Flue Gas Temp for a Gasifier

Not quite sure if I am asking a valid question but I am wondering what people are seeing for flue gas temps with their gasification boilers? I have a 2.5+ story chimney that gets a low draft (based on Manometer) but also generating a high flue gas temp. 250C+, the boilers upper limit was set at 250C so the software was controlling the burn. I have recently increased the upper limit to get a sense of where it wants to go but Murphy's Law. I am now in a part of my wood pile that is made up of a different wood and it has gotten much colder - not sure of what caused it but I am now staying under 240C. The wood that was causing the hign flue gas temp is super dry (3+ years for the most part). The boiler has a 180mm (just of 7" diameter) exhaust and I feed it into 8" stovepipe that feeds into a clay lined flue that has outer diameter of 7.5x11 (wondering if a liner is needed). Having the chimney cleaned tomorrow to be be sure I know what I am working with but I think it is clear. Thought I would check to see what others are seeing out there.

maple1 said:

I'm usually right around there (250c) if burning decently hard. Mine is a bit different though since it is natural draft, it is said to burn with better combustion efficiency with somewhat higher flue temps than induced draft boilers. But I don't think I get quite as high as that point of best combustion efficiency is, I rarely fill it full and full is when it is really humping.

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Thanks for the info. On my system their is a fan in the back that creates the "downward" fire into the lower gasification chamber. Am I correct in thinking yours does not have this fan and that it is all via natural draft? I am pretty knowledgeable about how my boiler works but not so much on industry terms, etc. Thanks for the help!

Thanks

Nofossil said:

I'm right in the same range (measured with a thermocouple in the center of the flue at the boiler exhaust).

My brother's home-made gasifier, on the other hand, runs at 120 flue temp - a condensing gasifier, of all things.

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Thanks, when you say that range I'm not sure which post you responded to are you in the 250 or 180 range? I wish I kept track of what I was at last year, I swapped to a bigger model boiler over the summer so I don't know if my old one didn't have a top temp shut off or if I was running lower flue gas temp. I'm not sure it is bad, if anyone has thoughts I am open to listening.

AroostookDave said:

My Tarm gasification boiler flue temp runs about 180c when it is running well. If it gets much above 200c it means that it is time to clean the dust out of the tubes. It has only one method of downdraft fan, always running. It does require a good size water tank. My wood is mixed variety and always under 20% moisture.

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Thanks for the info, my temp has been down the last few days 200 - 230. I was thinking it was because I wanted to get rid of some standing dead that I just took down and split (plenty dry just my other wood was drying for 3+ years) so the fuel source switched but now that you mention it I cleaned out the tubes a few days ago as well. I'm having the chimney cleaned tomorrow, long story but I had about a dozen really bad fires at the beginning of this year and last year was the rookie mistake - way to moist & way to big, had a bunch of bad fires at the start of that year as well. That will take care of all the variables I can think of. After that I am left to the low draft question, I am going to start a new thread on it but I am getting a .07 reading, with a chimney this tall I expected the opposite problem. I had installed a Draft Reducer but removed it because the only thing I got from it was dust. Not suer if I have a problem that needs a liner to address or if I should go make a Cappuccino and enjoy the warmth....

huffdawg said:

mines at 205c at this moment .. about 20 mins into the second burn for the day

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Thanks. Seems that #'s do vary but even my reduced #'s are on the high side, took a look a little while ago and was at 212 about an hour in (last few days I have been seeing 200 - 235). It is when I was maxing out at 255 that had me wondering. My boiler has a max flue temp control that was set for 250 (I wasn't initially aware of that) which was causing the primary damper to shut down to smother the fire. This would result in boiler temp hovering around 80 until the tail end of the fire (when flue gas volume was reduced) and then ramp up to max temp. The mixing pump was always running so my tanks were getting fully charged. I wasn't sure if maybe the programming was written that way to keep the fire from getting a bit out of control and having a premature overheat (pardon the expression).

woodsmaster said:

I run around 185 to 205 C when the tubes are clean and up to around 285 C when dirty.

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That is interesting, thanks! I had some fan motor issues at the start which resulted in about a dozen fires where the fan kept going on and off - a creosote nightmare. With everything I had going on at the time I didn't give any thought to cleaning the tubes as the boiler was new. They seemed fine when I cleaned them but I understand it doesn't take much.

huffdawg said:

mines at 205c at this moment .. about 20 mins into the second burn for the day

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Thanks. Seems that #'s do vary but even my reduced #'s are on the high side, took a look a little while ago and was at 212 about an hour in (last few days I have been seeing 200 - 235). It is when I was maxing out at 255 that had me wondering. My boiler has a max flue temp control that was set for 250 (I wasn't initially aware of that) which was causing the primary damper to shut down to smother the fire. This would result in boiler temp hovering around 80 until the tail end of the fire (when flue gas volume was reduced) and then ramp up to max temp. The mixing pump was always running so my tanks were getting fully charged. I wasn't sure if maybe the programming was written that way to keep the fire from getting a bit out of control and having a premature overheat (pardon the expression).

jebatty said:

I also have a 250C draft fan shutdown controller which I added to my Tarm Solo Plus 40. That is never reached during burn periods, except sometimes while I add wood during a roaring fire after only a partial burn. Or, if I forget to close the damper after loading a wood, or forget to close the firebox . I installed the control as a safety measure implemented to thwart my stupidity.

Normal high to mid burn flue temps run 200-235C. I have the chain turbulators. Without those the flue temps are higher by about 38C. At the higher end of the 200-235C range btuh output will exceed the 140,000 btuh rating of the Tarm. My wood is well dried mostly pine.

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Thanks, those are the #'s I am seeing - good to know you are seeing the same. I am going to keep an eye on things but attribute the high temp to dirty tubes as Woodmaster suggested in his Reply.

TCaldwell said:

Good morning, the flue temp at the back of the garn at high burn is currently reading 272degf, meaning I need to do a cleaning as its a little high!

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Thanks, I think I learned that was my issue. My boiler was brand new so I underestimated the effect of those 1st dozen or so bad fires. When the boiler was manufactured they forgot to install a small impeller that cools my fan motor. Because of the position of the impeller I never knew one was supposed to be there. The manufacturer never had this problem before so they didn't think of it. They were great though. They sent countless hours on email and phone helping me diagnose. Sent me a replacement motor from Europe basically they did everything and anything to help. Luckily woodburners stick together and in a conversation with another Effecta user in Alaska I stumbled upon the impeller.

velvetfoot said:

I find it hard to believe you're getting low draft in a 2.5 story chimney with cold outside temperatures and hot boiler. My baro damper was going crazy with a pellet boiler running lower flue temps. It has a 5" outlet into a full length 6" insulated liner though. In addition to cleaning, I think you might be onto something as far as exhausting into that big clay lined masonry chimney.

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I'm with you. It is not so low that it is a problem (reads at 0.07 on the Manometer, which is right on the low end of the manufacturers recommended target range), I just expected it to be much higher. As I mentioned i had installed a draft reducer that I pulled out, did not for me but allow dust to float in.

maple1 said:

I was waiting for that 'other thread' to appear about the flue draft thing. But anyway.

I also think you should see lots of draft with that chimney. I can maintain around 0.1" on a calm day with my 30'+/-, 7" Class A stuff. It will spike to 3 or 4 times that with wind gusts if my baro is kept shut.

But if that is what you're measuring, it must be due to the coldness of the clay liner, and the big cross section size. An insulated liner should help immensely. Ideally 7" but that might not fit what is there now. It might if you get the clay out first but that sounds like a project. I would check with Effect to see if a 6" would be accpetable - it likely would be being that tall.

But - what is the draft spec for the boiler? If it isn't more than 0.07", you're likely OK as is?

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Thanks for all the info, I held off on starting the new thread - being a new guy here I was trying not to post out of control. The spec is 0.07 to 0.1. If my flue gas temp stays down (I now think it will, since the tubes have been cleaned it seems to be staying lower - will be back to the same kind of wood I was using in a day or day) I should be OK. Getting the chimney cleaned and will have the guy measure the inside dimensions to see what I can fit in there if needed.

velvetfoot said:

What's a draft reducer? Baro damper? Most boiler mfrs require them, no?

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Fields calls is "Draft Control", I think it is a Baro Damper - not required, just suggested. I pulled it off because I was "closing it as much as possible to try to get the draft up.