Thermal destruction methods are often applied to odorous emissions. Rendering plants are prime examples of an odorous emission. In these plants, animal and poultry byproducts are processed to produce fallow, grease, and protein meals. Batch and continuous processes are used. Other sources (National Council of the Paper Industry for Air and Stream Improvement 1985; Prokop 1985) describe these processes in detail.
In a batch cooker system (the basic rendering process), cookers are charged with raw material; a cook is made under controlled time, temperature, and pressure conditions; the cooked material is discharged; and the cycle is repeated. Under continuous conditions, the raw material is charged to the cooker.
The principal odorous emissions from these processes are N- and S-containing organic compounds, which are listed in Table 5.21.4. Additional compounds include higher molecular weight organic acids, pyrazines, alcohols, and ketones. These compounds are mostly noncondensi-bles and arise under the cooking conditions (~220°F). The type of raw material and its age have a significant effect on the odor intensity. Continuous systems tend to be enclosed and have a greater capability of confining odors. In the batch process, the steam rate varies between 450-900 ft3 min-1. Steam is subsequently condensed and cooled to below 120°F. Odorous noncondensibles range in odor intensity from 5000-106 odor units/scf; the volumetric emission rate of noncondensibles varies between 25-75 ft3 min-1.
Since rendering plants use boilers for steam generation and drying, odor control by incineration usually uses the existing boilers. The following factors should be considered when boiler incineration of odor-intense effluents is implemented:
Excess air at odor pickup points should be avoided If possible, the odor-containing stream should be used as primary combustion air Moisture and particle concentrations should be low High-intensity odors must contact the furnace flame Sufficient residence time at T > 1200°F must be provided in existing boilers.
In addition, the use of an existing boiler for odor incineration must conform to engineering, safety, and insurance requirements. Sections 5.26 and 5.27 provide more information on odor and its control.
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