RE: GAS-L: NOx and gasifiers

[Mike Norris <mnorris@dekaresearch.com>]

I don't believe there is much point in "cleaning" the wood gas as the
original question focused on.  My simplified view of a gasifier for NOx
formation is the fuel is burned very rich, where the low temperature and
oxygen starved atmosphere suppress the formation of NOx. Any NOx that is
formed is likely to be reduced to N2 as carbon and hydrogen radicals
complete for the O atom within the gasifier.

Some of the nitrogen in the fuel or air will be reduced (as opposed to
oxidized) to NH3 and HCN.  These species are less stable than N2 and more
likely to form NOx, when the wood gas is burned with additional air in an
engine.  I can not think of a good way to extract N2 from NH3 and HCN
without oxidizing the whole mixture.  By the way, NH3 should make a dandy
fuel as it has 40% of the heating value of kerosene. 

The main source of NOx in the exhaust of the engine is likely to be "Thermal
NOx" resulting from relatively long residence times at high temperature in
the presence of oxygen.  The thermal NOx is formed in the secondary
combustion process, when the wood gas is burned with air.    I believe the
Thermal NOx problem using woodgas
is the same as with any other fuel.  Thermal NOx can be controlled by
burning at a fairly cool temperature (Adiabatic flame temps < 1300 deg C).
This can be accomplished by adding significant amounts of excess air or
recycling significant amounts of exhaust gas.  Actually I would expect wood
gas to burn at a fairly 
low temperature due to the low energy density of the fuel.  


Dr. Mike Norris
Staff Scientist 
DEKA Research and Development
Manchester NH

> -----Original Message-----
> From:	Claus Hindsgaul [SMTP:claush@et.dtu.dk]
> Sent:	Tuesday, June 25, 2002 10:26 AM
> To:	gasification
> Subject:	Re: GAS-L: NOx and gasifiers
> 
> tir, 2002-06-25 kl. 14:05 skrev Tom Reed:
> > First, there is very little "high temperature NOx"  -  probably less
> than 1
> > ppm - in wood gas because the flame temperature is lower than that of
> > hydrocarbons.
> 
> I don't quite agree on that.
> 
> Our measurements of emissions from an IC engine[1] fueled by superficial
> wood gas (mixed from flask without e.g. tars and NH3) showed NOx
> emissions for lambda<1.5. Indeed they were lower than for natural gas
> with this engine, but not negligible.
> 
> At the Biomass Conference in Amsterdam last week, a poster claimed that
> 80-100% of NH3 was converted to NOx during flame(!) combustion.
> 
> At the same conference Markus Kleinhappl presented measurements of NH3
> levels in the gas from their double fire gasifier in Graz in Austria to
> 150-800 mg/Nm3. He claimed that a limit of <55 mg/Nm3 should be met for
> engine operation.
> 
> Here at the DTU two-stage gasifier we also have massive amounts of NH3
> in our gas condensate. We have not yet measured the NH3 concentration in
> our gas. Tar is no longer an issue here, but it may be necessary to
> remove some NH3 from the gas in order to meet NOx-regulations.
> 
> Fortunately NH3 is miscible in water so I expect it to be easy obtain
> lower levels using water scrubbers. Only if there are tars in the gas,
> the NH3 contaminated water needs special treatment. Otherwise it may be
> processed at (Danish) biological surridge plants.
> 
> Can anybody elaborate on NH3 and NOx in wood gas and engine exhaust?
> Measurements of either would be very interesting.
> 
> Claus Hindsgaul
> 
> 
>     [1] Jesper Ahrenfeldt, Torben Kvist Jensen, Ulrik Henriksen and
>     Jesper Schramm: "Experiments with Wood Gas Engines". SAE paper
>     2001-01-3681, September 2001
> 
> 
> -- 
> Research Assistant M. Sc. Claus Hindsgaul
> MEK, DTU, Building 120 - DK-2800 Lyngby, Denmark
> Phone: (+45) 4525 4174 - FAX: (+45) 4593 5761
> claush@mek.dtu.dk, http://www.et.dtu.dk/Halmfortet
> 
> 
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