Above Images Source: NETL | EPA | DoE

Underground Coal Gasification (UCG)

As per National Energy Technology Laboratory (NETL) part of the U.S. Department of Energy (DOE) National Laboratory System, Underground Coal Gasification (UCG) takes advantage of the same chemical reactions of coal to produce product gases, as those occurring in conventional gasifier reactors. The main difference is that in UCG the underground coal seam itself becomes the reactor, so that the gasification of the coal takes place underground instead of in a manufactured gasification vessel at the surface.

How UCG Works?

Gasification is a technological process that can convert any carbonaceous (carbon-based) raw material such as coal into fuel gas, also known as synthesis gas (syngas). Gasification occurs in a gasifier, generally a high temperature/pressure vessel where oxygen (or air) and steam are directly contacted with the coal or other feed material causing a series of chemical reactions to occur that convert the feed to syngas and ash/slag (mineral residues). Syngas is so called because of its history as an intermediate in the production of synthetic natural gas. Composed primarily of the colorless, odorless, highly flammable gases carbon monoxide (CO) and hydrogen (H2), syngas has a variety of uses.

UCG Process

In UCG Process, injection wells are drilled into an unmined coal seam, and either air or oxygen is injected into the seam along with water. The coal face is ignited, and the high temperatures (about 1,200°C) from the combustion and limited oxygen causes nearby coal to partially oxidize into hydrogen, carbon monoxide (CO), carbon dioxide (CO2), and minimal amounts of methane (CH4) and hydrogen sulfide (H2S). These products flow to the surface through one or more production wells located ahead of the combustion zone. As the face is burned and an area depleted, the operation is moved to follow the seam. The graphic below illustrates the general process.

UCG History

UCG has been identified as a potential process for utilizing Unmineable Coal since the late nineteenth century. The process was first developed, during 1925, by Franz Fischer and   Hans Tropsch at Kaiser-Wilhelm Institute for Kohlenforschung in Mulheim Germany. Former Soviet Union invested heavily in UCG research. By 1939 the Soviets had successfully begun operating a UCG plant in the Ukraine. Later (and to this day) the Skochinsky Institute of Mining Moscow became a center for UCG expertise. The UCG technology developed by the Institute was implemented in three brown coal and two black coal power stations in the 1960s. One of these facilities, the power station at Angren, Uzbekistan, still operates, producing about a million standard cubic feet of syngas per hour.

During late 1970s and 1980s, the U.S. government instituted several research projects and trials of UCG. China also has a significant history of coal gasification and current installed capacity, and continues to add more gasification capacity at substantial rates, using a wide range of gasifier choices including both Chinese technology and foreign gasifier types.

Syngas

The Syngas can be further converted (or shifted) to nothing but hydrogen and carbon dioxide (CO2) by adding steam and reacting over a catalyst in a water-gas-shift reactor. When hydrogen is burned, it creates nothing but heat and water, resulting in the ability to create electricity with no carbon dioxide in exhaust gases. Furthermore, hydrogen made from coal or other solid fuels can be used to refine oil, or to make products such as ammonia and fertilizer. More importantly, Hydrogen enriched Syngas can be used to make gasoline and diesel fuel. 

Current Status

With increasing global demand of natural gas, interest in UCG has revived around the world. Australia, Pakistan, South Africa are actively working on pilot projects.