Analysis of Fire and Explosion Properties of LNG Analiza właściwości pożarowych i wybuchowych LNG

Aim: The aim of this article is to analyse fire and explosion properties of LNG along with the identification of hazards that may arise during emergency incidents involving it. The article is based on an analysis of the available literature and a full-scale experimental study involving a 200-liter LNG tank leading to a jet fire. Introduction: Safe use and proper transport of flammable and harmful substances, together with the analysis of the effects of threats, enable the reduction of the number of accidents and provide possible conditions for the evacuation of people and property in a hazard zone. The compilation and systematization of knowledge on the safe use of the environmentally friendly LNG fuel will allow for an increase in the scope of its use. It is consistent with the state’s sustainable development policy consisting in identifying threats or adjusting technical solutions that minimize losses in transport or industry. Methodology: There are many legal acts in the world regarding safe storage and transport of LNG. One of the most important is Directive 2012/18/EC known as “Seveso III”. This document contains requirements for the prevention of major accidents involving hazardous substances – including LNG – and ways to reduce their negative effects on human health and the environment. Relevant requirements have also been specified in standards, tests, articles and other international acts, including in the European agreement on the international carriage of dangerous goods by road (the so-called ADR Agreement). The article compares flammable and explosive parameters of LNG. Possible scenarios occurring during the release and ignition of the LNG vapour cloud have been shown. The change of pressure of LNG vapour in the 200 l tank as a function of its heating time in the burning spill of a mixture of gasoline and diesel fuel is presented. In such a thermal exposure, a jet fire with a flame length of up to 5 meters was obtained. Conclusions: The proper use of flammable gases should be a priority in ensuring fire and explosion safety in facilities, during transport, etc. Hence, recognizing the threats and comparing them, or matching technical solutions that minimize the effects of LNG failures will allow active inclusion of knowledge in this field in the process of protection against fire and explosion. In case of LNG storage, attention should be paid to the types of materials in the immediate vicinity of this liquefied gas in order to have sufficient mechanical properties at the lowest liquefied gas temperature.


Introduction
Constant human need to maintain an adequate standard of living generates a greater or lesser degree of demand for energy.
Saving, optimal use and the search for renewable energy sources should be the subject of action in order to ensure continuity of energy supply. Certainly, the demand for energy and sources that can provide it, along with its safe use, must be consistent with the country's sustainable development policy [1]. The analysis of the safe use of flammable gases in the context of sustainable development, consisting in learning about the risks or adjusting technical solutions that minimize losses in transport or industry, allows to increase the level of safety of all rescuers and the people in the vicinity of flammable gases. Currently, the concept of sustainable development is increasingly entering the mainstream discussion on socio-economic development, becoming a horizontal principle reflected in the development policies of many countries, including fire safety. According to the U.S. Energy Information Agency [2], energy consumption by 2040 will increase by 28% of the previous level. All over the world, intensive efforts are made to develop renewable energy sources and nuclear energy. The production of this type of energy is prospective, but it is forecast that most of the energy generated in the world will be dominated by the use of crude oil and natural gas [2]. Among the various methods of energy production, the use of LNG (liquefied natural gas) as an energy source is considered to be more environmentally friendly than coal-fired or nuclear energy [3]. Power plants for regasification of liquefied natural gas, e.g. integrated with liquid air energy storage (LAES), due to their flexibility, seem to be a favourable technological solution (they adjust the electricity demand profile to the increased operating profits from energy arbitrage) [4].
The construction of the LNG terminal in Poland in Świnoujście in 2016 made it possible to receive liquefied natural gas by sea from virtually any direction in the world, which contributed to the SFT VOL. 58 ISSUE 2, 2021, PP. 58-73 diversification of gas supply sources and strengthened Poland's energy security. Distribution of the natural gas via a gas pipeline network is problematic due to the distance between the existing deposits and the potential recipients, as well as the significant costs and time of the investment. As a result, the LNG transport market is becoming more and more popular, as it enables the supply of natural gas to places where network connections are not available. The benefits of this solution mean that there are more and more supporters of the development of the infrastructure that allows for road and rail transport of natural gas processed into a liquid form. In land transport, liquefied natural gas is delivered by road and rail tankers. The possibilities offered by road transport include, above all, speed, availability and flexibility of the deliveries. Unfortunately, also during transport dangerous incidents can occur with LNG. An example of an incident involving LPG was recorded in 2011 in the province of Murcia in Spain -the driver lost control of the vehicle, drove off the road and got stuck in a ditch between the embankment and the roadside. Probably as a result of a fuel leak and the simultaneous presence of an effective energy stimulus, the tanker burst into flames, and then exploded [5].
One of the likely events involving LNG, which can cause high losses, is the boiling liquid expanding vapour explosion (BLEVE).
In 1940-2005, more than 1,000 people died as a result of more than 80 BLEVE incidents of flammable substances, more than 10,000 people were injured, and property losses amounted to billions of dollars [6]. Additionally, during the BLEVE explosion, toxic compounds such as chlorine and phosgene may be released, and the infrastructure surrounding the explosion site is destroyed.
Hence, getting to know the chemical properties of LNG and the analysis of the possible threats is necessary to predict and limit the effects of its release.

Physicochemical properties of LNG
One of the key energy resources used in the household and industry is natural gas [7][8][9]. In the industry,this gas is used in two forms -in a liquefied form as LNG and in a compressed form as CNG (compressed natural gas). Natural gas consists of over 90% methane, which is the simplest hydrocarbon belonging to the alkanes. Methane is produced naturally under the conditions of anaerobic decomposition of organic matter, and such processes are favoured by wet lands, e.g. marshes (hence it is called "mud" gas) or it can be produced synthetically [10]. The world's largest deposits of the natural gas are located primarily in Russia, the Middle East, the United States, Canada and Mexico.
In Western European countries, the largest deposits are found in the Netherlands and under the bottom of the British and Norwegian sectors of the North Sea. Natural gas in room conditions (i.e. approx. 23°C and 1024 hPa) is a colourless and odourless z arbitrażu energii elektrycznej) [4]. Powstanie
W przemyśle gaz ten stosuje się w dwóch postaciach -w formie skroplonej jako LNG oraz w postaci sprężonej jako CNG (ang. compressed natural gas). Gaz ziemny składa się w ponad 90% z metanu czyli najprostszego węglowodoru należącego do alkanów. Metan powstaje naturalnie w warunkach beztlenowego rozkładu materii organicznej, a takim procesom sprzyjają tereny podmokłe np. mokradła (stąd nazywany jest gazem "błotnym") lub może być wytwarzany syntetycznie [10] substance which, after cleaning and meeting the quality requirements, is liquefied at a temperature of approx. -162°C (the boiling point of LNG depends on its composition and amounts to from -166°C to -157°C) at normal atmospheric pressure [9][10]. This form of natural gas reduces its volume and facilitates transport and storage in vacuum-insulated tanks (i.e. tanks on LNG carriers, on LNG-powered ships, in permanent storage tanks and cryogenic tanks). Due to its very low temperature, LNG can cause both cracks in the materials that make up the walls of the tank and the ship's structure, as well as cause frostbite in people who come into contact with it. Hence, LNG tanks or fittings must be made of special materials resistant to cryogenic liquids and resistant to low temperatures [11]. LNG leaks can cause water to freeze in the air, creating a white fog. Liquefied natural gas is non-corrosive. The absolute density of LNG in liquid form at a temperature of about -160°C, depending on its chemical composition, ranges from 430 kg/m 3 to 470 kg/m 3 , and under extreme conditions it can even reach 520 kg/m 3 [9][10][11]. Hence, LNG spilled on water, the density of which is about 1000 kg/m 3 , floats on its surface as a lighter one. LNG -like methane -does not dissolve in water. Liquefied natural gas has a volume approx. 600 times smaller than in the gaseous state, which means that after regasification, 100 m 3 of LNG produces 60,000 m 3 of natural gas [1,9].

Fire and explosion hazards of LNG
Accidents involving LPG take various forms and depend on the conditions of gas operation, storage, and the type of event.
The combustion of methane gas, or when released from the liquid phase, is related to the type of emission source, the application of an effective ignition source within its emission, and the time it takes to form a fuel-air cloud. The density of gaseous methane at low temperature, close to its condensation (-160°C), is approx. 1.751 kg/m 3 , so its absolute density is higher than that of air [9]. Hence, when it is released, LNG initially has a temperature close to its condensation temperature and accumulates just above the ground or above the water surface and evaporates using energy from the environment. At this stage, after evaporation, LNG becomes a heavy gas, and its heating to approx. -123°C causes the LNG vapour to become gas with a density similar to air. Then, as the temperature rises to a value of about -110°C (-113°C for methane), it becomes lighter than air and mixes easily with it. The initial violent evaporation is continued until the evaporation rate reaches a constant value depending mainly on the thermal properties of the substrate into which LNG is released and the heat obtained from the emission environment -most often from the air. The substrate heats LNG, causing it to evaporate due to heat conduction from the surface, convection from the surrounding air and its humidity [14][15][16][17].
Water from air humidity is a latent energy store and supports the process of LNG evaporation. When LNG vapours are mixed with air, the latter is cooled and water from its moisture condenses as a result of the heat released from the phase change of water. If LNG leaks from pressure equipment or pipelines, it will be streamed into the atmosphere. This process is related to the intense physical mixing of LNG with air. At the initial stage of mixing, particles of liquid natural gas in the form of aerosols can be identified in the released cloud of the mixture, which will then gradually evaporate as a result of mixing with air. Ignition of the methane-air mixture formed as a result of LNG release occurs when an appropriate stimulus appears in the range of methane concentration in the air between the lower and upper explosion limits (LEL and UEL) [18]. The explosion limits of methane, like that of other flammable gases, are not constant and depend on pressure and temperature. The presence of inert components affects the ignition of a flammable natural gas mixture. As their concentration increases, the ignition conditions deteriorate and the GGW value drops significantly. Based on the research [16][17][18][19][20][21][22], it was found that the LEL of methane is 4.6 ± 0.3%, and the UEL of methane -15.8 ± 0.4%, when methane is ignited in the air at a temperature of 20°C and 100 kPa (relates to ambient temperature and pressure), while in oxygen they are 5.1-61.0 vol.% [19][20][21][22]. When the released LNG cloud, Wraz ze wzrostem ich stężenia pogarszają się warunki zapłonu i znacznie obniża się wartość GGW. Na podstawie badań [16][17][18][19][20][21][22] stwierdzono, że DGW metanu wynosi 4,6 ± 0,3%, a GGW metanu -15,8 ± 0,4%, gdy metan jest zapalany w powietrzu w temperaturze 20°C i 100 kPa (odnosi się do temperatury i ciśnienia otoczenia), natomiast w tlenie wynoszą 5,1-61,0% obj. [19][20][21][22]. The dispersion of LNG vapours into the environment has been extensively studied both numerically and experimentally [23][24][25][26][27][28].
The research was performed in order to analyse hazardous events involving LNG and to obtain a comprehensive database for numerical research. A wide range of experiments were conducted for different leak rates, spill sizes and terrain characteristics. However, most of them were carried out in desert areas with high air temperature and low humidity (temperature > 30°C and relative humidity < 30%). Air humidity (RH) influences the dispersion of LNG vapours and should be included in the research, as LNG shipping and receiving terminals are usually located in coastal areas, where the ambient air is usually very humid (RH > 50%) [12]. The probability of fog formation SFT VOL. 58 ISSUE 2, 2021, PP. 58-73 following an accidental release of LNG in these areas is significantly high. A cloud of fog that mixes with LNG vapours is commonly confused with a trace of an LNG cloud of vapours. The experimental studies by Cormier et al. [17] show different sizes of the visible gas cloud captured by a VHS and infrared camera.
The actual size of the LNG vapour cloud as recorded by the infrared camera was much larger than that of the fog cloud, meaning a larger area of LNG flammable gas. Fog is an aerosol consisting of airborne droplets of water or ice crystals when water vapour condenses or solidifies due to low temperature. Often, in the simulations, the fog is treated only as liquid water droplets, and the ice formation process is not taken into account as  Liquefied natural gas does not pose a threat in the form of environmental contamination. Upon contact with air, LNG evaporates and is thinned in the air. Therefore, it is much less harmful and dangerous fuel than crude oil or LPG. The average heat of combustion of LNG is about 39.26 MJ/m 3 , which in terms of mass amounts to 54 MJ·kg -1 . This value is close to the value of heat of combustion of LPG (approx. 54 MJ·kg -1 ) or acetylene (50 MJ·kg -1 ) [33][34]. LNG as a fuel is characterized by a relatively high volume of energy density compared to typical fuels such as crude oil or LPG, and in addition, during combustion it emits relatively small amounts of nitrogen oxides compared to diesel or gasoline or fossil fuels [35].
The low temperature of LPG compared to the ambient temperature can cause frostbite when it comes into direct contact with the human skin. Materials exposed to low temperature must have