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Cold climate could give Alaska LNG edge Lower ambient temperatures let liquefaction process run more efficiently, an advantage of some 10% over facilities in Middle East Stan Jones Researcher/writer for the Office of the Federal Coordinator
What is it that brings misery, numb toes and frozen pipes to Alaska residents but warms the hearts of people designing a liquefaction plant?
The state’s cold weather.
Low temperatures would make the natural gas liquefaction process and related systems more energy-efficient for the Alaska LNG project. Higher efficiency and lower operating costs are an important advantage in an intensively price-competitive global market for liquefied natural gas sales.
Besides reducing fuel costs, the improved efficiency can cut capital costs as some of the equipment doesn’t need to be as big and powerful.
The Alaska LNG project as currently envisioned would include:
• A giant North Slope treatment plant to purify the gas as it comes out of the ground.
• 800 miles of pipeline and eight compressor stations to move the gas to Nikiski, on the Kenai Peninsula.
• An equally giant liquefaction plant at Nikiski to supercool the gas, along with storage tanks and marine shipping terminal.
Oil producers on the North Slope have long benefited from the same cold-weather phenomenon that would boost the bottom line of an LNG project. The huge compressors used at the oil production facilities are more efficient in cold weather, sending thousands of barrels of additional oil down the line each day during the winter.
The general colder-temperature-efficiency principles are the same throughout the LNG project, from the gas treatment plant to the pipeline compressor stations to the LNG plant. But the liquefaction plant will cost the most to build of any of the project components and will benefit the most from Alaska’s low temperatures.
Just like at home A liquefaction plant works a lot like the refrigerator in your home, except on a vastly larger scale - imagine Godzilla versus a gecko. The plant’s assignment is simple, but brutal: Take vaporous natural gas coming out of a pipeline at ambient temperatures - somewhere between 0 degrees and 100 degrees Fahrenheit over most of the Earth’s surface - and chill it until it condenses and liquefies at minus 260 so it can be shipped on tankers to overseas customers.
In the end, the liquefaction plant and your refrigerator are both affected by the surrounding temperature. When it’s cold in your house, the refrigerator doesn’t use as much power to keep your lettuce crisp. Similarly, the lower the temperature outside, the less power needed to run a liquefaction plant.
How much difference does temperature make?
Quite a bit, as it turns out. A detailed analysis of temperature effects on energy use was done in 2012 for Snohvit, an LNG plant in the Norwegian Arctic. The analysis (“Exergy Evaluation in the Arctic Snohvit Liquefied Natural Gas Processing Plant in Northern Norway - Significance of Ambient Temperature,” by Anne Berit Rian and Ivar S. Ertesvag, Department of Energy and Process Engineering, Norwegian University of Science and Technology, 2012) concluded that Snohvit - operating in an ambient temperature of 39 degrees - was 11 percent more efficient than if the temperature had been 68, and 20 percent more efficient than if it had been 97.
“That heat removed from the natural gas has to go somewhere,” said Jim Wilkins, a senior technical professional and process engineer with ExxonMobil in Houston. “And that typically goes into a heat sink, and ultimately the heat sink is ambient conditions.”
(A heat sink is an environment that absorbs heat from a higher-temperature source. In the case of a gas liquefaction plant, the receiving environment for the heat is normally air or water. The substance giving up heat is a liquid coolant the plant uses to chill natural gas until it liquefies. A device that actually transfers heat from a source to the heat sink is called a heat exchanger.)
How much of a break might a big liquefaction plant at Nikiski get from low-temperature energy efficiencies?
Steve Butt, an ExxonMobil employee managing the Alaska LNG team, has said that liquefaction equipment in Nikiski would be 10 percent to 15 percent more efficient than the same piece of equipment working in the Middle East. That means more LNG at a lower energy cost.
Still, a lot of design and engineering work remains for Alaska LNG before selecting specific equipment for the plant, filing plans with regulatory agencies, pinning down costs and making a final investment decision to go ahead with construction.
There is a rule of thumb for energy efficiency based solely on temperature differences, Wilkins said. The improvement at an LNG plant ranges from 0.5 percent to 1 percent each time the ambient temperature drops by 1.8 degrees.
This makes it possible to form some estimate of how much edge an Alaska plant might have over a rival in a representative Middle East LNG port with an average year-round temperature - typical of the region - of 80 degrees. At Nikiski, the favored LNG plant site for the Alaska project, it’s 36 degrees.
That’s a difference of 44 degrees.
Thus, Alaska’s efficiency advantage over the Middle East could range from 12 percent to 24 percent, in the same ballpark as the figures from Butt and from the Norwegian analysis for Snohvit.
Why it’s cool to be cold Some energy benefits arise inside the gas turbines that provide power to an LNG plant. These turbine engines draw in outside air to burn some of the incoming gas to produce heat. Then they convert the heat to either mechanical power or electric power, as needed. Using colder air makes any turbine a little more efficient.
The first step in running a gas turbine is to compress the incoming air so that it can be mixed with natural gas and burned in the combustion chamber. The hot combustion gases then expand through the turbine blades, spinning them to produce power. Because cold air starts out denser than warm air, less work is needed to compress the flow of cold air to the conditions required for combustion. That is, the turbine consumes less power doing its job, which translates into more power being available for useful work and higher efficiency.
However, the energy gain at this stage is not as pronounced as across the LNG plant as a whole - only 0.1 percent or so for every 1.8-degree drop in air temperature.
The big energy savings come at the heart of the plant, in the liquefaction process.
Because it takes less power to liquefy gas in cold weather, the turbines that drive the process don’t need to burn as much gas to produce LNG at minus 260 degrees. And that’s the definition of higher energy efficiency.
Details matter all through the process. The plant’s efficiency benefits from both the low temperature of the incoming gas and the low temperature of the surrounding air.
How cold would the incoming gas be at Nikiski?
The temperature of the gas in the buried pipeline from Prudhoe to Nikiski will be managed with one chief goal in mind: Making sure it is compatible with the temperature of the soil through which it passes. It’ll be kept below 32 in frozen soil and above 32 in non-frozen soil.
To do otherwise would invite disaster. In frozen soil, particularly permafrost, a warm pipeline would risk thawing the soil and causing the line to sag and possibly rupture. In un-frozen soil, a below-freezing pipeline would risk accumulating an ice bulb that could push it upward, possibly causing it to split or crack.
Thus, the gas is likely to show up at the Nikiski plant at a year-around average temperature in the neighborhood of 35 degrees, in sharp contrast to the Middle East, where soil temperatures can reach 86 degrees. So Nikiski could have something like a 50-degree head start in cooling its gas to minus 260.
Part 2 of this story will run in the Aug. 3 issue.
Editor’s note: This is a reprint from the Office of the Federal Coordinator, Alaska Natural Gas Transportation Projects, online at www.arcticgas.gov/alaska-frigid-climate-could-give-state-edge-lng-market
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