Ozone: What does NOx have to do with it? -- Part 2

Part 1 of this article in the last issue of From the Source summarized what nitrogen oxides (NOx) are, how they are formed, and why NOx is an environmental concern. The article also discussed the three major strategies for reduction of NOx emissions at the source. These strategies are: changing the nitrogen content of the fuel, minimizing peak combustion temperature, and minimizing excess oxygen in the combustion process. There are several combustion modifications employing one or more of the above strategies that can be used to reduce NOx generation. The following table summarizes these modifications.

Burner out of service - This technique can be used to retrofit oil or gas fired units that have multiple levels of burners. This is a staged combustion technique, where fuel flow from upper level burners is diverted to lower level burners, thus creating a fuel rich zone in the vicinity of the primary flame (lower level burners), and an air rich zone in the area around the upper level burners. The combustion system thus consists of lower level burners that are operated fuel rich, and upper level burners that are operated on air only. This method causes less than stoichiometric air to be available for combustion in the fuel rich zone, resulting in lower peak temperatures (due to incomplete combustion of the fuel). Combustion is completed in the air rich zone when additional air is introduced into the system through the upper level burners. This type of staged combustion system approach is referred to as "air staging". This results in a reduced temperature in the air rich (upper level) zone as well.

Fuel Biasing - This is a retrofit technique which has been approved only for oil and gas fired utility boilers. Fuel biasing works on the same (staged combustion) concept as the burner out of service technique and also applies an air staging approach; however, in the fuel biasing technique adjacent levels of burners are operated in a fuel rich and fuel lean configuration instead of the fuel rich and air only configuration employed in the burner out of service technique. This technique is less effective than the burner out of service technique for reducing NOx generation, but it is also typically easier to operate successfully.

Overfire air - This is also a staged combustion technique where all burners are operated in a fuel rich mode, with additional combustion air supplied through special overfire ports (a.k.a. NOx ports) located above the primary combustion zone. The overfire air technique therefore constitutes an air staging approach. This technique is typically not employed in a retro-fit application, since effective mixing of overfire air with the primary combustion gases is critical to successfully reducing NOx emissions and is highly dependent on furnace design parameters such as residence time, inside geometry, and pressure drop.

Fuel Reburning - Fuel reburning is another staged combustion technique which uses a fuel staging approach versus the air staging approach seen in the previously described techniques. In the fuel reburning technique, the primary (lower) combustion zone is operated in an excess air mode, while the secondary (middle) combustion zone is operated in a fuel rich mode. Approximately 10-20 % of the total fuel used for combustion is used in the secondary combustion zone. The secondary combustion zone is followed by a third (upper) combustion zone, in which only air is added to the process. The air added in the third combustion zone allows the combustion to be completed. The oxygen rich atmosphere of the primary combustion zone allows for the formation of NOx, which is then converted into molecular nitrogen (N₂) in the fuel rich (reducing) atmosphere present in the secondary combustion zone. Molecular nitrogen does not participate directly in the formation of NOx outside of the flame front (where prompt NOx can be formed), and does not participate in the formation of ground-level ozone when emitted into the atmosphere. Natural gas, distillate fuel oil, or some other low nitrogen content fuel is typically used for fuel in the secondary combustion zone, to reduce the amount of fuel NOx that can be formed in the oxygen rich atmosphere that exists there. A relatively small amount of thermal NOx will also be formed in the third combustion zone; however, the overall quantity of NOx emitted from a reburn combustion system will be much less than from a unmodified system.

Many of the staged combustion techniques mentioned above have been successfully employed to reduce NOx emissions; however, it should be noted that each of these techniques require that a higher level of combustion process control be exercised to prevent process upsets that can cause increased smoke, carbon monoxide, and/or hydrocarbon emissions.

Low excess air firing - The objective of this approach is to operate with the lowest level of excess air that is safe, efficient, and practical. This modification requires an investment in improved process monitoring and control systems, rather than changes to the furnace hardware. This approach reduces NOx generation by minimizing the amount of oxygen available for NOx formation in the post combustion zone. This approach has been integrated into all modern low-NOx burner systems.

Flue gas recirculation - This technique for reducing NOx generation is accomplished by taking typically 10-30% of the flue (exhaust) gas, mixing it with incoming combustion air and injecting the mixture directly into the combustion chamber. The recirculated flue gas acts as an inert thermal diluent (i.e. it dilutes the oxygen content of the combustion air) and provides for better mixing of combustion air and fuel (via increased turbulence), allowing for lower flame temperatures. Both of these results contribute to decreased generation of thermal NOx. Higher levels of recirculation can lead to flame instability as well as other operational problems such as condensation on internal heat transfer surfaces. This process also requires significant capital expenditures and requires a relatively large amount of space for the necessary ductwork and fans.

Low NOx Burners - Low NOx burners have been developed which can be used to retrofit almost any type of combustion process. Low NOx burners use modified air and fuel entry to slow the mixing rate, reduce the oxygen available for NOx formation in critical NOx formation zones, and/or reduce the amount of fuel burned at peak flame temperatures. Low NOx burners operate at much lower oxygen levels than conventional burners, and therefore generate less fuel and thermal NOx. The installation of low NOx burners generally leads to higher operation and maintenance costs due to the increased complication of the systems required to operate them.

Water/steam injection - This technique is commonly used to reduce NOx generation in gas turbines. The injection of steam or water reduces the peak flame temperature of the combustion process, thus reducing the creation of thermal NOx. This technique requires that the water/steam be treated to prevent corrosion, and may result in increased generation of carbon monoxide and/or hydrocarbons at low load conditions. The addition of water also slightly increases fuel consumption, because additional energy is consumed by the system for converting the water into water vapor (steam).

All of the above described techniques have been used to successfully control NOx generation; however, selection of the best technique for a particular combustion system depends on many factors, including system design, process control capabilities, fuel characteristics, and the degree of stability in system demand requirements. The combustion system designer and/or vendor can be a good initial source of information regarding the types of NOx reduction techniques that will work best for a given combustion unit. Many firms also specialize in retro-fitting combustion units with NOx reduction systems.