Questions re: large L-tube rocket stove for making tortillas

[nixsee]

I have some questions regarding making a large, low-cost, rocket cook stove being used to make lots of corn tortillas.

I need to heat up 21" diameter, parabolic-dish-like ceramic "comal", which needs to maintain a pretty high temp (about 250-300C) in order to get good tortillas. Right now I have a standard L tube stove made from pumice stone concrete blocks (lightweight and relatively insulative) with a feed tube that is about 6" wide and 8" high, with the bottom 3" being below the feedtube grate, which extends all the way to the back - this seems to work better than having the grate stop at the riser opening because it keeps the embers close to the fresh wood while allowing additional air to come in from below. The heat riser has a similar CSA (6x8") as the feed tube for the first 24" of height, before opening to a much wider, but still enclosed, 8" high area to allow the flames to disperse under the whole comal.

I'm having trouble uploading photos, but they are stored in this album here along with a video clip of the fire. The full-size view is with a pot inset, but the comal sits on top of the blocks as can be seen in the first pic. 

The only way I can get enough flame/heat to really get the comal up to temp is to put a couple medium sized logs into the heat riser from above, once the initial fire is roaring, and then put the comal on to heat up. Additionally, if we're making tortillas for a while (sometimes up to 2 hours), I can shift an upper block and slide additional logs in. I get a very clean, smoke-free, soot-free, tear- and cough-free fire like this, so it seems to be working well. I just want to try to make it as efficient as I can!

Do these rough dimensions make sense? Should the grate have more or less space below it? Should it extend to the back or not?

If I am losing energy by having logs in the riser, the only solution, really, is to make the feedtube larger to accommodate more wood. But I suspect that the riser CSA is already at its upper limits given the user ergonomics that limit the height of the stove.

So, is it ok to have a different feedtube and riser CSA? (also, when I say feed tube, it's really just an opening in the wall, not an actual tube).

Could I make the feedtube opening larger (8x8 or even 8x10?) while maintaining the same riser dimensions?

Would I perhaps even benefit from shrinking the riser tube CSA to get a more "rockety" effect? It is already relatively "rockety", with an audible and visible suction of air and pretty violent flames up top - literally shooting out from the sides of the comal. 

Most fundamentally, in an L shaped rocket stove, what things truly matter? Insulative materials, heat riser CSA:height ratio (what is the ideal ratio?), and adequate airflow from underneath the feedtray? Does the feed tube CSA really matter at all (within reason)?

Thanks so much and please feel free to ask any clarifying questions or make any and all criticisms. 

[fuegos]

hello Nixsee i'm new to rocket stoves but one thing i would advise is that you fill the joints of your build with a clay/sand mortar.as for the problem of height maybe a J tube design with some buried in the ground ? some sort of insulation around the riser would also help

[nixsee]

Thanks for the ideas fuegos.

Yes, filling the joints would surely improve it, but I am not going to do so until I am done tweaking the general design.

A J tube would definitely be helpful with both feeding fuel and getting more riser height. However, I haven't yet had much success with making a J tube - I suspect my designs likely had an excessively long burn tunnel whereas I've seen successful ones where the feedtube is basically on the other side of the wall of the riser. Perhaps I'll give that a shot soon!

The riser is pretty well insulated already given that it is made from double walled (hollow cavity, but sealed on the upper side), lightweight concrete blocks made from pumice stone (similar to vermiculite) - even after over an hour, there's essentially zero warmth on the outside.

I would love any insights that anyone has regarding putting wood in the riser, riser vs feedtube sizing, riser diameter vs height, and grate/feedtray type and positioning!