It is known from the prior art that carbonaceous feedstocks can be converted to synthesis gas by gasification and gas cleaning processes. The desired components of synthesis gas, hydrogen and carbon monoxide, can be used in chemical industry to produce a wide range of products such as liquid hydrocarbon fuels, alcohols, synthetic natural gas and fertilizers. One of the most important conversion processes is the Fischer-Tropsch (F-T) synthesis. In the basic form of the F-T synthesis the synthesis gas is converted to straight chain hydrocarbons in the presence of catalysts at temperature of 200-250° C. and pressure of 20-40 bar. The catalytically active metals, such as Fe, Co, Ru, Ni and Rh, can be used as catalyst. Mainly iron and cobalt catalysts are used commercially, and cobalt is dominating the market.
High-temperature Fischer-Tropsch is operated typically at temperatures of 330-350° C., and it uses an iron-based catalyst. Low-temperature Fischer-Tropsch is operated at lower temperatures which are typically 200-240° C., and it uses a cobalt-based catalyst.
Originally the raw material for the synthesis was brown coal but later natural gas (Gas-To-Liquid, GTL), coal (Coal-To-Liquid, CTL) and increasingly biomass (Biomass-to-Liquids, BTL) have been used. In principle, synthesis gas reactions are not dependant on the raw material. Technically and economically the biggest difference is the scale of the plants. BTL plants are principally at least one order of magnitude smaller than coal or natural gas plants. Gasification and gas cleaning constitute a decisive part of the investment cost, and technical solutions developed for the CTL- and GTL plants may not be suitable for BTL plants.
Many impurities are always present in the synthesis gas stream, and efficient gas purification is necessary before the synthesis gas can be used in typical applications. The requirements for gas purity in the F-T synthesis are very stringent because both Co-catalyst and Fe-catalyst are easily poisoned by the impurities. In this respect all typical contaminants, such as mechanical particles, acidic and basic agents, alkali metals and tars are problematic, and only very low concentrations can be tolerated. However, it has been reported that iron catalysts are more tolerant to sulphur than cobalt catalysts.
There are commercial absorption processes available for the removal of harmful contaminants. However, these processes are typically complex, and the very high investment cost may be an obstacle for their use in the small scale of typical BTL plants. As a whole, gas cleanup is the most critical problem in the development of advanced gasification based processes for most applications.
A separate zinc oxide guard bed is usually used to reduce the total sulfur content of the synthesis gas to less than 10 ppbv. The sulphur removal can be enhanced with another bed of nickel material. This ultra-low level of sulphur in the synthesis gas is useful for the applications of hydrocarbon synthesis.
Final cleanup of biomass gasification gas to the purity level required in cobalt catalyst beds is very expensive necessitating very large production units to be economic.
In the F-T synthesis diesel and other middle-distillates may be produced using a two-step process. In the first stage synthesis gas is converted to long-chain hydrocarbon wax. In the second step these heavy paraffins are selectively converted into desired middle distillates, kerosene and gas oil. The second step is a mild hydrocracking process using a dual-functional catalyst. New GTL-, CTL- and BTL-projects are almost exclusively based on a corresponding technology. The obvious drawbacks of this kind of concept are complexity of the process leading to a higher investment cost as well as need of hydrogen in the cracking step.
Typical challenges of the BTL-plant can be listed as follows: biomass cost and logistics, cost of processes, gasification and gas cleaning in small scale, processing and upgrading of products and waste-water treatment in a stand-alone plant. Therefore there is no commercial scale BTL-plants in operation. The high investment cost compared with the fairly low capacity has been an obstacle for commercial projects. Technical solutions suitable for much larger plants seem to prove too expensive for the scale of the BTL-plant.
The choice of the catalysts as well as the reaction conditions has a significant effect on the product distribution of the process. Low temperature, high pressure and the use of cobalt catalyst generally produce a heavy, highly n-paraffinic hydrocarbon product and has thus been the preferred choice for most of the F-T-projects.
The objective of the invention is to disclose a new type method and apparatus for producing a hydrocarbon fraction. Further, the objective of the invention is to produce a new hydrocarbon product.
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