Energy-Tech Magazine

The VS III Burner low NOX venturi coal nozzle assembly is illustrated in Figure 3.

The existing air register of the original OEM burner is reused for control of the secondary air delivered to the furnace. The register is typically divided into two streams: secondary (inner) and tertiary (outer) air. The split between secondary and tertiary air streams will be controlled using the existing dampers and swirl vanes. Previous testing conducted in a 100 MMBtu/hr combustion test facility has shown the flow split between SA & TA annuli to have a strong influence on burner NO_X performance [1]. Damper and swirl vane settings for optimum flow splits are determined and preset based on the CFD modeling of the burner. The existing tertiary air swirl vanes are typically adjustable from the burner front plate and must be functional at the time of the retrofit. If not already present, a set of fixed swirl vanes will be installed inside the inner air annulus, enhancing the gradual mixing of fuel and air.

A new secondary air diverter located inside the inner air annulus

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causes the air stream to initially flow away from the primary air combustion zone, thus creating an oxygen-lean zone immediately at the burner discharge. It also creates recirculation zones downstream of the secondary air diverter that are necessary to produce a well-attached and stable low NOX coal flame throughout the boiler load range. The tertiary air diverter is located in the outer air annulus and diverts tertiary air away from the primary combustion zone, further controlling the near field burner zone stoichiometry. This dual air stream and air diverter concept enhances the gradual mixing of air and fuel, controls the stoichiometry in the primary combustion zone, reduces the peak flame temperature and provides an environment for rapid devolatilisation of the coal in a reducing atmosphere, all of which are crucial elements of low NOX combustion.

The benefits demonstrated from this component retrofit approach on multiple installations have been thoroughly discussed previously [2-4].

Performance benefits from these component upgrades can include:

• Improved reliability and wear life of primary air side burner components
• Lower NOX emissions under the same load conditions
• Lower unburned carbon (UBC) in the flyash
• Improved flame length and CO emissions control
• Improved flame scannability
• Reduced furnace exit gas temperature (FEGT)
• Reduced attemperator spray flow

Economic benefits from these components can include:

• Reduced outage time (more than 50 percent reduction) during installation as compared to a complete burner replacement. None of the proposed modifications require burner removal or windbox alterations. All modifications are completed from the furnace or burner deck. The existing air register system remains intact, saving significant demolition time if completely new burners were to be installed.
• Low capital cost – typically 30-50 percent of the cost of a complete burner replacement.
• SCR ammonia consumption savings

CFD burner modeling
CFD is a powerful tool and has been used by RPI for more than 25 years to assist with the design of burner replacement or burner retrofit activities. CFD modeling permits customized burner hardware and also minimizes start-up and setting time in the field by identifying initial burner settings. In terms of customizing the burner hardware and minimizing on-site initial start-up setting time, RPI employs single burner CFD modeling to determine the desired near-field flow patterns for best flame behavior (e.g., flame length and attachment). The single burner model uses aero-dynamics only to establish nearfield recirculation zones, which are essential to produce good low NO_X combustion. In this approach, single burner air flow is simulated in an idealized tunnel furnace representing the equivalent firing region of the burner. The model tunnel diameter is similar in size to the actually firing environment, but without flame-to-flame interactions that might affect the flow behavior several throat diameters from the firing wall [5].