Sep-2018

Burner retrofit increases capacity and cuts costs

Replacement burners in a hydrogen reactor feed heater saved the cost of retrofit within a week.

ROY BISNETT
Zeeco

Article Summary

A large refiner on the East coast of the United States was experiencing problems meeting maximum heat release on a natural draft hydrogen reactor feed heater. Due to the refiner’s previous positive experience with burner retrofit projects, they contacted US based combustion equipment manufacturer Zeeco and asked for guidance. Zeeco burner experts completed an inspection and noted that several of the burner tips in the heater’s existing burners were plugged.

This tip plugging limited the heat release because attempting to increase the heat release under the current conditions triggered high fuel gas pressure alarms. A recommendation was made to clean the tips to allow the burners to operate at the higher end of the capacity curve, thus allowing the heater to operate at an increased heat release and improving feed rates overall for the facility. However, the refiner reported that the existing burner tips had been cleaned within the past two months and that, in fact, the refiner had to clean them nearly every month in order to operate at an acceptable feed rate. The refiner reported that tip cleaning is a time consuming task requiring a scaffold to be built, plus the manifold design of the existing burners required the removal of individual stainless steel tubing with compression fittings that fed into each individual burner tip, followed by removal of the eight burner tips for each of the six burners. The complexity and frequency of the process added a considerable maintenance cost burden to the facility as pipe-fitters and carpenters were required each time a cleaning was performed. The refiner further reported that the existing burners were an ultra low NOx design, were less than 10 years old, and operated on an average fuel gas composition of 923 Btu/SCF LHV.

Elsewhere in the refinery, Zeeco GB Single Jet burners had been retrofitted into other heaters in the plant operating on the same fuel gas supply and the refiner noted these burners did not require this rigorous maintenance due to inherent design differences. In fact, one heater had operated post-retrofit for a full six months with no tip plugging issues and another heater had just passed the one-year mark of operation post-retrofit with no tip cleaning required. Both heaters were reporting better than specified capacity and emissions.

The challenge
The refinery team and burner expert team calculated the amount of lost opportunity cost in their current operation and concluded it was in excess of $10000/day. The refiner determined that replacing the existing burners with GB Single Jet burners made operational sense, but wanted the retrofit accomplished as quickly as possible and with as little interruption to the refinery operation as possible. A CFD and burner test was not required due to empirical operation and emissions data on file from a similar application on-site. The burners had to meet emissions guarantees of 0.030 lb/MMBtu or 26 vppm while firing an average fuel gas of 923 Btu LHV, and operate to specification in a natural draft vertical cylindrical type heater with six up-fired burners. The burner circle diameter was 5.5 ft with a tube circle diameter of 15.3 ft. The radiant section height from floor to convective section measured 29.5 ft.

The solution
The GB Single Jet burners were designed to have a maximum flame height of 19.8 ft, a flame width of 2.8 ft, and 11.68 MMBtu/hr heat release for a total of 70 MMBtu/hr for the furnace. The burners needed to meet a turndown requirement of 4:1 and meet emissions guarantees of not to exceed 26 vppm, or 0.030 lb/MMBtu, while firing an average 923 Btu LHV fuel gas. The burners were to be produced and delivered within a 15-week window to meet the refinery’s desired schedule, and the team of refinery and burner manufacturer professionals developed a plan to install them while the heater remained in service. When the burners arrived, the installation team replaced one burner at a time over the course of the week without taking the furnace out of service (see Figure 1).

Operating characteristics of the selected burner:
• Reduces NOx through internal flue gas recirculation (IFGR), staged fuel, and staged air
• Natural, forced, induced, balanced draft, or turbine exhaust combustion air induction
• Can be configured for up-fired, side-fired and down-fired uses
• Plenum mounted or individual wind-box
• Heat release range – natural draft: 1-20 MM Btu/hr (0.293-5.860 MW)
• Heat release range – forced draft: 1-20 MM Btu/hr (0.293-5.860 MW)
• Turndown: 10:1 or greater for most cases
• Design excess air range: 8% or greater – for most cases
• Combustion air pressure drop – natural draft: 0.2-1in (5-25 mm) water column
• Combustion air pressure drop – forced draft: 0.5-4in (12.7-100 mm) water column.
 
The technology
The Zeeco GB Single Jet round flame burner chosen for this retrofit project uses a non-symmetrical single tip and cone design to boost internal fuel gas recirculation. The design utilises staged air and gas techniques to reduce NOx emissions (20-49 vppm for most applications) and increases opportunities for new applications and retrofits. The burner uses a single gas tip firing on a cone assembly, but instead of firing on the centreline of the burner, the tip and cone are offset to fire nearer to the inside diameter of the burner tile (see Figure 2).

Stable flame
The single tip and cone is a simple, stable design. The tip drilling includes ignition ports used to ignite the burner, firing ports used to create the secondary (rich) combustion zone, and a centre staged fuel port, which enhances the secondary (rich) combustion zone. Because all ports are in close proximity, the ignition gas travels a short distance – less than 30 mm – compared to the 380 mm travel distance for many staged fuel burner designs. Since gas travels a very short distance, there is much less chance of adverse influences affecting the burner stability compared to a standard burner design.

Compact design
With only one tip and cone, the burner’s compact design is often chosen for new applications and retrofits with limited space.

Low probability of flame interaction
Because the burner has a small footprint and only one tip located in the burner throat, the gas fired from one burner is further away from the adjacent burner when compared to a conventional staged fuel burner with gas tips on the periphery of the burner tile. This design means the gas fired from burner to burner is further apart, significantly reducing the probability of flame interaction and the possibility of a ‘flame cloud’.

Low maintenance
With a single tip and cone, there are fewer maintenance needs. The basic tip and cone design has been field-proven for decades, and is the preferred design by many operations groups due to lower overall maintenance requirements.

Accurate combustion air control
Controlling combustion air to each burner is crucial to proper operation. The GB Single Jet uses a dual blade opposed motion damper system to control the combustion air. The damper blades are mounted on 304SS shafts to prevent rust. The damper shafts are then mounted on bearings for easy movement.