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Old 03-01-06, 12:53 PM   #1
kayakboy
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Propane

So the other day I was at working (im as suprised as you are) and lunch comes around. They have no stove but they say there is a microwave at the co-op. I express my disgust and she replied that how do we know that propane isnt sketchy as well. I mean, to people hundreds of years ago, it would seem like sourcery. Alright maby not, but they would probally burn you at the steak for good measure. So I would like to know what the equation for burning propane or compressed natural gas (is there any difference?) is? I looked it up on google and got alot of stuff about weapons, but could find this;

Generally, the chemical equation for combusting a hydrocarbon in oxygen is as follows:
CxHy + (x + y/4)O2 → xCO2 + (y/2)H2O

For example, the burning of propane is:
C3H8 + 5O2 → 3CO2 + 4H2O

Anybody smarter thatn me?
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Old 03-01-06, 01:04 PM   #2
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Where is jschen on this one?
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Old 03-01-06, 01:18 PM   #3
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Old 03-01-06, 01:26 PM   #4
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Old 03-01-06, 01:35 PM   #5
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Well first off there's a difference, yes. Propane is a specific chemical, C3H8, natural gas (compressed or not) is a mixture of a number of gases including propane, methane, butane, and several others.

Your equation produces carbon dioxide (CO2) and water (H20) which is correct, except it assumes perfect combustion. If not enough oxygen is present you get carbon monoxide (CO) which is very poisonous and is what kills people who heat their house with charcoal because their power went out.

If the temperatures at combustion are high enough you can get other compounds, for example oxides of nitrogen (when the nitrogen in the air combines with oxygen) which is a major smog-producing pollutant produced by gasoline engines.
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Old 03-01-06, 01:41 PM   #6
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Where is jschen on this one?
I'm here. Thought this thread would be about buying propane, and that's why I didn't check earlier.

Your equations are fine, and Eggplant Jeff has basically answered your question. Natural gas is predominantly methane, CH4. Propane is by definition C3H8. Perfect combustion of either with sufficient oxygen yields only water and carbon dioxide. Either gas is, in pure form, odorless. The smell of "gas" is actually the smell of a trace of a thiol (tert-butyl thiol, I believe) to help people realize the danger of a gas buildup since otherwise, it's not obvious when there's a gas buildup.
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Old 03-01-06, 01:44 PM   #7
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Old 03-01-06, 01:48 PM   #8
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looked it up on wikipedia (http://en.wikipedia.org/wiki/Natural_gas). Natural gas is primarily Methane (CH4).

As far as how "clean" a fuel burns, you can get a good estimate by the number of hydrogen molecules vs. carbon molecules. Carbon molecules become carbon monoxide (poisonous) or carbon dioxide (greenhouse gas, although not harmful in small amounts). Hydrogen molecules become water vapor (essentially harmless in the quantities produced, although theoretically if we eliminated all carbon pollutants we might have to worry about it as a greenhouse gas).

Methane is very clean, 20% carbon 80% hydrogen. Propane isn't bad at 27% carbon 73% hydrogen. Gasoline is really big molecules with like 6-12 carbon atoms and usually a similar number of hydrogen atoms (so like 50%/50%, but it varies a lot).

What's your beef with microwaves?
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Old 03-01-06, 02:51 PM   #9
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The other byproducts of cooking with propane (actually any heated air, BBQ, oven, etc) is the transformations in the food you're eating. Combustion of hydrocarbons reaches their highest temperatures at stochiometric ratios when every CH is cleanly burned up with ample Oxygen. This can reach up to 5000F with propane at the flame-front.

Animal fats have several concerns in cooking with fire sources like BBQs, propane stoves/grills. One being the conversion of unsaturated and poly-unsaturated cis- fats to trans- configurations at temperatures above 500F. This is caused by breaking carbon C=C double-bonds in unsaturated fats, converting them into partially-hydrogenated fats which behaves similarly to saturated fats. This has been shown to result in increasing cholesterol levels.

At higher temps than that around 800F, you start getting the dark crispy edges to meats. These break down poly-unsaturated fats into radicals which are then metabolized into peroxides by your liver. A build-up of which can lead to various mutations and cancer in replicating cells.

As for microwaves, the frequency of 2.45 ghz is just right to excite the H- bonds in water. It doesn't have the energy to excite any other higher molecular-weight atoms in foods. You're basically cooking your food by heating up the just water that's contained in the food.
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Old 03-01-06, 02:55 PM   #10
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As for microwaves, the frequency of 2.45 ghz is just right to excite the H- bonds in water. It doesn't have the energy to excite any other higher molecular-weight atoms in foods. You're basically cooking your food by heating up the just water that's contained in the food.
If only it were so simple. Actually, the detailed effects of microwave heating is an issue of ongoing and raging debate among organic chemists right now. We've discovered (in hindsight, it's not too surprising) that nuking our reactions in the microwave can sometimes be extremely beneficial. And there's a lot of evidence that the result is not the same as that obtained from simply heating the reaction mixture. But at the same time, the evidence isn't conclusive.
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Old 03-01-06, 04:15 PM   #11
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I usually just use the oven(electric). Microwaves are pretty good for melting chocolate...as long as you're careful. Actually, my nuker probably sees more chocolate chips than anything else. Leftovers are next in line, I'd say.
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Old 03-01-06, 04:58 PM   #12
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If I'm not mistaken, "nuking" is a misnomer when talking about using a microwave oven.
It doesn't cook with radiation.
It cooks with radiant heat.
Similarly, the sun casts radiant heat on the earth.
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Old 03-01-06, 05:06 PM   #13
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Microwaves most definitely do not cook using radiant heat. The microwave radiation (as in non-visible light waves, not as in radioactivity) interacts with the subject (ie your leftovers), and in doing so, generates heat. Depending on the electronic (as in electrons, not as in electricity, though the two concepts are linked) nature of the material, some things are much more susceptible to microwave radiation than others. Water is one of the things particularly susceptible to the particular wavelength of microwave radiation used in commercial microwaves.

A signature characteristic of microwave heating is, as a result, an apparent heating from the inside out rather than from outside in. Another characteristic, which follows from the first, is very even heating (surface contacts excluded) if there is sufficient microwave power. And the potential for wild superheating. Methanol boils at 60-odd degrees Celsius, but in a microwave reactor, transient temperatures on the order of 200 Celsius can be achieved.

This last point, the wild superheating, is one reason why one should be very careful with heating water in a microwave that doesn't use a turntable. With virtually no turbulence, superheating can occur, and then the slightest turbulence from taking the glass of water out of the microwave can result in instantaneous generation of dangerously hot steam rushing at the user.
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Old 03-01-06, 05:15 PM   #14
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Thanks for the clarification.
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Old 03-01-06, 05:28 PM   #15
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Glad to be of service.
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Old 03-01-06, 05:58 PM   #16
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This last point, the wild superheating, is one reason why one should be very careful with heating water in a microwave that doesn't use a turntable. With virtually no turbulence, superheating can occur, and then the slightest turbulence from taking the glass of water out of the microwave can result in instantaneous generation of dangerously hot steam rushing at the user.
Unless you put a wooden stirrer in the cup/mug/glass. I forget why, but Alton Brown explained how it keeps the superheated water from turning into steam.
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Old 03-01-06, 06:05 PM   #17
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^^^Yes, that will work. The rough surface is sufficient to induce boiling. Superheating or supercooling only occurs with no disturbances. The presence of rough edges allows nucleation, or the generation of miniature gas (ie steam) bubbles or ice crystals, which then allows boiling or freezing.
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Old 03-01-06, 09:39 PM   #18
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Ahhh...Mythbusters just proved the myth true with provisions. Tap water usually has impurities so it'll boil and bubble in a microwave. Distilled water has no impurities so it won't bubble or boil when superheated. Thus distilled water can explode when disturbed, in this case, by a sugar cube.
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Old 03-01-06, 09:47 PM   #19
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Geeez ! who gives a crap what the scientific structure is . It burns & if you have a purpose for it use it.
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Old 03-01-06, 10:04 PM   #20
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Old 03-02-06, 03:26 AM   #21
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Originally Posted by MERTON
pfft. you don't need propane. just fart in a can and hose it up to a bunsen burner.
Ok, here you go:



Although I'd be suspicious of the resultant flavor...
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Old 03-02-06, 06:10 AM   #22
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Ok, here you go:



Although I'd be suspicious of the resultant flavor...
paintin a picture of "blow it out your ass"
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