Charcoal stove expriment

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TacAir
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Charcoal stove expriment

Post by TacAir » Tue Dec 11, 2012 12:04 pm

Charcoal powered, sorta hobo stove experiment.

While in Korea, I used a hobo stove that burned charcoal or the soft coal sold locally. The stove was used to cook beans for our BBQs, but it was really good for any food cooked in a larger pot.

That stove was made from a coffee can and a smaller internal can, either a soda can or a 303 can. This stove will get hot enough to melt a soda can and will quickly cause the 303 can to glow red.

So, could I make a smaller stove that worked as well as the full-sized unit?


Image
Here are the 3 stoves I made. I full sized using a coffee can (middle), the others used common 303 steel cans.

ETA- I took the photos before the burn tests - the coffee can had holes in it, just not when I took the photos - chasing the sun sort of thing...


Image
This somewhat ugly stovette is a cut off 303 can, opened at top and bottom. Inside I placed one and later two charcoal briquettes. I 'burned' each after starting the briquette with a shot of white gas.
The container held 1.5 cups of water at about 50F. I tracked the heat rise over time.
I won't post the data as this little wreak of a burner never boiled the water - so - FAIL
I suspect that the lack of a good draft is the reason this failed so badly.


Image
This is the taller of the stoves, so more of a chimney effect and should have worked better than the stubby wreck. I burned both one and two briquettes. Again, no posted data as this one also failed to boil the water.
I can only conclude that there is some critical mass of charcoal needed to produce enough heat to boil water.


Image
Here is an inside look at the large stove, you can see there are 6 briquettes - in Korea, we would layer these two or three deep - again, to cook a pot of beans.
When I lit this off, the water was boiling in under 10 minutes.

So? Lessons -
Small charcoal stoves are not a good option - several briquettes are needed to generate enough heat to cook with for any size pot.
Large size will burn nicely and with the internal chimney, will quickly put out the heat.

An alcohol stove is smaller, cleaner and frankly, will work better for cooking small pots of food.

Bottom line - if stuck in a grid-down situation and you have access to a bag of charcoal, a couple of cans will give you the option to cook without tearing the back stairs apart for wood to burn....
I started the charcoal with a shot of white gas, you could use auto fuel, in a pinch, but let it burn for a bit before putting the food on to cook.

Hope some of you might find this useful - and if you have an expedient stove, I'd like to see what you have built or cobbled together.
Last edited by TacAir on Tue Dec 11, 2012 7:05 pm, edited 1 time in total.
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Re: Charcoal stove expriment

Post by BullOnParade » Tue Dec 11, 2012 12:20 pm

Awesome!

So the smaller can draws air in from the top to feed the flames?
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Re: Charcoal stove expriment

Post by TacAir » Tue Dec 11, 2012 12:34 pm

BullOnParade wrote:Awesome!

So the smaller can draws air in from the top to feed the flames?
It provides a - call it a secondary chimney effect. I've made the internal 'chimney' out of hardware cloth (wire mesh) before but once it gets hot, the stuff I used was prone to melting.

The idea is to get as much air into the 'stove' as possible. Larger holes at th bottom help, in this case, I just used a church key to punch the inlets. If you have a GreenLee punch, you can make larger, 3/4 or 1 in holes.

In essence, all I've really done is make a cheaper and more efficient one of these
Image

The charcoal starters work well for the same reason - the chimney effect causes more air to feed into the burn chamber.

I built the first of these stoves in 1976, so it is something that has worked well for me over the years.

As always, YMMV.
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Re: Charcoal stove expriment

Post by maldon007 » Tue Dec 11, 2012 12:54 pm

Maybe break up the charcoal into smaller pieces? That might let the greater surface area burn better... Thank you for the interesting experiment!
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Re: Charcoal stove expriment

Post by docdredd » Tue Dec 11, 2012 12:57 pm

but with greater surface area you have reduced burn time. Im not sure you need more heat in this situation, more than a med heat over a longer time span.
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Re: Charcoal stove expriment

Post by maldon007 » Tue Dec 11, 2012 1:00 pm

I figured that, but also figured the burn time of whole charcoal was maybe excessive... Anyway, I might give it a shot, if I get a chance.
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Re: Charcoal stove expriment

Post by JayceSlayn » Tue Dec 11, 2012 1:48 pm

The principle of how the largest one burns is a little confusing to me, but the other two look solid in theory at least.

We all know hot air rises, and the reasoning for a length of "chimney" is to ensure that rising heated air (1) goes where we want it to heat the pot, and (2) creates an uninterrupted upward draft in the chimney that greatly increases the "suck" of natural convection, drawing in more fresh air at the bottom to our fuel. The two smaller cans have air entrance holes at the bottom for fresh air, and after burning with the fuel, the air exits through "leak by" holes at the top, passing all around the pot after leaving the chimney. To a short extent, the longer a chimney is, the more efficiently it performs this function, which is why maybe the little one didn't do as well as the taller small can.

There are also two conflicting convection mechanisms at work in these (or any) stoves, which involves the heated air trying to heat the water, and cold air from the environment trying to cool the water. A windscreen that helps to ensure the pot is surrounded by mostly heated air, and that any heated air is forced to pass by the pot, could help a lot. Also, cold air drafts can come directly downward from above the pot, and hot air can rise from the pot into the environment (these actually happen at the same time), so a lid is always a good idea too. A reflective windscreen also helps to ensure any thermal radiation that is given off by the heat source has at least a second chance of impinging on the water pot (better too if the pot is black/dark). The radiation loss from the pot of water that is near boiling is not a huge contributor, but has some effect.

I feel like the larger stove which uses an in-tact outer can is sort of trying to be a windscreen for the inner can, but without air holes at the bottom, fresh air must be inducted around the sides of the top, the same place where combusted hot air is trying to leave. This results in a kind of inefficient air transfer, and also likely entrains some of the oxygen-depleted air back down into the fuel where it is less effective at burning again than before. I would suggest putting holes in the bottom of the outer can as well. The inner can may be serving some effect, but since it would need to work as a down-draft chimney, sucking air from near the center of the pot, I don't see how it is significantly contributing to fresh air induction near the base of the briquettes.

There is a little something to be said about heating the pot of water faster, that it may be more efficient. Since the heat source must be hotter to increase the heat transfer rate into the water pot, this portion of the efficiency may increase slightly. More importantly though, the less time the pot spends at elevated temperature, the less time it has to lose heat you put into it to the environment. There's a lot of fine details behind whether heating faster is more efficient or not, but I think it should hold true in this case.
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Re: Charcoal stove expriment

Post by TacAir » Tue Dec 11, 2012 7:01 pm

My bad - i did punch holes in the large can after the photo shoot - again with just the church key, larger holes are better of course.
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Re: Charcoal stove expriment

Post by TacAir » Tue Dec 11, 2012 7:07 pm

maldon007 wrote:Maybe break up the charcoal into smaller pieces? That might let the greater surface area burn better... Thank you for the interesting experiment!
Hadn't thought of that! Next I play around, I'll try that.

Thanks
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Re: Charcoal stove expriment

Post by bltjr1951 » Sat Dec 15, 2012 9:12 am

TacAir wrote: In essence, all I've really done is make a cheaper and more efficient one of these
Image

The charcoal starters work well for the same reason - the chimney effect causes more air to feed into the burn chamber.
As always, YMMV.
More efficient??
The starters don't have inside chimney, they work just fine stuffed with charcoal.
Some youtubes show people using pots on them and wood for fuel.

I bet if you replaced inside can with more charcoal it would still work.
Maybe hotter sooner and longer burn time.

Most grills have charcoal on rack/grill, so air comes underneath.
Maybe if you had charcoal on "rack", it would burn hotter?
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Re: Charcoal stove expriment

Post by mark9atq » Fri Apr 24, 2015 6:26 pm

Running out of charcoal as would happen in the ZPAW for this stove type is a problem to overcome.

We've all probably seen those biomass briquette makers - press a sludge of sawdust + shredded paper + leaves
in solid fuel briquettes that need to dry before burning.

These work fine for applications where the smoke is remote from the air you are breathing. Example
an outside boiler shed which pipes hot water into your separate home for winter heating.
But they stink for cooking fires.

A technique to pre-process the sawdust and leaves and paper shreds is shown in this youtube.
Basically its just partially burning the material first, then dumping water on it to stop combustion. Then do the
standard bio briquette making routine, except
add some finely sifted clay soil into the mix. This is the Kingsford charcoal recipe.

Bio briquette making in the jungles of low lands of Nepal. A cleaner burning DIY briquette.
https://www.youtube.com/watch?v=HQfp-XIxNq0

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Re: Charcoal stove expriment

Post by Trippy » Sat Apr 25, 2015 8:37 pm

Did you necro a 2 1/2 year old post just to say we'll run out of charcoal in a ZPAW?

As long as there is wood, we won't run out of charcoal. Production is a simple process of burning wood is a reduction environment. We would eventually run out of briquettes, but not actual charcoal (until deforestation occurs).

A stove like this should also be able to burn wood quite well. Wood won't burn as hot as charcoal, but if you're just cooking, that doesn't matter.

Side note spurred by thought of running out of charcoal: Charcoal became popular due to iron production. it burns hotter than wood, which is necessary to smelt or melt iron. Once Europe ran out of trees, they had to switch over to using coal, which burns cooler than charcoal.

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Re: Charcoal stove expriment

Post by duodecima » Sun Apr 26, 2015 9:32 am

I for one am quite appreciative of the necro, I hadn't seen this thread, but I do indeed have extra charcoal on hand for a small grill (that hasn't always worked well.) I also have a bunch of old #10 cans, I will have to try this!

I wonder if using a smaller punch to put holes in the bottom of the can in addition to using the church key on the bottom sides might give more air (and therefore hotter burn, right?) and only let a little bit of ash out. I will have to play with this this summer.

I also appreciate posts that provide a solution to the problem they point out - I hadn't heard of bio briquettes before, thanks!
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Re: Charcoal stove expriment

Post by TacAir » Mon Apr 27, 2015 12:05 pm

duodecima wrote:I for one am quite appreciative of the necro, I hadn't seen this thread, but I do indeed have extra charcoal on hand for a small grill (that hasn't always worked well.) I also have a bunch of old #10 cans, I will have to try this!

I wonder if using a smaller punch to put holes in the bottom of the can in addition to using the church key on the bottom sides might give more air (and therefore hotter burn, right?) and only let a little bit of ash out. I will have to play with this this summer.

I also appreciate posts that provide a solution to the problem they point out - I hadn't heard of bio briquettes before, thanks!
In looking around the web, I've seen several variations of the stove in my OP. It would seem that if holes were added to the bottom and the can raised a bit, it would burn 'better'.

My original thought was how or is it even possible to build a 'stove' that could produce useable heat (boil water, cook food) using just one or two commercial charcoal brickettes. In a post disaster event, folks may have access to charcoal and not other fuels - city folk mostly. Most lumber used in buildings isn't safe to burn for cooking.

I'm going to give this another shot - now that it was warmed up. This time I'm going to make a 'holder' out of screen cloth - to hold just two brickettes, and then have a set of rods (coat hanger wire) to hold the 'fuel mass' up off the bottom of the can.

Objective - simple, be able to make with a swiss army knife or multi-tool, using common house hold 'trash'.

Win or fail, I'll post the photos....
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Re: Charcoal stove expriment

Post by Mall Ninja » Mon Apr 27, 2015 3:46 pm

So I wonder if a rocket-stove type configuration would work better? There are designs that use a large can/coffee can and a couple of smaller cans for the burn chamber and chimney. A bit more work build and improvising the insulator might be tough (maybe attic fiberglass insulation?)
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Re: Charcoal stove expriment

Post by TacAir » Mon Apr 27, 2015 4:55 pm

Got all the parts for my improved version, now have to wait till spouse gets home to tell me where she hid stored the bag of charcoal ; )
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Re: Charcoal stove expriment

Post by TacAir » Fri May 01, 2015 4:15 pm

Got everything together and made a couple of burns to see if what I had tried earlier might work. For those that just tuned in --

Question (to me)

Is it possible to make a windscreen/potholder/stove sort of thing out of common household scrap that uses one or two common charcoal briquettes to produce enough heat to cook with? Or even just get something hot enough to be considered eatable?
.
This scrap stove does not have to complete with a Trangia or Esbit burner, but those two systems do set the low end of acceptable

Image
Pieces parts for the different setups.


This is a larger pot, a bottom grid of hardware cloth and a larger piece made to hold a couple or three briquettes. I used a food safety thermometer to confirm water temp. The tent pegs are used to hold the fuel up off the bottom. I cut multiple holes on side and bottom of the can.

Image
Single briquette finally heated up and ready to go

I soaked the briquettes with kero and a bit of white gas to ensure ignition. After several minutes, it was going.

Image
Briquette fuel cell, this can hold up to 3 common briquettes

Image
Two-briquette burnoff.

This is a two-briquette fuel cell setup – these sit lower into the can.

Image
Testing setup. Two cups of water sitting on the burning fuel.

Result?

FAIL once again. After 10 min, the single briquette had only warmed the water about 7 degrees.
With the double briquette , after 10 min, the water has warmed by just of 10 degrees.
A commercial 'charcoal starter' with a handful (say, a dozen) of briquettes can and will cook nicely n a pinch. One or two is a no go.

To compare, a single Esbit tab would have heated 2 cups of water to about 180 degrees in 5 minutes and be at or nearly at a boil in 10 minutes.

And now you know. I was taught in college that any experiment, properly documented, is not a failure if something is learned. I've learned I'm keeping my Svea 123....
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