Low-NO, burners

This justifies all the work undertaken to arrive at fuel denitrification which, as is well known, is difficult and costly. Moreover, technological improvements can bring considerable progress to this field. That is the case with low NO burners developed at IFF. These consist of producing separated flame jets that enable lower combustion temperatures, local oxygen concentrations to be less high and a lowered fuel s nitrogen contribution to NOj. formation. In a well defined industrial installation, the burner said to be of the low NO type can attain a level of 350 mg/Nm, instead of the 600 mg/Nm with a conventional burner. 

Pig. 5. Schematics of (a) direct-firing hot-fan system for pulverized coal and (b) low NO burner for coal firing. 

Similar to oil-fired plants, either low NO burners, SCR, or SNCR can be appHed for NO control at PC-fired plants. Likewise, fabric filter baghouses or electrostatic precipitators can be used to capture flyash (see Airpollution controlmethods). The collection and removal of significant levels of bottom ash, unbumed matter that drops to the bottom of the furnace, is a unique challenge associated with coal-fired faciUties. Once removed, significant levels of both bottom ash and flyash may require transport for landfilling. Some beneficial reuses of this ash have been identified, such as in the manufacture of Pordand cement. 

New units can be ordered having dry, low NO burners that can reduce NO emissions below 25 ppm on gaseous fuels in many cases, without back-end flue-gas cleanup or front-end controls, such as steam or water injection which can reduce efficiency. Similar in concept to low NO burners used in boilers, dry low NO gas turbine burners aim to reduce peak combustion temperatures through staged combustion and/or improved fuel—air mixing. 

NOj Control. NO 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 SO2 requirements. As of 1996, EPA had estabHshed Tide 4 NO limits only for tangentially fired and waH-fired, dry-bottom boilers that would be impacted by Phase I of the CAAA SO2 regulations (Tide 4). Limits of 0.22 kg/10 kJ (0.5 lb/10 Btu) and 0.19 kg/10 kJ (0.45 lb/10 Btu) have been set for wall-fired and tangentially fired units, respectively. The EPA based these levels on what was achievable using low NO 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 product gas after cleanup consists of primarily CO and H2. Combustion of coal gas in high firing-temperature gas turbines converts virtually all of the CO to CO2, and gas turbine exhaust is expected to contain no more than 10 ppm CO when operating at design conditions. Carbon monoxide emissions from a CGCC plant are thus expected to be around one-tenth those of a modem coal-fired plant equipped with low NO burners. 

Environmental. Stringent environmental laws require that nitrogen oxides (NO ) and sulfur oxides emission from furnaces be drastically reduced. In many parts of the world, regulations require that NO be reduced to 70 vol ppm or lower on a wet basis. Conventional burners usually produce 100 to 120 vol ppm of NO. Many vendors (McGill, John Zink, and North American) are supplying low NO burners. 

Control of NO under the CAAA of 1990 will be accomphshed through the issuance of a revised NSPS in 1994, with the objective of reducing emissions by 2 miUion tons a year from 1980 emission levels. The teemission standards will not apply to cyclone and wet bottom boilers, unless alternative technologies are found, as these cannot be retrofitted with existing LNB technologies. 

New low-NO, burners are effective in reducing emissions from both new power plants and existing plants that are being retrofitted. Low NO, burners limit the formation of nitrogen oxides by controlling the mixing of fuel and air, in effect... 

TTte most cost-effective methods of reducing emissions of NO are the use of low-NO burners and the use of low nitrogen fuels such as natural gas. Natural gas has the added advantage of emitting almost no particulate matter or sulfur dioxide when used as fuel. Other cost-effective approaches to emissions control include combustion modifications. These can reduce NO emissions by up to 50% at reasonable cost. Flue gas treatment systems can achieve greater emissions reductions, but at a much higher cost. 

Use low-NO, burners to reduce NO, emissions from burning fuel in ancillary operations. 

Reduce nitrogen oxide emissions by use of natural gas as fuel, use low-NO, burners, and use hydrogen peroxide and urea in stainless steel pickling baths. 

There are several problems with these standards for older coal-fired plants that limit a sensible NO,-reduction policy. First, they are relatively lenient. Because they require only that low-NO, burners be used at coal-fired plants, relatively inexpensive NO,-reduction technologies are not being used, because the plants already meet the minimum standards. Second, standards are established on a technology-by-technology basis, and have resulted in more lenient requirements for dirtier technologies. This has created little or no incentive to switch to cleaner processes in the past decade. Thirdly, these standards are input-based, which means they provide no incentive for efficiency within any technology category. 

Combustion modifications and postcombustion processes are the two major compliance options for NO., emissions available to utilities using coal-fircd boilers. Combustion modifications include low-NO burners (LNBs), overfire air (OFA), reburning, flue gas recirculation (FGR), and operational modifications. Postcombustion processes include selective catalytic reduction (SCR) and selective noncatalytic reduction (SNCR). The CCT program has demonstrated innovative technologies in both of these major categories. Combustion modifications offer a less-expensive appiroach. 

The NO t constituent in the exhaust of machines firing natural gas is some 150 to 160 ppm, and for distillate fuels typically 260 ppm. In order to reduce these levels to the targets quoted above, catalytic filters can be used, but the systems currently available are expensive. As an alternative, certain manufacturers are developing low-NO burners but these limit the user to natural gas firing. 

Use of low-NO, burners to reduce NO, emissions from burning fuel in ancillary operations use of dry SO-, and dust removal systems in hue gases... 

The amount of prompt NO produced in combustion systems is relatively small compared with the total NO formation. However, prompt NO is still formed at low temperatures and is one of the features in producing ultra-low NO burners. The nitric oxide reacts with other species in the atmosphere to give various other nitrogen oxides, namely NO2 and nitrogen pollutants. 

Low-NO 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 apphcations and for some coals. Overfire air is another technique for... 

Air-staged burners Low-NO air-staged burners for firing gas (or oil) are shown in Fig. 24-28. A high-performance, low-NO, burner for high-temperature furnaces is shown in Fig. 24-32. In this design, both air-staging and external flue-gas recirculation are used to achieve extremely low levels of NO emissions (approximately 90 percent lower than conventional burners). The flue gas is recirculated by a jet-pump driven by the primary combustion air. 

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