I made a few alterations to my “pocket charcoal chimney”, and also tried it with natural charcoal instead of briquettes.
I haven’t really described the making of the stove, not that it’s very different from a million others. I guess it might be useful to give a complete how-to sometime, but I’d rather tweak a bit more yet to get it better.
New Stove Can
The bottom can is new, without a fuel door, but with big holes down below the level of the grate instead. I’ve given both a few test runs, and this seems a better option for my current intentions – if I can get the charcoal stove efficient enough I’ll avoid burning twigs, simply for the reduced smoke, tar and messing about. The fact that you have to feed a stove like this little twigs and stuff constantly means you’re constantly breathing in the smoke, whereas a normal camp fire you’ll only tend now and then. There is the option of packing it with more wood in one go, but that makes a lot of smoke, soot and mess on the kettle, burns like a torch for a few minutes and then you’re back to square one. This is the beauty of charcoal. I’ve gone right off sitting around stinking of woodsmoke.
The downside of having holes all the way round is that when there’s a wind blowing, you can’t point it into the wind to help fan the fire, but if there aren’t holes all the way round it will reduce the air, which is critical when it’s a still day. The optimum might be to have a door that can be closed on 2/3 or so of the air intake holes to force it with the wind. I suppose just wrapping foil round some of them would be an option too. I find it useful to put the whole stove on a layer of aluminium foil, and that can be brought up round the base as a windshield. It also reflects heat to improve efficiency and protect the ground from scorching.
I got the feeling that when I added the second can on top as a chimney, the fire was a little choked with the kettle in (which helps to boil the water in it!) due to the small area between kettle and chimney sides. The same thing happens when the kettle is located in the lower can rather than sitting on top of it on wires. For getting the most heat out of dying embers, this latter position is probably fine, because there’s less draught required, but when burning hot, the gap is rather small – only about 2.5 mm. The second can is important because it extends the height, creates more updraught and keeps the heat next to the kettle, but it has another disadvantage: it doesn’t pack up with the rest, being the same diameter as the stove can. These are your everyday baked-beans size holding about 420 g. They’re 7.5 cm diameter and 10.5 cm high.
So I’ve tried using a sweetcorn tin for the upper section – these are about 8.5 x 8.5 cm. – and fitted it on top of the other simply by cutting a hole a little smaller than the bean tin and bending it down evenly all around so that it locates on the top lip. The idea is that when the stove is on full blast and needs more draught up the chimney, you support the kettle only inside this upper, wider, can. [Edit: Now I’ve found that real charcoal doesn’t burn with flames leaping up the chimney, I’m not sure if this is better or not. Further testing required.]
Improved Kettle Support
You could drill or punch holes in the sides of this upper can, and put thick wire through to support the kettle (people often use spare tent pegs), but I came up with a new holder made of two strips of steel cut from a larger can and wired together with thin wire where they cross. This is probably a little bit lighter than the wires, and more easily adjustable for fit. It can be bent to length and hung over the top of the can. Here it is inside the sweetcorn can:
I haven’t perfected it yet, and I’m not sure whether to continue using the pegs through holes in the stove at the lower level or work something else out. It might be good to get the hangar piece so that it works in both cans and just needs lifting from one to another. I’ve put too many bends in this one as I botched it together.
This wire-mesh grate is better than the old computer fan grill, particularly to keep smaller particles of charcoal from dropping through. Once I’ve decided how high to have it I’ll cut the legs to size (which are just parts of the mesh bent down and under), and then it’ll stow upside-down to allow more room for the kettle.
That’s about it apart from a slightly wider base (stray embers catcher and another layer of protection for the ground), but I’m not sure if that’ll survive later hacks. Again, it’s the problem of storing this. At the moment the sweetcorn size of tin, 8.5 cm, fits nicely inside my water filter. This wider one has to be stowed separately.
Natural Charcoal (Bar-be-quick Lumpwood)
This experiment turned out much better than expected. I thought it might be difficult to start, but it seemed pretty much the same as briquettes. It burned without the smell, without the smoke, and with hardly any flame, just red-hot coals. I think there was maybe a slight smoky smell right up close. The main advantage of briquettes for barbequeing is they often give a longer slower burn: for the camping stove this was a disadvantage. The lumpwood continued a fair old time too, but was a quicker, hotter burn, as far as I can judge without a suitable thermometer.
Hot or Not?
My testing at this stage is mostly subjective and proof-of-concept. I’ll get to boil times later. I noticed there’s a standard developed for this kind of thing – 2 cups – so I’ve since doubled up the water from my first experiment. [Edit – just realised this is still only about 440 ml, which is a bit less than 2 cups, 473 ml.]
I’m still using about the same slug of fuel by weight as a briquette, and even trying to reduce it. I used about 40-45 g. of charcoal this time, broken up into pieces about 2-3 cm across, plus the powder and intermediate stuff that comes from tapping charcoal with a hammer. I still expect that smaller pieces of a more even size might give better results.
My water took a fairly long time to get hot enough to use, somewhere in the region of 15 minutes, and didn’t reach a rolling boil. Personally, I’m happy with that. I’m not “fast and light”, just light. I don’t mind waiting a wee bit for a nice cuppa if the stove works well, doesn’t need too much tending, is clean enough (by which I mean mainly the air quality and lack of soot on pans), reasonably safe, cheap and fairly lightweight. I haven’t found out yet what weight of propane and butane it takes to boil 2 cups of water, and that’s my target, but
a) charcoal will always be slower, probably, and
b) it won’t explode, probably. (Powders, however, can do so.)
[Edit: There’s a great comparison of stove fuels at Zen Backpacking, and charcoal isn’t too shabby on the old BTUs, about 14000 BTU per pound weight compared to about 19000 for propane. Methanol comes in at under 8000. There’s a lot of information on that page for estimating the weight of fuel and associated equipment required for camping trips, based on boiling a US pint of water a day, 16 oz, 2 cups, 473.18 ml – ish? Charcoal only gets a passing mention again, which I’m increasingly puzzled about. From the graph, I estimate my rough target as 50 g of fuel per “meal” or per “day”, i.e. to boil 2 cups. Pimps. Lemon squeezy.]
I personally don’t require a rolling boil at camp. Near boiling takes a fraction of the calories and is great for most beverages and pouring over my super-noodles or whatever. For my tea and coffee I’d add powdered milk, so it’ll be too hot if it’s boiling. There are a lot of videos on youtube of stove tests, where the object is to time super-accurately 2 cups of water reaching that magical rolling boil, but not often much science behind it – in particular, often the starting temperature is not taken, which makes a vast difference. Boiling some water straight out of a glacial stream is going to take a lot more fuel than from room temperature. But it’s those last couple of degrees to boiling that take vast amounts of fuel. Changing state is energy expensive.
Probably the only negative side to this test was that the lump charcoal showered a fair amount of sparks, something I didn’t notice at all with the briquettes. These were tiny little points of light pinging as the charcoal caught light, and they didn’t fly very far, but it’s a reason to keep the stove further from a nylon tent or other flamable material than with the briquettes. In the average English summer, I don’t expect those little sparks would set light to anything, but it’s something to be aware of if it’s been particularly dry. I’ve only tested it once, so it could be a mere fluke and not happen again, or it could be a regular feature.
One other big difference is the ash – there’s very little of it with proper charcoal, just white wood ash as you’d expect. The briquettes produced a big heap of motly ash, and I don’t know if it’s common, but I found a fair amount of little pebbles in mine. That’s a trick charcoal men have been criticised for since the 17th Century, hiding stones among their wares. It’s not easy to get away with it now, unless you pulverize it, mix it with all sorts of other things, and then shape it into briquettes.
Natural charcoal is actually very pleasant to deal with. The lumps just look like black pieces of wood, of course, complete with grain. It’s mucky stuff to handle, but, as the saying goes, “good clean muck”. And if you’re ever bored at camp, you can draw something with it.