[Stoves] [biochar] Charcoal as space filler in TLUD reactors

[Saastamoinen Jaakko]

Dear all,

I once tried to burn a mixture of dry pine needles (which are quite reactive) and char particles in TLUD from wood chips (produced in TLUD). Dry pine needles alone burn nicely and cleanly giving high power but in a relative short time due to low density of the needles and the bed. My idea was to  lengthen the burning time of the batch, lower the power level and also try to burn some of the added char for cooking.

This test resulted in problems in the propagation of the ignition front downwards and great release of emissions (smog of volatiles not burned completely). The proportion of the char should have been lower, but I did not try again.

I have studied factors affecting the velocity of the ignition front in beds of wood particles. As Crispin, referring to Tom Reed, says air rate greatly affects the amount of char left. Fuel moisture also affects the amount of char, because it decreases the velocity of the ignition front. Longer time and more energy from the burning particles are required to heat up and dry moist particles to sustain ignition. Then more char is consumed. Thus both air velocity and the velocity of the ignition front affect the stoichiometry (oxygen-volatiles) in the ignition flame front and the amount of char left. One would expect that adding inert particles may have a similar effect as moisture:

-          the inert particles must also be heated but they are not contributing to the heat generation

-          in addition the inert particles form a partial radiation shield between ignited and non-ignited fuel reducing heat transfer
Both of these decrease the velocity of the ignition front. Then air velocity should be decreased to maintain the same volatiles - oxygen ratio in the ignition front but this may lead to extinction.

Heat is required to heat the mixture of fuel and inert particles and this heat comes from the ignited particles, which means that more char must be consumed due to inert material (compared to fuel only). There is also an energy penalty in using a mixture of fuel and inert particles. Inert particles are heated to high temperature and this heat is lost (and no char is produced). Of course, one could utilize the sensible heat in the hot solid product in some preheating or drying process, but it Is not very practical in small scale cooking.

So my prediction is that mixing fuel and char or some more inert material fuel will decrease the total yield of char. One must also find the proper range of applicable mixture ratio  for good operation (low emission). The effect of changing fuel to mixture (fuel+inert or char) on air rate is also affecting.

Regards

Jaakko

From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of Crispin Pemberton-Pigott
Sent: 6. toukokuuta 2013 19:21
To: 'Discussion of biomass cooking stoves'
Subject: Re: [Stoves] [biochar] Charcoal as space filler in TLUD reactors

Dear Friends

Please consider a lesson learned long ago from Tom Reed which is that if you regulate the superficial air velocity in a stove you can burn the carbon completely, partly or not at all.

There may be many people who wish to burn the char chips they have lying (literally) around the kitchen by placing them into the fuel in a TLUD or other burner. A TLUD stove that accepts chopped wood (pieces, not chips) can function much better with the charcoal chips dropped into the spaces between the pieces of wood. It is worth a try. We cannot rule out whole classes of combustor because of someone's personal opinion of what other people ought to do.

Regards
Crispin



Dear Paul

Thanks very much for your detailed explanation.

Fundamentally, a "Perfect" TLUD will work perfectly if it has perfect fuel, perfect, required air volume, and perfect fan pressure (or stack vacuum) to deliver the perfect air flow.

Given that you have "off-standard" fuel, with greater average void space diameter, the pressure loss across the fuel bed will be lower. The fan or stack will thus deliver more air through the bed. Thus, you will get excess primary air flow, and if your secondary air porting was designed for a bed with a greater pressure drop, you will get less secondary air delivery; this will mess up your intended Secondary/Primary air ratio.

You can burn virtually any fuel in a TLUd, as long as it is uniform, and the stove was designed to handle it. Change the fuel significantly, and you need to change the design, to maintain the desired SA/PA ratio. If you "change the fuel", but don't change the design, then the only way to restore the system to "good operation" is to "modify the fuel" to one having similar a pressure drop across the bed, similar to one for which the system was designed. This is what you are effectively trying to do.

Assume, for example, that you have 4" long pieces of straw as fuel. This will likely give you all the problems you note. Adding char can, in theory, help increase the bed pressure drop. However, I am guessing that it will be a real stinker to get the char particles distributed through the bed. There will likely be "too much" in one area, and "too little" in another. Channelling is thus very likely. What you will probably end up with is a fuel bed having non-uniform flow properties, because of the large differences between straw properties and char properties. Try mixing various percentages of char with the straw, in a mixing bucket, then try to take "mixed fuel" from the "mixing bucket" and place it in the stove. I am guessing that you should see the non-uniformity of the fuel bed even before you ignite it.

My guess is that your best bet would be to chop the straw, so that it is free-flowing. This alone will make life very much easier for the operator, and will enable a greater weight of fuel to be added to the stove. Longer burn times between re-fuelling. Play with the air flow, simply by obstructing the fan intake with a piece of paper or cardboard. Then see if you can get your usual good combustion. If not, then consider re-drilling or partially plugging, the SA air holes to get back to the correct PA/SA ratio for good combustion.

Is chopping the straw (or whatever the fuel is) an option you can consider?

Best wishes,

Kevin

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