October / November 05 Volume 3 Number 5

New Ways to Ship Natural Gas

At offshore oil wells, natural gas is a nuisance. Instead of being a valuable energy source to be transported and sold, natural gas is simply vented to the atmosphere or "flared off." The global waste is estimated to be three trillion cubic feet of natural gas every year, which represents 3 percent of total natural gas production and 1 percent of global greenhouse gases. The reason natural gas is thrown out offshore is because natural gas—€”as a gas—€”must be transported through pipes and not ships designed to carry liquids. In the deep ocean there are obviously no pipes to transport natural gas to electric power plants and consumers.

The key to stopping the waste of flare offs would be an economic way to make natural gas shippable. Practically, this means liquefying the natural gas to 1/600th of its gaseous volume so natural gas can be transported in ships. Unfortunately, liquefying natural gas is difficult. Natural gas liquefies at negative 161°C at sea level. Currently, large and expensive land-based cryogenic plants liquefy natural gas for oversea transport, but the cooling systems used in these plants are far too large to be used offshore. Hence, the real problem to be solved is to find a way to cool natural gas cheaply, efficiently and on a small scale. With such a system, natural gas could be liquefied at each offshore oil rig and shipped to the market.

For the last 15 years at Los Alamos National Laboratory, a team led by Greg Swift has been working to capture the value lost in flare offs. Swift's work is in thermoacoustics—€”the science of the interrelationship between heat and sound. Much of Swift's work focuses on using sound waves to make engines and refrigerators with unprecedented reliability and efficiency. Thermoacoustic engines and refrigerators have no moving mechanical parts. In Swift's machines, the only moving part is the sound wave itself. Unlike mechanical bearings and pistons, sound never wears out. For his work in thermoacoustics, Swift has received two R&D 100 Awards, the Acoustical Society's Silver Medal in Physical Acoustics and the Department of Energy's prestigious E. O. Lawrence Award.

Starting with a collaboration with Ray Radebaugh of the National Institute of Standards and Technology, Swift and his team have been making successively larger thermoacoustic natural gas liquefiers. The largest thermoacoustic natural gas liquefier made to date has shown that the concept is sound, and Swift hopes that future potential corporate partnerships will allow a full-scale prototype to be built.

How Sound Liquefies Natural Gas
Although the effect is usually too small to notice, all sound is accompanied by temperature changes. The sound of normal conversation changes temperature by a mere 1/10,000 of a degree Celsius. These temperature changes are caused, in turn, by the pressure waves that comprise sound. We have more direct experience with how pressure changes cause temperature changes. Take, for example, a simple aerosol can such as one might use for spray painting. As we paint we release the pressure in the can, which grows increasingly cold. Similarly, within sound waves, areas where pressure drops become colder. Usually this effect is both small and mostly cancelled out (areas of high pressure in the sound wave become hotter), so we don't notice the temperature changes. What Swift's team has done with acoustic refrigerators is to control the sound in such a way that some areas of the device preferentially become hotter while others preferentially become colder. Hence, Swift's team uses sound to transfer heat from one portion of the thermoacoustic refrigerator to another.

Said Swift of the leap from thermoacoustic refrigeration to natural-gas liquefaction, "It was Ray [Radebaugh] who realized that natural gas was a good application for a heat-driven cryogenic refrigerator because you can use the heat from burning natural gas and get the energy to create the cryogenic natural gas."

What makes the thermoacoustic natural gas liquefier design revolutionary is that not only does it use sound to cool, the liquefier also uses heat to create the sound. In much the same way as magnetism and electricity are interrelated, sound and temperature can be interrelated. At LANL, Swift's collaborator Scott Backhaus has reversed the refrigerator to create a thermoacoustic engine, which Swift and Backhaus use as a giant audio speaker. At the top of the thermoacoustic natural gas liquefier is a furnace where natural gas creates heat, and Backhaus's engine turns that heat into the sound that drives the cooling at the bottom of the liquefier. The advantage of the system is obvious: When natural gas is being mined, natural gas is abundant and available to be burned to create energy.

The result is a simple and clever system:
1. Natural gas burns to create heat.
2. Heat is used at the top of the liquefier to create high-energy sound in helium.
3. Sound waves travel through the helium down the liquefier to the natural gas.
4. Sound waves cool and liquefy the natural gas and vent the waste heat to the surrounding environment (e.g., the ocean).
Not only does the thermoacoustic liquefier not have moving parts that can wear out, the entire machine is made without exotic and expensive alloys: Steel is the only structural metal needed. Thus, manufacturing is inexpensive.

"Bigature": The Large "Small-scale" Prototype
The set designers of the film The Lord of the Rings coined the term "bigature" to describe the huge "miniatures" they used in filming. In collaboration with Praxair, Inc., Swift's team has made their own "bigature": A prototype that is several stories tall.

The largest thermoacoustic liquefier made to date is more than 50 feet tall. The liquefier used a portion of the natural gas fed it to generate 30 kilowatts of acoustic power. Inside, the sound was 200 dB, which is enough to kill a human, though from outside the sound is a mere hum. The 40 Hz note (a tone found at the low end of subwoofers) was passed down the liquefier's helium where the pressure swings of 85 p.s.i. cooled the natural gas. For every 30 kilowatts of sound power, 7 kilowatts went directly into liquefying the natural gas. Daily, the prototype thermoacoustic liquefier generated 500 gallons of liquefied natural gas.

Swift learned important lessons from the large-scale prototype. Although there are not supposed to be moving parts to wear out in the system, the prototype was physically shaken by the intense sound waves. The shaking caused internal welds to jitter and eventually fail. Fortunately this issue can be solved, and Swift intends the full-size prototype to be acoustically balanced so the structure does not shake in use.

Swift looks forward to potential future commercial involvement to manufacture a full-size prototype. Says Swift, "The natural gas liquefier that we would like to build next is of a size that would be commercially interesting needs a megawatt of acoustic power." The full-size thermoacoustic liquefier would be 80 ft. tall and have a capacity to liquefy 10,000 gallons of natural gas per day. Importantly, the system would run solely on the energy produced by burning off only 10 to 20 percent of the natural gas. An advantage to a system of this size and not larger is that it could be transported by rail.