Hydrogen and Clean Fuels Basics

Hydrogen Production Alternatives

As noted above, the most common production process for hydrogen begins by chemically reacting high temperature steam with methane, the main constituent of natural gas, in a process called steam methane reforming (SMR).  The reaction forms synthesis gas, a mixture of hydrogen, carbon monoxide, and carbon dioxide with some leftover methane and water. The carbon monoxide is reacted further with steam to form additional hydrogen and carbon dioxide. The hydrogen then can be separated out for use by cooling the gas to a very low temperature, where the carbon dioxide solidifies while the hydrogen remains as a gas, a process called cryogenic separation. Most of the hydrogen produced by SMR is used for upgrading crude petroleum to gasoline and for making fertilizer containing ammonia (NH4), a compound of hydrogen and nitrogen.

 
 
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Hydrogen is produced from coal in a process that is similar to SMR but more complex because coal is not a single compound like methane. Coal is first gasified by reaction with oxygen and steam at high temperatures to form a gaseous mixture called synthesis gas or syngas, then the syngas is processed further to increase the hydrogen content, and finally other components are removed to produce a pure hydrogen stream. Gasification is much like combustion, except that not enough oxygen is provided to completely burn the coal to carbon dioxide. The figure at right shows the primary chemical reactions involved in coal gasification. Although not shown in the figure, coal is not simply carbon, but contains a complex mixture of hydrocarbons which first devolatilize to form tars and oils that decompose in the high temperature environment of the gasifier to form methane, carbon monoxide, and other compounds. The result of this overall process is the formation of a synthesis gas consisting primarily of hydrogen and carbon monoxide with small amounts of carbon dioxide, methane, and trace amounts of sulfur compounds and other contaminants. The overall process is exothermic (produces heat) and steam is added to control the temperature rise and enhance formation of hydrogen. Oxygen is used rather than air to avoid dilution of the syngas with nitrogen from the air. Gasification is usually carried out at high temperatures and pressures, and there are many gasifier designs and concepts based on developments since simple gasifiers were widely used to make “town gas” for streetlights in the 19th century.  Minerals in the feedstock (ash) separate and leave the bottom of the gasifier as an inert slag (or bottom ash), a potentially marketable solid product. Some gasifiers also yield devolatilization or pyrolysis products (e.g., coal tars, oils, phenols) that can and must be controlled. 

The synthesis gas leaves the gasifier at a high temperature.  With current technology, the gas has to be cooled to ambient temperatures to remove contaminants.  A scrubbing process is used to remove sulfur, nitrogen, and chlorine compounds and particulates.  The raw clean synthesis gas must be re-heated for the water-gas shift (WGS) reactors that produce additional hydrogen.  The reaction of carbon dioxide with steam to make more hydrogen is called the water gas shift reaction since it “shifts” hydrogen atoms from water to gaseous hydrogen. Hydrogen must be separated from the shifted gas containing CO2, CO, and other trace contaminants, and polished (cleaned further) to meet the requirements for various end-uses (e.g., fuel cell vehicles).

Instead of maximizing conversion of synthesis gas to hydrogen production, an alternate pathway prior to the introduction of a hydrogen infrastructure could be to convert the synthesis gas into hydrogen-rich liquids (e.g., Fischer-Tropsch (FT) liquids or methanol) for use as liquid transportation fuels or reformable fuels to produce hydrogen for fuel cell applications.  A similar approach would be to catalytically convert the synthesis gas to SNG for reforming into hydrogen at sub-central facilities or at small-scale distributed plants near the end-user.

"I believe that water will one day be employed as a fuel; that hydrogen or oxygen which constitute it, used singly or together, will furnish an inexhaustible source of heat and light."
~Jules Verne, 1874

The Hydrogen Economy
Hydrogen is a clean fuel that can be used in future fuel cell vehicles as well as in stationary power generation using fuel cells, once technological and economic issues are resolved with storage, production, and transport of hydrogen, and fuel cell cost reductions are achieved. Hydrogen can also be used in more conventional engines. It can be made from plentiful domestic resources, either in large central plants from coal or other resources, or locally from natural gas or other hydrocarbon hydrogen carriers. With cheap electricity from clean sources, it can even be made from water as envisioned many years ago by Jules Verne via a process called electrolysis, although electrolysis is not currently a commercial source of hydrogen due to its cost and inefficiency.

Hydrogen production from coal can be placed in the context of a national strategy for using hydrogen to alleviate transportation fuel issues arising from imported petroleum.

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