Catalytic Combustion Stove?

[Craig B]

Seeing all the recent activity in stove posts and talk of efficiency, I started wondering about burning fuel via catalytic combustion in a stove.  With catalytic combustion, you can basically get 100% efficiency out of your fuel.  The biggest hurdle (it seems to me) would be to make a catalytic matrix that is light enough to not eat up the weight savings from reduced fuel consumption.  It can also be tricky to get a catalytic reaction going and sustaining it, so you would waste a little bit of fuel on startup.  The lifetime of the catalyst might be an issue as well; even well designed catalytic reactors lose efficiency over time.

My initial thought on how to make the burner would be to coat something (directly on a pot maybe) with a thin layer of porous ceramic, then sputter some Platinum on it.  Maybe you would do these two steps a couple of times depending on the thickness of ceramic you could deposit, but end up with a 0.04-0.06″ thick layer of Pt infused ceramic.  Then you would need to have another layer of metal cover the ceramic and seal to it somehow.  Although it would need to be somewhat open to air to get oxygen for combustion. You would introduce the fuel (gaseous) at the center of the whole thing (bottom center of the pot), so it would travel radially out and up the sides within the ceramic layer.  You might be able to get light-off by holding a lighter next to the center of where the fuel is being introduced and heat it up until the reaction is self propagating.

A lot of potential engineering challenges in the manufacture, but it seems like it might be possible for such a thing to work at least.  What do you think, all you stove designers?

[Mike B]

Craig your thoughts intrigued me so I did a little googling and found this on the BPL site. I am at work atm and could not get too far down the rabbit hole. https://backpackinglight.com/forums/topic/72580/#comments

Maybe this thread hold some information for your quest

[Bruce Tolley]

Interesting question.

[James Marco]

Yeah, most of that stuff was with flame produced. I was always hoping for a hot, but not burning, stove with very high efficiency. A small amount of fuel would warm up a cup in a few minutes with no flame.

[R]

Cooper, acetone catalysis was mentioned on the last page.

Catalytic reactions have no guarantees of efficiency.  You may not get all of your fuel in contact with the catalyst and your exhaust stream will carry away heat, same as a flame.

Realized efficiency depends on your implementation, and conditions of operation.

A copper dish scrubber might be a good starting point.

[David Thomas]

To follow up on Rene’s post:

Thermal combustion needs to be well-mixed, for long enough, hot enough, with enough but not too much air. And to not be quenched on cold surfaces.

Catalytic combustion also needs time, temperature, and adequate concentrations of both fuel and oxygen.  The temperatures don’t have to nearly as high (600F versus 1400F) and it can occur with very low oxygen concentrations (such as in your automotive tailpipe) albeit at a lower rate.

I don’t see a reason for a stove to be catalytic versus simple thermal.  Thermal is easier, gets hotter (which is good for heat transfer), doesn’t need platinum which is costly and can sinter to a less active crystalline form, nor a ceramic substrate which is fragile.

Lots of thermal stoves need improvement. Better mixing, a bit leaner at sea level so they’re not too rich at elevation. Enough distance / time / volume to go to completion before hitting a cold surface. All of that would also reduce CO emissions. But some thermal stoves have pretty good heat rates, low CO emissions and combust almost 100% of the butane into CO2 and water.

[James Marco]

David, yes. I was aware of that. Mostly I was looking for a small 2 cup pot with a double wall that could be gotten up to around 300-400F rather quickly without a lot of fussing with conventional stoves. Mostly, catalysts do not effect the end results of any reactions. They just lower (usually) the ignition temp for the fuel.

Ideally, this would be in a double walled cup (similar to insulated cups) that I could simply pour some volume of fuel in to heat the contents. This would have a thin 1/8″ layer of nomex or similar heat resistant material as an insulator over it.

This is worth carrying the extra weight of the catalyst. This would eliminate most of the convectional heat losses, though you would still have some loss of efficiency in the heat transfers.

[R]

There’s a bit of a conundrum in a catalytic water heater. You want good thermal conductivity to move the generated heat in to your water.  But  that same conductivity inhibits keeping the catalyst at reaction temperature, which is well above water temp.  This is the equivalent of quenching combustion with a cold pot too close to the stove.

Stoves solve it by allowing space for combustion to complete before the hot gases contact the cold pot.  They use a hot gas flow for one-way thermal transfer.

You could employ a similar arrangement with a catalytic heater, separating the catalyst from the cold sink with a fluid transfer medium, but then it starts to look a lot more like a conventional stove, but at lower temperature.

Maybe there’s still a win if you design for radiant transfer. Something in a stack, like: cold sink, air gap, catalyst (wire mesh?), reflector (aluminum foil or cup?), insulation.

You’ve still got to solve for reaching reaction temp with out igniting your fuel.  Open flame is a very convenient way to heat the catalyst, but you could also use a battery if you’re ok with the weight.

If you arrange the above layers as concentric cups, you might remove the catalyst plus beverage cup, add fuel to the reflective cup, heat a corner of the catalyst with a lighter, put it all back together, wait for the cup to warm (you need to let the rest of the catalyst heat up, which will go faster with out the cold liquid), then add the cold water. This neglects air intake at the bottom.

Simpler than setting up a stove, but probably not as simple as one might hope. Also more tempting to do this in a closed tent in foul weather, so mind the exhaust ventilation.

This is all just off the top of my head. Hopefully there’s a simpler priming design to be found.  Maybe push-button electrical?  A latch switch using a shape memory alloy or bi-metallic element that opens the circuit once hot enough?

[James Marco]

Yes, still looking…and researching various bits…

 

[Craig B]

David, good point about the already relatively high combustion efficiency of conventional burners.  For some reason I had it in my head that it was on the order of 50%.  I must have been thinking of the total efficiency of some internal combustion engines that can approach that.

Rene, good point about conduction from the heated fluid potentially quenching the catalytic reaction.  I had not considered that in my thought experiment but it certainly adds more engineering challenges.

It would still be fun to play with in the lab…..

[David Thomas]

Typical efficiency of transferring heat to the pot is 35%. 50% with HX fins. But the combustion process itself is nearly 100% complete – it almost all goes to CO2 and water.

Now if the active catalytic layer were ON the outside of pot, that has potential to increase the heat transfer to the pot by a lot.

[R]

Yes, but that leads you to the  reaction temperature problem.  A strong thermal coupling to the cold sink (the water you want to hear) inhibits reaching and maintaining the reaction temperature.

A closed system, like in a CPU cooler’s heat pipes, should be pretty efficient, but at another weight penalty.

[Edward John M]

Isn’t that the point of the skirted pot tho Rene?

You get the cold surface away from the flame front but keep the hot combustion gases in contact with the pot for a longer time?

Ditto for any sort of close fitted windshield. Stoves are a pretty well established technology and seem to have reached a reasonable plateau with regards to fuel combustion efficiency, or at least that is how I interpret Rogers posts on stoves. What was the original rationale for the catalytic heaters? Lower temperature clean combustion or a longer burn time?

[Craig B]

The biggest benefit of using catalytic combustion is being able to engineer in better control of the combustion process and therefore what happens to the heat released.  As Dave pointed out, conventional stoves can maybe get to 50% thermal transfer efficiency.  The problem is the combustion gasses don’t get a long enough time in contact with the pot surface, especially as you crank up the gas flow of the stove, so half or more of the heat released is just wasted in the air.

With a catalytic reactor, the matrix the catalyst is embedded in will absorb more heat from the reaction, then be able to transfer that heat more efficiently to the thing you’re trying to heat. However, the matrix must maintain a minimum temperature to keep the combustion process going as Rene pointed out. So if your hot matrix is touching the side of a pot that is filled with water and therefore cold, the temperature of the matrix will drop, and the reaction will be “quenched”.  You would need to add an insulating layer (also as Rene mentioned previously) in between your matrix and heat sink (pot with water) to let a big enough gradient develop so the reaction is sustained.  But then of course your heat transfer rate into your water will be slowed somewhat, so it might take a while.

 

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