Effect of Air Preheat on Boiler Capacity

One of the first questions that my coauthor, Liz, asked me at one of my seminars was  

The answer to Liz s question was that air preheat requires less fuel gas firing, and thus, less flue gas and hence less heat pickup in 

Of course, 1 reminded Liz that she was also saving 10 p ercent of the fuel gas consumed for each poimd of steam production, so perhaps the 3 percent boiler capacity reduction was acceptable. 

At the Tenneco Oil Refinery in New Orleans, 1 first learned how to optimize the excess oxygen content of the flue gas from a boiler. The fired boiler was equipped with a forced-draft (FD) fan. Due to mechanical deficiencies of the FD blower, the combustion air supply was limited, but constant. The boiler produced 650-psig steam, which was in short supply in the refinery. Flence, the boiler was operated to maximize steam production. 

The utility plant operators would routinely adjust the fuel gas supply to maximize steam production. There was an accurate oxygen analyzer on the flue gas stack. It typically recorded SEVERAL percent oxygen in the boiler s stack. However, this value was ignored, and correctly so, by the operators. They simply adjusted the fuel-gas rate, which had a variable heating value, to maximize steam production with a fixed amount of combustion air. 

---