Urea Decomposition Duct for SCR Ammonia Generation
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Solution Overview
Problem
Current methods for reducing nitrogen oxide (NOx) emissions from small industrial and commercial boilers using urea as a reagent face challenges such as incomplete gasification, solid byproduct formation, and inefficiencies in scaling up urea injection rates, requiring complex vaporizer systems and long residence times.
Innovation Solution
A system utilizing an exhaust gas slipstream for transporting urea to a decomposition duct where temperature and flow are controlled, with a supplemental heater and precise urea injection, achieving complete gasification in under 1 second, reducing the need for external power and minimizing solid byproduct formation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If urea is injected into a vaporizer or hot combustion gases for gasification, then ammonia is produced for NOx reduction, but the process requires long residence times and complex vaporizer systems
Solution Approach 1:
The invention extracts the urea decomposition process from a separate vaporizer system and integrates it directly into the flue gas stream. Urea is injected directly into the hot flue gases where it decomposes, eliminating the need for complex dedicated vaporizer equipment while maintaining effective ammonia production for NOx reduction.
Solution Approach 2:
The invention merges the urea decomposition function with the existing flue gas flow path. By injecting urea directly into the hot flue gases that are already flowing through the SCR system, the decomposition process is combined with the gasification and distribution functions, simplifying the overall system architecture.
2Quantity of substance
If urea injection rate is increased to handle higher loads, then more ammonia is available for NOx reduction, but complete gasification becomes more difficult and solid byproducts increase
Solution Approach 1:
The invention changes the temperature parameter of the decomposition environment by injecting urea into flue gases at temperatures above 320°C (optimally 350-450°C). This temperature parameter change ensures complete urea decomposition and gasification even at higher injection rates, preventing solid byproduct formation while maintaining high ammonia production for effective NOx reduction.
3Reliability
If steam is used to vaporize aqueous ammonia or urea, then complete gasification is achieved, but steam removal from heat generation process and de mineralized water preparation are required
Solution Approach 1:
The invention uses the hot flue gases themselves, which are already present in the system, to provide the thermal energy needed for urea decomposition. The flue gases serve dual purposes: they are the medium through which NOx passes the catalyst and they provide the heat for urea gasification, eliminating the need for separate steam generation and water treatment systems.
4Productivity
If residence time is extended to ensure complete urea decomposition, then gasification efficiency improves, but system size and complexity increase
Solution Approach 1:
By changing the temperature parameter to above 320°C (optimally 350-450°C), the decomposition kinetics are accelerated, allowing complete urea gasification to occur within the existing short residence time of the flue gas stream. This eliminates the need to extend residence time or increase system size.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach effectively converts urea to ammonia for NOx reduction in small combustion systems, improving efficiency and reliability while reducing costs and complexity, achieving NOx reduction with shorter residence times and minimal external power requirements.
Implementation Method 1
A supplemental electric heater or burner is disposed in the slip stream portion of a duct following the blower and a temperature sensor is linked to the heater to maintain a gas temperature in the duct before the point of urea injection of at least 750° F.
Implementation Method 2
Method for urea decomposition and ammonia feed to a selective catalytic reduction system utilizes an exhaust gas slipstream for transporting urea to a decomposition duct where temperature and flow are controlled
Implementation Method 3
achieving complete gasification in under 1 second, reducing the need for external power and minimizing solid byproduct formation
Data Source
AI summary
A method of reducing NOx emissions from a lean burn combustion source employs an aqueous solution of reagent that is injected into a continuous decomposition duct at a rate of 0.2-10 gph with a flowing side stream of hot gas at a rate of 150-3000 scfm and a temperature of greater than 700° F. in the decomposition duct such that the aqueous reagent is converted to ammonia gas that is conveyed by the continuous decomposition duct to an ammonia injection grid that is placed in a primary exhaust stream from the combustion source upstream of a NOx reducing catalyst and NOx is reduced.