Boilers are essential equipment for many industries, such as power generation, chemical, food processing, and more. However, boilers also consume a lot of fuel and emit greenhouse gases, which can affect the environment and the bottom line. Therefore, it is important to optimize boiler efficiency and reduce fuel consumption and emissions.
One of the key factors that affect boiler efficiency is the amount of excess air in the combustion process. Excess air is the extra air that is supplied to the burners beyond the amount required for complete combustion of the fuel. Excess air is necessary to ensure complete combustion and prevent the formation of carbon monoxide (CO) and other harmful pollutants. However, excess air also has some drawbacks, such as increasing the stack temperature, reducing the heat transfer in the radiant section, and lowering the combustion efficiency.
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What is Stack Temperature and Why Does it Matter?
Stack temperature is the temperature of the flue gas that exits the boiler through the stack or chimney. It is an indicator of the heat loss in the boiler and the combustion efficiency. The lower the stack temperature, the higher the combustion efficiency and the less heat loss.
Stack temperature is directly related to the amount of excess air in the combustion process. The more excess air, the higher the stack temperature. This is because excess air increases the amount of nitrogen (N2) in the flue gas, which does not transfer heat by radiation and only gives up heat by convection in the stack. Moreover, excess air dilutes the concentration of carbon dioxide (CO2) and water vapor (H2O) in the flue gas, which are binary gases that can absorb and transfer heat by radiation. Therefore, excess air reduces the heat transfer in the radiant section of the boiler and increases the heat loss in the stack.
According to The Engineering ToolBox, a typical excess air level for natural gas combustion is 5-10%, which corresponds to a stack temperature of 200-300°F above the ambient supply temperature. However, if the excess air level increases to 44.9%, as measured in some boilers, then the stack temperature can rise to 400°F above the ambient supply temperature, resulting in a significant drop in combustion efficiency.
How to Reduce Excess Air and Optimize Boiler Efficiency?
To reduce excess air and optimize boiler efficiency, it is essential to monitor and control the combustion process using various instruments and devices, such as:
- Oxygen (O2) sensors or analyzers: These devices measure the amount of oxygen in the flue gas and indicate the level of excess air. The optimal O2 level depends on the type of fuel and boiler design, but generally ranges from 2-5% for natural gas and 3-6% for oil. O2 sensors or analyzers can be installed at various locations in the boiler system, such as near the burners, at the economizer outlet, or at the stack exit.
- Flue gas temperature sensors: These devices measure the temperature of the flue gas at different points in the boiler system and indicate the heat loss and combustion efficiency. Flue gas temperature sensors can be installed at various locations in the boiler system, such as near the burners, at the economizer inlet or outlet, or at the stack exit.
- Combustion controllers: These devices use feedback signals from O2 sensors or analyzers and flue gas temperature sensors to adjust the fuel and air flow rates to achieve optimal combustion conditions. Combustion controllers can be manual or automatic, depending on the level of sophistication and automation required.
- Burner management systems (BMS): These are integrated systems that control all aspects of burner operation, such as ignition, fuel supply, air supply, flame detection, safety interlocks, alarms, etc. BMS can also incorporate O2 sensors or analyzers, flue gas temperature sensors, combustion controllers, and other devices to optimize combustion efficiency and reduce emissions.
By using these instruments and devices to monitor and control excess air levels, boiler operators can achieve significant benefits, such as:
- Reducing fuel consumption and operating costs
- Increasing combustion efficiency and heat transfer
- Lowering stack temperature and heat loss
- Reducing CO2 and NOx emissions
- Improving boiler reliability and safety
Conclusion
Stack temperature is directly related to the amount of excess air in the combustion process. Excess air is necessary to ensure complete combustion and prevent CO formation, but too much excess air can increase stack temperature, reduce combustion efficiency, and increase heat loss and emissions. Therefore, it is important to optimize excess air levels and achieve the best balance between combustion quality and efficiency. This can be done by using various instruments and devices, such as O2 sensors or analyzers, flue gas temperature sensors, combustion controllers, and burner management systems, to monitor and control the combustion process and adjust the fuel and air flow rates accordingly. By doing so, boiler operators can improve boiler performance and save fuel and money.
