Page 205 - New Trends In Coal Conversion

168 New Trends in Coal Conversion

operate above these temperatures is a challenge to improve cleaning technology in the
future.

6.3.3.2 Hot gas desulfuration
During the last decades, many researchers have carried out several studies in desulfur-
ation sorbents, without obtaining a good result to create a new commercial product.
This is due to mainly two reasons:
• System requirements have changed every few years. More concretely, the sorbent operating
temperature has moved from the high-temperature (>600 C) to low-temperature
(350e550 C) applications, and technology developers require time to respond. Also, carbon

deposition remains a problem to be solved at these temperatures.
• There is an element of discontinuity in the development of sorbent systems from laboratory
to pilot and commercial testing.
Nevertheless, these previous years until today have been very important for the
development of capable solutions, such as these that will be explained from now
on. These researchers focus their resources on finding a new sorbent with adequate
properties to work at several operating conditions described earlier (Xiao et al., 2009).
The yields obtained during these studies have reached a removal of sulfur up to
5 ppm, usually making use of ZnO catalysts, which are able to resist temperatures

of about 400e500 C(Xiao et al., 2009; Denton et al., 2015). Some of these systems
have to also avoid the above mentioned deposition problem. Another important char-
acteristic is that the placement of the removal unit does not affect the present design of
the WS/PR system, but it makes it necessary to include an additional particulate
removal system to protect the combustion turbine from some solid sorbent particles
that could be dragged (Denton et al., 2015).
Another, for example, CRIEPI (Oki et al., 2017) has developed a sorbent that works
well at the required operating conditions with reasonable efficiency. After that, a prob-
lem with carbon deposition was encountered, which deteriorates the performance of
the hot gas cleanup system, and it was solved in the next stage of their project by
creating a new sorbent which is resistant to carbon deposition that was also valid under
high-pressure conditions.

6.3.3.3 Hot gas denitrification
Nitrogenous compounds, such as NH 3 contained in the gasified fuels, are not removed
in the hot/dry-type gas cleanup process. This NH 3 is then fed into the gas turbine where
it forms fuel-NO x in the combustion process (Hasegawa, 2010). The NO x emission can
be reduced by low-NO x combustion techniques in the gas turbine, deNO x devices
downstream of the turbine, or by reducing the NH 3 concentration in the syngas before
feeding the turbine.
Alternatives to reducing the NH 3 concentration in a hot syngas stream include: (1)
catalytic decomposition of ammonia, (2) combined NH 3 and H 2 S removal with regen-
erative sorbentecatalyst systems, (3) partial oxidation of ammonia to nitrogen.