[Stoves] Request for help on TLUD operating data

Ronal W. Larson rongretlarson at comcast.net
Sat Aug 16 08:56:03 CDT 2014


Crispin and list

1.   I look forward to your chart.

2.   For what I have in mind to control secondary air at the same time as primary, I guess this means that we need a way to also vary that ratio depending on the moisture content of the fuel.  I can see several ways to do that.  Of course I still recommend using dry fuels.  I heat partly with wood - but in Colorado, we don’t worry much about moisture content.   Mostly we’d welcome higher humidity.

3.  I don’t recall that this list has discussed the measurement of oxygen in the TLUD exhaust stream.  There are relatively low cost commercial hand-held sensors used by car buffs.  See http://www.carcraft.com/techarticles/116_0402_innovate_air_fuel_ratio_meter/  (found by googling for hot gas oxygen sensor)

4.  I think we can get by with something much cheaper, using just the sensor (the vast majority are made by Bosch - the original patent holder for these “passive fuel cell current generators”).  Anyone reading this up on these Bosch (or other) units?  (Some only need two wires - and presumably a cheap volt/amp/ohm meter.)  I see costs near $25 on E-Bay.

5.  I have not yet done anything personally beyond holding one that needed replacing.  Local garages throw them away regularly and some might still be usable for our purposes.

Ron


On Aug 16, 2014, at 12:16 AM, Crispin Pemberton-Pigott <crispinpigott at outlook.com> wrote:

> Dear Ron etc
> 
> If the discussion goes straight to theory it will be worth considering the effect of water vapour in the gas. 
> 
> I have been battling with the proper calculation of emissions and excess air for a while. What had happened so far in industry is the acceptance of 'good enough to for regulation' methods and the real answers are not available. The standard calculation of EA is fine for fuels that do not contain Oxygen bit biomass contains a lot: ~40%. The question then arises as to whether or not we are trying to get the effective EA as if there were gases with the Oxygen or if we want the actual air added to the fuel oxygen at the time. There are two different measures there. I have opted for the calculation of needed and available oxygen whether it came from the fuel or not. 
> 
> For gasifiers this matters a lot more than for other devices because the chances of have nearly no air's oxygen available is high, for the entire pyrolysation process. What reads as 'excess air' might just be oxygen from decomposing fuel. It matters that there is 'some' more than it matters where it came from. If one takes a 'carbon balance' approach to calculating EA and there is almost no carbon burning there is a problem. 
> 
> Similarly taking a volume based approach to what is happening in the fuel bed and ignoring the amount of water vapour involved leads to incorrect answers which leads to incorrect guidance and fudge factors. 
> 
> I have a chart of real time Sum-of-O2 in the gases that shows clearly the scale of the problem for your calculation of the (especially) upper section. I will post it when back on a computer. 
> 
> Regards 
> Crispin
> 
> 
> Crispin, list
> 
> I am part way through understanding the paper under discussion (and two other Finnish similar papers).  I now fear that the Figure 2  that I referenced was for a forced air situation.  No mention of viscosity - since their interest was mainly or only on the space below the pyrolysis front (termed a flame front).  So I am back to hoping for more weight-time history natural draft data from this list.  (on “B” and “C”).
> 
> There probably is considerable value still in understanding this work’s emphasis on theory.  Note that the radiative forcing for the movement (speed) of the pyrolysis front varies (at least approximately) as the cube (!) of the front’s temperature.  (and of course many other variables - especially moisture content.)
> 
> The viscosity data will require a temperature difference much larger than the 200 oC shown below.
> 
> I found a typo in the next to the last sentence Crispin quoted
> 
>> And (a question) stage 3 is the 95% of the time period…
> 
> should read 
>> And (a question) stage 2 is the 95% of the time period….
> 
> 
> More coming.
> 
> Ron
> 
> 
> On Aug 14, 2014, at 7:56 PM, Crispin Pemberton-Pigott <crispinpigott at outlook.com> wrote:
> 
>> Dear Ronal’n’All
>>  
>> Good find!
>>  
>> http://gekgasifier.pbworks.com/f/ignition+front+saasta.pdf
>>  
>>             b.  Because of my background, I think of the upward flow of primary air and pyrolysis gases as three resistances and a “current” (gas) generator.  The lowest chip/pellet region has a resistance RL  that continually gets smaller as its volume decreases.  The upper (char) region keeps gaining in height, but is losing weight rapidly as well;  for height (and other) reasons it (RU) presumably is increasing (at the end of a run, RL is zero).  The middle pyrolysis zone resistance would not seem to change much during a run.  And the “current” source also would not seem to change much during a run  (But maybe it does.)
>>  
>> I think that is a good description for the start of the conversation. The only think I would add is that the overall height changes with time because of shrinkage. It means the RU (region in the upper section that contains char) will not only be smaller than the original fuel it will also be evolving some CO2 and CO (by cracking CO2 from below).
>>  
>>             So my first (electrical analog) observation is that the only way that we can have a straight line is if the sum of RL and RU is a constant (call RC)
>>  
>> There is a reason why it should not be and that is because (assuming it is a natural draft device) the temperature of the vertical column varies with time giving an increase in draft as the colder RL (Region Lower) develops into a hot RU. The draft increase can be calculated using the Draft calculator available on this site using the section on the right. All things being equal, the draft will increase time, resulting in an slow increase in power which is what is observed using high frequency mass measurements (high in this case meaning per 10 seconds).
>>  
>>             Second - If I had to guess that the change in either would “exactly” balance the other, I would have said no way.  But for us, it is decidedly serendipitous/fortuitous.
>>             The reason for RU increasing must include viscosity changes.  
>>  
>> I have not investigated the influence of the temperature of the gases on the viscosity in an increasing temperature column. If the temperature in RL is 293°K and 493°K in RU then perhaps the increase in viscosity of the (different) gases will approximately balance. It seems you should put those two variables into your formula so you can investigate the overall effect of holding the char at a different temperature. Remember the height of RU as a % of total will change (as you describe) but also the ‘reference height’ will drop with time.
>>  
>> This doesn’t undermine your initial conceptual description.  
>>  
>> 1. Ignition at the top. It takes some time that the ignition front propagation reaches a steady velocity.
>>             [RWL:  Yes, but with controllable primary air, which most TLUDS allow, there can be large early primary and then a cut back.  And usually operated exactly that way.]
>>  
>> [CPP] Just a comment on the several failures to ignite TLUD’s in Ulaanbaatar this summer: there are some basic precautions to take. One is that the secondary air ports have to be closed to get a rapid ignition. We had several cases of people (a) having secondary air only (!) as the recommended lighting method, (b) leaving it open or not fully closed during ignition, (c) not providing a large dominance of primary air. A further refinement for ignition is that the kindling materials have to heat as much of the surface as possible as soon as possible. This is not readily accomplished by lighting a flat surface of fuel. In all cases the results of lighting at the bottom of a conical depression or a ‘vee’ pushed into the surface results in more rapid ignition of a larger surface per minute. Radiant heat from the side of the flames is a booster compared with trying to radiate heat downwards.
>> 
>> Also, if the air rate is too high, the burning is quenched due to cooling by the air so that the flame temperature goes down giving less radiation to preheat the non-ignited fuel and also keeping the non-ignited fuel cool.
>>             [RWL:  This is the first time I have seen this.  I can see a problem with a fan/blower, but also natural draft?
>> 
>> [CPP] We frequently see this when it is combined with the presence of secondary air. This season several stoves were ignited and ‘failed to thrive’ for more than an hour because of the combination. Fuel moisture (which can be as high as 33%) is a major issue in the early phase.
>> The velocity of the ignition front has a maxim at certain air rate. There is accumulation of char above the pyrolysis front. The maximum amount char is obtained with low primary air rate (but high enough to keep the front moving). If the air rate is high, also some char is burned above the pyrolysis front due to excess air especially if the fuel is moist.
>>             [RWL:  I am sure you can help us with fuel moisture issues.  Should we be “curing/drying” all fuel (maybe above the cookstove)?
>> 
>> [CPP] This is not going to fly In many cases, many places. It is better to have a good ignition method so once started, it dries the fuel continuously. There is already strong resistance at village level to trying to prepare fuel.
>> The excess oxygen that is not consumed in the pyrolysis front reacts with char giving less char. Even the ignition velocity is quite constant, the burning rate of the whole batch including the char may increase during the burning with high primary air rate, since the amount and thickness of char layer accumulating above the pyrolysis front is increasing and can react with excess oxygen. Then you would have a positive value for C in your formula (considering the whole weight loss of the batch), if there is much excess air.
>>             [RWL:  The design mod I have in mind requires small C, but keeping below a certain primary air flow rate should not be a major constraint.   By “excess air” in the last sentence,  I presume you mean excess primary air?  (we have been using “excess” with secondary air)
>>  
>> [CPP] I will add for Jaakko’s entertainment that we have been using the SeTAR HTP calculation of ‘excess air’ which is a chemically balanced calculation, not the usual
>>  
>> (O2-CO/2)/(21-(O2-CO/2))    [1]
>>  
>> We are using all the available molecules as measured, to consider the influence on the EA value of O2 present in the fuel. Performing a chemically balanced calculation provides a more realistic ‘EA equivalent’ telling us what is actually going on in the combustion and reaction zones. I can provide more details here if that is needed. It is in the lab manual which is on line at the WB site.
>> 
>> 3. In the end, the pyrolysis front reaches the bottom and this is may also be accounted by the term C*t^2 in your model.  In this stage the amount of pyrolysing particles at the bottom decrease leaving more excess air to react with the char.  It seems that sign of C depends on the air rate. With high air rate, the sign goes to more positive direction, since the rate of flame propagation is low in the stage 2, but the rate of char combustion becomes high at the stage 3.
>>             [RWL:   I need help with the terms “stage 2” and “stage 3”.   I think that with controllable primary air, that we can avoid the "high rate of char combustion” at the end of a run - assuming (as I do) that we want to maximize char production.   I presume stage 3 is this final stage as the pyrolysis front reaches the bottom.   And (a question) stage 3 is the 95% of the time period with a “constant” power level (constant rate of fuel conversion)?  So “stage 1” is the short start up period as the pyrolysis from moves.
>>  
>> [CPP]  Sort of related to the above: if the standard Excess Air is ‘x’ then a recalculated version including water vapour might provide some hints as to where to set the airflow (assuming you are not only finding it by trial and error).  You can maximise the char yield by dropping the system temperature. If you need more heat (Watts) just make it larger.
>>  
>> Regards
>> Crispin
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