i'm trying to explain the basics of gasification. as many of you know, this can be hard. so i'm trying to relate it to things that are generally known about combustion, then complicating matters from there.
here's where i'm at currently: http://www.gekgasifi...s/how-it-works/
does this help? suggestions for making it better? i'm trying to figure out the layers to take the newbie through.
as most of the mysteries of the universe are somewhere contained in the wider problem/opportunity of gasification, i can't start at its many ends. but what's the beginning before the freefall into the bottomless rabbit hole of biomass thermal conversion?
j
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3 replies to this topic
#2
Dharris894
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Posted 12 September 2009 - 12:16 PM
Jim,
As someone who is trying to learn the mysteries of wood to energy alchemy I think you have done the best job I have seen yet with this outline.
It is so important to understand the complete set of interactions and you did a very good job.
Here is a question I have:
I take a simple stove pipe and fill it with wood shavings, light it from the top and leave room at the bottom for air to be drawn in. The shavings burn slowly from the top down creating a moving front of a combustion layer, pyrolysis action directly below it and a glowing char layer above it.
The gas from this moves up the pipe and when it meets the air it forms a nice mellow flame. It seems to be a balanced, stable version of the process.
My question is whether that gas (if kept from mixing with air until it is cooled) is the producer gas or is the exhaust gas formed when it combusts with the air at the top of the pipe the producer gas?
As someone who is trying to learn the mysteries of wood to energy alchemy I think you have done the best job I have seen yet with this outline.
It is so important to understand the complete set of interactions and you did a very good job.
Here is a question I have:
I take a simple stove pipe and fill it with wood shavings, light it from the top and leave room at the bottom for air to be drawn in. The shavings burn slowly from the top down creating a moving front of a combustion layer, pyrolysis action directly below it and a glowing char layer above it.
The gas from this moves up the pipe and when it meets the air it forms a nice mellow flame. It seems to be a balanced, stable version of the process.
My question is whether that gas (if kept from mixing with air until it is cooled) is the producer gas or is the exhaust gas formed when it combusts with the air at the top of the pipe the producer gas?
#3
jimmason
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Posted 13 September 2009 - 02:13 AM
what you have described here is a TLUD, or "top lit updraft". this is mode of current interest for many stoves project. you can also think of it as a stratified downdraft gasifier, turned upside down and lit from the top, then pulled at a slow gas rate.
in the TLUD form, the gas that is coming off the top is a combination of pyrolysis gasses and some reduction gasses. the "flaming pyrolysis" front where the air meets the unburned fuel inside is a rather messy front, and you get a mix of drying, pyrolysis, combustion and reduction happening at the same time. it is not clearly a sitution where you burn your tar gasses and then later try to reduce them over hot coals. temps in TLUDs often don't even reach reduction temps, so you are more just partially combusting the pyrolysis gasses at the flaming pyrolysis front, then the rest come out the top as burnable gasses. given the low temps, these will be diluted with co2 and h20 from combustion, and more h2 from the drying below the flame front.
as you increase the pull rate you will generate higher temps and progressively create conditions where proper reduction will happen, as well as tar cracking around the flaming pyrolysis front. this is what you are trying to achieve in a stratified downdraft. the main problem is the flaming pyrolysis front wanders around as flow rate varies. it is difficult to keep it in one place, and thus maintain the needed distances and relationships for the 4 processes to set up properly. this is why many open top stratified downdraft types later went to a second stage for tar reburning, or multiple points of air injection so they could control where the pyro front was located, and the temps achieved. and by the end of all this, the results were more complicated than the original nozzle and throated types they were trying to replace, with still lesser success in tar conversion by most accounts. most open top stratifed downdraft units today are only achieving reasonable gas through a very complicated gas clean up train. all this tends to destroy the economics that made gasification attractive in the first place.
however, this architecture works fabulously for stoves work, where you are directly combusting the mostly pyrolysis gasses right at the top of the reactor tube. secondary air is introduced here to burn it.
so in summary, yes, it is possible to run a gasifier with nothing more than a straight pipe and close consideration of the pull rate through it. this is how a cigarette works (through as an updraft, not the cross-current wandering flamefront of the TLUD)
good question mr harris. thank you for asking.
j
in the TLUD form, the gas that is coming off the top is a combination of pyrolysis gasses and some reduction gasses. the "flaming pyrolysis" front where the air meets the unburned fuel inside is a rather messy front, and you get a mix of drying, pyrolysis, combustion and reduction happening at the same time. it is not clearly a sitution where you burn your tar gasses and then later try to reduce them over hot coals. temps in TLUDs often don't even reach reduction temps, so you are more just partially combusting the pyrolysis gasses at the flaming pyrolysis front, then the rest come out the top as burnable gasses. given the low temps, these will be diluted with co2 and h20 from combustion, and more h2 from the drying below the flame front.
as you increase the pull rate you will generate higher temps and progressively create conditions where proper reduction will happen, as well as tar cracking around the flaming pyrolysis front. this is what you are trying to achieve in a stratified downdraft. the main problem is the flaming pyrolysis front wanders around as flow rate varies. it is difficult to keep it in one place, and thus maintain the needed distances and relationships for the 4 processes to set up properly. this is why many open top stratified downdraft types later went to a second stage for tar reburning, or multiple points of air injection so they could control where the pyro front was located, and the temps achieved. and by the end of all this, the results were more complicated than the original nozzle and throated types they were trying to replace, with still lesser success in tar conversion by most accounts. most open top stratifed downdraft units today are only achieving reasonable gas through a very complicated gas clean up train. all this tends to destroy the economics that made gasification attractive in the first place.
however, this architecture works fabulously for stoves work, where you are directly combusting the mostly pyrolysis gasses right at the top of the reactor tube. secondary air is introduced here to burn it.
so in summary, yes, it is possible to run a gasifier with nothing more than a straight pipe and close consideration of the pull rate through it. this is how a cigarette works (through as an updraft, not the cross-current wandering flamefront of the TLUD)
good question mr harris. thank you for asking.
j
#4
canuck111
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Posted 25 February 2010 - 12:20 AM
I have just joined site as I have interest in gasification but want to try and couple with other fuel and power sources...Solar power(with battery) running blower motor and additional hydrogen added to the fuel mixture from altenator...
Is the producer gas better if it contains more hydrogen?
I am a fabricator and can build all the tanks lines etc and I have a cng pickup I am looking to convert. The reason I was thinking of adding the extra hydrogen so I can make the gasification portion smaller. Possibly hidden from view. The hydrogen production will be done with water electrolisis and added to the syngas mixture.
Have you ever used steam/air mixture to improve hydrogen content in you syngas....I am finding so many variables that it is confusing.....
Is the producer gas better if it contains more hydrogen?
I am a fabricator and can build all the tanks lines etc and I have a cng pickup I am looking to convert. The reason I was thinking of adding the extra hydrogen so I can make the gasification portion smaller. Possibly hidden from view. The hydrogen production will be done with water electrolisis and added to the syngas mixture.
Have you ever used steam/air mixture to improve hydrogen content in you syngas....I am finding so many variables that it is confusing.....
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