FAQs

What is Underground coal gasification (UCG)?

UCG is an alternative coal mining method that is operated remotely from the surface. The coal is extracted by gasification rather than the mechanical excavation in conventional mining. To achieve this, oxidising gases such as oxygen, steam or air are injected down a borehole into a gasification chamber in the coal, reactions convert the coal into a gas, and the UCG product gas is extracted through another borehole. Only the coal is removed and any rock mixed with the coal (generally referred to as ash) is left underground. The gas is cleaned on the surface and processed for its specific use at that site.

When coal is burned with limited oxygen and steam, initial combustion generates hot gases which then react further with the coal to form more gases. This is the same process that has been used above ground for over a hundred years in town gas works. Some of the coal's energy is consumed to provide the heat for the gas reactions, and about 80% of the energy in the coal is brought to the surface in the gas products.

Hydrogen and carbon monoxide and methane (natural gas) are the major gases present in UCG product gas – now generally referred to as syngas or synthesis gas because of its use to synthesise chemicals such as ammonia or liquid fuels. There is also carbon dioxide present which has no energy value and can be captured for storage in future.

UCG converts 80% of the coal energy into energy in the gas. UCG recovers more coal than conventional underground coal mining, because it gasifies the whole seam and avoids the loss of resource in coal washery rejects. When compared to coal seam gas energy recovery from the same coal seams, UCG produces more than 20 times the energy from the same area. UCG gives by far the highest energy recovery from deep coal of any extraction technique.

Underground reactions only continue as long as oxygen or steam is pumped into the underground gasifier. As the underground gasifier is a 'gas bubble' within the natural groundwater, when the gas injection is stopped, the bubble disappears, water returns and fills the voids created by removing the coal underground.

It is based on practical experience of our staff in conducting UCG trials in the US and New Zealand, and was further developed in a 10 year CSIRO research program that combined the skills of its scientists in coal gasification, coal mining engineering and sustainable environmental practices. The result is a modular UCG design to produce high quality syngas for large scale production that is suitable for use in power generation and commercial chemical plants, with built in environmental management.

Coal thickness

A thick coal seam, preferably 5m or greater. Thinner seams can be used but gas quality and hence the value of the gas is decreased.

Mineral matter content

Ash up to 55-60% can be tolerated but gas quality declines with higher values - optimally 45% or less.

Groundwater

Hydrostatic pressures must be maintained as they control the operating pressure in the gasifier – this favours depths greater than 200m to maintain a high hydrostatic pressure.

Limited geological structural disturbance of the coal seams, as extraction blocks need to be laid out in a way similar to longwall mines.

Geomechanical properties

Surrounding strata must allow controlled cavity formation and ground response – again this favours depths greater than 200m but is specific to a site and gasification design.

UCG accesses the coal through boreholes and recovers the coal transformed into a gas. It eliminates the surface disturbance associated with large mines, removes people from underground hazards, leaves rock and ash in the coal seam underground, preserves groundwater levels, and produces the energy in an efficient and easily processed form as gas. It is the lowest cost route to all the new clean coal technologies which are based on coal gasification, and provides the most economic method for carbon capture.

All fossil fuels, coal, oil and natural gas, produce CO2. The emerging 'clean coal technologies' do not change the amount of CO2 produced, however, these technologies; reduce the carbon emissions by increasing the efficiency of the process and reducing the cost of CO2 capture and storage. The major route to clean coal technologies currently being developed is through coal gasification, as this allows carbon dioxide in the gas stream to be captured by simple, efficient and low cost gas processing. UCG is the lowest cost coal gasification process available, and is therefore the most economic clean coal technology.

UCG syngas can be used in a number of ways, with carbon dioxide removed progressively to produce lower carbon emissions. Raw UCG syngas will generate electricity with 10% less CO2 than a conventional power station and at a lower cost. If carbon capture from UCG syngas is employed before combustion, carbon dioxide emissions are less than natural gas. Even further reductions are possible by converting the CO in UCG syngas to hydrogen and CO2, this results in a 2/3 reduction in CO2 emissions. The attractiveness of these options will be determined by the carbon trading system currently being developed.

CO2 and disposal is a common issue for all fossil fuels (coal, oil and natural gas), and UCG will access the best carbon sequestration opportunities as they become available.

One of the most exciting aspects of UCG is that it is the most efficient route to the new generation of clean coal gasification technologies and provides the lowest cost method to capture the CO2 for storage. This will advantageously position UCG operations in comparison to all other coal technologies. The high value chemical products created with UCG syngas will allow a UCG project the flexibility to include carbon management within the overall operations, and remain economically attractive.

It is important that whole-of-life analyses of carbon footprints are undertaken, and production, transport and processing are include in any comparison of the carbon production of alternate fuels.

Syngas can be used for its heat value in generating low emission electricity, but it's most valuable use is as a feedstock in chemical synthesis of products such as ammonia for fertiliser and explosives, methanol and liquid fuels. These value-adding industries will be established on the coal field, bringing new employment and opportunities to regional centres. For example, a UCG based 1,000 tonne per day ammonia plant will produce over $30 billion value of fertiliser and explosives over its 20 year life.

To date, most UCG sites have been fixed term trials not intended as commercial operations. With the extensive work by the US Department of Energy in the last quarter of the 20th century, the technical issues for commercial scale production were addressed and solved. The three critical changes now making UCG commercial have been the ability to characterise the geology of UCG sites using new geophysical techniques, the ability for directional drilling to allow precise underground gasifier construction with boreholes, and the increased value of energy now making UCG commercially profitable.