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

38 Iron Based Catalysts
C (s) + H2O CO (g) + H2 (g) (45)
As for the temperatures in the catalytic bed, the tar removal is enhanced from 750⁰C to 850 ⁰C
(Virginie,2012) and coke removal from the catalyst’s surface is possible.
At low temperatures, the adsorbed and activated aromatic ring systems polymerize to form coke
on the surface of the catalyst due to the low reactivity of steam and the high thermal stability of the
aromatic ring systems (Min,2011).
The iron oxidation state depends on the temperature (Virginie,2010a).
Effect of space velocity:
Space Velocity affects the gasification of biomass tar. To study the effect of residence time it is
possible to conduct experiments comparing the results of total tar content obtained from a catalyst
to those for an inert material (Simell,1992). Regarding iron oxide catalysts, as the SV decreases
the yield of H2 and CO2 increases whereas the yield of CO decreases. Because the lower the SV is
more time is given to the reactions to occur. When ferron dolomite was studied regarding the effect
of space velocity, it was concluded that when residence time was extended from 0.1 s to 0.3 s the
amount of PAHs was reduced.
Tar is decomposed over the iron oxide catalyst and the CO produced from tar is catalytically converted
to H2 and CO2 via WGSR (Azhar Uddin,2008).
Cyclic Use of Iron Catalyst:
To determine the stability of the catalyst it can be repeatedly used without regeneration. In a research
(Azhar Uddin,2008) it was reported that with the increase of cycle number the yield of hydrogen
and carbon dioxide decreased but the yield of carbon monoxide increased and the CH4 and
C2H4 yields were constant. In addition the surface area of the catalyst decreased too.
Calcination of Fe-based Catalysts:
Calcination is a thermal treatment process, in presence of air applied, to ores and other solid materials
to bring about a thermal decomposition, phase transition, or removal of a volatile fraction. The
calcination process normally takes place at temperatures below the melting point of the product
materials. It can occur into different temperatures and the main constituent of the catalyst is usually
hematite (Zhao,2002). It has been reported that the more active materials are best calcinated materials
(Simell,1992).
Mirella Virginie et al. (Virginie,2010b) calcinated Fe/olivine catalysts at different temperatures
(400°C, 900°C, 1000°C, 1100°C 1400°C) with different concentrations in iron. It seems according
to this research that the best results were obtained for 10 wt. % Fe/Olivine and calcination at
1000°C, achieving 91% conversion of toluene which was used as tar model. The same authors
suggested elsewhere (Virginie,2010a) that calcination needs to be performed at 1000°C to prevent
free iron oxides from the olivine surface. It is the best compromise between the amount of metallic
iron available after reduction and the lower energy required for calcination. Hence, the iron particles
are maintained and the absence of agglomeration is possible so the active sites during steam reforming
are enough.
To determine the calcination temperature and time Thermo gravimetric Analysis can be used. During
the procedure samples of 3g were kept in N2 atmosphere and heated up from room temperature
20°C to 900°C (15 °C/min). Then they were kept at this high temperature until the weight of the
sample was constant (approximately 70 min). CO2 content on the samples was calculated from the
obtained weight loss. It was concluded that the calcinated residues of the carbonate rocks were
higher than those obtained by Thermo Gravimetric Analysis (TGA) (Simell,1992).
In those studies the calcination was conducted in situ at 900°C for 1 ½ hours.
Therefore, many similar procedures are being applied in order to achieve calcination. The calcination
of catalysts can be achieved under an air flow rate of 200 mL/min for 2 h at 900 °C (Sarvaramini,2012).
Elsewhere, calcination of catalysts was applied in 20%O2/He at 800°C for 2 hours before
storage and further uses (Polychronopoulou,2006). For iron catalysts with CaO or MgO used
as a support were calcinated at 900°C for 4 hours at a 3°C/min heating rate to give (CaMg)O, CaO