Published on March 17th, 2018 | by James Ayre0
Efficient Wood Burning — Rocket Stoves, Masonry Heaters, & Dakota Fire Pits
March 17th, 2018 by James Ayre
The burning of wood and biomass has gotten something of a bad reputation amongst some in the environmental community in recent years, despite the fact that well-managed woodland biomass production and use is probably as close to carbon-neutral as you can get when factoring in full-lifecycle costs and impacts (especially those from the mining sector).
Obviously, at current population numbers, though, widespread reliance upon woodland/coppicing biomass and charcoal is an impossibility, but at much lower population numbers, such an approach is probably as close to environmentally benign as you can get while still maintaining reliable and constant access to heating/cooking tech (though, intelligent building siting and design can greatly limit needs in this regard as well).
With that in mind, I’m going to provide an overview here of most efficient ways out there currently to produce heat from wood and biomass burning — that is, the ways that involve complete or near-complete combustion, and thus higher heat production and oftentimes much lower particulate emissions.
Efficient Wood & Biomass Burning — Design, Secondary Combustion, & Particulate Emissions Levels
While the assumption of many people is probably that wood or biomass burning inevitably results in high levels of smoke and particulate emissions, the reality is that well designed systems need not result in the release of much of anything beyond CO2 (carbon dioxide) and water vapor.
Such systems need not be particularly complex or intensive to build — the so-called Dakota fire pit, for instance, is a means of producing a very high-heat and low-smoke campfire. Those wanting a permanent solution to home/building heating or portable outdoor heating, though, will benefit from systems designed for long-term use, such as masonry heaters; rocket stoves; and rocket stove mass heaters.
The basic effectiveness of all of these different systems are roughly down to the same factors — a design that allows for very high burn temperatures; extensive secondary combustion (re-burning of smoke/emissions); and, in the case of masonry heaters and rocket stove mass heaters, an extensive amount of thermal mass in combination with a long or labyrinth structure meant to absorb the heat from the outflow.
Below, I’ll go into the details of the different methods and technologies. Enjoy.
To start with the most basic and portable of such systems, let’s look at what a “rocket stove” is. Rocket stoves are simple but highly efficient and hot-burning stoves utilizing an insulated vertical chimney and a design ensuring high airflow around the fuel to allow for near-complete combustion prior to the flames reaching the end of the stove.
As a result of the near-complete combustion, rocket stoves need far less fuel to produce comparable levels of heat/light as less efficient forms of fires/stoves — and they also produce far less in the way of smoke/emissions.
To oversimplify, rocket stoves rely upon the same principles as Dakota fire pits do (and some other wood-burning modalities as well). Also to oversimplify, most rocket stove designs are intended as portable cooking and heating devices — which only utilize small pieces of wood or charcoal for fuel — and for outdoor use. If one was to adapt the rocket stove to indoor use, a number of changes would be required, which brings us to the rocket stove mass heater.
Rocket Stove Mass Heaters
Rocket stove mass heaters are essentially just rocket stoves that have been adapted for use as indoor heaters and stoves — through a scaling up of design elements, an exhaust outlet/system of some kind, and a large amount of thermal mass.
As with the rocket stove, the design is based around the use of an insulated combustion chamber to achieve near-complete combustion efficiencies through the use of extremely high heat — what differs is that rocket stove mass heaters utilize a large of amount of thermal mass that remains in contact with the exhaust gases in order to steal almost every last bit of heat that’s possible before venting (as is the case also with masonry heaters).
The design of rocket stove mass heaters can seem a bit counterintuitive to some, as at first glance it may seem that the smoke from the fire should simply rise and fill the room from the fuel-feed opening, but the reality is that if properly designed the fire burns sideways owing to the draft that’s generated.
There are a number of different design variations out there, but it’s probably fair to say that the basic design involves a combustion chamber shaped like a “J” which can be filled with wood at the short end, with the hot gases produced by combustion then traveling up the long end of the “J” chimney (via the draft generated) into a highly insulated secondary combustion chamber, with the exhaust gases then channeling through a large amount of thermal mass where the heat is lost to the system. The exhaust gases that then exit the flue are often fairly cool — in some designs being just over room temperature, in others in the 50° Celsius (120° Fahrenheit) range.
Differences as compared to masonry heaters include: higher initial radiant heat (when using designs with an exposed metal burn barrel, the greater affordability of the material (the cob thermal mass is literally dirt cheap), and a wider weight distribution owing to the use of cob (masonry heaters can be very heavy and dense items, as those that have moved them know). Also noteworthy of course is that rocket stove mass heaters can be fairly easily built without specialist skills, whereas masonry heater creation is a more involved process.
Potential dangers of poorly designed or built systems include: the release of smoke back into the building and thus carbon monoxide poisoning (this is why it’s important to be thorough with regard to construction and design), improper lighting leading to poor draft and thus interior smoke, and incomplete burn-chamber combustion leading to creosote buildup in the exhaust system (and thus possible chimney fires).
All of these potential problems can be easily avoided by being thorough with regard to design and construction.
Overall, rocket stove mass heaters represent an affordable, do-it-yourself alternative to masonry heaters (which can be quite expensive).
Masonry heaters are essentially radiant heaters based around the use of materials with high thermal mass in combination with a high-heat stove system. The technology goes pretty far back in time — at least 7,000 years, and probably a lot longer — with various designs seeing use throughout different parts of the Northern Hemisphere.
There are quite a lot of variations possible, but modern designs don’t necessarily differ much from earlier forms — whether of so-called “Russian stoves,” Roman hypocausts, or Chinese bed-stoves.
The advantage of such heaters — besides just greatly lowered fuel use and greatly reduced smoke/emission — is that they can be designed so as to continue radiating heat for many days after the fire has gone out. It’s possible, thus, to light a fire, for it to then go out a few hours later, and then for the masonry to continue radiating that heat out for 2–3 days afterwards. Though, the lengths of time involved will depend on the amount of thermal mass in the masonry heater in question and the amount of fuel used (heat generated).
The basic ideas behind the tech are simple: wring as much heat out of the exhaust gases as you possibly can before release, and burn the fuel as hot as you possibly can. These aims are accomplished primarily through the use of complex internal heat exchange flue channels in a high-thermal-mass masonry body, and a high-heat burn chamber. (Dampers are sometimes used to close off the flue when the heaters are not in use.)
Thus, it’s noteworthy that such designs have most often found use in areas and during time periods when wood/fuel is relatively scarce — during earlier periods of regional widespread deforestation in various parts of the world, for instance.
Despite the name “masonry heater” probably giving most modern people an expectation of brick or ceramic construction, the reality is that material use can vary quite a bit — with natural stones of various type, stucco, and earthen materials all being used in some circumstances. Something else to note here — owing to the great weight involved, it’s often necessary for there to be reinforcements in the floor underneath.
Potential stumbling blocks with regard to masonry heaters include: cracking in the masonry caused by the presence of different levels of heat stress at different points, and the slow speed at which initial heating occurs. This second factor is the reason for the exposed metal burn barrel in the design of many rocket stove mass heaters — a faster initial warm-up period (accompanied by the long-term radiant heat from the thermal mass material).
If well designed, and used with a bit of forethought, these issues aren’t truly problems, though. The greater stumbling block for most people is the relatively high price tag — and a good reason to consider a DIY rocket stove mass heater instead.
Dakota Fire Pits
To close things up here, I’ll provide a basic overview of what a so-called Dakota fire pit is. Dakota fire pits are simple-to-build fire pits that utilize a two-opening design that creates a strong draft to pull air under and through the fire that’s located in one of the holes.
To explain further: Two small holes are dug into the ground. In one, twigs are stacked and then covered in a easily combustible material. The other is built so as to draw air into the twig pile via a draft after the fire begins burning.
Owing to the high air/oxygen flow and the top-down burn flow, combustion of the materials in question is nearly complete — meaning that little to no smoke is produced and such fires require less fuel to produce a comparable amount of heat/light than other approaches. The approach is also well suited towards use in windy areas where lighting fires can otherwise be difficult — hence the origin and the name.
Top image by ChristiansburgWeld/eBay