Advanced Postcombustion Cleaning

Advanced postcombustion cleaning technologies encompass two approaches (1) using the existing flue gas ductwork to inject a sorbent and (2) inserting one or more separate vessels into the downstream ductwork where pollutant absorbents are added. These advanced technologies offer several 

In-duct sorbent cleaning occurs out, as the name indicates, inside the ductwork leading from the boiler to the smokestack. Sulfur dioxide absorbers (e.g., hydrated lime) are sprayed into the center of the duct. By controlling the humidity of the flue gas and the spray pattern of the sorbent, 50%-70% of the sulfur dioxide can be removed and the reaction produces dry particles that can be collected downstream. In-duct sorbent injection is an attractive option for retrofitting smaller, older plants where space requirements might be limited. 

When separate vessels are used, one or more process chambers are inserted in the flue gas ductwork, and various sorbents are injected to remove the pollutants. The separate vessels provide a longer residence time for the absorbent to react with the gas, and pollutant capture is greater. This approach, at some increase in cost over the in-duct injection procedure, has the potential of capturing more than 90% of the pollutants. Technologies such as the spray dryer and SCR represent approaches that use separate vessels. 

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Some of the advanced techniques used in postcombustion cleaning—such as the use of granular calcium oxide or sodium sulfite solutions—have already been described above. In the SNOX process, cooled flue gases are mixed with ammonia gas to remove the nitric oxide by catalytically reducing it to molecular nitrogen. The resulting gas is reheated and sulfur dioxide is oxidized catalytically to sulfur trioxide, which is subsequently hydrated by water to sulfuric acid, condensed, and removed. 

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