[Stoves] Saving the WBT
Marc-Antoine Pare
marcpare0 at gmail.com
Sun Aug 18 02:01:41 CDT 2013
Tom, the rice husk gas composition numbers are actually a wide range in
Belonio's handbook, as you suggested. The Belonio tabled are pulled from
Kaupp (1984), which are tables for gasifiers, not for TLUDs. As far as I
know, there isn't any TLUD rice husk gas composition data anywhere.
Tables copy-pasted below.
Table 8. Types and Percentage Composition of Gases Produced from the
Gasification of Rice Husk Gasifier at 1000 °C Temperature and at 0.3
Equivalence Ratio.
Gas
% Composition *
Carbon Monoxide, CO
26.1 – 15.0
Hydrogen, H2
20.6 – 21.2
Methane, CH4
0
Carbon Dioxide, C02
6.6 – 10.3
Water, H2O
8.6 – 24.0
* Rice Husk Moisture Content = 10 to 40%
Table 9. Composition of Gases Produced from Rice husk Gasifier at 1000 oC
Temperature and at Rice Husk Moisture Content of 30%.
Gas
% Composition *
Carbon Monoxide, CO
18.6 – 8.6
Hydrogen, H2
21.5 – 8.7
Methane, CH4
0
Carbon Dioxide, C02
9.5 – 12.6
Water, H2O
18.0 – 21.1
* Equivalence Ratio = 0.3 to 0.6
marc
notwandering.com
On Sat, Aug 17, 2013 at 6:03 PM, 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?
>
> 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.
>
> 3. Some reading this exchange may not realize that you light the
> pyrolysis gases before adding the burner assembly, then you drop the fan
> speed to extinguish the interior burning and can then relight the 80 flame
> lets. Other than Belonio, I don't know anyone else doing this. 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.
>
>
> 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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