1 August 2005

Monitoring NOx emissions

By John Prince and John Dilliott

Industry interest has piqued in monitoring nitrogen oxide (NOx) emissions in cogeneration facilities below two parts per million (ppm). Universities and government agencies have performed studies with inconclusive results. President Bush's administrative issue of the Clean Air Interstate Transport Rule, calling for a 70% reduction in NOx emissions, adds particulate matter, 2.5 micron (PM2.5), and mercury to the criteria pollutant control list. Expectations are reductions will reach 70% by 2010. This new rule will affect every power plant and internal combustion boiler and incinerator, requiring emitters to better maintain their continuous emissions monitoring systems (CEMS) because the rules will cause operations near the lower limits of emission permits.

But a field study report demonstrates successful monitoring of NOx below 1ppm over a 36-month period using industry-standard materials, commercially available for several years. Our team did not originally think of doing a study. The CEMS was supposed to be a garden variety system. But after installing the system, we realized it needed modifications because the NOx level was much lower than expected. Later, we learned of a study concluding you could not accurately continuously monitor a stack below 2ppm. We had been monitoring 1ppm for several months.

After a 30-month study, we proved it was indeed possible to monitor NOx in the 1ppm range for a long term with no more ongoing maintenance than a CEMS monitoring higher levels (5-20ppm) of NOx. This study is especially useful for operators of combustion processes where the expected NOx emissions concentration is below 2ppm. It would also benefit air pollution agencies responsible for ensuring accurate emissions testing.

The study

The University of California, San Diego (UCSD) is an educational and research campus with nearly 6 million square feet of occupied space within 60 buildings. The annual load is approximately 200,000 W-hr with a maximum demand of 28mW. It houses 20,000 students and 10,000 staff and faculty. Nearly 3.5 million square feet are conditioned with high temperature and chilled water generated via steam produced from the waste heat of gas turbine exhaust.

System availability has been 90% for the past year. The combination of the Solar Titan So-Lo NOx combustion technology with the post-combustion treatment has consistently produced NOx emissions in the .5-1.5ppm range. The previous 12 months saw 3.5 tons production of NOx with 190,000 W-hrs.

Cogeneration plant

UCSD's cogeneration plant generates 25 megawatts of electricity and enough steam for the campus. The results of using Solar Turbines' So-Lo NOx turbine generators and air emissions control technology are NOx emissions with daily cogeneration in the San Diego Air Pollution Control District.

The university's new emissions control system is one of the first commercial installations with this technology. The cogeneration facility, with its high-quality pollution control system, is the primary provider of electrical and steam power to the UCSD campus community. Our team reduced NOx emissions to 50% of San Diego Air Pollution Control District's required levels. The result saves UCSD nearly $250,000 a month.

APCD average vs. CEMS instantaneous reading at probe tip

Continuous emissions overview

The CEMS is a stand-alone automatically operated emissions monitoring system with all the components required for successful monitoring of NOx emissions less than 5ppm. The probe consists of one dry multi-point probe with heated filter element mounted on the stack.

A specially designed heated sample umbilical bundle transports the sample from the probe filter to the cabinet. Careful temperature control of the bundle occurs in the main cabinet. A programmable logic controller controls the system and automatically introduces calibration gases on a regular basis. The PLC controls calibration calculations as well as probes blow-back and gas stream switching.

The analyzers are located in the main cabinet with the main sample conditioning system, which consists of switching valves, a condensation removal assembly, and filters. The sample pumps, flow controls, and indicators are also located in the cabinet. The cabinet is located in an air-conditioned 6x8 enclosure. You can install dry/wet-basis analyzers.

Testing prior to the first APCD certification revealed a high degree of stratification in each of the emissions stacks. The results are a comparison of the test lab results with those of the CEMS using a standard single-point probe. (See figure).

Subsequent to this testing, we replaced the standard dry probe with a probe with multiple openings along the shaft. We installed no special balancing or mixing device. Daily NOx averages from December 2001 through May 2003 ranged from 0.1 to 2.1ppm NOx.

Behind the byline

John Prince is president of APG Analyzer Systems in Huntington Beach, Calif. John Dilliott is campus energy manager at the University of California Central Utilities and Cogeneration Plant in San Diego, Calif. For more information, visit www.calcems.com.