Etude d'un procédé de gazéification de biomasse en ambiance ...

Résumé
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ABSTRACT
A process for ligno-cellulosic biomass gasification is proposed: it allows to obtain a
synthesis gas (mixture of CO and H2) that may be used for the production of liquid fuels. The
current processes present drawbacks: they need a preparation of the biomass (drying and
grinding), they produce pollutants (CO2, CH4 and tars) and biomass ash extraction may
induce phenomena of fouling of the process.
The process, considered here, proposes an original way of heating to eliminate some of
these drawbacks or at least to strongly decrease them. It is based on an electric arc transferred
between two graphite electrodes above a glass melt. Biomass is introduced by gravity at the
top of the furnace and falls inside the arc volume on the melt surface.
The energy, needed for the endothermic gasification reactions is provided by the electric
arc and not by the consumption of a part of the biomass, that allows to increase the
conversion yield of the biomass. So the high temperature (more than 1200°C) favors the
gasification and decreases the formation of unwanted by-products by increasing tars
cracking.
The mineral melt has various functions. On one hand it allows "to store" inside the energy
of the electric arc to transfer it to the biomass, increasing the energy yield. On the other hand
it allows to increase the residence time of the particle in the hot atmosphere and therefore to
avoid the biomass crushing. Furthermore, the mineral melt allows the incorporation of the
remaining ashes.
Besides the usual humidity of the wood (~ 20 % (mass)) provides directly in the
atmosphere of the furnace the water necessary for the reaction of gasification:
C6H9O4 + 2 H2O → 6 CO + 6,5 H2; so the stage of preliminary drying is avoided.
Trials of beech wood gasification have been run with temperatures between 1200 and
1600°C:
- CO2 and CH4 are formed in small quantities, lower than 1 % (volume) for the first one
and on the order of 0,01 % for the second,
- the quantity of tars detected during trials is on the order of about 10 mg.Nm -3 ,
- the conversion yield of CO from the biomass is between 52 and 68 % and the one of H2
is between 60 and 68 %,
- the optimal temperature for the process is about 1300°C: the conversion yield of carbon
in dry biomass to CO is about 98 %(mass).
A simplified model for the thermal transfer to biomass particle shows that the
residence time corresponds with time obtained experimentally and allows to envisage a first
sizing for an industrial reactor.
A technico-economic approach was made from these results: the competitiveness of
this process depends strongly on the energy return on the reactor; but the additional cost
engendered by the lack of optimization is compensated by the decrease of the costs of
biomass preparation and of the gas cleaning.
Key words: Gasification, biomass, thermal plasma, mineral bath, high temperature, sized
centimeter particles.
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