Dear Andrew >Interesting theories Crispin. If, as you say kerosene was a simple >fraction in the past and is now a mix of oils with widely differing >carbon backbone then I can see how it will become more difficult to >control vapourising. I think the point you make is that oil must be >vapourised at a given temperature, which is normally controlled by >parameters of the stove design.
Historically kerosene is not a single thing like Butane or Ethane. It was all the junk between low boiling point liquids and diesel. At SASOL they make huge amounts of diesel and like biodiesel plants, there are a lot of 'mistakes'. In RSA diesel is about C16H34 to C22H44. As I mentioned, paraffin is the C9-C20 fraction so there is quite a bit of overlap. If the paraffin (kerosene) is too light, it tends to boil out of cheap crummy stoves and easily cause huge fires. In order to limit the flammability of paraffin they tend to put more of the heavier fractions into the paraffin however that said, it is highly variable. Paraffin/kerosene is a bunch of fractions that are easily collected in a fractional distillation tower tray. For all that is said about it, paraffin is a junk fuel into which whatever is left is poured. Jet-A is just paraffin with more control and narrower specifications on what goes in (plus some anti-freeze). Because paraffin lamps and stoves can burn a range of things it is not all that important what is in it (in terms of an exact ratio of component fractions). It is just a South African preference to make it towards the heavier end to limit domestic fires. Because of the variable composition it is not easy to make a 'perfect' paraffin burner because the fuel varies from country to country and season to season. >If you have a light fraction that boils off and then the temperature >rises before the heavier fraction boils then I suppose the lighter >fractions underneath can start pyrolysing, and hence char. Well, when the longer ones start breaking down, they are just short ones with ionised ends so I think you can assume it happens at the same time. The issue is how hot it is in the end. If there is a great deal of free carbon in the gas stream you are going to have problems. It is easy to show if you are building paraffin burners: just over heat the evaporator and it will soon clog the jet with carbon. The downside of running a burner at high power with droplets beautifully balanced (giving good combustion) is that if you turn the power down and let it stabilise, you can get under-heating of the fuel which means a streakier flame. Indian paraffin (which is widely used) is highly variable in 'quality' (read: 'content') making all sorts of problems. Part of the confusion about the content of paraffin is that a paraffin is just a shape of molecule (long carbon chain with hydrogen atoms all round). Calling it 'paraffin' is not calling it anything in particular. Paraffin wax is just a paraffin chain so long it is no longer a liquid at room temperature. As it burns it breaks into 'diesel' and 'paraffin' and butane and methane and so on, if you look at the chunks floating around. In the end it should be CO2 and water. The longer the chain to start with, the less water as a % of the result. That is why natural gas produces so much condensation and candles don't. For your burner, if you protect some of the evaporator with a metal shield to reduce the internal temperature you can probably limit your carbonaceous problem. You certainly want to overshoot to get a poor burn then let in more heat to see where the sweet spot is. Regards Crispin _______________________________________________ Stoves mailing list [email protected] http://listserv.repp.org/mailman/listinfo/stoves_listserv.repp.org http://stoves.bioenergylists.org http://info.bioenergylists.org
