A Study of NOx Reduction in Boilers at U.S. Power Plants

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Researched by Industrialinfo.com (Industrial Information Resources, Incorporated; Houston, Texas). NOx or nitrogen oxides is a generic term for a group of highly reactive gases, all of which contain nitrogen and oxygen in varying amounts. Many of the nitrogen oxides are colorless and odorless. Nitrogen oxide forms when fuel is burned at high temperatures, as in a combustion process. The primary sources of NOx are motor vehicles, electric utilities, and other industrial boilers. This report covers NOx emitted by boilers used for power generation.

There are three main reasons for concern about NOx emissions. Firstly, NOx is one of the main ingredients involved in the formation of ground-level ozone (a health hazard). Secondly, NOx reacts to form acid aerosols (a health hazard). Thirdly, NOx emissions contribute to the formation of acid rain

The EPA has issued regulations governing NOx emissions from electric utility boilers and other industrial boilers. The first phase, finalized in a rulemaking in 1995, aimed to reduce NOx emissions by over 400,000 tons per year between 1996 and 1999. The goal of the second phase is to reduce emissions by over two million tons per year beginning in 2000.

The Interstate Air Quality Rule, expected to be passed in 2005, requires states to reduce ground level ozone. Since NOx and ozone can be transported long distances, the act also requires "upwind" states to implement programs that will help "downwind" states meet the ozone standards. In the January 30, 2004 Federal Register, EPA proposed new air rules for reducing emissions of sulfur dioxide (SO2), nitrogen oxides (NOx), and mercury. EPA proposed the Interstate Air Quality Rule, which focuses on states whose SO2 and NOx emissions are significantly contributing to fine particle and ozone pollution problems in other downwind states. The proposed Interstate Air Quality Rule would cover 29 states in the Eastern United States and the District of Columbia. This would lead to extensive reduction of NOx, SO2, and mercury emissions in these states by 2010. The most likely control measures would be installation of SCR’s, scrubbers, and baghouses.

Industrialinfo.com maintains a database of boilers used for power generation, located at U.S. utility, private energy, and industrial sites. There are approximately 3,786 boilers listed by location in this database, including all boiler types - dry-bottom, wet bottom, heat recovery, boiling water reactor, fluidized bed, and others. The boilers listed in the database use a variety of different fuels, including coal, gas, wood, municipal refuse, and other fuels. For details Contact Industrialinfo.com's Member Center at 1-800-762-3361.

The attached chart (Click on image at right) shows an analysis of the number of NOx Controls installed on U.S. power plant boilers by type. About 40% (1,522) of the total of 3,786 boilers are equipped with some type of NOx control technology.

There are several different ways to control NOx emissions from boilers. The most common procedures are shown below in the glossary. Usually a combination of procedures is used - for instance a low NOx burner in conjunction with a SCR unit.

Glossary of NOx emission controls

BFAB - Biased firing (alternate burners)
LNB - Low NOx burners. Slow and control the rate of fuel and air mixing.
AOFA - Staged combustion of fuel and air
FGR - Flue gas recirculation
LEA - Low excess air
SCR - Selective catalytic reduction. Feed ammonia into flue gas to form nitrogen and water over a catalytic converter.
SNCR - Selective non-catalytic reduction. This is the reaction in the fire zone between ammonia and NOx.
DFA - Staged combustion. Late oxygen introduction.

NOx is produced during combustion of coal from the nitrogen contained in the coal, and from nitrogen in the air. Low NOx burners reduce NOx emission by about 35-50%. Adding an SCR (selective catalytic reduction) will reduce NOx emissions by 80-90%.

Variations in the burning process require minor changes to the fuel/air mixture to maintain the correct ratio for operation. The process of adjusting the fuel or air is called boiler trim. Methods of boiler trim vary by boiler and type, and control scheme. Some methods control the amount of air injected into the system, others control the fuel, and still others control both. However, in any scheme, it is critical, for safety and efficiency, to know the oxygen in the process. Most power plants use a measurement device that continuously measures oxygen in the combustion zone. These devices are referred to as zirconia oxygen controls.

Logically, errors in air measurement have direct effect on the control of the boiler, including process safety, fuel use, and plant emissions, including NOx. As a general rule of thumb, a 10% increase in excess air will increase fuel usage by 1%. Fuel use is greatly increased when the boiler operates in oxygen-deficient conditions. Excess air makes extra nitrogen available and requires additional heat to maintain combustion temperatures, creating favorable conditions for the formation of thermal NOx. Some excess air is necessary for safe and efficient boiler operation; fuel has surface area, and takes time to burn. Keeping the excess air volume near and slightly above stoichiometric values (quantitative relationship among reactants and products in a reaction) for the particular fuel minimizes formation of NOx, and keeps fuel use at a minimum.

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