Underground coal gasification
Lecture L4-1
Marek Ściążko, prof.
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Fundamental reactions for coal
gasification
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Example of an equilibrium
calculation for coal gasification
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• Underground coal gasification (UCG) converts coal in-situ into a
gaseous product, commonly known as synthesis gas or syngas
through the same chemical reactions that occur in surface gasifiers.
• Gasification converts hydrocarbons into a synthesis gas (syngas) at
elevated pressures and temperatures and can be used to create
many products (electric power, chemical feedstock, liquid fuels,
hydrogen, synthetic gas).
• Gasification provides numerous opportunities for pollution control,
especially with respect to emissions of sulfur, nitrous oxides, and
mercury. UCG could increase the coal resource available for
utilization enormously by gasifying otherwise unmineable deep or
thin coals under many different geological settings.
• A 300-400% increase in recoverable coal reserves is possible.
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UCG has been tested in many different experimental tests in many
countries. The U.S. carried out over 30 pilots between 1975 and 1996,
testing bituminous, sub-bituminous, and lignite coals. Before that, the
Former Soviet Union executed over 50 years of research on UCG, field
tests and several commercial projects, including an electric power plant in
Angren, Uzbekistan that is still in operation today after 47 years.
Since 1991, China has executed at least 16 tests, and has several
commercial UCG projects for chemical and fertilizer feedstocks. In 2000,
Australia began a large pilot (Chinchilla) which produced syngas for 3 years
before a controlled shut-down and controlled restart.
As present, multiple commercial projects are in various stages of
development in the U.S., Canada, South Africa, India, Australia, New
Zealand, and China to produce power, liquid fuels, and synthetic natural
gas.
The leading institution in Poland is Central Mining Institute which is testing
pilot scale unit.
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General features of UCG
• Initially, channel created in coal seam using special drilling
techniques
• As reaction proceeds, channel grows, creating underground ‘cavity’
• Volume of cavity increases progressively with progress of reaction
• Main features:
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Cavity growth is more in vertical direction than horizontal direction
Very little downward growth
Elliptical cylindrical shape symmetrical about injection well
Largest in vicinity of injection well
Thin char layer on the periphery of cavity
Slag nearer the base of cavity
Cavity bottom filled with ash, char and coal
Open void space at the top
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Components of UCG
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World-wide distribution of UCG
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Advantages of UCG
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Underground coal gasification
(UCG) is the in-situ conversion of
coal into combustible gases.
A complex process involving
chemical reactions, heat and
mass transfer, complex flow
dynamics & growing cavity
dimensions.
Advantages over conventional
process are:
– Low dust and noise.
– No ash handling at power stations
– No coal stocking and
transportation
– Larger coal resource exploitation
– Disadvantages–Surface
subsidence–Aquifer water
contamination
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Potential Limitations and
Concerns for UCG
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UCG can have significant environmental consequences: aquifer contamination, and
ground subsidence. While a framework can be constructed from current knowledge
that can eliminate or reduce these environmental risks, it is important to proactively
address this constraint on siting and operation of any UCG projects;
While UCG may be technically feasible for many coal resources, the number of
deposits that are suitable may be much more limited because some may have
geologic and hydrologic features that increase environmental risks to unacceptable
levels;
UCG operations cannot be controlled to the same extent as surface gasifiers. Many
important process variables, such as the rate of water influx, the distribution of
reactants in the gasification zone, and the growth rate of the cavity, can only be
estimated from measurements of temperatures and product gas quality and quantity;
The economics of UCG has major uncertainties, discussed later, that are likely to
persist until such times as a reasonable number of UCG-based power plants are built
and operated;
UCG is inherently an unsteady-state process, and both the flow rate and the heating
value of the product gas will vary over time. Any operating plant must take this factor
into consideration.
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Worldwide UCG operations experience with
respect to coal seam depth and thickness
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Conceptual Design of the Chinchilla
Project (Blinderman, 2003).
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Gas production history at
Chinchilla
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Typical plan view of the Soviet
process
• The dotted lines are meant
to show the location of the
underground linkage
channels formed in the coal
by a countercurrent
combustion step in
preparation for gasification.
• The production phase of
gasification is carried out by
concurrent combustion in
the channels. Concurrent
and countercurrent refer to
the flame front propagating
in the same or opposite
direction as the gas flow,
respectively (Gregg et al.,
1976).
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Production data for Angren
station (former Soviet Union)
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Minimal requirements for
UCG siting and operation
TDS – total dissolved solids
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Modeling of coal gasification
Underground coal seam
acts as a chemical reactor!
Mathematical models should contain:
•Kinetic constants
•Thermo-mechanical properties
•Cavity formation, evolution
•Mathematical models for UCG are useful to
visualize underground phenomena
•Examine safety aspects minutely
•Save on expensive technology
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Velocity Contours of Lab –scale Model
UCG Cavity
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Temperature development in a
coal seam
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