Deborah Fusselman, Dan Lipsher
Trinity Consultants Inc.
Dallas
A number of control technologies can be used to reduce NOx emissions from industrial boilers and heaters to acceptable levels.
These units-commonly found in petroleum refineries and petrochemical plants-are likely to soon be subject to tighter emissions standards in the U.S.
Other countries, such as Germany, already have extremely stringent NOx emissions limits. There the regulations on large furnaces ( 300 mw) contain a clause that requires industry to keep pace with the latest technology.
As a case in point, recently one German petrochemical installation was limited to total NOx emissions of 100 mg NOx/cu m flue gas from two ethylene crackers.
REGULATORY BACKGROUND
The 1990 Clean Air Act Amendments imposed the strictest controls vet on air pollutant emissions. The burning of fossil fuels in industrial boilers and heaters is a significant contributor of such regulated pollutants as nitrogen oxides (NOx), sulfur dioxide (SO2), carbon monoxide (CO), volatile organic compounds (VOCs), and particulate matter.
While VOC emissions have long been recognized as the primary contributor to atmospheric ozone, the role of NOx in ozone formation has been the subject of much recent debate.
A report published in 1991 by National Research Council concluded that, in most urban areas of the U.S., reductions in NOx emissions will lead to decreased levels of atmospheric ozone (OGJ, Jan. 6, p. 40). Other studies, however, indicate that NOx controls in some urban cores may actually result in increased ozone levels in those areas.
Nevertheless, it has become generally accepted that NOx is a precursor to atmospheric ozone, and as the NOx-ozone relationship has become more strongly established, regulations limiting NOx emissions have become more strict, leading to the development of technologies that reduce those emissions from industrial processes.
Many petroleum refineries are located in ozone nonattainment areas - that is, areas that do not meet national ambient air quality standards (Naaqs) for atmospheric ozone. Under Title 1 of the 4990 Clean Air Act Amendments, existing stationary sources of NOx in ozone nonattainment areas are subject to reasonably available control technology (RACT), which is being determined on a state-by-state basis.
Because of the subjective definition of RACT, establishing criteria that define a control technology as reasonable can be a partisan political process, with state regulatory agencies having to strike a balance between the interests of industry and environmentalist groups. Compounding the problem is the fact that each state is required to submit its State Implementation Plan (which, among other things, must incorporate RACT provisions) to U.S. Environmental Protection Agency (EPA) by Nov. 15, 1992. At press time, however, EPA had not issued any formal guidance on what constitutes RACT.
To obtain a construction permit in a nonattainment area, a facility must typically undergo a new source review and implement lowest achievable emission rate (LAER) controls to reduce emissions of nonattainment pollutants.
LAER controls are the most stringent controls developed for use by any facility in the world, and must be implemented regardless of cost. In addition, the facility must offset new emissions by reducing emissions of the same pollutants from existing sources in the same area.
Currently, the only NOx nonattainment areas in the country are in California. But as other states determine that NOx must be regulated in ozone nonattainment areas (which comprise most major metropolitan areas), LAER controls are likely to be required in these areas as well.
And because several air quality management districts in California already regulate NOx emissions, these emission rates are the actual LAER for NOx.
In attainment areas, new refineries with the potential to emit at least 100 tons/year of a regulated pollutant must obtain a prevention of significant deterioration (PSD) permit and use best available control technology (BACT) to reduce emissions. BACT is determined on a case-by-case basis that takes into consideration the environmental, economic, and energy impacts of prospective control technologies.
In practice, BACT is largely determined by the traditional technology that has been applied to a particular industrial process.
There are several types of control technologies available to reduce NOx emissions from industrial boilers and heaters. These technologies fall into three general categories, depending on where in the combustion process the control is implemented. In addition, some of these technologies can be combined to achieve even greater reductions than when used alone.
FRONT-END TECHNOLOGIES
NOx is formed by two mechanisms during the combustion of fossil fuels (Fig. 1):
- The oxidation at elevated temperatures of atmospheric nitrogen present in the combustion air (thermal NOx)
- The oxidation of a portion of the nitrogen bound in the fuel (fuel NOx).
Precombustion NOx reduction is accomplished by introducing substances that lower the flame temperature to reduce the amount of heat in the combustion zone. There are two such front-end technologies that can be used to reduce NOx emissions prior to combustion: injection of water or steam and recirculation of exhaust gas (Fig. 2).
DILUENT INJECTION
This method involves injecting water or steam into the combustion zone. Water is injected into the combustion air stream (at a water-to-fuel ratio of 0.21:1 to 1:1) via nozzles mounted in the windbox, vaporizing it before it enters the combustion chamber. If steam is injected directly, the diluent-to-fuel ratio is on the order of 1:1 to 2:1.
Diluent injection can reduce NO, emissions by as much as 75%. In general, steam injection results in greater NOx reduction than does water injection, and the decrease in NOx emissions increases with the diluent-to-fuel ratio.
For example, a water-to-fuel ratio of 0.5 will reduce thermal NOx formation by about 40%; if the ratio is increased to 1.0, NOx reduction will increase to between 60 and 70%.
As a retrofit technology, diluent injection requires modest modification to the combustion process. The system must, however, be designed so that the water is completely vaporized before it enters the combustion chamber to reduce the possibility of boiler corrosion caused by oxidation. The injection water must also meet the purity requirements of the boiler feedwater in order to avoid corrosion.
There are several factors that affect the amount of diluent needed to achieve desired reductions in NOx emissions.
First, manufacturers have been developing boilers with increasingly higher firing temperatures to increase overall thermodynamic efficiency (i.e., to reduce the amount of fuel needed). Second, regulatory agencies have been lowering the amount of allowable NOx emissions.
Both of these elements mean that greater amounts of water or steam must be injected to meet NOx limits, and the more diluent injected, the lower the thermodynamic efficiency of the boiler. In addition, CO emissions increase as the diluent injection rate increases.
Finally, as more water is injected into the combustion zone, flame stability may be affected; in the extreme case, the flame may even be extinguished by the water.
EXHAUST RECIRCULATION
Another way to reduce combustion zone temperature is by recycling a portion of the exhaust gas to a point where it joins (and thus dilutes) the inlet combustion air flow.
The benefits of this technology are two-fold:
- Diluting the effluent with inert material results in a lowered flame temperature, thereby reducing thermal NOx formation,
- The oxygen level in the flame is also decreased, which suppresses NOx formation in the first place.
Typically, 15-20% of the gas is recirculated, although levels as high as 45% are possible for gas-fired boilers (flame instability and high particulate loading limit the amount of gas that can be recycled in oil-fired boilers).
This technology can result in NOx reductions of 15% for oil-fired boilers, and as much as 85% for gas-fired units.
Exhaust gas recirculation poses some problems as a retrofit technology, however. Extensive modifications must be made to existing ductwork to reroute the exhaust, and the ductwork is subject to corrosion caused by the particulate matter in the effluent.
The relatively small reduction in NOx emissions, especially for oil-fired boilers, is another factor to be considered when examining retrofit NOx reduction solutions.
LOW-NOX BURNERS
The most popular and economical NOx reduction technology, at least for new boilers, is low-NOx burners, which reduce NOx emissions by internally staging the combustion process.
There are two types of staged-combustion burners. Staged-air burners (Fig. 3) involve the introduction of primary and secondary air so that the primary flame zone is substoichiometric (i.e., fuel-rich). Tertiary (staged) air is then injected after a brief delay. This type of combustion reduces the peak flame temperature because the flame burns for a short time under low excess air conditions.
In staged-fuel burners (Fig. 4), all of the combustion air is introduced at one time, but the fuel is staged, producing fuel-lean combustion in the primary zone. Unlike staged-air combustion, staging the fuel results in more excess air in the primary combustion zone. But there is less nitrogen in the mixture and thus, lower levels of NOx are produced.
The remaining fuel is then injected in the secondary combustion chamber, which ensures rapid mixing of the fuel and air. The lower flame temperature in the secondary zone and the decreased availability of oxygen in the staged zone serve to reduce NOx formation in a similar manner to exhaust gas recirculation.
Staged-air burners produce decreased NOx emissions of about 40% compared to conventional burners, while staged-fuel burners can reduce emissions by as much as 70%.
Additional reductions can be achieved by operating low-NOx burners using low excess air. (Low excess air operation can be accomplished by manually or automatically controlling the burner damper while monitoring the oxygen level in the combustion chamber.)
Another advantage of low-NOx burners, at least for new installations, is the fact that they are no more expensive than traditional burners. In fact, low-NOx burners are offered as standard equipment for many new boilers.
Costs for retrofitting existing equipment, however, may be substantial because significant physical changes may need to be made to accommodate the new burners. And because low-NOx burners are a depreciable capital cost, the NOx reduction economies will be affected by the age of the unit being retrofitted.
Low-NOx burners also may be detrimental to the longevity of boiler tubes because they create a reducing atmosphere that may promote slagging, thus exposing the tubes to accelerated corrosion; and flame impingement may overheat the tubes, resulting in premature failure caused by thermal stress.
In addition, there is the potential for CO emissions (generated by incomplete combustion with very low excess air), unburned hydrocarbons emissions (i.e., smoke), and problems associated with flame instability.
Still, these factors can be minimized with the proper construction materials, and the wide availability and low cost of this technology (especially for new boilers) makes low-NOx burners an excellent way to reduce emissions.
BACK-END TECHNOLOGIES
There are two primary methods of reducing NOx emissions from stationary sources after the pollutants have been formed by the combustion process.
These postcombustion, or back-end, technologies are rather expensive, but they generally reduce NOx emissions more than do low-NOx burners and combustion modification.
For this reason, they are best suited in retrofit situations or for use in combination with other processes to achieve maximum NOx reduction for LAER considerations or offset generation.
SCR
In selective catalytic reduction (SCR), ammonia vapor is injected into the flue gas in the presence of a catalyst-usually vanadium pentoxide, titanium dioxide, or a noble metal (Fig. 5). The ammonia selectively reacts with NOx (as opposed to hydrogen, CO, and methane, which readily react with oxygen as well) to form molecular nitrogen and water.
When used alone, SCR can reduce NOx emissions by as much as 90%. And because SCR works under relatively low combustion temperatures (as low as 300 C.), this technology can be used in conjunction with other (precombustion) processes to achieve maximum NOx reduction. Indeed, when used following steam or water injection, SCR can reduce NO, emissions by 95% or more.
The chemical reaction between ammonia and NOx, however, takes place within a very narrow temperature range. (For a vanadium pentoxide catalyst, the range is approximately 315-400 C.) If the combustion temperature is too high, both the catalyst and the ammonia may begin to decompose. If the temperature is too low, unreacted ammonia may be released from the stack (called "ammonia slip"). This can potentially cause even greater compliance problems than those faced with NOx emissions. (Ammonia is on the EPA list of extremely hazardous substances under Title 3 of the Superfund Amendments and Reauthorization Act of 1986, and on the list of regulated substances under the accidental release provisions of Title 3 of the 1990 Clean Air Act Amendments.)
If excess ammonia slips through the NOx-control system, it can react with sulfur trioxide to form ammonium bisulfate, which can contaminate the combustion air preheater and even jeopardize particulate matter emissions compliance.
Of even greater concern is the potential for an accidental release of ammonia during storage or transport to the boiler, which would result in a hazardous release, possibly affecting public safety. Moreover, vanadium and titanium can create a health and environmental risk. (Like ammonia, vanadium pentoxide is on EPA's list of extremely hazardous substances.)
Finally, SCR technology is quite expensive. In addition to the cost of the SCR unit, there are costs associated with cooling (or even heating) the exhaust gas prior to administering SCR, and disposing of the spent catalyst can be expensive.
Thus, while SCR can achieve significant NOx reductions, a large investment is required, especially when retrofitting an existing boiler. And consideration must be given to potential hazardous waste issues.
SNCR
Selective non-catalytic reduction (SNCR) is similar to SCR in that a reagent, typically ammonia or urea, is used to bond with NOx to produce inert gases that are released to the atmosphere (Fig. 6). But unlike SCR, the catalyst bed is omitted and the reaction is carried out at a higher temperature (900-1,100 C.). Because no catalytic agent is used and the technology can be employed at higher operating temperatures (the reducing reagent can be injected directly into the boiler's superheater), SNCR is generally less expensive than the catalytic process. In addition, hazardous waste disposal is not an issue with SNCR because no hazardous reactive agents are used. And while problems in storing and handling the reagent are similar to those encountered with SCR, these problems can be minimized by using urea.
Noncatalytic NOx removal, however, is also lower than that achievable with SCR (on the order of 40-70%). And, like SCR, SNCR is quite temperature-dependent; if the reductant is not introduced at the correct temperature range, unreacted ammonia could be released, or more, rather than less, NOx could be produced.
SNCR has been applied fairly extensively to refinery process heaters and waste heat boilers, using both ammonia and urea. Typical NOx reduction in these applications is about 60%. SNCR is particularly effective in retrofit situations because of its low capital cost, ease of installation, and small space requirement, or "footprint."
COMPLIANCE PLAN
The five technologies discussed in this article represent a wide range of NOx emissions reduction processes used by the oil and gas industry.
Because many refineries are in ozone nonattainment areas, it is important to be aware of the increasingly strict limits being placed on NOx emissions by federal and state regulators.
Fortunately, the technologies that exist to reduce NOx emissions to the allowable limits provide a considerable amount of flexibility in terms of efficiency and cost. And some technologies can be used in tandem to achieve maximum emissions reduction, provided the appropriate controls are employed to reduce the risk of an accidental hazardous release.
Moreover, most of the processes discussed in this article are applicable on a retrofit basis for existing facilities, as well as to new sources.
BIBLIOGRAPHY
Schorr, M. M., "NOx Control for Gas Turbines: Regulations and Technology," presented at Association of Energy Engineers, World Energy Engineering Congress, 1990.
"Nitrogen Oxide Control for Stationary Combustion Sources," U.S. Environmental Protection Agency, Office of Research and Development, Document No. EPA/625/5-86/020, 1986.
Copyright 1992 Oil & Gas Journal. All Rights Reserved.
