Liquid hydrogen: Difference between revisions

| 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>