[Gasification] slose coupling ic and gasifier (was Re: water injection of producer gas fired ic engines)

[jim mason]

On 7/9/06, Ken Boak <kenboak at stirlingservice.freeserve.co.uk> wrote:
> Jim and List
>
> You wrote
>
> relatedly, has anyone here yet to notice that all this fuel we are
> burning to make process heat to run the gasification reaction is
> largely the same thing happening next door in the ic motor?
>
> Yes - I posted something to this list last week but I fear that it was
> widely missed.
>
> I have CO2 rich exhaust gases coming out of my diesel at 350 C.   By
> increasing the load, the exhaust gas stream can be raised to 400 C or more.
>
> This process heat could be put to some use surely,  and is every atom of
> carbon not crying out for some hot CO2?.
>


i keep coming back to this question from ken.  there seems much
potential in exploring this.

as gasification is a process for separating out the stages of
combustion so we can mine and redirect its heat and chemical products
for desired effect, it seems we should note the curious presence of a
combustor when we are running a gasifier into an ic motor.  the ic
engine itself is a combustor producing the same products as the
combustion zone of a gasiifer.  heat and CO2.  these are of course are
the products we want for later stages of reduction C + CO2 = 2CO and
fuel drying.

the easy use of this would seem to be is to close couple the exhaust
of the ic motor to a heat exchanging coil inside the top of any fixed
bed gasifier.  300-400degC is plenty of useful heat to boil off water
in the fuel as vapor.  and really, exhaust gases can be much higher
than this, which is discussed below.  wasted exhaust heat out the ic
engine should be larger than the waste heat in the gasifier "exhuast",
the equations say.  though waste gasifier heat is of course relevant
too, and primed for preheating the gasifier air intake, or gasifier
fuel bed also.

the problem of too much water in the fuel is that it takes lots of
energy to boil off the water as vapor, which must happen before the
fuel can rise to pyrolysis temps.  and this energy needed to boil off
the water raises the process heat requirement for the total gasifier
to a point where there is not enough heat coming from the exothermic
combustion zone to run the endothermic reduction reactions.  thus the
CO2 from the combustion passes through the reduction zone and is NOT
completely reduced, as the temp falls too low.  thus the output with
too wet fuel is high CO2 gas.  and tars passing through are also not
fully cracked, so high tar comes out (in the case of a downdraft
gasifier.  updraft will have less change).

clearly the more heat we can return to the fuel drying part of hte
gasifier from the ic exhaust, the less heat the internal gasifier
combustion needs to provide.  allowing for wetter fuel.  or if dryer
fuel, we can start injecting steam to increase H2 and CH4 yields.  or
secondary ic exhaust injection into the reduction zone to provide more
CO2 for reduction.  like the kahle gasifier, which uses 20% recycled
CO2 from the ic motor, with nice gains in efficiency from doing so.
in general the more heat that can be added to the gasifier from
sources OTHER than the gasifier combustion, the more feed "fuels" to
the endothermic reactions can be supported.  thus more CO2 in or H2O
can be added.  and more CO and H2 will come out.  keep adding to the
point where the temp starts to drop below acceptable levels for the
endothermic reactions.  keep realizing efficiency gains.

ken says his exhaust gas is 350-400C.  not sure where his gauges are,
but this seems low to me.  manifold temp on a turbo diesel under full
load maxes at 700C.  over that there are problems.  any reasonable
load produces temp in the 400-600C range.  and as 1/3 of the power of
the engine is being converted to this temp, that is lots of heat in a
useful range for a gasifier.  this is not just drying temp, this is
pyrolysis temp.

we often talk of close coupling a tar producing gasifier to a open
flame application like a boiler.  but it seems we should consider some
close coupling in the other direction.  namely, treating the ic motor
as an important combustor producing process heat for the gasifier, and
close couple the ic to the gasifier.  good insulation wrap around the
manifold and exhaust pipe between the ic and gasifier can keep this
temp up until it gets into the heat exchange coils in the gasifier.

if you want more heat, recycle some of the product gas back into the
ic exhaust, and fire it as an afterburner inside the ic exhaust /
gasifier heat exchaing coils, both generating more heat, and burning
off "waste" hydrocarbons in the ic exhaust at the same time.  it would
be curious to see if there is enough waste hydrocarbons in the ic
exhaust to make a recycled producer gas after burner, inside the fuel
drying zone, a reasonable proposition.

such an afterburner might also be a reasonable way to extract the
extra 6-10% carbon left in the charash in a downdraft gasifier.  pack
it around the afterburner so its pyrolysis gases either enter the
afterburner stream, or off gas into the gasifier itself, to be
consumed in the combustion zone.

watch the heat, C, H and O go round and round.  the world is on fire . . .

j