Liquid Feed Incinerator

A large number of hazardous liquid waste incinerators used today are of this type. The waste is burned directly in a burner or injected into a flame zone or combustion zone of the incinerator chamber through atomizing nozzles. The heating value of the waste is the primary determining factor for the location of the injection point of the liquid waste. Liquid injection-type incinerators are usually refractory-lined chambers (horizontal or vertical flow, with up or down options), generally cylindrical in cross-section, and equipped with a primary burner (waste and/or auxiliary fuel fired). Often secondary combustors or injection nozzles are required where low heating value waste liquids are to be incinerated. Liquid incinerators operate generally at temperatures from 1000°C to 1700°C. Residence time in the incinerator may vary from milliseconds to as much as 2.5 seconds. The atomizing nozzle in the burner is a critical part of the system because it converts the liquid waste into fine droplets. The viscosity of the waste determines whether good atomization of the liquid is possible. Two-fluid atomizers, using compressed air or steam as an atomizing fluid, are capable of atomizing liquids with viscosities up to 70 centistokes. The physical, chemical, and thermodynamic properties of the waste must be considered in the design of the incinerator.

The method of injection of the liquid waste is one of the critical factors in the design and performance of these incinerators. The atomizer design is therefore an important design parameter of this system. The reasons for injecting the liquid waste as a fine spray are: (1) to break up the liquid into fine droplets, (2) to develop the desired pattern for the liquid droplets in the combustion zone with sufficient penetration and kinetic energy, and (3) to control the rate of flow into the combustion zone. Organic liquids pass through three phases before oxidation takes place. The liquid droplets are heated, vaporized, and superheated to ignition temperature. In a good atomizer, the droplet size will be small providing greater surface area and resulting in rapid vaporization. For example, the burning time for a 300 micron droplet is 150 ms while only 30 milliseconds for a 125 micron droplet. Depending on the type of liquid waste and the combustion conditions required, there are generally three types of atomizers. Proper mixing of air with the atomized droplet is very important for complete oxidation. If the liquids contain fine solids, the design must allow the particles to be carried to the gas stream without agglomeration to affect proper combustion. Sufficient time must be provided to permit complete burnout of the solid particles in the liquid suspension. Inorganic particles carried in the liquid waste stream may become molten and agglomerate into molten ash. The com-bustor must be designed to collect the molten ash without plugging the flow passages of the incinerator. Primary and secondary combustion chambers are used in liquid feed incinerators. Primary units are used to burn wastes, which have sufficient heating value to burn without auxiliary fuel. Secondary units require auxiliary fuel. Sufficient air must be provided at all times to oxidize the organics in the combustion chamber. Incinerators can produce soot when burning under insufficient oxygen and poor air mixing conditions. Soot can clog up nozzles, and accumulate in the chamber, impairing burning conditions.

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