Dipl. Ing. Matthias Mayerhofer Technische Universität München ...

Biomass Gasification 11
types of catalysts, though, are not very efficient are expensive so further investigation needs to be
done.
As for the sulfur removal the following procedures are the most common (Kohl,1997):
Sulfur Scavenging
Iron Oxide: Most widely used process is Iron Oxide adsorbent. The iron oxide captures the H2S by
forming Iron sulfide. It is possible to regenerate the iron by adding air to oxidize sulfide to elemental
sulfur. Operating temperature of iron Oxide sponge is 20-50°C
2Fe2O3+ 6H2S→2 Fe2S3+6 H2O (1)
2 Fe2S3+3O2→2Fe2O3+6S (2)
6 H2S+3O2→6 H2O+6S (3)
Zinc Oxide: It can be used as an adsorbent to form zinc sulfide. Its main advantage is that it is stable
and can be useful for hot synthesis gas streams.
The sorbents for the removal of traces of H2S are cylindrical extrudates of 3-4mm in diameter and
4-8mm in length. The maximum sulfur loading is 30-40 lb sulfur/ 100 lb sorbent and usually beds
are designed to last over a year. The operating temperature is 200-400°C.
ZnO+ H2S→ ZnS+ H2O (4)
2.4 Catalytic Tar Destruction
2.4.1 Tar Definition
‘Tar’ is defined by Milne and Evans (Milne,1998) as the organic compounds produced under thermal
or partial-oxidation regimes (gasification) of any organic material that condense under operating
conditions of boilers, transfer lines, ICE and inlet devices and are considered as largely aromatic.
As tar are considered all the hydrocarbons with molecular weight larger than that of benzene.
However, in some studies benzene is excluded and not taken in consideration as a tar molecule,
whereas in other cases it can be used as tar model compound for the examination of the effectiveness
of a catalyst.
The classification of Tar is based on the behavior of the tar compounds in downstream process.
The characterization of “tars” as primary, secondary, and tertiary is a first step in classifying these
materials and relating the composition of “tars” with formation conditions (2004).
Class 1: Components of tar that are the primary pyrolysis products and are virtually absent above
800°C. They can’t be detected with a GC and are close to the composition of the biomass itself.
Class 2: Aromatic compounds with hetero atoms (oxygen and nitrogen) that have high solubility
and are present in 500-1000°C with a maximum peak at 750°C. They are the decomposition of
primary tars.
Class 3: Light aromatics, toluene, xylene, styrene which do not contribute into tar related problems
and are present at 650-1000°C with a maximum peak at 900°C.
Class 4: Light polyaromatics such as naphthalene, acenaphthylene, anthracene and pyrene are
present above 750°C. They are contained of 2-3 aromatic rings and condense when the gas is
cooled down. For processes above 1000°C naphthalene is the primary component.
Class 5: Heavy polyaromatic tars that are produced either by the decomposition of heavy large
class 1 compounds or from lighter tar compounds due to growth reactions or pyrolytic aromatic
hydrocarbons. They contain 4-7 aromatic rings.