Oxyfuel Boiler Oxygen Control for NOx Reduction
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Solution Overview
Problem
Oxyfuel boiler systems face challenges in reducing fuel NOx emissions, as existing technologies do not effectively address the formation of nitrogen oxides during coal combustion with high oxygen concentrations, leading to increased combustion temperatures and unstable flame formation.
Innovation Solution
The oxyfuel boiler system incorporates an oxygen controlling apparatus that regulates oxygen concentrations in the burner and after-gas port, creating a reductive and oxidative combustion zone to suppress NOx formation, with an exhaust gas recirculation system that maintains high thermal efficiency and reduces nitrogen concentrations, thereby minimizing fuel NOx production.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If high oxygen concentration is supplied to the burner for efficient combustion, then combustion efficiency is improved, but fuel NOx formation increases
Solution Approach 1:
The combustion process is divided into two distinct zones: a reductive combustion zone near the burner where oxygen concentration is kept low to suppress fuel NOx formation, and an oxidative combustion zone above where oxygen concentration is high to ensure complete combustion. This spatial segmentation allows simultaneous achievement of low NOx formation and high combustion efficiency.
Solution Approach 2:
Different oxygen concentrations are supplied to different locations within the combustion chamber. The burner region receives low oxygen concentration (reductive atmosphere) to minimize fuel NOx, while the upper combustion zone receives high oxygen concentration (oxidative atmosphere) to maintain combustion efficiency. This local differentiation of oxygen supply quality resolves the contradiction between efficiency and emissions.
2Reliability
If oxygen concentration in after-gas port is increased to maintain combustion stability, then flame stability is improved, but NOx emissions increase
Solution Approach 1:
The oxygen supply system is segmented into two independent control paths: one for the burner and another for the after-gas port. This allows the after-gas port to receive high oxygen concentration for flame stability without causing proportional increase in fuel NOx formation at the burner, where oxygen concentration is deliberately kept low.
Solution Approach 2:
High oxygen concentration is locally supplied to the after-gas port region to stabilize the flame and ensure complete combustion, while the burner region maintains low oxygen concentration to suppress fuel NOx formation. This localized differentiation allows flame stability without compromising emissions control.
3Loss of energy
If recirculation flow rate of combustion exhaust gas is increased to control heat absorbing amount, then thermal efficiency is improved, but oxygen concentration control becomes more complex
Solution Approach 1:
Oxygen concentration meters are installed at both the burner and after-gas port to provide real-time feedback on oxygen levels. The control system uses this feedback to independently adjust oxygen supply to each zone, maintaining optimal oxygen concentrations for both NOx suppression and thermal efficiency despite variations in recirculation flow rate.
Solution Approach 2:
The oxygen supply system is made dynamically adjustable with independent control for the burner and after-gas port. This dynamic control capability allows the system to adapt to changing recirculation conditions while maintaining the dual objectives of low fuel NOx formation and high thermal efficiency, managing complexity through intelligent control.
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 significantly reduces fuel NOx emissions, enhances thermal recovery, and stabilizes combustion performance, allowing for efficient electric power generation while minimizing the need for NOx removal apparatuses and reducing operational costs.
Implementation Method 1
reductive combustion zone
Implementation Method 2
oxidative combustion zone
Implementation Method 3
exhaust gas recirculation
Implementation Method 4
dry dust-removing apparatus
Data Source
Figure 1
Figure 2
Figure 3
AI summary
The oxyfuel boiler system comprises: an oxygen generator 10; a coal mill 11; a burner 12 to burn pulverized coal; an after-gas port 13 to which oxygen generated at the oxygen generator 10 is supplied; a boiler 1 provided with the burner 12 and the after-gas port 13 on its wall; a flue 20 introducing combustion exhaust gas from the boiler 1 to the outside; a recirculation gas supply pipe 14 having an exhaust gas tapping port 22 in the midway of the flue 20 and supplying recirculation exhaust gas to the coal mill 11, the burner 12, and the after gas port 13; and an oxygen supply pipe 16 supplying oxygen from the oxygen generator 10 to the burner 12 and the after-gas port 13, wherein the exhaust gas tapping port 22 is disposed downstream of a dry dust-removing apparatus 4 arranged in the flue 20, and there is provided an oxygen controlling apparatus 25 for making a concentration of oxygen to be supplied to the after-gas port 13 lower than that of oxygen to be supplied to the burner 12.