The development of alternative technologies for the removal of gas pollutants at the integrated gasification combined cycle is considered an important aspect for the environmental friendliness of energy production. During coal gasification, N2 contained in coal is converted to NH3. As much as 50% of the ammonia in the fuel gas can be converted to nitrogen oxides (NOX) in the gas turbine when the gas is combusted to produce power. The decomposition of NH3 before it enters to the gas turbine is an acceptable solution, but the high concentration of hydrogen that contained at the gasification stream inhibits ammonia decomposition. A sulfur tolerant catalyst is also needed for ammonia decomposition, as hydrogen sulfide is another component of the gasifier outlet. Gobina et al. reported the potential application of inorganic membranes to coal gas clean up processes. Ammonia decomposition in a membrane reactor was investigated in the literature primarily theoretically. The authors concluded that membranes with high hydrogen selectivities are necessary to achieve high NH3 conversions. Therefore, the researches were mainly focused in metallic membranes, because of their high hydrogen selectivity. Collins and Way used a composite palladium-ceramic membrane reactor and achieved conversions of over 94% at a temperature of 600°C and pressure of 16 bar.
In the present study the potential of using a packed bed catalytic membrane reactor for NH3 decomposition was examined. The catalysts in this type of reactor are in granular form and are placed inside the reactor. For this purpose, Ni/AI2O3 catalysts were prepared, characterized and tested. Initial catalytic activity screening tests were carried out at several conditions with a feed stream that contains 3000 ppm NH3 and 20% H2 in N2. Experiments at differential conversions were also carried out in order to develop the kinetic equations required. Results showed that NH3 decomposition reaction favored by higher temperature and residence time and lower pressure. Catalytic activity screening tests showed that by using Ni/Al2O3 catalysts conversions up to 88% can be achieved at temperatures higher than 750°C.
Two types of membrane materials were tested in terms of their separation
a) a microporous ceramic membranes with a silica top layer and
b) a dense Pd-Cu-Ag / V composite metallic membrane.
The permeabilities and permselectivities of typical coal gasification gases (CO2, N2, H2) in the membranes were determined by the variable volume method. As it was obtained, the Pd-Cu/V membrane is 100% hydrogen selective, while the silica membrane presents Knudsen diffusion characteristics, with relatively low selectivities.
In order to evaluate the performance of the catalytic membrane reactor with both membrane materials and to compare its behavior with a conventional reactor, a mathematical model was applied. The model is based on the differential equations describing reaction mass transfer and permeation phenomena inside the reactor. The catalytic membrane reactor modeling indicates that the conversion of NH3 increases with the use of any of these membranes, compared with a conventional PFR. However, in the case of the ceramic membrane reactor the decomposition of NH3 is limited by the permeation of NHj through the membrane in the permeate side. With the use of the metallic membrane, NH3 decomposition higher than 90% can be achieved and the permeate stream consists of 100% pure H2.