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