PDF《2.2 Sewage Sludge Incineration》

1/95 Solid Waste Disposal 2.

2-1 2.2 Sewage Sludge Incineration There are approximately

170 sewage sludge incineration (SSI) plants in operation in the United States. Three main types of incinerators are used: multiple hearth, fluidized bed, and electric infrared. Some sludge is co-fired with municipal solid waste in combustors based on refuse combustion technology (see Section 2.1). Refuse co-fired with sludge in combustors based on sludge incinerating technology is limited to multiple hearth incinerators only. Over

80 percent of the identified operating sludge incinerators are of the multiple hearth design. About

15 percent are fluidized bed combustors and

3 percent are electric. The remaining combustors co-fire refuse with sludge. Most sludge incinerators are located in the Eastern United States, though there are a significant number on the West Coast. New York has the largest number of facilities with 33. Pennsylvania and Michigan have the next-largest numbers of facilities with

21 and

19 sites, respectively. Sewage sludge incinerator emissions are currently regulated under

40 CFR Part 60, Subpart O and

40 CFR Part 61, Subparts C and E. Subpart O in Part

60 establishes a New Source Performance Standard for particulate matter. Subparts C and E of Part 61--National Emission Standards for Hazardous Air Pollutants (NESHAP)--establish emission limits for beryllium and mercury, respectively. In 1989, technical standards for the use and disposal of sewage sludge were proposed as

40 CFR Part 503, under authority of Section

405 of the Clean Water Act. Subpart G of this proposed Part

503 proposes to establish national emission limits for arsenic, beryllium, cadmium, chromium, lead, mercury, nickel, and total hydrocarbons from sewage sludge incinerators. The proposed limits for mercury and beryllium are based on the assumptions used in developing the NESHAPs for these pollutants, and no additional controls were proposed to be required. Carbon monoxide emissions were examined, but no limit was proposed. 2.2.1 Process Description1,2 Types of incineration described in this section include: - Multiple hearth, - Fluidized bed, and - Electric. Single hearth cyclone, rotary kiln, and wet air oxidation are also briefly discussed. 2.2.1.1 Multiple Hearth Furnaces - The multiple hearth furnace was originally developed for mineral ore roasting nearly a century ago. The air-cooled variation has been used to incinerate sewage sludge since the 1930s. A cross- sectional diagram of a typical multiple hearth furnace is shown in Figure 2.2-1. The basic multiple hearth furnace (MHF) is a vertically oriented cylinder. The outer shell is constructed of steel, lined with refractory, and surrounds a series of horizontal refractory hearths. A hollow cast iron rotating shaft runs through the center of the hearths. Cooling air is introduced into the shaft which extend 2.2-2 EMISSION FACTORS 1/95 Figure 2.2-1. Cross Section of a Multiple Hearth Furnace 1/95 Solid Waste Disposal 2.2-3 above the hearths. Each rabble arm is equipped with a number of teeth, approximately

6 inches in length, and spaced about

10 inches apart. The teeth are shaped to rake the sludge in a spiral motion, alternating in direction from the outside in, to the inside out, between hearths. Typically, the upper and lower hearths are fitted with four rabble arms, and the middle hearths are fitted with two. Burners, providing auxiliary heat, are located in the sidewalls of the hearths. In most multiple hearth furnaces, partially dewatered sludge is fed onto the perimeter of the top hearth. The rabble arms move the sludge through the incinerator by raking the sludge toward the center shaft where it drops through holes located at the center of the hearth. In the next hearth the sludge is raked in the opposite direction. This process is repeated in all of the subsequent hearths. The effect of the rabble motion is to break up solid material to allow better surface contact with heat and oxygen. A sludge depth of about