Kraken stellt sich vor und bittet um Erklarung

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Pascal Kirchner
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Re: Kraken stellt sich vor und bittet um Erklarung

Beitrag von Pascal Kirchner »

Hallo Kraken
Die Crossdraftkanalen sind "blockiert", trotzdem fast alle Abgase fießen dadurch. Dort gibt es keine Reduktionszone und rohe Gas kommt heraus.
Ich bin nicht sicher, warum die untere Teil des Kohlebettes wird nicht erhitzt. Entweder die Wärme ohne Gasstrom wird sehr schlecht durch die Kohlebett transportiert und ganze Konzept ist fehlerhaft, oder ich muss die Drosseln dichter machen, dann am Anfant das ganze Kohlebett wird entzündet und als Reduktionszone dienen.
was willst du nun verändern am Konzept ?

lg Pascal
Islander
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Re: Kraken stellt sich vor und bittet um Erklarung

Beitrag von Islander »

Dear Octopus

Firstly, I hope you can understand English - and second that the translate function on this website has let me understand your problem ....

Attached is a paper on superficial velocity, which is the key to understanding gasification: -
SuperficialVelocity.pdf
. The authors are Reed, Walt and Das (the "Dasifier") so you know it is worth reading.

Look at the second graph in 4.7 and imagine you are starting you gasifier with maximum air - even more than is needed to burn 100% all the material. So you are at the right hand side of the graph. It does not matter what the design of your gasifier is: the theory applies to all designs. Now, gradually reduce the air flow - either by slowing down your blower, or by throttling the inlet air. You will get to the point where you are allowing exactly the correct air to burn the wood completely: this is called stoichiometric combustion and Lambda =1.0. For wood this is about 6.4 kg air per kg wood )but it can change a bit depending on the wood type and, of course, the moisture). At this point - Lambda=1.0 - you are burning all the wood completely so producing around 4.8kWt/kg wood in the gasifier.

Now, continue to reduce the air flow so there is not enough oxygen to fully burn the wood. This means that lambda is, say, 0.75 and some of the calories are left in the gas. So the burning in the gasifier goes down and the gas in the flare has some value - maybe carbon monoxide or methane. As you continue to reduce the air flow, the value of the exit gas will increase and eventually you will be able to light it. At this point you are gasifying the wood to a weak gas.

If you continue even further to reduce the air (oxygen), the calorific value of the gas will continue to rise but you will get to a point where the gasifier cools down due to insufficient burning. However, well before this point, you will start to produce a gas that is full of tar. This happens (depending on the wood) at about lambda = 0.25. The key to successful gasifier design - for wood that is - is taking the air flow down to the lowest level that is possible without producing too much tar. So a gasifier must be designed for a particular engine: it may be that your design works well with a 1 litre engine, but that with a 1.5 L it makes a weak gas and with 0.5L it produces tar. You have to work backwards from the endpoint when making your design.

It is difficult to explain but until you understand superficial velocity you will not understand wood gasification.

Islander :thumbup:
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kraken
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Re: Kraken stellt sich vor und bittet um Erklarung

Beitrag von kraken »

(Okay, this time in English.)

Dear Islander

I've read this paper few months ago and - to be honest - I did not understand anything. It applies to all other explanations of how gasification works. Even after watching Gasification 101 from APL I knew all the processes that are happening in the gasifier, yet still didn't know how to build one. Common practice of describing gasification as 'combustion in limited amount of air' is so totally misleading! It's combustion in limited amount of oxygen. Limited not by restricting air access but by burning part of the fuel in as high temperature as possible. For a long time I did not encounter the idea of gasification explained that way. I did not see it explained sufficiently in any place. Looks like people who are doing this either do not understand it or can't explain it, because the way it works is too obvious for them.
Luk did not only show me how it goes but explained the idea in two plain sentences!

Now I understand what superficial velocity is. But it's an engineering term which does not explain things. You have to know how things work prior to understand it. One has to get over the 'limited amount of air' curse. Once I know that it's not about limiting air access, but on the contrary: pushing even more air through the fuel bed to consume oxygen by burning tars, SV is pretty self-explanatory. The higher air flow rate is, the hotter combustion zone gets, more tars are burned and more cleaner gas is produced.


Regarding my crossdraft prototype, I tried to ignite charcoal bed starting from gas exit, leaving air flow as it was. Once it lit up fully, I added some pellets, but again it gave me lots of tarry gas and worked only few minutes. After that smoke was not burnable anymore. Looks like it's broken by design. Without enough char in fuel tube at the top it hardly works, so it seems that it is the fuel tube part that is doing almost all the job.

I put this prototype aside for a while and switch to more classic downdraft. As an interlude I took this old L-shaped thing made of refractory concrete I used previously.
WP_20150424_12_53_52_Pro.jpg
Insulated with rock wool, this time fed with fine-grained coal, was producing gas like this:
WP_20150911_17_42_00_Pro.jpg
Now I plan to build something similar to Spacepeter's gasifier but using refractory concrete instead of steel pipe for fire tube, making hearth diameter extending a bit downwardly for easier fuel flow.
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Islander
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Re: Kraken stellt sich vor und bittet um Erklarung

Beitrag von Islander »

kraken hat geschrieben: The higher air flow rate is, the hotter combustion zone gets, more tars are burned and more cleaner gas is produced.
Yes this is correct, BUT as you push the air flow up you are doing more and more burning in the gasifier and so less calorific value is left in the gas leaving the gasifier. The key to gasifier design is in providing just enough air to burn all the tars (for an engine) BUT NO MORE because then you are wasting energy by burning in the gasifier. So I think you understand this very well: if you gasifier is producing dirty gas you must pull it harder.

:thumbup: :thumbup:
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luk
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Re: Kraken stellt sich vor und bittet um Erklarung

Beitrag von luk »

Kraken, Josef,

I would try a simple things on your design.

I would completely close the opening where your pipe enters the bottem part. It will force your smoke through more charcoal (as seen from your drawing some 40 cm) and you might get a better reduction and somewhat better pull. Now the danger is that you suck already produced gas inside your glowing coal and burns most of it.

Then I would put your lighter a bit above your flare so the gas has the change to mix with some air. The gas will light better.

Try this and let hear us how that went.

Luk
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kraken
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Re: Kraken stellt sich vor und bittet um Erklarung

Beitrag von kraken »

I've abandoned that latest prototype and the idea of trying to use crossdraft principle. Simply not working as I expected (not to mention it wasn't typical crossdraft actually) and at this moment not seeing chance to fix it. I decided not to modify it because there's already lot of dirt inside, not suitable for welding anymore.

Now I'm slowly building something new from scratch:
nv2.jpg
Plain downdraft, hearth made of refractory concrete, tightened together with long screws, refractory ceramic cord between concrete and steel. One thing I'm not sure now is if this connection will remain airtight at high temperatures (although highest temps are supposed to be inside, but steel will surely heat up under insulation).
To remove ashes, I use screw auger of 9cm diameter, in a square channel, ~80cm long. It's far too long, but in this form it is ready to use and making it shorter would cost some effort, not worth it. This thing is originally used in automated coal furnaces, quite popular here. They sell this part in few sizes and this is the shortest version, custom width would be more expensive or not so durable (here's 24mm axle, scroll made of 6mm steel, meant to transport raw coal).
Ash will be transported to an airtight ash box, not drawn here.
Whole thing will be insulated with perlite. Meanwhile I tested perlite insulation with an old ceramic casting and, wow, over 1000C inside, 5cm of perlite, 1mm steel and I can touch the steel, it had maybe 70C.
Ceramic hearth is already there.
WP_20151007_14_47_08_Pro.jpg
I still need few days to weld the rest of this thing and put it together. Then I'll probably run it to make sure it won't disintegrate after heating up :lol:
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