May 15, 2007

Focus on O&M (May 2007)

Use of alternative fuels

In the past, burning fuel oil or another low-grade fuel in a combined-cycle plant was common only in countries where environmental regulations were not as strict as in the U.S. or EU. Today, higher-grade distillate fuels are being used during part of the year in many new plants. Firing waste fuels in gas turbines' duct burners has become more common, and firing gasified pet coke or coal could follow.

Using nondesign fuels can significantly increase the potential for gas-side problems, such as corrosion and deposition. Burning fuel oil in duct burners raises radiation heating and the potential for hot spots and liner damage. For example, stress-corrosion cracking of cold-end preheater tubes has become more prevalent. Although burning even clean natural gas can produce the phenomenon, burning fuel oils and gasified fuels raises the risk of occurrence. Figure 9 shows the deposits on the preheater tubes of a plant that burns a low-Btu, high-H2S gas.

9. Asking for trouble. Preheater deposits caused by burning low-Btu, high-H2S gas. Courtesy: Tetra Engineering

Extended lay-ups

During periods of low electricity demand, many combined-cycle plants are put on extended standby, using wet or dry lay-up procedures. HRSGs' experience with wet lay-up has generally been good because following modern industry guidelines can prevent serious corrosion problems.

For the other components of a combined-cycle plant, the greatest risk of an extended lay-up has been localized freezing, if the plant is in a cold climate. Many plants in extended lay-up have seen their tubes, headers, and piping freeze. Sometimes the damage was not immediately apparent and was discovered only during subsequent start-up. Figure 10 shows an area of economizer drains that is very difficult to protect from freezing.

10. Drains not in Spain. Components at high risk of freezing, such as these drains, must be identified and protected during extended lay-ups. Courtesy: Tetra Engineering

Increased use of wastewater

Many combined-cycle plants now take their cooling water from lower-quality sources, such as processed municipal wastewater, agricultural run-off water, or brackish wells. This makes it more difficult for the facility's water treatment plant to provide adequate supplies of water at various qualities to meet operating needs. Water treatment difficulties have increased waterside contamination and corrosion in evaporator tubes at several plants. Figure 11 shows underdeposit corrosion damage to the tubes of a high-pressure evaporator.

11. Hidden problem. Underdeposit corrosion damage in a high-pressure evaporator. Courtesy: Tetra Engineering

Tighter emissions control

Pollution standards haven't just been ratcheted down for operating plants. Increasingly, the lower levels also must be met during start-ups and shutdowns. The performance demands on selective catalytic reduction systems for NO_x control and on CO catalysts have increased, as has the risk of ammonia slip and deposition.

Supplemental firing systems also must comply with tighter pollution standards. The need to do so affects how they are fueled and their turndown ratios. The new focus on start-up and shutdown emissions is even affecting newer combined-cycle plants with steam-cooled gas turbines.

Figure 12 shows the dependence of CO emissions on load at a new power station. The high levels produced during start-ups limited the operating flexibility of the entire plant.

12. Roller-coaster ride. CO emissions are very dependent on turbine load. Courtesy: Tetra Engineering

—Contributed by Peter S. Jackson, PhD, PE (peter.jackson@tetra-eng.com), David S. Moelling, PE (dave.moelling@tetra-eng.com), and James W. Malloy of Tetra Engineering Group Inc.