[Stoves] Equipment required for testing stoves

Fri Dec 7 19:06:51 CST 2012

Josh and stoves list 

Thanks for the responses. I add a little below, but I have moved to a lower position everything from you that was now clear. AT first I had not realized that your response was sent only to me and not sent to the stoves list. Otherwise I would have just left everything unchanged . 

A few clarifying remarks/questions below from me. 

----- Original Message -----
From: "Josh Kearns" <yeah.yeah.right.on at gmail.com> 
To: rongretlarson at comcast.net 
Sent: Thursday, December 6, 2012 11:55:07 PM 
Subject: Re: [Stoves] Equipment required for testing stoves 

Ron et al., some responses below... 

On Thu, Dec 6, 2012 at 1:03 PM, < rongretlarson at comcast.net > wrote: 

Josh (cc list) 

<moved RL and JK parts a and b to a lower position> 

<blockquote>

RWL c. You had quite a tall chimney - so the device could not operate as a stove. Do you have any data on the maximum temperature with a shorter chimney in the ND cases (which might then better simulate a stove)? This further queried next. Did you have a means of controlling primary air other than through the chimney height? 

</blockquote>

The chimney is not too tall, it just depends on what height you like for cooking. You could put a pot on a ringstand at waist height if you like, up to you. 

[RWLc1: Yes, but the power level will then go way down - maybe/probably could operate only in FD mode, because you have such a tall fuel load. 

<moved JK para c1.5 on stove heights related to fans to spot below> 

Cutting the chimney height by, say, half, would make no measurable difference in the char. The real difference is induced by the fan. The fan adds something like the equivalent of 30 feet of well-insulated chimney. I made the taller chimney mainly to help contain the flames - people at the University are already sketched out by what I'm doing. 

[RWLc2: I can't yet agree on the " no measurable difference in the char " (and think you say the opposite below). I concur on the importance of adding extra chimney height for safety (and power/speed) reasons . 
Have you ever tried a speed controller on the fan - so as to simulate the 4-can (and other) chimney heights ? That would be most instructive on both the production temperatures and the char quantity and quality. I have seen no data of this type. 

Under ND conditions you are at the mercy of the size and shape, and thus bed porosity, of the feedstock. 

[RWLc3: Yes, agree partially. If one controls the primary air supply you should/might be able to control the (power) turn-down ratio over a 2:1 ratio. Controlling fuel-bed size and shape (diameter and height) is certainly available to the third-world cook (or at least the stove supplier). It would be interesting to see if that cook would also learn to choose different fuel shapes (porosities - for instance a spherical pellet with same total weight might cook differently.) Probably some big cook-time differences (and char temperatures) with (vertically oriented) grasses of different diameters. I have seen almost nothing on this topic also . 

[RWLc4: More on this below - but I still think the 4 upper cans do provide for more power (shorter fuel life) and I (now, still) think this parameter will change the char production temperature (and probably the properties - with which you agree below). I hope you or others can show more data along these lines - as this is an important new (to me) aspect of char-making stoves. Previously, I had believed (like you) that the power level did not impact the char temperature. Now I am (much) less sure. I have seen many stove videos showing increased power with taller chimneys - and am now guessing (based on your two graphs) that the char temperature is also (somehow) related. To repeat - this is a very important point that I hope others will comment on. 

<blockquote>

d. Can you supply the weight of the starting and ending weights of the load of pellets in each case. It appears that the FD power level was close to twice the power level (half the duration) of the ND case. It would be interesting to see the thermocouple measurements with 0 (?), 1, 2, and 3 extra cans above the secondary air inlets as well as for your case of four upper cans. 
</blockquote>

1, 2, or 3 cans would make very little difference in temperatures, probably substantial differences in char properties, and certainly nothing compared to the fan. 
[RWLd1 re 1-4 cans and T's: I still need to be convinced - with experimental data. I agree on use/value of a fan 

A gallon of pellets weighs about 2500 g. I get about 500 g of char under ND conditions and 250 g of char under FD conditions. 

[RWLd2 : your two power levels were probably about 

a. ND: 2.5-.5 = 2 kg of wood consumed at 18 MJ/kg over 1.2 hours (4320 secs). This gives 36 MJ/(1.2*3.6 ksec) = 8.3 kJ/sec = 8.3 kW. This is very high for a cook stove - of course, this is great for making char alone. The remaining issue is how low can the char temperature be. This is the key char parameter of interest to the soil scientists. 

b. FD: 2.5-.25 = 2.25 kg . Over 0.7 hours, this gives about 36*(1.125)/(0.7*3.6 ksec) = 8.3 *1.125/.7 = 8.3*1.6=13.3 kW 

These kW values are not helpful in comparing between stoves, unless the fuel beds (and probably more) happened to be identical or pretty similar. Still, I'll bet that we can give general guidelines for the likely char production temperature knowing power level (fuel exhaustion times) and a bit more - for any stove. I have not seen such an attempt. Anyone? 

Butsince the temperatures could be so helpful, I hope anyone else who has used thermocouples can also report in on their results - especially if anything was variable and the (relatively constant) interior fuel bed temperatures changed appreciably. I have certainly seen plots like yours - but nothing to show how temperatures changed - and as dramatically (600-900 C) as did yours with the same stove operated differently. I am guessing that your system could get as low as 400 or 500 C - with either fewer upper cans or a slower fan speed. Better understanding this ability to produce chars of different character should be a hugely valuable experimental result. I think this can be done with one thermocouple, not needing 3-4. 

<blockquote>

In other words - how low a temperature char could be obtained? I have seen no data on char temp as a function of power (primary air flow rate) - so your geometry offers a nice (non-stove) way to prepare chars which should have very different in-soil characteristics. 
In a stove, changing the power levels (fuel duration time) through changes in primary air supply (changing the turn-down ratio) would have a similar effect - and I have seen no previous data on this phenomenon. Is such data out there? 

</blockquote>

I have not seen or worked with a stove that has a primary air damper. I have worked with feedstocks that, if too fined grained, reduce draft too much. Then the TLUD doesn't work - it smolders and smokes badly. 
[RWLd3: Agreed. Not every combination of stove and fuel is going to work. I think the ability to change primary air supply is hugely important to a cook (and to optimize chances of excelling in the water boiling stove tests). This is not to say that in your (char production) situation that controlling air is important But you probably can help those of us trying to improve and understand char-making stoves. I don't understand enough of your work, but believe you also would like to know more about the production temperatures for the char you are producing. ] 

You can call our awkward laboratory experimental TLUD setup a "non-stove" if you want, but it produces chars that are physico-chemically representative of the range of chars that come out of TLUD cookstoves. 

[RWLd4: I am complimenting you on your work - which is not stove-optimization oriented. I believe you would only be able to cook satisfactorily if you did two things - drop to 1 or 2 upper cans (partly to get the power level down) and b) control primary air. You are presenting results that should be very helpful to both char-cook-stove designers and char-users - and that I have not previously seen. 

I hope others will join in if they have any char-bed (not flame) thermocouple data - preferable under different operating conditions - such as Josh has shown. 

Ron (end of Friday responses) 

Moved portions from Josh today (no need to respond to these) 

RWL 

On Thu, Dec 6, 2012 at 1:03 PM, < rongretlarson at comcast.net > wrote: 

<blockquote>

Josh (cc list) 

Nice data - thanks for sharing. 

Several questions: 

a. In the ND case, I am surprised that the upper thermocouple showed a sharp drop about 25 minutes prior to the almost identical drops of the lower two. It is almost as if it fell out - as the dark black curve came back near the end.. Your explanation? 

</blockquote>

Josh 

a. This is indicative of the thermocouple shorting out. When the probes malfunction the loggers will record nominal values of about 80C. Several firings eventually damage probes, even those rated for high temperatures, so unfortunately I see this kind of thing frequently. 

N.B.: One things I've found over the years, especially with monitoring traditional charcoal kilns, is that even high temperature probes with kevlar insulation around the wires break down quickly - not because of the heat but because chemicals in the smoke break down the fibers. (This is a serious bummer when you're doing research on a shoestring budget and probes cost $40 a pop.) 

RWL 
<blockquote>

b. There was somewhat similar behavior in the FD case, but no "falling out". I would have expected all probes to have dropped in temperature about the same time and manner - why the upper probe behaving differently? Faster cooling due to secondary air nearby? 

</blockquote>

Josh b 

This is because by the end of the run the top probe was no longer embedded in the char - the char had subsided below the level of the top probe. The other probes were still buried in char and so were insulated during the cooldown phase. 

<Above, in first part of this, are RWL responses to most of c and d, leaving just these two moved down> 

Josh 
c1.5 One reason the unit is tall is because I had to move the fan far enough away from the reactor so the heat wouldn't burn it out. For cooking you would possibly solve this by putting the fan off to the side. For me it doesn't matter if it's awkward or inconvenient to cook on, I just want to generate and test the char. 

RWL 
e. There were several cans below the fuel can with the 3-4 thermocouples. What was their purpose? 

Josh Answered above. 

End of RWL response 
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