Issues Magazine

New Feats from Fossil Fuels

By Greg Perkins

Underground coal gasification, in combination with other technologies, has the potential to meet the demands of energy security, efficiency and environmental protection.

A jet aircraft powered by fuel produced from coal in Australia? Linc Energy’s Chief Executive Officer, Peter Bond, recently made it a reality by flying in a jet 4270 km across Australia from west to east. It is a feat that many would not have believed possible.

The Jet A1 fuel used in the Citation CJ2 jet was produced by underground coal gasification (UCG) and gas-to-liquids (GTL) technology at Linc Energy’s facility at Chinchilla in Queensland. The Jet A1 Dash on 7–9 May 2012 involved a flight from Perth to Chinchilla in an event that took its name from the Jet A1 fuel that was used.

In doing so, Linc Energy demonstrated the potential of UCG. “It showed us that UCG is an energy pathway of the future, no longer a concept or dream. It is a reality available to people and countries that really want it,” Mr Bond said.

Balancing the demands of energy security, efficiency and environmental protection – the so-called three Es – presents a complex set of challenges for countries wanting to use resources within their borders to power their economies but not at the cost of the environment.

Within this scenario, traditional resources such as coal can be overlooked; however, UCG technology and energy demand are creating a renewed interest in coal, particularly as UCG uses coal seams that were considered “stranded” – too deep or uneconomic to mine.

UCG involves the partial oxidation of coal while it remains in the ground. The objective of UCG is to produce a synthesis gas that may be used as a feedstock for conversion into liquid fuels and chemicals via catalytic reactions, or into electric power by combustion of the syngas in a gas turbine connected to an electric generator. Synthetic crude for diesel and jet fuel production is generated by the GTL process.

There is general consensus that energy demand will continue to rise over the next few decades, particularly in countries such as India and China, while our ability to deliver that energy using conventional sources is becoming increasingly difficult and costly. Many emerging and developing nations have large coal resources and little indigenous oil reserves. Underground coal gasification offers a potential solution to provide environmentally friendly and economically sustainable energy on a large scale.

How UCG Works
In its simplest form, UCG works like this (Fig. 1):
Figure 1. Schematic of a UCG gasifier. Courtesy Linc Energy

Figure 1. Schematic of a UCG gasifier. Courtesy Linc Energy

• Vertical wells are drilled into the coal seam. These are linked together by horizontal drilling.

• The coal seam is heated, and then an oxidant consisting of air or oxygen or a combination of both is pumped into one of the wells (the injection well) to drive coal gasification.

• Through the production of heat, applied pressure and controlled water influx, the coal is converted to syngas.

• Syngas flows through the gasification chamber along the horizontal connection in the coal seam and flows to the surface through another well (called the production well).

• The syngas produced from the process is a high quality synthetic gas (syngas) containing a high percentage of carbon monoxide, hydrogen and methane.

In UCG, the “reaction vessel” is formed from the geology, with the seal provided by the formation and groundwater. Normal operation of the underground gasifier aims to maintain a small, positive flux of water into the gasifier from the surrounding coal seam and overburden in order to reduce gas loss and prevent the escape of hydrocarbons by steam flushing them up the production well.

Operational experience shows that the process is stable and that the gas composition is of a high quality and predictable.

A commercial UCG facility will consist of a number of UCG gasifiers supplying syngas to a downstream user, such as a power plant or gas to liquids plant. For a large-scale project, the number of individual gasifiers at any one time will be in the range of 10–50. During the life of the overall project, each gasifier would be started up, operated until the coal resource it targets is exhausted and then shut down. Depending upon the site’s location, a decommissioning step may be applied to ensure that residual hydrocarbons are removed from the gasification cavities after shutdown.

UCG-produced syngas can be economically used for a variety of purposes, including the production of liquid fuels (when combined with GTL technology), power generation in gas turbine-combined cycle power stations or for base load hydrogen fuel cell power systems, and as a feedstock for different petrochemical processes. Linc Energy is the only company in the world to combine UCG and GTL technologies.

Environmental Performance
Initial site selection is important to ensure that UCG operations are undertaken away from potential environmental and groundwater receptors.

Water produced in the UCG process is treated on-site and re-used for other purposes, including GTL. The treatment plant works like a sewage treatment plant in a city or a town, and basically at the end of the process the water can be re-used.

Linc Energy has conducted comprehensive water monitoring at the Chinchilla site over a number of years, and no adverse effects have been detected. The monitoring is conducted on a continual basis to ensure minimal impact on the environment.

Subsidence can effectively be eliminated by designing the gasifier to extract a set width of coal so as to avoid generating subsidence and induced stress fractures, both of which are detrimental to the UCG process.

With the aid of modern geotechnical assessment, careful site selection and modelling tools adapted from the coal mining industry, different gasifier configurations can be simulated before construction and then the appropriate one implemented to minimise subsidence. At Linc Energy’s Chinchilla site, no visible surface subsidence has been detected in any of the gasifiers over a 12-year period.

As with all coal-based processes, CO2 management is a high priority that must be addressed. The key to minimising CO2 emissions is to operate an efficient process. The efficiency of the UCG process is competitive with surface coal gasification processes. The thermal efficiency of the underground gasification process is typically 70–80%, while surface gasifiers have efficiencies typically between 75% and 85%.

Economics
The commercialisation of UCG is being driven by its potential to produce syngas at lower cost than conventional surface gasification methods and its potential to unlock coal resources that are too deep or uneconomical to extract using conventional mining methods. In many locations, UCG provides a solution for energy security as it uses abundant coal resources that are too deep to extract using any other means. In 2008, Linc Energy commissioned a report by PricewaterhouseCoopers which found that UCG technology was “sufficiently developed, and the economic conditions potentially suitable to favour the commercial development of a UCG industry”.

The same report identified that only 15% of the world’s 909 billion tonnes of coal were accessible. The report quotes estimates by the USA’s Lawrence Livermore National Laboratory “that recoverable reserves could be increased by at least 300 per cent to 400 per cent, that 1.6 trillion tons of un-mineable coal in the US may be recoverable with UCG, and that India has 467 billion tonnes of possible coal reserves, 66 per cent of which have UCG potential”. The clear message is that the potential of UCG as an energy source for the 21st century is very large indeed.

A major challenge for conventional coal-to-liquids projects is the initial capital cost of the coal mine and processing plant. In UCG–GTL plants there is no need for a conventional mining operation, resulting in significant savings.

Linc Energy is focusing on achieving further capital reductions via the modularisation of the GTL plant and equipment. Together in an integrated value chain, UCG, GTL and enhanced oil recovery have the capacity to produce oil with very low overall greenhouse gas emissions. It is expected that the lifecycle greenhouse gas emissions from such a project would be comparable to or better than conventional oil extraction from many existing reservoirs.

Linc Energy expects power generation from UCG to be a viable commercial business, particularly in regions that currently have severe power shortages or have few alternative resources for producing electricity. Examples include some regions in Africa and remote locations, such as Alaska. For power generation, UCG syngas would be used to feed a combined cycle gas turbine.

Conclusion
Underground coal gasification is a proven unconventional energy conversion process. It involves technical aspects associated with traditional coal mining, oil and gas extraction, and undertaking a chemical process underground.

There are a large number of benefits with UCG, including high efficiency and a low environmental footprint. The flexibility to produce a range of end products, such as ultra-clean fuels, power and chemicals, means it has enormous potential and energy security and energy independence from exports.

While the success of UCG is linked to the site in which it is operated, UCG technology and operational know-how has now evolved to the point where it can be economically and safely implemented in commercial projects. The flexibility of syngas from UCG is such that it is ready to deliver projects to meet the energy demands in a variety of regions.