

1. Outline of Equipment

The circulating fluidized bed furnace consists of a combustion chamber (riser), a cyclone, a loop seal and an ash classifier. Silica sand, the fluidizing medium, is blown upward by the combustion air from the bottom of the riser to the top. The gas velocity inside the riser is as high as 4-6m/sec and the air is in a turbulent state. In the riser, sludge and screen residue are dried and burned rapidly, turning into fine ash except the incombustibles. The combustion ash is sent to the cyclone together with the silica sand. The sand and the ash are separated by the difference in specific gravity and particle size, and the ash is sent to the flue-gas-treating unit downstream with the exhaust gas. The sand falls into the loop seal, and is returned to the furnace. Filtration residue and the incombustibles in the sand remain and accumulate in the furnace, and must be drawn out from the ash classifier.

The system flow of a dewatered sludge direct-feeding type is shown below. It reduces the use of auxiliary fuels by heating combustion air through heat exchanging with the combustion gas. In addition, it has a waste-heat boiler that enables the drying and self-sustained combustion without the use of an auxiliary fuel.

2. Features

1) Furnace I/D is halved
Compared with the bubbling fluidized bed with air speed of lm/sec, the circulating fluidized bed has an air speed of 4-6 m/sec. This can halve the inside diameter of the furnace.
2) The required static pressure of the fluidizing (combustion air) blower is low, consuming less power,
The bubbling fluidized bed needs 25-30kPa compared to 15-25kPa of circulating type.
3) Air ratio is low at 1.3 or less.
The medium intensifies contact, mixing and blending of air, waste and heat, raising the combustion efficiency and enabling low air ratio combustion.
4) Furnace temperature control is easy.
It is easy to adjust the amount of circulating particles by controlling the primary air. Local temperature drop or surge can be prevented.
5) Auxiliary fuel consumption is small.
The heat carried back by the circulating particles helps dry the sludge, preventing temperature drop in the lower part of the furnace, requiring no auxiliary fuel.
6) High ratio of filtration residue can be accommodated.
Circulating fluidized bed furnace has no such limitation because furnace temperature control is easy.

3. Performance

1) An example of flue gas composition while burning a mixture of high-molecule sludge and filtration residue.
Taken at the baghouse outlet, corrected to 12% 02
CO 5 ppm
NOx 37 ppm
Dioxins 0.001ng-TEQ/m³N

4. Application

- Sewage treatment plant
- Night soil treatment sludge and filtration residue
- Industrial waste sludge
- RDF