[Gasification] Reducing tar production

[Zietsman, Rex]

One way to overcome the tar problem is to separate the pyrolysis stage
from the gasification stage. The Danish Technical University (DTU) has
developed a two stage gasifier that does exactly that:
http://bgg.mek.dtu.dk/research/twostage/ Essentially they found that by
splitting pyrolysis from gasification and by adding air to the pyrolysis
gas in the combustion zone, they significantly cracked the high boiling
tars. The remaining tars then have to travel through the char where
additional conversion takes place so that the gas leaves the gasifier
with <25mg/m3 of tar. They then cool the gas down to about 90degC before
passing it through a bag filter. As the tars content is so low, the soot
and dust accounts for 95% and the tar for 5% of the solids removed by
the bagfilter. At this high dilution, the collected solids are not tacky
and therefore the bag can be pulsed to drop off the dust. I suspect that
that a truck engine filter would do the same but cost more to replace.
They then cool the gas further to drop out moisture. By the time the gas
gets to the engine, it typically has about 5mg/m3 of tar and virtually
no soot. 

The DTU have also developed a vortex gasifier that does not have a
separate pyrolysis step but uses aerodynamic principles to draw
pyrolysis products up the centre of the gasifier (straight sides, no
throat) much like a cyclone does. The central upward flow brings the
tars up into the high temperature combustion zone where they get cracked
and converted. I do not know how far they have progressed with such a
design but they have patented it.

Rex Zietsman