[Stoves] Some Basic Properties of ND-TLUD Gasification

Mon Jul 13 15:03:56 CDT 2015

Dear Julien, Stovers, 

Some nice work with well defined curves.  Not easy to get with so many variables.  I think we can get a lot of info from this work and I am trying to understand what we can take away from it.  This is how I am looking at it:

Box 1; (of 6-Box system)

You have controlled the variables of biomass by picking two very different types and working exclusively with them.

Box 2; (of 6-Box system)

You are adding the biomass in a consistent manor for each run.  The sticks are placed vertical and the pellets uniformly placed by pouring in and falling into place(?). 

Box 3; (of 6-Box system)

The combustion chamber is the same for each run aside from the noted variable being tested. (ie of Grate Aperture).

Comment: Based on Tom Reeds work it appears from his graphs of charring different biomasses in N2 that there is little weight loss with increasing temperatures once the weight has stabilized at temperatures above 450c.  And the ash value for each of the two biomasses chosen should be the same.

Suggest: You fill some 1.5”dia. X 6” long iron pipe with oven dry (105c) biomass chosen.  Tighten one end cap and tighten the other but back turn a turn to let gases escape.  Then heat to > 450c until all gases have escaped. Let cool and calculate the percent char+ash remaining.  Then ash (>550c with air) and calculate the percent maximum good quality char that can be produced.

So, I’m thinking, you have the total time of combustion and the total dry-ash free (DAF) weight of fuel in the stove.  You can calculate the expected rate of weight loss during the run (with even heating) of volatiles released just due to heat and internal oxygen.  Anything less means incomplete pyrolysis occurs and greater means loss of biomass due to combustion with added outside air during that snap-shot in time.  Of course you could end up with a mix of both torrefied biomass and overly combusted char should the flame front not being even during the run.  So some char quality of material left may be necessary.  Just pack it tight into the pipe, Heat > 450c and see what weight loss you get from any torrefied wood left.

Suggestion: Years ago I did some experiments in the shrinkage of Tongue Depressors (TD) at increasing temperatures in a nitrogen atmosphere. That work is not available to me anymore but it could easily be reproduced. I think having a TD placed vertical in the center of the fuel or cutting the TDs into inch sections and distributing in with the fuel.  Then measuring the change in width after a run could provide info regarding the combination of: 1)  temperature reached 2) time of duration of combustion and 3) oxidation from primary air –combination.  I’m thinking these are the three variables that control the outcome from a known fuel.

And you are nicely seeing all this; change in weight loss and temperature based on a constant (pick any) aperture setting in your experiment.

Looking at Figure 2: Again some very nice data to work with.

I would like to see the X axis as Residual DAF percentages and have an added vertical line showing the maximum good quality char that can be produced.  That way you can a) rule out points showing incomplete combustion (you have done this but I think can be better) and show loss of weight from char that had been combusted using primary air.  

In this Figure 2 you have ventured off from controlling the Fuel (Box 1) by only using two types for the entire experiment to trying different fuels.  Your list of variables you found to be necessary for controlling Box 1 is as follows:

Moisture

Particle Shape

Particle Size

Bulk carbon density

Particle carbon density

Void space

Air resistance  (not sure how to measure)

I would add a few more like a) water soluble Sugars, b) solvent soluble oils and c) weight loss at 450c (no air-pipe).  The sugars and oils are only needed for fuels that have a lot like dried bananas, pine cones or food waste, sugar cane etc.  Not the fuels you have used, I’m thinking.  I would also add E450c for others monitoring energy. 

These are all easy tests and would complete the Box 1 fuels characterization. 

I would like to see a picture of your combustion chamber you are using.

Great report

Regards

Frank

franke at cruzio.com

Frank Shields
franke at cruzio.com

> 
> ---------------------------- Original Message ----------------------------
> Subject: [Stoves] Some Basic Properties of ND-TLUD Gasification -- pdf
> From:    "Julien Winter" <winter.julien at gmail.com>
> Date:    Sun, 12 July, 2015 8:34 pm
> To:      "Discussion of biomass cooking stoves"
> <stoves at lists.bioenergylists.org>
> --------------------------------------------------------------------------
> 
> Hello all;
> 
> I have attached a handout that I prepared for a recent demonstration of
> natural-draft, top-lit updraft gasifiers.  I think you may find it
> interesting.  The content of Figure 1. I have shared with this list before
> in some form or another.
> 
> Figure 2., however is new.  It shows the linear relationship between the
> yield of char as a % of dry fuel, and the maximum temperature measured in
> the TLUD reactor.  The purpose of the regression is to estimate what the
> maximum temperature could have been, if you don't have a thermocouple, but
> you do have an inexpensive banance.
> 
> The explanation for the linear relation is based on the how more biomass is
> pyrolyzed to volatiles rather than char, as the rate of heating and maximum
> pyrolysis temperature increases.  While this phenomenon has been know for a
> long time (e.g. the Broido-Shafizadeh model), the chemical mechanism for
> partitioning between volatiles and char still remains obscure.
> 
> However, I have two linear relations for the fuels that I used.  The fuels
> are distinguished by their size, or the size of the pore space between
> particles.  I think we have to remember that when thermocouples are
> inserted into a fuel bed, they measure the radiant, convective, and
> conducted heat in the pore space and not the actual temperature within the
> particles, where pyrolysis occurs.  With large particles of cm in
> thickness, the center of the particle can be at ambient 25Â°C, until heat
> from the outside arrives.  A pyrolytic front enters the particle from the
> outside as it heats up, so the major loss of volatiles will occur at a
> lower temperature than outside the particle in the pore space. When the
> pores are large, as between sticks, pore space is more concentrated in the
> fuel bed.  With larger pores, there is more space for flaming, and the
> thickness of the pyrolytic front increases with particle thickness (because
> it takes longer for pyrolysis to reach the interior of larger particles
> than smaller ones).  So, if flaming is less evenly distributed in fuel beds
> of large particles, the pore space temperature should be higher.  Following
> from the above, the reason why there are two regression lines could be
> largely due to differences in pore space temperatures, and not so much
> pyrolysis temperatures inside the particles.
> 
> Fuel moisture content didn't have a substantive effect on the Tmax vs.
> %Char relationship.  I did not see a large effect of moisture on the
> maximum temperature of flaming pyrolysis, and read a paper where there were
> similar finding. ... but I other studies to read.
> 
> Cheers,
> Julien.
> 
> -- 
> Julien Winter
> Cobourg, ON, CANADA
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