|MadSci Network: Chemistry|
Hi Joe, I'm not a chemical engineer, but what the heck, its all just chemistry. So I thought Id spend Sunday evening surfing the net and see what I could find. You're asking about the water gas reaction. This is the reaction of steam with carbon or coke (coal with all the tars cooked out) to produce carbon monoxide and hydrogen: C + H20 -> C0 + H2O.
Dr James Plambeck of the University of Alberta has a great series of pages on chemistry including a page on the chemistry of coal. This reaction has been known since 1700s. Treating burning coke burning in air with water gives, more precisely, "producer gas" which, even with the best conversion, is at least half nitrogen. (The nitrogen being left over from the air). But heating coke with water in the absence air gives "water gas", which is mostly a mixture of CO (carbon monoxide) and H2. Water gas has a higher heating value than producer gas since it is not diluted with nitrogen. You can see Dr. Plambeck's site for great history and science of this and many other subjects: http://www.chem.ualberta.ca/~plambeck/che/p101/p01264a.htm
You ask what temperature does the carbon need to be for the reaction to proceed. Here's a company in Indonesia that is selling rotary kilns which are used to make activated charcoal and process water gas from coconut shells. I never cease to be amazed at the things you can find on the internet! If you need one of these gizmos see: http://www.nusakarbon.co.id/high.html
This site gives the formula for the reaction and says the kiln is run at 800° C to 1100° C. That temperature is rather high, but they are also trying to make actvated charcoal. The enthalpy (H) for this reaction is +131 Kj. The free energy (G) is the enthalpy (H) minus the entropy (TS). The H is the amount of heat released or needed to make or break the chemical bonds. The TS is the entropy term. Entropy is literally the tendency for things to spread out. And in the reaction, solid carbon is turned into CO, a gas.
Entropy "likes" for things to spread out and going from solid to a gas is the direction favored by entropy. Thus, the hotter it is the more the gas form is favored. So 800° to 1100° C seems about right. But since gas is being produced, high pressure is not needed for the reaction. Extremely high pressure would tend to cause gas (the CO) to be absorbed and the reaction would go back to forming carbon. So the water gas reaction is a low pressure reaction. By contrast, the Haber process turns nitrogen into ammonia, N2 + 3H2 -> 2NH3. In this reaction four molecules turn into two molecules. That goes "against entropy". So the Haber process uses high pressure, 80 to 350 atmospheres, but not so high temperatures, 600° C - 800° C, or less depending upon the catalyst.
You ask about hydrogen and oxygen igniting in the reaction. In the producer gas reaction, the coal is burning to make C02. But there is usually an insufficiency oxygen, much of the CO2 formed reacts with more carbon: CO2 + C -> 2CO. And if any of the hydrogen produced does react with oxygen it will make water which will react with carbon to continue the producer gas reaction. In the water gas reaction the carbon is heated with steam in the absence of air (or oxygen) so just CO and H2 are formed. There are many laboratory demonstrations of flammable gas being produced from water on burning carbon. And well, yes, I guess if you do it wrong (or right?) it can explode! See the following site. But note, this is one of those "don't try this at home experiments"! http://genchem.chem.wisc.edu/demonstrations/Gen_Chem_Pages/06thermopage/ water_gas_reaction.htm
OK. Lets get quantitative. I surfed some more and found that Dr Light, at FSU, used this question one of his chemistry tests:
1.The following reaction describes the preparation of water gas from coal. Water gas is used as a fuel. Calculate delta Go and Kp for this reaction at both 500° C and 1000° C. Which is the more favored temperature for producing the water gas?
C(s) + H2O(g) CO(g) + H2(g)
Dr Light was good enough to send me the information npage and answer key.
delta H is 131 Kj/mole and S is 0.134 entropy units
delta G = 131 Kj/mole - T x 0.134
0 = 131 Kj/mole - 979 x 0.134
I calculate that at 979° K (706° C), delta G for the reaction is zero. That means at 706° C (at constant pressure), the products = the reactants. Any hotter and the reaction goes towards products and any less and the reaction goes to the reactants.
Of course in the real world of water gas reactors, the products (CO and H2O) are constantly being swept away, so the reaction can produce water gas at temperatures below 706° C. Coal burns in air at about 500° - 600° C. Burning coal is hot enough to produce water gas in good yield. Blasting air onto coke gets the temperature over 1000° C which gives a more efficient production of water gas. Mike Conrad.
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