Operated and maintained by Dow Chemical Co., Freeport, Texas

Owned by Dynegy (50%) and American National Power (50%)

Sub-10 ppm NOx has been the dream of gas turbine users for many years, and currently there is only one OEM that can deliver on that promise. But OEMs are no longer the only suppliers of NOx retrofit hardware. Power Systems Manufacturing has developed a sub-5 ppm NOx retrofit kit for the GE 7EA that has taken dry low-NOx technology to the next level. Dow Chemical took a chance with the new technology and now operates the world’s cleanest—in terms of NOx—gas turbine.

By Troy Bearden and Curtis Banks, Dow Chemical Co.’s Texas Operations

Power 8 at Dow’s Oyster Creek site consists of three natural gas–fired GE Frame 7EAs and a 200-MW double extraction full condensing steam turbine originally commissioned between July and November 1994. The units are baseloaded and operate under power production contracts, although their primary use is to generate steam for Dow Chemical. The plant is capable of producing 440 MW. Dow supplies steam and power to the other plants at Oyster Creek and Plant A. Oyster Creek’s Power 8 is the newest of four powerhouses Dow operates at Freeport.

Oyster Creek Power 8 is not only the newest plant in the Dow Chemical fleet; it also has the lowest NOx emissions. The original dry low-NOx (DLN) 1.0 system ran about 12.6 ppm NOx and 3.3 ppm CO during normal operation at baseload. With the auxiliary duct burner–fired HRSGs, Power 8 is just barely going to make the NOx reductions required by state air pollution control authorities, assuming planned duct burner NOx reductions continue on schedule. The state NOx reduction plan requires a plantwide 80% reduction in NOx by 2008.

Lower NOx limits drove project

The project began over two and a half years ago when new Houston-area emission regulations were introduced to reduce NOx in incremental steps through 2008. For Dow, the lower limits meant it would have to make significant improvement in NOx reductions from its fleet of gas turbines and gas-fired HRSGs. After considering a variety of alternatives, Dow finally settled on standard selective catalytic reduction (SCR) with some form of ammonia injection. This approach was selected because the technology is well understood and has low technical risk. However, its installation and maintenance costs would be high, especially due to Power 8’s small site.

Power Systems Manufacturing (PSM) first began pitching its proprietary Low Emissions Combustor III (LEC III) early in 2001. But because the technology had no track record, the proposals didn’t get very far with Dow’s pragmatic operating staff. Dow’s refinery must operate reliably, and the possibility of any interruption of power and steam production was just too large a risk to take on an unproven technology.

In the meantime, PSM won a contract with TransAlta Cogeneration LP’s Fort Saskatchewan plant in Alberta, Canada. Fort Saskatchewan is a combined-cycle cogeneration plant that produces 120 MW of electricity and 100 tons/hr of process steam for a Dow Chemical facility. Prior to the turbine retrofit, the plant was having trouble getting NOx any lower than 17 ppm, and CO was fixed around 14 ppm. After installation of the LEC III system, EnTech Environmental certified 6-ppm NOx emissions and an average of 2.5 ppm of CO.

1. Low-emission combustor.

2. Quick change.

By the end of 2001, the replacement hardware was installed and producing 5.8-ppm dry NOx with no tweaking of fuel/air ratios. By September 2002, the hardware had run for 8,000 hours, maintaining 5.8 ppm NOx. A 24,000-hour major overhaul removed the hardware and returned it to PSM for refurbishment. Reinstallation is planned for September 2004. TransAlta was ecstatic not only about the results—the life increased to 16,000 hours of operation before refurbishment, compared with the 12,000-hour OEM limit—but also about the system’s 40% cost savings.

That was the kind of operating data that Dow needed to see to make a commitment to PSM for its LEC III technology. By January 2002, Dow invited PSM back for another sales pitch. This time PSM offered an 8-ppm guarantee for the simple reason that GE guarantees 9 ppm on the 7EA with DLN 1.0. Although the designs are significantly different in the way fuel and air are mixed prior to combustion; GE’s venturi in the liner is not 100% premix, whereas PSM’s low-emission combustor (LEC) is.

A deal was finally struck in November 2002 with several interesting requirements: the performance contract will include 8 ppm NOx as the guarantee point, but the design target will be 5 ppm, and PSM will work to reach that goal until both parties agree that no further reductions are practical. The schedule was tight; parts were due on the site on January 2 of this year. But because Dow requires that at least two of the three Frame 7EAs be operational at any time, other issues delayed the major outage until March 20, at which time the PSM hardware installation would be piggybacked on other planned major repairs. By April 8 the other repairs were completed and the LEC assemblies were installed. The turbine was buttoned up on April 11 and placed on turning gear that night, with startup scheduled for the morning.

Results count

Startup was uneventful, but the NOx readings caused quite a stir. Using default fuel splits of 81% primary/19% secondary, NOx readings were 6.75 ppm and CO was approximately 9 ppm at baseload power and 2,055ºF conditions. Tuning the splits over the next couple of days gave the engineers a better handle on the sensitivity of the system, and they arrived at a final split of 84%/16% with 4.75 ppm NOx and negligible CO using EPA-certified emissions test instruments within a week of initial installation of the LEC III—and meeting PSM’s design target of 5.0 ppm NOx.

This is an industry first for GE’s Frame 7EA turbine emissions control. Full turndown capability was maintained, and, at low part-load operation, the LEC system achieved emissions levels similar to baseload operation. Even more important, the performance of the LEC gave Dow the option to cancel the SCR, saving millions of dollars in capital costs and eliminating the complexity of an additional piece of hardware.

Dow Chemical Co.’s Curtis Banks says, “In the future, we intend to use PSM’s next-generation fuel nozzle technology and expect to see an additional 10% reduction in NOx, perhaps down to 4.2 ppm. I was also very impressed with PSM and its dedicated staff and the strict attention to detail displayed during the design and installation of this system. There was no extra down time associated with the LEC installation during our hot gas path inspection.”

Banks adds that, “as a matter of fact, I found out about PSM’s new LEC-III system by reading about it in POWER.” Perhaps that was the first step in Oyster Creek Power 8 becoming a Top Plant for 2003.

3. Low emissions.

4. Key staff.