Thanks to the improved exploring, boring, and retrieving skills, the extremely abundant nontraditional natural gas resources such as shale gas and coal-bed methane are recently being discovered and utilized. This abundance in natural gas makes the energy paradigm promptly shift from petroleum to natural gas in some regions including US and China, and the shift will be worldwide. At this moment, it is very important to render the GTL technology more efficient because we do not need to disturb ourselves to change the energy-related infrastructure and the transportation vehicles already fitted to petroleum if we can effectively convert natural gas to the clean liquid fuels and useful basic chemicals.

—Zhang et al.

Broadly, GTL processing using natural gas as feedstock entails three stages: reforming of methane to produce syngas (a mixture of H₂ and CO); conversion of the syngas using F-T synthesis to produce a broad range of hydrocarbons; and upgrading of the F-T products to naphtha, diesel, liquefied petroleum gas, etc., through catalytic hydrocracking.

Typically, iron (Fe) or cobalt (Co) catalysts are used in the F-T reactors. Although Co-based catalysts have higher activity and selectivity to long chain hydrocarbons compared to Fe-based catalysts, Fe-based catalysts are much less expensive than Co-based catalysts and active for the reverse water gas shift (RWGS) reaction. Thus, note the researchers, Fe-based F−T synthesis catalysts are beneficial for CO₂ conversion.

Unfortunately, in conventional low temperature F−T process using Fe-based catalysts, the optimum H₂/CO molar ratio is around 1.7. In this case, about 30% CO is converted to CO₂ due to WGS reaction. Consequently, carbon loss resulting from CO₂ formation cannot be avoided and large amounts of CO₂ are emitted back from the process. If syngas with high concentration of CO₂ is provided in the F−T synthesis unit using Fe-based catalyst, then not CO₂ formation, but CO₂ consumption can occur…

Therefore, on the basis of our previous work based on Co catalysts for F−T synthesis, we are now suggesting two new processes using Fe-based F−T catalysts, which can convert CO₂ to CO by not only dry reforming but also RWGS. It was found that increased process efficiency and significantly reduced CO₂ emission could be realized by recycling some unreacted syngas to reforming and F−T synthesis units.

—Zhang et al.

The team developed two process models for CUGP mainly producing light olefins and Fischer–Tropsch (F–T) synthetic oils with Aspen Plus software. Both models are mainly composed of a reforming unit, an F–T synthesis unit and a recycle unit;the main difference is the feeding point of fresh CO₂.

• In option 1, fresh CO₂ is fed to the reforming unit combined with natural gas and steam to produce CO by CO₂/steam-mixed reforming first and then to the F−T synthesis unit to produce targeted hydrocarbons through RWGS and F−T synthesis reactions;

• For option 2, fresh CO₂ is directly fed to the F−T synthesis unit to produce targeted hydrocarbons through RWGS and F−T synthesis reactions without entering into the reforming unit.

After F–T synthesis, the unreacted syngas is recycled to F–T synthesis and reforming units to enhance process efficiency. From the simulation results, the researchers found that the carbon efficiencies of both CUGP options successfully improved, and total CO₂ emissions were significantly reduced, compared with the conventional GTL processes.

While both CUGP options performed well, Option 2 delivered better results reducing total CO₂ emissions to 4.03g CO₂/MJ F-T product, with carbon efficiency of 86.9% and thermal efficiency of 69.5%.

Process efficiency was sensitive to recycle ratio; more recycle seemed to be beneficial for improving process efficiency and reducing CO₂ emissions.

Resources

• Chundong Zhang, Ki-Won Jun, Kyoung-Su Ha, Yun-Jo Lee, and Seok Chang Kang (2014) “Efficient Utilization of Greenhouse Gases in a Gas-to-Liquids Process Combined with CO₂/Steam-Mixed Reforming and Fe-Based Fischer–Tropsch Synthesis,” Environmental Science & Technology doi: 10.1021/es501021u