Emerson Exchange 365

XcelEnergy Comanche Generating Station is the largest power plant in Colorado, with a power production capability of 1,410 megawatts. The facility generates power with steam-driven turbine generators using low-sulfur coal.

During the 2016 Emerson Exchange Americas in Austin, David Fern, with XcelEnergy, joined Trevor Anderson and Matt Brummer, from Emerson, to discuss feedwater heater efficiency principals and the importance of upgrading heater level controls to improve efficiency.

They also shared information about the positive environmental influences the facility made by adding baghouses – fabric filter air pollution control devices –  to all three of its generators, to remove particulate emissions from the flue gas. All units also have low-nitrogen oxide burners to control NOX and lime-spray dryers to control sulfur dioxide emissions. “In addition, the facility uses activated carbon injection to control mercury emissions on all three units,” Fern said. “In fact, the Comanche Station is the first plant in Colorado to control mercury emissions.”

Fern also described how heat rate is the measure of energy put into a system used to calculate how efficiently a coal-fired power plant uses the heat content of the fuel. This heat rate is expressed as the Btu of fuel heat required to produce a kilowatt-hour of electricity in units of Btu/kWh. A feedwater heater draws a portion of the steam from the turbine and uses it to preheat the water entering the boiler, and raising the temperature of the feedwater reduces boiler fuel consumption.

“Feedwater heater level control is critical for heat rate efficiency and to prevent costly equipment damage,” Fern said. A 1-percent reduction in heat rate can generate $500,000 in annual savings for a 500 megawatt power plant. “Improving feedwater level controls will also help you realize increased reliability and accuracy,” he added.

A recent ASME standard recommends triple redundancy for the level measurement of feedwater heaters, and level measurement capabilities tied into the main control system. Once the Comanche Generating Station installed an Ovation DCS, with a standard guided wave radar (GWR) and global performance advisor (GPA), as well as a high-performance GWR with vapor compensation, it met ASME recommendations. XcelEnergy also recognized a 2-degree F increase in a single heater per train. The plant has plans to upgrade its remaining two heaters in each train to allow for expected 6-degree F increase. This will significantly reduce heat rate in the final feedwater temperature –resulting in a substantial fuel cost savings, as well as an 8-month ROI for the project.

Using a level measurement with GWR means the level measurement is independent of density changes. “It can handle high pressure and temperature, has no moving parts so there is no need for calibration and zeroing, and is generally unaffected by changes in the vapor space,” Fern added.

Benefits of the GPA allows operators to track equipment against design specifications, calculate heater terminal temperature differences, and drain cooler approach temperatures when needed.

Since upgrading to this new system, XcelEnergy has reduced risk of equipment damage and dramatically improved heat rate.