LOW NOX SYSTEM FOR GAS TURBINES IN COGEN BEING DEVELOPED

A catalytic combustion system that reduces NOx emissions from natural-gas turbines used to generate electricity is being developed for cogeneration systems built by AES Manufacturing Services Inc., Broken Arrow, Okla.

Catalytica Inc., Mountain View, Calif., and Tanaka Kikinzoku Kogyo K.K., Tokoyo, will collaborate with AES to tailor the companies' combustion system for Kawasaki MIA-13 gas turbines used in AES' mobile cogeneration units.

Each compact unit is mounted on an enclosed semitrailer and contains two Kawasaki turbines with shaft-driven generators and a single heat-recovery boiler. Its net output is 3 mw of electricity and more than 28,000 lb/hr of high-pressure steam.

At an industrial or commercial site where electrical capacity needs exceed 3 mw, several units may be installed in parallel.

Currently, AES units can control NOx to about 25 ppm with traditional steam-injection technology.

CONVENTIONAL FIRING

A catalytic combustion system reacts fuel with oxygen in the air on a catalyst surface to form hot combination gases that can be used to power a turbine.

Unlike conventional flame combustion, which produces high NOx emissions, catalytic combustion virtually eliminates the formation of NOx according to Catalytica.

In a conventional gas turbine, natural gas and air are mixed at an air/fuel ratio sufficient to sustain flame combustion, producing a temperature near 1,800 C. Catalytica says that current materials of construction require the turbine's inlet temperature to be reduced to 1,300 C. or less.

This reduction is accomplished through a by-pass of part of the air and cooling of the combustion gases. At the reaction times in conventional flame-combustion turbines, typical NOx emissions range from 100 to 200 PPM.

Catalytica says its catalytic combustion system can reduce NOx emissions substantially below the 25-ppm level of AES units without diverting steam from the cogeneration unit.

In catalytic combustion for gas turbines, natural gas and air react on the surface of a catalyst to produce thermal energy in the form of hot gases.

With a catalyst, the temperature of the combustion process can be kept to less than about 1,500 C., and for short reaction times (

TESTING UNDER WAY

Fig. 1 presents a schematic of catalytic combustion in gas turbines.

Air and fuel are fed to the catalyst at a ratio that generates only the required turbine inlet temperature (1,300 C.). For adiabatic combustion temperatures up to 1,500 C., NOx formation is

The Catalytica technology avoids exposure of the catalyst to the high temperatures responsible for deactivation and thermal shock fracture. The combustion process is initiated by the catalyst and is completed by homogeneous combustion in the post-catalyst region where the highest temperatures are attained.

Catalytica has tested its system in a high-pressure catalytic combustion facility in fountain View. More than 200 hr of stable catalytic operation were achieved in tests at simulated turbine operating conditions, says the company.

Fig. 2 shows the results of an earlier 100-hr test in which catalyst stability was also maintained. Emissions throughout the test were NOx

Catalytica says the combustion system has also been successfully tested at actual turbine operating conditions by General Electric Co., a major gas turbine manufacturer.

Results under actual flow and pressure conditions of a modern gas turbine are summarized in Table 1.

This past October, General Electric Co. agreed on large-scale testing of Catalytica's catalytic combustion technology. GE will design a catalytic combustion system using the Catalytica-Tanaka technology and perform sub-scale tests at GE's corporate research and development center and full-scale tests in GE Power Generation labs, both in Schenectady, N. Y.

WHO'S INVOLVED

Catalytica develops catalytic products and processes for pollution prevention in three areas: combustion systems, process technologies, and chemical products.

AES is a unit of AES Project Development Inc., Tulsa, one of 12 affiliated companies which design, manufacture, market, operate, and maintain power-generation technologies, such as the mobile cogeneration unit.

Tanaka Kikinzoku Kogyo K.K., Tokyo, develops, manufactures, and sells products based on precious metals and alloys for industrial use in electronics, automotive applications, catalysis, an other industries.

GE Power Generation is a division of GE Industrial & Power Systems, Schenectady, N.Y. GE manufactures and supplies power-generation systems and services worldwide, including steam turbines, gas turbines, generators, controls, combined-cycle systems, cogeneration power equipment, and various uprate, upgrade, and power-plant maintenance programs.