[Stoves] Request for help on TLUD operating data

[Ronal W. Larson]

Jaakko cc List

	This is hugely helpful. Thanks.

	Yours below here is the third from you today - but the other two are easier to respond to, so I am starting here and do those next.

	See inserts below.


On Aug 18, 2014, at 10:06 AM, Saastamoinen Jaakko <Jaakko.Saastamoinen at vtt.fi> wrote:

> Dear Ron,
>  
> my previous postings considered forced primary air condition. The air rate was kept constant with a fan. The velocity of the ignition front depends on the primary air rate also in the case of natural draught, but then the air rate is not given and it does not remain constant but depends on the balance equation
>  
> draught = pressure losses (including inlet to the  device, fuel bed and outlet)

	[RWL1:  I believe there is another reason for draught, that I hope you will comment on.  When the main pyrolysis gases CO and H2 react with secondary air just above the hot char bed, there is a reduction of the number of particles since

	2 CO + O2 = 2 CO2
	2 H2 + O2 = 2 H2O 
	
	In both cases 3 particles before the combustion (left side) turn into 2 particles after (right side) - with a consequent lowering of pressure, encouraging both primary and secondary air flows.  Of course, reduced by the presence of Nitrogen in both the primary and secondary air streams, but bolstered by significant non-nitrogen gas release at the pyrolysis front.  Is there a well-known “Law” to help put this additional helpful pressure drop into perspective?  Note this doesn’t happen with a methane flame where we have:	

	CH4 +2 O2 = CO2 + 2 H2O    (3 particles on each side of the equation)

>  
> A similar situation is when one heats a home with a stove. I have district heating but use sometimes also a masonry stove.  The draught  =  gravity * chimney height * density difference. The density difference  =density of air outdoors minus density of air (averagely )in the chimney. When one ignites the wood logs in a stove, the draught is initially quite low, because the chimney is cool. One can increase the draught by burning some paper in the chimney. One can decrease the pressure loss by opening the damper of the stove and the windows of the building. Then after some time the chimney gets hotter and draught becomes good and one can adjust proper air rate by the damper.

	[RWL2:  Understood.  I believe this happens quite quickly in a small cookstove of 30-40 cm height.  I do not believe TLUD operators have complained about insufficient draft.
>  
> In TLUD (with natural draught) the draught and the pressure losses (thickness of the non-ignited fuel bed and the thickness of the char layer) change with time (but they are equal according to the above equation). So the air rate changes with time.
	[RWL3:  My question on the linearity of the stove operation (after setting the primary air for a final time) involved the parameters A, B, and C - seemed to be saying that C is generally small - and the “air rate” doesn’t change very much.  That is, perhaps it is increasing viscosity effects (greater depth of char) offsetting the impact of decreasing fuel volume/depth.

> In principle it is possible to solve the air rate (or air velocity through the bed) from the above balance equation in dynamic conditions (as function of time), because the pressure loss is proportional  ~ air velocity ^2 or more precisely from a developed equation for a bed of particles ( Ergun’s equation, 1952).
	[RWL4:  This was easy to find via Wiki - and then other similar equations - including Darcy’s equation.  But none of these seem to handle the TLUD geometry and my issue of “C” normally being a small quantity (as experimentally observed).

> In practice it is difficult, because the situation is transient heating and the draught in TLUD is also changing (in analogy with initially cool chimney). 
	[RWL5:  Except it seems that the draught is pretty constant - perhaps for the “viscosity” reason given above.  I am not at all expecting a full balancing - but rather asking what conditions can we have (fuel, etc) that make the constant drought result possible (again for my reason of wanting to combine primary and secondary air control).
	
> It has been noticed that the pressure losses are higher for moist fuel bed due to drying so drying of fuel in sunshine before combustion is beneficial to get lower pressure losses.  In the case of TLUD, the hot char layer gives also some draught because the gas is hotter in this layer than ambient air and gas density is lower (than ambient)  but it causes also some additional pressure loss. Then, if no damper is applied along the burning, the air rate (and the burning rate) will evidently increase during the burning (thickness of cool bed decreases and draught increases) and the ignition front velocity changes.
	[RWL6:  Again my own personal testing experience is that the power level (weight loss per unit time) stays quite constant.  I am looking for cases where this is true.  Of course, stopping stove operation when the maximum amount of char has been achieved.

> The ignition front velocity may be low at first, then reach a maximum and then again could become lower if the air rate becomes high over the maximum situation (discussed in previous post). However, in this case quenching with too high air rate is not likely to take place, because if the temperature begins to drop, then also air velocity decreases. This balances the air rate to a certain level.     
	[RWL7:  Not quite sure what you are saying here - but the last sentence “ balancing” is what I am after - how much can there be?  How constant can the air flow be in a properly operating TLUD?  I am asking the whole list for experimental data on linear decrease of stove weight.
>  
> So in the case of natural draught, the construction of the device and especially how the draught is obtained, determines the operation. I use wood chips, pellets, small branches, wood blocks and pine needles in the  TLUD (developed by Tom Reed) which has a small blower. All these fuels burn well, but is difficult to use a broad range of fuel types in a natural draught device.
	[RWL8:  Yes,  Tom’s fan-powered TLUD was never intended to make char.  In his fan stove, char combustion is occurring simultaneously with char production because of the vigorous air flow - no separation into primary and secondary components, I believe.  When you say you have been cooking with a TLUD, is Tom’s the unit?

> The operation of TLUD with natural draught depends much on its construction and fuel type and it is difficult to develop any general theory or model unlike to a forced flow TLUD. It could be possible to simulate the operation numerically with theories of combustion and heat transfer (including combustion of fuel bed, transient heating of different parts of the device etc.), but the calculations would be device-dependent, quite cumbersome and probably not very accurate and reliable so that experiments would be required for a specific stove construction.
	[RWL9:  I agree.  But we need more theory than we have had and you seem most qualified to help in this regard.  Possibly also Prof. Horttanainen (cc’d) can chime in.  My main interest now is in how we can equate the loss of flow resistance below the pyrolysis from with the gain above it - so that total flow resistance stays relatively constant.  Seems that might be a suitable theoretical topic - that assumes relatively constant draft from other effects.
	If there are any simple measurements you know of to help establish a best excess (secondary) air quantity, that would be helpful.
	I am unaware of any natural draft stove where it seemed excess primary air was present. (Draft was never too much.)  Anyone able to cite experimental TLUD work with that character?

>  
> Some cheat device (fan) to replace the natural draught with a forced flow would be a major step to develop efficient cooking devices. Somebody should invent a cheap fan that uses for example gravity to rotate the fan like in a clock using heavy weights. If high enough air rate is reached, it is then easy to use a damper to control the air rate suitably and get efficient burning.
	[RWL10:  Paul Olivier (cc’d) is not a regular contributor on this list, but he has been using a small fan costing only a few $, with a nice speed controller.  Perhaps he can comment for us on fan use.  His regular fuel is rice husks - with a low energy density, so his units tend to be tall - not conducive to natural draft.  His is one of the few TLUDs with a cap and external supply of (non-fan-driven) secondary air hitting the exhaust gases after they exit through a hundred or more small holes.  His work builds on that of Prof. Alexis Belonio.  One Olivier presentation is at 
	http://www.slideshare.net/Jupiter276/paul-oliver-presentation-in-dalat

	In sum on fans -  I am certainly in favor of exploring them, but I also feel we might sell more char-making stoves if we can get acceptable operation without fans.

	Again, many thanks for your many helpful comments.  A bit more coming on an earlier message today.

Ron
>  
> Jaakko
>  
> From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of Ronal W. Larson
> Sent: 16. elokuuta 2014 8:16
> To: Discussion of biomass
> Subject: Re: [Stoves] Request for help on TLUD operating data
>  
> 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
> _______________________________________________
> Stoves mailing list
> 
> to Send a Message to the list, use the email address
> stoves at lists.bioenergylists.org
> 
> to UNSUBSCRIBE or Change your List Settings use the web page
> http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org
> 
> for more Biomass Cooking Stoves,  News and Information see our web site:
> http://stoves.bioenergylists.org/
>  
> _______________________________________________
> Stoves mailing list
> 
> to Send a Message to the list, use the email address
> stoves at lists.bioenergylists.org
> 
> to UNSUBSCRIBE or Change your List Settings use the web page
> http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org
> 
> for more Biomass Cooking Stoves,  News and Information see our web site:
> http://stoves.bioenergylists.org/

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20140818/e34c7d5a/attachment.html>