Heating & Cooling

 
Low-NOx Burners
  
 

 

Definition
Natural gas burners with improved energy efficiency and lower emissions of nitrous oxides.

 
Building Use
bullet highrise office
bullet lowrise office
bullet highrise apartment
bullet lowrise apartment
bullet retail
bullet food service
bullet institutional
bullet arena
 
 
Building Type
bullet new
bullet retrofit

 
Development
Status

bullet new technology

Description
When fossil fuels are burned, nitric oxide and nitrogen dioxide are produced. These pollutants initiate reactions which result in the production of ozone and acid rain. The nitrous oxides (NOx) come from two sources: high-temperature combustion (thermal NOx) and nitrogen bound to the fuel (fuel NOx). For clean-burning fuels like natural gas, fuel NOx generation is insignificant.

In most cases, NOx levels are reduced by lowering flame temperature. This can be accomplished by modifying the burner to create a larger (and therefore lower temperature) flame, injecting water or steam into the flame, recirculating flue gases, or limiting the excess air in the combustion process. In many cases a combination of these approaches is used. In general, reducing the flame temperature will reduce the overall efficiency of the boiler. However, recirculating flue gases and controlling the air-fuel mixture can improve boiler efficiency, so that a combination of techniques may improve total boiler efficiency.

Natural-gas fired burners with lowered NOx emissions are available for commercial and residential heating applications. One commercial/residential boiler has a burner with inserts above the individual burners; this design reduces NOx emissions by 30%. The boiler also has a "wet base" heat exchanger to capture more of the burner heat and reduce heat loss to flooring.

NOx production is of special concern in industrial high-temperature processes because thermal NOx production increases with temperature. These processes include metal processing, glass manufacturing, pulp and paper mills and cement kilns. Although natural gas is the cleanest-burning fossil fuel, natural gas can produce NOx emissions as high as 500 ppm or more. 

A burner developed by MIT and and the Gas Research Institute combines staged introduction combustion air, flue gas recirculation and integral reburning to control NOx emissions. These improvements in burner design result in a low-temperature, fuel-rich primary zone, followed by a low-temperature, lean secondary zone; these low temperatures result in lower NOx formation. In addition, any NOx emission present in the recirculated flue gas is reburned, further reducing emissions. A jet pump recirculates a large volume of flue gas to the burner; this reduces NOx emissions and improves heat transfer.

   
Benefits
bullet Lowers NOx and CO emissions
bullet increases energy efficiency
 
Limitations
bullet higher cost
bullet higher maintenance
 

Application
The low-NOx burner used for commercial and residential space heating is larger in size than conventional burners, although it is designed for ease of installation.

Experience
A low-NOx burner in a cement kiln reduced fuel consumption by 5% and increased output of the kiln by 10%. More dramatic were the reductions in NOx emissions: a 75% reduction in emissions (to levels about 100 to 200 ppm). Carbon monoxide levels were also low. Product quality was increased. Other industrial applications include a pulp and paper plant in Oklahoma, and cement plants in Oregon and British Columbia. A gas burner developed by MIT was tested in a metal forging plant in California. NOx emission levels were 25 to 35 ppm (80 to 90% lower than emissions from other burners at the plant), and CO emissions were below 30 ppm.

Example Manufacturers
Viessmann Manufacturing Company
750 McMurray Road
Waterloo ON 
Canada  N2V 2G5
tel 1 519 885 6300
fax 1 519 885 0887
www.viessmann.com

Fuel & Combustion Technology
283 Great Valley Parkway
Malvern PA 
USA  19355
tel 1 610 725 8840
fax 1 610 725 8846
www.fctinternational.com

DB Riley
P.O. Box 15040
Worcester MA
USA  01615-0040
tel 1 508 852 7100
fax 1 508 852 7548
www.dbriley.com

Todd Combustion
2 Armstrong Road
Shelton CT
USA 06484
tel 1 203 925 0380
fax 1 203 925 0384
www.toddcombustion.com

Information Sources
American Gas Association
400 North Capital St. NW
Washington DC
USA 20001
tel 1 202 824 7000
fax 1 202 824 7115
www.aga.org

Ontario Natural Gas Association
77 Bloor Street West
Suite 1104
Toronto ON
Canada M5S 1M2
tel 1 416 961-2339
fax 1 416 961 1173
www.onga.org

 
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