Hybrid SCR/SNCR process

Details Hybrid SCR/SNCR process Hybrid SCR/SNCR process

1997-04-17

1998-12-29

Lewis, Michael L.

Chemistry of inorganic compounds

Modifying or removing component of normally gaseous mixture

Nitrogen or nitrogenous component

423235, B01J 800

Patent

active

058536833

ABSTRACT:
A process for reducing the level of nitrogen oxides (NO.sub.x) in a fossil fuel (e.g. coal) combustion flue gas has a selective non-catalytic reduction (SNCR) treatment stage followed by a selective catalytic reduction (SCR) treatment stage. The level of NO.sub.x produced by a power plant, particularly a coal fired power plant, varies depending upon the plant load. A set predetermined non-zero amount of first nitrogenous treatment agent is constantly introduced for the SNCR stage during all periods of plant operation. Additional first nitrogenous treatment agent is introduced for the SNCR stage only during periods of relatively higher NO.sub.x production, such as during peak load periods. A second nitrogenous treatment agent is added for the SCR stage up to the maximum SCR NO.sub.x reducing capability of the SCR stage based upon the total amount of NO.sub.x present in the flue gas, subject to limitations on the amount of allowable ammonia slip exiting the plant. Once the total amount of NO.sub.x in the flue gas surpasses a level which the SCR stage can effectively treat, additional first nitrogenous treatment agent is added to the SNCR stage. The amount of additional first nitrogenous treatment agent added is incrementally raised as the NO.sub.x level increases. During relatively lower NO.sub.x producing periods when the set non-zero amount of SNCR plus the SCR stage alone are effectively capable of reducing the level of NO.sub.x to the desired level without surpassing ammonia slip limits, no additional treatment agent is added for the SNCR stage.

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