Technical Note: Fischer-Tropsch Process

The Fischer-Tropsch process often comes up in the discussion of new technologies for producing liquid fuels from solids or gases.  The Fischer-Tropsch process produces high value, clean-burning fuels.  FT fuels can be used in conventional engines with no modification and have improved combustion which reduces emissions, but may have a lower fuel economy.  The resulting fuels are colorless, odorless and low in toxicity.   FT fuels have less sulfur, nitrogen oxide, carbon monoxide and particulate matter emissions than petroleum fuels. 

The Fischer-Tropsch (FT) process was first developed in Germany in the 1920's.  It was used in Germany during WWII and was brought to prominence by Sasol in South Africa to produce oil and gasoline from coal, referred to as coal to liquids (CTL), when they were boycotted during apartheid and where it is still used.  Exxon, Rentech, Sasol and Shell offer commercial processes.  Shell has a commercial plant in operation converting natural gas to diesel fuel, referred to as gas to liquids (GTL), and several smaller plants are in operation.

The FT process requires a feed stream consisting largely of carbon monoxide and hydrogen.  Thus gasification is the first step of coal liquefication or production of Fischer-Tropsch fuels from biomass such as corn stover (corn stalks), wood or switch grass.  The feed gas, referred to as syngas, is produced in a gasifier by heating the gas to a temperatures greater than than 700^oC.  By carefully controlling the oxygen content the hydrocarbons in the feedstock are broken down to carbon monoxide and hydrogen. 

The Fischer-Tropsch process converts the feed gas into liquid organic compounds, carbon dioxide and water.  The conversion takes place in the presence of a catalyst, usually iron or cobalt.  The temperature, pressure and catalyst determine whether a light or heavy syncrude is produced.  For example at 330C mostly gasoline and olefins are produced whereas at 180 to 250C mostly diesel and waxes are produced.

Each company has proprietary FT technology, but a common theme is that most use a slurry-phase reactor with a cobalt-based catalyst. Exceptions are Shell and BP, whose processes use a fixed-bed reactor, and Rentech, Inc. (Denver, CO; www.rentechinc.com), which uses an iron-based catalyst. Most companies use autothermal reforming (ATR) rather than steam reforming because it is less expensive, especially when scaled up. "When you double the capacity of a steam reformer, you need double the number of tubes, so you pay about twice the price,"

The partial oxidation route of reforming provides the desired 2:1 ratio and is the preferred route in without consideration of other needs. There are two routes: one uses oxygen and produces a purer syngas without nitrogen; the other uses air creating a more dilute syngas. However, the oxygen route requires an air separation plant that increases the cost of the investment.

Steam reforming is carried out in a fired heater with catalyst-filled tubes that produces a syngas with at least a 5:1 hydrogen to carbon monoxide ratio. To adjust the ratio, hydrogen can be removed by a membrane or pressure swing adsorption system. Helping economics is if the surplus hydrogen is used in a petroleum refinery or for the manufacture of ammonia in an adjoining plant.

There are mainly two types of F-T reactors. The vertical fixed tube type has the catalyst in tubes that are cooled externally by pressurised boiling water. For a large plant, several reactors in parallel may be used presenting energy savings. The other process uses a slurry reactor in which pre-heated synthesis gas is fed to the bottom of the reactor and distributed into the slurry consisting of liquid wax and catalyst particles. As the gas bubbles upwards through the slurry, it is diffused and converted into more wax by the F-T reaction. The heat generated is removed through the reactor's cooling coils where steam is generated for use in the process.

The resulting organic compounds are a synthetic form of petroleum, analogous to a crude oil, and can be converted into many petroleum products including diesel and gasoline.  Alternatively hydrogen can be recovered by further processing, resulting in only carbon dioxide and hydrogen with no hydrocarbons in the product stream.  The primary interest at the present time is to produce low sulfur diesel fuel.  Production of diesel fuel requires little processing from the FT crude, has low sulfur and aromatic content, high cetane number and and it burns exceptionally clean in a diesel engine.  The process is known for high capital cost and high operating and maintenance costs.  Recent refinements in the process by the commercial suppliers and government research have improved the process and reduced these costs. It becomes practical when "green" transportation fuels are desired and/or when petroleum fuels become high priced.  With current prices of gasoline and diesel, FT fuels are competitive, Hirsch, P43, when produced on a large scale. 

Updated to include more details 6/20/05

Technocrati tags: Fischer-Tropsch, FT diesel

Comments

[Tony]

I don't understand where the oxygen goes in making synthetic gasoline and diesel fuels. One carbon monoxide molecule and two hydrogen molecules make one methanol molecule. But alkanes don't contain oxygen. Either the wax mentioned acts as a sink for carbonyl and hydroxide groups or the fuel is dominated by oxygen containing sythetic carbohydrates such a ethers and esters. I could find nothing on the Internet that said exactly what is in synthetic diesel. If society is going to evaluate synfuels processes, including Fischer Tropsch and Bergius, we need more information.

[Pete Johnson]

The Wikipedia article on Fischer-Tropsch shows the reaction as

(2n+1)H2 + nCO --> CnH2n+1 + nH2O.

In other words, the oxygen and excess hydrogen go into water as a byproduct.

[Loek]

Hi there,

I wanted to comment on the part you quoted from Hirsch. That document is filled with uncertainties and is really not an independent study. It was done and funded by the U.S., and we Europeans all know that America does not readily want to change. This report only shows the willingness and not the real routes to Renewable Energy Sources. That's all.

Furthermore, this is a great website which I will be visiting more in the future. I am doing my master thesis on the future of FT-fuels, so that's how I find this website.

[Ray]

I wish to know what happens to the sulphur in the process if the coal used has high content.

[John Sandford]

What are the tail gas constituents from the FT especially % CO2 . I assume the catalyst is either FE2O3 or other.
J Sandford

[Lala]

What are then the possible disadvantages of the FT process?

Lala

[Rama.J]

I very much enjoyed the energy blog, it is nice. But I think the site has a shortage of detail technical notes.

[Ehimen Harrison]

I am doing my master thesis on clean energy source from coal.I will want to know while most countries around the world have not adopted the technology on deriving synthetic gasoline,diesel from coal,bearing in mind from available literatures that the cost of conversion will be cheaper than that derived from crude oil.
I will also want to have a comperism of cost between crude oil derived fuels and coal derived synthetic fuel.
Harrison

[mohit]

I am doing my master thesis on clean energy source from coal.I will want to know while most countries around the world have not adopted the technology on deriving synthetic gasoline,diesel from coal,bearing in mind from available literatures that the cost of conversion will be cheaper than that derived from crude oil.
I will also want to have a comperism of cost between crude oil derived fuels and coal derived synthetic fuel
mohit

[Kit p]

Some advice on you thesis. Find out what your professors want to hear and tell it to them. So much for available literature.

Ten years ago when I was working on my thesis, FT based on natural gas was future; coal and nuclear power was dead. All but the most economical US nukes would close down yearly. Natural gas was cheap and would never go above $4/MMBTU.

Now we are looking at nukes and coal again. The price of natural gas and oil have come down before. Will we find more cheap oil or gas? Will the economies China and India collapse or continue to grow?

[Sumit Lekhi]

Does the process contions when we change from fixed bed reactor to Slurry phase reactor?

[jim]

HI
A LOT OF HOPE IS PINNED ON THE Fischer-Tropsch process TO PRODUCE LIQUID FUEL FROM ALMOST ANYTHING (BIOMASS) , THERE ARE 2 CONSIDERATIONS OF IMPORTANCE HERE:
1-MACHINES WITH PERPETUAL MOTION DO NOT EXIST.. IE.HOW MUCH ENERGY DO YOU NEED TO SPEND (X) TO PRODUCE (Y) AMOUNT OF ENERGY IN THE FORM OF LIQUID FUEL..AND AT WHAT COST ?
VERY OFTEN X IS > Y
2-THE DREAM OF FINDING THE IDEAL CATALYST THAT WILL CHANGE EVERYTHING IS JUST THAT A DREAM.

[Yosi Kasavi]

Hi,

What is the cost of gasoline produced from coal by using the FT process compared to real gasoline.
As far as I know USA has big coal reserves. How likely is that this process will end the dependence of the US economy to middle-east oil.

[chin]

The FT catalyst seems to be the key to the conversion. Yet there is so little information on this. Are these proprietary and closely held secrets? Do this have to be engineered to the particular feed or will generic catalysts do? Does anyone know the costs of these catalysts if they are iron based?

[Christopher Paul]

Interesting article. More interesting is the offtake from FT Fuels in the U.S. The U.S. Air Force and Airliners are buyers for the Diesel to be produced via this process. Already Ohio Clean River, through BAARD has begun production of a facility to accomplish FT Fuel production using Coal and Bio-mass with Carbon Dioxide gas to be recycled to more buyers down river to inject into oil wells for further energy production. In all, this process looks pretty promising for future energy supplies.

[Tunde]

Hi,
I'd like to know how much energy would be released by a decane oxidation reaction in a 50,000 barrel/day Fischer tropsch GTL plant. Thank you.

[Esa]

Hey!

I would like to know how much energy (MW) does Fischer Tropsch slurry bubble column reactor typically produce (the heat from the exothermic reaction) ? Thank you.

[Esa]

Hey!

I would like to know how much energy (MW) does Fischer Tropsch slurry bubble column reactor typically produce (the heat from the exothermic reaction) ? Thank you.

[Matt]

To Sumit Lekhi:

This is not a perpetual motion machine. You are taking a highly energetic, but inconvenient fuel (coal), and converting it into a slightly less energetic, but much more convenient fuel (diesel). The overall process consumes energy, so you get less energy out than you would if you just burned the coal.

[Sandman]

Thanks for this very comprehensive piece :)

[g whellams]

Has any one applied an RF cataliser in the FT process? Seems to me , a magnetically retained micro wave plasma would raise the temperature and reduce the required volume and pressure of a conventional reactor and at the same time increase the output.

[digital dissertation]

Blogs are good for every one where we get lots of information for any topics nice job keep it up !!!

[Dissertation Writing]

interesting post, i enjoyed it because it's quite informative.

[gerald in MD]

Does anyone have data on the yields of the Fishcher Tropp process? I seem to recall that it is very inefficient and endothermic ( requiring energy in) with yields down at 60% which would affect economics of this process.

[danial]

good blog