Selective catalytic reduction of NO, enabled by side stream urea decomposition

Abstract

A preferred process arrangement utilizes the enthalpy of the flue gas, which can be supplemented if need be, to convert urea ( 30 ) into ammonia for SCR. Urea ( 30 ), which decomposes at temperatures above 140 ° C., is injected ( 32 ) into a flue gas stream split off ( 28 ) after a heat exchanger ( 22 ), such as a primary superheater or an economizer. Ideally, the side stream would gasify the urea without need for further heating; but, when heat is required it is far less than would be needed to heat either the entire effluent ( 23 ) or the urea ( 30 ). This side stream, typically less than 3% of the flue gas, provides the required temperature and residence time for complete decomposition of urea ( 30 ). A cyclonic separator can be used to remove particulates and completely mix the reagent and flue gas. This stream can then be directed to an injection grid ( 37 ) ahead of SCR using a blower ( 36 ). The mixing with the flue gas is facilitated due to an order of magnitude higher mass of side stream compared to that injected through the AIG in a traditional ammonia-SCR process.

Description

technical field [0001] The present invention relates to a new NO x The selective catalytic reduction of urea can be achieved by a side stream separated from the main flow or a side stream of hot air used to decompose urea into its active components including ammonia. Background technique [0002] Efforts have been made in many jurisdictions to reduce nitrogen oxides (NO x ) emissions. The technologies include those that modify the combustion conditions and fuel (called primary measures) and those that treat the exhaust gases after combustion (called secondary measures). When effective primary measures are adopted, auxiliary measures can still be used to further reduce them. It appears that to best reduce NO x , both primary and secondary measures are necessary. [0003] Known auxiliary measures are selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR). Both methods have been performed with ammonia and urea. See for example US 3 900 554 where...

AI Technical Summary

Problems solved by technology

While achieving the goal of removing water from the described process, the specification introduces solid urea into the gas stream - potentially depositing urea on the SCR catalyst

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