Preparing for LNG by Rail Tank Car: A Readiness Review (2022)

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27 Although the prospect of shipping liquefied natural gas (LNG) by rail tank car is a new development, other commodities that are regulated as hazard- ous because of their cryogenic and flammable properties have been shipped in tank cars for decades. This chapter describes the composition of LNG and the hazards associated with its cryogenic and flammable properties. The chapter then compares the properties of LNG to those of other cryo- genic and flammable materials that are transported by rail in tank cars in the United States. WHAT IS LIQUEFIED NATURAL GAS? Natural gas is transported by the non-pipeline modes as a liquid because LNG takes up about 1/600 of the volume of natural gas uncompressed, making LNG advantageous when containment capacity is limited. In com- parison, compressed natural gas (as transported by pipeline) takes up about 1/200 to 1/250 of the volume of natural gas uncompressed. Natural gas is primarily composed of methane and ethane, plus small amounts of propane, butanes, carbon dioxide, and nitrogen. Before puri- fication for shipping, raw natural gas often contains additional impurities, including sulfur compounds, mercury, heavier hydrocarbons, water vapor, and oxygen.1 To create LNG, natural gas goes through a liquefaction 1 Penn State College of Earth and Mineral Sciences, “National Gas Composition and Speci- fications,” n.d., https://www.e-education.psu.edu/fsc432/content/natural-gas-composition-and- specifications. 3 Liquefied Natural Gas’s Hazardous Cryogenic and Flammable Properties

28 PREPARING FOR LNG BY RAIL TANK CAR process that removes impurities that can result in corrosion and freezing problems during transportation and storage. A natural gas liquefaction plant, such as the peak-shaving plants and ex- port facilities discussed in Chapter 2, performs four main processes: pretreat- ment, acid gas removal and dehydration, heavy hydrocarbon separation, and finally liquefaction.2 Pretreatment removes the first set of impurities, includ- ing dust, sulfur compounds, mercury, and slug, which is a combination of water and low-density hydrocarbon liquids. The next process uses an amine absorber and an adsorbent to remove carbon dioxide and water, which pre- vents ice from forming during liquefaction. Heavy hydrocarbons—pentane or heavier—are then separated from the remaining natural gas. During the liquefaction process, natural gas passes through a heat exchanger, where it is liquefied and cooled to approximately −260°F (−162.2°C). After purification and liquefaction, LNG is composed primarily of methane and ethane. The molecular composition of natural gas, and thus LNG, typically varies by origin. For example, in 2012, the average molecu- lar composition (i.e., molar content3) of LNG from the North West Shelf of Australia consisted of 87.33 percent methane, 8.33 percent ethane, 3.33 percent propane, 0.97 percent butanes, and 0.04 percent nitrogen, while the average composition of LNG from Alaska consisted of 99.71 percent methane, 0.09 percent ethane, 0.03 percent propane, 0.01 percent butanes, and 0.17 percent nitrogen.4 Based on the average composition of LNG re- ported by different locations, Table 3-1 shows the minimum and maximum molar content of each component in LNG. TABLE 3-1 Typical Content Limits of LNG Components5,6 Component Minimum Molar Content Maximum Molar Content Methane 87% >99% Ethane <1% 10% Propane <1% 5% Butanes <1% 1.5% Nitrogen <0.1% 1% 2 Cameron LNG, “LNG and Liquefaction,” n.d., https://cameronlng.com/lng-facility/lng- and-liquefaction. 3 In this context, the molar content, or molarity, is a measure of the number of molecules of a chemical species in a mixture. For example, on the Northwest Shelf of Australia, an average 87.33% of molecules in the LNG were methane. 4 International Group of Liquefied Natural Gas Importers, “The LNG Industry: GIIGNL Annual Report 2018,” 2018, https://giignl.org/wp-content/uploads/2021/08/rapportannuel-2018pdf.pdf. 5 Ibid. 6 S. Kuczyński, M. Łaciak, A. Szurlej, and T. Włodek, “Impact of Liquefied Natural Gas Composition Changes on Methane Number as a Fuel Quality Requirement,” Energies, vol. 13, no. 19, 2020, 5060, http://dx.doi.org/10.3390/en13195060.

LNG’S HAZARDOUS CRYOGENIC AND FLAMMABLE PROPERTIES 29 This variation in LNG composition will affect the material’s chemical and physical properties, including density, liquid-to-gas expansion ratio, and gas gross caloric value (GCV)—the energy content and quantity of heat released during the combustion of a unit volume of gas. Table 3-2 lists the minimum and maximum values of these physical properties based on the differences in LNG composition across origins. These variations primarily correspond to the percentage of methane in the LNG, with higher methane content resulting in a lower density, higher expansion ratio, and a lower gas GCV (MJ/m3).7,8,9 LNG PROPERTIES THAT CREATE HAZARDS The cryogenic and flammable properties of LNG can create hazards during transportation and storage. Among all cryogenic and flammable materi- als10 carried by rail, only ethylene and LNG are regulated as both types of hazards. The relevance of different cryogenic and flammable properties to particular hazard concerns is summarized in Table 3-3. LNG’s cryogenic temperatures can mean that the material’s inadvertent release from containment can be in the form of a very cold liquid. Exposure to the liquid can cause the embrittlement of materials. In cases where a small amount of the liquid is released, it will usually vaporize immediately. However, when released in sufficient volumes, the liquid state may be maintained, and the product may pool. Upon evaporation, a dense vapor TABLE 3-2 Select Chemical and Physical Properties of LNG at Atmospheric Conditions 11 Property Minimum Value Maximum Value LNG Density (kg/m3) 421.4 467.35 Gas Density (kg/m3) 0.72 0.83 Liquid-to-Gas Expansion Ratio 562.46 585.75 Gas Gross Caloric Value (MJ/m3) 39.91 45.32 7 International Group of Liquefied Natural Gas Importers, “The LNG Industry: GIIGNL Annual Report 2018,” 2018, https://giignl.org/wp-content/uploads/2021/08/rapportannuel- 2018pdf.pdf. 8 S. Kuczyński, M. Łaciak, A. Szurlej, and T. Włodek, “Impact of Liquefied Natural Gas Composition Changes on Methane Number as a Fuel Quality Requirement,” Energies, vol. 13, no. 19, 2020, 5060, http://dx.doi.org/10.3390/en13195060. 9 If you convert the gas GCV to MJ/kg, higher methane content will result in a higher gas GCV. This is because methane has higher energy content by weight compared to ethane, but methane has a lower density than ethane. 10 LNG is categorized as a hazardous gas (Hazard Class 2.1) but is transported as a cryo- genic liquid. Cryogenic ethylene falls within the same category. 11 International Group of Liquefied Natural Gas Importers, “The LNG Industry: GIIGNL Annual Report 2018,” 2018, https://giignl.org/wp-content/uploads/2021/08/rapportannuel-2018pdf.pdf.

30 PREPARING FOR LNG BY RAIL TANK CAR TABLE 3-3 Cryogenic and Flammable Properties Associated with Hazards Property Definition Potential Hazard Boiling Point The boiling point (BP) is the temperature at which a liquid transitions from the liquid to vapor state. Upon reaching its boiling point, a liquid will evaporate into a vapor and thus expand. Evaporation, if not controlled, can result in overpressurization of a container. Cryogenic Liquid A cryogenic liquid is a liquid having a boiling point below −130˚F (−90˚C). A flammable material that is cryogenic can be released as a cold liquid that potentially pools and partially vaporizes to form a dense liquid-vapor cloud that will sink until it warms and rises. Exposure to the release can result in cryogenic burns on people and embrittlement of materials. In addition, high concentrations of vapor can cause asphyxiation if enough oxygen is displaced. Liquid-to-Gas Expansion Ratio The expansion ratio of a liquefied and cryogenic substance is the volume of a given amount of that substance in liquid form compared to the volume of the same amount of substance in gaseous form, at room temperature and normal atmospheric pressure. A given amount of a liquid with a higher liquid-to-gas expansion ratio will expand into a larger volume upon evaporation compared to a given amount of a liquid with a lower liquid-to-gas expansion ratio. Adiabatic Flame Temperature The adiabatic flame temperature is the temperature at which a material burns in open air without a loss or gain of heat from the system. A flammable material with a higher adiabatic flame temperature will be hotter upon ignition compared to another flammable material that has a lower adiabatic flame temperature. Auto-Ignition Temperature The auto-ignition temperature is the lowest temperature at which a material will spontaneously ignite. If a flammable material is heated to a point above its auto-ignition temperature, it can spontaneously combust. Heat Flux The heat flux is the flow of energy per unit of energy per unit of time. Heat flux is commonly measured as W/m2 or Btu/(h × ft2). A flammable material that creates higher heat flux will result in greater energy transfer from the fire to surrounding materials, resulting in potentially greater thermal damage to people and property.

LNG’S HAZARDOUS CRYOGENIC AND FLAMMABLE PROPERTIES 31 Property Definition Potential Hazard Flash Point The flash point is the temperature at which a flammable material will flash from an ignition source, but not necessarily continue combustion. When a flammable material reaches its flash point, it can ignite upon contact with a source of ignition. Lower Flammable Limit (LFL) and Upper Flammable Limit (UFL) The lower flammable limit is the lowest concentration of a gas or vapor (percentage by volume in air) below which a flame will not spread in the presence of an ignition source. Concentrations lower than LFL are “too lean” to burn. The upper flammable limit is the highest concentration of a gas or vapor (percentage by volume in air) above which a flame will not spread in the presence of an ignition source. Concentrations higher than UFL are “too rich” to burn. These are also known as the lower and upper explosive limit (LEL and UEL). A material at concentrations between its lower flammable limit and higher flammable limit can ignite and burn upon contact with an ignition source. cloud may form above the pool and initially remain concentrated near the ground until it warms. The density of methane vapor as it evolves from an LNG pool is approximately 1.8 kg/m3, which is heavier than air.12 The du- ration of the pool can depend on factors such as the terrain, meteorological conditions, and pool size.13 People who are exposed may suffer cryogenic burns, and, until the vapor cloud disperses, there can be a risk of asphyxi- ation from displaced oxygen. In addition, the vapors’ presence could go undetected because the cryogenic temperature of the liquid precludes the 12 National Institutes of Standards and Technology, “Thermophysical Properties of Fluid Systems,” n.d., https://webbook.nist.gov/chemistry/fluid. 13 Based on experimental data of LNG by R. C. Reid and R. Wang (Cryogenics, 1978, pp. 401–404), the mass flux, in this case the boil-off rate, decreases with one over the square root of time on substrates such as concrete and soil; that is, mass flux = constant/sqrt(time). Reid and Wang determined the constant to be about 0.5 for soil. For example, an estimate for a 0.3-m-deep (1-ft-deep) LNG pool would take about 5 hours to evaporate without ignition. This rate will vary depending on the soil and the surface-area-to-volume ratio. TABLE 3-3 Continued

32 PREPARING FOR LNG BY RAIL TANK CAR addition of odorants in shipments. Odorants are normally added to gas transported under pressure to signal a leak.14 Methane vapor, which is the majority component of LNG vapor, is flam- mable but does not become a combustion hazard until it reaches concentra- tions in air of 5 to 15 percent by volume. At concentrations below the lower end of this range (or flammability limit), there is not enough fuel to sustain a combustion reaction, while at concentrations above the upper end of the range, there is not enough oxygen for combustion. Indeed, these flammability limits are key factors in designating the size of flammable vapor dispersion exclusion zones,15 which are areas surrounding an LNG storage container, transfer system, or facility in which an operator or government agency legally controls all activities for safety reasons. For LNG facilities, the vapor disper- sion exclusion zone is established by modeling where the vapor cloud from a leak would have an average fuel concentration at 2.5 percent or higher, or half of LNG’s lower flammability limit (5 percent concentration). While a spark or flame is needed to ignite LNG vapor in concentrations between its lower and upper flammability limits, concentrations of LNG vapor will not disperse as quickly as releases of natural gas, increasing the potential for the concentrations to spread and encounter an ignition source.16 Once ignited to create a pool fire, LNG has a high flame temperature and high heat flux. The latter is defined as the thermal energy transferred between a fire and any surrounding materials. PROPERTIES IN COMPARISON WITH OTHER FLAMMABLE AND CRYOGENIC MATERIALS TRANSPORTED BY RAIL Table 3-4 compares LNG’s flammable properties with those of other flam- mable materials transported by tank car. The properties of ethylene are provided along with those of the main components of LNG and liquid petroleum gas (LPG). Methane and propane are the predominant compo- nents of LNG and LPG, respectively. However, because small amounts of ethane are in LNG and small amounts of butane are LPG, their properties are also shown. Note that these properties are not absolute quantities, as heat and mass transfer depends on the scale, geometric configuration, and atmospheric conditions. When compared to propane, methane has a lower boiling point and higher liquid-to-gas expansion ratio. This means that LPG will evaporate 14 49 CFR § 192.625, “Odorization of gas.” 15 49 CFR § 193.2059, “Flammable vapor-gas dispersion protection.” 16 National Transportation Safety Board, “Pipeline Accident Report: Columbia Liquefied Natural Gas Corporation Explosion and Fire, Cove Point, Maryland, October 6, 1979.” Washington, DC: National Transportation Safety Board, April 16, 1980, https://ntrl.ntis.gov/ NTRL/dashboard/searchResults/titleDetail/PB80185721.xhtml.

LNG’S HAZARDOUS CRYOGENIC AND FLAMMABLE PROPERTIES 33 and expand less rapidly upon heating than LNG. With regard to these two properties, LNG is most similar to ethylene. Ethylene’s boiling point and liquid-to-gas expansion ratio are between those of methane and ethane, the two dominant components of LNG. TABLE 3-4 Chemical and Physical Properties of Ethylene and Hydrocarbons Present in LNG and LPG at Atmospheric Conditions17,18 Ethylene19,20 Methane21,22,23 Ethane24,25,26 Propane27,28 Butane29,30 Boiling Point in ˚F (˚C) at 1 atm −155.5 (−104.2) −258.7 (−161.5) −127.5 (−88.6) −43.8 (−42.1) 31.1 (−0.5) Flash Point in ˚F (˚C) −213 (−136.1) −306 (−187.8) −211 (−135) −156 (−104.4) −76 (−60) Auto-ignition Temperature in ˚F (˚C) 914 (490) 1004 (540) 940 (504.4) 842 (450) 550 (287.8) 17 W. M. Haynes, D. R. Lide, and T. J. Bruno, CRC Handbook of Chemistry and Physics: A Ready-Reference Book of Chemical and Physical Data, 97th Edition (Boca Raton, FL: CRC Press, 2016), pp. 15–49. 18 National Fire Chiefs Council, “Liquefied Petroleum Gas (LPG),” n.d., https://www.ukfrs. com/guidance/search/liquefied-petroleum-gas-lpg. 19 Airgas, “Material Safety Data Sheet: Ethylene,” February 2004, https://terpconnect.umd. edu/~choi/MSDS/Airgas/ETHYLENE.pdf. 20 National Institute of Standards and Technology, “Ethylene,” NIST Chemistry WebBook, SRD 69, n.d., https://webbook.nist.gov/cgi/cbook.cgi?ID=C74851&Mask=4. 21 National Oceanic and Atmospheric Administration, “Chemical Datasheet: Methane,” CAMEO Chemicals, n.d., https://cameochemicals.noaa.gov/chemical/8823. 22 Princeton University, “Cryogenic Liquids,” n.d., https://ehs.princeton.edu/book/export/ html/184. 23 National Institute of Standards and Technology, “Methane,” NIST Chemistry WebBook, SRD 69, n.d., https://webbook.nist.gov/cgi/cbook.cgi?ID=C74828&Units=SI&Mask=4. 24 National Oceanic and Atmospheric Administration, “Chemical Datasheet: Ethane,” CAMEO Chemicals, n.d., https://cameochemicals.noaa.gov/chemical/8619. 25 Airgas, “Material Safety Data Sheet: Ethane,” February 2004, https://www.mandtsystems. com/documents/MSDS_Ethane.pdf. 26 National Institute of Standards and Technology, “Ethane,” NIST Chemistry WebBook, SRD 69, n.d., https://webbook.nist.gov/cgi/cbook.cgi?ID=C74840&Units=SI&Mask=4. 27 National Oceanic and Atmospheric Administration, “Chemical Datasheet: Propane,” CAMEO Chemicals, n.d., https://cameochemicals.noaa.gov/chemical/9018. 28 National Institute of Standards and Technology, “Propane,” NIST Chemistry WebBook, SRD 69, n.d., https://webbook.nist.gov/cgi/cbook.cgi?ID=C74986&Units=SI&Mask=4. 29 National Oceanic and Atmospheric Administration, “Chemical Datasheet: Butane,” CAMEO Chemicals, n.d., https://cameochemicals.noaa.gov/chemical/5668. 30 National Institute of Standards and Technology, “Butane,” NIST Chemistry WebBook, SRD 69, n.d., https://webbook.nist.gov/cgi/cbook.cgi?ID=C106978&Units=SI&Mask=4. continued

34 PREPARING FOR LNG BY RAIL TANK CAR Ethylene Methane Ethane Propane Butane Adiabatic Flame Temperature in Air in ˚F (˚C) 3815 (2102) 3565 (1963) 3580 (1971) 3590 (1977) 3587 (1975) Lower Flammability Limit (LFL) 2.75% 5% 2.9% 2.1% 1.9% Upper Flammability Limit (UFL) 28.6% 15% 13% 9.5% 8.5% Liquid-to-Gas Expansion Ratio 489 650 437 274 233 While LNG (methane and ethane) has a lower flash point than LPG (propane and butane), its lower flammability limit is higher, thereby reducing the risk of ignition. Ethylene has a lower flash point than methane and one that is similar to ethane. However, the higher auto-ignition temperatures of methane and ethane decrease the chance (relative to LPG) of an LNG fire given a source of heat. Another flammability property, the adiabatic flame temperature,31 indicates the temperature of the combustion products if no heat is lost to the surrounding environment. LPG’s propane and butane components have higher adiabatic flame temperatures than LNG’s methane and ethane components. Based on this value, one might expect more thermal damage from an LPG fire; however, a material’s actual flame temperature will be the temperature after heat is lost to the environment and is typically significantly lower than the adiabatic flame temperature. Compared to LPG’s propane and butane, LNG’s methane and ethane have a wider range in their flammability limits—a larger difference between their lower and upper flammability limits. This suggests a higher potential for LNG to be in concentrations that will catch fire if exposed to an ignition source. When compared with ethylene, however, LNG’s upper and lower flammability limits are not as wide. When considering the radiative heat flux generated by a pool fire, LNG’s average experimental surface emissive power (SEP)—the amount of heat radiated outward from a flame per unit surface area—is three to five 31 Adiabatic indicates a situation in which heat does not enter or leave a system. The adia- batic flame temperature is thus the temperature during a combustion reaction in which no heat is gained from or lost to the surrounding environment. TABLE 3-4 Continued

LNG’S HAZARDOUS CRYOGENIC AND FLAMMABLE PROPERTIES 35 times higher than that of many other commonly transported hydrocarbons. Hydrocarbons with lower SEPs include LPG, diesel, gasoline, kerosene, and crude oil.32,33,34,35,36,37,38 Due to its much higher SEP, an LNG fire will result in a larger region of thermal damage at any given time. Using average SEP values, a solid flame model39 can be used to predict the heat flux from a pool fire of LNG or LPG. Based on this model, the thermal radiation pro- tection zone (where thermal damage to property and people can occur) of an LNG pool fire is approximately three times greater than that of LPG.40 While it would also be of interest to compare the SEP of LNG with that of ethylene (the only other cryogenic and flammable commodity transported by rail), comparable data for an ethylene pool fire could not be found. Table 3-5 compares LNG’s cryogenic properties with those of other cryogenic liquids transported by tank car. These cryogens—argon, nitrogen, and oxygen—all have a lower boiling point and higher liquid-to-gas expan- sion ratio than LNG. Because of their lower boiling points, they are more likely than LNG to evaporate rapidly upon heating. Moreover, their higher liquid-to-gas expansion ratios indicate that the product will expand into a larger volume upon evaporation. In addition, a lower boiling point indi- cates that the cryogenic liquid will be transported at a lower temperature, increasing the risk for cryogenic burns or damage if inadvertently released to cause exposure. 32 LPG, diesel, gasoline, kerosene, and crude oil have surface emissive powers between 40 and 80 kW/m2. LNG is reported to have a surface emissive power ranging between 150 and 290 kW/m2. 33 M. Munoz, E. Planas, F. Ferrero, and J. Casal, “Predicting the Emissive Power of Hydro- carbon Pool Fires,” Journal of Hazardous Materials, vol. 144, pp. 725–729, 2007. 34 A. Luketa, B. Blanchat, D. Lord, J. Hogge, A. Cruz-Cabrera, and R. Allen, “Pool Fire and Fireball Experiments in Support of the US DOE/DOT/TC Crude Oil Characterization Research Study,” Sandia National Laboratories, Albuquerque, NM, SAND2019-9189, 2019. 35 G. Mizner and J. A. Eyre, “Large-Scale LNG and LPG Pool Fires,” EFCE Publication Series (European Federation of Chemical Engineering) 25, pp. 147–163, 1982. 36 T. Blanchat, P. Helmick, R. Jensen, A. Luketa, R. Deola, J. Suo-Anttila, J. Mercier, T. Miller, A. Ricks, R. Simpson, B. Demosthenous, S. Tieszen, and M. Hightower, “The Phoenix Series Large Scale LNG Pool Fire Experiments,” SAND2010-8676, 2011. 37 D. Nedelka, J. Moorhouse, and R. Tucker, “The Montoir 35 m Diameter LNG Pool Fire Experiments,” in Proceedings of LNG IX, 9th International Conference & Exp on LNG, Nice, France, 1989. 38 P. K. Raj et al., “Experiments Involving Pool and Vapor Fires from Spills of Liquefied Natural Gas on Water,” ADA 077073, Arthur D. Little, June 1979. 39 K. Mudan, “Thermal Radiation Hazards from Hydrocarbon Pool Fires,” Progress in Energy and Combustion Science, vol. 10, pp. 59–80, 1984. 40 For example, the heat flux of LPG and LNG can be compared as a function of distance from the center of a 50-m-diameter pool fire using a solid flame model for different values of average SEP. A SEP of 50 kW/m2 and 280 kW/m2 reflect values representative of an LPG and LNG pool fire, respectively. Using the solid flame model, the LNG pool fire results in a heat flux of 35 kW/m2 at a distance approximately three times greater than that of LPG.

36 PREPARING FOR LNG BY RAIL TANK CAR TABLE 3-5 Chemical and Physical Properties of Nonflammable Cryogenic Liquids41 Nitrogen42 Argon43 Oxygen44 Boiling Point in ˚F (˚C) at 1 atm −321 (−196.1) −303 (−186.1) −297 (−182.8) Liquid-to-Gas Expansion Ratio at 1 atm and 20°C 710 860 875 SUMMARY Unlike all cryogenic commodities commonly transported in bulk by rail in the United States, with the exception of ethylene,45 LNG combines the hazards of a cryogen with the hazards of a flammable gas. Being flammable, LNG vapor may ignite when released to reach concentrations in air of 5 to 15 percent. In addition, LNG’s combustion in a pool fire will create high flame temperatures and high heat flux to surrounding materials. LNG’s heat flux is three to five times higher than that of other commonly transported hydrocarbons, including LPG; hence, its combustion will result in a larger region of thermal damage. LNG’s cryogenic temperatures can mean that the material’s inadvertent release from containment can be in the form of a very cold liquid, exposure to which can cause the embrittlement of materials. When LNG is released in sufficient volume, the liquid state may be maintained to form a vapor– liquid pool that can cause cryogenic burns and asphyxiation by people exposed. Because most cryogenic commodities (i.e., argon, nitrogen, and oxygen) have lower boiling points than LNG, they must be transported at even lower temperatures. As a result, these cryogens pose an elevated risk for cryogenic burns and material embrittlement if inadvertently released to cause exposure. An exception is ethylene, the only other cryogen that is also flammable. Its higher boiling point allows it to be transported at higher temperatures that pose lower risk of embrittlement to materials if released. 41 Northeastern University Office of Environmental Health and Safety, “Cryogenic Liquids,” March 2004, https://www.northeastern.edu/ehs/ehs-programs/laboratory-safety/fact-sheets/ cryogenic-liquids. 42 National Institute of Standards and Technology, “Nitrogen,” NIST Chemistry WebBook, SRD 69, n.d., https://webbook.nist.gov/cgi/cbook.cgi?ID=C7727379&Units=SI&Mask=4. 43 National Institute of Standards and Technology, “Argon,” NIST Chemistry WebBook, SRD 69, n.d., https://webbook.nist.gov/cgi/cbook.cgi?ID=C7440371&Units=SI&Mask=4. 44 National Institute of Standards and Technology, “Oxygen,” NIST Chemistry WebBook, SRD 69, n.d., https://webbook.nist.gov/cgi/cbook.cgi?ID=C7782447&Units=SI&Mask=4. 45 Hydrogen is also authorized but is not currently shipped by rail in the United States.