Burner Retrofit Considerations

Flame Envelope

1. Conventional raw gas and premix burners have luminous flames. The combustion reaction occurs within the visible flame boundaries. The flame envelope is defined as the visible combustion length and diameter. Ultra-Low NOx and latest generation burners have non-luminous flames. Much of the combustion reaction is not visible. The flame length and diameter are often determined by inserting a CO probe into the firebox and defining the flame envelope as CO concentrations greater than 50 ppm.

2. Ultra-Low NOx and latest generation burners have longer flame lengths than conventional burners. Longer flame lengths change the heat transfer profile in the firebox. Longer flame lengths can result in flame impingement on the tubes and mechanical supports.

3. The flame diameter is often defined in terms of ratios of the burner tile outside dimension. Many burners have flame diameters that are1 to1-1/2 times the diameter of the burner tile. Since the tile diameters are often larger for Ultra-Low NOx and latest generation burners, the flame diameters at the base of the flame many be slightly larger. The flame diameter often necks outs, giving a wider flame at the top.

Physical Dimensions of Firebox

1. Optimized designs have burner spacing that is designed to have gaps between the flame envelopes. Since the tile diameters are often larger for Ultra-Low NOx and latest generation burners, retofits can result in closer Burner-burner spacing and flame interaction. Flame interaction can produce longer flames and higher NOx values. Flame interaction and congealing can interupts the flue gas convection currents in the firebox, reducing the amount of entrained flue in the flame envelope. This condition increases the NOx levels. Ultra-Low NOx and latest generation burners should be spaced far enough apart to allow even flue gas recirculation currents to the burners.

2. The burner centerline to burner centerline dimension is one of the most important dimensions in the firebox. Many tube failures are causes by flame and hot gas impingement. When Ultra-Low NOx and latest generation burners are being retrofitted, the larger size of the flame envelope must be evaluated. Firebox convection currents can push the slow burning flames into the tubes.

3. Flame impingement on refractory often causes damage. When Ultra-Low NOx and latest generation burners are being retrofitted, the larger burner diameter may result in the burners being spaced closer to the refractory. Unshielded refractory may require hot face protection.

4. Many heaters are designed for flame lengths that are 1/3 to ½ the firebox height. Ultra-Low NOx and latest generation burners typically have flame heights of 2-2.5 ft/million Btu (2-2.5 m/MW). Longer flame heights from Ultra-Low NOx and latest generation burners may change the heat transfer profile in the firebox. The longer flames may result in flame or hot gas impingement on the roof and shock tubes. These tubes may require protection to prevent failures. Protection may include metallurgical upgrades, increased tube thickness or tube shielding. Some older heaters have very short firebox heights and may not be suitable for retrofits to Ultra-Low NOx and latest generation burners.

5. When retrofitting burners, may companies test prototype burners in a test furnace with similar geometry and fuels as the heater to be retrofitted.

Fuel Treatment

1. While many conventional burners have orifices 1/8”(3 mm). and larger, Ultra-Low NOx and latest generation burners often have tip drillings of 1/16”(1.5 mm). These small orifices are extremely prone to plugging and require special protection. Most fuel systems are designed with carbon steel piping. Pipe scale forms from corrosion products and plugs the burner tips. Tip plugging is unacceptable for any burner, but it is even more important not to have plugged tips on Ultra-Low NOx and latest generation burners because plugged tips can result in stability problems and higher emissions. Many companies have installed austenitic piping downstream of the fuel coalescer/filter to prevent scale plugging problems.

2. Coalescers or fuel filters are required on all Ultra-Low NO_x and latest generation burner installations to prevent tip plugging problems. The coalescers are often designed to remove liquid aerosol particles down to 0.3 to 0.6 microns. Some companies install pipe strainers upstream of the coalescer to prevent particulate fouling of the coalescing elements.

3. Piping insulation and tracing are required on fuel piping downstream of the coalescer/fuel filter to prevent condensation in fuel piping. Some companies have used a fuel gas heater to superheat the fuel gas in place of pipe tracing.

4. Unsaturated hydrocarbons can quickly plug the smaller burner tip holes on Ultra-Low NOx and latest generation burners.

5. Higher hydrogen content in the fuel gas results in higher NO_x production. It also increases the stability of the flame.

6. Natural gas fuel has often produced unstable flames and flameouts on Ultra-Low NOx and latest generation burners. Its use should be avoided if possible.

Air Control

1. Ultra-Low NOx and latest generation burners must be operated at design excess air levels to control NOx emissions.

2. Most refinery general service heaters are natural draft heaters. It is important to control the draft at the design value, usually 0.1”(3 mm) H2O at the top of the radiant section. High drafts increase tramp air ingress and often result in higher excess air levels at the burners. This condition results in higher NOx levels on Ultra-Low NOx and latest generation burners. Automated draft control has been installed on retrofits to obtain better excess air control.

3. Ultra-Low NOx and latest generation burners are usually supplied with individual plenum boxes and individual damper controls. Because excess air control is so important on these burners, some companies have installed individual actuators on each burner damper for better control. Others have connected all the burner dampers on a jack shaft to control the excess air levels.

4. New heaters are designed with seal welded construction to prevent tramp air ingress. Many older heaters have bolted panel design. High temperature silicon and foil tape have been used on these heaters to reduce tramp air. Observation openings should be designed to minimize excess air ingress. Observation openings should be closed when not in use.

5. Ultra-Low NOx and latest generation burners are usually supplied with individual plenum boxes. Many of these burners are supplied with mufflers to control noise emissions. The mufflers are often an effective devise to eliminate excess air fluctuations due to wind. Windscreens are often installed to eliminate wind effects when burner mufflers are not used. A 15 mph wind can cause a ± 0.11” H₂O draft variation at the burner, resulting in a ± 15%change in excess air level for a burner designed at 0.4” H₂O draft.

6. Forced draft systems should be considered for Ultra-Low NOx and latest generation burner retrofits. The forced draft system provides better excess air control, eliminates wind effects, and the increased burner pressure drop often results in a smaller flame envelope.

7. Ultra-Low NOx and latest generation burners may be installed in a common air plenum. Internal baffles may be required to obtain even air distribution.

Structural Considerations

1. Hole for Hole Replacement is the optimum situation for retrofits. However, since many Ultra-Low NOx and latest generation burners have larger burner tiles and larger burner cutouts, hole for hole replacement cannot occur. It is often more economical to replace the floor when hole for hole replacement is not an option.

2. Ultra-Low NOx and latest generation burners often weight more than conventional burners. Retrofits may require additional structural bracing. Floor Levelness/ Refractory Thickness

3. Heaters floor steel should be level. Bowed sections should be repaired or replaced.

4. The floor refractory thickness should be checked to ensure the heater floor steel is an acceptable temperature.

5. Physical constraints below the firebox floor should be checked. There should be sufficient space underneath the burner plenum for tip removal.

Process Related Parameters

1. Ultra-Low NOx and latest generation burners often have longer flames that change the heat flux profile. This is especially important on cracking heaters such as cokers and visbreakers. The longer flames may increase the bridgewall temperature and change the duty split between the radiant section and convection section.

2. When the heat flux profile changes, the location of the maximum tube metal temperature changes. Retrofitting Ultra-Low NOx and latest generation burners in short fireboxes can result in high roof and shock tube metal temperatures.

3. Ultra-Low NOx and latest generation burners may have less turndown capability than conventional burners. High CO levels can occur when firebox temperatures are below 1240ºF. Flame instability and flameout can occurred when firebox temperatures are below 1200ºF.

4. Conventional raw gas burners can handle a wide variation in fuel gas composition. Since Ultra-Low NOx and latest generation burners are often designed at the limit of stability, a fuel composition change may cause a stability problem. Since methane fuel is the hardest fuel to burn, many companies specify burn test using methane as the test fuel.

5. The proper design basis for the burner retrofit is extremely important. Sometimes the process requirements have changed significantly since the furnace was designed. Important design basis items include:

1) Emission Requirements

2) Process Duty Requirements

3) Heater General Arrangement Drawings

4) Turndown Requirements

5) Fuel Composition Ranges

6) Fuel Pressure

7) Startup Considerations

6. It is important to review existing plant data accurately. Heater tube fouling may result in high bridgewall temperatures. Fouled convection sections may result in higher firing rates. Tramp air may result in high excess air levels. Stack dampers are often frozen in place.

Instrumentation

1. When retrofitting Ultra-Low NOx and latest generation burners, addition heater instrumentation is often required. Most companies will install analyzers to determineO₂ and NO_x levels. Some companies will install CO and combustibles analyzers.

2. When retrofitting burners, may companies install the firebox draft indication and damper control on the DCS system to obtain better excess air control.

3. When retrofitting Ultra-Low NOx and latest generation burners, Flame Scanners may be require to protect against flameouts during turndown and startup situations.

4. When retrofitting burners, minimum fuel gas pressure instrumentation may be require to protect against flameouts during turndown and startup situations.

5. The heater should have a bridgewall temperature indicator on the DCS system.

Operations

1. Many operators have been trained to observe luminous conventional flames. After retrofitting Ultra-Low NOx and latest generation burners, the operators will have to be trained to observe non-luminous flames. A bright yellow flame on an Ultra-Low NOx burner may be an indication of a burner setup problem.

2. Ultra-Low NOx and latest generation burners have non-luminous flames that are hare to detect. Bright burner tile or flame holder color is often an indication that the primary tips are operating properly.

3. Special startup procedures may be required for Ultra-Low NO_x and latest generation burners.

4. Special premix gun inserts and pilot designs may provide additional stability during startup and turndown conditions.

5. Ultra-Low NOx and latest generation burners may have less turndown capability than conventional burners. High CO levels can occur when firebox temperatures are below 1240ºF. Flame instability and flameout can occurred when firebox temperatures are below 1200ºF.

5. Ultra-Low NOx and latest generation burners are designed to operate within closely controlled oxygen levels to obtain the lowest levels of NO_x. This may require more operator attention and interaction.

Installation Checkout

1. Correct burner installation is extremely important on Ultra-Low NOx and latest generation burners. It is often beneficial to have burner company representatives assist in checkout before initial operation.

2. Tip orientation and tip height should be checked

3. The burner tile must be installed properly. Check the diameter in different locations to ensure proper diameter dimensions and concentricity.

4. The damper should be checked to ensure freedom of movement through the entire range of operation.

Instability Issues Specific with Low NOx Burners

CFD Modeling

1. CFD modeling (Computational Fluid Dynamic modeling) is a useful tool in modeling firebox conditions. It has been used in multi-burner systems to analyze problems such as flame interactions, firebox currents, and localized high heat fluxes.

2. CFD modeling capability is relatively expensive and should be limited to special situations.

3. CFD modeling capability is improving as companies gain experience. However, field results may vary significantly from the model results.

Posted in: Chemical Engineering Processing

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