What I’m about to discuss here has been covered in other CP articles, e.g., “Improve Sustainability on a Shoestring." Nevertheless, I still feel it’s important enough to talk about based on some first-hand practical experiences my colleague, Subodh Chaudhari, and I have had over the past two years doing BoilerMACT energy assessments. We assessed more than 100 process heaters and steam generation equipment along with their historical operating data (hourly averages for one year). These units varied significantly in design, heat duty and operational characteristics, but still had commonalities when it came to excess air control, heat recovery and operating efficiencies.
In a nutshell, our energy assessment results relating to improving excess air control showed that:
• Steam systems are usually better set up and controlled than process heaters;
• Process heaters can save ~2–5% of the energy, whereas steam systems can save ~1–3%;
• Simple payback on these projects can be anywhere from a few months to 1.5 years; and,
• Excess air control is dictated by environmental regulations and also sometimes by unique operating constraints determined by management through analysis (e.g., less frequent decoking required by using higher excess air).
Large deviations from the targeted, optimized excess air (and flue gas oxygen content) are possible over time and varying maintenance schedules. Additionally, changing production demands and fuel mix, heating value and firing rate will cause the optimized excess air percentage to fluctuate.
From a layman’s perspective, and per my mentor and good friend Greg Harrell, excess air control and energy efficiency come down to two principles:
• Have enough air to ensure complete combustion as well as avoid combustibles in the flue gas; and
• Minimize the amount of air needed so that all extra mass (nitrogen specifically — 79% of air) is not heated from the ambient and released at stack temperature.
So, how do we manage excess air in our process heaters and steam generation equipment? Several different methodologies exist ranging from manual (physical dampers managed by operators as they do their walk-throughs or on an as-needed basis) to a sophisticated variable-frequency-drive (VFD) forced-draft fan and an automatically controlled balancing damper. With the advent of highly reliable and longer service life high-temperature in-situ sensors, cascade control technologies and data historians, implementation of automatic excess air control (oxygen trim control) has become very cost-effective; every plant should investigate automating the process. Furthermore, while the primary driver is oxygen, I have seen several places successfully implement a cascade system that combines oxygen and carbon monoxide (CO) in the flue gas to achieve a very tight and optimized combustion. In addition, you must consider the emission permits and limits relating to NOx when configuring the excess air controller for each process heater and all steam generation equipment.
As a simple best practice, install a positional controller with a periodic (quarterly or monthly) tune-up activity for the burner(s). A positional controller uses a fixed relationship (physical linkage mechanism) between the fuel valve opening (firing rate) and the damper opening. This controller requires periodic tuning because the fuel mix may change; linkages develop hysteresis and drift; ambient temperature changes with seasons, thereby changing the mass of air through the fan — remember, a fan is a fixed volume machine but the combustion process is dictated by mass; etc.
On the other hand, a state-of-the-art automatic oxygen trim controller with a VFD continuously monitors the flue gas oxygen with an in-situ sensor and trims the excess air with speed control on the fan to optimize the combustion process. In addition to the fuel energy, this also saves fan electrical energy!
In closing, I recommend you review the operations of your process heaters and steam generation equipment and evaluate if you can economically justify upgrading excess air control mechanisms. I am not going to suggest any “rules of thumb,” but instead direct you to an energy savings calculator from the United States Department of Energy called PHAST (Process Heating Assessment and Survey Tool), where you can model your process heater and see the energy and economic benefit of controlling excess air.