[Stoves] TLUD Test with Switchgrass Pellets

Tue Dec 20 13:32:28 CST 2011

Dear Ron

Sorry to have been so delayed in my reply. I have been preparing a kitchen and a wedding for this Saturday. Some parts are missing and I get a break!

Sent: Friday, November 18, 2011 10:15 PM

Crispin and list:

   1.  I think we are getting closer together.  But some differences remain.    Before getting into them,  is your "grasifier" design now the same as shown 20 months ago at:    http://www.bioenergylists.org/en/crispin_25-kw-grasifier

I think that should be a 2.5 kW Grasifier. It is a TLUD that costs $1.00 if made from stainless steel thin sheet. I made it when Roger Samson was visiting from Montreal and there was talk about sending wood pellets to Haiti as an emergency fuel instead of sending in Propane. It sounds like, long term, subsidised propane will win over the renewable. Oh well. Too many big interests involved.

>    I had not previously read this description.    What do you consider the differences from other TLUDs?   Yours looks like pretty much like designs we talked about 15 years ago.  Stated differently - what have you optimized?

I don’t think it is optimised though I did mare three different ones before deciding on the current design. It has preheated secondary air injection that is full-height, same as the 1984 Tsotso so that is not new. The outer shell is a secondary air preheater. It is conical in form. David Hancock (the famous) made his grate conical and the preheater cylindrical. I have done the opposite as it is easier to make a conical preheater than a conical grate with perforations. The narrowing gap (as it rises) accelerates the air. The entry holes are not punched or drilled, but pressed inwards to create a ‘blurted lip’ with a small radius on the edge of the holes.

I have several times written about the importance of getting the secondary air to reach the centre of the rising gases in order to burn properly in a short vertical distance. This is one way to give the incoming air velocity/inertia.

The CO/CO2 ratio was checked to see if it was combusting well and that is OK (i.e. extremely low as per the report). I found it helpful to have fixed primary air holes and openable ones that supply air during ignition. I also found that making a depressed conical open space in the centre assisted ignition.

.
   2.   One main difference we still have is the use of a lid.   I think we agree that one would want to use a lid for normal cooking tasks - to achieve higher efficiency.   But the formula you are using here gives higher efficiency for higher water evaporation.  

I do no agree with that. The formula gives the efficiency. Period. The thermal efficiency of boiling water has nothing to do with the final mass of the water. This is an error propagated by some who feel that it is the ‘delivered’ water that is the object of the exercise. This is a very important point for those testing  stoves. The amount of water remaining at the end of a boiling test might be of interest to somebody, but it is not of interest to an engineering testing the thermal efficiency. It is not possible to make a correct reconciliation of the energy if you do not use the initial water mass.

The thermal efficiency of a system is heat used divided by heat applied. If water is evaporated during the boiling procedure, the energy has been used. 

Put another way, the use of the final water mass pretends that the missing water was boiled out using zero energy which is certainly not true. I do not know the origin of the ‘final mass’ concept but it is technically incorrect and is not used in industry because it does not give the correct answer. If zero water was boiled (for example using a pressure cooker to raise water to 100 C) or if ½ the water was boiled away, the thermal efficiency of the system would be about the same (keeping in mind some tiny considerations to do with heat radiation from the pot and water). The formula used to determine the system’s thermal efficiency (and please remember this is not the heat transfer efficiency to the pot) must correctly account for all work done. The WBT as published, even the one used at PCIA now, uses the final mass of water boiled and applies all the energy used on the initial mass, some of which is completely evaporated. As a result you arrive and the conclusion that the evaporation of water appears to give a higher efficiency.

If I have misunderstood your interpretation about the boiling phase, and you really meant the simmering phase, let me continue:

The use of the lid affects both the boiling and simmering phase of the test. The boiling phase attempts to correctly determine the thermal efficiency of the stove heating water. Yes there is a small error in the calculation (the use of the final instead of initial mass as the ‘mass of water boiled’) but at least it is on the right track. The simmering phase is not a test of thermal efficiency, it is a simmer task. At task and a system performance measurement are not the same thing. Simmering is an arbitrarily defined task: keep water hot. 

One could determine the ‘heat transfer efficiency’ (HTE) during that simmering task, however the WBT method does not determine the HTE. What it used to do is take the final mass of water simmered and the fuel used to ‘keep it hot’ and see how much water was boiled away. (!) Now think about that. Work done during simmering is only determined by measuring the water missing. If you make more water go missing, it appears that more work was done. This means that the more perfectly you run the stove, the less water you boil away, and the lower the apparently thermal efficiency! That is crazy!

>From and engineering perspective, no work is anticipated when asking for water to be ‘kept hot’. If, at the end of ‘a perfect simmer’ there was no water missing, that would be simmering perfection. The efficiency according to the formula in WBT3 would say the efficiency was zero. The stove would get a terrible ‘efficiency’ rating because the heating efficiency from the first phase would be divided by 2.

(1+0)/2 = ½.

This point has been brought forward by me until I was sick of it for three years and through the whole of the WBT4 consultation. Some listened, some did not. 

Lids: you raise the point about the lid seeming to change the efficiency of the stove (lowering it) because without the lid, more water is evaporated. It only seems to be more efficient because of the defective formula that provides the number. It was never more efficient. It will not be more efficient in future. It is just that the formula was written by someone who considers simmering to have ‘an efficiency’. That thought was due to two misconceptions: first, that the measurement of the thermal efficiency of the whole system is the same as measuring the heat transfer efficiency, and second, that ‘missing water’ is a valid measure of work done during simmering. 

If anyone still has an understanding that the that transfer efficiency is measured using WTB3 or 4, consider this: [Heat transferred from the heat source to the pot] divided by [the heat generated by the heat source] (considering all chemical and particulate losses) gives the heat transfer efficiency.

The heat that gets into the pot takes several paths:

1.       Raises the bulk water temperature if it is below the local boiling point

2.       Evaporates water reducing the mass of water remaining

3.       Radiates from the pot, lid and handles if present

4.       Convects off the surface of the pot, lid and handles if present

5.       Radiates from the water surface of the water if the pot is partially or total exposed to the environment

6.       Conducted to the air from the pot, handles and lid if present

7.       Conducted to the pot support or other items touching the pot

8.       Raises the temperature of air between the water surface and the lid

9.       Raises the temperature of air passing over the surface of the water

10.   Raises the temperature of moisture in the air passing over the water, even if this is under the (leaky) lid

Of these 10 losses, the WBT only accounts for the temperature rise (or fall, which is really misleading) and the missing water (Items 1 and 2 above).

The fact that any WBT (simmering phase) reports a ‘thermal efficiency’ at all tells us that the author(s) did not consider the entire system and the difference between a measure of thermal performance and cooking task.

Jim Jetter informed that the current WBT he is using does not include the simmering phase ‘efficiency’ number in the reporting of the stove performance. This is real progress. Thank you Jim.

     I found this report from Aprovecho quite informative on the really high heat losses due to evaporation - and how they can be stifled with a lid:
   <http://www.aprovecho.org/lab/pubs/rl/perf-stud/doc/61/raw> www.aprovecho.org/lab/pubs/rl/perf-stud/doc/61/raw

Perhaps you will consider the engineering aspects of that commentary. Heat loss from an un-lidded pot is very real. The WBT formula does not ‘think’ that way. The calculation of the ‘thermal efficiency of simmering’ is not a measurement of heat loss.  If you operate the stove at any given power, say 1.5 kw, and the pot ‘simmers’ (maintains a constant temperature) with no lid, you have a ‘certain situation’. Call it case 1.

Case 2 is putting a lid on the pot and operating the stove at the same 1.5 kW will have what result? It will boil off much more water. Why? 

The loss of heat from radiation, conduction and convection from the surface of the water (leaving aside the evaporation for a moment) is significant. How significant? One can easily find out. Measure the evaporation with the lid off. Put on a lid and measure evaporation again. The difference between the two values is the heat that was otherwise being lose by non-evaporative heat transfers. Putting on the lid controls those non-evaporative losses allowing the stove power to be reduced to, say, 1.1 kW to maintain the same water temperature.

All this is quite separate from the miscalculation of the system efficiency which I hope I have fully dealt with above.

       I stand by my assertion that if you want to measure/calculate high efficiency using the water boiling test technique you are using, the numbers will look better (higher) without a lid.  

Yes they do ‘look better’ because the method of calculation is defective, and in addition, the concept is incorrect to begin with. The true efficiency of simmering is zero % because there is no change in enthalpy for a perfectly efficient simmer. 

There are stove operations where you want to remove water from a pot  - and one would certainly remove the lid to accomplish that.

Yes, and that would not be part of an experiment during which the system efficiency or heat transfer efficiency could be measured. It could be part of an arbitrary task.

    Even though we normally would want a lid,  I think the test is valuable without a lid in comparing stoves and transfer of heat to the pot.  

This is exactly what I am talking about. The transfer of heat to the pot cannot be determined by the method used in the Berkeley-hosted WBT’s. The amount of heat passing between the gases and the pot is not measured or calculated at any time during the test. I have developed a method for making such a determination and wrote about it here so I won’t repeat is now. It was not adopted for WBT4.

If I want to skew the results in my favor, I would certainly avoid a lid.  Same for picking a color of the pot (much smaller effect).

The colour of the pot (and handles) is part of Item 3, 4 and 6 above.

   There are parts of your argument on lids I don't follow.  But most interesting is this next apparent after-thought comment - which I don't know what to do with:

    [It occurs to me that an understanding that the lid coming off would increase the thermal efficiency may be rooted in the error in the WBT3 that calculates an ‘efficiency’ for simmering which is itself a conceptual error. Increasing the evaporation during a simmering test appears to increase the system efficiency which is impossible. It is an artefact of the math error. Any WBT3 efficiency number is incorrect for this reason. Only the high power heating efficiency is valid.]
     You also have comments below about influencing the computation using skirts, etc.   How does your last sentence improve things.  I want to emphasize that my only initial complaint was that your concluding the test gave 22% (now less than 20%) was being terribly unfair to all char-making stoves.  This last comment by you is one that needs further exploration by all of us.

Few more inserted comments below  [Labeled RWL2]   To get at differences in definitions, I have bold-underlined. every use of the term "efficiency"

  _____  

>>I am answering your post first because of its currency but I am not forgetting that I need to respond to Paal and Paul from the weekend and Vetle’s last very information-filled message. I have been extremely busy over the past week preparing for the COP 17 demo and beating Old Man Winter with some brickwork and also today’s demonstration of three burning modes and fuels for the visiting delegation from China (which went well).

>I like the computations you have provided, but think there are additional efficiencies needing computation. Yours below gives zero benefit to the (very sizeable) charcoal production.- which obviously will entail some howls from those (like me) interested in Biochar.

>>I went through the spreadsheet first and it seems you have captured the numbers well. The column D numbers are only very slightly different from the ones I used and they are because I had to round slightly to give the mass data. 

>>I do immediately have a question as to whether the ‘efficiency of char production’ has anything to do with ‘thermal efficiency. Yes they can bot he viewed as efficiencies but they are not related, of course. 

     [RWL2a:   I disagree.  They are intimately related. Especially in the revised formulas, which you seem to have agreed with..]

>>On what basis is the production of char, as I suppose a % of initial fuel mass or % of carbon content of the fuel, related to how a stove applies the heat generated to the pot?

>First, I think this sort of efficiency measurement should be done without a pot lid. 

Why?

   [RWL2b:  All this on "lids" already responded to above.]

I have covered this above.

(re the spreadsheet SS)

>That SS shows that about 50% of the initial energy remains in the produced char

     [RWL2c:  Sorry; "SS" stood for "Spread sheet".   We are agreeing on the 50% statement.]

>…I say that makes the overall stove efficiency, making no other changes in Crispin's work, about 75% (or 25% "loss" – not 75% loss). 

>>Well we have to be careful what is claimed here. The boiling efficiency of the pot from beginning to end was indeed 22% of the available heat. It was not 22% of the heat offered to the pot. That is for sure. It is also not correct to make the calculation of the char-subtracted value without pointing out the implications of a slightly different moisture content because in these cases where the char mass is high, a change in moisture gives a disproportionately large increase in the apparent efficiency. In fact, I tossed in the 22% because I did not have a high heat efficiency which I would much have preferred. I will try to get one in the next few days if anyone thinks it is important. It is certainly much higher than 22%.

    [RWL2d:  I am confused.   Obviously moisture content is important in efficiencies - but we have small moisture here and everything can be calculated.  I look forward to differences you find with a different "high heat efficiency"]]

I am referring to the system efficiency when the stove is operating on high power. This is used as a measurement by the people at BTG in the Netherlands. Run the stove flat out with a boiling pot, usually at the end of the boiling test. The missing water is a good measure of the thermal efficiency at that power level. It does not indicate anything else about the stove at other power levels. There are very good reasons for the SeTAR Centre’s Heterogeneous Test Protocol to use 3 or 4 different power levels during the evaluation of a stove. The stove is operates at different power levels and the pots are changed if the water reaches 80 C. From the temperature or 30-70 or 40-70 the system efficiency is very constant and can be determined quite accurately. Again, it is not the heat transfer efficiency which is invariably higher. 

>That is, I claim that just adding the boiling and charcoal-making efficiencies together is valid.  The missing 25% is energy that was not captured usefully.

>>The char-making efficiency is related to the theoretical maximum char possible and has nothing to do with heat transfer efficiency. I am surprise you would want to add them together.

    [RWL2e:   We will have to agree to disagree.  The amount of char has an enormous influence on what you are measuring and calculating.   ]

Well, it will have an effect on the number, but the result of the calculation is not a valid measure of anything I know about. What does this consolidated figure tell us? Did you follow my calculation to show that it is quite possible to have an answer that is more than 100%? A system efficiency of more than 100%? Are you kidding? That takes us into the world of ‘zero point energy’. http://www.halexandria.org/dward154.htm 

…

>>Heat retained by the pot (as described above as system efficiency heat) H1 divided by the Heat potential, minus heat value of the char remaining H2 = system efficiency.

>>H1 / H2 x 100 =  Eff %

     [RWL2g:  I gave the different efficiencies numerical labels.  I think this is Eff3.   I don't see calling H1 "heat retained" - as it is mostly a measure of heat in the evaporated water.   H2  ("Heat potential") seems to be the initial energy minus the energy in the char.]

No, heat retained is retained. Heat lost in evaporated water is lost and is calculated separately (mass times latent heat of evaporation).

To repeat - I calculate that about half the initial available switchgrass energy is still available in the produced char, and that this MUST be accounted for when comparing a pyrolysis stove with a combusting stove. 

It is possible to determine the heat remaining as unburned fuel. It has nothing to do with the determination of the efficiency of the stove and therefore has no place in the calculation. I will give two examples of where things will go wrong:

1.       The stove in Namibia known as the Namibian Tsotso

2.       The Champion gasifier

The Namibian Tsotso is basically a metal Rocket stove with a metal plate where a grate should be with a few small air holes. It uses a great deal of fuel to accomplish any task because the airflow under the fire is so bad, it turns almost all the fuel into charcoal. In what way is this stove ‘more efficient’? It can’t burn the fuel collected and generates masses of waste charcoal that will not burn I the stove. The user has to go and collect a whole lot more from the local (desert) environment.

(Paul don’t have a fit!) The Champion gasifier, if loaded full of pellets, creates enough ash (with some fuels) to block off the primary air flow through the chamber, cutting off the fire. It goes out. That eventuality could be anticipated by the cook and compensation made so the cooking was done by the time the fire died down and the cooking and simmering could be done by then. No problem. Now, according you your approach as I understand it, the unburned fuel is an ‘efficiency’. That is, apart from the fuel burned and the heat calculated to give a system efficiency, the unburned char produced should be an ‘additional efficiency’. On what basis? It is just unburned fuel. Same as raw unburned fuel with charcoaled ends removed from a Rocket stove at the end of a test. A stove that produces more charcoal from the initial fuel is not doing something ‘more efficiently’. It is just using the heat from the fire at a certain, determinable efficiency.

Is the char at the end of a Rocket stove burn an efficiency?

Is the char at the end of a TLUD gasifier burn an efficiency? 

With reference to the Champion stove test above: Is the unburned mass of pellets lying under the char in the self-extinguished stove an efficiency? If unburned pellets (which never get heated) are not to be considered  and ‘efficiency’ in your method, why not? They are also left over and could be added to the soil to improve it, and would probably bring more benefit than plain char because there is much more energy in unburned pellets than char. 

Would you consider the char portion of the leftovers to be a contributor to the ‘overall efficiency’? The truth is that leftover fuel has nothing to do with efficiency, save that you deduct it as an inefficiency for wasting it. As far as the stove is concerned, leftover unburnable fuel is a loss, not a benefit. If you want to credit the leftovers, either char of a blend of char and unburned fuel, you are welcome to do so, but it is not a stove efficiency.

I mean, why not take the unused fuel from a Rocket stove test and bury that in the ground to assist agriculture? Would that increase the reported efficiency of the stove?

>>I do not see that it gets special treatment. The stove type is not affecting ordinary engineering calculations. If a stove make 5 g or [RWL:  "or"?] 500 g of char, determining the system efficiency requires that the heat value of the remaining char be deducted.

      [RWL2h:  But the whole point of this exchange is that you did not initially make this eff4 computation - which approximately doubles the number from 20-25% up to (now) 57% .   I think it is very misleading to talk about 20-25% efficiency for this (or any charcoal-making) stove.]

I have no problem calculating the heat used / heat generated as the measure of efficiency. It is one of the ‘real’ efficiencies one could report. It is fair to say that if the remaining fuel cannot be used by the stove, it is a loss.

     [RWL2i:  Make that "thrice" rather than "twice".   We are disagreeing also on the word "system".   I am not disagreeing on reporting the 20-25% number.  I just am saying one has to report something on the char-making efficiency as well - even if the char is thrown away (which is not very likely).   

Well you can choose to look at the whole energy package and determine that there are several aspects of the system which a worthy of mention . I doubt it is a good idea to add them together, and more than one would add the fuel mileage of a car to the volume of the tank. They are both valid measures but they are not related.

I do not propose to add "%char" to anything.  I do say that eff2 = %energy in the char" is meaningful and can be added.  

Added in what way? Summed? 

If one were doing this only for later use of the char, it would be THE key number.   The first work on char-making stoves was to avoid the common horrible loss in making char in remote pits - not for placing char in the ground.]

Agreed. The calculation of the ‘horrible loss’ shows the loss is large. If you take the whole system into consideration (which I have tried to do here for years, with solid numbers provided) the horribleness becomes much reduced. This has been strongly objected to by some because they do not like charcoal at all. Well, reality bites! Charcoal produced through conventional means, distributed and used is not nearly as inefficient as people pre-suppose – meaning people supposing that they have considered all the necessary components that make up the energy chain. Almost invariably they have not. While I fully support the idea of making use of the charcoaling energy, I am not willing to throw out the charcoal baby with the charcoal energy waste bathwater. The reason why charcoal gives wood-based alternatives a run for their money is because charcoal stoves can be a lot more efficient than wood stoves. A really good charcoal stove is 60% efficient without a skirt! It is very difficult to make a wood stove that works that well. Current technology for charcoal production (even with wasting the heat instead of generating electricity locally) and charcoal stoves are about equal to a wood distribution and wood stove combination. If one generates electricity with the gases, there is no contest. Charcoal is a far better fuel for cooking.

>This comes closer to the efficiency on the water boiling side, but still does not fully capture the value of the char.   

>>The system efficiency is not intended to capture the value of the char because it has nothing to do with the combustion or the heat transfer. It is a completely separate subject with its own rules and understandings. It would by quite legitimate to point out how much of the carbon in the original fuel was in the char, for example, because someone might want to optimise the retention of carbon. 

    [RWL2j:  I still cannot fathom how you decide/believe that the (weight or energy) "value of the char ... has nothing to do with the combustion or the heat transfer".  ] 

It is used to calculate the thermal performance of a device if one is only interested in the ‘raw’ number. If you are talking about how much fuel a stove ‘uses’ then (according to the KPT) you measure how much fuel the family collects, buys and uses. If that amount does not go down, then the stove is not saving fuel. If the stove produces unusable char, it is interesting, but it is not ‘fuel saving’.

>The lost energy is about half of the non-char energy (not 3/4 of the input energy - as implied with a 22 or 25% efficiency computation).

>>I did not state any losses or describe them. You have inferred that I said the char as a loss. It is left over and I said so. It might be fuel, it might be fertiliser. In the light of the comments I presented about the thermal efficiency having to overcome the carbon subtracted from the fuel-stove energy chain, I think it is relevant to present all the information so the user can interpret the test in a way that is applicable to their intended purpose.

     [RWL2l:  It won't be just me that will infer something about a (single, unaccompanied) 25% efficiency statement (when others like me see it as closer to 75%).   

I don’t see a problem with that. You can make your own interpretations.

>To restate, with simple numbers that might result with a re-test without a pot lid, that hypothetically finds that 25% of the energy was productively used in boiling the water and also found that half the energy remained in the char:

I do not see how taking off the lid would do anything other than increase the radiation losses (which are not counted in a standard test) and increase the evaporation (which is). The result of taking off a pot [RWL:  "lid"?] ALWAYS results in a lower system efficiency number. The reason is that water has a emissivity coefficient of about 0.99, similar to black oil. If you take off the pot [RWL:  "lid"?], a lot of heat is lost by radiation to the sky/ceiling. This loss is not countable by present methods and appears to be an inefficiency, that is it lowers the system efficiency number in all cases. There is no case where you can increase the uncounted loss, and increase the system efficiency number.

Yes, ‘lid’, not ‘pot’. Thanks for understanding.

    [RWL2m:  All of this paragraph is contradicted by your sentence below starting: "[It occurs to me..."   ]

I can’t see the contradiction. If you take off the lid, the actual system efficiency decreases. The errors in the calculations contained in the WBT spreadsheet show that the efficiency increases, when it does not. That major reason is radiative losses from the water surface which are in the hundred watt range.

>…My preference: eff3 = eff1 +eff2 = eff1+ 2*eff1 = 3* eff1 = ¾ 

You cannot add efficiencies that if they are producing the same thing like heated water, but not if they do not have the same units. Steam and char are not the same units.

    [RWL2n:  I didn't and wouldn't;  my units were all compatible - all in energy terms.]

I have a car. The fuel take contains energy (Joules). The tires are made from rubber. They contain energy (Joules). You cannot meaningfully add them just because they have the same units.

>Of the ¾ useful output 2/3 is in the char and 1/3 is in the boiled water; one quarter of the input energy was not captured.)

It is important to note that char is not a ‘useful output’ from heat generation point of view. It is unburned carbon. It went in unburned and came out unburned. That is not an efficiency.

   [RWL2p:  Disagree.   Putting char in the ground will have to directly compete with the energy value.  

The energy value of what? Ron, the unused energy content of stove fuel is not a cooking efficiency.

>But about half the energy offered to the pot DID enter the pot. How well it made char is irrelevant to the system efficiency. I am tired of repeating that so I will end now.

   [RWL2q:  We both are tired of the repetition.  However to repeat:  your system efficiency is directly impacted by the magnitude of the char production.

Only if you view the energy efficiency of the production of trees or crops in a field, and you can show that adding the char somehow increases this energy production. As the char is not actually directly contributing to the energy in the trees (an example crop) as it remains inert for the most part for thousands of years, then the energy in the char is irrelevant to the production of crops.

The char itself may have some agricultural effect on the biome, it make promote growth, but the energy in the char is not liberated during this process. The proof of this is that the char is not ‘used up’.  It is my understanding that those who wish to sequester carbon in the soil choose char precisely for this reason: that the char will not be releasing its carbon. If it does not release its carbon, then the energy value of that char is never going to be realised. Thus the energy in the char has not place being included in an energy calculation reflecting ‘stove efficiency’. 

It is quite clear that you are looking for some method of adding the char left from a stove to the efficiency determination in order to elevate the ‘usefulness’ of stoves that do that. I have no problem with you wanting to have such an opinion, but it is not going to fly as part of an ‘efficiency’ calculation. 

The spreadsheet:…

>I have added the correct heat value for dry switchgrass and calculated the heat available.

>The line about pot lid off has been removed. [It occurs to me that an understanding that the lid coming off would increase the thermal efficiency may be rooted in the error in the WBT3 that calculates an ‘efficiency’ for simmering which is itself a conceptual error. Increasing the evaporation during a simmering test appears to increase the system efficiency which is impossible. It is an artefact of the math error. Any WBT3 efficiency number is incorrect for this reason. Only the high power heating efficiency is valid.]

     [RWL2s:  This last parenthetical [...] seems to leave everything in this dialog in doubt.   I hope we don't drop the topic at this point.  Something has to be meaningful as it relates to total char production.] 

I think I have covered the WBT calculation points early in the post.

The Average system efficiency is 56.66%. It also produced char which could theoretically have been 100% of the carbon and 0.176 as much ash and volatiles (which is 15% of the total mass). That char could have been 617.6 which I calculated as 1050g x 50% Carbon / 0.85 = 617.6 g.

   [RWL2u:  I am surprised it got this high.  But glad you are no longer talking 22.3%.  With the huge swings we are seeing in the various efficiencies, I suggest calling 56.66% as 57%.]

The efficiency for the whole test (all power ranges) was 22.3%. There was leftover char. The system efficiency accounting for unburned char (which the stove cannot burn) was 57%. Yes it is high.

      [RWL2w:  I think you did a nice job on the construction.  The use of stove pipe is good - and I like the conical shape for a preheater.  I am anxious to hear more about how results change with different amounts of primary air (through nails/bolts in the eight holes).

It makes smoke and has higher CO/CO2 if there is too much primary air. 

>>The edited spreadsheet is attached.

    [RWL2x:  and a new version, with some comments was going to be attached to this response.    But I stopped went I got to this pair of lines 15 and 16 - which don't look at all right.  Would you check with REAP again?   Fortunately,  I don't believe you used these numbers at all later, so I continued with the old-new comparison.

15.   Ash content of the initial fuel: 

grams

283.5

16.    Ash content of the char: 

grams

94.5

All the ash in the beginning is contained in the char that remains. Do you agree? If you don’t agree that the ash is in the char, where do you think it goes?

   RWL cont'd   I think there is sufficient uncertainty now about the meaning of this test (since you are now disowning the results) 

What are you talking about?? I hope you are not quoting yourself on other fora. What ‘uncertainty’? You sound like Gavin. It is an arguing tactic.

that I am going to not supply a revised spreadsheet.  I do have to emphasize that now everything is based on assuming that this switchgrass char has an energy content of 29.47 MJ/kg (presumably from Roger Samson and REAP).   Earlier, I was assuming a considerably smaller number based in part on your then-assumed larger ash content.

You can make you own calculation of the heat content of the char. I determined that based on my calculations from the chemistry of the reactions of the elemental composition. I was trying at the time to discuss the matter with Nordica at Aprovecho. Don’t think Roger has investigated it. James at the SeTAR Centre has several recent char heat values for the char remaining from various stove tests, including elemental analysis to go with it. I believe that will come out in a paper next year.

   Starting with 18.70 MJ/kg for dry switchgrass, you (Crispin) also this time calculated  (not assumed) an energy content of the switchgrass pellet at 10% moisture of 16.56 MJ/kg, whereas you had earlier used 16.  This caused only a small change.

   To summarize, the old and new eff1's  (for boiling) are about 22%.   The old and new eff2's (for char production) are 50 and 62%.  The old and new eff3's  (sums of the previous two) are 72 and 83%.  [Crispin does not accept the meaningfulness of, and did not compute, eff2 and eff3].    The old and new eff4's  (the ones we seem to agree areappropriate to define the stove- and which take account of the energy in the char) are 42 and 57%.   Crispin did not calculate in the last spreadsheet that the percent capture of the energy generated is about 22%/57%

    One lesson for me out of this is that the assumed energy content of the fuel and char can cause serious changes in the various efficiencies.  Now it appears that the stove is capturing only about a third of the available energy (after subtracting that in the char), rather than half.  

    Apologies to all - as it now appears that Crispin feels none of this is meaningful.  I hope we can start with firm definitions - which should go well beyond this list.    Maybe, hopefully, we did accomplish something - but I am not sure what.    Ron]   

I think you have already concluded  that you will have no meaningful response to my proper calculation of the efficiency of stoves, char producing or not. 

My conclusion is that deducting the energy in char remaining from a stove test is a valid way to determine the system efficiency related to heating a pot. Also, that adding the heat value in the remaining char to the system efficiency is double-counting. It will reward char-producing stoves with higher efficiencies to the point where the overall efficiency, calculates as you propose, will exceed 100%. 

That is ridiculous.

Regards

Crispin

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