ERD50 said:
As I understand it, the US does not have big reserves of natural gas, so converting NG to Methane (while it may be the most efficient method) isn't really addressing our energy problem.
Maybe I didn't clarify the right things, ERD (may I call you ERD?
). Natural gas is mostly methane, CH4, plus a few odds and ends that leak past the clean-up plant. Methane can be made into methanol, CH3OH, the lightest of the alcohols.
(Lecture continues. Plug iPod into ears. Select brew of choice.)
Making methanol from methane (AKA natural gas)--the movie:
Natural gas is heated up under pressure with steam, really hot. The mix is passed over a nickel-based catalyst inside a number of large (40 ft long x 6 or 8 inches diameter with a tube wall about half an inch or so thick), basically Ni-Chrome (metal similar to the wires in electric heaters) tubes that look like long cannons. These tubes are hanging straight down in rows in a huge insulated box filled with fire. When I was still in the business, I helped design a single such furnace that burned 1.2 billion BTUs per hour of mostly natural gas for fuel alone--the biggest in the world at that time. (We didn't tell the customer that. He would have panicked.) (As I recall, there were 770 tubes in that furnace. A real forest!)
Inside the tubes, the gases are heated much hotter and the steam reacts with the methane to form hydrogen and carbon monoxide: H2O + CH4 = 3 H2 + CO. We need extra water to push things along (which doesn't react, it just passes through) and we don't convert all the hydrocarbon (methane) to syngas, but that is the basic idea. This reaction absorbs heat, which is being supplied by the furnace-side of the tubes.
We capture all the heat in the flue gas and the hot syngas by making high pressure steam with it. (At about 1,500 psi. Very big medicine.) The steam drives steam turbines which drive big (the main one is ~ 50,000 HP) compressors.
The cold syngas is compressed by our big momma compressor up to around 100 atmospheres of pressure (and this is the modern LOW pressure process!). The syngas (= the make-up gas) is warmed up and passed through a large bed of a different catalyst (copper on zink oxide pellets). Here, the hydrogen and carbon monoxide react to make methanol, a reaction that gives off heat: 2 X H2 + CO = CH3OH. (Maybe you noticed that we only used two of the three hydrogen molecules. The excess H2 is eventually burned in the furnace, unless someone recovers it for some other use.) Only part of the CO is reacted each pass over the catalyst, maybe 4%, because of equilibrium. The effluent from the reactor is cooled until the methanol condenses out of the gases and is drained out of the 'loop' (as we call it). The unreacted gases go through a small compressor to make up for all the pressure drop in the loop and sent back to the reactor, together with the make-up gas.
The crude methanol (this process makes some 'cats and dogs' along with the good stuff) has to be distilled to remove the odds and ends and become pure enough to sell.
Oddly enough, all the monkey-motion is in balance. All of the waste heat is recovered so that generally, no extra fuel need be imported.
A hydrogen plant has the same front end, but the syngas is processed a little differently. Hydrogen plants generally export steam. There is more excess heat than we can use.
Methane is easy to use for feedstock. Other hydrocarbons up through gasoline can be made into syngas basically the same way. Anything heavier, up through coal, has to be gasified. This means partially burning it at high pressure in a bottle with steam and oxygen. The downstream stuff and the loop are the same as above.
End of lecture. Test at 11:00.
Professor Gypsy