Side Stream Urea Decomposition for SCR NOx Reduction
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Solution Overview
Problem
Current methods for reducing nitrogen oxide (NOx) emissions in industrial and small utility boilers face challenges due to the impracticality of urea injection in tight furnace convective zones, high costs, and inefficiencies in urea conversion to ammonia, particularly in systems designed for larger boilers, which are not suitable for smaller units, and issues with particulate control devices.
Innovation Solution
A system employing a side stream of hot exhaust gas to convert urea to ammonia using a gas-to-gas heat exchanger and injector, positioning the inlet downstream of the furnace and upstream of particulate control devices, and reintroducing the gasified ammonia downstream of these devices to ensure all exhaust gas passes through particulate control devices, optimizing heat usage and reducing fouling.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If urea is injected directly into the furnace convective zone, then NOx reduction is achieved, but urea deposition on boiler tube surfaces and corrosion of water wall surfaces occurs
Solution Approach 1:
The system divides the exhaust gas flow into two separate streams: a primary exhaust gas stream that passes through the furnace and particulate control devices, and a side stream that is diverted through a heat exchanger for urea decomposition. This segmentation allows urea to be converted to ammonia in a controlled environment without direct contact with boiler tube surfaces, eliminating deposition and corrosion problems while maintaining NOx reduction effectiveness.
Solution Approach 2:
The patent introduces a heat exchanger as an intermediary device that transfers heat from the primary exhaust gas stream to the side stream containing urea. This intermediary mechanism enables indirect heating and decomposition of urea without direct contact between the urea solution and hot boiler surfaces, preventing both deposition and corrosion while achieving the desired ammonia generation for NOx reduction.
2Object-generated harmful factors
If urea injection is performed in tight tube spacing zones, then NOx reduction is achieved, but adequate distribution of reagent into furnace gases is prevented
Solution Approach 1:
The system extracts the urea decomposition process from the constrained furnace environment and relocates it to a side stream heat exchanger system. By taking out the decomposition function from the tight tube spacing zone, the system eliminates the distribution problem while maintaining the chemical conversion function. The ammonia is then reintroduced into the primary exhaust stream downstream where proper mixing occurs naturally.
Solution Approach 2:
The patent moves the urea decomposition process from the spatial dimension of the furnace (constrained by tube spacing) to a different dimensional space - the side stream heat exchanger system. This dimensional transition allows urea to be decomposed in an open, unconstrained environment where heat transfer and chemical reaction can occur efficiently, and the resulting ammonia can be properly distributed into the exhaust stream.
3Object-generated harmful factors
If ammonia is introduced upstream of particulate control devices, then NOx reduction is achieved, but fouling of particulate control devices occurs
Solution Approach 1:
The system performs preliminary decomposition of urea to ammonia in the side stream heat exchanger before the ammonia is introduced into the primary exhaust stream. This preliminary action ensures that the decomposition occurs in a controlled environment upstream of the particulate control devices in terms of process sequence, but the physical introduction of ammonia occurs downstream of these devices, preventing fouling while maintaining NOx reduction effectiveness.
Solution Approach 2:
The patent inverts the conventional approach by not introducing urea or ammonia upstream of the particulate control devices. Instead, it decomposes urea in a separate side stream and reintroduces the ammonia downstream of the particulate control devices. This inversion of the introduction timing and location prevents ammonia from interfering with particulate control device operation while still achieving the desired NOx reduction.
4Object-generated harmful factors
If systems designed for large utility boilers are applied to small utility boilers, then NOx reduction capability is provided, but capital costs become prohibitively high
Solution Approach 1:
The system changes the operational parameters of the heat exchanger and decomposition process to be suitable for smaller boiler applications. By adjusting the scale and heat transfer characteristics of the side stream system, the patent adapts the technology from large utility boiler applications to small utility boilers, reducing capital costs while maintaining effective NOx reduction capability appropriate for smaller units.
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 reduces NOx emissions by utilizing the heat from the primary exhaust stream for urea conversion, minimizing fouling of particulate control devices, and ensuring efficient passage of all exhaust gases through these devices, thereby improving NOx reduction rates and reducing capital costs.
Implementation Method 1
a gas-to-gas heat exchanger to heat the side stream
Implementation Method 2
heat the side stream by passing the hot exhaust gas through a heat exchanger
Implementation Method 3
convert urea to ammonia for use in NOx reduction by selective catalytic reduction (SCR)
Implementation Method 4
selective catalytic reduction (SCR)... through which a portion of the primary exhaust stream flows, the second side stream... an SCR catalyst
Data Source
AI summary
A lean burn combustion source includes a first side stream comprising an inlet and an outlet, both positioned downstream of a furnace and upstream of a particulate control device, and a second side stream comprising: an inlet positioned downstream of the particulate control device and upstream of the catalyst, a heat exchanger section passing through the first side stream, whereby heat from hot exhaust gas flowing through the first side stream is transferred to hot exhaust gas flowing through the second side stream, an injector positioned in the second side stream injecting aqueous based reagent into the hot exhaust gas flowing through the second side stream such that the aqueous based reagent decomposes to ammonia gas, and an outlet in fluid communication with a reagent distribution device positioned in the primary exhaust gas stream downstream of the particulate control device and upstream of the catalyst.