On-Demand Ammonia Generation for Small Boiler NOx Reduction
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Solution Overview
Problem
Small industrial and commercial boilers face challenges in reducing NOx emissions due to the difficulties in handling and storing ammonia, and existing urea-based systems are inefficient and costly, especially for low-temperature applications with short residence times, leading to high costs per ton of pollutant removed.
Innovation Solution
A system that injects aqueous urea solution into a heated air stream within a continuous duct, where it evaporates and decomposes into ammonia gas, which is then injected into the exhaust duct upstream of a NOx reduction catalyst, eliminating the need for supplemental heaters and ammonia storage, and optimizing the duct design for efficient heat transfer and residence time.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If aqueous urea solution is used instead of ammonia for SCR, then safety and handling issues are improved, but the conversion efficiency to ammonia is reduced due to lower decomposition rates at small boiler temperatures
Solution Approach 1:
The system pre-heats the urea solution before injection into the exhaust stream. By preliminarily heating the urea to a temperature where it decomposes efficiently (above 400°F), the system ensures complete conversion to ammonia even in the short residence time available in small boilers, thus resolving the contradiction between safety (using urea instead of ammonia) and conversion efficiency
Solution Approach 2:
The invention changes the temperature parameter of the urea solution from ambient to elevated (above 400°F) before injection. This parameter change enables complete decomposition of urea to ammonia within the short residence time of small boilers, maintaining high conversion efficiency while using the safer urea compound
2Productivity
If supplemental heaters or burners are added to improve urea decomposition, then conversion efficiency is improved, but system complexity and cost increase
Solution Approach 1:
The system uses the existing hot exhaust gas from the boiler to heat and decompose the urea solution. The exhaust gas, which would otherwise be wasted, provides the thermal energy needed for urea decomposition. This self-service approach achieves high conversion efficiency without adding supplemental heaters or burners, thus avoiding increased system complexity
Solution Approach 2:
The exhaust gas acts as an intermediary medium that transfers thermal energy from the boiler to the urea solution. Instead of directly heating urea with supplemental burners, the system uses the exhaust gas as a heat transfer medium to accomplish decomposition, simplifying the overall system design
3Productivity
If residence time is extended to improve urea decomposition, then conversion efficiency is improved, but the compact design advantage of small boilers is lost
Solution Approach 1:
The system changes the temperature parameter of the urea solution before injection, heating it to above 400°F. This parameter change accelerates the decomposition rate, allowing complete conversion to ammonia within the short residence time (seconds) available in small boilers, thus maintaining compact design while achieving high efficiency
4Productivity
If ammonia is stored on-site for SCR, then immediate availability is improved, but safety and regulatory compliance worsen
Solution Approach 1:
The system extracts the hazardous ammonia storage requirement from the SCR system by using urea as the stored compound. Urea is decomposed to ammonia on-demand in the exhaust stream, eliminating the need for on-site ammonia storage while maintaining immediate availability of ammonia for NOx reduction
Solution Approach 2:
The system stores urea (a safe compound) in advance and converts it to ammonia on-demand through thermal decomposition. This preliminary storage of a safe precursor material resolves the contradiction between having ammonia immediately available and maintaining safety during storage
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 method effectively reduces NOx emissions in small boilers by generating ammonia on-demand, minimizing equipment costs, and avoiding the handling and storage issues associated with ammonia, while maintaining efficiency and safety, even at low temperatures and short residence times.
Implementation Method 1
injects aqueous urea solution into a heated air stream within a continuous duct, where it evaporates and decomposes into ammonia gas
Implementation Method 2
injects aqueous urea solution into a heated air stream within a continuous duct, where it evaporates and decomposes into ammonia gas
Data Source
AI summary
A system and method for reducing NOx emissions from a lean burn combustion source is provided. The system includes a blower passing air through a continuous duct having a hot portion and a reaction portion. The hot portion of the duct is positioned in the convective zone of the combustion source to heat the passing air for the reaction portion of the duct. An injector attached to a urea storage container is positioned in the reaction portion of the duct and sprays urea from the storage container into the heated air in the reaction duct for evaporation and decomposition into ammonia gas. The ammonia gas is then supplied to an injection grid in the exhaust duct of the lean burn combustion source upstream of a NOx reduction catalyst. The injection grid supplies the ammonia gas to the exhaust gas in the exhaust duct.