FLUE GAS RECIRCULATION FOR REDUCTION OF NITROGEN OXIDES (NOx) EMISSIONS IN INDUSTRIAL BOILERS
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| Overview: | Flue gas recirculation (FGR) significantly
reduces nitrogen oxides (NOx) emissions (up to 60 percent) in industrial boilers by
recirculating a portion of the boiler flue gas (up to 20 percent) into the main combustion
chamber. This process reduces the peak combustion temperature and lowers the percentage of
oxygen in the combustion air/flue gas mixture; thus retarding the formation of NOx caused
by high flame temperatures (thermal NOx). Nitrogen oxides (NOx) emissions are a significant pervasive pollutant that causes a wide variety of diseases, contributes to ozone and smog formation, causes 20 to 30 percent of acid rain, and is the basis for visibility problems because of the formation of aerosols. Thermal NOx is produced from the oxidation of nitrogen (N2) at temperatures above 1500°F. Thermal NOx is the primary source of NOx formation from natural gas and distillate oils because these fuels are generally lower or devoid of nitrogen. Fuel NOx, on the other hand, results from oxidation of nitrogen organically bound in the fuel. Therefore, FGR is not very effective on boilers that use fuels containing large amounts of fuel bound nitrogen. Department of Defense installations have large numbers of single burner water tube and fire tube package boilers that supply steam and hot water to the installation. These boilers range in size from 0.4 million British thermal units per hour (MMBtu/hr) to 250 MMBtu/hr. The majority of these boilers are old, less than 50 MMBtu/hr, package boilers that lack any pollution control devices. This equipment is the major source of nitrogen oxide (NOx) emissions at most military installations. To modify an existing boiler, ducting must be run from the stack to the boiler air supply fan. Space limitations can make routing new ductwork difficult and costly. More powerful fans, oxygen monitors, and air flow controllers are usually required. |
| Compliance Benefit: | The use of FGR decreases the amount of NOx formation at the facility and therefore may
help facilities meet state air pollution control requirements (40 CFR 52).
Additionally, this technology may help facilities meet the standards of performance for
industrial-commercial-institutional steam generating units in 40 CFR 60.
A decrease in a facility's NOx emissions may decrease the possibility
that a facility will meet the NOx emission threshold for an air permit under 40
CFR 70 and 71. The compliance benefits listed here are only meant to be used as general guidelines and are not meant to be strictly interpreted. Actual compliance benefits will vary depending on the factors involved, e.g., the amount of workload involved. |
| Materials Compatibility: | FGR can almost always be used safely and effectively with existing burner hardware. FGR
works particularly well with boilers which use clean fuels (e.g., natural gas, kerosene, and
distillate oils). Any change in boiler configuration or operation should be checked to
ensure that no flame impingement or other adverse change in operation occurs. |
| Safety and Health: |
No significant changes in safety or health issues should result from the installation and
implementation of FGR. Consult your local industrial health specialist, your local health
and safety personnel, and the appropriate material safety data sheet (MSDS) prior to
implementing this technology. |
| Benefits: |
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| Disadvantages: |
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| Economic Analysis: |
Two 8.37 MM Btu/hr package #2 oil fired boilers at the Naval Consolidated Brig Marine
Corps Air Station (MCAS) were retrofitted with a new FGR system and low-NOx burners,
resulting in average NOx emissions of 130 parts per million (ppm) or 0.16 lb/MMBtu at full
load when burning #2 oil. No adverse combustion conditions or boiler operating problems
were encountered. However, the boiler efficiency dropped from 92 percent to about 89.5
percent due to the conversion to natural gas from #2 fuel oil. The cost attributed to
retrofitting one boiler with ductwork, controls, and an uprated fan motor was $20,000
(1992 dollars). Each boiler has its own unique operating characteristics. Boilers of the same size and same equipment may have different operating requirements and combustion properties. Each boiler should be economically evaluated for FGR on an individual basis. The $20,000 cost included substantial effort on pre- and post-retrofit testing of NOx emissions and combustion conditions and the purchase and installation of oxygen (O2) and carbon monoxide (CO) instrumentation. Additional operation and maintenance (O&M) costs associated with the system are expected to be minimal. If the FGR is installed as an integral system, built into the boiler front; maintenance costs may be significantly reduced. The dampers and the ductwork provided should present no additional operating costs and require only minimal maintenance. Any instrumentation and controls supplied will require the usual periodic calibration and repair associated with those devices. The annual operating cost for maintenance will probably be decreased because of the increased reliability of the new equipment. Fuel costs increased due to the drop in efficiency by [((92-89.5)/92) x fuel use of 159,350 gallons] or 4330 gallons or about $4,330. Fuel consumption increased due to reduction in efficiency; however, fuel costs were reduced as natural gas prices were lower. Implementation of a FGR system is not likely to result in an economic benefit, indeed it is typically very expensive. However, if regulations change or there is a need to obtain NOx reductions, it is among the first alternatives that should be considered as it is often cheaper than many other alternatives. Economic Analysis Summary:
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| NSN/MSDS: | None identified.
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| Approving Authority: |
Appropriate authority for making process changes should always be sought and obtained prior to procuring or implementing any of the technology identified herein. |
| Points of Contact: | For more information |
| Vendors: | This is not meant to be a complete list, as
there may be other manufacturers of this type of equipment.
Gordon-Piatt Group
Entropy Technology & Environmental Consultants, LP
Coen Company, Inc. |
| Related Links: | None. |
| Sources: | Bayard de Volo, Nick,
Energy Technology Consultants, Inc., December 11, 1995, correspondence to John R. Guerra,
Brooks Air Force Base, TX. Evaluation of Air Pollution Control Technologies for Industrial Boilers, prepared by HSC/YAL, December 1995. Steam: Its Generation and Use, The Babcock & Wilcox Company, 40th edition, 1992. NOx Control Technology Data Source Book, EPA-600/2-91-029, NTIS PB91-217364. Evaluation and Costing of NOx Controls for Existing Utility Boilers, EPA-453/2-92-010. |