0. Introduction
Thermal power plants remain a cornerstone of global electricity generation, but their environmental footprint continues to draw attention, particularly concerning nitrogen oxides (NOx). NOx, as a key atmospheric pollutant, contributes significantly to smog formation and acid rain. For coal-fired power plants, adhering to stringent environmental regulations requires highly efficient flue gas treatment systems.
One of the most widely adopted technologies for reducing NOx emissions is Selective Catalytic Reduction (SCR). However, SCR technology imposes operational constraints: it demands a specific range of flue gas temperatures (310°C–420°C) for effective catalytic reactions. Challenges arise when coal-fired plants engage in peak shaving, a growing necessity in modern power grids with increasing renewable energy penetration. Under low-load conditions, flue gas temperatures often fall below the required range, compromising denitration efficiency.
This paper examines the operational performance of the #5 boiler at a coal-fired power plant after retrofitting with an economizer flue gas bypass. The bypass technology stabilizes SCR inlet temperatures during deep peak shaving, ensuring compliance with emission standards. The findings serve as a reference for similar units seeking to adapt to the demands of flexible grid operations.
1. General Overview of the Boiler System
The #5 boiler of National Energy Heze Power Generation Co., Ltd. is a subcritical-pressure, tangentially fired, natural circulation boiler developed by Dongfang Boiler Works. Designed for high efficiency and reliability, the boiler features several advanced engineering characteristics:
- Combustion and Fuel: The boiler is fueled by bituminous coal and employs a single furnace for complete combustion. Its tangential firing system optimizes combustion efficiency and minimizes NOx generation during the process.
- Structural Design: The boiler incorporates a full steel-frame suspension design with a metal waterproof cover, ensuring durability and safety.
- Exhaust and Ash Removal: It uses balanced ventilation and dry slag removal to improve thermal efficiency and operational simplicity.
In terms of performance:
- The boiler achieves a maximum continuous steam flow of 1025 t/h under BMCR conditions at 332MW.
- At rated conditions (318.2MW), it delivers a steam flow of 904.4 t/h.
The SCR system employs a “2+1” catalyst configuration arranged in the high ash section of the flue gas path. This layout ensures an effective denitration rate exceeding 80%, meeting regulatory emission standards of 50mg/Nm350 mg/Nm^350mg/Nm3 (6% dry oxygen basis). The catalyst operates efficiently within the 310°C–420°C temperature range.
2. Design and Functionality of the Economizer Flue Gas Bypass
The economizer flue gas bypass retrofit was introduced to address the limitations of the SCR system under low-load conditions. By selectively bypassing a portion of the economizer, the system ensures that the flue gas temperature at the SCR inlet remains within the optimal range.
Key Design Features
Bypass Flue Integration:
The bypass flue is a critical innovation that addresses the challenge of maintaining optimal flue gas temperatures for SCR operations under low-load conditions. This feature works by diverting a calculated portion of the flue gas before it passes through the economizer. The bypassed gas, being significantly hotter, is then mixed with the cooler gas exiting the economizer. This mixing process allows for precise control of the flue gas temperature at the SCR inlet, ensuring it remains within the ideal operating range of 310°C to 420°C. The integration of this bypass mechanism is particularly effective in overcoming the temperature drop that naturally occurs in the economizer at reduced boiler loads. By allowing selective diversion of flue gas, the system ensures that heat energy is strategically retained where it is most needed, without compromising the efficiency of other boiler components.
Control Mechanisms:
The bypass system is equipped with advanced control mechanisms, including regulating dampers and shut-off dampers, which provide operators with precise control over gas flow dynamics. The regulating dampers allow for gradual adjustment of the bypass flow rate, enabling the system to respond dynamically to fluctuating boiler loads. The shut-off dampers act as safety and isolation devices, ensuring that the bypass flue can be entirely closed off during high-load operations when bypassing is unnecessary. Together, these components enable the bypass system to maintain SCR inlet temperatures above the critical threshold of 310°C, even when boiler loads are as low as 30% (approximately 100MW). This degree of flexibility ensures that the SCR system remains effective across a wide range of operational scenarios, enhancing the overall reliability and compliance of the unit.
Operational Range:
The operational range of the bypass system is carefully designed to accommodate the increasingly stringent demands of modern power grids. During deep peak shaving conditions, when boiler loads are significantly reduced, the bypass system plays a pivotal role in stabilizing the economizer outlet temperature. By diverting sufficient flue gas, the system ensures that the economizer outlet temperature is consistently maintained at 300°C or higher. This capability not only meets the operational requirements of the SCR system but also prevents catalyst deactivation and ensures effective NOx reduction. The bypass system’s adaptability allows it to seamlessly transition between bypass and normal modes of operation, ensuring that boiler efficiency and emission performance are optimized under all load conditions.
This combination of design features makes the economizer flue gas bypass a robust and efficient solution for improving SCR performance and operational flexibility in coal-fired power plants.
3. Operational Adjustments for Flue Gas Bypass
Efficient operation of the economizer bypass system requires careful adjustment of dampers to regulate flue gas flow and temperature. The following describes the adjustment methods used under varying load conditions:
3.1 High-Load Operation
At high loads, the bypass damper remains closed to ensure maximum heat recovery in the economizer. The SCR inlet temperature naturally remains within the optimal range, negating the need for bypass intervention.
3.2 Low-Load Operation
Under low-load conditions, the SCR inlet temperature tends to drop below the minimum threshold (310°C). To counteract this:
- The bypass damper is gradually opened to divert flue gas around the economizer.
- Simultaneously, the main flue dampers are adjusted to optimize the mix of bypassed and economizer-processed gas.
3.3 Detailed Adjustment Procedure
- When SCR Inlet Temperature Falls Below 305°C:
- Manually open the bypass damper until the temperature exceeds 310°C.
- If the bypass damper is fully open and temperatures remain insufficient, partially close the main flue damper to increase bypass flow.
- When SCR Inlet Temperature Exceeds 310°C:
- Gradually close the bypass damper while reopening the main flue dampers.
- Continuous Monitoring:
- The induced draft fan current and furnace pressure must be closely monitored during adjustments to prevent operational instability.
4. Operational Precautions
To ensure safe, efficient, and reliable operation of the economizer flue gas bypass system, it is critical to follow a set of carefully defined operational precautions. These measures are designed to minimize potential drawbacks while optimizing the system’s performance during load adjustments, particularly under deep peak shaving conditions.
Minimizing Efficiency Losses
One of the primary trade-offs associated with operating the bypass system is a reduction in boiler efficiency. When the bypass damper is opened, a portion of the flue gas is diverted away from the economizer, reducing its ability to recover heat from the gas stream. This results in an increase in the exhaust gas temperature and a corresponding decrease in thermal efficiency. Over extended periods, this loss can accumulate into significant operational inefficiencies. Therefore, it is important for operators to minimize bypass usage during periods when SCR inlet temperatures are stable and within the optimal range. By carefully managing bypass operations, the overall efficiency of the boiler system can be preserved without compromising the effectiveness of the SCR system.
Monitoring Key Parameters
Effective monitoring of key operational parameters is essential for the safe and efficient functioning of the bypass system. Operators should maintain constant vigilance over the following critical variables:
- Furnace Negative Pressure: Abrupt changes in furnace pressure can indicate instability in the combustion process or improper adjustments to the bypass damper. A stable negative pressure ensures proper draft conditions, safe combustion, and efficient flue gas flow.
- Main and Reheat Steam Temperatures: Fluctuations in these temperatures may indicate heat transfer imbalances caused by bypass operations. Maintaining steady steam temperatures is crucial for ensuring that the turbine operates efficiently and within its design parameters.
- Induced Draft Fan Current: The bypass system can impact the flow resistance of the flue gas path, which in turn affects the operation of the induced draft fan. Monitoring the fan current ensures that the system remains within safe operational limits.
- SCR Inlet Temperature: Regular monitoring of the SCR inlet temperature ensures that it stays above the minimum threshold required for effective ammonia injection and NOx reduction, typically 310°C.
Timely Adjustments
Prompt and deliberate adjustments to the bypass damper are essential for responding to changing boiler loads and maintaining operational stability. Operators should adhere to the following best practices for damper adjustment:
- During Load Reductions: As the boiler load decreases, the economizer outlet temperature naturally drops. To compensate, the bypass damper should be opened promptly to divert a portion of the hotter flue gas into the SCR system. This ensures that the SCR inlet temperature remains within the required range and prevents the deactivation of the catalyst.
- During Load Increases: When the boiler load increases, the economizer outlet temperature rises, and the need for bypassed flue gas diminishes. In these scenarios, the bypass damper should be closed in a controlled manner to restore normal economizer operation, improve heat recovery, and enhance overall boiler efficiency.
Gradual Adjustments and Safety Considerations
Sudden or extreme adjustments to the bypass damper can create pressure imbalances or thermal shocks in the system, leading to potential damage or operational instability. Operators should ensure that damper adjustments are made gradually and in small increments to maintain system balance. Additionally, the following safety considerations should be observed:
- Regularly check the position and functionality of the bypass dampers to ensure they respond correctly to control signals.
- Monitor the induced draft fan’s performance closely during damper adjustments to avoid excessive negative pressure or overloading of the fan.
- Be prepared to revert to default configurations or isolate the bypass system entirely if abnormalities are detected in furnace pressure, steam temperature, or SCR performance.
By adhering to these operational precautions, operators can maximize the effectiveness of the bypass system, maintain compliance with emissions standards, and ensure the long-term reliability of the boiler. These measures also help balance the trade-off between NOx reduction and boiler efficiency, which is critical in modern coal-fired power plants operating under stringent environmental regulations and dynamic grid conditions.
5. Application in Deep Peak Shaving
The retrofit’s effectiveness was tested during deep peak shaving operations. When the unit load was reduced to 100MW, the bypass system was engaged, demonstrating the following benefits:
- Improved SCR Performance:
The bypass ensured SCR inlet temperatures remained above 300°C, maintaining denitration efficiency even at low loads. - Enhanced Operational Flexibility:
The unit’s minimum stable load was reduced from 50% to 30%, significantly enhancing its ability to accommodate grid demands. - Operational Data Comparison:
Statistical data comparing bypassed and non-bypassed conditions showed a clear advantage in maintaining temperature stability and emission compliance during low-load operation.
6. Conclusion
The economizer flue gas bypass retrofit has proven to be a transformative upgrade for the #5 boiler. By stabilizing SCR inlet temperatures under low-load conditions, the bypass system ensures consistent compliance with NOx emission standards while enhancing the unit’s peak shaving capabilities.
This innovation enables coal-fired plants to adapt to the challenges posed by modern grid dynamics, characterized by fluctuating demand and increased reliance on renewable energy. The successful implementation of this retrofit provides a valuable reference for similar units seeking to balance operational efficiency with environmental compliance.
