Excess Air Control for Fired Heaters: Optimizing Oxygen Analyzers and Combining Feedback and Feedforward Control Loops
Fired heaters are integral components of many industrial operations, and it is important that they be operated in a safe and efficient manner. To accomplish this, proper excess air control is essential. Excess air control is the process of controlling the amount of air that is used to combust fuel, which can have a large impact on the efficiency and safety of the heater. This article will discuss the use of oxygen analyzers and the combination of feedback and feedforward control loops for optimizing excess air control in fired heaters.
Using Oxygen Analyzers for Excess Air Control
Oxygen analyzers are an important tool for controlling excess air in fired heaters. These analyzers generally use a zirconium oxide cell that is either inserted directly into the stack or placed in an external cell with a short sample system. This system has relatively low maintenance and fast response times, making it ideal for excess air control.
The oxygen sample point should be situated in the fired heater in order to get the most accurate sample and avoid flue gas dilution from air leakage. Depending on the amount of air leakage in the convection section, the sample point can be placed before or after the convection section.
In addition to permanent oxygen analyzers, portable oxygen analyzers can be used to check the accuracy of the permanent analyzer and to detect air leaks. Portable oxygen analyzers are preferred over carbon dioxide analyzers because they are more sensitive to the exact amount of excess air present.
Combining Feedback and Feedforward Control Loops
For optimal excess air control, a combination of feedback and feedforward control loops is recommended. In the feedforward loop, the air and fuel rates are monitored and the air rate is adjusted to maintain the fired heater near its optimal operating level.
In the feedback loop, an oxygen analyzer is used to measure the oxygen level in the flue gas. This helps to reduce errors due to changes in air humidity, atmospheric pressure, and fuel quality, as well as instrument error. The combination of these two control loops helps to ensure that combustible breakthrough will not occur unless there are hardware malfunctions.
Excess air control is an important factor in the safe and efficient operation of fired heaters. Oxygen analyzers and the combination of feedback and feedforward control loops are two of the most important tools for optimizing excess air control. By using these tools properly, the risk of combustible breakthrough can be minimized and the efficiency of the heater can be maximized.
Energy Saving Opportunities Through Control of Draft and Heat Recovery in Process Heaters
The utilization of process heaters for energy conservation has a great potential for cost savings. This article will focus on two important opportunities for energy conservation in process heaters: control of draft and heat recovery. By understanding and maximizing the process heater’s potential for energy conservation, businesses can realize significant cost savings.
Understanding Draft
The difference in pressure between inside the fired heater and the outside atmosphere at the same elevation is known as draft. A positive draft is defined as the pressure being less in the fired heater than the atmosphere. This is the normal operating mode for process fired heaters. It is recommended to avoid very high draft, as this could cause excessive air leakage, and to also avoid a condition of negative draft, which could cause hot flue gases to escape through any cracks or openings, resulting in structural damage and safety issues to personnel.
Fired heater draft is typically maintained at about 0.1 inches of water column just below the convection section by adjustments to the stack and burner dampers. In natural draft fired heaters, the stack damper can be under remote control, but the burner air registers must be manipulated manually, making it difficult to independently control both air flow and draft. However, in forced draft furnaces, direct control of both box pressure and air flow is fairly common. In this situation, the air flow is usually controlled by the forced draft fan, and the draft pressure can then be controlled by manipulating the stack dampers or the induced draft fan, if one is used.
Heat Recovery
Furnace stack waste heat recovery and combustion air preheating are two major and common opportunities for efficiency enhancement in fired heaters. Both processes recover heat available in the stack flow by lowering the stack temperature. While air preheating results in direct fuel fired reduction, waste heat boilers produce steam that can be used for other process applications, leading to reduced fuel consumption in the plant’s boilers.
Air preheating systems save fuel by transferring heat from flue gas to the combustion air, resulting in a fired heater stack gas temperature reduction and an increase in operating efficiency. By lowering the stack temperature to about 300 to 350°F, which is typical for air preheaters’ installations, fired heater efficiency can reach more than 90%.
Air preheating systems can be added to any furnace, natural or forced draft. The only considerations governing each installation are economic return on investment, plot plan available for the new equipment, and fired heater and burner construction that must be compatible with the required hot air ductwork and plenums.
Types of Air Preheaters
Generally, there are four types of air preheaters: rotary/regenerative; tubular; circulating fluid and heat pipe. Each type has inherent advantages and disadvantages. Pressure drop for both air stream and the flue gas stream should be limited to approximately 3 inch water each and the minimum flue gas outlet temperature is normally in the range of 300 to 350°F (sulfur content of the fuel fired, type of the preheater and the material of construction may limit the outlet temperature to a slightly higher level). Soot blowing facilities are required with fuel oil and dirty gas firing services.
The main constraint for maximum benefit is the cold end temperature corrosion protection. The temperature of the flue gas leaving the preheater will determine the system efficiency. The flue gas temperature should be as low as possible without risking significant low temperature corrosion of the air preheater elements. Additionally, consideration must be given to corrosion in downstream ducting. The minimum flue gas temperature in the downstream ducting should be above the actual dew point with a reasonable safety factor.
Methods for Negating Effects of Cold End Metal Temperatures
The flue gas temperature leaving the preheater will be affected by the inlet end temperature and the firing rate of the equipment. In general, lower inlet air temperature and firing rate will result in lower flue gas exit temperature. The following methods can be utilized to negate the effects of cold end metal temperatures due to variations in ambient air temperatures and firing rates:
- Use of a steam/air heater to preheat the cold air ahead of the main air preheater
- A cold air bypass around the preheater which limits the minimum flue gas temperature at the cold end when necessary
- A hot air bypass back to the inlet of the forced draft fan to raise the air temperature entering the preheater in much the same way as the steam/air heater.
Conclusion
By utilizing the process heaters’ potential for energy conservation, businesses can realize significant cost savings. Understanding and controlling draft, as well as optimizing heat recovery, are two of the most important opportunities for energy conservation. By utilizing air preheating systems, fired heater stack gas temperature can be reduced, resulting in an increase in operating efficiency. However, consideration must be given to corrosion in downstream ducting, and various methods can be utilized to negate the effects of cold end metal temperatures due to variations in ambient air temperatures and firing rates.