Downtime duration dictates different ‘agility’ decisions

The common theme through every recent visit to a combined-cycle (CC) facility is agility—adapting to the real-time pricing dynamics of grid markets and growing renewable-energy penetration by investing in agility options. But the specific goals for each site, and how they are achieved, can be markedly different. Because the state has the toughest emissions laws in the land, California stands apart.

But at least the emissions limits are fixed. Everything else about operations at today’s CCs is unpredictable.

CCJ ONsite recently visited Pacific Gas & Electric Co’s (PG&E) Gateway Generating Station, near Antioch, Calif, and chatted with staff from headquarters and the utility’s Colusa Generating Station about a two-hour drive northeast.

Like most CCs designed and installed about five or more years ago, high efficiency was the goal because the units were expected to run at high capacity factors (CF). Based on the last few years, “expected” has all but disappeared from the plant lexicon.

Last year, 2015, was a low hydro year for PG&E—less than half of an average water year. Gateway and Colusa picked up the slack. Colusa’s capacity factor was 62% (full capacity, with duct burners), Gateway’s slightly higher. The GTs at Colusa had only about 50 starts each, and Gateway had about double that number. In some months, the units started up only once or twice and remained on for days. In other words, the units ran more like they were originally intended.

During the prior two years, Colusa’s CF was more like 43% or 44%, and in 2013 the Colusa machines had double the number of starts that they did in 2015. This year, report Alvin Thoma, director of fossil and solar O&M, Tim Stafford, Gateway’s acting plant manager, Ed Warner, Colusa plant manager, and Steve Royall, director of hydro generation O&M, the common pattern is the Colusa units are running regularly but are frequently scheduled to cycle off during the middle part of the day—exactly the inverse of what is traditionally thought of as the most important time to be online.

One common (though not consistent or predictable) load profile has been to start up and ramp to full load between 3 and 5 p.m. as the sun wanes and people return home, and then back off to minimum load or 1 × 1 operation in the middle of the night. Starting about 5 or 6 in the morning, the GTs are ramped up at between 14 and 17 MW/min up to full output, only to be shut off entirely by 9 or 10 a.m.

More recently, the dispatch schedule from the ISO has been wrong as often as it is right, with units called on exceptional dispatch to run when not scheduled, and to shut down when scheduled to run.

Colusa and Gateway aren’t yet experiencing two-start days, but the PG&E operations staff expects that mode is coming as the utility keeps pace with the state’s push towards the renewables mandate of 50% in 2030. Today, PG&E’s renewable energy supply is already nearly 30% (including hydro).

Emissions before electricity. No one awake in the industry would be surprised to learn that both PG&E plants have to meet annual, quarterly, and daily NOx and CO emissions levels. But the “event” limits may not be common knowledge.

Gateway and Colusa both have to adhere to separate hot-start, warm-start, and cold-start event emissions limits. On top of this, Colusa has to meet a hard limit for each hour of the startup period as a permissive to proceed through the next stage, or hour, of the start. Because Colusa is a more recent facility (coming online in 2010), its emissions are more constrained. “Colusa has one of the most restrictive permits ever written,” said Warner.

At Colusa, the maximum NOx emissions allowed during a hot start are 152 lb/hr; the cold-start limit at Gateway is 452 lb/start. The PG&E plant officials stress that if there is any indication during startup that this total will be violated, the plant incurs a forced outage instead.

“Under no circumstances do we violate the permit, even if the grid is strained,” insisted Thoma. In other words, emissions violations are treated like safety issues. Plants with the best safety “culture” give their facility staff the right to stop production if there is any safety concern, with no recriminations. PG&E also applies this rule to emissions.

NOx and CO emissions limits at Colusa are 2.0 ppm and 3.0 ppm, respectively.

Electric blanket comfort. GE presented PG&E with the usual menu of agility options—OpFlex, new seals pack, NOx reduction, advanced steam path design, steam turbine heating blanket, and others. Operations personnel found that considering the options in the context of different downtime periods between starts was helpful to the analysis. Different options have different value points depending on whether the unit is down for six hours, overnight, 24 hours, or over a weekend.

So far, Colusa has gone with OpFlex and the heating blanket (photo). The plant expects the technology saves up to an hour during cold and warm starts, when deployed, and keeps hot starts under an hour (from over 80 minutes prior to having the technology). The plant has only had a few opportunities to use the blanket to date, but it has “performed as expected” when in service, noted Warner.

Steam-turbine heating blanket at Colusa is key part of unit agility, shown at the high-pressure turbine section

Steam-turbine heating blanket at Colusa is key part of unit agility, shown at the high-pressure turbine section

In addition to keeping the turbine warm, the equally important purpose of the blanket is to heat the top and bottom casings uniformly, within 50 deg F of each other. To do this, heating is precisely controlled through a programmable logic controller (PLC) system. Other than that, “just think of it as an electric blanket (electric heating elements inside conductive material) underneath the turbine’s normal insulation layer,” Warner suggested.

It’s a little more complicated than that. There are 24 low-voltage/high-current control zones on the shell, such that heating starts at the high-pressure end at 650F, at 975F at the inlet/barrel section, and at 550F for the intermediate section. Those are some impressive temperatures for resistance heating.

The real value of the technology will come as ramping and cycling needs grow in line with meeting the ever higher renewable-energy mandate. For example, when the plant is shut down for an entire weekend, or over 60 hours, the blanket will keep the steam turbine between 500F and 700F. In essence, with the combination of the blanket and Opflex, the steam turbine is no longer the limiting factor to ramping to full output.

Keeping the turbine components warm also reduces stress caused by thermal cycling. Fuel savings are considerable too, 66-million Btu per start. At a $3/million Btu price for natural gas, that’s almost $200,000 in a 100-start year. And in a state where every molecule of emissions counts against you, avoiding an average of 52.5 lb of CO and 75 lb of NOx per start is a critical bonus. The price tag for the heating blanket was south of $1 million, suggesting that the benefits will quickly pay back the investment.

OpFlex, explains Royall, is a technique to take advantage of the conservative design factors included in the original automation package. In other words, the control sequence is tweaked to capture as much of the margin as possible without sacrificing component life. The GE engineers worked closely with the plant operations staff to refine and improve the startup process.

BOP considerations. PG&E undertook an exhaustive evaluation of cycling at Gateway and Colusa from a reliability perspective, especially under a two-start-per-day regime. Many recommendations fell out of that effort.

One aspect of the effort was to rank the components from a spare-parts perspective. No surprise, the boiler feed pumps came out on top. The original design of Colusa called for two 70% capacity pumps. As a result of the study, a full motor/pump/hydraulic coupling will be warehoused onsite. “We can’t run at full output with a 70% capacity pump,” said Thoma.

Colusa also is a zero liquid discharge plant (Gateway is not), which aggravates cycling objectives. One thing that resulted from a comprehensive review of the water/steam system was that when the plant breaks vacuum on a start, you have to re-establish the water/steam chemistry. The plant had additional steam drains to accommodate higher flow but found that they could surpass the temperature limits of the ultra-filtration units. Therefore, they added a fin fan cooler designed for the heat rejection from the additional blowdown.

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