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Show Solid feed preparation and control is very important for the reduction of kiln puffs. However, it may not be cost effective or practical for the incinerator operator to achieve a steady and uniform feed, especially in the case of commercial incinerators. Solid and non-pumpable wastes are batch fed to rotary kilns because this is often the only safe and efficient method of incinerating these highly variable streams. For example, repackaging of the the waste feed into small batches may require extensive personnel handling which could also create safety hazards for workers; mixing and blending of potentially incompatible materials could also cause unsafe chemical reaction conditions. Dynamic oxygen level control by the adjustment of the combustion air flow rate has been widely used in boiler control. However, its application in rotary kiln incinerators is rare because most combustion air systems have limited turndown ratios and slow response rates. Adjustment of the combustion air flow rate is further restricted due to the limitations of the induced draft fan, the air pollution control device (APCD) capacity, or the furnace gas residence time requirement. Also, this approach may adversely affect the incinerator temperature and pressure controls. For example, an increase in air feed to increase the excess oxygen level in the combustion chamber could carry enough associated nitrogen into the combustion zone to lower the residence time of the furnace gases, and could overload the flue gas handling system and cause the loss of vacuum. Another alternative is to keep air flow at an allowable maximum when burning problematic solid wastes, while modulating the fuel (liquid waste) feed rate to maintain a consistent total heat release. Although effective, the limitation of this measure is that a significant amount of fuel or liquid waste is required, which reduces the available solid waste capacity. It has been suggested that the reduction of kiln rotational speed is effective in diminishing kiln puffs. A potential drawback of this approach would be its negative impact on solid waste throughput. BENEFITS OF OXYGEN COMBUSTION SYSTEMS For many years, industrial furnaces have used oxygen-enhanced combustion systems to improve combustion performance. Engineers recognized that hazardous waste incineration systems could also benefit from the use of oxygen. Following is a brief discussion of how oxygen enrichment can alleviate many of the aforementioned failure modes. Oxygen enrichment can enhance flame stability, which can mitigate the occurrence of flameout. Although atomization of liquid waste depends primarily on the burner or nozzle design, oxygen enrichment has been shown to improve the evaporation of liquid waste by raising the intensity of the flame and, therefore, improving the destruction efficiency of waste (~). I t has been suggested that the higher partial pressures of excess oxygen in the combustion chamber available with oxygen enrichment, can alleviate the temporary oxygen-deficient conditions the lead to puffs (9-12). Studies show that this approach can be inefficient and may also cause high NOx emission levels (~). In addition, the high flame temperature associated with conventional - 3 - |
