Liquefied Natural Gas - Quality of LNG

Quality of LNG

LNG quality is one of the most important issues in the LNG business. Any gas which does not conform to the agreed specifications in the sale and purchase agreement is regarded as “off-specification” (off-spec) or “off-quality” gas or LNG. Quality regulations serve three purposes:

1 - to ensure that the gas distributed is non-corrosive and non-toxic, below the upper limits for H₂S, total sulphur, CO₂ and Hg content;

2 - to guard against the formation of liquids or hydrates in the networks, through maximum water and hydrocarbon dewpoints;

3 - to allow interchangeability of the gases distributed, via limits on the variation range for parameters affecting combustion: content of inert gases, calorific value, Wobbe index, Soot Index, Incomplete Combustion Factor, Yellow Tip Index, etc.

In the case of off-spec gas or LNG the buyer can refuse to accept the gas or LNG and the seller has to pay liquidated damages for the respective off-spec gas volumes.

The quality of gas or LNG is measured at delivery point by using an instrument such as a gas chromatograph.

The most important gas quality concerns involve the sulphur and mercury content and the calorific value. Due to the sensitivity of liquefaction facilities to sulfur and mercury elements, the gas being sent to the liquefaction process shall be accurately refined and tested in order to assure the minimum possible concentration of these two elements before entering the liquefaction plant, hence there is not much concern about them.

However, the main concern is the heating value of gas. Usually natural gas markets can be divided in three markets in terms of heating value:

• Asia (Japan, Korea, Taiwan) where gas distributed is rich, with an GCV higher than 43 MJ/m3(n), i.e. 1,090 Btu/scf,
• the UK and the US, where distributed gas is lean, with an GCV usually lower than 42 MJ/m3(n), i.e. 1,065 Btu/scf,
• Continental Europe, where the acceptable GCV range is quite wide: approx. 39 to 46 MJ/m3(n), i.e. 990 to 1,160 Btu/scf.

There are some methods to modify the heating value of produced LNG to the desired level. For the purpose of increasing the heating value, injecting propane and butane is a solution. For the purpose of decreasing heating value, nitrogen injecting and extracting butane and methane are proved solutions. Blending with gas or LNG can be a solutions; however all of these solutions while theorically viable can be costly and logistically difficult to manage in large scale.