OCR Text |
Show energy has been lost to the boiler walls in the primary combustion zone, secondary air is introduced to complete combustion to bring the overall mixture to an air-rich condition. This fuel-rich combustion and subsequent secondary combustion occurs in the radiant portion of the boiler and is completed before the combustion gases enter the convective pass where NOx formation is virtually zero. This appraoch reduces both oxygen availability and peak flame temperature. There are various methods of accomplishing 0/S combustion. The earliest method, "two-stage combustion," utilizes overfire air (OFA) ports (or NOx ports) over the burners to divert and introduce the secondary air into the furnace. Diverting a portion of the combustion air to the NOx ports forces the burners to operate more fuel-rich. A second method of 0/S combustion utilizes existing burners to supply the secondary air to complete the combustion. This is accomplished by terminating the fuel flow to selected burners, thus increasing the flow to the remaining burners to maintain the same total fuel flow. Air flows to the burners are left essentially unchanged so that burners-out-of-service (BOOS) are on air only, and the remaining burners are operated more fuel-rich. As with "two-stage combustion," the secondary combustion occurs when the air from the air-only burners mixes with the fuel-rich combustion products in the bulk gas zone. There are practical limits to the extent to which 0/S combustion can be applied. Additional air flow and FD fan capacity is needed as the degree of 0/S combustion is increased to raise the overall 02 level so that smoke and CO emissions can be avoided. In particular, this approach is only applicable to multiburner units. NOx ports or OFA ports alone are usually not as effective as BOOS. There are two reasons for this. The NOx ports are normally not as large as the burner throats; consequently the NOx ports do not provide as great a degree of staged combustion as BOOS; and the ports are usually located above the top row of burners which is not generally a good location in which to inject the second stage air for completing oil fuel combustion. 4-5 P-233 |