[Stoves] Thick wood and MC in micro-gasifiers was Re: Smoke-free biomass pellet fueled stove
Saastamoinen Jaakko
Jaakko.Saastamoinen at vtt.fi
Wed Nov 14 02:49:21 CST 2012
Dear Crispin,
the energy from char combustion goes partly to evaporate the moisture by radiation down and partly to heat the gas and it is obtained as usable energy for cooking. The velocity of the propagation of the ignition is slower with high moisture and the available time for cooking becomes then longer. However, above some limiting moisture, the ignition front does not propagate at all. So there seems to be some optimum moisture giving maximum total amount energy for cooking, if no value is given for the residual char (and one is not interested in the heating power but only in total energy).
Water gas shift reaction takes palace in gas CO+H2O = CO2+H2 and it does not have effect on char. H2 is oxidized to H2O rapidly if O2 is present.
However char is oxidised by gasification reactions H2O + C = H2 + CO and CO2 +C = CO. The rate of these is much lower than reaction with oxygen, but however at about temperatures about 850 oC they become influential. The reactions are endothermic, which adjusts the extent of gasification (high gasification rate results in low temperature decreasing the gasification rate). There is always some H2O present in the gas from ignition front since wood contains hydrogen and combustion air also contains some H2O. Char is also consumed by CO2 produced in the ignition front. So gasification of char takes place also for combustion products from pyrolysis of dry wood.
So the main reason for "missing char" for moist wood as Tom Reed and AJH have explained is that in addition to the part going down to dry the fuel, a considerable part is going up (and obtained as usable energy) and the cooking time is longer.
Jaakko Saastamoinen
----Original Message-----
From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of Crispin Pemberton-Pigott
Sent: 12. marraskuuta 2012 19:23
To: Stoves
Subject: Re: [Stoves] Thick wood and MC in micro-gasifiers was Re: Smoke-free biomass pellet fueled stove
Dear Dr Tom
That was a great account of a good experiment. So my main point is that as the energy needed to remove the moisture is so much less than the 'missing char' why does the car disappear in (apparently direct) proportion to the moisture?
Thanks
Crispin
PS Prof Henrick Wallman also says there is no water gas shift reaction going on but I remain unconvinced (though willing to be) because I find way too much H2 in the exhaust. I have measured 15,000 ppm H2(EF).
-----Original Message-----
From: Tom Reed <tombreed2010 at gmail.com>
Sender: "Stoves" <stoves-bounces at lists.bioenergylists.org>
Date: Mon, 12 Nov 2012 11:40:24
To: Paul Anderson<psanders at ilstu.edu>
Reply-To: Discussion of biomass cooking stoves
<stoves at lists.bioenergylists.org>
Cc: ajheggie at gmail.com<ajheggie at gmail.com>; morgan.defoort at colostate.edu<morgan.defoort at colostate.edu>; Hugh McLaughlin<wastemin1 at verizon.net>; Bob Fairchild<solarbobky at yahoo.com>; Discussion of biomass cooking stoves<stoves at lists.bioenergylists.org>
Subject: Re: [Stoves] Thick wood and MC in micro-gasifiers was Re:
Smoke-free biomass pellet fueled stove
Dear Crispin, Paul and Aul:
The water-gas reaction
C + H2O ===> CO + H2
DH. 420. 280. 280.
DH (delta H, but my I pad has no delta symbol), the heat produced by the reaction is -(560-420) =. -140 kJ/(12 g of C). In other words you would have to add 140 g of heat (plus enough heat to reach reaction temperature) to make the reaction go.
<><><>
My memory is that I dried wood chips in a 230 F oven for several hours, then prepared 7 bags of equal weight chips (100 g ?) and added enough water to each bag to bring total weight to 5,10,15, 20, 25, 30 % moisture, and then let the bags sit for a few days to equilibrate the water and chips. I then burned each sample until only charcoal remained (flame color changed fro yellow to blue) in our WoodGas battery blown stove.
I was delighted to find 30,25,20,15,10,5 and 1% charcoal remaining. From this it seemed obvious that each lowest layer of charcoal, in order to ignite the subsequent layer of chips below it, had to be partially burned in order to remove the water from the next layer of wood chips in order to ignite it. Dry wood burns; wet wood won't.
Since that experiment in my Denver lab, I have moved twice and would have difficulty finding my original notebook. But the experiments made so much sense, that I am sure the above description is in essence correct.
Onward...
Tom Reed
Thomas B Reed
280 Hardwick Rd
Barre, MA 01005
508 353 7841
On Nov 12, 2012, at 9:10 AM, Paul Anderson <psanders at ilstu.edu> wrote:
> Dear Andrew, with copies to Tom and Morgan
>
> I am sending a copy of this reply to Tom Reed <tombreed2010 at gmail.com> . This is the only email address he now uses, in case anyone else wants to note it down.
>
> Tom did those experiments with known moisture a few years ago. Perhaps he still has the notes. I think it was not prepared as a formal report. I believe he said it was nearly linear, with about 0% char when MC reached 30%.
>
> 150mm diameter is beyond anything I have ever tried. I marvel when the 70 mm branches pyrolyzed all the way through in the experiments done by my Ugandan associates. There is a photo of the loaded fuel in the Quad TLUD in the report of test results by CREEC on the Quad stove (see the report at www.drtlud.com ). Thickness (diameter) increases the length of time of the operation of the stove (quite logically). Jackson, the CREEC technician, did a great job with having 3 different sizes of the same wood in the three different runs. But we do not have funding for running repeated tests to get sufficient replications.
>
> The issues include being too thick so that the pyrolysis gases are too slow / too little to sustain a good flame at the upper combustion part of the TLUD.
>
> I hope that CSU can put that into their computer model. And I hope that CSU will write something to the Stove Listserv or tell us where they are writing something. So far they hold the biggest pot of funding for research about micro-gasifiers, except for the big companies like Philips (and what BP did in the past).
>
> Paul
>
> Paul S. Anderson, PhD aka "Dr TLUD"
> Email: psanders at ilstu.edu Skype: paultlud Phone: +1-309-452-7072
> Website: www.drtlud.com
>
> On 11/12/2012 5:53 AM, ajheggie at gmail.com wrote:
>> [Default] On Sun, 11 Nov 2012 20:01:02 -0600,Paul Anderson
>> <psanders at ilstu.edu> wrote:
>>
>>> Crispin and all,
>>>
>>> I think that the char being created in a TLUD is not hot enough to
>>> cause the water gas reaction.
>> I take this view too, and even if it were hot enough the endothermy
>> of the reaction would limit it.
>>
>> If this phenomena is important to us, e.g we want to maximise char
>> retention, then it's worth knowing the optimum fuel moisture content.
>> Also I had found there was a limit to the size of particle that could
>> be thoroughly charred. I found I could not successfully scale up a
>> tlud device to chare 150mm cross section longs which I hade carefully
>> dried. What I never got round to trying, as I lost the farm workshop
>> due to redevelopment, was seeing if such logs would char if embedded
>> in smaller material.
>>
>> I no longer have a useful tlud device that will consistently produce
>> measurable results but it may be worth doing some runs with oven
>> dried wood chips which have been then re wetted with the correct
>> amount of water to give samples of 0-25%mc wwb and weigh the char
>> produced when lit with the same amount of liquid fuel. I would
>> especially like to see the results when fed with primary air
>> maintained at a constant flow.
>>
>> AJH
>>
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