Fixed Hearth Incinerators

Fixed hearth incinerators, also called controlled air, starved air, or pyrolytic incinerators, are the third technology for hazardous waste incineration. These units employ a two-stage combustion process, much like rotary kilns.

100-200% Excess Air

Discharge to Quench or Heat Recovery

0.25-2.5 Seconds Mean Residence Time

100-200% Excess Air

Discharge to Quench or Heat Recovery

0.25-2.5 Seconds Mean Residence Time

■ Auxiliary Fuel or Liquid Waste

50-80% Stoichiometric air

Refractory

Transfer Ram

Ash Discharge ■ Ram

Ash Discharge

FIG. 11.14.4 Typical fixed hearth combustion chamber. (Reprinted, with permission, from Dempsey and Oppelt 1993.)

■ Auxiliary Fuel or Liquid Waste

50-80% Stoichiometric air

Refractory

Transfer Ram

Ash Discharge ■ Ram

Ash Discharge

FIG. 11.14.4 Typical fixed hearth combustion chamber. (Reprinted, with permission, from Dempsey and Oppelt 1993.)

As shown in Figure 11.14.4, waste is ram-fed or pumped into the primary chamber, and burned at roughly 50-80% of stoichiometric air requirements. This starved air condition causes the volatile waste to be vaporized by the endothermic heat provided in oxidation of the fixed carbon fraction. The resulting smoke and pyrolytic products consist primarily of methane, ethane, and other hydrocarbons; carbon monoxide and combustion products pass to the secondary chamber. Here additional air is injected to complete combustion, which occurs spontaneously or through the addition of supplementary fuels. Primary chamber combustion reactions and turbulent velocities are maintained at low levels by the starved-air conditions, minimizing particulate entrainment and carryover. With the addition of secondary air, the total excess air for fixed hearth incinerators is 100-200%.

Fixed hearth units tend to be smaller in capacity than liquid injection or rotary kiln incinerators because of the physical limitations in ram-feeding and transporting large amounts of waste material through the combustion chamber. Lower capital costs and reduced particulate control requirements make them more attractive than rotary kilns for smaller on-site installations.

Discharge to Cyclone

1.0-5.0 Seconds Mean Combustion Gas Residence Time

Preheat Burner

Liquid Sludge ■ Feed Fluidizing Combustion Air

Discharge to Cyclone

Solids Feed and Cyclone Ash Recycle

iSSPfi

Sand or Alumina m _

iSSPfi

Sand or Alumina m _

Ash/Bed Removal

FIG. 11.4.5 Typical fluidized bed combustion chamber. (Reprinted, with permission, from Dempsey and Oppelt 1993).

Auxiliary Fuel

Air Distribution Manifold

Solids Feed and Cyclone Ash Recycle

Auxiliary Fuel

Air Distribution Manifold

Ash/Bed Removal

FIG. 11.4.5 Typical fluidized bed combustion chamber. (Reprinted, with permission, from Dempsey and Oppelt 1993).

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