Some gas feeds contain 3-10% nitrogen content, and thus LNG plants will require dedicated Nitrogen Rejection Units, to ensure the resultant cargoes can be safely and efficiently transported. Nitrogen rejection units can add $0.3-0.7/mcf to the final delivered costs of LNG, based on calculations in this data-file.
Nitrogen content should be maintained below 1% of LNG cargoes, in order to maximize the calorific value of delivered gas, but also to prevent “tank rollover”. Tank rollover occurs when the different gases in an LNG tank stratify into layers, then these layers mix rapidly, causing a surge in pressure, which can be a safety risk and/or damage the tanks.
Hence what happens when the feedstock gas contains higher quantities of nitrogen? Most pipeline specs allow 3-4% nitrogen. Permian gas can be over 5% nitrogen. Pluto LNG produces from a gas field with 6% nitrogen.
Large-scale methods of separating nitrogen from gas/LNG mainly hinge on the lower boiling point of nitrogen (at -192ºC) than methane (at -162ºC). But different methods of nitrogen removal will apply, depending on the nitrogen content.
End-flash is the simplest solution. If a few percentage points of the LNG is “flashed” back into a vapor, nitrogen will boil off fastest, and its concentration will be enriched in the vapor. The vapor could then be blended in with the gas feeding the LNG plant’s compressor turbines. As a rule of thumb, this technique can “halve” the nitrogen content, e.g., from 1.5% to 0.75%.
A Nitrogen Rejection Unit is required to remove larger quantities of nitrogen. This involves splitting the LNG train into a separate pre-fractionation unit, where some of the gas is liquefied, leaving behind a gaseous stream that is enriched to 30-35% nitrogen. This enriched stream is then cooled further, to below -192ºC, then the nitrogen and methane are cryogenically separated. Although the nitrogen stream still contains 1-3% methane.
The costs of NRUs are estimated in this data-file, directly adding $0.2/mcf to the cost of LNG liquefaction. There is a paucity of data in the public domain, hence we have built up the numbers from our own engineering calculations, and broader models of LNG liquefaction costs and cryogenic air separation unit costs.
Another $0.1-0.5/mcf of cost will be added, depending on the nitrogen content of the feed gas. If an LNG plant is built with 10MTpa and 1.3bcfd of input capacity, but then 3-10% of the input capacity is taken up by valueless nitrogen, then a higher liquefaction spread must be charged on the remaining 90-97% throughput gas.
Hence overall, removing nitrogen from an LNG feed can add $0.3-0.7/mcf to the final delivered costs of LNG. There are also operational constraints.
For example, it could be challenging to use electrically driven compressors at a plant fed with high-nitrogen input gas. The 97-99% pure nitrogen stream exiting the NRU still contains 1-3% methane. You might burn off that methane by blending the stream in with the fuel at a gas-fired compressor. But if you don’t have gas-fired compressors, what do you do — vent it ?!
Nitrogen Rejection Units are offered by Chart, Linde, Conoco, Air Products (now Honeywell) and smaller suppliers. Please see our overview of the LNG supply chain. We reviewed white papers from Chart and Linde, who seem to be leaders, in this data-file. BCCK also highlights its involvement with NRUs in situ in the Permian basin, to meet pipeline specs.