Hello Forensicboy-ga,
The first aspect to consider about using hydrogen as a fuel is that
the "pure" hydrogen gas, i.e. the molecule of hydrogen which is H2, is
unstable enough not to be present on Earth in large quantities, in the
natural conditions of Earth. Whenever it is released by whatever
natural or artificial process, it either combines with other atoms to
form more stable molecules, or it climbs up the atmosphere since it's
the lightest gas. (article at Wikipedia --
http://en.wikipedia.org/wiki/Hydrogen )
Therefore, to obtain hydrogen gas to use as a fuel, another substance
has to be processed. It can be said that hydrogen gas needs to be
"manufactured" from "raw materials" -- as opposed to other fuels that
may need to be extracted, but are already present in nature.
Thus, an amount of energy is needed to create molecular hydrogen --
what leads us to the second aspect to consider: To produce hydrogen
there are two elements involved -- the "raw material" that will be
processed to obtain the hydrogen, and the true source that will
provide the energy for that conversion.
There are several methods to obtain hydrogen. One is out of water by
electrolysis. In this case, the energy is provided by electricity (1).
Thus, at the end of the process, we will have used an amount of
electricity, and "stored" its energy into the bonds of the hydrogen
molecule, easy to release by combustion -- and the water used will
have been turned into molecular hydrogen (the product we wanted to
manufacture) and molecular oxygen (the byproduct).(2)
---------
(1) To be rigorous, electricity is not a source either, because we
don't find it in nature in an readily manipulatable form -- so we also
"manufacture" electricity out of hydraulic energy, burning fossil
fuels such as coal, oil or natural gas, etc. (Consumer Energy Council
of America; "Differentiating Energy Sources and Carriers" --
http://www.cecarf.org/Programs/Fuels/SourcesCarriers.html )
(2) Of course, this is the schematic version, the actual process is a
bit more complex.
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(Since hydrogen can be obtained from water, there's a popular
misconception as if water itself could be considered as a fuel.
Nothing could be more wrong. A fuel is a substance that easily
releases energy, while to obtain a fuel -- hydrogen -- out of water,
an amount of energy must be taken from an external source.)
Specific heat:
Based on the definition at Wikipedia, for the measure unit you ask
specific heat "is the amount of energy [in BTU] required to raise the
temperature of one" pound "of the substance by one" degree Fahrenheit.
Thus, the unit of specific heat is Btu/(lb?°F)
(http://en.wikipedia.org/wiki/Specific_heat_capacity )
According to the Union Gas "Natural Gas Composition" webpage, "natural
gas is a naturally occurring gas mixture, consisting mainly of
methane", typically in about 95%
(http://www.uniongas.com/aboutus/aboutng/composition.asp ).
Hence, the specific heat of methane is the closest to the specific heat of the mix.
From the chart of "Physical Properties of Industrial Gases and Common
Industrial Chemicals" published by Universal Industrial Gases, Inc. we
can extract the specific heat for both gases:
Methane: 0.593 BTU/lb °F
Hydrogen: 3.425 BTU/lb °F
(http://www.uigi.com/physical_prop_e.html )
Thus, hydrogen's specific heat proves to be more than five times
higher than methane's.
Heat of Combustion:
Maybe you also want to compare for those gases the *heat of
combustion*, which is -- according to Wikipedia -- "the energy
released as heat when a compound undergoes complete combustion with
oxygen". (http://en.wikipedia.org/wiki/Heat_of_combustion )
The table at the useful "Little Red Book" website provides the values
for both hydrogen and methane:
Hydrogen: 61,095 Btu/lb
Methane: 23,875 Btu/lb
(http://www.littleredbook.com/fuels.html )
Hydrogen can indeed be produced out of natural gas as a raw material.
Moreover, about 95% of the hydrogen produced currently comes from that
feedstock, according to the document "Hydrogen Production and
Delivery" published by the International Partnership for the Hydrogen
Economy (http://www.iphe.net/ ). In the same paper there is a brief
description of a method to obtain hydrogen from natural gas:
"Steam reforming converts methane (and other hydrocarbons in natural
gas) into hydrogen and carbon monoxide by reaction with steam over a
nickel catalyst. The carbon separated from the hydrogen in the
reforming process may be captured and sequestered to avoid damage to
the environment."
(http://www.iphe.net/final%20fact%20sheets%20etc/Production%20Changes%201-3-05.pdf
)
For a detailed description of the process, please visit New York
State's getenergysmart.org "HYDROGEN FACT SHEET - Hydrogen Production
? Steam Methane Reforming (SMR)"
(http://www.getenergysmart.org/Files/HydrogenEducation/6HydrogenProductionSteamMethaneReforming.pdf
)
Because in all exchange of energy there are losses, the amount of
energy stored in the hydrogen obtained is less than the amount
employed in the process -- and yet, when the hydrogen gets burned,
there'll be some losses too, so the final energy actually profited
will be even less. The amount of work effectively produced out of a
source of energy is called efficiency, which is frequently expressed
as a percentage of the theoretical work expectable in the ideal
scenario of zero loss.
If you want to learn more about the efficiency related to the use of
hydrogen, please visit The Management Group's
(http://www.tmgtech.com/pages/1/index.htm ) 2003 document "The
Hydrogen Report", by John R. Wilson & Griffin Burgh, a well founded
critic paper on the hydrogen economy
(http://www.tmgtech.com/images/Energy_Economics_Rev_B.doc ), titles
1.7.2.; 1.8.3.1.; 1.9.1. I recommend you the reading of that article.
Also, you can find illustrative information at its Executive Summary
(http://www.tmgtech.com/images/Hydrogen_Report_ES_-_bio.doc )
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I believe that the answer provided should fulfill the question's
requirement. If any doubt is left, please ask for clarification and
I'll be glad to respond.
Regards,
Guillermo |
