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Low-NOX burners are very effective for wall-fired units, resulting in reductions of NOX generation by as much as 50 percent. Figure 2 illustrates one type of low NOX burner.
Although many positive results can be seen after installing a low-NOX burner, additional boiler modifications and fuel sizing considerations must be analyzed. Installation of the burners can take up to 30 days, which will require an extended boiler outage. Also, NOX reduction in T-Fired units is not as great as a wall-fired unit. This is because in a T-Fired unit the “burners” are more accurately described as coal nozzles injecting the fuel into the combustion zone in the boiler. Since it is not a true “burner,” the ability to control fuel and air distribution is not as great as burners on a wall-fired unit. This results in a lower percentage reduction of NOX. When upgrading a unit to accommodate low-NOX burners, many support devices might require upgrades as well, such as windbox modifications, burner tilts,
The addition of low-NOX burners also will increase the amount of LOI in combustion. To counter this undesired effect, upgrades to the milling system are required for better fineness. Control modifications and upgrades will be required to accommodate the new burner operational philosophy.
With combustion modification, other unintended side effects could be observed and may include:
- Increased furnace exit gas temperature, causing superheater/reheater metallurgical issues
- Increased attemperation rates, due to the higher gas temperature
- Higher economizer outlet temperature, if heating element surface area remains constant
The change in combustion dynamics also will increase the fly ash/bottom ash ratio, resulting in a higher ash loading on back-end particulate matter reduction equipment. Lastly, if burners are not implemented properly, or off-design fuels are burned, the potential for eyebrows (slag formations) to form on the burners themselves increases, and consequently decreases the effectiveness of their NOX reduction capabilities.
In addition to burner modifications, T-Fired units might also benefit from modifications to tilts, yaws and secondary air systems as part of a concentric firing system. This system will act as a form of staged combustion in the horizontal plane of the furnace. Combustion is distributed more evenly on the horizontal plane, giving a near-wall oxidation environment which, in addition to reducing NOX, also will reduce the generation of CO and decrease LOI. Figure 3 shows the operating philosophy of creating multiple combustion zones in the horizontal plane.
When installing a low-NOX burner system, it is highly recommended that staged combustion supplement this system. Staged combustion will further change the combustion air distribution by supplying a sub-stoichiometric ratio of combustion air to the primary and secondary air systems in the main combustion zone. The remainder of air required to complete combustion will be supplied above the main combustion zone over fire air ports. This modification of the air distribution will further alter combustion dynamics and reduce the generation of NOX. T-Fired units benefit greatly from this NOX reduction technique due to its high tolerance for sub-stoichiometric stable combustion. A single vertical stage in the furnace is typically referred to as close-coupled over fire air, or over fire air. Close-coupled over fire air is used in T-Fired units supplying the remainder of combustion air directly over the main combustion zone. Over fire air used in wall-fired units utilizes the same operating philosophy, but the location of this port is generally higher in the furnace.