by Marc Karell and Amit Chattopadhyay
The reduction of NOx emissions is a major goal of the Clean Air Act Amendments because of their known role in the formation of ground-level ozone. The U.S. EPA believes that facilities can reduce NOx emissions from their most common sources, combustion equipment, using reasonable means.
Many NOx-control technologies have been successfully applied to stationary combustion sources. Selection of an appropriate technique will depend on the type of facility (for example, industrial boiler, gas turbine, municipal-waste combustor), site-specific conditions, and regulatory and economic considerations.
NOx is formed by two primary mechanisms, resulting in thermal NOx and fuel-bound NOx. A third mechanism, "prompt NOx" accounts for a minor share of NOx formation.
Thermal NOx formation occurs only at high flame temperatures, when dissociated nitrogen from combustion air combines with oxygen atoms to produce oxides of nitrogen such as NO and NO2. Formation of thermal NOx increases exponentially with combustion temperature, and increases as a function of the square root of the quantity of oxygen in the combustion zone.
Fuel-bound NOx formation is not limited to high temperatures, but is dependent upon the nitrogen content of the fuel.
The best way to minimize NOx formation is to reduce flame temperature, reduce excess oxygen, and/or to burn low nitrogen-containing fuels.
Available NOx Reduction Strategies and Technologies
The following represents proven, available NOx-reduction strategies and technologies for combustion sources.
Fuel switching. Fuel switching is the simplest and potentially the most economical way to reduce NOx emissions. Fuel-bound NOx formation is most effectively reduced by switching to a fuel with reduced nitrogen content. No. 6 fuel oil or another residual fuel, having relatively high nitrogen content, can be replaced with No. 2 fuel oil, another distillate oil, or natural gas (which is essentially nitrogen-free) to reduce NOx emissions.
Flue-gas recirculation (FGR). Flue gas recirculation involves extracting some of the flue gas from the stack and recirculating it with the combustion air supplied to the burners. The process, by diluting the combustion air with flue gas, reduces both the oxygen concentration at the burners and the temperature. Reductions in NOx emissions ranging from 30 to 60% have been achieved.
Low NOx burners. Installation of burners especially designed to limit NOx formation can reduce NOx emissions by up to 50%. Greater reduction efficiencies can be achieved by combining a low-NOx burner with FGR—though not additive of each of the reduction efficiencies. Low-NOx burners are designed to reduce the peak flame temperature by inducing recirculation zones, staging combustion zones, and reducing local oxygen concentrations.
Derating. Some industrial boilers can be derated to produce a reduced quantity of steam or hot water. Derating will decrease the flame temperature within the unit, reducing formation of thermal NOx. Derating can be accomplished by reducing the firing rate or by installing a permanent restriction, such as an orifice plate, in the fuel line.
Steam or water injection. Injecting a small amount of water or steam into the immediate vicinity of the flame will lower the flame temperature and reduce the local oxygen concentration. The result is to decrease the formation of thermal and fuel-bound NOx. Be advised that this process generally lowers the combustion efficiency of the unit by 1 to 2%.
Staged combustion. Either air or fuel injection can be staged, creating either a fuel-rich zone followed by an air-rich zone or an air-rich zone followed by a fuel-rich zone.
Staged combustion can be achieved by installing a low-NOx staged combustion burner, or the furnace can be retrofitted for staged combustion. NOx reductions of more than 40% have been demonstrated with staged combustion.
Fuel reburning. Staged combustion can be achieved through the process of fuel reburning by creating a gas-reburning zone above the primary combustion zone. In the gas-reburning zone, additional natural gas is injected, creating a fuel-rich region where hydrocarbon radicals react with NOx to form molecular nitrogen. Field evaluations of natural gas reburning (NGR) on several full-scale utility boilers have yielded NOx reductions ranging from 40 to 75%.
Reduced-oxygen concentration. Decreasing the excess air reduces the oxygen available in the combustion zone and lengthens the flame, resulting in a reduced heat-release rate per unit flame volume.
NOx emissions diminish in an approximately linear fashion with decreasing excess air. However, as excess air falls below a threshold value, combustion efficiency will decrease due to incomplete mixing, and CO emissions will increase.
The optimum excess-air value must be determined experimentally and will depend on the fuel and the combustion-system design. A feedback control system can be installed to monitor oxygen or combustibles levels in the flue gas and to adjust the combustion-air flow rate until the desired target is reached. Such a system can reduce NOx emissions by up to 50%.
Selective catalytic reduction (SCR). Selective catalytic reduction (SCR) is a post-formation NOx-control technology that uses a catalyst to facilitate a chemical reaction between NOx and ammonia to produce nitrogen and water.
An ammonia/air or ammonia/steam mixture is injected into the exhaust gas, which then passes through the catalyst where NOx is reduced. To optimize the reaction, the temperature of the exhaust gas must be in a certain range when it passes through the catalyst bed.
Typically, removal efficiencies greater than 80% can be achieved, regardless of the combustion process or fuel type used. Among its disadvantages, SCR requires additional space for the catalyst and reactor vessel, as well as an ammonia storage, distribution, and injection system. Also, a Risk Management Plan (RMP) in compliance with Federal Accidental Release Prevention rules may have to be prepared and submitted for ammonia storage.
Precise control of ammonia injection is critical. An inadequate amount of ammonia can result in unacceptable high NOx emission rates, whereas excess ammonia can lead to ammonia "slip," or the venting of undesirable ammonia to the atmosphere.
Selective non-catalytic reduction (SNCR). Selective non-catalytic NOx reduction involves injection of a reducing agent—ammonia or urea—into the flue gas. The optimum injection temperature when using ammonia is 1850ºF, at which temperature 60% NOx removal can be approached. The optimum temperature range is wider when using urea.
Below the optimum temperature range, ammonia forms, and above, NOx emissions actually increase. The success of NOx removal depends not only on the injection temperature but also on the ability of the agent to mix sufficiently with flue gas.
Many facilities are being required to reduce NOx emissions as a result of legislation to limit emissions of precursors of ground-level ozone. Numerous options having varied success rates are available for NOx reduction and control. Therefore, careful thought must enter into the technical decision-making process.
As a general rule, reducing the flame temperature, reducing excess air, and/or burning low-nitrogen-containing fuels can minimize NOx formation. Post-formation control technologies are also available.
The various options must be reviewed in detail with respect to the level of NOx reduction necessary, the specific combustion source, the potential increase in emissions of other pollutants, site-specific constraints, and economic viability.
About the authors: Marc Karell and Amit Chattopadhyay are employed by Malcolm Pirnie, Inc. The former works at the company's White Plains, NY, office and has more than 15 years of experience in the air-quality engineering field; the latter, at the Mahwah, NJ, office, has more than 25 years of experience in combustion engineering. Both are registered professional engineers.