LNG As an Energy Efficient Eco-Friendly Cryogenic Fuel
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LNG as an energy efficient eco-friendly cryogenic fuel S C Sarkar Advanced Centre of Cryogenic Research, Jadavpur University, Kolkaata, India Abstract given gas field to a particular consuming area. The use of an old fuel in a new form has progressed Finally, pipeline natural gas cannot meet the fluctu- at a rapid space for the last couple of years, due to ating gas demand, if any. its several advantages. The fuel in question is natu- Liquefied Natural Gas (LNG), on the other ral gas and, its new form, is liquefied natural gas hand, is ideally transported in Cryogenic tankers by designated as LNG. LNG transported in cryogenic road, ships and rail wagons. There is a distinctive vessels offers several advantages over pipe line advantage on liquefaction as there is a volume transport of natural gas when the gas consuming reduction of about 630 times on liquefaction, and areas are far away from the gas producing areas. liquid natural gas handling is more like handling oil. Moreover, LNG as an automobile fuel has also a In recent times, the use of LNG has gained much definite edge over other fuel. However, the LNG popularity globally and it is the right moment to age in India is of very recent origin and, only in review the status of LNG development with regard January, 2004, the first LNG cargo had reached an to various resources of natural gas, gas liquefaction, Indian port in the state of Gujarat. storage, transport, utilization of LNG and to trace In this juncture, this paper presents an effective the desirability of LNG technology. study on the characteristics of LNG, advantages and disadvantages of various natural gas liquefaction 2 Special characteristics of liquefied cycles, the present state of affairs of LNG in India, natural gas its import and CNG versus LNG as an automobile Natural gas is a mixture of paraffinic hydrocarbons fuel, eco-friendliness of natural gas fuel etc. It also such as methane, ethane, propane, butane etc. discusses the potential of natural gas generation Small amounts of higher hydrocarbons such as eth- from different sources, and the need for indigenous ylene may be present (nitrogen may also be pres- development of LNG technology for import substi- ent) apart from carbon dioxide and a trace amount tution in the Indian context. of hydrogen sulphide. Since most natural gas con- tains over 90% methane, natural gas becomes syn- Keywords: liquefied natural gas, compressed natu- onyms to methane. ral gas, cryogenic fuel, automobile fuel Details of useful properties and applications of natural gas are given in the literature. Some impor- tant properties and applications are highlighted here. Natural gas is a low density (0.789 basis air), 1 Introduction low sulphur content fuel, and it is cleaner fuel when The LNG production schemes in different countries compared to gasoline and is practically free from have gained tremendous momentum in recent sulphur and carbon monoxide emission. times because of i) economics of energy generation, The various important properties of natural gas ii) the growing concern about environmental pollu- are given in Table 1. Typical composition of indige- tion, iii) the necessity of fuller utilization of natural nous natural gas from Bombay High and the resources, and iv) the objections raised against the Tripura Field as presented in Table 2, could be flaring of by product gas from crude oil operations. taken into consideration while studying different, Natural gas can be transported from the gas pro- existing natural gas liquefaction cycles, as well for ducing areas to the gas consuming areas, either by proposing any new design for the purpose. pipeline or in a container or tanker in liquid form. However, pipelines and piped gas transported have 2.1 LNG versus CNG a number of drawbacks. Among others, pipelines The energy density of LNG is 435Kg/m3 as com- cannot be laid at depths of more than 100m below pared to 175 Kg/m3 for compressed natural gas sea level and, in any case, under water pipelines are (CNG) at 200 bars. This means that for a given expensive and sometimes risky. Furthermore, capacity fuel tank, an LNG powered vehicle can pipelines are a permanent fixture leading from a travel up to 2.4 times the distance of the CNG Journal of Energy in Southern Africa • Vol 16 No 4 • November 2005 55 Table 1: Important properties of natural gas sieve, and only methane is liquefied. Moreover, it is Auto ignition point 811K always economical to purify the gas than to provide Flammable range 5-15% extensive separation facilities in the liquefaction sec- tion of the plant. Laminar burning velocity 0.4 m/sec Carbon dioxide and hydrogen sulphide can also Atmospheric boiling point 111.5K be removed in absorbers containing mono ethanol Latent heat of vaporization 29.04KJ/Kg amine, and water vapour can be removed in Al2O3 Critical temperature 190.9K dryers and a diethylene glycol absorber. Details of purification methods for natural gas are available in Critical pressure 45.8 bar the literature. The basic practical cycles for natural gas lique- Table 2: Indigenous natural gas composition faction are (Timnerhaus & Flynn 1994; Lom 1975; Sarkar 2001; Sarkar et al. 1992; Sarkar 1997): Components a) Tripura Field b) Bombay High (i) Classical cascade cycle based on separate pure Percentage Percentage components such as propane, ethylene and by Volume by Volume methane. Methane 98.75 77.72 (ii) Mixed Refrigerant Cycle (MRC) using a refrig- Ethane 1.07 6.74 erant comprising a mixture of nitrogen and Propane 0.08 4.54 hydrocarbons such as methane, ethane, propane, butane and pentane. Isobutane 0.04 0.84 (iii) Expansion cycle using either the expansion of N butane Traces 1.17 feed stock or nitrogen. I Pentane Traces 0.23 The alternative cycles have the following fea- N Pentane Traces 0.23 tures: Hexane Traces 0.18 i) A Classical cascade has low power consumption and uses several compressors. Carbon dioxide 0.1% 6.51 (ii) A Mixed refrigeration cascade has medium 1.87 power consumption with one compressor. iii) An Expander cycle consumes very low power to liquefy a large flow of feedstock gas. counterpart, or in another way, for a given vehicle A Nitrogen expander cycle has very high power range, an LNG powered vehicle needs up to 2.4 consumption and is suitable for small capacity times less fuel tank capacity than its CNG counter- plants with a low utilization rate. part. Again, an LNG powered vehicle costs less to manufacture than a CNG powered vehicle. 4 LNG as an automobile fuel The capital and maintenance costs of LNG refu- LNG is superior to gasoline and fuel oil in terms of elling stations are a fraction of their CNG counter- calorific value, which is evident from Table 3 parts and they do not require any electricity. LNG is (Sarkar & Bose 1992; Sarkar & Bose 1994; www. portable, so it is free from the pipes, and the fuelling petronas.com; The Statesman 2000 & 2004; stations can be built at any places and for large Petronet, LNG Limited 2004). vehicles, LNG would be preferred than CNG. LNG as a road fuel has already been introduced in the Table 3: Comparison of calorific values UK (www.petronas.com; The Statesman 2000 & Fuel Gasoline Fuel oil LNG 2004; Petronet, LNG Limited 2004). Wt Calorific value 3 Gas liquefaction cycles Kj/Kg 48,238 43,781 55,446 It is well known that most natural gas contains more that 90% methane along with some higher hydro- LNG has special characteristics to make it a pop- carbons, moisture, carbon dioxide, nitrogen and ular engine fuel in the future. It makes available trace amounts of hydrogen sulphide. To prevent 41KJ /Kg for refrigeration as compared to 21.2 KJ blockage during liquefaction, impurities such as /Kg for liquid nitrogen. The higher refrigeration moisture, carbon monoxide etc. are invariably obtainable with LNG can be used towards cooling removed prior to liquefaction. in water jackets or improving the inter-cooling Higher hydrocarbons have also to be removed between compressor stages. prior to liquefaction, if there is no such arrangement In a high output turbo-charged piston engine, in the liquefaction plants for their separation by the refrigeration effect lowers the overall intake condensation. If the amount of impurities are com- charge temperature. This improves power output paratively less, then it is possible to remove all the and reduces the tendency towards knock and pre- impurities by adsorption in a carbon molecular ignition. It could be efficiently used as aircraft 56 Journal of Energy in Southern Africa • Vol 16 No 4 • November 2005 engine and ships engine fuel owing to its high tion of LNG production, distribution and utilization. octane number and easier maintenance. The PETRONAS LNG Complex in Bintulu, LNG can be used for feedstock for various Malaysia, is currently the World’s largest integrated chemicals, fertilizers, petrochemicals and for iron LNG facility at a single location with a combined ore reduction to sponge iron. The cold available production capacity of about 23 MMTA. LNG is during regasification of LNG can be utilized in the being supplied from this installation to South Korea liquefaction of industrial gases like oxygen, nitrogen (The Statesman 2000 & 2004). etc. LNG technology and its uses in India is still in its LNG is the fuel of the future. Big power compa- infancy. India has just entered into the LNG Age nies like Tata Electric are moving away from coal with the formation of Petronet LNG Limited, and its towards LNG and, in future, it is likely to replace first LNG terminal had started functioning in naptha as the main fuel for the plants.