Continued Advances in Natural Gas-Fired Generation

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By Robert Evatt

Some of the most recent advances in natural gas-fired generation were highlighted during a presentation during POWER-GEN International 2017.

Michael College, supervisor of surface water – project review with the Susquehanna River Basin Commission, said dry cooling technologies used in place of traditional cooling towers can convey numerous advantages to power generation facilities.

Of course, dry cooling uses much less water. While traditional cooling towers require 270 to 330 gallons of water per MWh, dry cooling towers can use as little as 10 to 20 gallons. College said the 1 GW Moxie Freedom plant was able to cut its needs from 7 million gallons per day to 500,000 gallons per day.

Though dry cooling also helps reliability, especially in areas where water is in short supply.

“During times of peak production, it’s the hottest, driest times in the basin,” he said. “Sometimes, you can’t produce because there’s no water available.”

Less water also results in less infrastructure needs. Additionally, power producers aren’t as tied to areas with abundant water and can select a greater variety of plant sites.

One surprising side effect of dry cooling has been a more favorable reception to potential power plants from the public.

“If you’re using significant amounts of water, you’ll get a different reaction from the public,” College said.

There are some trade-offs with dry cooling, such as higher up-front costs and additional space requirements. Nevertheless, the Susquehanna River Basin now encourages developers to use dry cooling, and has streamlined the permitting process for dry cooling equipment.

Chris Corron, product manager of 60 Hz plant and integrated systems at GE Power, said single-shaft combined-cycle gas units have become much more popular in North America, especially as the technology has advanced. With bigger gas turbines with capacities reaching 600 MW, a single-shaft setup now has the same efficiency as 2x1.

“You can produce more steam and have more steam on that shaft, giving you more efficiency with F-class,” he said.

Not only are single-shaft units less expensive due to less hardware, they’re also capable of ramping up faster. Corron said developers need to take rapid cycling into account, especially with the rapid rise of renewables and their intermittent generation times.

“A plant that will be in service for 30 years will have to be able to adjust to changing market conditions,” he said.

The rise of electric vehicles could also influence future power generation needs, as Corron predicted most of them would be charged in the evening hours when people return home from work.

Since steam turbines require some time to match temperatures and limit stress on equipment, GE has developed a rapid response system that allows the gas turbine and the steam turbine to ramp up independently. As a result, both can reach baseload in as little as 22 minutes.

Bonnie Marini, director of product line management at Siemens, said her company has continually adjusted its product line offerings as generation demands change.

“The market’s changing. There’s all kinds of needs out there, so we need to make our power plants do things we haven’t done before.”

That includes rapid cycling. Marini said uneven heating and cooling of turbines can lead to different levels of material flexing and hard rub if operators aren’t careful. To eliminate that problem, Siemens has incorporated a hydraulic clearance optimization that moves toe rotors slightly aft to give them more room while ramping up. Once the optimal temperature has been reached, the rotors move back into their original position.

Another recent innovation is clean ramping. Emissions are typically heavier during ramping, which wasn’t a problem in the past when ramping happened less often. But with renewables, ramping happens more frequently – Marini said the Bayonne Energy Center performed 1,042 starts in 90 days due to frequent demands from New York’s subways.

Clean ramping carefully adjusts the amount of ammonia injected into systems during ramping to continue emissions control efforts without under or over-injecting the gas.

Siemens now has the option to deliver plant components in pre-assembled blocks that can snap together. This modularity reduces the need for skilled labor on-site, shortens production schedules and increases plant safety.

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