Alternative Fuels Essay, Research Paper
Ken Frencher
3/19/99
Chem 136
Liquid Fuels from Natural Gas
We have known for a long time that eventually our oil deposits will dry up, and it would be helpful at that time if we had some kind of alternative fuel to power all of the machines that we use. Over the past two decades more oil has been pumped than has been found in the world. For this reason scientists have been looking to potential alternative fuels for the future. One of these alternative fuels may come in the form of natural gas. Natural gas has been found to be more plentiful than oil, and it burns cleaner. It is worth so little in some places that it is just pumped back into the ground or burned. It is already being used in a more purified diesel fuel, and it also could be converted to methanol which could be used to power fuel cells in the future, but the big problem with natural gas is that it costs much more than oil to process.
Natural gas is mostly made up of methane, and at room temperature and pressure is a gas. This makes it expensive to transport because of its much lower energy density. It also means that if you want to use it as a traditional fuel source you are going to have to convert it to a liquid. This is where many problems arise. Even if you compressed natural gas into a liquid it would be difficult to handle because of its explosiveness. The ideal conversion of natural gas would be to convert it into an easily handled liquid at room temperature. In this form either pumping or using tanks could transfer natural gas.
To make natural gas into a liquid you want to first break the chemical bonds of methane. Using heat and a nickel-based catalyst traditionally does this. This breaks the hydrogen bonds to convert methane to carbon monoxide and hydrogen. The substance that is produced is called syngas. The second step is to convert the gas produced into a liquid by blowing it over a catalyst made of cobalt, nickel, or iron, which transforms it into various liquid hydrocarbons. A different liquid emerges depending on which temperature this is done at. Then a refining process is used so the final products are premium burning fuels. This is a very expensive process that uses a lot of energy.
This process uses a lot of oxygen to produce the syngas, so one way the process could be cheapened is by making oxygen cheaper to produce. This is being done in a number of different ways. The first and easiest is to try to use air directly, and bypass the use of pure oxygen. Another process uses ceramic membranes through which only oxygen can pass. This provides a filter effect in which the ceramic membranes separate the oxygen. Although the ceramic membranes would be expensive to make these materials could reduce the cost of syngas up to 25 percent. A similar type of ceramic membrane could also be produced to separate hydrogen, which could be later used in fuel cells.
Another way of lowering costs of conversion of natural gas is to convert it in a single step. Using this approach the cost of processing natural gas could be cheaper than the cost of processing crude oil. This process is achieved by using different catalysts and greater amounts of oxygen. Experimenting in early reactions the hydrocarbons were burning faster than they could be produced. So it was found that the single step conversion should be done at lower temperatures and use catalysts to stabilize the reactions. Further refining of this process could make a clean burning methanol and diesel fuel easy to produce.
Natural gas conversion will get cheaper as progress is made in technology and improvements in the Fischer-Tropsch reaction are made. Even now natural gas is relatively cheap to produce in comparison with crude oil. With its abundance and its environmentally friendly properties natural gas will be exploited more and more in the future. Fuel cells are already being used in the Necar produced by Mercedes that runs on methanol. It is also used to produce a cleaner burning diesel fuel that can be used in automobiles today. Eventually when the obstacle of the cost of conversion is overcome then the only thing limiting its use is the applications we can think of to apply it to.
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