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 a gross calorific value (GCV) higher than 43 MJ/m3(n), i.e. 1,090 Btu/scf, the UK and the US, where distributed gas is lean, with a 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 propane are proven solutions. Blending with gas or LNG can be a solution; however all of these solutions while theoretically viable can be costly and logistically difficult to manage in large scale. Lean LNG price in terms of energy value is lower than the rich LNG price. Liquefaction technology There are several liquefaction processes available for large, baseload LNG plants (in order of prevalence): AP-C3MR – designed by Air Products & Chemicals, Inc. (APCI) Cascade – designed by ConocoPhillips AP-X – designed by Air Products & Chemicals, Inc. (APCI) AP-SMR (Single Mixed Refrigerant) – designed by Air Products & Chemicals, Inc. (APCI) AP-N (Nitrogen Refrigerant) – designed by Air Products & Chemicals, Inc. (APCI) MFC (mixed fluid cascade) – designed by Linde PRICO (SMR) – designed by Black & Veatch AP-DMR (Dual Mixed Refrigerant) - designed by Air Products & Chemicals, Inc. (APCI) Liquefin – designed by Air Liquide As of January 2016, global nominal LNG liquefaction capacity was 301.5 MTPA (million tonnes per annum), with a further 142 MTPA under construction. The majority of these trains use either APCI