Page 108 - Wastewater Solids Incineration Systems

Combustion Technology 77

that occurred in the MHF resulted in uniform drying. The firebrick walls and hearths
were amply resistant to combustion temperatures (WEF, 1992).
Nichols began to help municipal consulting engineers use this design in the early
1930s. Twenty-three units were installed in 12 cities before World War II in the North-
east or Midwest. Feed solids handled included raw primary, digested primary, and
digested activated. Ferric and hydrated lime were the primary chemicals used in
solids conditioning for dewatering. In the late 1940s and 1950s, many other cities
adopted this approach, resulting in the installation of 44 more units by 1960.
Since then, several factors have affected the use of MHFs. In late 1973, the first oil
embargo by the Organization of Petroleum Exporting Countries caused many fur-
naces to shut down because of the cost and scarcity of oil and gas. Improvements in
dewatering technologies and development of polymers as dewatering aids resulted
in dryer feed solids and reduced fuel costs. Of the approximately 400 MHF units
installed in municipal wastewater plants, 250 to 260 remain in operation. The last
new unit was constructed in 1993. Since then, a number of units have been upgraded
to meet the Title 40 Part 503 of the Code of Federal Regulations (40 CFR Part 503) or
have been replaced with fluid bed incinerators.
New MHFs are unlikely to be constructed at wastewater plants. However, the
many operating MHFs have several years of significant useful life remaining. To
maximize the existing capital investment, many MHFs will require upgrades to
achieve their maximum useful life and to satisfy regulatory requirements. This sec-
tion describes MHF combustion technology and potential upgrades that are available
to achieve maximum life expectancy of existing facilities while meeting regulatory
requirements.

9.1 Process Design Considerations

The MHF is designed for continuous operation. Startup fuel requirements and the
extended time needed to bring the hearths and internal equipment to the correct tem-
perature from a completely cold condition typically preclude intermittent operations.
The MHF is a vertical, cylindrical, refractory-lined steel shell containing a series of
horizontal refractory hearths, one above the other (Figure 5.11). A central shaft,
hollow to allow the passage of cooling air through it, runs the height of the furnace
and rotates within the furnace at roughly one rotation per minute carrying the rabble
arms above each hearth with it. There are two or four rabble arms per hearth. Each
arm contains rabble teeth or plows that rake the dewatered cake spirally across the