[Stoves] Saving the WBT

Sat Aug 17 18:54:32 CDT 2013

It is challenging to try to understand what happens in a char-making TLUD.
My exposure to stoves has been entirely limited to the work of Belonio,
both from a practical and theoretical side. On the theoretical side, the
following is what I have gleaned from Belonio with the help of a young
engineer from the University of Delft. I throw this out to the list with
great trepidation, since I have only been working on this reflection for
about a week.

Temperature is very important, and it is generated as C reacts with O2
giving rise to CO2 (initial combustion that supplies heat to the process).
The O2 is supplied from the primary air and from the H2O within the
biomass. The temperature has to be high enough to optimize the endothermic
reactions that take place within the process. The endothermic reactions are
the water gas reaction (C combines with H2O to form CO and H2) and the
Boudouard reaction (C combines with CO2 and to form CO).  If the
temperature is high enough, C will not combine with H2 to form methane. If
the temperature is high enough, there will be little tar and oil formation.
The goal is to create a high percentage of CO and H2.

Then there is the moisture content of the biomass. A moisture content of
10% is ideal. If there is too much water in the biomass, water is
transformed from a liquid to a gas within the process, and the process
temperature is lowered. Also if there is too much water, the water gas
shift reaction is favored giving rise to CO2 and H2. So if the moisture
content increases beyond what is optimal, there is less CO, more CO2 and
more H2O in the gas.

Then there is the amount of oxygen being supplied to the process. If too
much oxygen is supplied, the amount of CO and H2 decreases, and the amount
of CO2 and H2O increases. Excess oxygen burns up CO and H2 within the
reactor. This translates into a big inefficiency, since the heat generated
here is generally quite far away from the bottom of the pot. Part of the
oxygen comes from the water, and the rest from the primary flow of air. An
air equivalency ratio of 0.3 is ideal.

But air must be supplied uniformly up through through the biomass.
Channeling (too much air in some places and not enough in other places)
severely disrupts the entire process. In such a case, the concept of an
ideal air equivalency ratio becomes somewhat meaningless. Some people
design TLUD stoves that handle all types of biomass. But I only know of
about 4 or 5 types of biomass that are sufficiently uniform to be run
through a TLUD in their raw state. Everything else has to be prepared
(splitting, cutting, chipping or pelletizing) to be rendered sufficiently
uniform. Of all forms of preparation, pelletizing appears to be the best.

If rice hulls are processed at 1000 C, at an equivalency ratio of of 0.3
and at a moisture content of 10%, the gas content consists of 26.1% CO,
20.6% H2, 0% CH4, 6.6% CO2 and 8.6% H20 (numbers from Belonio). This adds
up to 61.9% of the total gas. The remainder is mostly N2.

The presence of CO2 and H2O in the gas gives rise to a dirty gas. In a
stove test, it would be interesting to measure the CO2 and H2O content of
the gas prior to combustion at the burner. If CO is intimately mixed with
CO2 and H2O, the combustion of CO at the burner is compromised.

When the gas is burned at the burner, heat is generated by the combustion
of CO and H2. Air is about 21% oxygen and 79% nitrogen, and it takes
considerably less oxygen to burn CO and H2 than other more complex forms of
gas such as methane, propane or butane. The molar ratio of air to gas to
burn the CO and H2 in the above proportions is roughly 1.11 mol/mol. The
mixing ratio of air to gas by volume is roughly 0.42 m3/m3. Also if the gas
prior to combustion has a temperature in excess of 500 C, this facilitates
the combustion of CO and H2. If anyone would like to see these
calculations, I will supply the spreadsheet off-list.

This might explain why the Belonio burner with the burner housing I added
to it functions reasonably well in spite of the fact that the premixing of
air and gas does not take place. So little secondary air is required, the
gas is hot, and the mixing takes place all along the periphery of the two
off-set rings of burner holes. As the gas exits the 80 burner holes, it
does so under mild pressure and sucks in air from the burner housing.
http://www.youtube.com/watch?v=84qDsbBO9p8

I have seen several rice hull gasifiers where gas exits through one large
burner hole in the middle of the burner. This produces a single flame with
a long diffusion tail, and the transfer of heat to the pot under such
conditions cannot be optimal.

So in conclusion, the process temperature within the reactor should be
higher than 700 C, the moisture content of the biomass should be less than
12%, the air equivalency ratio should be about 0.3, the biomass should be
sufficiently uniform, the temperature of the gas prior to combustion should
be in the range of about 500 C, the gas prior to combustion should contain
little CO2 and H2O, and the mixing of secondary air with gas should as
thorough as possible.

Thanks.
Paul Olivier

On Sun, Aug 18, 2013 at 12:19 AM, Ronal W. Larson <rongretlarson at comcast.net
> wrote:

>
> http://www.et.byu.edu/~tom/classes/733/ReadingMaterial/Jenkins-Baxter.pdf
>
> *"Stoichiometric air fuel ratios …………..for biomass they are 4 to 7,"*
>
> I have seen "6" a lot, and the inverse (fuel to air weights) would be 17%
>
>
> On Aug 17, 2013, at 5:49 AM, Alex English <english at kingston.net> wrote:
>
>
>   Ron, Paul,
> Below; Paul refers to 'equivalency ratio'. This would be the amount of
> primary (under fuel air)
>
>          *[RWL:  Alex,  thanks  _ I wasn't thinking this way.  For your
> moving grate design, this term  "under fuel air" makes sense.  But for
> TLUDs,   I believe the term "under" makes less sense, as all the O2 is used
> up at the pyrolysis front, regardless of its magnitude in volume per unit
> time.  Since it would seem that CO needs about half the oxygen as CO2
>  (except some O2 is coming from the biomass and we have to account for H2
> going to H2O), maybe a number near half  (meaning the 30% and 60% numbers
> below) makes sense.  Or, maybe Paul's definition of equivalency ratio
> includes excess air - not stoichiometric air.  Paul - do you have a cite we
> can go to?*
> *
> *
>
> divided by the theoretical amount of air (stoichiometric) for complete
> combustion of that fuel. Then he speaks of CO2, CO and H2 production and
> syngas quality and variable fuel moisture contents.  It would be nice to
> see data that would correlate to his instance #2. I have yet to see "Syn"
> gas composition measurements from a TLUD.  "process temperature might be
> below 500C" Where does this number come from?
>
>       *[RWL:  I am going to stay away from this, due to press of other
> business.  The above cite with Tom Miles as co-author might have some of
> this.   I think the 500 C term means at the pyrolysis front.  Would you go
> higher?*
> *
> *
>
> "A lot of CO is emitted by the stove"
> Here he refers to CO that fails to be combusted in the burner portion of a
> stove making it sound like it is a consequence of conditions that occur in
> the fuel bed. "Syn"gas quality does affect burner performance but burner
> parameters also affect stack CO emissions.
>      *[RWL:  Maybe,  but I think Paul is repeating what I heard often at
> the Stove Camp.    All the stoves burning char (not done in TLUDs usually)
> suffer from very high CO production.  (emphasis added below in Paul's
> comment).*
>
>
>
> Instance #3 seems plausible.
>        *[RWL:  Agreed.  but there should be a paper to see the details
> and definitions.]      *Whew -  this is a good topic - but I need
> something more to read.   Thanks to both Paul and Alex.      Ron
>
>
>
> Alex
>
>
>
>
>
>
>
>
>
> Paul writes;
>
> Ron,
>
>  One should look at a stove according to what it is designed to use as
> fuel. Let us look, for example, at stoves that process rice hulls.
>
> In a first instance, the stove might simply burn rice hulls. Here we are
> talking about direct combustion where an air equivalency ratio situates
> close to 1. Such a stove will produce a lot of CO2 and H2O as well as
> relatively high levels of CO. The fuel for such a stove is rice hulls.
>
> In a second instance, the air equivalency ratio might be 0.6, the process
> temperature might be below 500 C, the moisture of the biomass might be 20%
> or more, and too much secondary air might be applied to the combustion of a
> dirty syngas containing a lot of CO2 and H2O. Since the production of CO
> and H2 is suboptimal, it might make sense in this instance to burn the char
> in order to maximize the production of energy. *But unfortunately burning
> the char has serious problems: a lot of CO is emitted by the stove,* and
> heat is generated far below the pot. If the char is burned within this
> second stove, the fuel for such a stove is rice hulls.
>
>  In a third instance, the air equivalency ratio situates close to 0.3, the
> process temperature rises above 800 C, the moisture content of the biomass
> situates at 10%, and the supply of secondary air is kept low, but still
> adequate, to achieve total combustion of the syngas. Here the production of
> CO and H2 is optimized, the temperature of the syngas prior to combustion
> at the burner reaches as high as 500 C, and not too much secondary air is
> mixed in with the syngas. In this instance, up to 30% of the weight of the
> rice hulls would still remain as biochar. But it would make no sense to
> burn this biochar, since the production and combustion of the syngas were
> optimized.
>
>
>
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-- 
Paul A. Olivier PhD
26/5 Phu Dong Thien Vuong
Dalat
Vietnam

Louisiana telephone: 1-337-447-4124 (rings Vietnam)
Mobile: 090-694-1573 (in Vietnam)
Skype address: Xpolivier
http://www.esrla.com/
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