[Gasification] Why is nitrogen dilution bad?

[Jonathan F. Pratt]

Jim,

For the simplest type of "Producer Gas to Synfuels" setup, your probably 
best attempting some kind of retort with good scrubbing and filtration and 
the most common iron catalysts used in the big fischer-tropsch plants.  You 
could start with pure fine rust or iron oxide fines.

The retort will produce a gas free of nitrogen and will mainly be a mix of 
methane, H2 and CO and a bunch of other hydrocarbons in small percentages.

Some of the literature from the late 40's didn't mention using retorts for 
the raw syn gas but did mention acheiving very good yields of hydrocarbons 
in the range suitable for producting synthetic gasoline in the lab by using 
a 1 inch vertical column "iron pipe" filled with iron oxide spaced at 8 cm 
intervals or so with small screens to enable the fluidization of the iron 
oxide as the gas was injected from the bottom under pressure of 10-25 
atmospheres if memory serves me correctly.  temperature of the column was 
maintained around 300C

Good first pass throughput and temperature stability was attained using this 
setup which is a small scale "fluidized bed reactor".  The output is mainly 
hydrocarbon vapors that is condensed.  Raw biomass may produce a higher CO 
to H2 ratio than is optimal, but the yield should still be pretty good

Generally a retort will produce a lot of tar and nasty constituents that 
would probably plug up the catalyst however if you use a "Riche" type retort 
and very good filtration this might actually work for a small scale 
biomass-synfuel setup.  The Riche style retort is essentially a long 
vertical tube heated from the bottom, the gasses generated in the retort 
pass through the glowing char formed at the bottom and to some extent 
breaktown the tars and other constituents in the raw pyrolysis gas.  Filter 
the gas through a cyclone and sawdust filter and run it through a compressor 
(optional) then through the "fluidized bed" of iron oxide.  Just make sure 
you have a pressure release valve on the retort!

Jonathan

>
> Message: 8
> Date: Sun, 28 Jan 2007 02:45:40 -0800
> From: "jim mason" <jimmason at whatiamupto.com>
> Subject: [Gasification] why do liquifaction proceses require
> nitrogen-less synthesis gas?
> To: gasification at listserv.repp.org
> Message-ID:
> <31a4f6f00701280245t3134f70fpb1f0c2218e2756ea at mail.gmail.com>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> i'm reviewing the literature on methanol and fischer-tropsch based
> liquifaction processes, with an interest in seeing if any of these are
> realistic to consider as DIY sorts of efforts.
>
> despite lots of details on catalysts and temp and pressure, i can't
> seem to answer a very basic question: why is the nitrogen such a
> problem in these processes?
>
> all the processes assume starting with a synthesis gas that is only CO
> and H2.  no tars and co2 is scrubbed out.  so usually you start with
> an oxygen blown gasifier, or a external combustor dual FLB sort of
> system, so that the produced gas is without nitrogen.  which is lots
> of work, obviously.
>
> is the problem that the nitrogen will tend to reform to ammonia (NH3)
> in the presence of cataylsts under temp and pressure.  or will it form
> NoX and poison the various catalysts?  or is there another problem?
>
> can someone here clarify this for me?
>
> j
>