Method for operating a Fischer-Tropsch synthesis
The invention relates to a method and a system for operating a Fischer-Tropsch synthesis, wherein a feed gas comprising CO and H 2 from coal gasification ( 1 ) is desulfurized and subsequently fed into a Fischer-Tropsch synthesis as an input gas, wherein hydrocarbons are formed from carbonic oxides and hydrogen by catalytic reactions. The hydrocarbons are separated as liquid products ( 4 ), and a gas flow comprising CO and CO2 exiting the Fischer-Tropsch synthesis unit ( 3 ) is compressed and fed into a conversion stage, wherein CO and steam are transformed into H 2 and CO 2 . In the method according to the invention, the gas exiting the conversion stage is fed back into the Fischer-Tropsch synthesis unit as a gas rich in H 2 , together with the desulfurized input gas, after the gas is prepared in that CO 2 and/or further components other than H 2 are removed.
1. A method of running a Fischer-Tropsch synthesis, the method comprising the steps of:
(a) desulfurizing a raw gas containing carbon oxides from coal gasification and then combining the desulfurized raw gas with an H 2 rich gas obtained downstream to obtain an input gas stream, having an H 2 /CO molar ratio of at least 1.5:1 and feeding the input gas stream to a Fischer-Tropsch synthesizer to produce, by catalytic reactions, hydrocarbons from the input gas;
(b) separating out the hydrocarbons as liquid products from the input gas to obtain a CO-containing and CO 2 -containing process gas stream in which the CO content is below 20% and removing the CO-containing and CO 2 -containing process gas stream from the Fischer-Tropsch synthesizer;
(c) compressing the CO-containing and CO 2 -containing process gas stream leaving the Fischer-Tropsch synthesizer, feeding the compressed gas to a converter in which the CO in the compressed gas reacts with steam to convert the CO to H 2 and CO 2 in the gas stream and removing said converted gas stream from the converter;
(d) carrying out a gas treatment following step (c), to remove the CO 2 and/or constituents other than H 2 from the converted gas stream to obtain an H 2 rich gas stream, and combining the H 2 rich gas stream together with the desulfurized raw gas according to step (a) as the input gas stream fed into the Fischer-Tropsch synthesizer; and
(e) diverting a partial stream of the desulfurized raw gas, obtained according to step (a), from the Fischer-Tropsch synthesizer, and prior to step (c), feeding the partial stream of the diverted desulfurized raw gas upstream of the compressor into the recirculated gas stream.
2. The method according to claim 1 further comprising the step of
(f) setting an H 2 :CO molar ratio of at least 2:1 in the input gas stream that is fed according to step (a) to the Fischer-Tropsch synthesizer.
3. The method according to claim 1 wherein the gas treatment of the gas stream leaving the converter according to step (d) is composed of a gas scrubber.
4. The method according to claim 3 , further comprising the step of
(f) discharging a partial stream from the H 2 rich gas stream obtained according to step (d) and feeding it to a gas turbine for energy recovery.
5. The method according to claim 1 in which a pressure-swing adsorber is used for the gas treatment of the converted gas stream according to step (d), wherein
essentially pure hydrogen is accumulated on a higher pressure side of the pressure swing adsorber according to step (d) and that is mixed with the raw desulfurized gas and combined as the input gas stream according to step (a) in the Fischer-Tropsch synthesizer, and
following step (d), a gas mixture comprising carbon monoxide, carbon dioxide and hydrogen is accumulated at a lower pressure side of the pressure swing adsorber, following and using the accumulated gas to generate steam in a waste-heat boiler.
6. The method according to claim 5 , in which the accumulated gas mixture leaving the lower pressure side of the pressure-swing adsorption stage is compressed and supplied to a gas turbine.