Chapter 2
Materials for Hydrogen Production
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• Hydrogen can be produced using diverse, domestic resources
including fossil fuels, such as natural gas and coal (with carbon
sequestration); nuclear; biomass; and other renewable energy
technologies, such as wind, solar, geothermal, and hydroelectric power.
• The overall challenge to hydrogen production is cost reduction.
For cost-competitive transportation, a key driver for energy
independence and therefore the hydrogen economy,
hydrogen must be comparable to conventional fuels and
technologies on a per-mile basis in order to succeed in the
commercial marketplace.
• The U.S. Department of Energy supports the research and
development of a wide range of technologies to produce
hydrogen economically and in environmentally friendly ways.
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DOE's hydrogen cost goal
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DOE's hydrogen cost goal
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Current Technology
The development of clean, sustainable, and cost-competitive hydrogen production
processes is key to a viable future hydrogen economy. Hydrogen production technologies
fall into three general categories: thermal processes, electrolytic processes, and photolytic
processes.
1. Thermal Processes
 Some thermal processes use the energy in various resources, such as natural gas,
coal, or biomass, to release hydrogen, which is part of their molecular structure. In
other processes, heat, in combination with closed-chemical cycles, produces
hydrogen from feedstocks such as water—these are known as "thermochemical"
processes.
 Reforming of Natural Gas
 Gasification of Coal
 Gasification of Biomass
 Reforming of Renewable Liquid Fuels
 High-Temperature Water Splitting
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2. Electrolytic Processes
 Electrolytic processes use electricity to split water into hydrogen and oxygen, a
process that takes place in an electrolyzer. Hydrogen produced via electrolysis can
result in zero greenhouse gas emissions, depending on the source of the electricity
used. The source of the required electricity—including its cost and efficiency, as well
as emissions resulting from electricity generation—must be considered when
evaluating the benefits of hydrogen production via electrolysis. The two electrolysis
pathways of greatest interest for wide-scale hydrogen production, which result in
near-zero greenhouse gas emissions, are electrolysis using renewable sources of
electricity and nuclear high-temperature electrolysis.
3. Photolytic Processes
 Photolytic processes use light energy to split water into hydrogen and oxygen.
Currently in the very early stages of research, these processes offer long-term
potential for sustainable hydrogen production with low environmental impact.
 Photobiological Water Splitting
 Photoelectrochemical Water Splitting
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Timing of R&D for Hydrogen
production technology
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Distributed Natural Gas Reforming
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Bio-Derived Liquids Reforming
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Coal and Biomass Gasification
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Thermochemical Production
(Using a Heat-Driven Chemical Reaction To Split Water)
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Issues in Hydrogen Production Using Gasification
• Gasifiers are used commercially to react a carbon-containing material with
water (or steam) and oxygen under reducing conditions (shortage of oxygen),
producing chemicals used as feedstock for other processes, fuel for power
plants, or steam for other processes.
• Gasifiers used in industry for chemical processing are high-temperature, highpressure reaction chambers, typically operating between 1,250 and 1,575 oC,
and with pressures between 300 and 1,200 psi.
• The gasification process produces CO and H2 as the primary products, along
with by-products of CO2 and minority gases. Because the gasification process is
intentionally conducted with a shortage of oxygen needed for complete
combustion of the feedstock carbon, the partial oxidation shown in equation
1.1 occurs:
C  H2O gas   O2  CO  H2  CO2  minoritygases by  products
(1.1)
 By-products include mineral impurities in the carbon feedstock that become ash or
slag.
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• A number of carbon feedstocks can be used as the carbon sources, with
the most common being coal, methane, or by-products/tails from the
petrochemical industry.
• Since gasification occurs in an environments with a shortage of oxygen
(reducing), the general balanced chemical reaction that occurs throughout
the gasification process for a hydrocarbon can be written as in equation
1.2:
x
y
C x H y  O2  xCO  H 2  heat, C *
(1.1)
2
2
* C originates from excess feedstock carbonin the reducing gasification environm ent.
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• Gasification is considered a noncatalytic process that involves a number of
endothermic and exothermic reactions, with the overall process being
exothermic.
• Ideally, the amount of excess carbon should be small, about 1.0 wt %, but is
dependent upon variables such as the gasifier type, carbon feedstock, O2/C
ratios, and the level of carbon beneficiation.
• Because of the controlled oxygen shortage, gasification produces a primary
product of CO and H2, call synthesis gas (shortened to syngas), that is
commercially valuable, along with a number of byproducts that depend on
the process and impurities in the carbon feedstock. The byproducts can
include excess C, sulfur, ash, soot, metal oxides, and gases (common
gaseous impurities include CO2 and H2S; low-level impurity gases include
CH4, NH3, HCN, N2 and Ar).
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Carbon Feedstock For Gasification
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Gasification Products
• Syngas (H2 and CO) ― Used to generate heat, chemicals, electricity, or
“town gas.”
• Chemicals ― H2, CO or both are used as chemical feedstock for the
production of ammonia, oxo-chemicals, methanol, acetic acid, hydrogen,
fertilizer, or synthetic hydrocarbon fuels (zero-sulfur diesel and other
transportation fuels) manufactured using Fischer-Tropsch processing, and
other chemicals.
• Electricity ― Produced from the combustion of syngas or from gasification
steam.
• Steam ― Gasification or combustion by-product used in plant applications,
power generation, and “over the fence” needs of nearby companies.
• Gasification by-products ― (used for enhanced oil and methane recovery,
the food industry, urea fertilizer, possible enhanced greenhouse
production, proposed geological disposal), S, N2, Ar, Ni, and V.
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• Iron metal production ― CO, H2 or both are used to reduce iron oxide
into metallic iron for steel production by the following reactions:
Fe2O3  3CO  2 Fe  3CO2
Fe2O3  3H 2  2 Fe  3H 2O
– Iron produced by the direct reduction of iron oxide is called direct
reduced iron (DRI) and is typically made using natural gas.
– Two gasification facilities are designed to use CO and H2 ―one has
been in operation since May1999 (Saldanha Steel, near Cape Town,
South Africa); the other is under construction in South Korea.
– These DRI facilities are designed to be syngas fuel flexible, capable of
using syngas CO and H2 combinations ranging from 100 % CO to 100
% H2.
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Environmental Advantages
•
•
•
•
•
•
•
Gaseous emissions
– Very low emissions compared to other processes ― NOx, SOx and particulate
emissions below current Environmental Protection Agency (EPA) standards.
– Organic compound emissions are below environmental limits.
– Mercury emissions can be reduced to acceptable environmental levels.
SOx can be processed into a marketable by-product.
Ash can be liquefied into a slag that passes toxicity characteristic leaching
procedure (also known as TCLP) testing in most instances.
CO2 can be contained and recovered in the closed loops of gasifiers for
remediation/reuse.
Low-value carbon materials with environmental issues are easily utilized as a
carbon feedstock.
Gasifiers have product flexibility that allows output to be market driven.
Gasification is a thermally efficient process.
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Hydrogen Generation by Gasification
•
In the U.S., the total H2 consumption during 2003 was about 3.2 trillion
cubic feet, with most utilized in petroleum refining and ammonia
production.
•
In the U.S., the most H2 is generated by steam methane reforming, which
constitutes about 85 % of the total production.
•
Gasification of hydrocarbon materials like coal, petcoke, and heavy oil,
however, is starting to play a larger role in the production of H2 .
•
The commercial production of typically involves one of the following
processes:
1. steam reforming
2. water shift gas reaction
3. partial oxidation
4. autothermal reforming.
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•
Steam reforming, also known as steam methane reforming, involves reacting a
hydrocarbon with steam at high temperature (700 to 1,000 ﾟC) in the presence of a
metal catalyst, yielding CO and H2. Of the processes used to make H2, steam reforming
is the most widely practiced by industry and can utilize a variety of carbon feedstocks,
ranging from natural gas to naphtha, liquid petroleum gas (LPG), or refinery off-gas.
Steam reforming, in its simplest form using methane as a feedstock, follows the general
reaction
(1.3)
•
Water shift gas reactions form CO2 and H2 using water and CO at elevated temperature,
as shown in equation 1.4. The reaction may be used with catalysts, which can become
poisoned by S if concentrations are high in the feed gas. The water shift gas reaction is
used as a secondary 4
means of2processing

gas  syngas when greater
2 amounts of H2 are
desired from gasification.
(1.4)
CH  H O
 CO  3H
CO  H2O  gas   H2  CO2
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•
Partial oxidation is the basic gasification reaction, breaking down a hydrogenated
carbon feedstock (typically coal or petroleum coke) using heat in a reducing
environment, producing CO and H2 (equation 1.2). A number of techniques are utilized
to separate H2 from the CO in syngas or to enrich the H2 content of the syngas. These
include H2 membranes, liquid adsorption of CO2 or other gas impurities, and the water
shift gas reaction (equation 1.4).
x
y
C x H y  O2  xCO  H 2
2
2
•
Autothermal reforming is a term used to describe the combination of steam reforming
(equation 1.3) and partial oxidation (equation 1.2) in a chemical reaction. It occurs
when there is no physical wall separating the steam reforming and catalytic partial
oxidation reactions. In autothermal reforming, a catalyst controls the relative extent of
the partial oxidation and steam reforming reactions. Advantages of autothermal
reforming are that it operates at lower temperatures than the partial oxidation reaction
and results in higher H2 concentration.
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• Types of Commercial Gasifiers
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