Liquid hydrogen: Difference between revisions

| ImageFile2 = Liquid_Hydrogen_pourFile:Liquid_hydrogen_bubblechamber.jpg

| Formula = {{chem2|H2}}_{([[Liquid|l]])}}}

| Density = {{convert|700.8507085|g/Lcm3|lb/ft3|abbr=on}}<ref>[http://webbook.nist.gov/cgi/fluid.cgi?Action=Load&ID=C1333740&Type=SatT&Digits=5&PLow=.5&PHigh=1.5&PInc=.1&RefState=DEF&TUnit=K&PUnit=atm&DUnit=kg/m3&HUnit=kJ/mol&WUnit=m/s&VisUnit=uPa*s&STUnit=N/m Thermophysical Properties of Hydrogen] , nist.gov, accessed 2012-09-14</ref>

| MeltingPt_ref = <ref name="h">[http://www.safety.seas.harvard.edu/services/hydrogen.html ''Information specific to liquid hydrogen''] {{webarchive|url=https://web.archive.org/web/20090717083849/http://www.safety.seas.harvard.edu/services/hydrogen.html |date=2009-07-17 }}, harvard.edu, accessed 2009-06-12</ref>

| BoilingPt_ref = <ref name="h"/>}}

| HPhrases = {{HPhrases|H220|H280}}

| PPhrases = {{PPhrases|P210|P377|P381|P403}}

| GHS_ref = <ref>GHS: [https://gestis.dguv.de/data?name=007010 GESTIS 007010]</ref>

| PEL = }}

'''Liquid hydrogen''' (H_{2{{chem2|H2(l)}}}) is the [[liquid state]] of the element [[hydrogen]]. Hydrogen is found naturally in the [[molecule|molecular]] H₂ form.<ref>{{cite web |title=We've Got (Rocket) Chemistry, Part 1 |url=https://blogs.nasa.gov/Rocketology/tag/liquid-hydrogen/ |website=NASA Blog |date=15 April 2016 |access-date=3 October 2021}}</ref><!-- this is true in Earth's atmosphere, but not generically -->

To exist as a liquid, H₂ must be cooled below its [[critical point (thermodynamics)|critical point]] of 33&nbsp;[[Kelvins|K]]. However, for it to be in a fully liquid state at [[atmospheric pressure]], H₂ needs to be cooled to {{convert|20.28|K|C F}}.<ref name="IPTS-1968">[http://media.iupac.org/publications/pac/1970/pdf/2203x0555.pdf IPTS-1968], iupac.org, accessed 2020-01-01</ref> A common method of obtaining liquid hydrogen involves a [[compressor]] resembling a jet engine in both appearance and principle. Liquid hydrogen is typically used as a concentrated form of [[hydrogen storage]]. Storing it as liquid takes less space than storing it as a gas at normal temperature and pressure. However, the liquid density is very low compared to other common fuels. Once liquefied, it can be maintained as a liquid for some time in thermally insulated containers.<ref>{{Cite web |title=Liquid Hydrogen Delivery |url=https://www.energy.gov/eere/fuelcells/liquid-hydrogen-delivery |access-date=2022-07-30 |website=Energy.gov |language=en}}</ref>

Hydrogen requires a theoretical minimum of {{convert|3.3 |kWh/kg|MJ/kg|abbr=on}} to liquefy, and {{convert|3.9 |kWh/kg|MJ/kg|abbr=on}} including converting the hydrogen to the para isomer, but practically generally takes {{convert|10-13 |kWh/kg|MJ/kg|abbr=on}} compared to a {{convert|33 |kWh/kg|MJ/kg|0|abbr=on}} heating value of hydrogen.<ref>{{Cite report |url=https://www.hydrogen.energy.gov/pdfs/9013_energy_requirements_for_hydrogen_gas_compression.pdf |title=DOE Hydrogen and Fuel Cells Program Record: Energy requirements for hydrogen gas compression and liquefaction as related to vehicle storage needs |last=Gardiner |first=Monterey |date=2009-10-26 |publisher=United States Department of Energy |issue=9013}}</ref>

Liquid hydrogen is a common [[liquid fuel|liquid]] [[rocket propellant|rocket fuel]] for [[spacecraft propulsion|rocketry]] applications&nbsp;—application bothand is used by [[NASA]] and the [[United States Air Force|U.S. Air Force]], which operate a large number of liquid hydrogen tanks with an individual capacity up to 3.8 million liters (1 million U.S. gallons).<ref name="Flynn2004">{{cite book|author=Flynn, Thomas |title=Cryogenic Engineering, Second Edition, Revised and Expanded|url=https://books.google.com/books?id=-XfMBQAAQBAJ&pg=PA401|date=2004|publisher=CRC Press|isbn=978-0-203-02699-1|page=401}}</ref> In most [[rocket engine]]s fueled by liquid hydrogen, it first [[regenerative cooling (rocket)|cools]] the nozzle and other parts before being mixed with the oxidizer&nbsp;— usually [[liquid oxygen]] (LOX)&nbsp;— and burned to produce water with traces of [[ozone]] and [[hydrogen peroxide]]. Practical H₂–O₂ rocket engines run fuel-rich so that the exhaust contains some unburned hydrogen. This reduces combustion chamber and nozzle erosion. It also reduces the molecular weight of the exhaust, which can actually increase [[specific impulse]], despite the incomplete combustion.

The first [[Thermonuclear weapon|thermonuclear bomb]], [[Ivy Mike]], used liquid [[deuterium]], also known as (hydrogen-2), for nuclear fusion.

The product of hydrogen combustion in a pure oxygen environment is solely water vapor. However, the high combustion temperatures and present atmospheric nitrogen can result in the breaking of N≡N bonds, forming toxic NOx if no exhaust scrubbing is done.<ref>{{Cite journal |last=Lewis |first=Alastair C. |date=2021-07-22 |title=Optimising air quality co-benefits in a hydrogen economy: a case for hydrogen-specific standards for NOx emissions |journal=Environmental Science: Atmospheres |language=en |volume=1 |issue=5 |pages=201–207 |doi=10.1039/D1EA00037C |s2cid=236732702 |issn=2634-3606|doi-access=free }}</ref> Since water is often considered harmless to the environment, an engine burning it can be considered "zero emissions". In aviation, however, water vapor emitted in the atmosphere contributes to [[global warming]] (to a lesser extent than CO₂).<ref>{{cite journal |last1=Nojoumi |first1=H. |title=Greenhouse gas emissions assessment of hydrogen and kerosene-fueled aircraft propulsion |journal=International Journal of Hydrogen Energy |date=2008-11-10 |volume=34 |issue=3 |pages=1363–1369 |doi=10.1016/j.ijhydene.2008.11.017}}</ref> Liquid hydrogen also has a much higher [[specific energy]] than gasoline, natural gas, or diesel.<ref name="almc.army.mil">[http://www.almc.army.mil/alog/issues/MayJun00/MS492.htm Hydrogen As an Alternative Fuel] {{webarchive|url=https://web.archive.org/web/20080808053811/http://www.almc.army.mil/alog/issues/MayJun00/MS492.htm |date=2008-08-08 }}. Almc.army.mil. Retrieved on 2011-08-28.</ref>

Due to its cold temperatures, liquid hydrogen is a hazard for [[Frostbite|cold burns]]. ElementalHydrogen hydrogen as a liquiditself is biologically inert and its only human health hazard as a vapor is displacement of oxygen, resulting in asphyxiation, and its very high flammability and ability to detonate when mixed with air. Because of its flammability, liquid hydrogen should be kept away from heat or flame unless ignition is intended. Unlike ambient-temperature gaseous hydrogen, which is lighter than air, hydrogen recently vaporized from liquid is so cold that it is heavier than air and can form flammable heavier-than-air air–hydrogen mixtures.