Just say NOx
Nitrogen oxides (NO_x) are precursor gases that react with volatile organic compounds (VOCs) in the presence of sunlight to form ground-level ozone. Controlling NO_x emissions from stationary point sources related to the chemical, petrochemical, and refining industries presents special challenges to both the industry and the regulators. Interest in understanding the potential NO_x reduction technologies as well as cost of controlling NO_x emissions from stationary point sources is growing.
Regulatory drivers
The Clean Air Act (CAA) (42 United States Code sections 7401-7671q), established national ambient air quality standards (NAAQS)-as originally enacted in 1970. The 1990 amendments to CAA (CAAA) further expanded the program. Each state is responsible for meeting the NAAQS for the six criteria air pollutants (nitrogen oxide, sulfur dioxide, carbon monoxide, ozone, particulate matter and lead) within a set timeframe. State implementation plans (SIPs) must include requirements for sources to reduce their emissions so that all areas of the state will reach attainment of the NAAQS within the timeframe laid out in CAAA.
The eight county Houston-Galveston area was classified as a Severe II nonattainment area and must reach attainment by November 15, 2007. Los Angeles area was classified as an extreme nonattainment area and must reach attainment by November 15, 2010.
Table 1. Potential NO_x reduction technologies
|
Table lists various technologies available for NO_x reduction. The table also lists the approximate percent reduction using each technology, and the approximate ppmv NO_x that can be achieved. |
|
|
|
|
|
|
Technology |
Approximate
reduction |
Approximate
LBS/MM BTU |
ApproximatePPMV at 3 percent O_2 |
|
Standard burners |
Base Case |
0.14 |
120 |
|
Low Nox burners (LNBs) |
60% |
0.06 |
45 |
|
Ultra low NOx burners (ULNBs) - 1st Generation |
80% |
0.03 |
25 |
|
ULNB - current guarantee level |
87%
|
0.018 |
15 |
|
ULNB - burners currently being developed |
95% |
0.007 |
6 |
|
Flue gas re-circulation (FGR)
|
55% |
0.025 |
20 (1) |
|
Advanced control systems (Compu NO_x), with FGR |
90% |
0.015 |
12 (2) |
|
Selective non-catalytic reduction (SNCR) |
40% |
. 033 (1) - .085 |
27 (1) - 70 |
|
Catalytic scrubbing |
70% |
.017 (1) - .044 |
14 (1) -36 |
|
Fuel reburn combined with SNCR |
50-70% |
.017 (1) (3) - .072 |
14(1) (3) - 60 |
|
Selective catalytic reduction (SCR |
90-95% |
.006 (1) -.015 |
5 (1) - 12 |
Notes:
1. Level that can be achieved assuming LNB installed. A lower level could be achieved if combined with ULNB
2. On boilers.
3. Can achieve as low as 30 ppmv in discharge from FCCU CO boilers, if combined with partial burning regenerators.
4. Burner performance tends to be better than listed above for boilers, because of the lower firebox temperatures in boilers.
The Texas Natural Resource Conservation Commission (TNRCC) recently drafted revisions to the state of Texas SIP, including a variety of measures to reduce NO_x and VOC emissions from industrial plants, automobiles and other sources such as construction and landscaping equipment. TNRCC's proposal requires industrial plants to cut their NO_x emissions by 90 percent. If the U.S. Environmental Protection Agency (EPA) concludes that the TNRCC's plan doesn't have enough pollution reductions, it can impose economic sanctions that include a cut in transportation funds and strict limits on industrial expansion. Recent regulatory actions by the state of Texas and tight control requirements that exist in the state of California have opened up a potentially huge market for NO_x control equipment. This is pushing vendors to develop improved technologies to reduce costs.
Burners
Burners have been undergoing a rapid state of development based on pressures to reduce NO_x formation. Stages of burner development include:
* Low NO_x burners (LNB) -- specifically designed for lower NO_x emissions. These burners typically utilized air staging to reduce NO_x. LNBs by themselves are not capable of meeting the new standards in the Houston area;
* First generation ultra low NO_x burners (ULNB) which tended to use fuel staging. The vendors say these burners are capable of meeting the 0.036 lbs/MM Btu of NO_x requirement for fired heaters in the Houston Area, under 40 million (MM) Btu/hr (British thermal units per hour). In many cases it may be possible to retrofit these burners into existing heaters without a shutdown; and
* Second generation ULNBs, which use staged fuel, combined with internal flue gas re-circulation (IFGR). Jacobs Engineering Group Inc. is installing the first burners of this type at a refinery in Southern California. This test installation will determine if these burners can meet the Houston area requirements of 0.015 lbs/MM Btu of NO_x for fired heaters in the 40 to100 MM Btu/hr range.
| Recent regulatory actions by the state of Texas and tight control requirements that exist in the state of California have opened up a potentially huge market for NOx control equipment. |
ULNBs currently under development utilize technologies such as steam injection into the fuel gas, flue gas re-circulation into the fuel gas and/or more advanced staging and IFGR. Jacobs will be installing this generation of burners, in the first quarter of 2001, at a refinery in the Houston area, to determine if these burners meet Houston's requirement of 0.01 lbs/MM Btu of NO_x for fired heaters > 100 MM Btu/hr.
These burners may or may not end up a substitute for selective catalytic reduction (SCR) systems on larger units in the Houston area. SCR systems use a preferential catalyst and ammonia to convert NO_x to nitrogen. The odds are they will replace SCRs on most smaller units. An exception may be boilers. Boilers are simpler than fired heaters because the firebox temperatures are lower. There is a strong possibility that burners can meet the requirements for large boilers and that SCRs will not be required for boilers.
Vendor guarantees for ULNBs are dependent on the following:
* No air leaks. Most of the ULNB use internal fuel gas re-circulation to dilute the flame. Air leaks result in a high oxygen level in the flue gas that is re-circulated and results in higher NO_x levels. Leaks into the furnace may need to be welded shut and gaskets may need to be installed to stop air leaks around view ports;
* Firebox temperatures. Low NO_x guarantees are easier to obtain if firebox temperatures are low; and
* Fuel gas - natural gas composition. Fuel gas systems containing hydrogen cause problems with some burners, because hydrogen burns hot and because of swings in the hydrogen concentration.
More advance control systems may be required to ensure the performance of the burners. Control modifications that may be required include:
* Installation of continuous emissions monitoring systems (CEMS);
* Use of oxygen analyzers within the firebox;
*Automatic controls on stack dampers or automatic modulation of the burner inlet registers; and
* Automatic control of fan speed using variable frequency modulation.
Selective Catalytic Reductions systems (SCRs).
SCRs utilize the following equations for the reaction between ammonia and NO_x at high temperatures:
6NO + 4NH3 = 5N2 + 3H2O
2NO + 4NH3 + 2O2 = 3N2 + 6H2O
6NO2 + 8NH3 = 7N2 + 12H2O
Types of SCR technology
Low temperature catalysts (ex. pellets). These catalysts have the advantage of low-pressure drop and of operating at lower temperatures, but are susceptible to sulfur and particulates.
| Optimizing the split between the different types of technologies, on a large facility that will use multiple technologies, will require a trial and error analysis of the various options. |
Advantages of low temperature SCRs include:
* Can be operated at a lower temperature because it has a higher surface area for a higher activity;
* Can be located at the end of the stack gas train in the low temperature area. This minimizes the need to run ducting from a high temperature area and then return the flue gas to the stack gas train;
* Operating temperature ranges from 300 to 680 degrees Fahrenheit;
* Low pressure drop. A typical pressure drop is two inches water, with ranges of 0.5 inches to 6 inches; and
* Can be used in installations firing natural gas and most installations firing plant/refinery fuel gas.
Disadvantages of low temperature SCRs:
* Susceptible to sulfur concentrations, although this can be overcome; and
* Susceptible to particulate loadings > 10 milligrams per cubic meter (mg/m^3), although a baghouse could be installed to protect the catalyst. The low particulate tolerance requires cleaning of the upstream system during installation. The presence of internal fiber insulation could also cause problems.
Medium to High temperature catalysts (ex. honeycomb units). These catalysts can tolerate sulfur and particulates but must operate at higher temperatures.
Advantages of honeycomb catalyst include:
* Has been in use for a number of years;
* Can stand high sulfur and high particulate loadings. Jacobs used a honeycomb catalyst for a Fluidized Catalytic Cracking Unit (FCCU) in the Netherlands. A soot blower was provided for particulate control (the particulates are stirred up and flushed through the honeycomb); and
* Zeolite catalysts can be used for high sulfur applications. The reaction with zeolite catalysts occurs inside the molecular sieve body, rather than on the surface of a metal catalyst. This eliminates the sulfur poisoning of metallic catalysts and reduces the conversion of sulfur dioxide (SO_2) to sulfur trioxide (SO_3).
Disadvantages of honeycomb catalysts include:
* Must rout ducting from a high temperature location in the flue gas system. A typical operating temperature range is 550 to 725 degrees Fahrenheit. High temperature catalysts are also available that operate in the 650 to 1100 degrees Fahrenheit range; and
* Increased pressure drop. Larger hole sizes can be used to reduce the pressure drop, but this requires a larger catalyst volume.
In addition to the choices between types of catalysts, the following types of choices are also required in designing SCR systems:
* Catalyst volume (space velocity) versus percent conversion. In addition, modules can be mounted in series for greater conversion;
* Where in the flue gas train to locate the SCR. There is some flexibility in operating temperature, but in general the higher the temperature, the lower the catalyst volume;
* Catalyst hole size versus pressure drop. Pressure drops are lower for bigger hole sizes (honeycomb catalysts) but the efficiency is lower so the catalyst volume is larger. For low temperature catalysts, the choice is the thickness of the cross-flow bed which impacts pressure drop; and
* Horizontal or vertical flow. Vertical flow is most common because it occupies less space. Vertical downflow is preferred because it allows particulates to drop through the catalyst.
The supporting facilities for an SCR system can include ammonia storage and vaporization facilities, ammonia distribution, an air compressor to provide dilution air for the ammonia injection, an ammonia analyzer in the stack to provide feedback for ammonia addition control and soot blowers or a baghouse for fouling service. Modeling of the air plenum may be required to ensure good air distribution.
Selective non-catalytic reduction (SNCRs) systems
SNCR units can achieve approximately 40 percent NO_x reduction without the requirement to install an SCR. Combined with other technologies, an SNCR can potentially produce low NO_x concentrations.
SNCRs utilize the reducing capability of ammonia and urea to reduce NO_x to nitrogen. The equations for the reaction of ammonia and urea with NO_x are:
2NO + 2NH3 + 2O2 = 2N2 + 3H2O (NH3/NO = 1/1)
CO(NH2)2 + 2NO + 1/2O2 = 2N2 + CO2 + H2O
The requirements for good conversion include:
* Proper temperature. The reactions occur over a very narrow temperature range;
* Ammonia reaction --1600 to 2200 degrees Fahrenheit (with versions down to 1400 degrees);
* Urea reaction -- 1600 to 2000 degrees Fahrenheit (with versions down to 1200 degrees);
* Good mixing. The ammonia is typically injected into holes drilled into the firebox;
* Adequate residence time; and
* No impingement of the injected chemical against tubes.
Catalytic scrubbing
Nitrogen dioxide (NO_2) can easily be scrubbed with caustic solutions, but nitric oxide (NO)cannot be scrubbed. Jacobs has installed a number of non-catalytic scrubbing systems on nitric acid plants and plants generating NO_2 in the process.
Most thermal NO_x is NO, so the NO_x in the vent from fired units cannot be scrubbed unless the NO is catalytically oxidized to NO_2. A number of vendors have proprietary technology for the catalytic oxidation of NO in scrubbing systems. Jacobs engineered a catalytic scrubbing system for a commercial incineration system that was an economical installation because the system could be retrofitted into the existing system with little capital investment, only a 70 percent NO_x reduction was required and caustic scrubbers existed which were used to remove acid gases (mainly HCl).
Flue gas re-circulation (FGR)
FGR has been used extensively on boilers to reduce NO_x. Re-circulating the flue gas reduces the oxygen concentration and thus reduces flame temperatures. Reported flame temperatures are:
* Zero percent FGR - 3500 degrees Fahrenheit
* 20 percent FGR - 2900 degrees Fahrenheit
Historically, re-circulation rates in the 15 to 20 percent neighborhood have been used, with NO_x reductions in the 40 to 55 percent range. Forty percent re-circulation is the maximum stochiometrically possible and can achieve NO_x reductions as high as 70 percent, but these systems require very tight control systems to maintain flame stability.
Summary
Recently proposed revisions to the state of Texas SIP will make NO_x control even tighter in the Houston area than in Los Angeles. However, technologies are available that can meet these regulations. Because of the regulatory drivers, NO_x reduction technology has been advancing rapidly.
A new generation of ULNB may evolve to the point that it can meet the regulatory requirement for most of the fired heaters in the Houston and Los Angeles area. If this happens, it will significantly reduce the requirement to install SCRs. SCRs will still have a place, and will undoubtedly be used in many of the larger fired heaters. "Other" technologies will have a niche, and will be used in special situations. Optimizing the split between the different types of technologies, on a large facility that will use multiple technologies, will require a trial and error analysis of the various options.
Enter 204 for more information.
Mike Bardford, PE, is an environmental manager at Jacobs Engineering Group Inc., a provider of professional technical services. Rajiv Grover is an environmental section manager at Jacobs. They can be reached via e-mail at [email protected] and [email protected] respectively.
This article originally appeared in the 12/01/2000 issue of Environmental Protection.