[Gasification] CO2 recycling

Mon Oct 15 19:07:46 EDT 2007

 Dan,

Now you are talking sense regarding: turning smoke in to fuel. The smoke :(products of non oxidized volatiles) can be readily reduced into clean gas by a plasma.
The energy wasted by the conventional pyrolysis process will be regained at the out put end of the reaction, because the waste heat of the engine can be made to perform much of the preheat needed for raising the fuel temperature.
The oxygen starved pyrolization area can"Surround" the "Plasma cell" which will adequately provide the heat necessary to volatilize the fuel. You may find that insufficient water content in the fuel can be supplemented by the addition of "super heated steam" provided by a stepper motor driven metering pump @ $75. using distilled water. giving a worthy addition, as every pound of steam will wind up as gas.? Another innovation would be, the production of an "ELECTRODE LESS" Plasma created by Micro waves @ 475GH which is close to a domestic microwave magnetron frequency.
Ducting microwaves into a a converted water heater cylinder is not as difficult or as dangerous as it sounds. the magnetron has to be kept cool and the business end has to be properly surrounded by wave guides leading to a Faraday cage, If you are worried about this, place a few fluorescent tubes around to give warning of micro wave leakage. I suggest you read up on this subject prior to experimentation.

GF

-----Original Message-----
From: Daniel Chisholm <dmc at danielchisholm.com>
To: Discussion of biomass pyrolysis and gasification <gasification at listserv.repp.org>
Sent: Mon, 15 Oct 2007 4:14 pm
Subject: Re: [Gasification] CO2 recycling

On Mon, 2007-15-10 at 19:33 +0100, Ken Boak wrote:
> ...is this plasma conversion process ... convert 
> municipal solid waste into syngas, steam and process heat?
[snip]

> Look at the plasma cell being the "catalyst" (a poor choice of word) which 
> allows engine grade gas to be produced from the tarry output of a 
> conventional gasifier.

Actually I think that thinking of the plasma cell as a "catalyst" for
cleaning up the gas is an excellent choice.

> Most of the exothermic heat needed to pyrolise the biomass has been created 
> from the oxidation of th charcoal. The plasma cell is not being asked to 
> supply the hole energy input, just sufficient to allow high temperature 
> ctacking of the tars.

Now this I agree could make good sense - you're using expensive
electricity, but you're only using it to "polish up" a gas that you've
created using cheap energy.  It could well be that the value added by
having the gas cleaned could justify the costs (energy + capital) of the
plasma treatment.

Keep in mind that if you are being paid $50 per ton to process garbage,
that the economics of using some electricity to run the process cleanly
might work out.  Whereas if you are paying $20-$50 per ton for some
biomass feedstock, trying to make money producing engine-grade gas, you
can't spend nearly as much on electricity.

> On a more practical note, I have been tinkering in the workshop.
[snip]
> It was so good I rapidly blew holes in the 316 stainless steel kitchen 
> container, where a multitude of small arcs established between the charcoal 

Hopefully there were no spousal consequences... ;-)

> With a power input of about 3/4 kilowatt, a small plasma cell  about the 
> size of a bean can could be used to clean up the sooty exhaust output  of 
> the diesel generator set,  and oxidise the carbon to CO and reduce the steam 
> to H2.

I disagree with these power estimates.  Let me run a rough calculation
here:

Assume a 1kW diesel genset.  It will consume about 10 pounds per hour of
air, and about 0.4 pounds per hour of fuel, producing about 10.4 pounds
per hour of exhaust.  Let's say the exhaust is at a temperature of 600C,
and that you must bring it to 6000C in order to treat it in your plasma
cell.

The heat capacity of air is about 30 J/mol/K, or roughly 1 J/g/K (for
water vapour it is about 2 J/g/K).  Our 10.4 pound per hour exhaust
stream is 4720 grams, so we need about 4720 Joules to heat our exhaust
by one degree C or K (if we use the lower figure for air.  At the same
time I will ignore the fact that the heat capacity increases at higher
temperatures when you dissociate gases, and then even moreso when you
ionize them - if you include these effects in the calculation, it will
make an even larger result).

To raise the exhaust stream from 600C to 6000C requires 5400deg-C times
4720 Joules/degree, or 25.5MJ/hr.  This is 7.1 kW.

So, assuming a 100% conversion from electricity into heat in the plasma
cell, you would need to feed 7.1kW of power into the plasma cell in
order to process the exhaust stream from a 1kW diesel engine.  This is a
lot.

Note that this process I have described has the exhaust leaving the
system at 6000C, which seems like a bit of a waste of some extremely
high quality heat.  You could attempt to capture and regenerate some of
this, and try to run it counterflow to the incoming exhaust.  But it
would be impractical to build a heat exchanger that could operate over
this entire temperature range (you would turn the hot end of it to
plasma!), so you're not going to be able to recover a particularly big
fraction of the output heat in order to preheat the incoming exhaust.

> The energy input for this comes from the plasma arc and some of the charcoal 
> that is oxidised by the plasma in the usual exothermic manner.
> 
> In the diesel exhaust there is no shortage of soot and water vapour, and 
> there is possibly enough excess air to feed the exothermic reaction. There's 
> close to 4kW of heat energy available in my diesel exhaust, which will 
> readily char the biomass and get it up toclose to 600C.   The next 
> experiment will be to try the experiment in a vessel through which the hot 
> engine exhaust passes over the charcoal.

-- 
- Daniel
Fredericton, NB  Canada

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