COOLflow™ Case Study 116

COOLflow Technology and Dynaswirl-LN Burners Prove an Unbeatable Combination to Dramatically Reduce NOx and Eliminate Costly Vibration

The Situation

The Ormond Beach power plant in Southern California experienced intense, combustion-induced boiler vibrations in two of its units after a series of modifications were made to meet the state’s NOx regulations. The vibrations occurred over a wide range of operating loads and resulted in costly repairs, frequent maintenance, reduced loads due to unstable flame formation, and poor turndown that limited the ability to maximize spinning reserves during off-peak periods.

Over a period of time, the plant tried a number of modifications to correct the vibration and further reduce emissions as California’s regulations tightened, but each attempt proved to be unacceptable. Forced outages occurred, equipment was damaged, and flame shapes became highly unstable and potentially hazardous. In addition, boiler vibrations increased dramatically. When selecting a solution to these problems the plant was looking to achieve several goals: eliminate the vibration, restore flame stability, further reduce NOx to meet or exceed the state’s future regulations, increase operational flexibility even at low levels, retain the current low levels of CO and excess O2 , and reduce the maintenance and repair requirements.

The Boilers

The two units share the distinction of being the largest utility boilers in California with one of the highest burner zone heat release (BZHR) rates in the country. Because NOx increases with BZHR, and the state’s emissions regulations are stricter than federal regulations, meeting the NOx requirements on these units without backend cleanup was a difficult task.

Both units are virtually identical, Foster Wheeler once-through supercritical boilers rated at 800 MW. Rated steam capacity is 5,740,000 lb/hr at 3,595 psig and 1,005°F. Each boiler fires natural gas as the main fuel and has low-sulfur- oil firing capability. The burner configuration was 4 rows by 4 columns located on two firing walls, for a total of 32 burners. The original design used Over Fire Air (OFA) ports as the only means of NOx control.

A series of modifications were initially performed on the units to address performance problems and to comply with changing NOx regulations. These included the introduction of FGR on one unit and a combination of FGR, OFA and Burners Out of Service (BOOS) on the other. Boiler vibrations, which were present throughout these modifications, grew both severe and damaging.

The TODD Solution

After performing a comprehensive evaluation of the two units and learning their history, our burner engineers recommended a strategic, two-part solution to address the plant’s numerous goals. COOLflow, a proven physical modeling technique used to optimize airflow distribution, was first implemented. Based on initial observations, the engineers installed baffles to straighten the airflow and provide uniform air velocity into and around the entrance of each burner. This also assisted in creating a stable flame centered in front of each burner outlet.

The engineers then retrofitted one of the units with 32 TODD® Dynaswirl-LN™ burners, designed for low-NOx operation. The principle behind these low-NOx burners is internally staged combustion. Combustion air is introduced in several distinct streams, producing thorough combustion at lower maximum flame temperatures and lower excess air levels. The Dynaswirl-LN burner operates on the principle of providing axial airflow through the burner and developing a controlled but limited vortex of primary air at the face of the swirler. This concept of swirling only the primary air maintains a stable flame at the core of the burner while limiting dilution at high turndown rates. The burner venturi design is characterized by a low pressure drop, but the venturi tends to equalize velocities across the air stream creating a more uniform flame.

The TODD Result

As a result of installing the Dynaswirl-LN burners and applying the COOLflow modeling technology, NOx was reduced by 23%-35% over a wide range of loads when operating under comparable BOOS conditions. In addition, for the first time the plant was able to fire all burners in service at full load without vibration, while providing significant NOx reduction. The power plant was now able to increase peak load from 740 MW to 790 MW and still remain within current NOx standards.

Compared to the uncontrolled NOx (1200-1500 ppm estimated for gas; 700 ppm for oil), the burner retrofit and windbox modifications, in conjunction with FGR and combustion air staging, resulted in NOx reductions of 92% for gas and 72% for oil. The retrofit also eliminated boiler vibration under all burner firing patterns, which substantially reduced repair and maintenance require-ments. As a result of applying the COOLflow modeling, flame shapes remained stable, well defined, and closely attached to the burners – even when firing with FGR rates as high as 45%. Previous low levels of CO (200 ppm) and excess 02 (1%) were also maintained. Based on the success of this unit, a TODD burner retrofit was performed on the other unit, producing comparable results.