What Is CEMS Used For?

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Thread Starter

Daneezy

What is CEMS used for and how does it operate in Power Generation gas turbines (GE Frame 9e)?

Does it have any relationship with the control system?
 
The continous emmissions monitoring system is used to monitor the output of the exhaust stack. The most common parameters monitored are NOx, SOx, CO and O2. There is no required direct link to the control system. However the values are used to modify the temperature control constants to acheive the required emissions compliance. The other common use is for monitoring and reporting emmissions violations to the EPA.
 
Just some clarification.... In general, a CEMS (Continuous Emissions Monitoring System) is a means of monitoring emissions from an engine or a stack exit or anywhere in between. Some sites monitor gas turbine exhaust (before it enters either a HRSG (Heat Recovery Steam Generator: a boiler) or a stack, or some kind of secondary emissions treatment/reduction system. Some sites with a secondary emissions treatment/reduction system have a second CEMS that monitors the stack exit. This way they can determine the effectiveness of the secondary system on overall emissions.

Many times some air monitoring/control/regulatory body requires submission or archival of emissions data, and the CEMS can do this or some other data archival and retrieval system can record the data from the CEMS for future reference/review. (The "EPA" is likely a reference to the US Environmental Protection Agency, just one of many such regulatory bodies in the US.)

In the past, CEMS data was NOT directly used by the GE-design heavy duty gas turbine control system for modification of control parameters affecting emissions. Data/values from the CEMS would be manually analyzed and emissions reduction parameters could be manually modified to achieve desired emissions levels (within limits).

Sometimes, CEMS outputs are connected to the Speedtronic turbine control system for monitoring/display purposes only--that is, they are not used for control purposes, but only to be able to see emissions data on a GT HMI display. (This is usually done using 4-20 mA signals, but can also be done via MODBUS or some other communication protocol depending on the vintage and options of the Speedtronic turbine control system.)

Newer DLN (Dry Low NOx) reduction systems use some CEMS outputs for direct modification and control of some operating parameters affecting emissions. But, in general, CEMS are very slow to output changes--that's because they have to draw a sample from their ports in the GT exhaust or stack through long tubing runs, analyze the sample and output the values, then purge the tubing before drawing another sample to be analyzed. Technological advances have improves response times from several minutes to less than a minute--but this is not sufficient for some operating conditions or some regulatory bodies.

CEMS are generally not provided by the packager of GE-design heavy duty gas turbines. They require lots of maintenance, calibration gases (which means changing high-pressure gas cylinders on a periodic basis--which requires purchasing these gases and lots of paperwork, and sometimes, recalibration of the CEMS depending on the gas sample composition(s)), and, usually, lots of software upgrades. They are temperamental and some are extremely unreliable, while others are very reliable, but still require maintenance and attention which consumes manhours (money).
 
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