An Assessment of the Long-Term Performance of Gas Continuous Emission Monitoring Systems

A field test program was conducted to evaluate the long-term performance of several gas continuous emission monitoring systems (CEMS).* This paper presents the results gathered on the long-term accuracy and calibration drift characteristics of ten CEMS installed at scrubber-controlled, coal-fired power plants. The program involved periodic accuracy audits and a review of available calibration drift data for selected CEMS. Accuracy audit results show that both SO2 and NOx CEMS are capable of providing accurate data on a long-term basis. However, frequent audits are necessary in order to verify the performance of an individual CEMS. The results of the calibration drift data evaluation show that despite infrequent occurrences of excessive drift, CEMS operated with a significant bias for extended time periods simply because corrective action was not taken in a timely manner.