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.