Minimization of Major Air Pollutants from Sewage Sludge Fluid Bed Incinerators

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ABSTRACT

Emissions of major air pollutants including carbon monoxide (CO), nitrogen oxides (NOx) and mercury (Hg) were studied at two sewage sludge fluidized bed combustion (FBC) plants. Both the North West Bergen (NWB) County Utility Authority and the Little Miami (LM) plants have air pollution control systems (APCS) that are of the wet type. Various operating conditions such as temperature, excess air, fuel and feed properties, concentration of calcium chloride, and the method of operating were investigated to determine their effect on the emissions.

Results of the study reveal that NOx increase with increased bed temperatures and excess air. CO is lower at higher freeboard temperatures. Both CO and NOx emissions are strongly dependent on the feed characteristics. CO and NOx fluctuate more with higher sludge solid content (TS). They increase not only with TS, but also with the sludge volatile solids (VS) and its heat content. NOx increase with the ratio of sludge oxygen over sludge nitrogen concentration. They are also higher with fuel oil than with natural gas when selected as the auxiliary fuel source.

CO and NOx emissions are also higher and fluctuate more when the operation is not run consistently; when the feed quality is inconsistent, or when the feeding is intermittent or interrupted, or when the roof sprays are activated. It is believed that the feed characteristics and mode of operating affect the flame temperature and the excess air surrounding the combusting materials, which in turn have an impact on CO and NOx emissions.

While some of the mercury (Hg) vapor is removed by condensation due to flue gas cooling, the addition of calcium chloride to the feed has shown a clear reduction in both emissions of NOx and Hg. It is believed that calcium chloride can generate strong oxidants, promoting the oxidation of both elemental mercury and NOx into more soluble forms, which can then be separated from the exhaust gas in a wet scrubber.

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