Rotary Kiln Incinerators

Rotary kiln incinerators (Figure 11.14.3) are more versatile, as they are used to destroy solid wastes, slurries, containerized wastes, and liquids. Because of this, these units are frequently incorporated into commercial off-site incineration facilities and used for Superfund remediation.

The rotary kiln is a horizontal, cylindrical, refractory-lined shell mounted on a slight slope. Rotation of the shell transports waste through the kiln and mixes the burning solid waste. The waste moves concurrently or counter-currently to the gas flow. The residence of waste solids in the kiln is generally 0.5 to 1.5 hrs. This is controlled by kiln rotation speed (typically 0.5 to 1 rpm), waste feed rate, and in some instances, internal dams to retard waste movement through the kiln. The feed rate is regulated, limiting the waste processed to 20% or less of kiln volume.

Rotary kilns are typically 5-12 ft in diameter and 10-30 ft in length. Rotary kiln incinerators generally have a length-to-diameter ratio (L/D) of 2:8. Smaller L/D ratios result in less particulate carryover. Higher L/D ratios and slower rotational speeds are used when waste materials require longer residence time.

The primary function of the kiln is converting solid wastes to gases through a series of volatilization, destructive distillation, and partial combustion reactions. An afterburner, connected directly to the discharge end of the kiln, completes gas-phase combustion reactions. Gases exiting the kiln are directed to the afterburner chamber.

Some recent systems have a "hot cyclone" installed between the kiln and afterburner to remove solid particles that might create slagging problems in the afterburner. The afterburner may be horizontally or vertically aligned, and functions on the same principles as a liquid injection in

Combustion Air

Waste Liquids Auxiliary Fuel

Waste Solids, Containers or Sludges

Discharge to Quench or Heat Recovery

Combustion Air

Discharge to Quench or Heat Recovery

Waste Liquids Auxiliary Fuel

Waste Solids, Containers or Sludges

120%-200% Excess Air

1.0-3.0 Seconds

Mean Gas Residence Time

Refractory

Rotary Kiln

FIG. 11.14.3 Typical rotary kiln/afterburner combustion chamber. (Reprinted, with permission, from Dempsey and Oppelt 1993.)

120%-200% Excess Air

1.0-3.0 Seconds

Mean Gas Residence Time

Refractory

Rotary Kiln

FIG. 11.14.3 Typical rotary kiln/afterburner combustion chamber. (Reprinted, with permission, from Dempsey and Oppelt 1993.)

cinerator. In fact, many facilities also fire liquid hazardous waste through separate waste burners in the afterburner. Afterburners and kilns are usually equipped with auxiliary fuel-firing systems to bring the units up to temperature and to maintain the desired operating temperatures. Some operators fire aqueous waste streams into afterburners as a temperature control measure. Rotary kilns are designed with a heat release capacity of up to 150 MM Btu/hr in the United States; Average units are typically around 60 MM Btu/hr.

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