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Show In existing equipment the retrofit of an SGR system might allow increased steaming capacity if the old boiler was limited by fan speed (I.D. or F.D.), opacity, particulates, or furnace pressure. This is because of the reduction of the gas flow requirements which takes some of the burden off the fans and the reduction in the particulate loading in the flue gas that goes to the stack. Reduced opacity is also achieved by the modifications to the overfire air system discussed below. The Application To apply Stoker Gas Recirculation, a series of flues is connected into the boiler system along with a gas recirculation fan. Figure 1 contains a cutaway of the boiler with a line drawing of the SGR system. Flue gas is extracted at a point downstream of the boiler firebox after it has been cooled in the boiler bank, economizer, or heat trap. The flue gas flows through the fan and is injected into the stoker undergrate air duct downstream of the combustion air (forced draft) fan. At the point of injection of the recirculated flue gas into the air stream, proper mixing of the two streams is achieved by a distribution nozzle system. Equipment and controls for stoker gas recirculation are installed in parallel with the existing stoker system. Therefore, there is no interference with operation of the stoker or boiler if gas recirculation is not utilized. A flow control damper on the discharge of the FGR fan is positioned by a local drive unit receiving a control signal from a controller mounted on the boiler system control panel. The boiler operator can remotely operate this damper by "hand" or "•automatic.'1 In the automatic mode the FGR combustion control signal follows the forced draft flow controller and can be biased to set the ratio of FGR to air. On manual the operator can set the percent FGR desired. Stoker Gas Recirculation will allow operation at reduced combustion air flow but the boiler operator must make adjustments. To adjust the relative air flow effectively, there must be a consistent means of measuring or detecting combustion air flow and boiler steaming rate. Continuously measuring excess oxygen content of the flue gas is a way to properly set the 23-4 |