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

Results and Discussion 63
7.3.3 Apparent Kinetic constant for Tar Elimination
The tar content was calculated at the inlet and the outlet of the catalyst. Therefore, Caballero et al.
(Caballero,2000) came up with an apparent kinetic constant which indicates the effectiveness of tar
elimination. The kinetic is related to the two main parameters that affect the tar conversion such as
the temperature and the gas residence time. As the tests for the iron based catalysts were run in
different residence times (τ) and different temperatures it is interesting to see how this constant
varies with the variation of the parameters. In the studies of Caballero the calculation was made for
nickel based catalysts, so a comparison between the results obtained from this work for iron based
catalysts, precious metal and nickel based catalysts is worth. The kinetic can be easily calculated
by the following equation (46):
[ ] (46)
The parameter is calculated for all the different operating conditions that were tested in this work.
For the total amount of tar conversion with iron catalysts, the parameter ranges from ~17 to ~56
mTb,wet³/kg×h. In Table 10 these results are presented. As expected the parameter’s value increases
with the increase of temperature and it decreases as the space velocity increases. The
value of the parameter is a lot lower respectively to the values for the nickel based catalysts, which
according to Caballero’s work the range from 50 to 130 mTb,wet³/kg×h. This is expected of course
as nickel catalysts are much more active regarding tar decomposition according to literature. The
range of the parameter for nickel catalysts is also affected whether it is crushed or not. As for the
iron based catalysts tested in this work they are fine particles because the long term aim is to use
them in situ with the fluidized in-bed material.
For the nickel based catalysts the results are much closer to the results found in literature. The
parameter is ~ 137 to ~ 152 mTb,wet³/kg×h.
Table 10: kapp,tar parameter for iron based catalysts
Temperature (°C) SV (1/h)
750 6500 20.63
860 6500 22.95
800 6500 55.82
800 4500 18.93
800 8500 23.70
800 10000 18.43
kapp,tar
(mTb,wet³/kg*h)
In the following Table 11, the parameter is presented for the other too catalysts tested at the different
operating conditions. As expected, the precious metal catalysts have lower parameter values
as their conversion is also a little lower. Still, they are much higher respectively to the iron based
catalysts.