US 5,853,683 AGranted

Hybrid SCR/SNCR process

AssigneePublic Services Electric & Gas Corporation

InventorsFrancis X. Gibbons, Alexander L. Huhmann, Albert J. Wallace

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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.

Claims10

Dependent claims

A process for reducing the amount of nitrogen oxides contained in a flue gas stream of a carbonaceous material combustion process, the process comprising the steps of:

providing means for introducing into the flue gas stream a first nitrogenous treatment agent effective in the selective non-catalytic reduction of nitrogen oxides;
providing means for introducing into the flue gas stream a second nitrogenous treatment agent effective in the selective catalytic reduction of nitrogen oxides, said means for introducing the second nitrogenous treatment agent located downstream from the means for introducing the first nitrogenous treatment agent;
providing a catalyst effective for the selective catalytic reduction of nitrogen oxides in the presence of said second nitrogenous treatment agent, said catalyst having a maximum effective nitrogen oxides level reducing capability;
selecting a non-zero baseline amount of first nitrogenous treatment agent to be the minimum amount of first nitrogenous treatment agent added during all periods the process is operating;
contacting the flue gas with at least said selected non-zero baseline amount of said first nitrogenous treatment agent causing at least a portion of the nitrogen oxides in the flue gas to be reduced by the selective non-catalytic reduction of nitrogen oxides during all periods when the flue gas stream contains nitrogen oxides;
contacting the flue gas with said second nitrogenous treatment agent;
then contacting the flue gas with said catalyst effective for the selective catalytic reduction of nitrogen oxides in the presence of said second nitrogenous treatment agent causing additional nitrogen oxides in the flue gas to be reduced by the selective catalytic reduction of nitrogen oxides;
and
wherein during periods when the level of nitrogen oxides in the flue gas exceeds the amount of nitrogen oxides which can be removed by the combination of selective non-catalytic reduction using said selected non-zero baseline amount of first nitrogenous treatment plus selective catalytic reduction using said maximum effective nitrogen oxides level reducing capability of said catalyst, additional first treatment agent is introduced into the flue gas stream in an amount effective to reduce the level of nitrogen oxides in the flue gas approximately to the level of said maximum effective nitrogen oxides level reducing capability of said catalyst.

6 more elements

In a process of reducing the amount of nitrogen oxides contained in a flue gas stream of a carbonaceous material combustion process, said process including:

providing means for introducing into the flue gas stream a first nitrogenous treatment agent effective in the selective non-catalytic reduction of nitrogen oxides;
providing means for introducing into the flue gas stream a second nitrogenous treatment agent effective in the selective catalytic reduction of nitrogen oxides, said means for introducing the second nitrogenous treatment agent located downstream from the means for introducing the first nitrogenous treatment agent;
providing a catalyst effective for the selective catalytic reduction of nitrogen oxides in the presence of said second nitrogenous treatment agent, said catalyst having a maximum effective nitrogen oxides level reducing capability;
contacting the flue gas with said first nitrogenous treatment agent reducing nitrogen oxides in the flue gas;
then contacting the flue gas with said catalyst effective for the selective catalytic reduction of nitrogen oxides in the presence of said second nitrogenous treatment agent reducing nitrogen oxides in the flue gas;
and
wherein, during periods when the level of nitrogen oxides in the flue gas exceeds the amount of nitrogen oxides which can be removed by the combination of said first nitrogenous treatment plus said maximum effective nitrogen oxides level reducing capability of said catalyst, first treatment agent is introduced into the flue gas stream in an amount effective to reduce the level of nitrogen oxides in the flue gas approximately to the level of said maximum effective nitrogen oxides level reducing capability of said catalyst;
the improvement comprising:
continuously introducing into contact with the flue gas stream at all periods during the process a preselected non-zero baseline amount of said first nitrogenous treatment agent causing at least a portion of the nitrogen oxides in the flue gas stream to be reduced by selective catalytic reduction of nitrogen oxides during all periods during which the flue gas stream contains nitrogen oxides.

6 more elements

Dependencies are inferred from the claim text.