Direct Urea Injection for SCR NOx Reduction
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Solution Overview
Problem
Existing methods for reducing nitrogen oxides (NOx) emissions in coal-fired power plants through Selective Catalytic Reduction (SCR) face challenges with the direct injection of aqueous urea, including plugging, incomplete decomposition, and inefficient mixing in solid fuel combustion units, which are costly and pose safety risks due to the need for complex ammonia handling and vaporization systems.
Innovation Solution
A method and apparatus for directly injecting a urea solution into the flue gas stream of a coal-fired power plant, utilizing turbulence-producing devices and an atomizing urea solution injection nozzle with blanketing air to ensure adequate mixing and prevent plugging, eliminating the need for ammonia and complex systems.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If aqueous urea is directly injected into the flue gas stream, then safety is improved and handling is simplified, but deposits form in the grid plugging it
Solution Approach 1:
The patent applies preliminary action by creating turbulence in the flue gas stream before the injection point to pre-mix the gas flow. This preliminary mixing action ensures that when aqueous urea is injected, it disperses uniformly throughout the stream without forming deposits in the grid, thus maintaining grid reliability while enabling safe aqueous urea handling.
Solution Approach 2:
The patent changes the physical parameters of the flue gas stream by introducing turbulence through static mixers or other mixing devices. This parameter change in flow pattern and mixing intensity allows aqueous urea to be injected and distributed uniformly without the incomplete decomposition that causes grid plugging, resolving the contradiction between ease of operation and reliability.
2Device complexity
If urea is injected without complex decomposition systems, then device complexity is reduced, but urea decomposition to ammonia is incomplete
Solution Approach 1:
The patent applies self-service by utilizing the existing high temperature flue gas stream (typically 300-600°C) to automatically decompose the injected aqueous urea into ammonia. The flue gas itself provides the thermal energy needed for decomposition, eliminating the need for external decomposition systems while maintaining high decomposition efficiency through the naturally occurring thermal conditions.
Solution Approach 2:
The patent uses the flue gas stream as an intermediary medium that serves dual purposes: it is both the carrier for urea injection and the heat source for decomposition. This intermediary role of the flue gas eliminates the need for separate decomposition equipment while ensuring complete urea conversion to ammonia through the thermal energy already present in the combustion process.
3Productivity
If injection occurs at high temperature, then urea decomposition is facilitated, but duct wall corrosion increases
Solution Approach 1:
The patent applies dimensionality change by shifting the injection location from near the duct wall to the center of the flue gas stream. This spatial repositioning allows urea to be injected into the bulk gas flow where decomposition occurs uniformly throughout the stream, preventing concentrated urea or ammonia contact with duct walls and thereby reducing corrosion while maintaining effective decomposition through the available thermal field.
Solution Approach 2:
The patent applies local quality by creating a localized decomposition zone in the center of the flue gas stream away from duct walls. The turbulence-induced mixing creates a specific region where urea decomposition occurs uniformly, separating the decomposition process from the duct wall interface and thus eliminating the local corrosive conditions that would otherwise occur at high temperatures.
4Reliability
If wall injection is used to avoid grid plugging, then reliability is improved, but mixing is insufficient causing inefficient catalysis
Solution Approach 1:
The patent applies preliminary action by introducing turbulence and mixing devices upstream of the injection point to pre-condition the flue gas flow. This preliminary mixing action ensures that when urea is injected at the wall, it is rapidly dispersed into the bulk stream through the pre-established turbulent flow patterns, achieving both wall injection reliability and sufficient mixing for efficient catalysis.
Solution Approach 2:
The patent uses pneumatic principles by utilizing the high-velocity flue gas stream itself as the mixing medium. The kinetic energy and turbulence of the gas flow create intense mixing that rapidly distributes injected urea throughout the stream, achieving uniform distribution and sufficient mixing for efficient catalysis without requiring additional mechanical mixing equipment.
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 achieves efficient NOx reduction with improved process efficiency, reduced urea usage, and enhanced safety by ensuring uniform distribution of ammonia at the SCR catalyst, overcoming previous limitations of plugging and decomposition issues in solid fuel combustion units.
Implementation Method 1
utilizing turbulence-producing devices and an atomizing urea solution injection nozzle
Implementation Method 2
Injection occurs downstream of the economizer, at a lower temperature range
Implementation Method 3
the need to vaporize the urea before it hits the walls of the duct to prevent corrosion
Implementation Method 4
an atomizing urea solution injection nozzle with blanketing air
Data Source
AI summary
This disclosure provides an apparatus and method for reducing emissions of nitrogen oxides (NOx) from a combustion source. For example, a method and apparatus for injecting a urea solution directly into the flue gas stream of a coal-fired power plant that utilizes Selective Catalytic Reduction (SCR) to lower NOx emissions.