Natural gas is inexpensive, burns cleaner than other fossil fuels and Pennsylvania has a lot of it. Although once difficult to bring to the surface, the growing popularity of hydraulic fracturing means that gas from shale is now being tapped at an accelerated rate. The potential for gas to displace other fossil fuels and be used in new applications is stronger and more economically attractive now that significantly more gas is being produced. Non-traditional applications like fueling locomotives, marine vessels, heavy equipment/trucks and even providing gas to areas where no pipeline service exists are now opening up.
Gas itself, however, is not economically feasible to transport large distances and is typically distributed through stationary pipelines. Compressed natural gas (CNG) and especially liquefied natural gas (LNG) are much more cost-effective to transport. LNG is created by cooling natural gas to cryogenic temperatures. Once liquid it is clear, non-toxic, non-explosive and takes up 600 times less volume than the equivalent amount of gas. It liquefies at -260 °F (-162 °C), must be kept in insulated, vented tanks and care must be used when handling to avoid contact with the body.
To become a part of the game-changing Pennsylvania shale gas play, Chester Engineers is designing equipment to process raw natural gas from transmission pipelines and convert over 1 million cubic feet of gas into approximately 10,000 gallons per day of LNG. This unit will be modular, skid-mounted, fully integrated and turn-key to be placed on-site and continuously operated with minimal supervision. Conceptually the process will consist of 6 major components – gas cleaning/pre-treatment, gas compression, pre-cooling, liquefaction and storage along with a micro-turbine to produce electric power and provide chilled water for cooling unit components.
Gas enters the pre-treatment/gas cleaning module to remove specific, inherent impurities such as carbon dioxide, heavy hydrocarbons, mercury and water that would profoundly interfere with the liquefaction process. The pre-cleaning process produces a clean stream of gas for LNG processing and a waste gas stream containing higher concentrations of the various contaminants which can be burned on-site power generation.
The now clean stream enters a reciprocating piston compressor to develop 800 psig pressure for the next stages of the process. The stream is then cooled in multiple stages to liquid form. The first stage is a pre-cooling module which takes all of the gas to -24°F using a closed circuit refrigeration unit. The next step is the process employs two heat exchangers and patented technology to liquefy the stream. Before entering this module, the gas stream is split in half with part of the stream being used to generate refrigeration and the second part of the stream becoming liquefied product. The process stream passes through two heat exchangers, is rapidly chilled and sent to storage tanks. The rest of the stream is passed through a turbo-expander which significantly drops the pressure and temperature of the stream creating both very cold gas as well as some liquid which provides part of the refrigeration in the heat exchangers. The LNG enters well insulated storage vessels that keep it at approximately 50 psi and -228°F.
None of the gas entering the system is vented or flared. Everything that enters is either liquefied or burned for energy to power the unit. Operations of the LNG plant will be fully automated to support around the clock operations with minimal maintenance and high reliability. Highly automated controls and alarms are incorporated in the design to safely and efficiently operate and monitor the equipment.