HIGH-EFFICIENCY ENERGY SYSTEMS WITH BIOMASS GASIFIERS AND SOLID OXIDE FUEL CELLS - Biomass as a Sustainable Energy Source for the Future - page 515

SOFC anodes depends on the type of SOFC employed and its operating conditions,
such as temperature, biomass-derived product gas moisture content, and cell voltage.
It also may depend upon the thermodynamic possibility of carbon deposition, the kinet-
ics of carbon formation, and subsequent reaction steps. Several recent publications
describe the influence of tar components on SOFC performance (Aravind et al.,
2008; Mermelstein et al., 2010; Singh et al., 2005). Most of these studies have presented
promising results regarding the tolerance of SOFCs during comparatively short expo-
sure to tar-loaded gases (Hofmann et al., 2009). However, highly detailed theoretical
and experimental studies are required to understand the fate of tar at SOFC anodes.

16.3.2.2 Particulate Matter Solid particulates are always present in the raw bio-
syngas generated in gasifiers. The size of the particulates present in biosyngas can
range from a few micrometers to the submicrometer level (Aravind et al., 2012).
Particulates generally include the inorganic material derived from mineral matter in
the biomass feedstock, unconverted biomass in the form of char, and material from
the gasifier bed if bed materials are employed in the gasification process (Aravind
et al., 2012).
Note that these particle sizes (Hindsgaul et al., 1999) match well with the pore sizes
of SOFC anodes, which also range from submicrometer level to a few micrometers
(Lee et al., 2003). Thus, particulates are likely to block the micropores of the anodes
if they are in the solid form at SOFC operating temperatures, thus decreasing the
SOFC performance. Hofmann et al. (2008) have reported the effects of particulates
from biomass gasifiers on SOFCs with Ni - GDC anodes and found that particulates

up to approximately 10 m in diameter deposited on the surface of the anode, decreas-

ing the performance of the SOFC during the measurement. However, more informa-
tion is needed on the impact of particulates on SOFC performance. It is likely that
particulates should be removed as much as possible, even to the level of a few
ppm, to enable smooth long-term SOFC operation with biosyngas.

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16.3.2.3 Alkali Compounds Biomass often contains significant amounts of alkali
compounds, mainly consisting of potassium and sodium, with the amount of potas-
sium considerably higher than that of sodium. Eutectic potassium and sodium salts
in the ash material can vaporize at gasification temperatures above 700 C. Unlike
the solid particulates that can be separated by physical means such as barrier filters,
the vaporized alkali compounds will remain in the product gas at high temperature.
For this reason, simple filtration cannot always remove these compounds. Condensa-
tion of the vaporized alkali compounds on particles in the gas stream typically begins
at approximately 650 C, with deposition subsequently occurring on cooler surfaces in
the system such as heat exchangers or turbine expansion blades.
Of the total alkali content of the biomass, only a minor fraction remains in the gas
phase after the gasification process. However, Nurk et al. (2011) reported that an
amount of KCl as small as 6 ppm in the SOFC feed can decrease the performance of
the SOFC, indicating that the alkali tolerance of SOFCs may be less than a few ppm.

16.3.2.4 Halides The halide gases obtained from gasification mainly contain HCl,
with HF and HBr as the other two main constituents. Measurements have indicated