Harnessing cold energy

Unlocking the cold energy potential in the LNG regasification process

LNG regasification is an energy-intensive process. During the production stage, natural gas undergoes liquefaction through a cooling process at extremely low temperatures, typically around -160°C, and at a pressure of 1 atm. This is done to reduce its volume for easier transportation and storage. However, liquefying natural gas requires a significant amount of energy; on average, about 500 kWh of electricity is consumed per tonne of LNG produced, which accounts for roughly 3.6% of the natural gas’ lower heating value (LHV).

After liquefaction, LNG needs to be pressurised, usually at 80 – 120 barg, and converted back to its gaseous state. This conversion takes place using vaporisers. Common vaporisation methods include submerged combustion (SCV), open-rack (ORV), and intermediate-fluid (IFV) vaporisers.

Nevertheless, in all these processes, a significant portion of the energy used during liquefaction (approximately 28%) is lost to the environment. Additionally, these systems require electrical energy for LNG and water pumps (in ORV) and/or regasified gas (in SCV).

LNG contains immense potential for power generation, known as the ‘cold energy potential’, with an available heat of approximately 725 kJ_th/kg_LNG (from -160°C – 15°C at 80 barg), and mechanical exergy from the gasification process alone amounting to about 348 kJ_ex/kg.

At present, the utilisation of cold energy taps into less than 1% of its overall potential, despite the possibility of generating around 2.5 GW of electricity from its exploitation. This capacity could be significantly enhanced by the implementation of climate change policies.

Harnessing LNG cold energy with a breakthrough in ORC technology

An interesting application of cryogenic LNG to enhance the efficiency of regasification terminals involves the generation of electricity by incorporating a power cycle between the environment and the vaporisation process. Essentially, a thermal engine can harness any significant temperature difference to produce mechanical energy, subsequently converting it into electrical energy via a turbogenerator. This generated electrical power becomes a valuable byproduct of the regasification process, serving internal LNG terminal needs with any surplus being supplied to the national grid.

The ORC technology presents itself as an efficient and readily available method for harnessing the cold energy potential from LNG. Similar to IFV, in the ORC system, seawater is directed into a sequence of heat exchangers where it vaporises a low-boiling working fluid. Within the ORC cycle, the high-pressure fluid vapour expands in the turbine before being discharged into the condensers where the heat is released to LNG, facilitating its vaporisation until it returns to a gaseous state.

In contrast to the traditional IFV approach, where the cold energy potential is often wasted, the ORC system effectively converts it using a turboexpander (Figure 2).

Conventional ORC systems typically feature a single level of condensation and employ propane, R13, R22, or R23 as the working fluid. These systems typically range in size from a few hundred kilowatts to 5 MW. However, the performance of single-level ORC systems is limited, as illustrated by Figure 4, which shows the temperature-heat exchanged diagram for the working fluid condensation process. This configuration’s condensation curve often fails to align with LNG vaporisation, significantly constraining turbine expansion and, consequently, power production.

To address these limitations, Exergy International has developed and patented a multi-level condensation ORC cold energy plant (CEP). This innovation aims to maximise the utilisation of the LNG heat sink along the vaporisation curve, thereby enhancing cycle efficiency and maximising electrical power production at the required regasification rate. The patented design accommodates up to four levels of condensation, employing a single feed pump in the ORC circuit and Exergy’s Radial Outflow Turbine (ROT).

The benefits of Exergy’s multi-level condensation with a single feed pump include:

• Achieving higher cycle efficiency, increased specific power production (SPP), and a greater cold energy recovery factor compared to single-level systems.
• Enabling a single pump configuration using a throttling valve at the outlet of high-pressure condensers. At the valve outlet, the fluid undergoes depressurisation and exists in a two-phase state, comprising liquid and vapour. This two-phase flow is directed to the lower-pressure condensers, where the liquid mixes with the low-pressure condensate, and the vapour is condensed along with the low-pressure vapour from turbine discharge. Utilising a single, low suction pressure pump results in lower ORC plant complexity and CAPEX, fewer critical equipment exposed to cryogenic temperatures, anticipated fewer maintenance stops, and increased plant annual availability.
• Integration with Exergy’s patented high-efficiency ROT, which naturally facilitates spillages between stages, allowing for both high-pressure and low-pressure expansion within a single turbomachine unit.
• Suitable for the use of propane as a working fluid, which is recognised in literature as one of the best performing among hydrocarbons and is widely available in the market at a low cost. Propane has already demonstrated success in conventional IFV and numerous cryogenic applications. Additional advantages of propane include its low global warming potential (GWP) of less than 10 and moderate vacuum capability in low-pressure condensers (> 0.1 barg).

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Read the article online at: https://www.lngindustry.com/special-reports/23052024/harnessing-cold-energy/