[Gasification] The Kalle Gasifier Chemistry
Thomas Reed
tombreed at comcast.net
Thu Jan 18 08:20:19 CST 2007
Dear Jims and All:
You said below "for some reason i remember the output gas being 60% more
volume than air input". Here's the answer in a large nutshell. It is
worth the effort to learn this because the Kalle (and other) charcoal
gasifier embodies the basic principles of all gasification. (Downdraft
wood gasifiers are basically wood pyrolysers feeding raw gas into a
charcoal gasifier.) (See and study the Kalle gasifier at
http://www.hotel.ymex.net/~s-20222/gengas/kg_eng.html, the best
description of any gasifier development that I have seen.)
I seem to be the only one that pays attention to the rather simple
chemistry of gasification and air/fuel ratios. It was a major part of
developing the BEF WoodGas cookstove. Bear with me and I'll make carbon
gasification chemistry as painless as possible.
------------------------------------------------------------------------
You said below "/for some reason, I remember the output gas being 60%
more volume than air input"/. Here's the answer in a large nutshell.
The fundamental reaction in the Kalle and all charcoal (and coal)
gasifiers is the *carbon partial oxidation (POX) reaction*:
* C + 1/2 (O2+4N2)* ==> CO** + 2 N2 *
so 2.5 moles into the gasifier gives 3 moles out (moles = volumes) or an
increase of 20%, not 60%. This is oversimplified, since the reaction
temperature for this partial oxidation is 1250C, much too hot for most
materials of construction and wasteful of energy.
------------------------------------------------------------------------
The partial oxidation reaction temperature for this reaction, Tpo =
1252C, is too hot for most materials and a great waste of energy. This
is moderated in the Kalle gasifier by "exhaust gas recycle" which
depends on injecting exhaust to convert excess temperature into more
heat. It depends on the *Boudouard reaction:*
* C + CO2 <==> 2 CO
*Note the double arrow. This reaction is VERY endothermic and so can
convert the *excess *thermal engine of the partial oxidation reaction to
more fuel and keep the gasifier cool. However, below ~900 C this
reaction goes into reverse, so we can't recycle all the CO2 in the
exhaust and have perpetual motion.
------------------------------------------------------------------------
In the real Kalle gasifier exhaust CO2 is added to moderate the
temperature, something like:
C + 1/2 (O2+4N2) + CO2 ==> 3CO + 2N2, so 3.5 moles produces 5 moles, an
increase of 42%.
The exhaust of the engine using charcoal gas is *C + (O2 + 4N2) ==> CO2
+ 4N2*, so if you are *recycling the engine exhaust*, each CO2 from
exhaust will bring in 4 N2. I'll leave it to you to see how much
exhaust you can add before the temperature drops below 900 C and the
Boudouard reaction goes into reverse. How do the Kalle gasifier know
how much CO2 to add? Today we would have a thermocouple measuring the
output gas temperature and an electronic control keeping the temperature
above 900C, but Kalle was probably smarter than that.
------------------------------------------------------------------------
Note that using engine exhaust you also get the same tempering effect
with steam and the *water gas reaction*:
* C + H2O <==> CO + H2*
and this is also a VERY endothermic reaction which quickly cools the
excessive heat of the partial oxidation reaction. All coal gasifiers
moderate the POX reaction with steam.
The reason that small charcoal gasifiers were well developed before WWII
and evolved into wood gasifiers very quickly during the war (as the
forests disappeared at an alarming rate) was that every city had coal
gasifiers making town gas from coal and coke and the basic reactions
were understood by many people. How many people today know how a
charcoal gasifier works?
Keep up your interest in the Kalle gasifier and we'll see if we can
resurrect that knowledge.
Yours truly,
TOM REED THE BIOMASS ENERGY FOUNDATION
* If you are being exact, air is more accurately "O2 + 3.76N2" (and even
more accurately you need to include ~1% argon which is inert like the
nitrogen and .03% CO2 which makes all life possible, but maybe warmer.
When I get to heaven I'm going to ask the chief chemist to adjust air to
my 20% O2 and 80% N2 for easy calculation.
**Carbon monoxide, CO, has exactly the same heat of combustion, 280
MJ/kg, as hydrogen, H2, so maybe we should be heading toward a CO
economy rather than a H2 economy. In fact we should be heading BACK to
a CO + H2 (producer gas) economy which was in place from 1820 to 1940
when methane pipelines went transcontinental. Only eventually we should
be making the CO and H2 from biomass, not coal.
------------------------------------------------------------------------
------------------------------------------------------------------------
jim mason wrote:
>> Tom Reed BEF
>>
>> There is a picture of the gasifier mounted on a sedan at the end of the
>> article. The height of the car is 5 cm on my screen and the diameter of
>> the gasifier is 2 cm. If the true car height is 2 m, then the gasifier
>> has a diameter of (2/5*2) 80 cm. It looks rather "fat". It was
>> probably made in different sizes for different engines, just as later
>> the Imbert/WWII gasifier had to be sized to the engine.
>>
>
> well yes, we can estimate the total hopper size in this manner, but
> none of the critical dimensions as you know. moreover, i would
> imagine the kalle is rather flexible on total hopper size, as the
> action is all in the center nozzle and uptake grate. in general i
> think they were usually squarish in cross section. more like a cross
> draft than typical up or down draft designs.
>
> i would imagine we could estimate nozzle size for x gas output from
> total nozzle area for multi tuyeres imbert types. those tables are in
> all the books. and direct delivered air for x gas should be around
> the same for the kalle and nozzle type downdrafts.
>
> the uptake grate could be estimated by making it at least equal to the
> area the gas flows through after the grate (in the space bwetween the
> two center tubes). but really, it would need to be rather larger than
> this, as char is going to build up on the grate and resist flow.
>
> there was a rule of thumb i read somewhere between volume of air in
> and volume of gas out. for some reason i remember the output gas
> being 60% more volume than air input. this would be corrected to
> standard conditions, which of course were not very standard. but
> approximately from there, you could add heat, and therefore get
> volumetric flow, and use the tables that show flow resistance for x
> volume. and size grate and center tube flow area accordingly. uggh!
>
> doesn't someone have all this documented out already? did every last
> kalle really end up in the scrap heap?
>
>
>> You are correct that this is probably the most explicit
>> explanation of a chemically and mechanically sophisticated
>> invention ever published. I also recommend it to all as
>> fascinating reading and inventing.
>
> i agree it is an inspiring read. but i still think my most inspiring
> read in the gasification world to date has been the "encyclopedia of
> biomass thermal conversion" in your BEF catalog. the various concerns
> and factors explored therein, in academic studies, design history,
> nature of fuels, indexes of research and companies, details tens, if
> not hundreds of similar thought trajectories as the kalle article.
> you can't read it and not similarly start imagining new designs and
> scenarios. all variables become clear and open for play. it is what
> hooked me. and its only 800 pages . . . ;-)
>
>
>
>
>> jim mason wrote:
>> > i am wondering if any of you might have ideas for where to find
>> > specific dimensions and documentation of the kalle gasifier.
>> >
>> > the best i can find it the article at
>> > http://www.hotel.ymex.net/~s-20222/gengas/kg_eng.html. the article is
>> > great. if you haven't read it, you should consider it. a great walk
>> > through of the logic that led to the design.
>> >
>> > nonetheless, the documentation on the dynamic nozzle/grate tube, as
>> > well as the cyclone/co2 recycle are not very specific. approximations
>> > of course could be made, but it would be much more attractive to start
>> > where kalle left off, and not replow his field.
>> >
>> > so, does anyone have or know of where there are specific dimensions or
>> > even good pictures? better yet, has anyone ever heard of one for
>> > sale? likely not.
>> >
>> > maybe someone knows someone who has one and could be bothered for some
>> > detailed documentation?
>> >
>> > thanks in advance.
>> >
>> > jim
>> >
>> > _______________________________________________
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>> > Gasification at listserv.repp.org
>> >
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>> > http://gasifiers.bioenergylists.org
>> >
>> >
>> >
>>
>>
>
>
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