Environmental Engineering Reference
In-Depth Information
that HCl is the prevalent chlorine product (Zevenhoven and Kilpinen, 2001), present
up to 200 ppm (Van der Drift, 2001) depending on the type of biomass and the oper-
ating conditions of the gasifier. The presence of HCl in the feed to SOFCs can result
in the adsorption of chlorine onto the surfaces of nickel particles in the anode causing
a loss of nickel particles, thus degrading the FC performance (Xu et al., 2010). How-
ever, the impact of halides on SOFC performance has not been studied in detail.
16.3.2.5 Sulfur Compounds
In general, biomass fuel contains much less sulfur
than coal. The sulfur in biomass is converted to hydrogen sulfide or sulfur oxides
during gasification, depending on the gasification system. Sulfur in the biosyngas
is likely to be present mainly as H
2
S (Aravind et al., 2012), and its content varies from
20 to 200 ppm (Johansson et al., 1999). The use of sorbents such as limestone can
reduce the amount of H
2
S present in the gas.
H
2
S is likely to cause considerable problems in FC operation as it can adsorb on
the active sites of the anode, thus inhibiting adsorption of the fuel molecules, which
negatively affects the fuel oxidation reaction rate. The effects of sulfur on SOFC
performance have been studied extensively, with particular emphasis on short-term
poisoning (Cheng et al., 2007), but some uncertainties remain regarding the long-term
behavior. For example, the effects of H
2
S poisoning may be reversible (Rasmussen
and Hagen, 2009). Furthermore, H
2
S may also be a fuel for SOFCs under certain
circumstances (Aguilar et al., 2004).
16.3.2.6 Nitrogen Compounds
The main nitrogen-containing contaminants in
biosyngas are NH
3
and HCN. The acceptable levels of ammonia in the outlet of flue
gas streams from power plants are typically dictated by local regulations. Thus,
ammonia is usually removed to satisfy local environmental regulations. In SOFCs,
however, ammonia can be used to produce electrical power. Ammonia dissociates into
N
2
and H
2
at the anode, and the H
2
is then electrochemically oxidized (Staniforth and
Ormerod, 2003; Wojcik et al., 2003).
HCN is another nitrogen-containing contaminant, which can be present at levels up
to a few hundred ppm (Aravind et al., 2012). The impact of HCN on the SOFC anode
has not been well studied and should be explored in the future.
16.3.2.7 Other Contaminants
A variety of contaminants are present in biosyngas
in very small quantities. These contaminants originate from the biomass or from
components of the gasifier system or possibly the cooling and the cleaning systems.
Some examples are mercury, cadmium, lead, manganese, cobalt, antimony, selenium,
beryllium, arsenic, chromium, nickel, and silicon. The presence of these contaminants
is often limited to a few ppm or sub-ppm levels (Salo and Mojtahedi, 1998). Their
impact on SOFCs has rarely been studied. Future studies are needed to understand
the impact of these contaminants on the long-term operation of SOFCs.
NO
x
emissions from SOFC systems are generally below 0.5 ppm (tinyurl.com/
qhswo7e) and are mainly generated in the afterburner. These low concentrations of
NO
x
emitted by SOFCs are far below the NO
x
emission limits (tinyurl.com/kdfbsbz)
for combustion plants in the EU and the United States.
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