Experience from many different types of coal to liquid fuel conversion techniques exists, while only some have been commercialized and proven feasible by industry. CTL-technology is an old concept that has to be fused with modern processes and machinery to be able to fulfil the economic and environmental constraints of today. Existing technological infrastructure from conventional crude oil processing and gas liquefaction can provide synergistic effects reducing costs and necessary research.
Research on synthetic fuels from non-coal feedstock exists in several places, where coal is too expensive or impractical to acquire for example. In theory, FT-synthesis can be used to create liquid fuels from very unconventional feedstock as long as hydrogen and carbon are available. For instance, carbon dioxide, carbon monoxide and other combustion exhaust have been used as feedstock in patent applications.
1. Pyrolysis The oldest method for obtaining liquids from coal is high temperature pyrolysis. Typically, coal is heated to around 950° C in a closed container. The heat causes decomposition and the volatile matter is driven away, increasing carbon content. This is similar to the coke-making process and accompanying tar-like liquid is mostly a side product.
The process results in very low liquid yields and upgrading costs are relatively high. Coal tar is not traditionally used as a fuel in the transportation sector. However, it is used worldwide for manufacturing roofing, waterproofing and insulation products and as a raw material for various dyes, drugs and paints.
Mild temperature pyrolysis uses temperatures of 450-650 °C. Much of the volatile matter is driven off and other compounds are formed through thermal decomposition. Liquid yields are higher than for high temperature pyrolysis, but reach a maximum at 20%. The main product is char, semi-coke and coke (all smokeless solid fuels). This technique has mostly been used to upgrade low-rank coals, by increasing calorific value and reducing sulphur content.
The Karrick process is a low temperature carbonization process that also yields liquids. The main product is, however, semi-coke. The tar liquids produced require further refining before they can be used as a transportation fuel.
Mild temperature pyrolysis uses temperatures of 450-650 °C. Much of the volatile matter is driven off and other compounds are formed through thermal decomposition. Liquid yields are higher than for high temperature pyrolysis, but reach a maximum at 20%. The main product is char, semi-coke and coke (all smokeless solid fuels). This technique has mostly been used to upgrade low-rank coals, by increasing calorific value and reducing sulphur content.
The Karrick process is a low temperature carbonization process that also yields liquids. The main product is, however, semi-coke. The tar liquids produced require further refining before they can be used as a transportation fuel.
In summary, pyrolysis provides low liquid yields and has inherently low efficiency. Furthermore, the resulting liquids require further treatment before they can be used in existing vehicles. A demonstration plant for coal upgrading was built in the USA and was operational between 1992 and 1997. However, there is little possibility that this process will yield economically viable volumes of liquid fuel. Consequently, further
investigation and analysis of coal pyrolysis is not undertaken.
investigation and analysis of coal pyrolysis is not undertaken.
2. Direct coal liquefaction (DCL) This process is built around the Bergius-process, where the basic process dissolves coal at high temperature and pressure. Addition of hydrogen and a catalyst causes “hydro-cracking”, rupturing long carbon chains into shorter, liquid parts. The added hydrogen also improves the H/C-ratio of the product.
Liquid yields can be in excess of 70% of the dry weight coal, with overall thermal efficiencies of 60-70%. The resulting liquids are of much higher quality, compared to pyrolysis, and can be used unblended in power generation or other chemical processes as a synthetic crude oil (syncrude). However, further treatment is needed before they are usable as a transport fuel and refining stages are needed in the full process chain.
Refining can be done directly at the CTL-facility or by sending the synthetic crude oil to a conventional refinery. A mix of many gasoline-like and diesel-like products, as well as propane, butane and other products can be recovered from the refined syncrude.
Liquid yields can be in excess of 70% of the dry weight coal, with overall thermal efficiencies of 60-70%. The resulting liquids are of much higher quality, compared to pyrolysis, and can be used unblended in power generation or other chemical processes as a synthetic crude oil (syncrude). However, further treatment is needed before they are usable as a transport fuel and refining stages are needed in the full process chain.
Refining can be done directly at the CTL-facility or by sending the synthetic crude oil to a conventional refinery. A mix of many gasoline-like and diesel-like products, as well as propane, butane and other products can be recovered from the refined syncrude.
Some smaller pilot-plants and testing facilities have provided positive results. In 2002, the Shenhua Group Corporation, the largest state-owned mining company in China, was tasked with designing and constructing the world’s first DCL commercial plant in Inner Mongolia Autonomous Region, which recently became operational.
3. Indirect coal liquefaction (ICL)
This approach involves a complete breakdown of coal into other compounds by gasification. Resulting syngas is modified to obtain the required balance of hydrogen and carbon monoxide. Later, the syngas is cleaned, removing sulphur and other impurities capable of disturbing further reactions. Finally, the syngas is reacted over a catalyst to provide the desired product using FT-reactions.
Alteration of catalysts and reaction conditions can create a wide array of different products. For instance, methanol is one possible product that can be produced directly or further converted into high quality gasoline via the Mobil process in additional stages. In general, there are two types of FT-synthesis, a high temperature version primarily yielding a gasoline-like fuel and a low temperature version, mainly providing a diesel-like fuel. More details on FT-synthesis via ICL-technology have been discussed by others.
Sasol in South Africa owns the only commercial-scale ICL plants currently in operation with well established and proven technology and together with a lot of operational experience. In total, Sasol has over 50 years of experience of ICL and has produced over 1.5 billion barrels of synthetic oil during its existence. A number
of different ICL-technologies have been developed by Sasol, the oldest ones date from the 1950s and was used to late 1980s. Today, advanced technologies from the 1990s are utilized, including the Sasol Advanced Synthol High Temperature FT-synthesis and the Sasol Slurry Phase Distillate Low Temperature FT-synthesis.
This approach involves a complete breakdown of coal into other compounds by gasification. Resulting syngas is modified to obtain the required balance of hydrogen and carbon monoxide. Later, the syngas is cleaned, removing sulphur and other impurities capable of disturbing further reactions. Finally, the syngas is reacted over a catalyst to provide the desired product using FT-reactions.
Alteration of catalysts and reaction conditions can create a wide array of different products. For instance, methanol is one possible product that can be produced directly or further converted into high quality gasoline via the Mobil process in additional stages. In general, there are two types of FT-synthesis, a high temperature version primarily yielding a gasoline-like fuel and a low temperature version, mainly providing a diesel-like fuel. More details on FT-synthesis via ICL-technology have been discussed by others.
Sasol in South Africa owns the only commercial-scale ICL plants currently in operation with well established and proven technology and together with a lot of operational experience. In total, Sasol has over 50 years of experience of ICL and has produced over 1.5 billion barrels of synthetic oil during its existence. A number
of different ICL-technologies have been developed by Sasol, the oldest ones date from the 1950s and was used to late 1980s. Today, advanced technologies from the 1990s are utilized, including the Sasol Advanced Synthol High Temperature FT-synthesis and the Sasol Slurry Phase Distillate Low Temperature FT-synthesis.
Source:
Hook, M., Aleklett, K., 2010, "A Review on Coal to Liquid Fuels and Its Coal Consumption," International Journal of Energy Research, 34, Issue 10, October 2010, pp. 848-864.
Hook, M., Aleklett, K., 2010, "A Review on Coal to Liquid Fuels and Its Coal Consumption," International Journal of Energy Research, 34, Issue 10, October 2010, pp. 848-864.
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