Thermal Nitrogen Oxide (NOx) Control
Table of Contents:
Colannino Associates, Oceanside, California
The control of nitrogen oxides (NOx) using thermal methods encompasses a variety of devices. This chapter focuses on NOx and its control using combustion modifications, post-combustion thermal and catalytic methods, and combinations thereof.
Typical Applications and Uses: Combustion Sources
Various combustion sources produce NOx. Boilers use a burner to combust the fuel and release heat. The heat boils water and generates steam. Larger boilers usually contain the water and steam inside tubes (water-tube boilers) surrounding a firebox. Some smaller boilers have a combustion tunnel surrounded by water (fire-tube boilers). The water-tube boiler has an analog in the petroleum refinery—the process heater.
The process heater is used to heat or transform a process fluid, for example, crude oil. Analogous to the water-tube boiler, the process fluid is pumped through tubes surrounding a firebox. Most boilers are heated with burners in the horizontal direction. Process heaters are often fired with the burners in the floor. However, some process heaters are wall fired, and some specialty reactors such as reformers are down fired from the roof. Process heaters may be tall, round floor-fired units (known as vertical cylindrical heaters) or rectangular units known as cabin type, which are often floor fired but may also be wall fired. Some specialty heaters, such as ethylene cracking furnaces and reformers, use heat to chemically transform the process fluid.
Gas turbines and reciprocating engines transform heat into mechanical motion. Hazardous waste incinerators use high temperatures to destroy waste products. All conventional combustion processes form NOx.
NOx are criteria pollutants as classified by the Environmental Protection Agency (EPA). Accordingly, the EPA has established National Ambient Air Quality Standards (NAAQS). Local air quality districts translate the NAAQS into local regulations for various combustion sources. These regulations vary widely from region to region. The purpose of this chapter is to show how NOx is formed and to discuss some methods for ameliorating it.
NOx is generated from combustion systems in three ways. The mechanisms are referred to as thermal (Zeldovich), fuel bound, and prompt (Fenimore).
Primary Mechanisms Used
NOx may be reduced at the source (combustion modification) or after the fact (post-combustion treatment). Combustion modifications comprise thermal strategies, staging strategies, and dilution strategies. Post-combustion methods comprise flue-gas treatment techniques described in Sections 15.5.2 and 15.6.
Different Forms of NOx
Nitric oxide (NO) is the most predominant form of NOx. Most boilers and process heaters generate more than 90% of NOx as NO. However, gas turbines and other combustion systems that operate with lots of extra air can generate significant quantities of visible nitrogen dioxide (NO,). NO, is reddish-brown in color and responsible for the brown haze called smog. NO, although odorless, oxidizes slowly to NO, in the atmosphere. Hence, most NOx requirements are given as NO, equivalents.
Hydrocarbons and NOx react to ground-level ozone. Ozone at high altitude is good because it filters out harmful ultraviolet rays. Ozone at ground level is bad because it interferes with respiration, especially for sensitive individuals such as asthmatics and the elderly. The complicated chemistry among ozone, NOx, and hydrocarbons is why hydrocarbons and NOx are strictly regulated. Carbon monoxide (CO) can also participate in the chemistry and is also a regulated pollutant.
NOx Measurement Units
NO* is measured in a variety of differing units depending on the source. For example, NO* from most boilers is regulated as volume concentrations at a reference oxygen condition, for example, 100 parts per million (ppm), dry volume, corrected (ppmdc) to 3% 02. Most NO* meters analyze their samples after water is condensed. Failure to condense the water before measurement in a dry analyzer could damage the analyzer. Such analyzers are known as extractive analyzers because they must first extract a sample from the stack, condense the water, and then send the dry conditioned sample to the analyzer. In situ analyzers read NO* directly in the hot wet stream. Figure 15.1 shows an analyzer designed to measure the NO* content in situ and report the result in meaningful NO* units. It uses a nondispersive infrared beam and optical measurement techniques.
The most popular type of post-combustion treatment is selective catalytic reduction (SCR). Ammonia or urea is injected in the flue gas near a catalyst. The net reaction is as follows:
Catalysts perform best within a narrow operating temperature range. In some cases, flue-gas tempering or conditioning is required. This may include
NO* analyzer (Air Instruments and Measurements, Inc.).
evaporative coolers, air tempering systems, heat exchangers, and so on. Catalyst activity may be adversely affected due to abrasion with ash, high sulfur in the flue gas, or metal poisons.
NOx is formed in combustion systems in three primary ways. The following provides an overview of each type.
The thermal NOx mechanism comprises the high-temperature fusion of nitrogen and oxygen. This reaction occurs when air is heated to high temperatures such as those that exist in a flame. The reaction is not very efficient. Air contains 79% nitrogen (N2) and 21% oxygen (02) by volume. Despite this, only 100 ppm or so of NOx is produced by the thermal NOx mechanism. Notwithstanding, NOx is currently regulated to less than 40 ppm in many localities and less than 10 ppm in some regions. Southern California and the Houston-Galveston area are two of the most highly restricted regions for allowable NOx emissions.
The overall reaction for thermal NOx formation is as follows:
However, the actual elemental mechanism is much more complicated. Nitrogen is a diatomic molecule held together with a triple covalent bond (№N). This bond takes a lot of energy to rupture, which accounts for the poor efficiency of the overall reaction. Oxygen, however, is a diatomic molecule held together by a double covalent bond (O = O). This bond is much easier to rupture. In fact, oxygen is the second most reactive gas in the periodic table (exceeded only by fluorine, which has a single covalent bond, F-F). These facts make combustion possible but also allow for some attendant NOx formation. At high temperature, diatomic oxygen forms atomic oxygen.
Atomic oxygen is very reactive. The fuel consumes virtually all the reacting oxygen in a combustion system. However, some free radical oxygen collides with diatomic nitrogen in the combustion air to produce NO.
We use the equals sign (=) to indicate that the reaction proceeds on a molecular level, as opposed to the arrow (—>), which indicates a net reaction that is a combined series of elemental steps. The atomic nitrogen is also extremely reactive and can attack diatomic oxygen to produce another molecule of NO.
The leftover atomic oxygen goes on to propagate the chain reaction via (15.3). Adding (15.3) and (15.4), we obtain the net reaction given by (15.1).
From this chemistry, we can write a rate law. If we presume that reaction (15.3) is the rate-limiting reaction and that oxygen is in partial equilibrium with its atomic form (У202 —> О), then the rate law becomes:
where the quantities in brackets are the volume concentrations of the enclosed species, A and b are constants, T is the absolute temperature, and t is time. Reaction (15.5) cannot be integrated over the tortured path of an industrial burner because the actual time-temperature-concentration path is unknown. However, the equation does tell us something useful about thermal NOx formation. Namely, NOx is exponentially related to temperature. A small temperature difference makes a big NOx difference. This means that hot spots in the flame can dominate NOx formation. Second, NOx is proportional to at least the square root of oxygen concentration. The nitrogen concentration is less important because it does not change much with little or lots of air. However, the oxygen concentration changes markedly with an increase in combustion air, as it is being consumed in the fuel-air reaction. Finally, the time at these conditions affects NOx. Therefore, the highest NOx will be formed by persistent hot spots in the flame and at high oxygen concentration.
For these reasons, a low-NOx burner is designed to operate at a temperature that reduces NOx formation, has a uniform temperature and oxygen pattern within that range, and has a residence time that is conducive to NOx control.
Special burners have been developed for the purpose of extracting the maximum heat from the fuel while emitting the lowest NOx. Figure 15.2 shows a modern low-NOx combustor and its principal components.
When nitrogen is bound in the fuel molecule itself, the fuel-bound mechanism operates. The nitrogen must be part of the chemical structure of the fuel. For example, natural gas containing a small percentage of nitrogen gas in the fuel does not produce NOx via the fuel-bound route because the nitrogen is not bound as part of the fuel molecule. Coal and certain fuel oils
Low-NOx burner and components (John Zink Co.).
have nitrogen as part of the fuel molecule, and in those cases the fuel-bound NOx mechanism may be the predominant NOx production mode.
As an illustration, consider a hydrocarbon like heavy fuel oil having a small percentage of nitrogen bound in its structure (CxHv.Nf), where the subscripts x and у indicate the number of carbon and hydrogen atoms in the molecule, respectively. As the fuel is heated and before it can even react with oxygen, it falls apart to generate some cyano intermediates (HCN, CN). The destruction of a fuel in the presence of heat but not oxygen is referred to as pyrolysis.
The pyrolysis reaction is a low-temperature reaction. However, the intermediate cyano species may then react with oxygen to form NO and other species.
The greater the weight percentage of fuel-bound nitrogen in the fuel, the greater the amount of associated NOx. However, there is a law of diminishing returns, and at higher nitrogen concentrations things are not as bad as they could be; not all the fuel-bound nitrogen will be converted to NOx. However, for small concentrations of fuel-bound nitrogen, for example, a few hundred ppm in the fuel, the conversion to NOx is quantitative. Because the pyrolysis reaction is a low-temperature reaction, the peak flame temperature plays a small role in fuel-bound NOx. The more important consideration is access of oxygen to the HCN and CN. Therefore, to reduce fuel-bound NOx, dilution strategies like flue-gas recirculation (FGR), staged air, and fuel dilution are superior to reducing peak flame temperature.
The use of a reference oxygen condition is required for all volume-based measurements. Otherwise, one could simply dilute the effluent stream with air and measure reduced concentrations while making no real reduction in emissions. The factor for dilution correction differs slightly from region to region but is generally of the following form:
For example, 100 ppm NOx measured at 5.3% 02 works out to be about 115 ppm corrected to 3% 02, for example, 100 x (20.9 - 3)/(20.9 - 5.3) = 114.7.
An alternative unit for NOx from boilers is pounds per million British thermal units (BTU), expressed as lb N02/MMBTU. With this unit we have several options to consider. Primarily, is the heat release the higher heating or lower heating value? The higher heating value considers the heat from the fuel presuming that the stack gas is cool enough to condense water vapor. For most boilers, the stack is not so cool, but the calculation is usually done on a higher heating value basis anyway.
The lower heating value is often used for process heaters. The lower heating value calculates fuel energy presuming that the stack gas does not condense. Since the lower heating value does not benefit from the heat of condensation, it is lesser by this amount than the higher heating value. For most hydrocarbons, the lower heating value is about 10% lower than the higher heating value. However, one should calculate the difference precisely. For CO (whose combustion generates no water), higher and lower heating values are identical. For hydrogen (whose combustion generates only water), there is a large difference between higher and lower heating value.
For natural gas combustion, presuming a higher heating value basis, 40 ppm at 3% 02 = 0.05 lb/MMBTU, and the relationship is linear. That is 0.10 lb/MMBTU = 80 ppm, ceteris paribus. Process heaters generally use a lower heating value basis, which means that the lb/MMBTU equivalent will be a larger number because we are dividing by a lesser heating value.
Gas turbines are generally regulated to a 15% oxygen reference, while reciprocating engines are regulated on a gram-N02 per brake-horsepower basis (g/bhp). Some utility boilers are regulated on the absorbed duty (i.e., the heat release less the heat lost out the stack). For these reasons, one must have knowledge of the customary units of the governing regulatory body.
Thermal-NOx Control Strategies
Thermal strategies are those that act to lower the peak flame temperature and thus reduce NOx from the thermal mechanism. One such thermal strategy is FGR. By recirculating a portion of the flue gas into the combustion air, the flame is cooled. A secondary effect of FGR is to reduce the oxygen concentration, again lowering NOx from the thermal mechanism. The increased mass flow from FGR also adds turbulence and homogenizes the flame, reducing hot spots. The disadvantage of FGR is that fan power is required to recirculate the flue gas. Flowever, FGR can cut NOx in half. A typical natural gas flame with FGR produces 50 ppm NOx, while the flame without FGR produces about 100 ppm. Generally, no more than about 25% FGR can be recirculated in a conventional burner before stability problems occur.
Steam or water can be added to the flame by means of an injection nozzle. The nozzle is moved to a location that does not interfere with combustion but cools off the flame. This strategy costs little in capital cost to implement. However, the water or steam carries heat away from the flame that is not recovered, so thermal efficiency losses result.
FGR acts primarily to cool the flame and secondarily as a dilution strategy for the oxygen in the combustion air. Recirculating flue gas to the fuel side for gas fuels can be more effective than FGR in reducing NOx for several reasons. First, gaseous fuels are usually supplied at pressures of 40 psig or above for industrial settings. This fuel energy may be used in an eductor arrangement to pull flue gas from the stack. When such a strategy is feasible, fuel dilution requires no external power. Second, diluting the fuel directly reduces concentrations of HCN and CN that occur on the fuel side, thus reducing fuel- bound and prompt NOx. Diluting the fuel or air stream with any inert agent, be it nitrogen, C02, noncombustible waste stream, or steam, reduces NOx from thermal and dilution mechanisms. Care must be taken not to reduce the fuel or oxygen near or below their flammability limits—otherwise the flame will become unstable or go out. In extreme cases, burner instability can result in an explosion if a flammable mixture fills the furnace and suddenly finds a source of ignition.
Staged combustion burner (John Zink Co.).
Rather than mixing all the fuel and air together at once in a hot combustion zone, either the fuel, air, or both may be staged along the length of the burner. The stepwise addition of fuel (two or three stages are sufficient) delays mixing and allows for some heat transfer to the surroundings before further combustion takes place. Air staging is generally considered more effective to reduce fuel-bound nitrogen, while fuel staging is more effective at reducing thermal NOx. Figure 15.3 shows a staged combustion burner designed specifically for NOx reduction.
Selective noncatalytic reduction (SNCR) uses ammonia (or an ammoniacal agent) to reduce NOx. At some temperature between 1400 and 1800°F, ammonia dissociates to form NFf2.
NH2 is a short-lived and very reactive species that reduces NO to nitrogen and water.
SNCR can reduce NOx to 50 ppm or lower. However, such reaction temperatures are found within the furnace itself. Therefore, to provide adequate mixing and residence time, SNCR requires a large furnace (e.g., coal-fired, and municipal solid waste systems and some large utility boilers). Most SCR catalysts are base metal oxides, especially vanadia and titania deposited on an alumina honeycomb surface. A typical honeycomb-type catalyst block containing exotic base metal catalysts is shown in Figure 15.4.
By adding a catalyst, one can lower the required temperature window to 500-750°F. These temperatures occur close to the stack in process heaters and within the air-preheaters of larger boilers. So, the size of the furnace is not such an important factor. The strategy is also more effective than SNCR, generating 90% NOx reductions or greater. The important steps are adsorption of ammonia and N02 onto the catalyst surface (X-Y). NO, may be formed rapidly from NO by oxygen on the catalyst surface or in
Post-combustion honeycomb catalyst (Bremco).
the gas phase. Water on the surface protonates the ammonia to NH4. The essential chemistry is:
The adjacent sites hold the ammonia and N02 in proximity, where they quickly react, restoring the catalyst surface for additional reactions. An electron from the surface is required to balance the reaction.
An intelligently designed NOx control system starts with the accurate determination (or estimation) of the NO and NO, that is or will be produced from the source.
Accurate sizing and specification of low-NOx burners requires consideration of fuel properties, furnace operating temperatures, excess oxygen conditions, and knowledge of the service application. This almost always requires detailed conversations between the burner vendor and the end user.
Likewise, SCR systems require detailed conversations between the end user and the SCR system supplier. The catalyst can be rendered ineffective by physical blinding with inert particulate, abrasion, or poisoning by certain heavy metals or sulfur. An inventory of any possible fouling or poisoning agents must be derived first by analyzing the fuel, its metals content, and its propensity to form oxides or produce partially burned or unburned carbonaceous compounds and comparing the result to known fouling agents for the proposed catalyst. Possible remedies include, among others, removal of fouling agents before the catalytic stage, use of a sacrificial precatalyst, or more frequent catalyst replacement.
In SCR or SNCR systems, unreacted ammonia that slips through the system is termed ammonia slip. Ammonia slip is more easily controlled on base- loaded (steady-state) operations. In such a case, the ammonia injection rate can be determined by experience and testing, then maintained in an optimum range. Feedback controls can sometimes be used to adjust the ammonia rate, however, to date, these have proven to be slow to respond. Usually, some ammonia slip is tolerated, and larger NOx reductions are possible if higher ammonia slip rates are acceptable. Some regulatory districts are putting limitations on the total allowable slip, thus complicating NOx control.