Knocking Out NOx
The U.S. Environmental Protection Agency (EPA), and the state and local agencies regulate a number of pollutants. Most of these pollutants have come into compliance in the time since the National Ambient Air Quality Standards (NAAQS) were established as a result of the Clean Air Act of 1970. The exceptions are ozone and particulate mater (PM). The group known as nitrogen oxides (NOx) is a precursor to both of these pollutants, and the Clean Air Act Amendments of 1990 requires that NOx be treated as non-attainment in any area that does not meet the NAAQS for ozone or PM. Accordingly, NOx is the primary pollutant of concern when permitting a gas turbine.
Gas Turbine NOx Control
A gas turbine operates by compressing incoming air, combining it with fuel, and combusting the mixture. The combustion process releases the fuel's energy, forming hot gases that power the turbine. In conventional combustion systems, a flame is used to combust the fuel. The temperature required to sustain a stable flame is significantly higher than the temperature at which the gas turbine is designed to operate. This excessive temperature causes the nitrogen and oxygen in the air to react, forming NOx, a major contributor to air pollution.
Catalytica Energy Systems Inc (CESI) has developed Xonon® Cool Combustion, a catalytic technology that combusts fuel flamelessly. This process releases the same amount of energy as flame-based combustion systems but at a lower temperature. Importantly, this lower temperature is below the threshold at which NOx is formed. The Xonon combustion system is the only on-engine combustion system that has demonstrated ultra-low emissions.
The Xonon combustion system is a pollution prevention technology that achieves low NOx emissions without the need for exhaust gas cleanup, and without degrading the performance of the gas turbine engine or adversely impacting the environment. This ultra-low NOx emission control technology achieves less than 3 ppm NOx while maintaining carbon monoxide (CO) less than 10 ppm and volatile organic compounds (VOCs) less than 3 ppm. This performance has been verified by EPA under their Environmental Technology Verification (ETV) program (www.epa.gov/etv/08/xonon_vs.pdf), and by over 8,000 hours of full load operation on a utility grid in a demonstration project, which is shown in Table 1.
U.S. regulations have gradually reduced the allowable NOx levels and, consequently, forced the development of gas turbines that emit lower quanitities of NOx. |
The system is totally contained within the combustor of the gas turbine and is not a process for cleanup of the turbine exhaust.
The overall combustion process consists of the partial combustion of the fuel in the catalyst module followed by completion of the combustion downstream of the catalyst. The partial combustion within the catalyst produces no NOx and the combustion downstream of the catalyst occurs in a flameless homogeneous reaction that produces almost no NOx.
In the mid 1980s, it was possible to permit gas turbines with NOx emissions in the 150 to 200 ppm range. However, U.S. regulations have gradually reduced the allowable NOx level and, consequently, forced the development of gas turbines that emit lower quantities of NOx. Today, large gas turbines must emit less than 3 ppm in most applications, and small turbines are commonly permitted in the less than 5 ppm range for units being installed in areas that are non-attainment for ozone (which accounts for most generator set applications).
Field Experience
A program to demonstrate reliability and durability of the new turbine technology was conducted at Silicon Valley Power, a municipal power company for the city of Santa Clara, California. The program was supported and recognized by a number of important third-party organizations including the California Air Resources Board, the California Energy Commission, the U.S. Department of Energy, the U.S. Environmental Protection Agency, the Electric Power Research Institute, and the Gas Technology Institute.
The program consisted of operating a Xonon equipped Kawasaki M1A-13X gas turbine generator set as a continuous duty unattended unit connected to the electric utility grid. Run in two separate segments of over 4,000 hours each, the unit produced the results shown in Table 1.
Table 1
Performance Results of the Xonon Catalytic System
|
Performance Criteria
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Summary of Results
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|
Operating Hours
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> 8100
|
|
NOx emissions (average at full load) |
< 2.5 ppm (corrected to 15 percent O2)
|
|
CO emissions (average at full load) |
< 6 ppm (corrected to 15 percent O2 )
|
|
VOC emissions (average at full load) |
< 3 ppm
|
|
Reliability**
|
> 98 percent |
** Reliability (based on ANSI/IEEE Standard 762-1987) = 1 - Forced Outage Rate
Compliance Alternatives
Before the introduction of the new turbine technology, gas turbine users had three primary options to comply with the NOx emission regulatory requirements.
The new turbine technology can mitigate many of the community concerns, resulting in a shorter permitting time, and permits that are less burdensome. |
- Lean pre-mix combustion systems are proven technology at 25 ppm. In areas where the application and local regulations will accommodate 25 ppm, this will continue to be the technology of choice, until the requirements change. Some gas turbine manufacturers have offered commercial guarantees at less than 15 ppm.
- In areas where regulations require less than 5 ppm, units have been permitted for continuous operation at 2.5 to 5 ppm, depending on the application, using a combination of lean pre-mix and an exhaust gas cleanup technology such as SCR.
- In some applications, the user can obtain a permit for operation at 25 ppm by agreeing to a permit limitation on operating hours. These limits usually restrict the unit to typically less than 500 hours annually, depending on the local agency requirements.
Exhaust Gas Cleanup Systems
Selective catalytic reduction (SCR) has been the primary technology used for exhaust gas cleanup with over 15 years of proven experience. However, exhaust gas cleanup systems have a number of factors that are difficult and costly to accommodate, and can be prohibitive in many distributed generation applications.
One of the new turbine technology's advantages is that no hazardous substances have to be delivered and stored at the site of operation. |
- They add significant capital and operating costs, and in many distributed generation cases, they render the project economically infeasible.
- They reduce system efficiency. They impose backpressure on the gas turbine, which decreases the power output and efficiency.
- They increase the system footprint, requiring space that is not available in many distributed generation applications.
- SCR requires the use of toxic and/or hazardous reactants (such as ammonia).
Some other technologies, such as SCONOx, have become commercially available recently. The SCONOx system is applicable to natural gas fueled gas turbines. It is based on a unique integration of catalytic oxidation and absorption technology. Like Xonon,the SCONOx system does not require the use of ammonia, eliminating the potential of ammonia slip conditions evident in SCR systems. But, according to a recent U.S. Department of Energy study comparing the costs of gas turbine emission control systems, it has the highest capital and operating cost of any NOx control system (see www.eren.doe.gov/der/chp/pdfs/noxreport.pdf).
This article originally appeared in the December 2001 issue of Environmental Protection, Vol. 12, No. 12, p. 22.
This article originally appeared in the 12/01/2001 issue of Environmental Protection.