Powering up with a let-down turbine

Installing a turbine where steam pressures are being dropped generates power and reduces carbon intensity. This article appears in the March issue of EPM with the headline, "Powering up by letting down."
By Susanne Retka Schill | February 02, 2016

Arkalon Energy has been generating nearly one-third of the electrical needs for the 110 MMgy plant since start-up eight years ago. “When they were planning the facility, ICM offered the option,” explains plant manager Richard Hanson. A let-down turbine was installed in the steam line between the boilers and the evaporators. “We’re using the steam twice,” he says. “Once to turn power and then to do its job in evaporation.”

The technology is not new, says John Graham, proposal development engineer with the Dresser-Rand business, part of Siemens Power and Gas. The turbines are widely used in refineries and combined-heat-and-power projects, and have been used in the ethanol industry since the early 2000s. “I think we’ve installed about 50 steam turbine generator sets in various corn-to-ethanol plants in the U.S. and a few in other countries.” He adds, the interest was higher for a while and dropped off, most likely because all those who benefited most had already installed turbines, but it once again is building. After a few years of few inquiries, interest is building again. “From my experience the plant has to produce at least 50 MMgy to have a large enough volume to make a steam turbine practical,” he says.

Most plants he’s worked with produce steam at 150 pounds per square inch, the highest pressure needed in the process, while the lowest pressure required is near-atmospheric for the evaporators. Rather than releasing pressure through a valve, those plants can install a turbine, called either a let-down or back-pressure turbine. A 50 MMgy plant generally has enough steam for a 1 MW turbine that can generate up to 30 percent of its electrical power. Graham adds that it isn’t exactly free, however. “All of the energy that the steam turbine takes out has to be made up with additional fuel burn in the boiler in order to have the energy available in the exhaust steam needed for the evaporator. It isn’t free power, but generally it’s very inexpensive.” The steam turbine takes 10 to 20 percent of the heat energy out of the steam, he explains.

One ethanol producer took the idea one step further, Graham adds. “They generate steam at higher pressure than they need in the process and use a topping steam turbine to drop all of their steam down to the higher process pressure needed. Then, they use a separate stream turbine to drop the steam to the lower process pressure required by the evaporator.” 

As plants expand, Dan Sonnek, principal with IntegroEnergy Group Inc., recommends they consider installing an oversized boiler. “Right now they’re running 150-pound boilers,” he explains. “We may spec out a 400-pound or 600-pound boiler so we have additional steam pressure that we can spin a turbine with and make more electricity down the road.” The boiler is run at a lower pressure until the time when electric costs cross a line at which it will pay to add the turbine.

How quickly a turbine gives a return on the investment (ROI) depends largely upon a plant’s cost of electricity. A 1 MW turbine costs about $1 million itself and the entire project, with auxiliary equipment and installation can range from a total of $1.5 million up to $3 million on the high end, Sonnek says. “If they’re still running on a 5-cent per kWh electric contract, it will take a long time to pay it back: It’s not making the electric bill go away, it’s just reducing it some. If you’re just dropping it 15 percent, it may not justify it. You may be able to spend that money elsewhere and make more ethanol.”

Typically, plants aren’t interested unless the ROI is 2.5 years or less, he adds, the exception being plants that are trying to reduce their carbon intensity score to ship ethanol into the California market. The number of facilities approaching that ROI is increasing as many plants are seeing their electric costs go up each year.

There is a difference between plants served by investor-owned utilities (IOUs) and those getting electricity from rural electric cooperatives (RECs), Sonnek continues. Many ethanol projects were welcomed by the RECs because the plants added to their base load, and evened out demand.

Because RECs generally do not generate their own power, they often view a plant’s interest in installing a turbine as a negative. Many IOUs have a different perspective, Sonnek says. “The IOUs have more incentives for this type of thing. They’re more open to it because they generate themselves. At a soybean facility, we actually got money back from the investor-owned utility as incentive to install the turbine, to reduce their electric load. They like having the distributed generation throughout their system.”

Among the services his engineering firm offers to plants is to work with the utility. “We never advocate making the utility go away, because if something happens to our system, we still need the electricity. We don’t want to shut down. We want to be able to work with the utility.”

Installation, Maintenance
Installing a back-pressure turbine requires proper planning. “It’s not just taking a valve and replacing it with a turbine,” Sonnek says. Integration with the plant and the interconnect with the utility are key to maximizing the electrical output. There are added piping, controls, safeties and switch gear, plus the system has to be designed to handle turbine condensate. It’s also important the turbine controls and the front end are set up correctly, he adds. “If the plant backs off its run rate, the turbine can become very, very inefficient, almost insignificant. We’ve addressed some issues with that, and there are ways to make them efficient on the lower end.”  

Maintenance on the turbines is much like anything else in the plant, says Hanson about Arkalon’s system. “It’s there turning seven days a week, 24 hours a day, so there’s things we check such as vibration analysis of bearings. There’s some yearly maintenance. We did a three-year tear down of the turbine and rebuilt whatever needed to be and we just completed our five-year turbine and generator rebuild. So, there is some expense, but it’s no different than anything else in the plant.”

Sonnek adds that back-pressure turbines are a good way for plants to get into generation. “It’s a fairly simple operation and fairly simple install and they can get used to having a generator on-site,” he says. He got his start working in the wet milling industry, where full-scale cogeneration systems are the norm. Ethanol plants, with their 24-7 power loads, are considered prime candidates for cogeneration through combined-heat-and-power installations. A back-pressure turbine is a good first step, Sonnek says. “They’re all going to be capable of it. You can see the maturity in the ethanol industry with operations, with management. They’re getting better control, better analysis of their system at all times, and that’s what you need when you get into generation.”

Author: Susanne Retka Schill
Senior Editor, Ethanol Producer Magazine
[email protected]