Efficient Method of Producing Heat Using LPG

[PraAnan]

I'm trying to heat a 55mm x 55mm piece of aluminium to a minimum temperature of 250 Celsius (max is 300C) using LPG at a rate of 300Btu/hr.

The issue I'm having is that because I'm using a flame the temperature across the entire surface isn't uniform. I've read about catalytic flame-less combustion of gases and after doing a little bit of research I came across the following products.

Infrared Honeycomb
and
Catalytic Heaters

First of all, are the materials being sold in the first link the same as the ones that are in the heaters in the second link?

Is it more efficient to convert LPG to heat using a catalytic method rather than a flame based one?

What is the starting procedure for these ceramic based materials? Is it as simple as starting the gas flow and then adding a spark or is it more complicated? Lastly, I'm curious to know about how long these ceramics can be used for before they start loosing efficiency and/or start breaking down from the heat.

 

[jambaugh]

As long as you are getting complete combustion, all methods are equally efficient in terms of conversion of energy. All mentioned methods can be max efficient... or less so if the design of equipment isn't optimal. Test for CO emissions and soot formation which indicate incomplete combustion.

 

[PraAnan]

I don't have access to any CO or soot measuring equipment but would a simple way to measure soot be to just keep a piece of metal on the flame and see if there is any build up? Because to calculate the rate of lpg use I'm actually using a gas cooker and I've never noticed any build-up or residue on the surface of anything that I used.

Also, the flames on the stove are blue in colour, doesn't a blue flame usually indicate that it is clean burning and therefore showing that the efficiency is high?

 

[jambaugh]

I don't have access to any CO or soot measuring equipment but would a simple way to measure soot be to just keep a piece of metal on the flame and see if there is any build up? Because to calculate the rate of lpg use I'm actually using a gas cooker and I've never noticed any build-up or residue on the surface of anything that I used.

Also, the flames on the stove are blue in colour, doesn't a blue flame usually indicate that it is clean burning and therefore showing that the efficiency is high?

Testing with the piece of metal may cause soot by causing incomplete combustion. If you see soot building up in your setup then that indicates you should adjust flame. I don't think you need to worry much about inefficienty w.r.t. complete combustion. One of the main concerns is safety. Catalytic burners can reduce CO danger when using LPG without a vent but one should still be cautious. The blue flame doesn't necessarily mean complete carbon combustion from a safety standpoint. The blue color is the hydrogen burning in the hydrocarbon LPG. You will notice a difference between pure hydrogen flame, methane, propane, and butane. Each having higher carbon ratio has a higher yellow component to the flame up to the paraffin of a candle which has little noticable blue.

But I am getting way off topic. It sounds like with your setup, combustion efficiency is close enough to 100% to not be an issue. You lose more heat via radiation and convection than you might be losing to incomplete burning.

 

[LURCH]

...The issue I'm having is that because I'm using a flame the temperature across the entire surface isn't uniform...

Before doing anything too complex, have you tried just setting the aluminum on a thin plate of steel? That might spread the heat out more evenly. Unfortunately, it may also diminish the efficiency you're looking for (the steel will radiate some of the enegry away to places other than the aluminum plate). You could also try sarounding the plate with an enclosure, making your setup work less like a stove top and more like an oven.

Also, since you only give two dimentions for the aluminum, I assume it is a thin plate, and not a block, yes?

 

[PraAnan]

@jambaugh Thank you for your detailed post, it was really informative and I was very glad to read the last part.

One issue that I might have is that in our stove at home we have a small central flame which by chance happens to output only a little over 300btu (302Btu) but none of the stoves in our school have flames which output a 300Btu minimum. Is there a simple way where I can extract 300Btu/hr from a gas cylinder without having to take our family stove from the kitchen and into school?

@LURCH I had a similar thought last night and I actually cut out a copper square which is the same size as the aluminium today and I'll be putting that between the fire and the aluminium piece.

The oven idea is also interesting, I do have some insulating material so I could use that to close up the flame a bit more.

Yes, the aluminium has a thickness of less than 5mm and the copper piece is a little less than 3mm.

Would simply resting the aluminium piece above the copper piece be good enough to transfer the heat energy? I looked into getting some thermally conductive epoxy (arctic silver) but the max temperature on that is 150C. Are there any thermal adhesives which will work without fault at temperatures close to 300C?