Are Low NOx Burners Really Low NOx?

Issue 8 and Volume 109.

There is an optimal air-to-fuel ratio in the piping of fuel to all boilers fired with pulverized coal. And there is another ratio for the additional air needed to support final combustion in the boiler. The function of low-NOX burners is to limit the formation of thermal NOX and control how fuel bound nitrogen is released. The addition of overfire air provides the required amount of O2 to complete delayed coal gas combustion while still preventing thermal NOX formation. Yet as good as it sounds, this only works in a perfect world. All low-NOX burners are susceptible to system changes and upsets that can impact performance. Segregation of coal and air, into what has is termed “coal ropes,” is one such condition.

Rick Wark, president of Sure Alloy Steel Corp., supplier of abrasion resistant steels, ceramics, and hard facing materials for various applications including the coal-fired power industry, notes that stoichiometric relationships between coal and air for every type of burner are based upon a homogeneous mixture of coal and air. “By homogeneous, we mean of similar structure,” says Wark. “At the inlet end of every burner is an elbow which creates a coal rope. At the point fuel is released into the boiler for combustion, the homogenous mixture of coal and air has changed and is not at the 2-to-1 ratio that is required for a low-NOX burner to work properly.”

Coal roping in the burner fuel piping creates areas of very high air/fuel ratios, allowing premature ignition just inside the burner or at the very burner tip. The ability of any burner to lower thermal NOX is diminished in these fuel lean areas.

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In the video image at the top, poor mixing of primary air and coal in a B&W burner flame with OEM conical diffuser and kicker produces delayed combustion. Partial ignition of the coal is taking place at the bottom of the burner while the majority of the coal has to travel much farther into the boiler before it mixes with air to start the combustion process. High LOI and boiler slagging are symptomatic of this condition. In the bottom photo, the use of the In-Line Diffuser shows a clean burn produced when air and coal are properly mixed. The entire outer area of the flame surrounds the contained inner fuel and air. This type of condition allows for controlling NOx with the secondary air and overfire air. Fuel bound nitrogen can be released from inside the flame, forming N2 and not NOx.
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The fuel rich area having a low air/fuel ratio will require more space and time for complete combustion. Overfire air, however, can react with the rich area of coal and generate thermal NOX. In many cases, the coal doesn’t have enough time to completely combust and can cause slagging, heat release in the backpass of the boiler, and increased loss on ignition (LOI).

Wark says the success of low-NOX burners is questionable when coal roping is present. Recent video footage confirms the poor quality of ignition at the burner face when coal roping is taking place (see photos).

However, a blender installed in the burner fuel piping can eliminate the coal ropes and provide for a homogenous mixture of fuel and air flowing to the boiler. “We developed a blender which is installed just after the last elbow or connection leading to the burner pipe,” says Wark. “Every type of burner manufactured will benefit from the In-Line Diffuser’s ability to transform coal ropes into a homogenous flow to the boiler.”

Shown at left is a CE burner tilt nozzle without the In-Line Diffuser, indicating premature combustion due to the segregation of coal and air. Coal roping has created an area of high air/fuel ratio at the tip of the burner nozzle. The ignition continues from the exit of the burner along the entire top of the unmixed coal and air column. Seen at right is a similar burner tilt nozzle with the In-Line Diffuser installed. A homogeneous mixture of fuel and air is delivered to the tangent point of center circle of the boiler. T-Fired units are designed to mix the air and coal from all the burners in the very center of the boiler. Notice the greater length of the flame, allowing it to extend completely into the center of the fireball. This flame is exactly where it is supposed to be and there will never be any concerns about burning or melting the burner nozzles. Secondary air and overfire air can now be used effectively for limiting NOx formation. Photos courtesy of Sure Alloy Steel.
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Bill Duncan of Reliant Energy’s Cheswick Station says, “Based upon the improvement we have seen with one elevation of burners, we decided to retrofit all 40 burners on our boiler with the In-Line Diffuser.” p