OCR Text |
Show secondary air is injected to give an excess oxygen level of 1.5 to 2 percent by volume on a dry basis in the fmal flue gases. The temperature is maintained in the 1800 to 2000-P range to limit formation of thermal NOx. The temperature and residence time used in the third stage are governed by the requirement that the carbon monoxide produced in the first stage is substantially destroyed (oxidized). A residence time of about one second is normally sufficient to accomplish this goal. The selection of temperature, excess oxygen, and residence time for this stage are chosen such as to strike the right balance between the need to destroy the CO and at the same time limit the formation of thermal NOx. Equipment The three stages mentioned above are carried out in a series of refractory-lined carbon steel vessels with a high-intensity burner at the head of the equipment train. This type of burner takes a relatively high pressure drop on the combustion air, causing intense turbulence which produces very good mixing between the air and the waste and fuel being burned. Good mixing and a high temperature (over 2600-F) in the burner result in good combustion characteristics. In addition, the high intensity-burner is desirable for this type of application because it operates well under sub stoichiometric conditions. The high temperature and turbulence in the burner help dissociate the nitrogen-bearing compounds in the waste. In the case of NOx-containing waste gases, it causes the hydrocarbons in the fuel to react with the oxygen in the NOx, producing elemental nitrogen. The first stage of the combustion process is funher carried out in the reduction furnace. The furnace is a cylindrical, refractory-lined vessel which provides a residence time of at least 2.0 seconds at an operating temperature of 2200-F. Because of the high-temperature and reducing conditions existing in the reduction furnace, a high-alumina firebrick and a low-iron castable refractory are generally preferred. The cooling step utilizes a venturi section to maximize mixing between the hot products of combustion from the reduction furnace and the quench medium. Depending on the cooling medium use~ water is injected through atomizing spray guns, steam through nozzles, and recycled flue gas through ports around the refractory-lined venturi section. Water and steam are usually injected downstream of the venturi while recycle flue gas is injected into the venturi. The quench section also is constructed of refractory-lined carbon steel Re-oxidation air to complete the combustion process in the third stage is introduced through a second set of ports discharging into another venturi section. The combustion process started here is completed in the re-oxidation furnace, which is a cas table refractory-lined carbon steel, cylindrical vessel. The fmal products of combustion are either vented through a refractory-lined stack of appropriate height or sent through a boiler before being vented through an unlined stack. 3 |