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Re: [WoodGas] Re: Why nozzles go dim with preheat

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  • sabbadess@aol.com
    David, You are correct but it s a pure cost reason not desire. I just didn t have money for more steel when I built my last rig. Everything I am doing now
    Message 1 of 20 , Mar 1, 2011
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      David,

      You are correct but it's a pure cost reason not desire. I just didn't have money for more steel when I built my last rig. Everything I am doing now has agressivve preheat to the centerfeed. The upper nozzles are just for burning the fuel away from the sides to keep the fuel flowing. In my mind the only thing that matters is that the tar gets burned before it goes through the restriction and the resulting gas is reasonably reduced.

      Stephen
      -----Original Message-----
      From: David Siedschlag <pellets4fuel@...>
      To: WoodGas <WoodGas@yahoogroups.com>
      Sent: Mon, Feb 28, 2011 11:06 pm
      Subject: RE: [WoodGas] Re: Why nozzles go dim with preheat




      Thanks for the encouragement Stephen. In looking at your unit it doesn’t look like you incorporate preheating, on less I am thinking of someone elses.

      David

      From: WoodGas@yahoogroups.com [mailto:WoodGas@yahoogroups.com] On Behalf Of sabbadess@...
      Sent: Monday, February 28, 2011 9:23 PM
      To: WoodGas@yahoogroups.com
      Subject: Re: [WoodGas] Re: Why nozzles go dim with preheat

      David,

      Don't get frazzled ... Just build. This arguement has gone on for years and will likely go on for years more. Both ways work. You aren't going to kill your gasifier one way or the other. Greg likes cold to warm air. I like smoking hot. Everyone else can speak for themself. We actually had a poll going around about it. Until we get some numbers on a specific design the debate will likely rage in the background.

      Stephen
      -----Original Message-----
      From: David Siedschlag <pellets4fuel@... <mailto:pellets4fuel%40yahoo.com>; >
      To: WoodGas <WoodGas@yahoogroups.com <mailto:WoodGas%40yahoogroups.com>; >
      Sent: Mon, Feb 28, 2011 8:38 pm
      Subject: RE: [WoodGas] Re: Why nozzles go dim with preheat

      So guys,

      Which is it preheat or not, and simply put why or why not. I talked with GM already and I know what he believes, JM in the gek throws as much heat at everything as he can it seems. I have a whole gasifier under construction with air preheat built in, and it makes me sick to have to think I welded together just a cool looking sculpture. Drop all the chemical equations, and parts of this and that to this or that, and say it how it is so a simple guy like me can understand. I understand when stuff gets hot it expands, and cold again it goes back. So if you preheat air, it expands, or takes up more volume, and then has less ox per that volume, and now your not getting enough of those ox into where you need it?

      And maybe you can’t explain it simple, and because I am just mechanically inclined, not chemically I just won’t get it and maybe shouldn’t be even building a gasifier. !$#@%#@$ Always did like books with lots of pictures more than books with lots of words. And the only reason why I passed high school chemistry was because the teacher was an old WWII vet and he spent more time telling stories about the Germans than he did teaching.

      Hey and I ain’t rippen on anyone, just getting quite frazzled!

      David

      From: WoodGas@yahoogroups.com <mailto:WoodGas%40yahoogroups.com>; [mailto:WoodGas@yahoogroups.com <mailto:WoodGas%40yahoogroups.com>; ] On Behalf Of Greg Manning
      Sent: Monday, February 28, 2011 6:46 PM
      To: WoodGas@yahoogroups.com <mailto:WoodGas%40yahoogroups.com>;
      Subject: RE: [WoodGas] Re: Why nozzles go dim with preheat

      Now I'm missing something.

      Reduction in a gasifier (in my neck of the woods) is chemical reduction, NOT physical.

      What physical reduction are you talking about Stephen ??

      1) Bunker (drying)

      2) Oxidation (tar burning, char making, small portion driven to ash in the process, IN the presence of an oxidizer)

      3) Reduction, Thermalitic devolution of charcoal into elements, NOT in the presence of an oxidizer.

      4) Gas evolution from those elements, small amount of spent char and ash as solids (turds if you will) in the grand scheme of things.

      including re-versions and bond-in's and out's

      Granted, 3 & 4 happen in 2, BUT we hope 2 does NOT happen in 3 or 4.

      OOT (one other thing), Gas contents, H2 in the area of 18% is good, 22% is better, CH4 in 4 % is good 8% is better. CO in 20%, 18% is better. Rest in "others".

      Greg

      -----Original Message-----
      From: WoodGas@yahoogroups.com <mailto:WoodGas%40yahoogroups.com>; <mailto:WoodGas%40yahoogroups.com>;; [mailto:WoodGas@yahoogroups.com <mailto:WoodGas%40yahoogroups.com>; <mailto:WoodGas%40yahoogroups.com>;; ]On Behalf
      Of sabbadess@... <mailto:sabbadess%40aol.com>; <mailto:sabbadess%40aol.com>;;
      Sent: Monday, February 28, 2011 5:31 PM
      To: WoodGas@yahoogroups.com <mailto:WoodGas%40yahoogroups.com>; <mailto:WoodGas%40yahoogroups.com>;;
      Subject: Re: [WoodGas] Re: Why nozzles go dim with preheat

      Hi Tom,

      Like I know what I'm talking about?!?! OK, I pretend to. The big take away message is 22.4L/hr out means a lot less than 22.4L/hr went in. Even less goes in the more you preheat because of the thoroughness of the reduction and expansion to CO. Weird but that's what the numbers say. Also the less that goes in, the less combustion there is and the dimmer the nozzles appear. The ratio of each chemical component will change with the pull rate.

      The thermal equilibrium driving of (4) has to happen or eventually the gasifier would either be a molten puddle on the floor or the hearth would lose temperature to the point that reduction wouldn't occur and it would just be a combustor. The rig always gains or loses temperature until it achieves a temperature balance.

      A rodent(mol) is ALWAYS 6.02X10^23 molecules or atoms of stuff regardless of thermal expansion. I used "standard temperature and pressure" just to compare everything uniformly. Once the gas goes through the cooler it's not a bad assumption. It will however, follow the ideal gas laws as the temperature goes up. The 6.02X10^23 molecules will take up more space.

      If somebody finds something I did wrong and can correct me, so much the better for all of us.

      Actually this is really just trivia because as Greg and Jay have pointed out, preheating does other stuff in the gasifier at the machine design level which drives other changes like thickening of the pyrolysis zone.

      Stephen
      -----Original Message-----
      From: TomC <ginfizz20@... <mailto:ginfizz20%40yahoo.com>; <mailto:ginfizz20%40yahoo.com>;; >
      To: WoodGas <WoodGas@yahoogroups.com <mailto:WoodGas%40yahoogroups.com>; <mailto:WoodGas%40yahoogroups.com>;; >
      Sent: Mon, Feb 28, 2011 6:03 pm
      Subject: [WoodGas] Re: Why nozzles go dim with preheat

      Stephen; Feel free to ignore my remarks because I know nothing of which I speak. I have tried to follow your premise, but I start off with one problem. We are pulling the gases through the gasifier at a constant rate, no matter the source. So if the source is 22.4 L/hr will it be 22.4L of CO or CO2 or 11.2L/hr of each. Now if we introduce H2O vapor will we get eight times 22.4L/hr or because of the constant vacuum will we get 2.24L/hr of CO; 2.24L/hr of CO2; and 17.92L/hr for a total of the constant 22.4L/hr.
      I did not understand the "throttle back". Under ideal conditions adding (1)&(3)and equaling about 4x(4)would be an equilibrium situation. Is it because under a 22.4L/hr pull, the pull on the CO and CO2 is reduced so dramatically with the introduction of the H2O vapor?
      In the formula I did not see any mention of the temperature of the incoming O2 other than you said a rodent (opps, I mean mol)is under standard conditions. How does the formula change if the mol is not at "standard" conditions. Using preheated air the mol would be less than 22.4L if the temperature were reduced to standard or there would be more O2if the volume was held constant and more O2 was added to fill the void.?
      If I am not making any sense it is ok because this whole thing has me a little confused. But I'm trying. TomC

      --- In WoodGas@yahoogroups.com <mailto:WoodGas%40yahoogroups.com>; <mailto:WoodGas%40yahoogroups.com>;; , sabbadess@... wrote:
      >
      >
      > Wood gassers:
      >
      > It's another snowy blustery day here in downeast Maine. Out of shear boredom due to a cold shop I decided to dig into why gasifiers with air preheat have nozzles that go dim. Here is what I've discovered:
      >
      > http://groups.yahoo.com/group/WoodGas/photos/album/118107394/pic/1808817628/view?picmode= <http://groups.yahoo.com/group/WoodGas/photos/album/118107394/pic/1808817628/view?picmode= <http://groups.yahoo.com/group/WoodGas/photos/album/118107394/pic/1808817628/view?picmode=&mode=tn&order=ordinal&start=41&count=20&dir=asc> &mode=tn&order=ordinal&start=41&count=20&dir=asc> &mode=tn&order=ordinal&start=41&count=20&dir=asc
      >
      > If you don't want all the science blah-blah, it's simple. Without the thermal drag of outputting hot gas the combustion section can "throttle back" and still hit the temperatures needed to make all the gas that the suction is trying to draw. There are big volume jumps when CO is created. Just 12 grams of carbon burned to create CO will turn 11.2 liters of oxygen into 22.4 liters of CO. Having the heat to make that go is a big deal. If you don't have the heat for reduction then 12g of carbon burned with 22.4 liters of oxygen will yield 22.4 liters of CO2. Your actual volumetric output will go way down.
      >
      > Carbon reacting with steam also doubles the volumetric output of H2 and CO compared to the initial steam(ignoring superheating expansion in this statement.)
      >
      > Greg...sounds like an answer for roaring to me.
      >
      > Stephen
      >
      >
      >
      >
      >
      > [Non-text portions of this message have been removed]
      >

      [Non-text portions of this message have been removed]

      ------------------------------------

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    • David Siedschlag
      Jim, Thanks for your reply, well both. The first one bordered on over the top, but I still got it. To realize this and post the second one was very kind. I
      Message 2 of 20 , Mar 1, 2011
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        Jim,

        Thanks for your reply, well both. The first one bordered on over the top, but I still got it. To realize this and post the second one was very kind. I am pretty good at getting things, but when lots of chemical equations and formal language starts getting used I think my ears shut off, and I wish I just had a picture, or animation. Like “thermal drag” to me is the little ox dudes will go in the cool air entrance tube no problem, but once they get in there “Man its hot in here” they say and start to slow down. The ones outside don’t know that, and they still want to get in but can’t because someone else is in their way not wanting to go to where they are suppose to cuz it’s even hotter there. This is just my guess. Maybe this isn’t what thermal drag is referring to at all.

        Anyway, simply put I am a visual learner.

        Thanks for trying to put it in laymans terms. You did well and your link to “beginners instructions” I will print out for reference.

        I would also still like to firm up a meeting time when we are in CA, with you or one of the other guys or gals that know about gasification. My wife and I have set all of Monday the 28th aside for this. Please let me know what channels to take to set this up. And to acknowledge your busy schedule, I don’t expect you to spend this time for free.

        Thanks again,

        David

        From: WoodGas@yahoogroups.com [mailto:WoodGas@yahoogroups.com] On Behalf Of jim
        Sent: Monday, February 28, 2011 10:41 PM
        To: WoodGas@yahoogroups.com
        Subject: [WoodGas] Re: Why nozzles go dim with preheat







        --- In WoodGas@yahoogroups.com <mailto:WoodGas%40yahoogroups.com> , sabbadess@... wrote:
        >
        > David,
        >
        > Don't get frazzled ... Just build. This arguement has gone on for years and will likely go on for years more. Both ways work. You aren't going to kill your gasifier one way or the other. Greg likes cold to warm air. I like smoking hot. Everyone else can speak for themself. We actually had a poll going around about it. Until we get some numbers on a specific design the debate will likely rage in the background.
        >
        > Stephen

        that was a better answer than my tome. sorry, i started writing and well, rewrote the whole thing again. not sure why. maybe greg will read it and send me a beer!

        the truth again is heating or not heating is not a make or break part of the solution. minor differences in fuel prep do more than differences in air preheating it seems to me. knowing the correct range to operate the gasifier in make more difference than both.

        running any imbert design is good and sufficient. tuning and finding the correct operating range is the big reckoning.

        i've tried to formalize the correct operating range, so as to reduce guessing. or at least create a vocabulary to discuss it. we all quibble about the specific numbers, but we are all progressively talking about similar numbers and can compare between our results.

        this tuning method is also what inhabits our automation brain. this is the system that defines what is good, bad and otherwise, and reduces the need for an expert operator. you can do the same things with a manometer, some thermocouples and your eyes.

        http://wiki.gekgasifier.com/w/page/27348181/The-Masonic-Method:-How-to-Operate-a-Downdraft-Gasifier

        if you know the pressure drop across the reactor, and the temp at the restriction, you can pretty well read off numbers that tell you whether you are making tar or not. this will work with any imbert type gasifier.

        success does not need to be such a mystery.

        j

        > -----Original Message-----
        > From: David Siedschlag <pellets4fuel@...>
        > To: WoodGas <WoodGas@yahoogroups.com <mailto:WoodGas%40yahoogroups.com> >
        > Sent: Mon, Feb 28, 2011 8:38 pm
        > Subject: RE: [WoodGas] Re: Why nozzles go dim with preheat
        >
        >
        >
        >
        > So guys,
        >
        > Which is it preheat or not, and simply put why or why not. I talked with GM already and I know what he believes, JM in the gek throws as much heat at everything as he can it seems. I have a whole gasifier under construction with air preheat built in, and it makes me sick to have to think I welded together just a cool looking sculpture. Drop all the chemical equations, and parts of this and that to this or that, and say it how it is so a simple guy like me can understand. I understand when stuff gets hot it expands, and cold again it goes back. So if you preheat air, it expands, or takes up more volume, and then has less ox per that volume, and now your not getting enough of those ox into where you need it?
        >
        > And maybe you can’t explain it simple, and because I am just mechanically inclined, not chemically I just won’t get it and maybe shouldn’t be even building a gasifier. !$#@%#@$ Always did like books with lots of pictures more than books with lots of words. And the only reason why I passed high school chemistry was because the teacher was an old WWII vet and he spent more time telling stories about the Germans than he did teaching.
        >
        > Hey and I ain’t rippen on anyone, just getting quite frazzled!
        >
        > David
        >
        > From: WoodGas@yahoogroups.com <mailto:WoodGas%40yahoogroups.com> [mailto:WoodGas@yahoogroups.com <mailto:WoodGas%40yahoogroups.com> ] On Behalf Of Greg Manning
        > Sent: Monday, February 28, 2011 6:46 PM
        > To: WoodGas@yahoogroups.com <mailto:WoodGas%40yahoogroups.com>
        > Subject: RE: [WoodGas] Re: Why nozzles go dim with preheat
        >
        > Now I'm missing something.
        >
        > Reduction in a gasifier (in my neck of the woods) is chemical reduction, NOT physical.
        >
        > What physical reduction are you talking about Stephen ??
        >
        > 1) Bunker (drying)
        >
        > 2) Oxidation (tar burning, char making, small portion driven to ash in the process, IN the presence of an oxidizer)
        >
        > 3) Reduction, Thermalitic devolution of charcoal into elements, NOT in the presence of an oxidizer.
        >
        > 4) Gas evolution from those elements, small amount of spent char and ash as solids (turds if you will) in the grand scheme of things.
        >
        > including re-versions and bond-in's and out's
        >
        > Granted, 3 & 4 happen in 2, BUT we hope 2 does NOT happen in 3 or 4.
        >
        > OOT (one other thing), Gas contents, H2 in the area of 18% is good, 22% is better, CH4 in 4 % is good 8% is better. CO in 20%, 18% is better. Rest in "others".
        >
        > Greg
        >
        > -----Original Message-----
        > From: WoodGas@yahoogroups.com <mailto:WoodGas%40yahoogroups.com> <mailto:WoodGas%40yahoogroups.com>; [mailto:WoodGas@yahoogroups.com <mailto:WoodGas%40yahoogroups.com> <mailto:WoodGas%40yahoogroups.com>; ]On Behalf
        > Of sabbadess@... <mailto:sabbadess%40aol.com>;
        > Sent: Monday, February 28, 2011 5:31 PM
        > To: WoodGas@yahoogroups.com <mailto:WoodGas%40yahoogroups.com> <mailto:WoodGas%40yahoogroups.com>;
        > Subject: Re: [WoodGas] Re: Why nozzles go dim with preheat
        >
        > Hi Tom,
        >
        > Like I know what I'm talking about?!?! OK, I pretend to. The big take away message is 22.4L/hr out means a lot less than 22.4L/hr went in. Even less goes in the more you preheat because of the thoroughness of the reduction and expansion to CO. Weird but that's what the numbers say. Also the less that goes in, the less combustion there is and the dimmer the nozzles appear. The ratio of each chemical component will change with the pull rate.
        >
        > The thermal equilibrium driving of (4) has to happen or eventually the gasifier would either be a molten puddle on the floor or the hearth would lose temperature to the point that reduction wouldn't occur and it would just be a combustor. The rig always gains or loses temperature until it achieves a temperature balance.
        >
        > A rodent(mol) is ALWAYS 6.02X10^23 molecules or atoms of stuff regardless of thermal expansion. I used "standard temperature and pressure" just to compare everything uniformly. Once the gas goes through the cooler it's not a bad assumption. It will however, follow the ideal gas laws as the temperature goes up. The 6.02X10^23 molecules will take up more space.
        >
        > If somebody finds something I did wrong and can correct me, so much the better for all of us.
        >
        > Actually this is really just trivia because as Greg and Jay have pointed out, preheating does other stuff in the gasifier at the machine design level which drives other changes like thickening of the pyrolysis zone.
        >
        > Stephen
        > -----Original Message-----
        > From: TomC <ginfizz20@... <mailto:ginfizz20%40yahoo.com>; >
        > To: WoodGas <WoodGas@yahoogroups.com <mailto:WoodGas%40yahoogroups.com> <mailto:WoodGas%40yahoogroups.com>; >
        > Sent: Mon, Feb 28, 2011 6:03 pm
        > Subject: [WoodGas] Re: Why nozzles go dim with preheat
        >
        > Stephen; Feel free to ignore my remarks because I know nothing of which I speak. I have tried to follow your premise, but I start off with one problem. We are pulling the gases through the gasifier at a constant rate, no matter the source. So if the source is 22.4 L/hr will it be 22.4L of CO or CO2 or 11.2L/hr of each. Now if we introduce H2O vapor will we get eight times 22.4L/hr or because of the constant vacuum will we get 2.24L/hr of CO; 2.24L/hr of CO2; and 17.92L/hr for a total of the constant 22.4L/hr.
        > I did not understand the "throttle back". Under ideal conditions adding (1)&(3)and equaling about 4x(4)would be an equilibrium situation. Is it because under a 22.4L/hr pull, the pull on the CO and CO2 is reduced so dramatically with the introduction of the H2O vapor?
        > In the formula I did not see any mention of the temperature of the incoming O2 other than you said a rodent (opps, I mean mol)is under standard conditions. How does the formula change if the mol is not at "standard" conditions. Using preheated air the mol would be less than 22.4L if the temperature were reduced to standard or there would be more O2if the volume was held constant and more O2 was added to fill the void.?
        > If I am not making any sense it is ok because this whole thing has me a little confused. But I'm trying. TomC
        >
        > --- In WoodGas@yahoogroups.com <mailto:WoodGas%40yahoogroups.com> <mailto:WoodGas%40yahoogroups.com>; , sabbadess@ wrote:
        > >
        > >
        > > Wood gassers:
        > >
        > > It's another snowy blustery day here in downeast Maine. Out of shear boredom due to a cold shop I decided to dig into why gasifiers with air preheat have nozzles that go dim. Here is what I've discovered:
        > >
        > > http://groups.yahoo.com/group/WoodGas/photos/album/118107394/pic/1808817628/view?picmode= <http://groups.yahoo.com/group/WoodGas/photos/album/118107394/pic/1808817628/view?picmode= <http://groups.yahoo.com/group/WoodGas/photos/album/118107394/pic/1808817628/view?picmode=&mode=tn&order=ordinal&start=41&count=20&dir=asc> &mode=tn&order=ordinal&start=41&count=20&dir=asc> &mode=tn&order=ordinal&start=41&count=20&dir=asc
        > >
        > > If you don't want all the science blah-blah, it's simple. Without the thermal drag of outputting hot gas the combustion section can "throttle back" and still hit the temperatures needed to make all the gas that the suction is trying to draw. There are big volume jumps when CO is created. Just 12 grams of carbon burned to create CO will turn 11.2 liters of oxygen into 22.4 liters of CO. Having the heat to make that go is a big deal. If you don't have the heat for reduction then 12g of carbon burned with 22.4 liters of oxygen will yield 22.4 liters of CO2. Your actual volumetric output will go way down.
        > >
        > > Carbon reacting with steam also doubles the volumetric output of H2 and CO compared to the initial steam(ignoring superheating expansion in this statement.)
        > >
        > > Greg...sounds like an answer for roaring to me.
        > >
        > > Stephen
        > >
        > >
        > >
        > >
        > >
        > > [Non-text portions of this message have been removed]
        > >
        >
        > [Non-text portions of this message have been removed]
        >
        > ------------------------------------
        >
        > Yahoo! Groups Links
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        > No virus found in this message.
        > Checked by AVG - www.avg.com
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      • Mike LaRosa
        Thanks Jim, Good read. ML
        Message 3 of 20 , Mar 1, 2011
        • 0 Attachment
          Thanks Jim, Good read. ML


          --- In WoodGas@yahoogroups.com, "jim" <jim@...> wrote:
          >
          >
          >
          > --- In WoodGas@yahoogroups.com, "David Siedschlag" <pellets4fuel@> wrote:
          > >
          > > So guys,
          > >
          > > Which is it preheat or not, and simply put why or why not. I talked with GM already and I know what he believes, JM in the gek throws as much heat at everything as he can it seems. I have a whole gasifier under construction with air preheat built in, and it makes me sick to have to think I welded together just a cool looking sculpture. Drop all the chemical equations, and parts of this and that to this or that, and say it how it is so a simple guy like me can understand. I understand when stuff gets hot it expands, and cold again it goes back. So if you preheat air, it expands, or takes up more volume, and then has less ox per that volume, and now your not getting enough of those ox into where you need it?
          > >
          > >
          >
          >
          > david,
          >
          > the simple answer is adding heat makes things hotter and taking away heat makes things colder. but this as most attempts at simplifying what is happening in gasification is misleading.
          >
          > no one here will argue that reducing the amount of heat going in will increase the amount of energy coming out. thermodynamics does not provide free energy. you have to earn each degree temp, and spend each btu wisely. none come free.
          >
          > what some here will argue is that colder air in will create a hotter fire in the hearth. the argument is that cold air is denser and it can get more oxy to the char and generate a more concentrated higher temp. somewhat like a cold charge into an engine will give you more power.
          >
          > at this point there will be some discussion of the differences between an IC engine and a gasifier. an ic engine is a volume constrained combustion device, so cooler air lets you get more charge in the cylinder before compression. this will give you more power out, or greater power density.
          >
          > a gasifier is not really a volume constrained device. it is variable flow, and we can increase or decrease flow to get the desired amount of reactants in it. furthermore, the main challenge of a gasifier is not power density, but tar conversion. we need to make certain temps and good coverage of those temps to crack tars. this is a very different problem of the charge density of an ic engine. thus a simple comparison between the two isn't very helpful.
          >
          > at about this point in the debate one of us will start debating what is actually being combusted-- tar or the char? this is called the "gas centric vs char centric" debate. it goes like this.
          >
          > if you have air coming into a mix tar/char environment, which one will be selectively combusted? i'll argue the tar will be more selectively burned, as gas to gas mixing has exponentially more reaction sites than gas to solids, which is why HCs are more explosive as gassess than liquids or solids. it is why a fire is big when you put on new wood and there is lots of pyrolysis gasses coming up, vs when it is just a char bed and little oxygen is getting to the char. the fluidyne camp will argue that the char is more selectively oxidized and most of the tar is cracked. that somehow the air hits the char and oxidized c to co2, without being effected much by all the surrounding tar gas. neither of us can provide numbers on this, so we each wave our hands and think our first principles are correct.
          >
          > about this point in the disucssion i'll ramble on about the 4 processes. to the degree we have combustion in a gasifier, it is only one of 4 processes, and the amount we have is variable. the main problem of biomass gasification is that pyrolysis produces much more tar gas than char. there is really too much tar gas to burn in its entirety and reduce over the amount of available char. i calc our amount of tar to be 2.2 times over what we can burn and have the needed amount of char to fully reduce. we can only burn a portion of the tar, and have to thermally crack the rest to co an h2. both the cracking and the reduction are making co and h2. it is important to keep these two different sources for our desired output gasses separate.
          >
          > thus what we call the "combustion zone" in a biomass gasifier is really a combustion and cracking zone. we burn only a portion of the tar gas and try to crack the rest by the heat produced. my calcs suggest we are in actuality burning less than half of the tar gas resulting from pyrolysis, the rest is being thermally cracked. this cracking is sensitive and at the toppish end of our available temps, thus the huge problem of completing the tar conversion.
          >
          >
          > after going through this, i'll usually then note that this is why things are so much easier with charocoal or coal in a gasifier. these fuels have most of their volatiles already eliminated, thus they produce much less tar to char after complete pyrolysis. the result is you can completely combust the tar gasses, not really leaving any to crack, and have plenty of char to reduce the results.
          >
          > but what determines now much combustion we have?
          >
          > combustion is really the "filler" process that expands and contracts to fill the needed gas volume and needed heat not provided elsewhere. if you had unlimited heat from elsewhere, you would not need any combustion. you'd just generate your gas volume via pyrolysis. pyrolysis is breaking down solids and making gas, along with the char. when you pull a vac on the gasifier, you are getting some amount of volume enabled by pyrolysis, plus some amount enabled by air coming in for combustion. you only get combustion to the degree that you are not generating volume by pyrolysis, and heat to drive it by something else. and hopefully the amount of combustion you get creates the needed heat and spread to cover the hearth so cracking works and tar is converted.
          >
          >
          > thus the amount of heat add or lose will determine the amount of combustion you have. this is the point of stephen's conjecture on why the char dims with preheated air. seems possible, but also the air density issue might be part of this too. i don't really know.
          >
          > what i do know is we will need less combustion with preheated air and preheated fuel. this will result certainly in less nitrogen dilution and a higher energy density gas. it will also result in higher top temps and general improvement in moisture handling.
          >
          > in the negative, it is also increasing the amount of tar you are having to crack vs burn. a massive heat loss gasifier might require so much combustion to keep up with heat and gas evolution needs, that all the tar could be burned to completion, and the char overrun with lots of c02 and h2o coming out the end.
          >
          > the fluidyne design is optimized for good fines purging of the reduction zone. the reduction zone is significantly reduced with the striaght tube while doing this. this is a fine tradeoff, but do note what we get and what we lose.
          >
          > with the truncated reduction tube, residence time is shortened and less reduction is completed. gas exits hotter and with more co2 and h2o not yet reduced. doug suggests tuning the fluidyne by moving the grate up until the gas goes fairly anemic. this is where the char fines will flow out without grate shaking. it is also where much of the reduction is prevented, and the less energy dense gas that results.
          >
          > note here that the gas volume making method is more tilted towards the cracking of pyrogasses than the reduction of combustion gasses. this is fine, as we again have such an excess of tar gas to crack that in principle, we could get combustible gas with no reduction whatsoever.
          >
          > all of this follows from a pursuit of good fines purging. the fluidyne design smartly focusses on this as it is a huge problem once you get to long runs. greg has similarly shown how well it works with his gasifier boiler.
          >
          > that got a bit off air preheating. i was trying to explain some of why this is not so simple. combustion is a "filler" process in a gasifier. the amount you have will vary depending on heat added or lost elsewhere in the system, and the gasses evolved by pyrolysis as a result. total preheat and you need no air whatsoever. that is also called a charcoal retort.
          >
          > realistically, we are always heat challenged in a gasifier and are struggling to maintain the 1000c and up temps over a big lobe needed to crack the tars. removing all parasitic loads that take heat and reduce top temp seem indicated.
          >
          > air on the away into the gasifier needs to be raised to the auto igntion point of tar gas before oxidation will start. something has to heat that air. it will either be local heating in the hearth, which takes away heat, or something external, which prevents this loss in the hearth.
          >
          > similarly drying, pyrolysis, heating the char to hearth temps also require heat. this heat is coming from somewhere, and reducing the temps of whatever the source is. if we are using the hearth as our heat source for these heating needs, we will be lowering the temps int he hearth.
          >
          > i argue we should try to eliminate all these parasitic loads. get all the heating loads of incoming air, drying and pyrolysis off of the combustion in the hearth. let all the heat generated there by combustion be used to max temp s and best crack the tars.
          >
          > this is why the gek totti has a big heat recycling system to eliminate the heating loads usually presented incoming air, fuel drying and pyrolysis. i'm trying to deliver already hot char and hot air to the hearth, making the gasifier more like a charcoal gasifier with an externally driven pyrolysizer sitting on top.
          >
          >
          > the total heat budget of this architecture will be superior to a non heat recylcing system. to argue not suggests that we get heat for free from somewhere else. heat is never free. adding more cold does not create more hot.
          >
          > nonetheless, it is possible that we might ahve some localized higher temps at some point in the process, either by change in combustion amount or reduction in temp elsewhere. super cold air might result in proportionally more combustion inside, and thus a higher spot temp.
          >
          > or, the cold air might really be saying something about the absolute water loading of the air on the way in. in the numbers we've passed around, the heating changes that result from humidity changes seem to trump the 100f or so air temp differences we are concerned with here. it might be that the impact of water differences in cold air is what is creating the effect. it might have nothing to do with temp.
          >
          > who knows, as no one is trying to answer this with controlled testing. so far we've had a video of a variably glowing ember and ensuing hysteria!
          >
          >
          > i'd say the best test of this we could do would be to measure in hearth temps in multiple spots with quick switch between heat and no preheated air intake. measure the temp differences and measure the resulting tar coming out the system. do a test like this, but with air in temp changing: http://wiki.gekgasifier.com/w/page/6123786/Multi-fuel%20Run%20Comparison
          >
          > problem is there are turning changes needed to optimize each mode. the arc of combustion propagation and temp rise/fall will be different, requiring nozzle and hearth redimensioning to take advantage of the differences. so just switching back and forth will handicap the potential of one or both modes.
          >
          > we'll do this at some point out here, but frankly, many other things have been higher priority. of the many known unknowns of a gasifier, this for me does not rise to one of them. thus i have directed limited and time and resources elsewhere. we're not going to gain any meaningful ground by putting cold air into a gasifier, despite the recurring strum and drang to the contrary.
          >
          >
          > jim
          >
        • TomC
          Mike L; Question; haven t you always said that you get more power in the fall or winter when you drive your truck? How does that correlate to preheat being
          Message 4 of 20 , Mar 1, 2011
          • 0 Attachment
            Mike L; Question; haven't you always said that you get more power in the fall or winter when you drive your truck? How does that correlate to preheat being better when you are saying the cooler air in the fall makes better gas.??TomC

            --- In WoodGas@yahoogroups.com, "Mike LaRosa" <ook187@...> wrote:
            >
            > Thanks Jim, Good read. ML
            >
            >
            > --- In WoodGas@yahoogroups.com, "jim" <jim@> wrote:
            > >
            > >
            > >
            > > --- In WoodGas@yahoogroups.com, "David Siedschlag" <pellets4fuel@> wrote:
            > > >
            > > > So guys,
            > > >
            > > > Which is it preheat or not, and simply put why or why not. I talked with GM already and I know what he believes, JM in the gek throws as much heat at everything as he can it seems. I have a whole gasifier under construction with air preheat built in, and it makes me sick to have to think I welded together just a cool looking sculpture. Drop all the chemical equations, and parts of this and that to this or that, and say it how it is so a simple guy like me can understand. I understand when stuff gets hot it expands, and cold again it goes back. So if you preheat air, it expands, or takes up more volume, and then has less ox per that volume, and now your not getting enough of those ox into where you need it?
            > > >
            > > >
            > >
            > >
            > > david,
            > >
            > > the simple answer is adding heat makes things hotter and taking away heat makes things colder. but this as most attempts at simplifying what is happening in gasification is misleading.
            > >
            > > no one here will argue that reducing the amount of heat going in will increase the amount of energy coming out. thermodynamics does not provide free energy. you have to earn each degree temp, and spend each btu wisely. none come free.
            > >
            > > what some here will argue is that colder air in will create a hotter fire in the hearth. the argument is that cold air is denser and it can get more oxy to the char and generate a more concentrated higher temp. somewhat like a cold charge into an engine will give you more power.
            > >
            > > at this point there will be some discussion of the differences between an IC engine and a gasifier. an ic engine is a volume constrained combustion device, so cooler air lets you get more charge in the cylinder before compression. this will give you more power out, or greater power density.
            > >
            > > a gasifier is not really a volume constrained device. it is variable flow, and we can increase or decrease flow to get the desired amount of reactants in it. furthermore, the main challenge of a gasifier is not power density, but tar conversion. we need to make certain temps and good coverage of those temps to crack tars. this is a very different problem of the charge density of an ic engine. thus a simple comparison between the two isn't very helpful.
            > >
            > > at about this point in the debate one of us will start debating what is actually being combusted-- tar or the char? this is called the "gas centric vs char centric" debate. it goes like this.
            > >
            > > if you have air coming into a mix tar/char environment, which one will be selectively combusted? i'll argue the tar will be more selectively burned, as gas to gas mixing has exponentially more reaction sites than gas to solids, which is why HCs are more explosive as gassess than liquids or solids. it is why a fire is big when you put on new wood and there is lots of pyrolysis gasses coming up, vs when it is just a char bed and little oxygen is getting to the char. the fluidyne camp will argue that the char is more selectively oxidized and most of the tar is cracked. that somehow the air hits the char and oxidized c to co2, without being effected much by all the surrounding tar gas. neither of us can provide numbers on this, so we each wave our hands and think our first principles are correct.
            > >
            > > about this point in the disucssion i'll ramble on about the 4 processes. to the degree we have combustion in a gasifier, it is only one of 4 processes, and the amount we have is variable. the main problem of biomass gasification is that pyrolysis produces much more tar gas than char. there is really too much tar gas to burn in its entirety and reduce over the amount of available char. i calc our amount of tar to be 2.2 times over what we can burn and have the needed amount of char to fully reduce. we can only burn a portion of the tar, and have to thermally crack the rest to co an h2. both the cracking and the reduction are making co and h2. it is important to keep these two different sources for our desired output gasses separate.
            > >
            > > thus what we call the "combustion zone" in a biomass gasifier is really a combustion and cracking zone. we burn only a portion of the tar gas and try to crack the rest by the heat produced. my calcs suggest we are in actuality burning less than half of the tar gas resulting from pyrolysis, the rest is being thermally cracked. this cracking is sensitive and at the toppish end of our available temps, thus the huge problem of completing the tar conversion.
            > >
            > >
            > > after going through this, i'll usually then note that this is why things are so much easier with charocoal or coal in a gasifier. these fuels have most of their volatiles already eliminated, thus they produce much less tar to char after complete pyrolysis. the result is you can completely combust the tar gasses, not really leaving any to crack, and have plenty of char to reduce the results.
            > >
            > > but what determines now much combustion we have?
            > >
            > > combustion is really the "filler" process that expands and contracts to fill the needed gas volume and needed heat not provided elsewhere. if you had unlimited heat from elsewhere, you would not need any combustion. you'd just generate your gas volume via pyrolysis. pyrolysis is breaking down solids and making gas, along with the char. when you pull a vac on the gasifier, you are getting some amount of volume enabled by pyrolysis, plus some amount enabled by air coming in for combustion. you only get combustion to the degree that you are not generating volume by pyrolysis, and heat to drive it by something else. and hopefully the amount of combustion you get creates the needed heat and spread to cover the hearth so cracking works and tar is converted.
            > >
            > >
            > > thus the amount of heat add or lose will determine the amount of combustion you have. this is the point of stephen's conjecture on why the char dims with preheated air. seems possible, but also the air density issue might be part of this too. i don't really know.
            > >
            > > what i do know is we will need less combustion with preheated air and preheated fuel. this will result certainly in less nitrogen dilution and a higher energy density gas. it will also result in higher top temps and general improvement in moisture handling.
            > >
            > > in the negative, it is also increasing the amount of tar you are having to crack vs burn. a massive heat loss gasifier might require so much combustion to keep up with heat and gas evolution needs, that all the tar could be burned to completion, and the char overrun with lots of c02 and h2o coming out the end.
            > >
            > > the fluidyne design is optimized for good fines purging of the reduction zone. the reduction zone is significantly reduced with the striaght tube while doing this. this is a fine tradeoff, but do note what we get and what we lose.
            > >
            > > with the truncated reduction tube, residence time is shortened and less reduction is completed. gas exits hotter and with more co2 and h2o not yet reduced. doug suggests tuning the fluidyne by moving the grate up until the gas goes fairly anemic. this is where the char fines will flow out without grate shaking. it is also where much of the reduction is prevented, and the less energy dense gas that results.
            > >
            > > note here that the gas volume making method is more tilted towards the cracking of pyrogasses than the reduction of combustion gasses. this is fine, as we again have such an excess of tar gas to crack that in principle, we could get combustible gas with no reduction whatsoever.
            > >
            > > all of this follows from a pursuit of good fines purging. the fluidyne design smartly focusses on this as it is a huge problem once you get to long runs. greg has similarly shown how well it works with his gasifier boiler.
            > >
            > > that got a bit off air preheating. i was trying to explain some of why this is not so simple. combustion is a "filler" process in a gasifier. the amount you have will vary depending on heat added or lost elsewhere in the system, and the gasses evolved by pyrolysis as a result. total preheat and you need no air whatsoever. that is also called a charcoal retort.
            > >
            > > realistically, we are always heat challenged in a gasifier and are struggling to maintain the 1000c and up temps over a big lobe needed to crack the tars. removing all parasitic loads that take heat and reduce top temp seem indicated.
            > >
            > > air on the away into the gasifier needs to be raised to the auto igntion point of tar gas before oxidation will start. something has to heat that air. it will either be local heating in the hearth, which takes away heat, or something external, which prevents this loss in the hearth.
            > >
            > > similarly drying, pyrolysis, heating the char to hearth temps also require heat. this heat is coming from somewhere, and reducing the temps of whatever the source is. if we are using the hearth as our heat source for these heating needs, we will be lowering the temps int he hearth.
            > >
            > > i argue we should try to eliminate all these parasitic loads. get all the heating loads of incoming air, drying and pyrolysis off of the combustion in the hearth. let all the heat generated there by combustion be used to max temp s and best crack the tars.
            > >
            > > this is why the gek totti has a big heat recycling system to eliminate the heating loads usually presented incoming air, fuel drying and pyrolysis. i'm trying to deliver already hot char and hot air to the hearth, making the gasifier more like a charcoal gasifier with an externally driven pyrolysizer sitting on top.
            > >
            > >
            > > the total heat budget of this architecture will be superior to a non heat recylcing system. to argue not suggests that we get heat for free from somewhere else. heat is never free. adding more cold does not create more hot.
            > >
            > > nonetheless, it is possible that we might ahve some localized higher temps at some point in the process, either by change in combustion amount or reduction in temp elsewhere. super cold air might result in proportionally more combustion inside, and thus a higher spot temp.
            > >
            > > or, the cold air might really be saying something about the absolute water loading of the air on the way in. in the numbers we've passed around, the heating changes that result from humidity changes seem to trump the 100f or so air temp differences we are concerned with here. it might be that the impact of water differences in cold air is what is creating the effect. it might have nothing to do with temp.
            > >
            > > who knows, as no one is trying to answer this with controlled testing. so far we've had a video of a variably glowing ember and ensuing hysteria!
            > >
            > >
            > > i'd say the best test of this we could do would be to measure in hearth temps in multiple spots with quick switch between heat and no preheated air intake. measure the temp differences and measure the resulting tar coming out the system. do a test like this, but with air in temp changing: http://wiki.gekgasifier.com/w/page/6123786/Multi-fuel%20Run%20Comparison
            > >
            > > problem is there are turning changes needed to optimize each mode. the arc of combustion propagation and temp rise/fall will be different, requiring nozzle and hearth redimensioning to take advantage of the differences. so just switching back and forth will handicap the potential of one or both modes.
            > >
            > > we'll do this at some point out here, but frankly, many other things have been higher priority. of the many known unknowns of a gasifier, this for me does not rise to one of them. thus i have directed limited and time and resources elsewhere. we're not going to gain any meaningful ground by putting cold air into a gasifier, despite the recurring strum and drang to the contrary.
            > >
            > >
            > > jim
            > >
            >
          • Mike LaRosa
            Hi Tom, I just get better condensation in the winter and the resulting gas is more dense. The main thing I notice with some preheat is that it takes less time
            Message 5 of 20 , Mar 1, 2011
            • 0 Attachment
              Hi Tom, I just get better condensation in the winter and the resulting gas is more dense. The main thing I notice with some preheat is that it takes less time to get a new hopper full of wood to run steady state. The first few miles the fire is pretty much just cooking the moisture out of the wood and then the process moves onto the other volatiles. Every bit of heat put in speeds those processes up. I can observe the rate of temperature increase some at the tcouple after my cyclone. ML


              --- In WoodGas@yahoogroups.com, "TomC" <ginfizz20@...> wrote:
              >
              > Mike L; Question; haven't you always said that you get more power in the fall or winter when you drive your truck? How does that correlate to preheat being better when you are saying the cooler air in the fall makes better gas.??TomC
              >
              > --- In WoodGas@yahoogroups.com, "Mike LaRosa" <ook187@> wrote:
              > >
              > > Thanks Jim, Good read. ML
              > >
              > >
              > > --- In WoodGas@yahoogroups.com, "jim" <jim@> wrote:
              > > >
              > > >
              > > >
              > > > --- In WoodGas@yahoogroups.com, "David Siedschlag" <pellets4fuel@> wrote:
              > > > >
              > > > > So guys,
              > > > >
              > > > > Which is it preheat or not, and simply put why or why not. I talked with GM already and I know what he believes, JM in the gek throws as much heat at everything as he can it seems. I have a whole gasifier under construction with air preheat built in, and it makes me sick to have to think I welded together just a cool looking sculpture. Drop all the chemical equations, and parts of this and that to this or that, and say it how it is so a simple guy like me can understand. I understand when stuff gets hot it expands, and cold again it goes back. So if you preheat air, it expands, or takes up more volume, and then has less ox per that volume, and now your not getting enough of those ox into where you need it?
              > > > >
              > > > >
              > > >
              > > >
              > > > david,
              > > >
              > > > the simple answer is adding heat makes things hotter and taking away heat makes things colder. but this as most attempts at simplifying what is happening in gasification is misleading.
              > > >
              > > > no one here will argue that reducing the amount of heat going in will increase the amount of energy coming out. thermodynamics does not provide free energy. you have to earn each degree temp, and spend each btu wisely. none come free.
              > > >
              > > > what some here will argue is that colder air in will create a hotter fire in the hearth. the argument is that cold air is denser and it can get more oxy to the char and generate a more concentrated higher temp. somewhat like a cold charge into an engine will give you more power.
              > > >
              > > > at this point there will be some discussion of the differences between an IC engine and a gasifier. an ic engine is a volume constrained combustion device, so cooler air lets you get more charge in the cylinder before compression. this will give you more power out, or greater power density.
              > > >
              > > > a gasifier is not really a volume constrained device. it is variable flow, and we can increase or decrease flow to get the desired amount of reactants in it. furthermore, the main challenge of a gasifier is not power density, but tar conversion. we need to make certain temps and good coverage of those temps to crack tars. this is a very different problem of the charge density of an ic engine. thus a simple comparison between the two isn't very helpful.
              > > >
              > > > at about this point in the debate one of us will start debating what is actually being combusted-- tar or the char? this is called the "gas centric vs char centric" debate. it goes like this.
              > > >
              > > > if you have air coming into a mix tar/char environment, which one will be selectively combusted? i'll argue the tar will be more selectively burned, as gas to gas mixing has exponentially more reaction sites than gas to solids, which is why HCs are more explosive as gassess than liquids or solids. it is why a fire is big when you put on new wood and there is lots of pyrolysis gasses coming up, vs when it is just a char bed and little oxygen is getting to the char. the fluidyne camp will argue that the char is more selectively oxidized and most of the tar is cracked. that somehow the air hits the char and oxidized c to co2, without being effected much by all the surrounding tar gas. neither of us can provide numbers on this, so we each wave our hands and think our first principles are correct.
              > > >
              > > > about this point in the disucssion i'll ramble on about the 4 processes. to the degree we have combustion in a gasifier, it is only one of 4 processes, and the amount we have is variable. the main problem of biomass gasification is that pyrolysis produces much more tar gas than char. there is really too much tar gas to burn in its entirety and reduce over the amount of available char. i calc our amount of tar to be 2.2 times over what we can burn and have the needed amount of char to fully reduce. we can only burn a portion of the tar, and have to thermally crack the rest to co an h2. both the cracking and the reduction are making co and h2. it is important to keep these two different sources for our desired output gasses separate.
              > > >
              > > > thus what we call the "combustion zone" in a biomass gasifier is really a combustion and cracking zone. we burn only a portion of the tar gas and try to crack the rest by the heat produced. my calcs suggest we are in actuality burning less than half of the tar gas resulting from pyrolysis, the rest is being thermally cracked. this cracking is sensitive and at the toppish end of our available temps, thus the huge problem of completing the tar conversion.
              > > >
              > > >
              > > > after going through this, i'll usually then note that this is why things are so much easier with charocoal or coal in a gasifier. these fuels have most of their volatiles already eliminated, thus they produce much less tar to char after complete pyrolysis. the result is you can completely combust the tar gasses, not really leaving any to crack, and have plenty of char to reduce the results.
              > > >
              > > > but what determines now much combustion we have?
              > > >
              > > > combustion is really the "filler" process that expands and contracts to fill the needed gas volume and needed heat not provided elsewhere. if you had unlimited heat from elsewhere, you would not need any combustion. you'd just generate your gas volume via pyrolysis. pyrolysis is breaking down solids and making gas, along with the char. when you pull a vac on the gasifier, you are getting some amount of volume enabled by pyrolysis, plus some amount enabled by air coming in for combustion. you only get combustion to the degree that you are not generating volume by pyrolysis, and heat to drive it by something else. and hopefully the amount of combustion you get creates the needed heat and spread to cover the hearth so cracking works and tar is converted.
              > > >
              > > >
              > > > thus the amount of heat add or lose will determine the amount of combustion you have. this is the point of stephen's conjecture on why the char dims with preheated air. seems possible, but also the air density issue might be part of this too. i don't really know.
              > > >
              > > > what i do know is we will need less combustion with preheated air and preheated fuel. this will result certainly in less nitrogen dilution and a higher energy density gas. it will also result in higher top temps and general improvement in moisture handling.
              > > >
              > > > in the negative, it is also increasing the amount of tar you are having to crack vs burn. a massive heat loss gasifier might require so much combustion to keep up with heat and gas evolution needs, that all the tar could be burned to completion, and the char overrun with lots of c02 and h2o coming out the end.
              > > >
              > > > the fluidyne design is optimized for good fines purging of the reduction zone. the reduction zone is significantly reduced with the striaght tube while doing this. this is a fine tradeoff, but do note what we get and what we lose.
              > > >
              > > > with the truncated reduction tube, residence time is shortened and less reduction is completed. gas exits hotter and with more co2 and h2o not yet reduced. doug suggests tuning the fluidyne by moving the grate up until the gas goes fairly anemic. this is where the char fines will flow out without grate shaking. it is also where much of the reduction is prevented, and the less energy dense gas that results.
              > > >
              > > > note here that the gas volume making method is more tilted towards the cracking of pyrogasses than the reduction of combustion gasses. this is fine, as we again have such an excess of tar gas to crack that in principle, we could get combustible gas with no reduction whatsoever.
              > > >
              > > > all of this follows from a pursuit of good fines purging. the fluidyne design smartly focusses on this as it is a huge problem once you get to long runs. greg has similarly shown how well it works with his gasifier boiler.
              > > >
              > > > that got a bit off air preheating. i was trying to explain some of why this is not so simple. combustion is a "filler" process in a gasifier. the amount you have will vary depending on heat added or lost elsewhere in the system, and the gasses evolved by pyrolysis as a result. total preheat and you need no air whatsoever. that is also called a charcoal retort.
              > > >
              > > > realistically, we are always heat challenged in a gasifier and are struggling to maintain the 1000c and up temps over a big lobe needed to crack the tars. removing all parasitic loads that take heat and reduce top temp seem indicated.
              > > >
              > > > air on the away into the gasifier needs to be raised to the auto igntion point of tar gas before oxidation will start. something has to heat that air. it will either be local heating in the hearth, which takes away heat, or something external, which prevents this loss in the hearth.
              > > >
              > > > similarly drying, pyrolysis, heating the char to hearth temps also require heat. this heat is coming from somewhere, and reducing the temps of whatever the source is. if we are using the hearth as our heat source for these heating needs, we will be lowering the temps int he hearth.
              > > >
              > > > i argue we should try to eliminate all these parasitic loads. get all the heating loads of incoming air, drying and pyrolysis off of the combustion in the hearth. let all the heat generated there by combustion be used to max temp s and best crack the tars.
              > > >
              > > > this is why the gek totti has a big heat recycling system to eliminate the heating loads usually presented incoming air, fuel drying and pyrolysis. i'm trying to deliver already hot char and hot air to the hearth, making the gasifier more like a charcoal gasifier with an externally driven pyrolysizer sitting on top.
              > > >
              > > >
              > > > the total heat budget of this architecture will be superior to a non heat recylcing system. to argue not suggests that we get heat for free from somewhere else. heat is never free. adding more cold does not create more hot.
              > > >
              > > > nonetheless, it is possible that we might ahve some localized higher temps at some point in the process, either by change in combustion amount or reduction in temp elsewhere. super cold air might result in proportionally more combustion inside, and thus a higher spot temp.
              > > >
              > > > or, the cold air might really be saying something about the absolute water loading of the air on the way in. in the numbers we've passed around, the heating changes that result from humidity changes seem to trump the 100f or so air temp differences we are concerned with here. it might be that the impact of water differences in cold air is what is creating the effect. it might have nothing to do with temp.
              > > >
              > > > who knows, as no one is trying to answer this with controlled testing. so far we've had a video of a variably glowing ember and ensuing hysteria!
              > > >
              > > >
              > > > i'd say the best test of this we could do would be to measure in hearth temps in multiple spots with quick switch between heat and no preheated air intake. measure the temp differences and measure the resulting tar coming out the system. do a test like this, but with air in temp changing: http://wiki.gekgasifier.com/w/page/6123786/Multi-fuel%20Run%20Comparison
              > > >
              > > > problem is there are turning changes needed to optimize each mode. the arc of combustion propagation and temp rise/fall will be different, requiring nozzle and hearth redimensioning to take advantage of the differences. so just switching back and forth will handicap the potential of one or both modes.
              > > >
              > > > we'll do this at some point out here, but frankly, many other things have been higher priority. of the many known unknowns of a gasifier, this for me does not rise to one of them. thus i have directed limited and time and resources elsewhere. we're not going to gain any meaningful ground by putting cold air into a gasifier, despite the recurring strum and drang to the contrary.
              > > >
              > > >
              > > > jim
              > > >
              > >
              >
            • bigotes_bigotes
              Jim, You might want to sit down first, but I m going to compliment you on this post. Very well thought out and straightfoward. One thing on the fluidyne
              Message 6 of 20 , Mar 2, 2011
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                Jim,
                You might want to sit down first, but I'm going to compliment you on this post. Very well thought out and straightfoward.

                One thing on the fluidyne design that I think many are missing is that there is no pressure drop (as in every other hearth design) when the gas enters the reduction phase. That should be equatable to higher temperatures which might be helping to achieve full reduction. Maybe.
                Other than that I see no major difference really because you can drop the grate as far as you want to get more char bed.

                Higher temps down there will also be helping in cracking (if the tars get that far). I had my reduction tube extremely well insulated also to maintain those higher temps.

                Bigotes

                --- In WoodGas@yahoogroups.com, "jim" <jim@...> wrote:
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                > --- In WoodGas@yahoogroups.com, "David Siedschlag" <pellets4fuel@> wrote:
                > >
                > > So guys,
                > >
                > > Which is it preheat or not, and simply put why or why not. I talked with GM already and I know what he believes, JM in the gek throws as much heat at everything as he can it seems. I have a whole gasifier under construction with air preheat built in, and it makes me sick to have to think I welded together just a cool looking sculpture. Drop all the chemical equations, and parts of this and that to this or that, and say it how it is so a simple guy like me can understand. I understand when stuff gets hot it expands, and cold again it goes back. So if you preheat air, it expands, or takes up more volume, and then has less ox per that volume, and now your not getting enough of those ox into where you need it?
                > >
                > >
                >
                >
                > david,
                >
                > the simple answer is adding heat makes things hotter and taking away heat makes things colder. but this as most attempts at simplifying what is happening in gasification is misleading.
                >
                > no one here will argue that reducing the amount of heat going in will increase the amount of energy coming out. thermodynamics does not provide free energy. you have to earn each degree temp, and spend each btu wisely. none come free.
                >
                > what some here will argue is that colder air in will create a hotter fire in the hearth. the argument is that cold air is denser and it can get more oxy to the char and generate a more concentrated higher temp. somewhat like a cold charge into an engine will give you more power.
                >
                > at this point there will be some discussion of the differences between an IC engine and a gasifier. an ic engine is a volume constrained combustion device, so cooler air lets you get more charge in the cylinder before compression. this will give you more power out, or greater power density.
                >
                > a gasifier is not really a volume constrained device. it is variable flow, and we can increase or decrease flow to get the desired amount of reactants in it. furthermore, the main challenge of a gasifier is not power density, but tar conversion. we need to make certain temps and good coverage of those temps to crack tars. this is a very different problem of the charge density of an ic engine. thus a simple comparison between the two isn't very helpful.
                >
                > at about this point in the debate one of us will start debating what is actually being combusted-- tar or the char? this is called the "gas centric vs char centric" debate. it goes like this.
                >
                > if you have air coming into a mix tar/char environment, which one will be selectively combusted? i'll argue the tar will be more selectively burned, as gas to gas mixing has exponentially more reaction sites than gas to solids, which is why HCs are more explosive as gassess than liquids or solids. it is why a fire is big when you put on new wood and there is lots of pyrolysis gasses coming up, vs when it is just a char bed and little oxygen is getting to the char. the fluidyne camp will argue that the char is more selectively oxidized and most of the tar is cracked. that somehow the air hits the char and oxidized c to co2, without being effected much by all the surrounding tar gas. neither of us can provide numbers on this, so we each wave our hands and think our first principles are correct.
                >
                > about this point in the disucssion i'll ramble on about the 4 processes. to the degree we have combustion in a gasifier, it is only one of 4 processes, and the amount we have is variable. the main problem of biomass gasification is that pyrolysis produces much more tar gas than char. there is really too much tar gas to burn in its entirety and reduce over the amount of available char. i calc our amount of tar to be 2.2 times over what we can burn and have the needed amount of char to fully reduce. we can only burn a portion of the tar, and have to thermally crack the rest to co an h2. both the cracking and the reduction are making co and h2. it is important to keep these two different sources for our desired output gasses separate.
                >
                > thus what we call the "combustion zone" in a biomass gasifier is really a combustion and cracking zone. we burn only a portion of the tar gas and try to crack the rest by the heat produced. my calcs suggest we are in actuality burning less than half of the tar gas resulting from pyrolysis, the rest is being thermally cracked. this cracking is sensitive and at the toppish end of our available temps, thus the huge problem of completing the tar conversion.
                >
                >
                > after going through this, i'll usually then note that this is why things are so much easier with charocoal or coal in a gasifier. these fuels have most of their volatiles already eliminated, thus they produce much less tar to char after complete pyrolysis. the result is you can completely combust the tar gasses, not really leaving any to crack, and have plenty of char to reduce the results.
                >
                > but what determines now much combustion we have?
                >
                > combustion is really the "filler" process that expands and contracts to fill the needed gas volume and needed heat not provided elsewhere. if you had unlimited heat from elsewhere, you would not need any combustion. you'd just generate your gas volume via pyrolysis. pyrolysis is breaking down solids and making gas, along with the char. when you pull a vac on the gasifier, you are getting some amount of volume enabled by pyrolysis, plus some amount enabled by air coming in for combustion. you only get combustion to the degree that you are not generating volume by pyrolysis, and heat to drive it by something else. and hopefully the amount of combustion you get creates the needed heat and spread to cover the hearth so cracking works and tar is converted.
                >
                >
                > thus the amount of heat add or lose will determine the amount of combustion you have. this is the point of stephen's conjecture on why the char dims with preheated air. seems possible, but also the air density issue might be part of this too. i don't really know.
                >
                > what i do know is we will need less combustion with preheated air and preheated fuel. this will result certainly in less nitrogen dilution and a higher energy density gas. it will also result in higher top temps and general improvement in moisture handling.
                >
                > in the negative, it is also increasing the amount of tar you are having to crack vs burn. a massive heat loss gasifier might require so much combustion to keep up with heat and gas evolution needs, that all the tar could be burned to completion, and the char overrun with lots of c02 and h2o coming out the end.
                >
                > the fluidyne design is optimized for good fines purging of the reduction zone. the reduction zone is significantly reduced with the striaght tube while doing this. this is a fine tradeoff, but do note what we get and what we lose.
                >
                > with the truncated reduction tube, residence time is shortened and less reduction is completed. gas exits hotter and with more co2 and h2o not yet reduced. doug suggests tuning the fluidyne by moving the grate up until the gas goes fairly anemic. this is where the char fines will flow out without grate shaking. it is also where much of the reduction is prevented, and the less energy dense gas that results.
                >
                > note here that the gas volume making method is more tilted towards the cracking of pyrogasses than the reduction of combustion gasses. this is fine, as we again have such an excess of tar gas to crack that in principle, we could get combustible gas with no reduction whatsoever.
                >
                > all of this follows from a pursuit of good fines purging. the fluidyne design smartly focusses on this as it is a huge problem once you get to long runs. greg has similarly shown how well it works with his gasifier boiler.
                >
                > that got a bit off air preheating. i was trying to explain some of why this is not so simple. combustion is a "filler" process in a gasifier. the amount you have will vary depending on heat added or lost elsewhere in the system, and the gasses evolved by pyrolysis as a result. total preheat and you need no air whatsoever. that is also called a charcoal retort.
                >
                > realistically, we are always heat challenged in a gasifier and are struggling to maintain the 1000c and up temps over a big lobe needed to crack the tars. removing all parasitic loads that take heat and reduce top temp seem indicated.
                >
                > air on the away into the gasifier needs to be raised to the auto igntion point of tar gas before oxidation will start. something has to heat that air. it will either be local heating in the hearth, which takes away heat, or something external, which prevents this loss in the hearth.
                >
                > similarly drying, pyrolysis, heating the char to hearth temps also require heat. this heat is coming from somewhere, and reducing the temps of whatever the source is. if we are using the hearth as our heat source for these heating needs, we will be lowering the temps int he hearth.
                >
                > i argue we should try to eliminate all these parasitic loads. get all the heating loads of incoming air, drying and pyrolysis off of the combustion in the hearth. let all the heat generated there by combustion be used to max temp s and best crack the tars.
                >
                > this is why the gek totti has a big heat recycling system to eliminate the heating loads usually presented incoming air, fuel drying and pyrolysis. i'm trying to deliver already hot char and hot air to the hearth, making the gasifier more like a charcoal gasifier with an externally driven pyrolysizer sitting on top.
                >
                >
                > the total heat budget of this architecture will be superior to a non heat recylcing system. to argue not suggests that we get heat for free from somewhere else. heat is never free. adding more cold does not create more hot.
                >
                > nonetheless, it is possible that we might ahve some localized higher temps at some point in the process, either by change in combustion amount or reduction in temp elsewhere. super cold air might result in proportionally more combustion inside, and thus a higher spot temp.
                >
                > or, the cold air might really be saying something about the absolute water loading of the air on the way in. in the numbers we've passed around, the heating changes that result from humidity changes seem to trump the 100f or so air temp differences we are concerned with here. it might be that the impact of water differences in cold air is what is creating the effect. it might have nothing to do with temp.
                >
                > who knows, as no one is trying to answer this with controlled testing. so far we've had a video of a variably glowing ember and ensuing hysteria!
                >
                >
                > i'd say the best test of this we could do would be to measure in hearth temps in multiple spots with quick switch between heat and no preheated air intake. measure the temp differences and measure the resulting tar coming out the system. do a test like this, but with air in temp changing: http://wiki.gekgasifier.com/w/page/6123786/Multi-fuel%20Run%20Comparison
                >
                > problem is there are turning changes needed to optimize each mode. the arc of combustion propagation and temp rise/fall will be different, requiring nozzle and hearth redimensioning to take advantage of the differences. so just switching back and forth will handicap the potential of one or both modes.
                >
                > we'll do this at some point out here, but frankly, many other things have been higher priority. of the many known unknowns of a gasifier, this for me does not rise to one of them. thus i have directed limited and time and resources elsewhere. we're not going to gain any meaningful ground by putting cold air into a gasifier, despite the recurring strum and drang to the contrary.
                >
                >
                > jim
                >
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