(188c) Catalytic Hot Gas Cleaning with Monoliths in Biomass Gasification in Fluidized Bed. Modeling of the 2nd Generation Two Layers Monolithic Reactor

Monoliths containing nickel
or other compounds can be used to eliminate tar and ammonia in a real biomass
gasification gas. They can work with a fuel gas containing important amounts of
particulates, as in the case of the fuel gas produced with fluidized bed
gasifiers. They avoid then the use of ceramic filters under a tar-containing
atmosphere. The use of monoliths in biomass gasification is a very recent and
promising technology which has not yet reached its commercialisation stage and
requires experimental studies at pilot scale. Experimental studies at UCM
indicate that tar and ammonia conversions (eliminations) with monoliths depend
on so many experimental variables that a model is needed to understand,
correlate and compare the results obtained with monoliths. The presentation of
such a model is the main objective of this communication.

An advanced macrokinetic
model is presented here both for the monolith itself and for the whole
monolithic reactor. The overall model is based on two microkinetic models for
the tar and NH₃ elimination reactions (which use effective or
apparent kinetic constants for the overall tar and NH₃ elimination),
two mass balances for tar and NH₃ and a heat balance in the
monolith. The macrokinetic model is developed according to the basic rules of
Chemical Reaction Engineering.

Several important and
noticeable facts appear in these monoliths, such as the non-Arrhenius
dependence on temperature of the effective kinetic constants, and the big
delta-T across the monolith. The fist fact is due to the control by the
external diffusion (mass transfer) in the channels of the monolith which is
here proved both experimental and theoretically. It makes that the superficial
gas or face gas velocity has to be higher than 1 m/s to have important tar
conversions. It, in turn, originates the need of using several layers of
monoliths to get an enough gas residence time in the monolith. The big axial DT in the monolith is due to the
predominancy of the endothermal reactions in the existing reaction network. It
makes that at the monolith exit temperatures lower than 750 ºC may appear which
would originate the formation of a whisker-type coke with the consequent
irreversible deactivation of the monolith.

The equations developed for
the macrokinetic model are easy to handle and allow a correct analysis of the
experimental data, obtained at small pilot plant scale. The model has also been
used to design a 2nd generation monolithic reactor for this new application
which is already being used at UCM.