what is UCG, and how to use it. benefits of coal gasification

1. Introduction:
Underground coal gasification (UCG) is an industrial process, which converts coal into product gas.
UCG is an in-situ gasification process carried out in non-mined coal seams using injection
of oxidants, and bringing the product gas to surface through production wells drilled from the surface.
The product gas could to be used as a chemical feedstock or as fuel for power generation. The
technique can be applied to resources that are otherwise unprofitable or technically complicated to
extract by traditional mining methods, and it also offers an alternative to conventional coal
mining methods for some resources.
Underground coal gasification converts coal to gas while still in the coal seam (in-situ). Gas is
produced and extracted through wells drilled into the unmined coal–seam. Injection wells are used to
supply the oxidants (air, oxygen, or steam) to ignite and fuel the underground combustion process.
Separate production wells are used to bring the product gas to surface. The high
pressure combustion is conducted at temperature of700–900 °C (1290–1650 °F), but it may reach up
to 1,500 °C (2,730 °F).
The earliest recorded mention of the idea of underground coal gasification was in 1868, when Sir
William Siemens in his address to the Chemical Society of London suggested the underground
gasification of waste and slack coal in the mine. Russian chemist Dmitri Mendeleyev further
developed Siemens' idea over the next couple of decades.
Process:
The process decomposes coal and generates carbon dioxide (CO2), hydrogen (H2), carbon
monoxide (CO), methane (CH4). In addition, there are small quantities of various contaminants
including sulfur oxides (SOx), mono-nitrogen oxides (NOx), and hydrogen sulfide(H2S). As the coal
face burns and the immediate area is depleted, the oxidants injected are controlled by the operator.
The Basic UCG Process - has two wells drilled into the coal, one for injection of the oxidants, another
to bring the product gas to surface. Coal has considerable variation in its resistance to flow, depending
on its age, composition and geological history, so simply relying on the natural permeability of the
coal to transport the gas is generally not satisfactory. High pressure break-up of the coal with water
(hydrofracing), electric-linkage and reverse combustion have all been used with success in both pilot
and commercial scale operations. The technique is best suited to deep coal seams, 500 metres plus and
can be undertaken both on and offshore.

2. Different Methods - Two different methods of UCG have evolved, both are commercially available.
The first, based on technology from the former Soviet Union, uses vertical wells and a 'reverse'
combustion to open up the internal pathways in the coal. The process was successfully tested (1999-
2003) in Chinchilla, Australia using air and water as the injected gases.
The second, tested in European and American coal seams, creates dedicated in-seam boreholes, using
drilling and completion technology adapted from oil and gas production. It has a moveable injection
point known as CRIP (controlled retraction injection point) and generally uses oxygen or enriched air
for gasification.
In-seam and Directional Drilling
In-seam drilling was identified at an early stage as an option, but steerable drilling in coal only started
to become available in the latter stages of the US programme of UCG (1975-1990). The breakthrough
came when directional in-seam drilling was combined with CRIP. This arrangement provided an
unobstructed path for the departing gases.

3. Benefit of UCG
 Compared with traditional coal mining, UCG eliminates the need for large scale soil and
overburden removal and coal wash plants. This eliminates issues such as acid mine drainage,
spoil dumps, coal fines storage dams, mining rehabilitation, fly ash disposal and methane
emissions, when compared with coal seams that are traditionally mined.
 Once a UCG operation is finished, the soil profile remains intact and the land can be used
again afterwards. This reduces the land use conflicts that often arise with traditional coal
mining.
 Compared with conventional coal to liquids or surface gasification for power, UCG eliminates
the need for a surface gasification plant. Such plants are capital and energy intensive.
 Compared with conventional gas or coal seam gas production, UCG creates significantly more
energy for the same land area, meaning UCG takes up a smaller space.
 When used for power generation, UCG syngas results in a 25 per cent reduction in greenhouse
gas emissions compared with coal-fired power stations1.
 Exhausted UCG cavities are capable of absorbing significant quantities of carbon dioxide –
potentially up to 400 times more carbon than traditional carbon capture and storage options.
Drawback of UCG:
Aquifer contamination is a potential environmental concerns Organic and often toxic materials (such
as phenol) remain in the underground chamber after gasification and, therefore, are likely to leach into
ground water, absent appropriate site selection. Phenol leachate is the most significant environmental
hazard due to its high water solubility and high reactiveness to gasification. Livermore conducted a
burn at Hoe Creek, Wyoming, producing operating pressure in the burn cavity greater than the
surrounding rock, forcing contaminants (including the carcinogen benzene) into potable
groundwater.[12] However, some research has shown that the persistence of such substances in the
water is short and that ground water recovers within two years.