NOx BACT Review

Combustion Turbines and Duct Burners

NO_x is produced through two mechanisms: high temperature processes, which create thermal NO_x (products of the reaction of nitrogen and oxygen gases in the air) and combustion of nitrogen‐containing materials, which produces fuel NO_x. Table 6.16 lists the technologies that were identified for controlling NO_x emissions from gas turbines and their effective emission levels.

SCONO_X

SCONO_X is an emerging proprietary catalytic and absorption technology that has shown some promise for turbine applications. Unlike selective catalytic reductions (SCRs), which requires ammonia injection, this system does not require ammonia as a reagent; its parallel catalyst beds are alternately taken off line through means of mechanical dampers for regeneration.

Despite its advantages, however, the process SCONO_X catalyst is subject to the same fouling or masking degradation that is experienced by any catalyst operating in a turbine exhaust stream. There is also a small energy loss from the performance loss due to the pressure drop across the catalyst.

Table 6.15 Technologies and their approximate control efficiencies.

Source: From USEPA (2016b).

Pollutant	Technology	Potential control efficiency (%)
NOx	SCONOX	70–95
	Selective catalytic reduction (SCR)	50–95
	Dry low NOx combustors	40–60
	Selective non‐catalytic reduction (SNCR)	40–60
	Water/steam injection	30–50
	Good combustion practices	Base case
CO	Catalytic oxidation	60–80
	Good combustion practices	Base case
PM10	Good combustion practices	10–30
	Fuel specification: clean‐burning Fuels	Base case
VOC	Catalytic oxidation	60–80
	Good combustion practices	Base case

Table 6.16 NO_x control technologies and effective emission levels.

Source: From USEPA (2016b), Catalytica Energy Systems Inc. (2004), and California Air Resources Board (2000).

Technology	Typical control range (% removal)	Typical emission level (ppm)
SCONOX	90–95	2–2.5
XONONTM flameless combustion	80–90	3–5
Selective catalytic reduction (SCR) with low‐NOx combustor or SCR with water injection	50–95	2–6
SCR with water/steam injection or advanced low‐NOx combustor	50–95	6–9
Dry low‐NOx combustor and/or aggressive water injection	30–70	9–25
Water/steam injection or low‐NOx burners	30–70	25–35

The vendor of SCONO_X guarantees performance to all owners and operators of natural gas‐fired combustion turbines, regardless of size or gas turbine supplier. The system is designed to reduce both CO and NO_x emissions from natural gas‐fired power plants to levels below ambient concentrations. Indeed, the EPA considers SCONO_X a technically feasible and commercially available air pollution control technology and expects its emission levels for criteria pollutants such as NO_x, CO, and VOC to be comparable or superior to previously applied technologies for large combined cycle turbine applications.

SCONO_X has been demonstrated successfully on smaller power plants, including a 32 MW combined‐cycle General Electric LM2500 gas turbine in Los Angeles (Das 2003). This facility uses water injection in conjunction with SCONO_X to achieve a NO_x emissions rate of 0.75 ppm on a 15‐minute rolling average. The SCONO_X technology has also been successfully demonstrated on a 5 MW Solar Turbine Model Taurus 50 at the Genetics Institute in Andover, Massachusetts. The system is reducing NO_x down to 0.5 ppm NO_x, on a one‐hour rolling average. The permit for the power plant was originally issued for 2.5 ppm NO_x.

The manufacturer guarantees CO emissions of 1 ppm and NO_x emissions of 2 ppm. According to one set of figures, when NO_x is reduced from 12.18 ppm (gas turbine with duct burner firing) to 2 ppm, the cost effectiveness is $13 627/T of NO_x removed (Catalytica Energy Systems Inc. 2004).

XONON

Several companies are reported to be working on a second technology for the control of NO_x. Introduced commercially by Catalytica Combustion Systems, it is being marketed under the name XONON. This technology replaces traditional flame combustion with flameless catalytic combustion. NO_x control is accomplished through the combustion process using a catalyst to limit the temperature in the combustor below the temperature where NO_x is formed. The XONON demonstrated to achieve near‐zero emissions. The XONON combustion system consists of four sections: (i) the preburner, for start‐up, acceleration of the turbine engine, and adjusting catalyst inlet temperature if needed; (ii) the fuel injection and fuel‐air mixing system, which achieves a uniform fuel‐air mixture to the catalyst; (iii) the flameless catalyst module, where a portion of the fuel is combusted flamelessly; and (iv) the burnout zone, where the remainder of the fuel is combusted.

The single field installation of the XONON technology at a municipal power company is being used to perform engineering studies of the technology at Silicon Valley Power, in Santa Clara, California. NO_x emissions are well below 2.5 ppm on the 1.5 MW Kawasaki M1A‐13A gas turbine. Catalytica has a collaborative commercialization agreement with General Electric Power Systems, committing to the development of XONON. In conjunction with General Electric Power systems, the XONON system was specified for use with the GE 7FA turbines at the proposed 750 MW natural gas‐fired Pastoria Energy Facility, near Bakersfield, California. The project entered commercial operations in 2003. Because the NO_x emissions limitations of 2.5 ppm have been demonstrated in practice by a commercial facility, this technology is considered commercially available at this time (Catalytica Energy Systems Inc. 2004).

Selective Catalytic Reduction

SCR systems selectively reduce NO_x by injecting ammonia (NH₃) into the exhaust gas stream upstream of a catalyst. NO_x, ammonia, and oxygen react on the surface to form molecular nitrogen (N₂) and water. The overall chemical reaction can be expressed as

Parallel plates or honeycomb structures, permeated with the catalyst, are installed in the form of rectangular modules, downstream of the gas turbine in simple‐cycle configurations, and into the heat recovery steam generator (HRSG) portion of the gas turbine downstream of the superheater in combined‐cycle and cogeneration configurations.

The turbine exhaust gas must contain a minimum amount of oxygen and be within a somewhat narrow temperature range in order for the SCR system to operate properly. The temperature range is dictated by the catalyst: if it is too low, the reaction efficiency drops and increased amounts of NO_x and ammonia are released from the stack; if it becomes too high, the catalyst may begin to decompose. Turbine exhaust gas is generally too hot to be passed through the catalyst, so it is cooled by the HRSG, which extracts energy from the hot turbine exhaust gases and creates steam for use in other industrial processes or to turn a steam turbine. In simple‐cycle power plants where no heat recovery is accomplished, high temperature catalysts (e.g. zeolite) are an option. SCR can typically achieve NO_x emission reductions in the range of about 80–95%.

SCR is the most widely applied post‐combustion control technology in turbine applications and is currently accepted as LAER for new facilities located in ozone non‐attainment regions. It can reduce NO_x emissions to as low as 4.5 ppmvd for standard combustion turbines without duct burner firing and as low as 2–2.5 ppmvd when combined with lean‐premix combustion (again without duct burner firing). SCR uses ammonia as a reducing agent in controlling NO_x emissions from gas turbines. The portion of the unreacted ammonia passing through the catalyst and emitted from the stack is called ammonia slip. The ammonia is injected into the exhaust gases prior to passage through the catalyst bed. There is also a potential for increased particulate emissions from formation of ammonia salts. SCR may also results in the generation of spent vanadium pentoxide catalyst, which is classified as a hazardous waste. In addition, there is an energy loss from the performance loss due to the pressure drop across the SCR catalyst.

Gas turbines using SCR typically have been limited to 10 ppmvd ammonia slip (emissions of ammonia that has not reacted with nitrogen) at 15% oxygen. However, levels as low as 2 ppmvd at 15% oxygen have been proposed and guaranteed by control equipment vendors. In addition, Massachusetts and Rhode Island have established ammonia slip LAER levels of 2 ppmvd. Massachusetts has permitted at least two large gas turbine power plants using SCR reduction with 2 ppmvd ammonia slip limits. California recommended that the establishment of ammonia slip levels below 5 ppmvd at 15% oxygen on the basis of guarantees from control equipment vendors of single‐digit levels for ammonia slip.

Data supplied by the vendor show that when NO_x is reduced from 15 ppm (gas turbine with duct burner firing) to 3.5 ppm, the cost effectiveness is $9473/T of NO_x removed.