Mercedes Mullins
The process of converting coal into liquid hydrocarbons, or liquid fuels and petrochemicals, is known as coal liquefaction. This process was first developed in the early 20th century and has been used to create synthetic liquid fuels from coal and other carbon sources. One of the most well-known methods of coal liquefaction is the Fischer-Tropsch synthesis (FT), which was invented in the 1920s and is still used today to power most large vehicles in South Africa. FT synthesis produces high-quality diesel, lubrication oil, and waxes, as well as smaller amounts of lower-quality motor gasoline. Other methods of coal liquefaction include the Exxon Donor Solvent Process, the Imhausen High-pressure Process, and the Conoco Zinc Chloride Process. While the cost of producing diesel fuel from coal through the FT synthesis process has historically been high, it may become more competitive as oil prices rise.
| Characteristics | Values |
|---|---|
| Process | Coal liquefaction |
| Other names | "Coal to X", "Carbon to X", "Coal to liquid fuels" (CTL) |
| Example | Fischer-Tropsch synthesis (FT) |
| FT synthesis output | High-quality diesel, lubrication oil, waxes, and small amounts of lower-quality motor gasoline |
| FT synthesis benefits | Emits fewer particulates and less carbon monoxide than conventional diesel fuels |
| FT synthesis drawbacks | High cost |
| Example of carbonization | Karrick process |
| Karrick process temperature range | 680 °F (360 °C) to 1,380 °F (750 °C) |
| Karrick process air conditions | Absence of air |
| Karrick process output | Liquids, semi-coke |
| Other processes | COED, COGAS, TOSCOAL |
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What You'll Learn
Fischer-Tropsch synthesis
Fischer-Tropsch (FT) synthesis is a process of converting coal into liquid hydrocarbons, specifically diesel fuel. The process is named after its inventors, Franz Fischer and Hans Tropsch, who developed the technology in the 1920s at the Kaiser Wilhelm Institute. FT synthesis involves a set of chemical reactions that convert a mixture of carbon monoxide and hydrogen, known as syngas, into liquid hydrocarbons. These reactions occur in the presence of metal catalysts, at temperatures ranging from 150°C to 340°C and pressures of 15 psi to 500 psi.
The FT process can be adjusted to produce different types of hydrocarbons, depending on the catalyst used, the temperature, and the type of process employed. For example, nickel tends to promote methane formation and is not desirable for diesel production, while cobalt is more active and generally preferred for this application. Iron catalysts are also commonly used and offer the advantage of lower costs and higher water-gas-shift activity, making them suitable for lower hydrogen/carbon monoxide ratios.
FT synthesis has been used for many years, playing an important role in supplying liquid hydrocarbon fuels for the German war effort during World War II. Following the war, South Africa adopted FT synthesis to address its isolation during the apartheid era and to meet its hydrocarbon fuel and chemical needs. The process has also received attention as a potential solution to the diminishing supply of crude oil-derived hydrocarbons and the need for low-sulfur diesel fuel.
The design of commercial reactors for FT synthesis is critical due to the highly exothermic nature of the reaction. Three main types of reactor designs are typically used: fixed-bed, fluid-bed, and slurry-bed reactors. Slurry-bed reactors are considered the state-of-the-art technology for low-temperature FT synthesis, offering better temperature control and higher conversion rates.
Overall, Fischer-Tropsch synthesis is a well-established process for producing liquid hydrocarbon fuels, particularly diesel, from coal via gasification and syngas conversion. The process has been refined and adjusted over the years, and it continues to be an important technology for fuel production and addressing energy security concerns.
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The Karrick process
The coal is heated at 680 °F (360 °C) to 1,380 °F (750 °C) in an airless environment. The carbonization temperature is lower than the 800 °C (1,500 °F) to 1,000 °C (1,800 °F) range used for producing metallurgic coke. This lower temperature optimises the production of coal tars, which are richer in lighter hydrocarbons than normal coal tar. The produced liquids are mostly a by-product, and the main product is semi-coke, a solid and smokeless fuel.
The Karrick LTC process does not generate carbon dioxide, but it does produce a significant amount of carbon monoxide. One short ton of coal can yield up to one barrel of oils and coal tars (12% by weight), 3,000 cubic feet (85 cubic metres) of rich coal gas, and 1,500 pounds (680 kilograms) of solid smokeless char or semi-coke. Yields by volume of approximately 25% gasoline, 10% kerosene, and 20% good quality fuel oil are obtainable from coal.
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Sulphur removal
Coal liquefaction is a process of converting coal into liquid hydrocarbons, such as liquid fuels and petrochemicals. The most common process chain is coal to liquid fuels (CTL). Sulphur in coal is found in both inorganic and organic forms. Inorganic sulphur, in the form of pyrite (FeS2), can be removed from coal relatively easily by simply washing the coal. This method can result in a reduction of 10–50% of the total sulphur content.
The primary desulphurization methods include physical, physico-chemical, chemical, and microbial desulphurization. Among these methods, chemical-related desulphurization is considered the most effective for both inorganic and organic sulphur. This includes processes such as acid/alkaline leaching, H2O2 oxidation, electrochemical reduction, solvent extraction, pyrolysis, air oxidation, and microwave/ultrasonic-assisted desulphurization.
The level of desulphurization is largely independent of particle size, but demineralization can adversely affect sulphur removal. Selective oxidation may be an effective pretreatment for subsequent desulphurization reactions. For instance, the desulphurization of four samples of Central Anatolian lignites was examined by treating them with aqueous NaOH. The NaOH concentration and temperature were the most significant parameters, while the extraction time had only a slight effect. In this case, organic sulphur was not removed from three of the samples, but 12.5% of the organic sulphur was removed from the fourth sample by treatment at 70 °C for 8 hours with a 20% NaOH solution.
Another study investigated the use of ultrasound in reagent-based coal de-ashing and desulphurization. Three different ultrasonic frequencies (25 kHz, Dual (58/192 kHz), and 430 kHz) and three reagents (HCl, HNO3, and H2O2) were used. The model derived from this study incorporates cavitational intensity, reagent concentration, sonication time, coal particle size, and coal concentration as key parameters.
The rules mandating the removal of sulphur down to 15 parts per million were put in place to make diesel fuel burn cleaner and produce lower levels of sulphur dioxide and sulphur trioxide gases, which contribute to acid rain and smog pollution. Sulphur contained in fuel causes emissions of sulphur dioxide (SO2) and contributes to the formation of secondary particulate matter (PM), which is particularly harmful to humans and the environment.
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Coal liquefaction
Direct liquefaction processes convert coal into liquids directly, without the need for intermediate steps. This is achieved by breaking down the organic structure of coal with the application of a hydrogen-donor solvent, often at high pressures and temperatures. In contrast, indirect liquefaction processes involve gasifying coal to a mixture of carbon monoxide and hydrogen, known as synthesis gas or syngas. This syngas is then converted into liquid hydrocarbons using the Fischer-Tropsch process.
Specific liquefaction technologies generally fall into two categories: direct and indirect liquefaction. Direct liquefaction converts solid coal directly into liquid form, resulting in only the partial dismantling of the coal structure. Coal is exposed directly to hydrogen at high temperatures (450°C) and high pressures (14000-20000kPa) for approximately one hour in the presence of a solvent that breaks down the hydrocarbon structure. Indirect liquefaction, on the other hand, requires an intermediate gasification step, resulting in the complete dismantling of the coal structure.
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$52.98
Dual-catalyst system
Coal liquefaction is a process of converting coal into liquid hydrocarbons, such as liquid fuels and petrochemicals. This process is often referred to as "coal to X" or "carbon to X", where X can be a variety of different hydrocarbon-based products. The most common process chain is "coal to liquid fuels" (CTL).
The Fischer-Tropsch synthesis (FT) process, named after its inventors, is the basis for indirect coal liquefaction (ICL) technology. This process chiefly produces high-quality diesel, lubrication oil, and waxes, along with smaller amounts of lower-quality motor gasoline.
Direct coal liquefaction (DCL) was invented by German chemist Friedrich Bergius in 1913 as a method to convert lignite into synthetic oil. This process was an important part of German industry during World War II.
To produce diesel fuel from coal, gasification of coal can be performed to produce methanol, which can then be used in dual-fuel combustion. Dual-fuel combustion involves the use of alternative fuels, such as natural gas or methanol, in combination with diesel fuel. In this process, the alternative fuel is induced into the manifold and mixed with fresh air to form a homogeneous mixture, which is then ignited by pilot diesel or spark plugs.
The dual-fuel mode has been found to significantly reduce NOx and PM emissions compared to conventional diesel engines. However, at low and medium loads, the fuel combustion efficiency of dual-fuel engines is lower, resulting in increased unburned hydrocarbon and carbon monoxide emissions.
To address these emissions, catalytic converters and diesel catalysts are employed. Catalytic converters are exhaust emission control devices that use catalysts to convert toxic gases and pollutants from internal combustion engines into less harmful emissions. Diesel catalysts were introduced in the 1970s for underground mining to reduce carbon monoxide and hydrocarbon emissions. These catalysts have evolved into specialized diesel oxidation catalysts (DOCs) that contain palladium or platinum supported on alumina. DOCs convert particulate matter, hydrocarbons, and carbon monoxide into carbon dioxide and water, operating at up to 90% efficiency.
Emission regulations, such as the US 2010 and Euro V standards, have mandated highly efficient NOx reduction catalysts for diesel engines. These catalysts, such as SCR catalysts and NOx adsorbers, have been crucial in reducing NOx emissions from diesel engines.
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Frequently asked questions
The process of converting coal into diesel fuel is called coal liquefaction. This process was developed in the early 20th century and is often referred to as "coal to X" or "carbon to X", where X can be various hydrocarbon-based products. The Fischer-Tropsch synthesis (FT) method, developed in the 1920s, is commonly used to create synthetic liquid fuels from coal.
Converting coal into diesel fuel can reduce a country's dependence on oil imports. Additionally, Fischer-Tropsch fuels emit fewer particulates and less carbon monoxide compared to conventional diesel fuels, making them more environmentally friendly.
Yes, the process of converting coal into diesel fuel can be expensive. Additionally, coal liquefaction may not always be economically viable, especially when compared to extracting natural petroleum.
