Boiler NOx Reduction via Upstream Heat Exchanger
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Solution Overview
Problem
Current methods for reducing nitrogen oxides in steam-generating boilers, such as chemical recovery boilers, face inefficiencies and high costs due to the need for optimal temperature control and material adjustments, which are challenging to achieve, especially in larger and more expensive boilers.
Innovation Solution
A method involving a heat exchanger, or screen, is placed in the flue gas flow upstream of the superheater zone to decrease the temperature of flue gases, creating a suitable temperature window for the introduction of a reducing agent like ammonia or urea, allowing for effective nitrogen oxide reduction without the need for oversized furnaces or expensive materials.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the temperature of flue gases is decreased to create a suitable temperature window for reducing nitrogen oxides, then the effectiveness of nitrogen oxide reduction is improved, but the heat recovery efficiency of the boiler is reduced
Solution Approach 1:
The flue gas flow path is segmented into distinct zones: a first zone for heat recovery and a second zone for nitrogen oxide reduction. By spatially separating these functions, the system can optimize temperature for heat recovery in the first zone while maintaining lower temperatures suitable for reduction chemistry in the second zone, thus resolving the contradiction between heat recovery efficiency and nitrogen oxide reduction effectiveness.
Solution Approach 2:
A reducing agent (such as ammonia or urea) is introduced as an intermediary substance that facilitates nitrogen oxide reduction at lower temperatures. This intermediary enables the reduction process to occur effectively in the second zone without requiring the entire flue gas stream to be cooled uniformly, thereby preserving heat recovery efficiency while achieving nitrogen oxide reduction.
2Object-generated harmful factors
If a reducing agent is introduced into the flue gas flow to reduce nitrogen oxides, then nitrogen oxide emissions are reduced, but the complexity of the boiler system is increased
Solution Approach 1:
The reducing agent injection system is merged with the existing flue gas flow path and combustion control system. The reducing agent is introduced through existing nozzles or injection points in the flue gas stream, and the control system integrates with the combustion control, thereby reducing nitrogen oxide emissions without significantly increasing system complexity.
Solution Approach 2:
The introduction of the reducing agent changes the chemical parameters of the flue gas stream, enabling nitrogen oxide reduction through chemical reactions. By controlling the injection timing, concentration, and mixing conditions, the system achieves effective nitrogen oxide reduction while maintaining relatively simple hardware configurations.
3Reliability
If the boiler is designed with larger size to accommodate optimal temperature control for nitrogen oxide reduction, then nitrogen oxide reduction effectiveness is improved, but the material cost and boiler height are increased
Solution Approach 1:
Instead of increasing the vertical height of the boiler to accommodate separate temperature zones, the system utilizes the horizontal dimension by introducing a first zone and second zone arranged in sequence along the flue gas flow path. This dimensional approach allows optimal temperature control for nitrogen oxide reduction without proportionally increasing boiler height, thereby reducing material costs while maintaining reduction effectiveness.
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 enables efficient nitrogen oxide reduction by ensuring proper temperature and mixing conditions for the reducing agent, reducing emissions while minimizing boiler height and material costs, and protecting superheater surfaces from corrosion.
Implementation Method 1
the temperature of the flue gases is decreased by means of a heat exchanger that is a screen and that is located in the flue gas flow upstream of the introduction of the reducing agent
Implementation Method 2
heat is transferred into the feed water or boiler water flowing inside the elements
Implementation Method 3
a method of decreasing an amount of nitrogen oxides from flue gases... by introducing a reducing agent into the flue gases
Data Source
Figure 1
Figure 2a~2c
AI summary
The present invention relates to a method and a boiler for decreasing the amount of nitrogen oxides in flue gases of a boiler, which flue gases are generated in the combustion of fuels and air. The boiler has a water circulation system comprising superheaters (8) and a furnace (1) for combustion of fuel and for generating flue gases that contain nitrogen oxides, which flue gases mainly flow upwards in the furnace and further to the superheater zone and via other heat recovery surfaces of the boiler out of the boiler, and a nitrogen oxides reducing agent is introduced into the flue gases. It is essential that the nitrogen oxides reducing agent is introduced into the flue gases prior to the superheater zone, before which the temperature of the flue gases is decreased by means of at least one heat exchanger (15) that is located in the flue gas flow upstream of the introduction of the reducing agent, for obtaining a suitable temperature window in the flue gas flow in order to reduce nitrogen oxides.