The San Joaquin Valley of California is a large geographic basin with a serious air pollution problem. In December 2001, The San Joaquin Valley Air Basin (SJVAB) was reclassified from serious to severe nonattainment (the EPA designates an area as nonattainment if it has violated the national 8-hour ozone standard).

Because of the severe nonattainment reclassification, the San Joaquin Valley Unified Air Pollution Control District is required by Section 172 of the Federal Clean Air Act Amendments of 1990 to implement measures to attain the National Ambient Air Quality Standard (NAAQS). This can be achieved by reducing ozone-forming emissions of volatile organic compounds (VOC) and oxides of nitrogen (NOx). The California Clean Air Act also requires nonattainment areas to adopt all feasible control measures for stationary sources of air pollution, to reduce NOx emissions.

NOx & CO Limits Table

The two primary methods of controlling NOx emissions from boilers, steam generators and process heaters are:

1. Change the combustion parameters (i.e. combustion modification) to reduce NOx formation or

2. Treat the NOx formed before it is emitted into the atmosphere (i.e., exhaust gas treatment)

Combustion Modifications

Combustion modification systems are designed to reduce thermal NOx formation by changing the flame characteristics to reduce peak flame temperature. Combustion controls include low excess air operation, staged combustion, over fire air ports, biased firing, and burners out of service.

Combustion modification is also achieved by different burner designs such as Low NOx and Ultra Low NOx burners. Some of the design principles used in Ultra Low NOx and Low NOx burners include staged air burners, staged fuel burners, pre-mix burners, internal recirculation, and radiant burners.

Combustion control systems may be used by themselves or in combination with Flue Gas Recirculation (FGR). FGR recycles a portion of the exhaust stream back into the burner windbox, mixing low oxygen air with combustion air prior to entering the combustion chamber. This technique reduces thermal NOx formation by reducing oxygen in the combustion zone and by reducing the peak temperature.

Exhaust Gas Treatment

Exhaust Gas Treatment (EGT) is another way to reduce NOx. NOx is reduced to molecular nitrogen by adding flue gas treatment systems located after the boiler firebox. EGT includes Selective Catalytic Reduction (SCR) and Selective Noncatalytic Reduction (SNCR). Both EGT's operate at a certain temperature range to effectively reduce NOx in the exhaust gas by injecting either urea or ammonia into the post combustion zone of the boiler.

Another control technology is Low Temperature Oxidation (LTO). This technology is similar to SCR but uses ozone rather than ammonia as the reducing agent. It requires either a liquid oxygen storage tank or an oxygen generator to provide feedstock for the ozone generator. The ozone is generated from oxygen using a corona discharge system and then injected into the exhaust gas stream. The ozone reacts with NOx to form nitric acid. A packed tower scrubber with two scrubbing chambers then removes the nitric acid, including any excess ozone, from the exhaust stream.

Based on information provided by vendors and manufacturers of NOx control devices, the technology that would likely be used is Ultra Low NOx burner systems either with or without FGR. LTO is currently being used in only one facility in the South Coast Air Quality Management District.

Compliance Schedule

Rule 4306 prescribes a minimum percent of the total number of subject units at each stationary source would have to comply with the rule by June 1, 2005, June 1, 2006 and June 1, 2007. In addition, recent Amendment have been included to Rule 4306, which allows some units to comply by December 1, 2008, under an enhanced option compliance schedule (click here for a complete compliance schedule). The enhanced option is an option for users who would like to extend their compliance date, while meeting stricter NOx limits.

It is estimated that the potential NOx emissions reduction from Rule 4306 is about 8 tons per day.

Resource Documents

Compliance Schedule

Emissions Limits

Rule 4306 (Published by the SJVAPCD)

The EPA Ozone Report (2003)

Resource Links

California Air Resource Board

San Joaquin Valley Air Pollution Control District

United States Environmental Protection Agency (Office of Air & Radiation)

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