Page 206 - New Trends In Coal Conversion

Technologies for control of sulfur and nitrogen compounds and particulates 169

NH 3 is removed by catalytic decomposition into nitrogen and hydrogen at high tem-

peratures under hot coal gas conditions (600e900 C). Several catalysts with various
metal compositions have been evaluated, including nickel, molybdenum, ruthenium,
calcium-iron, and manganese-supported catalysts. These catalysts were fairly effective

at 800 C, depending on the coal gas composition (Lim et al., 2017). It was found that
ruthenium is the most active among them. The cost of ruthenium is a major concern for
commercial use. Therefore, iron-based catalysts have been investigated for NH 3
decomposition. It has been stated that ferrous minerals are promising catalysts for
NH 3 , reporting decomposition rates of 85% (Durak-C ¸ etin et al., 2017). Sulfur tolerance,
steam resistance, and deterioration of the catalysts reduce catalyst activity, and it is
necessary to increase catalyst durability under high-temperature conditions (Hasegawa,
2010). The combined removal of H 2 S and NH 3 by sorbentecatalyst regenerative sys-
tems has been proposed to decrease the energy loss between the processes and initial
cost. The main issue is to keep a high NH 3 decomposition activity after several regen-

eration cycles operating at temperatures around 500e750 C. Recently, Co-Mo oxides
systems have shown good performance in contrast with more studied ZnO 2 systems due
to the lower regeneration capacity of the former (Lim et al., 2017).
The selective catalyst oxidation (SCO) of ammonia involves the controlled addition
of an oxidizer (O 2 , NO, NO/O 2 ) to the gasification gas to quantitatively and selectively
transform NH 3 into N 2 and H 2 O. The catalytic oxidation of ammonia usually occurs

between 400 and 600 C(Durak-C ¸ etin et al., 2017). The SCO have been successfully
tested under modeled syngas conditions but long-term tests under real syngas condi-
tions are still needed (Tuna and Brandin, 2013).
Selective oxidation, which involves the addition of oxygen and NO as oxidizers to
promote the decomposition of NH 3 , is unlikely to be developed for IGCC applications
because of the complexity of the gas-phase reaction chemistry at high temperatures

(>800 C). In contrast, Selective Catalytic Oxidation (SCO), which uses a catalyst
to increase the rate of NH 3 decomposition at significantly lower temperatures

(400e450 C), offers considerable potential for IGCC applications. To achieve high
levels of NH 3 decomposition, both oxidants must be present in combination with
the catalyst. Acid catalysts, such as alumina and aluminosilicate catalysts, have been
shown to adsorb NH 3 preferentially on the surface, with a resulting increase in the
rate of reactions of NO, O 2 , and NH 3 . At higher temperatures, NH 3 decomposition
to nitrogen is reduced as a result of the increased reactions of NO and oxygen with
carbon monoxide, hydrogen, and methane. The prospects of selective oxidation may
be increased by the development of highly selective oxidation catalysts, but ultimately,
a greater understanding of the reaction mechanisms and kinetics involved in selective
oxidation is required to minimize unwanted side reactions. As a stand-alone unit, SCO
appears to be the most promising approach as it operates within the current temperature
range for other gas-cleaning operations. However, selective oxidation is clearly not an
easy solution to the NH 3 problem, while low-temperature catalysts may improve the
prospects of SCO and catalytic decomposition.