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R LNG TECHNOLOGY

By Greg Swift Thermoacoustics for Los Alamos National Laboratory and John Wollan Liquefaction of Natural Praxair

A prototype device for liquefying natural gas using thermoacoustics is capable of producing 500 gallons per day of LNG, consuming 35 percent of the incoming gas in the process. Larger capacities operating at higher efficiencies are on the drawing board.

ne ordinarily thinks of a In this article, we introduce the the half- resonance present in the wave as consisting only of basic principles of thermoacoustics apparatus illustrated by the schematic O coupled pressure and position and describe progress toward their in (c), where the is at the top oscillations. In fact, temperature use for liquefaction of natural gas. and the refrigerator is at the bottom. oscillations accompany the pressure Thermoacoustic natural-gas liquefiers exchangers (HX) and a regenerator oscillations and when there are spatial are surprisingly simple: They use no in the engine convert some of the heat gradients in the temperature exotic materials, require no close power (QH) from burning natural gas at oscillations, oscillating heat flow tolerances, and are little more than a hot temperature (TH) into acoustic occurs. The combination of these welded pipe and heat exchangers power (W), rejecting power oscillations produces a rich variety of filled with pressurized helium. This (Q0) to a water stream at ambient “thermoacoustic” effects. In everyday simplicity, along with the reliability temperature (T0). Acoustic power is life, the thermal effects of sound are too and low maintenance inherent in consumed by the refrigerator, which small to be easily noticed; for example, thermoacoustic technology, suggests uses it to pump heat (QC) from a the of the temperature that thermoacoustic liquefiers could liquefying natural-gas load and rejects oscillation in conversational levels of enable economic recovery of marginal waste heat (Q´0 + Q˝0) to the ambient sound is only about 0.0001°C. gas resources such as associated water stream. Each of the heat However, in an extremely intense sound gas from offshore oil wells, gas exchangers may be of finned-tube or wave in a pressurized gas, these accumulations at remote locations, and shell-and-tube construction, as open to thermoacoustic effects can be even the recovery of landfill gas and helium flow as possible. Each harnessed to create powerful heat marginal coal seam gas accumulations. regenerator usually consists of a pile of and refrigerators. Whereas In addition, the technology could stainless-steel screens, supporting the typical engines and refrigerators rely on find an application in areas where smooth temperature profile between the crankshaft-coupled pistons or rotating smaller-scale gas liquefaction is two adjacent heat exchangers. turbines, thermoacoustic engines and needed: liquefaction at seasonal Thermodynamically, acoustic power refrigerators have no moving parts (or at peak shaving facilities and at fleet- is just as valuable as other forms of most only flexing parts without the need vehicle fueling stations. “” such as electric power or for sliding seals). This simplicity, rotating-shaft power. The first law of coupled with reliability and relatively Thermoacoustic Basics thermodynamics determines that low cost, has highlighted the potential Many varieties of heat-driven W+Q0 = QH in the engine. The second of thermoacoustic devices for practical thermoacoustic systems law shows that the engine efficiency use. As a result, thermoacoustics is exist, but in this article we consider W/QH is bounded by the Carnot maturing quickly from a topic of basic only a toroidal thermoacoustic-Stirling efficiency, 1 – T0/TH. The most efficient scientific research through the stages of hybrid engine driving a thermoacoustic thermoacoustic engine to date has applied research and on to important orifice (Figure achieved 40 percent of the Carnot practical applications. 1). Parts (a) and (b) of Figure 1 show efficiency, while the most powerful has

Fall 2002 • GasTIPS 21 \ produced 17 kW of acoustic ...... ~~ .... , ...... II power. Similarly, in the II refrigerator, the first law of (a) Position ., thermodynamics determines 1 that W + Qc = Qo'+ Q/; the second law shows that the efficiency Q&W,' known regenerator as the coefficient of performance, is bounded by the Carnot expression '-' TJ(T, - Tc). The most moving down, thermaUy / ,-i A expandim efficient thermoacoustic orifice pulse tube refrigerator to date has achieved 25 percent of this Carnot bound. One of the most important large dimensions in a I thermoacoustic device is the length of its resonator, which (together with the helium sound speed) determines the operating frequency, just as the length of an organ pipe determines its pitch. This I: ..l ...... length typically ranges ...... from ...... 10 cm for the simplest ...... experimental systems to 10 ...... m for today's most efficient and mature systems. The Figure 1: Schematic of a Heat-driven Thermoacoustic Refrigeration System resonator shown in Figure 1 uses a half-wavelength , How the Engine Mrks thermoacoustic engines and shown schematically in parts (a) and (b) To understand the conversion of heat to refrigerators, the amplitude of the (but without details of the wave within acoustic power by this simple engine, pressure oscillation is 10 percent of the the engine and refrigerator). This wave consider the magnified view of part of mean pressure, and the amplitude of appears spontaneously whenever the the regenerator shown in Figure 1, motion is a similar percentage of the temperature in the engine's hot heat part (d), which shows a typical parcel length of the regenerator. Thermal exchanger is high enough, and the of helium at four instants of time as contact between the oscillating helium amplitude of the wave increases as it oscillates in position, pressure, and the wall of the pore, plus the the heat supplied to the hot heat temperature, and density, exchanging externally imposed temperature exchanger increases. In parts (a) and heat with the nearby solid in the gradient, add a new feature to what (b), the pressure and position regenerator. The tiny pore of the would otherwise be a simple acoustic are shown at two times: the red curves regenerator is shown as a smooth-walled oscillation - oscillatory heat transfer show these variables when the helium channel for simplicity. between the helium and the solid. is at the uppermost extreme of its The wave carries the helium up and While the heliuq is moving downwards, position in the resonator, with density down along the pore, compressing and it encounters evdr warmer portions of and pressure highest at the top of the expanding it, with time phasing such the regenerator, so it absorbs heat and resonator and lowest at the bottom, that it is most pressurized while it is expands; while the helium is moving while the blue curves show them 180" moving down and most depressurized upwards, it rejects heat and contracts. later in the cycle. while it is moving up. In typical Figure 1, part (e), a pressure-volume

22 GasTlPS Fall 2002 ( p–V ) diagram for the parcel of helium density, cp is its isobaric specific heat low pressure time of the , illustrated in part (d), shows that the per unit mass, and f is the frequency of and the thermal contraction, attending ∫ ␦ helium does net work ( p dV) on its the acoustic oscillation; K is roughly its upward motion, occurs during the surroundings because expansion takes the distance heat can diffuse through high pressure time. The resulting during the high-pressure time of the the helium during a time 1/πf. In acoustic power absorbed by the helium ␦ cycle and the contraction during the today’s thermoacoustic systems, K is is supplied by the thermoacoustic low-pressure time. This process typically a fraction of a millimeter. engine, transmitted to the refrigerator depends on the correct time phasing (Pores too tight impose too much through the wave in the resonator. between motion and pressure, which is viscous drag on the helium.) maintained by inertial and compressive Development History effects in the ductwork near the How the Refrigerator Works Heat driven acoustic oscillators have regenerator. The net work that the The basic principle of operation of the been known for over a century — the helium does on its surroundings is thermoacoustic orifice pulse tube earliest and simplest was discovered produced at the resonance frequency. refrigerator is very similar to that of the accidentally by European glassblowers. Thus, the parcel of helium shown in (d), thermoacoustic engine. A magnified But an accurate theory applicable to and all others like it within the view of part of the refrigerator’s thermoacoustic phenomena was not regenerator, deliver acoustic power to regenerator in Figure 1, part (f), developed until the 1970s, through the the wave, while the wave sets the illustrates one typical parcel of helium efforts of Nicholas Rott and coworkers frequency of the power production. as it oscillates in position, pressure, at ETH-Zurich. Rott’s theory is based Each parcel of helium also deposits temperature, and density, exchanging on a low-amplitude linearization of the a little heat (not shown in Figure 1) at heat (dq) with the nearby solid in the Navier-Stokes, continuity, and energy one location in the regenerator while regenerator, moving that heat up the equations, with sinusoidal oscillations the pressure is rising and the parcel is temperature gradient. As the helium of all variables. relatively stationary near the upper oscillates along the refrigerator’s In the early 1980s, the thermal- extent of its motion. It absorbs that heat regenerator, it experiences changes in team at Los Alamos, supported near the lower extreme of its motion, at pressure. At the lower extreme of its by BES in DOE’s Office of Science, a warmer location in the regenerator, motion, the typical parcel of helium was frustrated by the large number when the pressure is falling. With rejects heat (dq) to the regenerator, of precision moving parts required respect to heat, all parcels act like because the pressure rises while the for their experiments on the members of a bucket brigade, with the helium is relatively stationary at that thermodynamic behavior of near-critical overall effect being absorption of heat at location. Similarly, at the upper extreme in heat engines. While looking the hot and rejection of of its motion, it absorbs heat (dq) from for simpler engine designs, they read heat at the ambient heat exchanger. the regenerator, because the pressure the publications of Peter Ceperley at rises while it is relatively stationary George Mason University, who had Pore Size there. Thus, the parcel of helium moves realized that the timing between The pore size in the regenerator a little heat along the regenerator, up pressure changes and motion in determines the of the thermal the temperature gradient, during each Stirling engines is the same as in a contact between the regenerator solid cycle of the acoustic wave. All the other traveling sound wave (Ceperley, 1979). and the moving helium. parcels in the regenerator behave Inspired by his insight, the Los Alamos Good thermal contact is needed to similarly, so that the overall effect, researchers began considering accomplish the cycle shown in again like in a bucket brigade, is the acoustic technology to eliminate Figure 1, because the temperature of net transport of heat from the cold moving parts. Eventually, they brought the helium should match the local solid heat exchanger to the ambient together a thermodynamic point of view, temperature while the helium moves. heat exchanger. acoustic techniques, explicit heat Analysis shows that a spacing between The helium also consumes acoustic exchangers, and Rott’s theory, plates of a fraction of a thermal power from the wave (not shown in producing the first powerful ␦ √ π ␳ penetration depthd K = K/ f cp is Figure 1), because the thermal thermoacoustic engines and the first best, where K is the thermal expansion of the helium, attending its thermoacoustic refrigerators. conductivity of the helium, ␳ is its downward motion, occurs during the Fundamental research on

Fall 2002 • GasTIPS 23 Figure 2: First Thermoacoustically Driven Pulse Tube Refrigerator Figure 4: First Acoustic Liquefier thermoacoustics has grown ever liquefaction of since, at Los Alamos and throughout natural gas, using the world. combustion of gas In the late 1980s, a partnership as the heat source. between the Los Alamos team and Ray A typical modern Radebaugh at the National Bureau of gas liquefaction Standards (now National Institute of plant costs a Standards and Technology) in Boulder billion dollars, combined a thermoacoustic engine with liquefies 104 an orifice pulse tube refrigerator to m3/day, and has create the first cryogenic refrigerator substantial with no moving parts (Figure 2). This operating and Figure 3: Illustrations of an Acoustic Liquefier device was dubbed the “Coolahoop” maintenance costs. and Offshore Applications because the bent brass portion of the The need for half-wavelength acoustic resonator relatively small, reliable, inexpensive this technology in 1994. The following (extending upward in Figure 2) liquefaction equipment seemed year, DOE’s Office of Fossil Energy resembled a hulahoop (Radebaugh, clear and an “acoustic liquefier” (through NETL) began supporting Los et al., 1991). In the photo, the pulse seemed to fit the need perfectly. The Alamos team’s partnership with tube refrigerator is the silver-colored goal of an acoustic liquefier with a Cryenco. Hardware development “U” at bottom center, while the two capacity of 10,000 gallons per day (gpd) continued in Denver through many thermoacoustic engines are under the followed, eventually including transitions, most recently as Praxair bulky white insulation to the right and economic analysis for arrays of such acquired the project. While working on left of the refrigerator. liquefiers on floating LNG production/ the Denver development, the Los Even though this early system had storage vessels and oil/gas separation Alamos researchers have continued only 5 W of cooling power at 120 vessels (van Wijngaarden, 1999; research on fundamentals, increasing Kelvin, Radebaugh believed from the Figure 3). engine efficiency and bringing outset that the best application for this Cryenco, a small manufacturing thermoacoustic improvements to orifice heat-driven refrigerator would be company in Denver, began working on pulse tube refrigerators.

24 GasTIPS • Fall 2002 Prototype Acoustic Liquefier Hardware The first natural-gas-fired thermoacoustic liquefier was completed in Denver in 1997 (Figure 4). It achieved a liquefaction capacity of 140 gpd of LNG, producing 2 kW of refrigeration power at –140°C. The second of hardware development, which began in mid 1999, has been the development of an efficient 500 gpd system (Figure 5). The thermoacoustic portion of the system is prominently visible in Figure 5, with the engine on top and refrigerators on the bottom, linked by a half-wave resonator. The natural gas burner is at the very top, under the blue banner. The engine is in the large bulge below the burner. The refrigerators are hidden inside the large, cylindrical vacuum insulation can near the bottom, but two of their slender inertances and compliances are visible above the vacuum can. The thermoacoustic working helium is at an average pressure of 450 psi, with oscillations up to ±45 psi in amplitude at a frequency of 40 Hz. In this system, three refrigerators are used, driven in parallel by the Figure 5: Current Protoype 500 GPD Acoustic Liquefier thermoacoustic wave but connected in series with respect to the natural-gas 65 percent of a natural-gas stream Next Steps stream so that the first acts as a natural- while burning 35 percent. The next step in capacity will target gas precooler, the second removes the In 2001, the 500-gpd system was 10,000 gpd, the largest size that we rest of the sensible heat and some of operated at 60 percent of its design believe can be factory produced en the latent heat, and the third removes pressure amplitude, with the engine masse and transported by rail. Initial the rest of the latent heat. The design producing 12 percent of its design brainstorming is underway and serious calls for the engine and resonator to power and each of the three engineering design will begin soon. deliver 30 kW of acoustic power to the refrigerators running separately at 25 This effort will be financed by Praxair refrigerators, whose combined cooling percent of their design powers. All and by the Advanced Technology power is 7 kW. The burner delivers heat thermoacoustic phenomena were Program of the National Institute of to the engine, and is made more working as expected, but a crack in an Standards and Technology. efficient by a traditional recuperator to inaccessible weld prevented testing at However, the development of an preheat the incoming fresh air by higher powers. During 2002, this efficient, low-cost acoustic liquefier is capturing heat from the flue. Waste heat system is being rebuilt, including challenging. Even the 500 gpd system is removed from the engine and the dramatic improvements to the burner is a scaleup of a factor of 1600 in refrigerators by circulating water at and burner-engine heat exchanger. cooling power over the first laboratory ambient temperature. Overall system Financial support for this effort is demonstration, which used simple efficiency should yield liquefaction of provided by Praxair and NETL. electric heat to power the engine and an

Fall 2002 • GasTIPS 25 electric-heat test load on the “inertance” are also local accentuators For more information contact the refrigerator, and had such poor of inertial mass, enforcing the correct authors, Greg Sw$, with Los Alamos efficiency that it would have liquefied amplitude and time phasing of the gas National Laboratory at swi@[email protected] only 9 percent of a natural-gas motion in the nearby regenerators. or via telephone at 505-665-0640, or stream while burning the other 91 Portions labeled “compliance” John Wollan with Praxair at percent. Nevertheless, the 10,OOO gpd accentuate compressibility. [email protected] or by system is expected to liquefy 80 Another key aspect of the telephone at 303-549-7204A more percent of its throughput, and we elimination of moving parts from complete, animated version of Figure I expect that further improvements can Stirling systems is the use of pulse Vor PCs, not Macs) can be obtained by eventually bring the efficiency close tubes and thermal buffer tubes in place downloading TashOpZp.exefrom to 90 percent without compromising of cryogenic or red-hot pistons. These httpd/www.lanl.gov/thennoacoustics/mov the low cost and reliability of the portions of the system maintain ies.html. Further background on the thermoacoustic approach. thermally stratified adiabatic oscillating fundamentals of thennoacoustics is flow, thereby transmitting acoustic available at http:,%uww.lanl.gov/ Back to Basics power from the cryogenic temperature thermoacousti& which includes lids to Readers familiar with Stirling engines (in a refrigerator) or the red-hot journal publications and a book. or refrigerators will recognize that the temperature (in an engine) to ambient InJonnation about the natural-gas processes in the regenerators and heat without suffering from convective heat liquefir is also available at exchangers discussed above in the leak. Some current fundamental httpd/iuunu.lanl.gov/mstJ context of Figure 1 are identical to the research in thermoacoustics is engidecon. html. processes in Stirling devices. Hence, directed toward understanding and another way to view thermoacoustics is maintaining this thermally stratified References as one chapter in the story of the condition in the presence of violent Ceperley, P.H., 1979. ”Apistonless Stirling engine- The traveling- elimination of moving parts and sliding oscillating flow. wave heat engine,“ Journal of the seals fiom Stirling devices - a story in Efficiency and power density are two Acoustical Society of America, Vol. which earlier chapters include Beale’s key figures of merit for any energy- 66, pp. 1508-1513. invention of the free-piston Stirling conversion technology. The power of engine and Gifford and Longsworth‘s thermoacoustic devices is roughly Radebaugh, R., McDermott, K.M., invention of the basic pulse tube proportional to pav$la(vosJpavg)2,with Swifi, G. W., andMartin, R. A., refrigerator (Beale, 1969; Gifford and pavgthe average pressure, A the cross 1991. “Developmentof a Longsworth, 1965). A key aspect in the sectional area of the regenerator, a the thermoacoustically driven orifice thermoacoustics chapter is the sound speed of the helium, and post the pulse tube refrigerator,“Interagency deliberate use of inertial effects in the amplitude of the oscillating pressure. Meeting on Cryocoolers, Plymouth oscillating helium. A moving slug of Helium has the highest sound speed of MA, October 24. helium can behave inertially much like the inert , so high-pressure helium Van Wijingaarden, W. C., 1999. a moving solid piston, bouncing against is used in most thermoacoustic systems, “Therrnoacousticrefrigeration - A the compressibility of nearby helium to including the acoustic liquefier. This stirring concept for offshore associated act like a spring-mounted mass. From leaves p,Jpmg as the primary variable gas liquefaction,“Monetizing Stranded this point of view, the half-wave which might be increased in order to Gas Reserves Conference, Houston, resonator of Figure 1 can be thought of increase power per unit area. December 1999. as if the mass of the helium in the Unfortunately, increasing posJpavg Beale. W. T., 1969. “Free-piston central third of the resonator bounces generally reduces efficiency, as a StirlinP PnPinPs - wmp mnhl tmtc resonantly against the compressibilities variety of higher loss processes such as and simulations,” S.A.E. paper of the helium in the upper and lower turbulence grow in importance, and as number 690203.