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 <[email protected]> wrote: > See comments below. > > > On Sun, Aug 18, 2013 at 8:03 AM, Ronal W. Larson <[email protected]> > 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 <[email protected]> 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 >> <[email protected]> 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 <[email protected]> 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. >>> >>>> >>> >>> _______________________________________________ >>> Stoves mailing list >>> >>> to Send a Message to the list, use the email address >>> [email protected] >>> >>> to UNSUBSCRIBE or Change your List Settings use the web page >>> http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org >>> >>> for more Biomass Cooking Stoves, News and Information see our web site: >>> http://stoves.bioenergylists.org/ >>> >> >> >> _______________________________________________ >> Stoves mailing list >> >> to Send a Message to the list, use the email address >> [email protected] >> >> to UNSUBSCRIBE or Change your List Settings use the web page >> http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org >> >> for more Biomass Cooking Stoves, News and Information see our web site: >> http://stoves.bioenergylists.org/ >> >> >> >> >> >> -- >> 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/ >> _______________________________________________ >> Stoves mailing list >> >> to Send a Message to the list, use the email address >> [email protected] >> >> to UNSUBSCRIBE or Change your List Settings use the web page >> http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org >> >> for more Biomass Cooking Stoves, News and Information see our web site: >> http://stoves.bioenergylists.org/ >> > > > > > -- > 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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