Plugging air leaks
Now that we have gone through the calculations, let's review why air in-leakage can really siphon points off a plant's heat rate, performance, and unit reliability. As you can see in Figures 9 and 10, excess air entering the furnace or convection path has a large impact on "true" excess air. Data for these figures were taken upstream of the air heater and prior to the excess O2 probes.
A mere 7% air infiltration upstream of the excess O2 probes with accepted air and fuel imbalances correlates with a burner stoichiometry of about 0.85, or 15% excess air required to complete combustion (Figure 9). When fuel or airflow is imbalanced more than 10% and/or overfire air is deep staged to +20% for NOx control, stoichiometry will be even lower. Units retrofitting flue gas desulfurization systems and/or firing high-sulfur coals understand that sulfur and chlorine are harmful corrosion compounds and accelerate water wall corrosion in a reducing atmosphere.
Users should consider periodic water-cooled high-velocity thermocouple probe measurements of furnace exit flue gas excess oxygen. Total airflow measurements of primary airflow, secondary airflow, and overfire duct airflows should also be periodically verified for calibration. Calibrations should be completed to complement acceptable mill performance testing that ensures desirable air-fuel ratios and acceptable coal fineness.
Another problem: Today's low-NOx burners with multiple stages of overfire air and flame-attachment burners are designed to create fuel-rich flame cores and result in less NOx production. Often, burners of scientifically proven good designs self-destruct due to overheating and metal deformation. Seldom is this destruction due to the burner design itself.
Our experience has been that burner reliability and NOx reduction performance are largely related to the fuel balance, combustion airflow balance, accuracy of flow indications, residence time (some furnaces have more time by design than others), air in-leakage, burner line pluggage, burner type, and primary airflow velocities—among a number of other factors.
Some original equipment manufacturers of burners utilize underfire air, curtain air, side wall air, and/or multiple overfire air injection ports throughout the boiler. Sometimes these ports are designed for good scientific reasons. However, most of the time they are used as a back-up source for NOx reduction to reduce the burner belt flame intensity and stoichiometry while delivering uncontrolled and unmeasured airflow. By introducing unmeasured, uncontrolled airflow, precision in measuring and control of the stoichiometry is lost.
We also routinely observe that imprecise measurement and control of combustion airflow, coupled with problematic pulverizers, is the root cause of localized reducing atmospheres in the burner belt zone. All too often the result is aggressive fireside tube wastage, especially with higher–iron content and higher-sulfur bituminous coals.
—R.F. (Dick) Storm (rfstormsea@aol.com) is president of Storm Technologies (www.stormeng.com). Stephen K. Storm (skstorm1@aol.com) is a vice president of the company and its manager of technical field services. Stephen G. Hall (stephen.hall@stormeng.com) is a field service engineer for Storm Technologies.
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