Originally Originally appeared in: CHEMICAL ENGINEERING PROGRESS
January 1994 issue, pgs 46-49 Reprinted with publisher’s permission.
AIR POLLUTION CONTROL
Specify Better Low NOx Burners For Furnaces Air staging, fuel f uel staging, and internal flue gas recirculation are among the design fea- tures that help reduce NO x emissions
Ashutosh Garg, Kinetics Technology
temperature. Whether for a new furnace or a retrofit application, these burners must meet five major requirements. Operation with lower NO x formation: A flame pattern compatible with furnace geometry; Easy maintenance and accessibility; A stable flame at turndown conditions; and the ability to handle a wide range of fuels. Burner Types
Table 1 lists the types of burners currently in use in chemical process International Corp. industries (CPI) plants and petroleum refineries. Figure 1 compares stagedcombustion burners with standard gas S pecifying the right require ments for low and oil burners. NOx burners can significantly reduce nitrogen oxides (NO x) emissions from a Staged-Air Burners. Combustion air is furnace. Ultra Low NO x burners that can split and directed into primary and meet even the most stringent emission secondary zones, thus creating fuel-rich control limits imposed by some states, and fuel-lean zones. are now available and offer a very These burners are most suitable attractive route to NO x reduction. for forced-draft However, burner selection and liquid-fuel-fired specification should be done very applications. carefully, because burner operation has a Combustion air direct effect on furnace performance. pressure energy This article describes the various types of lends itself to low NOx burners and outlines the main better control of design parameters that must be the staging air considered when selecting a burner flows. It ensures system. a high enough air Low NOx burners generally modify velocity to the means of introducing air and fuel to produce good airdelay the mixing, reduce the availability fuel mixing and a of oxygen, and reduce the peak flame good flame.
Staged-air burners lend themselves very well to external flue gas recirculation (FGR). In such designs, flue gas is generally introduced into the primary combustion zone. Staged Fuel Burners. The fuel gas is injected into the combustion zone in two stages, thus creating a fuel-lean zone and delaying completion of the combustion process. The fuel supply is divided into primary fuel and secondary fuel in a ratio that depends on the NO x level required. The flame length of this type of burner is about 50% longer than that of a standard gas burner. Staged-fuel burners are ideal for fuel gas fired natural draft applications. Low Excess Air Burners. These burners reduce NOx emissions by completing combustion with the lowest amount of excess air possible, usually no more than 5-8%. Increases in excess air result in increases in NO x formation (Figure 2a).
Most forced-draft burners have the ability to operate at very low levels of excess air. In a multiple-burner installation, it is essential that all burners receive equal amounts of air. This can be achieved by simulating the flow profiles in the ducts and burners. Flow deficiencies and other irregularities can then be detected and corrected using splitters and vanes, ensuring equal air distribution within +1%. Flue Gas Recirculation Burners. In these burners, 15-25% of the hot (300500oF) flue gas is circulated along with combustion air. The flue gas acts as a diluents, reducing flame temperature and suppressing the partial pressure of oxygen, thus reducing NO x formation. Flue gas can be injected into burners through a separate scroll into the primary zone or mixed with incoming air. External FGR can be used with natural draft burners, although it is mostly used with forced-draft preheated air burners. In some new burner designs, flue gas is internally re-circulated using the pressure energy of fuel gas, combustion air, or steam. This makes the operation of burners simple and eliminates the FGR fan and its controls, although burner size becomes large. Ultra Low NOx Burners. Several designs are available today that combine two NOx reduction steps into one burner without any external equipment. These Figure 1: 1: Off-stoichiometric Off-stoichiometric burners typically inappropriate staged air combustion can be achieved by air staging or fuel staging. with internal FGR or staged fuel with Courtesy of John Zink Co internal FGR. In the former design, fuel is mixed with part of the combustion air, creating a fuel-rich zone. High pressure viscosity, and heating value. atomization of liquid fuel or fuel gas Fuel Specification. Correct and accurate creates FGR. The secondary air is routed fuel specifications are essential for Atomization Medium. For low NOx by means of pipes or ports in the burner predicting NOx emissions. burners, steam is preferred as the block to complete combustion and For gaseous fuels, the complete atomization medium over compressed optimize the flame profile. analysis listing all the constituents is air, because higher quantities of steam In staged-fuel gas burners with required, as well as any possible decrease the amount of NO x in the flue internal FGR, fuel gas pressure induces variations in gas composition. Major gas. Increases in steam temperature recirculation of flue gas, creating a fuel components affecting NO x emissions are increase NOx emissions. lean zone and a reduction in oxygen hydrogen and hydrocarbons in the C3-C4 partial pressure. range. Other physical properties, such as Fuel Filters. Staged fuel gas burners The former design can be used with pressure, temperature, and heating value have more gas tips and rises than the liquid fuels, whereas the latter design are required for burner design. standard burners, and the fuel gas flow is used mostly for fuel gas applications. For liquid fuels, the most important per tip is reduced to as low as one-fourt h. parameter is the fuel’s nitrogen content – It is important that these burners be used about 40-90% of the fuel nitrogen shows with clean fuel gases. To accomplish this, Design Parameters The following parameters require up as NOx in the flue gas. Other liquid installation of fuel gas filters and attention during system design. fuel parameters required by burner knockout pots to remove particulates and vendors are pressure, temperature, condensate is recommended.
Some plants have opted for low fuel gas pressures or double orifice designs for the gas tips to keep the tip size large enough to avoid plugging when firing dirty gases. These options generally do not give good results, and they also produce longer flames (flame length is discussed later). Heat Release And Turndown. Plant engineers have typically specified a margin on the heat release rate as high as 30-50% over the design heat release. Furthermore, on most standard burners, the turndown for gas fuel is generally 5:1 and for oil it is 3:1. These two parameters offered virtually unlimited flexibility to overfire and underfire the furnaces. However, to ensure optimum performance of low NOx burners, it is important to limit the overdesign margin to only 10%. In most cases, the turndown should be limited to 3:1 for gas and 2:1 for oil. This will require more attention from the operators and minimization of burner outages. The result, though, will be better performance from the low NO x burners. Heater Draft. The available heater draft is a very important design parameter, especially for natural-draft burners, because it directly indicates the air pressure energy available for air/fuel mixing. It is, therefore, important that available draft be specified correctly. In some cases, it may be advantageous to increase the available draft by increasing the stack height or Figure 2. 2. NOx emissions are a function of various diameter. Increased draft availability can furnace parameters. reduce the size of the burners. However, increased draft at the hearth also increases the likelihood of air leakage Combustion Air Temperature. prevent flame impingement damage to into the furnace, so the furnace should be Combustion air temperature has a direct the furnace tubes. The key to getting a made leak-tight to prevent such air bearing on flame temperature, and the short flame has been to increase excess infiltration. higher the flame temperature, the more air until the flames are blue sand short. thermal NOx is formed, as shown in This practice has been curbed to some Firebox Temperature- In the past, Figure 2c. If the heater is already extent by the installation of oxygen standard burners specified independent of equipped with an air preheater, then analyzers. the furnace design parameters. However, burners utilizing flue gas recirculation The basic design principle of low the performance of low NO x burners is offer a good degree of NO x reduction. In NOx burners calls for staged combustion closely linked with furnace design and new heaters, alternative methods of waste and cooler flames. This is in direct firing arrangements. NO x formation is heat recovery should be investigated. conflict with the good mixing of air and dependent on firing density and firebox fuel required for efficient combustion. temperature. The burner vendor needs the Flame Length- This parameter has the Thus, a balance needs to be struck firebox temperature and geometry to most important effect on the operation of between the two requirements so as to predict NOx emissions correctly. Higher the furnace. achieve acceptable NO x levels and flame firebox temperature leads to higher NO x Traditionally, furnace operators are dimensions. formation, as depicted in Figure 2b. accustomed to short, crisp flames, which A typical low NOx burner has a
Acknowledgement The author is thankful to KTI management for permission to publish this article. Thanks are also due to Rose Williams for repeatedly typing the manuscript.
Further Reading Bell, C.T., and S. Warren, Warren , “Experience with Burner NOx Reduction,” Hydrocarbon Processing, 62(9), pp.145-147 (Sept. 1983). Garg A., A., Trimming NOx from Furnaces”, Chem. Eng. 99 (11), pp.122pp.122130 (Nov. 1992) Johnson, W.M., and R.R. Martin, Martin , “Staged Fuel Burners for NOx Control in Fired Heaters”, presented at the 1984 Winter National Meeting of AIChE, Atlanta, GA (Mar. 1984) Kunz, R.G., et al., al., “Control NOx from Gas Fired Hydrogen Reformer Furnaces” presented at the National Petroleum Refiners Association.
burner throats. flame that is about 50-100% longer than the flame in a standard burner (when operated design conditions). Any Internal flue gas recirculation calls for larger burner tiles and re-circulation flue variation in operating condition tends to gas ports. increase the flame length in low NO x burners, thereby increasing the chance of Thus, it is becoming very difficult to fit the flame impingement. new low NOx burner in an existing heater The expected flame length must be floor without sacrificing some degree of kept in mind when specifying the heat operational and maintenance flexibility. It is release rate and the total number of essential that the engineering contractor be burners. It is also recommended that the given drawings of the general arrangement of maximum heat release rate per burner be the heater and the steelwork to work out the limited to 10 MMBtu/h. Furthermore, the installation details. burner flame length should be kept to a third of the firebox height for low-roof Burner Testing cabin heaters. Typical clearances for low Burner design is mostly empirical and NOx burners are recommended in Table 2. predicted design and operating conditions can only be verified through performance tests. Burner Size. Today’s low NOx burners are Thus, burner testing is strongly recommended much larger than standard burners for several for all new low NO burners. x reasons: Testing of these burners should be handled with care. The flue gas flow and the Air staging has led to the use of expected temperature profile in the furnace secondary and tertiary air controls. usually cannot be reproduced exactly in the test furnace. For this reason, emission test Fuel staging had led to the segregation of results should be considered estimates, and emission calculations should gas tips and, thus, larger diameter actual incorporate a margin to account for this. burners. It also requires more gas piping Low NOx burners have been installed in a and separate gas controls. variety of applications in both new facilities Recirculation of flue gases requires and in revamped plants. Table 3 summarizes separate gas tubes, and the increased several installations. volumes of gas and air require larger
Waibel, R., et al., al. , “Fuel Staging Burners for NOx Control”, presented at the 1986 Symposium on Industrial Combustion Technology, sponsored by Gas Research Institute, U.S. Dept. of Energy, American Flame Research Committee, and American Society for Metals (now ASM International), Chicago, IL (Apr. 1986)
A. Garg is manager of thermal engineering at Kinetics Technology International Corp. (KTI), Houston, TX (713/974 5581; 713/974 6691). He has more than 19 years of experience in process design, sales, and commissioning of fired heaters and combustion systems. Previously, he worked for Engineers India Ltd., and for KTI in India. He received a B.Tech in chemical engineering from the Indian Institute of Technology. He is a registered professional engineer in California and a member of AIChE.
