Jan-2018

Retrofitting crude furnace burners

A burner retrofit has delivered more robust operation with reduced NOx emissions from a crude process furnace

RYAN ROBERTS
Zeeco

Article Summary

Zeeco recently had the opportunity to work with a large refinery on the US Gulf Coast on a burner retrofit project for a crude heater process furnace. The existing burners were a conventional emissions design using carbon steel rotary registers designed to operate on ambient, forced draft combustion air. There was a total number of 16 existing burners installed in the furnace, utilising a common air plenum.

The refinery sought to replace the existing burners because of operating challenges:
1. The original burner rotary registers had become inoperable and were frozen at one setting.
2. Even after returning the registers to an operable condition, they would again freeze in a short period of time due to the carbon steel material oxidising in the high humidity and salinity of Gulf Coast air.

The existing burners used a multiple tip design for gas firing operation, and also had the ability to fire liquid fuels. During discussions prior to the start of the project, the refiner indicated that it would be removing the capability for liquid firing. The refiner also wanted to employ a new fuel gas burner design that would be easier to maintain and produce less NOx.

There would be insufficient time during the planned turnaround when this retrofit project was scheduled to occur to perform any floor modifications to the furnace. Therefore, the proposed solution could not involve any heater floor and refractory modifications, and the retrofit burners would need to fit the existing burner mounting in the furnace.

As a summary, the following were the main priorities and design objectives for the crude heater burner retrofit:
• Use a burner register design that would mount in a common air plenum
• This burner register design must be constructed of a material that would resist rusting in the Gulf Coast atmosphere
• The burner register must be sufficiently robust that it could be operated for the foreseeable future without freezing in place
• The burner that would be supplied must fit into the existing furnace refractory floor opening without any floor modifications
• It was the preference of the refiner to use the same quantity of burners (16) to prevent any floor modifications that would make the project economically unfeasible
• The burner must be easy to maintain for operations/maintenance personnel
•  The burner should be designed to provide some reduction in NOx emissions versus the existing conventional emissions burners currently installed in the furnace.

After reviewing all of the mechanical requirements from the refiner, along with all of the process requirements for the burner operation, the GB Single Jet burner from Zeeco was selected as the best solution.

GB Single Jet burner design features
The GB Single Jet burner is based upon an existing conventional emissions burner design, with the incorporation of staged air, staged fuel, and internal flue gas recirculation (IFGR) to reduce emissions. The burner uses a single gas tip firing on a cone assembly, but instead of firing on the centre line of the burner, the tip and cone are offset to fire nearer to the inside diameter of the burner tile (see Figure 1).

The offset gas tip and cone design allows the burner to stage a percentage of the combustion air in the burner throat and generate IFGR into the base of the burner flame. The location of the gas tip and cone assembly increases the amount of IFGR and helps create a stable low pressure zone to maximise the amount of IFGR into the combustion zone. The introduction of IFGR allows for the peak flame temperature in the flame core to be reduced dramatically. As shown in Figure 2, reducing the peak flame temperature reduces thermal NOx emissions. The GB Single Jet burner configuration’s single tip, offset design simplifies operation and maintenance and reduces emissions when compared to a raw gas conventional emissions burner.

Another design feature of the GB Single Jet burner is the compact size of the burner components. Most low NOx burners utilise a larger number of gas tips, complex tile geometry, and flame holders in order to provide a stable burner flame that still meets emission requirements. The GB burner uses only a single gas tip and cone assembly to achieve the emissions requirements. Also, the tile geometry for this burner is normally a straight-sided tile. In addition to being a more cost effective tile shape, it is a smaller tile footprint than that required for a typical low NOx burner. The smaller tile footprint simplifies retrofit applications into existing furnace burner mountings by removing the need for expensive floor steel and refractory modifications.

Mechanical features of the burner
Different air register design
From discussions with the refiner, the main mechanical feature on the burner that required an upgrade in design and materials of construction was the air register assembly. As indicated earlier, the existing rotary air register assembly had become frozen in position, preventing the refiner from operating the burners as designed. The refinery’s maintenance personnel had no way of controlling the burners to achieve long term, efficient operation of the furnace. Also, the frozen rotary registers presented a safety hazard, as some burners had the rotary registers frozen in a position where there was insufficient air for complete combustion entering through the burner.

After meeting on site with refinery personnel, the Zeeco burner design team selected an air register with rotary inlet vanes instead of rotary registers for the replacement burners. The rotary inlet vane provided the best design because it would fit into the existing furnace floor opening for the burner and did not rely on a rotating register mounted to a stationary air register. The rotary inlet vanes would rotate about the centre line of the vane on a stationary cylinder inside the common air plenum. This stationary cylinder would be welded to the burner front plate, where the register shafts, linkage arms, connecting gears, and vane shaft bearings would be mounted (see Figure 3).

Six inlet vanes were chosen for this design as this provided the optimum open area to flow sufficient combustion air and excess air to insure complete combustion of the fuel. As shown in Figure 3, there is a single damper handle provided on the burner to allow for simultaneous adjustment of all register inlet vanes on the burner. Each inlet vane has a damper shaft that is welded to the vane centre line, and this damper shaft projects through the burner front plate. Each individual damper shaft is connected to the damper handle by the use of linkage arms and gears. Each damper shaft protrusion also has a packing bearing that can be lubricated to insure smooth operation of the vane inlet damper over the lifetime of the burner. Figure 4 shows the bottom view of the burner front plate to further illustrate the configuration of the vane inlet register and the linkage arms driving the movement of the register. It is important to note that this is the view of the burner front plate looking directly up from below when it is mounted into the common air plenum assembly.

All of the vane inlet registers can be operated easily with the single damper handle assembly provided (see Figure 4). The damper handle assembly is spring loaded and can be locked into place. The damper has 32 individual settings between full open (setting number 8) and full closed (setting number 0). This would provide the refinery’s operations personnel with a better method of controlling combustion air entering the burner, and the individual lockable settings would allow for consistency in the combustion air register settings of all 16 burners in the furnace.