FLUE GAS RECIRCULATION FOR REDUCTION OF NITROGEN OXIDES (NOx) EMISSIONS IN INDUSTRIAL BOILERS

Revision Date: 8/01
Process Code: Navy/Marines: SR-10-03; Air Force: FA03;Army N/A
Usage: Navy: High; Marines: High; Army: High; Air Forcce: High
Compliance Impact: Medium
Alternative for: Normal Combustion Practices
Applicable EPCRA Targeted Constituents: N/A

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 (DoD) 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 RACT or BACT (40 CFR 52) requirements. Additionally, this technology may help facilities meet standards of performance for industrial-commercial-institutional steam generating units in 40 CFR 60, Subpart Db. 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 a general guideline 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, 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:
  • Typically costs less to implement than low NOx burners, if installing as separate unit.
  • In most situations, would be sufficient to satisfy state NOx RACT (Reasonably Available Control Technology) regulations or other NOx emissions requirements.
  • Provides potential for emission reduction credits Provides potential for increased boiler flexibility.
  • If part of an integral system, may reduce energy and space requirements.


Disadvantages:
  • May cause space limitations for recirculation ducts, fans, and additional air ports if FGR is installed separately
  • May require additional energy to run the recirculation fans if FGR is installed separately
  • Oxygen concentration must remain above 17 percent.
  • Requires additional controls and instruments to control air flow over the desired operating range.


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 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

Annual Savings for FGR System: -$4,330
Capital Cost for Equipment/Process: $20,000
Payback Period for Investment in Equipment/Process: Does Not Payback

Note:  With an integral system, maintenance savings may result in a less than 5 year payback, based on fuel cost estimates.

 

Approving Authority: Approval is controlled locally and should be implemented only after engineering approval has been granted.  Major claimant approval is not required.


NSN/MSDS:
Product NSN Unit Size Cost MSDS*
FGR Euipment None Identified N/A $N/A  

*There are multiple MSDSs for most NSNs.
The MSDS (if shown above) is only meant to serve as an example.


Points of Contact: Navy:
Mr. Robert W. Humphreys
Commander Naval Region Southwest
Tactical Planning Code N44.PP2
4635 Pacific Highway, Bldg. 1
San Diego, CA  92145-2135
Phone: (619) 524-3100 x 179
FAX: (619) 524-2988
Email: humphreysrw@pwcsd.navy.mil


Vendors: This is not meant to be a complete list, as there are other manufacturers of this type of equipment.

Gordon-Piatt Energy Group, Inc.
P.O. Box 650
22725 C Street
Winfield, KS 67156-0650
Phone: (620) 221-4770
or (800) 638-6940
FAX: (620) 229-2298
Contact: Mr. Dan Christenson

Entropy Technology & Environmental Consultants, Inc.
12337 Jones Road, #414
Houston, TX 77070
Phone: (218) 807-7007
FAX: (218) 807-1414
URL: www.etecinc.net


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.



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