(Sorry for the double post, here. Dunno how my browser fritzed on that post a second ago.
Also looks like a couple other folks were able to respond before I was able to get my reply posted, and theirs looks like good input too.)
Julie Reed wrote:the main premise of rocket heater’s efficiency is the extreme temperatures in the riser burning all of the gasses completely.
This is true, but with a couple caveats. The temperature required to completely burn (oxidize) all of the carbon compounds in wood’s pyrolysis gasses is 800°C (1472°F) and above.
(See:
https://www.bios-bioenergy.at/images/bios/downloads/publikationen/Pellets/091-Paper-Brunner-Primary-measures-for-low-emission-wood-combustion-EUBCE2009.pdf)
The riser on a rocketstove can reach temps much, much higher than that… particularly the largest JTubes 8+ inches in diameter, fed thinly split wood (which burns faster than cordwood) and in risers heavily insulated with ceramic fiber (which can accumulate incredibly high internal temperatures) — especially during long burning cycles to heat large thermal storage masses.
Traditional, wood-fired, natural-draft ceramics kilns, for example, can regularly reach temps at least as high 1510°C when continuously fed thinly-split wood in the same manner a JTube is. (The highest officially recorded is 1563°C…and that’s without a ceramic-fiber-insulated combustion chamber. See:
https://www.guinnessworldrecords.com/world-records/428387-highest-temperature-in-a-wood-fired-kiln )
Achieving vastly higher temps than 800°C wouldn’t be an issue, except for the formation of NOx emissions — from nitrogen (N2 gas) admitted as “ballast gasses” in combustion air — and from the organic nitrogen compounds naturally present in the biomass used as woodfuel.
The higher the combustion temperature, the more NOx is formed. (Production of Nitric Oxides [NOx] from ballast gas Nitrogen [N2] starts at around 1100°C, steadily climbs with increasing temperature, and greatly accelerates above 1300°C) … so getting the temperature high enough to burn all the pyrolysis gasses is important, …but for the cleanest burn, it’s also important to avoid excessively high temperatures which increasingly encourage NOx.
The ultra-hot-firing formation of NOx also becomes more of a problem when there’s lots of excess oxygen — beyond the amount of O2 which is necessary to oxidize all the carbon to CO2. The excess O2 in the combustion zone(s) is then available to oxidize nitrogen. (At lower combustion temps, nitrogen has less affinity for oxygen than carbon does— which some Kachelgrundöfen designs take advantage of, by providing a “reduction zone” where excess O2 gas is lean, and so residual carbon monoxide scavenges oxygen molecules off of the nitric oxides, reducing them to N2 while oxidizing the CO to CO2.)
NOx production also increases when air is aggressively mixed through the fuel bed — as with a JTube that sucks a lot of the ash through the burn tunnel and riser — or with a bottom-grate design that admits a large portion of the burn chamber’s primary air through bottom of the fuel bed.
(Bottom air greater than about 5% also encourages carbon monoxide formation, and it creates more particulate matter [PM 2.5] in the exhaust, by mixing and vaporizing more of the ash components —particularly the potassium and sodium fraction, which vaporize (boil) at ~759°C and ~883°C, respectively — forming secondary compounds and micronized particles as they cool and solidify in the exhaust stream.)
(From
https://www.babcock.com/home/about/resources/learning-center/nitrogen-oxides-nox-primer)
“NOx formation is promoted by rapid fuel-air mixing. This produces high peak flame temperatures and excess available oxygen which, in turn, promotes NOx emissions. Combustion system developments responsible for reducing NOx formation include low NOx burners, staged burning techniques (overfire air), and flue gas recirculation (FGR). The specific NOx reduction mechanisms include controlling the rate of fuel-air mixing, reducing oxygen availability in the initial combustion zone, and reducing peak flame temperatures.”
But the Kachelofen, as I understand it, has a primary and secondary combustion the same as a typical epa stove has. No extremely hot riser.
The riser in a rocketstove is its secondary combustion zone. In the “riserless” batchbox rocketstove designs like the Walker Riserless Core, Double Shoebox Rocket, and Vortex stove, the “riser” is turned sideways, effectively making it a secondary combustion zone very much like Kachelgründöfen, (albeit with a greater reliance on the shape and size of the ports than on the sharp-angled turns and taper of the flue pathway.)
EPA steel or cast iron box stoves are designed to emit heat directly from the primary combustion in the firebox, which neither Kachelgrundöfen nor rocketstoves are designed to do.
Being made of thermally-emissive steel, (which robs heat from the combustion chamber before secondary combustion is complete) typical EPA stoves do not operate at temperatures as high as rocketstoves or Kachelgrundöfen. Because of this, they typically require a catalytic combustor in the secondary or tertiary burn zone to clean up their emissions (the rare mineral elements like platinum in the catalytic combustor are what “catalyze” the burning of soot/creosote-forming hydrocarbons at lower temperatures, usually between 210°C-600°C, up to a max. of 816°C … the lower combustion temperatures also being needed within the steel box to prevent metal fatigue, warpage, cracking, spalling, and/or (at the very highest temps) beginning to melt.
In contrast, both Rocketstoves and Kachelgrundöfen are made of high-duty firebrick composed of kaolinic fireclay and “chamotte” (grog particles made by calcining high-alumina fireclay, instead of the silica sand used in standard building bricks and the low-duty firebrick splits that sometimes line EPA steel box stoves.) Chamotte firebricks and fire tiles retain heat in the primary and secondary combustion zones, allowing for enough heat accumulation to burn off pyrolysis gasses above 800°C, so they don’t need a catalytic combustor.
So… if that’s true, and yet European standards for emissions are even stricter than USA, how are the Kachelofen burning so clean? And by extension, burning possibly even more efficiently than an rmh?
I’m not sure that Kachelgrundöfen burn cleaner than *batchbox* rocketstoves in terms of CO emissions, but they do burn a little cleaner than J-Tubes.
I’m pretty sure that Grundöfen designs (which intentionally lack a bottom grate, and don’t have a narrow “port” or the upright riser which create aggressive suction on the firebox) produce less NOx and particulate matter, because they don’t stir up the fuel bed or disturb the ash in the firebox. The Grundöfen design kinda-sorta(ish) works like a TLUD in the way it pyrolyses the fuel and burns the gasses without aggressively mixing fuel and air in the fuel bed of the primary combustion zone. …At least, that’s my understanding.
