In a series of technical articles we introduce you to the Nordsol technology for liquefaction of biogas to bio-LNG. This first article discusses the cooling process and in particular the role of the biomethane turbo expander.
Michiel van Aken
Technical Director & Partner
Nordsol’s technology explained: The Turbo Expander
Oct 25 2024
Introduction
Summary video
From biogas to bio‑LNG in two steps
With the Nordsol technology, bio-LNG is produced from biogas in just two steps. In the gas treatment step, the biomethane in the biogas is separated from other components, such as CO2. With our patented Flash-2-Sweep technology we only need membranes to produce biomethane pure enough to liquefy. This technology is described in a different article.
In the biomethane liquefaction step, the temperature of the biomethane must drop to -145°C or lower. Several refrigeration technologies can do this, each with its own operational challenges, efficiency and complexity.
Energy consumption determines a significant part of the operational costs and impacts the carbon intensity of the biofuel. But, small-scale bio-LNG production also requires a compact installation and an easy-to-control, preferably unmanned process. Therefore, the right balance between energy efficiency and process manageability is key for the economic viability of a bio-LNG plant.
Choice of the refrigeration cycle
The following basic refrigeration cycles for bio-LNG can be distinguished, listed in increasing order of complexity and energy efficiency.
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- Liquid nitrogen (LIN) open-cycle
- Stirling cycle
- Linde cycle
- Reverse Brayton cycle
- Mixed Refrigerant (MR) cycle
- Cascade, mixed fluid or pure component cycles
These processes have many variants and permutations, but they can all be categorized according to the six groups mentioned above. To approach a theoretically perfect thermodynamic process, a highly complex and costly installation would be necessary. Therefor, Nordsol has designed its biomethane liquefaction process based on the reverse Brayton cycle, which is a good compromise between efficiency and complexity.
Nordsol implementation of the reverse Brayton cycle
Our reverse Brayton cycle is based on methane gas expansion through a single turbo expander and is a simplified version of the efficient AP-C1 process developed by Air Products. This market leader in LNG has demonstrated that gas expansion cycles with flash cooling can provide significant power savings.
In addition, Nordsol uses biomethane as the refrigerant, rather than nitrogen. This makes it the most efficient reverse Brayton cycle in the required temperature range.
But, our implementation of the reverse Brayton cycle with flash cooling also offers other important advantages besides energy efficiency.
Reduced risk of freeze-out
Sub-cooling to a low temperature (less than -150°C) in a heat exchanger carries the risk of local freeze-out during transients and brief upsets (e.g. increased CO₂ concentrations). This risk is significantly reduced in a gas expansion – flash cooling process with a relatively high temperature (-120°C) of the heat exchanger.
Smooth cooldown
Cold shocks are very common with liquid LNG or liquid Mixed Refrigerant (MR) in heat exchangers. Longer-term, they can result in leaks in heat exchangers due to thermal fatigue. Since there are no liquids in the heat exchanger in the iLNG process, there is no risk of cold shocks, no liquid mal-distribution (unbalanced cooling) nor any risk of thermal fatigue.
Integration with Boil-Off Gas
Low-pressure flash gas, combined with boil-off gas (BOG) from the tank and return gas from truck loading, serves as the sweep gas for the membranes in the biogas treatment step. The gas is recycled to the biogas compressor, which now combines several functions into one efficient machine: biogas compressor as well as BOG, flash gas, and truck return gas compressor.
Simplified startup procedure
During the startup, sub-cooling is initially not possible since there is no liquid. The cooldown procedure of a gas expansion – flash process is easier and faster, with the option to recycle a relatively large flow of flash gas during a steady, well-controlled equipment cooldown.
The biomethane turbo expander
The unique Nordsol application of the turbo expander, equipped with foil bearings and with methane as the refrigerant, further improves the simplicity and efficiency of the reverse Brayton cycle process in the required temperature range, compared to the nitrogen expander.
The turbo expander running at 60,000 rpm is derived from so-called air cycle (air conditioning) machines in commercial aircraft, like Boeing 777. These Air Cycle Machines (ACM) operate on air and are very reliable, robust, capable of performing 1000’s of start-stops and based on ‘simple’, robust gas foil bearings.
The biomethane turbo expander with foil bearings is currently running at high efficiency, reliably, 24/7 at Nordsol’s own bio-LNG plant in Amsterdam.
The Nordsol implementation of the reverse Brayton cooling cycle, combined with flash cooling, biomethane as the refrigerant, and a biomethane turbo expander, combines the best of two worlds: low equipment count, low refrigerant inventory, easy to operate while maintaining a high efficiency.
Michiel van Aken
Technical Director & Partner
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