CEMS that were primitive by today’s standards started becoming standard equipment in coal-fired power plants in the 1970s following passage of the Clean Air Act. In the early 1990s another 1,500 CEMS were placed into service to comply with amendments to the Act to control acid rain. Now, a decade and a half later, the U.S. coal-fired fleet is once again facing the prospect of replacing or upgrading CEM systems.

Major components of existing systems are wearing out, becoming outdated, or both. New reporting requirements promulgated by EPA and local regulators - coupled with opportunities related to the cap-and-trade approach to emissions control - are also incentives to acquire better CEMS. And looming mercury (Hg) control requirements mean utilities will not only have to reduce mercury emissions, but measure, record and quantify those reductions using the best technology available.

“If you have a good SO2 system installed, you can reduce your lime cost on an FGD,” says Rich Hovan, environmental products marketing manager for Forney Corporation, which has about 500 CEM systems installed at power plants in the United States.

“If you have a good NOx monitoring system on an SCR, you can control your ammonia injection and control temperatures better, which will extend the life of your SCR. That’s important,” says Hovan, noting that many state regulations have put an end to the ‘old NOx season’ (starting the SCR in April and shutting it down in October).

Some of the systems placed into service when SO2 limits were 500 ppm can’t reliably and accurately measure current SO2 emission limits, which can be as low as 50 ppm. NOx detection limits are falling as well. “Ten years ago you were looking at 30 to 100 ppm NOx levels,” says Dave Vojtko, national sales manager for Horiba Instruments. “Now people are concerned with sub-10 ppm levels - more like 2 to 3 ppm for NOx. New analyzers are now fabricated with much more inert materials with increased detection capabilities and employ surface mount electronics.”

Frank Duckett, project manager for Thermo Electron, one of the world’s largest suppliers of continuous emission monitoring analyzers, says that while most companies are not presently installing new systems per se, they are replacing old analyzers at a fairy robust rate. “The analyzers installed in the early 1990s are 15 years old and that’s about the lifetime for an analyzer,” he says. “We’ve reached a period where technology advancements in analyzer sensitivity combined with lack of spare parts for old ones are driving a need for replacements. Operators can’t maintain what they have so we see a big push to get newer instrumentation.”

Maintenance represents a significant amount of the expense associated with owning and operating a monitoring system. One of the most time-consuming tasks involved in maintaining a CEMS is reviewing and recording daily CEMS checks. An estimated 50 to 70 percent of manpower requirements for maintaining a CEMS are consumed recording daily system readings such as dilution air pressure, sample flow rates and analyzer lamp voltages. In addition, pumps must be repaired and filters replaced.

Probes must be maintained and absorbent chemicals replenished. Calibration checks must be performed daily, which requires the use of cylinders containing pure concentrations of whatever gases an analyzer must measure. Depending on the gases being measured, cylinders cost from $150 to as much as $3,000 for mercury calibration gas, and last about three months.

Many plants employ a maintenance staff dedicated to CEMS because they can’t afford the downtime if its CEMS is not working. Regulations forbid plant operation without fully functional continuous monitoring equipment. “It’s essential to keep them in top shape so you can keep the plant running,” says Mike Corvese, also a project manager at Thermo Electron. “Furthermore, the cap-and-trade programs make monitoring a bit more critical because it can determine how many credits and allowances a plant produces for its owners. That can equate to many thousands of dollars.”