Multiplehearth Incineration

Multiple-hearth incineration was developed in 1889 and was first applied to sludge incineration in the 1930s. It is the most widely used method of sludge incineration (Sebastian 1974b). The multiple-hearth furnace consists of a steel shell lined with a refractory (see Figure 10.10.2). Horizontal brick arches separate the interior into compartments. The sludge is fed through the roof by a screw feeder or a belt feeder and flapgate at a rate of about 7 to 12lb per sq ft. Rotating rabble arms push the sludge across the hearth to drop holes, where it falls to the next hearth. As the sludge travels downward through the furnace, it turns into a phosphate-laden ash (see Tables 10.10.1 and 10.10.2).

The sludge is dried in the upper, or first, operating zone of the incinerator. In the second zone, it is incinerated at a temperature of 1400 to 1800°F (760 to 982°C) and deodorized. In the third zone, the ash is cooled by the in

FIG. 10.10.1 Sludge drying and incineration using a deodorized flash-drying process.

coming combustion air. The air, which travels in coun-terflow with the sludge, is first preheated by the ash, then participates in the combustion, and finally sweeps over the cold incoming sludge drying it until the moisture content is about 48%. At this percentage of moisture content, a phenomenon called thermal jump occurs as the sludge enters the combustion zone. The thermal jump allows the sludge to bypass the temperature zone where the odor is distilled. The exhaust gases are 500 to 1100°F (260 to 593°C) and are usually odor-free. The sludge temperature profile across the furnace is shown in Table 10.10.3.

The pollution control equipment usually includes three-stage impingement-type scrubbers for particulate and sulfur dioxide removal and standby after-burners, which de-

FIG. 10.10.2 Multiple-hearth incineration of sludge.

stroy malodorous substances such as butyric and caproic acids. The need for afterburners is a function of the exhaust gas temperature. Usually at temperatures above 700 to 800°F (371 to 427°C) in a well-controlled incinerator where the combustion process is complete, afterburners are not necessary for odor-free operation. If combustion is not complete, however, the exhaust gas temperature might have to rise to 1350°F (732°C) before the odor is distilled. In such cases, installing an afterburner is less expensive than using auxiliary fuel to achieve such high exhaust temperatures.

If the incoming sludge contains 75% moisture and if 70% of the sludge solids are volatile, the incineration process produces about 10% ash. The ash can be used as a soil conditioner and as the raw material for bricks, concrete blocks, and road fills, or it can be landfilled. In the United States the supply of phosphates is sufficient for less than a century (Sebastian 1974b), so the phosphate content of sludge ash is important. If the ash also contains

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