RE: GAS-L: maximum pressure

["Gavin Gulliver-Goodall" <Gavin@roseplac.worldonline.co.uk>]

Jack and all

My understanding of the reason for pressurising gasifiers is to reduce gas
clean up:

Compressing air is easy: the resultant pressurised gas can be burnt in a
pressurised burner (say Gas turbine) directly, or stored under that pressure
for consumption at a varying rate .

If you try to compress producer gas you have to clean it first or the
compressor gets gummed up with tars etc. If you intend to burn the gas
directly then the suspended tares have useful CV so why remove them?

Gav (the old dog)

-----Original Message-----
From: JackProot@aol.com [mailto:JackProot@aol.com]
Sent: Wednesday, February 06, 2002 3:36
To: gasification@crest.org
Subject: Re: GAS-L: maximum pressure

In a message dated 02-02-01 14:25:07 EST, LINVENT@aol.com writes:

<< The major benefit to high pressure is not having to compress the gas to
 feed into a turbine which operates at 150-350 psi. Compressing the gas
after
 gasification is a significant parasitic load. >>

I am still trying to understand the advantages of working under pressure
in a hot gasifier. Reflections :

- compressing the gas after gasification requires energy, OK. But so does
the compression of air before gasification. Slightly smaller volume maybe
but then consider the unavoidable leaks ...

- I keep thinking something is missing in the reasoning. Or why would
people spend so much money in Hawai to replace a stupid compressor
after the gasifier ?

- In a first message, I wrote that Le Chatelier was pushing the reactions
the wrong way : when pressure goes up, the reactions are displaced
towards the least volume.   Ex : 2CO <-> CO2 + C goes to the right.
Not exactly what we want.
Well, actually, I didn't get the whole picture : we can also see something
like   C + 2H2 <-> CH4    which is favored by an increase in pressure.
Generating methane is quite exciting !
I ran a quick thermodynamic model with a 20% moisture wood at 800°C.
No methane at atmospheric pressure of course, it is largely decomposed
above 600°C. But at 15 bar, I had about 5% in the gas. (Could not go any
higher as the wood began leaving an important carbon residue in the
vessel. Need more moisture to increase the pressure.)
Interestingly enough, the yield increases slowly with the pressure.
(I mean the ratio of heat capacity in the gas vs. the wood used.)

- If we take the gas produced at one atmosphere and we compress it
to 15 bar at the same temperature, we get approximately the same
composition as the gas produced at 15 bar in the furnace. This latter
operation is of course a lot easier. (A heat exchanger could cool down
the gas before the blower and reheat it after, to soften the cost.)

Ain't that fun, to run a furnace on paper ?
We can still burn the paper when we are done   :-)

Still suspicious : rich idea in hiding or mad scientist ?

Jacques Proot
(metallurgical process - and gas amateur after hours)

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