[Gasification] Fuel sizing and throats

[Ken Calvert]

Dear Jim etal   I have been watching and reading a lot of theorizing  on 
folks trying to optimize on the
best fuel sizes and maximum surface areas etc.   Two of the gasifiers  that 
we ran 25 years ago in the Pacific islands
had 100mm 4" throats  and we fed them on  wood blocks  approximating to 2" 
cubes.  They were down draft models and the tuyers
would have been around  8-9" above the throat area.  Now in theory that 
sounds horrendous but they worked just fine and one of them worked for 15 
years.
We burnt out three throats on that one and that was expensive because the 
throat was a metal casting made out of  the highest temperature rated
alloy we could get. The last one cost us $1,500 because the cost of the 
Nickel or Manganese or something went up.   It was actually a truncated cone 
about 8" ID accross at the top,  12" tall and 4" ID wide at the bottom. Then 
the grate was about 6" below that again.  That design ensued that the gas 
had to pass down through at least that depth of  incandesant charcoal.before 
leaving the combustion area.   However the point that I am making is that 
even with a down draft configuration, the fuel is well and truly carbonised 
and fragmented and probably that charcoal is well activated long before it 
gets level with the tuyers.  You could almost hear the fuel exploding as the 
gases ripped it apart.   The gas gets pulled down but the heat is radiated 
upwards as well as downwards.And there is a lot of heat   Keep Gasifiying 
Ken C.

----- Original Message ----- 
From: "jim mason" <jimmason at whatiamupto.com>
To: "doug.williams" <Doug.Williams at orcon.net.nz>
Cc: <gasification at listserv.repp.org>
Sent: Sunday, April 01, 2007 4:26 PM
Subject: Re: [Gasification] woodgas truck report for last night


> well, we got distracted today and didn't finish the truck, so i'll write
> some email instead.
>
>
>
>>
>> First you need to recognise the difference of a charcoal gasifier and a
>> wood
>> system. A central tube is OK for charcoal, as a single nozzle, but if you
>> put nozzles horizontal on the end, you will have a big gap between the
>> lobes. It will also have to lick the wall, not stop short with char
>> insulation. Central nozzles are not good performers on wood, as they
>> impede
>> the fuel flow.
>
>
>
> you are assuming the wood is chunk wood and a large reaction area is 
> needed
> as the total surface area of chunk wood is low.  if the fuel is granular,
> the reaction suface is much higher, and reaction area volumes can be much
> smaller.  such is the case with a fluidized bed.  and such can be the case
> with a kalle or crossdraft as long as the pyrolysis is above the
> combustion/reduction zone and delivering char to the reaction zone and tar
> circulation is thoughtfully handled.
>
> such a concentrated reaction zone works in a kalle because of the high
> reaction surface of granular fuels.  same with a cross draft.  granular
> fuels will not work in most downdraft designs, due to packing, as you 
> know.
> air and gravity work together and packed fuel is a result.  but granular
> fuels will work in updrafts, as well as fluidized beds.  noting this is
> important when designing for fuel flexibility.
>
> for me, the "only chunk wood fuel" limitation of most downdrafts doesn't
> fulfill the promise of gasification.  i was attracted to all this by the
> "any waste biomass" line.  the truth has been more like "any waste biomass
> in x size and y density, but not above z moisture, in type A gasifier
> specifically designed for x, y and z".  there is a highly dependent series
> of fuel variables that all must be in line before there is any show.  such
> is not a reasonable use scenario for most situations of waste biomass
> gasification.  which is why just reproducing the best wwII type designs is
> not going to be adequate for small scale gasifcation to gain contemporary
> traction in my opinion.
>
>
>> and really, things moving forward from the kalle design seem able to
>> > combine
>> > ease of fabrication with good insulation without exotic materials, as
>> the
>> > hot reaction zones are in the center of your char, not against any 
>> > walls
>> > of
>> > a vessel.  fuel shape flexibility is also much higher.
>>
>> This gasifier is a charcoal system, and you cannot mix and match just
>> because of simplicity.
>
>
>
> well, i beg to differ.  at least for experimentation.  the history of
> gasification is little other than a mix and match, test and iterate sort 
> of
> endeavor.  i'm not sure if i have ever stumbled across another endeavor so
> flexible in the routes to get to the desired end product.  this is a big
> part of why it has fascinated me so.  never have i met an engineering
> problem less respecting of dogmatic solutions.  it is wide open and new
> things can still be discovered, 100 years after its heyday.
>
>
>
>> this might be a result of the poor insulation on my quadrifier
>> test rig, so that once the combustion and reduction zones get too
>> separated,
>> the reduction zone does not stay hot enough.
>
> Reduction is always immediately beginning at the point the fire stops,
> having consumed all the oxygen. Even a smoulder well have micro reduction,
> but only incandescent oxidation temperatures will create the depth of
> reduction you need.
>
> ---------------------
>
> yes, but if you are running a stratifed, and the glowing char is properly 
> at
> the bottom, and if you significantly reduce gas flow, the flame can climb
> out the top quite quickly, burning the fuel surface, but not fully
> converting chunks to char all the way through.  so the char bed can remain
> at the bottom, but likely not hot enough to do much good.  and the little 
> at
> the point where the fire runs out of oxygen is not enough or hot enough to
> do much, as you note.
>
>
>> the result is always that my test flare goes out when my fire tops out in
>> the hoppper.  when it bottoms out in the hopper, it tends to burn out the
>> grate if i'm using a marginal grate material.
>
> The reduction zone is supposed to be in the char up from the grate, say
> 4-5", and the oxidation zone should be sitting on top of that, so the gas
> making bed is only about 9-19" deep. If the fire climbs out of the fire
> tube, you end up with low temperature charcoal following the flame front 
> of
> the oxidation. When this happens, you need to auger out the bottom 
> charcoal,
> to reduce the pressure drop of the ever deepening reduction char. This is
> done in some systems, and is a crazy way to make gas.
>
> -------
>
> that is a great operating summary doug.  thank you.  and yes, i agree the
> control of these is very touchy and the difficulties of keeping them in
> equilibrium state quickly cancel the ease of building.
>
>
>> well, this is the same thing that is going to happen in a choke plate
>> design
>> or imbert constriction or V hearth with fixed air in.  there is still a
>> exapnsion after the choke plate.
>
> This is incorrect, as the oxidation zone in these type of gasifiers 
> deliver
> incandescent CO2 to the reduction zone. The size of the reduction zone is 
> in
> a way restricted, because the gas passes through the char closest to the
> casting, not through the central char, which is inert down to the grate.
> The gas exits the sides of the char pile on the grate, not through the
> grate.
>
> ---------------
>
> i do not understand this.  how is the center char inert?  the gas is going
> to travel the path of least resistance, which is usually going to favor 
> the
> sides.  but to the degree that ash is not clogging the center, it is going
> to travel there too.
>
> whether controlled air or open hopper, it is still a pile of char after 
> the
> choke plate (assuming the end of the combustion is fixed at the choke
> plate.)  so i'm not sure how the situation is much different.  well, i 
> guess
> it is different in that the co2 will be hotter to the reduction zone after 
> a
> controled and focussed combustion area, given it is more concentrated and
> hotter, unlike the open tube where the combustion is more diffuse.
>
> such benefits are particularly noted in crossdraft and kalle type units.
> very high heat and very good self-insulation of both the combustion and
> reduction zones.  why these have been so ignored in contemporary efforts i
> find odd.  the main problems of keeping nozzles and grates together under
> the intense heat are easily solved with contemporary materials, 
> unavailable
> during the war.
>
>
>>so if there is poor insulation in that
>> area, it will get cooler.  if there is good insulation, it will equalize
>> at
>> relevant temps, in equal opposition to the energy moving into the zone
>> from
>> combustion.
>
> The heat needs to be in the char bed, not insulated to prevent losses 
> after
> it exits the char. If you hold the has temperature after it is made, you 
> get
> reversion to soot and CO2.
>
> ----------
>
>
> yes, the insulation i am referring to is in all zones until the product
> gas.  then the point is to mine such back to the air intake first, and 
> fuel
> feed second.   get the gas cool quickly, recovering as much as possible in
> the process.
>
>
>> it is starting to seem to me that insulation is close to as important as
>> many dimension issues.  the heat loss can be tremendous both internally
>> and
>> externally.  the temp levels of all these reactions are obviously
>> critical.
>> bad insulation can quickly drop them below what will work.
>
> If you have inadequate oxidation temperatures, then you need to conserve
> heat. Insulation is not a fix to try to keep a gasifier hot enough for the
> job. Go back to the core issue, of not enough exothermic heat to drive the
> reduction zone.
>
> ----------
>
> yes, the heat needs to exist first.  but achieving and keeping the proper
> temp is of course greatly helped by good insulation.
>
>
>> .
>>
>> No, you had a 8" fire tube velocity at the plate, discharging its
>> potential
>> heat generation into a 12" reduction zone.
>>
>>
>> how is pressure drop going to show me where the bed stabilizes?  that is
>> unclear to me.  do you mean to note a vacuum reading where gas production
>> is
>> good, and the fire doesn't top out the hopper, then run it at that point?
>
> You should be starting this thing from the bottom, and char only should 
> fill
> the first say 15". the fire will burn up towards the incoming air until it
> reaches a point where the gas will light, this is the point you set the
> pressure drop. If it increases it means the fire is climbing out of the
> tube.
>
> ----------------------
>
> that is a good method of tuning flow of a stratified type.  i hadn't 
> thought
> of that.  but i'd think in practice there is a pretty big variation in
> pressure as the char bed is ashing and internal resistance is changing by 
> a
> whole mess of factors during fuel movement.  in real operating
> circumstances, does such pressure differential monitoring really help to
> control the bed location?
>
> also, and more importantly, what is the range of water inches you think is
> ok to be pulling on downdraft type gasifiers?  when mine are running well 
> on
> pellets, it seems to be about 1-2" of water.  much above there suggests 
> the
> bed is packing in my experience.  i have not read much info on vacuum
> readings of gasifiers.
>
> thanks again doug.   that was very helpful.
>
> j
>
>
>
>
> Hope this helps.
> Doug Williams,
> Fluidyne Gasification.
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