[Stoves] Saving the WBT
Paul Olivier
paul.olivier at esrla.com
Sun Aug 18 19:18:28 CDT 2013
Ron,
You write:
*[RWL2: I hope that they will jump into this dialog if they have seen
anything on this ratio in tests or literature. I am still unsure what
"equivalency" means, does it include excess air concepts?*
The process needs oxygen to fuel the initial combustion reaction (C + O2 =
CO2) that provides heat to the process. But only a fraction of the oxygen
needed to completely combust the biomass should be supplied. With an air
equivalency ratio of 1, there is enough oxygen to completely combust the
biomass.
You write:
*[RWL3: This is what most TLUD users live with (and same for rockets). A
tall diffusion flame*
But a tall diffusion flame is so inefficient.
Your write:
*RWL4. This was point I was making- because only you and Belonio are doing
this (a top surface with lots of small holes - that seems to work very
well), I think.*
Yes, there is a top surface with a lot of small holes, and all of these
hole are situated along the periphery of the burner. Belonio followed the
logic of many gas burners where there are rings of burner holes. But
Belonio did not premix air with gas before the gas exits the burner holes.
So the burner I inherited from him produced a long diffusion tail. When I
added a burner housing to the Belonio burner, this long diffusion tail
disappeared, and all flames stay nicely under the pot. The burner housing
forces air along a horizontal plane to mix with 80 vertical jets of gas. I
might venture to guess that each jet draw or sucks air according to the
velocity of the exiting gas. Instead of one big hole in the middle of the
burner, there are 80 little holes, each with access to secondary air, as we
see here: http://www.youtube.com/watch?v=84qDsbBO9p8
Without the burner holes I would be completely lost in trying to spot the
presence of CO2 in the gas. Whenever I see just a few burner holes that do
not support a flame, I know that too much CO2 is being produced at some
point within the reactor, usually as a result of channeling. I can look
through the burner holes that do not support a flame and see open flames
within the reactor. If CO and H2 burn within the reactor, this is highly
inefficient. They should be burning as close as possible to the pot.
When channeling occurs, it becomes almost meaningless to talk about the
importance of an air equivalency ratio at about 0.3. If at one point within
the reactor, there is an air equivalency ratio of 1, and if at another
point, there is an air equivalency ratio of 0.1, then the entire process is
totally derailed. Channeling occurs the moment the biomass is not
sufficiently uniform or not uniformly situated within the reactor. Since
air takes the easiest path through the biomass, we have to make sure that
there are no easy paths, either by working with biomass such as rice hulls
and coffee husks that are uniform, or by preparing the biomass through
cutting, chipping or pelleting. Working with chunks of wood of all sizes is
so complicated. Here the flow of air cannot be uniform, and burning embers
can break loose and fall below the main process front, and create multiple
process fronts. The process front within the reactor has to be as thin as
possible, and it has to descend in a uniform manner.
If the air equivalency ratio is not right, then the TLUD should not be
understood as a gasifier or a pyrolizer. Rather it should be understood, at
best, as an apparatus where the direct combustion of biomass has been
optimized. Yes, it might leave some char behind, but this could be
understood as an inefficiency in the combustion process.
You write:
*[RWL5. They are "happy" with the flame as is. For one thing, the
"chimney" they are using provides draft that you (with a fan) don't need.*
We should not be "happy" with open flames within the reactor. This puts
heat too far away from the pot. If we rely on a chimney to provide draft,
then this chimney has to be of a certain height to have an effect, and if
there are open flames within this high chimney, heat is being generated too
far away from the pot. If the chimney would not have open flames, and if
not too much heat would be lost through the sides of the chimney, then the
chimney concept would be OK.
You write:
*[RWL: I look forward to hearing (actually I did hear - and not so good,
for reasons we have discussed - which I will let you report). *
Here you are referring to my attempt yesterday at making a premix burner.
The result in the beginning was not so bad:
https://www.youtube.com/watch?v=ZIzphuZA9cg
But as the burn proceeded, the rice hull char began to offer more
resistance to the flow of air through the reactor (as is always the case
with rice hull biochar). This caused an increase in the amount of air being
premixed with the gas, and most of the gas combusted right below the burner
holes:
https://www.youtube.com/watch?v=P1NEC7T8tdU
Once again, gas has to get combusted as close as possible to the pot. There
are many ways to reduce the flow of secondary air to the burner as the burn
proceeds. I am afraid to go in the direction of air entrainment, since the
gas has to be put under a lot more pressure, and this might cause gas to
leak out at the point where the burner rests on the reactor.
Many thanks.
Paul Olivier
On Mon, Aug 19, 2013 at 1:57 AM, Ronal W. Larson
<rongretlarson at comcast.net>wrote:
> Paul: Your and my last were also to the stove list, so I re-insert the
> list
>
>
> On Aug 17, 2013, at 8:06 PM, Paul Olivier <paul.olivier at esrla.com> wrote:
>
> See comments below.
>
>
> On Sun, Aug 18, 2013 at 8:03 AM, Ronal W. Larson <
> rongretlarson at comcast.net> wrote:
>
>> Paul and List:
>>
>> Three comments/questions:
>>
>> 1. The gas analysis from Belonio was apparently at 1000C in the hot
>> char, but you believe you are closer to 500 C?
>>
>
> This 1000 C is not the temperature of the gas that exits the reactor. It
> is the process temperature in the zone where C combines with O2 to form
> CO2. This reaction supplies the heat for the endothermic reactions that
> follow. These endothermic reactions cool down the gas as it exits the
> reactor. The temperature of the gas as it exits the reactor (and prior to
> combustion) reaches as high 500 C. This we were able to measure.
>
> *{RWL1: I am comfortable with the 500 C number, but I remain
> concerned that 1000 C may be too high for the (or most) pyrolysis from max
> temps. Numbers I recall seeing with thermocouple readings have never been
> that high.*
>
>
>> 2. Is there any way to know what the air equivalency ratio is as you
>> are operating? even if you are above or below the optimum (of 0.3)? I
>> guess this is determined by the CO measurements, but I haven't seen any
>> data for either TLUDs or rockets on that.
>>
>
> If too much oxygen is supplied to the process as is the case of an air
> equivalency ratio of 0.6, the amount of carbon monoxide produced can drop
> by over 50% and the amount of H2 produced can drop by almost 60%. The
> reactor will heat up to dangerously high levels. We can try to correlate
> the specific rate of solids consumption to the flow of primary air to
> determine the equivalency ratio. But I have never done this.
>
> *[RWL2: I hope that they will jump into this dialog if they
> have seen anything on this ratio in tests or literature. I am still unsure
> what "equivalency" means, does it include excess air concepts?*
>
>
>
>>
>> 3. Some reading this exchange may not realize that you light the
>> pyrolysis gases before adding the burner assembly,
>>
>
> Before adding the burner assembly, I do not light the gases. I am merely
> lighting the biomass. When the biomass lights, a large flame rises out of
> the reactor.
>
> *[RWL3: This is what most TLUD users live with (and same for
> rockets). A tall diffusion flame*
>
> When the burner is placed on the reactor, this large flame within the
> reactor must go out. There should never be open flames within the reactor,
> otherwise I get burner holes that do not support a flame.
>
>
>> then you drop the fan speed to extinguish the interior burning and can
>> then relight the 80 flame lets.
>>
>
> Yes. At this point, I do not "relight" but "light" the burner holes for
> the first time.
>
> *[RWL4. This was point I was making- because only you and Belonio
> are doing this (a top surface with lots of small holes - that seems to
> work very well), I think.*
>
>
>
>> Other than Belonio, I don't know anyone else doing this.
>>
>
> I am totally confused. How then do they get the open flames within the
> reactor to go out?
> *[RWL5. They are "happy" with the flame as is. For one thing, the
> "chimney" they are using provides draft that you (with a fan) don't need.*
>
>
>> In your final sentence, people may not realize that your flamelets are
>> still diffusion type, not premixed. I know no-one getting premixed
>> flames, either rockets or TLUDs.
>>
>
> This morning I will test a 250 unit with a secondary air pipe that runs
> from the reactor grate at the bottom of the reactor into the burner at the
> top. This pipe is situated fully inside the reactor. This burner has 50%
> more burner holes than a normal burner to account for the added flow. If
> this works, this will be true premix burner.
>
> *[RWL: I look forward to hearing (actually I did hear - and not
> so good, for reasons we have discussed - which I will let you report).
> Ron*
>
>
> Thanks.
> Paul
>
>>
>>
>> On Aug 17, 2013, at 5:54 PM, Paul Olivier <paul.olivier at esrla.com> wrote:
>>
>> 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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>>
>
>
> --
> 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/
>
>
>
--
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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