I need to generate clean (smokeless) carbon dioxide with a rocket stove

Greeting, first time permies guy.  I have a large greenhouse with a large underground room attached to it that is intended to produce fodder from barley.  In the winter my difficulty is somewhat that it's difficult to keep temps at 70+F, but as I've experimented I have a bigger problem providing enough carbon in the air to the plants.  If I simply increase outside venting in the winter I am pulling air as low as -15 degrees (I live at 6500') so I am looking at building a small rocket stove with the ability to provide clean (smokeless) carbon in the air for the plants.  

The exhaust has to be quite clean because I don't want to slowly cause a sooting buildup on the inside of the greenhouse.

As I understand it, smaller wood in a smaller burn chamber will come to temp faster and burn cleaner, but I'd like to have a slightly larger feed chute so I can load it up and let it burn for several hours unattended.

I'm not opposed to using a small 12v computer fan or similar to create a good burn faster, but I'd like to use as little electric power as possible.

Thoughts?

Hi Sc;  Welcome to Permies!
Well, I don't want to discourage you but...
In my opinion this will not work well for you.
Here's why.
1)  All rocket stoves have some smoke during startup and die down.
2) Enlarging the feed tube will create a better draft than the riser... your feed tube will become a chimney.
3) No j tube will burn several hours unattended.
4) Soot will get on everything.

Yes small wood will burn fast and hot.
Small fans can be used to push start your draw.

So having told you why I believe you will not get the results your hoping for...
Give it a try!  We encourage thinking outside the box!

Hi, and welcome! I have to confess I'm a bit confused by your post.

I know that some plants like tomatoes go crazy in an enhanced CO2 atmosphere. You can even buy CO2 generators (using fossil fuels) for commercial tomato greenhouses.

But everything I can find about barley says it is stressed by high CO2. Not good.

Perhaps I have misunderstood what you are trying to do?

Venting combustion gases into a closed human-accessible space is generally not a good idea, as all devices produce carbon monoxide at some point in the burn cycle.

Dc Stewart wrote:Venting combustion gases into a closed human-accessible space is generally not a good idea, as all devices produce carbon monoxide at some point in the burn cycle.

Very true, and carbon dioxide is not good for people either. I believe that when CO2 generators are used in commercial greenhouses, those spaces are considered "confined spaces," with locks, signs, and specific safety procedures for entry.

Direct injection of flue gasses into a greenhouse is an extremely dangerous idea and I would encourage you to fundamentally rethink it. Carbon monoxide is sketchy stuff and you don't want to mess around with it.

Some non-combustion methods of enhancing greenhouse CO2:

Alcohol fermentation
If you've got barley, make beer. If you have fruit trees, make cider or fruit wine. If you've got bee hives, make mead. The gas that bubbles out of it during fermentation is CO2. As a bonus, the "spent" material post fermentation can be used as animal fodder, which is what you're after in the first place. See here.

Compost
Aerobic compost gives off CO2 during the decomposition process. You need to be a bit wary here too, since CO2 isn't the only gas produced. Small amounts of CO, H2S and other sulphur compounds, and N2O can sometimes come off, and if you don't turn it enough and it goes anaerobic you might get CH4 too. But it's a whole lot safer than running a wood stove into the place. All of these other gasses are reduced the more you aerate the pile, so setting it up in one of those rotating drum composters and then turning it religiously would be the best bet. Gives off some heat too.

Animals
If you've got chickens/ducks/geese/goats etc, make them a little pen inside the greenhouse to sleep in at night. Instead of going to the gym in the winter, put a stationary bike in there and get on it and pedal. Bonus points if it's a bike like this. Double bonus points if it's also rigged up to spin the aforementioned compost tumbler.

Mushroom Cultivation
Fungal metabolism is more similar to animals than plants - they breath in O2 and exhale CO2. You might not get a huge quantity out of them, but every little bit helps.

These methods, even in combination may not generate enough for your needs, so some degree of air exchange with the outside will still be necessary. But you don't want to lose the heat by pumping in cold winter air, so you'll need to look into heat recovery ventilation.

Thanks all, some additional info-

I've been building and running fodder operations for about 10 years, we have several of them feeding cattle herds, a small pig farm, small back-yard horse/cattle and this one in my greenhouse provides for my 5 cows, 50 chickens, a few pigs, etc.

We have found that the limiting factor for production is CO2 content in the room, so we have invested heavily in propane fired CO generators (basically an inefficient propane heater) with automated controls, sensors and alarms.  What we have found is that we can't produce enough to even register on a meter, the high density of the barley consumes it too rapidly.  The problem with the propane generators is they produce a large amount of humidity as a byproduct, and the costs are far too high, especially now.

I have also used the small RV diesel heaters which work well to keep the temps up, but simply don't produce enough exhaust to benefit the plants as much as I'd like.  Also, diesel costs are far higher now.

So I was thinking of a small/mid sized rocket stove.  The greenhouse itself is 20' tall inside and I've used a small rocket stove in there in the past, but in the fodder room, which is an underground concrete room built onto the greenhouse and they share ventilation and heat transfer, I need more carbon.

Here's my thoughts-
A small rocket stove that has a divisible burn chamber that has a removable panel isolating 1/3 of the burn volume.  Start some charcoal outside, when it's hot it can be placed in the small burn chamber and a small fire with small fuel to get the stove hot. If more heat is needed, the divider can be pulled and the entire burn chamber used.

Fair enough. Safety issues can be managed and mitigated.

I'm curious. How did you come to the conclusion about  CO2 as the factor limiting production?

Douglas Alpenstock wrote:Fair enough. Safety issues can be managed and mitigated.

I'm curious. How did you come to the conclusion about  CO2 as the factor limiting production?

Actually that was an interesting experience.  We were testing our first fodder system about 10 years ago in a garage.  We had been monitoring the temp, humidity, production rates (barley seed : fodder) and as winter hit we took a huge hit, even though all of our metrics were 'good'.  I happened to have pneumonia at the time and was really suffering with a lot of chest pain, but after spending 30 minutes in the garage with the fodder I felt great for the first time in weeks.  We decided to allow the temp and humidity to suffer in exchange for more fresh air and it was a complete success.  Since then we have set up our fodder rooms with venting, fans, heaters and/or CO generators.  With the CO generators we found it was impossible to dump an excess of CO into the room, as fast as we could add it in we'd get improved fodder production and no net change in O2 or CO content in the room, but with the increasing expense of propane and increased humidity.  Even allowing the room to wildly fluctuate in temp the fodder produces far better with plenty of access to CO.  My greenhouse was designed with this in mind and the geothermal air line have a natural chimney effect through the top of the greenhouse to constantly draw fresh air, even in the winter.

The current room has several hundred feed of underground air lines coming in, but in the winter we get well below zero and it's tough to maintain temps and CO content.  A clean burning stove would be nice to replace the propane or diesel options.  The smaller fodder room directly vents into the main greenhouse, so in the winter I could use all the CO I can get.

So , a very cool arrangement you have there!
A few pictures would be nice.

As far as rocket stoves.
A J tube design is just not going to burn long enough for you.
Trying to modify a proven design is  not recomended, (but lots of folks try anyway).
A batchbox design ,  will burn hotter / have coals for several hours.

Is there a way you could vent this batch outdoors while it gets hot then switch a damper and vent it indoors?

thomas rubino wrote:So , a very cool arrangement you have there!
A few pictures would be nice.

As far as rocket stoves.
A J tube design is just not going to burn long enough for you.
Trying to modify a proven design is  not recomended, (but lots of folks try anyway).
A batchbox design ,  will burn hotter / have coals for several hours.

Is there a way you could vent this batch outdoors while it gets hot then switch a damper and vent it indoors?

Yes, I have quite a bit of venting to the outside the could be used. I've hesitated to use it because it vents into an area where my cows live, so I'll have to build a support that is sufficient for cows to scratch against, but that's ok in the long run.

Here is a video of the greenhouse taken over a year ago for a friend of mine.  A lot has changed as we've added grow beds, finished the walls and roof, increased ventilation, etc.. but it's still a good place to start.