Last updateSun, 06 Oct 2019 10am

Case Study: Yokogawa TDLS Helps AEP Coal-fired Power Plant Comply with Strict US Regulations

The American Electric Power (AEP) Clinch River power plant is located near Cleveland, VA which is approximately 500 km west of Richmond, VA. It generates electricity with three 237 MW coal-fired units. Owned and operated by AEP’s Appalachian Power Company subsidiary, the plant started operating in 1958.

Since 2004, AEP has invested more than $5 billion to retrofit the environmental controls at a number of its coal-fired power plants. Of this sum, about $2 billion has been allocated to this and other plants run by the Appalachian Power Company. This aims to reduce the emission of nitrogen oxide (NOx) and sulfur dioxide (SO2) from the burning of coal to generate electricity.

The Clinch River power plant is equipped with selective non-catalyst reduction (SNCR) systems. SNCRs are a refitted post-combustion technology that reduces the amount of NOx released into the atmosphere by injecting an ammonia (NH3) reagent into the flue gas. This induces a chemical reaction that breaks the NOx down into harmless nitrogen and water vapor.

Ammonia is a toxic gas and its emission contributes to ozone depletion. Ammonia slip, or the emission of ammonia gas into  the atmosphere, can occur due to incomplete mixing of the ammonia reagent with the NOx or the injection of excessive amounts of ammonia.  To avoid fines and shutdowns, it is desirable to keep fugitive ammonia emissions at or below the minimum levels mandated by the U.S. Environmental Protection Agency (EPA) and other regulatory bodies.

Downstream from the SNCR systems, it is also essential to hold the NH3 below 5 ppm and preferably to 2–3 ppm as this minimizes the formation of ammonium sulfate [(NH4)2SO] and bisulfate (NH4HSO4) on the preheater surfaces. Such deposits are hard to remove with sootblowers. A fouled preheater operates less efficiently and corrodes at a higher rate, leading to higher plant operating costs. In the worst case, the deposits can trap fly ash and cause a pressure drop in the preheater that can lead to a plant shutdown.

Additionally, while Clinch River does not sell their fly ash (for drywall, gypsum board, cement, etc.), some generating plants do, and the ammonia slip can contaminate the fly ash, giving it a strong odor and making it unfit for commercial sales. Excess NH3 can also increase exhaust opacity beyond regulatory limits, resulting in complaints and fines.

In 2006, the Clinch River power plant installed nine tunable diode laser (TDL) analyzers to measure ammonia slip from the SNCRs. However, these analyzers, which came from another vendor, did not provide sufficient availability to ensure that the compliance monitoring requirements could always be met. High maintenance costs and the accuracy of their readings were also operational concerns.

After conducting a months-long evaluation of several vendors’ TDL analyzers mounted on-site, AEP and Clinch River decided to install nine Yokogawa TDLS200 TruePeak analyzers to improve its monitoring of ammonia slip.

Study in Detail