include fire hazards, equipment damage, and environmental impact. The manufacturers are valuable sources of information in this connection.
A continuous feed system is needed to eliminate the hazards now encountered with manual introduction of waste into the flames in the firebox. For purposes of ease of feeding, the solids are first shredded. This also provides for the homogenization of the waste stream and for more uniform burning rates. Some systems have gravity feed. A screw feed system might be more appropriate for Navy application where the height of the unit and the number of decks involved would be an issue. Also, in modernization of existing incinerators, single-deck installations are more readily adapted. Both paper and plastic have good fuel value. They would not present any problem to incinerators as long as they formed part of the mixed waste stream, avoiding surges in high-temperature gases or volatiles that might be produced if waste consisting solely of plastics was burned.
Burning rates on modern grates are in the range of 60 to 100 pounds of waste per square foot per hour. A modest grate size of 4 ft × 4 ft would therefore be capable of providing a capacity greater than 1,000 lb/h, adequate to handle the wastes from even the largest ships. A moving or reciprocating grate would be desirable to move the residue from the feed to the ash removal chute.
The rule of thumb for good combustion is a residence time of 2 seconds at 1,800ºF with oxygen contents of at least 2 percent. Although such temperatures can be sustained with the average heating value of the waste streams, the use of burners fired with auxiliary fuel in both the primary and secondary combustion chambers is advantageous to ensure burning of low heating value wastes. Good combustion at much shorter residence times is achievable if good mixing is provided and advantage is taken of the much higher temperatures present at diffusion flame fronts produced at the interface of the volatiles generated by the waste and air. The size of the incinerators can therefore be reduced considerably using advanced combustion theory, modern instrumentation, and computer controls. Design of compact incinerators using some of the advances in combustion science is under way as part of the Navy program administered by China Lake (Schadow, 1995).
The gases from combustion chambers should be cooled prior to their exhaust. Where there is use for supplementary heat, the gases could be cooled in a waste heat boiler or heat exchanger. The amount of energy that can be recovered is not large, however, and it would probably be more cost effective to cool the gases by dilution with excess air (NRC, 1977). This kind of cooling system is used in the commercial incinerator in service aboard the USS Theodore Roosevelt. Excess air is injected into hot combustion products by use of an eductor. The high velocity from the eductor is also used to provide partial particle removal through the centrifugal forces imparted to the particles in the effluent gas stream.
International Maritime Organization regulations for incinerators (IMO 73/78) are not very restrictive at this time. The following specifications must be met: (1) CO levels must be lower than 200 g/m3; (2) the smoke number must be below Bacharach 3; and (3) carbon in the ash must be below 10