I am ignorant - has there been research done into the pollutants given off by RMHs? Is there a product on the market already that might comply with these clean air guidelines but be considered - in principle at least - an RMH?
Peter van den Berg has tested many RMH examples with professional (and very expensive) equipment and found properly built ones to have very low levels of some pollutants, but he doesn't have a particulate tester as far as I know. With the typical long horizontal ducts or stratification chambers, I would expect considerably lower particulates simply from the greater opportunity for them to settle out. I would be very interested in an actual test of the finest particulates, which are least likely to settle out.
Theoretically, I would expect RMH particulate emissions to be lower than typical woodstoves simply because there are almost no unburned hydrocarbons escaping from the combustion chamber. Those tend to be sticky and generate particulates. A well-built RMH with highly insulated combustion core will generally get up to efficient combustion within 5 to 15 minutes, so might pass the first-hour particulate standard that one person in the article complains about.
Glenn Herbert wrote: I would be very interested in an actual test of the finest particulates, which are least likely to settle out.
I second the motion. Without data on ultra-fine particulates, which are the most damaging, it's going to be hard to sell the RMH concept in urban environments. It may also be necessary to incorporate an electrical pre-heat system to keep the startup burn as clean as possible.
What you are looking for I haven't seen anywhere in a reliable form. And I have been watching for it for several years.
Personal belief is that RMH are big emmiters of the small particles that can't be burned. Everyone talks about how clean they are and how little ash they generate. To me that is the key point saying they can't be clean. Every cord of wood contains X amount of material that can't be burned. Minerals etc. It is physically impossible to burn it. So either it ends up in the stove as ash or it goes up the stack. When an entire year's ash is say 10% of the minimum amount the stove can make the rest had to go up the stack. And the super fine particles can't be seen at the stove pipe out.
Now this is one I expect when the rocket stove community catches on to be easily solved. Needle type electrostatic filters have been in use for basically a century and they would be incredibly easy to tack onto an RMH. Fairly low power needs and easily made self cleaning because of the batch nature of RMH use. The rows of needles would be right Where the stove pipe exited the building point down into the air flow. Half getting high voltage low current positive and half same negative. The dust would collect on them thru electrostatics. The stove gets cold and a temperature sensor momentarily reverses the polarity causing all the dust to fall before it turns off till the next stove start. If it was over a good collection pan it wouldn't restrict airflow much and because the stove doesn't run steady it could easily batch clean into a nearly stagnant air stream and from there to an ash box. With a stove producing lots of unburned stuff it would collect on the needles like tar and they couldn't self clean. But the rocket because of burning everything it can would have almost none of this so the only thing left to remove would be dust. And the method of collection would cause it to clump making it cleaner and safer to handle too.
This is not to diss RMHs, not at all. Only to acknowledge that air quality impacts are a concern, as with any solid fuel burning appliance. Concentrated in an urban area, in their thousands, there is an issue to be addressed.
It seems to me that urban RMHs (URMS?) will need enhanced capabilities on either end, some sort of electrical or gas preheat and an electrostatic or mechanical filter on the exhaust. (EURMS?) The benefits of such a hybrid are worth contemplating.
(BTW, I'm in the country, and really wish my neighbours, who claim permaculture cred, would install a RMH instead of the crap wood stove that they run incompetently, belching particulate and setting off the smoke detectors inside my house.)
Interestingly in denmark there was now a magazine released defending firewood and woodstoves: https://www.lob.dk/tidsskrift/artikler-februar-2021-braende-miljo-og-sundhed/. It saying, amonst other things, that the damage wood smoke particulate does to humans has been overestimated, at least in denmark and probably most european countries. Part of that is because particles have been assumed to be as bad a from burning diesel, but studies have found that the particles are very different and less damaging to humans. And in copenhagen, they also said that woodstoves where the number 1 source of pollution, but here they also found this not to be true when they actually went out on the streets to measure this (and I think cars where found to be most polluting). They mentioned a few other reasons but I can not remember all the details.
It would certainly be interesting to see detailed measurements on particles released by different RMHs, but some points in its defense, isn't part of the deal that because of the effeciency and thermal mass together it does not have to burn as much wood? If people do use 1/10-1/4 of the wood, wouldn't that mean there is a very high chance it would release less particles overall? Purely because there is less burnt biomass.
Also I know of some RMHs that have been approved in the EU, for example this french one: https://www.uzume.fr/post/les-fruits-de-la-pers%C3%A9v%C3%A9rance and it is saying that it pollutes 7 times less than the EU 2020 standard for mass stoves and 3.5 times less than the upcoming 2022 standard. But yeah I am not sure if for these standards all particulate being released are measured, but it still sounds very promising to me.
Lastly, here in Denmark some people have developed a "smoke washer" (røgvasker), which is a particulate filter that can be easily self-built. I don't know all the details but I know it does contain a biochar filter as part of the system. Then you should clean the filter once a year and you get some fertilizing salt you can put into your garden. It is still in a quite experimental phase, but at least one model has been accepted by local authorities and is in current use.
The important paragraph (translated), which comes from a report from a dutch government agency:
"In addition to the quantity in an absolute sense, the composition and toxicity of the emitted dust also differs. With good combustion, the dust emission can be limited to approximately 20-50 mg / m3. The composition of the emitted dust then mainly consists of the inorganic components present in the biomass (salts), the toxicity of which is not significant. In contrast, poor combustion not only drastically increases the amount of dust, but also the content of organic condensable tars and soot. These are often toxic or carcinogenic components, which are of great importance for public health."
And this particular RMH has been categorized as <20mg/m3 dust emission. Reading this, I would be very curios to see what would happen if that whole area would switch over to RMHs and using them correctly.
The curious RMH enthusiasts among us might want to gather some data. Here's what I would try:
1. Get a quantity of fuel of a single type (e.g. all pine or all oak) and dry it well, then split it into several batches of equal weight.
2. Clean the RMH thoroughly so that there is no ash buildup in the system.
3. Burn one of the batches under normal operating conditions. When done, remove all the ash from the system and weigh it.
4. Repeat step 3 for all the batches.
5. Compare the fraction of ash recovered from the system versus fuel mass with a published analysis for the type of wood you are burning. If you're really curious you can conduct your own Loss on ignition sample tests.
If the ash fraction is the same or close to the standard, then this tells us that solid particulates are not going up the stack in any appreciable amounts. If it's lower, you're emitting particulates and a trap of some sort might be indicated. If it's higher, something is probably wrong with your RMH because that is a sign of incomplete combustion.
Of course, fitting an electrostatic precipitator as C. Letellier describes above wouldn't be all that hard, either, and you could collect and weigh the captured fly ash after each burn for the sake of science. I'm pretty content after a few years of observation that mine isn't putting out anything objectionable.
Anything solid that does get out the stack of my 4" RMH is going to have to be really tiny and lightweight, because the stack gas velocity is so low by the time it reaches the flue cap. All I ever see coming out is steam. When It's time to tear down and rebuild the core (hopefully April) I will inspect the last cleanout and see what has settled out where the flue exits the mass.
permaculture is largely about replacing oil with people. And one tiny ad: