Page 203 - New Trends In Coal Conversion

166 New Trends in Coal Conversion

a liquidesolid separation process can be recycled to the gasification stage to increase
the coal conversion efficiency. The final liquid effluent must be treated before spilling
(Kosstrin, 2017).
The alternative to direct scrubbing is the use of a dry barrier filter system placed
downstream of the syngas cooling, followed by a wet scrubbing system to remove
the chlorine. Filter elements can be made of both ceramic and metal. Some IGCC ex-
periments have shown little reliability with ceramic filters and obtained better reli-
ability than 95% by using metal-based filter elements under the same operational
conditions. The use of a barrier filter for solid particulate removal has several advan-
tages with respect to the direct scrubbing approach. The percentage of unconverted
coal recovered in the filter is higher than in the scrubber (barrier filter: almost
100%, scrubber: around 70%). Also, the lower humidity of the stream recycled to
the gasifier section involves a higher increment in the overall efficiency when the
char is recovered in the filter instead of from the effluent stream of the scrubber
(Kosstrin, 2017). In addition, the use of a filter for dedusting simplifies the perfor-
mance of the scrubber, avoiding solid deposits and sludge handling.
Following the scrubber, it is necessary to clean the gas by applying the following
steps: the elimination by hydrolysis of HCN/COS in a catalytic bed and desulfuration
and elimination of H 2 S from synthesis gas by chemical or physical absorption. This
last process is capable of achieving eliminations of up to 99%. To achieve the required
emission levels, a water wash is carried out in several phases to eliminate gaseous ni-
trogen compounds.
When considering a case of IGCC blown with oxygen, N 2 produced by the air sep-
aration unit has a double purpose: to increase the thermal efficiency of the plant and
reduce the NO x emissions produced by the high temperature of the flame produced
by gasifying with oxygen (Hasegawa, 2010).

6.3.2 Gas cleaning in IGCC with CCS

The gas cleaning section is extended by a CO shift unit when CCS is included in
IGCC. The possible sequences for H 2 S removal and CO 2 are shown in Fig. 6.11.
One option would be to keep the H 2 S and CO 2 removal completely separate by placing
the H 2 S removal system before the water gas shift reactor. Another option would be to
install a Selexol unit that could cocapture both CO 2 and H 2 S. This option eliminates
the need for the COS hydrolyzation unit because most COS is converted to H 2 Sin
the water-gas-shift reactor (Chiesa et al., 2005).
Another option would be to have two adjacent but separate Selexol units after the
water-gas-shift reactor; the first is designed to separate H 2 S and the second to target
CO 2 . To prevent CO 2 removal in the H 2 S Selexol unit, the solvent will have to be pre-
loaded with CO 2 in a previous step (EPRI, 2000).
The International Energy Agency ensures that the cocapture of H 2 S and CO 2 would
be able to increase the efficiency of an IGCC plant by up to 20% due to the savings that
would be produced by eliminating the sulfur recovery unit (IEA, 2003). On the other
hand, the presence of sulfur in its gaseous state makes the total gas volume greater,
besides making the use of more expensive special materials necessary to avoid