The 1990 amendments to the U.S. Clean Air Act forced process plants and other industrial facilities throughout the country to examine every aspect of their process heating operations to reduce cumulative emissions of nitrous oxides (NOx). In San Francisco, the 1994 Regulation 9, Rule 10 of the Bay Area Air Quality Management District imposed the most stringent NOx emissions at the time anywhere in the world.
In 1998, the U.S. Environmental Protection Agency (EPA) refined the Act's amendments into the National Emission Standards for Hazardous Air Pollutants from Petroleum Refinery Vents (referred to as Refinery MACT II), which cover emissions from catalytic cracker, catalytic reformer and sulfur plants. These rules were promulgated in late 2000 and have now impacted the majority of refineries. Some areas of the country have until 2008 to fully meet emissions targets, but EPA has already teamed up with the Department of Justice to force multimillion-dollar emission-reduction deals with major oil producers.
In California, the Los Angeles and San Francisco air districts have imposed limits of about 0.030 lb/Mm Btu average NOx emissions (as a group average) for all current heaters in an existing refinery. New heaters typically are required to include secondary catalytic reduction (SCR), using ammonia and a catalyst to react with the nitrous oxides, to reduce NOx emissions to the 5- to 10-ppm range. The San Joaquin Valley Air Pollution Control District, which oversees a "severe non-attainment area" for smog emissions, recently approved regulations forcing boilers and combustion gas turbines to limit NOx emissions to below 9 to 15 ppm, depending upon size and service, over the next several years. The impact of these deadlines is severe. Basically, a plant must comply or shut down the high NOx-producing furnaces and boilers.
The regulations can be most difficult to meet during startup, shutdown or upset conditions. Yet, short NOx emission excursions can lead to expensive fines and create community ill will.
Typical Vertical Cylindrical Process Heater
- Conventional units produce 75 to 120 ppm NOx, while ultra-low-NOx designs now can cut emissions to as low as 13 ppm.
- These regulations have spurred the replacement of aging furnace burners with newer low-NOx designs. However, experience has shown that sensitive online oxygen monitoring and draft controls are equally critical for reducing NOx emissions in process heaters.
Thermal NOx results from the fixation of molecular nitrogen and oxygen present in the combustion air. NOx emissions increase rapidly at peak flame temperatures exceeding 1,540 Degrees C (2,800 Degrees F) and with the time reactants remain within the area of peak flame. The more oxygen present in the eye of the flame, the more NOx made. The industry norm is to burn with residual 3% oxygen going out the stack. Above that oxygen level, NOx emissions rise. In addition, the burn becomes less efficient because excessive amounts of air are being heated. Reducing oxygen levels below 3% is desirable but difficult to achieve, especially in older furnaces prone to air leaks through the furnace walls and before the oxygen sensing equipment.
An often-overlooked variable is the heating value of the fuel. High Btu-value fuel gas rich in propane and butanes can increase NOx production by 25% to 40%, depending upon composition.
Vertical Process Heater with SCR Retrofit
- Thanks to ammonia injection followed by secondary catalytic reduction, flue-gas exits with only 5 to 10 ppm NOx.
The most common methods to lower NOx emissions are use of low-NOx burners (LNB) alone or in combination with flue gas recirculation (FGR) through the burner. According to EPA technical document No. 453/R-93-034, which identifies alternative controls for NOx emitters, FGR combined with LNBs can lead to total NOx reduction of 55% compared to uncontrolled emissions. State-of-the-art burners can now cut this much further, to as much as 80% to 85%, and, in some cases, possibly more.
FGR recycles 15% to 30% of the inert products of combustion back to the primary combustion zone. This dilutes the reactants and boosts the mass flow through the burners and flames. At a fixed heat release from the fuel gas, the higher mass flow of gases leads to a lower peak flame temperature, which, in turn, reduces local oxygen concentrations to levels below 3% and inhibits thermal NOx formation.
Manipulation of several variables can cut NOx emissions:
- reducing the oxygen concentration in the flame zones;
- stretching the flame out, to reduce the peak flame temperature and force the flame to burn in zones with reduced oxygen concentration; and
- increasing the flue-gas mass flow so that a given flame must heat up more flue gas, thus decreasing peak flame temperature.
A great amount of proprietary effort has gone into burner designs to "stage" the fuel release ports that spread the flames out, to induce FGR by the action of the fuel-gas jets, and to spray the fuel into areas lean in oxygen. Self-induced FGR is preferable and much less costly than external systems.
External systems typically involve the use of a motor- or turbine-driven fan, ductwork, dampers, etc. Optimal operation requires accurate oxygen analyzers, well-placed sample points and precise damper controls for both negative draft fireboxes of simple process heaters and positive fireboxes such as boilers. So, for every retrofit, always consider installing an analyzer and precision damper drives, linkages, damper blades, bearings, etc., to effect smooth control of the draft without any sloppy play (hysteresis). Even with SCR techniques, controlling the firebox environment is essential.
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.