Coal properties critical when determining suitable gasification technology for CTL

26th October 2012 By: Chantelle Kotze

The use of suitable gasification processes to convert different coals from a solid to a liquid is often overlooked and mistakes in the choice of gasifier design have been made in South Africa over the years at considerable cost and loss of efficiency, says Clean Coal Technology chair and Fossil Fuel Foundation director, Professor Rosemary Falcon.

The future of coal-to-liquids (CTL) in South Africa and in the Southern Africa region is good, given the area’s considerable coal reserves, which are estimated at between 28-billion tons and 33-billion tons, she points out.

However, the technical know-how to install and operate the chosen gasification process and the Fischer-Tropsch downstream process to produce the various carbon-based liquids, solids and other products may be a challenge, says Falcon.

Other challenges include the cost effectiveness of the CTL process – even though it may be cheaper or strategically more beneficial to produce products locally from coal rather than importing crude oil to manufacture petrol, diesel, oil and related petroleum products – and environmental controls that may hamper the future development of CTL in the country.

South Africa’s coal reserves rank among the top ten countries worldwide, with coal suitable for CTL in Limpopo, in Mozambique and in Zimbabwe, notes Falcon.

Botswana is also contemplating the potential for CTL, given the country’s considerable coal reserves and current landlocked position.

Mining Weekly reported in June that Mozambique hosted substantial coal deposits situated in Moatize and the Mucanha-Vuzi subbasins, in the prospective Zambezi coal basin, in Tete.

The Moatize subbasin contains seven coal seams and has reserves estimated at 750-million tons, while the Mucanha-Vuzi subbasin is said to contain as much as 3 600-million tons in coal reserves, despite the subbasin being severely block-faulted.

Meanwhile, Zimbabwe has an estimated 26-billion tons of in situ coal reserves, par- ticularly in the north-west and southern parts of the country, reveals the Zimbabwe Chamber of Mines.

However, apart from some sections of the Waterberg coalfield in Limpopo, which hosts about 40% of South Africa’s remaining coal resources, there is little, if any, coal remaining that has the reactive organic materials in coal necessary to produce fuel from CTL processes in direct gasification processes, such as pyrolysis or liquefaction.

The most successful route for CTL in the Southern African context, to date, is the indirect route – namely to gasify the coal first and then reconstitute the gaseous products into the valuable liquid downstream products as required.

Critical in the first step of the CTL process, the gasification of the coal, is the correct matching of the coal properties to one of the numerous gasifi- cation processes.

“Coal properties can vary extensively from one geo- graphic site to the next; therefore, the properties of the targeted coal have a major impact on the choice of a suitable gasification technology,” says Falcon.

The type of coal found in South Africa, known as a Gondwana-type coal, is similar to that found in Australia, India and South America, yet different from the coal found in Europe and the US.

For these reasons, technologies designed for Europe and the US are not always suit- able in the Southern African or Gondwana context, says Falcon.

In the CTL process, one of the most suitable gasification technologies for Gondwana- type coal, which is low-grade coal with a high ash content, involves the Lurgi gasification process, which is a precursor technology producing the gas for the Fischer-Tropsch chemical conversion process.

The Fischer-Tropsch process was developed in Germany in the late 1930s and adopted by South African energy and chemicals producer Sasol in the mid 1950s.

CTL using the Fischer-Tropsch Conversion Process

The process involves coal being fed into a large gasifier where, through the process of gasification, a raw gas is produced that is purified into the synthesis gas (syngas) needed for the next step of the process – the Fischer-Tropsch synthesis.

Through the Fischer-Tropsch conversion process, numerous chemical reactions convert the syngas, which is a mixture of carbon monoxide and hydrogen, into a variety of hydrocarbons.

The hydrocarbons can then be upgraded using various petrochemical steps to produce liquid and gaseous fuels in a variety of forms and qualities.

Coal Formation

Different coal-forming periods produce different types of coal, owing to the differences in organic-matter content, explains Falcon.

“Southern hemispheric coals were formed after an ice age in a cool climate and under highly variable environmental conditions, whereas coals in the northern hemis- phere were formed in wide water-logged swamps under hot equatorial conditions.”

Falcon says much of South African coal has been partially oxidised before being formed into coal and is, therefore, not very reactive. South Africa also has more mineral matter in its coal, which causes higher ash content after combustion or gasification.

Northern hemispheric clean swamp coals, however, feature more volatile-rich and highly-reactive organic material that degasifies quickly when heated.

Falcon asserts that, owing to our low-reactive coal, South Africa has to rely on the indirect gasification process from among the CTL suite of pro- cesses – converting the coal as much as possible into gas and heavier hydrocarbons and then chemically reconsti- tuting and splitting the gases and heavy hydrocarbons into various downstream products to produce petrol, diesel, oils, coke, bitumen and other carbon-based materials.

Sasol has been successful in its CTL venture because it uses the indirect gasification process on the typically less reactive coals found in relative abundance in the country, says Falcon.

Environmental Issues

The Sasol CTL process produces a significant quantity of carbon dioxide (CO2) and Sasol is recognised as the largest single point-source for CO2 in the world, says Falcon.

The advantage, however, is that the concentration of CO2 produced during Sasol’s CTL processes is about 95%, which enables the greenhouse gas to be stored directly in identified geological storage sites in what is known as carbon capture and storage (CCS).

Other coal-using processes, such as coal-fired power-genera- tion plants, produce gaseous emissions of which the CO2 constitutes between 12% and 15%.

“In this case, the CO2 has to be separated from the remaining gases in the air by a carbon capture process before being stored, which is expensive and has a large footprint that adds significantly to the complexity and cost of power generation,” says Falcon.

Above-ground gasification and underground coal gasification (UCG) produces CO2, which will need to be stored through CCS processes.

CCS will help reduce CO2 emissions, an initiative that has become a necessity worldwide.

South African Centre for Carbon Capture and Storage (SACCCS) head Dr Tony Surridge says coal is the biggest contributor to CO2 emissions in South Africa.

He says the SACCCS is progressing well with plans to perform a carbon storage test injection in 2016, which will determine the potential for carbon storage in South Africa.

“The main aim of the test injection is to determine whether South Africa hosts the appropriate geological formations needed for carbon storage. If not, further research will be needed to find alternative solutions, such as renewable energy and fuel switching,” he adds.

If the appropriate geologi- cal formations are present, the CO2 produced from UCG can be pumped back into the seams and into the pores in the rock structure at great depth, from which the underground coal gas has come.

This would,however, not be possible in conventionally mined coal which is used for gasification purposes, as this sort of mining would be done at shallow depths and contain large caverns for the mine shafts that would allow CO2 to escape, says Falcon.

Alternative Gasification

UCG is a clean coal technology whereby coal is gasified within the coal seam on site. By inject- ing a gaseous oxidising agent into the seam, usually oxygen or air, the resulting product gas is brought to the surface through production wells drilled from the surface. The resulting gases are then directed either to power stations or to a CTL plant where the Fischer-Tropsch process is used to convert the gas into liquid fuels.

The UCG technique allows the exploitation of seams that are too deep or too difficult to mine using conventional methods, or low-grade seams that are uneconomical to mine, says Falcon.

Local UCG has been trialled on a pilot scale by State-owned power utility Eskom for the past five years.

Eskom’s Majuba power station, in Mpumalanga, began a conceptual study in 2002 on the possible cofiring of UCG with coal for power generation. In 2007, the pilot plant was commissioned and a demonstration plant and commercial plant is yet to be started.

The benefits of UCG include leaving the coal in the seam and reducing the need for extensive mining, as only drilling is needed to bring the product to the surface.

As a result, there is no stockpile of discarded materials, no acid mine drainage or danger to miners and, therefore, no or minimal envi- ronmental issues, provided the seam is 800 m or more below the surface to ensure the process does not contami- nate any underground water resources usually about 200 m to 300 m below surface.