Liquid Natural Gas: A reveiw
Dept. Of Automtoive Engineering, University of Antwerp, Belgium
The objective of this paper is to review Liquid Natural gas (LNG) as an alternate fuel by comparing it to other complex fuels like gasoline, diesel and CNG in the face of increasing energy demands, changing climatic conditions and strict legislative norms for emission reduction. LNG is a form of natural gas like CNG which is obtained by liquefying the natural gas by reducing the temperature of gas to -1620C. The properties of LNG changes with its composition but usually remains constant since the major component is methane (93-98%).
The paper also compares the advantages and disadvantages of the fuel in contrast to conventional fuels and concurs that LNG while having its advantages has its drawbacks in terms of technical and legislative challenges. The major advantage of LNG is its composition. Methane being the simplest hydrocarbon, contains only one carbon atom which when stoichiometrically combusted, give only 1 molecule of CO2 making it second to hydrogen in least polluting fuels.
Through this paper, we also compare the legislative challenges in various parts of the world and how LNG can be used to overcome those challenges. Thus, LNG can be used as a transitioning fuel to cleaner and better fuel management system in the automotive industry.
Keywords—LNG, Properties of LNG, Advantages and Disadvantages, Challenges of LNG)
In recent years, due to the increased use of oil and its by-products by developed and developing countries, the risk of depleting planet’s natural resources is increasing. Even in these current situation, a huge percentage of total transportation is fueled by Oil and its product. This continued use of fossil fuels is leading to increased emission levels around the world, resulting in greenhouse effect leading to climate change and increased average worldwide temperature. Thus, to rectify such dreadful result and to comply with the stringent emission norms issued by governments in order to control the emission percentages, in recent times a lot of research is being done in the field of energy to find alternate sources of fuels for transportation purposes. One such Alternate fuel is Natural gas (NG). NG in general is termed as cleanest and safest alternative fuel to oil based fuels currently in use.
Liquid Natural Gas (LNG) is Natural gas converted to liquid form for the ease of transportability. It is a colorless, odorless, non-corrosive and non-toxic in nature. The LNG is obtained by cooling the natural gas to a very low temperature. The process cooling of Natural gas (NG) is called liquefaction. Liquefaction of NG releases oxygen, carbon dioxide, Sulphur compounds and water contained within it. Due to this process, we can reduce the bulk or volume of NG by a factor of 600. This means that a particular mass of NG could be stored at a 600 times smaller container when liquefaction as compared to the gaseous state of the fuel 1.
1.1Properties of LNG:
To accurately forecast the properties of LNG, the composition of should be clearly distinguished. Depending on the property, LNG is stored at -162oC to -165oC. Due to the low temperature of LNG, the fuel is considered to be a cryogenic liquid. To remain a liquid, LNG must be stored in a thermos-like container that helps the liquid isolate from ambient temperature and reverting the process of liquefaction 1. Since LNG is considered as a cryogenic liquid, special care is taken to select the containers, as it can result in loss of strength turning the material brittle 2. The Most important of properties are as stated in the table below.
Properties of LNG
Boiling Point (oC)
-162 to -165
430 – 470
Flammability Limit (%)
5 – 15
Calorific Value (MJ/Kg)
Auto-Ignition Temperature (oC)
Table 1. Properties of LNG 2, 3
From the above table, we can see that LNG is majorly comprised of Methane (CH4), a hydrocarbon (HC) of least carbon content. The ratio of carbon to hydrogen is the lowest in it thereby making it only second to hydrogen in terms of carbon dioxide (CO2) production. Therefore, it is termed as the cleanest and safest alternative fuel available in current situation 4, 5. From the above table we also see that the density of LNG is lower than that of water, therefore when spilled over, LNG floats above water 4. Furthermore, the calorific value and auto-ignition temperature of LNG, when compared to diesel or gasoline are very high thus making is clear as an alternative to Diesel and Gasoline 6.
When compared to conventional fuels, LNG has a higher-octane number ranging from 110-130 which allows the higher compression ratio in a regular engine. Due to the relation between octane number and Auto-ignition temperature (essentially temperature withstanding capabilities), the fuel can be compressed to a higher pressure without autoigniting, thereby utilizing the working capacity of an engine to its maximum without loss of cylinder pressure leading in increase the efficiency of the engine 5.
2. Worldwide Applications of LNG:
The use of LNG in transport sector is rapidly increasing in many parts of the world. It is most commonly used as a clean alternative fuel for heavy duty trucks and environment friendly transportation systems in comparison to passenger vehicles. Due to comparative high idle timing in the passenger vehicle, increased heat leads to higher evaporation losses. This increased loss poses a threat in terms of safety as the flammability range for the fuel is as low as 5%, which could result in a fire. Therefore, to use LNG as a fuel, special cryogenic apparatus should be used to avoid evaporation and implied safety hazards.
While the above apparatus makes LNG an expensive fuel, many developing countries are adopting it as their ‘go-to fuel’ do to its availability, price and advantages over other conventional fuels. As of 2009, 11.4 million Natural gas vehicles(NVGs) are present in the market which run on some form of Natural gas (i.e., CNG or LNG) and is projected to reach about 30 million by 2017-2018. Figure 1. compares various countries based Number of NGVs in over the years until 2015.
Figure1. Worldwide NGV growth by region7
From the figure, we concur that the largest consumers of LNG are the Asia-pacific region which comprises of maximum developing nations in the world. Therefore, it can be concluded that that Natural gas transportation and technology has maximum penetration in countries like India, Pakistan, China, Iran and significant lower penetration in Europe for the amount of investment in the technology.
2.2 Electricity Generation:
While transportation is a major aspect of any type of fuel, electricity generation accounts for a substantial amount in terms of energy consumption and emissions. The fuel used for electricity generation can result in a change of supply-demand chain, thereby changing the pricing and availability of the fuel. A substantial amount of research has been done in the area of LNG utilization for electricity generation. According to Okamura et al. 8, LNG is the largest contributor to electricity generation in Japan. According to them, use of LNG contributed greatly in improvising the atmospheric environment by reducing CO2 emissions. According to many research conducted, the simplest method of using LNG is through a cold Rankine cycle that absorbs the evaporation heat from the environment and rejects the heat of condensation to preheat and evaporate LNG 4. This reduces the energy required for liquefaction of LNG thereby increasing the efficiency of the fuel. Querol et al. 9 examined various power generation method based on thermos-economic aspect and concluded that use of LNG in power generation is effective and economic.
2.3 Other applications:
In addition to transportation and electricity generation, LNG is also being used fertilizer industry. It performs as a crucial element in form of urea and ammonia which are key ingredients for fertilizer production. Furthermore, LNG is slowly replacing LPG and gaining a foothold in cooking and household heating systems all over the world.
3.1 Lowest Hydrocarbon emissions:
Since LNG is majorly comprised of CH4, it produces the least amount of carbon emissions in the atmosphere. Owing to a single Carbon atom in CH4, it is the simplest form of hydrocarbon available in nature. Therefore, it is only second to hydrogen in carbon emission % cumulatively. Hence, with increasing environmental standards, to curb pollution and improve the air quality and reducing the impact of global warming by reducing carbon emissions, LNG is gaining popularity and foothold in every aspect of industry.
3.2 Ease of Transportability:
Since LNG is produced by liquifying the natural gas removing Sulphur and water content, a higher quantity of Natural gas can be transported when compared to CNG or NG. Through liquefaction, it is possible to reduce the volume of gas by a factor of 600 which facilitates its transporting on ships, trucks and pipelines. This guarantees the global distribution of “green fuel” with increased “well to wheel” efficiency. It can also be easily converted back to gas which can be transported for domestic use.
3.3 Higher Energy Content:
As seen from the properties table, the energy content for LNG is higher than other conventional fuels. The energy content for LNG is 50.2 MJ/Kg in comparison to 46.4 MJ/Kg for gasoline and 48MJ/Kg for diesel. The above data suggests that comparatively reduced quantity of fuel is required for LNG in contrast to fossil fuels for producing the same amount of energy. This increases the efficiency of the fuel, resulting in reduced emissions for the same amount of energy.
3.4 Optimized Quality:
LNG is produced by liquifying Natural gas which eliminates impurities making it purer. This combined with higher energy content and higher CH4 percentages makes it chemically stable and increases the quality of the fuel.
In comparison to fossil fuel, which have reached their peak and have started to decline in terms of supply-demand aspect, LNG is still in its developing phase which allows it open to the market. Since LNG is not used in the same concentrations as fossil fuels, the cost of fuel is lower when compared to fossil fuels. This is an additional advantage to the economy for developing countries like India, Iran, which result in maximum consumption of natural gas. Figure 2. provides an overview of countries importing Natural Gas which gives an insight on the impact of LNG on the economy of developing and developed countries.
Figure 2. Import data for countries 4
4.1 Regulation standards:
Since LNG is a relatively new in the field of automotive industry, it does not have as stringent standards and regulations for L-NGV or the fuel quality as conventional fossil fuel vehicles or fuel standards. Due to lack of regulation, cost of the technology is impeded and creates a hinderance in manufacturing cost of equipment. Furthermore, various countries having different agendas and policies regarding the same prohibits the development of standardized technology.
Since a complete and wholesome equipment to handle LNG as a fuel is still not popularly available, manufacturers retrofit, a certain type of vehicle which leads in changing of the dynamics of vehicle leading fluctuating efficiencies. This also create an obstacle in creating a network of harmonized LNG fueling station since the parts would depend on the manufacturers and they are at a liberty to change the design of their parts until the technology standardized.
Due to the absence of standards for the fuels, the consistency of the quality of fuel is taking a serious hit. This consistency in fuel quality, especially in cryogenic processing of biogas and in LNG fueling station face a special challenge that needs to be addressed and rectified in order to create steady flow and growth of the fuel in the market.
4.4 Technological apparatus:
LNG is a cryogenic liquid, which means that it needs to be cooled to a very low temperature in order to liquify it. This serious reduction in temperature can be easily done on a stationary site through refrigeration. Nut to integrate LNG as a fuel on a NGV, the fuel should be stored in a special container at special conditions. For transportation purposes, the fuel should be stored at a pressure of 8 bar, in two cylinders separated by vaccum for safety purposes. Therefore, creating the apparatus to carry the fuel would be difficult.
5.Technological and legislative challenges:
5.1 Technological challenges:
With LNG having its advantages and disadvantages, the technology and fuel is not without its challenges. As mentioned earlier, LNG is a cryogenic fuel and requires continuous refrigeration to maintain the liquid form. This is best suited for large scale storage and fixed installation since the apparatus for refrigeration is not easily portable. Hence, the major challenge for LNG as a fuel is to maintain its liquid form.
This could be achieved by keeping the gas in a refrigerated and pressurized container instead of just atmospheric container. This will have a continuous heat leak, and therefore there will be a continual evaporation of the liquid. When there has been a take-off of gas in excess of the quantity that would evaporate by the heat leak, the cylinder pressure will fall. Once the take-off stops, the heat leak will raise the temperature of the liquid, causing the vapor pressure to rise again and pressurize the cylinder. Providing the take-off and heat leak are reasonably balanced, there will be no need to vent the excess vapor, as variations in the pressure would be accommodated within the pressure rating of the vessel. Thus, it is suited better for large vehicles such as Trucks and domestic buses.
But while considering a passenger or light utility vehicle, the functioning of vehicle is quite different in comparison to heavy vehicles and would tend to spend a long period out of use therefore, leak off of LNG would account for a substantial design consideration while designing the vehicle. For example, when a vehicle is garaged overnight, the leak off is likely to be sufficient to give rise to flammable atmosphere within the garage. This could be serious safety and fire hazard. Although the leak of gas cannot be prevented, it can be dealt with by installing an oxidizing catalytic converter, which could oxidize LNG to CO2 and H2O, thereby reducing the content of flammable fuel in air. But the major disadvantage of catalytic oxidizer is that it can be deliberately removed or could be poised which would increase the flammable content of fuel in the air. Therefore, it cannot be considered as a permanent solution to the hazard in question.
5.2 Legislative challenges:
LNG being a fairly new fuel in terms of its recent developments in applications, requires a lot more research to take place in order to standardize the technology and the fuel quality. In recent past, due to the lack of international standardization, the regulations and guidelines have been fairly regional and limited to areas such as Asia, North America and Europe. Hence to attain globalization of the fuel and its implied technology, internationally accepted standards have to be agreed upon by all the major stake holding countries in LNG energy like, Japan, America, India, Pakistan, Iran etc.
Furthermore, as mentioned earlier, the research and development in the field of application of LNG in automotive industries have to increase exponentially to standardize the fuel. The standardization of fuel can have a major impact on the LNG trading and market worldwide, since the fuel cannot be monopolized to a particular region and would have a specific and standard properties according to the international standards.
Also, LNG could prove useful in realizing the objectives of Euro6 emission legislation of reducing the pollution levels of below 3% and 5% for CO2and NOx. Figure 3 shows the evolution of European emissions legislation while Figure 4 shows the evolution of worldwide emission legislations in comparison to European standards.
Figure 4, portrays the emission levels as a function of color in the graph. The darker shades of green signify (India, China), more pollution or emissions in the countries in comparison to lighter shade of green (Europe, USA) which signify less pollution due to its stringent emissions legislations. As of 2014 from the graph we see, Europe to be transitioning from Euro5 to Euro6 whereas countries like India and china in BSIII and China IV respectively.
Figure 3. Evolution of European Emissions legislation 5.
From Figure 4 we can also concur that developing countries like India and China are about 10-12 years behind in emission rectification by legislative actions.
Figure 4. Emission Legislation Comparison Worldwide.
Figure 5. Legislative Projection comparison between India and Europe
From Figure 4 and Figure 5 we can concur that a developing country like India will need about 10 years to be at par with European Union. BS 6 emission legislature for India is projected to be instated nationwide by 2026. Thus, it is paramount for developing countries like India to adapt to a comparatively greener fuel like LNG in order to achieve the objectives for the proposed legislature. Although LNG wouldn’t affect the carbon emissions, it would have a significant impact on the HC and NOx.
Furthermore, with the deteriorating air quality in the aforementioned countries, it is a logical option to switch to LNG as an Automotive fuel since it has a negligible amount of Particulate matter emissions which would impact significantly in improving the air quality of the countries, thereby improving the living condition of the populace.
6. Personal Thoughts on Subject:
LNG is a promising fuel and could one day very well may be an alternative to complex fossil fuel in immediate future. But since it is a fossil fuel, it cannot be considered as a permanent solution to the rising emission and global warming problems the world encounters today. Due to its properties and reduced emission levels in comparison to complex fossil fuels like gasoline or diesel, it could very well be used as a transitioning fuel. It could be used as a bridge to reduce the differences between complex fossil fuels and completely clean fuels like hydrogen or electric vehicle in the automotive industry. This can be done by slowly phasing out of complex fuels in various stages. The first of many stages would be to use LNG as an additive with complex fuels to improve the characteristics of the fuel. After which, LNG can be used in higher % of mixtures to not only improve the characteristics but also improve the fuel efficiency and emission cycle of the fuel as a combination and finally using LNG as a pure fuel with no additives.
While this kind of procedure can prove to take time, the transitioning is half way through due to use of CNG as fuel. LNG can be used to obtain CNG by expanding the liquid in a safe manner so as not to make it unstable or flammable in nature which could prove to be a huge fire hazard inside the automotive system. Since the space occupied by LNG is 600 times less than CNG, a proposed solution to this could be to use 2 storage tanks in the system, one to store the LNG and the other to store the expanded CNG as and when required based on the pressure gradience of the tanks, instead of using 1 large tank.
However, the above-mentioned solution is a hypothetical approach to the problem and a vast amount of research is required to be conducted in the area of using LNG as a pure fuel in an Automotive system leading to standardization of fuel and its accompanying technology.
Like all Natural Gases, LNG is cleaner than its counterparts like coal, gasoline and diesel and also offers an opportunity to diversify our energy supplying capabilities. LNG will prove to be a more viable option when compared to CNG in long range use as the fuel offers special advantages over CNG through its transportation and storage capabilities and also provides flexibility through Liquid-Compressed Natural Gas (L-CNG) interface. LNG can be used to produce CNG by heating the liquid above its atmospheric liquid conditions (-1620C) or expanding the liquid from (8Bar) a pressurized container. This can prove to be very useful in using the fuel to produce energy, since the technology to use CNG as a fuel already exists in present time.
The major factors that could thrust LNG to be a transitioning fuel are, rapidly changing climatic conditions due to global warming, escalating oil prices due to increased consumption and demand of the fossil fuels, decreasing LNG costs due worldwide acknowledgement and trade of the fuel and availability of natural gas through coal and biomass.
With more developing countries like India, tuning to LNG for their energy needs to satisfy the legislative objectives, LNG demands are increasing and have the power to steer the supply chain for the fuel.
The existing cryogenic gas industries can suitably diversify in the LNG ?eld because of their historical depth of knowledge and width of expertise in handling and storage of cryogenic liquids. Moreover, the implementation of new safety measurements, increasing demand of LNG and expending LNG technology will drive LNG to become a globally promising fuel alternative.
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