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

30 Iron Based Catalysts
Real gas has not been very commonly used in order to investigate the effectiveness of the iron
catalyst. Synthetic gas is more likely used, so a tar model compound is injected inside the reactor
to imitate real gas conditions. Two are the mostly used tar model compounds; toluene or naphthalene.
Steam reforming of toluene:
C6H8 + 14H2O→ 18H2 + 7CO2 (35)
Steam reforming of 1-methyl naphthalene:
C11H10 + 11H2O 11CO + 16H2 (36)
CO + H2O→4CO2 + H2 (37)
Tar conversion depends on the ratio of the reductive gases (H2 and CO) to the oxidative gases
(H2O and CO2). The result of tar decomposition depends on the temperature, the catalyst material
and the oxygen potential (Nordgreen,2012). The more active the material, the lower the temperature
at which the best results are obtained as the tar is decomposed sufficiently (Simell,1992).
The activation of tar molecules is accomplished either in the gas phase or via chemisorption on the
catalyst surface (Min,2011). In the first case, the volatiles produced from biomass pyrolysis contain
abundant radicals that react with tar molecules to form activated tar fragments. Thus, H- and CH3-
radicals are abstracted in the gas phase. In the second case, the procedure depends on the temperature.
At higher temperatures, the steam reforms the gas or/and the coke that is deposited on
the catalyst surface. At lower temperatures, the coke yields are higher due to low reactivity of
steam. Hence, tar fragments combine together easily on the surface of the catalyst to form coke
which is difficult to gasify at low temperatures.
4.2 Catalyst Deactivation
The main reason for catalyst deactivation is attrition and of course carbon deposition. Also due to
physical changes such as sintering, phase changes and component volatilization
(Nordgreen,2012). The main problem is that once coke is deposited on a catalyst’s surface the
reforming of large aromatic compounds is difficult as there are less vacant continuous active sites
(Min,2011).
4.2.1 Carbon deposition
Boudouar Reaction: 2CO⟶ CO2+CO (38)
Andrzej Machocki mainly focused his research on the formation of carbonaceous deposit and its
effect on carbon monoxide hydrogenation on iron-based catalysts. He tested Iron-Cobalt catalyst
and Iron catalysts promoted with potassium (K) and concluded that carbon deposition occurs regardless
the ratio Fe/Co. Also the deposition doesn’t depend on the initial metallic phase composition,
the particle size or on the amounts and quality of metal carbides. Temperature and H2/H2O
ratio though play an important role in carbon deposition (Sarvaramini,2012). At 800⁰C though, as
H2/H2O ratio increases the coke formation is enhanced and at 720 ⁰C the deposition decreased
slowly with the increase of the ratio. As for Fe/olivine catalysts, carbon formation is relatively low
and after 750 ⁰C no carbon deposition is evident (Virginie,2012). The carbon deposition also increases
with the increase of Fe amount in the catalyst (Polychronopoulou,2006).
Also, it has been reported that coke is less active in dry CO/CO2 reforming conditions.
The fundamental properties of coke depositions are the amount, the type and the location of the
coke on the catalyst which depend on the operating conditions, catalyst type and feed composition
(Le Minh,1997).