Catalytic hot gas cleaning with monoliths in biomass gasification in fluidized beds. 3. Their effectiveness for ammonia elimination

Biomasses with relatively high N contents generate a gasification gas with a NH3 content between 500 and 6000 ppm, with 2000 ppm of NH3 as the value that can be selected as a reference. This NH3 would generate NOx contents above the legally accepted limits; this is the reason why very often NH3 has to be removed from a gasification gas that also contains tar and particulates. The present paper only focuses on the performance of Ni-based monoliths for NH3 elimination from a realistic gasification gas coming from a bubbling-fluidized-bed biomass gasifier, at small pilot-plant scale. Besides using NH3 conversions, analysis of the results was also made using effective kinetic constants (k(eff)) not only for NH3 but also for the simultaneous and competitive tar removal reaction. Correlations were found between the k(eff,NH3) and k(eff,tar) values, included in effective Sherwood numbers, and the Reynolds and Schmidt numbers, the pitch of the channels of the monolith, and the temperature. The effect of the temperature is of the potential type, not Arrhenius, with an exponent of 2.75 +/- 1.75, because of the control by the external mass transfer. k(eff,NH3) is also somewhat smaller (0.8 +/- 0.3) than the equivalent one for tar elimination (k(eff,tar))Data on the deactivation of the monoliths, their causes, and their possible solutions are also provided. A final discussion on the poor performance of these monoliths in biomass gasification in a fluidized bed is also included in this paper.