Source: Henmi, Okazawa, and Sota 1986.

Source: Henmi, Okazawa, and Sota 1986.

heated by these screws. As shown in Table 10.10.4, the cake is dried to a substantial degree (some 20% of the inlet flow is evaporated) as the sludge cake passes through the cake dryer. The heating oil circulates in a closed cycle and is maintained at about 480°F (250°C) inside the screw-conveyor dryers by the throttling of two three-way valves. One valve can increase the outlet oil temperature from the cake dryers by blending in warmer inlet oil; the other can lower the outlet temperature by sending some of the oil through an oil cooler.

The operators of the Tokyo incinerator feel that the total capital cost of the plant is unaffected by the addition of the heat-recovery feature because the cost of the heat-transfer equipment is balanced by the reduced capacity requirement. The operating costs, on the other hand, are cut in half with the heat-recovery system (Table 10.10.5).

Another interesting feature of this system is the method of cleaning the accumulation of ash and soot from the heat-transfer surfaces. This automated system uses 3- to 5-mm-diameter steel-shot balls that are dropped every three to six hours from the top of the hot-air heaters. The random movement of the balls removes the dust from the heater tubes. The dust is removed at the bottom, while the balls are collected and returned to the top.

Table 10.10.6 gives the composition of the ash residue and the stack gases (after they have been cleaned by wet electrostatic precipitation); both meet Japanese regulations. Table 10.10.7 gives the composition of the wastewater produced by this process. According to the operators, the process produces almost no odor.

Adapted from Municipal Waste Disposal in the 1990s by Béla G. Liptâk (Chilton, 1991).

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