2. Important features of lng

Natural gas is a mixture of paraffinic hydrocarbons such as methane, ethane, propane and butane, etc. Small amounts of higher hydrocarbons such as ethylene may be present apart from carbon dioxide, a trace amount of hydrogen sulfide and nitrogen. Since, LNG is the cleanest form of natural gas and contains more than 98% methane therefore, it becomes synonyms to methane. Natural gas is a low density (0.789 basis air) and low sulfur content fuel as compared to gasoline, and is practically free from carbon monoxide emission.

Natural gas is converted to LNG by cooling it down to −162 °C [5], at which it becomes a liquid and this process reduces its volume [6] by a factor of more than 600. The ability to convert natural gas to LNG, which can be shipped on specially built ocean-going ships, provides consumers with access to vast natural gas resources worldwide.

LNG is a clear, odorless, non-toxic, non-corrosive, cryogenic liquid at atmospheric pressure. The density of LNG is approximately 0.4–0.5 kg/L, depending on temperature, pressure and composition, compared to water at 1.0 kg/L. Thus LNG, if spilled on water, floats on top and vaporizes rapidly. In the absence of an ignition source, LNG evaporates quickly and disperses, leaving no residue. Hence, no environmental cleanup needed for LNG spills on water or land. LNG, when vaporized in gaseous form, will only burn in concentrations of between 5% and 15% mixed [7] in the air. The removal of acid gases (gas sweetening) such as CO₂ and H₂S, from natural gas before liquefaction is an important process for producing pure methane. The comparison of various important properties of LNG with other liquid fuels is given in Table 1[8].

Table 1.

Comparison of physical and chemical properties of LNG with diesel, gasoline and LPG.

Properties	LNG	Diesel	Gasoline	LPG
Auto ignition point (°C)	540	316	257	454–450
Flash point (°C)	−187	60	−45	−104
Boiling point (°C)	−160	204	32	−42
Flammable range (%)	5–15	N/A	1.3–6	2.1–9.5
Stored pressure	Atmospheric	Atmospheric	Atmospheric	Pressurized
Toxic	No	Yes	Yes	No
Carcinogenic	No	Yes	Yes	No
Health hazards	None	None	Eye irritant	None

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2.1. LNG vs. CNG and LPG

CNG and LPG are often confused with LNG while CNG and LPG have quite different properties than LNG at similar conditions of temperature and pressure.

CNG is a mixture of hydrocarbons consisting of approximately 80–90% methane in gaseous form, and it is colorless, non-carcinogenic, non-toxic, inflammable and lighter than air. Because of its low energy density, it is compressed to pressure of 200–250 kg/cm² (to enhance the vehicle on-board storage in a cylinder). Superior to petroleum, it operates at one-third the cost of conventional fuel and is hence, increasingly becoming popular with automobile owners.

As far as LPG is concerned, it is a clean, high octane, abundant and eco-friendly fuel. It is obtained from natural gas through fractionation and from crude oil through refining. It is a mixture of petroleum gases like propane and butane. LPG is a gas at atmospheric pressure and normal temperatures, but it can be liquefied when moderate pressure is applied or when the temperature is sufficiently reduced (−42 °C). This property makes the fuel an ideal energy source for a wide range of applications, as it can be easily condensed, packaged, stored and utilized. When the pressure is released, the liquid makes up about 250 times its volume as gas, so large amounts of energy can be stored and transported compactly.

In liquefied form, the volume of LNG is 600 times less than the same amount of natural gas at room temperatures while the volume of CNG is 1% less of its original volume. LNG shipping is therefore an economic way of transporting large quantities of natural gas over long distances as compared to other natural gases such as CNG and LPG [9] and [10]. LNG is transported and stored at normal atmospheric pressure and LNG carriers are purpose-built tank vessels for transporting LNG at sea.

The energy density of LNG is 435 Kg/m³ as compared to 175 kg/m³ for compressed natural gas (CNG) at 200 bar. This means that for a given capacity fuel tank, an LNG powered vehicle can travel up to 2.4 times the distance of the CNG counterpart, or in another way, for a given vehicle range, an LNG powered vehicle needs up to 2.4 times smaller fuel tank capacity than its CNG counterpart. Again, an LNG powered vehicle costs less than a CNG powered vehicle for manufacturing. The capital and maintenance costs of LNG refueling stations are a fraction of their CNG counterparts and they do not require any electricity.

Hence, LNG offers special advantages over CNG and LPG in terms of easier transportation, storage and better density than gaseous methane. LNG also offers additional flexibility as liquefied to compressed natural gas (L-CNG). Additionally, LNG has a role of contributing to the development of biogas-to-biomethane as a vehicle fuel, both for gas purification and transport.

3. Worldwide use of LNG

3.1. Transportation

The use of LNG in transport sector is increasing rapidly in many parts of the world [11]. It is the most commonly used as a clean burning alternative vehicle fuel in thousands of heavy duty trucks, buses and waste collection trucks as compared to passenger cars because on average passenger cars stand idle more often, which would give rise to high evaporative losses. The use of LNG requires storage facilities for the cold (-162 °C) liquid natural gas at the roadside refueling stations and special fuelling equipment which can handle cryogenic temperatures [12]. In addition, the trucks must be equipped with special dual fuel engines to use LNG. Moreover, the fuel tank on board of the truck needs to be adapted for LNG usage. These requirements make the use of LNG relatively expensive.

LNG is superior to gasoline and oil in terms of calorific value, which is evident from Table 2[13] and [14]. It makes available 41 KJ/kg for refrigeration as compared to 21.2 kJ/kg for liquid nitrogen. This characteristic of being able to obtain high refrigeration enables LNG to be used towards cooling in water jackets or improving the inter-cooling between compressor stages. In a high output turbo-charged piston engine, the refrigeration effect lowers the overall intake charge temperature. This improves power output and reduces the tendency towards knock and pre-ignition.

Table 2.

Comparison of energy contents of LNG vs. CNG, diesel, gasoline and LPG.

Fuel	CNG	LNG	Diesel	Gasoline	LPG
Energy content (gross heating value)	37–40 MJ/m3	25 MJ/L	38.3 MJ/L	34.5 MJ/L	25.4 MJ/L
	46–49 MJ/kg

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It could be efficiently used as aircraft engine and ships engine fuel owing to its high octane number and easier maintenance. A recent study has revealed that LNG would be the preferred feedstock for power, chemical, fertilizer [15] and petrochemical plants in future. Big power companies are moving away from coal towards LNG and, in future, it is likely to replace naphtha as the main fuel for the plants.

Its consumption is expected to grow from, presently, 12.5 million tons to 50 million tons by 2016–2017 [16]. Moreover, LNG has good antiknock characteristics therefore it can be used with higher compression than gasoline inside the motor without premature ignition of the fuel/air mixture [17]. Natural gas (LNG) offers a higher thermal efficiency and lower specific energy consumption than gasoline and oil, hence, it is expected as a promising fuel for the future. In addition because of more stringent standards against environmental emissions and its regulations as well as economic reason, natural gas is considered as a clean-burning, alternative fuel for the transportation sector.

A brief review on the use of LNG as a transport fuel is reported below.

Natural gas has been used as fuel for transportation for decades and currently, about 11.4 million NGVs of different types are running either on CNG or LNG worldwide, which correspond to a share of 1% of the total vehicles population [11] and [17]. The leading users of NGVs are Pakistan (2.7 million), Iran (1.95 million), Argentina (1.9 million), Brazil (1.6 million), and India (1.0 million) [17] as represented in Fig. 2, with the Asia–Pacific region leading with a global market share with 5.7 million NGVs, followed by Latin America with almost 4 million vehicles. In Latin America and Asia, the increase of natural gas vehicles has been particularly strong in recent years when oil prices escalated [18]. These countries together have more than half of the worldwide existing stock of natural gas vehicles. In Iran and India, the stock of natural gas vehicles amounted to more than 800,000, followed by Italy (5, 80,000).

Fig. 2. 

World’s leading NGVs users.

Source: GVR Gas vehicle report 2009.

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However, LNG as a road fuel has already been introduced in the UK [16]. There are about 3000 LNG vehicles in US that run on LNG and most LNG vehicles are government owned; there are 40 governments – owned 40 LNG fuelling stations at the same time. Today, approximately 50 LNG vehicle fueling stations are available worldwide [19].

The 10 countries following the front runners – among them are China, Russia and the US–have obviously lower stock figures. However, they are well above the German stock figure (64,454 NGVs). With regard to the total car population of a given country, Bangladesh has more than 20%, Armenia 13%, Pakistan14%, Argentina and Brazil have 10% natural gas fueled cars. In contrast, the share of natural gas fueled cars in Europe is very low.

Even in Italy, where CNG was already used as fuel for cars in the1930s; natural gas vehicles have a share of only 1.1% of the total car population. In Germany, the corresponding share is only 0.1%. In addition to cars, buses and trucks the two-and three-wheel vehicles are also driving with natural gas. However, cars dominate the natural gas vehicles fleet in those countries where natural gas is widely used as a vehicle fuel. One exemption is India, where natural gas cars have a share of 38% natural gas vehicles stock and two and three-wheeler natural gas vehicles play an important role.

Another exception is the Ukraine with a share of natural gas cars amounting only to 6%. In Ukraine, buses as well as truck shave a share of 25%, while, other vehicles even reach a share of about 44%. To compare this with European countries, Germany’s natural gas cars have a share of 79% of the total natural gas vehicles population, while the share of trucks amounts to 18%, and the share of buses is very low (2%).

LNG is included as a substitute of crude oil in automotive fuel chains in a number of studies [20], [21],[22] and [23]. Williams et al. [20] and Gover et al. [21] studied only a small number of natural gas based chains. The focus of Gover et al. [21] was specifically on future fuel chain options for the UK.

The use of LNG as a transportation fuel in the heavy trucking industry has been reported by William et al. and Litzake et al. [24], [25] and [26]. According to their report LNG can directly substitute the diesel fuel and for heavy-duty vehicle applications, such as haul trucks, LNG is the most viable option for long range use. The amount of fuel that can be stored in a liquefied state with cryogenic tanks greatly exceeds a CNG system.

Further, the usefulness of LNG as a motor fuel and refrigerant has been reported by Kirillov [27]. As per their report, LNG is effective not merely as a cheap and ecologically clean fuel. It may, at the same time, be a source of refrigeration as well.