Radiant Wall Burners
These burners, which were installed in a steam-reforming furnace for hydrogen production, hold NOx emissions to below 30 ppm using heavy refinery fuel gas.
The role of oxygen control and dampersIn a simple process heater subject to tight NOx-emission limits, a slight negative pressure must be maintained inside the furnace with a stack damper. The exact excess-oxygen levels should be fine-tuned via automatic or manual dampers/registers at each burner, based on real-time monitoring of oxygen levels. Smooth sensitive operation of the dampers is essential. In addition to the damper blades, damper controls and linkage are proving increasingly important to NOx reduction efforts. Nowadays, retrofits typically include installation of a new distributed control system, advanced firing control logic and oxygen analyzers in the stack. Such sophisticated instrumentation requires mechanical upgrades in the damper cabling, linkages, pulleys, joints, etc. These all need to move easily and smoothly and to reverse direction immediately without binding.
Sometimes excess oxygen gets into the firebox and burner, air preheaters, etc., because many older furnaces have cracks and leaks from overuse, rust, and damage from past explosions. Cold O2 leaks in, finds its way to the flame, and creates excess NOx. It is amazing how cool gas currents can drop to the floor and burner in large fireboxes. On an existing furnace, it is very difficult to eliminate such air leaks. However, manipulating the firebox draft with very fine damper control can reduce the amount of air leaks. Typically, the top of the radiant section of the furnace should be kept at a slight negative pressure, usually -0.10 in. water pressure. To achieve that at both the high and, especially, the low firing range requires precise damper movements and damper blades that can go nearly closed. Stack dampers may need to be replaced with tighter-sealing damper blades, so low draft can be maintained and controlled at reduced firing rates.
Modern process heaters and their associated automated control systems tend to monitor a particular process variable very closely and to quickly adjust the firing to hold the set point(s). To maintain a steady-state excess oxygen and draft at the burners, a furnace damper might move 600 to 2,000 times per day to accommodate process swings and adjustments. So, the damper blade and drive should be capable of very small smooth movements. To provide accurate control of the damper blade, it is imperative that damper drives respond to 0.25% demand signal changes. The drive and connecting-rod linkage movement must operate smoothly and without any unnecessary backlash and deadband.
Another compelling reason for precise excess oxygen and draft controls involves the bottom line. Some complexes such as refineries and chemical plants typically burn plant-generated mixed fuel gas and light-ends offgases as well as expensive purchased natural gas. This practice means that fuel composition can change from minute to minute and, with it, the amount of air required. In such environments, it becomes nearly impossible to maintain low excess-oxygen levels without using online oxygen analyzers, sensitive draft monitoring and controls, and automated fresh-air dampers. Such equipment will reduce the amount of purchased gas consumed as well as minimize NOx emissions.
The burners pictured above can reduce NOx emissions significantly, down to levels approaching 10 ppm.
Retrofit realitiesManaging a retrofit project is always a challenge. Much of the difficulty in retrofitting a unit lies in understanding which items are important and in identifying what restrictions will be posed by operations, maintenance, construction, safety, and other company groups. Preparing for a retrofit can take months or years of work: writing specifications, selecting qualified vendors, conducting factory tests of burners and equipment, coordinating so that everyone understands what's going on, clarifying requirements to the sub-vendors, the sub-sub-vendors, and then giving these to the construction contractor. Finally comes the task of installing equipment correctly per the drawings, which can be confusing at times, especially with relatively new types of combustion equipment.
Today, one plant engineer or manager often does work previously handled by two or three individuals; so, the need for turnkey solutions, particularly in areas involving fast-evolving technology, has clearly increased. For NOx-reduction projects, it is very desirable to work with vendors, engineering specialists and installation contractors who are experienced with the technology and who can easily and effectively interface with suppliers and constructors. Equipment vendors should be queried on their ability to provide upfront engineering, CAD installation and electrical interface drawings, and even field supervision and startup assistance. Bringing the vendor to the field to view site-specific needs first hand is usually worth the effort.
This damper drive, which might move up to 2,000 times per day, is capable of very small movements.
In California, hundreds of process heaters have been retrofitted with low-NOx burners, oxygen analyzers, dampers, SCR systems and sophisticated controls. Many of the furnaces were equipped with new dampers and new or relocated damper drives. At first, some of us did not recognize the importance of oxygen-level monitoring and precise damper control -- and the dramatic effect they have on the NOx performance and overall efficiency. Nor did we realize that sealing leaks in an existing furnace is an art where inexperience can become very costly and ineffective. We now tend to standardize on particular vendors for specific applications of burners, oxygen analyzers, dampers and damper drives, boiler burners, and SCR systems.
Operators of process plants elsewhere can benefit from the technology pioneered and the experience gained in California. The development of low-NOx and ultra-low-NOx burners in process heaters, boilers, radiant wall furnaces and SCR systems has paved the way for today's NOx reduction projects.
Damper Drive Retrofitting
The need for turnkey solutions has increased because of the fast-evolving nature of the technology.
Proper design, installation and operation of NOx-reduction equipment in process heaters not only reduce nitrous oxide emissions but also offer significant payback in reduced fuel costs and decreased operator surveillance of process furnaces.
Don Nelson is a senior project engineer for ConneXsys Engineering, Richmond, Calif. In recent years, he has specialized in NOx reduction projects, and has been involved in advanced designs of ultra-low-NOx burners for furnaces.