Development of a thermoacoustic natural gas liquefier. (2002)

Praxair, in conjunction with the Los Alamos National Laboratory, is developing a new technology, thermoacoustic heat engines and refrigerators, for liquefaction of natural gas. This is the only technology capable of producing refrigeration power at cryogenic temperatures with no moving parts. A prototype, with a projected natural gas liquefaction capacity of 500 gallons/day, has been built and tested. The power source is a natural gas burner. Systems will be developed with liquefaction capacities up to 10,000 to 20,000 gallons per day. The technology, the development project, accomplishments and applications are discussed. In February 2001 Praxair, Inc. purchased the acoustic heat engine and refrigeration development program from Chart Industries. Chart (formerly Cryenco, which Chart purchased in 1997) and Los Alamos had been working on the technology development program since 1994. The purchase included assets and intellectual property rights for thermoacoustically driven orifice pulse tube refrigerators (TADOPTR), a new and revolutionary Thermoacoustic Stirling Heat Engine (TASHE) technology, aspects of Orifice Pulse Tube Refrigeration (OPTR) and linear motor compressors as OPTR drivers. Praxair, in cooperation with Los Alamos National Laboratory (LANL), the licensor of the TADOPTR and TASHE patents, is continuing the development of TASHE-OPTR natural gas powered, natural gas liquefiers. The liquefaction of natural gas, which occurs at -161 C (-259 F) at atmospheric pressure, has previously required rather sophisticated refrigeration machinery. The 1990 TADOPTR invention by Drs. Greg Swift (LANL) and Ray Radebaugh (NIST) demonstrated the first technology to produce cryogenic refrigeration with no moving parts. Thermoacoustic engines and refrigerators use acoustic phenomena to produce refrigeration from heat. The basic driver and refrigerator consist of nothing more than helium-filled heat exchangers and pipes, made of common materials, without exacting tolerances. The liquefier development program is divided into two components: Thermoacoustically driven refrigerators and linear motor driven refrigerators (LOPTRs). LOPTR technology will, for the foreseeable future, be limited to natural gas liquefaction capacities on the order of hundreds of gallons per day. TASHE-OPTR technology is expected to achieve liquefaction capacities of tens of thousands of gallons per day. This paper will focus on the TASHE-OPTR technology because its natural gas liquefaction capacity has greater market opportunity. LOPTR development will be mentioned briefly. The thermoacoustically driven refrigerator development program is now in the process of demonstrating the technology at a capacity of about 500 gallon/day (gpd) i.e., approximately 42,000 standard cubic feet/day, which requires about 7 kW of refrigeration power. This capacity is big enough to illuminate the issues of large-scale acoustic liquefaction at reasonable cost and to demonstrate the liquefaction of about 70% of an input gas stream, while burning about 30%. Subsequent to this demonstration a system with a capacity of approximately 10{sup 6} standard cubic feet/day (scfd) = 10,000 gpd with a projected liquefaction rate of about 85% of the input gas stream will be developed. When commercialized, the TASHE-OPTRs will be a totally new type of heat-driven cryogenic refrigerator, with projected low manufacturing cost, high reliability, long life, and low maintenance. A TASHE-OPTR will be able to liquefy a broad range of gases, one of the most important being natural gas (NG). Potential NG applications range from distributed liquefaction of pipeline gas as fuel for heavy-duty fleet and long haul vehicles to large-scale liquefaction at on-shore and offshore gas wellheads. An alternative to the thermoacoustic driver, but with many similar technical and market advantages, is the linear motor compressor. Linear motors convert electrical power directly into oscillating linear, or axial, motion. Attachment of a piston to the oscillator results in a direct drive compressor. Such a compressor has two distinct advantages over rotary motor compressors. One, it is a completely dry system. Because there are no gearbox and roller bearings, there is no requirement for lubricants, which eliminates the cleanup issues associated with lubricants in cryogenic refrigerators driven by conventional compressors. Two, the oscillator is suspended by flexure bearings. Flexure bearings have no wearing parts and have essentially infinite lifetime. Linear motors can also be run in reverse as linear generators and can be driven by acoustic engines. Although most natural gas is still carried from well to user as gas in pipelines, the use of liquefied natural gas (LNG) has been increasing. A typical modern, large liquefaction plant costs a billion dollars, liquefies 10{sup 9} scfd, uses 10-15% of its throughput to power itself, and has substantial operating and maintenance costs.