[Stoves] Saving the WBT
Ronal W. Larson
rongretlarson at comcast.net
Sat Aug 17 20:03:54 CDT 2013
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
>>
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>> 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
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