The market for liquefied natural gas (LNG) is booming. International LNG trade is expected to exceed $120 billion this year, making it second only to oil as the most valuable world commodity, according to Goldman Sachs Group.
Demand for the product positions the United States – with its abundant natural gas — nicely to build an LNG export market. This is a big switch from a decade ago when the U.S. was experiencing tight energy supplies and thought by now it would rely on foreign LNG imports.
As a result of the industry shift, we are seeing keen market interest in development of liquefaction plants in the U.S. (and other parts of the world). The nation now has plants under construction to produce 44.1 million tons per annum (MTPA) of LNG and has proposed an additional 268 MTPA of capacity.
LNG plants can cost in excess of $8 billion to site, develop and build. So efforts are underway to make these new plants as cost-effective as possible – which is one of the reasons why the LNG industry is examining Turbine Inlet Air Chilling (TIAC).
Before we can discuss the application of TIAC in LNG processing, it’s important to understand what it takes to convert natural gas to a liquid.
Natural gas (primarily composed of methane) comes out of the ground at a temperature of about 50 to 80 degrees Fahrenheit. In order to make transportation viable (profitable), there is a need to maximize the amount of methane in a fixed volume. One way to accomplish this is to compress the natural gas (known as CNG) to roughly 3,000-4,000 psia. This reduces the volume of methane by about 200 times. The other way to accomplish this is to liquefy the gas by cooling it down to minus 260 degrees Fahrenheit. This reduces the volume of methane by about 600 times, plus allows the tank (storage vessel) to be near atmospheric pressure. Called ‘cascading refrigeration’, the cooling process involves sequential steps of cooling the natural gas. Once delivered, the LNG is converted (re-gasified) back to a gas for use.
The Role of Combustion Turbines in LNG Plants
A high level of reliability is typically important to every end-user, regardless of what the plant produces. The differentiator for an LNG plant is that few other plants can claim losses in the millions-of-dollars-per-minute range due to unplanned outages or performance degradation. Consequently, safety, reliability, predictability, and consistency are critical in the LNG industry. In that respect, combustion turbines are frequently employed in LNG plants due to their high level of reliability, among other reasons.
The compressors used to create the cascading refrigeration process can be very large. It takes a lot of energy to turn these compressors and that energy typically comes in one of two ways: Mechanical or electrical. There are pros and cons to each method and, unfortunately, there is no answer that is right all the time. This becomes a parametric analysis that is typically performed (and resolved) in or before the FEED study phase of the project. In the mechanical example a combustion turbine can be directly coupled to a compressor where the turbine would provide the mechanical energy required to turn the compressor. This setup functions similarly to a typical gas turbine generating plant except the generator is replaced by a compressor and the operative word becomes horsepower (instead of kilowatts for a gas turbine generator). Inputs required to turn the compressor are AIR and fuel for the combustion turbine. In the electrical example the compressor is coupled to a (typically large) electric motor which takes electricity as an input and converts that to mechanical (or rotational) energy and that energy is used to turn the compressor. Combustion turbines can have a place in either scenario – whether the plant uses combustion turbines to directly drive the compressors or the plant employs a power island (essentially a power plant inside the property designed to provide the required electricity to turn the compressors). These power islands are fully functional power plants, which are typically gas turbine generators (open or closed cycle).
Combustion turbines find their way into LNG plants in different ways, but the technology has an inherent weakness: It is susceptible to performance degradation at high ambient temperatures. This means as ambient temperature changes, the overall performance (and capability) of the entire plant fluctuates. Since we already acknowledged that consistency is a critical criterion for an LNG plant, they frequently look for ways to eliminate variability, or fluctuation. Turbine Inlet Air Chilling (TIAC) is a proven technology that not only improves hot weather performance of combustion turbines, but, and sometimes more importantly, normalizes production and offers a level of predictability in an industry that is accustomed to putting a high value on consistency.
Check back for our next article which will get into the details of how a TIAC solution is applied to these different scenarios and the benefits that can be realized as a result.
 “Shale Set to Pummel Another Market as U.S. LNG Plants Arrive,” Bloomberg Business, May 19, 2015