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Kinetic Study of Ammonia Decomposition on Tungsten Carbide for Cleaner Production of Hydrogen from Biomass Gasification

Sourabh S. Pansare, Walter Torres, and James G. Goodwin Jr. Department of Chemical and Biomolecular Engineering, Clemson University, 127 Earle Hall, Clemson, SC 29634-0909

 

Biomass gasification is one of the most flexible and energy efficient technologies for production of hydrogen, syngas, low-to-medium energy fuels and also for power generation. Presence of impurities such as tars, H2S, and NH3 pose major barriers in the utilization of the biomass gas. When H2 from this stream is fed to proton exchange membrane fuel cells, impurity NH3 molecules would block the active Pt sites of the anode as well as the acid sites of the Nafion membrane leading to poor performance. Moreover, when the gasification stream is used for power generation, undesirable conversion of NH3 into NOx through gas turbine combustion can occur with conversion level as high as 50%. Hence, NH3 needs to be decomposed to N2 and H2 at higher temperatures making NH3 decomposition an important reaction to be studied. Ru-based catalysts have proved to be particularly active and stable for NH3 decomposition, but are expensive. Other promising types of less expensive catalysts are carbides and nitrides of Mo, V, and W. Among carbide materials, tungsten carbide (WC) is of particular interest as it exhibits catalytic properties similar to those of platinum [1] as well as has extreme hardness and greater thermal stability. This paper reports on a kinetic study of NH3 decomposition on WC and its comparison with reaction on tungstated zirconia (WZ) and on commercially available NH3 synthesis catalyst (Amomax-10). The effect of the presence of H2 and CO, the two main components in syngas, on the behavior of WC is also reported. The catalysts were characterized by BET, XRD, SEM, EDX, and temperature programmed reaction (TPRx). The NH3 decomposition reaction was carried out at 1 atm with temperatures ranging from 475-800C. The concentration of NH3 in the inlet stream was 4000 ppm. The time-on-stream experiments showed that WC was characterized by an induction period which in general decreased in time with increase in temperature. Complete decomposition of NH3 was observed at 550C for the reaction conditions used. In the presence of syngas components (H2 and CO), higher temperatures (ca. 750C) were required for the complete decomposition of the NH3. The reasons for this behavior of WC in the presence and absence of syngas components as well as its power law dependence will be discussed. A comparison of reaction rate data for WC with that for WZ and for Amomax-10 will be presented. 1. Levy, R. B., Boudart, M., Science 181 (1973) 547.