HYDROGEN
FROM COAL
“This section presents the basics of making hydrogen from coal in large
centralized plants. Many of
the issues and technologies associated with making hydrogen from coal are
similar to those associated with making electric power from coal. These
subjects are closely linked to one another and should be considered in
concert. This is particularly the case for gasification, a clean coal
technology, which will be required for making hydrogen and which also
offers the best opportunity for making low-cost, high-efficiency, and
low-emission power production through the integrated gasification combined
cycle (IGCC) process. The lowest-cost hydrogen coal plants are likely to
be ones that coproduce power and hydrogen.
Coal is a viable option for making hydrogen in very large, centralized
plants when the demand for hydrogen becomes large enough to support an
associated very large distribution system. The United States has enough
coal to make all of the hydrogen that the economy will need for more than
200 years, a substantial coal infrastructure already exists, commercial
technologies for converting coal to hydrogen are available from several
licensors, the cost of hydrogen from coal is among the lowest available,
and technology improvements are identified to reach the future DOE cost
targets. The major consideration is that the CO2 emissions from
making hydrogen from coal are larger than those from any other way of
making hydrogen. This puts an added emphasis on the need to develop carbon
sequestration techniques that can handle very large amounts of CO2
before the widespread use of coal to make hydrogen is implemented.
Gasification
Technology
The key to the efficient and clean manufacture of hydrogen from coal is to
use gasification technology, which is a clean coal technology, as opposed
to the combustion process used in conventional coal-fired power plants.
Gasification systems typically involve partial oxidation of the coal with
oxygen and steam in a high-temperature and elevated-pressure process. This
creates a synthesis gas, a mix of predominantly carbon monoxide (CO) and H2
with some steam and CO2. This synthesis gas (syngas) can be
further reacted with water to increase H2 yield. The gas can be
cleaned in conventional ways to recover hydrogen and a high-concentration
CO2 stream that is easily isolated and sent for disposal.
Syngas produced from current gasification plants can be used in a variety
of applications, often with multiple applications from a single facility.
These applications include use as a feedstock for chemicals and
fertilizers, use for making hydrogen for hydro-processing in refineries,
or use for generating electricity by burning the syngas in a gas turbine.
Research
and Development Needs
In terms of its stage of development, coal gasification is a less mature
commercial process than other coal processes and other hydrogen generation
processes using other fossil fuels, especially with respect to capturing
CO2 and providing flexibility in both H2 and
electricity production. In the committee’s analysis, the current
production cost of making hydrogen from coal in central station (i.e.,
large, centralized) plants is estimated to be $1.03/kg. The potential for
improvement through technology development is significant, as indicated
below:
R&D for
current technology should be directed at the following: capital cost
reduction; standardization of plant design and execution concept; and
improvements in reliability, gas cooler designs, process integration,
oxygen
·
plant
optimization, and acid gas removal technology. With success in these
areas, the production cost of hydrogen from coal is estimated to drop to
$0.90/kg.
·
The
potential also exists for new technologies to make larger improvements in
the efficiency and cost of making hydrogen from coal. For new gasification
technologies, the best opportunities for R&D appear to be for new
reactor designs (entrained bed gasification) and improved gas separation
(hot gas separation) and purification techniques (membrane purification).
These new technologies and the concept of integrating them with one
another into a complete operating plant are in very early development
phases and will require longer-term development to verify the true
potential and to reach commercial readiness. With success, the estimated
hydrogen production cost can be reduced to $0.77/kg.
·
David
Gray and Glen Tomlinson, Mitretec Systems, “Hydrogen from Coal,”
Environmental Impacts of Coal Consumption and Transportation
Using more coal to produce hydrogen will have a number of environmental
consequences. Coal mining itself causes numerous environmental issues,
ranging from widespread land disturbance, soil erosion, dust, biodiversity
impacts, waste piles, and so forth, to subsidence and abandoned mine
workings. Once coal has been extracted, it needs to be moved from the mine
to the power plant or other place of use.
The main pollutants resulting from conventional combustion of coal are
sulfur oxides (SOx), nitrogen oxides (NOx),
particulates, CO2, and mercury (Hg). SOx is
dealt with through lower-sulfur-content coal as well as flue gas
desulfurization (FGD). Approximately 30 percent of
Potentially the most significant future issue for coal combustion is CO2
emissions, since on a net energy basis coal combustion produces 80 percent
more CO2 than the combustion of natural gas does, and 20
percent more than does residual fuel oil, which is the most widely used
other fuel for power generation (EIA [2001], Table B1). Likewise, the CO2
emissions associated with making hydrogen from coal will be larger than
those for making hydrogen from natural gas. Using currently available
technology, the CO2 emissions are about 19 kg CO2
per kilogram of hydrogen produced, compared with approximately 10 kg CO2
per kilogram of hydrogen manufactured from natural gas.
Atmospheric emissions from coal-fired generating plants are of concern to
various bodies—national (criteria pollutants [CO, particulates,
O3, NO2, SO2, and Pb], are defined and
regulated by the EPA under the National Ambient Air Quality Standards) and
international (greenhouse gases, considered under the UN Framework
Convention on Climate Change, are mainly CO2, CH4, N2O,
hydrofluorocarbons, perfluorocarbons, and SF6). Since the
1970s, the
Current Coal Technologies
Conventional coal-fired power generation uses a combustion boiler that
heats water to make steam, which is used to drive an expansion steam
turbine and generator. Various designs of coal combustion boilers exist,
the most modern and efficient of which use pulverized coal and produce
supercritical (high-pressure/high-temperature) steam. Overall efficiencies
are typically in the 36 to 40 percent range. Although a staple for power
generation for decades, this conventional combustion technique is not
suitable for making hydrogen. Hydrogen-making technologies employ a
conversion process rather than a combustion process. These conversion
processes, such as gasification, are suitable for making power and/or
hydrogen.
Clean
Coal Technologies
Clean coal technologies use alternative ways of converting coal so as to
reduce plant emissions and increase plant thermal efficiency, leading to
an overall cost of electricity that is lower than the cost for electricity
from conventional plants. Systems under development include low-emission
boiler systems (LEBSs), high-performance power systems (HIPPSs),
integrated gasification combined cycle (IGCC), and pressurized
fluidized-bed combustion (PFBC) (Ness et al., 1999). The goal is to attain
thermal efficiencies in the 55 to 60 percent range (higher heating value [HHV])
(Ness et al., 1999). With the exception of the IGCC systems, all of the
others rely on increasingly sophisticated emissions control systems; IGCC
uses a different conversion system to reduce emissions at the outset. It
is this gasification technology that is best suited to making hydrogen
from coal.
Gasification
Technology
Gasification systems typically involve partial oxidation of the coal with
oxygen and steam in a high-temperature and elevated-pressure reactor. The
short-duration reaction proceeds in a highly reducing atmosphere that
creates a synthesis gas, a mix of predominantly CO and H2 with
some steam and CO2. This syngas can be further shifted to increase H2
yield. The gas can be cleaned in conventional ways to recover elemental
sulfur (or make sulfuric acid), and a high-concentration CO2
stream can be easily isolated and sent for disposal. The use of high
temperature and pressure and oxygen minimizes NOx
production. The slag and ash that is drawn off from the bottom of the
reactor encapsulate heavy metals in an inert, vitreous material, which
currently is used for road fill. The high temperature also eliminates any
production of organic materials, and more than 90 percent of the mercury
is removed in syngas processing. Syngas produced from current gasification
plants is used in a variety of applications, often with multiple
applications from a single facility. These applications include syngas
used as feedstock for chemicals and fertilizers, syngas converted to
hydrogen used for hydro-processing in refineries, production, generation
of electricity by burning the syngas in a gas turbine, and additional heat
recovery steam generation using a combined cycle configuration.
There are currently at least 111 operating gasification plants running on
a variety of feedstocks. These include residual oils from refining crude
oil, petroleum coke, and to a lesser extent, coal. The syngas that is
generated has typically been used for subsequent chemicals manufacture;
making power from IGCC systems is a more recent innovation, successfully
demonstrated in the mid-1980s and commercially operated since the
mid-1990s. Gasification is, therefore, a well-proven commercial process
technology, and several companies offer licenses for its use.
Oxygen-Blown
Versus Air-Blown Gasification
Gasification plants exist that use either air-blown or oxygen-blown
designs. Air-blown designs save the capital cost and operating expense of
air separation units, but the dilution of the combustion products with
nitrogen makes the separation of CO2, in particular, a much
more expensive exercise. In addition, the extra inert nitrogen volume
going through the plant increases vessel sizes significantly and increases
the cost of downstream equipment. Oxygen-blown designs do not introduce
the additional nitrogen, so once the sulfur compounds have been removed
from the syngas, what is left is a high-purity stream of CO2
that can be more easily and cheaply separated. Because of the need to
consider CO2 capture and sequestration for future hydrogen
generation plants, only oxygen-blown designs are feasible for
consideration.
Estimated Costs of Hydrogen Production and Carbon Dioxide Emissions
Most gasification plants produce syngas for chemical production, and often
for steam. IGCC plants then burn the syngas to produce power. The
flexibility to polygenerate multiple products to suit a given situation is
one of the strengths of the gasification system. Thus, relatively few
gasification plants are dedicated to producing hydrogen only (or indeed
any other single product). The future large-scale hydrogen generation
plant will likely also generate some amounts of power because of the
advantages provided through polygeneration. It is necessary therefore to
preface any remarks concerning the costs of producing only hydrogen or the
costs of sequestering CO2 with this caveat.
All of the technology needed to produce hydrogen from coal is commercially
proven and in operation today, and designs already exist for hydrogen and
power coproduction facilities. However, technology advances currently in
development will continue to drive down the costs and increase the
efficiency of these facilities. Hydrogen-from-coal plants combine a number
of technologies including oxygen supply, gasification, CO shift, sulfur
removal, and gas turbine technologies. All of these technology areas have
advances under development that will significantly improve the plant’s
capital and operating costs and thermal efficiency. Examples of these
pending technology advances include Ion Transport Membrane (ITM)
technology for air separation (oxygen supply); advances in gasifier
technology (feedstock preparation, conversion, availability); warm gas
cleanup; advanced gas turbines for both syngas and hydrogen; CO2
capture technology advances; new, lower-cost sulfur-removal technology;
and slag-handling improvements.
It is estimated that today a gasification plant producing hydrogen only
would be able to deliver hydrogen to the plant gate at a cost of about
$0.96/kg H2 with no CO2 sequestration. If CO2
capture were also required, it would cost $1.03/ kg H2. This
pricing reflects costs for producing hydrogen from very large, central
station plants at which hydrogen will be distributed through pipelines. In
these plants a single gasifier can produce more than 100 million scf/day H2.
It is envisioned that a typical installation would include two to three
gasifiers.
The economics of making hydrogen from coal is somewhat different from that
for making it from other fossil fuels, in that the capital costs needed
per kilogram of produced hydrogen are larger for coal plants, but the raw
material costs per kilogram of produced hydrogen are lower. Coal is
inexpensive, but the coal gasification plant is expensive. If the coal
price is changed by 25 percent, the hydrogen cost is changed by only
$0.05/kg. If the cost of the plant is changed by 25 percent, the hydrogen
cost is changed by $0.16/kg. This should lead to a very stable cost of
hydrogen production that can be lowered through future improvements in
technology.
In addition to the CO2 produced from making the electricity
consumed in producing hydrogen, CO2 emissions result from the
carbon in the coal. The emissions depend on the type and quality of coal,
but for typical Western coal with 2 percent sulfur and 12,000 Btu/dry lb,
approximately 18.8 kg CO2 are emitted per kilogram of hydrogen
produced. With a CO2 capture system in place, it is estimated
that this figure could be reduced by as much as 80 to 90 percent, the
exact amount depending on capital efficiency and cost-benefit analysis.
Although the economics of hydrogen production from coal does vary somewhat
with the quality of coal being gasified, essentially any coal can be
gasified to produce hydrogen. Coals with ash content greater than 30
percent are already being gasified. The main effects of coal-quality
variance on hydrogen production are the amount of by-products produced
(primarily slag and elemental sulfur) and the capital cost, which would be
affected mostly by the amount of additional inert material in the coal
that has to be handled. For a gasification plant producing maximum
hydrogen from coal, the variance in potential feed coal quality is
estimated to produce a variance of less than 15 percent in the amount of
CO2 generated per ton of hydrogen produced. The lower-quality
coals generate lower amounts of CO2 per ton of hydrogen. Other
effects of coal quality are less significant.
Research and Development Needs
In terms of its stage of development, coal gasification is a less mature
commercial process than coal combustion processes and other hydrogen
generation processes using other fossil fuels, especially in the aspects
of capturing CO2 and providing flexibility in hydrogen and
electricity production. In that sense the potential for improvement
through technology development is significant. The main issues are capital
cost and reliability (the latter is usually addressed through including
standby equipment). Both are major reasons why IGCC technology has not
been widely adopted for power generation, which is a very competitive
business. The flexibility to vary between hydrogen production and power
production will cost extra capital, which has to be recovered.
For the commercial processes available from several different licensors,
the R&D needs should be directed at capital cost reduction,
standardization of plant design and execution concept, gas cooler designs,
process integration, oxygen plant optimization, and acid gas removal
technology. The potential efficiency and capital cost improvements in
these areas could combine to lower the overall cost of hydrogen from coal
by about 10 to 15 percent from today’s costs. Since many parts
of the coal-to-hydrogen process are the same as for coal-to-power
processes, similar improvements in power costs from IGCC should be
possible. These areas are improvements to existing technology, so they
should be able to be achieved in the near term.
The potential also exists for new technologies to make larger improvements
in the efficiency and cost of making hydrogen from coal. For new
gasification technologies, the best opportunities for R&D appear to be
for new reactor designs (entrained bed gasification), improved gas
separation (hot gas separation), and purification techniques. These
technologies, and the concept of integrating them with one another, are in
very early development phases and will require longer-term development to
verify the true potential and to reach commercial readiness. Recent
studies have indicated that the combined potential of these new
technologies could lower the cost of making hydrogen from coal by about 25
percent.
Future Costs
Evolutionary improvements in current technology can lower the cost of
hydrogen from coal from the estimated $0.96/kg to about $0.90/kg. The
evolution of future costs will be a function of the number of units
constructed over time, since each subsequent plant gives an additional
opportunity to apply the experience derived from prior plants, as well as
economies of scale for process unit production.
The introduction of new technologies can lower costs even further. New
gasification technologies along with new syngas cleanup and separation
technologies hold potential for further improving efficiencies and
lowering the costs of producing hydrogen to about $0.71/kg (see Chapter
5 and Appendix
E). Separating and capturing CO2 will increase these costs
to $0.77/kg.
Department of Energy Programs for Coal to Hydrogen
The DOE programs for making hydrogen from coal reside in the Office of
Fossil Energy and are related to programs to make electricity from coal.
The overall goal of the Hydrogen from Coal Program is to have an
operational, zero-emissions, coal-fueled facility in 2015 that coproduces
hydrogen and electricity with 60 percent overall efficiency (DOE, 2003c).
Major milestones for reaching this goal include these:
·
2006—Advanced
hydrogen separation technology, including membranes tolerant of trace
contaminants, identified;
·
2011—Hydrogen
modules for coal gasification combined-cycle coproduction facility
demonstrated; and
·
2015—
Zero-emission, coal-based plant producing hydrogen and electric power
(with sequestration) that reduces the cost of hydrogen by 25 percent
compared with the cost at current coal-based plants demonstrated.
To reach these milestones, R&D activities within the Hydrogen from
Coal Program are focused on the development of novel processes that
include these:
·
Advanced
water-gas-shift reactors using sulfur-tolerant catalysts,
·
Novel
membranes for hydrogen separation from CO2,
·
Technology
concepts that combine hydrogen separation and water-gas shift, and
·
Fewer-step
designs to separate impurities from hydrogen.
Associated coal gasification R&D programs in which success is
dependent on efficiency improvements and lower cost include these:
·
Advanced
ITM technology for oxygen separation from air,
·
Advanced
cleaning of raw synthesis gas,
·
Improvements
in gasifier design, and
·
CO2
capture and sequestration technology.
Summary
The United States has enough coal to make all of the hydrogen that the
economy will need for a very long time, a substantial coal infrastructure
already exists, commercial technologies for converting coal to hydrogen
are available from several licensors, the cost of hydrogen from coal is
among the lowest available, and technology improvements are identified to
reach the future DOE cost targets. As such, coal is a viable option for
making hydrogen in large, central station plants when the demand for
hydrogen becomes large enough to support an associated distribution
system.
The key to the efficient and clean manufacture of hydrogen from coal is to
gasify the coal first, to produce a synthesis gas—a mixture of hydrogen
and CO—and then to further process the CO with water to produce
additional hydrogen and CO2.
Combinations of coal gasifiers and gas cleanup processes have been built,
tested, and used to produce electric power in the integrated gasification
combined cycle (IGCC) process. While IGCC power plants have been built and
operated on a commercial scale, further process improvements to lower
costs and to improve reliability are both possible and desirable.
Accordingly, a number of years ago the DOE initiated a related R&D
program called Vision 21, which is up and running and has been reviewed by
the National Research Council, most recently in early 2003 (NRC, 2003b).
Major aspects of this program will be applicable to making hydrogen from
coal and will lead to more efficient and lower-cost hydrogen production
designs.
Making hydrogen from coal produces a large amount of CO2 as a
by-product. At present, the
Beyond the Vision 21 program, the DOE recently announced its intention to
proceed with a large, coal-to-electricity-and-hydrogen verification plant
with coupled sequestration. This plant, called FutureGen, is now in the
early stages of detailed planning. In addition to demonstrating
coproduction of electricity and hydrogen with sequestration, the system is
intended to act as a large-scale testbed for innovative new technologies
aimed at reducing systems costs.”