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The premix premise

Published by , Editorial Assistant
Hydrocarbon Engineering,

For almost 40 years, fired heater and furnace operators around the world have been using staged fuel technology to reduce nitrogen oxide (NOX) emissions to comply with continuously more stringent emissions regulations.

Today, they are dealing with problems associated with the original burner designs for retrofit applications, as well as new heaters and furnaces. Typical issues include, but are not limited to, fuel discharge tips located in the high temperature radiant section, extremely small fuel discharge ports, long flames, flame stability, general flame quality, variable NOX emissions, the ability to satisfactorily burn a wide range of fuels, startup in cold operating environments and excessive carbon monoxide (CO) in lower temperature radiant section operations.

Staged fuel design
The typical ultra-low NOX burner uses the staged fuel design concept, which usually has a burner tile with two sets of the fuel injection nozzles. These tips are ‘nozzle mix,’ meaning the fuel does not begin to mix with the combustion air until it enters into the combustion zone. All of the combustion air passes through a central opening in the tile. Here, the first set of tips (or tip) injects some small portion of fuel, referred to as the primary fuel, into the central burner tile opening. The second set of tips, commonly called staged tips, discharges 90% or more of the fuel around the outside perimeter of the tile. The primary tip or tips can have up to eight discharge ports, while the staged tips can have anywhere between 16 and 24 ports for a total of 24 to 32 discharge ports. Each port can be as small as 1/16 in. dia.

Premix technology
Users around the world are faced with retrofitting existing units to reduce NOX emissions in order to comply with local current and anticipated emissions regulations. Many existing heaters and furnaces that require burner replacement for NOX control have premix burners and other small burner types. These units cannot be retrofitted using existing low NOX burner technology as the burners and flame volume are too large.

Callidus Technologies has developed a burner that uses proven technologies to address the issues normally encountered with a conventional staged fuel burner. The Callidus CUBL-CF Compact Flame burner provides 30 – 40% more capacity than a similar physical size ultra-low NOX burner in addition to a flame length that is approximately 60% of the length of those burner flames. The burner incorporates ‘premix’ technology, where 100% of the fuel is premixed with combustion air prior to the initiation of combustion instead of delaying the fuel/air mixing until the high pressure fuel is injected into the combustion air.

The overall burner design is quite similar to the Callidus CUBL burner. The major difference is the fuel injection system. Instead of a nozzle mix fuel injection system, which relies on the high velocity fuel jet for mixing with the combustion air after the fuel enters the primary and secondary combustion zones, the Callidus CUBL-CF Compact Flame burner has a premix fuel injection system. The fuel and a portion of combustion air are thoroughly mixed prior to entering the combustion zones. Due to the premix design, the ports in the tips that discharge the fuel/air mixture into the combustion zones are much larger than the tips used in the nozzle mix design. Furthermore, the burner is significantly smaller than a similar capacity nozzle mix staged fuel burner.

This article was originally published in the December issue of Hydrocarbon Engineering. To read the full version, sign in or register for a free trial subscription.

Written by Dr Richard Martin, Honeywell UOP Callidus, USA.

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