(586e) Predicting Biomass Fuel Bound Nitrogen (FBN) Conversion in an Oxygen/Steam-Blown Fluidized Bed Gasifier

Karl M. Broer (kbroer@iastate.edu)

Robert C. Brown (rcbrown@iastate.edu)

When biomass is processed in a fluidized bed (FB) gasifier, its nitrogen content, also known as the fuel bound nitrogen (FBN), is converted primarily into ammonia (NH₃) and nitrogen gas (N₂). Smaller amounts are also present in the raw syngas as hydrogen cyanide (HCN), char-bound nitrogen, and tar-bound nitrogen. NH₃ is of the most interest because it is typically found in the syngas in high enough concentrations to form problematic amounts of NO_X when the syngas is combusted for integrated gasification-combined cycle applications. This NH₃ can also be responsible for poisoning catalysts in biomass-to-liquid applications. Chemical equilibrium calculations indicate that for typical FB biomass gasification temperatures, pressures, and equivalence ratios, nearly all FBN should be converted to harmless N₂. These results contradictexperimental results, which show significant NH₃ concentrations. More sophisticated biomass gasification models based on fundamental gasification kinetics have been developed that are capable of predicting NH₃ concentrations. The model developed by de Jong et al. [1] exhibits modest ability to accurately predict NH₃ concentration, but relatively poor ability to predict the concentrations of other important syngas constituents, such as CO and H₂. Additionally, the de Jong model was designed for steam-air gasification, whereas steam-oxygen blown gasification presents a new range of operating conditions that are characterized by much higher initial oxygen concentrations at the bottom of the fluidized bed, and different relative proportions of permanent gases in the resulting syngas.

A revised kinetic model involving fundamental homo- and heterogeneous combustion reactions has been generated to predict the final concentration of the nitrogen compounds in syngas, along with the other major constituents, such as H₂, CO, CO₂, and CH₄, for steam/oxygen-blown gasification. In addition to predicting the syngas composition, the model suggests which reaction pathways of nitrogen evolution are most important under FB gasification conditions. Knowledge of these major mechanisms allows insight into which methods may be worth further investigation for in situ reduction of NH₃ concentration in syngas in favor of N₂ and char-bound nitrogen, which are less problematic for downstream syngas applications.

1. de Jong, W., Unal, O., Andries, J., Hein, K., & Spliethoff, H. (2003). Biomass and fossil fuel conversion by pressurized fluidised bed gasification using hot gas ceramic filters as gas cleaning. Biomass and Bioenergy, 25, 59-83.