In 1963, the U.S. Congress began funding air quality research programs initiated in response to public concern about increasing levels of smog and air pollution in metropolitan areas [1]. Government agencies realized that nitrogen-oxide (NOx) emission levels were increasing due to industrial expansion and that high flame temperatures required for industrial heating processes were a large contributor to NOx emissions. However, it was quickly noted that there was "insufficient information on all aspects of nitrogen-oxide emission and ground level concentration to be able to draw any useful conclusions" [2]. As data accumulated, it became clear that NOx emission levels were increasing and posed a health risk to both the public and environment. Finally, in 1970, the Environmental Protection Agency (EPA) officially recognized NOx as an air pollutant that must be controlled and regulated.
Today, local environmental agencies are imposing increasingly stringent regulations on NOx emissions. As NOx regulations continue to become more stringent, process burners in the refining, petrochemical and steel industries, as well as in boiler burners have been improved to meet the challenge using various NOx control strategies.
Refining, petrochemical and boiler burners seem to have led the field as targets for NOx reduction, and burners used in steel industry have followed close behind. Until recently, geographical location and other factors contributed to NOx reduction requirements not being as aggressive in the steel industry as in the other process heating industries mentioned. But that has begun to change.
New technology
A relatively new NOx control technology [COOLfuel(tm)] developed by John Zink Co. has seen fairly widespread application in refining/petrochemical and boiler applications, and is being used in steel industry applications for the first time.
The NOx control technology was tested for a major U.S. heat treating plant that initiated a project aimed at reducing NOx emissions from radiant tube burners. At its current production levels, the company met air permit requirements, but required better NOx control at higher production levels. Before considering John Zink's technology, the only option available was to install low NOx radiant tube burners.
Incorporating COOLfuel would allow the existing burners to achieve lower NOx emissions with only minor modifications to the burners themselves. John Zink developed a COOLfuel retrofit kit for the existing burners with the target of reducing NOx emissions by more than 50% without deterioration in flame and heat-transfer performance. Full-scale tests conducted at John Zink's combustion test facility under simulated furnace conditions showed that the retrofit kit could substantially reduce NOx emissions from the existing radiant tube burners. The retrofit kit requires neither modifications to the burner-tube assembly nor any expensive controls.
NOx formation
It is important to understand the mechanisms responsible for NOx formation to be able evaluate the methods used to control NOx emissions. NOx formation in combustion processes is attributed to three distinct chemical kinetic mechanisms: (1) NOx produced by fuel-bound nitrogen x, (2) prompt NOx and (3) thermal NOx. NOx from fuel-bound nitrogen is formed when burning fuels that contain nitrogen molecules bound into the fuel, such as ammonia (NH3) or fuel oils. As these fuels react within the combustion process, the nitrogen in the fuel, which becomes immediately available in the initial part of the flame as the fuel dissociates, readily combines with oxygen to form NOx. Natural gas and other "clean" fuel gases, which contain no fuel-bound nitrogen, form NOx only via the prompt and thermal NOx mechanisms.
Prompt NOx (as the name suggests) forms rapidly in the very early stages of the combustion process and occurs mainly under fuel-rich conditions in a process that is very similar to the behavior of fuel-bound nitrogen. As the fuel heats within the flame zone, it breaks apart and forms highly reactive radicals (CH, OH, etc.), which combine with nitrogen. Eventually, these radicals (now bound with nitrogen) react with oxygen resulting in the formation of NOx. Typically, prompt NOx is not the major contributor of NOx in combustion related equipment. In a conventional combustion process that has no NOx control strategies implemented, almost all of the NOx formed is due to the thermal NOx mechanism. However, in ultralow-NOx burners of the recent past, prompt NOx may represent a larger fraction of the NOx produced, but is still much smaller than the thermal NOx.
Thermal NOx formation is strongly related to the temperature of the combustion process. High-temperature zones within the flame and surrounding environment cause nitrogen molecules in air to break apart and combine with oxygen and lead to the formation of NOx.
Because the thermal mechanism is usually the dominant source responsible for almost all NOx formation in combustion systems that burn clean fuel, low NOx burners are designed to mitigate thermal NOx production. Reduction of thermal NOx is typically achieved by designing burners that lower peak flame temperatures. Although there are various methods used to lower flame temperature, low-NOx burners rely on the same fundamental principle to accomplish this; that is, dilute the fuel/air mixture with flue gas prior to ignition. The presence of flue gas within the combustion zone provides an additional mass to absorb heat from the flame. This thermal ballasting (heat sink) reduces peak flame temperatures and overall flame temperature, and results in reduced thermal NOx formation.
NOx reduction techniques
Five techniques commonly used in the combustion industry to dilute the fuel/air mixture with furnace flue gas to lower the flame temperature are air staging, fuel staging, flue-gas recirculation, internal flue-gas recirculation and fuel modification.
Air staging consists of supplying the air required for the combustion process in two locations some distance apart. All of the fuel is mixed with part of the air in the first stage and part of the fuel is burned. The resulting products of combustion serve to dilute the remaining fuel before it reaches the second stage of air and fully combusts, thereby accomplishing a form of fuel modification.
Fuel staging also consists of supplying the fuel in two different locations some distance apart. The products of combustion from the first stage serve to dilute the air stream similar to flue-gas recirculation (FGR) and the fuel delivered in the second stage undergoes the process of internal flue gas recirculation (IFGR). The combination of the processes makes fuel staging more effective at NOx reduction than air staging if the burner process is designed properly. Figures 1, 2 and 3, respectively, illustrate how FGR, IFGR and fuel modification are accomplished.
Flue-gas recirculation has long been used as a NOx reduction technique in various types of combustion processes [3]. FGR systems extract a portion of the flue gas from the exhaust of the furnace using a fan and reintroduce it into the combustion process by mixing it with the air supply. This method effectively reduces NOx, but requires the additional cost of a high-temperature fan to recirculate the flue gas. The next two methods do not require the use of a fan, but instead, depend on the energy available from the fuel pressure to mix flue gas with the fuel.
Internal flue-gas recirculation systems use the pressure energy of the fuel to entrain and mix flue gas inside the furnace. It is important that rapid mixing occurs between the fuel and flue gas prior to ignition to achieve reduced NOx emissions. The dilution achieved in designs where this mixing period is short will not yield NOx performance comparable with the fuel-modification technique.
Fuel modification uses the pressure energy of the fuel to extract flue gas from the exhaust of a furnace and thoroughly mix it with the fuel prior to combustion. In theory, the gas used to dilute the fuel can be any nonparticipating gas such as carbon dioxide, nitrogen, steam, etc. However, in practice, flue gas drawn from the furnace exhaust generally is the most economical and convenient diluent. COOLfuel technology is a fuel-modification technique, which together with its earlier generation variants account for over 200 commercial installations on boilers and process heaters in the past 10 years.
In the case of radiant-tube burner NOx reduction, disadvantages of FGR and IFGR techniques, respectively, compared with COOLfuel technology were the need for a high temperature fan and for major modifications to the burners.
Retrofit Kit
The retrofit kit uses the pressure of the fuel gas to transport the flue gas from the exhaust of the radiant tube and to mix the fuel and flue gas effectively prior to delivery to the burners. The amount of flue gas that can be educted using the COOLfuel system is limited by available fuel pressure and pressure losses associated with the flow of flue gas from the exhaust of the radiant tube to the inlet of the burner. The available fuel pressure can have a significant impact on the design of the system; higher fuel pressure provides more energy to educt flue gas, which allows for a more compact retrofit kit design.
The plan for retrofitting the heat-treat furnace consisted of applying a retrofit kit to each burner/tube assembly (Fig.4). The flue-gas piping length was minimized by tapping the exhaust of a neighboring burner to supply the flue gas to a given burner. Figures 5 and 6 illustrate the test arrangement used to evaluate the retrofit kit performance. The arrangement consists of a conventional radiant burner and tube assembly surrounded by a refractory lined enclosure to simulate actual furnace conditions. An air gap between the radiant tube and refractory lined enclosure permits cooling air to be circulated along the length of the tube allowing for temperature control of the simulated furnace. Several thermocouples are positioned along the length of the tube; some attached to the surface of the radiant tube and others within the air gap between the tube and refractory lined enclosure. A long radius elbow located downstream of the radiant tube directs the exhaust flow into the vertical stack section.
A damper located near the exit of the stack allows for control of the pressure drop through the burner assembly. This arrangement provides the simulated pressure drop that would exist under actual furnace conditions. Located a few feet below the stack damper is a sample port connected to a gas analyzer that provides a real-time readout of the concentration of oxygen, NOx and carbon monoxide in the flue gas. Located between the vertical stack and end of the radiant tube is a flue-gas recirculation line, a 2-in. (50 mm) pipe that extends from the radiant tube outlet to the inlet of the COOLfuel system. An air-cooled heat exchanger allows full control of the flue gas recirculation temperature. Located downstream of the heat exchanger is an orifice-metering run that provides the information necessary to determine the flow rate of flue gas being drawn back (educted) into the system. The flue gas flow rate is controlled by a gate valve located a few feet downstream of the orifice-metering run.
The hot flue gas educted by the system thoroughly mixes with the fuel prior to entering the back end of the burner, flowing through the burner and mixing with combustion air near the entrance to the radiant tube. The temperature of the combustion air is controlled using an air preheater located just upstream of the burner, which provides air temperatures ranging from ambient to 400 F (205 C).
Test results
Figure 7 shows the effects of NOx emissions for various quantities of flue gas mixed with the fuel. NOx emissions decrease substantially as the amount of flue gas mixed with the fuel increases. For example, using 70 F (~20 C) combustion air, NOx emissions for the standard burner (without COOLfuel) were approximately 100 ppmdv (parts per million by volume, dry) compared with 39 ppmdv when mixing 3 lb of flue gas with 1 lb of fuel using the retrofit kit-a 60+% decrease in NOx emissions.
Figure 7 also shows the effects of preheated air temperature on NOx emissions. NOx emissions increase with increasing combustion air inlet temperature. For example, the standard burner (without retro fit kit) produced NOx emissions of about 100 ppmdv (ambient air) compared with 190 ppmdv when firing under identical conditions using 400 F preheated air-an increase of about 90%. The percent increase in NOx emissions due to preheated air temperature appears to remain fairly constant regardless of the amount of flue gas mixed with the fuel.
The NOx reduction achieved with the COOLfuel retrofit kit is attributed primarily to the reduction in the flame temperature. Flame temperature can be reduced by several hundred degrees when flue gas is mixed with the fuel prior to combustion, minimizing the contribution from thermal NOx. Figure 8 shows the NOx data in relation to corresponding calculated values of adiabatic flame temperatures. The increase in NOx emissions correlates very well with the adiabatic flame temperature regardless of the combustion air preheat temperature or the amount of flue gas mixed with the fuel prior to combustion, indicating that thermal NOx is the governing mechanism responsible for NOx reduction using COOLfuel technology.
Figure 9 shows the effect of various concentrations of flue gas in the fuel-flue gas mixture on flame appearance, clearly showing that the flame becomes more translucent as the concentration of flue gas in the mixture increases. A gradual change in flame appearance occurs from a zero concentration of flue gas to a concentration of approximately three pounds of flue gas per pound of fuel. The yellow flame color observed in the unmodified burner's flame is attributed to radiating carbon particles created when fuel does not quickly mix with combustion air. This allows the fuel to start to heat up in the flame before it can combust. As the fuel extracts heat from the flame and increases in temperature, it begins to undergo pyrolysis-a chemical reaction without the presence of oxygen. Pyrolysis of the fuel typically results in the formation of an agglomeration of carbon particles referred to as soot. The yellow color in the flame is a result of soot radiating at a temperature above approximately 2200 F (1205 C).
Figure 7 demonstrates that the formation of soot can be significantly reduced when flue gas is mixed with the fuel prior to combustion. The reduction in soot formation can be largely attributed to the slower rate of pyrolysis due to a lower flame temperature. However, the lower flame temperature does not adversely affect the radiation heat transfer required for the process because the reduction in radiation from the soot is offset by the radiation from the additional mass of flue gas. Moreover, the flue gas radiation is more uniformly distributed along the length of the tube providing a more uniform heat transfer to the process.
Figure 10 shows the tube skin temperatures at various locations along the tube length for preheated air temperatures of 70, 200 and 400 F (20, 95 and 205 C). The data show that the temperature profile along the length of the tube is not altered when flue gas is mixed with the fuel, and, in some cases, the addition of flue gas lowered the peak tube temperature and the high/low temperature spread (referred to as the hot spot over average, or HSOA). These trends indicate that the heat transfer mechanism is dominated not by radiation from the soot particles, but rather by radiation from the products of combustion-namely carbon dioxide and water vapor.
The photograph at the far left in Fig. 11 corresponds to the burner operating with the COOLfuel system with a flue gas-to-fuel mass ratio of three at an oxygen level of 2%. The rest of the photograph sequence shows flames corresponding to turndown conditions achieved by adjusting the fuel pressure, but not the air flow rate or the COOLfuel system. Test results show that a stable flame can be achieved beyond a turndown ratio of 2 to 1 without having to adjust flue gas or airflow rates. IH