Optimized design provides high returns
The next step was adding the SNCR injectors to the model and viewing the results. The CFD NOx calculation with chemical kinetics was performed, and the results are presented in Figure 6. Here, the NO concentration at the exit of the radiant furnace is compared with and without urea injection. This figure shows that urea injection provides a substantial reduction in NOx. Blue and teal regions and the surrounding green areas indicate where the greatest NOx reduction occurs. The total predicted NOx reduction is 32% for this case. This reduction is concentrated in a localized area, representing less than 50% of the cross section of the boiler. On a practical level, there is no injection system that can cover the rest of the area for this large unit.
6. Visual results. NO concentration (top) without and (bottom) with urea injection. Source: Nalco Mobotec
Based on the CFD simulations, Nalco Mobotec engineers decided to inject urea at 50 injectors. Urea flow to each injection port was controlled as a function of boiler load. After the SNCR was installed, the CFD predictions for SNCR NOx reduction were compared with the full load field data, as shown in Table 2.
Table 2. Accurate predictions. CFD predictions of NOx reduction show good agreement with field measurements. Source: ANSYS/Nalco Mobotec
Field measurements showed that 0.54 lb/million Btu of baseline NOx was produced by the furnace prior to the SNCR installation, compared with 0.50 lb/million Btu predicted by the CFD model. Field measurements also showed that 0.35 lb/million Btu of NOx was generated after the SNCR installation, compared with the 0.34 lb/million Btu predicted by CFD. The remarkable accuracy of the CFD predictions demonstrates the power of this technology and why it is popular in the power generation industry.
Table 2 also shows that, at a full load of 3,952 million Btu/hr, the SNCR reduced NOx from 0.54 lb/million Btu to 0.35 lb/million Btu. Over the North Carolina 12-month ozone season, with a capacity factor of 80%, this results in a reduction of 3,426 tons of NOx emissions per year. With NOx credits trading at $2,000 per ton, $6.9 million per year of revenue is generated.
The recurring costs of operating the SNCR, primarily urea, are approximately $1.5 million per year. The capital cost associated with installing the SNCR was approximately $2.5 million. Both operating costs and capital costs were kept low on the project by utilizing CFD to optimize placement of the injection ports. As a result, the SNCR paid back its cost in less than six months, generating an additional $5.7 million in net revenue per year for the utility owner since that time.
—Contributed by Dr. Gui-su Liu (gliu@nalcomobotec.com), senior combustion scientist and Dr. Brian S. Higgins (bhiggins@nalcomobotec.com), VP of technology at Nalco Mobotec.
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