Practical Flameless Combustion

Paper from the AFRC 2014 conference titled Practical Flameless Combustion by R. Isaacs.

In this paper, we will discuss Zeeco's development and application of flameless combustion technology in process heaters over the past approximately 13 years. Flameless combustion is the use of internal flue gas recirculation to change the composition of the fuel gas by entrainment of flue gasses to lower peak flame temperature and reduce thermal NOx. This "conditioning" of the fuel gas causes the fuel gases to combust with a much lower luminosity, creating "flameless" combustion. This "conditioning" of the flue gas creates a host of burner design problems, however. This new fuel gas composition burns at lower temperatures, making it more difficult to stabilize. Several methods of stabilizing this conditioned fuel mixture are discussed, during both normal and abnormal operating conditions. Some methods rely on furnace temperatures sufficient to bring the fuel to auto ignition temperature. These designs often have a "startup" and a "run" mode. Zeeco's GLSF Free-Jet burner utilizes hot refractory ledges to stabilize the flame during all operating conditions, eliminating the need for different operational modes. Other design concerns such as overall cost of ownership, flame dimensions, and burner spacing will also be discussed, as well as how the Zeeco GLSF Free-Jet burner addresses these concerns.

PRACTICAL FLAMELESS COMBUSTION Jonathon E. Barnes DESIGN ENGINEER, PROCESS BURNER DIVISION, ZEECO, INC. Seth A. Marty DESIGN MANAGER, PROCESS BURNER DIVISION, ZEECO, INC. Rex K. Isaacs DIRECTOR OF BURNER PRODUCTS, ZEECO, INC. ABSTRACT In this paper, we will discuss Zeeco's development and application of flameless combustion technology in process heaters over the past approximately 13 years. Flameless combustion is the use of internal flue gas recirculation to change the composition of the fuel gas by entrainment of flue gasses to lower peak flame temperature and reduce thermal NOx. This "conditioning" of the fuel gas causes the fuel gases to combust with a much lower luminosity, creating "flameless" combustion. This "conditioning" of the flue gas creates a host of burner design problems, however. This new fuel gas composition burns at lower temperatures, making it more difficult to stabilize. Several methods of stabilizing this conditioned fuel mixture are discussed, during both normal and abnormal operating conditions. Some methods rely on furnace temperatures sufficient to bring the fuel to auto ignition temperature. These designs often have a "startup" and a "run" mode. Zeeco's GLSF Free‐Jet burner utilizes hot refractory ledges to stabilize the flame during all operating conditions, eliminating the need for different operational modes. Other design concerns such as overall cost of ownership, flame dimensions, and burner spacing will also be discussed, as well as how the Zeeco GLSF Free‐Jet burner addresses these concerns. INTRODUCTION Over the past several decades, environmental regulations have increased pressure on the petrochemical and refining industries to reduce harmful emissions such as nitrogen oxides (NOx) and carbon monoxide (CO) from fired equipment. This regulatory pressure has motivated the development of advanced burner technologies to reduce NOx and CO emissions while maintaining or improving flame characteristics. In order to meet this challenge, new technologies have been developed, including flameless combustion technologies. Flameless combustion technologies have been successfully applied to various process heating furnaces for well over a decade. Special burner designs utilizing this technology have successfully lowered NOx emissions below 9ppm in process furnaces while maintaining acceptable "flame" or oxidation envelopes, turndown and other performance requirements. Flameless combustion has proven to be a challenging technology to successfully implement in process heaters without addition of complex operational strategies and additional equipment. Often these technologies are only used in high temperature furnaces with high preheat air temperatures, beyond the conditions that are normally found in process heaters. When these conditions are present, achieving an acceptable oxidation envelope for a process heater has also proven difficult. In 2001, Zeeco developed the Free‐Jet burner to utilize this novel combustion technology in a simple, easy to operate manner for a wide variety of process furnaces. The burner utilizes a novel stabilization method to provide a compact oxidation envelope which allows for application of flameless combustion technology in many traditional process heater applications. THE BASICS In order to understand how flameless combustion works, formation of thermal NOx emissions must first be examined. For gaseous fuels with no fuel‐bound nitrogen (N2), thermal NOx is the primary contributor to overall NOx production. Thermal NOx is produced when flame temperatures reach a high enough level to "break" the covalent N2 bond apart, allowing the "free" nitrogen atoms to bond with oxygen to form NOx. Stoichiometric Equation describing typical combustion in a natural gas fired burner Methane and Air with Excess Air 2CH4 + 4 (XA) O2 + 15 (XR) N2 ‐‐‐> 2CO2 + 4H2O + (XA) 15N2 + (XR) O2 Natural air is comprised of 21% O2 and 79% N2. Combustion occurs when O2 reacts and combines with fuel (typically hydrocarbon). However, the temperature of combustion is not normally high enough to break all of the N2 bonds, so a majority of nitrogen in the air stream passes through the combustion process and remains diatomic nitrogen (N2) in the inert combustion products. Very little N2 is able to reach high enough temperatures in the high intensity regions of the flame to break apart and form "free" nitrogen. Once the covalent nitrogen bond is broken, the "free" nitrogen is available to bond with other atoms. Basic chemistry dictates that "free" nitrogen, or nitrogen radicals will react to other atoms or molecules that can accept them to create a more stable atom. Of the possible reactions with the products of combustion, free nitrogen will most likely bond with other free nitrogen to form N2. However, if a free nitrogen atom is not available, the free nitrogen will react with the oxygen atoms to form thermal NOx. As the flame temperature increases, the stability of the N2 covalent bond decreases, allowing the formation of free nitrogen and subsequently increasing thermal NOx. Burner designers can reduce overall NOx emissions by decreasing the peak flame temperature, which can reduce the formation of free nitrogen available to form thermal NOx. The varied requirements of refining and petrochemical processes entail the use of numerous types and configurations of burners. The method utilized to lower NOx emissions can differ by application. However, thermal NOx reduction is generally achieved by delaying the rate of combustion. Since the combustion process is a reaction between oxygen and fuel, the objective of delayed combustion is to reduce the rate at which the fuel and oxygen mix and burn. The faster the oxygen and the fuel gas mix, the faster the rate of combustion and the higher the peak flame temperature. Figure 1 plots Peak Flame Temperature against Thermal NOx created. NOx emissions increase as the adiabatic flame temperature increases. Slowing the combustion reaction reduces the flame temperature, which results in lower thermal NOx emissions. Figure 1: Thermal NOx formation vs. Adiabatic Flame Temperature The industry's standard method to reduce thermal NOx is to mix the fuel gas together with the inert products of combustion to recondition the fuel before combustion occurs. Since the reconditioned fuel is mainly comprised of inert components, the resulting composition burns at a lower peak temperature. To best utilize the inert products of combustion (flue gas) within the furnace, the fuel gas is introduced along the outside perimeter of the burner tile in an area where flue gas is present while the furnace is in operation. As the fuel gas passes through the inert products of combustion, mixing naturally occurs. This changes the composition of the fuel, and stabilization occurs at the tile exit. Since the reconditioned fuel mixture is 15 to 50% inert or more in most cases, the resulting flame burns at a lower peak temperature and generates less thermal NOx. The mixing of the fuel gas with flue gas prior to combustion is called Internal Flue Gas Recirculation (IFGR). In flameless combustion, this concept of Internal Flue Gas Recirculation is taken to its extreme to provide ultra‐low NOx emissions. In most flameless combustion burners, all of the fuel gas is introduced on the outside of the burner tile and is conditioned by passing through flue gas. As a result of this increased IFGR, the conditioned fuel mixture oxidizes with little visible radiation, making the flame completely or nearly completely transparent, i.e. flameless. This flameless condition indicates that a maximum amount of IFGR is achieved and the peak flame temperature is lowered considerably. As indicated in Figure 2, as IFGR is increased to levels seen in flameless combustion, adiabatic flame temperature is reduced considerably, resulting in much lower NOx emissions. Figure 2: Impact of Fuel Type and IFGR on Adiabatic Flame Temperature APPLYING FLAMELESS COMBUSTION TO PROCESS FURNACES As a result of this high level of IFGR, special considerations must be taken to ensure that burner stability, flame length and other characteristics are maintained. A primary concern in flameless combustion is stability. Flame stability (even in transparent flames) depends on three factors: time, temperature and turbulence. The conditioned fuel gas must spend sufficient time mixing with oxygen in a zone of adequate temperature and turbulence to initiate and sustain the combustion reaction. The conditioned fuel gas mixture, as a result of high amounts of inert components in its composition, has an elevated auto ignition temperature, making it more difficult to stabilize. 100% H2 100% CH4 80% IFGR W/CH4 In conventional burners, a small amount of the fuel gas is directed to a low pressure zone, providing a region of intense mixing that creates stability. This method is illustrated in Figure 3. This region of intense mixing also produces high local flame temperatures, creating high NOx. In order to reduce NOx in these types of burners, expensive external methods such as flue gas recirculation are required to achieve NOx levels required by the industry today. Figure 3: Conventional Burner Stabilization Method In many flameless combustion burners, the method to stabilize the burner flame is to rely on local furnace temperatures to heat the conditioned fuel gas/combustion air mixture to auto ignition temperature (Figure 4). Once the mixture reaches sufficient temperature, the reaction will initiate and sustain itself. Ideally fuel reaches this temperature quickly enough to create a controlled oxidation envelope. If not heated quickly enough, the conditioned fuel mixture will oxidize in a less controlled manner, causing non‐ideal operating conditions. This stabilization method works well when the furnace is sufficiently hot, but these conditions are often not present in typical process heaters, but even when the design conditions allow for this method to be used, these conditions are generally not present at turndown or off design conditions. As a result, these types of burners often have a startup mode which increases burner stability at reduced firing rates and furnace temperatures at the cost of increased emissions (Figure 5). This also adds operational complexity and cost to the operation of the burners. Flameless Mode Startup Mode Figure 4: Comparison of "Startup" and "Flameless" modes of operation A preferred method to stabilize flameless combustion burners is by passing the fuel gas over several hot refractory ledges on the perimeter of the burner tile, before reaching the combustion air. These hot refractory ledges provide additional time, temperature and turbulence to ensure that the fuel gas oxidizes in a stable manner over a wide range of operating conditions, without the need for special startup modes. This stabilization method is illustrated in Figures 6, 7, and 8 and is the stabilization method used in Zeeco's GLSF Free‐Jet burner. During startup, the furnace is typically full of ambient air. At this stage, very little flue gas is available for entrainment, so the burner is not flameless, and a flame can be observed to stabilize on the lowest refractory ledge. Figure 5: Startup FUEL Air Air As the burner rate increases, and oxygen content in the flue gas is decreased, the flame stabilizes on the second refractory ledge. As the burner rate approaches design rates and the oxygen content in the flue gas approaches design levels, the burner enters the flameless mode of operation. A small portion of the conditioned fuel gas is trapped on the top refractory ledge, and provides sufficient temperature to initiate and sustain the combustion reaction. The bulk of the conditioned fuel gas proceeds to participate in the combustion reaction in a flameless manner. Figure 6: Design Rate, Flameless Combustion This stabilization method also allows for a more compact burner design, as no stabilization metal is required in the burner throat for the startup operation. This allows for easier retrofit into existing furnaces which results in reduced downtime and installation costs. The design also reduces operational and capital costs by simplifying operation of the burner and reducing quantity of associated equipment. OPERATIONAL CONSIDERATIONS TR A M P AI R Because the NOx reduction method of IFGR and flameless combustion depends on inert flue gas entrainment and conditioning of the fuel gas stream, it is important to control the composition of the flue gas local to the burner tile. This is done by ensuring that all the combustion air enters the burner through the burner throat. Commonly, process furnaces operate under a negative pressure. In the case of natural or induced draft furnaces, this negative pressure is the motive force for introducing combustion air to the combustion chamber and therefore is required for operation. While this condition is not required for forced draft systems, it is still common for these furnaces to operate under a slight negative pressure. This slight negative pressure will result in air ingress to the combustion chamber through any opening in the heater shell. This is commonly known as "tramp air". In a conventional burner, this tramp air typically causes the operator to believe less air is required thorough the throat of the burner, resulting in sub stoichiometric burner operation, poor flame quality and burner instability. The burner emissions are typically affected as well, depending on the burner type and the operating condition. In a flameless combustion burner, this tramp air typically results in oxygen enrichment of the flue products local to the burner. This oxygen enrichment tends to increase the adiabatic flame temperature of the conditioned fuel mixture resulting in increased NOx emissions. Therefore, in order to maintain the benefits of flameless burner technology, it becomes critical to ensure tramp air ingress is minimized to the heater. BU R N E R SPAC I N G Burner spacing is especially important when using flameless combustion burners in multiple burner installations in process heaters. If the oxidation envelope is not tightly controlled, burner to burner interaction can be magnified and flame clouds can occur. In the Zeeco Free‐Jet burner, the unique stabilization method, as well as the large number of gas ports on the exterior of the burner tile allow for a compact oxidation envelope, which enables burners to be spaced closely together without flame interaction. FL A M E SU P E R V I S I O N Flameless combustion presents a problem for traditional flame supervision methods as UV radiation is limited in flameless combustion burners. If the burner relies on separate operational modes, typically the burner management philosophy relies on furnace temperature to allow the burner to operate in a flameless mode. When operating in the flameless mode there is no flame supervision required, as the temperature of the furnace is above auto ignition temperature and the flame cannot extinguish. The burner is then limited to flameless operation only when the furnace is at conditions which allow for no flame supervision. When the burner must operate with lower furnace temperatures, the "startup" mode is utilized, and traditional flame supervision is used. This can severely limit the effectiveness of the flameless combustion technology in a typical process furnace, as this "window" in which the flame supervision is not required may be limited, or in some cases, never present at all. The Zeeco Free‐Jet burner avoids this complex operational methodology by providing sufficient UV radiation even while operating in a "flameless" mode. A small amount of the conditioned fuel gas stabilizes on the top hot refractory ledge which sustains a visible flame, allowing for traditional flame supervision to be utilized even while the bulk of the fuel gas oxidizes in flameless mode. This allows for much simpler operation as well as a much wider range of operability than other flameless combustion burners. This allows the same emissions reduction principles to be utilized even in conditions which flameless combustion burner traditionally cannot operate. CONCLUSIONS While initially developed for high temperature applications, flameless combustion technology offers an attractive NOx reduction method in process furnaces and has been successfully applied for over 13 years with thousands of installations worldwide. Special care must be taken for the burner design to adapt this technology for the relatively cooler temperatures found in process furnaces to avoid unnecessarily complex and difficult to operate designs. The Zeeco Free‐Jet burner successfully utilizes flameless combustion technology in a simple, easy to operate design for a wide range of process heater applications.