Development of a compact batchrocket core.

As andrewb and independentenergy at donkey's forum suggested, I took out the superwool left and right from the port. And added another layer at the back, looking like this:



After closing it up I loaded the firebox with softwood, about 3.2 kg in total.



It started up quickly, the burn commenced in short order and went totally wrong. The O² kept coming down, all the way to about 3.5%. At the cross-over of 4.5% O², the CO went sky-high, way beyond the scale of the diagram. Regrettably, I am unable to produce a diagram because the software of the Testo crashed, close to the end of the burn and I wasn't able to retrieve the data. Someday, I have to figure out how to do that, provided it is possible, of course.

The next run, cold again in about 8 hours, I tried again using a wider port. To 70% this time, just to make sure the combination of 60% port and new internal layout of the riser box wasn't the culprit. I loaded up again and started with a slightly more modest load, without the pallet bobbins at the bottom.



It turned out I was right, the above mentioned combination wasn't why it went wrong. In other words, the burn commenced as quick as before and the O² got even lower this time, to 2.7%. As one may believe, I wasn't pleased, at all. The only change was the layout of the liner in the riser box, combined with the wider port in the second run.



Oh, forgot to mention there was black smoke from the chimney during the CO double peak. In order to show how high above the acceptable level the CO came, I changed the scale for that line from 5000 ppm to 12000 ppm. Maximum acceptable level is 1200 ppm on average, without peaks. This turned out to be 1713 ppm on average with a double peak up to 11264 ppm.



Tomorrow, wheather permitting, I'll run another test using a slightly different layout. The plan is to cut the sides of the liner out so that only the double layer at the back remains. Frankly, I think the pattern will repeat itself.

The effect of the thinner firebox' back wall kept nagging me. I reinstated the superwool on four sides of the riser box, a bit over an inch (30 mm) higher and also crossing the port.



I tried four testruns in total, some were really nice but half the runs were not cutting the cake. All were loaded with the same fuel and the same pattern was mimicked.



The following diagram happened to be a good one, the results of the one next day were disappointing, to put it mildly.





What the heck was going on? Still, the thought of why the less deeper port was yielding bad results kept following me. But today it dawned on me: since the shallower port generated bad results, would a deeper port generate better ones? After all, such an effect would be a sliding scale, rather than an on-off switch. So the inplementation of a deeper port was suddenly a thing to do right now.
In order to keep the bottom of the riser box as a square, I took out the wool at the rear wall and added the same amount at both sides of the riser port. What actually happened was that the bottom half of the afterburner shifted one inch (25.4 mm) to the back. Now the depth of the port rose to 8 cm (3.15"), exactly what Independentenergy ( a poster on Donkey's forum) was asking for, although the riser floor remained a square.



I used the exact same amount and type of fuel as before, arranged in the same manner. There was more wind today, 5 Bft with strong gusts, normally not a good omen for a succesful run.
And this were the results, surprising to say the least.



As can be seen, the O² went down rather quick. I closed the stove's door at three minutes into the diagram and refrained from doing anything else than looking at it, biting my nails (figuratively speaking) and hoping for the best. At the 3.5 minutes mark the CO dropped lower than 500 ppm and stayed there until 45 minutes. A run with a length of 57 minutes and CO below 500 ppm for 41.5 minutes is something that I rarely see, I tell you that. On top of that, the white Testo filter came out very light grey instead of very black!
Average numbers for this run: O² 12.09%, eff. 87.47%, CO 307.9 ppm, TR 146.27 ºC. The CO level for an entire run, ended according to the EU norm and compensated for to the 13% oxygen level would be 286.35 ppm. Now waiting for a day with calmer weather conditions in order to verify whether this was a one-off or new, stable behaviour.

Despite the fact that yesterday wasn't a calm day, I simply couldn't wait to re-test the succesfull configuration of the day before. Wind force 5 Bft this time, no gusts. The new Testo is able to measure chimney drag during the whole of the run. Which shows that the draft limiter which is still there isn't really able to keep the draft within limits. What I am aiming for is 12 Pa, plus and minus 2 Pa, equavalent to 0.12 millibar. But the draft tend to increase upto 22 Pa in gusts, the limiter is ehrrmm... limited.

Anyway, the firebox was loaded with 2.8 kg (6.17 lbs) of soft wood species, same pattern and so on, lit on top as nearly always. (This part tend to get a bit boring.)



Upto 18 minutes into the burn the pattern of the day before seems to be repeated. No high start peak in the CO line and the O² dropped quite steep. But around the 20 minutes mark the oxygen level went lower than the 5% mark and the CO level came up. As could be expected, critical point in almost every run is around that mark. But it didn't get very high, corrected itself and was back again below 500 ppm in three minutes. This peak was almost certainly due to tunnel formation through the fuel pile, always lots of CO in that situation. Good run, not a stellar example, but a good run.
Average numbers for this run: O² 11.37%, eff. 88.45%, CO 542.6 ppm, TR 143.77 ºC.



After the test was ended I checked the Testo filter as I always do, and this came out as very, very light grey. Having oak hardwood at hand, recently purchased but still 25% moist, I couldn't resist the temptation to do a small reload, just to see how the core would react to this. So I shoved four (smallish) pieces into the firebox onto a glowing coal bed, closed the door, took a snapshot of the fire and started the Testo again.



The new load catched fire almost instantly, but probably since the pieces were so damp inside the burn didn't get very hefty. But still, quite nice and it was a joy to see the oxygen level dropping down while the carbon monoxide level reacted moderately and went on to lower levels. Surprisingly enough, this run was still within the limits of the current EU norm.
Averages for this run: O² 13.47%, eff. 84.01%, CO 741.3 ppm, TR 168.81 ºC.



Today's a windy day again, I'll try the same softwood run again and see how it goes. I'll report back about what the results are.

Thanks so much for documenting your results.  What is the technology you used to measure everything?  I read that post but didn't see the name (or understand it).  Where did you put the device to take your measurements?

Thanks, Tina

The device is a Testo 300, a professional gas analyzer. It is measuring by means of a probe, inserted into the chimney pipe behind the stove. It measures the temperature, oxygen and carbon monoxide in the flue gas.

Efficiency is calculated by using the exhaust temperature and oxygen by the device itself. The CO level is a way to determine the quality of the burn, the lower the better. Coupled to a computer, it is also able to create its own diagrams and spreadsheets.

In the graphic: from top to bottom the red line is representing efficiency in percentages, green is representing oxygen in percentages, blue is exhaust temperature in degrees Celsius and purple is carbon monoxide in parts per million.

I like to play with fire, I did that on and off for roughly the past 40 years in one form or another.

To cut a long story short: the results are outstanding, stability is excellent. Windforce 6 Bft with gusts is not a good omen for a succesful burn but the core remained rock stable.

Not-so-excellent is the stability of the Testo's software, it crashed and I was not able to retrieve the data (again). I lost probably the first 15 minutes of the burn because of this, mainly because the vacuum pump of the Testo kept running. I had to decouple it, turned it over, 6 Torx screws had to be removed, opened up the device and decoupled the battery from the main board. This is the only way to stop the pump in this situation.

Then the battery needed to be connected to the main board again, the lid had to be closed and the screws placed back, coupled it to the probe again and started it up. Pfhoei! But anyway, not every expensive piece of equipment is bug free, let's call it that.

Back to the results then, I was very pleased with what I saw. The gusts are visable in the green oxygen line, it wobbles quite a bit. But the average numbers were eventually good enough to pass the EU norm. Bar the numbers for fine dust, I am not able to measure those because the device to do just that is horrendously expensive.



I'll do some more tests, using oak when that's dry enough. And configuration-wise, a different exit from the core, also known as end port, but that's for later on.

Rummaging through the data I stumbled upon the notion that with less wind the core seems to struggle with what could lead to a fuel overload. On December 23rd, 5 Bft with gusts the diagram looked like this:



Too much CO to my liking in the startup phase but the filter came out light grey. December 25th (can't stop thinking about it, you know) happened to be a day with 3 Bft. Same configuration as two days earlier, same fuel load as well. But a very different result. The core seems to struggle with the middle part of the burn although no fuel overload occured.



So, what to do? All the latter testruns with the new liner configuration in the riser box were done with a 70% riser port and a 100% end port. In order to check, I changed the riser port to 60% again and ran it on the second day of Christmas, 2 Bft this time. The warming up hill in the first part is still there but the entire burn stayed above 8% of oxygen.



The same burn was repeated on December 27th, 2 Bft again and it showed again the behaviour I was trying to avoid. This burn wasn't a good one, not to the point of smoke from the chimney but it was too close for my liking.



Most burns show this CO hill in one form of another. It seems it is nearly always there during a cold start. The question is now, could I do something in order to mitigate the effect or change to a quicker heating up afterburner? Ehrmm... maybe by shifting the end port to the front of the riser box? I mentioned this idea before but didn't use it, yet. This seems to be the right time to do it and so I did. Here are two pictures of the changed port location, it looks a bit rough around the edges but it's the correct 100% chimney csa opening.





Always a bit tensive, a new step, so I loaded up the firebox with a smaller load, with more air between the pieces than before. Together with the 6 Bft which was going on outside  it could overload, without question. And to my surprise, it didn't. The CO hill in the first half remained absent, the O² went a bit too low for my liking but it did cope quite well.



Next step will be: run another test, same configuration and hopefully the same wind circumstances. It might be that the riser port is too narrow now, but that remains to be seen. I might try hardwood now, the two bought nets of oak seems to be dry enough.

Nice Peter! Very interesting to follow your endeavors. Thank you for sharing and looking forward to the next steps.

Yesterday I did another testrun, not sure whether or not this was the last one for this year. I felt a bit adventurous, the small oak pile next to the heater looked good so I filled it with some fitting logs for this small firebox. Two small softwood planks on top and a handful of tinder plus a barbeque lighter. The layer of fresh charcoal underneath is there intentionally, it provides for better lighting of the lowest logs. The sloppy looking pieces of aluminum tape left and right in the door frame were ripped out early in the burn, this was part of another (failed) test.



The fire came on slow, the start bulb of CO was there once again although not alarmingly high. On the 24 minutes line I fiddled a bit with a piece that was still on top but not really burning well. While the burn was underway and on 30 minutes, I felt the small oak logs weren't as dry as I hoped for. So, I took another one larger piece from the pile and splitted it. The fresh split side brought 22% humidity on my simple moist measuring instrument. Far too wet, but the burn went on nevertheless quite well. Towards the end of the burn the CO went up sky-high, to be expected with damp wood, forming lots of coal. Despite all this, the burn was still within the confinement of the current EU regulations. The Testo filter came out lightish grey, which means there was just a tiny amount of soot formed and exhausted through the chimney. Throughout the burn, no smoke at all from the chimney and not even water vapour. Outside temperature was a little bit over 10 ºC, equivalent to 50 ºF.



The relative ease of burning wet wood while maintaining reasonable burn quality is remarkable, just my opinion.

Something interesting and surprising with every post, still exciting to tag along.

I know this experiment is focused on the core itself at least for now, what is the situation from the core to the chimney? Is there any way to introduce heat output into living space measurements once you are satisfied with the core design?

Ezra Beaton wrote:I know this experiment is focused on the core itself at least for now, what is the situation from the core to the chimney? Is there any way to introduce heat output into living space measurements once you are satisfied with the core design?

In our living space a batchrocket mass heater is providing warmth in sufficient quantities. We live in a passive house, my workshop is inside the insulated envelope. As you might have read, the way I extract enough heat so the Testo won't be overheated is by using two barrels on top of each other. My workshop is essentially unheated most of the time, it's 36' x 13' floorspace and 8.5' high. This space is heated from 61 ºF to 75 ºF in about half an hour by that miniature monster. So during the test I'll open windows in order to get rid of the excess heat.

It's hard to tell what power the thing delivers, the best way to do so is by using the weight of the fuel. Burned by an overall efficiency of 90% lower heating value, each kilogram is yielding about 4 kWh. The firebox is able to hold about three kilogram so that would be roughly 12 kWh . That number converted to Btu would be 40,900. Is this something you are able to translate into your own situation, for example?

Peter van den Berg wrote:

Ezra Beaton wrote:I know this experiment is focused on the core itself at least for now, what is the situation from the core to the chimney? Is there any way to introduce heat output into living space measurements once you are satisfied with the core design?

In our living space a batchrocket mass heater is providing warmth in sufficient quantities. We live in a passive house, my workshop is inside the insulated envelop. As you might have read, the way I extract enough heat so the Testo won't be overheated is by using two barrels on top of each other. My workshop is essentially unheated most of the time, it's 36' x 13' floorspace and 8.5' high. This space is heated from 61 ºF to 75 ºF in about half an hour by that miniature monster. So during the test I'll open windows in order to get rid of the excess heat.

It's hard to tell what power the thing delivers, the best way to do so is by using the weight of the fuel. Burned by an overall efficiency of 90% lower heating value, each kilogram is yielding about 4 kWh. The firebox is able to hold about three kilogram so that would be roughly 12 kWh . That number converted to Btu would be 40,900. Is this something you are able to translate into your own situation, for example?

I do not live in an efficient house (I've done what I can afford for now), and cannot in this place use a rocket stove mass heater at all (for the living space). All of my experience with rocket stoves is in non-living space structures or just outdoors. The stove I use is an older airtight stove, chosen because of it's increased proportion of firebrick construction, and I run it as efficiently as I possibly can, using brick mass close by to spread the time out as best as it will. I do not load it in the night, I just have learned to live with the swings in temperature from hottest to coldest (about 20F-25F between fires). I would say my situation is probably very close to the average American experience of heating with wood. This being said, based on your experience and this calculation estimate, I would say your stove is at minimum 15 times more efficient at turning wood into room heating than the average American wood heating system.

The constant in the American situation is trying to find a way to certify for insurance and building code purposes. This settles out into two current options - whole steel units (ugly, expensive, and without a true mass storage system), and masonry heaters installed by certified masons (extremely expensive but well worth it if you can afford it). I'm really enjoying your experiments because it lifts my spirits that maybe eventually we could provide enough data about a standardized design that it would become acceptable to our bureaucracies.

Since last year, there are some new developments. Yes, really, there are! First run of the new year was done with small planks, 22 of them, with a layer of charcoal beneath it. The port configuration was still the same as before, 60/100%. The 60% for the riser port, 100% for the end port. Too much wind, 5 Bft and gusts, resulting in a just so-so burn.



Not a very succesful testrun, not bad either. I let it sink in for a day, what to do next? It would be nice to get a more consistent and stable burn, something one could rely on. Maybe the warming up phase could be shorter and to this end, the top of the riser box should be hot earlier. Still no insulation on that top, nor the sides of the box that rises above the firebox.

Best I could come up with is to shift the end port down a bit, possibly more heat could be captured in a shorter time span. Maybe even better mixing of the burnables and fresh air.
Puzzling around for some time, how to do it with the resources I've got in the workshop. Bits and bobs, strips and small pieces of the ceramist's kiln shelfs. I managed to get it done and I was quite content with the result. End port is now 100% of chimney csa, and in the same changing mood, the riser port was changed back to 70% as well.



The end port shifted down about 40 mm (1.57"), it could be more but that remains to be seen. Actually, it did look quite smart, sitting in the barrel like that. Not an idiot's contraption, anyway.



I loaded the thing up today in the morning after coffee break, had some difference of opinion with the Testo 300, lit the fire and waited what would happen. Wind force 3 Bft, nothing to be worried about.



As can be seen, there's small planks all over, overlapping and pairing two at the time in order to simulate larger pieces. The burn was a bit adventurous, first I forgot to set the draft limiter in action so the draft of the chimney got pretty strong. At 15 minutes into the diagram I noticed the error and set the limiter in action. This resulted in a lower draft but the oxygen level started to decrease almost immediately. This is a bit strange, normally more draft means the fire is ramping up so less draft should deminish the fire somewhat. But oh well, the CO got down as well so no worries here.



In order to check whether or not my conclusion was valid, I'd put the limiter out of action at 17 minutes. The O² level reacted almost immediately and also the end temperature. So I freed the limiter and both went down again. From that moment on I did nothing to influence the process, I just let it go. At about 37 minutes the top of the burn was reached, the O² went down briefly to 5.8% and that was it.

So what happened? Delivering more fresh air to the fire did not result in a more fierce burn, rather the opposite. This could mean that a stronger draft than the 12 Pascal of the EU test could trigger a cleaner burn. And also a lower efficiency as a consequence. The white Testo filter came out light grey as usual, CO has been below 500 ppm for 32 minutes, more than half of the entire burn. Starting up from cold and completed with a tailout to 3.6% CO².

I am very satisfied, it looks like this could be a turning point. Don't hold your breath, tomorrow another burn is planned to verify this wasn't a one-off. And I need to verify this results using hardwood species as well. Interesting times again!

Since Januari 4th, I've ran the development model three times. On the 5th, a full load of softwood planks. Nothing remarkable, smoothly and maybe a little bit too low in oxygen at some point but no overfuel at all. No surprises here, just a burn without worries.

On the 6th, I tried a testrun with hardwood again. Still not dry enough, 20% moist. The burn couldn't get upto speed and I had to poke now and then, turning pieces over. Probably too tightly packed, not a nice run and the average CO level was beyond the EU norm. Not good, the thing is clearly much happier with really dry fuel. Unless... the moist pieces were added on a glowing hot coal bed.

On the 7th, I decided to have another try with the moist oak. More loosely loaded this time and I kept the fuel just not higher than the port level. This measured testrun happened to be number #51.  All three testruns described here produced light grey instead of black Testo filters.



It looked good but the core wouldnt play ball. Coming up very slow, no poking required though. It definitely isn't a good idea to use moist fuel for a cold start and than expect to turn out into a good burn. Because the burn was spoiled anyway, I added two pallet bobbins on the large, hot coal bed.



Keep in mind, the effect of adding fuel in a standard batchrocket would send the CO level to the sky in a towering spike, accompanied by visable smoke from the chimney. And also, correcting itself in about 5 minutes, sometimes much more.

Not this time though, both oxygen and carbon monoxide went down, steeply. See the following diagram, the refill was done at the 55 minutes mark.
Watching this, the thought of "this is too good to be true" came up very quickly. So a check was in order, another bobbin was added at the 71 minutes mark. Same behaviour, both lines came down again, although half as much as the first time. Seems logical to me, half the amount of fuel, half the wood gas production.



In conclusion: adding fuel is working extremely well with this core. The riser box in this configuration seems to be capable of handling a temporarily overload without problems. Something similar happened just a month ago, published on December 20th. Four pieces of moist oak were added and the CO and O² lines came down, although not as obviously as yesterday.

The devil is in the details, it surely is.
One testrun a day is possible, not more than that because of the passive nature of our house. Otherwise all doors and windows should be open for hours in order to get rid of the excess heat.
The testrun of Januari 12th was done the classical way. Small fire in front of the port and once that was going a full load on top of that. Not bad at all, so another test could be coupled, adding three big pallet bobbins while the coal bed was at its hottest. It shows that refilling could be overdone, a high and narrow peak of CO rose up. It lasted for two minutes and corrected itself without a hiccup.



One item on my to-do-list was a narrower end port, just to check what effect that would have. A modification also suggested by a member of the Dutch Ecologieforum. With some effort I could manage it to restrict the port to 89%, close enough to the 90% goal.


It didn't look like much less as compared to the former situation, though. The batch was loaded a bit sloppy, not as compact as I was used to. Remember, this is softwood and very dry, less than 10% so it did catch fire very quickly.



The O² went down all the way to 4% and stayed there. I tried to supply more oxygen by opening the door, to no avail. This clearly was not what I wanted, although this wasn't an overfuel situation it looks like to be on the very brink of it. Not good, so the following day I opened the barrels up again and took the modification out.

Measuring the formerly used pieces it dawned on me that the opening hasn't been 100% of chimney pipe csa, just 95%. In order to get it right this time, with the help of some firebrick crumps and stove caulk I managed to end up with an exactly 100% end port. Pfhew!



The following run was exactly what I was aiming for, very stable, O² just above 6% and no peaks. A little bit high end CO but oh well, part of the game sometimes.



Today I planned another testrun, a smaller load, same bone dry fuel, and more densily packed. Also good, no trouble at all. Good numbers and no overfuelling, far from it. Measured testrun #56.



This is the latest state of affairs. Normal top lit batches should be no higher than the top of the port and some space between the fuel and the rear wall. Next attempt would be a testrun using hardwood species. The moisty oak might not be the best choice but that's what I have laying around.