Fuel Cells Measure Low NOx

AIR-WORLD Environmental, Inc.>, (http://www.airworldenviro.com) has announced three new projects for its AW-2K CEM system. The AW-2K uses fuel cell (a.k.a. electrochemical cell) technology for its analyzers. TECO Energy in Apollo Beach, FL has ordered two AW-2K CEM systems with O2, NOx and CO to be installed at Tampa Electric's Big Bend coal-fired boiler units 1 and 2 and BP Oil in Texas City, TX will install an AW-2K CEMS with O2, CO and NOx to measure emissions from natural gas heaters at an oil refinery. An AW-2K with NOx, CO, CO2 and O2 has been selected for installation at FIU's new advanced combustion lab in Miami, FL. The lab conducts research on the clean and efficient use of fossil fuels.

Bob Mullowney, President of AIR-WORLD Environmental, spoke on "Field Experience Using Low Cost Fuel Cells for CEMS" at the Electric Utilities Environmental Conference (EUEC) held in Tucson, AZ in January. Mullowney discussed the origins of fuel cells and their application to CEMS. He also discussed the unique aspects of the AW-2K CEMS, especially its low cost, compact size and accuracy.

Fuel cells got their start about 1981. Mullowney recalled that a few companies tried using early electrochemical cells for CEMS monitoring but found they had a short life, were non-linear and drifted. However, in 1991 a British company made a major change to the design of fuel cells by using the counter electrode which stabilized the fuel cell by balancing the output of the sensing electrode and the reference electrode. The results were cells with a longer life of up to two years or more, linear output and very little drift.

Mullowney began development on a fuel cell-based CEMS in 1994 and his company installed the first two systems on Westinghouse gas turbines in early 1997. The installed cost was less than $75,000 per sample point. The Version 1 units were 3 ft. x 4 ft. x 1 ft. and weighed nearly 200 pounds. Changes in electronics, sampling technology and software over the last four years have reduced the size of the box so that the Version 4 unit is only 27 in. x 30 in. x 12 in. and weighs under 85 pounds.

Because of the very small size and simplicity of the fuel cells analyzers Mullowney was able to design a CEMS that reduced the installation cost to just a few thousand dollars, reduced equipment costs from one-third up to two-thirds of those of traditional CEMS, and reduced maintenance costs to less than $1000/year. Mullowney claims that most on site plant maintenance personnel, even those with very little training and minimal CEMS knowledge, can perform the required maintenance.

As we have reported in the newsletter (e.g., February 1999, page 1) low-level concentrations have become of very real concern to gas turbine users and others faced with monitoring NOx at very low levels. Most gas turbines now have a NOx limit of 25 ppm or less with some permits requiring emissions under three ppm. The challenge facing the owners of these systems lies in passing the yearly RATA test.

Mullowney reminded conference attendees that NOx is actually NO plus NO2. (Under certain conditions NO2 will exceed NO in gas turbines.) Fuel cell (electrochemical) technology measures NO and NO2 separately in two different analyzers and the two measurements are added to yield NOx. The problem comes in certifying any NOx monitoring system using EPA's Relative Accuracy Test Audit (RATA) procedure known as Method 7E. This method requires using a chemiluminescence analyzer and does not require that calibration and error corrections be applied using protocol NO2 calibration gases.

Chemiluminescence analyzers do not measure NO2; they only measure NO and use thermal or catalytic oxidizers to convert NO2 to NO. In addition, the efficiency of thermal oxidizers varies and is not 100 percent. In order to get an accurate method 7E test, Mullowney recommended that testers use protocol gas with cal gases blended with CO2 at the approximate concentrations found in the stack gases and calibrate and correct the errors using both NO and NO2 cal gases.

Mullowney presented the results of an actual field RATA certification whose accuracy demonstrated how well fuel cells using electrochemical technology can pass RATA tests. The certification was for an oil-fired steam boiler. The fuel cell system installed replaced a chemiluminescence NOx analyzer and a zirconium oxide O2 analyzer. The system had been in operation for about eighteen months and had passed all its RATA tests.