Hybrid Rocket Stove

Its been awhile since I've built a stove. I found that the gravity feed that I used in my outdoor rocket boiler, while it worked and was reliable, it was inconvenient to use. Bundling wood and dropping it down a 7 ft tall stack isn't fun when there is snow and its freezing cold outside.

I set out to build a stove that would be clean burning, not need constant attention, and be built with common materials. What I've ended up with is a hybrid between a wood gasification stove and a rocket stove. Even if smoke, soot, and smell is absent from exhaust, there can still be combustible gases such as CO, H2, and CH4 that are invisible and odorless. This became evident to me through a small rocket stove I built from a exhaust elbow inside a metal paint can. The inside was insulated with perlite. I noticed flame jets popping up around where the exhaust pipe came out the lid of the paint can. I believed this to be caused by heated air inside the paint can combining with unburnt fuel being exhausted, much like a wood gas stove.

This observation inspired me to add a secondary air injector to the heat riser of a rocket stove.

To get longer load times, I've constructed a larger firebox designed to burn entire logs at a time. The logs gasify inside the firebox and the wood gases are then channeled to the heat riser. They mix with air from a secondary air injector where they spontaneously ignite, thus providing clean combustion. Air temperatures from the air injector range between 650 and 700 C.

Here's a video playlist I put together on my prototype stove: https://www.youtube.com/playlist?list=PLq-ETrB2scGG_tQBeuGbiVTXGIDTEnFuX

This small stove puts out a peak of 31K BTU and lasts 1.5 hours between needing to be reloaded. When you consider the only insulation this stove has is the concrete block that makes up its body, the clarity and lack of odor of the exhaust is quite impressive. It definitely works well as a outdoor cookstove.

In my full sized version, I will be insulating the firebox and heat riser to hopefully obtain even better results. Calculated run time with the larger firebox is 6 to 8 hours. Waste heat from the stove body will be reclaimed to further preheat combustion air, both primary and secondary. Heat will be extracted from the exhaust gas at the top of the heat riser by means of a firetube boiler (non pressurized) and pumped to an exchanger.

I hope you enjoy the videos on my little project. I'll be doing more videos on the larger version that I plan to heat my house with this year.

Geo Schoonmaker : You can't argue with Success ! You have delivered a good set of videos on making a Rocket Cook Stove, and as advertised it
seems that we can see gasification taking place !

I Am surprised with the longevity you are getting from your concrete blocks, as you said some of them came from one of your earlier (forced air?)
Rocket Mass Heater RMH Builds -could you comment on the percent of your concrete blocks that were reusable from that earlier project ?!

A local Scoutmaster and his Webelos, start off with the tin can and perlite rocket stoves that they make and use on group outings, and do small
Pocket Rockets, that which a suitable wind break make outdoor ice fishing adventures a fond memory of the young and the old!

While I really don't consider myself a 'Doomsday Scenario Prepper' I do think that being prepared for local disruptions of power / Public Service
is always a good idea ! Your simple design is a big step in that direction ! For the Goodof the Crafts Big AL !

Thanks!

allen lumley wrote:

I Am surprised with the longevity you are getting from your concrete blocks, as you said some of them came from one of your earlier (forced air?)
Rocket Mass Heater RMH Builds -could you comment on the percent of your concrete blocks that were reusable from that earlier project ?!

On my outdoor rocket boiler, I used 8 solid blocks, 7 of which are reusable. Those all came from an homemade wood stove I built three years ago used to heat a steam boiler.

Being ready for the power outages and personal financial problems is about all the prepping I do myself.

cool deal you made there! thank you for sharing it.

"a small rocket stove I built from a exhaust elbow inside a metal paint can"

Small rocket mass heaters are notoriously difficult to make work well, and the same is probably true of rocket stoves. Size matters; with smaller volumes, there is too much surface robbing heat and not enough length for complete combustion. You appear to have neatly addressed problems created by going with a hybrid rather than a standard rocket style of stove. If you enclose the wood to reduce draft and use big logs, you will need secondary air, though if the steel lasts any length of time deep in the combustion zone, you are not getting the high temperatures that accompany complete combustion. More insulation may well help this as you mention. The point of the RMH is to have a short, fast, hot, efficient burn. I have a concern that a similar efficiency from the extended burn may require much more technical apparatus to maintain.
What is your climate like? Do you have extended deep freeze conditions that require constant major heat input?

I live in Missouri, so our winters are really unpredictable. Last winter was horribly cold, of course, but most are up and down. A week or so of cold, then it warms up to 60 degrees, and then comes the snow or cold rain Wash, rinse, repeat lol

Double post, but I wanted to share an update on my stove project. I've managed to slow the btu production while still producing pretty clean exhaust. With a bit more tuning, I think it can get back down to odorless exhaust. Here's a video of some of the things I've been trying:



The vortex generator, crude though it may be, seems to promise a shorter riser length. I'm planning on putting one in each of the firetubes of the boiler I'll be building over the next couple of months. The idea is to swirl the hot exhaust gases in contact with the firetubes, hopefully promoting better heat transfer. I'll test it with and without to see if it makes a difference.

I've got the burn time up to 2.5 hours, with the btu output down to 18k, so it makes a killer outdoor cookstove now.

Hope you all liked that update, I'm doing videos start to finish on the larger insulated build as well.