Low-NOx burners

NOx stands for a variety of nitric oxides. Many heaters in the United States have been retrofitted with staged burners in the last decade or so. These staged burners combust the fuel in two or three stages. For example, in the burner shown in Fig. 30.6, 50 percent of the fuel is burned with 100 percent of the air. The flame produced by this first stage of combustion radiates heat to the process tubes and refractory walls. Next, the remaining 50 percent of the fuel is added around the circumference of the first-stage burner. This second stage of combustion also liberates radiant heat. But because the radiant heat is liberated in two steps, the maximum flame temperature is reduced. This has two favorable results  

Nitric oxide (NOx) production is reduced, as the oxidation of nitrogen is a strong function of the flame temperature. Hence the term low-NOx burners. 

Heat liberation from the low-NOx burner is more uniform than with a conventional burner. This permits a higher average firebox temperature to be sustained, without promoting 

Premix, or primary-air, burners do a great job of mixing air and fuel gas. But they also produce a high flame temperature and hence, higher concentrations of NOx in the heater s effluent flue gas. 

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Typically costs less to implement them low NOx burners, if installing as separate unit. 

Gas turbines and power stations are particularly prone to generate NOx and the search for the low-NOx burner that will operate at high efficiency (i.e. with low hydrocarbon emissions) continues. The principle of the low-NOx burner is to slow the rate of combustion by dividing it into several stages by the gradual mixing of the combustion gases with the stoichiometric air volume. 

Options typically include a wide range of pressure ranges and steam outputs, fully automatic operation, full modulation (variable FW or steam supply, rather than on-off operation), low NOx burners, and high tumdown-ratio burners to reduce cycling and improve fuel utilization. 

More modem industrial boiler designs also provide facilities for easy fuel changeover with simultaneous firing, low NOx burners, high turndown ratios, and low excess air capabilities (down to only 10% excess air). 

Environmental considerations may require boiler designers to provide units with low NOx burners. Specific designs may also necessitate an enlarged furnace and overfire air or NOx ports. 

Selective catalytic reduction (SCR) and selective noncatalytic reduction processes (SNCR) are widely employed in large industrial and utility boiler plants, as well as in municipal waste incineration plants and other combustion processes. They are used to complement mechanical improvements (such as low NOx burners and furnace design modifications) as an aid to reducing the emission levels of NOx, S02, and other noxious gases into the atmosphere. 

Garg A (1994) Specify Better Low-NOx Burners for Furnaces, Chem Eng Prog, Jan 46. 

The burners are located between tube rows. A larger number of burners reduces the heat release per burner and allows a smaller flame diameter and a reduced lane spacing. A ratio of one burner for every 2 to 2.5 tubes provides a uniform heat release. Most burners are a dual-fired design, firing both PSA offgas and supplemental makeup gas. Low NOx burners are used to meet environmental requirements. Makeup gas can be used to induce flue gas into the flame, reducing the flame temperature and NOx level. In a well functioning unit NOx levels as low as 0.03 lb/MMBtu are possible. 

FIG. 24-32 Low-NOx burner with air-staging and flue-gas recirculation for use in high-temperature furnaces. Hauck Manufacturing Company. Developed and patented by the Gas Research Institute.)... 

NOx Control. NOx control limitations are described in both Tide 1 and Tide 4 of the CAAA of 1990. Tide 4 requirements affect only coal-fired boilers and take effect at the same time that the boilers are impacted by CAAA S02 requirements. As of 1996, EPA had established Tide 4 NOx limits only for tangentially fired and wall-fired, dry-bottom boilers that would be impacted by Phase I of the CAAA S02 regulations (Tide 4). Limits of 0.22 kg/106 kj (0.5 lb/106 Btu) and 0.19 kg/106 kj (0.45 lb/106 Btu) have been set for wall-fired and tangentially fired units, respectively. The EPA based these levels on what was achievable using low NOx burners. However, plants can employ a number of different front- or back-end emissions controls, including a combination of options, to achieve these levels. EPA plans to announce Tide 4 NO requirements for 300 additional boilers by late 1996 or eady 1997. 

The Act also calls for a 2 million ton reduction in NOx emissions by the year 2000. A significant portion of this reduction will be achieved by coal-fired utility boilers that will be required to install low NOx burner technologies and to meet new emissions standards. 

Model-based boiler optimization schemes have proved successful in many power plant and industrial boiler applications. Successful NOx reduction through this kind of optimization can avoid or postpone large capital expenditures for low NOx burners, over-fire air modifications, and selective catalytic reduction/selective noncatalytic reduction (SCR/SNCR). 

Control of NO under the CAAA of 1990 will be accomplished through the issuance of a revised NSPS in 1994, with the objective of reducing emissions by 2 million tons a year from 1980 emission levels. The technology being considered is the use of low-NOx burners (LNBs). The new emission standards will not apply to cyclone and wet bottom boilers, unless alternative technologies are found, as these cannot be retrofitted with existing LNB technologies. 

Low-NOx burners are designed to delay and control the mixing of coal and air in the main combustion zone. A typical low-NO, air-staged burner is illustrated in Fig. 24-16. This combustion approach can reduce NO, emissions from coal burning by 40 to 50 percent. Because of the reduced flame temperature and delayed mixing in a low-NO, burner, unburned carbon emissions may increase in some applications and for some coals. Overfire air is another technique for... 

Reduction of amount of the primary air by utilization of low NOx burner pipe and by PF sourced by an indirect firing system. 

The concentration of NOx in the exhaust gases reflects the temperatures in the flame and/or in the kiln. In kilns such as rotaries, the flame influences the NOx level, and a low NOx burner may be used to reduce the NOx level [33.5, 33.6]. Further experience is probably required to asses the effects of such burners on other aspects of kiln operation. Production of dead-burned dolomite in rotary kilns requires temperatures of up to 1900 °C. This results in very high NOx levels (Table 33.2). 

In many shaft kilns, the fuel is burned as an ill-defined flame and, in consequence, the gas temperatures within the kiln are generally low. As a result, the NOx levels are well within required emission limits (Table 33.2). Low NOx burner technology may, however, be of some benefit in those shaft kilns which have combustion chambers, and in which there is a well-defined flame. 

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