[Gasification] CO2 recycling

Tue Oct 16 20:06:17 EDT 2007

 Dan .
I did not mention nitrogen. for there is none.
The biomass is being pyrolized? in the absence of oxygen or nitrogen.
Only steam is being provided for the production of hydrogen and oxygen.in addition to the "BIOMASS products within this plasma reaction.
Should extra oxygen be required,it will be separated from nitrogen and provided as necessary.
Perhaps you feel that St Lucie county (my home) is about to be defrauded by a fly by night company 
promising? to provide surplus electrical energy from the pyrolization of garbage.
Not from comparable high grade Biomass ,but from GARBAGE? 
As previously stated this is not a new technology and is being successfully used in Japan and Spain. 
So why do your mathematics add up to a no can do ZERO?
GF

-----Original Message-----
From: Daniel Chisholm <dmc at danielchisholm.com>
To: Discussion of biomass pyrolysis and gasification <gasification at listserv.repp.org>
Sent: Tue, 16 Oct 2007 12:26 pm
Subject: Re: [Gasification] CO2 recycling

On Tue, 2007-16-10 at 16:25 +0100, Ken Boak wrote:
> Daniel, List,
> 
> Thanks for the mass flow numbers.
> 
> Clearly heating 10.4 lbs of mass up to 6000C is impractical.  It appears 
> that the problem is the nitrogen loading of the exhaust gas. Apart for its 
> role as a heat transfer fluid, it serves little or no other useful purpose 
> in the gasification process, and is indeed a burden to the calorific value 
> of the resultant producer gas stream.

You could run your diesel engine on pure oxygen; this would produce
about 2-2.5 lbs of exhaust gas flow per kWh of shaft power.  The cost of
this would be the least of your worries, the practical problems would
likely overwhelm this from being a sensible consideration (e.g. without
redesign for pure oxygen, your engine would likely melt pistons, burn
valves, etc).

FWIW I think I recall reading that the Swedish submarine that uses a
Stirling cycle engine uses fuel oil plus pure O2 (though this is
starting to get off-topic! ;-)

> Producer gas, generated from drawing air through hot charcoal will always 
> have this nitrogen loading, which limits it's calorific value.  Syngas, made 
> from injecting superheated steam onto incandescent charcoal, will not have 
> this additional nitrogen burden.
> 
> What is needed is an accurate mass analysis of the diesel exhaust, to 
> determine the main components and to make a judgement on their suitability 
> for any exhaust gas cleaning/recycling process.

Roughly, from memory:

- diesel exhaust has excess air.  Instead of the stoichiometric ~14.7lbs
fuel/air ratio, a diesel engine runs 20-25 pounds of air per pound of
fuel, when operating at maximum power (just short of the smoke point).
My round-number figure of 10 pounds per kWh is based on a  brake
specific fuel consumption of 0.4 pounds of fuel per kWh of shaft power
(which is perhaps optimistic, since this is about 0.3 pounds per
bhp-hr).  10lbs-air/0.4lbs-fuel = 22:1 air:fuel ratio.

- to a first approximation, all the fuel is burnt.  (if it weren't,
there would be economic justification for fixing this).

- to a second approximation, there is some leftover unburnt fuel (not
much) and some soot.  This is more of a pollution problem than a cost
efficiency problem.  This is dealt with as an exhaust emissions
treatment issue.

- because the exhaust is air-rich and fuel poor, it is more difficult
(compare to a gasoline engine) to treat NOx emissions (think of them as
"over-oxidized" exhaust components) than it is to treat the
underoxidized exhaust components (soot, unburnt HCs).

- the exhaust has the water that you'd expect from burning CH2 -> CO2 +
H2O.  So we get 0.4lbs of CH2 + 10lbs air -> 1.25# CO2 + 0.51# H2O +
(10lbs air, missing 1.36# O2)

> If the hot exhaust contains sufficient excess air (or more specifically O2) 
> to support the torrefaction or pyrolysis of woodchips, then this would be a 
> good use of the thermal energy in the exhaust, and the woodchips and 
> charcoal bed would act as a filter for the greasy/sooty particulates that 
> are present in the diesel exhaust when running the engine on waste vegetable 
> oil.

I have been told (thanks Doug!), if I remember correctly, that when
directly drying/heating wood chips with engine exhaust, you start to get
smoke forming at about 200C.  You can certainly use this to preheat and
to completely dry your fuel, if your gasifier design needs this or
benefits from this.  Beyond this temperature you can still use free
waste heat from the engine exhaust, but you would have to do so
indirectly (so you will need greater area for your heat transfer
surfaces).

> If there is sufficient heat available in the exhaust gases to fund this 
> first stage of gasification, without the addition of more heat, other than 
> that of the combustion of the biomass in the excess air, then the resulting 
> gas stream will be a tarry, nitrogen laden, raw gas, which in this state is 
> no good for engines. The tars need to be cracked and the percentage of 
> nitrogen reduced significantly. It is probably best to burn this first-stage 
> gas, to provide the thermal energy to assist with the secondary stages of 
> syngas production.

At some point in your process you need ultimately to exhaust your
exhaust.  You probably don't want a lot of it recycling into your input,
since it is nitrogen-rich and CO2-rich.

> Some of this raw gas could be sacrificially burned in pre-heated secondary 
> air, to produce temperatures in excess of the original 600 C (from the 
> diesel exhaust). This could be used for water heating or steam generation or 
> for elevating the steam temperature to above 600C.

You could, but this hot tarry gas is further along the food chain than
your char, which is much less reactive and therefore more difficult to
gasify.  You might be better off using your preheated air (using only
waste process heat, not burning your tarry gas) to burn your char.  This
will give you very hot CO2 (above the CO2 + C -> CO back-reaction
temperature) that you can then combine with your 600C hot tarry gas, to
reduce some of the tars.  There might already be enough intrinsic water
in the hot tarry gas that you won't have to explicitly add superheated
steam.

> The next question is how much energy you need to put into the incandescent 
> charcoal to produce syngas from superheated steam?

A good way to get a feel for this is to run various scenarios on the DKK
"gasifier.exe" program (gosh I wish I got a royalty for every time I
promote that program!!! ;-).  You can adjust the parameters to answer
all sorts of interesting questions.  To answer what I think is your
question, I would change its fuel formula to straight C (i.e. not the
default biomass formula), set the fuel preheat to the amount desired,
and then fiddle with the process's input steam flow rate and temperature
until the amount of air needed for the process closes to zero.  This
will give you the complete mass/energy balances for steam gasification
of char.

-- 
- Daniel
Fredericton, NB  Canada

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