A general GTL process consists of a feeding unit; a pre-treatment unit; a reforming unit; an FT (Fischer-Tropsch) synthesis unit; an upgrading unit; and a product separation and rectification unit comprising a series of distillation and absorption columns.

The reforming unit in the study comprises both steam reforming of methane (SRM) and carbon dioxide reforming of methane (CDR) processes to form syngas for the FT unit.

SRM: CH₄ + H₂O → CO + 3H₂

CDR: CO₂ + CH₄ → 2CO + 2H₂

After FT synthesis, the unreacted syngas mixture is recycled to the reforming unit or the FT synthesis unit or both to enhance process efficiency.

The composition flexibility of the syngas mixture due to the two different types of reforming reactions raises an issue that in order to attain the optimized feed composition of FT synthesis the amount of flow rate of each component in the fresh feed mixture should be determined considering the effects of the recycle and its split ratio: the recycle flow rate to the FT reactor over the recycle flow rate to the reforming unit.

Their work shows that a greater recycle to the reforming unit is less effective than that to the FT synthesis unit from the standpoint of the net heat efficiency of the process, since the reforming reactions are greatly endothermic and greater recycle to the reformer requires more energy.

With the recycling of CO₂ generated during the GTL process, they found that zero emission of production CO₂ could be realized and the required amount of methane (natural gas, NG) could be significantly reduced.

A 2008 study by researchers at CMU (Jaramillo et al.) concluded that CO₂ emissions from the production of one liter of FT-diesel using imported natural gas as a feedstock range from 3 to 3.8 kg per liter. (Earlier post.) Using domestic natural gas only resulted in only a slight decrease. Thus they argue, the KRICT authors point out, that as a result conventional GTL processes do not help reduce GHG emissions and they conclude that the production of GTL-based fuel does not seem a reasonable path to follow.

There is, however, another option to reduce GHG emission if the synthetic fuels are produced from the distinct GTL process in which a GHG gas is reused as a reactant. According to our study, about 15-17 g CO₂/MJ of synthetic fuel is directly consumed to produce synthetic fuels, and this value is equivalent to 0.5-0.7 kg CO₂ per one liter of the synthetic fuel, which means that in the least optimistic scenario of Jaramillo et al. the level of GHG emission from this kind of GTL process is comparable to that from the conventional petroleum based process and in the most optimistic scenario, the level of GHG emission is lower than 0.5-0.7 kg CO₂ per one liter of the synthetic fuel.

It is also pointed out that the CO₂ generated from combustion of vent gas to heat up the reformer can be properly reused as a carbon source for the reformer. Our calculation shows that the amount of CO₂ from vent gas incineration at about 77% recycle is nearly close to the net amount of consumed CO₂...And this means that CO₂ emission is nearly zero from the process of our concern. If recycle ratio is raised above 77%, then more CO₂ can be consumed than the process gives off.

In addition, the required NG is greatly reduced since CO₂ also plays a role as one of the carbon sources. According to Choudhary et al., autothermal reforming process requires about 0.53-0.61 kg NG to produce 1 kg syngas (H₂/CO ratio is about 2.0). In comparison to that, the SCR (combinational method of SRM and CDR) requires 20-30% less NG, 0.42-0.46 kg per 1 kg syngas (H₂:CO÷2.0)...If we recycle the unreacted syngas mixture to save more NG, then the required NG/syngas ratio is greatly reduced to 0.23-0.37...Also, it can be seen...that more recycling flow rate to the reformer, i.e., the lower split ratio requires less NG mainly because of the CDR in the reformer.

—Ha et al.

Resources

• Kyoung-Su Ha, Jong Wook Bae, Kwang-Jae Woo and Ki-Won Jun (2010) Efficient Utilization of Greenhouse Gas in a Gas-to-Liquids Process Combined with Carbon Dioxide Reforming of Methane. Research Institute of Chemical Technology (KRICT), Daejeon 305-600, Korea Environ. Sci. Technol., Article ASAP doi: 10.1021/es902784x