Updated: July 2002

1. Outline of the Unit

At NKK we have developed a circulation fluidized bed sewage sludge incinerator based on the advantages of a conventional fluidized bed incinerator, which helps reduce the toxic substances in exhaust gas and the power required to operate the unit. The unit is described below.
With a conventional fluidized bed incinerator the hearth area and the air volume are set so that a good bubble fluidization floor is formed at the bottom of the incinerator. The sludge added into the incinerator is crushed, dried, and combusted (mostly). Then the uncombusted sludge is completely combusted in the freeboard in the middle and upper sections of the incinerator and exhaust gas is discharged from the top of the incinerator.
On the other hand, since the circulation fluidized bed sewage sludge incinerator has a smaller hearth area and the gas flow velocity is maximized, unlike a conventional incinerator, the bubble fluidization floor is not formed. Quartz sand is fluidized and dispersed with the gas and a thin layer of quartz sand is formed up to the top of the incinerator. With this mechanism the contact efficiency of sludge, quartz sand, and air inside the incinerator is improved, allowing evaporation, drying, and combustion of sludge using the entire incinerator. Figure 1 shows the concepts behind a conventional bubble type fluidized bed incinerator and the newly developed circulation fluidized bed incinerator.

Figure 1 Concept

2. Features

The circulation fluidized bed sewage sludge incinerator offers the following three distinctive advantages:
1. Reduction in the space required for incinerator installation
2. Optimized two-stage combustion
3. Reduction in the power for the blower used for combustion
(1) Reduction in the space required for incinerator
Installation --
The cake load (cake combustion volume per hearth area) is high and the incinerator diameter is 2/3 or less that of a conventional incinerator.
(2) Optimized two-stage combustion --
The adoption of the two-stage combustion method allows for a reduction in NOx concentration. The high temperature quartz sand that circulates inside the incinerator helps maintain a high temperature (around 850ºC) inside the incinerator high, enabling a reduction in CO and N2O concentrations in the exhaust gas.
(3) Reduction in the power for the blower used for combustion --
Since the secondary air is blown into a thin layer of quartz sand there is little pressure loss, reducing the power required for the blower. Although the power reduction rate varies depending on the design conditions, generally the power used is 10%-30% less than that of a conventional blower.

Figure 2 Structure

3. Performance

The high temperature inside the incinerator can be maintained and the incinerator can be operated while reducing CO, N2O and NOx. Table 1 below shows example data.

Table 1 Sewage sludge dewatered cake combustion
(Example exhaust gas data)

4. Fields of application and target waste

The target waste for the incinerator is dewatered sewage sludge with water content of about 70-85%. Other waste is treated individually.