A Stirling liquefaction tech deployed at VafabMiljö

In September 2022, Swedish municipal waste management, recycling, and biogas cooperative VafabMiljö Kommunalförbund (VafabMiljö) held the official opening of a novel biomethane liquefaction unit at its Gryta anaerobic digestion (AD) plant in Västerås.

Although a first-of-its-kind bioLNG plant in Sweden, the Stirling Cryogenics liquefaction technology deployed at VafabMiljö is not new.

Actually, Stirling Cryogenics can trace its technology heritage back to 1954 as Philips Cryogenics, a spin-off from technology major Philips, explained Arjan Coenradie, Managing Director, Stirling Cryogenics.

Part of Italy-headed engineering company HYSYTECH s.r.l., Stirling Cryogenics designs, builds, and supplies cryogenic solutions based on the reversed Stirling thermodynamic cycle and has over 3 000 installations around the world.

All of our Cryogenerators are based on this reversed Stirling cycle and have a long history, highly reliable record, and high efficiency. Our main products are stand-alone liquid nitrogen production systems and bespoke cryogenic cooling systems based on our Stirling Cryocoolers, said Arjan Coenradie.

Ideal for small-scale liquefaction

In more recent years, the company looked into applying technology to the biomethane (aka renewable natural gas – RNG) liquefaction space and has installed several such liquefaction units at biogas plants in the Netherlands and Italy.

By feeding upgraded and polished biomethane gas to the liquefier (in a range of 0-20 barg) bioLNG is produced and directly available for use or storage.

A single StirLNG-1 can produce around 200 kg/day of bioLNG at 4 barg (140 gal/day or 550 ltr/day) and a single StirLNG-4 unit around 600 kg/day at 4 barg (550 gal/day or 2.200 ltr/day).

Actual capacity varies on gas composition, temperature, and pressure. As multiple units can operate in parallel which provides redundancy and flexibility, the generic range we can offer is from 100 kg/day to around 15 000 kg/day (65–12.000 gal/day), said Arjan Coenradie, adding the VafabMiljö installation contains two StirLNG-4 units that operate at a pressure of 8 barg to produce around 1 tonne of bioLNG per day per unit.

Reversed Stirling cycle

The core element in all Stirling Cryogenics equipment is the Stirling Cycle Cryogenerator. The Stirling Cycle alternately compresses and expands a fixed quantity of helium (He), the working gas, in a closed cycle.

The compression takes place at room temperature to facilitate the discharge of heat caused by compression, whereas the expansion is performed at the cryogenic temperature required by the application, which in the case of biomethane at VafabMiljö is 146 K (-127 degrees Celsius) at 8 barg.

The Stirling cycle efficiently produces cooling power at cryogenic temperatures by the input of shaft power from an electric motor.

One main advantage of the process is that the gas to be liquefied is not part of the cycle to create the cold, it never comes into contact with the helium.

The gas will just flow through a cold heat exchanger, where energy is extracted so the gas will cool down and then condensate against the cold surface.

This eliminates any risk of contamination of the liquefaction process as well as of the Stirling Cycle working gas, resulting in long continuous operating periods and longevity.

Furthermore, this is a phase change at saturated equilibrium so there is no pressure change, the bioLNG flows out downwards by gravity. As mentioned, the (re-) liquefaction capacity of the StirLNG-4 depends on the specific process conditions.

The main parameters are the gas inlet temperature, gas inlet pressure, and gas composition. The influence of gas inlet temperature is straightforward – with lower inlet gas temperature, the less energy the StirLNG-4 needs to extract, and the liquefaction rate increases.

The influence of pressure is, however, more complicated. The pressure of the gas determines the liquefaction temperature. At higher pressure the liquefaction temperature increases.

Higher liquefaction temperature results in a higher production rate, due to two reasons: firstly, less energy needs to be extracted to reach liquefaction temperature, and; secondly, at higher temperatures, the Stirling Cycle will generate more cooling power, while also using less input power.

Over time other non-condensable gases (NCGs) such as oxygen and nitrogen might be liquefied only partially in the bioLNG flow, dependent on their solubility. The remainder needs to be vented from the liquefaction heat exchanger.

This will be a mixture of methane/oxygen/nitrogen gas that needs to be processed. This venting has a minor effect on the liquefaction rate, but it will increase the rate of gas consumption against liquid production, depending on the quantity of NCGs.

At VafabMiljö the non-condensable gases are routed back to the bioCNG unit.

Low carbon dioxide a supply chain criteria

The purity of the incoming non-odorized biomethane (RNG) feed gas is also critical – after all, different gases liquefy at different temperatures.

Compared to upgraded biogas and compressed biomethane (bioCNG), it is the carbon dioxide (CO₂) content that differs the most, and this is why a polishing unit is needed to bring the CO₂ content down to around 50 ppm at 0 barg.

Notable too is that the CO₂ level is not a specific requirement for the StirLNG-4 or for bioLNG powered trucks or engines, but for the entire liquefied natural gas (LNG) logistics chain at that given pressure.

It must be considered that when, downstream in the logistics chain, the LNG pressure is decreased, CO₂ will deposit as a solid. This ”dry ice” deposit will collect in vessels and potentially block or even damage valves and pumps. Therefore, it is the lowest (bio)LNG temperature in the logistics chain that determines the maximum CO₂ content of the feed gas, ended Arjan Coenradie.