Over the past two year, my colleague, Subodh Chaudhari, and I have been 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.