Underground Coal Gasification - Process

Process

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 of 700–900 °C (1290–1650 °F), but it may reach up to 1,500 °C (2,730 °F).

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 (SO_x), mono-nitrogen oxides (NO_x), and hydrogen sulfide(H₂S). As the coal face burns and the immediate area is depleted, the oxidants injected are controlled by the operator.

There are a variety of designs for underground coal gasification, all of which are designed to provide a means of injecting oxidant and possibly steam into the reaction zone, and also to provide a path for production gases to flow in a controlled manner to surface. As coal varies considerably in its resistance to flow, depending on its age, composition and geological history, the natural permeability of the coal to transport the gas is generally not adequate. For high pressure break-up of the coal, hydro-fracturing, electric-linkage, and reverse combustion may be used in varying degrees.

The simplest design uses two vertical wells: one injection and one production. Sometimes it is necessary to establish communication between the two wells, and a common method to use reverse combustion to open internal pathways in the coal. Another alternative is to drill a lateral well connecting the two vertical wells. UCG with simple vertical wells, inclined wells, and long directional wells was used in the Soviet Union. The Soviet UCG technology was further developed by Ergo Exergy and tested at Linc's Chinchilla site in 1999–2003, in Majuba UCG plant (2007–present), in Cougar Energy's UCG plant in Australia (2010). The same technology is being applied by several developers in UCG plants in New Zealand, Canada, USA, India and other countries.

In the 1980s and 1990s, a method known as CRIP (controlled retraction and injection point) was developed (but not patented) by the Lawrence Livermore National Laboratory and demonstrated in the United States and Spain. This method uses a vertical production well and an extended lateral well drilled directionally in the coal. The lateral well is used for injection of oxidant and steam, and the injection point can be changed by retracting the injector.

Carbon Energy was the first to adopt a system, which uses a pair of lateral wells in parallel. This system allows a consistent separation distance between the injection and production wells while progressively mining the coal between the two wells. It provides access to the greatest quantity of coal per well set and also allows greater consistency in production gas quality.

A new technology has been announced in May 2012 by developer Portman Energy wherein a method called SWIFT (Single Well Integrated Flow Tubing) uses a single vertical well for both Syngas recovery and oxidant delivery. The design has a single casing of tubing strings enclosed and filled with an inert gas to allow for leak monitoring, corrosion prevention and heat transfer. A series of horizontally drilled lateral oxidant delivery lines into the coal and a single or multiple syngas recovery pipeline(s) allow for a larger area of coal to be combusted at one time. The developers claim this method will increase the syngas production by up to ten (10) times prior design approaches and the single well design mean development costs are significantly lower and the facilities and wellheads are concentrated at a single point reducing surface access roads, pipelines and facilities footprint.

A wide variety of coals are amenable to the UCG process. Coal grades from lignite through to bituminous may be successfully gasified. A great many factors are taken into account in selecting appropriate locations for UCG, including surface conditions, hydrogeology, lithoglogy, coal quantity, and quality. According to Andrew Beath of CSIRO Exploration & Mining other important criteria includes:

• Depth of 100–600 metres (330–2,000 ft)
• Thickness more than 5 metres (16 ft)
• Ash content less than 60%
• Minimal discontinuities
• Isolation from valued aquifers.

According to Peter Sallans of Liberty Resources Limited these criteria are:

• Depth of 100–1,400 metres (330–4,600 ft)
• Thickness more than 3 metres (9.8 ft)
• Ash content less than 60%
• Minimal discontinuities
• Isolation from valued aquifers.