Small-scale and micro LNG plants

The economics have nothing to do with export. Small-scale LNG competes with diesel. A remote mine burning 50 million litres of diesel a year in haul trucks and power generation is a customer; so is a trucking fleet doing Perth–Port Hedland; so is a ferry operator facing emissions rules. LNG delivered by road tanker is cheaper per unit of energy than diesel in most years, cleaner-burning, and the trucks and engines to use it exist off the shelf.

The other driver is stranded gas. A small field with no pipeline, or an oil well flaring associated gas, can be turned into a product with a plant that fits on a few skids. The concept is called a "virtual pipeline" — the trucks are the pipe.

Most small-scale plants avoid hydrocarbon refrigerants entirely. Nitrogen is cheap, inert, and doesn't need make-up from fractionation. The cycle is a reverse Brayton cycle:

1. Compress. Nitrogen is compressed to around 50–60 bar in a multi-stage centrifugal or screw compressor and cooled against air or water.
2. Pre-cool. High-pressure nitrogen passes through the warm end of the cold box, cooled by returning low-pressure nitrogen.
3. Expand. The nitrogen goes through a turbo-expander — a small turbine — where it drops to perhaps 8–10 bar and gets very cold (down to −170 °C). The work it produces drives a booster compressor on the same shaft ("compander").
4. Chill the gas. The cold nitrogen flows back through the cold box, liquefying the natural gas in adjacent passages.
5. Recycle. Warm low-pressure nitrogen returns to the compressor suction. The loop is closed — no refrigerant is consumed.

Dual-expander versions (two expanders at different temperatures) improve efficiency by 15–20%. Even so, a nitrogen plant uses about 0.5–0.7 kWh per kg of LNG, versus roughly 0.3 kWh/kg for a baseload C3MR train. At small scale nobody minds — simplicity, safety and fast start-up matter more than fuel cost.

The same contaminants must go, but the equipment changes:

• CO₂ removal: amine is still used, but membranes and pressure-swing adsorption (PSA) are popular below about 100 tpd because they have no solvent, no reboiler and no chemical handling. Some micro plants simply run a mol sieve big enough to catch CO₂ as well as water, if the inlet CO₂ is low.
• Dehydration: molecular sieve, same as big plants but often a two-bed skid.
• Heavies: pipeline gas is usually lean enough that a simple separator after pre-cooling, or a small heavies removal column, is enough. Biogas needs siloxane and sulphur removal first.
• Mercury: a small carbon bed, frequently in a single vessel.

Small plants use vacuum-insulated double-wall tanks — a stainless inner vessel inside a carbon steel outer, with the gap under vacuum and packed with perlite or multilayer insulation. Heat leak is a fraction of a percent per day. LNG is loaded through vacuum-insulated hoses or small arms into road tankers, which are themselves vacuum-insulated and can hold product for days to weeks before pressure builds to the relief setting.

At the customer, a satellite station receives the LNG into another bullet tank and vaporises it on demand — usually through ambient air vaporisers, which are just tall finned aluminium tubes that frost up and are swapped over in pairs. A gas engine or generator downstream can't tell the difference from pipeline gas.

These plants are often run by a very small crew — sometimes one operator per shift with remote monitoring, or unattended with call-out. The daily work is loading trucks, managing tank inventory against a delivery schedule, mol sieve changeovers, and watching the expander bearing temperatures and vibration. Truck loading is the highest-risk routine task: connecting and purging cryogenic hoses, earthing, and making sure the tanker's pressure and level are right before transfer.