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The article titled, ‘LNG export: Boom or bust?’ explored the impact of U.S. LNG imports on gas turbine operation and infrastructure.
The second part of this series assesses turbomachinery and process cycle selection for LNG projects. In evaluating turbomachinery in the context of the overall plant design, the project team must consider factors such as machinery degradation rates, operator experience, spare parts availability, expansion planning for additional trains and the machinery emissions signature. However, to shorten the present discussion, the two primary factors influencing the LNG project net present value (NPV), specific energy consumption and availability, are discussed here.
The conclusion is that availability is more important to NPV than energy consumption. Availability will influence LNG project NPV more drastically than small percentage changes in efficiency or the process cycle efficiency. In an environment of LNG plants with higher capital cost, therefore, validation of compression train drivers or improvements to the refrigeration cycle are difficult to justify given the risk to NPV if plant availability suffers.
Specific energy consumption
Thermal efficiency is often regarded as one of the most important decision criteria in deciding between competing refrigeration cycles and turbomachinery. However, the critical issue for LNG plants is specific energy consumption (SEC) or the amount of energy expended to produce LNG, which includes all of the plant processes (not only the major equipment). This metric also provides an indication of fuel consumption and process efficiency.
SEC is a preferred efficiency metric for an LNG plant as it accounts for all of the plant processing and machinery efficiencies; it can be measured in real time; and it is normalized such that different LNG processes or plant sizes can be compared fairly. SEC is the amount of energy consumed by the plant (feed gas heating value and electricity used), minus the amount of LNG and extracted Natural Gas Liquids (NGLs) delivered by the plant, divided by the energy consumed. This can be calculated for either the “holding mode” or the “loading mode” of the plant. The energy consumption will differ slightly because of the greater energy expended by the loading pumps when LNG is being transferred from the tanks.
(Read more in the Sept/Oct issue of Turbomachinery International)