NOx reduction is important in lowering the levels of NOx being released into the atmosphere during combustion. When a form of NOx reduction is used, it is important to have accurate measurements of not only NO but also NO2 because the ratio of NO to NO2 is changed. The traditional NO2 amounts as a percent of total NOX (10% or 1.1 factor) cannot be used when NOx reducing methods are used since it will lead to very significant errors in the calculations. When NOx reduction methods are used, the values for NO2 can be greater than 50% of the total NOx. It is important to measure True NOx (NO + NO2) when using NOx reducing methods to maintain the integrity of the measurements.
NOx Reduction Methods
- Staged Combustion
- In this NOx reducing method, only a portion of the fuel is burned in the main chamber. All of the fuel is eventually burned, producing the same amount of energy, but this method greatly reduces the temperature in the main chamber. As temperatures decreased, the amount of thermal NOx is reduced.
- Catalytic Converters
- These devices are utilized to lower the toxicity of the emissions of many combustion processes such as stationary engines, boilers, heaters and internal combustion engines. Catalytic converters break down nitrogen oxides into separate nitrogen and oxygen particles. Some catalytic converters are also used to reduce the high CO levels produced when reducing NOx, as low CO levels are important to ensuring complete combustion.
- Flue Gas Recirculation
- Flue Gas Recirculation, FGR, is a method of NOx reduction that lowers the temperature of the flame, and therefore reduces thermal NOx. A portion of the exhaust gas is re-circulated into the combustion process, cooling the area. This process may be either external or induced, depending on the method used to move the exhaust gas. Flue Gas Recirculation may also minimize CO levels while reducing NOx levels.
- Reducing O2 levels
- By reducing the amount of O2 that is available to react with the nitrogen, NOx is reduced. This is achieved through the use of oxygen trim controls. To minimize the O2 levels, a combustion analyzer is used to adjust the fuel and air mixture. This method can reduce the level of NOx produced by up to 10%, but it may increase the emissions of Carbon Monoxide (CO) very significantly. This method is widely used in many processes such as in rich burn engines.
- Low NOx Burners
- By changing the shape and formation of the flame by using plates to control airflow, a more elongated flame is created in the burner. The temperature is decreased due to the extended flame and surface area, and the lower temperature reduces the amount of thermal NOx. CO levels may be elevated when using low NOx burners. It is important to monitor CO and True NOx levels to better control Low NOx burners.
- Low Nitrogen Fuel Oil
- The use of low nitrogen oils, which can contain up to 15-20 times less fuel bound nitrogen than standard No. 2 oil, can greatly reduce NOx emissions as fuel bound nitrogen can contribute anywhere between 20-50% of total NOx levels.
- Water/Steam Injection
- Water or steam injection reduces the amount of NOx produced by lowering the temperature of the flame during combustion. The lower temperature allows for the decrease of thermal NOx.This method can result in an increase of 3-10% boiler efficiency losses and excess amounts of condensation may form.Some advanced designs of steam injection technology do not have significant impact on boiler efficiency.
- SCR (Selective Catalytic Reduction)
- SCR is a process where a reductant, most often ammonia, is added to the flue. The reductant then reacts with the NOx in the emissions and forms H2O and N2 (ambient nitrogen). This process may take place at anywhere between 500°F and 1200°F depending on the catalyst used. SCR may reduce NOx emissions by up to 90%. SCRs are mainly used in large industrial and utility boilers.
- SNCR (Selective non-catalytic Reduction)
- SNCR is a process that involves a reductant, usually urea, being added to the top of the furnace and going through a very long reaction at approximately 1400-1600 °F. This method is more difficult to apply to boilers due to the specific temperature needs, but it can reduce NOx emissions by 70%.