Continuous Emissions Monitoring System (CEMS)

By Michael Lee, PE | Senior Engineer, Electrical Engineering, Instrumentation and Controls, Dallas

Upgrading aging Continuous Emissions Monitoring Systems (CEMS) in fossil generation power plants is important for maintaining environmental compliance and operational efficiency. This technical article details best practices for planning, design, and installation intended to deliver a reliable, long-lasting system while limiting disruption to plant infrastructure and operations.

The Monitoring Mandate

Generating electrical power is an essential utility that facilitates the daily operation of our modern-day society. Without it, the present day first-world standard of living, as currently structured, would not exist. From communications and transportation, to manufacturing and healthcare, our society is built upon the provision of reliable, inexpensive energy. Along with maintaining successful facility operations, electrical utilities are mandated to provide a record of greenhouse gas emissions for government review, oversight, and recordkeeping. Electrical generation utilities can maintain commercial operation only by complying with this federal requirement, which includes using a Continuous Emissions Monitoring System (CEMS).

Maintaining CEMS compliance for emissions monitoring allows fossil generation plants to focus on maintaining and upgrading critical facility infrastructure to serve the community effectively. As with any system that provides monitoring capabilities, it is paramount to be mindful of the average lifecycle of CEMS equipment components and obsolescence schedules. Specifically, as equipment within fossil generation plants ages, challenges may arise in maintaining compliance with the mandated environmental requirements. To mitigate these challenges and keep systems in optimal operating condition, upgrades made to existing CEMS are pre-planned during scheduled construction outages, to avoid unexpected component failures. Upgrades also provide an opportunity to harness the most recent developments in emissions monitoring technology. Therefore, thorough planning and evaluation ahead of time is required upfront to deliver an accurate, long-lasting CEMS.

Embarking on a Complete CEMS Overhaul

Older CEMS in plants can become problematic as replacement parts grow scarce or unavailable due to obsolescence. Rather than attempting to locate these individual components to maintain the existing system, it is more cost-effective for utilities to upgrade the entire CEMS with new equipment, including components such as the exhaust gas umbilical, programmable logic controller (PLC), and sample conditioning panel. Using an Issue for Bid (IFB) specification, an owner can competitively bid equipment procurement cost among multiple CEMS vendors. When written effectively, this IFB specification can clearly outline the material requirements and equipment installation constraints associated with the existing facility infrastructure. In addition to alleviating existing maintenance issues, a new system can maintain the power plant’s compliance with current reporting and certification requirements as governed by the U.S. Environmental Protection Agency (EPA).

Complete system upgrades require significant coordination and planning to achieve success. Given the commercial impacts of extended downtime, a thorough IFB specification, an accurate engineering detail design package, and a comprehensive project milestone schedule are vital for reducing outage duration and optimizing system functionality during the transition to in-service operation. Preparation begins with understanding system deficiencies and determining if increased monitoring capabilities are required. Whether the project scope dictates an in-kind equipment replacement or a system functionality augmentation, it must be clearly defined and agreed upon by all project stakeholders at the start of the detail design phase.

For example, if an existing system has an outlet stack port monitored by the CEMS, but no existing turbine exhaust port monitoring capability, the utility may want to augment monitoring capabilities. This means there would now be two different monitoring components, the stack and the turbine exhaust, and each would require CEMS equipment (one sampling conditioning panel and an exhaust fan per connection point). Oftentimes, the newer quick start gas combustion generation units will have both turbine exhaust and stack monitoring ports present. Depending on the number of monitoring point connections, the CEMS equipment vendor will have to furnish the appropriate quantities. Also, evaluating available unoccupied space inside the existing CEMS shelter is critical to confirm sufficient room for installing new equipment, especially when adding monitoring point connections.

Best Practices for Coordination and Installation

To identify potential scenarios which could impact design scope, a comprehensive walkthrough of the CEMS shelter, stack, and turbine exhaust port locations of interest is required. This walkthrough is accompanied by a full review of existing CEMS design drawings, including plan views, electrical schematics, I/O lists, loop drawings, instrument lists, and electrical panel schedules.

During this process, existing shelter dimensions, proposed design components, data communication preferences, existing infrastructure such as cable trays, calibration gas bottle racks and slab dimensions, and future power requirements are verified.

This Review Approach Involves Considerations, Including:

• Developing an accurate and detailed equipment specification package, covering hardware and software. This includes the provision of required analog and digital signals for process conditions such as temperature, pressure, flow, and chemical concentration. Determining the existing hard and soft I/O types and quantities and including any new I/O that will be added. Any new PLC will have to be specified to account for all necessary monitoring and control signals, including signal conversion for any existing systems that will need to communicate with the new PLC. Taking a full inventory of the condition of all existing process instrumentation and include instrument replacement in the project scope, as needed.
• Investigating the existing stack and turbine exhaust flange orientations. If existing orientations differ from the newly furnished probe boxes, new flange adapters or welding modifications to the existing flanges will be required. If the utility client or construction contractor does not have the capability for specialized welding services, coordinate with a third party that can provide the required welding service support.
• Verifying the current condition of the existing stack and turbine exhaust ports. If screw threads exist, but are not in a satisfactory state, the threads will need to be reconditioned for the installation of the new probe boxes to be successful.
• Verifying the new umbilical (sample) line lengths, the new cable tray design as required, and specifying the proper sample line fastening materials, which must be outdoor rated.
• Sample lines will be pre-manufactured with a thick outer casing of insulation material and an inner diameter composed of tubing lines for transporting the exhaust gas while confirming the new sample line outer diameter (of insulation) removal is minimized for the termination of the polytetrafluoroethylene (PTFE) tubing and electrical conductors to the sample conditioning plate inside the CEMS shelter. This will minimize the chance of condensation forming within the new sample line when in operation.
• Performing a load calculation for the existing CEMS shelter power panel, transformer, and motor control center (MCC) cubicle to account for the power requirements of the new CEMS equipment (sample conditioning panel, umbilical line, Nox exhaust fans, PLC). Once calculations are performed, a new, upsized power panel may be required to accommodate increased current load and additional circuit capacity.
• The full run of the existing power cable will require a sizing calculation to verify that it is sufficient for the new, potentially increased, CEMS equipment load ampacity. If sizing calculations indicate the need for additional capacity, new power cable will need to be furnished, routed, and terminated accordingly. Be mindful of any required civil work for trenching if new upsized feeder cable must be routed from the MCC to the CEMS shelter.
• Taking adequate measures to secure new nitrogen oxide (Nox) exhaust pumps, located inside the CEMS shelter, to limit vibration and possible risk of damage to nearby equipment.
• Determining the requirements for the existing CEMS shelter bulkhead includes evaluating whether the existing bulkhead can be reused or must be replaced. During the design phase, verifying the bulkhead dimensions and penetration hole diameters is essential to confirm that any new vent, sample line, gas tubing, or instrument cable can be routed through the bulkhead.
• Determining whether the generation utility prefers a specific calibration gas bottle configuration layout, such as using individual solenoids for each bottle or routing multiple quarterly (linearity) gases to a single solenoid. The selected approach can reduce routine maintenance activities for plant personnel. Calibration gas bottles must also be protected from direct sunlight, which may require fabricating and installing a new sunshade if one is not already in place. Depending on the types and quantities of bottles required, structural scope is recommended to extend the CEMS shelter concrete pad to accommodate more bottles. A modified bottle rack will then need to be designed, furnished, and installed to secure all bottles in close proximity to the CEMS shelter.
• Confirming that the client IT personnel coordinate the timely configuration of ethernet switch ports and communication protocols before commissioning to facilitate the transmission of reporting data between multiple control systems. Failure to complete this step in advance will negatively impact the construction schedule.
• Creating comprehensive supporting design documentation, such as a control narrative, alarm responses, and a CEMS system description. This documentation provides critical guidance for plant personnel during maintenance and troubleshooting and serve as a reference for future engineering service providers, equipment vendors, and contractors during subsequent CEMS upgrades.

By successfully executing these design elements and completing all deliverable documents, the installation, commissioning, and start-up of new CEMS can be achieved effectively. Following best practices during project kickoff, the IFB specification effort, and the detail design phase allows engineering service providers to deliver long-lasting, safe, and efficient CEMS capable of reliable environmental monitoring for electrical utilities.

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