Download Underground Coal Gasification (UCG) PDF

TitleUnderground Coal Gasification (UCG)
Tags Gasification Natural Gas Carbon Capture And Storage Carbon Sequestration
File Size285.4 KB
Total Pages10
Table of Contents
                            Introduction –
Benefits of UCG –
Process of Underground Coal Gasification (UCG) -
Gasification circuit and Cavity behaviour -
Carbon capture and sequestration (CCS) –
Monitoring and Control –
Technical requirements –
Environmental Impact and its Control –
Conclusion –
Summary –
                        
Document Text Contents
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convert unminable underground coal/lignite into combustible gases (i.e., combustible
syngas – a combination of hydrogen and carbon monoxide) by gasifying. UCG uses a
similar process to surface gasification. The main difference between both gasification
processes is that in UCG the cavity itself becomes the reactor so that the gasification of
the coal takes place underground instead of at the surface.

Despite considerable research and testing, no commercially viable project has yet been
demonstrated anywhere. Research has been conducted principally in Western Europe,
USA, China, the former Soviet Union and Australia.

Benefits of UCG –

As a method of exploiting coal, UCG represents an environmental improvement on the
combination of coal mining and surface combustion of coal. It is also safer and intuitively
more efficient.

Environmental benefits of UCG over underground coal mining for fuelling power
generation include:

(i) Lower fugitive dust, noise and visual impact on the surface

(ii) Lower water consumption

(iii) Low risk of surface water pollution

(iv) Reduced methane emissions

(v) No dirt handling and disposal at mine sites

(vi) No coal washing and fines disposal at mine sites

(vii) No ash handling and disposal at power station sites

(viii) No coal stocking and transport

(ix) Smaller surface footprints at power stations

(x) No mine water recovery and significant surface hazard liabilities on abandonment.

Additional benefits include:

(i) Health and safety

(ii) Potentially lower overall capital and operating costs

(iii) Flexibility of access to mineral

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exit boreholes and is carried to the surface where it is cleaned and upgraded for use. The
whole aspect is elaborated in next paragraphs.

In fact, gasification differs from combustion which takes place when coal is burned in
excess oxygen to produce carbon dioxide and water. Another important difference
between coal combustion and coal gasification is in pollutant formation. The reducing
atmosphere in gasification converts sulphur (S) from coal to hydrogen sulphide (H2S)
and nitrogen (N) to ammonia (NH3), whereas combustion (oxidation) produces sulphur
dioxide (SO2) and oxides of nitrogen (NOx).

The principal processes can be divided into two stages, namely (i) pyrolysis (also known
as carbonisation, devolatilisation or thermal decomposition) and (ii) gasification. During
pyrolysis coal is converted to a char releasing tars, oils, low molecular hydrocarbons and
other gases. Gasification occurs when water, oxygen, carbon oxide and hydrogen react
with the char.

The main gases produced are carbon dioxide, methane (CH4), hydrogen and carbon
monoxide (CO) and oxygen. CH4 is essentially a product of pyrolysis, rather than
gasification. Its formation is favoured by low temperature and high pressure.

In a theoretical appraisal of the gasification process, the Autothermal Chemical
Equilibrium (ACE) condition exists. This is a condition at which the heat value of the
product gas and the conversion efficiency of the gasified coal (chemical energy of
product gas/chemical energy of gasified coal) is a maximum. At high temperatures and
pressures (say 5MPa, 900°C), ACE conditions are approached rapidly but at lower
temperatures and pressures the time to attain equilibrium greatly exceed the residence
time of the gases in the gasifier and therefore ACE will not be attained.

The basic reactions can be generalised to a simple empirical form:

C + O2 → CO2 (+heat)

C + CO2 (+heat) → 2CO

C + H2O (+heat) → H2 + CO

C + 2H2 → CH4 (+heat)

During pyrolysis coal, subjected to high temperatures, yields higher heat value gases than
ACE gasification products for a relatively small consumption of O2. Pressure increases
the proportion of coal pyrolysed to form methane thus raising the heat value of the
product gases. There is also some evidence to suggest that elevated pressures cause
pyrolysis processes to penetrate in situ coal, further enhancing the gasifier yield.

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Gasification circuit and Cavity behaviour -

The gasification circuit requires a flow link to be achieved between an injection and a
production well. Methods of achieving the link are:

* Accurate drilling assisted by a target device in the vertical well if necessary.

* Reverse combustion, involving ignition at the base of the production well.

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.

Installation of well pairs (injection and production wells) is costly and therefore it is
desirable to gasify the maximum volume of coal between a well pair. As gasification
proceeds, a cavity is formed which will extend until the roof collapses. This roof collapse
is important as it aids the lateral growth of the gasifier. Where the roof is strong and fails
to break, or where the broken ground is blocky and poorly consolidated, some fluid
reactants will by-pass the coal and the reactor efficiency could decline rapidly.

The most successful gasifier or reactor control process, developed in the USA, involves
the use of a burner attached to coiled tubing. The device is used to burn through the
borehole casing and ignite the coal. The ignition system can be moved to any desired
location in the injection well. This ‘controlled retraction of ignition point’ (CRIP)
technique enables a new reactor to be started at any chosen upstream location after a
declining reactor has been abandoned.

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Monitoring and Control –

In order for the gasification process to be controlled, it is essential that continuous
analytical measurement of the product gas stream is available.

Injection flow rate and composition, temperature and pressure were measured at various
parts of the circuit to facilitate control of the gasifier and to ensure pressure and
temperature design limits of system components were not exceeded. The manipulation of
the following variables allowed the reaction rate and the gas quality to be adjusted within
certain limits:

(i) Injected gas flow rate and composition

(ii) Reactor back pressure

(iii) Production well base temperature

(iv) Safety monitoring and alarm control

Technical requirements –

Important technical requirements and considerations in designing a commercial gas
production scheme:

(i) A cost-effective means of acquiring high-resolution coal seam geological data

(ii) Reproducible drilling accuracy

(iii) Multiple, independent gasifier units (with separate injection and production wells) to
ensure systems failures do not totally halt gas production

(iv) Integrated surface plant using readily available off-the-shelf equipment wherever
practicable.

The most critical element of deep UCG is arguably the directional drilling. Technologies
exist which are capable of achieving the required precision but there is considerable
uncertainty about the general drillability of coal seams in other than ideal conditions.

Environmental Impact and its Control –

The main environmental issues concerning UCG are:

(i) Atmospheric emissions;

(ii) Surface water;

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(iii) Drinking water pollutants;

(iv) Noise;

(v) Site operations;

(vi) Groundwater;

(vii) Subsidence.

Conclusion –

Today, high prices of oil and gas and uncertainties about political stability in most of oil
producing countries, have renewed interest in all kinds of fuel. A renewed interest in coal
gasification is therefore not surprising. Further-more, hydrogen is now a welcome by-
product because of the current interest in alternatively fuelled vehicles. UCG is
potentially the most important clean coal technology of the future with worldwide
application. Ultimately, it could be a substitute for deep mining coal for power generation
use.

Applying improved UCG technology to gasify deep, thin, and low grade coal seams
could vastly increase the amount of exploitable reserves. The coal could be converted to
gas for a variety of uses and emissions of sulphur, nitrous oxides and mercury could be
dramatically reduced. UCG could increase recoverable coal reserves by as much as 300
to 400 percent. Another benefit of UCG is that hydrogen accounts for nearly half the total
gas product which can be separated and actively used as automotive fuel or as feed-stock
for the Chemical Industry.

Moreover, investment in CCS technologies is growing rapidly; however, challenges
facing this technology include developing policy to create incentives for deployment, the
creation of a regulatory framework, securing funding for large-scale projects to help
refine the technology, and managing the unresolved environmental and public safety
risks.

Countries are turning to UCG to fully utilize their coal resources in an economically
viable and environmentally acceptable manner. Using UCG technology even without a
carbon-capture-and-sequestration plan could also be eligible for carbon credits.

Summary –

Mining coal has a tremendous environmental impact, so extracting the energy out of coal
while it still is in the ground makes a lot of sense. Gasifying coal below ground
eliminates the need to mine the coal. Underground coal gasification (UCG) has the
potential to provide a clean convenient source of energy from coal seams where
traditional mining methods are either impossible or uneconomic to develop. Underground

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