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Electrical Generating Equipment - Gas Turbines

Post-Combustion Catalytic Control

  • Selective Catalytic Reduction (SCR)

    Selective catalytic reduction (SCR) systems selectively reduce NOx emissions by injecting ammonia (NH3) into the exhaust gas stream upstream of a catalyst. Nitrogen oxides, NH3, and O2 react on the surface of the catalyst to form N2 and H2O. The exhaust gas must contain a minimum amount of O2 and be within a particular temperature range (typically 450°F to 850°F) in order for the SCR system to operate properly.

    The temperature range is dictated by the catalyst material, which is typically made from noble metals, including base metal oxides such as vanadium and titanium, or zeolite-based material. The removal efficiency of an SCR system in good working order is typically from 65-90%. Exhaust gas temperatures greater than the upper limit (850°F) cause NOx and NH3 to pass through the catalyst unreacted. Ammonia emissions, called NH3 slip, may be a consideration when specifying an SCR system and are often limited by air permitting.

    Ammonia, either in the form of liquid anhydrous ammonia, or aqueous ammonia hydroxide is stored on site or injected into the exhaust stream upstream of the catalyst. Although an SCR system can operate alone, it is typically used in conjunction with water-steam injection systems or a lean-premix system to reduce NOx emissions to their lowest levels (less than 10 ppm at 15 percent oxygen for SCR and wet injection systems).

SCR flow diagram for gas engine system

Some SCR installations incorporate CO catalytic oxidation modules along with the NOx reduction catalyst for simultaneous CO/NOx control. Carbon monoxide oxidation catalysts are typically used on turbines to achieve control of CO emissions, especially turbines that use steam injection, which can increase the concentrations of CO and unburned hydrocarbons in the exhaust. CO catalysts are also being used to reduce VOCs and organic HAPs emissions. The catalyst is usually made of a precious metal such as platinum, palladium, or rhodium. Other formulations, such as metal oxides for emission streams containing chlorinated compounds, are also used. The CO catalyst promotes the oxidation of CO and hydrocarbon compounds to carbon dioxide and water as the emission stream passes through the catalyst bed. The oxidation process takes place spontaneously, without the requirement for introducing reactants. The performance of these oxidation catalyst systems on combustion turbines results in 90-plus percent control of CO and about 85-90% control of formaldehyde. Similar emission reductions are expected on other HAP pollutants. This could become an important control mechanism as the new MACT formaldehyde standard for new gas turbines is 91 ppb (parts per billion) at 15% O2.

  • New Catalytic Reduction Technologies
    New catalytic reduction technologies have been developed and are currently being commercially demonstrated for gas turbines. Such breakthrough technologies include, but are not limited to, the SCONOX and the XONON systems, both of which are designed to reduce NOx and CO emissions.

    SCONOX
    The SCONOX system is applicable to natural gas-fired turbines. It is based on a unique integration of catalytic oxidation and absorption technology. CO and NO are catalytically oxidized to CO2 and NO2. The NO2 molecules are subsequently absorbed on the treated surface of the SCONOX catalyst. The system manufacturer guarantees CO emissions of 1 ppm and NOx emissions of 2 ppm. The SCONOX system does not require the use of ammonia, eliminating the potential of ammonia slip conditions evident in existing SCR systems. Only limited emissions data are available for a gas turbine equipped with a SCONOX system. This data reflects HAP emissions and currently is not sufficient to verify the manufacturer's claims.

    XONON
    The XONON system is applicable to diffusion and lean-premix combustors and is currently being demonstrated with the assistance of leading gas turbine manufacturers. The system utilizes a flameless combustion system where fuel and air reacts on a catalyst surface, preventing the formation of NOx while achieving low CO and unburned hydrocarbon emission levels. The overall combustion process consists of the partial combustion of the fuel in the catalyst module followed by completion of 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. The system is totally contained within the combustor of the gas turbine and is not intended as a process for cleaning the turbine exhaust. The catalyst manufacturer claims that gas turbines equipped with the XONON catalyst emit NOx levels below 3 ppm and CO and unburned hydrocarbons levels below 10 ppm.

Commercialization programs with several turbine manufacturers are underway to market XONON and SCONOX technologies. A rule change to exempt new gas turbines that use lean premix and diffusion flame combustion from the MACT standard for formaldehyde is proposed.

Microturbines, due to their relatively small size and low operating temperatures have not been required to add any post-combustion controls.