Urea Decomposition Slip Stream for SCR Ammonia Generation
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Solution Overview
Problem
Current methods for reducing nitrogen oxide (NOx) emissions in small industrial, commercial, and electric utility boilers, especially combined cycle gas turbines, face challenges in efficiently converting urea to ammonia for selective catalytic reduction (SCR) due to issues like incomplete gasification, solid byproduct formation, and inefficiencies in heat utilization, particularly at higher urea injection rates and low loads.
Innovation Solution
A system and method that involves spatially separating heat exchanger sections, using a slip stream of hot exhaust gases to decompose aqueous urea to ammonia in a continuous duct with controlled gas flow and temperature, and injecting the ammonia gas upstream of the SCR catalyst, minimizing residence time and bypassing no heat exchanger surfaces to maintain efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If urea is injected into a heated vaporizer or flowing side stream for gasification, then ammonia is produced for NOx reduction, but solid byproducts form and foul surfaces reducing chemical utilization
Solution Approach 1:
The patent changes the temperature parameter in the decomposition chamber to maintain it above 650°F, ensuring complete urea decomposition without solid byproduct formation. This temperature control prevents fouling while maintaining high ammonia production rates for effective NOx reduction
Solution Approach 2:
The patent replaces traditional vaporizer heating systems with a slip stream of hot exhaust gases that flows through the decomposition chamber. This substitution eliminates the need for separate heating equipment and heat exchangers that could become fouled, while maintaining the necessary thermal conditions for complete urea decomposition
2Productivity
If flue gas is bypassed around heat exchanger sections to provide heat for urea gasification, then complete gasification is achieved, but heat enthalpy is lost as the bypassed gases do not pass through the heat exchanger
Solution Approach 1:
The patent makes the slip stream of exhaust gases serve multiple functions: it provides the necessary heat for urea decomposition in the continuous duct, and then the decomposed gases are returned to the primary flue gas stream to pass through the heat exchanger sections. This eliminates energy loss while ensuring complete gasification
Solution Approach 2:
The patent merges the decomposition chamber with the existing flue gas flow path by creating a slip stream that branches off, undergoes urea decomposition, and then rejoins the main flue gas stream. This integration ensures that all gases eventually pass through the heat exchanger sections, maintaining thermal efficiency while enabling complete urea gasification
3Productivity
If residence time of 1-10 seconds is used for urea decomposition in large-scale combustors, then complete decomposition is achieved, but larger decomposition reactors are required which are not advantageous for small combustion sources
Solution Approach 1:
The patent maintains a compact decomposition chamber volume but achieves complete decomposition by controlling the slip stream flow rate and temperature to ensure sufficient residence time. The continuous flow of hot exhaust gases through the relatively small chamber provides both the necessary thermal conditions and adequate exposure time for complete urea decomposition without requiring a large reactor volume
4Productivity
If steam from the boiler is used to vaporize aqueous ammonia or urea, then vaporization is achieved, but steam is removed from the heat or power generation process and de-mineralized boiler makeup water is required
Solution Approach 1:
The patent uses the waste heat from the exhaust gases themselves to vaporize and decompose the urea reagent in the continuous duct. The hot slip stream of exhaust gases provides the necessary thermal energy, eliminating the need to extract steam from the boiler for this purpose and maintaining full steam generation capacity for heat and power production
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 reduces NOx emissions effectively by ensuring complete urea gasification in under 1 second, minimizing heat and power losses, and scaling for higher urea injection rates, while maintaining system efficiency and preventing solid byproduct fouling.
Implementation Method 1
A slip stream of hot exhaust gases is withdrawn from a location downstream of the catalyst at a temperature of 500° F. to 900° F. and directed through a fan or blower to a continuous duct. An aqueous based reagent is injected into the slip stream flowing through the continuous duct such that the aqueous based reagent decomposes to ammonia gas.
Implementation Method 2
A slip stream of hot exhaust gases is withdrawn from a location downstream of the catalyst at a temperature of 500° F. to 900° F. and directed through a fan or blower to a continuous duct.
Implementation Method 3
a catalyst effective for NOx reduction is provided downstream of the second heat exchanger section
Data Source
AI summary
A method for reducing NOx emissions in the exhaust of a combined cycle gas turbine equipped with a heat recovery boiler and a catalyst effective for NOx reduction, wherein a slip stream of hot flowing exhaust gases is withdrawn from the primary gas flow after the catalyst at a temperature of 500° F. to 900° F. and directed through a fan to a continuous duct into which an aqueous based reagent is injected for decomposition to ammonia gas and the outlet of the continuous duct is connected to an injection grid positioned in the primary exhaust for injection of ammonia gas into the primary exhaust stream at a location upstream of the catalyst.