Gasification of four biomass feedstocks (leucaena, sawdust, bagasse, and banagrass) with significantly different fuel-bound nitrogen (FBN) content was investigated to determine the effects of operational parameters and nitrogen content of biomass on the partitioning of FBN among nitrogenous gas species. Experiments were performed using a bench-scale, indirectly heated, fluidized-bed gasifier. Data were obtained over a range of temperatures and equivalence ratios representative of commercial biomass gasification processes. An assay of all major nitrogenous components in the gasification products was performed for the first time, providing a clear accounting of the evolution of FBN. Important findings of this research include the following:(1) NH 3 and N 2 are the dominant species evolved from fuel nitrogen during biomass gasification; >90% of FBN in feedstock is converted to NH 3 and N 2 ; (2) relative levels of NH 3 and N 2 are determined by thermochemical reactions in the gasifier; these reactions are affected strongly by temperature; (3) N 2 appears to be primarily produced through the conversion of NH 3 in the gas phase; (4) the structural formula and content of fuel nitrogen in biomass feedstock significantly affect the formation and evolution of nitrogen species during biomass gasification.
Experiments were performed to examine the structure of a chemically reacting, gas-phase, two-stream plane mixing layer. Temporally and spatially resolved measurements of streamwise velocity and of the concentrations of a reactant and product species and a conserved scalar were recorded across the mixing layer at streamwise locations between Redelta; = 730–2520. Non-reacting flow experiments were conducted to establish the entrainment and mixing characteristics of the layer. Reacting flow experiments were performed using dilute concentrations of the reactants, NO and O3, to ensure that the flow field remained isothermal. The probability density functions (p.d.f.s) and associated statistical quantities of the conserved and reactive scalars are compared with results from previous analytical and experimental studies. The data suggest, in concert with the Broadwell-Breidenthal model, that fluid in the mixing layer exists in three states: tongues of unmixed free-stream fluid which, on occasion, stretch across the layer; finite-thickness interfacial diffusion zones of mixed fluid which border the parcels of unmixed fluid; and regions comprising fluid of nearly homogeneous composition. The data also confirm previously reported asymmetry in entrainment rates from the two feed streams and show the important role of molecular diffusion in the mixing process. A fast-chemistry assumption, applied to predict reactive-species concentrations from the measured conserved-scalar p.d.f.s, overestimates the extent of reaction, indicating the importance of finite-rate chemistry for the present conditions. A Damköhler number, based on large-scale mixing times, is shown to be useful in determining the applicability of a fast-chemistry analysis to reacting mixing layers.
A better understanding of oil droplet formation, degradation, and dispersal in deep waters is needed to enhance prediction of the fate and transport of subsurface oil spills. This research evaluates the influence of initial droplet size and rates of biodegradation on the subsurface transport of oil droplets, specifically those from the Deepwater Horizon oil spill. A three-dimensional coupled model was employed with components that included analytical multiphase plume, hydrodynamic and Lagrangian models. Oil droplet biodegradation was simulated based on first order decay rates of alkanes. The initial diameter of droplets (10-300 μm) spanned a range of sizes expected from dispersant-treated oil. Results indicate that model predictions are sensitive to biodegradation processes, with depth distributions deepening by hundreds of meters, horizontal distributions decreasing by hundreds to thousands of kilometers, and mass decreasing by 92-99% when biodegradation is applied compared to simulations without biodegradation. In addition, there are two-to four-fold changes in the area of the seafloor contacted by oil droplets among scenarios with different biodegradation rates. The spatial distributions of hydrocarbons predicted by the model with biodegradation are similar to those observed in the sediment and water column, although the model predicts hydrocarbons to the northeast and east of the well where no observations were made. This study indicates that improvement in knowledge of droplet sizes and biodegradation processes is important for accurate prediction of subsurface oil spills.
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