Batch Box Rocket Stove: Ergonomic, Efficient Wood Combustion | Hacker News Reader

Batch Box Rocket Stove: Ergonomic, Efficient Wood Combustion

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88 pointsShroudedNight44 comments

An annual problem I have every summer is dealing with smoky campfires when camping. Your bog standard cicle-of-rocks or metal ring fire pit is crazy inefficient, and unless you run continuously in the offgassing phase (bright, tall orange flames, as the wood gas burns off), with lots of convection, you inevitably produce a ton of smoke. This is terrible for all folks, but my asthma-prone lungs get especially cranky. So you either have to continuously feed wood (inefficient and costly), or play the game of shuffling upwind constantly.

I usually end up tinkering and modifying the fire pit to try to eek some better air flow patterns, but it's usually a lost cause with open pit fires.

I might just cave and get one of those stainless inserts, but what I'd really like is something flatpack that I could assemble into a more efficient burner. I've seen small ones [1] that slot together to make a rocket stove, but nothing campfire sized.

I wonder how much demand there would be for such a thing. Seems like there are several products for general "firepit" designs, but without a chimney, these suffer from most of the same problems as open pits. [2][3]

Edit: oh, maybe a "tent stove" is what I'm looking for.

[1] https://www.amazon.com/Marsh-Kettles-Flat-Rocket-Stove/dp/B0...

[2] https://www.amazon.com/Flat-pack-firepit-grill-carry/dp/B086...

[3] https://www.etsy.com/listing/799723294/flatpack-firepit-port...

For those metal ring pits with a cooking grate, you can build the fire on top of the grate for better airflow. If you can get air coming in underneath the fire from all sides, it'll greatly reduce the smoke. And you also reduce the chances of smoke "chasing" the person that's sitting the closest, since they're not blocking a lot of airflow from the sides. Still not peak efficiency, but a lot less smoke.

https://www.amazon.com/Quick-Grill-Large-Original-Stainless/...

This is dirt cheap, light-ish, and excellent.

To get mostly-smokeless performance, leave out the bottom metal plate when you assemble it so it gets continuous air flow right through the device. I used to use https://www.solostove.com/en-us (car camping only) and it just burns too* clean - no smoke, hot fire, goes through fuel very fast, and all the heat goes straight up. Still love that thing, but actually for an "improved campfire experience" that collapsible grill thing is excellent. Particularly useful: the heat reflects off the inclined metal plates right on to the people sitting around the fire, in exactly the way it does not on a solo stove.

It's really good. Makes me happy.

Promising, thanks!

I had a charcoal grill like this once and it worked quite well, but something stole it off my lawn literally between cooking burgers and cleaning up. Must have still been hot. Anyway, I tried replacing it with a similar model, and convection was terrible. So it seems subtleties in the design which make a big difference.

Well, at least you got the burger even if they got the grill.

That particular device is two completely different animals with and without the bottom plate. Mediocre and smokey with it, really kind of fantastic without it. The wire mesh stops the big lumps from falling through but lets the ash go, and the whole thing burns like a chimney stove. Really good.

Something like this? It's not backpack portable, although I think there are some out there that are smaller. https://www.amazon.com/dp/B09DKTM6NF

If you're in the US, take a look at Firebox [1] that have a bunch of options through the price range all of which are fairly light. I would suggest going with a biomass stove with some thermal mass (easier to keep the burn chamber hot and also to stop the wind from pushing your stove around) if you're not constrained by backpacking/bikepacking constraints though (e.g. car camping or just going to a park/yard for some outdoor bbq.) You can make one with tin cans and some filler material (there's several guides online) or you can buy something like the EcoZoom [2].

Lots of people don't realize that wet wood generates a lot more smoke and that it takes wood a while after falling off a tree to dry up, so if I'm going by car or by train I often take some dry wood with me to keep the smoke dry.

[1]: https://www.fireboxstove.com/index.php?route=common/home

[2]: https://ecozoom.com/

I repurposed an unused grill lighting thing as a perfect miniature outdoor fireplace. I don’t know the exact name of the device but it is a metal tube 8” wide and 16” tall with a metal grille at one end, inserted up the tube a couple of inches:

https://image.dhgate.com/0x0/f2/albu/g9/M00/38/77/rBVaVV7PBK...

Yes, you have to feed it sticks like cookies into a Cookie Monster, but it’s really good for having a snap fire with lots of eat and lots of combustion. Because they are a ubiquitous garden accessory they are also very cheap.

I've used and researched a lot of portable wood stoves. If you want a portable rocket stove, the one I would recommend is the Virestove. There are some video reviews on youtube that you can check out before buying it. I haven't used a more practical one yet. If you have the funds, they have it in titanium right now. Should be quite lighter than the stainless version. The mini version is more portable of course, but doesn't achieve the smokeless combustion that the larger version does. I guess it's the length of the chimney, but not sure.

For a flat pack grill that's practical and portable, I'd recommend you check out the ones made by UCO. They come in three sizes as of now. The only downside I find with flat pack style grills is that on most of them you have to raise the grill to add wood.

Somebody recommended the Firebox line. They're pretty awesome, but are regular wood stoves. They will smoke just like any fire pit or flat pack grill. No rocket smokeless combustion. The stainless version is very well built and will last forever. Haven't tried the titanium version, but seem to be well like by users. I'd also recommend the Bushbox, by Bushcraft Essentials. These towo are the best of the folding style portable wood stoves (not rocket style).

links to this stuff (some are available on amazon):

virestove.com

ucogear.com/portable-grills/

bushcraft-essentials.com/en/outdoor-stoves/

fireboxstove.com

I have to say though that I've been using wood stoves to cook less and less, mostly because of health concerns. It seems every year I find more and more reasons not to be around them.

This is up there with one of my favorites from Hacker News: The Microhydro Plant

https://news.ycombinator.com/item?id=6895582

https://news.ycombinator.com/item?id=20075110

https://ludens.cl/paradise/turbine/turbine.html

> Higher temperatures result in cleaner combustion.

Higher temperatures lead to lower amount of unburned particles, but also lead to NOx production. That's what the WW scandal was about by the way.

If you want clean combustion, you need high temperature + SCR[1]

[1]: https://en.wikipedia.org/wiki/Selective_catalytic_reduction

What temperatures does this trade off between fewer unburied particles and more NOx production begin to take place? Is it a situation where unburned particles reduce quickly and dramatically as temperature increases and NOx increases slightly until much higher temperatures or is it closer to 1:1?

It's definitely not 1:1 and for a while, every degree you gain results in less particles and barely any NOx. But to get rid of the remaining particles, you need to reach higher temperatures and then NOx emissions is going to increase.

So there is a “sweet spot”, which is what's targeted by car engine manufacturers since air pollution norms have been enacted, but we can agree that the result is not “clean”, it's just “the least bad you can do”.

If one were willing to build a rather more complex device, likely requiring some electric pumps, one could use an oxygen concentrator to supply nearly pure oxygen as the input gas. With much less N2 available, production of nitrogen oxides would be reduced.

Or you could do this:

https://en.m.wikipedia.org/wiki/Chemical_looping_combustion

Seems like a bit of a stretch for a household device.

Your first idea is great. Unfortunately, combustion in pure oxygen runs extremely hot, so hot that it's difficult to find materials to build a stove.

https://en.wikipedia.org/wiki/Oxy-fuel_combustion_process

Oxygen plus recirculated flue gas in an appropriate ratio would burn cooler.

Only if you cool the flue gas beforehand, otherwise it doesn't change the equation.

> one could use an oxygen concentrator to supply nearly pure oxygen as the input gas. With much less N2 available, production of nitrogen oxides would be reduced.

I suspected the amount of energy needed to concentrate oxygen would dwarf the amount of energy of the combustion so I did the calculation:

I took the following device[1]: 350W to produce up to 5L of Oxygen per minute.

5L of dioxygen means roughtly 6 gram or 0.18 mol per minute.

Pure carbon combustion produces 393 kJ or energy per dioxygen molecule, whereas dihydrogen produces 572kJ per dioxygen molecule.

Wood has roughtly as much Carbon as di-hydrogen (glucose, the basic block for cellulose is 6 carbon for 12 hydrogen, I'm voluntary ignoring the oxygen here since I want an upper bound), so when burning wood, for each of our dioxygen molecule we can take ~500kJ/mol as an approximation.

That means your device can only produce enough oxygen to sustain a fire generating around 1,5kW of power. (I was wrong)

So, even though my intuition was wrong, spending 350W of electricity to get less than 1,5kW of heat out of the wood you're burning doesn't sounds like a great deal.

You're much better off with a heat pump at that point…

> Or you could do this:

> https://en.m.wikipedia.org/wiki/Chemical_looping_combustion

I had never heard about that, this is so cool!

[1]:https://oxystore.fr/concentrateurs-d-oxygene-stationnaires/5...

1.5kW for a room heater isn’t bad.

In any case, I imagine that an industrial scale oxygen concentrator would be dramatically more efficient.

That's 1.5kW in exchange for 350W + the fuel. Whereas a heat pump gives you the same without the wood at all. (And a basic stove gives you the 1.5W for just wood without electricity nor complex and expensive machine).

An industrial scale thing may be more energy efficient, but why bother when you can just get rid of the NOx with catalysis instead…

Your average heat pump doesn’t burn wood, and wood is, in several respects, an excellent fuel. It’s renewable, it’s easy and safe to store, it’s dense, and it’s easy and safe to transport.

(Yes, on a large scale, it’s better for everyone to use heat pumps. But heat pumps plus wood for backup in unusually cold weather may also be a good choice.)

On an industrial scale, some form of nitrogen-free combustion may simplify CO2 capture. And, if a CO2-capturing plant needs to remove nitrogen from a gas stream, removing it from the intake stream instead of the exhaust stream may eliminate the need for catalytic reduction. Also, the intake gas is clean and cool, and there’s no risk of contamination the zeolite adsorption bed (or whatever gets used) with soot.

> Your average heat pump doesn’t burn wood, and wood is, in several respects, an excellent fuel. It’s renewable, it’s easy and safe to store, it’s dense, and it’s easy and safe to transport.

You don't understand: your heat pump roughly generate the same amount of heat without burning anything. Instead of feeding electricity to an oxygen concentrator, you feed it to your heat pump and tada! you have your heat without burning anything!

> But heat pumps plus wood for backup in unusually cold weather may also be a good choice.)

Sure, but in this case you just need a good stove, if that's only used occasionally then you don't care about a little bit of NOx.

> On an industrial scale, some form of nitrogen-free combustion may simplify CO2 capture.

Sure, but then it's a completely different topic, at least because a concentrator won't give you nitrogen-free gas, just 8 times less nitrogen, which may not be enough for the needs. CLC is probably better suited for that, or regular air distillation to get pure oxygen.

There are at least one misconception here that I can see (I work in gasification technology).

You only need the pure oxygen for producing enough heat to pyrolysize the fuel (wood/biomass), once it's broken down into tar gas, you can feed it through your charcoal layer to further reduce it into syngas. The syngas can then be burned at a separate space with air at a lower temperature where nitric oxides aren't created.

Utilizing the residual heat in the exhaust gas (flue gas) is another means of feeding the pyrolysis phase. As you mentioned its also a great source of CO2 and H2O as a feedstock for producing CO and H2 from the hot charcoal.

What I'd like to see happen is usage of the "waste" oxygen produced while producing hydrogen from electrolysis of water. The oxygen could be used to produce syngas, and the hydrogen could be fed into this fuel stream or used separately for off hour electricity production.

There is at least one misconception here that I can see. You don't need pure oxygen to initiate pyrolysis, combustion with atmospheric air is suitable to initiate pyrolsyis as well as gasification systems that pyrolyze biomass without any oxygen/combustion using electric resistive elements eg powered by solar pnaels if you want to learn more about gasificatoin theres soem great resources in the internet

You're correct. But the whole initial conversation was based around not wanting to create NOx from usage of atmospheric air which contains Nitrogen.

As I've noted, I work in gasification technology.

What scale what feedstock?

To complete the analysis, you should (a) use the high temperature combustion heat to run a heat engine to power the oxygen concentrator, and (b) include the waste heat from the oxygen concentrator in the overall heat output of the device. Then you have a box with (wood, air) in and (combustion products, heat) out.

You could do (a) but I don't think there exists oxygen concentrator running on anything else than electricity on the market, so if you need to design you own custom oxygen concentrator and get it built, your CapEx are going to be ludicrous. Or you can make electricity from the heat then power a regular concentrator, that would be more reasonable but that's still another complex and costly machine to add to the system. that's the thing with thermodynamic machines, you can always increase the yield by increasing temperature and getting closer to the Carnot cycle, but every step is more costly than the previous one.

In the real world you always have CapEx + OpEx as additional input.

Regarding (b), here waste heat is hardly usable as it is low temperature and diffuse. You could still you the residual heat from the engine in (a) since it would still be hot enough, but the 350W dissipated by the concentrator can't realistically be collected.

I assume there's too much airflow to use a urea-based filter like adblue or something in a home stove.

Though, if you could get the stove exhaust to pass through some liquid anyway (like the bubbler in a bong, but larger), wouldn't that deal with particulates effectively?

> I assume there's too much airflow to use a urea-based filter like adblue or something in a home stove.

Why, these can be manufactured at the scale you need, and a home stove isn't especially powerful compared to a car or truck engine so even such a filter made for cars would work.

> Though, if you could get the stove exhaust to pass through some liquid anyway (like the bubbler in a bong, but larger), wouldn't that deal with particulates effectively?

Not really, first of all you'd need your bubbles to be really small for that to work, and then you'd need something (a pump) to push the smoke through the water.

> Higher temperatures lead to lower amount of unburned particles, but also lead to NOx production.

Honest question: compared to which pollution if the wood is let to rot in nature? For it'll eventually decay too.

NOx

Natural decay doesn't produce any oxides of nitrogen because it's at a different temperature. It does extract nitrogenous compounds, which are returned to the soil, but not in the acid form of NOx.

I really like the idea of using a dual bell system, where one is small and steel and the other’s large and masonry. The thing that triggers me to build/add to a fire is feeling cold, so having quick response is nice. But then of course you want heat mass to avoid dumping all of the extra heat out the chimney.

If my Hearthstone stove ever goes out, I’d like to build one of these.

In the US, the Liberator rocket heater is a great option. It's UL listed if you care about that sort of thing. https://rocketheater.com/

Is this the same as a rocket mass heater? Appears to be a somewhat different design, but same physics. A rocket mass heater is going to have the burner box outside of the bell.

Essentially, yes. The instructions / calculations on the building page are all oriented towards building one for the purposes of space heating.

That being said, I'm intrigued to try the 'core' design in something like a pizza oven to see how it performs.

If you scroll down on the completed projects page, someone made a combo pizza oven and pool heater. It apparently excels at both. Once it's up to temperature, 4 minutes to cook a pizza.

I had missed that, thanks for pointing it out!

Any reason you couldn't have bells on separate floors? I.e. core and a bell on floor one that leads to a bell on floor two before exiting through a chimney?

It's not immediately clear what the difference is between this and regular rocket stoves.

I think (I could not find a clear description that was short and easy to understand, maybe I missed it) that it has a door on the box where the wood goes in, and a pipe that feeds fresh air into that box to keep the combustion going.

Did I get that right?

The difference is that in a normal rocket, you need to keep feeding the fire at regular intervals because the feed tube is so small. In the batch box you load it once and let it burn.

To add a bit of context to the sibling comment, rocket stoves can work when made out of pretty much any material, because the air flow is not tightly regulated.

Batch box's purpose is to allow a more full-sized fire with the efficiency of a rocket stove, and there are several key differences to hit that:

- It should be made of a refractory material. Rocket stoves function perfectly well with plate metal or anything, really

- All dimensions, especially the air inlets and heat riser, must match a specific set of ratios to preserve the specific airflow needed to maximize burn efficiency. Too much means air will move too quickly, preventing full burn

- A choke point between the burn chamber and riser exists to slow down and funnel gasses and heat together. Without this, you don't have enough heat to get full combustion

Batch box burners tend to be quite a bit more fussy than a rocket stove to get going, though if a good portion of your exhaust rise is inside and pre-warmed, it should have a strong enough natural draft that starting it isn't much extra effort. I toyed around with the dimensions a bit myself and have something that works really well (cylindrical chamber rather than block), though I don't think I'm getting optimal efficiency out of it.

It looks a lot like my father in law's sauna.

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