Matt Taher

Bechtel Fellow and Technology Manager

With the world’s energy needs continuing to grow and climate change a rising concern, liquefied natural gas (LNG) stands out as a dependable transitional fuel for the next decade. Composed primarily of methane — the simplest and lightest hydrocarbon with just one carbon atom — LNG is the cleanest-burning fossil fuel, producing less carbon dioxide (CO₂) per unit of energy than other hydrocarbons. LNG liquefaction accounts for part of the total CO₂ emissions in the LNG lifecycle, with its share depending on factors like feed gas composition, train size, and design features.

In mid- to large-scale LNG facilities, carbon intensity — measured as tonnes of CO₂ equivalent (t CO₂e) per tonne of LNG produced — is ideally achievable at levels as low as 0.05 t CO₂e per tonne of LNG, though actual values vary depending on design constraints and operating conditions. To improve carbon intensity, the most effective approach is to reduce CO₂ emissions while increasing the efficiency of LNG production. Lowering emissions through efficient energy use, waste heat recovery, and advanced process integration directly contributes to lowering the carbon intensity of LNG.

Simultaneously, efficiently maximizing LNG output ensures that emissions are spread over a greater volume of product, further improving the carbon intensity index.

LNG liquefaction is energy-intensive, mainly due to the power needed for refrigeration compressors. The choice and configuration of the liquefaction process, which determines how many refrigeration compressors are required, significantly impacts the carbon intensity index. For instance, utilizing a single refrigeration compressor without a pre-cooling cycle can cut CO2 emissions by reducing the number of drivers. However, this may indirectly increase power consumption due to less efficiency, which can offset carbon intensity gains.

Over the past five decades, the LNG industry has seen significant evolution in the types of drivers used for refrigeration compressors, ranging from steam turbines and gas turbines to electric motors and hybrid configurations. Today, this trend is increasingly shaped by the need to reduce greenhouse gas emissions and minimize auto-consumption, which — particularly in feed gas-constrained scenarios — can help maximize LNG output. Utilizing electric motors to drive refrigeration compressors, especially when powered by low-carbon or renewable electricity (e.g., hydropower), can substantially reduce carbon intensity.

In gas turbine-driven LNG liquefaction plants, up to 80% of CO₂ emissions originate from the gas turbines used for compressor drive and power generation. The exhaust from these gas turbines — containing about 60% of the fuel’s lower heating value (LHV) at high temperatures (425°C–550°C) — offers significant potential for waste heat recovery to enhance plant’s thermal efficiency. This heat can be recovered using waste heat recovery units (WHRUs), integrated power recovery systems such as Organic Rankine Cycles, or heat recovery steam generators (HRSGs) that produce process steam and, in some cases, drive steam turbines for power generation or mechanical drive.

In an LNG liquefaction plant, most of the process heat requirements originate from the regenerator reboiler in the acid gas removal unit (AGRU), with its design duty primarily influenced by the maximum CO2 content present in the feed gas. Additional process heat is needed for regeneration gas used with dehydration molecular sieve beds. Process heat demand can be supplied by recovering energy from gas turbine exhaust through WHRUs. If WHRU is not feasible, electric heaters can be utilized as an alternative to fired heaters, which may contribute to higher CO2 emissions. Medium-voltage electric heaters are relatively recent developments; they offer a potential option for carbon-free process heating at large scale.

When used in LNG liquefaction plants, turbo-expanders contribute to improving energy efficiency and reducing carbon intensity. Turbo-expanders provide a greater temperature drop than Joule-Thomson valves. The colder fluid exiting the turbo-expander helps in reducing the refrigeration load, thus enhancing the overall plant thermal efficiency. At the same time, recovered energy from the expansion is carbon-free, improving the carbon intensity of the liquefaction plant.