[Gasification] Adding water and Stoichiometrics?

[Ken Calvert]

Doug, thankyou for all those comments.  However, I wasn't getting at you!
>> but NOT for all the reasons that have been offered up so far.
What I was getting at were the comments immediately previous,  about  why 
one should
blow steam, or heaven forbid, spray water into ones air intakes in order to 
get all that extra energy
and  and make a lot of hydrogen etc!
More strength to your arm!
Ken C.


----- Original Message ----- 
From: "doug.williams" <Doug.Williams at orcon.net.nz>
To: "Ken Calvert" <renertech at xtra.co.nz>; "Gasification" 
<gasification at listserv.repp.org>
Sent: Saturday, February 10, 2007 10:49 PM
Subject: Re: [Gasification] Adding water and Stoichiometrics?


> Hello Ken, and Colleagues,
>
> Well Ken, you raise a few interesting points:
>
>> Doug is so very right about using only the driest wood that you can get,
>> but NOT for all the reasons that have been offered up so far.
>
> Unless you can be very specific about how to ask a question, the answer 
> can be quite ambiguous. So will try to tidy up a few loose ends that this 
> discussion has revealed.
>
>>     Coal and charcoal are pretty well pure carbon, and so can only make
>> carbon monoxide.
>
> This is more correct for coal, which in a general sense, does not take up 
> atmospheric moisture, unlike charcoal which can easily come up to 10% 
> moisture, if stored in poor conditions.
>
>> That process produces a lot of surplus heat which will ultimately melt 
>> down
>> your hearth.
>
> This is only true if the oxidation lobe from the incoming air touches the 
> metal. In the case of updraft, where the air can come in under the grate, 
> the oxidation comes back hard to the grates, burning them out. Many older 
> coal gasifiers had water baths under the grates, and this steamed away to 
> cool the grate, and provide additional H2.
>
> Adding water to charcoal gasifiers via steam generation was very common, 
> but not necessary to control the bed temperature, as many suggest. All gas 
> making principles in a high temperature packed carbon bed apply. First you 
> create exothermic heat, which if you want to make H2, should be not less 
> than 1,000C, creating a incandescent CO2 gas, which then gives it's heat 
> to the reduction charcoal, converting the CO2 into CO. This reaction stops 
> at about 850C, so the actual gas making pathway should never over heat. 
> The oxidation zone however, can be blasted to much higher temperatures, 
> but all that does is to make more endothermic heat, which means more CO2, 
> more exothermic heat for reduction, more gas, more waste heat, BUT still 
> an exit grate temperature of 850C.
>
> If gas exiting the grate begins to increase in temperature, it means that 
> the reduction zone is channelled, or begun to allow incandescent CO2, to 
> pass through without reduction. This usually means that the char bed is 
> not flowing correctly, replacing the reduction char as it is consumed, 
> along the exothermic heat.
>
> Correctly designed, there is no surplus heat actually from the gas making 
> itself, but there is the heat still in the gas after the gas making stops. 
> This is the waste heat that's available (<700C), and this is what you try 
> to preserve with insulation etc, if you wish to use it.
>
>> So, the good thing to do is to introduce a bit of moisture, produce some
>> more combustible gases
>> like hydrogen and carbon monoxide and keep the temperatures down in the 
>> safe
>> range for a long life of your equipment.
>
> Yes absolutely correct, BUT, only if your gasifier can make the gas at the 
> lowest operating output with a grate outlet temperature of 850C. The top 
> end output and the increased oxidation temperature, then determine the 
> parameters of your gasifier, because the oxidation can move and burn out 
> the walls. Anyone got red hot walls on their gasifier ???
>
>> Hydrogen is also good because it ups the flame speed of the combustion
>> charge.  Flame speed of straight
>> monoxide is abysmal and you will never get a high reving motor to run 
>> well
>> on a high level of monoxide.
>
> Worth all the effort to have H2 in the gas.
>
>>   Now consider wood, it is primarily cellulose which is a carbohydrate. 
>> And
>> what that means is that for
>> every molecule of carbon there is also one of water built into the 
>> chemistry
>> of the wood.
>
> This is an aspect of wood fuel that many forget about in gasification. 
> Oven dry wood has "about" 20% of it's weight locked ups chemically bound 
> water.
>
> >So, even the
>> driest wood that you can get has more steam already in there and a higher
>> hydrogen content and a better flame speed
>> than anything made from straight coal.
>
> Or fresh charcoal without steam addition.
>
>>I would love to go on and pronounce that cellulose
>> has just the right chemical ie., 'stoichiometric' mixture to provide the
>> maximum amount of chemical energy
>> when it is gasified with air, but  that would be simply shooting my 
>> mouth.
>> I don't know!
>
> I would totally concur with you on this, so long as you specify the fuel 
> conditioning. Gasifiers are not magical machines that turn any wood, of 
> any size and moisture content into high quality gas.
>
>> So,  Tom Miles, Tom Reed,  Doug Williams, all you experts out there?
>> The challenge!  how about a pronouncement on how much steam can we add or
>> subtract to a wood fired gasifier to optimise all the equations?
>
> Ken, I can smell a trick question, especially as my photographic memory 
> reminds me of your Report on the successful gasification of wet wood. The 
> Fluidyne letter files are still complete, and all your letters and photos 
> are on file, so I will ask you to tell us instead. (:-)
>
> My experience of operating the same gasifier in many different countries, 
> altitudes, climatic conditions, wood types, moisture content, tells me, 
> that you need to have wood fuel dried to about 15-20% moisture. In these 
> conditions, the gas making process retains it's temperature profiles while 
> making a reasonable amount of H2 (14-18%), with surplus water passing 
> uncracked to condense in the gas cooling system. I see no reason to add 
> water or steam to a wood gasifier, and I would certainly like to be 
> convinced that it was possible.
>
>>Can we mix wood and charcoal to get a better  mixture?
>
> This is only reducing the moisture content of the fuel load. Why waste all 
> the wonderful hydrocarbons making charcoal just to get lower moisture? It 
> alters nothing of the design requirements needed to crack tar from the 
> wood, and the higher oxidation temperatures would still be needed.
>
>> All those recipies for 'torrified' wood?   Did that improve anything?
>
> Only if you have a gasifier design that cannot handle anything less than 
> really dry wood, or you have not the ability to dry bulk bins of prepared 
> fuel. For the average person, torrification is not easy to organize, and 
> in our PC World, creates an emission problem.
>
> On a closing note, I must point out, that every thing I have offered as an 
> explanation, can and will be, taken out of context.
> I can only watch for your comments,  and or assumptions, but cannot put 
> any more time into writing. Only luck tonight gave me time to offer the 
> above pathetic offering, to describe the wonderful phenomena called 
> gasification.
> Doug Williams
> Fluidyne Gasification.
>