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Researchers Suggest That Integrating CO2 Reforming of Methane into GTL Process Can Significantly Reduce GHG Emissions and Natural Gas Feedstock Requirements

Ha
Process scheme for a gas-to-liquids process utilizing CDR. Credit: ACS, Ha et al. Click to enlarge.

A new process model developed by researchers at the Petroleum Displacement Technology Research Center, Korea Research Institute of Chemical Technology (KRICT) suggests that incorporating carbon dioxide reforming of methane (CDR) into a gas-to-liquids (GTL) process for the production of synthetic fuels can significantly reduce greenhouse gas emissions by recycling some portion of the unreacted syngas mixture and CO2 generated from combustion at the reformer burner.

The model also reduces the amount of natural gas required, and eliminates the conventional CO2 absorber and separation units. A paper on their work was published online 14 January in the ACS journal Environmental Science & Technology.

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

Comments

SJC

This looks significant. It is interesting to analyze whether we make methanol or do F/T and make synthetic gasoline. Methanol can be used in cars with little or no modification. It has about half of the energy per gallon of gasoline, but the yields are higher. If we can take 100 units of energy and make 70 units of methanol energy but only 50 units of synthetic gasoline energy, which is better over a 10 year period?

DaveD

This is why it's criminal for us not to be flex fueling EVERY vehicle sold in the US. Who knows which one will be the winner over the next 2-5 years. Why would we be limited to gasoline and some tiny mixture of things???

This makes no sense. It is too cheap not to do the flex fuel considering the options it gives us.

SJC

Thank you DaveD, this is what I have been saying all along. Some have posted that it should be an option like a sunroof, but economies of scale say that it could be put on EVERY car very inexpensively and then we have more options at very little cost.

I would buy only a flex fuel car if it was a choice. It could become a major selling point. For NO extra money, this brand has ALL flex fuel cars and you have more choices. People like choices, it is freedom. There is no reason to be locked into ONLY gasoline, when I can chose cellulose E85 or M85 at a lower price with better energy security.

Henry Gibson

The use of fuel M100, pure methanol, is possible for all automobiles because the reduction of range is not great enough to cause problems for most uses. Even short distance passenger jet aircraft operation is possible. An additional system that directly injected methanol into the intake manifolds in addition to the standard gasoline fuel system is possible for many cars to reduce the use of gasoline but not eliminate it. There is an easy process to convert methanol into gasoline with zeolites. Massive stores of methanol should always be kept on hand by any nation. It does not cost much to make a modern car with a fuel system and a computer program to deal with any combination of ethanol, methanol, butanol or gasoline. Methanol burns with many fewer particulates produced. A "fuel cell" for homes that uses electricity to produce methanol from natural gas should be a high priority invention for many governments. Until then and after, plug-in-hybrids will reduce the need for oil imports. We already have good enough batteries. ..HG..

SJC

The first thing that some mention when they hear methanol is toxic. Methanol is no more toxic then gasoline and it degrades when leaked from tanks much more easily than gasoline. It is odd that when we have a solution like M85, some people talk it down as if they were working for the oil companies.

Engineer-Poet

The downside of E85 (and probably M85 also) is that the bacteria which degrade hydrocarbons prefer alcohols, so leaks cause a more persistent pollution problem.

Imposing flex-fuel systems on all vehicles is a waste of money unless the amount of alcohol is quite large.  Even if the USA had 30 billion gallons/year of EtOH, that would still only satisfy about 17% of the volumetric demand for gasoline after accounting for E10 (and even less of the energy demand) and most vehicles would never encounter it.  It would make far more sense to keep alcohol as a separate fuel stream and use it for e.g. ultra-boosting of downsized engines and reduce overall fuel demand by cutting friction and pumping losses in engines.

SJC

Well E-P has spoken, no sense giving that any more thought.

You get 50 million FFVs out there at a very small increased cost per unit. NOW you have a HUGE market for M85 and cellulose E85, that is how you break the log jam.

I am not going to debate the point any further. The Congress is about to mandate FFVs in the next few years and you can say it will never work and will be a waste of money and then it WILL work and then you will act as if you never said anything against it.

Engineer-Poet

The log jam is not on the demand side.  There are enough FFVs out there to consume every bit of EtOH we currently make, but consumers aren't buying.

Sam

The FFV's that are in use are not necessarily near E85 stations. Thats because most of our ethanol comes from corn in the midwest and most of our FFVs are on the coasts. Distribution is a major issue for biofuels. But biomass resources to produce biogas and syngas are available in most places, these could be catalytically upgraded to liquid fuels or used for CHP/trigen systems for electricity/light manufacturing and/or food storage.

Sam

The FFV's that are in use are not necessarily near E85 stations. Thats because most of our ethanol comes from corn in the midwest and most of our FFVs are on the coasts. Distribution is a major issue for biofuels. But biomass resources to produce biogas and syngas are available in most places, these could be catalytically upgraded to liquid fuels or used for CHP/trigen systems for electricity/light manufacturing and/or food storage.

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