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By Mike Millikin on Wind

Posted on: Wed, Dec 28 2005 11:44 AM

Rentech has been awarded a new patent (its 20th in this area) on a process for co-producing Fischer-Tropsch (FT) liquids and electrical power with CO2 emissions reduced through carbon capture.

The plant described in the patent combines an air separation unit, a syngas generator, a Fischer-Tropsch unit, a CO2 removal unit and a combined cycle electricity generation unit. Although each of these individual components are well known, Rentech is knitting the units together in its process.

The process described in the patent—Integrated Fischer-Tropsch and Power Production Plant with Low Carbon Dioxide Emissions—uses primarily hydrogen from the FT tail gas (gas that is not converted inside the FT reactor) and bypassed syngas (a combination of hydrogen and carbon monoxide (CO) produced from a hydrocarbon feedstock) to fuel the combustion turbine of a combined cycle unit.

The combined streams of FT tail gases and bypassed syngas are fed to a shift reactor in which the carbon monoxide is reacted with water to produce hydrogen and carbon dioxide (CO2). CO2 removed from the shift reactor outflow can then be used in commercial process such as chemical and fertilizer production, bottling for carbonated drinks and tertiary recovery in oil fields.

The remaining effluent from the shift reactor (mainly H2) flows to power a combustion turbine to produce electricity. Fueling the turbine with primarily hydrogen greatly reduces the amount of CO2 produced in the stack gases.

Hot exhaust gases from the turbine are cooled in a heat recovery steam turbine, which is also coupled to the generator. Exhaust steam is cooled and recirculated as process water.

Producers can opt to divide the syngas into two streams as it leaves the sulfur removal unit, with one stream going to the FT reactor, the other directly to the shift reactor. This implementation allows producers to optimize the output between FT fuels and electricity.

A computer simulation of the process yielded the following:

  • 5,550 tons per day (tpd) coal gasified with 3,091 tpd water and 4,806 tpd oxygen. The coal (Pittsburgh #8) is 74.16% by weight carbon.
  • After quenching and cleaning, 47.2% of the syngas flows to an FT reactor to produce 6,000 barrels per day of liquids and tail gases.
  • FT tail gases and the other 52.8% of the syngas are mixed with 233 MMSCFD of steam and sent to a low-temperature shift reactor.
  • Gases leaving the shift reactor are on a volume basis approximately 38.0% hydrogen, 37.5% CO2, 23.0% H2O and less than 1% of CO, CH4 and N2.
  • Carbon capture removes 11,300 tpd of CO2.
  • The H2-rich gas used as fuel in the combustor produces approximately 349 Mwe in the combined cycle unit.
  • 3.5% by weight of the carbon in the coal fed to the gasifier is present in the flue gases emitted into the atmosphere.


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