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What is a Continuous Emissions Monitoring Systems (CEMS)?

September 11, 2020

Table of Contents
Smoke stacks
Air emissions like CO, CO2, NOx, SOx are some of the pollutants that contribute to global warming and acid rain.

What is a CEMS?

CEMS, or Continuous Emissions Monitoring Systems, are a traditional hardware-based solution for monitoring and emissions, where air samples are collected from the source using sensors and measured by physical analyzers. The different types of CEMS can be categorized into Dilution, In-situ and FTIR CEMS.

The Components of a CEMS
Diagram 1: Displays the typical components of a CEMS.

No matter the type of CEMS, they all have similar components as depicted in the diagram above. The sampling system usually includes:

Probe: A blunt-ended instrument used to sample the exhaust gas exiting the smokestack.

Heated Sample Line: A chemically inert line used to direct the sampled gas from the probe to the gas analyzer. The sample line is usually heated to maintain the temperature of the sampled gas to be as close to the temperature in the smokestack. This is necessary to ensure an accurate reading of the pollution concentration.

Calibration Line: According to US EPA CFR 40 Part 60 and 75, sensors of the CEMS are required to be calibrated daily. Calibration helps ensure the accuracy, linearity and stability of the sensors are maintained. The calibration line is connected from the standardized calibration gas tanks to the probe at the smokestack.

Pump: A built-in or separate pump brings in the sampled gas to the gas analyzer from the smokestack via the sample line.

The next component is the actual measurement system. The gas sample from the smokestack is passed through a conditioning unit. The conditioning unit helps remove any excess moisture in the gas depending on the measurement system used - wet or dry. The concentration of the conditioned gas sample is then measured and reported to an UI Dashboard on a monitor if there is one.

Distributed Control System (DCS): A platform for automated control and operation of a plant or industrial process. A DCS combines the following into a single automated system: human machine interface (HMI), logic solvers, historian, common database, alarm management, and a common engineering suite.

Data Acquisition & Handling System (DAHS): Measurements of the sensors are sampled, converted to digital values, and stored in a computer, or by a standalone device for further storage and analysis.

The DCS and DAHS are interconnected with the CEMS. The CEMS may come with its own DAHS or it can tap into the facility's DAHS via the DCS to store the emissions data. It is pertinent that the CEMS is compatible with the communication protocol of the DCS for smooth data transfer. The common communication protocols use OPC DA, OPC UA, Modbus, OSI PI.

CEMS has been the industry standard for decades, but these systems require highly specialized hardware and trained technicians to implement, operate and maintain. This results in prohibitively high capital and operating expenses,  presenting a major barrier of entry for smaller facilities.

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